Tailoring the Functionality of ZnO via Highly Lattice Mismatched and Lattice Matched Alloying
NON-TECHNICAL DESCRIPTION: Zinc oxide (ZnO) is emerging as one of the materials of choice for ultraviolet (UV) applications, but only at a narrow energy range. ZnO is attractive because it is a very efficient light-emitter in a wide range of temperatures, and has a benign chemical nature. Creating an alloy is one route for tailoring optical properties and achieving additional light emissions at desired energies. Thus alloying ZnO with certain atomic constituents can add new optical and electronic functionalities to ZnO. The optical properties of the alloys depend on their material quality, and on how amenable to mixing, or soluble, are the atomic constituents in the ZnO matrix. This project focuses on the study of two ZnO-based alloy systems, Mg(x)Zn(1-x)O and ZnS(1-x)O(x), with the objective of achieving high-quality alloys with known solubility and material properties that enable optical properties by design at energy ranges above and below that of pure ZnO, respectively. In particular, the alloys may provide materials with tunable bandgaps and new optical emissions in the deep-UV as well as in the blue-green spectral ranges. This research has broader impact in the potential use of these alloys in the highly vital field of blue and UV semiconductor light sources and sensors, as well as in coating technologies for the protection of sensitive electronic devices operating in harsh environments. As part of this research effort, an educational outreach program is also being initiated that presents a series of lectures to the local community on the role of materials in consumer technology. The educational effort is coordinated with a professor from the Department of Philosophy at the University of Idaho, whose expertise is in enhancing communication in cross-disciplinary research and on the transfer of scientific and technical knowledge to the general public.
TECHNICAL DETAILS: In this research, Mg(x)Zn(1-x)O with a high percentage of Mg composition that promotes the cubic phase is investigated with the goal of achieving bandgap engineered alloys in the UV range of 4 - 6 eV, while ZnS(1-x)O(x) is studied with the objective of creating materials with bandgap in the blue-green part of the visible spectrum. ZnS(1-x)O(x) is a highly-lattice mismatched alloy system, and may prove useful for enabling new electronic and optical properties, such as strong bandgap bowing into the visible spectrum, and modification of valence band to acceptor level separation for the case of doped samples. For this research, sintered ceramics, as well as films that are grown via a sputtering technique, are synthesized. Due to the different thermal equilibrium conditions of these two growth techniques, studying both types of materials is expected to yield comprehensive knowledge into key aspects of the alloys such as solubility limits, sample homogeneities, and metastabilities. The material properties are studied via several high-resolution atomic imaging techniques and X-ray diffraction, while the optical properties are studied via photoluminescence, Raman scattering, absorption, and infrared spectroscopy. Additionally, Hall effect measurements are employed for the investigation of doped ZnS(1-x)O(x) for understanding the valence - acceptor levels relation. The experimental studies are complemented with analytical modeling. The outcome of this research enables the creation of materials with a broad range of bandgaps and new emission lines in an important part of the spectrum. Another significant impact of this research is that for the advancement of ZnO as a viable material in quantum-well based devices, knowledge concerning its alloy systems is necessary, as the alloy constitutes the barrier component. The research is a collaborative effort between two PIs from the University of Idaho and from nearby Washington State University. A postdoctoral researcher, one graduate student, and an undergraduate student are supported by and participate in this research. As the PIs are strongly committed to educational efforts, several other undergraduates from diverse backgrounds take part in the research. The two laboratories provide an excellent opportunity for the students and the postdoctoral researcher to gain basic knowledge in optical materials as well as be trained in cutting-edge research techniques.