Ohmic contacts to Gallium Nitride materials

Ohmic contacts to Gallium Nitride materials

Accepted Manuscript Title: Ohmic contacts to Gallium Nitride materials Author: Giuseppe Greco Ferdinando Iucolano Fabrizio Roccaforte PII: DOI: Refere...

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Accepted Manuscript Title: Ohmic contacts to Gallium Nitride materials Author: Giuseppe Greco Ferdinando Iucolano Fabrizio Roccaforte PII: DOI: Reference:

S0169-4332(16)30756-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.04.016 APSUSC 33017

To appear in:

APSUSC

Received date: Revised date: Accepted date:

22-1-2016 2-4-2016 4-4-2016

Please cite this article as: Giuseppe Greco, Ferdinando Iucolano, Fabrizio Roccaforte, Ohmic contacts to Gallium Nitride materials, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.04.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ohmic contacts to Gallium Nitride materials Giuseppe Greco 1, Ferdinando Iucolano 2 and Fabrizio Roccaforte 1 , * 1

Consiglio Nazionale delle Ricerche – Istituto per la Microelettronica e Microsistemi (CNR-IMM), Strada VIII, n. 5 – Zona Industriale, 95121 Catania, Italy 2

STMicroelectronics, Stradale Primosole 50, 95121 Catania, Italy *

Corresponding author: [email protected]

Graphical abstract

Highlights: 

Ohmic contacts are a crucial issue for wide band gap semiconductors devices



This paper reviews the mechanisms of Ohmic contact formation on GaN-based materials



Ti/Al-based contacts and multilayers (Ti/Al/X/Au) are used for n-type GaN



Ni/Au-based bilayers are used for Ohmic contacts to p-type GaN



Several parameters affect Ohmic contact formation to AlGaN/GaN heterostructures



“Au-free” contacts are important for the integration of GaN technology on Si-fabs

1

Abstract In this review article, a comprehensive study of the mechanisms of Ohmic contact formation on GaNbased materials is presented. After a brief introduction on the physics of Ohmic contacts, a resume of the most important results obtained in literature is reported for each of the systems taken in consideration (n-type GaN, p-type GaN and AlGaN/GaN heterostructures). The optimal metallization schemes and processing conditions to obtain low resistance Ohmic contacts are presented, discussing the role of the single metals composing the stack and the modification induced by the thermal annealing, either on the metal layers or at the interface with GaN. Physical insights on the mechanism of Ohmic contact formation have been gained by correlating the temperature dependence of the electrical parameters with a morphological/structural analysis of the interface. In the case of the AlGaN/GaN systems, the influence of the heterostructure parameters on the Ohmic contacts has been taken into account adapting the classical thermionic field emission model to the presence of the two dimensional electron gas (2DEG). Finally, the state of the art of “Au-free” metallization to AlGaN/GaN heterostructures is also presented, being this latter an relevant topic for the integration of GaN technology on large scale Silicon devices fabs. Keywords: n-GaN, p-GaN, AlGaN/GaN heterostructures, Ohmic contacts P.A.C.S.: 73.40.Cg; 73.40.-c; 73.40.Ns

TABLE OF CONTENTS I. Introduction ................................................................................................................................. 2 II. Basic physics of Ohmic contacts................................................................................................. 4 III. n-type GaN................................................................................................................................ 7 IV. p-type GaN ............................................................................................................................. 16 V. AlGaN/GaN heterostructures.................................................................................................... 24 VI. Au-free Ohmic contacts to AlGaN/GaN heterostructures......................................................... 37 VII. Summary and Outlook ........................................................................................................... 42

I. Introduction Nowadays, wide band gap (WBG) semiconductors, like Silicon Carbide (SiC) and Gallium Nitride (GaN), are considered the most promising materials for the next generation of power electronic devices. In fact, their extraordinary physical properties, such as a wide band gap ( > 3 eV), a high critical electric field (2-4 MV/cm) and a low intrinsic carrier concentration (10-9-10-10 cm-3), give the possibility to fabricate devices operating at much higher voltages, temperatures and frequencies than in Silicon. In addition, devices based on WBG materials enable a significant reduction of the power losses and an increase of the energy efficiency. 2

While SiC already possesses a considerable maturity with respect to the present material quality and device performances, GaN materials and devices still suffer of limited performances and reliability issues. In fact, the large lattice and thermal expansion coefficient mismatch with the substrates used for GaN growth (Al2O3, SiC, Si…) generates a high defects density (mostly dislocations), which represent a limiting factor for the full exploitation of the electric field strength of the material in power devices. A peculiar aspect of GaN-based materials is the possibility to grow AlGaN/GaN heterostructures, generating a two dimensional electron gas (2DEG) at the interface by the piezoelectric polarization gradient. Thanks to the presence of the 2DEG, high electron mobility transistors (HEMTs) based on GaN can provide a low channel resistance with respect to Si and SiC devices. In this context, Ohmic contacts are fundamental building blocks of power devices. In particular, since Ohmic contacts provide the link from the device to the external circuitry and vice-versa, their resistance must be negligible with respect to that of the semiconductor drift layer, in order to minimize the device specific on-resistance (Ron) and, hence, the power losses of the system. Typically, electronic devices based on WBG semiconductors require specific contact resistance (c) values in the range 10-5-10-6 cm2. However, obtaining good Ohmic contacts in these materials is inherently difficult, due to the wide band gap (3.4 eV for GaN) which typically leads to Schottky barrier height values in the order of 1 eV on n-type and even of 2 eV on p-type material. Even more critical can be the situation in AlxGa1-xN alloys, where the band gap increases with increasing the Al content. This review paper summarizes significant results achieved in the last two decades on Ohmic contacts to GaN-based materials and devices. After a brief introduction on some fundamental concepts on metal/semiconductor junctions, the most common schemes for Ohmic contacts to GaN will be discussed. Particular emphasis will be given to alloyed Ti/Al/Ni/Au and Ni/Au stacks, for ntype and p-type GaN, respectively. The mechanisms of current transport at the metal/GaN interface will be discussed considering the temperature dependence of the specific contact resistance and correlated with the microstructure of the interface and of the reacted metal stack. In the second part of the paper, the common approaches for Ohmic contacts to AlGaN/GaN heterostructures (i.e., for source-drain in HEMT devices) will be presented. Besides that, the impact of the heterostructure properties (like the thickness of the AlGaN barrier, the 2DEG sheet carrier density, or the Al concentration) on the specific contact resistance is discussed by means of a semiempirical analytical description, derived modifying the thermionic field emission model. In the final section, “Au-free” Ohmic contacts will be also discussed, as they represent an important requirement for the integration of GaN technology into Si devices fabs.

3

II. Basic physics of Ohmic contacts An Ohmic contact is a metal/semiconductor contact in which the potential barrier at the interface is absent or transparent for carrier tunnelling. Ohmic contacts exhibit linear current-voltage (I-V) characteristics and are fundamental building blocks of any semiconductor device, providing the signal transfer from the devices to the external circuitry or vice versa. The contact resistance is an important parameter characterizing the metal/semiconductor interfaces. It is the total resistance of the metal/semiconductor junction (expressed in ) and, hence, it depends on the area of the contact. A more useful physical parameter describing the performance of Ohmic contacts is the specific contact resistance c, which is independent of the contact geometry, and is typically expressed in cm2. The relation between the contact resistance and the specific contact resistance in a metal/semiconductor contact can be viewed analogously to that between the resistance and the resistivity in a resistor. Often, in the presence of lateral devices (as in the case of GaN-based HEMTs), the parameter used to described the performance of the Ohmic contacts is the so called contact resistance RC, which is referred to the contact length and, hence, is expressed in mm. In general, the specific contact resistance c can be defined as [1,2]: 1

 J  c     V V 0

Eq. 1

where J is the current density and V is the applied bias. In Ohmic contacts, the specific contact resistance c depends on the metal/semiconductor Schottky barrier height B and on the doping concentration of the semiconductor. For a n-type semiconductor, according to the Schottky-Mott theory, the metal/semiconductor Schottky barrier height B obeys to the relation:

q B  q m   s

Eq. 2

where m the work function of the metal, and χs is the electron affinity of the semiconductor. Analogously, for a p-type semiconductor, the Schottky barrier height B can be written as: q B  E g  q m   s 

4

Eq. 3

Let us consider, as an example, the case of a n-type semiconductor. Depending on the semiconductor doping level ND, different mechanisms of carrier transport at the interface can occur, owing to the different thickness of the space charge region W formed at the metal/semiconductor interface (see schematic illustration in Fig.1).

Low ND (<

Intermediate ND

11017 cm-3)

(11017 ÷

Thermionic Emission

High ND

11019 cm-3)

(> 11019 cm-3)

Thermionic Field Emission

metal

metal semiconductor

Field Emission

metal semiconductor

a)

b)

semiconductor

c)

Fig.1:Schematic illustration of the different carrier transport mechanisms through a metal/semiconductor barrier for different doping levels ND.

Consequently, the specific contact resistance c will have a different dependence on the barrier height

B , on the carrier concentration ND and also on the temperature T. In general, for lightly doped semiconductors (ND<11017 cm-3), the thermionic emission (TE) dominates the current transport and the specific contact resistance can be described by the relation:

c 

k  q B  exp  ** qA T  kT 

Eq. 4

where k is the Boltzmann constant, A** is the Richardson constant, and q is the elementary charge. Under this condition, c is independent of the doping ND and, due to the fact that the space charge region W is sufficiently thick, the carriers must be thermally excited over the barrier B to flow through the interface. For intermediate doping levels, generally in the range of 11017
5

 c  exp

B E  E00 coth 00   kT 

Eq. 5

where E00 is a characteristics energy defined as [3]:

E 00 

qh 4

ND  0 m*

Eq. 6

being ε0 the vacuum permittivity, ε and m* the permittivity and the electron effective mass of the considered semiconductor, respectively. Finally, for heavily doped semiconductors (ND>11019 cm-3), the field emission (FE) is the dominant mechanism, i.e., the barrier becomes enough narrow to allow a direct tunnel of electrons through the interface. In this case, the specific contact resistance c is proportional to an exponential of B divided by E00, that can be written as:    c  exp B  N D 

   

Eq. 7

As can be seen in Eq 7, in the FE mechanism the specific contact resistance of tunnel Ohmic contacts is strongly dependent both on the Schottky barrier B and on the semiconductor doping ND level. For fixed values of the barrier height B and carrier concentration ND, the temperature T determines the carrier transport mechanism through the metal/semiconductor interface. In particular, when the thermal energy kT is much lower compared with the characteristic energy E00, the tunnelling of the carriers by FE mechanism is predominant. At higher temperatures, i.e., when the thermal energy kT is comparable with E00 (kT ≈ E00), the carriers can pass through the barrier by TFE mechanism. Finally, at much higher temperatures, i.e., under the conditions kT >> E00, thermionic emission over the barrier becomes relevant for the current transport. According to all the above considerations, metals with a low Schottky barrier height are recommended to obtain an Ohmic contact with a low specific contact resistance c. Fig. 2 reports a survey of experimental literature results of the Schottky barrier height B values as a function of the metal work function m for different metals on both n-type and p-type GaN,.

6

Schottky Barrier Height, B (eV)

3.0

n-GaN p-GaN

Al Ti

2.5

W

2.0

Au

Ni Pd

1.5

Pt

1.0 Cr

0.5

Ag

Pt

Ni Pd Au

Ti Al

0.0 4.0

4.5

5.0

5.5

6.0

Metal Work function, m (eV) Fig. 2: Survey of literature results of Schottky barrier height B values as a function of the metal work function m for different metals on both n-type and p-type GaN. Data are taken from Ref. [4,5] for n-type GaN and Ref. [6,7,8] for p-type-GaN.

As can be seen, Titanium (Ti) and Aluminium (Al) allow to obtain experimental values of Schottky barrier height in the range of 0.4-0.5 eV. Hence, these metals are in principle the materials of choice for Ohmic contacts to n-type GaN. On the other hand, in the case of p-GaN, as can be also deduced from Eq. 2, it is inherently more difficult to obtain such low values of B, since this would require metals with work function higher than 6 eV [9]. As can be seen in Fig. 2, for p-type GaN, the most suitable metals for Ohmic contacts should be Ni, Au, Pt, Pd, resulting in a Schottky barrier height around 1.6-1.9 eV. As a matter of fact, in the next sections, it will be shown that the most important findings on Ohmic contacts to n-type and p-type GaN have been reported for Ti- and Ni- based systems, respectively.

