Epitaxial Lift-Off Process for Triple-Junction Solar Cells Using Internal Push–Pull Stressors

Abstract

An epitaxial lift-off process utilizing an internal push–pull stressor scheme for the release of thin-film multijunction solar cells is reported. The process involves a sidewall protection step to localize the etching to the sacrificial layer. The sidewall protection allows etching of the sacrificial layer utilizing typical hydrofluoric and hydrochloric acid solutions, while preventing etching of Al and P containing layers forming the active region of the III–V tandem stack. The proposed thin-film process was validated by releasing a ~ 8-μm thick GaInP/GaAs/GaInNAsSb triple-junction solar cell structure grown on a GaAs substrate. The integrity of the released solar cell structure was confirmed by focused ion beam scanning electron microscopy. The crystalline quality and strain relaxation in the thin-film after the i-ELO process was studied by reciprocal space mapping with x-ray diffractometry, revealing a high integrity of the released film. The presented approach is primarily targeted for the lift-off of relatively thick multijunction solar cell films, for example including three or more junctions, alleviating possible etchant-related restrictions for the choice of III–V materials forming the multijunction structure.