Virtual Substrate Synthesis for Low-Cost High Efficiency III-V

The use of the low-cost thin-film vapor-liquid-solid (TF-VLS) method in the manufacturing of III-V solar cell substrates has the potential to provide a lightweight, flexible, and cheaper alternative to traditional III-V substrates typical of state-of-the- art power generation technology. The TF-VLS process has been shown to produce high optoelectronic quality polycrystalline InP on lightweight flexible metal foils. In this work, this novel method is applied to the growth of binary and ternary III-V materials which include: InP, InAs, and InGaP on Mo foils and/or sputtered Mo on Si wafers. As a result of InP trials, powder XRD measurements have identified the presence of polycrystalline InP peaks and the absence of pure unbound In peaks, signifying full consumption of In by InP formation. Photoluminescence measurements show InP samples emit in close agreement to the InP bandgap of 1.34 eV and share similar FWHM values with single crystal InP, indicating the optical properties of the TF-VLS grown material is similar to that of single crystal InP. Cross-section SEM of InP grown on Mo/Si demonstrate crystal growth in a planar format without defects such as pinholes or voids throughout the InP layer. A series of studies were performed to investigate the effects of varying phosphorization parameters such as temperature and partial pressure of phosphorous gas. Temperature studies show that varying phosphorization temperatures do not seem to have a pronounced effect on crystallinity but they do have an effect on the optical quality of the material. For both studies, at two different partial pressures, the PL intensity for the InP grown at higher temperatures is greater than InP grown at lower temperatures, thereby indicating a temperature dependence on the optical quality of the material. Partial pressure studies revealed that samples phosphorized at lower pressures demonstrate greater PL intensities indicating higher radiative recombination efficiency and partial pressure dependence on the optical quality of the material. The TF-VLS method was expanded to polycrystalline InAs growth with trials indicating multiple InAs XRD peaks but with a number of unidentified peaks. Photoluminesce of TF-VLS grown InGaP show five distinguishable peaks corresponding to bandgaps ranging from 1.28-1.65 eV, possibly indicating different phases of InGaP or materials other than the intended InGaP. Initial In layer optimization efforts conclude that In sputtered at a low pressure (1 mTorr) show more surface coverage than at higher pressures. Electron beam studies show that surface morphology of the In layer becomes more planar and continuous with simultaneously increased deposition rate and layer thickness. Electron beam vs. RF sputtering comparison prove the former method to be vastly superior to the latter, validating electron beam deposition as the preferred In deposition method for the growth of high quality polycrystalline III-V materials. Collectively, these efforts aim to improve the novel TF-VLS growth process to provide low-cost substrates for next-generation III-V photovoltaic technology