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| 2. |
- Arnaudov, B., et al.
(författare)
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Free-to-bound radiative recombination in highly conducting InN epitaxial layers
- 2004
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Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 36:4-6, s. 563-571
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical simulation of near-band-edge emission spectra of highly conducting n-InN assuming the model of 'free-to-bound' radiative recombination (FBRR) of degenerate electrons from the conduction band with nonequilibrium holes located in the valence band tails. We also study experimental photoluminescence (PL) spectra of highly conducting InN epitaxial layers grown by MBE and MOVPE with electron concentrations in the range (7.7 × 1017-6 × 1018) cm-3 and find that the energy positions and shape of the spectra depend on the impurity concentration. By modeling the experimental PL spectra of the InN layers we show that spectra can be nicely interpreted in the framework of the FBRR model with specific peculiarities for different doping levels. Analyzing simultaneously the shape and energy position of the InN emission spectra we determine the fundamental bandgap energy of InN to vary between Eg = 692 meV for effective mass mn0 = 0.042m0 and Eg =710 meV for mn0 = 0.1m0. © 2004 Elsevier Ltd. All rights reserved.
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| 3. |
- Darakchieva, Vanya, 1971-, et al.
(författare)
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Deformation potentials of the E1 (TO) and E2 modes of InN
- 2004
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:18, s. 3636-3638
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Tidskriftsartikel (refereegranskat)abstract
- The determination of deformation potentials of E1(TO) and E 2 modes of InN were discussed. The deformation potentials were evaluated for two sets of stiffness constants using x-ray diffraction, IR spectroscopic ellipsometry (IRSE), Raman scattering, and Grüneisen parameter values. The InN layer were grown on GaN buffer layers on (0001) sapphire by molecular beam epitaxy. It was found that the strain-free values of the InN E1(TO) mode was 477.9 cm-1 and 491.9 cm -1 for the E2 modes.
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| 4. |
- Darakchieva, Vanya, et al.
(författare)
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Infrared ellipsometry and Raman studies of hexagonal InN films : Correlation between strain and vibrational properties
- 2004
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Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 36:4-6, s. 573-580
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Tidskriftsartikel (refereegranskat)abstract
- The vibrational properties of InN films with different strain have been studied using Infrared ellipsometry and Raman scattering spectroscopy. We have established a correlation between the phonon mode parameters and the strain, which allows the determination of the deformation potentials and the strain-free frequencies of the InN E1(TO) and E2 modes. The LO phonons and their coupling to the free-carrier plasmon excitations are also discussed in relation to the carrier concentration in the films. © 2004 Elsevier Ltd. All rights reserved.
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| 5. |
- Fu, Y, et al.
(författare)
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Electron mobilities, Hall factors, and scattering processes of n-type GaN epilayers studied by infrared reflection and Hall measurements
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 67:11
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the drift and Hall mobilities of electrons in metal-organic chemical-vapor-deposited wurtzite GaN thin films on sapphire substrate by infrared (IR) reflection and Hall measurements. By analyzing the Hall factor (the ratio between the drift mobility obtained from IR reflection spectra and the Hall mobility from the Hall measurements), it has been concluded that the electron mobility in GaN epilayer is determined by the ionized impurity when the electron concentration is low. At a high carrier concentration of 3.2x10(18) cm(-3), electronic states of more than 70 meV become populated taking into account the thermal excitation, so that the optical-phonon-scattering process becomes activated (the optical-phonon energy is 69.43 meV obtained from IR reflection measurements). Thus, in highly doped wurtzite GaN epilayers, ionized-impurity- and optical-phonon-scattering processes jointly determine the carrier transport properties.
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| 6. |
- Fu, Y, et al.
(författare)
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Photocurrents of 14 mu m quantum-well infrared photodetectors
- 2003
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Ingår i: Journal of Applied Physics. - : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 93:12, s. 9432-9436
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Tidskriftsartikel (refereegranskat)abstract
- We study the factors that determine photogenerated carriers and response wavelengths of photocurrents of long wavelength (similar to14 mum) quantum well (QW) infrared photodetectors (QWIPs). The material structures of QWIPs are first characterized by the photoluminescence measurements (PL). By calculating the density of photogenerated carriers in the continuum above the energy barriers using the PL calibrated QWIP structures, we have demonstrated that due to the sample quality, the photocarriers can be either in miniband states (Bloch states in the multiple quantum wells), or they transport from one quantum well to the next in the form of running waves. By including possible scattering processes at the QWIP working temperature to link the theoretically calculated photocarrier density with the experimentally measured photocurrent, it is shown that the width of the photocurrent peaks of 14 mum GaAs/AlGaAs QWIPs under investigation is determined by the optical phonon emissions of photocarriers. We have further calculated the densities of photocarriers in the QWIPs reported in the literature. It is shown that the Bloch wave boundary conditions are appropriate for QWIPs with narrow QWs, whereas running wave boundary conditions are appropriate for wide QWs.
