Hydrogen passivation of vacancies in diamond: Electronic structure and stability from ab initio calculations
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Hydrogen passivation of vacancies in diamond: Electronic structure and stability from ab initio calculations Kamil Czelej1 and Piotr Śpiewak1 1
Materials Design Division, Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland ABSTRACT Point defects in diamond such as vacancies act as a strong donor compensation center; therefore, remarkably reduce electron conductivity of diamond-based devices. Artificial synthesis methods of n-type diamond utilize the hydrogen-containing precursors enabling its diffusion into diamond crystal and subsequent formation of hydrogen-vacancy complexes. Here we employ spinpolarized, hybrid density functional theory calculations, in order to characterize the electronic properties and stability of hydrogen-passivated vacancies in diamond. We found strong thermodynamic preference for hydrogen passivation of four vacancy-related dangling bonds. An analysis of formation energy vs Fermi level diagrams indicate, that strong donor compensation effect associated with vacancies can be entirely neutralized by hydrogen incorporation. Thus, a careful control of hydrogen partial pressure in the growth process might be crucial to improve the electron conductivity of n-type diamond. INTRODUCTION Diamond is a transparent, wide band gap semiconductor with unique combination of electronic and optical properties. In particular, very high electron and hole mobilities of 4500 cm-2V-1s-1 and 3800 cm-2V-1s-1, respectively [1], thermal conductivity >2000 Wm-1K-1 and breakdown voltage of 107 Vcm-1 [2] enable diamond to surpass other wide band gap semiconductors applied for high power and high frequency electronic devices. However, these properties are highly sensitive to the presence of lattice defects, such as vacancies, dopants or impurities introduced into the diamond during the material synthesis. For this reason, a thorough understanding of the electronic structure, equilibrium geometry and stability of the defects is of paramount importance for high power electronic and optoelectronic fields. Hydrogen is the smallest element in the periodic table; therefore, it can penetrate the diamond lattice and form hydrogen-related defects. Over the last decades, the impact of isolated hydrogen and hydrogen pairs on the electronic structure, formation energy and equilibrium geometry of hydrogen defects in diamond have been investigated using different theoretical methods [3–15]. The center bond site as the most stable geometry of interstitial H impurity was predicted by several groups [3–5,8]. In the case of hydrogen pairs, various equilibrium geometries were reported in literature [8]. The most stable geometry is so-called H2* complex with one H atom located at the C–C bond and the second H atom located at the anti-bonding site, behind the C atom, along C–C axis [8]. It has been well established that interstitial hydrogen defects induce a number of defect states in the band gap of diamond and compensate both n-type and p-type dopants [3,5,7,8]. Recently, compl
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