A SAXS Study of Hydrogenated Nanocrystalline Silicon Thin Films

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1245-A13-02

A SAXS Study of Hydrogenated Nanocrystalline Silicon Thin Films K.G. Kiriluk1, D. L. Williamson1, D. C. Bobela2 , P. C. Taylor1, B. Yan3 , J. Yang3, S. Guha3, A. Madan4, F. Zhu4 1

Colorado School of Mines, Golden, CO 80401 National Renewable Energy Laboratory, Golden, CO 80401 3 United Solar Ovonic LLC, Troy, MI 48084 4 MVSystems Inc. Golden, CO 80401 2

ABSTRACT We have used small-angle x-ray scattering (SAXS) in conjunction with X-ray diffraction (XRD) to study the nanostructure of hydrogenated nanocrystalline silicon (nc-Si:H). The crystallite size in the growth direction, as deduced from XRD data, is 24 nm with a preferred [220] orientation in the growth direction of the film. Fitting the SAXS intensity shows that the scattering derives from electron density fluctuations of both voids in the amorphous phase and H-rich clusters in the film, probably at the crystallite interfaces. The SAXS results indicate ellipsoidal shaped crystallites about 6 nm in size perpendicular to the growth direction. We annealed the samples, stepwise, and then measured the SAXS and ESR. At temperatures below 350◦C, we observe an overall increase in the size of the scattering centers on annealing but only a small change in the spin density, which suggests that bond reconstruction on the crystallite surfaces takes place with high efficacy. INTRODUCTION Hydrogenated nanocrystalline silicon (nc-Si:H) is starting to be used in multi-junction solar cells with thin film based materials. nc-Si:H has been shown to suffer less light induced degradation than a-Si:H [1], however like a-Si:H, the degradation mechanism is not currently understood. nc-Si:H is a complicated, inhomogeneous material and understanding its nanostructure will help to understand the degradation mechanism. During the nc-Si:H deposition process, silane gas is diluted with hydrogen to promote the growth of crystallites in an amorphous matrix. Varying the amount of hydrogen dilution during growth maintains near uniform density and size of the crystallites [2]. The hydrogen also serves to passivate Si dangling bonds found in the amorphous phase of the nc-Si:H film and on the surfaces of crystallites. To understand the growth and size of the crystallites, X-ray diffraction (XRD) and smallangle x-ray scattering (SAXS) measurements were performed on several different nc-Si:H samples. For the as-deposited nc-Si:H samples, the average crystallite size in the direction perpendicular to the diffraction planes and parallel to the growth direction is related to the width of the diffraction peaks in XRD by the well-known Scherrer formula [3]. The average crystallite size in the direction perpendicular to the growth direction is obtained by analyzing the shape and intensity of the SAXS data [4]. The evolution of the nanostructure can be probed by isochronally annealing the samples, then measuring the resulting SAXS scattering intensity. When the anneal temperatures are

sufficiently high to drive out hydrogen, the SAXS profile changes therefore indicating a change in the over

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