III. n-type GaN Several alternatives have been proposed to fabricate Ohmic contacts to n-type GaN. Table I reports a survey of literature results obtained for different metal schemes and annealing conditions. Evidently, the reported values of the specific contact resistance ρc can vary in a wide range (10-2 - 10-7 Ωcm2) and depend on several parameters, such us the metal work function and thickness of the metal layers, the doping concentration of the substrate ND, the annealing conditions (temperature, atmosphere, time, etc.).

7

Metal layers Ti/Al (20/100 nm)

-3

Annealing Conditions

Doping, ND (cm ) 1×10

17

Al (150nm) Ti/Al (15/115 nm) Ti/Al/Ti/Au (30/100/30/30 nm) Ti/Al/Mo/Au (15/60/35/50 nm) Ti/Al/TiAl3 (35/115/50 nm) Ti/Au/Pd/Au (20/60/40/50 nm) Ti/Al/Ti/Au (30/100/30/30 nm) Ti/Au (20/200 nm) Ti/Al/Au (20/20/200 nm) Ti/Al/Ni/Au (20/20/20/200 nm) Ti/W/Au (12/20/50 nm) Ti/Al/Re/Au (15/60/20/50 nm) Ta/Ti/Ni/Au (5/50/20/15 nm) Ti/Al/Ti/Au (30/100/30/30 nm) Ti/Al/Ti/W/Au (30/100/30/30/30 nm) Ti/Al/Mo/Au (15/60/35/50 nm) ITO (Indium Tin Oxide) (110 nm) Ti/Al/Ni/Au (15/200/50/50 nm) Ti/Al/ZrB2/Ti/Au (20/100/50/20/80 nm) Ti/Al/Ti/Au (20/50/20/250 nm) Cr/Au (50/250 nm) Cr/Pt/Au (50/20/250 nm) V/Al/V/Au (15/80/20/200 nm) Ti/Al (35/115 nm) Hf/Al/Ni/Au (20/100/25/50 nm) Ti/Al/Ni/Au (20/100/25/50 nm)

-6

Ref. 10

900 °C for 30 sec in N2

8×10

not annealed

2.2×10 -3 3.0×10 -5 6.5×10

17

Ti/Ag (15/150 nm)

2

ρc (cm )

-2

1.5×10 18 1.0×10 19 1.7×10

11

7×10

17

600 °C for 15 sec in Ar

5×10

-6

12

5×10

16

600 °C for 15 sec

2×10

-5

13

750 °C for 30 sec

6×10

-7

14

18

850 °C for 30 sec in N2

4.7×10

-7

15

18

700 °C for 60 sec in Ar

2.1×10

-5

16

1×1018

800 °C for 30 sec in air

3.2×10

-7

17

800 °C for 30 sec

3×10

-6

-3

1.4×10 1 ×10 3×10

5×10

20

17

18

2.2×10

18

700 °C for 10 min in N2

1×10

2.2×10

18

800 °C for 10 min in N2

7.86×10

-5

2.2×10

18

800 °C for 30 sec in N2

9.65×10

-7

19

4.07 ×10

18

900 °C for 60 sec in N2

8.4×10

-6

20

4.07 ×10

18

750 °C for 60 sec in N2

1.3×10

-6

21

900 °C for 45 sec in Ar

5×10

750 °C for 30 sec

5.8×10

-6

750 °C for 30 sec

5.0×10

-6

16

850 °C for 30 sec in N2

4.7×10

-7

24

2×1017

600 °C for 15 min in N2

2.8×10

-6

25

2×1018

750 °C for 60 sec in Ar

1.1×10

-5

26

2×1018

700 °C for 60 sec in N2

3×10

800 °C for 5 min in N2

4.2×10

-5

400 °C for 5 min in N2

2.9×10

-5

400 °C For 5 min in N2

1.4×10

-4

650 °C for 30 sec in N2

2.2×10

-6

700 °C in Ar 700 °C in Ar (with cap layer)

1.1×10

-5

4.2×10

-6

650 °C for 60 sec in vacuum

1×10

-6

9×10

-7

5×10

17

9.2×10

5×10

17

1×1020 20

3.5×10

22

23

1×1018

1×10

-6

-6

27

28

29

30

18

31

850 °C for 60 sec in vacuum 8

Ti/Al/Ti/Au (20/100/45/55 nm)

900 °C for 30 sec in N2 900 °C for 30 sec in N2 (pre-treatment at 400°C for 1 min in O2)

3×1018

2.1×10

-5

1.5×10

-5

32

Table I: Survey of literature data on Ohmic contacts to n-type GaN.

The first studies on Ohmic contacts to n-type doped GaN have been focused on metals with a low work function, such as Ti (4.33 eV [33]) or Al (4.28 eV [33]), and were carried out on very highly doped materials (with ND > 1018 cm-3) [34,35]. In addition, beside being itself a low work function metal, Ti is also extremely reactive with GaN upon annealing, and it can form low work function TiN compounds at the interface. Nevertheless, single metal layers, like Ti or Al, are typically not suitable to form low resistance Ohmic contacts to n-type GaN. In fact, their high propensity to oxidation makes these simple systems unsuitable, especially in power devices applications where high temperatures operation is required [5]. Moreover, the use of Ti alone can also lead to the formation of voids at the interface upon annealing, resulting into a bad mechanical contact [36,37]. Hence, the preferred alternative is using Ti/Al bilayers, where the top Al layer allows to form stable low work function phases with the underlying Ti layer upon annealing. The phases formed upon annealing of

2

c (cm )

Ti/Al bilayer are also more resistant to the oxidation.

10

-1

10

-3

10

-5

10

-7

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

0

200

400

600

[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

800

Annealing Temperature (°C) Fig. 3: Representative plot of the general trend of the specific contact resistance c as function of the annealing temperature T for Ohmic contacts to n-type GaN. The optimal annealing temperature range is marked by the dashed region. The data are taken from Table I.

Fig. 3 shows a selection of experimental ρc data as a function of the annealing temperature T, taken from Table I. This plot is representative of the general trend observed in most of the metal schemes on n-type GaN, i.e., the specific contact resistance c is quite high for as-deposited contacts (> 10-3 Ω cm2) and decreases with increasing annealing temperatures down to 10-6 Ω cm2 and below. Indeed, 9

few authors reported on the possibility to obtain a reasonable Ohmic contact without any thermal annealing, and only for high doping levels [11,38]. Ohmic contact achieved at low annealing temperatures (400°C) have been investigated by Lee et al. [28] using Cr-based contact. Although their proposed solution did not show values of ρc lower than 310-5 Ω cm2, it enabled to improve the thermal stability and the surface morphology. In general, although a low specific contact resistance can be obtained after annealing of Ti/Al bilayers [10], the ease of oxygen contamination during annealing can lead to an increase of the resistivity of these metal upon annealing at high temperatures. Moreover, the upper Al layer tends to ball-up during annealing, thus increasing the roughness of the contact surface [5]. Hence, to avoid these problems, more complex multilayers have been considered at higher annealing temperatures. As an example, multilayers employing V [29,39,40] or Hf [31] as low work function metals in contact with the GaN demonstrated an Ohmic behaviour after annealing at 650°C. In the annealing temperature range 600-700°C, also other Ti-based schemes, such as Ti/Al, Ti/Al/TiAl3 or TiN/Ti/Al [13,16,30, 41,42,43,44] were able to give low values of ρc (between 2.8 10-6 and 2.110-5 Ωcm2 ). In general, as can be seen in Fig. 3 the optimal annealing temperature range to achieve values of c below 10-5 cm2 is between 650 and 850°C (dashed region in Fig. 3) . A further increase of the annealing temperatures (850-950°C) leads to slight increase of the values of c. It is worth noting that most of the data reported in the literature (Table I) refer to multilayers in which a capping layer is introduced over a Ti/Al stack (Ti/Al/Ni/Au, Ti/Al/Ti/Au, Ti/Al/Pd/Au, Ti/Al/Mo/Au,….) [14,15,16,17,18,19,21,22,23,24,26,27,32,41,45,46]. A schematic of these metallization is reported in Fig. 4.

Au

Cap layer Barrier layer

Ni, Ti, Pt, Pd, Mo, Re, Ir

Al

Overlayer

Ti, Ta

Contact layer

n-GaN Fig. 4: Schematic of the multilayer typically used to fabricate Ohmic contact to n-GaN.

10

Mohammad [47] discussed the role of the single metal layers in these complex systems, proposing some guidelines for the choice of stack to obtain a low resistance Ohmic contact after annealing. The contact layer must have a low work function and a negligible resistance. Moreover, it should limit the diffusion of the upper metals onto the GaN surface. Generally, refractory metals (Ti,Ta,…) are used as first metal layer, since the reaction with GaN and the formation of nitrides compounds (TiN, TaN,…) is thermodynamically favoured [48]. The formation of TiN leads several important advantages. In fact, the out-diffusion of N-atoms from GaN leads to the formation of nitrogen vacancies that act as donors and increase the net carriers concentration below the interface. In addition, upon annealing a Ti-based contact has the advantage to reduce the possible native oxide (Ga2O3) formed at the GaN surface [49,18]. The second metal layer, referred as overlayer, should be able to form low work function compounds with the other metals. Aluminium (Al) is normally used, as it well satisfies this requirement. The third layer (Ni, Ti, Pt, Pd, Mo, Re or Ir) is referred as barrier layer. It has generally a high melting point (> 1400°C), in order to stabilize the multilayer during annealing, limiting the inter-diffusion of the first two metal layers, with the upper fourth layer. Mohammed et al. [50] suggested that the barrier layer is not a mere diffusion barrier, but it also plays a role in the reaction between the metals, influencing the specific resistance and the surface morphology of the contact. Finally, the fourth metal layer, refereed as cap layer, acts as protective layer to minimize or prevent the oxidation of the underlying metals. Gold (Au) is commonly used for this purpose. It must be pointed out that also this metallization scheme typically obeys to the general trend of a decrease of ρc with increasing annealing temperature, with a minimal contact resistance achieved at approximately 850°C. As reported in section II, the specific contact resistance c is expected to be dependent also on the doping concentration ND of GaN layer. Fig. 5 reports the values of specific contact resistance c as function of doping concentration ND, both for as deposited contacts (non-annealed) and annealed contacts. In particular, Ti/Al based layers annealed in a temperature range between 700-850°C have been considered on this graph. Clearly, as already highlighted in Fig. 3, the thermal annealing induces a significant lowering of the c values. However, while for as deposited contacts the values of c strongly decrease with increasing ND, no clear correlation is observed in the annealed contacts for the typical doping levels (in the range 51017-51018 cm-3). This behaviour can be ascribed to the thermal reactions occurring after annealing, which modify not only the metal/GaN barrier height but also the electrical properties of the GaN substrate in the proximity of the interface.

11

As deposited Annealed

-1

2

c (cm )

10

-3

10

-5

10

-7

10 17 10

10

18

19

10 -3

Doping, ND (cm )

Fig. 5: Specific contact resistance c as a function of the doping concentration ND for both as deposited (nonannelaed) and annealed (700-850°C) Ti/Al-based layers Ohmic contacts to n-type GaN. The data are taken from Table I.

In order to gain further insights into this topic, it is useful to monitor the modifications of the metal/GaN interfacial region during annealing and the temperature dependence of the electrical properties. Iucolano et al. [26] studied the electrical evolution of the Ti/Al/Ni/Au multilayer with the annealing temperature. The electrical characterization of the contact resistance was performed by means of current-voltage (I-V) measurements using TLM structures. As can be seen from the I-V characteristics reported in Fig. 6a, the as deposited sample exhibits a non-Ohmic behaviour, while a gradual improvement of the electrical properties is observed with increasing annealing temperatures. Such an improvement is associated to a reduction of c, as shown in Fig. 6b. The best result has been obtained at the annealing temperature of 750 °C, while a further increase of the annealing temperature resulted in a small increase of c.

