Sulfur doping of diamond films: Spectroscopic, electronic, and gas-phase studies

Abstract

Chemical vapor deposition (CVD) has been used to grow sulfur doped diamond films on undoped Si and single crystal HPHT diamond as substrates, using a 1% CH4/H2 gas mixture with various levels of H2S addition (100–5000 ppm), using both microwave (MW) plasma enhanced CVD and hot filament (HF) CVD. The two deposition techniques yield very different results. HFCVD produces diamond films containing only trace amounts of S (as analyzed by x-ray photoelectron spectroscopy), the film crystallinity is virtually unaffected by gas phase H2S concentration, and the films remain highly resistive. In contrast, MWCVD produces diamond films with S incorporated at levels of up to 0.2%, and the amount of S incorporation is directly proportional to the H2S concentration in the gas phase. Secondary electron microscopy observations show that the crystal quality of these films reduces with increasing S incorporation. Four point probe measurements gave the room temperature resistivities of these S-doped and MW grown films as ∼200 Ω cm, which makes them ∼3 times more conductive than undoped diamond grown under similar conditions. Molecular beam mass spectrometry has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for H2S doping levels (1000–10 000 ppm in the gas phase) in 1% CH4/H2 mixtures, and for 1% CS2/H2 gas mixtures, for both MW and HF activation. CS2 and CS have both been detected in significant concentrations in all of the MW plasmas that yield S-doped diamond films, whereas CS was not detected in the gas phase during HF growth. This suggests that CS may be an important intermediary facilitating S incorporation into diamond. Furthermore, deposition of yellow S was observed on the cold chamber walls when using H2S concentrations >5000 ppm in the MW system, but very little S deposition was observed for the HF system under similar conditions. All of these results are rationalized by a model of the important gas phase chemical reactions, which recognizes the very different gas temperature profiles within the two different types of deposition reactor.