Interband Absorption Spectra Research Articles

Weakly confined silicon nanodiscs as material system for THz absorption: analytical study

A weakly confined silicon based nano structure as a THz absorbing material is introduced. Using an effective mass approximation and 2D hydrogenic solution along with 1st order perturbation correction for truncated potential, we calculated inter-band and intra-band energy spectrum of weakly confined silicon nanodisc system. Variation of inter-band and intra-conduction-band absorption spectrum with the different sizes of the nanodisc are presented for various diameter from 10 nm to 32 nm and constant thickness of 4 nm. Inter-band and intra-band absorption spectrum are simulated using the oscillator strength. These calculation shows that intra-band absorption spectrum of the weakly confined nanodisc can cover a large THz (∼ 0.3–6.5 THz) spectrum range for these set of nanodiscs. The proposed silicon based THz absorbing material can be utilized for CMOS compatible THz technology if design on silicon-on-insulator system.

Calculation of the Density of the Distribution of Electronic States in the Conduction Band from the Fundamental Absorption Spectra of Amorphous Semiconductors

The region of fundamental absorption in the optical spectra of amorphous semiconductors is theoretically studied using the Davis-Mott approximation according to the Kubo-Greenwood formula. As is known, three types of optical transitions of the electron can be observed in the fundamental absorption region; from the tail of the valence band to the conduction band, from the valence band to the conduction band and from the valence band to the tail of the conduction band. For all these electronic transitions, analytical expressions of the partial absorption spectra are obtained from two different types of the Kubo-Greenwood formula. The width of the optical mobility gap and the proportionality coefficient were determined in the analytical form of the interband absorption spectrum by fitting them to the experimental interband absorption spectrum. A new method is presented for calculating the density of distribution of electronic states in the conduction band of amorphous carbon based on the experimental interband absorption spectrum and the analytical expression of the Kubo-Greenwood formula written for the interband absorption spectrum.

Size Quantization of the Electron Energy Spectrum in a Microscopic Semiconductor Crystal

The interband absorption spectrum of microscopic CdS crystals ranging in size from ~30 to 800 Å and dispersed in a transparent insulating matrix has been studied. There is a significant (~0.8-eV) shift of the fundamental absorption edge in the short-wavelength direction, and there are oscillations in the interband absorption spectrum caused by quantum size effect.

Investigation of type-II Ga(As)Sb/GaAs quantum dots embedded in an InGaAs quantum well for solar cell applications

We theoretically investigate the electronic and optical properties of charge carriers in type-II Ga(As)Sb/GaAs quantum dots (QDs) embedded in an InGaAs quantum well (QW), to determine their viability for use in intermediate band solar cell (IBSC) devices. This hybrid structure has the advantage of allowing a large flexibility to alter wave function distribution of the electron ground state, which is often considered as an intermediate band (IB) in QD-IBSC. The polarized transitions dipole moment (TDM), the polarized absorption coefficients and the radiative lifetime were studied as a function of QD height. Our theoretical results show an enhancement of inter-band absorption spectrum with a redshift of the peak corresponding to h0→IB transition and a blueshift of the continuum→IB peak, as the height of QD increases. The intra-band light absorption spectrum exhibits a redshift and its magnitude remains almost unchanged, as the height of QD increases. Furthermore this structure provides strong intra-band light absorption comparable to that of type-I QDs and equivalent to h0→IB transition strength. A long carrier lifetime in IB of about 30ns was found, but remains more than an order of magnitude less than the inter-band radiative time due to the low confinement potentials of the electron along the ρ-axis in this type-II dot-in-well (DWELL) structure.

A Theoretical Study of Interband Absorption Spectra of Spherical Sector Quantum Dots under the Effect of a Powerful Resonant Laser.

We explore the variation of interband absorption spectra of GaAs spherical sector quantum dots (QDs) in response to a strong resonant laser, using the renormalized wave function method. Even though a spherical sector QD appears identical to a section cut from a spherical QD, it contains a controllable additional spatial parameter, the apical angle, which results in radically different wave functions and energy levels of particles, and is anticipated to exhibit novel optical properties. The obtained findings reveal that the apical angle of the dot has a considerable effect on the interband absorption spectrum. With the increase in the dot apical angle, a significant redshift of the interband absorption peaks has been identified. Increasing the pump laser detuning and dot radius yields similar results. Especially when a powerful resonant laser with tiny detuning is utilized, a dynamical coupling between electron levels arises, resulting in the formation of new interband absorption peaks. These new peaks and the former ones were similarly influenced by the aforementioned parameters. Furthermore, it is thought that the new peaks, when stimulated by a suitable laser, will produce the entangled states necessary for quantum information.

