SURFACE AND INTERFACE INVESTIGATIONS OF INSB, GASB, CDTE, INGAN AND GAN EPITAXIAL SEMICONDUCTOR THIN FILMS

Abstract

This thesis employs XPS, AES, SIMS, AFM, XRD, and RBS techniques to perform comprehensive surface and interface studies on III-V and II-VI semiconductor heterostructures. The systems studied have bandgaps covering the IR to UV wavelengths: InSb/GaAs and GaSb/GaAs in the infrared region, CdTe/CdS in the visible region, and InxGa1-xN/GaN/sapphire and Mg-doped GaN/sapphire in the blue/UV region. Surface and interface investigations reveal that the surfaces of InSb and GaSb are oxygen and carbon contaminated due to ambient exposure. The thickness of the surface oxide layer consisting of two oxide components can be determined based on the XPS data, using a modified Carlson's equation. AES and RBS studies show that the interdiffusion at the interfaces of InSb/GaAs and GaSb/GaAs is evident. The width of the interdiffusion region is dependent on the lattice mismatch, since at relatively low growth temperatures (around 420 °C), interdiffusion at the InSb/GaAs and GaSb/GaAs interfaces occurs mainly within the distorted interfacial region. Thus a 7.5% mismatch between GaSb and GaAs results in a 34 nm wide interface, whereas a 13.6% mismatch between InSb and GaAs results in a 85 nm wide interface. A correlation between the interface properties and the photovoltaic conversion efficiency of CdTe/CdS solar cells has been demonstrated. A mixed CdTe/CdS interface is beneficial to the improvement of CdTe/CdS solar cell efficiency due to reduced interfacial stress and thus lesser interfacial states. Interface roughness significantly influences the conversion efficiency; CdTe/CdS solar cells with rougher interfaces display better performance by promoting multiple light reflection which enhances the solar cell efficiency. The stoichiometry of InxGa1.xN epilayers have been determined by a combined XRD and RBS investigation. XPS reveals that In-Ga alloy species are present at the surface and increase with indium content. The interface between In,Ga1.xN and GaN ( 4 ± 3 nm wide) is much sharper than those of InSb/GaAs and GaSb/GaAs. One major reason is believed to be the small lattice mismatch between lnxGa1-xN and GaN (< 3.5 % for x < 0.3). Mg doping plays an important role in influencing the surface chemical states of GaN. It significantly broadens the XPS peaks of GaN, possibly through the electrical passivation of Mg acceptors by hydrogen atoms incorporated into GaN during the MOCVD deposition process.