Theoretical study of the optical absorption and refraction index change in a cylindrical quantum dot

Analytical expressions of the optical absorption coefficient and the change in refractive index associated with intraband relaxation in a cylindrical quantum dot are obtained by using the density matrix formalism. Energy levels in conduction band were calculated with finite confining potential in the framework of the effective-mass envelope-function theory. Numerical calculations on a typical GaAs/AlβGa1−βAs QD are performed. It is found that the absorption and refraction index change sensitively depend not only on the incident optical wave but also on the dot size and the Al mole fraction β in the AlβGa1−βAs material.     

Impurity optical absorption in parabolic quantum well

Optical absorption in GaAs parabolic quantum well in the presence of hydrogenic impurity is considered. The absorption coefficient associated with the transitions between the upper valence subband and donor ground state is calculated. The impurity ground state wave function and energy are obtained using the variational method. Dependence of the absorption spectra on impurity position in quantum well was investigated. It is shown, that along with quantum well width decrease the absorption threshold shifts to higher frequencies. Results obtained within frames of parabolic approximation are compared with results for rectangular infinite-barrier quantum well case. The acceptor state → conduction band transitions considered as well.     

Deep-center photoluminescence in undoped semi-insulating GaAs: 0.68 eV band due to the main deep donor

The 0.68 eV photoluminescence band present in undoped semi-insulating GaAs crystals has been studied with the change of temperature. It is shown that the 0.68 eV band is due to the radiative transition involving a main deep donor and the valence band. The origin of the donor is of an intrinsic origin and may involve an As antisite defect. It is found that the donor level does not change in energy with respect ot the valence band at T = 4–300 K. The donor level is found to be at 0.73 eV from the conduction band at T = 4 K.     

Donor binding energies in GaAs quantum wells considering the band nonparabolicity effects and the wavefunction elongation

The donor binding energies in finite GaAs/Ga_xAl_1 − xAs quantum wells have been calculated by considering the confinement of electrons, which increases as the well width increases. The variational solutions have been improved by using a two-parameter trial wavefunction, and by including the conduction band nonparabolicity. It is shown that the method used gives results in agreement with those obtained in the experiments on the effective mass and the donor binding energy, both of which are strongly dependent on the well width.     

Effect of laser intensity on the semiconductor–metal transition in a doped Quantum Well

We demonstrate laser induced semiconductor–metal transition through an abrupt change in diamagnetic susceptibility of a donor at critical concentration in a GaAs/Al_xGa_1−xAs Quantum Well for finite barrier model in the effective mass approximation using variational principle. We have considered Anderson‘s localization due to the random distribution of impurities in our calculation. The nonparabolicity of the conduction band is also considered. Our results without laser field agree with the earlier theoretical results and also with the recent experimental results.     

On the maximum in hall coefficient temperature dependence in medium-doped n-GaAs

Hall coefficient and conductivity are measured in a wide temperature interval from 14.5 to 295 K in order to characterize n-type GaAs bulk monocrystals with moderate donor concentration 10¹⁷–10¹⁸ cm^–3. The weak temperature dependence of the Hall coefficient, showing a slight maximum at 120 K, is analyzed for the first time in terms of the model developed by Klotynsh and Bariss of discrete local levels degenerate with the continuum. For that purpose the presence of a discrete donor level degenerate with the conduction band is supposed and its energy position is calculated using the model. The concentration and the degeneracy factor of this level are determined by fitting the theoretical temperature dependence of the free electron concentration to the experimental one, the former being calculated using the charge neutrality equation. In addition, a qualitative interpretation of the Hall mobility temperature dependence is given.     

Diffusion coefficient of hot electrons in GaAs

The longitudinal and transverse diffusion coefficient of hot electrons in GaAs at a lattice temperature of 300 K has been calculated by the Monte Carlo technique. The calculations showed that drift velocity and diffusion coefficient of hot electrons in GaAs can be fitted to available experimental data if the three-valley Γ-L-X model is used. The estimates of some parameter values of GaAs conduction band have been made.     

Electronic structure of GaAs1−xNx under pressure

The electronic band structures of GaAs_1−xN_x for x=0.009, 0.016, 0.031 and 0.062 are calculated ab initio using a supercell approach in connection with the full-potential linear muffin-tin orbital method. Corrections for the ‘LDA gap errors’ are made by adding external potentials which are adjusted to yield correct gaps in pure GaAs. Even small amounts of nitrogen modify significantly the conduction bands, which become strongly non-parabolic. The effective mass in the lowest conduction band thus exhibits strong k-vector dependence. Calculated variations of gaps and effective masses with x and externally applied pressure are presented and compared to a variety of experimental data. There are significant error bars on our results due to the use of the supercell approach. These are estimated by examining the effects of varying the geometrical arrangement of the N-atoms substituting As. However, the calculations show that the electron mass for x>0.009 is much larger than that of pure GaAs, and that it decreases with x.     

