The Influence of Thermophoresis Effects During Deposition of Hydrogenated Amorphous Silicon Thin Films with Nanocrystall
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0910-A07-07
The Influence of Thermophoresis Effects During Deposition of Hydrogenated Amorphous Silicon Thin Films with Nanocrystalline Silicon Inclusions C. Blackwell1, C. Anderson2, J. Deneen3, C. B. Carter3, U. Kortshagen2, and J. Kakalios1 1 School of Physics and Astronomy, University of Minnesota, Minneapolis, MN, 55455 2 Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN, 55455 3 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, 55455 ABSTRACT Silicon cluster formation is known to occur within silane plasmas when a capacitivelycoupled deposition reactor is operated at high gas chamber pressures. These clusters are sensitive to the thermophoretic forces that will, depending on the sign of the thermal gradient, direct them toward or away from the silicon film’s growing surface. We have developed a dualchamber deposition system that produces nanocrystalline silicon particles (roughly 3-5 nm in diameter) in a flow-through reactor, and injects these particles into a separate capacitivelycoupled plasma chamber where the amorphous film is produced. The structural, optical and electronic properties of these mixed-phase materials are investigated as a function of the controllable thermal gradient applied across the silane plasma during deposition. INTRODUCTION Mixed-phase thin film materials, consisting of crystalline silicon particles, with diameter ranging from 1-2 nm to as large as several hundred nm, embedded within a hydrogenated amorphous silicon matrix, have recently attracted considerable attention.[1,2] Hydrogenated amorphous silicon with nanocrystalline silicon inclusions (a/nc-Si:H)-based photovoltaic devices grown at high deposition rates display high solar conversion efficiencies and an enhanced resistance to light-induced defect creation.[3-6] Typically, crystalline silicon particles are synthesized in a Plasma Enhanced Chemical Vapor Deposition (PECVD) system operated at gaschamber silane pressures of 1-2 Torr. At these pressures silicon-hydrogen radicals coalesce and form particulates within the silane plasma. In a single-chamber deposition system, the size of the particulates is controlled by dilution of the silane with hydrogen.[7] A drawback of this process is that the plasma deposition conditions that optimize nanoparticle formation are far different from those that yield high electronic quality a-Si:H. We have recently developed a dualchamber deposition system, where nanocrystalline particles are synthesized in an upstream flowthrough tube reactor, and are then injected into a second capacitively-coupled plasma deposition system in which the a-Si:H is grown.[8] Particles in a plasma rapidly acquire a negative charge (owing to the higher energy and mobility of electrons compared to positively charged ions within the plasma) and are therefore repelled from the negatively charged sheath regions. These trapped charged particles can be moved either toward or away from the growing a-Si:H film surface by either convective drag
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