Coke, ML; (2017) Electronic properties of zinc oxide and related materials grown by molecular beam epitaxy. Doctoral thesis , UCL (University College London).

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Abstract

Thin film heterostructures have been at the heart of advances in electronics. The use of material interfaces with confined electron spaces is one of the key areas of research in thin film deposition An example of such a thin fi lm system is ZnMgO/ZnO where confi ned electron spaces occur at the interface between these two materials. The nature of the electron con finement depends upon the nature of the interface. Abrupt interfaces between ZnMgO and ZnO are strained due to lattice mismatch. This strain is relaxed if there is a gradual incorporation of Mg during growth, resulting in a dffi use interface. This strain relaxation is however accompanied by reduced confi nement and enhanced Mg-ion scattering of the confi ned electrons at the interface. Here we experimentally study the electronic transport properties of the diff use heteroepitaxial interface between single-crystal ZnO and ZnMgO fi lms grown by molecular beam epitaxy (MBE). The spatial extent of the interface region is controlled during growth by varying the zinc flux. We show that, as the spatial extent of the graded interface is reduced, the enhancement of electron mobility due to electron confi nement more than compensates for any suppression of mobility due to increased strain. In order to better understand the relationship between the material composition of the interface and the electronic properties of the con fined electron space, variations in gradient of Mg are investigated. We use the angular dependence of the magneto resistance to con firm the two dimensional nature of the space. We also looked at the electron scattering times and the associated eff ective mass. Both classical and quantum scattering times are consistent with the dominating eff ect being alloy scattering. Zn/ZnO/Zn heterostructures are a system of interest for application as Josephson junctions. We present initial studies into the superconductivity of MBE deposited zinc and subsequent morphological studies.

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