Interfacial issues in wide-bandgap II-VI materials and related multiple quantum well structures

Abstract

This thesis is devoted to exploring two main processes potentially relevant to the physics and technology of wide-bandgap II-VI materials: :br:i) fabrication of Zn1-yCdySe quantum wells on lattice-matched substrates to implement strain-free blue-green emitters; :br:ii) optimization of the properties of metal/II-VI contacts through modification of the local interface environment. In the first area, we showed that graded composition InxGa1-xAs buffer layers on GaAs (001) wafers can be used to match the Zn1-yCdySe quantum well lattice parameter. Detailed studies of the electronic and structural properties of Zn1-zMgzSe alloys and Zn1 yCdySe/Zn1-zMgzSe heterojunction interfaces allowed us to design and fabricate strain-free Zn1 yCdySe/Zn1-zMgzSe multiple quantum well structures on the novel InxGa1-xAs substrates. We found that the surface corrugations of the III-V buffer do not hinder the structural quality of the quantum wells, and give only rise to long-period coherent undulations of quantum well and barrier layers. In the area of metal/II-VI semiconductor junctions, our experimental studies of Zn/ZnSe (001) interfaces stimulated new theoretical calculations of interface formation energies and electronic structure. Both experiment and theory indicate that this interface represents a unique example of ideally unreactive metal/ZnSe contact. Conversely, we found that Au/ZnSe contacts show evidence of interface reactions, in contrast with most earlier reports. Significant lateral inhomogeneities in the interface parameters are likely related to preferential reaction at steps and kinks of the ZnSe(001) surface. By inserting ultrathin layers of Zn at the interface between Au and ZnSe(001), we demonstrated the possibility of inducing large modifications in the Schottky barrier height, together with an improved lateral homogeneity and ideality of the resulting metal/ZnSe contacts.