Doping Effects in Co-deposited Mixed Phase Films of Hydrogenated Amorphous Silicon Containing Nanocrystalline Inclusions
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1066-A06-08
Doping Effects in Co-deposited Mixed Phase Films of Hydrogenated Amorphous Silicon Containing Nanocrystalline Inclusions C. Blackwell1, Xiaodong Pi2, U. Kortshagen2, and J. Kakalios1 1 School of Physics and Astronomy, University of Minnesota, 116 Church Street S.E., University of Minnesota, minneapolis, MN, 55455 2 Mechanical Engineering, University of Minnesota, 111 Church Street S.E., University of Minnesota, minneapolis, MN, 55455 ABSTRACT Hydrogenated amorphous silicon films containing silicon nanocrystalline inclusions (a/nc-Si:H) that have been n-type doped have been synthesized using a dual-plasma co-deposition system. We report the structural and electronic properties of n-type doped a/nc-Si:H as a function of phosphine doping level and nanocrystalline concentration. The volume fraction of nanocrystals in the doped a/nc-Si:H thin films is measured using Raman spectroscopy, and the hydrogen binding configurations are characterized using infra-red absorption spectroscopy. In undoped a/nc-Si:H, the inclusion of low and moderate nanocrystalline concentrations results in an increase in the dark conductivity, compared to a-Si:H films grown without nanocrystalline inclusions. In contrast, the addition of even a low concentration of silicon nanoparticles in doped a/nc-Si:H thin films leads to a decrease in the dark conductivity and photoconductivity, compared to pure a-Si:H films. INTRODUCTION Interest in hydrogenated amorphous silicon with nanocrystalline silicon inclusions (a/ncSi:H)-based photovoltaic devices stems from reports of an enhanced resistance to light-induced degradation, coupled with high solar conversion efficiencies and high deposition rates.[1-4] There has been considerable effort in determining the deposition processes and conditions that would yield optimal electronic properties of undoped a/nc-Si:H mixed-phase thin films.[1-7] However, there has not been to date comparable effort in synthesizing and studying the properties of doped a/nc-Si:H films. An elucidation of the opto-electronic properties of doped a/nc-Si:H films is a necessary step in the development of a mixed-phase pin a/nc-Si:H-based solar cell. Mixed phase a/nc-Si:H thin films are typically synthesized in a single PECVD reactor chamber, using high gas pressures and a high hydrogen concentration.[1,2,5] The deposition parameters necessary for silicon nanocrystalline formation in the silane plasma are very far from those known to yield high electronic quality a-Si:H. While one can control the concentration of nanocrystals embedded within the a-Si:H matrix through a thermophoretic force in a single chamber plasma deposition system, the electronic conductivity of the resulting a/nc-Si:H has been found to be decreased for thermal gradients imposed across the silane plasma during film growth that enhance nanoparticle incorporation.[8] These results have motivated the construction of a dual chamber co-deposition to better control the quality of the a-Si:H film.[9,10] In this system silicon nanocrystals are synthesized in one p
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