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STRUCTURE AND STABILITY OF MICROVOIDS IN a-Si:H R. BISWAS* and I. KWON** *Microelectronics Research Center, Iowa State University, Ames, IA 50011 **Los Alamos National Laboratory, Los Alamos, NM 87845

ABSTRACT Microvoids have been-observed in a-Si:H as demonstrated by small angle Xray scattering. We have studied the structural properties of these microvoids with molecular dynamics simulations. Using molecular dynamics simulations with classical potentials, we have created microvoids by removing Si and H atoms from a computer generated a-Si:H network. The internal surfaces of the microvoids were passivated with additional H atoms and the microvoids were fully relaxed. Microvoids over a limited range of sizes (5-90 missing atoms) were examined. We obtained a relaxed microvoid structure with no dangling bonds for a microvoid with 17 missing atoms, whereas other sizes examined produced less relaxed models with short H-H distances at the microvoid surface. The strains near the microvoid surface are described. The microvoid model was stable to local excitations on weak bonds in the vicinity of the microvoid. INTRODUCTION Microvoids have been observed in a variety of a-Si:H films by small angle X-ray scattering (SAXS) studies [1,2,3]. After correcting for the Laue monotonic scattering, the most recent measurements [3] indicate that the interpretation for microvoids in device-quality glow-discharge a-Si:H is not certain. These devicequality films were grown at a substrate temperature of 300 'C and at a deposition rate of 0.14 nm/s. However when the films are grown at a faster deposition rate of 1.35 nm/s and at the same substrate temperature, the films are of poorer quality and the SAXS data [3] clearly exhibit a void volume fraction of about 4% for microvoids that are approximately 1 nm in diameter. An excessive density of microvoids has been generally believed to correlate with poorer quality material and an increasing density of hydrogen and also dihydride bonding configurations [4]. An open question is what is the correlation between the microvoid content and electronic quality of the a-Si:H films? Another extremely relevant question is what is the correlation between lightinduced instability in a-Si:H and the occurrence of the microvoids. Recent measurements [3] indicate a correlation between the initial and degraded efficiencies of solar cells with microstructure. This was evidenced by depositing i-layers of p-i-n solar cells at different deposition rates with all other cell conditions remaining the same and measuring the solar cell efficiency. This suggests a correlation between voids and light-induced degradation, arguing for the importance in understanding the structure and electronic properties of the microvoids. The structural properties of microvoids are addressed in this paper.

Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993 Materials Research Society

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PREVIOUS SIMULATIONS Our previous molecular dynamics simulations have provided important insights into microvoids in amorphous silicon networks. These studie