Interband Light Research Articles

The study of the structure of the electronic states of the FeGa3 and RuGa3 compounds by optical spectroscopy method

Investigations of the optical properties of intermetallic compounds FeGa3 and RuGa3 in the wavelength range of 0.22−14 μm have been conducted. The spectra of the interband light absorption are interpreted based on comparative analysis of calculated and experimental dispersion dependences of optical conductivity. Experimental data confirm the existence of the energy gaps at the Fermi level with a width of ~0.6 eV in the density of states of these materials, which was previously predicted in the band calculations.

Exciton states and direct interband light absorption in the ensemble of toroidal quantum dots

One electron and exciton states in toroidal quantum dot (QD) have been considered. The convenient coordinate system has been defined and the Schrodinger equation has been solved in these coordinates. The electron energy spectrum and wave function dependence on the geometrical parameters of toroidal QD have been obtained. The comparison with the results obtained by numerical methods has been done. Optical absorption has been considered in both cases of “small” toroid when the interaction of an electron and a hole is neglected and cases of relatively “large” toroid when the correlation between the particles has been taken into account. Interband optical transitions have been considered in the ensemble of toroidal QDs. The selection rules for quantum transitions and absorption edge dependence on the geometrical parameters have been obtained.

Spectral properties of RuAl2 and RuGa2 compounds: Ellipsometric analysis

The optical properties of RuAl2 and RuGa2 intermetallic compounds have been investigated in the spectral range of 0.22–14 μm. The nature of interband light absorption has been interpreted based on a comparative analysis of calculated and experimental frequency dependences of optical conductivity. The data obtained confirm the existence of pseudogaps with a width of ~0.8 eV localized at the Fermi level in the electron densities of states of these materials, which was predicted in previous energy-band calculations.

Electronic structure of RSn1.1Ge0.9 (R = Dy, Ho) ternary compounds: Band calculation and optical properties

The results of investigations of the electronic structure and optical properties of the compounds DySn1.1Ge0.9 and HoSn1.1Ge0.9 are presented. Our spin-polarized calculations of the electronic structure are carried out in the local spin density approximation with correction for strong electronic correlations (LSDA+U method) in the 4f shell of the rare-earth ion. In the wavelength range 0.22–15µm, the optical constants of the intermetallic compounds were measured by the ellipsometric method, and a number of spectral and electronic characteristics are determined. Based on the calculated densities of states, the structural features of the optical conductivity in the region of interband light absorption are interpreted.

Coefficient of Interband Light Absorption by InAs/GaхIn1-хAs Quantum Dot Superlattice at Low Temperatures

In the paper the InAs/GaxIn1-xAs superlattice system of small size cubic QDs (10 nm) has been considered. Dispersion relations for electron and hole subbands have been calculated for superlattices of different dimensionality. The dependences of the interband absorption coefficient on light frequency, quantum dot size and interdot distance have been researched.It is shown, that the dimension of the superlattice has influence on the shape of the absorption bands and the increasingof the distance between quantum dots is followed by narrowing of the absorption peaks for all three superlatticetypes.

Electronic structure and optical spectroscopy of the GdRhGe compound

The optical properties of the GdRhGe compound have been investigated in a wide spectral range by ellipsometry. Self-consistent calculations of the electronic structure have been performed within the local electron spin density approximation with a correction to strong electron interactions in the 4f shell of gadolinium ions (LSDA+U method). The experimental dispersion relation of the optical conductivity in the region of interband light absorption is interpreted based on the results of calculating the electron densities of states.

Landau quantization, Aharonov–Bohm effect and two-dimensional pseudoharmonic quantum dot around a screw dislocation

We investigate the influence of a screw dislocation on the energy levels and the wavefunctions of an electron confined in a two-dimensional pseudoharmonic quantum dot under the influence of an external magnetic field inside a dot and Aharonov–Bohm field inside a pseudodot. The exact solutions for energy eigenvalues and wavefunctions are computed as functions of applied uniform magnetic field strength, Aharonov–Bohm flux, magnetic quantum number and the parameter characterizing the screw dislocation, the Burgers vector. We investigate the modifications due to the screw dislocation on the light interband absorption coefficient and absorption threshold frequency. Two scenarios are possible, depending on if singular effects either manifest or not. We found that as the Burgers vector increases, the curves of frequency are pushed up towards of the growth of it. One interesting aspect which we have observed is that the Aharonov–Bohm flux can be tuned in order to cancel the screw effect of the model.

Optical properties of narrow band prolate ellipsoidal quantum layers ensemble

Electronic states and direct interband light absorption in the ensemble of prolate spheroidal quantum layers are considered. The problem of finding the one-electron wave function and energy spectrum have been solved exactly. Absorption edge dependence on the thickness of the layer in the strong size quantization regime has been obtained. The effect of nonparabolicity of the dispersion law of energy levels and optical absorption have been taken into account and calculations are carried out for the cases of both parabolic and Kane’s dispersion laws. Selection rules have been revealed. Absorption coefficient dependence on the frequency of incident light has been obtained, taking into account dispersion of nanolayer thicknesses for the cases of both symmetric and asymmetric distribution functions.

