In this study the reaction of In/n-GaAs during thermal annealing has been investigated by transmission electron microscopy. It was expected that thermal annealing could result in the formation of $In_{1-x}Ga_xAs$ epitaxial phases at the interface which can make the interface ohmic by lowering the potential barrier for electron transport.
Indium was deposited on n-type (001) GaAs substrate by thermal evaporation at room temperature. Then the In/n-GaAs structures were annealed at 350℃ and 750℃ in flowing $N_2$ ambient. These temperatures were above- and below the critical temperature which is related to the compositional miscibility gap of In-Ga-As ternary system. And these temperatures were above the melting point of indium, so the thermal reaction and phase transformations were expected to be carried out from molten states.
The results of this study showed that no epitaxial $In_{1-x}Ga_xAs$ phases were formed at the interface between indium and n-GaAs substrate before thermal annealing nor after thermal annealing in flowing $N_2$. As-deposited indium films were polycrystalline body-centered tetragonal with random orientation to the GaAs substrate. After 350℃ -30 min annealing, recrystallized polycrystalline indium grains from the molten state were observed, and their orientations to the substate were random. During 350℃ -120 min annealing, polycrystalline cubic $In_2O_3$ phases were formed, which had lattice parameter 1.012 nm. And their {111} planes were approximately parallel to the {001} or {111} planes of GaAs substrate. At 750℃, large voids were observed in GaAs substrate which were considered to be the result of As out-diffusion induced decomposition of GaAs. And near the voids cubic γ-Ga_2O_3 with lattice parameter 0.822 nm was formed with an epitaxial orientation relationship to the substrate. In addition, an unknown cubic phase with a large lattice parameter was observed at upper regions of films. Its shape was faceted and it had a particular orientation relationship with repect to the substrate. At 750℃ inhomogeneous decomposition of GaAs and oxidation were severe during thermal annealing of In/n-GaAs.
In conclusion, no epitaxial $In_{1-x}Ga_xAs$ phases were produced by thermal annealing of In/n-GaAs in $N_2$ ambient. These results are thought to be attributed to the annealing condition, for it caused the phase transformation and cystallization from the molten states to be initiated at the surface, not at the interface. And at higher temperatures it was suggested that inhomogeneous decomposition of GaAs and severe oxidation made it difficult to produce epitaxial $In_{1-x}Ga_xAs$ phases by thermal annealing.
