| 1. |
- Armakavicius, Nerijus, et al.
(författare)
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Electron effective mass in GaN revisited: New insights from terahertz and mid-infrared optical Hall effect
- 2024
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Ingår i: APL Materials. - : AIP Publishing. - 2166-532X. ; 12:2
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Tidskriftsartikel (refereegranskat)abstract
- Electron effective mass is a fundamental material parameter defining the free charge carrier transport properties, but it is very challenging to be experimentally determined at high temperatures relevant to device operation. In this work, we obtain the electron effective mass parameters in a Si-doped GaN bulk substrate and epitaxial layers from terahertz (THz) and mid-infrared (MIR) optical Hall effect (OHE) measurements in the temperature range of 38-340 K. The OHE data are analyzed using the well-accepted Drude model to account for the free charge carrier contributions. A strong temperature dependence of the electron effective mass parameter in both bulk and epitaxial GaN with values ranging from (0.18 +/- 0.02) m(0) to (0.34 +/- 0.01) m(0) at a low temperature (38 K) and room temperature, respectively, is obtained from the THz OHE analysis. The observed effective mass enhancement with temperature is evaluated and discussed in view of conduction band nonparabolicity, polaron effect, strain, and deviations from the classical Drude behavior. On the other hand, the electron effective mass parameter determined by MIR OHE is found to be temperature independent with a value of (0.200 +/- 0.002) m(0). A possible explanation for the different findings from THz OHE and MIR OHE is proposed. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)
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| 2. |
- Gopalan, Prashanth, et al.
(författare)
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The anisotropic quasi-static permittivity of single-crystal beta-Ga2O3 measured by terahertz spectroscopy
- 2020
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Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 117:25
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Tidskriftsartikel (refereegranskat)abstract
- The quasi-static anisotropic permittivity parameters of electrically insulating beta gallium oxide (beta -Ga2O3) were determined by terahertz spectroscopy. Polarization-resolved frequency domain spectroscopy in the spectral range from 200GHz to 1THz was carried out on bulk crystals along different orientations. Principal directions for permittivity were determined along crystallographic axes c and b and reciprocal lattice direction a *. No significant frequency dispersion in the real part of dielectric permittivity was observed in the measured spectral range. Our results are in excellent agreement with recent radio frequency capacitance measurements as well as with extrapolations from recent infrared measurements of phonon mode and high-frequency contributions and close the knowledge gap for these parameters in the terahertz spectral range. Our results are important for applications of beta -Ga2O3 in high-frequency electronic devices.
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| 3. |
- Hilfiker, Matthew, et al.
(författare)
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Anisotropic dielectric function, direction dependent bandgap energy, band order, and indirect to direct gap crossover in α-(AlxGa1−x)2O3 (0≤?≤10≤x≤1)
- 2022
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 121:5
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Tidskriftsartikel (refereegranskat)abstract
- Mueller matrix spectroscopic ellipsometry is applied to determine anisotropic optical properties for a set of single-crystal rhombohedral structure alpha-(AlxGa1-x)(2)O-3 thin films (0 <= x <= 1). Samples are grown by plasma-assisted molecular beam epitaxy on m-plane sapphire. A critical-point model is used to render a spectroscopic model dielectric function tensor and to determine direct electronic band-to-band transition parameters, including the direction dependent two lowest-photon energy band-to-band transitions associated with the anisotropic bandgap. We obtain the composition dependence of the direction dependent two lowest band-to-band transitions with separate bandgap bowing parameters associated with the perpendicular (b(Eg,⊥) = 1.31 eV) and parallel (b(Eg,||) = 1.61 eV) electric field polarization to the lattice c direction. Our density functional theory calculations indicate a transition from indirect to direct characteristics between alpha-Ga2O3 and alpha-Al2O3, respectively, and we identify a switch in band order where the lowest band-to-band transition occurs with polarization perpendicular to c in alpha-Ga2O3 whereas for alpha-Al2O3 the lowest transition occurs with polarization parallel to c. We estimate that the change in band order occurs at approximately 40% Al content. Additionally, the characteristic of the lowest energy critical point transition for polarization parallel to c changes from M-1 type in alpha-Ga2O3 to M-0 type van Hove singularity in alpha-Al2O3.
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| 4. |
- Hilfiker, Matthew, et al.
