Structural Processes in Gold-Based Metallization During the Formation of Ohmic Contacts to GaSb
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Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
with the thickness Au(20 nm)/Zn(10 nm)/Au(60 nm) and Au(20 mm)/Sb(280 nm)/Au(30 nm), respectively. Contact annealings were performed at temperatures up to 400'C for 3 min each under flowing H 2 gas. The electrical properties were characterized by I-V measurements after patterning the contacts into arrays of circular dots (200 pm or smaller in diameter) by photolithographic methods. The specific contact resistance was evaluated using a four-point probe method. The microstructure and phase formation were investigated by XRD, RBS and TEM. XRD was performed using an automated DRON diffractometer with Co Kox radiation, RBS measurement used 2 MeV 4He+ ion beam produced by Van de Graff accelerator, while TEM was done employing a JEOL 2000 EX high-resolution electron microscope. For these experiments, planview specimens were thinned by chemical etching. RESULTS Electrical Properties As-deposited p-GaSb/Au contacts exhibited ohmic behavior and were characterized by specific contact resistance rc of Ix10 4 1 cm2. Upon annealing at 300'C, rc decreased to the value of lx10 5 n cm2. Addition of zinc or of antimony into the Au metallization did not have much influence on the contact resistance. Microstructures 1. GaSb/Au contacts The principal results of microstructure analysis of GaSb/Au contacts are reported in Fig. 1. No interfacial reaction between the Au and GaSb has been observed in as-deposited samples, within experimental resolution. In the RBS spectrum of the unheated sample (Fig. 1.a) the Au signal is well separated from the substrate signal. The x-ray diffraction patterns and electron diffiraction patterns show the gold reflections only (Fig. l.b,c). Both diffraction techniques indicated that these films were polycrystalline. Au grains were randomly oriented and the grain size varied from 100 to 500 nm. Heat treatment causes a progressive interaction between gold and GaSb. The early stage of reaction is clearly seen on RBS spectra of contact annealed at 100°C. The gold signal decreases indicating interdiffusion of Au, Ga, and Sb. Also XRD and TEM analysis indicate the structural transformations in the contact region. At this stage two phases were present in GaSb/Au contact: AuSb 2 intermetallic compound, and an orthorhombic MnP type phase with lattice parameters a=0.6401_+0.0003nm, b=0.6253±0.00006 nm, c=0.3447±0.0005 nm, identified as AuGa phase. The grain size of metallization decreased to about 20-50 nm. Under annealing at higher temperatures the interfacial reaction, involving the diffusion of Ga and Sb into Au is progressing. 2. GaSb/AuZn contacts The principal results of microstructure analysis of GaSbAu(Zn) contacts are reported in Fig.2. In spite of sequential deposition of Au and Zn, metallization constituents did not form separate layers in as-deposited samples. Apparently, the originally deposited Zn layer has
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Fig. l.a. 2 MeV 4 He+ RBS spectra of as deposited and annealed GaSb/Au contacts.
Cn (A7 aj .-. JLi I/) Li
450 Channel Nu
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