Prolate spheroidal quantum dot: Electronic states, direct interband light absorption and electron dipole moment

Excitonic states and direct interband light absorption are considered in prolate spheroidal quantum dot. The problem of finding the one electron wave function and energy spectrum have been solved exactly. Three regimes of size quantization have been considered. Absorption edge dependence on the small semiaxes of the spheroid has been obtained. The probability of the direct interband transitions has been calculated and the selection rules for quantum numbers have been obtained. It has been shown that in prolate spheroidal quantum dot the exact value of the electron ground state with high accuracy coincides with the value of ground state in strongly prolate ellipsoidal quantum dot obtained in the framework of the adiabatic approximation. The dependence of z component of the electron dipole moment has been studied on the small semiaxes of the prolate spheroidal quantum dot.

Oblate spheroidal quantum dot: electronic states, direct interband light absorption and pressure dependence

Exitonic states and direct interband light absorption are considered in an oblate spheroidal quantum dot. The problem of finding the one electron wave function and energy spectrum has been exactly solved. Three regimes of size quantization have been considered. Absorption edge dependence on the small semiaxes of the spheroid and its hydrostatic pressure dependencies have been obtained. It has been shown that the exact value of the electron ground state in the oblate spheroidal quantum dot coincides with high accuracy to the value of ground state in strongly oblate ellipsoidal quantum dot obtained in the framework of the adiabatic approximation. The dependences of the electron ground state energy on the hydrostatic pressure and temperature have been studied.

Evolution of the electronic structure and optical spectra of intermetallides DyNi5 − x Cu x under changes of concentration

Spin-polarization calculations of the electronic structure of intermetallic compounds DyNi5 − x Cu x (x = 0, 1, 2) are performed in the local spin density approximation with strong electron correlations in the 4 f shell of a rare-earth ion taken into account (the LSDA + U). Spectral properties of these materials are studied by the optical ellipsometry method in the wavelength range of 0.22–16 μm. It is established that the optical absorption spectra are significantly modified upon partial substitution of nickel by copper atoms. The experimental dispersion dependences of optical conductivity in the region of interband light absorption are interpreted on the basis of calculations of electronic state densities.

Effect of Interdiffusion on Optoelectronic Properties of In1-xGaxAs/InAs/In1-yAlyAs Quantum Well

The effect of interdiffusion of Al, Ga and In atoms on confining potential, ground state energy level of electron and interband light absorption coefficient in In1-xGaxAs/InAs/In1-yAlyAs quantum well is investigated in the framework of variational method and Wood-Saxon potential model. It is shown that the interdiffusion leads to the smearing of potential rectangular profile, to the rising up of ground state energy level and to the significant decrease in the interband absorption coefficient.

Effect of copper and cobalt impurities on the electronic structure and optical spectra of the intermetallic compound PrNi5

The electronic structure and optical properties of the intermetallic compound PrNi5 and their evolution during the substitution of copper or cobalt atoms for nickel atoms have been investigated. The band spectra of the studied compounds have been calculated in the local spin density approximation corrected to account for strong electron-electron interactions in the 4f shell of the rare-earth ion (LSDA + U method). The dispersion relations of the optical conductivity in the interband light absorption region have been interpreted using the results of calculations of the electron density of states.

Impact ionization of excitons in Ge/Si structures with Ge quantum dots grown on the oxidized Si(100) surfaces

Photoconductivity (PC) of Si/Ge structures with Ge quantum dots (QDs) grown on the Si(100) surfaces covered with the ultrathin, about 0.3–0.5 nm thick, SiO2 films is studied as a function of the interband light intensity under various lateral voltages. The structures exhibit PC with steps and a step with a peak at the step edge for low- and high-temperature grown structures, respectively. These PC features are associated with the impact ionization of QD-related excitons. The PC at step edges increases by several orders of magnitude for a certain value which is governed by the balance between rates of photo-generation, recombination, and impact ionization of excitons. The electron localization deeper in Si from the Ge QD layer in conjunction with a narrow binding-energy distribution of excitons is suggested to be the main reason that provides the sharpness of PC steps. The PC appears to be very sensitive to the impact ionization and QD preparation conditions. This allows revealing the specific characteristics of QD structures, related to their electronic and structural properties.

