Opto-electronic properties of co-deposited mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films
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Opto-electronic properties of co-deposited mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films James Kakalios,1 U. Kortshagen,2 C. Blackwell,1 C. Anderson,2 Y. Adjallah,1 L. R. Wienkes,1 K. Bodurtha,1 and J. Trask2 1
School of Physics and Astronomy, Department of Mechanical Engineering University of Minnesota, Minneapolis, MN 55455
2
ABSTRACT Mixed-phase thin film materials, consisting of nanocrystalline semiconductors embedded within a bulk semiconductor or insulator, have been synthesized in a dual-chamber co-deposition system. A flow-through plasma reactor is employed to generate nanocrystalline particles, that are then injected into a second, capacitively-coupled plasma deposition system in which the surrounding semiconductor or insulating material is deposited. Raman spectroscopy, X-ray diffraction and high resolution TEM confirm the presence of nanocrystals homogenously embedded throughout the a-Si:H matrix. In undoped nc-Si within a-Si:H (a/nc-Si:H), the dark conductivity increases with crystal fraction, with the largest enhancement of several orders of magnitude observed when the nanocrystalline density corresponds to a crystalline fraction of 2 – 4%. These results are consistent with the nc donating electrons to the surrounding a-Si:H matrix without a corresponding increase in dangling bond density for these films. In contrast, charge transport in n-type doped a/nc-Si:H films is consistent with multi-phonon hopping, possibly through extended nanocrystallite clusters with weak electron-phonon coupling. The flexibility of the dual-chamber co-deposition process is demonstrated by the synthesis of mixed-phase thin films comprised of two distinct chemical species, such as germanium nanocrystallites embedded in a-Si:H and Si nanocrystallites embedded within an insulating a-SiNx:H film. INTRODUCTION The opto-electronic properties of semiconducting materials are primarily determined by their chemical composition and atomic bonding arrangements. In contrast, in nanostructured systems the low-dimensionality and short length scales can have a significant influence on the material’s characteristics, such as the energy gap [1]. The ability to synthesize mixed-phase materials, consisting of nano-scale materials embedded within a bulk semiconductor or insulator has enabled the development of novel materials with properties not easily realized in homogeneous thin films. Solar cells utilizing silicon nanocrystallites embedded within an hydrogenated amorphous silicon matrix (a/nc-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 [2-4]. These mixed phase films are typically synthesized in a capacitively-coupled singlechamber plasma system, using high gas pressures and a heavily hydrogen-diluted silane precursor [5-9]. This single-chamber process poses limitations for the production of a/nc-Si:H, since (1) the properties of the crystalline particles
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