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Öğe Behavior of photoluminescence energy in polar ZnCdO/ZnO quantum wells under hydrostatic and uniaxial pressures: Linear and nonlinear piezoelectric contributions(Academic Press Ltd- Elsevier Science Ltd, 2020) Yildirim, HasanDependence of the photoluminescence (PL) energy (E-PL) in polar ZnCdO/ZnO quantum wells (QWs) on hydrostatic and uniaxial pressures are investigated as regards the Cd concentration of the structures and their physical dimension. The calculations are carried out within the framework of the effective mass and envelope function approximations. The built-in electric field is incorporated into the calculations using a piezoelectric polarization expression up to a nonlinear regime. It is found that the nonlinearity in piezoelectric polarization, under the effect of the hydrostatic pressure, leads to a large built-in electric field up to 5 MV/cm in consequence of the high lattice mismatch existing in the structures. The resulting quantum confined Stark effect causes a decreasing dE(PL)/dp from 23.6 meV/GPa down to even negative values, such as -6.46 meV/GP, for the QWs up to 50 angstrom in width. A linear relationship between E-PL at ambient pressure E-PLO and dE(PL)/dp is obtained in both cases of the hydrostatic and uniaxial pressure applications. It is seen that the relationship is universal in view of the Cd concentration.Öğe Binding energies and oscillator strengths of impurity states in wurtzite InGaN/GaN staggered quantum wells(Amer Inst Physics, 2012) Yildirim, Hasan; Aslan, BulentUsing the variational methods, we have calculated the binding energies of the lowest donor states, 1s and 2p(+/-), in wurtzite InGaN/GaN staggered quantum wells. The binding energies in narrow wells are larger in magnitude than the values in bulk GaN due to the quantum confinement effects. However, the energies decrease sharply in wider wells because of the weakening confinement due to the strong built-in electric field inside the well. The binding energies of donors placed at the opposite edges of the well are quite different as the built-in electric field forms an asymmetric, triangular potential inside the well. The oscillator strength of the possible transitions between the donor states is then computed by modelling them as the states of a two-level atom. A magnetic field applied along the growth direction splits up the degenerate 2p(+/-) states. The amount of splitting in the quantum well is found to be small possibly due to the heavy electron effective mass inside the well. The oscillator strength of the transition between the donor states becomes greater with the increasing magnetic field. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4751438]Öğe Binding energies of shallow donors in polar ZnO/ZnBeO quantum well(Pergamon-Elsevier Science Ltd, 2024) Cakir, Rasit; Yildirim, HasanThis study examines a single wurtzite ZnO/ZnBeO quantum well structure oriented in the polar c -direction. The focus is on investigating the binding energies associated with an impurity donor atom within this system. To achieve this, a self -consistent solution to the Schrodinger and Poisson equations is obtained using the finite difference method. The framework employed involves the effective mass and envelope function approximations. The impurity is represented using a hydrogenic-type wave function, and donor binding energies are determined via a variational approach. The research analyzes the binding energies of the 1s and 2p +/- states, along with the transition energy between them. These quantities are explored as functions of the well width and Be concentration, considering donor positions at the right and left interfaces as well as at the well's center. Furthermore, the impact of an external magnetic field oriented along the growth direction is assessed, spanning up to 10 T, in order to quantify changes in the binding energies. The presence of a built-in electric field induces an asymmetric band profile and a triangular well configuration. This asymmetry results in a loss of symmetry within the binding energy curves. Ultimately, the investigation culminates in the computation of the oscillator strength governing transitions between the donor states. When the donor is situated at the right interface, the energy values remain relatively constant as the well width increases, and the oscillator strength values demonstrate a consistent linear rise.Öğe Dispersion relations of interface and quasi-confined phonon modes in ZnO/BeZnO quantum wells(Elsevier, 2021) Yildirim, HasanThe long-wave optical phonon frequencies of BexZn1-xO alloys are determined within the modified random-element isodisplacement model. The results indicate a two-mode behavior for A(1) and E-1 branches of the optical phonons. The dispersions of the interface and quasi-confined modes in the polar ZnO/BexZn1-xO quantum wells are calculated within the dielectric continuum model up to x = 0.2. In the absence of strain, the quantum wells yield as many as three distinct frequency ranges for the IF modes. However, the quasi-confined modes have two separate frequency ranges with boundaries determined by the ZnO-like and ZnO phonons. With the inclusion of strain, the boundaries of the frequency ranges for each mode are modified but without any change in the overall picture. (C) 2020 Elsevier B.V. All rights reserved.