GaN heterostructures

GaN heterostructures

Microelectronic Engineering 87 (2010) 2208–2210 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier...

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Microelectronic Engineering 87 (2010) 2208–2210

Contents lists available at ScienceDirect

Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee

C–V characterization of SF6 plasma treated AlGaN/GaN heterostructures J. Osvald a,*, T. Lalinsky´ a, G. Vanko a, Š. Hašcˇík a, A. Vincze b a b

Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratislava, Slovakia International Laser Centre, Ilkovicˇova 3, 841 04 Bratislava, Slovakia

a r t i c l e

i n f o

Article history: Received 24 September 2009 Received in revised form 4 January 2010 Accepted 3 February 2010 Available online 11 February 2010 Keywords: Capacitance measurement Heterostructures Plasma etching

a b s t r a c t We studied influence of SF6 plasma treatment on electrical parameters of AlGaN/GaN heterostructures by C–V, I–V and SIMS measurement. We found the C–V measurements as an effective tool that is able to help to analyze electrical manifestations of charge changes in heterostructures during the semiconductor processing. In contrary to previously published results we found out that for diminishing of the two-dimensional electron gas concentration in the channel layer positively charged ions implanted during the plasma treatment into the semiconductor and not F ions are responsible. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor heterostructures based on GaN material, especially heterostructures AlGaN/GaN are still in the focus of semiconductor research teams because many of physical and technological questions connected with these heterostructures are still not solved satisfactorily. One of such issues is plasma etching of GaN and AlGaN and its influence on electrical parameters and characteristics of the structures made from these materials. It is a general problem to etch GaN with wet etchants and that is why reactive ion etching (RIE) with various gases is studied for gate recess formation. Chloride based gases were widely studied for gate recess etching [1]. Chloride etchants have low selectivity to other materials and cause damage of electrical parameters, especially enhancement of the leakage current of the structures that must be recovered by rapid thermal annealing (RTA) [2]. If SiN passivation is used fluorine-based plasma etching is used after lithography step for patterning nitride layers. There is also another problem connected with high density of traps in the gate region of the transistor, that increases leakage current through the gate and contribute to the current collapse of the transistor [3]. To minimize the effect of surface traps an insulating dielectric layer under the gate electrode [4], epitaxial cap layer [5] or gate-terminated metallic field plate [6] were used. A new approach to reduce the influence of surface traps in the gate region appeared recently. It consists in a selective plasma treatment of the HEMT gate and its region using CF4 plasma. This procedure reduces the gate leakage current [7] and also current collapse [8]. * Corresponding author. Fax: +421 2 5477 5816. E-mail address: [email protected] (J. Osvald). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.02.004

We report on experiments where we explored the influence of plasma of other gas – SF6 on electrical parameters of AlGaN/GaN heterostructures. Etching in SF6 has an advantage comparing to Cl2 because it is not corrosive and toxic. However, etching in fluorine plasma is known to create a thin GaFx layer on the surface. In order to remove this layer, argon or nitrogen is sometimes added to SF6 and GaFx is by this way physically removed by Ar (N2) sputtering [9]. We show that the effect of plasma influence/damage of the structures may be described to the large extent by C–V measurement supported by I–V measurement. As a main diagnostic technique C–V measurement was used. This measurement is applied in heterostructures prevalently when sheet carrier density in two-dimensional electron gas (2DEG) is of interest. We have shown that from C–V characteristics also the doping density in the barrier (AlGaN) or channel (GaN) layer, barrier layer thickness and Schottky barrier height may be assessed. We simulated experimental C–V curves by the method described in Ref. [10]. Using simultaneous solution of the Poisson and continuity equations, which gives electrons and holes concentrations in the whole structure, we calculate the differential capacitance of the structure as a derivative of the charge in the structure according to the applied voltage. In the simulation process, we were able by changing five parameters of the structure, namely the thickness of the AlGaN layer, doping concentration of AlGaN a GaN layers, Schottky barrier height and certainly the sheet concentration of two-dimensional electron gas (2DEG) to fit in experimental curves.

