Development of GaN surface roughening techniques and investigation of the related solar-cell characteristics

• Main Authors: Yen-hsiang Huang, 黃彥翔
• Other Authors: Pinghui Sophia Yeh
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/70978836770706867051

碩士 === 國立臺灣科技大學 === 電子工程系 === 101 === In this work, GaN surface roughening techniques were developed and used in solar cells to increase the photo-current density. Commercial GaN LED wafers we used to fabricate GaN solar cells have fewer numbers of quantum well we need. Moreover, to avoid QCSE (quantum-confined Stark effect), the thickness of each quantum well is only about 20-30 Ǻ resulting in a very thin active layer and hence low light absorption when operated as solar cells. Therefore we added a scattering layer by using surface roughening to increase the optical length of the incident light in the active layer and expect to increase light absorption and the photocurrent density. First, we tried wet etching on GaN wafer to generate light scatters. According to our simulation using FRED software, the ideal size of spherical scatters for GaN-based solar cells is about 0.15+/-0.05 μm. But the width and depth of the scatters produced by wet etching were about 0.26-0.65 μm and 0.06 μm, respectively. Additionally, the area coverage of scatters was only about 1.5% that cannot facilitate light scattering efficiently. Secondly, dry etching method was conducted as follows, SiO2 sub-micron spheres were used as mask, followed by ICP-RIE (inductively-coupled plasma reactive ion etching) dry etching to produce textured GaN surface. The width and depth of the scatters were about 0.10-0.22 μm and 0.03-0.05μm, respectively. The area coverage of scatters reached more than 70%. The dry etching produced scatters were applied to GaN solar cells. We observed that quite many devices were affected by piezoelectric polarization effect, which resulted in significant decrease in short circuit current density. The ratio of decreasing short circuit current density caused by polarization could be larger than the gain of increasing short circuit current density caused by scattering. In addition, the roughening of GaN surface also increased Schottky contact and the surface defects/surface recombination center, which lead to low quantum efficiency. Besides the transparent conducting layer has quite rough surface originally. In results, the photo current density was not increased as expected.