Fabrication of single crystal semiconductor films by the pore transformation and its applications

Abstract

III-V solar cells have attracted intensive attention as a promising technology for renewable energy, having excellent light conversion efficiency, power densities, temperature coefficients, mechanical flexibility, and proven stability. Nonetheless, III-V solar cells are limited in their use due to their high manufacturing costs. The high production costs partially stem from expensive template for epitaxial growth of III-V devices. Herein, we studied the layer transfer technology using Ge substrate, which is commonly used as an epitaxial template, for minimizing the production cost of III-V devices. Our layer transfer technology utilized reorganization of porous Ge by high temperature annealing. At high temperature, Ge pores are reorganized into thin Ge film on voids, which can be used as epitaxial template for III-V materials. The III-V device on the Ge film can be transferred to a handling substrate by utilizing a void layer as a separation layer. This enables recycling of the rest of the substrate, resulting in reduction of the production cost. In this thesis, we have studied the Si layer transfer technique prior to the development of Ge thin film technology. We demonstrate the formation of an ultrathin (100) single crystal Si film based on silicon on nothing (SON) technology which utilizes the reorganization of well-ordered cylindrical Si pores at high temperature. Squarely-arrayed cylindrical Si pores are fabricated by nanoimprint lithography and deep reactive ion etching. By modifying and optimizing the initial geometry of Si pores, we successfully fabricate a defect-free and ultrathin Si film with thickness up to 330 nm on a plate-shaped void. In addition, we investigated the formation of oxygen-related defects during hydrogen annealing and suppressed the defect formation by modifying gas phase diffusion of oxygen during annealing. Finally, we successfully demonstrated the transfer of Si film on SON onto PDMS pad and shows optical semitransparency of 30 – 70 % in visible and near-infrared light and mechanical flexibility. We next studied the formation of Ge film based on SON technology for ultrathin and flexible III-V solar devices. We have developed a (100) 6$^\circ$ (111) single crystal Ge film based on the germanium on nothing (GON) technology which utilizes the reorganization of the cylindrical Ge pores at high temperature. By controlling the geometry of initial Ge pores and using Br passivation, we successfully fabricated a defect-free and ultra-flat Ge film on a plate-shaped void. In addition, we realized the formation of GaAs solar cell with light conversion efficiency of 14.4 % by utilizing a GON film as an epitaxial template. Finally, we demonstrated the transfer of GaAs cell onto handling substrate by using the plate-shaped void under the Ge film as a release layer. Finally, we have developed a GON structure by electrochemically etched porous Ge to minimize the production cost of Ge film. In order to fabricate a porous Ge with uniform porosity and a thickness over a micrometer, we employed bipolar electrochemical etching which consists of anodic etch step and cathodic passivation step. By using two step bipolar electrochemical etching, double layer porous Ge which have low porosity on top and high porosity on bottom layer is realized. We next demonstrated the formation of GON using double layer porous Ge by annealing in hydrogen ambient. Owing to segregation of voids by Ostwald ripening, we successfully fabricated a GON structure having uniform Ge layer on large voids.