Diamagnetic susceptibility of hydrogenic donor impurity in low-dimensional semiconducting systems

The binding energies of a hydrogenic donor both in the parabolic and non-parabolic conduction band model within the effective mass approximation have been computed for the low-dimensional semiconducting systems (LDSS) like quantum well, quantum well wire and quantum dot taking GaAs/Al_xGa_1−xAs systems as an example. It is observed that the effect of non-parabolicity is not effective when the system goes to lower dimensionality. The diamagnetic susceptibility of a hydrogenic donor impurity has also been computed in these LDSS in the infinite barrier model. Since no theoretical or experimental works on the diamagnetic susceptibility of LDSS are available in the literature, as a realistic case the diamagnetic susceptibility has been computed in the finite barrier model (x=0.3) for a quantum well and the results are discussed in the light of semiconductor-metal transition.     

Effect of conduction band nonparabolicity on the optical properties in a single quantum well under hydrostatic pressure and electric field

The effect of conduction band nonparabolicity on the linear and nonlinear optical properties such as absorption coefficients, and changes in the refractive index are calculated in the Al_0.3Ga_0.7As/GaAs heterostructure-based symmetric rectangular quantum well under applied hydrostatic pressure and electric field. The electron envelope functions and energies are calculated in the effective mass equation including the conduction band nonparabolicity. The linear and nonlinear optical properties have been calculated in the density matrix formalism with two-level approximation. The conduction band nonparabolicity shifts the positions of the optical properties and decreases their strength compared to those without this correction. Both the optical properties are enhanced with the applied hydrostatic pressure. While the absorption coefficients are bleached under the combined effect of high pressure and electric field, the bleaching effect is reduced when nonparabolicity is included.     

Experimental determination of electron distribution functions in degenerate GaAs at high electric fields

The change of the optical absorption due to applied electric fields up to 450 V/cm was measured on n-type GaAs having a carrier concentration of 10¹⁸ cm^?3 at 77 K. The fundamental absorption in degenerate n-type semiconductors is proportional to the number of unoccupied places in the conduction band at an energy, which is determined by the phonon energy and the bandstructure. Hence, the measured change in absorption allows the evaluation of the distribution function. Results are given as a function of the energy above the conduction band edge, which was determined assuming parabolic bands.     

First-principles investigation of impurity concentration influence on bonding behavior,electronic structure and visible light absorption for Mn-doped BiOCl photocatalyst

We performed first-principles calculation to investigate the bonding behavior, electronic structure and visible light absorption of Mn_xBi_1−xOCl (x=0, 0.0625, 0.09375 and 0.125) using density functional theory (DFT) within a plane-wave ultrasoft pseudopotential scheme. The relaxed structural parameters are consistent with the experimental results. The bonding behavior, bond orders, Mulliken charges and bond populations as well as formation energies are obtained. The calculated band structures and density of states show that Mn incorporation results in some impurity energy levels of Mn 3d states in forbidden band as well as valence band and conduction band, and that Mn 3d states, for the modest Mn doping concentration, not only can act as the capture center of excited electrons under longer wavelength light irradiation, but also may trap the photo-excited holes, improving the transfer of photo-excited carriers to the reactive sites. Our calculated optical absorption spectra exhibit that the spectral absorption edge is obviously red-shifted and extends to the visible, red and infrared light region due to the incorporation of Mn. Our calculated absorption spectra are in excellent agreement with the experimental results of Mn-doped BiOCl photocatalyst.     

Electronic structure of QD arrays: application to intermediate-band solar cells

Intermediate band solar cells (IBSC) have been proposed as a potential design for the next generation of highly efficient photo-voltaic devices. Quantum nanostructures, such as quantum dots (QD), arranged in super-lattice (SL) arrays produce a mini-band (IB) that is separated by a region of zero density of states from other states in the conduction band. Additional absorption from the valence band to the IB and IB to the conduction band allows two photons with energies below the energy gap to be harvested in generating one electron-hole pair. We present a theoretical study of the electronic and optical properties of the IB formed by an InAs/GaAs QD array. The calculations are based on an 8-band k · p Hamiltonian, incorporating mixing between valence and conduction states, strain and piezoelectric field. Theoretical results of the the mini-band width variation with the period of the QD array in the z direction are presented. For one particular spacer distance, d _z = 4 nm, we report detailed variation of the optical dipole matrix elements through the mini-band and identify the character of the states involved. This approach captures the essential physics of the absorption processes in a realistic model of the IBSC structure and will be used to provide input parameters for predictive modelling of transport properties.     

