Van Der Waals Growth of III-V Semiconductors on Graphene

Abstract

Recent discovery of graphene and its unique properties has attracted a great deal of interest in implementing this material in a variety of new devices, targeting applications in ultra-fast electronics, quantum information, carbon-free energy conversion, optoelectronics, and bio-integrated technologies [1,2]. However, graphene lacks a controlled non-zero bandgap, which prevents its large-scale application in electronic and energy conversion devices. To overcome this limitation, we exploit direct integration of III-V semiconductors on graphene through van der Waals epitaxy. Known for their efficient light emission and high charge carrier mobilities, III-V semiconductors are at the core of numerous technologies including high-efficiency solar cells, lasers, light emitting diodes, and ultra-fast transistors, to name a few [3].Due to lack of dangling bonds on graphene’s surface, in epitaxial growth of III-V semiconductors on graphene the crystals follow the order of graphene lattice through van der Waals forces. Initially, two nucleation and growth processes were suggested for such van der Waals epitaxy: (1) nucleation starts at defect sites on graphene [4], and (2) nucleation starts on specific sites on graphene lattice, which are the favorable adsorption sites for III/V adatoms [5]. The latter growth mechanism limits the epitaxial growth of the III-V crystals in four different relative orientations (with different strain values) [5]. Recently, density functional theory calculations followed by homoepitaxial growth of InP, GaP, and GaAs through several layers of graphene suggested another determining factor in van der Waals epitaxial growth: long-range interaction between the III-V substrate under graphene and the III-V structures grown on graphene [6].In this talk, we demonstrate the low-pressure metal-organic vapor phase deposition (MOCVD) of InAs, GaAs, InP, and GaP islands on single layer graphene sheets grown on copper foils and transferred on SiO2/Si (100), as well as graphene sheets directly grown on Ge (100), (110), and (111) substrates. We show that regardless of the nature of the substrate under graphene, III-V crystals grow mostly on graphene defect sites and their morphology can be somewhat controlled through controlling the growth parameters. We then show the effect of long-range forces between Ge and III-V structures on growth morphology, which results in significant coalescence and growth of tens of micron large islands on graphene/Ge as opposed to parasitic crystals grown on graphene/SiO2. In what follows, we provide a brief summary of our observations in both systems.All III-V semiconductors grown on graphene/SiO2/Si showed very similar growth morphology and crystal structure. Plan-view and cross-sectional scanning electron microscopy (SEM), micro-Raman spectroscopy, powder and high-resolution X-ray diffraction (XRD), along with cross-sectional transmission electron microscopy (TEM) and nanoscale cathodoluminescence analysis revealed formation of 1D, 2D, and 3D structures in zinc-blende, wurtzite, and polytypic (mixed zinc-blende/wurtzite) phases. We have recently demonstrated that polytypic InP crystals form a type II homojunction with potential applications in optoelectronic devices [7]. XRD analysis of the growth time evolution of texture, as well as growth rate and crystal size evolution analysis of the crystals using atomic force microscopy (AFM) suggested a growth mechanism similar to selected area MOCVD growth [7], which is widely used to grow nanostructures and devices. This enabled us to have some degree of control over the growth morphology (single crystals, polycrystals, nanowires) through controlling the growth parameters such as growth temperature, rate and time.Similar 1D, 2D, and 3D structure were grown on graphene/Ge (100), (110), (111). However, the size and crystalline quality of the islands have significantly improved for growth on Ge substrates. Moreover, two-step growth (nucleation at low temperature (450 °C) and growth at higher temperature (600 °C)) lead to coalescence of the crystals and formation of tens of microns large islands with smooth surface (roughness ~ 1nm from AFM data) on graphene/Ge, while no significant improvement in growth morphology was observed for growth on graphene/SiO2. These observations suggest a strong sensitivity of the morphology of the deposited films on the nature of the substrate below the graphene monolayers. This provides insight to control the properties of such hybrid systems and will pave the path to engineering a new class of electronic and optoelectronic devices by combining the advantages of semiconductors and graphene.