Optimization of Photon-Assisted Microwave CVD with Application to Thin Film Transistors and Solar Cells
Wayne Anderson Principal Investigator
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9902437<br/>Anderson<br/><br/>Amorphous silicon ( a-Si ) and Microcrystalline silicon (mc-Si) are technologically important materials for thin film transistors ( TFT's ) used in flat panel displays and for photovoltaics ( PV ). Both materials are prone to degradation when subjected to light exposure or applied voltages. This instability leads to non-optimum device performance, even though methods of partially stabilizing the material are known to exist. This study will seek to improve upon these instabilities by introducing special processing techniques. The improved a-Si and mc-Si will be used in fabrication of TFT's and PV devices.<br/><br/>Microwave electron cyclotron resonance ( MECR), proven to produce more stable materials, will be used for deposition, with an assist by illuminating the substrate during the deposition to improve long-term stability. Further processing steps to improve upon device stability will include H- dilution, and special dopants. In a preliminary sense, the PI's have explored these approaches with encouraging results. The use of a Ti dopant has increased photoconductivity, without increasing dark conductivity, to give an improved ratio. Illumination of the substrate with an intense light beam during MECR ( called PAMECR ) provides a material which has much improved stability, when later exposed to light, and greatly improved electrical properties. Use of photon assist has increased carrier lifetime ten-fold in a-Si and doubled crystal diameter in mc-Si. Use of this stabilized material should produce stabilized devices.<br/><br/>PA-MECR with H-dilution and various dopants will be thoroughly studied to produce more stable a-Si and mc-Si for TFT's and PV devices. In-situ closed ion source mass spectroscopy (CIS-MS) will be used to correlate the composition of process gases with variations in process conditions. Films will be evaluated by a variety of techniques including FTIK, AFM, TEM, SIMS, AES, XPS, etc. Devices will be fabricated from the best materials and tested for long-term stability. The use of stabilized thin-film Si should lead to TFT's with reduced voltage-induced instability and PV devices with reduced photo-induced instability. Special buffer layers will be utilized on low-cost substrates to permit fabricating large-area devices at a reasonable cost. The SUNY beamline at Brookhaven National Lab will be used to explore the microscopic interfacial roughness between layers by grazing incidence X-ray scattering ( GIXS ) and the correlation with electrical performance. Thus, the primary goal will be developing improved mc-Si and aSi with an understanding of the role of the PA process using CIS-MS, FTIR, GIXS and other techniques to establish an optimum deposition procedure. The plasma chemistry will be correlated with bonding information in a-Si and H-passivation in mc-Si which relate to electrical properties, optical properties and stability.