Mutual passivation of donors and isovalent nitrogen in GaAs

We study the mutual passivation of shallow donor and isovalent N in GaAs. We find that all the donor impurities, SiGa, GeGa, SAs, and SeAs, bind to N in GaAs:N, which has a large N-induced band-gap reduction relative to GaAs. For a group-IV impurity such as Si, the formation of the nearest-neighbor SiGa-NAs defect complex creates a deep donor level below the conduction band minimum (CBM). The coupling between this defect level with the CBM pushes the CBM upwards, thus restoring the GaAs band gap; the lowering of the defect level relative to the isolated SiGa shallow donor level is responsible for the increased electrical resistivity. Therefore, Si and N mutually passivate each other's electrical and optical activities in GaAs. For a group-VI shallow donor such as S, the binding between SAs and NAsdoes not form a direct bond; therefore, no mutual passivation exists in the GaAs:(S+N) system.     

Optical properties of the alkaline-earth fluorohalides matlockite-type structure SrFX (X=Cl, Br, I) compounds

The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0). We find that the value of ε₁(0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data.     

Effect of Γ-X band crossover and impurity location on the diamagnetic susceptibility of a donor in a quantum well

We present the calculation of diamagnetic susceptibility (χ_dia) of a hydrogenic donor in GaAs/Al_xGa_1−xAs quantum well for various compositions of Al and for different impurity locations within the well. The effect of Γ-X band crossing due to hydrostatic pressure on χ_dia is also investigated taking into account the non-parabolicity of the conduction band.     

Simultaneous Effects of External Electric Field and Conduction Band Nonparabolicity on Optical Properties of a GaAs Quantum Dot Embedded at the Center of a GaAlAs Nano-Wire

An investigation of the optical properties of a GaAs spherical quantum dot which is located at the center of a Ga1-xAlx As cylindrical nano-wire has been performed in the presence of an external electric field. The band nonparabolieity effect is also considered using the energy dependent effective mass approximation. The energy eigenvalues and corresponding wave functions are calculated by finite difference approximation and the reliability of calculated wave functions is checked by computing orthogonality. Using computed energy eigenvalues and wave functions, the linear, third-order nonlinear and total optical absorption coefficients and refractive index changes are examined in detail. It is found that （i） Presence of electric field causes both blue and red shifts in absorption spectrum; （ii） The absorption coefficients shift toward lower energies by taking into account the conduction band nonparabolicity; （iii） For large values of electric field the effect of conduction band nonparabolieity is less dominant and parabolic band is estimated correctly; （iv） In the presence of electric field and conduction band nonparabolicity the nonlinear term of absorption coefficient rapidly increases by increasing incident optical intensity. In other words, the saturation in optical spectrum occurs at lower incident optical intensities.     

Binding energies of wannier excitons in GaAs-Ga1−xAlxAs quantum well structures

Binding energies of Wannier excitons in a quantum well structure consisting of a single slab of GaAs sandwiched between two semi-infinite slabs of Ga_1?xAl_xAs are calculated using a variational approach. Due to reduction in symmetry along the axis of growth of these quantum well structures and the presence of band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed leading to two exciton systems, namely, the heavy hole exciton and the light hole exciton. The variations of the binding energies of these two excitons as a function of the size of the GaAs quantum wells for various values of the heights of the potential barrier are calculated and their behavior is discussed.     

Experimental evidence of the hybridization of the electron states of an impurity and the conduction band in the HgSe:Fe system

X-ray absorption spectra from iron donor impurities in mercury selenide have been analyzed in a concentration range where the Fermi energy of conduction electrons is close to the energy of the donor d level. At high impurity concentrations, the resulting spectrum corresponds to the completely filled donor state and coincides with the spectrum of a bivalent iron ion. A transition to an intermediate-filling state is observed with decreasing the concentration. The spectra are quantitatively analyzed in a model implying the existence of a mixture of ions that contain and do not contain a donor electron in a bound state. It has been found that such a model is significantly inconsistent with the experimental data. It has been shown that the concentration dependence of the x-ray spectra corresponds to the manifestation of the significant hybridization of localized and delocalized donor electron states in the conduction band.