Compound Semiconductor-Based Thin-Film and Flexible Optoelectronics.

Abstract

Compound semiconductors are the basis of modern optoelectronics due to their intrinsically superior optical and electronic properties compared with elemental semiconductors. However, their applications remain limited due to a prohibitive substrate cost. This limitation has driven the development of epitaxial lift-off (ELO) technology that separates the thin-film epitaxial layer from the substrate by selectively removing a sacrificial layer between them. However, ELO has its own limitations including a long process time, complicated transfer to a secondary, low cost host substrate, and wafer surface degradation which prevents wafer recycling. In this thesis, we address all of these limitations by developing a new, non-destructive ELO (ND-ELO) process. When combined with adhesive-free cold-weld bonding of the wafer directly to a plastic substrate, ND-ELO provides an approximately 100 times reduction in process time, and a considerably simplified transfer process compared with conventional ELO. Furthermore, it allows indefinite wafer reuse by employing the epitaxial protection layers, eliminating surface degradation of the parent wafer encountered in conventional ELO. We demonstrate the feasibility and generality of this process by applying it to optoelectronic devices including photovoltaic cells, LEDs, MESFETs and photodetectors on two compound semiconductor systems, InP and GaAs. Furthermore, we present an approach that can achieve an estimated cost of only 3% that of conventional GaAs solar cells using an accelerated ND-ELO wafer recycling process, and integrated with lightweight, thermoformed plastic, truncated mini-compound parabolic concentrators (CPC) that avoid the need for active solar tracking. Using solar cell/CPC assemblies, without daily solar tracking, the annual energy harvesting is increased by 2.8 times compared with planar solar cells. This represents a drastic cost reduction in both the module and balance of systems costs compared with heavy, rigid conventional modules and trackers that are subject to wind loading damage and high installation costs. The demonstration of cost-efficient and high performance compound semiconductor-based flexible thin-film optoelectronics is a critical step toward allowing their widespread deployment in mainstream state-of-the-art applications including wearable, flexible and conformal devices.