(författare)
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Anisotropic dielectric functions, band-to-band transitions, and critical points in alpha-Ga2O3
- 2021
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Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 118:6
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Tidskriftsartikel (refereegranskat)abstract
- We use a combined generalized spectroscopic ellipsometry and density functional theory approach to determine and analyze the anisotropic dielectric functions of an alpha -Ga2O3 thin film. The sample is grown epitaxially by plasma-assisted molecular beam epitaxy on m-plane sapphire. Generalized spectroscopic ellipsometry data from multiple sample azimuths in the spectral range from 0.73eV to 8.75eV are simultaneously analyzed. Density functional theory is used to calculate the valence and conduction band structure. We identify, for the indirect-bandgap material, two direct band-to-band transitions with M-0-type van Hove singularities for polarization perpendicular to the c axis, E 0 , perpendicular to = 5.46 ( 6 ) eV and E 0 , perpendicular to = 6.04 ( 1 ) eV, and one direct band-to-band transition with M-1-type van Hove singularity for polarization parallel to E 0 , | | = 5.44 ( 2 ) eV. We further identify excitonic contributions with a small binding energy of 7meV associated with the lowest ordinary transition and a hyperbolic exciton at the M-1-type critical point with a large binding energy of 178meV.
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| 5. |
- Hilfiker, Matthew, et al.
(författare)
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Elevated temperature spectroscopic ellipsometry analysis of the dielectric function, exciton, band-to-band transition, and high-frequency dielectric constant properties for single-crystal ZnGa2O4
- 2022
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 120:13
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Tidskriftsartikel (refereegranskat)abstract
- We report the elevated temperature (22 degrees C <= T <= 600 degrees C) dielectric function properties of melt grown single crystal ZnGa2O4 using a spectroscopic ellipsometry approach. A temperature dependent Cauchy dispersion analysis was applied across the transparent spectrum to determine the high-frequency index of refraction yielding a temperature dependent slope of 3.885(2) x 10(-5)K(-1). A model dielectric function critical point analysis was applied to examine the dielectric function and critical point transitions for each temperature. The lowest energy M-0-type critical point associated with the direct bandgap transition in ZnGa2O4 is shown to red-shift linearly as the temperature is increased with a subsequent slope of -0.72(4) meV K-1. Furthermore, increasing the temperature results in a reduction of the excitonic amplitude and increase in the exciton broadening akin to exciton evaporation and lifetime shortening. This matches current theoretical understanding of excitonic behavior and critically provides justification for an anharmonic broadened Lorentz oscillator to be applied for model analysis of excitonic contributions.
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| 6. |
- Hilfiker, Matthew, et al.
(författare)
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High-frequency and below bandgap anisotropic dielectric constants in alpha-(AlxGa1-x)(2)O-3 (0 <= x <= 1)
- 2021
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 119:9
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Tidskriftsartikel (refereegranskat)abstract
- A Mueller matrix spectroscopic ellipsometry approach was used to investigate the anisotropic dielectric constants of corundum alpha-(AlxGa1-x)(2)O-3 thin films in their below bandgap spectral regions. The sample set was epitaxially grown using plasma-assisted molecular beam epitaxy on m-plane sapphire. The spectroscopic ellipsometry measurements were performed at multiple azimuthal angles to resolve the uniaxial dielectric properties. A Cauchy dispersion model was applied, and high-frequency dielectric constants are determined for polarization perpendicular (epsilon(infinity,perpendicular to)) and parallel (epsilon(infinity,parallel to)) to the thin film c-axis. The optical birefringence is negative throughout the composition range, and the overall index of refraction substantially decreases upon incorporation of Al. We find small bowing parameters of the highfrequency dielectric constants with b(perpendicular to) = 0:386 and b(parallel to) = 0:307. Published under an exclusive license by AIP Publishing.
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| 7. |
- Hilfiker, Matthew, et al.
(författare)
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Zinc gallate spinel dielectric function, band-to-band transitions, and Gamma-point effective mass parameters
- 2021
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Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 118:13
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Tidskriftsartikel (refereegranskat)abstract
- We determine the dielectric function of the emerging ultrawide bandgap semiconductor ZnGa2O4 from the near-infrared (0.75eV) into the vacuum ultraviolet (8.5eV) spectral regions using spectroscopic ellipsometry on high quality single crystal substrates. We perform density functional theory calculations and discuss the band structure and the Brillouin zone Gamma-point band-to-band transition energies, their transition matrix elements, and effective band mass parameters. We find an isotropic effective mass parameter (0.24m(e)) at the bottom of the Gamma-point conduction band, which equals the lowest valence band effective mass parameter at the top of the highly anisotropic and degenerate valence band (0.24m(e)). Our calculated band structure indicates the spinel ZnGa2O4 is indirect, with the lowest direct transition at the Gamma-point. We analyze the measured dielectric function using critical-point line shape functions for a three-dimensional, M-0-type van Hove singularity, and we determine the direct bandgap with an energy of 5.27(3) eV. In our model, we also consider contributions from Wannier-Mott type excitons with an effective Rydberg energy of 14.8meV. We determine the near-infrared index of refraction from extrapolation (1.91) in very good agreement with results from recent infrared ellipsometry measurements (root epsilon(infinity)= 1.94) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)]. Published under license by AIP Publishing.