100 50

2

Current (mA)

150

-3

10

As deposited 600 °C 650 °C 700 °C 750 °C 800 °C 850 °C

c (cm )

200

0 -50

-4

10

-100 -150 -200 -2

-5

10

a) -1

0

1

2

Voltage (V)

b) 600

700

800

900

Annealing Temperature (°C)

12

Fig. 6: (a) Current-voltage (I-V) characteristics measured between adjacent TLM pads in Ti/Al/Ni/Au contact to n-type GaN at different annealing temperatures. (b) Specific contact resistance c as a function of the annealing temperature. The data are taken from Ref. [26].

This electrical evolution occurring with increasing annealing temperatures is accompanied by morphological and structural changes of the Ti/Al/Ni/Au multilayer. Fig. 7 shows the AFM images of the contact surface before and after annealing. In particular, the surface roughness (RMS) of the as deposited contact is 3.1 nm, i.e., very similar to that of the underlying GaN material used for the experiments (2.6 nm). However, this value significantly increases after the annealing treatments, to 40.6 and 46.6 nm, after annealing at 600 °C and 800 °C, respectively. The results of these morphological analyses, carried out on smaller areas, suggested that the average grain size of the annealed metal contact increases with increasing annealing temperature.

a)

2 nm

As dep 20 nm b)

600 °C 200 nm

2 nm

c)

800 °C 200 nm

2 nm

Fig. 7: Evolution of the surface morphology of Ti/Al/Ni/Au contacts with the annealing temperature: as deposited contact (a) and contacts annealed at 600°C (b) and 800 °C (c).

To describe the evolution of the electrical behaviour with the annealing temperature, several possible explanations have been reported in literature. In some papers on Ti-based contacts [12,17,18,20,24,26,43,45,51,52] the decrease of c upon annealing was attributed to the formation of an epitaxial TiN layer at the interface, indicating the lower Schottky barrier height of TiN, with respects to Ti, as the key factor for the reduction of c. This hypothesis has been confirmed by Jeon et al. [53] who compared TiN/Al and Ti/Al Ohmic contacts on the same n-type GaN layer without any annealing treatment. Indeed, the better electrical behaviour of TiN/Al contacts was attributed to the lower barrier height formed at interface TiN/n-GaN. Conversely, other authors reported that the reduction of c cannot be attributed to the lower barrier height of an interfacial TiN layer. As an example, Lin et al. [10] observed that the Schottky barrier height of Ti on GaN increases after annealing at 700°C (where an Ohmic contact is formed) with respect to the value measured in the non-annealed contact (where still a non-Ohmic behaviour is detected). Therefore, they concluded that 13

the Ohmic contact formation is due to an increase of the carrier concentration of the semiconductor below the interface. Luther et al. [12] confirmed that the formation of TiN is necessary to achieve an Ohmic contact, demonstrating that the reaction between GaN and Ti accompanies the reduction of

c. Several authors [18,26,43,45,51,52,1010] reported that the reaction between GaN and Ti during annealing induces the formation of N vacancies in GaN, favouring an Ohmic contact behavior. In fact, N vacancies act as donors in n-type GaN [18,26,43,45,51,52,1010], increasing the net carrier concentration in the proximity of the interface and resulting in a thinner barrier for electron tunnelling. This interpretation is very often invoked to explain the Ohmic behaviour of Ti-based contacts to nGaN. In this context, Motayed et al. [22] pointed out that only 2 mono-layers of TiN should be able to generate 100 nm of heavily doped GaN at the interface, with a doping concentration around 1020 cm-3. However, the occurrence of such a strong near-interface modification of the substrate was not demonstrated by direct experimental analytical techniques. In our case, interesting information have been obtained correlating the temperature dependence of c with the structural analysis of Ti/Al/Ni/Au Ohmic contacts [26]. The analyses were performed on two relevant samples, annealed at 600 °C (just below the onset of Ohmic behaviour) and at 800 °C (Ohmic behaviour with low c). Fig. 8 reports the temperature dependence of c for Ti/Al/Ni/Au contacts to n-type GaN annealed at 600°C and 800°C. Evidently, although in both samples the specific contact resistance c decreases with increasing the measurement temperature, different mechanisms of current transport have been identified. -3

600 °C 800 °C TFE (B=1.21 eV)

2

c (cm )

10

10

-4

FE (B=0.81 eV)

10

-5

300 350 400 450 Measurement Temperature (K) Fig. 8: Temperature dependence of the specific contact resistance c of Ti/Al/Ni/Au contacts to n-GaN for two different annealing temperatures, 600 °C (a) and 800 °C (b), reported by Iucolano et al. [26]. The fits of the experimental data with the TFE and FE models are also reported as continuous lines.

14

In fact, for the sample annealed at 600 °C, the experimental data can be described by a TFE mechanism, while in the sample annealed at 800 °C the FE mechanism better fits the temperature dependence of c. From these fits (reported in Fig. 8), the authors determined both the Schottky barrier height B and the net doping concentration ND below the interface. In particular, a reduction of the Schottky barrier from 1.21 eV after annealing at 600 °C down to 0.81 eV at 800 °C was observed, accompanied by an increase of the carrier concentration, i.e., from 21018 cm−3 in the as-prepared sample to 4.61019 cm−3 in the annealed contacts. Hence, these experimental measurements let us to conclude that the improvement of the specific contact resistance c is due to to the combined effect of a reduction of the barrier height and an increase of the carrier concentration, enhancing the tunnelling of the carriers though the thinner barrier (FE mechanism). A similar result has been found by Fontsere et al. [54] for Ti/Al Ohmic contacts (without the Ni/Au cap layer) to n-type implanted GaN. In their work, a combination of FE and TFE has been assumed to describe the temperature dependence of ρc [54]. The increase of the carrier concentration upon annealing deduced by the modelling of the temperature dependence of c is also consistent with the results obtained from cross section TEM analyses. Fig. 9 shows the cross section TEM images of the samples annealed at 600°C and 800°C. Clearly, an intermixing of the metal layers occurred already at 600 °C, with the presence of a 4 nm non-uniform TiN epitaxial layer at the interface. On the other hand, this interfacial TiN layer becomes thicker (~ 9 nm) after annealing at 800 °C, and the presence of TiN intrusions penetrating inside the GaN is observed in some regions at this annealing temperature. Moreover, a uniform layer of Al-AuTi is observed over the formed TiN. A local chemical analysis showed that this layer mainly contains Au and Al atoms, while Ti atoms are only present in a small amount, thus indicating that the interfacial Ti has been almost completely consumed for the formation of TiN.

Fig. 9: TEM cross section of Ti/Al/Ni/Au contacts to n-GaN annealed at 600 and 800 °C.

15

Although the observed temperature dependence of the specific contact resistance c is quite common in n-type GaN, in recent years some works [55,56,57] reported an anomalous behaviour of ρc that cannot be explained by the conventional current transport mechanisms, like TFE or FE. As an example, an increase of ρc with increasing temperature has been observed in In-n-GaP and In-n-GaN materials with a high dislocations density [55,56,57]. Blank et al. [57] proposed an alternative approach to describe the unusual behavior of Ohmic contacts in defective materials, considering the space charge region at metal/semiconductor interface as shunted by metal shunt resistances formed, as an example, by the preferential diffusion of metal atoms inside the dislocations. Under these conditions, since the metal resistance increases linearly with temperature above the Debye temperature, a linear temperature dependence of the contact resistance is expected. Later, Sachenko et al. [58,59,60] refined the model considering also a diffusion mechanism in order to explain the temperature behavior of c in metal/GaN (or metal/GaP) at low temperatures and for different doping levels of the semiconductor. In the section V, it will be shown that a similar “metal-like” behavior of c can occur in the case of Ohmic contacts to AlGaN/GaN heterostructures grown on silicon substrates, i.e., where a high density of treading dislocations are present in the material. Besides its low work function, another advantage attributed to the use of Ti as contact layer is the reduction of the native oxide upon annealing at high temperature, that favours the Ohmic contact formation [22]. In fact, GaN surfaces are particularly sensitive to the oxygen adsorption [61]. Hence, using Ti-based alloyed contacts can reduce the thin oxide layer originally present at the surface of GaN, dissolving small amounts of oxygen while maintaining a stable α-Ti phase with oxygen in solid solution [22]. Alternatively, other authors followed the approach of specific surface cleanings, such us wet etched in KOH solutions or aqua regia [62], or physical plasma etch in SiCl4 [63] or Cl2/H2 [64], to reduce the specific contact resistance. However, the possibility to use such a surface treatments prior to metal deposition strongly depends on their possible integration in the fabrication process of the device. For that reason, the formation of Ti/Al/Ni/Au contacts by lift-off techniques, carrying out the surface cleaning with gentle HF-based wet etches, remains the easiest and most common way to form Ohmic contacts to heavily doped n-type GaN substrates.

IV. p-type GaN There are some inherent physical difficulties in the formation of good Ohmic contacts to p-type GaN. Firstly, as pointed out in section II, it is difficult to obtain a low metal/p-GaN barrier height B, due to the wide bang gap of the material (3.4 eV), and to the typical values of the metal work functions 16

(in the range of 4 – 5 eV). Secondly, another important issue is related to the difficulty to achieve a high free holes concentration in p-type doped GaN. Magnesium (Mg) is the most efficient p-type dopant specie for GaN. Although significant progresses on the p-type doping techniques and electrical activation have been reported by Amano and Nakamura [65,66,67], the high ionization energy of the Mg-acceptors (in the range 150-200 meV), which depends on the active acceptor concentration N A [68]), strongly limit the free holes concentration in p-type GaN at room temperature [69,70]. Moreover, the formation of Mg-H complexes [71] further reduces the free holes concentration. Hence, while a 19

high Mg concentration (> 10

cm-3) can be incorporated during p-GaN growth, the free holes

concentration typically achieved in the material is in the range 1017 – 1018 cm-3. To partially overcome the above issues, the use of alloyed metallic compounds and specific annealing conditions are adopted to reduce the effective metal/p-GaN barrier height and locally increase the active carrier concentration below the interface. Table II summarizes the most relevant results reported in literature for Ohmic contacts to p-type GaN, obtained with different metal schemes and annealing conditions.

Metal layers Ni/Au (10/5 nm) Ni/Au (10/40 nm) Ta/Ti (60/40 nm) Pt/Ni/Au (20/50 nm) Ni/Pd/Au (20/20/100 nm) Ti/Pt/Au (5/8/4 nm) Ni/Au (20/20 nm) Ni/Au (20/50 nm) Ni/ITO (10/250 nm) Ru (10 nm) Ir (10 nm) Ru/Ni (5/5 nm) Ir/Ni (5/5 nm) Ni/Au (5/5 nm) Au/Ni/Au

-3

2

Doping, p (cm )

Annealing Conditions

ρc (cm )

21017 cm-3

400 °C for 10 min in Air

3×10

500 °C for 5 min in N2 at 500 °C for 5 min in N2/O2 800 °C for 20 min in vacuum

7×10 -3 2×10 3.2×10

-5

74

31017 cm-3

350 °C for 1 min in N2

5.1×10

-4

75

4.11017 cm-3

550 °C for 1 min in air 550 °C for 1 min in N2 550 °C for 1 min in O2

1.1×10 -3 1.5×10 -4 1.0×10

31017 cm-3

800 °C for 2 min in N2

4.2×10

1.21017 cm-3

500 °C for 5 min in air

2×10

1.21017 cm-3

600 °C for 30 sec in N2

6.1×10

-4

79

11018 cm-3

600 °C 2 for min in air

8.6×10

-4

80

500 °C for 1 min in N2 500 °C for 1 min in O2 500 °C for 1 min in N2 500 °C for 1 min in O2

1.3×10 -2 4.1×10 -4 7.1×10 -2 5.1×10

500 °C for 1 min in O2

4.5×10

-5

500 °C for 1 min in O2

3.4×10

-5

500 °C for 1 min in O2 500 °C for 10 min in air

10×10 -5 5.0×10

51017 cm-3 71017 cm-3

31017 cm-3

51017 cm-3

17

-4

Ref. 72

-3 73

-2

-5

-4

76

77

78

-3

81

-5

82

(5/8/4 nm) 1.01×10 -6 8.46×10 -6 6.23×10 -2 1.13×10 -3 6.23×10 -3 3.43×10 -3 6.73×10 -3 2.63×10 -4 1.23×10 -1 1.8×10 -5 6.6×10

