Combinatorial Approach to Thin-Film Silicon Materials and Devices
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Combinatorial approach to thin-film silicon materials and devices Qi Wang, Leandro R. Tessler1, Helio Moutinho, Bobby To, John Perkins, Daxing Han2, Dave Ginley, and Howard M. Branz National Renewable Energy Laboratory (NREL), 1617 Cole Blvd., Golden, CO 80401 U.S.A. 1 Instituto de Física “Gleb Wataghin,” Unicamp, C. P. 6165, 13083-970 Campinas, SP, Brazil 2 Department of Physics & Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599 U.S.A. Abstract We apply combinatorial approaches to thin-film Si materials and device research. Our hot-wire chemical vapor deposition chamber is fitted with substrate xyz translation, a motorized shutter, and interchangable shadow masks to implement various combinatorial methods. For example, we have explored, in detail, the transition region through which thin Si changes from amorphous to microcrystalline silicon. This transition is very sensitive to deposition parameters such as hydrogen-to-silane dilution of the source gas, chamber pressure, and substrate temperature. A material library, on just a few substrates, led to a three-dimensional map of the transition as it occurs in our deposition system. This map guides our scientific studies and enables us to use several distinct transition materials in our solar-cell optimization research. We also grew thickness-graded wedge samples spanning the amorphous-to-microcrystalline Si transition. These single stripes map the temporal change of the thin silicon phase onto a single spatial dimension. Therefore, the structural, optical, and electrical properties can easily be studied through the phase transition. We have examined the nature of the phase change on the wedges with Raman spectroscopy, atomic force microscopy, extended x-ray absorption fine structure (EXAFS), x-ray absorption near-edge spectroscopy (XANES), ultraviolet reflectivity, and other techniques. Combinatorial techniques also accelerate our device research. In solar cells, for example, the combinatorial approach has significantly accelerated the optimization process of p-, i-, n-, and buffer layers through wide exploration of the complex space of growth parameters and layer thicknesses. Again, only a few deposition runs are needed. It has also been useful to correlate the materials properties of single layers in a device to their performance in the device. We achieve this by depositing layers that extend beyond the device dimensions to permit independent characterization of the layers. Not only has the combinatorial approach greatly increased the rate of materials and device experimentation in our laboratory, it has also been a powerful tool leading to a better understanding of structure-property relationships in thinfilm Si. Introduction The combinatorial approach (high-throughput experimentation) has been applied with success in recent years to thin-film materials research [1-4]. However, the initial work on this approach was begun by Joseph Hanak almost 30 years ago [5]. Hanak made a continuously varied composition sample to study binary
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