2. Experiment We used undoped AlGaN/GaN heterostructure grown by metal– organic chemical vapor-phase deposition (MOCVD) on (0 0 0 1)

J. Osvald et al. / Microelectronic Engineering 87 (2010) 2208–2210

sapphire. The thickness of AlGaN and GaN layers according to the provider were 32 nm and 3 lm, respectively. The aluminum mole fraction of the AlGaN layer was nominally 0.25. Then the device ‘‘MESA” isolation was performed using a reactive ion etching (RIE) in CCl4 gas. Source and drain alloyed ohmic contacts with Nb/Ti/Al metallic system were formed by annealing at 850 °C for 35 s. Then the gate windows with 2 lm length were opened. We used a two-step surface plasma treatment of the AlGaN barrier layer before the deposition and patterning the Schottky gate contacts. Firstly, a shallow recess-gate etching of AlGaN (1–2 nm) was performed in CCl4 plasma. Then the recess-gate etching was followed in situ by SF6 plasma treatment at 250 W power. Self-bias formed by RIE etching on the cathode was 200 V. Ni/Au electron beam evaporation was carried out subsequently to form 100 lm wide gate contacts. We prepare also a reference samples without plasma treatment. The structures were electrically characterized by I–V and C–V measurement and the distribution of implanted species into heterostructures was studied by secondary ion mass spectroscopy (SIMS). The samples were analyzed using time of flight based SIMS instrument (Ion-TOF, SIMS IV) with high-energy Bi+ primary source with combination of low energy sputter gun (Cs+). The time of flight based SIMS investigations were provided to get surface chemical information and also from the depth, where the interface was clearly resolved. 3. Results and discussion C–V characteristics of the samples (Fig. 1) have been measured at 1 MHz frequency by Boonton bridge and I–V curves by HP 4145B Semiconductor Parameter Analyzer. It is seen that the characteristic of the sample influenced by SF6 plasma is shifted to the right in voltage, the capacitance plateau is at higher value and a capacitance increase in the low forward voltage occurs. This increase is not observed in the reference sample where the capacitance is constant also in the low forward voltage. The capacitance decrease at voltage where the charge from the 2DEG is totally depleted and the depletion of the GaN layer starts is little bit steeper for the sample without plasma treatment. We simulated both experimental C–V curves by the method described in Ref. [10]. Five structure parameters were extracted from the curves; AlGaN barrier layer thickness, doping concentration in AlGaN layer, doping concentration in GaN channel layer, sheet carrier density in 2DEG, and the barrier height of the Schottky contact between metal gate and AlGaN layer. The results of the simulation are in Table 1.

experimental curves simulated curves

-7

2

Capacitance (F/cm )

3x10

-7

2x10

with plasma

without plasma

-7

1x10

0 -8

-7

-6

-5

-4

-3

-2

-1

0

1

Voltage (V) Fig. 1. C–V characteristics of the sample influenced by the plasma treatment and the sample that was not treated by plasma.

