Incorporation of nitrogen in diamond by using a modulated growth technique

Abstract

Owing to negative electron affinity (NEA), diamond has emerged as one of the most important materials for a low threshold cold cathode in vacuum microelectronics and in flat-panel display technology [1]. N-type doping of diamond is an important problem to be solved for effective emission of electrons. Nitrogen is one of common impurities in both natural and synthetic diamond. A recent report showed the possibility of nitrogen for n-type doping in the diamond matrix [2]. Substitutional nitrogen produces a deep-lying donor level below the conduction band minimum. The activation energy is roughly 1.7 eV. Therefore, the level is too deep to be ionized at room temperature. A dramatic improvement of field emission characteristics has been reported for a heavily nitrogen-doped diamond [3]. Urea [(NH3)2CO], N2 and melamine [C3H6N6] have been utilized as nitrogen doping sources [4–6]. Specifically, Okano et al. achieved the lowest turn-on field of ∼0.5 V/μm with nitrogen-doped diamond films by employing urea as a doping source [7]. However, a significant degradation of diamond quality has been observed during the nitrogen incorporation [5, 8]. In particular, the nitrogen incorporation substantially affects the surface morphology and thereby results in a spiked surface morphology with very low growth rate. This result means that there is a trade-off between surface morphology and doping density. The trade-off has been a technical hurdle in achieving heavy nitrogen doping while maintaining a reasonable surface morphology. In this paper, we introduce a modulated growth technique to overcome the hurdle. A home-built hot filament chemical vapor deposition (HFCVD) system was used for deposition of diamond films. Twisted W wires (0.45 mm diameter) were used for a filament. Prior to deposition, the filament was carburized at least for 7 h while monitoring the resistivity of W wires. The filament temperature was measured by an optical pyrometer. The separation between the substrate (n-type Si(111), 5× 5 mm2) and filament was 6.5 mm. The substrate surface was scratched with 1 μm diamond suspension. After dipping into a diluted