Controlled Spalling-Based Mechanical Substrate Exfoliation For Iii-V Solar Cells: A Review

Controlled spalling is a fast process that can mechanically exfoliate III-V semiconductor layers from their host wafer substrates and has the potential to produce high power-density, flexible, III-V solar cells at large scale. The controlled spalling system is typically composed of three parts: a brittle substrate with or without epitaxial growth layers, a tough stressor layer deposited on top, and a handle layer. Controlled spalling is achieved by tuning the stress and thickness of the stressor layer and the peeling force applied by the handle layer to overcome the cohesive energy of the substrate, resulting in propagation of a crack parallel to the surface of the substrate. Proof-of-principle device demonstrations, at wafer scale and in multiple configurations, show no loss of performance compared to conventionally processed devices while preserving the wafer for reuse, reclaim, or recycling for cost and material savings. This review summarizes advances in controlled spalling as it relates specifically to III-V solar cells, covering advances in spalling-related methods development, process modeling and control, fracture surface morphology, and device processing and performance. Opportunities for controlled spalling of III-V materials and remaining challenges for achieving reliable process integration are also discussed.