In-situ Incorporation of Lithium and Nitrogen into CVD Diamond Thin Films
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In-situ Incorporation of Lithium and Nitrogen into CVD Diamond Thin Films M. Zamir Othman, Paul W. May and Neil A. Fox School of Chemistry, University of Bristol, Bristol, United Kingdom ABSTRACT Experiments were performed to incorporate Li and N simultaneously into the diamond lattice during hot-filament chemical vapour deposition in an attempt to produce n-type semiconducting diamond with useful electronic characteristics. Microcrystalline diamond films were grown using a mixture of methane/ammonia/hydrogen gases with tantalum as the filament. The Li was added by placing crystals of lithium nitride (Li3N) on the substrate and allowing them to melt and then slowly diffuse into the film. SIMS depth profiles showed that this process produced high levels of Li and N (0.05% - 0.5%) situated in the same region within the diamond film. The crystallinity and morphology of diamond crystals produced were confirmed using laser Raman spectrometry and scanning electron microscopy. INTRODUCTION Despite recent developments in doping diamond films with phosphorus, nitrogen, antimony, arsenic and sulfur, the production of n-type semiconducting diamond with good electronic properties remains elusive [1-4]. Lithiated diamond surfaces were suggested to produce low work-function materials [5, 6] and theoretical studies have predicted that interstitial lithium will act as a shallow donor and will enhance the electrical properties of diamond [7]. The energy to excite an electron from the lithium donor level to the conduction band of diamond is calculated to be less than 0.3 eV [8], however this has proved difficult to obtain experimentally. Research has been focused on trying to force Li into the diamond lattice by implantation, diffusion and insertion during growth, but in all cases the Li remained inactive inside the diamond lattice [9, 10]. This is partly due to the low solubility of Li, but also to its high mobility in diamond at high temperature which causes diffusion and aggregation of the Li into unwanted Li clusters [7]. It has been suggested [11] that this Li diffusion can be prevented by simultaneously adding nitrogen together with Li, with the N acting as a trap to pin down the Li in the diamond lattice and reduce its mobility. In theory, the electrons from an interstitial Li are transferred through a substitutional N atom directly into the diamond lattice. Thus, it is suggested that if a suitable co-doping process can create defect sites with 1:1 ratio of Li:N, this could produce n-type semiconducting diamond with a shallow donor level and high electron mobility. In this paper, we present the results of a study to incorporate both Li and N while growing diamond thin films using hot-filament chemical vapour deposition (HFCVD) system. Secondary Ion Mass Spectrometry (SIMS) depth-profile analysis is presented to show the concentration of both dopants and the thickness of the dopant layer inside the diamond film.
EXPERIMENT To grow lithium-nitrogen-codoped diamond, lithium nitride (Li3N) and ammonia gas were used as lithium an
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