Low Temperature Plasma Synthesis of Nanocrystals and their Application to the Growth of Crystalline Silicon and Germaniu
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Low Temperature Plasma Synthesis of Nanocrystals and their Application to the Growth of Crystalline Silicon and Germanium Thin Films P. Roca i Cabarrocas1, K.H. Kim1,2, R. Cariou1, M. Labrune1,2, E.V. Johnson1, M. Moreno1, A. Torres Rios1, S. Abolmasov1, and S. Kasouit1,2 1 LPICM, CNRS, Ecole Polytechnique, 91128 Palaiseau, France 2 Total S.A., Gas & Power – R&D Division, Courbevoie, France ABSTRACT We summarize our research studies on the synthesis of silicon and germanium nanocrystals and their application to the growth of a variety of thin films, spanning the range from fully disordered amorphous up to fully ordered crystalline. All these films are deposited in a standard radio-frequency glow discharge system at low temperature (~200 °C). We show how the plasma synthesis of silicon nanocrystals, initially a side effect of powder formation, has become over the years an exciting field of research which has opened the way to new opportunities in the field of materials deposition and their application to optoelectronic devices. Our results suggest that epitaxy requires the melting/amorphization of the nanocrystals upon impact on the substrate, the subsequent epitaxial growth being favored on (100) c-Si substrates. As a consequence, the control of the impact energy is a critical aspect of the growth which will require new strategies such as the use of tailored voltage waveforms. INTRODUCTION Plasma Enhanced Chemical Vapor Deposition (PECVD) based on parallel plate capacitively coupled RF glow discharge systems has become the mainstream technology for large area deposition of hydrogenated amorphous (a-Si:H) and microcrystalline silicon (μc-Si:H) thin films. It has allowed the development of flat panel displays and tandem a-Si:H/μcSi:H solar cells [1]. The standard model to describe the process leading to low defect density a-Si:H and μcSi:H films is based on the surface diffusion of silicon radicals, in particular SiH3 [2,3]. However, when trying to increase the deposition rate by tuning process parameters such as the RF power, the total pressure, the dilution of silane, the excitation frequency,…the common issue is the formation of powders [4,5]. This has been an active subject of research, mostly aiming at avoiding powder and cluster formation [6]. Yet, numerous groups have reported on the formation of silicon nanocrystals in silane discharges [7,8,9]. Moreover, we have discovered that silicon nanocrystals produced in SiH4-H2 gas mixtures can be incorporated in the growing film, leading to a nanostructured material that we have called hydrogenated polymorphous silicon (pm-Si:H) [10]. Interestingly, pm-Si:H films display improved transport properties and stability with respect to a-Si:H [11,12] which makes them attractive for stable solar cells and thin film transistors [13,14]. This approach has been further extended to SiF4- H2-Ar gas mixtures [15] leading to the deposition of microcrystalline silicon films, as well as to GeH4-H2-Ar gas mixtures [16] for germanium nanocrystals synthesis. In this paper we review some
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