| 1. |
- Horng, Ray-Hua, et al.
(author)
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Structure Effect on the Response of ZnGa2O4 Gas Sensor for Nitric Oxide Applications
- 2022
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In: Nanomaterials. - : MDPI. - 2079-4991. ; 12:21
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Journal article (peer-reviewed)abstract
- We fabricated a gas sensor with a wide-bandgap ZnGa2O4 (ZGO) epilayer grown on a sapphire substrate by metalorganic chemical vapor deposition. The ZGO presented (111), (222) and (333) phases demonstrated by an X-ray diffraction system. The related material characteristics were also measured by scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. This ZGO gas sensor was used to detect nitric oxide (NO) in the parts-per-billion range. In this study, the structure effect on the response of the NO gas sensor was studied by altering the sensor dimensions. Two approaches were adopted to prove the dimension effect on the sensing mechanism. In the first approach, the sensing area of the sensors was kept constant while both channel length (L) and width (W) were varied with designed dimensions (L x W) of 60 x 200, 80 x 150, and 120 x100 mu m(2). In the second, the dimensions of the sensing area were altered (60, 40, and 20 mu m) with W kept constant. The performance of the sensors was studied with varying gas concentrations in the range of 500 ppb similar to 10 ppm. The sensor with dimensions of 20 x 200 mu m(2) exhibited a high response of 11.647 in 10 ppm, and 1.05 in 10 ppb for NO gas. The sensor with a longer width and shorter channel length exhibited the best response. The sensing mechanism was provided to explain the above phenomena. Furthermore, the reaction between NO and the sensor surface was simulated by O exposure of the ZGO surface in air and calculated by first principles.
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| 2. |
- Bairagi, Samiran, et al.
(author)
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Formation of quaternary Zn(AlxGa1−x)2O4 epilayers driven by thermally induced interdiffusion between spinel ZnGa2O4 epilayer and Al2O3 substrate
- 2023
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In: Materials Today Advances. - : Elsevier. - 2590-0498. ; 20
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Journal article (peer-reviewed)abstract
- Zinc aluminogallate, Zn(AlxGa1−x)2O4 (ZAGO), a single-phase spinel structure, offers considerable potential for high-performance electronic devices due to its expansive compositional miscibility range between aluminum (Al) and gallium (Ga). Direct growth of high-quality ZAGO epilayers however remains problematic due to the high volatility of zinc (Zn). This work highlights a novel synthesis process for high-quality epitaxial quaternary ZAGO thin films on sapphire substrates, achieved through thermal annealing of a ZnGa2O4 (ZGO) epilayer on sapphire in an ambient air setting. In-situ annealing x-ray diffraction measurements show that the incorporation of Al in the ZGO epilayer commenced at 850 °C. The Al content (x) in ZAGO epilayer gradually increased up to around 0.45 as the annealing temperature was raised to 1100 °C, which was confirmed by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy. X-ray rocking curve measurement revealed a small full width at half maximum value of 0.72 °, indicating the crystal quality preservation of the ZAGO epilayer with a high Al content. However, an epitaxial intermediate �–(AlxGa1−x)2O3 layer (� - AGO) was formed between the ZAGO and sapphire substrate. This is believed to be a consequence of the interdiffusion of Al and Ga between the ZGO thin film and sapphire substrate. Using density functional theory, the substitution cost of Ga in sapphire was determined to be about 0.5 eV lower than substitution cost of Al in ZGO. Motivated by this energetically favorable substitution, a formation mechanism of the ZAGO and AGO layers was proposed. Spectroscopic ellipsometry studies revealed an increase in total thickness of the film from 105.07 nm (ZGO) to 147.97 nm (ZAGO/AGO) after annealing to 1100 °C, which were corroborated using TEM. Furthermore, an observed increase in the direct (indirect) optical bandgap from 5.06 eV (4.7 eV) to 5.72 eV (5.45 eV) with an increasing Al content in the ZAGO layer further underpins the formation of a quaternary ZAGO alloy with a tunable composition.
