Towards III-V solar cells on Si: Improvement in the crystalline quality of Ge-on-Si virtual substrates through low porosity porous silicon buffer layer and annealing

Abstract

Abstract.A comparison between the crystalline quality of Ge grown on bulk Si and on a low porosity porous Si (pSi) buffer layer using low energy plasma enhanced chemical vapor deposition is reported. Omega/2Theta coupled scans around the Ge and Si (004) diffraction peaks show a reduction of the Ge full-width at half maximum (FWHM) of 22.4% in presence of the pSi buffer layer, indicating it is effective in improving the epilayer crystalline quality. At the same time atomic force microscopy analysis shows an increase in root means square roughness for Ge grown on pSi from 38.5 nm to 48.0 nm, as a consequence of the larger surface roughness of pSi compared to bulk Si. The effect of 20 minutes vacuum annealing at 580°C is also investigated. The annealing leads to a FWHM reduction of 23% for Ge grown on Si and of 36.5% for Ge on pSi, resulting in a FWHM of 101 arcsec in the latter case. At the same time, the RMS roughness is reduced of 8.8% and of 46.5% for Ge grown on bulk Si and on pSi, respectively. The biggest improvement in the crystalline quality of Ge grown on pSi with respect to Ge grown on bulk Si observed after annealing is a consequence of the simultaneous reorganization of the Ge epilayer and the buffer layer driven by energy minimization. A low porosity buffer layer can thus be used for the growth of low defect density Ge on Si virtual substrates for the successive integration of III-V multijunction solar cells on Si. The suggested approach is simple and fast -thus allowing for high throughput-, moreover is cost effective and fully compatible with subsequent wafer processing. Finally it does not introduce new chemicals in the solar cell fabrication process and can be scaled to large area silicon wafers.

A review of concentrator silicon solar cells

“…The efficiency of the cell was 19.8% under 1 sun, and the efficiency of the same cell grown on GaAs substrate was 23.6% [101]. Calabrese et al made porous structure on Si substrate to improve the crystalline quality of Ge virtual substrate [102]. Grassman et al grew a GaAsP cell on Si substrate directly using MBE [103], García-Tabarés et al grew a GaP cell on Si substrate directly using metalorganic vapor phase epitaxy (MOVPE) [104] and Wilkins et al grew a GaAs cell on Si substrate which was also made porous structure directly using chemical beam epitaxy [105], but above researchers did not report the efficiencies of the cells, thus it is still hard to grow high quality III-V multi-junction solar cell on Si substrate directly.…”

Binder-free nanostructured germanium anode for high resilience lithium-ion battery

“…Here, a top-down process based on HF electrochemical etching was chosen to create nanostructured Ge [43,44]. In particular, a custom made PTFE (PolyTetraFluoroEthylene) single-tank electrochemical cell was used for this purpose [45]. Specifically, a graphite rod (Alfa Aesar 040765, 99.9995% purity) acting as a counter electrode was immersed in a solution composed by HF (CARLO ERBA Reagents, 50% diluted, MOS grade) and ethanol (CARLO ERBA Reagents, RS grade) (3:1 v/v), while the backside of the sample was pressed on an aluminium plate to ensure the electrical contact.…”

Advances in the Use of Atomic Force Microscopy as a Diagnostic Tool for Solar Cells Characterization: From Material Design to Device Applications

“…Furthermore, AFM studies on the crystallinity and surface roughness of a 5 μm‐thick Ge epilayer on bulk Si and on a low porous Si buffer layer have shown that low‐porosity buffer layer is favorable for the deposition of low‐defect‐density Ge on Si virtual substrates for the subsequent incorporation of III–V multijunction solar cells on Si. [ 145 ] C‐AFM has been described as a reliable technique for analyzing structural defects such as the existence of antiphase domains which acts as nonradiative recombination centers during GaAs heteroepitaxial deposition of Ge for photovoltaic applications. [ 146 ]…”