Incommensurate Heteroepitaxy by van der Waals and Weak Chemical Interactions for Epitaxial Thin Film Transfer

Abstract

High quality crystalline semiconductor films are a key component in the production of electronic and opto-electronic devices, however, the requirement of latticed-matched single crystal substrates for the epitaxy of a thin film limits the available material systems which can be developed commercially. This strict lattice matching requirement is relaxed for two-dimensional layered materials grown via van der Waals epitaxy. Unfortunately, the same low surface energies of these layered materials also suppress the growth of three-dimensional materials upon them, preventing direct large area single-crystal growth. The work presented in this thesis will demonstrate and investigate the spontaneous van der Waals epitaxy, driven by weak chemical interaction, of a three-dimensional material on a three-dimensional material system. Despite a 3.7% lattice mismatch, high quality CdTe can be heteroepitaxially deposited on α-Al2O3 but with an incommensurate interface which demonstrates weak adhesion between the film/substrate. This weak adhesion is exploited by developing a strain driven epitaxial thin film transfer and handling method, which causes deposited layers and structures to separate at the substrate interface for transfer to secondary carrier substrates without effecting the film properties and leaving the original substrate for subsequent use. Simple transferred thin film crystalline II-VI heterostructure devices on flexible substrates are demonstrated, without the need for selective chemical etch layers, ion-implantation or complex post-processes as required by conventional fabrication techniques. Following a growth study of GaAs on three oxide substrates, the phenomena of epitaxial registry with apparent weak interface adhesion is demonstrated for another system, GaAs/α-Al2O3, where a layer transferred heterostructure device on a flexible substrate is also demonstrated.