TFT's and Solar Cells on Flexible Substrates Using Microwave-deposited Silcons

High-quality nanocrystalline silicon (nc-Si) films will be deposited by microwave electron cyclotron resonance (MECR) chemical vapor deposition with application to thin film transistors (TFT's) and heterojunction solar cells. An in-situ residual gas analyzer (RGA) will monitor the process and achieve reproducible conditions. Recent work has given a field effect mobility of amorphous (a-Si:H) films of 3 cm2/V-s compared to the usual values of 1 cm2/V-s and over 15 cm2/V-s has been achieved so far with nc-Si films. Hydrogen dilution (HD) and photon assist (PA) during deposition permit lower substrate temperatures during deposition. The MECR process places the substrate away from the plasma to avoid plasma damage. Thus far, TFT's have been quite good using oxide-coated Si, quartz, glass and plastic substrates. The process transfers readily to a variety of substrates with little loss in performance. Concerning intellectual merit, the MECR process with HD and PA will be devoted to achieve high quality Si films at temperatures < 250 C on lower cost plastics. Oxide formation for the gate of TFT's will be done at temperatures < 200 C. Film analysis will utilize high resolution transmission electron microscopy, atomic force microscopy, secondary ion mass spectroscopy and other methods. Grazing incidence X-ray scattering will evaluate substrate-film interfaces to minimize carrier recombination at hetero-interfaces and results correlated with deep level transient spectroscopy. Smaller gate dimensions , using e-beam pattern generation at Cornell University and a new system to be installed in Buffalo, will give much better values of field effect mobility and better devices. The all Si heterojunction solar cell will utilize MECR-deposited a-Si:H and nc-Si on a Si wafer or thin film to achieve much higher currents than with normal cells due to the bandgap difference in the layers. Past efficiency values of 10.5% should be increased to 15% by improved films interfaces. Concerning broader impact issues, the graduate student will interface with another graduate student supported on a NASA Space Grant and another supported part-time as a teaching assistant. An undergraduate student and a minority high school student, with support from the University's BEAM program, will also work on the project. Minorities and women will be encouraged to participate. Research tasks and educational courses will be linked to mutually benefit from the project. This work will also interface with Cornell University for nano-fabrication, Brookhaven for GIXS work , the team in Buffalo setting up the new e-beam system, NREL for materials and photovoltaic measurements, SUNY at Stony Brook for modeling and technical expertise, and Taitech, Inc. for measurements. Technology transfer mechanisms are in place at the university.