Oxygen vacancy defects in tantalum pentoxide: a density functional study
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Oxygen vacancy defects in tantalum pentoxide: a density functional study a
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R. Ramprasad , Michael Sadd , Doug Roberts , Tom Remmel , Mark Raymond , Eric b b b a Luckowski , Sriram Kalpat , Carole Barron , Mel Miller a Motorola Semiconductor Products Sector, 2100 E. Elliot Rd., Tempe, AZ 85284 b Motorola Semiconductor Products Sector, 3501 Ed Bluestein Blvd, Austin, TX 78721 ABSTRACT First principles total energy calculations were performed in order to characterize O vacancy defects in Ta2O5. A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. Results indicate that O vacancies in Ta2O5 can be broadly classified based on their location in the lattice. One type of vacancies (occupying the “in-plane” sites) displays deep or mid gap occupied states, and shallow unoccupied states, while a second type (occupying “cap” sites) results in shallow occupied states. For a wide range of local Fermi level or chemical potential, the neutral and +2 charged states of the in-plane type vacancy and the +2 charge state of the cap type vacancy are found to be most stable. Migration energies of the two types of vacancies in the neutral and +2 charge states are markedly different, with the “cap” type of vacancies displaying very high barriers to migration (~ 5 eV) compared to the “in-plane” type (~ 0.5-1.0 eV). INTRODUCTION Although tantalum pentoxide (Ta2O5) has been studied both experimentally and theoretically over the past three decades, its real emergence as a dielectric material that can be integrated with conventional CMOS has happened only in the last decade [1]. While interest in high dielectric constant materials, in general, is primarily due to a need to scale down device sizes, the renewed interest in Ta2O5 is due to the ability to deposit it using conventional methods compatible with equipment and processes already available in the semiconductor industry. Nevertheless, concerns exist with Ta2O5 (and other alternative dielectric materials), one of them being defect densities, and their impact on the leakage currents (via defect or “trap” levels created in the band gap of the dielectric). The present work attempts to theoretically characterize oxygen vacancy defects in Ta2O5. The atomistic structure of Ta2O5 was chosen so that it is computationally tractable, while at the same time is representative of deposited films. Different types of O vacancies with qualitatively different types of coordination environments were considered within this model, and the defect or “trap” levels due to these defects were determined using first principles total energy calculations. Correlations between the coordination environment and the location of the defect levels were identified. The stability of variously charged vacancies was also considered, as a function of the vacancy type and the local chemical potential. Activation barriers to migration of vacancies in different charge states were also studied.
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