A review on epitaxial lift-off for III-V solar cells

Epitaxial lift-off (ELO) as a step in the fabrication of III-V devices offers a significant cost-reduction by non-destructive removal of the growth substrate, which can subsequently be reused. Specifically in solar cell production, ELO facilitates the creation of thin-film configurations that surpass the performance of substrate-based cells. This process involves a selective lateral etch of a sacrificial layer, typically a high-aluminum content AlGaAs layer, with hydrofluoric acid (HF). In the reaction, various aluminum-fluoride compounds, arsenic gas, fluoride ions, and water are formed. However, challenges arise in ensuring unhampered etching due to geometric constraints and side reactions leading to solid material deposits, including aluminum ions, hydrogen gas, and solid arsenic. This review provides an overview of all major aspects involving the theoretical understanding and practical application of epitaxial lift-off. An analysis is presented of various studies of the process parameters that influence the sacrificial layer etch rate under various experimental conditions. This includes factors such as the release layer's aluminum fraction, thickness, and doping concentrations, as well as experimental conditions, such as HF concentration and temperature. The influence of stress and strain on the ELO process, and the challenge to study this comprehensively, is also addressed. This work is concluded with remarks regarding substrate reuse and the challenges and opportunities for ELO, such as thin-film fragility, alternative release layers and multi-release ELO.