11017 cm-3

500 °C for 10 min in air

2.73×10

[Mg] 11019 cm-

300 °C for 1 min in N2 300 °C for 2 min in N2

7.5×10 -3 4.2×10

21017 cm-3

500 °C for 1 min in N2

2.4×10

51017 cm-3

330 °C for 2 min in air

2.74×10

500 °C for 1 min in N2 500 °C for 10 min in air 800 °C for 1 min in N2 600 °C for 1 min in N2

7.6×10 -4 5×10 -4 2.5×10 -4 4.0×10

600 °C for 1 min in N2

2.5×10

-4

600 °C for 1 min in N2

2.0×10

-4

31017 cm-3

450 °C for 1 min in air

8.2×10

-6

11017 cm-3

500 °C for 10 min in O2

4.35×10

6.81017 cm-3

600 °C for 1 min in Ar 600 °C for 1 min in N2/O2

2.85×10 -3 2.×10

51017 cm-3

Ni/Au (5/5 nm)

21017 cm-3

Ni/Ag (5/120 nm) Ni/Au (20/20 nm) Ni/Au (20/20 nm) Pd/Ni/Au (10/20/30 nm) Ag (160 nm) Ni/Au (20/20 nm) Ni/Au/TaN/Ti/Au (50/80/50/20/80 nm) Ni/Au/TiN/Ti/Au (50/80/50/20/80 nm) Ni/Au/ZrN/Ti/Au (50/80/50/20/80 nm) Ni/Ag/Ni (1/150/50 nm) Ni/Ag/Au (1/150/50 nm) Ni/Au (20/80 nm)

-5

450 °C for 2 min in air 550 °C for 2 min in air 550 °C for 5 min in air 300 °C N2 400 °C N2 500 °C N2 300 °C O2 400 °C O2 500 °C O2 500 °C N2 500 °C O2

Ni/AlZnO (5/450 nm)

31017 cm-3

3

11017 cm-3

11017 cm-3

-4

83

84

85

86

-3

-5

-4

87

88

89

-3

-4

90

91

92

93

-1 94

Table II: Survey of literature data on Ohmic contacts to p-type GaN.

As can be seen, several metals (Ag, Ru, Ir,…) or multilayers (Cr/Au, Pt/Au, Pd/Au, Pt/Ni/Au, Pd/Ni/Au, Ni/Au,…) have been studied as Ohmic contacts to p-type GaN. Among them, the systems based on high work function metals, like Ni, Pd or Pt, are preferred, since they should be able to give a lower barrier height on p-GaN (see Fig. 2). However, a clear correlation between the metal work function and the resulting values of c on p-type GaN cannot be established. In fact, independent of the used metal, the values of the specific contact resistance c are in the range 10-2-10-5, about 1-2 orders of magnitude higher that those found for n-type GaN. The most widely used metal for Ohmic contact to p-type GaN is Ni, generally in combination with a Au capping layer. Fig. 10 reports the values of the specific contact resistance c as a function of the annealing temperature for Ni-based Ohmic contacts to p-type GaN, obtained by annealing processes 18

in the temperature range between 400 and 550 °C, in oxidizing (air, O2 or N2/O2) or in neutral ambient (Ar or N2). Two peculiar aspects can be deduced from this plot. Firstly, differently from the case of Ohmic contacts to n-GaN, in this case a clear decreasing trend of the specific contact resistance c with the annealing temperature is not observed. Secondly, it is interesting to observe that the experimental values of c obtained after annealing in oxidizing atmospheres are generally lower than that obtained in neutral ambient. As a matter of fact (see Table II), many authors report on the use of thermal annealing process in oxidizing atmospheres (O2, N2/O2, air, ..)

-1

2

c (cm )

10

-3

10

-5

10

Neutral ambient (N2,Ar) Oxidizing ambient (air,O2,N2/O2)

-7

10

400

450

500

550

Annealing Temperature (°C) Fig. 10: Specific contact resistance c of Ni/Au Ohmic contacts to p-GaN as a function of the annealing temperature in neutral (N2, Ar) or oxidizing ambient (air, O2, N2/O2). The data are taken from Table II.

The mechanism of Ohmic contact formation to p-type GaN, and the role of oxygen have been widely debated in the GaN community. Several authors attributed the formation of an Ohmic contact to the presence of nickel oxide (NiO) at the interface, formed in oxidizing atmosphere [72,95]. In particular, Ho et al. [72] suggested that the formation of a thin p-type semiconducting NiO layer at the interface with the p-GaN is responsible of a low Schottky barrier height B (0.19 eV). However, a later work from Yu et al. [96] reported a significantly higher calculated value of the barrier height (B = 2.28 eV) for the p-NiO/p-GaN interface. Basing on these controversial findings, it remains unclear the correlation between the formation of NiO and the reduction of c. .

19

1

10

0

60 Ar N2/O2

Current (A)

10

-1

10

-2

As deposited Ar N2/O2

40

2

c (cm )

10

20 0 -20 -40

10

-3

a) 400

500

600

b)

-60 -0.2

700

-0.1

0.0

0.1

0.2

Voltage (V)

Annealing Temperature (°C)

Fig. 11: (a) Specific contact resistance c as a function of the annealing temperature and (b) I-V characteristics of a TLM structure of Au/Ni contacts to p-GaN before (as deposited) and after annealing at 600°C in Ar and N2/O2 ambient. The data are taken from Ref. [94].

In this context, we have investigated the electrical behaviour of Ni/Au contacts to p-type GaN, annealed either in Ar or in N2/O2 atmosphere [94]. The I-V measurements on TLM structures showed a rectifying behaviour after the deposition. On the other hand, an improvement of the I-V curves occurred already after annealing above 400°C for both atmospheres (Ar or N2/O2). Fig. 11a shows the evolution of the specific contact resistance c with the annealing temperature. It is noteworthy, that for each annealing temperature the annealing in N2/O2 always showed better electrical characteristics than in Ar. This trend is summarized by the I-V characteristics acquired at the annealing temperature of 600°C (Fig. 11b). At this annealing temperature, the sample treated in N2/O2 showed a specific contact resistance of 210-3 cm2, 1-2 orders of magnitude lower that in the sample

Au

Au

Au-Ni

annealed in Ar (i.e., in the range of 100 - 10-1 cm2).

Ar NiO

Au

NiO

Ar

Au N2/O2

30

40

100 nm

b)

Au

NiO

NiO

a)

P-GaN

Ni

Au

Intensity (a.u.)

600°C

N2/O2

50

2 degrees

20

P-GaN

c)

100 nm

Fig. 12: XRD patterns (a) and cross section TEM images for Au/Ni contacts to p-GaN, annealed in Ar (b) and N2/O2 (c) at temperature of 600°C.

The evolution of the electrical behaviour observed upon annealing was correlated with the contact microstructure, studied by XRD and TEM analysis (Fig. 12) . Fig. 12a reports the XRD patterns for the samples annealed in Ar and in N2/O2 at annealing temperature of 600°C, where the lowest c was achieved. The contacts annealed in Ar show a peak related to the formation of a Au-Ni solid solution. The presence of a Ni-Au solid solution in similar systems was also observed by other authors, indicating the diffusion of Ni trough the Au layer [79,84]. On the other hand, in the sample annealed in N2/O2, the presence of the peaks related to NiO are detected. TEM analyses carried out on cross sectional samples annealed in the two different conditions allowed to better clarify this scenario. In fact, in the sample annealed in Ar the original deposition sequence of the metals is preserved, i.e., with a Ni layer in contact with the p-GaN and a Au layer on the top (Fig. 12b). Local EDX analysis revealed the presence of Ni inside this Au layer, indicating that in-diffusion of the Ni into the Au layer occurred and confirming the formation of a Au-Ni solid solution. The thin NiO present on the metal surface is related to the partial oxidation of Ni atoms that reach the surface, due to the high propensity of Ni to oxidize even with by the residual oxygen present in the annealing chamber. On the other hand, the TEM image of the sample annealed in N2/O2 shows an inversion of the original sequence of the metal layers (Fig. 12c). In fact, the Au layer is now in contact with the pGaN, while Ni is completely oxidized at the surface, forming a continuous layer of NiO (the systems appears finally as NiO/Au/p-GaN). Also in this case, the Ni atoms diffused in the Au layer. However, due to the negative value of the Gibbs free energy (G = -83.7 kJ/mol), the NiO formation is favoured respect to the Au-Ni solid solutions (from -0.6 to +4.3 kJ/mol according to the Ni:Au ratio) and all the Ni can out-diffuse and transform into an oxide (NiO) at the surface [85,97]. As a result of this process, the reversal of the original sequence of the metal layer occurred. The role of the “inversion sequence” of the metal layers during oxidizing ambient annealing has been discussed by several authors to justify the Ohmic contact formation. Ishikawa et al. [98] pointed out that a thin amorphous layer can be present at the p-GaN surface prior to metal deposition. This layer, consisting of about 2 nm of Ga2O3 and adsorbed carbon or hydrocarbon contaminations, is formed during the exposure to air of the GaN surface immediately after MOCVD growth. The presence of this layer is obviously detrimental for the metal/p-GaN contact properties. Indeed, theoretical calculations indicate that the presence of a thin (~2 nm) insulating layer at the interface with the p-GaN leads to an increase of B of about 0.2-0.3 eV, which in turn can cause a strong increase of c. According to their interpretation, the annealing in oxidizing ambient and the 21

consequent out-diffusion of Ni from the p-GaN surface can lead to a complete or partial removal of the contamination layer from the p-GaN surface (“self-cleaning effect”), and a lowering of the c value [73,78,98]. This interpretation is corroborated by the fact that different wet chemical etches (buffered HF, boiling aqua regia (HNO3:HCl = 1:3), or in HCl) [79,99], able to reduce oxygen or hydrocarbon surface contaminations, have been successfully employed to reduce the specific contact 2

resistance in Ni- and Pd-based Ohmic contacts to p-GaN down to the 10-4-10-5 cm range [79,99]. Another possible interpretation is related the presence of hydrogen in the p-GaN layer. In fact the residual hydrogen is known to electrically passivate the Mg acceptors in the p-GaN, by forming stable and neutral Mg-H complexes that strongly reduces the effective acceptors concentration NA [69]. In this context, it has been reported that the annealing treatments could induce the dissociation of the Mg-H complexes [71], causing a reactivation of Mg-atoms as acceptors. Specifically, Hull et al. [100] demonstrated that an annealing in oxidizing ambient strongly reduces the H concentration inside the p-GaN layer, increasing the effective doping concentration NA and resulting in a reduction of the specific contact resistance of the Ohmic contacts. Additional insights into this controversial topic have been obtained from the study of the temperature dependence of the metal/p-GaN specific contact resistance c. In our study, the specific contact resistance c of Ni/Au Ohmic contacts annealed in Ar or N2/O2 at 600°C has been monitored in a temperature range between 25 and 150 °C (see Fig. 13). In both cases a decrease of c with increasing the measurement temperature has been observed. By fitting the data with the TFE model it was possible to determine the barrier height B of the metal/p-GaN interface and the effective doping concentration NA. In particular, while a similar doping concentration (~ 5-6×1019 cm-3) was extracted in both cases, the sample annealed in Ar showed an experimental metal/pGaN barrier height (B = 1.07 eV) higher than that found in the sample annealed in N2/O2 (B = 0.71 eV). Similar values of B have been reported in literature for Ni/Au metal systems annealed in N2/O2 atmosphere [73]. Hence, the lower c obtained for the contacts annealed in oxidizing ambient can be associated to the lower B, since no significant difference in the doping concentration could be determined in the two cases.

22

2

c (cm )

6.0x10

-1

4.0x10

-1

2.0x10

-1

6.0x10

-3

4.0x10

-3

2.0x10

-3

Argon N 2 O2

TFE ( B=1.07 eV)

TFE (B=0.71 eV)

0.0

300

350

400

450

Measurement Temperature (K) Fig. 13: Temperature dependence of the specific contact resistance c of Ni/Au contacts to p-GaN annealed at 600 °C in Ar and N2/O2 ambient. The continuous lines are the fits obtained using the TFE model, from which the values of the barrier height B were determined.

