Probing embedded structural inhomogeneities in MgZnO alloys via selective resonant Raman scattering

Thapa, Dinesh; Huso, Jesse; Che, Hui; Huso, Michelle; Morrison, John L.; Gutiérrez, Diana; Norton, M. Grant; Bergman, Leah

doi:10.1063/1.4805005

Cited by 21 publications

(23 citation statements)

References 42 publications

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“…As commented before, previous optical absorption spectroscopy studies [15,17] tration a low-energy absorption tail appears overlapping to the main absorption edge of the sample. In Fig.…”

Section: Resultsmentioning

confidence: 77%

“…However, the study of the optical properties of Mg x Zn 1−x O thin films with phase separation has been scarce [15][16][17]. Those previous studies find with optical absorption spectroscopy [15,17] a lowenergy absorption tail overlapped to the main absorption edge for Mg concentrations above a critical concentration when phase separation occurs. The origin of this absorption tail observed in Mg x Zn 1−x O thin films grown by different methods, has been tentatively explained by some authors [17] as due to the beginning of the absorption edge of the coexisting rock-salt phase.…”

Section: Introductionmentioning

confidence: 99%

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Phase segregation in MgxZn1–xO probed by optical absorption and photoluminescence at high pressure

Marín-Borrás

Ruiz‐Fuertes

Segura

et al. 2017

Journal of Applied Physics

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The appearance of segregated wurtzite MgxZn1−xO with low Mg content in thin films with x > 0.3 affected by phase separation, cannot be reliably probed with crystallographic techniques owing to its embedded nanocrystalline configuration. Here we show a high-pressure approach which exploits the distinctive behaviors under pressure of wurtzite MgxZn1−xO thin films with different Mg contents to unveil phase segregation for x > 0.3. By using ambient conditions photoluminescence (PL), and with optical absorption and PL under high pressure for x = 0.3 we show that the appearance of a segregated wurtzite phase with a magnesium content of x ∼ 0.1 is inherent to the wurtzite and rocksalt phase separation. We also show that the presence of segregated wurtzite phase in oversaturated thin films phase is responsible for the low-energy absorption tail observed above x = 0.3 in our MgxZn1−xO thin films. Our study has also allowed us to extend the concentration dependence of the pressure coefficient of the band gap from the previous limit of x = 0.13 to x ≈ 0.3 obtaining dEg/dP = 29 meV/GPa for wurtzite with x ≈ 0.3 and 25 meV/GPa for the segregated x ≈ 0.09 wurtzite phase.

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“…As commented before, previous optical absorption spectroscopy studies [15,17] tration a low-energy absorption tail appears overlapping to the main absorption edge of the sample. In Fig.…”

Section: Resultsmentioning

confidence: 77%

Section: Introductionmentioning

confidence: 99%

Phase segregation in MgxZn1–xO probed by optical absorption and photoluminescence at high pressure

Marín-Borrás

Ruiz‐Fuertes

Segura

et al. 2017

Journal of Applied Physics

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“…This band-edge for the alloys can be explained in terms of in-gap density of states due to structural and morphology defects as well as alloy inhomogeneities [20,21]. We previously found that the derivative method yields a good measure of the alloy's bandgap; sometime referred to as the optical gap rather than the fundamental gap [19]. The energies of the optical gaps are usually somewhat lower than those of the fundamental gaps.…”

Section: Resultsmentioning

confidence: 99%

“…Figures 1 and 2a, b present the XRD and the transmission spectra, respectively, of the Mg 0.17 Zn 0.83 O and Mg 0.34 Zn 0.66 O samples. 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. Usually, the transmission spectrum of the alloy is not decreasing in a sharp manner at the spectral-range of the bandgap, but rather exhibits an Urbach-type band-edge tailing.…”

Section: Resultsmentioning

confidence: 99%

Thermal stability of ultra-wide-bandgap MgZnO alloys with wurtzite structure

Thapa

Huso

Lapp

et al. 2018

J Mater Sci: Mater Electron

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Mg x Zn 1−x O 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.

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“…The crystal structure of ZnO is hexagonal wurtzite, while that of MgO is cubic rocksalt. The different crystal structure of the two end-members impedes the realization of a complete solid-solution, of the Mg x Zn 1-x O alloy system [11,12]. Achieving good solubility depends generally on the growth techniques and on post-growth treatments such as annealing.…”

Section: Introductionmentioning

confidence: 99%

UV-luminescent MgZnO semiconductor alloys: nanostructure and optical properties

Thapa

Huso

Miklos

et al. 2016

J Mater Sci: Mater Electron

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MgZnO is emerging as a vital semiconductoralloy system with desirable optical properties that can span a large range of the UV spectrum. Due to its benign chemical character, MgZnO is considered to be an environmentally friendly material. This paper presents studies on annealing as a useful and straightforward approach for the enhancement of the optical and crystal quality of Mg 0.17 Zn 0.83 O nanocrystalline films grown via DC sputtering. The alloys were studied via several imaging and optical techniques. It was found that high-temperature annealing, *900°C, in Argon atmosphere, significantly improves the solubility of the alloy. This temperature range is consistent with the thermal diffusion temperature of Mg needed for the creation of a soluble alloy. Moreover, the annealing process was found to minimize the undesirable visible luminescence, attributed to Mg and Zn interstitials, while significantly enhancing the bandgap sharpness and the efficiency of the UV-luminescence at *3.5 eV. The analysis indicated that these optical attributes were achieved due to the combined effects of good solubility, an improved morphology, and a reduction of native defects. The annealing was also proven to be beneficial for the reduction of the compressive stress in the alloy: a relaxation *1.8 GPa was calculated via Raman scattering. The inherent stress was inferred to originate mainly from the granular morphology of the alloys.

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Probing embedded structural inhomogeneities in MgZnO alloys via selective resonant Raman scattering

Cited by 21 publications

References 42 publications

Phase segregation in MgxZn1–xO probed by optical absorption and photoluminescence at high pressure

Phase segregation in MgxZn1–xO probed by optical absorption and photoluminescence at high pressure

Thermal stability of ultra-wide-bandgap MgZnO alloys with wurtzite structure

UV-luminescent MgZnO semiconductor alloys: nanostructure and optical properties

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