Comparison of Inorganic and Organic Acid Etching Processes on Germanium(100)

Abstract

Germanium is currently used in semiconductor devices alloyed with silicon and shows promise as an alternative material which could eventually replace silicon. One challenge associated with implementing germanium is rapid oxidation after cleaning, which results in degraded electrical performance because germanium oxides contain many defects. Optimization of cleaning and etching processes are critical to the development of devices based on Ge. This study compares the use of inorganic and organic acids to etch germanium oxides, such as Ge(+1)2O, Ge(+2)O and Ge(+4)O2. X-ray photoelectron spectroscopy (XPS) was used to characterize the Ge(100) surface composition after hydrochloric and citric acid treatments. Immersion in concentrated HCl (12 M) completely removed both GeO2 as well as sub-stoichiometric oxides. The surface remained oxide free when etched inside an Ar purged glovebox attached to an XPS chamber, but suboxides were detected within 3 minutes when samples were exposed to air. A short immersion (60 s) in 2.4 M citric acid in ambient conditions yielded a visible film on the surface even after drying. Two rinse strategies were employed to remove residual citric acid from the surface. After a short rinse (30 s) in ethanol peaks attributed to carboxylic acid were detected in the C 1s XPS region. Residual citrate species were minimized by sonication in ethanol, however citrate species were still detected. Germanium surfaces etched in HCl showed less oxide and adsorbed carbon than those etched in citric acid however both surfaces are unstable in air and suboxides form readily. We investigated the two different acid cleans before passivation of the surface with sulfur.