Grating-coupled mid-infrared light emission from tensilely strained germanium nanomembranes

Mechanically stressed nanomembranes are used to demonstrate mid-infrared interband light emission from Ge within the 2.1–2.5 μm atmospheric transmission window. Large biaxial tensile strain is introduced in these samples to convert Ge into a (near-) direct-bandgap semiconductor and to red-shift its luminescence. A diffractive array of Ge pillars is used to outcouple the long-wavelength interband radiation, which is otherwise primarily emitted in the sample plane. An order-of-magnitude strain-induced enhancement in radiative efficiency is also reported, together with the observation of luminescence signatures associated with photonic-crystal cavity modes. These results are promising for the development of silicon-compatible lasers for mid-infrared optoelectronics applications.

Specific features of the electronic structure and spectral properties of NdNi5 − x Cu x compounds

The spectral properties of the intermetallic compounds NdNi5 − x Cu x (x = 0, 1, 2) have been studied using optical ellipsometry in the wavelength range 0.22–16 μm. It has been established that substitution of copper atoms for nickel leads to noticeable changes in the optical absorption spectra, plasma frequencies, and relaxation frequencies of conduction electrons. Spin-polarized calculations of the electronic structure of these compounds have been performed in the local spin density approximation allowing for strong electron correlations (LSDA + U method) in the 4f shell of the rare-earth ion. The calculated electron densities of states have been used to interpret the experimental dispersion curves of optical conductivity in the interband light absorption region.

Optical spectroscopy and electronic structure of compounds HoNi5 − x Al x (x = 0, 1, 2)

The optical properties of the compounds HoNi5 − x Al x (x = 0, 1, 2) have been investigated using the ellipsometric method in the wavelength range from 0.22 to 16 μm. The electronic structure of these intermetallic compounds has been calculated in the local electron-spin density approximation with the correction for strong electronic interactions in the 4f shell of the holmium ions. The experimental dispersion dependences of optical conductivity in the region of interband light absorption have been interpreted based on the results of the calculation of the electron density of states. The plasma and relaxation frequencies of electrons have been determined.

Vanishing of interband light absorption in a persistent spin helix state.

Spin-orbit coupling plays an important role in various properties of very different materials. Moreover efforts are underway to control the degree and quality of spin-orbit coupling in materials with a concomitant control of transport properties. We calculate the frequency dependent optical conductivity in systems with both Rashba and Dresselhaus spin-orbit coupling. We find that when the linear Dresselhaus spin-orbit coupling is tuned to be equal to the Rashba spin-orbit coupling, the interband optical conductivity disappears. This is taken to be the signature of the recovery of SU(2) symmetry. The presence of the cubic Dresselhaus spin-orbit coupling modifies the dispersion relation of the charge carriers and the velocity operator. Thus the conductivity is modified, but the interband contribution remains suppressed at most but not all photon energies for a cubic coupling of reasonable magnitude. Hence, such a measurement can serve as a diagnostic probe of engineered spin-orbit coupling.

Effects of the magnetic field direction and anisotropy on the interband light absorption of an asymmetric quantum dot

In this paper, the direct interband transition and the threshold frequency of absorption in a two-dimensional anisotropic quantum dot are studied under the influence of a tilted external magnetic field. We first calculate the analytical wave functions and energy levels using a transformation to simplify the Hamiltonian of the system. Then, we obtain the analytical expressions for the light interband absorption coefficient and the threshold frequency of absorption as a function of the magnetic field, magnetic field direction, and anisotropy of the system. According to the results obtained from the present work, we find that (i) the absorption threshold frequency (ATF) increases when the magnetic field increases for all directions. (ii) When anisotropy is increased, ATF increases. (iii) At small anisotropy, the magnetic field direction has no important effect on the ATF. In brief, the magnetic field, magnetic field direction, and anisotropy play important roles in the ATF and absorption coefficient.

Optical TCAD on the Net: A tight-binding study of inter-band light transitions in self-assembled InAs/GaAs quantum dot photodetectors

A new capability of our well-known NEMO 3-D simulator (Ref. Klimeck et al., 2007 [10]) is introduced by carefully investigating the utility of III–V semiconductor quantum dots as infrared photodetectors at a wavelength of 1.2–1.5 μm. We not only present a detailed description of the simulation methodology coupled to the atomistic sp3d5s∗ tight-binding band model, but also validate the suggested methodology with a focus on a proof of principle on small GaAs quantum dots (QDs). Then, we move the simulation scope to optical properties of realistically sized dome-shaped InAs/GaAs QDs that are grown by self-assembly and typically contain a few million atoms. Performing numerical experiments with a variation in QD size, we not only show that the strength of ground state inter-band light transitions can be optimized via QD size-engineering, but also find that the hole ground state wavefunction serves as a control factor of transition strengths. Finally, we briefly introduce the web-based cyber infrastructure that is developed as a government-funded project to support online education and research via TCAD simulations. This work not only serves as a useful guideline to experimentalists for potential device designs and other modelers for the self-development of optical TCAD, but also provides a good chance to learn about the science gateway project ongoing in the Republic of Korea.