Browsing by Author "Bulutay, C."

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Open Access
Analysis of strain fields in silicon nanocrystals
(American Institute of Physics, 2009) Yilmaz, D. E.; Bulutay, C.; Çaǧın, T.
Strain has a crucial effect on the optical and electronic properties of nanostructures. We calculate the atomistic strain distribution in silicon nanocrystals up to a diameter of 3.2 nm embedded in an amorphous silicon dioxide matrix. A seemingly conflicting picture arises when the strain field is expressed in terms of bond lengths versus volumetric strain. The strain profile in either case shows uniform behavior in the core, however, it becomes nonuniform within 2-3 Å distance to the nanocrystal surface: tensile for bond lengths whereas compressive for volumetric strain. We reconcile their coexistence by an atomistic strain analysis.
Open Access
Auger recombination and carrier multiplication in embedded silicon and germanium nanocrystals
(The American Physical Society, 2008) Sevik, C.; Bulutay, C.
For Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices, the Auger recombination and carrier multiplication (CM) lifetimes are computed exactly in a three-dimensional real space grid using empirical pseudopotential wave functions. Our results in support of recent experimental data offer other predictions. We extract simple Auger constants valid for NCs. We show that both Si and Ge NCs can benefit from photovoltaic efficiency improvement via CM due to the fact that under an optical excitation exceeding twice the band-gap energy, the electrons gain lion's share from the total excess energy and can cause a CM. We predict that CM becomes especially efficient for hot electrons with an excess energy of about 1 eV above the CM threshold.
Open Access
Bound-state third-order optical nonlinearities of germanium nanocrystals embedded in a silica host matrix
(The American Physical Society, 2008) Yıldırım, H.; Bulutay, C.
Embedded germanium nanocrystals (NCs) in a silica host matrix are theoretically analyzed to identify their third-order bound-state nonlinearities. A rigorous atomistic pseudopotential approach is used for determining the electronic structure and the nonlinear optical susceptibilities. This study characterizing the two-photon absorption, nonlinear refractive index, and optical switching parameters reveals the full wavelength dependence from static up to the ultraviolet spectrum, and the size dependence up to a diameter of 3.5 nm. Similar to Si NCs, the intensity-dependent refractive index increases with decreasing NC diameter. On the other hand, Ge NCs possess about an order of magnitude smaller nonlinear susceptibility compared to Si NCs of the same size. It is observed that the two-photon absorption threshold extends beyond the half band-gap value. This enables nonlinear refractive index tunability over a much wider wavelength range free from two-photon absorption.
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.
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.
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.
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.
Open Access
Computational modeling of quantum-confined impact ionization in Si nanocrystals embedded in SiO2
(2007) Sevik, C.; Bulutay, C.
Injected carriers from the contacts to delocalized bulk states of the oxide matrix via Fowler-Nordheim tunneling can give rise to quantum-confined impact ionization (QCII) of the nanocrystal (NC) valence electrons. This process is responsible for the creation of confined excitons in NCs, which is a key luminescence mechanism. For a realistic modeling of QCII in Si NCs, a number of tools are combined: ensemble Monte Carlo (EMC) charge transport, ab initio modeling for oxide matrix, pseudopotential NC electronic states together with the closed-form analytical expression for the Coulomb matrix element of the QCII. To characterize the transport properties of the embedding amorphous SiO2, ab initio band structure and density of states of the α-quartz phase of SiO2 are employed. The confined states of the Si NC are obtained by solving the atomistic pseudopotential Hamiltonian. With these ingredients, realistic modeling of the QCII process involving a SiO2 bulk state hot carrier and the NC valence electrons is provided.
Open Access
Dc-switchable and single-nanocrystal-addressable coherent population transfer
(2010) Gunceler, D.; Bulutay, C.
Achieving coherent population transfer in the solid-state is challenging compared to atomic systems due to closely spaced electronic states and fast decoherence. Here, within an atomistic pseudopotential theory, we offer recipes for the stimulated Raman adiabatic passage in embedded silicon and germanium nanocrystals. The transfer efficiency spectra display characteristic Fano resonances. By exploiting the Stark effect, we predict that transfer can be switched off with a dc voltage. As the population transfer is highly sensitive to structural variations, with a choice of a sufficiently small two-photon detuning bandwidth, it can be harnessed for addressing individual nanocrystals within an ensemble. © 2010 American Institute of Physics.
Open Access
Disorder-free localization around the conduction band edge of crossing and kinked silicon nanowires
(A I P Publishing LLC, 2015) Keleş, Ü.; Çakan, A.; Bulutay, C.