The reduction of the effective Schottky barrier height determined by our analysis is consistent with the barrier lowering (0.2–0.3 eV) expected by the elimination of the native contaminants at metal/pGaN interfaces reported by Ishikawa et al. [98]. As already discussed in section III for the n-type material, also in the case of p-GaN the role of the defects density on the Ohmic contact formation has been discussed in literature. As an example, Kwak et al. [101,102] reported that the presence of dislocations has a minimal influence on the specific contact resistance of Ohmic contacts to p-GaN. In this context, one of the possible interpretations given to explain the behavior of Ohmic contacts to p-GaN is the presence of the deep level defects, related to the Mg-acceptors and located in the lower part of the gap, that assist the current flow from the metal into the p-type semiconductor [103,104]. These defects have been associated either to the presence of nitrogen vacancy-Mg pair defect (VN-MgGa) [105] or to an an impurity band (with an activation energy of 300-360 meV) formed by Mg-acceptors [106]. Accordingly, while the dependence of the contact resistance on the Mg-doping can be justified in terms of Mg-related defects [101,102], a decreasing temperature dependence of c is expected, similar to the one observed in Fig. 13. Clearly, the formation of Ohmic contacts to p-GaN remains continuously under discussion. In this context, the compatibility of such annealing treatments in oxidizing atmospheres with the fabrication flow chart of GaN-based power devices remains still an open issue.

23

V. AlGaN/GaN heterostructures As mentioned in the introduction, owing to the presence of a two-dimensional electron gas (2DEG), AlGaN/GaN heterostructures play an important role in GaN-materials and devices technology. In particular, heterostructures are used for the fabrication of High Electron Mobility Transistors (HEMTs). In a standard HEMT device, the current flows between source and drain in the 2DEG channel and is modulated by the application of a bias to a Schottky metal gate The most straightforward approach for the development source-drain Ohmic contacts in HEMTs has been to transfer the metal schemes already used for n-type GaN to the AlGaN/GaN system. However, for a deep understanding of Ohmic contact formation to AlGaN/GaN heterostructures some important factors must be considered. As an example, the AlGaN barrier layer has a larger band gap than GaN (that depends on the Al concentration in the layer), thus making the formation of an Ohmic contact inherently more difficult. Moreover, AlGaN/GaN heterostructures are typically fabricated using undoped layers, in order to limit the scattering phenomena and optimize the mobility of the 2DEG. Hence, the current conduction is guaranteed by the high sheet carrier density of the 2DEG, that is located at the AlGaN/GaN interface, i.e., typically 15-30 nm below the surface. On the other hand, the sheet carrier density of the 2DEG depends on both the AlGaN thickness and on the Al concentration, Clearly, the situation is much more complex that in a simple n-type GaN material. A survey of relevant literature results reported on Ohmic contacts to AlGaN/GaN heterostructures is given in Table III. As can be seen, only few works proposed the use of simple Ti/Al bilayers [107,108,117], but most of the authors used Ti/Al/X/Au multilayers (X = Ni, Ti, Ta, Mo, Ir, Nb, Pt,…), following the scheme proposed by Mohammed for n-GaN (see Fig. 4),.

Metal layers (nm)

dAlGaN (nm)

χAl (%)

Ti/Al (30/71 nm)

33 nm

15 %

Ti/Al (20/150 nm) Ti/Ta/Al (thickness N.A.) Ta/Ti/Al (thickness N.A.) Ti/Al/Ni/Au (15/220/40/50 nm) Ti/Al/Ni/Au (30/180/40/150 nm) Ti/Al/Pt/Au (15/85/50/50 nm) V/Al/Pt/Au (15/85/50/50 nm)

Annealing conditions 950 °C for 20 sec in N2

34 nm

22 %

16 nm

35 %

10 nm

33 %

30 nm

24 %

18 nm

25 %

950 °C for 20 sec in N2 (with pre-annealing) 950 °C for 10 min in N2 950 °C for 10 min in N2 (with implantation)

2

ρc (cm ) -5 2.2×10

107 3.2×10

-6

5×10

-5

5×10

-6

30 %

108

5.1×10

-4

5.3×10

-7

800 °C for 60 sec in Ar

2.8×10

-6

110

900 °C for 30 sec in N2

7.3×10

-7

111

2.2×10

-6

1.0×10

-5

950 °C for 4 min in N2

7.5 nm

Ref.

109

850 °C for 60 sec in N2

24

112

Ti/Al/Pt/Au (20/100/40/80 nm) Ti/Al/Pt/W/Ti/Au (20/100/40/50/80 nm) Ti/Al/Pt/WSi/Ti/Au (20/100/40/50/80 nm)

40 nm

Ti/Al/Ir/Au (30/200/50/20 nm)

40 nm

30 %

35 nm

30 %

Ti∕Al∕Nb∕Au (15∕60∕35∕50 nm) Ti/Al/Mo/Au (15/60/35/50 nm) Mo/Al/Mo/Au (15/60/35/50 nm) V/Al/Mo/Au (15/60/35/50 nm) Ti/Al (30/120 nm) Ti/Al/Ni/Au (30/120/40/50 nm) Ta/Ti/Al/Ni/Au (10/30/120/40/50 nm) Ti/Al/Mo/Au (15/60/35/50 nm) Ta/Al/Mo/Au (15/60/35/50 nm)

2×10

25 nm

27 nm

21.5nm

30 %

1.6×10

-5

1.0×10

-5

850 °C for 30 sec in N2

4.6×10

-5

850 °C for 100 sec in N2

5×10

-6

800 °C in N2

3×10

-6

650 °C in N2

9×10

-7

700 °C in N2

2.7×10

-6

500 °C in N2

4.7×10

-5

700 °C in N2

6.5×10

-6

700 °C in N2

7.5×10

-7

850 °C for 45 sec in N2

20 %

25%

30 %

Nb/Ti/Al/Ni/Au (10/20/100/40/50 nm) Nb/Ti/Al/Ni/Au (20/20/100/40/50 nm) Ti/Al/Ti/Au (20/80/40/100 nm) Ti/Al/Ni/Au (20/80/40/100 nm)

24 nm

-7

1.09×10

-6

117

118

1.5×10

-6

2.8×10

-6

1.5×10

-6

4.5×10

-6

3.5×10

-6

950 °C for 30 sec in N2

3.5×10

-6

850 °C for 30 sec in N2

2.96×10

50

850 °C for 30 sec in N2

21 nm

32 nm

28 %

25%

-7

1.5×10

-5

3.7×10

-6

850°C fpr 35 sec in N2

119

120

6.50×10

-6

2.50×10

-6

7.27×10

-7

2.13×10

-4

850 °C for 30 sec in N2 23 nm

30 % 850 °C for 30 sec in N2 (with 50nm SiN encapsulation layer)

Ti/Al/Ta/Au (20/80/40/100 nm) Ti/Al/Ni/Au (20/180/55/45 nm) Ti/Al/Ni/Au (20/120/55/45 nm)

2.96×10

-6

30 %

114

116

1.0×10 750 °C for 30 sec in N2

113

115

800 °C for 30 sec in N2

Ti/Al/Mo/Au (15/60/35/50 nm) Ti/Al/Pt/Au (15/60/35/50 nm) Ti/Al/Ir/Au (15/60/35/50 nm) Ti/Al/Ni/Au (15/60/35/50 nm) Ti/Al/Ta/Au (15/60/35/50 nm) Ti/Al/Nb/Au (15/60/35/50 nm) Ti/Al/Ti/Au (15/60/35/50 nm) Ti/Al/Mo/Au (15/60/35/50 nm)

-5

22 nm

21.5 %

121

870 °C for 50 sec in N2

122 9.7×10

25

-7

Ti/Al/Ni/Au (15/200/50/50 nm)

50 nm

22 %

850 °C for 60 sec in Ar

7.0×10

-5

123

Table III: Survey of literature data on Ohmic contacts to AlGaN/GaN heterostructures.

The typical values of the specific contact resistance c are in the range 10-5-10-6 cm2 for annealing temperature of 800-900 °C. Lower annealing temperatures (650-750°C) are reported only by few authors [50,116,117]. As can be seen from the comparison with the data reported in Table I, similar metallization schemes are used in n-GaN and AlGaN/GaN heterostructures. However, there are important differences between these two systems. Typically, when using Ti/Al-based contacts annealed at high temperatures (800-900 °C), the formation of TiN is considered an important indicator for Ohmic contact, both on n-GaN or AlGaN/GaN. However, Van Daele et al. [52] observed that the reaction of Ti with GaN, leading to the formation of TiN, can result in the formation of voids below the TiN layer. On the other hand, in AlGaN/GaN heterostructures, the presence of Al in the AlGaN alloy partially mitigates the kinetics of this reaction with respect to the situation occurring on GaN. In fact, considering the enthalpies of formation of AlN (-318.1 kJ/mol), GaN (-110.9 kJ/mol) and TiN (-265.5 kJ/mol), the extraction of N from GaN is always energetically favoured. Due to the higher bond strength, the formation of Al-N is thermodynamically favoured with respect to Ti-N [52]. Hence, with respect to the formation of TiN, an AlGaN barrier layer becomes more stable with increasing the Al concentration, as the driving force for the metal/AlGaN reaction becomes smaller. Another important aspect that has been discussed in some studies on Ti/Al/X/Au Ohmic contacts is the role of the Ti/Al ratio. In fact, an appropriate Ti/Al ratio can promote the reaction between metal and AlGaN, improving the properties of the contact. Crespo et al. [124],compared different Ti/Al ratios, using Ti/Al/Ni/Au metallization annealed at different temperatures. They fixed the thickness of Al (200nm), Ni (50nm) and Au (20nm), varying the thickness of the Ti contct layer on the AlGaN barrier. Their results reported in Ref. [124] are summarized in Fig. 14, showing the contact resistance (in mm) of the Ti/Al/Ni/Au contacts annealed at different temperatures (700, 800 and 850°C) as a function of the Ti/Al ratio. Evidently, at the lowest annealing temperature (700°C), the contact resistance is lowered when reducing the Ti/Al ratio (i.e, the Ti thickness, in this case). A similar behaviour at lower annealing temperatures (550-600 °C) has been observed also in other works, that verified this trend using simplified metal schemes (Ti/Al only) [125] and/or simple n-GaN substrates [126] to reduce the complexity of the reaction between the metals and to avoid the influence of 2DEG, respectively. In this range of annealing conditions, the importance of Al in the Ti/Al layer consists in the mitigation of the reaction kinetics of Ti with the underlying AlGaN, in contrast to an excess of Ti that would lead to an aggressive reaction with the formation of voids below the TiN formed at the 26

interface [52]. Roccaforte et al. [127,128] studied the behaviour of Ti/Al(200nm)/Ni(50nm)/Au(50nm) as a function of the annealing temperature for two significantly different values of the Ti thickness, 15nm and 100nm. Also in this case, at a fixed annealing temperature, the contact with the thinner (15 nm) Ti layer exhibited a lower specific contact resistance than the thicker one (100 nm Ti). However, this difference was much higher at lower temperatures ( 700°C-750°C), i.e., where the reaction process of this stack was not completed yet [127]. As a matter of fact, also in the data reported in Fig. 14 [124], the dependence of the contact resistances on the Ti/Al ratio become weaker at higher annealing temperatures (800-850°C). At these temperatures, however, the contact resistance decreases with increasing the Ti/Al ratio, reaching a minimum at a ratio of 0.15. A similar behaviour has been also observed by other authors [129,122], who hypothesized that the excess of Ti is not detrimental for the contact, but enhances the formation of TiN islands, increasing the generation of

Contact resistance (mm)

N-vacancies acting as donor states below the contact. 30 25

700 °C 800 °C 850 °C

20 15 10 5 0 0.0

0.1

0.2

Ti/Al ratio Fig. 14: Contact resistance as function of the Ti/Al ratio in Ti/Al/Ni/Au metal schemes annealed at the temperatures of 700°C, 800°C and 850°C. After annealing at 700°C, the contact resistance is lowered with decreasing the Ti/Al ratio. At higher temperatures (800-850°C) only smaller variations are measured, with the contact resistance decreasing towards a minimum reached at a Ti/Al ratio of 0.15. The data are taken from Ref. [124]

Clearly, basing on the controversial evidences reported in literature, the contact properties cannot be uniquely explained by the relative Ti/Al thickness. In fact, also the thickness of the single metal layers composing the stack and the annealing conditions determine the final electrical behaviour. In this sense, it cannot be ruled out the diffusion of Au towards the interface (limited at lower temperatures and/or using thicker Ti diffusion barriers) plays also a beneficial role for Ohmic contacts by the formation of Al-Au or Ti-Al-Au phases at the interface [24,130,131]

27

Of course, like in the case of Ohmic contacts to n-GaN, also in AlGaN/GaN heterostructures the annealing temperature and the resulting interfacial microstructure play an important role in the reduction of the specific contact resistance c. To get a clearer scenario of the system, it is particularly useful to monitor the evolution of the structural properties with the electrical characteristics of Ti/Al/X/Au contacts during the annealing processes. Fig. 15a shows the current-voltage (I-V) curves of Ti/Al/Ni/Au (15/200/50/50 nm) TLM patterns on AlGaN/GaN heterostructures, for different annealing temperatures (750 °C, 800 °C and 850 °C). In Fig. 15b the specific contact resistance c values extracted at each annealing temperature by the TLM analysis are reported. Evidently, a decrease of c with increasing annealing temperatures is occurring in this temperature range.

