Optimal Oxide Passivation of Ge for Optoelectronics

Abstract

Oxide passivation is an active field of investigation for reducing surface state densities on Ge surfaces, namely below high-K gate stacks. However, the quality of Ge oxide passivation has not been as widely investigated in the context of performance in minority carrier photonic devices. p-Ge substrates with a foundry-applied oxide passivation are compared with oxide-stripped substrates passivated by dry thermal oxides grown between 400 • C-650 • C in O 2 . By using p-Ge, only the behavior of minority conduction electrons and their associated traps become the relevant focus of this study. Passivation quality, as evaluated by photoluminescence (PL), is found to vary as a function of temperature between 400-650 • C. Changes in PL intensity are attributed to differences in surface termination of the p-Ge substrate. X-ray photoelectron spectroscopy (XPS) studies find that the presence of suboxides (GeO x ) increase with oxidation temperature, with GeO 2 dominant at 400 • C and mixed suboxides at higher temperatures. Nonuniform desorption of Ge oxide is confirmed above 550 • C. Thermal oxidation is also shown to improve the quality of ion-damaged Ge surfaces. We conclude that suboxides confer better passivation than a GeO 2 interface, as determined by PL intensity.