Monolithic Growth of Crystalline III-Vs on Non-Epitaxial and Heteroepitaxial Substrates for Solar Energy Conversion

One of the key challenges in III-V growth for energy applications remains the high cost of manufacturing. This is driven by a variety of factors, but one of the most significant costs remains the epitaxial substrate. While polycrystalline materials grown through traditional vapor phase techniques offer the opportunity to eliminate the high-cost epitaxial substrates, the resulting materials quality is significantly lower than that obtained through III-V homoepitaxy. Thus, a major focus for the development of III-V energy harvesting devices has been (i) the use of epitaxial transfer approaches--to amortize the cost of the substrate over many devices--or (ii) growth of reduced dimensional structures on heteroepitaxial or non-epitaxial substrates--to reduce the defectivity of the resulting III-V materials. Here we will talk about a third approach enabling growth of high quality thin-film III-V's on non epitaxial substrates, Templated Liquid Phase (TLP) Growth.<br/>We will first discuss the basics of this growth approach, which involves deposition of the group III element (e.g. indium) and a capping layer (e.g. SiO₂) on a non-epitaxial substrate. This is then heated to the desired growth temperature under vacuum, and then a group V precursor such as AsH₃, PH₃, or NH₃, may be flowed and reacted with the group III element. Uniquely, at the growth temperatures used, the deposited group III element is in the liquid phase, but through control over the surface energies as well as structure, it is possible for the liquid to maintain the geometry in which it was deposited. Eventually, this group III liquid becomes supersaturated and a III-V nucleus forms. By controlling the nucleation rate and density, it is possible to define large single crystal domains on the substrate, effectively minimizing the effect of grain boundaries, yielding significantly better material properties than vapor phase approaches. Critically, this enables the use of lower cost substrates such as metal foils, silicon wafers, and polymers.<br/>Next, we will discuss the quality achievable with this approach for III-V materials on different substrates, and highlight the causes of varying quality on disparate substrates, despite the large single crystal domains being similar.in size. Using a combination of structural and electronic approaches, such as transmission electron microscopy, electron backscatter diffraction, photoluminescence, and Hall measurements, we will show that the current quality achieved by TLP approaches the quality exhibited by single crystalline counterparts, but without the epitaxial substrate. As a follow-up, we will show that the TLP grown III-V's can act as a 'remote substrate' for follow up metal organic chemical vapor deposition (MOCVD) growth of lattice matched hetrostructures, quantum wells, etc. Finally, we will discuss a variety of energy devices realized using the TLP grown materials.