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| 8. |
- Knight, Sean Robert, et al.
(författare)
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Terahertz permittivity parameters of monoclinic single crystal lutetium oxyorthosilicate
- 2024
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 124:3
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Tidskriftsartikel (refereegranskat)abstract
- The anisotropic permittivity parameters of monoclinic single crystal lutetium oxyorthosilicate, Lu2SiO5 (LSO), have been determined in the terahertz spectral range. Using terahertz generalized spectroscopic ellipsometry (THz-GSE), we obtained the THz permittivities along the a, b, and c? crystal directions, which correspond to the ea; eb, and ec? on-diagonal tensor elements. The associated off diagonal tensor element eac? was also determined experimentally, which is required to describe LSO's optical response in the monoclinic a-c crystallographic plane. From the four tensor elements obtained in the model fit, we calculate the direction of the principal dielectric axes in the a-c plane. We find good agreementwhen comparing THz-GSE permittivities to the static permittivity tensors from previous infrared and density functional theory studies.
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| 9. |
- Korlacki, Rafal, et al.
(författare)
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Strain and Composition Dependencies of the Near-Band-Gap Optical Transitions in Monoclinic (AlxGa1-x)2O3 Alloys with Coherent Biaxial In-Plane Strain on Ga2O3(010)
- 2022
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Ingår i: Physical Review Applied. - : AMER PHYSICAL SOC. - 2331-7019. ; 22:6
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Tidskriftsartikel (refereegranskat)abstract
- The bowing of the energy of the three lowest band-to-band transitions in beta-(AlXGa1-X)2O3 alloys is resolved using a combined density-functional theory (DFT) and generalized spectroscopic ellipsometry approach. The DFT calculations of the electronic band structure of both beta-Ga2O3 and theta-Al2O3 allow the linear portion of the energy shift in the alloys to be extracted, and provide a method for quantifying the role of coherent strain present in the beta-(AlXGa1-X)2O3 thin films on (010) beta-Ga2O3 substrates. The energies of band-to-band transitions are obtained using the spectroscopic ellipsometry eigenpolarization model approach [A. Mock et al., Phys. Rev. B 95, 165202 (2017)]. After subtracting the effects of strain, which also induces additional bowing and after subtraction of the linear portion of the energy shift due to alloying, the bowing parameters associated with the three lowest band-to-band transitions in monoclinic beta-(AlXGa1-X)2O3 are found.
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| 10. |
- Matson, Joseph R., et al.
(författare)
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The Role of Optical Phonon Confinement in the Infrared Dielectric Response of III–V Superlattices
- 2023
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Ingår i: Advanced Materials. - : WILEY-V C H VERLAG GMBH. - 0935-9648 .- 1521-4095.
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Tidskriftsartikel (refereegranskat)abstract
- Polar dielectrics are key materials of interest for infrared (IR) nanophotonic applications due to their ability to host phonon-polaritons that allow for low-loss, subdiffractional control of light. The properties of phonon-polaritons are limited by the characteristics of optical phonons, which are nominally fixed for most “bulk” materials. Superlattices composed of alternating atomically thin materials offer control over crystal anisotropy through changes in composition, optical phonon confinement, and the emergence of new modes. In particular, the modified optical phonons in superlattices offer the potential for so-called crystalline hybrids whose IR properties cannot be described as a simple mixture of the bulk constituents. To date, however, studies have primarily focused on identifying the presence of new or modified optical phonon modes rather than assessing their impact on the IR response. This study focuses on assessing the impact of confined optical phonon modes on the hybrid IR dielectric function in superlattices of GaSb and AlSb. Using a combination of first principles theory, Raman, FTIR, and spectroscopic ellipsometry, the hybrid dielectric function is found to track the confinement of optical phonons, leading to optical phonon spectral shifts of up to 20 cm−1. These results provide an alternative pathway toward designer IR optical materials.
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