Real-time spectroscopic ellipsometry for III-V surface modifications - Hydrogen passivation, oxidation and nitridation by plasma processing

Passivation of GaAs surfaces through an initial cleaning step by hydrogen plasmas followed by the formation of an oxide layer by UV-light-assisted oxygen plasma anodization or by the formation of a wide gap GaN layer by N-2 plasma nitridation is presented. The emphasis is on the chemical and microstructural modifications which surfaces undergo during the above passivation processes. In situ spectroscopic ellipsometry is used to monitor the surface in real-time. Specifically, the H-2 remote plasma is operated at 230 degrees C to remove native oxide from GaAs and InP surfaces. Real-time ellipsometry well discriminates the cleaning end-point, the structural damage and the passivation of shallow defects operated by H-atoms, so explaining the observed change of the photoluminescence intensity as a function of hydrogen exposure. O-2 plasma anodization carried out at 130-250 degrees C under UV-light irradiation of the sample is able to change the chemistry of the growing GaAs oxide layer, in that an As2O5-rich oxide with reduction of elemental arsenic is obtained. Segregation of As at the GaAs surface is also encountered during N-2 plasma nitridation of GaAs to form a GaN passivating layer. This problem is overcome by low-temperature (250-350 degrees C) nitridation of GaAs with N-2-H-2 plasmas. An attempt is made to correlate the surface chemistry as determined by ellipsometry with the Fermi level unpinning as determined by X-ray photoelectron measurements.