Author: Yuda Wang
Publisher:
Published: 2016
Total Pages: 134
ISBN-13:
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With the rapid evolution of semiconductor technologies, the size of the fundamental device components is already approaching nanometer scale. In order to fabricate even smaller and faster yet more power efficient devices, new materials or designs are required. As one of the best candidate for future electronic and photonic applications, semiconductor nanowires have created substantial interest in the last decade. Variety of researches has been conducted to understand its growth and fundamental properties. Among the nanowires with different materials and designs, hetero-structure nanowires are especially attractive due to their capability of realizing band gap engineering without forming interface defects. In Chapter 2, we use a combination of optical, electronic and electron-beam measurements as well as theoretical simulation to obtain a clear picture of a GaP/GaAs core/shell nanowire hetero-interface strain distribution and relaxation. Micro-Raman spectroscopy is primarily used to map the high resolution strain distribution. A compressive strain is observed on GaAs, while a tensile strain is observed on GaP. The tension on GaP becomes smaller as core/shell size ratio grows. Selected-area electron diffraction (SAED) is also performed to study the strain, which is consistent with Raman. Due to the strain and stress, the band structure of either GaP or GaAs is modified. A band structure calculation along the core/shell nanowire is performed based on strain measured by Raman, which is consistent with photo-current measurement. Finally, comparing the experimental strain and the finite-element method simulation strain, a relaxation of the strain is observed and it is correlated to the hetero-interface dislocation densities observed by TEM measurements. When designing new electronic or photonic devices based on nanowires, the understandings of carrier dynamics are critical in optimizing their performance. In Chapter 3, transient Rayleigh scattering (TRS) experiment is performed to study the carrier dynamics of complex band structure InP nanowires. Different band structures of zinc blende and wurtzite InP nanowires are clearly observable. More interestingly, a fitting model based on band to band transition theory is developed to extract the carrier densities and temperatures as a function of time after initial excitation. Based on the carrier density or temperature relaxation, electron/hole recombination or thermalization process could be analyzed respectively. Comparing the carrier thermalization behavior of InP nanowires to other materials, like GaAs nanowires, a unique hot phonon effect is observed due to InP's special phonon band structures (huge band gap between optical and acoustic branches). In addition to the visible to near-IR wavelength range we have been studying for long time, near~mid IR wavelength materials nanowires become interesting recently due to their potential opto-electronic applications. In Chapter 4, an infrared modified TRS system is developed and optimized to obtain high quality ultra-fast TRS data across wavelength range 500~2500nm with a simple diode (InGaAs or InSb). The electronic band structures and carrier relaxation dynamics are obtained for a variety of nanowires (i.e. Zn3As2, GaAs1-xSbx, GaSb). For bare Zn3As2 nanowire data, a substantially long carrier relaxation process is observed, which indicates low Zn3As2 surface recombination velocity. For GaAs11-x/subSbx samples, the nanowire obtains 2-order of magnitude longer carrier lifetime after InP surface passivation. All of these measurements provide informative feedback to the growth and design of near~mid IR nanowires for future applications.