Öğe Donor binding energies in a GaN/ZnGeN2 quantum well(Academic Press Ltd- Elsevier Science Ltd, 2017) Yildirim, HasanBinding energies of a donor atom within a GaN/ZnGeN2 quantum well structure have been investigated. Hydrogenic type wave functions are assumed and the Schrodinger and Poisson's equations are solved self-consistently. The binding energies of the donor states is and 2p(+/-) associated with the first subband in the quantum well and the transition energies between them are represented as a function of the quantum well width, the donor position and the external magnetic field. The ground state is and the excited states 2p(+/-) have maximum binding energies as much as 64 and 11 meV, occurring in quantum wells of widths 10 and 18 angstrom, respectively. The binding energies are found to be an asymmetric function of the donor position with respect to the quantum well center because of the asymmetry in the band profile introduced by the built-in electric field in the structure. An external magnetic field up to 10 T is included into the calculations and it is seen that the excited states show a small Zeeman splitting, very close to the bulk GaN value, because of the heavy effective mass of electron. (C) 2017 Elsevier Ltd. All rights reserved.Öğe Donor-related third-order optical nonlinearites in GaAs/AlGaAs quantum wells at the THz region(Iop Publishing Ltd, 2011) Yildirim, Hasan; Aslan, BulentGaAs/AlGaAs quantum wells doped with donor atoms are investigated for nonlinear optical applications in the THz range. The electronic properties of the quantum wells are obtained numerically by applying an iterative shooting algorithm. Donor binding energies are computed through the evaluation of variational wavefunctions. The solution of the density matrix equations of motion for non-interacting two-level atoms within the rotating wave approximation is used to formulate the third-order optical nonlinearities. Transitions between the 1s and 2p+ impurity states because of an incident light polarized perpendicularly to the growth direction are considered as the origins of optical nonlinearity. Following a set of computations for a quantum well doped at the center, it is found that the nonlinear susceptibility decreases when the well becomes wider or the Al concentration increases. Additionally, when the doping center is shifted to the well edge, the nonlinear susceptibility decreases too. A large nonlinear figure of merit is obtainable in wider wells compared to the narrower wells although the latter delivers larger nonlinear susceptibilities.Öğe Effects of built-in electric field on donor binding energy in InGaN/ZnSnN2 quantum well structures(Elsevier Science Bv, 2019) Yildirim, HasanInxGa1-xN/ZnSnN2 quantum well structures are studied in terms of a binding energy of a donor atom. 1s and 2p +/- impurity states are considered. The Schrodinger's and Poisson's equations are solved self-consistently. A hydrogenic type wave function to represent each impurity state is assumed. The calculations include band-bending in the potential energy profile introduced by the built-in electric field existing along the structures. The binding energy and the energy of the transition between the impurity states are represented as a function of the quantum well width, the donor position, and the indium concentration. An external magnetic field up to 10 T is included into the calculations to compute the Zeeman splitting. The maximum value of the transition energy is around 30 meV (nearly 7.3 THz) which occurs in a 15-angstrom In0.3Ga0.7N/ZnSnN2 quantum well. Being strong, the built-in electric field makes the transition energy drop quickly with the decreasing well width. For the same reason, the energy curves are found to be highly asymmetric function of the donor position around the well center. Compared to the bulk value, the transition energy in the quantum well structures enhances nearly two-fold. (C) 2019 Elsevier B.V. All rights reserved.