應用原子層沉積技術於奈米級粗紋化矽晶太陽能電池與高介電常數金屬閘極堆疊之研究

Abstract

Atomic layer deposition (ALD) systems has shown great promise in emerging semiconductor and energy conversion technologies. It is because the ALD technique offers many benefits, including easy and precise thickness control, low defect density excellent step coverage and conformality, good reproducibility, high uniformity over a large area, low deposition temperature, and the pinhole-free structure, as a result of the self-limiting reaction and layer-by-layer growth. In addition, the ALD thin films can be deposited on thermal fragile substrates because of the low deposition temperature. Moreover, the plasma-enhanced ALD (PEALD) can provide more selection of materials and higher ALD growth rate. In this thesis, it mainly focuses on the application of ALD of the p-type silicon solar cells and high-K/metal gate dielectrics stacks devices. In this thesis, the gate stack composed of a crystalline ZrO2 high-K dielectric and an AlN buffer layer treated with remote NH3 plasma was developed to achieve low CET, Dit, and Jg. The high dielectric constant of the crystalline ZrO2 with tetragonal/cubic phase was formed by the low-temperature post metallization annealing at 450oC. The AlN buffer layer was introduced between the crystalline ZrO2 high-K dielectric and the Si substrate, by the PEALD technique, to suppress the formation of the low-K silicate interfacial layer, leading to the reduction in CET. The Jg was also suppressed by the insertion of the amorphous AlN buffer layer by three orders of magnitude. In addition, the decrease of Dit could be accomplished because of the hydrogen passivation from the remote NH3 plasma used as the precursor for AlN deposition. Furthermore, the remote NH3 plasma treatment upon the AlN buffer layer prior to the ZrO2 deposition further reduces the CET, Dit, and Jg due to deactivation of the nitrogen vacancies. Accordingly, a low CET of 1.21 nm, Dit of 5.32×1011 cm−2 eV−1, and Jg of 1.09×10-5 A/cm2 were achieved in the crystalline ZrO2/AlN buffer gate stack treated with the remote NH3 plasma. The result indicates that the crystalline high-K dielectrics/AlN buffer layer is a promising gate stack structure to improve the sub-nanometer CET scaling in the future. In the part of the efficient solar cells in this thesis, the nanotextured black Si (NBSi) wafers fabricated by the silver-catalyzed wet chemical etching are used as the base of the fabrication of Si solar cells. However, a lot of defects were created at the surface of the Si nanowire arrays during the fabrication process, resulting in severe surface recombination. Thus the deposition of a surface passivation layers was used to solve this problem, and the surface recombination could be reduced by the following two mechanisms: (1) decrease in interfacial state density, i.e., the so called chemical passivation, and (2) the reduction in minority carrier concentration near the interface by built-in electric field which is referred to as the filed-effect passivation. In this thesis, the Al2O3 passivation layer prepared by ALD contributes to efficiency enhancement of NBSi solar cells with an n+-emitter/p-base structure. Moreover, not only the single passivation layer has been used in the research, but also the Al2O3/TiO2 dual-layer passivation stack is demonstrated. The high positive oxide charge density and low interfacial state density could effectively suppresse the surface recombination. At the end, the Jsc and  of the NBSi solar cell with the FGA(forming gas annealing)-treated Al2O3/TiO2 dual-layer passivation stack were increased by 11% and 20%, respectively, as compared with the NBSi solar cell without any surface passivation. Hence, a high-efficiency up to 18.5% NBSi solar cell was achieved by applying the Al2O3/TiO2 dual-layer passivation stack. Finally, we also fabricated the ZnO nanorod array (NRA) using the aqueous chemical method, as the antireflective structure on the flat surface of the n+-emitter/p-base Si solar cells. This one-dimensional ZnO nanostructures prepared by the bottom-up synthesis approach can prevent the defect generation caused by the top-down chemical etching. A highly conformal Al2O3 shell layer was deposited upon the surface of ZnO NRA by ALD. The ZnO/Al2O3 core-shell NRA could further reduce the total reflection and the Al2O3 shell layer can also act as the effective anti-corrosion protective layer for ZnO NRA. Because of the low and wideband reflectance, simple fabrication and low processing temperatures, free from defect generation, considerable efficiency enhancement, anti-corrosion, and high uniformity, the ZnO/Al2O3 core-shell NRA can be further applied very practically as the antireflective structure in efficient photonic devices including light-emitting diodes, solar cells, and photodetectors.