Effect of RF or VHF Plasma on Nanocrystalline Silicon Thin Film Structure: Insight from OES and Langmuir Probe Measureme
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Effect of RF or VHF Plasma on Nanocrystalline Silicon Thin Film Structure: Insight from OES and Langmuir Probe Measurements Lala Zhu1,2, Ujjwal K Das1 , Steven S Hegedus1, Robert W Birkmire1,2 1. Institute of Energy Conversion, University of Delaware, Newark, Delaware, USA 19716; 2. Department of Physics and Astronomy, University of Delaware, Newark, Delaware, USA 19716. ABSTRACT Optical emission spectroscopy (OES) and Langmuir Probe were used to characterize RF and VHF plasma properties under conditions leading to nanocrystalline silicon film deposition. Films deposited by RF plasma at low pressure (3 Torr), even with high crystalline volume fraction, show weak X-ray diffraction signals, suggesting small grain size, while RF films at higher pressure (8 Torr) and VHF films at both high and low pressure have larger grain sizes. The preferential growth orientation is controlled by the H2/SiH4 ratio with RF plasma, while the film deposited by VHF shows primarily (220) orientation independent of H-dilution ratio. Langmuir Probe measurements indicate that the high energy electron population is reduced by increasing pressure from 3 Torr to 8 Torr in RF plasma. Compared with RF plasma, the VHF plasma shows higher electron density and sheath potential, but lower average electron energy, which may be responsible for the larger grain size and crystal orientation. The growth rate and crystalline volume fraction of the film is correlated with OES intensity ratio of SiH* and Hα/SiH* for both RF and VHF plasmas. INTRODUCTION Nanocrystalline silicon (nc-Si) is an attractive material for thin film photovoltaic application due to its lower band gap and better stability compared with amorphous silicon (a-Si). It also performs better in thin film transistors (TFT) due to higher mobility. nc-Si films are commonly deposited by plasma-enhanced chemical vapor deposition (PECVD) with silane gas (SiH4) highly diluted with hydrogen (H2). Due to the indirect band gap of nc-Si, a much larger thickness is required for light absorption, so growth rate is critical for high throughput in industry processing. The best intrinsic nc-Si film for solar cell application is deposited at the amorphous/nanocrystalline transition region [1]. It has been reported that better nc-Si cell performance is obtained with (220) oriented films and the grain size around 25 nm [2]. Optical emission spectroscopy (OES) provides the intensity of emission species. The plasma’s electrical properties, such as electron density, average electron energy, electron energy distribution [3] and sheath potential, were measured by Langmuir Probe. In this study, nc-Si films were deposited by RF (13.56 MHz) and VHF (40.68 MHz) plasma. The combination of OES and Langmuir Probe measurements were used to evaluate the plasma properties, which can be correlated to film properties of growth rate, crystalline volume fraction, grain size and crystal orientation. EXPERIMENT nc-Si:H films were deposited in a PECVD system either using capacitively coupled RF or VHF plasma. SiH4 and H2 were delive
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