Aluminum-Catalyzed Lateral Growth of Spherulite-like GeS Thin Films on Insulating Substrates Using Vapor Transport: Implications for Electro-optic Applications

Abstract

Due to the presence of lone-pair electrons, it is challenging to attain lateral growth of thin films of Group IV monochalcogenides. In this study, lateral growth of germanium monosulfide (GeS) thin films with a minimum thickness of approximately 20 nm was attained on SiO₂/Si and quartz substrates at a growth temperature of 420 °C, employing a combined method that uses an aluminum (Al) catalyst and a predeposited amorphous GeS layer. Time-dependent changes in grown GeS thin films using a 5 nm-thick Al layer on SiO₂/Si and quartz substrates were shown to exhibit a lateral growth rate of 2–3 μm/s. The birefringent properties of grown GeS with a Maltese extinction cross pattern were confirmed by using cross-polarized optical microscopy, indicating the formation of a spherulite-like structure. It appears that Mullins–Sekerka instability dominates the growth front at the periphery of circular domains, causing a dendrite morphology. A transition region dominated by kinetic-limited diffusion occurs, forming the spherulite-like structure of GeS. X-ray diffraction and atomic force probe investigations reveal the grown spherulite-like GeS, on insulating substrates, to consist of a layered structure with a flat surface. The single-crystalline area of GeS appears to be larger than 200 nm in size, as evaluated by selected area electron diffraction via transmission electron microscopy. This study reveals a potential method of achieving the lateral growth of GeS on insulating substrates. This method may facilitate the use of GeS as functional semiconductors for developing next-generation electro-optic applications, such as in-memory sensing and computing devices, with potential for programming via electric and optical control.