Electronic Transport in Co-deposited Hydrogenated Amorphous/Nanocrystalline Thin Films

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1066-A02-03

Electronic Transport in Co-deposited Hydrogenated Amorphous/Nanocrystalline Thin Films Y. Adjallah1, C. Blackwell1, C. Anderson2, U. Kortshagen2, and J. Kakalios1 1 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 Mixed-phase hydrogenated amorphous silicon thin films containing nanocrystalline silicon inclusions have been synthesized in a dual chamber co-deposition system. A PECVD deposition system produces small crystalline silicon particles (3-5 nm diameter) in a flow-through reactor, and injects these particles into a separate capacitively-coupled plasma chamber in which hydrogenated amorphous silicon is deposited. Raman spectroscopy is used to determine the volume fraction of nanocrystals in the mixed phase thin films, while infra-red spectroscopy characterizes the hydrogen bonding structure as a function of nanocrystalline concentration. At a moderate concentration of 5 nm silicon crystallites, the dark conductivity and photoconductivity are consistently found to be higher than in mixed phase films with either lower or higher densities of nanocrystalline inclusions. INTRODUCTION There has been considerable recent interest in the properties of mixed phase materials consisting of nanocrystallites embedded within an amorphous matrix, owing to their potentially superior properties for a wide range of technological applications. Hydrogenated amorphous silicon thin films containing silicon nanocrystalline inclusions (a/nc-Si:H) have been investigated as potential materials for photovoltaic devices [1,2], while silicon nanocrystals within an insulating matrix are employed in non-volatile memory and electroluminescent devices [3,4].These materials are typically synthesized in a Plasma Enhanced Chemical Vapor Deposition (PECVD) system operated at high silane gas chamber pressures, where silicon cluster formation is known to occur [5,6]. However, the plasma conditions that yield silicon nanocrystals are far from those that are known to produce high electronic quality a-Si:H, and the concentration of silicon particles embedded within the a-Si:H matrix is not easily controlled. Moreover, when synthesized using in a single capacitively-coupled plasma chamber, one is limited to growing silicon nanocrystalline particles in a-Si:H. We have consequently constructed a dual chamber codeposition system, where the silicon nanocrystals are formed in one plasma deposition system, and are then entrained in a carrier gas and injected into a second PECVD system, where hydrogenated amorphous silicon is deposited [7]. A preliminary report on this dual chamber system has been published previously [7]. This paper reports the structural and electronic properties of undoped a/nc-Si:H films containing silicon nanocrystals of 5-6 nm in diameter, as a function of nanocrystalline concentration.

MATERIALS PREPARATION The undoped a/nc

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