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Item Open Access Carrier-induced refractive index change and optical absorption in wurtzite InN and GaN: Full-band approach(The American Physical Society, 2010) Bulutay, C.; Turgut, C. M.; Zakhleniuk, N. A.Show more Based on the full band electronic structure calculations, first we consider the effect of n -type doping on the optical absorption and the refractive index in wurtzite InN and GaN. We identify quite different dielectric response in either case; while InN shows a significant shift in the absorption edge due to n -type doping, this is masked for GaN due to efficient cancellation of the Burstein-Moss effect by the band gap renormalization. Moreover, for high doping levels the intraband absorption becomes significant in InN. For energies below 1 eV, the corresponding shifts in the real parts of the dielectric function for InN and GaN are in opposite directions. Furthermore, we observe that the free-carrier plasma contribution to refractive index change becomes more important than both band filling and the band gap renormalization for electron densities above 1019 cm -3 in GaN, and 1020 cm -3 in InN. As a result of the two different characteristics mentioned above, the overall change in the refractive index due to n -type doping is much higher in InN compared to GaN, which in the former exceeds 4% for a doping of 1019 cm -3 at 1.55μm wavelength. Finally, we consider intrinsic InN under strong photoexcitation which introduces equal density of electron and holes thermalized to their respective band edges. The change in the refractive index at 1.55μm is observed to be similar to the n -doped case up to a carrier density of 1020 cm -3. However, in the photoexcited case this is now accompanied by a strong absorption in this wavelength region due to Γ5v → Γ6v intravalence band transition. Our findings suggest that the alloy composition of Inx Ga1-x N can be optimized in the indium-rich region so as to benefit from high carrier-induced refractive index change while operating in the transparency region to minimize the losses. These can have direct implications for InN-containing optical phase modulators and lasers.Show more Item Open Access Carrier-induced refractive index change in InN(Wiley, 2008) Bulutay, Ceyhun; Zakhleniuk, N. A.Show more Rapid development of InN technology demands comprehensive assessment of the electronic and optoelectronic potential of this material. In this theoretical work the effect of free electrons on the optical properties of the wurtzite phase of InN is investigated. The blue shift of the optical absorption edge by the free-carrier band filling is known as the Burstein-Moss effect for which InN offers to be a very suitable candidate as has been recently demonstrated experimentally. Due to well known Kramers-Kronig relations, a change in absorption is accompanied by a change in the index of refraction. Considering n-type InN samples with free electron concentrations ranging from 5x10 17 to 5x1020 cm-3, and employing a nonlocal empirical pseudopotential band structure, it is shown that this leads to a few percent change of the index of refraction. These carrier-induced refractive index changes can be utilized in optical switches, futhermore it needs to be taken into account in the design of InN-based optical devices such as lasers and optical modulators.Show more Item Open Access Comparative analysis of zinc-blende and wurtzite GaN for full-band polar optical phonon scattering and negative differential conductivity(American Institute of Physics, 2000) Bulutay, C.; Ridley, B. K.; Zakhleniuk, N. A.Show more For high-power electronics applications, GaN is a promising semiconductor. Under high electric fields, electrons can reach very high energies where polar optical phonon (POP) emission is the dominant scattering mechanism. So, we undertake a full-hand analysis of POP scattering of conduction-hand electrons based on an empirical pseudopotential band structure. To uncover the directional variations, we compute POP emission rates along high-symmetry directions for the zinc-blende (ZB) crystal phase of GaN. We also compare the results with those of the wurtzite phase. In general, the POP scattering rates in the zinc-blende phase are lower than the wurtzite phase. Our analysis also reveals appreciable directional dependence, with the Γ-L direction of ZB GaN being least vulnerable to POP scattering, characterized by a scattering time of 11 fs. For both crystal phases, we consider the negative differential conductivity possibilities driven by the negative effective mass part of the band structure. According to our estimation, for the ZB phase the onset of this effect requires fields above ∼ 1 MV/cm. © 2000 American Institute of Physics.Show more Item Open Access Electron momentum and energy relaxation rates in GaN and AlN in the high-field transport regime(The American Physical Society, 2003) Bulutay, C.; Ridley, B. K.; Zakhleniuk, N. A.Show more Momentum and energy relaxation characteristics of electrons in the conduction band of GaN and AlN are investigated using two different theoretical approaches corresponding to two high electric-field regimes, one up to 1-2 MV/ cm values for incoherent dynamics, and the other at even higher fields for coherent dynamics where semiballistic and ballistic processes become important. For the former, ensemble Monte Carlo technique is utilized to evaluate these rates as a function of electron energy up to an