Growth Mechanism of Compound Semiconductor Nanostructures on 2-D Materials Studied by Improved Direct Growth and Observation Techniques

Abstract

Atomically thin layered crystals isolated by mechanical exfoliation method have exhibited new physical properties and provided novel applications. Moreover, hybrid structures of these 2-dimensional (2-D) layered materials with semiconductor thin films and nanostructures offer additional functionalities, such as flexibility and transferability, thereby greatly extending the applicability to the electronic and optoelectronic devices. Accordingly, many efforts have focused on the growth of nanomaterials using 2-D materials as substrates. In order to fabricate such nanomaterials with desired shapes and physical properties, the study on the initial growth mechanisms, such as nucleation, nuclei growth, and orientational relationship with substrate, should be accompanied in detail. This work mainly focuses on exploring the growth mechanisms of compound semiconductor nanomaterials on graphene, as a representative material among various 2-D layered materials, using transmission electron microscopy (TEM). In order to avoid unintentional damages arising from conventional TEM sample preparation processes, direct growth and observation method was developed to observe nanomaterials at the early growth stages. In this method, electron-beam transparent graphene was exploited as a supporting layer for TEM measurements as well as a substrate for nanomaterials growth. Compound semiconductor nanomaterials including ZnO, InAs, and GaAs were grown on graphene which had been transferred onto a TEM grid, followed by TEM measurements conducted without TEM sample preparation processes. Reflection high-energy electron diffraction (RHEED) transmission mode was also employed to analyze the structural information of nanostructures in real time during the growth. Contrary to its conventional usage in reflection mode, RHEED was used in transmission mode. This new technique allows us to obtain diffraction patterns containing the structural information of nanomaterials, which is analogous to the principle of electron diffraction in TEM. Using these newly developed methods, the growth mechanisms of compound semiconductor nanomaterials were thoroughly investigated. First, the growth behavior of ZnO nanomaterials was clearly observed with atomic-resolution and high-sensitivity using a graphene template for the direct growth and observation method. This method successfully disclosed the growth behavior of ZnO nanomaterials on graphene, such as nucleation of ZnO cubic phase and formation of ∑7 coincidence site lattice boundary, which are previously unknown. Second, the growth mechanisms of GaAs/single-layer graphene (SLG)/InAs double heterostructures were unraveled with an aid of a further improved direct growth and observation method. This study showed that InAs nanorods grown on SLG can influence on the nucleation and growth behavior of GaAs nanomaterials on the other side of SLG. Lastly, the growth behavior of InAs nanorods were investigated in real time using the RHEED transmission mode. Time-resolved observation using RHEED transmission mode revealed the transition in local growth condition from In-rich to As-rich at the very early stage of InAs nanorods growth and the strain relaxation process of GaAs/InAs coaxial nanorods during the shell layer coating.