Öğe Effects of magnetic field on the terahertz nonlinear optical properties in donor-doped GaAs/AlGaAs quantum wells(Wiley-V C H Verlag Gmbh, 2012) Yildirim, Hasan; Aslan, BulentEffects of the magnetic field on nonlinear optical properties at THz range in GaAs/AlGaAs quantum wells doped with donor atoms are investigated. Expressions for the third-order nonlinear optical susceptibilities are obtained through the solution of the density matrix equations of motion within the rotating wave approximation. Donor binding energies are calculated variationally by means of an iterative shooting algorithm. Magnetic field has strong effect on the nonlinear susceptibility: it removes the degeneracy in energies of 2p +/- impurity states and increases the absolute value of the nonlinearity. It is also shown that a large and tunable optical nonlinear figure of merit is possible with the magnetic field applied in the growth direction. The nonlinear optical quantities are also calculated for donor distributions with different full width at half maximum values in the absence of magnetic field and the observed features at low energy part are attributed to the increasing homogeneity in the donor distribution.Öğe Electronic structure and surface properties of cubic perovskite oxide BaMnO3(Elsevier, 2011) Gokoglu, Gokhan; Yildirim, HasanWe present the electronic, magnetic, and structural properties of the cubic perovskite oxide BaMnO3 in both bulk and surface geometry. BaMnO3 is reported as keeping the cubic phase even at low temperatures. The calculations are based on the density functional theory (DFT) within plane-wave pseudopotential method and spin-polarized Generalized Gradient Approximation (GGA) of the exchange-correlation functional. The systems studied are treated in ferromagnetic order. The structures of electronic bands and density of states of the systems show half-metallic behavior in both bulk and BaO- and MnO2-terminated (001) surfaces of E2(1) structure. The calculated magnetic moment of bulk structure is 3.00 mu(B), which is largely conserved at surface geometries. Average surface and relaxation energies are also calculated. The rumpling of atoms in relaxed surfaces is determined. It is seen that the relaxation of oxygen relative to metal ion is always in outward direction for both terminations indicating a positive rumpling. (C) 2010 Elsevier B.V. All rights reserved.Öğe Electronic structure of antiferromagnetic PbCrO3 (001) surfaces(Elsevier Science Sa, 2011) Yildirim, Hasan; Agduk, Savas; Gokoglu, GokhanSurfaces of cubic perovksite PbCrO3 in (0 0 1) plane are investigated through density functional theory. The plane wave pseudopotential method is applied with generalized gradient approximation scheme. Hubbard U correction (GGA + U) is included in all calculations in order to simulate on-site Coulomb interactions between Cr-d states. Two types of terminations, namely, PbO-and CrO2-terminations are considered in construction of the surfaces. Surfaces of both terminations show convergence at 9-layer slab geometry. The density of states calculations on the converged slab geometry yield a metallic behavior for both PbO-and CrO2-terminations. Both metal atoms, Pb and Cr, in the uppermost layer of the respective terminations, have inward atomic relaxations much larger in magnitude than the oxygen atoms of the respective layer. However, Cr atoms which are labeled as up and down according to their spin orientation show different relaxations. The interlayer distance between the uppermost layer and the first one next to it decreases in both PbO- and CrO2-terminated surface geometries. The calculations of the relative movement of the oxygen atom with respect to the Pb or Cr atom in each terminations give a positive rumpling in the uppermost layer. (C) 2011 Elsevier B.V. All rights reserved.Öğe Exciton binding and excitonic transition energies in wurtzite Zn1-xCdxO/ZnO quantum wells(Academic Press Ltd- Elsevier Science Ltd, 2018) Yildirim, HasanBinding energy of the 1s exciton state in the Zn1-xCdxO/ZnO quantum wells, changing between 10 and 50 angstrom in width, has been calculated for x up to 0.2. The exciton wave function is represented in terms of hydrogenic wave functions and the effects of dielectric mismatch, exciton-phonon interactions and built-in electric field on the binding energy are considered. The calculated built-in electric field is around 19.6x MV cm(-1) in a 20-angstrom Zn1-xCdxO/ZnO quantum well. The numerical results indicate that the exciton binding energy stays below its bulk value virtually for all x because of the built-in electric field dominating the quantum confinement effect. The binding energy is nearly 26 meV at maximum, barely above its bulk value, and about 13 meV at minimum. The contribution of the dielectric mismatch is up to 40% while that of the exciton-phonon interactions is negligible. The exciton binding energy increases up to 66 meV, well above the bulk value, in case the built-in electric field is neglected. A qualitative agreement between the calculated excitonic transition energies and the low temperature and low excitation intensity photoluminescence peak data is obtained although some ZnCdO material properties, such as low temperature band gap, are not available.