Hopping transport in doped co-deposited mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films
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Hopping transport in doped co-deposited mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films L. R. Wienkes, C. Blackwell, and J. Kakalios School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 ABSTRACT Studies of the electronic transport properties of n-type doped hydrogenated amorphous/nanocrystalline silicon (a/nc-Si:H) films deposited in a dual-plasma co-deposition reactor are described. For these doped a/nc-Si:H, the conductivity increases monotonically for increasing crystal fractions up to 60% and displays marked deviations from a simple thermally activated temperature dependence. Analysis of the temperature dependence of the activation energy for these films finds that the dark conductivity is best described by a power-law temperature dependence, σ = σo (T/To)n where n = 1 – 4, suggesting multiphonon hopping as the main transport mechanism. These results suggest that electronic transport in mixed-phase films occurs through the a-Si:H matrix at lower nanocrystal concentrations and shifts to hopping conduction between clusters of nanocrystals at higher nanocrystal densities. INTRODUCTION Photovoltaic devices employing mixed-phase silicon thin films, consisting of silicon nanocrystals embedded in hydrogenated amorphous silicon (a-Si:H) as the photovoltaic material, have recently attracted considerable attention due to their high solar conversion efficiencies, high deposition rates and improved resistance to light-induced defect creation (the Staebler-Wronski effect) without a degradation of the nominal opto-electronic properties [1-3]. These mixed-phase films are typically synthesized in either a capacitively-coupled single-chamber plasma system, using high gas pressures and a heavily hydrogen-diluted silane precursor [4-8], or in a dualchamber co-deposition system [9,10], which enables the separate optimization of the deposition conditions for nanocrystallite formation and growth of the surrounding a-Si:H matrix. While intrinsic mixed-phase silicon plays an important role as the absorber layer in photovoltaic devices, an understanding of the transport in doped mixed-phase films is also crucial for device optimization. In this report we present detailed electronic transport measurements on n-type doped hydrogenated amorphous/nanocrystalline silicon (a/nc-Si:H) synthesized in a dualchamber reactor. EXPERIMENTAL DETAILS The films used in this study were grown in a two-chamber co-deposition system [10], in which the nanoparticles are synthesized in a plasma reactor using silane (SiH4), argon and phosphine (PH3), with the phosphine-silane ratio set at [PH3]/[SiH4] = 6 x 10-4 for all of the films studied here. The nanocrystallites are grown in the particle reactor chamber, consisting of a 3/8 inch quartz tube with ring electrodes, where the pressure and power of the particle reactor (1.7 Torr and 70W) are chosen to promote the growth of crystalline nanoparticles. The nanocrystallites are then entrained by the argon and injected into a second PECVD chamber (a traditional, capa
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