III-V Thin Films for Flexible, Cost-Effective, and Emerging Applications in Optoelectronics and Photonics

Conspectus Semiconductor thin films possess a unique set of characteristics, making them highly suitable for a number of optoelectronic and photonic applications. The III-V semiconductors, in particular, are lightweight, flexible, durable, and suitable for high-efficiency devices. For example, flexible and lightweight III-V thin-film solar cells have been demonstrated with a solar conversion efficiency of 37.8%. Besides photovoltaics, III-V semiconductor thin films are also suitable for LEDs, lasers, detectors, sensors, and frequency-converting devices. These characteristics make the III-V thin films excellent candidates for applications in space, Internet of things, drones, autonomous vehicles, and wearable devices. Despite these advantages, the high cost of fabrication has hindered the uptake of III-V thin films. On the other hand, recent developments in multilayer epitaxial lift off (MELO) may drastically improve the cost-efficiency of device fabrication. These developments are discussed in the Account, and they alone may improve cost-efficiency by a factor of 4. Even further cost improvements may be achieved with the use of ultrathin solar cells. For ultrathin cells, a promising architecture is discussed, where an epitaxially grown III-V absorber layer is sandwiched between nonepitaxial carrier-selective contact layers thus forming a double-heterojunction. This structure minimizes epitaxial thickness and promises low-cost and high-efficiency devices, with theoretical cost-efficiency improvement reaching as high as a factor of 10 when combined with MELO. Perhaps even more interestingly, recent developments in III-V thin films demonstrate a wholly novel device types. One interesting device architecture uses single-crystalline nanofilms with a thickness of only tens of nanometers, thereby reducing device cost significantly. These ultralow thicknesses have two consequences: the surface properties become dominant, and the optical properties, such as absorption, are decoupled from bulk material values. This Account discusses an example of UV-sensitive GaAs photodetectors, contrasting typical GaAs devices that are sensitive in infrared. Another important device class that has attracted significant attention recently is metamaterials. Metamaterial-based devices offer novel solutions to a whole range of applications in photonics and optics. They are planar structures with nanoscale resonators that are used to focus, bend, and modify light. III-V thin films are excellent candidates for metamaterial fabrication due to their high refractive indices, high nonlinear-optical coefficients, and direct band gaps that allow the fabrication of optoelectronic metamaterials. III-V metamaterials with a focus on frequency conversion are discussed. As a whole, this Account discusses various facets of III-V thin-film technology, from cost-efficient fabrication to novel and emerging device types. The improved cost-efficiency makes them attractive for a number of increasingly important application areas where lightweight, flexibility, and high performance are critical. Simultaneously, the novel device types open new avenues for the use of III-V thin films. Nanofilm devices can be ultralow cost and offer an interesting platform for applications in sensing and detection. Metamaterials on the other hand, are versatile photonic devices that are thought to play a key role in the fourth industrial revolution. These aspects suggest that III-V thin films will attract significantly increasing interest in the near future in research of novel devices as well as in many real-world applications.