Control of Medium Range Order in Amorphous Silicon via Ion and Neutral Bombardment
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Control of Medium Range Order in Amorphous Silicon via Ion and Neutral Bombardment Jennifer E. Gerbi,1 Paul. M. Voyles,2,3 Michael M. J. Treacy,3 J. Murray Gibson,4 Wangchun Chen,5 Brent J. Heuser,5 and J. R. Abelson1 1
Department of Materials Science and Engineering and Coordinated Science Laboratories, University of Illinois at Urbana-Champaign, Urbana, IL 61801 2 Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801 (currently at Bell Laboratories, Lucent Technologies, Murray Hill, NJ, 07974) 3 NEC Research Institute, Princeton, NJ 08540 4 Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 5 Department of Nuclear, Plasma, and Radiological Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 ABSTRACT We have observed the existence of medium range order via fluctuation microscopy in amorphous silicon grown at 230oC. We hypothesize that this structure develops during the highly non-equilibrium growth process; high densities of ordered surface nuclei are produced which are subsequently buried and forced into an unfavorable energy state. These nm sized regions are distorted in the bulk due to strain, but remain topologically crystalline. In this work, we alter the growth energetics both at the surface and sub-surface during magnetron sputter film deposition with two kinds of particle bombardment, respectively: a controllable flux of low-energy (20eV) Ar+ ions, and higher energy (100eV) D vs. H neutrals. With this method, we demonstrate for the first time control over the intensity of this medium-range structural order at a constant substrate temperature as seen primarily with fluctuation electron microscopy, but also Raman scattering, spectroscopic ellipsometry, and SAXS. We suggest that these bombardments can increase adspecie surface mobility or drive local sub-surface restructuring (“kinetic annealing”), thus increasing or decreasing the size, density and/or strength of the ordered regions. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) is an important semiconductor for applications such as thin film transistors and photovoltaics. As the most widely-used material for low-cost solar cells, the propensity for it to degrade in light via the formation of metastable electronic defects - the Stabler-Wronski (SW) effect - limits its use [1]. The mechanism for this effect is still not understood, but recent studies show that the film structure as a whole, as opposed to only local defect creation, is affected by light soaking. Prolonged illumination changes the film density, stress, internal friction, and Si-H bonding, for example; a review is provided in [2]. Such changes must involve a significant fraction of the atoms in the film, and thus any structural order in a-Si:H may play an important role [3]. It is not clear if the SW effect mechanisms are primarily due to hydrogen motion, changes in the film structure, or both. A new microscopy technique, fluctuation electron microscopy (FEM) [4], has revealed that a-Si:H is not truly amorphous; i
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