Microstructural Evolution of Co 2 GaAs Thin Film on GaAs Substrate
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ABSTRACT Interfacial reactions and epitaxial growth of the microstructure between cobalt thin film and (100)-oriented GaAs substrate have been studied, using X-ray diffraction, transmission electron microscopy and EDS analysis. Cobalt thin films were deposited by e-beam evaporation, and were subsequently annealed at 400 'C. The reaction layer evolved due to the annealing was identified as the ternary phase, Co 2GaAs, that was grown epitaxially on the substrate. The new orientation relationship between C0 2GaAs and GaAs substrate was found as Co2GaAs H/ GaAs and {001)Co 2GaAs // {011 }GaAs. These results can be well explained upon matching the unit cells of C0 2GaAs and GaAs. Upon the annealing at 600'C, the Co 2GaAs phase is usually decomposed to stable binary phases, CoGa and CoAs.
INTRODUCTION Interfacial reactions between metal films and GaAs have been investigated in order to understand the electrical properties of metal-semiconductor interface. Since the electrical properties of metal-GaAs contact are sensitive to the microstructure evolved at the interfacial region, fundamental information of the structure, morphology and chemistry of the reaction layer is important to design a contact scheme. There have been a number of investigations reported on thin film studies for metals, such as Pd[1], Ni[2], Pt[3] on GaAs. Interfacial reactions of Co thin film with GaAs have also been investigated by several researchers[4-1 1]. They have reported that Co 2GaAs was initially observed at the annealing temperature of about 400 OC, and that a subsequent annealing at higher temperatures resulted in the formation of binary phases, such as CoGa and CoAs. Although the interfacial reaction is well understood phenomenologically, detailed information on the microstructure or morphology of the reaction layer, such as Co 2GaAs, is insufficient. The objective of the present investigation is to find out the epitaxial growth relationship of the microstructure evolved at the interfacial region in Co/GaAs system. The morphology and orientation relationship of the epitaxially grown ternary phase, C0 2GaAs, on the GaAs substrate were investigated based on the information obtained by XRD, TEM and EDS.
EXPERIMENT Chrome oxide doped semi-insulating GaAs substrates with (100) orientation were used in the present study. Thin film of 300nm Co was deposited by e-beam evaporation and Si0 2 thin film 159
Mat. Res. Soc. Symp. Proc. Vol. 399 8 1996 Materials Research Society
was sputter-deposited for the capping layer on the top of Co film. The base pressure was lower than 3 x 10-6 Torf and the deposition rate was about 4 A/sec. The sample was annealed at 400 "C in an evacuated (
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