Preparation and properties of clean Si3N4 surfaces

In situ chemical methods for preparing atomically-clean surfaces of Si3N4 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(100) wafers by low-pressure chemical vapor deposition) were characterized using primarily infrared reflection-absorption spectroscopy. A combination of annealing in NH3 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 O2 chemisorption show adsorbate-induced features in the Si3N4 band gap and also evidence for changes in surface potential due to adsorption. (Relevance to films as passivation layers in integrated circuits, protective coatings for magnetic disk drives, etc. noted.)