本論文研發氮化鎵表面粗糙化技術並嘗試應用於提升氮化鎵太陽能電池光電流。由於商用氮化鎵晶圓是為做LED元件設計，主動層多重量子井數目較少。為了避免QCSE (quantum-confined Stark effect)效應，其主動層量子井厚度約只有20-30 A。過薄的主動層厚度將使得太陽能電池吸收效率不佳，因此希望透過散射體的應用，增加主動區的光程長度，以提升光吸收率與光電流密度。
首先透過濕式蝕刻方式嘗試將晶片表面產生適當地散射體尺寸。根據FRED模擬，理想的氮化鎵太陽能電池散射體尺寸為0.15+/-0.05 μm，但濕式蝕刻所產生的散射體寬度約0.26-0.65 μm，深度為0.06 μm，面積覆蓋率約1.5%左右，預期無法有效的將光散射。若使用乾蝕刻方式，以二氧化矽次微米球當遮罩，並透過ICP-RIE(inductively-coupled plasma reactive ion etching)乾蝕刻的方式將氮化鎵表面粗糙化，產生散射體寬度約0.10-0.22 μm，深度約0.03-0.05μm，其覆蓋率可達到70%以上。
我們將乾蝕刻的散射體應用至氮化鎵太陽能電池上，並比較有無散射體之光電流密度。在元件量測上，觀察到許多元件受到壓電極化影響使短路電流密度下降，其幅度大過散射體可增加的比例。另外，氮化鎵表面蝕刻增加了蕭特基接觸與表面缺陷密度/表面復合中心，導致量子效率下降，以及透明導電層已有相當粗糙的表面，造成光電流密度未達到預期增加。

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.

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