Near IR interband transitions and optical parameters of metal-germanium contacts

A theoretical and experimental study of the zero-bias quantum efficiency eta(0) for metal (Au, Cu, Ag)-Ge Schottky barrier photodetectors in the near IR range (1.1 microm < lambda < 1.8 microm) has been performed. By an interactive computer programming technique, the optical parameters of the metal thin film electrodes (index of refraction n and extinction coefficient k) as a function of wavelength and of film thickness are determined. Starting with a two-layer calculation of the reflectance R, transmittance T, and absorptance A of the metal electrode, it is found that the eta(0) in this near IR range is dominated by the band-to-band excitation of electrons in the Ge substrate. Using the minority carrier diffusion length L(p) as an adjustable parameter, good agreement between theoretical and experimental results was found for L(p) approximately 150 microm; this value was obtained independent of choice of metal or metal thickness justifying the above procedure.