We explore ballistic regime quantum transport characteristics of oxide-embedded crossing and kinked silicon nanowires (NWs) within a large-scale empirical pseudopotential electronic structure framework, coupled to the Kubo-Greenwood transport analysis. A real-space wave function study is undertaken and the outcomes are interpreted together with the findings of ballistic transport calculations. This reveals that ballistic transport edge lies tens to hundreds of millielectron volts above the lowest unoccupied molecular orbital, with a substantial number of localized states appearing in between, as well as above the former. We show that these localized states are not due to the oxide interface, but rather core silicon-derived. They manifest the wave nature of electrons brought to foreground by the reflections originating from NW junctions and bends. Hence, we show that the crossings and kinks of even ultraclean Si NWs possess a conduction band tail without a recourse to atomistic disorder.
Open Access
Dynamic Nuclear Spin Polarization in Resonant Laser Spectroscopy of a quantum dot
(American Physical Society, 2012-05-09) Hogele, A.; Kroner, M.; Latta, C.; Claassen, M.; Carusotto, I.; Bulutay, C.; Imamoglu, A.
Resonant optical excitation of lowest-energy excitonic transitions in self-assembled quantum dots leads to nuclear spin polarization that is qualitatively different from the well-known optical orientation phenomena. By carrying out a comprehensive set of experiments, we demonstrate that nuclear spin polarization manifests itself in quantum dots subjected to finite external magnetic field as locking of the higher energy Zeeman transition to the driving laser field, as well as the avoidance of the resonance condition for the lower energy Zeeman branch. We interpret our findings on the basis of dynamic nuclear spin polarization originating from noncollinear hyperfine interaction and find excellent agreement between experiment and theory. Our results provide evidence for the significance of noncollinear hyperfine processes not only for nuclear spin diffusion and decay, but also for buildup dynamics of nuclear spin polarization in a coupled electron-nuclear spin system.
Open Access
Dynamical correlations in quasi-one-dimensional electron gas
(E D P Sciences, 1998) Bulutay, C.; Tanatar, Bilal
We study the short-range correlations in a quasi-one-dimensional electron gas within the dynamical version of the self-consistent field theory. The static structure factor exhibits a peak structure at low densities. The zero-frequency limit of the dynamic local-field factor has structure not encountered in the previous applications of the present method. The large oscillations at low densities observed in the pair-correlation function indicates a transition to a partially ordered state.
Open Access
Efficiency and harmonic enhancement trends in GaN-based Gunn diodes: Ensemble Monte Carlo analysis
(American Institute of Physics, 2004) Sevik, C.; Bulutay, C.
Gallium nitride can offer a high-power alternative for millimeter-wave Gunn oscillators. Hence, an ensemble Monte Carlo-based comprehensive theoretical assessment of efficiency and harmonic enhancement in n-type GaN Gunn diodes is undertaken. First, the effects of doping notch/mesa and its position within the active channel are investigated which favors a doping notch positioned next to cathode. It is then observed that the width of the notch can be optimized to enhance the higher-harmonic operation without degrading its performance at the fundamental mode. Next, the effects of dc bias and channel doping density are investigated. Both of these have more significant effects on the higher-harmonic efficiency than the fundamental one. The lattice temperature is observed to have almost no influence up to room temperature but severely degrades the performance above room temperature. As a general behavior, the variations of temperature, channel doping, and the notch width primarily affect the phase angle between the current and voltage wave forms rather than the amplitude of oscillations. Finally, the physical origin of these Gunn oscillations is sought which clearly indicates that the intervalley scattering mechanism is responsible rather than the Γ valley nonparabolicity or the effective mass discrepancy between the Γ and the lowest satellite valleys.
Open Access
Electron initiated impact ionization in AlGaN alloys
(Institute of Physics, 2002) Bulutay, C.
Detailed impact ionization (II) analysis of electrons is presented for AlGaN alloys as a vital resource for solar-blind avalanche photodiode and high power transistor applications. Necessary ingredients for the II characterization are supplied from a recent experiment on the GaN end, and a Keldysh analysis for the AlN end, of the alloy AlGaN. High-field electron dynamics are simulated using an ensemble Monte Carlo framework, accounting for all valleys in the lowest two conduction bands, obtained from accurate empirical pseudopotential band structure computations. The effect of alloy scattering on II is considered and observed to be significant. For any AlxGa1-xN alloy, the electron II coefficients are found to obey the form, A exp(-K/F), for the electric field, F.
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.
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.
Open Access
Electronic and Optical Properties of Silicon Nanocrystals
(Wiley-VCH, 2010) Bulutay, C.; Ossicini S.
[No abstract available]
Open Access
Electronic structure and optical properties of silicon nanocrystals along their aggregation stages
(Elsevier B.V., 2007) Bulutay, C.
