Preparation and properties of clean Si 3N 4 surfaces

Abstract

In situ chemical methods for preparing atomically-clean surfaces of Si 3N 4 thin films in ultra-high vacuum (UHV) have been studied using X-ray and ultraviolet photoemission, electron energy loss and Auger electron spectroscopies. Prior to the UHV studies, the films (grown ex situ on Si(1 0 0) wafers by low-pressure chemical vapor deposition) were characterized using primarily infrared reflection–absorption spectroscopy. A combination of annealing in NH 3 to remove C and deposition of Si (followed by thermal desorption) to remove O is found to be an effective cleaning procedure. Other potential cleaning methods, such as annealing in UHV without in situ chemical treatment and annealing in a flux of H atoms, were also considered and found to be only partly effective. The clean surfaces are disordered but show no evidence of SiSi bonding (which would indicate N vacancies) in the Si LVV Auger spectrum or in surface-sensitive Si 2p photoemission data. Evidence for surface-related features is seen in the N 1s photoemission and in energy loss spectra in the region of valence excitations; however, no indication of occupied surface states near the valence band maximum is seen in ultraviolet photoemission spectra. Preliminary results for O 2 chemisorption show adsorbate-induced features in the Si 3N 4 band gap and also evidence for changes in surface potential due to adsorption.