Metalorganic Chemical Vapor Deposition of III-V Materials for Low-Cost Photovoltaic Applications

Gallium Arsenide (GaAs) is a III-V material that offers the opportunity for high efficiency photovoltaic (PV) devices due to its attractive properties as well as compatibility with other III-V materials to extract the most energy out of the solar spectrum. However, challenges such as high cost have posed a high barrier of entry and have limited these technologies to niche applications. The use of an inexpensive, single-crystalline-like germanium (Ge) template deposited on a metal tape has been identified as a candidate in reducing the cost of manufacturing III-V-based PV devices. Metalorganic chemical vapor deposition (MOCVD) was used to epitaxially grow GaAs PV devices on these Ge templates. Solar conversion efficiencies up to 13.2% were obtained on these metal foils. In addition, these flexible metal tapes accommodate for roll-to-roll epitaxial growth, which can also reduce costs on the production side. An in-line 2-dimensional x-ray diffraction tool was used to evaluate GaAs tapes for quality control purposes. The ability to monitor GaAs tapes as they are produced offers another means to reduce the cost of PV devices by reducing waste from unsuccessful processing. Another means towards the cost reduction of GaAs PV production was by developing a water-assisted epitaxial lift-off process (H2O-ELO) that allows for the reuse of a single-crystalline GaAs wafer without the need for chemical-mechanical polishing (CMP) processing. A sandwich structure composed of (Ca,Sr)F2/BaF2/(Ca,Sr)F2 was deposited on a GaAs wafer, and used towards epitaxialvii GaAs growth. This approach yielded devices with 10% efficiencies. The devices were lifted off and transferred to a host wafer to show minimal degradation in device performance. As a proof of concept, once the devices were lifted off, and the same substrate was reused twice for device growth. Each device yielded a less than 2% drop in performance. Issues such as defects stemming from antiphase domains (APDs) limited device performance. The use of a vicinal substrate offered a means to reducing the effect of APDs. Devices grown on vicinal substrates featuring the ELO structure improved device performance to 15.2%. The work done on these approaches can potentially pave the way towards making high-efficiency III-V-based PV devices more accessible for everyday use