Improvement in separation rate of epitaxial lift-off by hydrophilic solvent for GaAs solar cell applications

In this study, the epilayer structure of a GaAs solar cell was quickly separated from the GaAs substrate by performing the epitaxial lift-off (ELO) process in hydrofluoric acid (HF) solutions mixed with hydrophilic substances, and the epilayer structure was then transferred to a Cu substrate. The lateral etching rate (7.4-14.3 μm/min) of the AlAs sacrificial layer, obtained by adding hydrophilic substances - namely acetone (ACE), isopropanol (IPA), and methanol (MA) - to the HF solution was much higher than that obtained using a pure HF solution (3.6 μm/min). We attributed the increase in the lateral etching rate to the decrease in both the surface tension of these solutions and the contact angle between the etching-induced bubble and substrate, which enabled efficient venting of bubbles during the ELO process. By contrast, when the HF:ACE solution was used in the ELO process, few solid products were deposited on the sample surface, indicating that there was little obstruction for bubble venting. Therefore, although HF:ACE exhibited the highest surface tension and contact angle among these three solutions, it still resulted in the highest lateral etching rate (14.3 μm/min). When the hydrophilic substances were added to HF solution, the cell efficiency was not degraded after the ELO process. Furthermore, the cell device requiring the shortest removal time of the GaAs substrate exhibited the highest efficiency, because of the low etching-induced damage to the GaAs epilayer. Additionally, the GaAs substrate can be rapidly removed and reused via this ELO technique, which is beneficial for photovoltaic applications.