Date of Award

August 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Paul Lyman

Committee Members

Peter Schwander, Min Gyu Kim, Prasenjit Guptasarma, Michael Weinert

Abstract

Zinc oxide (ZnO) has gained wide technological interest due to its direct bandgap of ~3.37 eV and high exciton binding energy of ~60 meV and has exhibited promise for numerous electronics and opto-electronics applications. ZnO can also be alloyed with materials like magnesium oxide (MgO) to tailor the bandgap. Such heterostructures (Zn, Mg)O can be used in optoelectronic devices like quantum well lasers, photodetectors, etc.In this work, we studied the physical properties of zinc oxide (ZnO), magnesium oxide (MgO) and the heterostructures of zinc and magnesium oxide (Zn,Mg)O grown by atomic layer deposition (ALD) using a homemade viscous flow type ALD reactor with in-situ quartz crystal microbalance (QCM) sensor. The (Zn,Mg)O heterostructures were grown in two different ways; the conventional superlattice growth technique, and an intermixed alloy structure using a novel sub-saturation deposition technique. To our knowledge, this is the first successful attempt to grow (Zn,Mg)O using a sub-saturation method. Our investigation showed that the ZnO films grow in the hexagonal wurtzite structure while the MgO films grow in the rock-salt cubic structure with poor crystallinity, with the crystallinity increasing with the increase in film thickness. The structure of (Zn, Mg)O films might grow with a different structural phase, which needs further investigation to confirm. The optical bandgap of the (Zn,Mg)O films, as confirmed by UV-vis analysis, showed the increase with increasing the Mg content in the films. The structural properties as well as crystallite size of the films were investigated using x-ray diffraction (XRD) on samples grown on Si substrate. The XRD pattern revealed ZnO was polycrystalline with hexagonal wurtzite structure and showed intensity dominant in the (002) peak. The crystallinity of MgO was very poor yet showed the stable rock salt phase. In the case of (Zn,Mg)O, the dominant (002) peak gradually disappeared with the increase in Mg content. The ellipsometry measurement found that the thickness of the (Zn,Mg)O films decreases with increase in Mg content, more rapidly than what is expected from the rule of mixture. The refractive index of the (Zn,Mg)O, on the other hand, also decreased with the increase in Mg content in the film. Moreover, sheet resistance of the (Zn,Mg)O films, as determined by four-point probe measurement technique, increases quickly with the increase in Mg content.

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