Morphological Study of InAs0.75P0.25/InP Nanowires Grown onto Si Wafer Towards Higher Efficiency

Department of Materials Science EngineeringWhile most of the world's current power supply is generated by fossil fuels such as oil, natural gas and coal, These traditional fossil fuels faces a number of challenges, including increasing prices, security concerns from the limited number of countries which have a significant dependence on imported fossil fuel supplies, and growing environmental problem over the global warming associated with energy production using fossil fuels. Because of these concerns and other challenges to traditional fossil fuel. Recently, solar energy is considered solution of these energy problem[1]. Solar energy, due to its most abundance among renewable energy and wide distribution, has attracted much attention[2]. Because of that, many people have been research on solar cell. Solar cell device is classified by materials, which is silicon based solar cell, organic based solar cell compound semiconductor based solar cell. Traditional solar cells are made from single-crystal silicon[3]. Silicon is very abundant and inexpensive element in the earth's crust. However, silicon, has a narrow band gap of 1.1 eV, resulting in a maximum efficiency of 29%[4]. And silicon has disadvantage of increasing wafer price with increasing demand. Organic based solar cell is easy to fabricate[5]. But, organic based solar cell has low conversion efficiency and fast degradation time[6]. Lastly, compound semiconductor based solar cells have many advantages. III-V semiconductors, with their direct band gap and high electron mobility, are excellent materials for advanced optoelectronic device[7]. And band gap can be controlled from 0.3 eV to 2.5 eV by ratio of compound materials[8]. However compound semiconductor based solar cell has high manufacturing cost[9]. And solar cell device is also classified by device structure. In planar structure, solar cells have disadvantages of high level of impurities and crystalline defects. These disadvantages of compound semiconductor lead to reduce minority carrier diffusion length and decrease conversion efficiency. But, planar structure solar cells are less expensive than nanostructure solar cell. In nanowire based solar cell, it has many optical and electrical advantages such as ray-optics light tapping absorption short diffusion lengths for the minority carrier high aspect ratio. However, nanowire based solar cells need high process cost[10]. To combine advantages, which are compound semiconductor and nanowires, many researchers have studied III-V semiconductor nanowire based solar cell. As their efforts, many kinds of III-V semiconductor nanowire based solar cells are reported. III-V compound semiconductor nanowire based solar cells have advantages of high electron mobility[11], high absorption coefficient[12] as III-V compound semiconductor, and low reflectance[13], large surface area[14] as nanowire structure. It shows excellence electrical and optical performance. However previous III-V semiconductor nanowires should be grown by noble process such as vapor-liquid-solid (VLS)[15] and selective area epitaxy (SAE)[16][17]. In this study, we try to fabricate solar cell of low cost and high efficiency. As a results, we fabricate solar cells device using InAs0.75P0.25/InP nanowires on silicon wafer grown as strain induced method. And also, we optimized morphology of InAs0.75P0.25/InP nanowires. InAs0.75P0.25/InP nanowires using strain-induced growth method has different geometry, diameter, height, and density, varies significantly across the wafer. We identify that solar cells using densely grown InAs0.75P0.25/InP nanowires have low conversion efficiency for agglomeration effect. Additionally, we find solar cells using InAs0.75P0.25/InP nanowires have best efficiency at density of 45 × 106/cm2. And also, we construct process for fabrication of InAs0.75P0.25/InP nanowires based solar cells for high conversion efficiency. Optical and electrical properties are optimized by ITO layer condition deposited by e-beam evaporator, exposed nanowire tips by reactive-ion etching (RIE) process. Moreover, back surface field is used for reduce carrier recombination. And also, we adopt grid pattern as top contact metal for decrease series resistance. Finally, we fabricated solar cells, which have conversion efficiency of 11.4%.ope