2209

First difference between the samples is in the thickness of the AlGaN layer. The thickness of the plasma treated sample has about 2 nm thinner barrier layer. Untreated sample had AlGaN layer thickness 34 nm according to our simulation and after the plasma exposition it was 32 nm. The doping density of AlGaN barrier layer of the reference sample was found to be 1  1017 cm 3, but to fit in the C–V curve of plasma treated sample the doping concentration had to be higher – 1  1018 cm 3. The carrier concentration in untreated GaN sample was found to be 1  1015 cm 3 and after SF6 plasma treatment increased similarly as in AlGaN layer to 1  1016 cm 3. We expect that it is a result of F+ ions embodiment into the layers. The capacitance of the structure is not very sensitive to these parameters, but the order of the concentration can be reliably ascertained. In opposite to this observation, the sheet carrier density in plasma inserted sample decreased from 8.4  1012 cm 2 from the reference sample to much lower value 4.6  1012 cm 2. Finally, Schottky barrier height has been assessed as 1.35 and 0.9 V for the reference and the plasma treated sample, respectively. The explanation of the above experimental facts and their modelling is of interest for better understanding of the process of heterostructure plasma treatment. We have also made SIMS measurement and found out fluorine and other elements depth profile (Fig. 2). On the plasma treated sample surface, a thin layer of fluorine has been detected and the fluorine concentration decreases toward the AlGaN/GaN interface. For thinner AlGaN layer (20 nm), an accumulation of fluorine was observed [11]. Similar decrease in direction to the semiconductor bulk is observed for sulfur atoms. Fluorine is electronegative element and that is why it is assumed that it creates acceptor levels in semiconductors. But by RIE plasma etching, the sample was placed at negatively biased electrode and that is why the sample was bombarded by positively charged ions. It is reasonably to assume that the particles that bombard the AlGaN surface are positively charged SFx (x = 1–5) radicals or F+ ions. This fact is confirmed by already mentioned decrease of fluorine and sulfur concentration from the surface towards the bulk. The ionized radicals are implanted to the AlGaN layer that is depleted from electrons because of the Schottky barrier formed at the surface. Since concentration of free electrons in the surface depletion layer is by many orders of magnitude lower than fluorine and sulfur concentration, these positively charged radicals are not able to recombine with electrons. They remain in positively charged state and that is why we found by the C–V measurement higher positively charged ionized donors in AlGaN layer after the plasma treatment than in the samples untouched by SF6 plasma. Since the concentration of implanted ions is much higher close to the surface than deeper in AlGaN layer, additional electric field formed diminishes electric field formed by polarization in AlGaN layer. Result of this effect is the decrease of electron concentration in 2DEG that is generally observed after the plasma treatment of AlGaN/GaN heterostructures. From the C–V measurement, we extracted also the Schottky barrier height and compared it with the barrier parameters calculated from I–V measurement (Fig. 3). The barrier height of the structure without plasma treatment was 1.35 V and diminished to 0.9 V after the plasma etching. The barrier heights from I–V measurement were 0.59 V and 0.54 V without and with plasma treatment, respectively. For both methods the barrier height for plasma untreated sample is higher than the plasma etched structure. The large difference between the barrier values is with a large probability caused by tunneling of electrons across the metal AlGaN interface that diminishes effective barrier height. This is expressed also in very large value of the diode ideality factors – 4.99 for untreated sample and 3.80 for plasma treated sample. On the other hand, the capacitance measurement is not influenced by the form of electron

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Table 1 Parameters extracted from the reference and plasma treated samples by the simulation of experimental C–V curves. d (nm) Reference sample After plasma

ND AlGaN (cm

3

)

17

34 32

1  10 1  1018

ND GaN (cm

3

)

15

1  10 1  1016

NS 2DEG (cm 12

8.3  10 4.6  1012

2

)

u (V) 1.30 0.95

4. Conclusion 5

10

Au F F Al S Cl Ni Ga GaN Au

Intensity (counts/s)

4

10

3

10

2

10

Ni

S

GaN Al

Ga

1

10

Cl

In conclusion, we have shown by analyzing and simulation of the C–V curves and SIMS measurement that positively charged radical of SFx or F+ ions are built in AlGaN barrier layer during the SF6 plasma etching of AlGaN/GaN heterostructures. These positively charged ions increase the charge of positively charged ionized donors in AlGaN layer and partially compensate electrical polarization in AlGaN layer that results in the diminishing of the 2DEG sheet concentration in the channel layer which is experimentally observed. Relatively large Schottky barrier lowering after plasma treatment was also detected by the C–V measurement. Acknowledgments

0

10

0

20

40

60

80 100 120 140 160 180 200 Depth (nm)

Fig. 2. Depth profile of elements in the SF6 plasma treated sample.

The authors are thankful the financial support received during the development of this work from Slovak Grant Agency for Science under Contract No. 2/0163/09 and 1/0787/09, Agency for Research and Development APVV-0655-07, and The ResearchEducational Centre of Exellence VVCE-0049-07.

4

10

2

2

J (A/cm )

10

References

without plasma treatment plasma treated simulated curves with extracted parameters

0

10

-2

10

-4

10

-4

-2

0

2

Voltage (V) Fig. 3. I–V curves of the sample influenced by the plasma treatment and the sample not treated by the plasma from which Schottky barrier heights were also calculated.

transport across the barrier because the barrier height is deduced only from width of the depletion layer [12].

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