Influence of sulfur incorporation on field-emission properties of microcrystalline diamond thin films

Abstract

Results are reported on the electron field emission properties of microcrystalline diamond thin films grown on molybdenum substrates by the sulfur (S)-assisted hot-filament chemical vapor deposition technique using methane (CH4), hydrogen sulfide (H2S), and hydrogen (H2) gas mixtures. Electron field-emission measurements revealed that the S-incorporated microcrystalline diamond thin films have substantially lower turn-on fields and steep rising currents as compared to those grown without sulfur. The field-emission properties for the S-incorporated films were also investigated systematically as a function of substrate temperature (TS). Lowest turn-on field achieved was observed at around 12.5 V/μm for the samples grown at TS of 700°C with 500 ppm H2S. To establish the property-structure correlation, we analyzed the films with multiple characterizations include scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy (RS), and x-ray photoelectron spectroscopy (XPS) techniques. It was found that sulfur addition causes significant microstructural changes in microcrystalline diamond thin films. S-assisted films show smoother, coarse-grained surfaces (non-faceted) than those grown without it (well-faceted) and a relatively higher content of non-diamond carbon (primarily sp2-bonded C). RS and investigations on the morphology by SEM and AFM indicated the increase of sp2 C content with increasing TS followed by a morphological transition at 700°C in the films. XPS investigations also showed the incorporation of S in the films up to a few atomic layers. It is believed that the electron-emission properties are governed by the sulfur incorporation during the chemical vapor deposition process. Although most of the S is expected to be electrically inactive, under the high doping conditions hereby used, it is shown rather indirectly through multiple characterizations that there may be some amount of S in donor states. Therefore the results are discussed in terms of the dual role of S whereby it induces the structural defects in the form of enhanced sp2 C content at the expense of diamond quality and a possibility of availability of conduction electrons. In fact the latter finding is supported through room temperature electrical conductivity measurements.