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| 3. |
- Bairagi, Samiran, et al.
(author)
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Zinc gallate (ZnGa2O4) epitaxial thin films : determination of optical properties and bandgap estimation using spectroscopic ellipsometry
- 2022
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In: Optical Materials Express. - : Optica Publishing Group. - 2159-3930 .- 2159-3930. ; 12:8, s. 3284-3295
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Journal article (peer-reviewed)abstract
- Electronic grade ZnGa2O4 epitaxial thin films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition and investigated using spectroscopic ellipsometry. Their thickness, roughness and optical properties were determined using a Multiple Sample Analysis based approach by the regression analysis of optical model and measured data. These samples were then compared to samples which had undergone ion etching, and it was observed that etching time up to four minutes had no discernible impact on its optical properties. Line shape analysis of resulting absorption coefficient dispersion revealed that ZnGa(2)O(4 )exhibited both direct and indirect interband transitions. The modified Cody formalism was employed to determine their optical bandgaps. These values were found to be in good agreement with values obtained using other popular bandgap extrapolation procedures. Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published articles title, journal citation, and DOI.
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| 4. |
- Horng, Ray-Hua, et al.
(author)
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Effects of UV-ozone treatment on the performance of deep-ultraviolet photodetectors based on ZnGa2O4 epilayers
- 2022
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In: Materials Chemistry and Physics. - : ELSEVIER SCIENCE SA. - 0254-0584 .- 1879-3312. ; 292
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Journal article (peer-reviewed)abstract
- ZnGa2O4 epilayers have been grown on sapphire using the metalorganic chemical vapor deposition system. However, there is a trade-off between high conductivity and large defect density (oxygen vacancies) with the growth time of the growth of ZnGa2O4 epilayers. The ultraviolet (UV)-ozone treatment on the ZnGa2O4 epilayer at 100. C was proposed to reduce the number of oxygen vacancies in ZnGa2O4. The effect of UV-ozone treatment on the performance of ZnGa2O4 metal-semiconductor-metal (MSM) photodetector (PD) was evaluated. X-ray photoelectron spectroscopy analysis showed a decrease in the number of oxygen vacancies after UV-ozone treatment of ZnGa2O4. The measured lattice parameter near the surface around 10 nm of untreated ZnGa2O4 was 8.3434 +/- 0.0120 angstrom and increased slightly to 8.3775 +/- 0.0083 A. after UV-ozone treatment due to the decrease in oxygen vacancies. The dark current (at 5 V) of ZnGa2O4 PD was significantly reduced from 251 to 20.2 pA before and after UV-ozone treatment; it resulted in a substantial one-order enhancement in the on/off ratio of the PDs from 2.7 x 10(5) and 2.15 x 10(6) after the UV-ozone treatment. Furthermore, the rejection ratio also improved between 240 and 470 nm from 35 to 84 after UV-ozone treatment. The relationship between photocurrent and light intensity and the improvement in raising and falling time also showed the reduced density of trap states by UV-ozone treatment. This indicates that UV-ozone treatment can enhance the characteristics of ZnGa2O4 PDs for UV sensing applications.
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| 5. |
- Horng, Ray-Hua, et al.
