Solid-State Electronics Vol. 30, No. 10, pp. 1039-1042, 1987
0038-1101/87 $3.00+0.00 Copyright © 1987PergamonJournals Ltd
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OHMIC CONTACTS ON p-TYPE Gao47Ino53As/InP C. E. ALLEVATO,J. SELDERS,F. SCHULTE and H. BENEKING Institute of Semiconductor Electronics, Aachen University of Technology, Sommerfeldstral3e, D-5100 Aachen, FRG (Received 25 July 1986; in revised form 23 March 1987)
Abstract---Ohmic contacts of Au and Ag based Zn containing alloys on p-type Ga0.47In0.53As have been studied using intermediate layers of Ti and Ni, respectively. Low specific contact resistance in the order of 10-5 D,cm2 are achieved. In case of AuZn alloy, the Ti intermediate metal layer causes higher contact resistances together with a worse contact morphology in contrast to Ni intermediate layers. However for AgZn contacts Ti adherent layers improve the contact resistances, especially for lower alloying temperatures. Moreover these contacts exhibit significant smoother interfaces as revealed by TEM micrographs. Thus AgZn contacts apply best to low resistive contacting of very thin p-layers forming e.g. the base of a ballistic device.
The ternary alloy Ga0.47In053As has attracted special attention for application to optoelectronic and high speed devices[I,2], including quantum well structures. To obtain a satisfying performance of these devices ohmic contacts have to show linear behaviour over a wide voltage range, low contact reistance and smooth surface. Non dispensable are small penetration depth and planarity. Especially the last two requirements are important for producing ohmic contacts on very thin p-layers e.g. for HBTs or self aligned devices. Because of the low barrier height of n-GaInAs low specific contact resistances pc = 1 x 10-7~clI12 on 10-1Scm -3 material have been realized. Usually these contacts are ohmic without a heat treatment. In contrast Au based contacts on p-material of the same doping level only reach values of 3 x 10 -5 f~cm2 after a solid state reaction occurring above 400°C. For p-GaAs however it has been shown that Agln and AgZn contacts provide a low specific contact resistance[5-7]. These results have motivated to compare Ag-based and Au-based alloyed contacts on p-type GalnAs, also in respect of their planarity. In this letter we present the dependence of the specific contact resiStance and surface morphology on the annealing temperature for Au and Ag based contacts on p - G a l n A s using different intermediate layers.
2. EXPERIMENTAL PROCEDURE
The Gao.47Ino.53As layers used in this work have been grown by liquid phase epitaxy (LPE) on semiinsulating (100) InP with a carrier concentration of 1 x 10Is cm-318]. After defining resistor structures, as
shown in Ref. , the metal layers have been deposited under a pressure of 10-4Pa by electron beam evaporation. The thicknesses of the evaporated layers are 25 nm Ni-150 nm AuZn-5 nm Ni, Ti respectively and 25 nm-150 nm AgZn-5 nm Ni, Ti respectively. Before evaporation the surface has been cleaned with 2 keV Ar ions for 1 min to remove the oxide layer on GalnAs. Then the samples have been alloyed at temperatures between 350°C and 500°C for 2 min in a nitrogen atmosphere. The specific contact resistances are measured using the transmission line model (TLM). The surface morphology has been analysed by SEM. In addition the N i - A g Z n - T i specimen has been analysed in a 2 MeV transmission electron microscope. A special cross-sectioning technique after Ref.  has been applied to thin the samples.
3. RESULTS AND DISCUSSION AS indicated in Table 1, all contacts show nonlinear I - V characteristics before annealing. In the case of linear I - V characteristic the lowest specific contact resistance of 1.1 x 10 5f~cm2 has been obtained for N i - A g Z n - T i annealed at 400°C. Figure 1 shows that there is a tendency for systems using AgZn and AuZn to have about the same value of the specific contact resistance for alloying temperatures between 450°C and 500°C. This fact indicates that Zn probably takes the leading role in determining the electrical properties of those contacts. The combination of AuZn together with a Ti intermediate layer leads to very rough surfaces and a high contact resistance for annealing temperature above 350°C. The other Zn containing contacts be1039
C . E . ALLEVATO et al,
Table 1. Measured values of the specific contact resistance Contact system
(350°C) After annealing
(400"C) After annealing
(450°C) After annealing
(500°C) After annealing
9.9 x 10 -5
3.0 x 10 - 5
3.7 x 10 5
3.9 x 10 5
4.2 x 10 -4
2.6 x 10 _4
2.2 x 10 4
3.9 x 10 -5
3.0 x 10 4
8.1 x 10 - 4
4.4 x 10 5
1.1 x 10 -5
1.1 x 10 - 5
2.6 × 10 5
2.4 x 10 5
Ni-AuZn-Ni 25-150~5 nm Ni-AuZn-Ti 25-150-5 nm Ni-AgZn-Ni 25-156-5nm Ni-AgZn -Ti 25-150-5 nm
Specific contact resistances in Dcm2.
have better in surface morphology and specific contact resistance after alloying. For such contacts Pc is in the range of 1.1 x 10-5 f~cm2 to 3.9 x 10-Sf~cm2. The N i - A g Z n - T i contact, in contrast to the Aubased contact, has not so drastically changed its surface morphology after annealing at 400°C for 2min. This is demonstrated in Fig. 2, where N i - A u Z n - T i contacts are shown in comparison to N i - A g Z n - T i contacts before and after annealing, respectively. A TEM micrograph of its cross-section is shown in Fig. 3. In contrast to an Au-based contact onto GalnAs no significant reaction or precipitate formation can be observed at the interface, see Fig. 4.
The experimental results reported in this paper
indicate that Zn containing ohmic contacts can be formed on p-type GalnAs with low contact resistances in the range of 1.1-3.9 x 10-5~cm 2. The best contact morphologies after alloying are obtained with AgZn alloys. These are correlated with good contact resistance. TEM micrographs also indicate that Ag contacts show no crystal formation or observable diffusion of the metal into the matrix material in contrast to Au-based contacts. Therefore AgZn containing contacts seem to be preferable to Au-based contacts, if thin p-type GalnAs layers have to be contacted.
Acknowledgements--The authors are grateful to E. Woelk and Dr L. Vescan for her valuable comments and discussion to this work and to J. Knauf, B. Jansen and R. Wiiller for technical assistance. Also the authors would like to thank Dr Jouffrey and his coworkers at the Centre National de la Recherche Scientifique in Toulouse for their assistance at the T E M micrographs.
x Ni-AuZn-Ni Ni-AuZn-Ti1 p-GalnAm + Ni -AQZn-Ni P=(i'4-1"g)xlO1%m-3 Ni-A~Zn-Ti
10-3 @".... 1 10-4 o_,
. e ..... ~-.~--~ I"
Fig. 1. Specific contact resistance vs annealing temperature for different metal multilayers based of Au and Ag.
Ohmic contacts on p-type Ga047In053As/InP
Fig. 2. Contact morphology of: (a) Ni AuZn Ti as deposited; (b) Ni-AuZn Ti alloyed at 400°C for 2 min; (c) Ni-AgZn-Ti as deposited; (d) Ni AgZn-Ti alloyed at 400°C for 2 min or GalnAs/lnP.
Fig. 3. Cross-section of Ni-AgZn Ti~GalnAs/InP after heat treatment (400°C, 2 min).
C.E. ALLEVATO et al.
Fig. 4. Cross-section of Ni-AuSn-Ni43alnAs/InP after heat treatment (400°C, 2 min).
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