Ab initio simulation of the electronic structure of Ta 2 O 5 crystal modifications
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ONIC PROPERTIES OF SOLID
Ab Initio Simulation of the Electronic Structure of Ta2O5 Crystal Modifications T. V. Perevalov* and A. V. Shaposhnikov Rzhanov Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *email: [email protected] Received December 2, 2012
Abstract—Ab initio simulation of the electronic structure crystalline β and δ phases of tantalum(V) oxide (Ta2O5), representing a promising dielectric material for microelectronics, has been carried out. Both ideal crystals and those with neutral oxygen vacancies in various coordination positions have been studied. The simulation has been performed using the density functional theory with hybrid functionals involving the Har tree–Fock exchange energy. This approach gives a correct description of the bandgap width: 4.1 eV for βTa2O5 and 3.1 eV for δTa2O5. The energy levels related to oxygen vacancies in various positions have been determined for the spectra of electron states in β and δTa2O5 polymorphs. It is established that the presence of oxygen vacancies in Ta2O5 crystal modifications leads to the formation of characteristic absorption peaks in their electron energy loss spectra. DOI: 10.1134/S1063776113040158
1. INTRODUCTION In recent years, tantalum(V) oxide (Ta2O5) films have been objects of extensive experimental and theo retical investigation, related to their importance for micro and nanoelectronics [1]. A high dielectric per mittivity and compatibility with conventional VLSI technology processes make Ta2O5 a promising candi date material for solving problems of large tunneling leakage currents encountered in further scaling of dynamic randomaccess memory (DRAM) devices [2–4]. In addition, Ta2O5 films have very promising for use as a recording medium in flash memory devices [5–7]. These new memory devices exhibit a better relationship between the charge storage and repro gramming times as compared to that in conventional flash memory. At present, tantalum oxide is also con sidered a promising material for nextgeneration non volatile randomaccess memory devices based on resistive switching [8]. One of the main problems encountered in imple menting Ta2O5 into the technological process is the unacceptably high conductivity of this material, which is related to a large defect concentration. It has been found that the conductivity of Ta2O5 films obtained by various methods significantly decreases upon anneal ing in oxygencontaining media such as ozone and NO2 [9–12] and O2 plasma [13]. This effect was explained by assuming that conduction in the initial Ta2O5 film, which is depleted of hydrogen, proceeds via oxygen vacancies—in agreement with the fact that these vacancies are the predominant intrinsic defects in most dielectrics with high permittivity [14].
The most widely used methods of growing Ta2O5 films are based on chemical vapor deposition (CVD) and atomic layer deposition (ALD). Both these tech nologies involve postgrowth treatment (in particular, hightemperature annealing
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