UC San Diego

This dissertation presents works on two significant application areas of semiconductor nanowire. The first is nanowire photodetectors and their applications for image sensing and retinal prosthesis. The second is nanowire solar cells for renewable energy application and both Si and III-V nanowire devices are extensively discussed. Three structures of Si nanowire photodetectors are presented, including nanowire photoconductor, axial and radial junction nanowire photodiodes. Using the design of a crossbar structure, an individually addressable vertical Si nanowire photoconductor array was presented, followed by discussion on integration to CMOS for image sensing applications. In addition, the Si axial junction nanowire photodiodes were also demonstrated, which showed excellent detectivities in visible spectrum at zero bias, with a peak value of 2.14×1013 Jones at wavelength of 636 nm. Furthermore, the radial junction nanowire photodiodes were discussed as well, which exhibited enhanced photo responsivity due to shorter carrier separation and collection paths. These nanowire photodiode arrays promises potential application for retinal prosthesis by replacing the dysfunctional photo receptors in human retina In addition to photodetectors, I also demonstrated Si nanowire solar cells including both planar Si solar cells with nanowire absorber and Si radial junction nanowire solar cells. A low-cost chemical etching method was introduced to fabricate large-scale Si nanowire array as light absorber to improve the light absorption of planar Si solar cells. The efficiency of this device is over one order of magnitude higher than the one without nanowires. In addition, the Si radial junction nanowire solar cells were studied. A systematic study on design parameters, including nanowire core and shell doping concentrations, band structures and surface passivation methods, were discussed to achieve optimal device performance. With optimal design, a power conversion efficiency of 8%was demonstrated InAs based nanowire core/ multi-shell heterojunction solar cells were investigated as well. A systematic study of catalyst-free MOCVD growth of InAs nanowires on Si substrates was performed. Various growth parameters and surface treatment methods were studies to achieve optimal nanowire growth and reveal the growth mechanism. Based on the InAs nanowire growth, the III-V core/multi-shell nanowires were successfully fabricated and solar cells have been demonstrated