Unconventional structured semiconductors and their applications in optoelectronics and photovoltaics

???Unconventional structured semiconductors??? have novel structures that provide improved optical and electrical properties compared with the conventional crystalline semiconductors. Two kinds of semiconductors are discussed within this thesis: gallium nitride (GaN) and silicon (Si). A novel Pt-assisted electroless etching technique is used to produce porous GaN (PGaN), which is of particular interest for optoelectronics due to its large direct bandgap (3.4 eV). PGaN is also promising for use as a substrate for epitaxial growth and for chemical and biosensing. Several possible applications for PGaN have been explored. PGaN is able to be functionalized for use as a surface enhanced Raman spectroscopy (SERS) substrate by solution-based electroless deposition and vacuum evaporation of Au and Ag. SERS enhancement factors of up to 108 have been observed for Ag-coated porous GaN. Other efforts are focused on using the porous film as conductivity sensors. The improved optical properties observed in porous GaN make it a promising candidate to replace crystalline GaN in ultraviolet photodetector. Si, an indirect bandgap semiconductors (1.1 eV), is currently used in >90% of the photovoltaic (PV) production. The main limitation of Si solar cell is the high cost of the wafer production. One attractive strategy to reduce the substrate cost is to use ultrathin Si films (1-5 ??m thick) combined with effective light trapping methods. We have developed a new capillary-driven self-assembly process to generate three-dimensional (3D) Si structures in millimeter range from a 2D precursor. Based on these 3D Si structures, we have constructed spherical and cylindrical shaped Si solar cells. These 3D constructs allow the microcells to harvest both the directly incident and diffuse components of sunlight, thereby improving the solar energy conversion efficiency. The output power of the 3D solar cell was observed to be about 2 times as compared to that of a conventional planar Si solar cell with the same thickness and equivalent mass. We also developed a mechanics model that successfully predicts the critical conditions for folding of thin foils with complicated shapes. Further interests are focused on using the micro-structured single crystalline Si sheets/ribbons as the basic structure for novel light trapping photovoltaic devices, including plasmonic solar cells and deployable solar cells