Öğe Exciton binding energies in CdSe/MgSe quantum well structures(Academic Press Ltd- Elsevier Science Ltd, 2018) Yildirim, HasanBinding energies of heavy- and light-hole excitons in CdSe/MgSe quantum wells have been determined in terms of quantum well width. Hydrogenic wave functions are assumed to represent exciton wave functions. Contribution of the polarization charges originating from the dielectric mismatch between the well and the barrier materials to the Coulomb interaction is calculated by means of image charge method. The effect of the exciton-phonon interaction on the exciton binding energies are included by means of an effective potential obtained through an exciton-bulk-optical-phonon Hamiltonian. The results indicate that both the modified Coulomb interaction due to the dielectric mismatch and the exciton-phonon interaction considerably change the exciton binding energy. The heavy-hole exciton binding energy in a 10-angstrom quantum well is seen to be enhanced as much as 87% compared to the value found when only the Coulomb interaction without dielectric mismatch is taken into account. The resulting binding energies of heavy-hole excitons range approximately between 57 and 41 meV for the quantum well width changing between 10 and 35 angstrom. Compared to the bulk CdSe exciton binding energy 15 meV, the exciton binding energies in CdSe/MgSe quantum wells are increased almost four times. (C) 2017 Elsevier Ltd. All rights reserved.Öğe Excitons in nonpolar ZnO/BeZnO quantum wells: Their binding energy and its dependence on the dimensions of the structures(Elsevier, 2022) Yildirim, HasanBinding energy of the 1s exciton in the nonpolar ZnO/BexZn1-xO quantum well for x up to 0.3 has been determined by means of fractional dimension approach. Electron and hole wave functions have been computed by solving Schrodinger equation for each particle. The results have been represented as a function of the dimension of the structure. The effects of the optical phonon interactions and the image charges on the binding energy have been considered also. Their contribution leads to a fractional change as much as 0.53 and the binding energy increases up to nearly 140 meV within the considered ranges of the parameters. The effect of the changes in the barrier width has been found negligible, except for the case of very thin quantum wells.Öğe Intersubband transitions in InxGa1-xN/InyGa1-yN/GaN staggered quantum wells(Amer Inst Physics, 2014) Yildirim, Hasan; Aslan, BulentIntersubband transition energies and absorption lineshape in staggered InGaN/GaN quantum wells surrounded by GaN barriers are computed as functions of structural parameters such as well width, In concentrations, and the doping level in the well. Schrodinger and Poisson equations are solved self-consistently by taking the free and bound surface charge concentrations into account. Many-body effects, namely, depolarization and excitonic shifts are also included in the calculations. Results for transition energies, oscillator strength, and the absorption lineshape up to nonlinear regime are represented as functions of the parameters mentioned. The well width (total and constituent layers separately) and In concentration dependence of the built-in electric field are exploited to tune the intersubband transition energies. (C) 2014 AIP Publishing LLC.Öğe Many-body effects on intersubband transitions in polar ZnO/ZnMgO multiple quantum wells(Elsevier, 2019) Yildirim, HasanZnO/ZnMgO multiple quantum wells are investigated in terms of intersubband transitions within the conduction band. The Schrodinger's and Poisson's equations are solved self-consistently. The intersubband transition energy is represented as a function of their physical dimension and Mg concentration. The effects of the depolarization and excitonic shifts on the transition energy are considered. A high level of doping, such as 1 x 10(25) m(-3), is required to screen the built-in electric field, which is around 0.355 MV/cm at most. In case of screening, it is possible to have quantum wells aligned in energy and a homogeneous electron distribution over the quantum wells. The transition energy is between 51 and 403 meV if the width of the constituting quantum wells is between 20 and 100 angstrom. The contribution of the many-body effects is 29 meV at most. However, this contribution strongly depends on the well width: it can be as low as -1 meV.Öğe Non-polar ZnCdO/ZnO step-barrier quantum wells designed for THz emission(Elsevier, 2021) Yildirim, HasanSymmetric and asymmetric quantum wells of ZnCdO structures grown on ZnO in non-polar orientations are investigated for their potential in THz emission due to the intersubband transitions between the subbands in the conduction band. The Schro?dinger and Poisson equations are solved self-consistently for the square and step barrier quantum wells of the Zn1-xCdxO layers for x up to 0.25. The results are represented as functions of the Cd concentration in the well and step-barrier layers and their dimensions. The role of the many-body effects on the intersubband transitions is discussed. It is found that emission wavelengths between 4.5 and 8.55 THz in the step-barrier quantum wells are possible in the case of an optical pumping at a wavelength in the range of 8.27-15.7 mu m. The depolarization and excitonic shifts are found to contribute to the bare intersubband transition energies as much as their 15% at maximum.