Electronic Structure and Dynamics of Defect in a-Si:H By AB-Initio Molecular Dynamics
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ELECTRONIC STRUCTURE AND DYNAMICS OF DEFECT IN a-Si:H BY AB-INITIO MOLECULAR DYNAMICS N. ORITA*, T. SASAKI** AND H. KATAYAMA-YOSHIDA** *Electrotechnical Laboratory, Tsukuba 305, Japan **National Research Institute for Metals, Tokyo 153, Japan ***Department of Physics, Tohoku University, Sendai 980,
Japan
ABSTRACT Electronic structure and dynamics of defects in hydrogenated amorphous silicon (a-Si:H) are investigated based upon ab-initio molecular-dynamics simulations. It is shown that (i) the hydrogen-passivated dangling bond (Si-H), (ii) the positively-ionized three-centered bond (SiH÷-Si), (iii) the negatively-ionized three-coordinated dangling bond (D-) and (iv) the fivecoordinated floating bond (Fs) are the intrinsic defects in a-Si:H. Based upon the calculated result, we discuss the role of hydrogen and the origin of the photo-induced defect in a-Si:H. INTRODUCTION It is well known that the solar cells fabricated by hydrogenated amorphous silicon (a-Si:H) are degraded during the exposure to the light. The degradation by the light is believed to be due to the creation of the photo-induced defects including paramagnetic dangling bonds in the band gap of a-Si:H in which dangling bonds are non-magnetic before the exposure of the light. This is so called Stabler-Wronski effect [1]. Stabler-Wronski effect reduces the conductivity and photoconductivity by a few orders of magnitude, suggesting the creation of paramagnetic dangling bonds in the band gap. Regarding the role of hydrogen in a-Si:H, it is believed that the paramagnetic dangling bond in the band gap is passivated by a hydrogen atom. However, the role of the hydrogen in a-Si:H is still not clear because of the lack of microscopic experimental data. The purpose of the present paper is to elucidate the role of hydrogen and the microscopic origin of the defects in a-Si:H based upon an ab-initio molecular-dynamics simulation [2]. We investigate the electronic structure and dynamics of defects in a-Si:H for this purpose and give a unified physical picture to this system. CALCULATION METHOD We generate a-Si:H by rapid cooling and annealing using the dynamical simulated-annealing scheme proposed by Car and Parrinello [2] with the periodic boundary conditions of supercells containing 64 Si and 8 H atoms. To determine the metastable atomic coordinates (Rn) (n = 1,...,N, where N is the number of atoms) and the electronic structure of a-Si:H according to the Car-Parrinello method, the equations of motion as a function of time (t) and electron coordinates (r) are written as, gq~i(r; t) -- -H(t) Ti(r;t) + I e ij T j(r; t) ,
(1)
Mn i (t) = - Vn E [(TPi), (Rn)],
(2)
where gi and Mn are masses of wavefunctions and the n-th atom (while the atomic mass Mn is the real physical quantity, giis a fictitious parameter to realize the classical-mechanical motion of the wavefunction 'Pi(r; t) as an analog to the atomic motion, we choose gi = 3200 a.u. and the time step is At=10 a.u.), H is the one-electron hamiltonian appearing in the Kohn-Sham equation within the local-density
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