Influence of Annealing on Crystallinity and Conductivity of p-type Nanocrystalline Si films
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0910-A08-03
Influence of Annealing on Crystallinity and Conductivity of p-type Nanocrystalline Si films Durga P. Panda1, and Vikram Dalal2 1 Microelectronics Research Center, Iowa State University, ASC1, Ames, Iowa, 50011 2 Electrical and Comp. Engr., Iowa State University, Coover Hall, Ames, Iowa, 50014 ABSTRACT Nanocrystalline p-type Si:H layers are important for photovoltaic and MOSFET devices. A high conductivity in these layers leads to better voltages in PV devices and to lower series resistance in both PV and MOSFET devices. The lower resistance results in higher channel mobility in MOSFET devices and higher fill factors in PV devices. In this work, we discuss the improvement in conductivity and crystallinity of p-type nanocrystalline Si:H layers by the use of post-deposition annealing. The p layers were deposited at ~ 180°C from mixtures of silane, hydrogen, helium and diborane using ECR plasma deposition techniques. It was found that addition of He to H at first improved both the conductivity and crystallinity, but too much He led to an amorphous phase and lower conductivity. The as-grown films were measured for their crystallinity using both Raman spectroscopy and x-ray diffraction. The conductivity and activation energies were also measured. The films were then successively annealed at temperatures of 250, 300, 350 and 400°C. The crystallinity and grain size were found to increase as the annealing temperature increased. The greatest relative increase was during the initial annealing stages. The conductivity of the films increased significantly as a consequence of the annealing. Conductivities as large as 20 S/cm were obtained in very thin films (~50-150 nm). The corresponding activation energies were in the range of 0.03 eV. When these annealed layers were used for MOSFET and PV devices, there was an appreciable increase in performance characteristics. INTRODUCTION Recently there is a considerable interest in the growth of hydrogenated nanocrystalline silicon at low substrate temperatures with emphasis on low cost and flexible plastic substrates for application to large area and roll-roll production of thin-film transistors, photovoltaic and optoelectronic devices[1-4]. Hydrogenated nanocrystalline silicon has been shown to be much better than hydrogenated amorphous silicon in terms of device stability under light soaking in solar cells and gate bias stress in thin film transistors, in addition to possessing higher mobilities and lesser defects. Particularly, highly phosphorous doped and boron doped nc-Si:H have been shown to have much higher conductivities and better doping efficiencies than their amorphous counterpart. For these reasons, p+ nc-Si:H contacts for solar cells and thin film transistors are expected to provide low series resistance and better quality ohmic contacts. Doping of amorphous silicon was demonstrated by Spear and Lecomber[5]. Kanicki et al[6], He et al[7], and Kuo and Latzko have demonstrated highly conductive n+ uc-Si:H as source/drain contacts in a-Si:H TFTs. For optimized depos
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