Femtosecond Carrier Dynamics in Nanocrystalline Silicon Films: The Effect of the Degree of Crystallinity
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Femtosecond Carrier Dynamics in Nanocrystalline Silicon Films: The Effect of the Degree of Crystallinity K.E. Myers,1 Q. Wang,2 and S.L. Dexheimer1 1
Department of Physics and Materials Science Program, Washington State University, Pullman, WA 2 National Renewable Energy Laboratory, Golden, CO
ABSTRACT We present studies of the ultrafast dynamics of photoexcited carriers in HWCVD nanocrystalline silicon thin films to address the underlying physics of carrier relaxation and recombination processes in this heterogeneous material. The degree of crystallinity is controlled by varying the H-dilution during deposition, yielding materials with increasingly larger grain size and crystalline fraction at higher dilution values. Time-resolved measurements of the carrier dynamics were made using a femtosecond pump-probe method, in which a short pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in the optical properties as a function of the pump-probe delay time. Photoexcitation of carriers with pulses 35 fs in duration centered at 1.55 eV results in a net induced absorbance signal in the near-infrared that is analyzed in terms of a multi-component response that includes contributions from the silicon crystallites and the amorphous matrix.
INTRODUCTION Thin film hydrogenated nanocrystalline silicon (nc-Si:H), comprised of nanoscale crystallites of silicon embedded in a hydrogenated amorphous silicon (a-Si:H) matrix, has attracted interest due to its potential optoelectronic applications as well as the physical properties that result from its heterogeneous nature. This material is promising for photovoltaic applications, especially given its high efficiency and resistance to the photoinduced degradation characteristic of thin-film amorphous silicon. An important issue in the physics of this heterogeneous material is the extent to which its optical and electronic properties can be understood simply as a combination of the properties of the separate phases, and in particular, the role of the grain boundary regions. The characteristics of nc-Si:H, including the size of the crystallites and the crystalline fraction, or ratio of the volumes of the crystalline and amorphous components, can be directly controlled via the film deposition conditions. We have carried out systematic studies of photoexcited carrier dynamics in nc-Si:H as a function of its composition, and we find that the fast optical response can be understood in terms of contributions from each of the separate phases in the material.
A16.5.1
MATERIALS Thin film nc-Si:H samples were grown by the hot-wire assisted chemical vapor deposition (HWCVD) technique, and the composition of the films was controlled by variation of the hydrogen dilution ratio R = H2 / SiH4 during deposition. The films were deposited at a substrate temperature of 240 °C on Corning 7059 glass, and the hydrogen dilution ratio was varied at a gas pressure of 30 mTorr. X-ray diffraction measurements indicate that the threshold for the transition fro
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