Comparative Study of Electronic Properties of Point Defects in Monoclinic Hafnium Dioxide
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Comparative Study of Electronic Properties of Point Defects in Monoclinic Hafnium Dioxide Valerie Cuny, and Nicolas Richard CEA, DIF, BP 12, BruyËres-le-Ch‚tel, 91680, France ABSTRACT Continuous downscaling of transistor leads silicon dioxide constituting the gate in typical metal oxide semiconductor field effect (MOSFET) to its limits. One possibility is to replace SiO2 by a material of higher dielectric constant (high-k). Hafnium dioxide seems to be the most promising one. However, high-k transistor performances are often affected by the presence of defects creating charge traps or diffusion centers. In this paper, using a pseudopotential plane wave code in the density-functional total theory framework, we calculate in a monoclinic HfO2 supercell the structure, formation and ionization energies, electron affinities of intrinsic defects (oxygen vacancy and interstitial, and hafnium vacancy). We consider different charge states of these defects. The positions of defect levels with respect to the bottom of silicon conduction band are determined. Our results will be discussed and compared to literature data. INTRODUCTION The miniaturization in the microelectronic industry leads to the end of the use of amorphous SiO2 gate dielectric in metal-oxide-semiconductor field-effect transistors. To continue this miniaturization, it would need to have a low thickness of the gate oxide. But a such decrease would involve mainly a reproducibility problem and a leakage current through the gate oxide increasing at an exponential rate. High-k gate dielectrics will be necessary to maintain a low tunneling leakage current. And also an oxide with a high dielectric constant would allow to have Equivalent Oxide Thickness (EOT) values of about 1 to 1.5 nm. Several candidates are susceptible of replace SiO2. Hafnium dioxide (HfO2) seems to be the most promising material because of mainly its high permittivity, a thermal stability and a low leakage current at the interface with silicon [1, 2]. Hafnia exists under different crystallographic structures. At low temperature, HfO2 has a 5 15 monoclinic C2h phase. Increasing temperature, it transforms into the tetragonal D4h phase (> 2000 K), and then into the cubic Oh5 phase (> 2870 K). We consider the monoclinic structure firstly for this stability at room conditions [3], and secondly because after anneling in oxygen at 500-1200∞C, the crystalline films remain near to a monoclinic structure [4]. The performance of thin hafnia film as gate dielectric seems to be affected by defects. Notably, film annealing will involve the formation of oxygen interstitial [4]. And the threshold voltage shift of MOS transistor will be due to charge trapping in the gate dielectric and oxygen vacancy will be one of the major traps contributing to charge trapping in HfO2 [5]. Effect of these defects in monoclinic hafnia (mHfO2) has stimulated these calculations submitted in this paper. Several theoretical investigations on electronic properties of point defects in HfO2 have been carried out [4, 6-10]. Fo
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