First Principles Study of Band Offsets and Band Bending of InxGa1-xN/GaN and Zn1-xBexO/ZnO Heterostructures and Quantum Wells

InxGa1-xN (InGaN) and Zn1-xBexO (ZnBeO) are compound semiconductor solid solutions that display a wide band gap tuning range and strong spontaneous and piezoelectric polarizations due to the wurtzite crystal structure. They have gained significant importance in electronic and optoelectronic devices where the active layers are often fabricated as InGaN/GaN and ZnBeO/ZnO multiple quantum well (MQW) structures to take the advantage of an enhanced quantum confinement. The performances of devices based on such polarizable MQWs are significantly affected by band offsets and band bending, two critical interlayer interfacial parameters closely related to the electronic structure of the barriers and the wells. In this thesis, first principles calculations based on density functional theory are carried out to study how the properties of each layer in InGaN/GaN and ZnBeO/ZnO MQWs and their band offsets and band bending can be altered by different heteroepitaxial growth conditions. The results show that for bulk InGaN and ZnBeO, the band gap energy, spontaneous polarization, and elastic properties are nonlinearly dependent on the In and Be concentrations, respectively. For heteroepitaxial InGaN and ZnBeO thin films on suitable (0001) substrates, their band gap energy is significantly reduced for a tensile in-plane misfit strain and for a compressive misfit strain larger than −6%. For InGaN/GaN and ZnBeO/ZnO MQWs, it is found that a combination of mechanical boundary conditions, epitaxial orientation, and the In (or Be) concentration can be used as design parameters in optoelectronic devices. Taken InGaN/GaN MQWs as an example, the valence band offset (ΔEV) increases with In concentration in InGaN/GaN superlattices regardless of the mechanical boundary conditions and growth epitaxy. For low In compositions (xEV values. The valence band offset is more enhanced in polar c-plane conditions for In concentrations larger than 0.5. For non-polar m-plane InGaN/InN MQWs, the role of epitaxial strains that increase with increasing In concentration according to Vegard’s law have very little effect on ΔEV for xc-plane MQWs is substantial, especially for larger In concentrations (x\u3e0.5)