| 1. |
- Alijan Farzad Lahiji, Faezeh, et al.
(author)
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Growth and optical properties of NiO thin films deposited by pulsed dc reactive magnetron sputtering
- 2023
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In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : A V S AMER INST PHYSICS. - 0734-2101 .- 1520-8559. ; 41:6
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Journal article (peer-reviewed)abstract
- NiO thin films with varied oxygen contents are grown on Si(100) and c-Al2O3 at a substrate temperature of 300 degrees C using pulsed dc reactive magnetron sputtering. We characterize the structure and optical properties of NiO changes as functions of the oxygen content. NiO with the cubic structure, single phase, and predominant orientation along (111) is found on both substrates. X-ray diffraction and pole figure analysis further show that NiO on the Si(100) substrate exhibits fiber-textured growth, while twin domain epitaxy was achieved on c-Al2O3, with NiO(111) k Al2O3(0001) and NiO[1 (1) over bar0]k Al2O3[10 (1) over bar0] or NiO[(1) over bar 10]k Al2O3[2 (1) over bar(1) over bar0] epitaxial relationship. The oxygen content in NiO films did not have a significant effect on the refractive index, extinction coefficient, and absorption coefficient. This suggests that the optical properties of NiO films remained unaffected by changes in the oxygen content.
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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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Glancing Angle Deposition and Growth Mechanism of Inclined AlN Nanostructures Using Reactive Magnetron Sputtering
- 2020
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In: Coatings. - : MDPI. - 2079-6412. ; 10:8
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Journal article (peer-reviewed)abstract
- Glancing angle deposition (GLAD) of AlN nanostructures was performed at room temperature by reactive magnetron sputtering in a mixed gas atmosphere of Ar and N-2. The growth behavior of nanostructures shows strong dependence on the total working pressure and angle of incoming flux. In GLAD configuration, the morphology changed from coalesced, vertical nanocolumns with faceted terminations to highly inclined, fan-like, layered nanostructures (up to 38 degrees); while column lengths decreased from around 1743 to 1068 nm with decreasing pressure from 10 to 1.5 mTorr, respectively. This indicates a change in the dominant growth mechanism from ambient flux dependent deposition to directional ballistic shadowing deposition with decreasing working pressures, which is associated with the change of energy and incident angle of incoming reactive species. These results were corroborated using simulation of metal transport (SiMTra) simulations performed at similar working pressures using Ar and N separately, which showed the average particle energy and average angle of incidence decreased while the total average scattering angle of the metal flux arriving at substrate increased with increasing working pressures. Observing the crystalline orientation of GLAD deposited wurtzite AlN nanocolumns using X-ray diffraction (XRD), pole-figure measurements revealedc-axis growth towards the direction of incoming flux and a transition from fiber-like to biaxial texture took place with increasing working pressures. Under normal deposition conditions, AlN layer morphology changed from {0001} to {10 (1) over bar1} with increasing working pressure because of kinetic energy-driven growth.
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| 4. |
- Bairagi, Samiran, 1990-
(author)
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Optical studies of AlN and GaO based nanostructures using Mueller matrix spectroscopic ellipsometry
- 2023
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Doctoral thesis (other academic/artistic)abstract
- This thesis explores the diverse optical properties manifested when light interacts with various materials, with an emphasis on circular polarization- and bandgaprelated phenomena. The studies in this work are centered around Mueller matrix spectroscopic ellipsometry, with the objective of synthesizing and characterizing nanostructured and high-quality thin films to expand our understanding of the optical properties arising from their underlying structure and electronic transitions, respectively.Papers I, II, and III of the research address the optical properties associated with circular polarization, emphasizing the importance of the morphology and structure of the sculptured thin films used. To clarify this, AlN-based chiral sculptured thin films are synthesized using glancing angle deposition and magnetron sputtering. The discussion explores the impact of different growth parameters on the morphology and crystal structure of the films. By examining these thin film samples, it is shown how their structure and crystallographic orientation can be designed to reflect narrow spectral bands of circularly polarized light at specific wavelengths. The research also tackles how thin films preferentially reflect one handedness of circularly polarized light over the other with a high degree of circular polarization. A combination of theoretical and experimental studies offers insights into the nuances of growth and light-material interactions, particularly in complex photonic structures.Papers IV and V investigate the optical properties that arise from electronic transitions in thin films, focusing on the complex dielectric function and optical bandgap phenomena. These properties are explored using high-quality single crystalline homogenous thin films of ZnGaO, grown using metal-organic chemical vapor deposition. Various formalisms to calculate bandgap values are evaluated for their precision and applicability. The modified Cody formalism stands out as the preferred choice due to its ability to provide the most linear region for extrapolating bandgap energy values. Through both theoretical calculations and experiments, a critical analysis is provided on the evolution of the crystal structure and optical properties of these thin films when exposed to elevated temperatures. These findings explain the interplay between the structural characteristics of thin films and their subsequent influence on bandgap properties.Altogether, this thesis provides a fundamental understanding of the structural and intrinsic properties of materials that govern light-matter interactions. This research paves the way for the further development of thin film-based polarization filters and advanced optoelectronic device technologies.