Calculation of the Density of Electronic States in the Valence Band from Experimental Interband Absorption Spectra of Amorphous Semiconductors

Calculation of the Density of Electronic States in the Valence Band from Experimental Interband Absorption Spectra of Amorphous Semiconductors

Excitonic Effects and Impurity–Defect Emission in GaAs/AlGaAs Structures Used for the Production of Mid-IR Photodetectors

A series of undoped GaAs/AlxGa1 –xAs multiple quantum well heterostructures, whose doped analogs are used for the production of photodetectors operating in the spectral range 8–12 μm, is fabricated by molecular-beam epitaxy. For the heterostructures, the spectral position of absorption lines corresponding to the allowed transitions between quantum-confined electron and hole levels in GaAs layers is established. The influence of impurity–defect states on the luminescence and absorption spectra of quantum wells is studied. The excitonic corrections for the allowed transitions are determined in relation to the quantum-well width and the aluminum content in the barrier layers. The role of excitonic effects in restoring the structure of single-electron states from interband-absorption spectra (luminescence-excitation spectra) and the relationship between these states and the working region of IR photodetectors based on GaAs/AlxGa1 –xAs quantum wells are discussed.

Effect of Thermal Annealing on Absorption and Hole Escape Processes in Type II GaSb/GaAs Quantum Dots: Implications for Solar Cell Design

We present a theoretical study of the rapid thermal annealing process and strain on the electronic, optical, and charge dynamical properties of type II GaSb/GaAs quantum dots (QDs). Our theoretical results show a blueshift and the enhancement of interband absorption spectrum because of inter-diffusion processes and the annealed samples. We have identified that increased annealing temperature makes the hole escape times from such QD shorter while the effect of strain on the hole confinement potential becomes weaker. At the same time, in both structures, unstrained and strained QDs, the intra-band absorption in the valence band (IVBA) is redshifted, and its magnitude is decreased after annealing when compared with the as-grown sample. To explain those effects we discuss how the electron–hole wave function overlap varies with thermal treatment and strain and discussed those effects on radiative and thermal processes. Our results explain and are in agreement with the recent experimental observations on similar structures. We have identified that holes could thermally escape out of the type II GaSb/GaAs QD before they are recombined and will contribute to the enhancement in absorption of QD solar cells. The emission and tunneling times of holes were estimated to be $ 10^{-12}$ to $10^{-14}$ s about one order of magnitude faster than the escape times because of radiative recombination processes, $ 10^{-11}$ to $10^{-13}$ s.

Экситонные эффекты и примесно-дефектное излучение в GaAs/AlGaAs-структурах, применяемых для изготовления детекторов среднего ИК-диапазона

AbstractA series of undoped GaAs/Al_ x Ga_1 –_ x As multiple quantum well heterostructures, whose doped analogs are used for the production of photodetectors operating in the spectral range 8–12 μm, is fabricated by molecular-beam epitaxy. For the heterostructures, the spectral position of absorption lines corresponding to the allowed transitions between quantum-confined electron and hole levels in GaAs layers is established. The influence of impurity–defect states on the luminescence and absorption spectra of quantum wells is studied. The excitonic corrections for the allowed transitions are determined in relation to the quantum-well width and the aluminum content in the barrier layers. The role of excitonic effects in restoring the structure of single-electron states from interband-absorption spectra (luminescence-excitation spectra) and the relationship between these states and the working region of IR photodetectors based on GaAs/Al_ x Ga_1 –_ x As quantum wells are discussed.

Effect of interdiffusion and external magnetic field on electronic states and light absorption in Gaussian-shaped double quantum ring

The effect of interdiffusion and magnetic field on confined states of electron and heavy hole as well as on interband absorption spectrum in a Ga1−xAlxAs/GaAs Gaussian-shaped double quantum ring are investigated. It is shown that both interdiffusion and magnetic field lead to the change of the charge carriers' quantum states arrangement by their energies. The oscillating behavior of the electron ground state energy as a function of magnetic field induction gradually disappears with the increase of diffusion parameter due to the enhanced tunneling of electron to the central region of the ring. For the heavy hole the ground state energy oscillations are not observable in the region of the values of magnetic field induction B=0−10T. For considered transitions both the magnetic field and the interdiffusion lead to a blue-shift of the absorption spectrum and to decreasing of the absorption intensity. The obtained results indicate on the opportunity of purposeful manipulation of energy states and absorption spectrum of a Gaussian-shaped double quantum ring by means of the post growth annealing and the external magnetic field.