750 °C 800 °C 850 °C

30

10

10

-3

10

-4

10

-5

2

c (cm )

Current (mA)

20

0 -10 -20

a)

-30 -3

-2

-1

0

1

2

3

b) 750

800

850

Annealing Temperature (°C)

Voltage (V)

Fig. 15: (a) I-V characteristics of Ti/Al/Ni/Au on AlGaN/GaN heterostructure, acquired on adjacent TLM patterns at different annealing temperatures; (b) Specific contact resistance c determined by the TLM analysis as a function of the annealing temperature. The data are taken from ref [123].

In order to explain the improvement of the electrical characteristics of Ti/Al/Ni/Au contacts with the annealing temperature, a structural investigation of the interfacial regions has been carried out by TEM, XRD and EDX analyses. Cross section TEM images of a Ti/Al/Ni/Au contact on AlGaN/GaN heterostructure after annealing at 750°C and 850°C are shown in Fig. 16a and Fig. 16b. As can be seen, already at 750 °C an intermixing of the original metal layer occurred, forming Al-Ni and Al-Au phase and a thin layer of AlAu4 have been observed at the interface. On the other hand, at the annealing of 850 °C, the TEM image clearly shows the formation of a non-uniform TiN layer at the interface. In some regions, this layer exhibits protrusions penetrating the AlGaN barrier layer and directly contacting the 2DEG at the AlGaN/GaN interface. This observation explains the improvement of the specific contact resistance c with increasing the annealing temperature from 750

28

°C to 850 °C. In fact, an “intimate contact” metal-2DEG is more efficient than a simple tunnelling through the thin AlGaN layer [132]. To get further insights into the transport mechanisms in this system, we have studied the temperature dependence of the specific contact resistance for Ti/Al/Ni/Au contacts on AlGaN/GaN heterostructures annealed at 750°C (without TiN protrusions) and 850°C, where TiN protrusions directly in contact with the 2DEG. As can be seen in Fig. 16c at the annealing temperature of 750 °C, the specific contact resistance c decreases with increasing the temperature. From the fit of the experimental data with the TFE model (described in section II) a barrier height B = 0.89 eV and a ND = 1.751019 cm-3 have been determined. The high value of ND determined by the fit can be attributed to the presence of the 2DEG. Interestingly, at the higher annealing temperature (850 °C), the specific contact resistance c slightly increases with the increase of the measurement temperature, similarly to a “metal-like” behaviour. In this case, c was fitted by a power low of the temperature, ρc~Tα, with α=1.8. As a matter of fact, this behaviour has been already observed in metallic system where the resistance varies following a similar power law with 1<<5 [133].

35

a) Al-Au

30

Al-Ni

c)

750°C 850°C

750 °C

AlGaN

100nm

b)

Al-Ni

RC (mm)

25 AlAu4

Al-Au AlAu4

TiN 850 °C

20 15 10 +1.8

5

AlGaN

0 2DEG GaN

TFE (B=0.89 eV)

100nm

RC~T

300

350

400

450

Measurement Temperature (K)

Fig. 16: Cross section TEM micrographs of Ti/Al/Ni/Au contacts to AlGaN/GaN heterostructures after annealing at 750°C (a) and 850°C (b) and contact resistance RC as a function of the measurement temperature T for the same samples (c). The results are extracted from Ref. [123].

Clearly, the current transport is dominated by the experimentally observed “spike mechanism”, in which the presence of TiN protrusions directly contacting the 2DEG creates preferential paths for the

29

current flow, thus making the current transport even more efficient than in a simple TFE or tunnelling mechanism. These effects have been also highlighted by local current measurement, performed by conductive atomic force microscopy analysis (C-AFM). C-AFM analysis carried out on Ohmic contacts to AlGaN/GaN heterostructures revealed an inhomogeneous current map, where the current flows mostly through preferential spots created by metal protrusions that directly contact the 2DEG. This behaviour supports our interpretation of the “spiking mechanism” in the contacts annealed at 850°C [123,134,135]. On the other hand, it must be pointed out that also the presence of pre-existing material defects in the AlGaN layer can influence the contact properties, promoting the formation of such metallic protrusions [110,119136]. Wang et al. [136,137] studied the impact of threading dislocations in Ohmic contact on AlGaN/GaN heterostructures, observing the formation of discrete TiN islands along the threading dislocations upon annealing of Ti/Al contacts. In fact, dislocations can be considered as fast diffusion channels for the metal (Ti), favouring preferred growth regions for TiN formation. [137]. Also other surface defects, like “V-defects” at the termination of threading dislocations, largely present at the surface of nitrides heterostructures [138,139,140,141,142,

143 144

,

], can determine local

preferential conduction effects [145]. In this context, as already discussed in section III, dislocations can act as metal shunts for the current conduction, acquiring a dominant role in the transport properties of Ohmic contact on AlGaN/GaN heterostructures [56,58]. This effect has been experimentally visualized by means of a surface analysis of AlGaN/GaN heterostructures grown on Si substrates., Fig. 17 shows the morphological (AFM) and two dimensional current maps (C-AFM) of two AlGaN/GaN heterostructures having different dislocations densities (12109 cm-2 and 4109 cm-2 ) [146,147]. As can be seen, while a flat morphology and a uniform current map is observed for the sample with a lower dislocations density (Fig. 17a and Fig. 17b), the sample with a higher dislocations density shows preferential conduction spots in the current map (C-AFM) corresponding to the surface defects observed by AFM (Fig. 17c and Fig. 17d). The electrical analysis of Ti/Al contacts fabricated on these materials confirmed a different behaviour. In fact, the temperature dependence of the specific contact resistance c of Ti/Al contacts fabricated on these materials followed a TFE mechanism in the sample with a low defect density, while a slight increasing trend of c with the temperature was observed in the sample with a higher defect density. As a matter of fact, according to Blank et al. [56] and Sachenko et al. [58], if the current in an Ohmic contact flows preferentially over metal shunts (e.g., like the conductive defective regions observed in the C-AFM in Fig. 17), the contact resistance increases with the temperature, in a way that is characteristic of a metallic-type of conductivity. 30

This behaviour has been further supported by TEM images that showed metal protrusions penetrating the AlGaN barrier layer in the proximity of the surface defects [146,147].

Fig. 17: AFM image of the morphology (a and c) and the corresponding C-AFM two dimensional current maps (b and d) acquired on the AlGaN/GaN heterostructures having different dislocations density (DD), i.e., DD = 4109 cm-2 and DD = 12109 cm-2.

As specified at the beginning of this section, the mechanism of Ohmic contact formation to AlGaN/GaN heterostructures is more complicated with respect to the formation of Ohmic contact to n-GaN, due to the presence of the 2DEG formed at AlGaN/GaN interface. In particular, the properties of Ohmic contacts fabricated on AlGaN/GaN heterostructures depend not only on the metal/AlGaN interface, but also on the heterostructure properties, like the thickness of the AlGaN barrier layer, or

2

c (cm )

the Al concentration [148]. For this reason, these two parameters have been also reported in Table III.

10

-1

10

-2

10

-3

10

-4

10

-5

10

-6

10

-7

[50] [110] [111] [112] [113] [114] [115] [116]

0

10

[117] [118] [119] [120] [121] [122] [123]

20 30 40 dAlGaN (nm) 31

50

60

Fig. 18: Representative plot of the trend of the specific contact resistance c as function of the AlGaN thickness dAlGaN. The data are taken from Table III. The dash line is not a fit but is reported to guide the eye only.

Analogously to the case of Ohmic contacts to n-type GaN (see Fig. 3), also in the case of AlGaN/GaN heterostructures we have tried to find a correlation with the annealing temperature. In this case, however, while most of the data refer to annealing temperatures between 600 and 850°C, a significant dispersion of the data points was observed, i.e., a clear trend with the annealing temperature could not be found. This latter suggests that a special attention must be put on the role of the heterostructures parameters, rather than on the processing conditions. Fig. 18 reports experimental data of the specific contact resistance c (taken from Table III) as a function of the AlGaN thickness. For seek of clarity, the plot reports only the data relative to “non-recessed” contacts. As can be seen, in this case it is possible to identify a trend of the experimental data. In particular, the values of c first decrease with decreasing the AlGaN barrier thickness dAlGaN down to a minimum value. Then, for thinner AlGaN barrier layers (lower than 10 nm) the few available literature data show an inversion of this trend, i.e., with an increase of the c value. The role of the AlGaN/GaN heterostructures parameters on the Ohmic contacts properties has been discussed in literature since the early 2000s [149,150]. In particular, Singh et al. [149] first noted that Ohmic contacts to AlGaN/GaN heterostructures cannot be treated in the same way as the contacts formed on GaN. In particular, it has been observed that the tunnel probability, not only increases by lowering the metal/AlGaN Schottky barrier height, but also by reducing the AlGaN thickness. Lately, in an experimental study reported by Qiao et al. [150], the possibility to improve an Ohmic contact by reducing the AlGaN thickness below the contact pad, with an advancement of the metal/AlGaN interface induced by a thermal reaction, has been discussed. In that study, a Ta/Ti/Al stack has been used, considering three different thicknesses of the Ta contact layer. Depending on the Ta thickness, the reaction occurring after annealing treatments led to a partial consumption of the AlGaN barrier layer (“advancing interface”), i.e., leaving a thinner (20 nm) or thicker (25 nm) AlGaN barrier layer below the contact. It has been observed that a thinner residual AlGaN layer led to a lower c value, attributing this behaviour to the thinner barrier that the electrons have to tunnel to reach the 2DEG [150]. Very recently, Takei et al. [151] tried to explain the dependence of c on the heterostructure parameters. In particular, they argued that in AlGaN/GaN heterostructures the contact resistance RC arises from the series of different components, as we have schematically illustrated in Fig. 19.

32

Metal

Ω

Ω

RC 2DEG

AlGaN

AlGaN

RCAlGaN

RC

RSH

2DEG

GaN

RCmetal/AlGaN

RC2DEG

GaN

a)

b)

Fig. 19: (a) Schematic of an AlGaN/GaN HEMT structure with the different resistive components encountered in the current path . (b) Particular of the different contributions to the metal/AlGaN contact resistance RC.

A first contribution (RC metal/AlGaN) is related to the metal/AlGaN interface and does not depends on the properties of the heterostructures. The two other components are correlated to the AlGaN thickness (RC

AlGaN)

and to the sheet carrier density of the 2DEG (RC

2DEG).