electric-field value of 1 MV/cm (2 MV/cm) for GaN (AlN). Momentum and energy relaxation rates within this incoherent transport regime in the presence of all standard scattering mechanisms are computed as well as the average drift velocity as a function of the applied field. Major scattering mechanisms are identified as polar optical phonon (POP) scattering and the optical deformation potential (ODP) scattering. Roughly, up to fields where the steady-state electron velocity attains its peak value, the POP mechanism dominates, whereas at higher fields ODP mechanism takes over. Next, aiming to characterize coherent dynamics, the total out-scattering rate from a quantum state (chosen along a high-symmetry direction) due to these two scattering mechanisms are then computed using a first-principles full-band approach. In the case of POP scattering, momentum relaxation rate differs from the total out-scattering rate from that state; close to the conduction-band minimum, momentum relaxation rate is significantly lower than the scattering rate because of forward-scattering character of the intravalley POP emission., However, close to the zone boundary the difference between these two rates diminishes due to isotropic nature of intervalley scatterings. Finally, a simple estimate for the velocity-field behavior in the coherent transport regime is attempted, displaying a negative differential mobility due to the negative band effective mass along the electric-field direction.Show more Item Open Access Full-band polar optical phonon scattering analysis and negative differential conductivity in wurtzite GaN(American Physical Society, 2000-12-15) Bulutay, C.; Ridley, B. K.; Zakhleniuk, N. A.Show more GaN has promising features for high-field electronics applications. To scrutinize these transport-related properties, primarily the dominant scattering mechanism in this material needs to be well characterized. In the quest for Bloch oscillations in bulk GaN, our aim is to conduct a full-band scattering analysis requiring very high energies where parabolic approximation is far from applicable. For this purpose, we first obtain an accurate band structure for the conduction band of wurtzite GaN based on the empirical pseudopotential method, using the most recent experimental data as the input. We compute the scattering rate, relevant up to room temperatures, due to longitudinal-optical-like and transverse-optical-like polar phonon modes along several ~high-symmetry! directions, from the conduction band minimum at the zone center to the half of the reciprocal lattice vector in each direction. We observe that the location and the symmetry of the neighboring valleys to the route play a decisive role on the scattering rates. The observation of Bloch oscillations in bulk wurtzite GaN is doomed by the very large value of the polar scattering rate. However, there exists the possibility of a negative differential conductivity driven by the negative effective mass part of the band structure for fields above 2.3 MV/cm for wurtzite GaN.Show more Item Open Access High-energy electron relaxation and full-band electron dynamics in aluminium nitride(Elsevier, 2002) Bulutay, Ceyhun; Ridley, B. K.; Zakhleniuk, N. A.Show more Material properties of AlN, particularly its wide band gap around 6 eV, warrant its operation in the high-field transport regimes reaching MV/cm fields. In this theoretical work, we examine the full-band scattering of conduction band electrons in AlN due to polar optical phonon (POP) emission, which is the main scattering channel at high fields. First, we obtain the band structure for the wurtzite phase of AlN using the empirical pseudopotential method. Scattering rates along the full length of several high-symmetry directions are computed efficiently through the Lehmann-Taut Brillouin zone integration technique. In order to shed light on the behaviour of the velocity-field characteristics at extremely high electric fields, in the order of a few MV/cm, we resort to an Esaki-Tsu estimation. Comparison of these results for AlN is made with our similar work on GaN. With typically more than 50% higher POP scattering rate compared to GaN, AlN has poorer high-field prospects. Availability of these data for AlN and GaN paves the way for practical assessment of the high-energy electron dynamics for the ternary alloy, AlGaN.Show more Item Open Access Polar optical phonon scattering and negative Kromer-Esaki-Tsu differential conductivity in bulk GaN(Elsevier, 2001-09) Bulutay, C.; Ridley, B. K.; Zakhleniuk, N. A.Show more GaN is being considered as a viable alternative semiconductor for high-power solid-state electronics. This creates a demand for the characterization of the main scattering channel at high electric fields. The dominant scattering mechanism for carriers reaching high energies under the influence of very high electric fields is the polar optical phonon (POP) emission. To highlight the directional variations, we compute POP emission rates along high-symmetry directions for the zinc-blende and wurtzite crystal phases of GaN. Our treatment relies on the empirical pseudopotential energies and wave functions. The scattering rates are efficiently computed using the Lehmann-Taut Brillouin zone integration technique. For both crystal phases, we also consider the negative differential conductivity possibilities associated with the negative effective mass part of the band structure. (C) 2001 Elsevier Science B.V. All rights reserved.Show more