Surface Processes during Growth of Hydrogenated Amorphous Silicon
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Surface Processes during Growth of Hydrogenated Amorphous Silicon Eray S. Aydil1, Sumit Agarwal1, Mayur Valipa1,2, Saravanapriyan Sriraman1, and Dimitrios Maroudas2, 1 Chemical Engineering Department, University of California Santa Barbara, Santa Barbara, CA 93106-5080, U. S. A. 2 Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003-3110, U. S. A. ABSTRACT Hydrogenated amorphous silicon films for photovoltaics and thin film transistors are deposited from silane containing discharges. The radicals generated in the plasma such as SiH3 and H impinge on the surface and lead to silicon film growth through a complex network of elementary surface processes that include adsorption, abstraction, insertion and diffusion of various radicals. Mechanism and kinetics of these reactions determine the film composition and quality. Developing deposition strategies for improving the film quality requires a fundamental understanding of the radical-surface interaction mechanisms. We have been using in situ multiple total internal reflection Fourier transform infrared spectroscopy and in situ spectroscopic ellipsometry in conjunction with atomistic simulations to determine the elementary surface reaction and diffusion mechanisms. Synergistic use of experiments and atomistic simulations elucidate elementary processes occurring on the surface. Herein, we review our current understanding of the reaction mechanisms that lead to a-Si:H film growth with special emphasis on the reactions of the SiH3 radical. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) films deposited fom SiH4 containing plasmas are widely used in solar cells and thin film transistors (TFTs) for flat panel displays [1]. The microstructure and electronic properties of plasma-deposited a-Si:H films depend strongly on the reactions of reactive radicals produced in the discharge (SiHx: 0≤x≤3, H) with the film surface. Although post-deposition processing, such as thermal or laser annealing, can improve electronic properties, much of the film’s structure and electronic properties are determined during film growth. Thus, the film growth mechanism has been of great interest since the deposition of first amorphous silicon films. Much of our knowledge of the deposition mechanism is inferred from macroscopic observations, such as the radical concentrations in the plasma during deposition and the variation of the deposition rate or film properties with plasma conditions [2-5]. In the last decade, direct measurement of surface species and gas phase radicals during deposition has advanced our understanding of the specific reactions that lead to a-Si:H film growth. Such measurements were made possible by advances in surface sensitive infrared spectroscopies and plasma diagnostics, such as cavity ring down spectroscopy [6-24]. While it is possible to determine the surface compositions and species fluxes impinging on the surface through a variety of plasma and surface diagnostics, it is impossible to observe experimentally the elementary surface re
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