According to this

interpretation [151], a reduction of the AlGaN thickness allows to reduce the resistive contribution of the AlGaN barrier layer, reducing the c. In fact, as reported in the earlier works [149,150], a thinner AlGaN increases the probability for electrons tunnelling through the AlGaN barrier layer. On the other hand, according to the model of Ambacher et al. [148], a thicker AlGaN layer leads to a higher sheet carrier density of the 2DEG, reducing the resistive contribution of the 2DEG. Hence, since the total contact resistance is given by the sum of these contributions, the specific contact resistance is expected to decrease down to a certain value when reducing the AlGaN thickness (dAlGaN), and increase at very low values dAlGaN, as the 2DEG becomes very poor [151]. While this expected trend is similar to what we have reported in Fig. 18, only a qualitative description was given in Ref. [151]. Recently some authors [152,153,154] described the mechanism current transport in Ohmic contact to AlGaN/GaN heterostructures invoking the presence of two barriers, as schematically shown in Fig. 20. A first barrier ΦB1 is the one seen by the electrons to tunnel from the metal to the high temperature annealed AlGaN layer. In particular, during high temperature annealing treatments ( > 800 °C) a high density of donor-like N-vacancies is formed. Hence, it is possible to assume the presence of a very thin “modified AlGaN” layer at the surface, where the local electron conduction band is well below the Fermi level and a very thin Schottky barrier is formed at the interface. Since the electrons can easily tunnel through this very thin barrier, it is possible to neglect the contribution 33

of this barrier to the total contact resistance Rc. The second barrier ΦB2 is the one that the electrons injected in the AlGaN layer have to overcome in order to reach the 2DEG at AlGaN/GaN interface. Since only a very thin surface layer has been modified by the annealing treatment (d'), the AlGaN thickness that electrons have to overcome to reach the 2DEG (d''), is given by d'' = dAlGaN - d' ≈ dAlGaN.

E

Modified AlGaN

AlGaN

B1

Metal

B2

d'

GaN

ΔEF

d

ΔEc

Fig. 20: Schematic band diagram to describe the mechanism of current transport in Ohmic contacts to AlGaN/GaN heterostructures, according the description proposed in Refs. [152,153,154].

Starting from all the aforementioned considerations, we formulated a simple analytical description to take into account the dependence of the specific contact resistance c on the heterostructures properties, e.g., on the AlGaN thickness (dAlGaN). According to the TFE, the specific contact resistance c depends on the Schottky barrier height B and on the characteristic energy E00, being this latter related to the doping concentration ND of the bulk semiconductor (see Eqs. 5 and 6). However, in the presence of an AlGaN/GaN heterostructure, it is reasonable to assume that characteristic energy E00 is related to the polarization field-induced carrier density generated from the 2DEG. As a matter of fact, Nam et al. [155] described the current transport in Schottky contacts to AlGaN/GaN samples introducing a polarization field effect-based thermionic field emission (PFE-TFE) model, replacing the donor density ND with a polarization fieldinduced carrier density ND-2DEG , written as:

N D 2 DEG 

nS d AlGaN , x Al  d AlGaN

Eq. 8

Eq. 8 provides a simple analytical expression which explicitly considers the dependence on both the sheet carrier density and on the AlGaN thickness. Applying this expression, the characteristic energy E00 for an AlGaN/GaN heterostructure can be rewritten as : 34

E00 d AlGaN , x Al  

qh 4

nS d AlGaN , x Al  d AlGaN  AlGAN 0 m*

Eq. 9

where dAlGaN, xAl and AlGaN are the thickness, the Al concentration and the permittivity of the AlGaN barrier layer, respectively. The other parameters have the same meaning as in Eq. 6. In Eq. 9, the dependence of the sheet carrier density nS on the heterostructure properties can be assumed considering the well known model of Ambacher et al. [148],

nS d AlGaN , x Al  

 x Al   0 AlGaN x Al   2  qB1 ( x Al )  EF x Al   EC x Al  q q  d AlGaN

Eq. 10

where σ is the polarization sheet charge density created by the spontaneous and piezoelectric polarization, B1 is the Schottky barrier height at the metal/AlGaN interfaces, EF is the Fermi level position with respect to the GaN conduction band edge, and ΔEc is the conduction band discontinuity between GaN and AlGaN. The dependence of these parameters on the Al concentration xAl is given in Ref. [148]. According to the previous assumptions, the standard TFE formulation (given in Eq. 5) can be modified in a “TFE-2DEG model”, considering the dependence of E00 on the AlGaN thickness (dAlGaN) and Al concentration (xAl) as :

 c  exp

 B2 , x   E d E00 d AlGaN , x Al coth 00 AlGaN Al  kT  

Eq. 11

where ΦB2 represents the barrier that electrons have to overcome to be injected on the 2DEG, as schematically shown in Fig. 20. Fig. 21 reports the curves calculated with the proposed TFE-2DEG model, as function of the AlGaN thickness (dAlGaN), considering a typical value of the Al concentration (xAl = 25%) and different Schottky barrier height values from 0.3 eV to 0.5 eV. Such low values of the Schottky barrier height have been found by some authors using the TFE model to fit their data and justified invoking a scenario like that depicted in Fig. 20 [152,153,154]. As can be seen in Fig. 21, the calculated c curves steeply decrease with increasing dAlGaN for very low values of the AlGaN barrier thickness, reaching a minimum for AlGaN thickness lower than 10 nm. For thicker AlGaN barrier layers, the specific 35

contact resistance increases again with increasing dAlGaN. It is worth noting that the curves calculated assuming the TFE-2DEG approach well reproduce the general trend of the experimental values

10

-2

10

-4

10

-6

10

-8

B2=0.5 eV

xAl=25%

2

c (cm )

observed in Fig. 18, confirming the validity of the proposed model.

B2=0.4 eV

0

B2=0.3 eV

10

20

30

40

50

60

dAlGaN (nm) Fig. 21: Specific contact resistance c as a function of the AlGaN thickness (dAlGaN) for Ohmic contacts to AlGaN/GaN heterostructures, calculated according to the proposed TFE-2DEG model. The calculation is done for an Al concentration of 25% and different values of the barrier height B2.

Fig. 22 shows the specific contact resistance c, calculated using the proposed TFE-2DEG model,

as a function of the Al concentration xAl (Fig. 22a) and of the sheet carrier density nS (Fig. 22b). The calculations have been done for two different AlGaN thickness values (16 and 26 nm) and for a barrier height B2=0.4 eV. Obviously, the specific contact resistance c decreases with increasing the Al concentration xAl (in the typical range of 15% - 30%) and with increasing the sheet carrier density nS (in the typical range of 1012-1013cm-2). Here, it is very important to point out that, as can be easily deduced by the calculated curves, even small variations of the heterostructure parameters xAl and nS result into significant changes of the c values. As an example, considering a variation of the sheet carrier density from 81012cm-2 to 11013cm-2, that can be normally occur within an AlGaN/GaN heterostructure, the specific contact resistance can change from 8.710-5 Ωcm2 to 3.710-5 Ωcm2 in case of 26 nm thick AlGaN of and from 1.210-5 Ωcm2 to 4.510-6 Ωcm2 in case of 16 nm of AlGaN barrier layer.

36

10

-4

10

-6

dAlGaN=16 nm dAlGaN=26 nm

-8

10

-2

10

-4

10

-6

a)

10 0.15

-8

0.20

dAlGaN=16 nm dAlGaN=26 nm

2

c (cm )

B= 0.4 eV

2

c (cm )

10

-2

0.25 Al

0.30

0.35

B= 0.4 eV

b)

10 12 10

-2

ns (cm )

10

13

Fig. 22: Specific contact resistance c, calculated using the proposed TFE-2DEG model, as a function of the Al concentration xAl (a) and as function of the sheet carrier concentration nS (b) for Ohmic contacts to AlGaN/GaN heterostructures. The calculations have been done for two different values of dAlGaN (16 nm and 26 nm) and for a fixed value of the barrier height B2 = 0.4eV.

This formulation let us conclude that the typical variations of Al concentration xAl , AlGaN thickness dAlGaN, and sheet carrier density nS, either from wafer-to-wafer but also within the same wafer, can be responsible for a significant variability often detected in the measurements of the specific contact resistance c in AlGaN/GaN heterostructures. Consequently, even an optimum metal stack, with optimized thickness ratio of the metals and optimized alloying conditions, can produce different values of c. These findings highlight the importance of accurately monitor the source of GaN materials for a better understanding of the Ohmic contact formation on AlGaN/GaN heterostructures.

VI. Au-free Ohmic contacts to AlGaN/GaN heterostructures As reported in the previous sections, one of the most common solutions for Ohmic contacts, both to GaN and AlGaN/GaN heterostructures, is based on annealed Ti/Al/X/Au multilayers (X = Ni, Ti, Ta, Mo, Ir, Nb, Pt,…). In these schemes, the Au cap layer has the role to prevent surface oxidation [47,52] and to improve the contact resistance by the formation of conductive phases inside the entire metal stack [117,156] and at the interface [110,157]. However, although good Ohmic contacts with low specific contact resistance values can be obtained using such multilayers, several practical technological concerns arise from their use in HEMT devices fabrication. In particular, the presence of Au is responsible for the high surface roughness of the contacts after annealing [157], and for the poor edge acuity, which in turn can cause device failure associated with long-term diffusion

37

phenomena [158]. Additionally, the reduction of the surface roughness becomes mandatory when scaling down the device dimensions [159]. The interest to growth GaN-based heterostructures on large area Si substrates (200mm) is continuously increasing. In fact, Si represents a cheaper alternative to more expensive substrates as Sapphire or SiC. Moreover, the possibility to integrate GaN technologies with the well-developed Si devices technologies should open bright perspectives for the introduction of more efficient devices (based on GaN) in consumer electronics [160,161,162]. In this context, to make GaN devices fully compatible with Si CMOS technology, “Au-free” metallization schemes are necessary. In fact, it is well known that Au is an undesired source of “cross-contaminations” in Si devices industry. Last, but not least, a Au-free technology would reduce the overall manufacture cost of the HEMT devices. Different approaches have been proposed to obtain low resistance Au-free Ohmic contact to AlGaN/GaN heterostructures. Ohki et al. [163] proposed the use of an intentionally doped AlGaN barrier layer. However, this solution leads to an increase of the leakage current and to a drastic degradation of the breakdown voltage of the devices [164]. Hence, few works reported on the use of selective ion-implantation of Si-ions in AlGaN/GaN heterostructures, in order to locally increase the n-type doping in the AlGaN layer only below the metal pad and favour the Ohmic contact formation [30,165, 166]. In this case, however, the electrical properties of the heterostructure can be significantly degraded by the high doses and temperatures required for a reasonable electrical activation of n-type dopant [167,168]. Alternatively, the use of a selective n+-GaN grown layer below the contact has been also proposed, although this process approach significantly increases the complexity of the device fabrication process [169.]. Clearly, all the aforementioned solutions are not able to fully preserve the electrical proprieties of AlGaN/GaN heterostructure (2DEG). For that reason, also for a Au-free technology, the use of metals with a low work function like Ta or Ti [170], enabling to obtain a low Schottky barrier height value, is the starting point to form Ohmic contacts [170]. In fact recently, Ta or Ti based Au-free Ohmic contacts on AlGaN/GaN heterostructures with low specific contact resistance (RC = 0.28 ÷2.00 Ω mm) have been reported in literature [125,153,171,172,173,174,175]. One of the advantages shown by a Au-free metallization consists in the improvement of the surface morphology. As an example, the surface roughness (RMS) of our Ti/Al Ohmic contacts (formed at 600°C) on AlGaN/GaN heterostructures, measured by AFM analysis, was 11.7 nm, in a 5050 μm2 scan area [170]. This value is much lower than the RMS value measured on Ti/Al/Ni/Au contacts formed at 800°C (RMS = 46.5 nm) [26,128]. The higher surface roughness of Ti/Al/Ni/Au contacts has been attributed to the reaction of Au with Al, i.e., to the formation of Al2 Au phase [157, 176]. Similar benefits by the elimination of Au, in terms of surface roughness, have been obtained also by 38

Lee et al.[171] and Malmros et al. [173], in Ti/Al/W (870 °C) and Ta/Al/Ta (550 °C) contacts with respect to standard Ti/Al/Ni/Au (annealed in the range 830-870 °C).

