Improvement of the Refractory Metal/n-GaAs Interface by Low Temperature Anneal
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EXPERIMENTAL PROCEDURE A variety of Zn doped (100)-oriented GaAs substrates, ranging in doping from 5x 1016 to 8×10' 8 cm 3 , were used in these studies. To follow the thermally activated changes in 013, contacts were formed on lightly doped GaAs so that the thermionic emission controls the transfer of carriers over the barrier. Substrates with higher doping concentrations were used to verify the importance of DB modifications for the formation of low-resistance metal/p-GaAs contacts. Zn and Au were deposited by thermal evaporation in oil-free vacuum, on unheated substrates. The AuZn metallization was formed by sequential deposition of the Au(4Onm)/Zn(4Onm)/("d"nm)Au structure. The thickness "d" of the second Au layer was adjusted to provide the desired concentration of Zn. For electrical characterization, the metallization was patterned by conventional photolithography, using AZ 1350 JSF resist and the soft- and postbake steps at 90 0 C for 30 min. and at 120 0C for 20 min. respectively. Before annealing contacts were encapsulated with 200 nm thick, rf-sputtered Si0 2 film. The substrate temperature during sputter deposition was estimated not to exceed 100°C. Heat treatments were carried out in flowing H2 at temperatures ranging from 200 to 420 0C, for 3 min. X-ray diffraction (XRD) was employed to identify phase transformations in the metallization. The contact resistivity rc was measured by modified Terry-Wilson method and transmission line method (TLM). 0B at different stages of the formation of pure Zn and Au(Zn) ohmic contacts was determined from I-V characteristics under forward bias for rectifying contacts, and from r,-T measurements between 80 and 423 K in the case of ohmic contacts. RESULTS AND DISCUSSION Phase transformations in the Au(Zn) metallization during the formation of ohmic contacts The results of XRD analysis of Au(Zn)/p-GaAs contacts after particular steps of the fabrication of ohmic contacts i.e.: deposition of metallization, patterning, and annealing at 200, 320 and , 4200 C are presented in Table 1. The as-deposited Au(Zn) metallization, in spite of sequential deposition of Au and Zn, in the whole range of Zn concentration, consisted of two cubic phases: P3-AuZn (approximately 50 at.%Zn) and a-AuZn (solid solution of Zn in Au), with lattice parameter varying from 0.3150 to 0.3165 nm and from 0.4052 to 0.4071 nm respectively. According to Elliot [5], at temperatures below 500"C, for Zn concentration in Au up to 40 at.%, the equilibrium Au-Zn phase diagram consists of the following phases: ct-AuZn, ot3-AuZn (Au 4Zn), OC 2-AuZn or x1-AuZn (low or high temperature Au 3Zn phase), Au 5Zn 3, and O3-AuZn. Surprisingly, in AuZn thin film structures, in which the content of Zn ranges from 10 to 35 at.%, phase formation paths are similar. Thermally induced transformations start at around 100-200 0 C and proceed through the gradual decomposition of 3-AuZn phase and formation of ct2-AuZn phase. Rising the temperature to above 3000 C causes further transformations into oa3-AuZn for Zn content less than 2
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