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| 7. |
- Fu, Ying, 1964, et al.
(författare)
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Photoluminescence spectra of doped GaAs films
- 2004
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Ingår i: Applied Physics A. - : Springer Science Business Media. - 0947-8396 .- 1432-0630. ; 79:3, s. 619-623
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the dependence of the photoluminescence (PL) spectrum on the doping level and the film thickness of n-GaAs thin films, both experimentally and theoretically. It has been shown theoretically that modification of the PL spectrum of p-type material by p-type doping is very small due to the large valence-band hole effective mass. The PL spectrum of n-type material is affected by two factors: (1) the electron concentration which determines the Fermi level in the material; (2) the thickness of the film due to re-absorption of the PL signal. For the n-type GaAs thin films under current investigation, the doping level as well as the film thickness can be very well calibrated by the PL spectrum when the doping level is less than 2 x 10(18) cm(-3) and the film thickness is in the range of the penetration length of the PL excitation laser.
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| 8. |
- Fu, Y, et al.
(författare)
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Quantum mechanical model and simulation of GaAs/AlGaAs quantum well infrared photodetector-I optical aspects
- 2002
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Ingår i: Hongwai yu haomibo xuebao. - : Science Press. - 1001-9014. ; 21:5, s. 321-326
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Tidskriftsartikel (refereegranskat)abstract
- A complete quantum mechanical model for GaAs/AlGaAs quantum well infrared photodetectors(QWIPs) is presented here. The model consisted of four parts: (1) Starting with the description of the electromagnetic field of the infrared radiation in the QWIP, effective component of the vector potential [\A(z)\] along the QWIP growth direction (z-axis) due to the optical diffraction grating was calculated. (2) From the wave transmissions and the occupations of the electronic states, it was discussed that the dark current in the QWIP is determined by the drift-diffusion current of carriers thermally excited from the ground sublevel in the quantum well to extended states above the barrier. (3) The photocurrent was investigated by the optical transition (absorption coefficient between the ground state to excited states due to the nonzero-[\A(z)\]). (4) By studying the inter-diffusion of the At atoms across the GaAs/AlGaAs heterointerfaces, the mobility of the drift-diffusion carriers in the excited states was calculated, so the measurement results of the dark current and photocurrent spectra can be explained theoretically. With the complete quantum mechanical descriptions of (1-4), QWIP device design and optimization are possible.
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| 9. |
- Fu, Y, et al.
(författare)
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Quantum mechanical model and simulation of GaAs/AlGaAs quantum well infrared photodetector-II electrical aspects
- 2002
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Ingår i: Hongwai yu haomibo xuebao. - : Science Press. - 1001-9014. ; 21:6, s. 401-407
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Tidskriftsartikel (refereegranskat)abstract
- A complete quantum mechanical model for GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) was presented. The photocurrent was investigated by the optical transition( absorption coefficient) between the ground state and the excited states due to the nonzero component of the radiation field along the sample growth direction. By studying the inter-diffusion of the Al atoms across the GaAs/AlGaAs heterointer faces, the mobility of the drift-diffusion carriers in the excited states was calculated. As a result, the measurement results of the dark current and the photocurrent spectra are explained theoretically.
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| 10. |
- Jiang, Jun, et al.
(författare)
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Length dependence of coherent electron transportation in metal-alkanedithiol-metal and metal-alkanemonothiol-metal junctions
- 2004
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 400:4-6, s. 336-340
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Tidskriftsartikel (refereegranskat)abstract
- We have applied the elastic-scattering Green's function theory to study the coherent electron transportation processes in both metal-alkanedithiol-metal (gold-[S(CH2)(n)S]-gold, n = 8-14) and metal-alkanemonothiol-metal (gold-[H(CH2)(n)S]-gold, n = 814) at the hybrid density functional theory level. It is shown that the current decreases exponentially with the molecular length. At the low temperature limit the electron decay rate, beta, for alkanedithiol junction is found to be around 0.30/CH2 at 1.0 V bias, much smaller than the calculated value of 0.60/CH2 for alkanemonothiol junction. The decay rate for alkanedithiol junction at the room temperature is neither sensitive to the activation of the Au-S stretching vibrational mode nor to the external bias. The calculated current-voltage characteristics and decay rates for both junctions are in excellent agreement with the corresponding experimental results.
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