The structural control of silicon nanocrystals is an important technological problem. Typically, a distribution of nanocrystal sizes and shapes emerges under the uncontrolled aggregation of smaller clusters. The aim of this computational study is to investigate the evolution of the nanocrystal electronic states and their optical properties throughout their aggregation stages. To realistically tackle such systems, an atomistic electronic structure tool is required that can accommodate about tens of thousand nanocrystal and embedding lattice atoms with very irregular shapes. For this purpose, a computationally efficient pseudopotential-based electronic structure tool is developed that can handle realistic nanostructures based on the expansion of the wavefunction of the aggregate in terms of bulk Bloch bands of the constituent semiconductors. With this tool, the evolution of the electronic states as well as the polarization-dependent absorption spectra correlated with the oscillator strengths over their aggregation stages is traced. The low-lying aggregate nanocrystal states develop binding and anti-binding counterparts of the isolated states. Such information may become instrumental with the maturity of the controlled aggregation of these nanocrystals.
Open Access
Enhancement of optical switching parameter and third-order optical nonlinearities in embedded Si nanocrystals: A theoretical assessment
(Elsevier, 2008) Yildirim, H.; Bulutay, C.
Third-order bound-charge electronic nonlinearities of Si nanocrystals (NCs) embedded in a wide band-gap matrix representing silica are theoretically studied using an atomistic pseudopotential approach. Nonlinear refractive index, two-photon absorption and optical switching parameter are examined from small clusters to NCs up to a size of 3 nm. Compared to bulk values, Si NCs show higher third-order optical nonlinearities and much wider two-photon absorption-free energy gap which gives rise to enhancement in the optical switching parameter.
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.
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.
Open Access
Gain and temporal response of AlGaN solar-blind avalanche photodiodes: An ensemble Monte Carlo analysis
(A I P Publishing LLC, 2003) Sevik, C.; Bulutay, C.
A study was performed on temporal and gain response of AlGaN solar-blind avalanche photodiodes (APD). The ensemble Monte Carlo method was used for the purpose. It was found that without any fitting parameters, reasonable agreement was obtained with the published measurements for a GaN APD.
Open Access
Ground-state properties of quasi-one-dimensional electron systems within dynamic local-field correction: Quantum Singwi-Tosi-Land-Sjo lander theory
(American Physical Society, 1999) Tanatar, Bilal; Bulutay, C.
Dynamic local-field correction (LFC) brings a richer picture about the description of a many-body system than the standard mean-field theories. Here we investigate the ground-state properties of a quasi-one-dimensional electronic system using the quantum version of the Singwi-Tosi-Land-Sjölander (STLS) theory and present a critical account of its performance. The results are markedly different than those theories based on static LFC and the random-phase approximation; an example is the static structure factor, which develops a significant peak at low densities, signaling a developing ordered phase. An indication of growing instability at low densities is seen on G(q,0), the static behavior of the dynamic LFC, which has an oscillatory character with a magnitude exceeding unity, peaking exactly at 4k F. The pair-correlation function comes out as positive for the densities considered in this work. The correlation energy and the compressibility curves are seen to be quite close to the static STLS results. A flaw of the theory is the significantly negative values of the dynamic structure factor around the plasmon frequencies, also the lifetime of the plasmons turns out to be negative away from the single-pair continuum. In summary, the major shortcomings of the dynamic STLS scheme are the violation of the compressibility sum rule (as in the static STLS case) and the misrepresentation of the plasmons in the dynamic structure factor. © 1999 The American Physical Society.
Open Access
High dielectric constant and wide band gap inverse silver oxide phases of the ordered ternary alloys of SiO2, GeO2, and SnO2
(The American Physical Society, 2006) Sevik, C.; Bulutay, C.
High dielectric constant and wide band gap oxides have important technological applications. The crystalline oxide polymorphs having lattice constant compatibility to silicon are particularly desirable. One recently reported candidate is the inverse silver oxide phase of SiO2. A first-principles study of this system together with its isovalent equivalents GeO2 and SnO2 as well as their ternary alloys is performed. Within the framework of density functional theory both the generalized gradient approximation and local density approximation (LDA) are employed to obtain their structural properties, elastic constants, and electronic band structures. To check the stability of these materials, phonon dispersion curves are computed which indicate that GeO2 and SnO2 have negative phonon branches whereas their ternary alloys Si0.5 Ge0.5 O2, Si0.5 Sn0.5 O2, and Ge0.5 Sn0.5 O2 are all stable within the LDA possessing dielectric constants ranging between 10 and 20. Furthermore, the lattice constant of Si0.5 Ge0.5 O2 is virtually identical to the Si(100) surface. The GW band gaps of the stable materials are computed which restore the wide band gap values in addition to their high dielectric constants.