Browsing by Subject "Optical phonons"

Now showing 1 - 5 of 5

Open Access
Anharmonicity in GaTe layered crystals
(Wiley-VCH Verlag GmbH & Co. KGaA, 2002) Aydınlı, Atilla; Gasanly, N. M.; Uka, A.; Efeoglu, H.
The temperature dependencies (10-300 K) of seven Raman-active mode frequencies in layered semiconductor gallium telluride have been measured in the frequency range from 25 to 300 cm -1. Softening and broadening of the optical phonon lines are observed with increasing temperature. Comparison between the experimental data and theories of the shift of the phonon lines during heating of the crystal showed that the experimental dependencies can be explained by contributions from thermal expansion and lattice anharmonicity. Lattice anharmonicity is determined to be due to three-phonon processes.
Open Access
Anharmonicity of zone-center optical phonons: Raman scattering spectra of GaSe0.5S0.5 layered crystal
(IOPscience, 2002) Gasanly, N. M.; Aydınlı, Atilla; Kocabas, C.; Özkan H.
The temperature dependencies (10-300 K) of the eight Raman-active mode frequencies and linewidths in GaSe0.5S0.5 layered crystal have been measured in the frequency range from 10 to 320 cm-1. We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison of the experimental data with the theories of the shift and broadening of the interlayer and intralayer phonon lines showed that the temperature dependencies can be explained by the contributions from thermal expansion, lattice anharmonicity and crystal disorder. The purely anharmonic contribution (phonon-phonon coupling) is found to be due to three-phonon processes. It was established that the effect of crystal disorder on the broadening of phonon lines is greater for GaSe0.5S0.5 than for binary compounds GaSe and GaS.
Open Access
Microheater-integrated spectrally selective multiband mid-infrared nanoemitter for on-chip optical multigas sensing
(American Chemical Society, 2023-11-10) Rahimian Omam, Zahra; Ghobadi, Amir; Khalichi, Bahram; Güneş, Burak; Özbay, Ekmel
Traditional optical gas sensors often require multiple components such as broadband infrared sources, detectors, and band-pass filters to detect various target gases, resulting in bulky and expensive sensor designs. A streamlined optical gas-sensing platform utilizing a narrowband thermal emitter with a spectrally selective response, capable of accommodating various target gases, has the potential to supplant current bulky designs. Through the on-chip integration of a narrowband metamaterial perfect absorber with a microelectromechanical system (MEMS) heater, a selective infrared source emitter could be designed. In this paper, a multiband metamaterial absorber with resonance modes located at different gas absorption signatures is developed for optical multi-gas-sensing applications. The proposed nanoemitter supports penta-band light absorption through the simultaneous excitation of phononic modes (within the hexagonal boron nitride (hBN) topmost layer) and plasmonic modes (with the spectrally selective underlying metal-insulator-metal (MIM) absorber stack). It achieves five near-perfect sharp absorption resonance peaks compatible with the H2S, CH4, CO2, NO, and SO2 gas absorption signatures in the mid-infrared (MIR) spectral range. This spectrally engineered multiwavelength absorption behavior is achieved by simultaneously coupling the optical phonons (OPhs) and the plasmonic modes in the vicinity of the OPh region of hBN and by exciting plasmonic modes with the help of the spacer (ZnTe: zinc telluride) and the metallic nanogratings. Finally, this low-cost and efficient penta-band absorber is combined with a MEMS-based microheater. The microheater uses a Peano-shaped configuration to provide a highly uniform surface temperature, which is crucial for accurate and reliable gas sensing. The proposed platform demonstrates excellent potential in terms of cost-effectiveness, source-free operation, and suitability for multi-gas-sensing platforms.
Open Access
Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel
(IOP Publishing, 2010-03-16) Gökden, S.; Tülek, R.; Teke, A.; Leach, J. H.; Fan, Q.; Xie, J.; Özgür, Ü.; Morkoç, H.; Lisesivdin, S. B.; Özbay, Ekmel
The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)-1 with corresponding sheet carrier densities of 2.3-1.3 × 1013 cm-2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In 0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)-1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm-2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.
Open Access
Theoretical assessment of electronic transport in InN
(Elsevier, 2004) Bulutay, C.; Ridley, B. K.
Among the group-III nitrides, InN displays markedly unusual electronic transport characteristics due to its smaller effective mass, high peak velocity and high background electron concentration. First, a non-local empirical pseudopotential band structure of InN is obtained in the light of recent experimental and first-principles results. This is utilized within an ensemble Monte Carlo framework to illuminate the interesting transport properties. It is observed that InN has a peak velocity which is about 75% higher than that of GaN while at higher fields its saturation velocity is lower than that of GaN. Because of the strongly degenerate regime brought about by the high background electron concentration, the electron-electron interaction is also investigated, but its effect on the steady-state and transient velocity-field characteristics is shown to be negligible. Finally, hot phonon generation due to excessive polar optical phonon production in the electron scattering and relaxation processes is accounted for. The main findings are the appreciable reduction in the saturation drift velocity and the slower recovery from the velocity overshoot regime. The time evolution of the hot phonon distribution is analysed in detail and it is observed to be extremely anisotropic, predominantly along the electric force direction.