Expanding the Scalability and Applications of III-V Optoelectronic Devices by Evolution of Thin-Film Vapor-Liquid-Solid Growth

Two significant drivers to innovation in electronics and electronic materials in recent/nhistory have been electronic device scaling and the pursuit of high efficiency photovoltaic/ncells at low costs. The motivations of these two fields have been interestingly parallel, as/n“density” has been a key metric for both – areal energy density in the case of photovoltaics,/nand component density in the case of electronic devices. So strong is this motivation to/nlower device costs by packing more performance into a smaller area that “laws” have been/ndevised to inspire innovation in each field, with Moore’s law to describe the periodic/ndoubling of transistor density and Swanson’s law to outline the steady drop in solar cell/nmodule costs over time. As each law approaches a wall erected by fundamental physical/nlimitations, science must identify roadblocks and solutions that can allow innovation to/ncontinue./nSemiconductor materials are a key limiting factor for each application, as their physical/nproperties determine ultimate functionality of a device and the challenges involved in/ndevice design. In both electronic and optoelectronic applications, scalable manufacturing/nof III-V materials has been a promising avenue to improvement, as while they are/ntraditionally expensive to produce, they use a larger portion of the solar spectrum for/nphotovoltaic devices, and are easily utilized in the fabrication of high performance optical/nand electronic devices. In recent years, a more scalable method for the growth of III-V/nmaterials without costly epitaxial substrates has been developed, by utilizing the vaporliquid-/nsolid growth (VLS) process to grow structures confined by a metal catalyst./nStructures such has nanowires have been fabricated with this technique and studied/nextensively, but a recent expansion of the approach has also allowed for growth of highquality/nthin films using planar templates for nucleation control. In this dissertation, I/ndiscuss the use of this approach in a number of applications, including the development/nof large-area photovoltaic devices and an evolution of the technique to greatly expand its/napplication space through lower process temperatures./nFirst, I will discuss an ideal preliminary application of this technique, with the/ndevelopment of p-body InP photoelectrochemical cells for the direct production of/nchemical fuel using sunlight. This application shows the utility of large-scale/npolycrystalline growth with larger than normal grain sizes enabled by the technique, and/nthe fundamentals of the growth process and usable doping methods are explored in/ntandem. This study also demonstrates the successful application of an efficient selective/nelectron contact to the poly-InP system, enabling promising device performance and/nenhancing device stability under harsh photocathode operation conditions. Hydrogen/nfuel production from simulated sunlight is also directly and quantifiably observed from/nthe device as a capstone to this experiment./nFollowing the investigation of larger area thin-film growth, the microscale templatedliquid-/nphase (TLP) crystal growth method is explored and expanded to target a wider/nrange of applications. This method, a modification to the thin-film vapor-liquid solid (TFVLS)/nprocess initially studied, has previously enabled growth of defined patterns of single/ncrystal domains on amorphous substrates. While this is an impressive result with great/npromise for integration of III-Vs into highly scaled electronics, growth temperatures/npreviously explored would need to be lowered significantly for facile integration to be a/nreality. Using a simple modification to the existing TLP process, I demonstrate growth/ntemperatures well within the silicon CMOS thermal budget, with proof-of-concept devices/nfabricated at temperatures as low as 270ºC with the InP system. With applicability to a/nvariety of substrates, this study has neatly expanded the application space of III-Vs, with/ncomplex methods and material requirements replaced with simple direct growth.

Advisor: Javey, Ali