Metal layers (nm) Ta/Al/Ta (10/280/20 nm) Ti/Al/Ti/TiN Ta/Al (70/200 nm) Ti/Al (70/200 nm) Ta/Si/Ti/Al/Ni/Ta (5/5/20/120/40/30 nm) Ti/Al/W (60/100730 nm) Ti/Al/W (20/100/20 nm) Ti/Al/TiN (0.02x/x/60 nm) Ti/Al/ TiN (0.10x/x/60 nm) Ti/Al/ TiN (0.20x/x/60 nm)

dAlGaN (nm) 22 nm 25 nm 10 nm

χAl (%)

Annealing conditions

14 % 25 % 25 %

550 °C for 60 sec in N2 600 °C for 60 sec in N2 550 °C for 90 sec in N2 700 °C for 60 sec in Ar 500 °C for 60 sec in Ar

Contact resistance, RC (mm) 0.06 0.28 1.25 36.3 1.8

Ref. 173 172

25 nm

25 %

170

18 nm

26 %

850 °C for 30 sec in N2

0.22

152

17.5 nm

26 %

870 °C for 30 sec in N2

0.49

171

10 nm

25 %

600 °C for 60 sec in N2

0.65

175

0.62 15 nm

20 %

550 °C for 90 sec in N2

1.63

125

2.00

Table IV: Survey of literature data on “Au-free” Ohmic contacts to AlGaN/GaN heterostructures.

Table IV shows the most relevant results reported on “Au-free” metallization schemes onto AlGaN/GaN heterostructures. Among them, Malmros et al. [173] achieved an extremely low Rc (0.06 Ω mm) using Ta/Al/Ta metal layers annealed at 550 °C and attributed the Ohmic behaviour to the formation of TaN at the interface. More complex stacks have been also used to obtain “Au-free” Ohmic contacts to AlGaN/GaN heterostructures. As an example, Li et al. [177] obtained a contact resistance Rc = 0.22 Ω mm using a Ta/Si/Ti/Al/Ni/Ta metal scheme. In this case, the reduction of the Rc was attributed to the formation of TixSiy or TixSiyTa alloys at the interface. The low work function of these metals allow to achieve extremely low Schottky barrier height (0.11-0.12 eV), as determined assuming a TFE mechanism of current transport. In many cases, specific processing steps (wet cleaning, exposure to plasma, partial or total recession of the AlGaN layer, etc.) have been employed prior to metal deposition in order to reduce the value of Rc. As example, Lee et al. [171] demonstrated that the fabrication of “recessed” Ti(60 nm)/Al(100 nm)/W(30 nm) Ohmic contacts allowed to decrease the contact resistance down to 0.49 Ω mm, i.e., about a factor of 4 lower than the that measured in “non-recessed” contacts (RC = 2.02 Ω·mm). In their work, AlGaN/GaN heterostructures with a 17.5 nm thick AlGaN barrier layer covered by a 2 nm GaN cap layer, were used. It is interesting to note that the recessed contacts exhibited a clear Ohmic behaviour after annealing at 800 °C, in contrast to the non-recessed ones that 39

became Ohmic only after annealing at 900 °C. On the other hand, the optimal annealing conditions for conventional Ti/Al/Ni/Au contacts was 870°C (with RC = 0.38 Ω mm) [171]. The main results of that work on Au-free Ti/Al/W contacts are summarized in Fig. 23. In particular, Fig. 23a reports the total resistance as function of the pad distance in the TLM structures, both for recessed (30 nm of recessed depth) and non-recessed contacts. The inset shows a schematic of the two fabricated contacts. In this case, the recessed contact was obtained with an “over-etch” of the AlGaN barrier, down to the GaN buffer. Under these conditions, as observed by TEM analyses (not shown here), although the over-etch led to the removal of the 2DEG below the metal pad, the Ohmic contact formation was ensured by the direct contact of the metal with the 2DEG on the side [171]. Fig. 23b the contact resistance RC determined from the TLM analysis on these samples is reported as function of the recessed depth. The data relative to a sample obtained using only a partial recession of the AlGaN layer (recessed depth of 15 nm) is also reported in Fig. 23b.

250 200

Ω

Recessed

Ω

AlGaN

GaN

AlGaN

Non-recessed Partially recessed Recessed

3.0

Ω

GaN

RC (mm)

Ω

Resistance ()

3.5

Non-recessed

150 100

2.5 2.0 1.5 1.0

50 0

Non-recessed Recessed

a) 0

5

10

15

20 25 Pad distance (m)

30

0.5 0.0

b) 0

10

20

30

Recessed depth (nm)

Fig. 23: (a) Total resistance as a function of the pad distance measured in TLM structures on “recessed” and “non-recessed” Ti/Al/W Ohmic contacts to AlGaN/GaN heterostructures. The inset shows a schematic of these two situations. (b) Contact resistance as function of the recessed depth. The data are extracted from [171].

Clearly, the reduction of the AlGaN thickness below the contact is beneficial for the contact resistance RC. A similar approach have been reported by Firrincieli et al. [125] using Ti/Al/TiN metal contacts annealed at 550 °C. Also in that work, an over-etch of the AlGaN barrier layer has been employed, to form a direct contact of the metal with the 2DEG on the side. Although a direct link between the metal and 2DEG may be expected to improve the Ohmic contact properties, similarly to the situation observed in the presence of metal protrusions described in the previous section, the mechanisms of current transport in totally recessed Au-free Ohmic contacts have

40

been not yet clarified. As a matter of fact, also Qiao et al. [150] already pointed out that TFE model is not applicable for totally recessed contact configuration. For this aim, we investigated the temperature behaviour of Ti(20nm)/Al(100nm)/W(50nm) Ohmic contacts on AlGaN/GaN heterostructures, with a total recession below the contacts. AlGaN/GaN heterostructures with a 16 nm thick AlGaN layer have been used in our experiments. The recession of the AlGaN barrier layer (~ 20 nm) have been performed with an over-etch prior to metal deposition. A subsequent annealing at low temperature (550 °C) has been performed in N2 atmosphere to obtain Ohmic contact. The contact resistance, determined by TLM measurements, was RC = 1.02 Ωmm. Then, the temperature dependence of the contact resistance RC has been monitored varying the substrate temperature between 25 °C and 150 °C. Fig. 24a reports the I-V characteristics measured on adjacent TLM pads of “recessed” contacts, as function of the temperature. Evidently the higher current observed at room temperature is an indication of a lower specific contact resistance.

5

0

2.0

25 °C 50 °C 75 °C 100 °C 125 °C 150 °C

1.5

RC (mm)

Current (mA)

10

-5

-10 -1.0

RC~T

+0.8

1.0

0.5

a) -0.5

0.0

0.5

0.0

1.0

Voltage (V)

b) 300

350

400

450

Measurement Temperature (K)

Fig. 24: (a) I-V characteristics, acquired on adjacent TLM patterns at different measurement temperatures, for recessed Ti(20nm)/Al(100nm)/W(50nm) Ohmic contacts to AlGaN/GaN heterostructures (dAlGaN = 16 nm) . (b) Contact resistance RC as a function of the measurement temperature T for the same samples.

The values of Rc , extracted at each measurement temperature, are reported in Fig. 24b. Clearly, the values of Rc exhibit a slight increase with increasing the measurement temperatures. This situation is very similar to what has been experimentally observed in the case of Ti/Al/Ni/Au contact annealed at high temperatures (see Fig. 16). It can be observed that the contact resistance increases from RC = 1.02 Ωmm measured at room temperature to RC = 1.4 Ωmm measured at 150 °C. Also in this case, the experimental data were fitted by a power low of the temperature, RC  Tα, with α=0.8. Hence, similarly to the case of Ohmic contacts with protrusions directly contacting the 2DEG (described in the previous section), also in this situation a “metal-like” behaviour rules the current transport. However, in this case, a lower coefficient of the power law is found by the fit of the experimental data ( = 0.8) with respect to the case of non-recessed contacts with protrusions ( =1.8). This 41

difference can be associated to the different morphology of the contact between the metal and the 2DEG, i.e., in protruded areas (in reacted contacts) or on the side of the 2DEG (in totally recessed ones). Clearly, even if under the practical point of view the fabrication of totally recessed contacts can be a more reproducible process and, hence, more suitable for an industrialization, the behaviour of the totally recessed contacts remains still unclear and requires in the future an accurate modelling.

VII. Summary and Outlook In this paper, the key issues for Ohmic contact formation to GaN-based materials have been reviewed, focusing on the cases of n-type GaN, p-type GaN and AlGaN/GaN heterostructures. For each substrate, a critical analysis of the main literature results reported in the last two decades allowed to identify the most suitable metal stacks and annealing conditions for Ohmic contact formation. In particular, the use of a Ti/Al/X/Au multilayers (X = Ni, Ti, Ta, Mo, …) annealed at approximately 800°C is a suitable solution for both n-type GaN and AlGaN/GaN heterostructures. On the other hand, Ni/Au bilayers, annealed under oxidizing ambient at about 500-600°C, can be adopted to form Ohmic contacts to p-type GaN materials. However, the integration of these annealing processes in power devices technology can be a concern. For that reason, alternative solutions (like the use of heavily p-type doped sub-contact layers) must be always taken in consideration. Monitoring the temperature dependence of the specific contact resistance, both in Ti/Al/Ni/Au contacts to n-type GaN and in Ni/Au contacts to p-type GaN allowed to identify the thermionic field emission (TFE) as the main carrier transport mechanism at the metal/GaN interfaces, and to correlate the electrical behaviour with the evolution of the interface microstructure upon annealing (intermixing of the metal layers, formation of new phases, etc.). The parameters determined by these analyses (barrier height, carrier concentration) were particularly useful to explain the mechanism of Ohmic contacts formation. On the other hand, the formation of Ohmic contacts to AlGaN/GaN heterostructures is inherently more complex, due to the presence of the 2DEG at the interface, whose properties depend on several factors (like the AlGaN thickness and Al concentration). In this system, although Ti/Al/X/Au metal schemes can be used for Ohmic contacts, the electrical properties ultimately depend on the characteristics of the heterostructure, on the defect density and processing conditions. Clearly, in this case the situation is more complex that in simple n-type or p-type GaN substrates. In particular, while TFE reasonably explains the physics of non-recessed Ohmic contacts, in the presence of metal protrusions reaching the 2DEG, formed upon high temperature annealing, or of electrically active near-surface defects, a “metal-like” behaviour must be invoked to describe the temperature 42

dependence of c. In this context, in order to explain the dependence of the specific contact resistance on the heterostructure parameters, a modified TFE model (TFE-2DEG) has been presented in this review. This semi-empirical description was able to quantify the possible influence of the AlGaN thickness, Al concentration and 2DEG sheet carrier concentration on the variations of c. This approach also allowed us to explain why, even in “optimized” metal contacts, non negligible variations of c from wafer to wafer or even within the same wafer are often observed from the device makers. Finally, the state of the art and the mechanism of formation of “Au-free” Ohmic contacts to AlGaN/GaN heterostructure have been discussed. In fact, this technology represents an important step forward for the integration of GaN devices into Si devices fabs, using AlGaN/GaN heterostructures grown on large area Si wafers. In this case, partially or even totally recessed Ti-based Ohmic contacts give better results in terms of specific contact resistance and capability to control the process. However, the mechanism of Ohmic contacts formations in heterostructures remain still under debate. In conclusion, in spite of the enormous efforts devoted to the study of Ohmic contacts to GaN-based materials in the last decades, this topic includes still several open scientific issues with relevant technological implications. In particular, it appears of fundamental importance the accurate knowledge/selection of the properties of the used GaN-based materials (especially in the case of heterostructures) to predict and optimize the reproducible fabrication of Ohmic contacts. For that reason, we are of the opinion that the fundamental research activities on Ohmic contacts to GaN will proceed in the next years parallel to the evolution of the quality of GaN materials.

Acknowledgements

The authors would like to acknowledge the technical and scientific contribution of all the colleagues of CNR-IMM of Catania, Italy. Among them, F. Giannazzo, P. Fiorenza, and R. Lo Nigro are acknowledged for the fruitful discussions and for past contributions on this topic. A special thank is also for S. Di Franco and C. Bongiorno for their valuable support in devices fabrication and TEM analyses. Moreover, the authors thank the colleagues of STMicroelectronics of Catania for the support during sample processing of recessed contacts. Finally, M. Leszczyński, P. Prystawko and P. Kruszewski (Institute of High Pressure Physics, Warsaw, Poland) are acknowledged for the samples growth and discussions on Ohmic contacts to p-type GaN.

43

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