Abstract

According to the commonly accepted theory,¹ AuGe forms an ohmic contact with n-type GaAs by having Au combining with Ga to form various AuGa compounds, thereby leaving Ga vacancies in GaAs. Ge then occupies these Ga vacancies and as a result dopes heavily the underlying GaAs region. The width of the surface depletion region caused by the characteristic surface Fermi level pinning is thus reduced, and electrons are therefore able to tunnel through this barrier resulting in a linear I-V relationship. In this experiment, thin AuGe and Au contacts are made to Si doped n⁺GaAs. Polarized Raman spectra are taken in the backscattering geometry to determine the possible changes in doping level under the contacts. We find that both samples show an increase in doping directly underneath the contacts. By analyzing the ratios of LO and L⁻ lines, the doping concentration is estimated to have increased from ~2.3 × 10¹⁸ to 5 × 10¹⁸ cm^-3, which is however not enough to produce efficient tunneling and ohmic behavior.^1,2 Since this increase is also present in the contact without Ge, it is presumably caused by the excess Si in the n⁺GaAs occupying the Au- induced vacancies. This experiment suggests that the function of Ge in AuGe contact is more subtle than previously thought, and the commonly accepted theory needs to be modified.

© 1989 Optical Society of America