Abstract
MgxZn1−xO thin films were grown as metastable alloys via a sputtering technique in order to achieve single-phase wurtzite alloys with deep-UV optical bandgaps. As-grown alloys with Mg composition range 0–72% resulted in optical bandgaps spanning the UV-range of 3.3–4.4 eV. The thermal stability of the alloys was studied via post-growth controlled annealing experiments up to 900 °C. Alloys with low Mg up to 34% were found to be highly stable and retained their optical and material properties; however, alloys with higher Mg, up to 72%, were found to be unstable and were phase separated into wurtzite and cubic structural phases with respective optical bandgaps at ~ 3.5 and 6.0 eV. Both the as-grown and annealed alloys were studied using X-ray diffraction for structural identification, transmission spectroscopy for bandgap analysis, and Raman scattering for mapping the phonon mode-behavior. The experimental value for the solubility limit was found to be ~ 30%. A straightforward model calculation based on the Raman-mode saturation behavior yielded a similar value for the solubility limit of the alloys. The results are discussed in terms of available phase-diagrams for stable-state ceramics alloys that were grown under thermodynamics equilibrium conditions.
Highlights
Ternary semiconductor alloys are important materials in device technologies, enabling bandgaps by design with tailored values for a specific application [1,2,3]
In this research we focus on the thermal stability study of single-phase MgxZn1−xO thin films with the wurtzite structure. MgxZn1−xO alloys up to x = 0.72 were grown via a sputtering technique at a relatively low temperature of 250 °C
The bandgaps were estimated via the transmission derivative method, dT/dE, that was shown to be a useful approach for ascertaining bandgaps of alloys [19], and in particular provides a suitable definition of bandgap for the alloys studied in this paper
Summary
Ternary semiconductor alloys are important materials in device technologies, enabling bandgaps by design with tailored values for a specific application [1,2,3]. The novelty of our research is a comprehensive experimental study that includes XRD, transmission, and Raman scattering which was conducted for samples that span almost the entire composition range in order to pinpoint their thermal stability and to acquire a reliable measure to the solubility limit. This was done in conjunction with a straightforward model calculation based on the Raman-mode saturation behavior which enables the analytical determination of the solubility limit of the alloys
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