Epitaxial Growth of Single-Crystalline-Like Ge Templates with Conductive Buffers on Low-Cost, Flexible Substrates for Thin Film III-V Solar Cells, and Methods for Texture Improvement and Defect Reduction

Date

2018-08

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Abstract

In an effort to fabricate high-efficiency III-V solar cells on low-cost, flexible metal substrates, a novel single-crystalline-like Ge template is developed with all electrically-conductive buffer layers, after which various techniques are applied to improve quality of Ge. The new template has a biaxially-textured Ge/NiSi2/TiN architecture, with epitaxy orientations of (001)<100>Ge || (001)<100>NiSi2, and (001)<100>NiSi2 || (001)<110>TiN. The buffers show metal-scale resistivity and dramatically low sheet resistance. However, single junction III-V solar cell fabricated on this Ge template shows a low efficiency which is probably caused by the high defect density of Ge. So this work focused on improving the quality of Ge templates on flexible metal substrates. Significant texture improvement is seen in the Ag layer growth on single-crystalline-like IBAD MgO and IBAD TiN substrates, resulting from the minimization of grain boundary energy. With the Ag layer applied as a buffer, Ge exhibits improved in-plane texture Δφ of ~3.15° and out-of-plane texture Δω of ~1.62°. Further texture improvement is seen in the two-step Ge grown on the Ag buffer, with a Δφ of ~1.51° and a Δω of ~1.29°. Defect reduction by two-step Ge growth is directly observed in the cross-sectional TEM image, with only a few defects identified in the top part of high temperature (HT) Ge on IBAD TiN substrate with an Ag buffer. Besides, a reduced volume fraction of twins (2.21%), a larger grain size (0.5-1 μm) and a lower Ge-Ge Raman peak spread (4.24 cm^-1) are found in the HT Ge compared to low temperature (LT) Ge. High temperatures are essential for energy minimization and, in turn, texture improvement and defect reduction. Device-quality Ge is demonstrated by an Liquid Phase Epitaxy (LPE) technique on IBAD MgO templates, with the defect density roughly estimated to be 5 × 10^6 cm^-2. The significantly reduced defect density of the LPE Ge results from: the strain-free layer by LPE; the partially horizontal grain boundaries preventing diffusion from buffers; the reduced area of grain boundaries by large grain size; and the low misorientation among grains due to the improved texture. These Ge templates with improved quality can potentially lead to high-efficiency III-V solar cells on low-cost, flexible metal substrates.

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Keywords

Single-crystalline-like, Texture improvement, Defect reduction, Conductive buffers, Thin film solar cells

Citation

Portions of this document appear in: Li, Yongkuan, Yao Yao, Ying Gao, Sicong Sun, Pavel Dutta, Monika Rathi, Jae-Hyun Ryou, and Venkat Selvamanickam. "Biaxial-textured Titanium Nitride thin films on low-cost, flexible metal substrate as a conductive buffer layer for thin film solar cells." In 2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), pp. 2368-2371. IEEE, 2017.