Nitrogen incorporation in nanocrystalline diamond thin films

Abstract

Diamond is a promising material for applications in high-temperature, high-power and high-speed electronic devices due to its exceptional physical and electrical properties. However, the difficulties in achieving the efficient n-type doping of diamond with controllable high conductivity form a serious obstacle in realizing the practical applications of diamond in the electronic field. In contrast to diamond single crystals or microcrystalline films, nanocrystalline diamond (ND) films have some specific properties, such as great surface smoothness, low friction coefficient, and low threshold for electron field emission, which are in some aspect even superior to single or polycrystalline diamonds. Moreover, due to the high density of grain boundaries, incorporation of nitrogen impurities (n-type dopants) into the ND films is demonstrated easier. The works herein are based on the synthesis of a series of nitrogen-incorporated ND films by microwave plasma-enhanced chemical vapor deposition (MP CVD) in a gas mixture of CH4/H2/N2. The nitrogen concentration is varied in a wide range (0-75 %) in the plasma. The phase composition, structure, and morphology of these ND films are studied systematically by using UV and visible Raman spectroscopy, x-ray diffraction (XRD) and scanning electron microscopy (SEM). It is revealed that the sp2/sp3 ratio of carbon bonds increases, and the grain size decreases with the increase of nitrogen concentration in the plasma. A peak located at ~1190 cm-1 is observed in the Raman measurement. Based on the dependence of its intensity on the nitrogen concentration, this peak is demonstrated to be possibly originated from the graphitic C−N bonding. Moreover, the current-voltage (I-V) measurements show a dramatic decrease of the resistivity, about six orders of the magnitude, of the nanocrystalline diamonds due to the nitrogen incorporation. The dependence of the resistivity on the nitrogen concentration is revealed. To study the role of nitrogen on the improvement of electrical conductivity of ND films, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were carried out to investigate the variations of composition and band structures induced by nitrogen incorporated in ND films. The nitrogen content in the films was found to be only about around 1% for all of the samples, and the depth profile analysis indicated that the nitrogen content maintained constant with the depth. However, the phase content of sp2 and sp3-hybridized C−C and C−N phases in the films depended obviously on the nitrogen concentration in the plasma. Combined with Raman spectroscopic observations, the variation of conductivity of ND films is suggested to be mainly due to the development of highly-ordered graphitic sp2 phase induced by the nitrogen incorporation. Furthermore, the UPS results revealed that new electronic states were created within the diamond band gap by the nitrogen incorporation, implying the hopping or thermal activated conduction in ND films as well. Another attractive property of diamond is its intrinsic negative electron affinity (NEA). With the improved electrical conductivity of ND films with nitrogen incorporation, ND films would be one of the most promising materials for fabricating the novel cold cathode field electron emission (FEE) devices. To enhance the FEE performance, e.g., to decrease the threshold electrical field and to improve the emission stability, a large field enhancement factor (geometry-dependent) is a key factor. In this works, the flat as-deposited, nitrogen-incorporated ND films were reconstructed successfully to ND cones and pillars with high aspect ratios via simple reactive ion etching (RIE) in hydrogen and argon plasmas with the assistance of metal masks. The effects of etching conditions (bias-voltage, gas composition, and pressure etc) and the thickness of metal mask on the formation of conical/pillar structures of ND was investigated in detail.

Date of Award	3 Oct 2006
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wenjun ZHANG (Supervisor)