Öğe Nonlinear optical absorption in wurtzite ZnCdO quantum wells(Iop Publishing Ltd, 2019) Yildirim, HasanNonlinear optical properties of a Zn1-xCdxO/ZnO quantum well structure originating from the transitions between the subbands within its conduction band are studied in terms of its dimension, Cd concentration and doping level. The intersubband transition energies are calculated within the effective mass and envelope function approximations. The Schrodinger and Poisson equations are self-consistently solved by considering the free and the bound charge distributions. The many-body effects, such as depolarization and excitonic shifts, are taken into account. The results are represented as functions the Cd concentration, limited between 0.05 and 0.2, and the well width, changing from 10 to 50 angstrom. The calculations indicate an intersubband transition energy range from 40 (9.67) to 138 meV (33.4 THz) and a blue-shift in it, as a result of the many-body effects, up to 11 meV in case of a doping level of 1 x 10(25) m(-3). It is shown that the transition energy increases with the widening quantum well within the range of the parameters as a result of the strong built-in electric field. The nonlinear absorption lineshape is found be to asymmetric and has the peak positions depending on the light intensity and the doping level inside the quantum well.Öğe On the donor states in double InxGa1-xN/InyGa1-yN/GaN staggered quantum wells(Iop Publishing Ltd, 2013) Yildirim, Hasan; Aslan, BulentWe have calculated the binding energies of the donor states, 1s and 2p +/-, with respect to the lowest sub-band energy in a double quantum well composed of wurtzite InGaN staggered quantum wells with GaN barriers. All the energies and the wavefunctions were calculated by applying the variational methods. We have found that the binding energies of donors placed in the right quantum well are larger and independent of the middle barrier width of up to 40 angstrom. This is because of the strong built-in electric field which brings more confinement to the donor wavefunctions in the right staggered quantum well. The binding energies are found to be strong functions of the donor position in the double quantum well system which is the consequence of the large asymmetry introduced by the built-in electric field.Öğe Second harmonic generation in asymmetric MgSe/CdSe/ZnCdMgSe quantum well structures(Wiley-V C H Verlag Gmbh, 2017) Yildirim, Hasan; Aslan, BulentMgSe/CdSe/ZnCdMgSe step quantum well structures and coupled quantum well structures under an applied electric field have been investigated for the process of second harmonic generation (SHG) with the double-resonant condition. The structural parameters of the quantum wells having equally spaced three consecutive energy levels within the conduction band have been determined by solving Schrodinger equation. It is shown that the energy level separation can be continuously tuned between 214 and 472meV by changing the thickness and the material composition of the layers. The product of dipole matrix elements used in determining the second-order nonlinear susceptibility is calculated to be as much as 473 angstrom 3. This is lower than the values obtained in widely known GaAs based material systems because of the high effective mass of electron in these selenides. However, the second-order nonlinear susceptibility 2 that controls SHG is found to be of the order of 10-7 V-1 at its maximum; this is comparable to the values found in similar systems.Öğe Strain in BInGaN thin layers grown in nonpolar and semipolar directions(Springer Heidelberg, 2022) Yildirim, HasanStrain properties of BInGaN layers assumed to be grown on GaN, AlN and ZnO substrates in nonpolar and semipolar directions have been calculated. The strain components in the laboratory system have been presented as a function of boron and indium contents of the layer for each substrate. We have found that the in-plane strain components go up to approximate to 7, 10, and 6% in magnitude in the cases of GaN, AlN, and ZnO substrates, respectively. The piezeolectric properties of the BInGaN layers assumed to be grown in semipolar direction have been computed, and the outcomes for selected values of the boron content have been plotted against the indium content. Finally, the built-in electric field values inside the well layers of BInGaN/GaN and BInGaN/AlN quantum wells, considered to be grown in the semipolar direction, have been figured out. According to the results, the field in both types of the quantum wells reaches the values as high as approximate to 10 MVcm(-1) in magnitude.
