STM Characterization of Low Dimensional Surface Electronic Properties of Undoped Diamond in Buffer Solutions
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1039-P14-03
STM Characterization of Low Dimensional Surface Electronic Properties of Undoped Diamond in Buffer Solutions Nianjun Yang, Hiroshi Uetsuka, Takatoshi Yamada, and Christoph E. Nebel Diamond Research Center/AIST, Tsukuba, 305-8568, Japan ABSTRACT Diamond is a promising semiconductor which shows some unique surface electronic features if grown optimized with low bulk and surface defect densities. The appearance of a highly conducting surface layer if immersed into electrolyte solution is maybe the most striking feature. Scanning tunneling microscopy experiments on diamond in electrolyte solutions are applied to determine the electronic properties governing these transition. These experiments reveal the formation of unoccupied quantized electronic states in the valence band close to the surface. A two-dimensional density of state distribution with three levels from light-, heavy-, and split-off-band holes is detected. Removal of the electrolyte causes a reversible transition into the insulating state and vice versa. INTRODUCTION In science and technology diamond is well known as gemstone, but hardly as promising semiconductor, although exciting new electronic phenomena have been reported over recent years [1-6]. The surface of diamond shows superior features unique to diamond which attracted significant attention during the last 15 years [7,8]. For example, insulating diamond becomes conductive like a metal (“insulator metal transition”) if it is immersed into electrolyte solution [9]. The change from insulating to conductive arises by valence band electrons which transfer into the electrolyte [8], thereby generating a hole accumulation layer in diamond. This layer is electronically confined, located at the surface, and extends only several tens of Ångstroem into the bulk [10]. Numerical calculations using the Schrödinger and Poisson equations simultaneously, predict quantized electronic properties of this hole accumulation layer due to the formation of a two-dimensional (2D) quantum well [11,12]. All applied conductivity and Hall-effect experiments up-to now, however, failed to reveal these quantization effects [10]. This has been attributed to disorder effects like surface roughness and scattering at ionized impurities so that macroscopic experiments cannot reveal the 2D character. Only recently, Gan and co-workers reported about 2D quantization effects as detected by electron field emission experiments on H-terminated diamond [13]. This finding may be due to the fact that field emission of electrons originates from nano-meter size areas. In this paper we report about scanning tunneling microscopy experiments (STM) applied on hydrogen terminated undoped diamond in electrolyte solution to characterize the local electronic density of state distribution of diamond in contact with electrolyte solution. In the past, STM experiments have been performed on boron doped diamond [14] and H-terminated undoped diamond [15] in ultra-high vacuum to characterize the arrangement of atoms on the surface. In case of undope
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