(author)
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Growth mechanism and characteristics of beta-Ga2O3 heteroepitaxailly grown on sapphire by metalorganic chemical vapor deposition
- 2022
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In: Materials Today Advances. - : ELSEVIER. - 2590-0498. ; 16
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Journal article (peer-reviewed)abstract
- In this study, monoclinic gallium oxide (beta-Ga2O3) epilayer was successfully grown on c-plane, (0001), sapphire substrate by metalorganic chemical vapor deposition (MOCVD) with interplaying growth temperature, TEGa flow rate, and growth time. X-ray diffraction 20 scans show only three narrow diffraction peaks referred to beta-Ga2O3 ((2) over bar 01), ((4) over bar 02), and ((6) over bar 03) in all epilayers, indicating a superior crystalline quality. Current-voltage (I-V) measurement reveals that these beta-Ga2O3 films are insulating and exhibit high resistance in a range of 10(12)-10(14) Omega. The crystallization characteristics of the epilayers can be effectively improved with thickness through increasing TEGa flow rate and growth time, which was evidenced by X-ray rocking curves and I-V measurements. However, the surface roughness of beta-Ga2O3 film increases with growth time and TEGa flow rate. When the growth temperature increases above 825 degrees C, the thickness of beta-Ga2O3 film decreases clearly. Furthermore, it can be found that the growth rate decreased as the growth time increasing. The growth mechanism based on first-principles calculation was proposed as that 3D growth induced by the lattice mismatch between beta-Ga2O3 and sapphire starts at nucleation stage, and follows up a lateral growth promoting a 2D growth after the thick epilayer being grown. In addition, the complex chemical reaction between TEGa and oxygen precursors was unraveled by density function theory calculation. (c) 2022 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
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| 6. |
- Horng, Ray-Hua, et al.
(author)
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Ion implantation effects on the characteristics of 8-Ga2O3 epilayers grown on sapphire by MOCVD
- 2022
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In: Ceramics International. - : ELSEVIER SCI LTD. - 0272-8842 .- 1873-3956. ; 48:24, s. 36425-36432
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Journal article (peer-reviewed)abstract
- In this study, the Si-ions implantation technique with different doses from 1 x 1014 to 1 x 1015 cm-2 and dose energy 30, 40 and 50 keV was used to tune the electrical properties in unintentionally doped (UID) 8-Ga2O3 epilayers grown on the sapphire substrates by metalorganic chemical vapor deposition (MOCVD). A high quality UID 8-Ga2O3 epilayers were fabricated using the optimized growth parameters of MOCVD. The UID and Si-ions implanted 8-Ga2O3 epilayers were examined and results were compared with the help of X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. Si-ions implantation parameters were also simulated by stopping and range of ions in matter software (SRIM) and actual Si-ions concentration was measured by secondary ions mass spectroscopy. The electrical properties of the implanted 8-Ga2O3 epilayers were measured by transmission length method and Hall measurements. The sheet resistivity for the 8-Ga2O3epilayers with Si-ion dose of 1 x 1014, 6 x 1014 and 1 x 1015 cm-2 were found as 2.047, 0.158 and 0.144 Cd cm, respectively measured by Hall measurements and the electron carrier concentrations for the above doses were 4.39 x 1018, 6.86 x 1018 and 7.98 x 1019 cm-3. From the above results, the ion implantation was demonstrated to effectively reduce the resistivity with the high carrier concentrations.
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| 7. |
- Lu, Chan-Hung, et al.
(author)
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beta-Ga2O3 MOSFETs electrical characteristic study of various etching depths grown on sapphire substrate by MOCVD
- 2023
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In: DISCOVER NANO. - : SPRINGER. - 2731-9229. ; 18:1
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Journal article (peer-reviewed)abstract
- beta-Ga2O3 thin films with both a 45 nm Si-doped conductive epilayer and unintentionally doped epilayer were grown on c-plane sapphire substrate by metalorganic chemical vapor deposition. beta-Ga2O3 based metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated with gate recess depths of 20 nm and 40 nm (it indicated gate depth with 70 nm and 50 nm, respective), respectively, and without said recessing process. The conductivity of beta-Ga2O3 epilayers was improved through low in situ doping using a tetraethoxysilane precursor to increase MOSFET forward current density. After recessing, MOSFET operation was transferred from depletion to enhanced mode. In this study, the maximum breakdown voltage of the recessed 40 nm transistor was 770 V. The etching depth of a recessed-gate device demonstrates its influence on device electrical performance.
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