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| 5. |
- 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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| 6. |
- Horng, Ray-Hua, et al.
(author)
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Growth and Characterization of Sputtered InAlN Nanorods on Sapphire Substrates for Acetone Gas Sensing
- 2024
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In: Nanomaterials. - : MDPI. - 2079-4991. ; 14:1
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Journal article (peer-reviewed)abstract
- The demand for highly sensitive and selective gas sensors has been steadily increasing, driven by applications in various fields such as environmental monitoring, healthcare, and industrial safety. In this context, ternary alloy indium aluminum nitride (InAlN) semiconductors have emerged as a promising material for gas sensing due to their unique properties and tunable material characteristics. This work focuses on the fabrication and characterization of InAlN nanorods grown on sapphire substrates using an ultra-high vacuum magnetron sputter epitaxy with precise control over indium composition and explores their potential for acetone-gas-sensing applications. Various characterization techniques, including XRD, SEM, and TEM, demonstrate the structural and morphological insights of InAlN nanorods, making them suitable for gas-sensing applications. To evaluate the gas-sensing performance of the InAlN nanorods, acetone was chosen as a target analyte due to its relevance in medical diagnostics and industrial processes. The results reveal that the InAlN nanorods exhibit a remarkable sensor response of 2.33% at 600 ppm acetone gas concentration at an operating temperature of 350 degrees C, with a rapid response time of 18 s. Their high sensor response and rapid response make InAlN a viable candidate for use in medical diagnostics, industrial safety, and environmental monitoring.
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| 7. |
- Lo, Yi-Ling, et al.
(author)
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Determination of effective Ga/N ratio to control GaN growth behavior in liquid-target reactive magnetron sputter epitaxy
- 2024
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In: Materials Science in Semiconductor Processing. - : ELSEVIER SCI LTD. - 1369-8001 .- 1873-4081. ; 176
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Journal article (peer-reviewed)abstract
- The optimization of magnetron sputter epitaxy (MSE) for the high -volume production of high -quality GaN films is increasingly important. This study concerns the influence of key MSE process parameters - including the partial pressure of process gas, target -to -substrate distance (TSD), and growth temperature (TG) - for the synthesis of GaN thin films using a liquid Ga target. It is observed that the effective Ga/N ratio on the substrate surface determines the film's growth behavior and affects material's composition and luminescence properties. A lower Ar/N2 partial pressure ratio substantially enhances the crystalline quality, evidenced by the reduction in peak width of x-ray rocking curves from approximately 1.25 degrees (N -rich regime) to 0.35 degrees (Ga-rich regime) and improved GaN bandgap emission. While target sputtered in a highly Ga-rich condition significantly reduces the GaN growth rate (R), primarily due to Ga desorption in nitrogen -limited condition at elevated TG. Ion mass spectrometry and rate monitor measurements demonstrate that the Ga/N ratio can be controlled by adjusting Ar/N2 pressure ratio in MSE process. A reduction in TSD from 9.3 cm to 7 cm resulted in an increased R from 541 nm/h to 731 nm/h, corroborated by Simulation of Metal Transport (SIMTRA) analysis. Temperature -dependent studies revealed that films grown above 900 degrees C exhibited flat surface with high crystalline quality.
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