Deterministic Nucleation of InP on Metal Foils with the Thin-Film Vapor−Liquid−Solid Growth Mode

A method for growth of ultralarge grain (>100 μm) semiconductor thin-films on nonepitaxial substrates was developed via the thin-film vapor−liquid−solid growth mode. The resulting polycrystalline films exhibit similar optoelectronic quality as their single-crystal counterparts. Here, deterministic control of nucleation sites is presented by substrate engineering, enabling user-tuned internuclei spacing of up to ∼1 mm. Besides examining the theory associated with the nucleation process, this work presents an important advance toward controlled growth of high quality semiconductor thin films with unprecedented grain sizes on nonepitaxial substrates. ■ INTRODUCTION Photovoltaic devices fabricated from III−V semiconductors offer the highest efficiencies of all classes of materials available, a direct result of the high external luminescence efficiencies of III−V semiconductors. However, until recently, growth of high quality III−V semiconductors has required expensive epitaxial substrates and metal−organic chemical vapor deposition (MOCVD) processes, offering significant scaling challenges, and relegating high-efficiency III−V devices to niche applications. Recently, we demonstrated growth of poly crystalline InP thin films with grain sizes >100 μm on nonepitaxial substrates using the thin-film vapor−liquid−solid (TF-VLS) growth mode. TF-VLS growth occurs by passing the phosphorus precursor gas over an In film, which has been capped with SiOx. Phosphorus diffuses through the SiOx cap at the growth temperatures, causing supersaturation of the liquid In and inducing precipitation of solid InP. As shown schematically in Figure 1 and reported previously, the process enables the transformation of an entire In film into InP. The mechanisms that enable TF-VLS growth are (i) the inhibition of In dewetting by the template, comprised of the Mo substrate and SiOx capping layer and (ii) the reduction of incident phosphorus flux by the SiOx capping layer, enabling lower nucleation densities. While similar in concept to the vapor−liquid−solid (VLS) growth mode utilized for nanowire growth, the TF-VLS mode enables growth of ultralarge grain continuous thin films, a morphology previously unattainable via the VLS method. Unlike traditional vapor phase growth of polycrystalline materials, TF-VLS growth decouples the lateral nucleation density and film thickness for continuous polycrystalline film growth, enabling lateral grain sizes orders of magnitude larger than film thickness. Due to the large grain sizes, TF-VLS grown polycrystalline InP exhibited optically measured Voc (quasi-Fermi level splitting) ∼95% of single crystalline InP. The near single crystal performance of TF-VLS polycrystalline films suggest that this method could play a key role in future thin film optoelectronic devices. Due to the extreme sensitivity of optoelectronic device quality to defects such as grain boundaries and interfaces, developing a method for deterministic control of nucleation, in polycrystalline films, is of significant interest for device applications. Previously, we showed that nucleation density in TF-VLS grown films could be controlled by manipulating the phosphorus flux; the nuclei positions, however, were random. Here, we present a general scheme for engineering nucleation in the TF-VLS mode. Specifically, a patterned nucleation promoter (evaporated MoOx) is utilized to control the position and density of InP nuclei on Mo foil substrates. Furthermore, a simple model showing quantitative agreement with experiment is presented, leading to a set of design rules for nucleation engineering in TF-VLS grown materials. Received: October 15, 2013 Revised: December 11, 2013 Published: January 17, 2014 Article