Energy spectra and optical transitions in germanene quantum dots

The band gap of buckled graphene-like materials, such as silicene and germanene, depends on external perpendicular electric field. Then a specially design profile of electric field can produce trapping potential for electrons. We study theoretically the energy spectrum and optical transitions for such designed quantum dots (QDs) in graphene-like materials. The energy spectra depend on the size of the QD and applied electric field in the region of the QD. The number of the states in the QD increases with increasing the size of the dot and the energies of the states have almost linear dependence on the applied electric field with the slope which increases with increasing the dot size. The optical properties of the QDs are characterized by two types of absorption spectra: interband (optical transitions between the states of the valence and conduction bands) and intraband (transitions between the states of conduction/valence band). The interband absorption spectra have triple-peak structure with peak separation around 10 meV, while intraband absorption spectra, which depend on the number of electrons in the dot, have double-peak structure.

A quantum dot in topological insulator nanofilm

We introduce a quantum dot in topological insulator nanofilm as a bump at the surface of the nanofilm. Such a quantum dot can localize an electron if the size of the dot is large enough, ≳5 nm. The quantum dot in topological insulator nanofilm has states of two types, which belong to two (‘conduction’ and ‘valence’) bands of the topological insulator nanofilm. We study the energy spectra of such defined quantum dots. We also consider intraband and interband optical transitions within the dot. The optical transitions of the two types have the same selection rules. While the interband absorption spectra have multi-peak structure, each of the intraband spectra has one strong peak and a few weak high frequency satellites.

Interband optical absorption in a circular graphene quantum dot

We investigate the energy levels and optical properties of a circular graphene quantum dot in the presence of an external magnetic field perpendicular to the dot. Based on the Dirac–Weyl equation and assuming zero outward current at the edge of the dot we present the results for two different types of boundary conditions, i.e. infinite-mass (IMBC) and zigzag boundary conditions. We found that the dot with zigzag edges displays a zero-energy state in the energy spectra while this is not the case for the IMBCs. For both boundary conditions, the confinement becomes dominated by the magnetic field, where the energy levels converge to the Landau levels as the magnetic field increases. The effect of boundary conditions on the electron- and hole-energy states is found to affect the interband absorption spectra, where we found larger absorption in the case of IMBCs. The selection rules for interband optical transitions are determined and discussed for both boundary conditions.

Interband absorption in square and semiparabolic near-surface quantum wells under intense laser field

The exciton effects on the interband absorption spectra in near-surface square and semiparabolic quantum wells under intense laser field are studied taking into account the correct dressing effect for the confinement potential and electrostatic self-energy due to the repulsive interaction between carriers and their image charges. We found that for near-surface quantum wells with different shapes the laser field induces significant effects on their electronic and optical properties. The numerical results for the InGaAs/GaAs system show that the red-shift of the absorption peak induced by the increasing cap layer can be effectively compensated using the blue-shift caused by the enhanced laser parameter. In square quantum well without laser field our theoretical values for the absorption peak position are in good agreement with the available experimental data. As a key result, we conclude that the optical properties in near-surface quantum wells can be tuned by tailoring the heterostructure parameters: well shape, capped layer thickness and/or dielectric mismatch as well as the external field radiation strength.

Intense laser field effect on the interband absorption in differently shaped near-surface quantum wells

The 1S-exciton properties and interband absorption spectra in differently shaped near-surface quantum wells (NSQWs) with symmetrical/asymmetrical barriers, under intense laser field, are investigated taking into account the correct dressing effect for the confinement potential and electrostatic interaction between carriers and their image-charges. We found that: i) the 1S-exciton binding energy is significantly reduced by the laser intensity in InGaAs NSQWs of different asymmetrical shape; ii) the red-shift of the absorption peak induced by the asymmetry diminution or by increasing cap layer thickness can be effectively compensated using the blue-shift caused by enhancing laser parameter. Therefore, the optical properties of the differently shaped NSQWs could be tuned by proper tailoring of the heterostructure parameters (well shape, barrier asymmetry) and/or dielectric mismatch as well as by varying the laser field intensity.

Coexistence and coupling of zero-dimensional, two-dimensional, and continuum resonances in nanostructures

Quantum dots (QDs) embedded in a matrix exhibit a coexistence of ``zero-dimensional'' (0D) bound electron and hole states on the dot with ``three-dimensional'' (3D) continuum states of the surrounding matrix. In epitaxial QDs one encounters also ``two-dimensional'' (2D) states of a quantum well-like supporting structure (wetting layer). This coexistence of 0D, 2D, and 3D states leads to interesting electronic consequences explored here using multiband atomistic pseudopotential calculations. We distinguish strained dots (InAs in GaAs) and strain-free dots (InAs in GaSb) finding crucial differences: in the former case ``potential wings'' appear in the electron confining potential in the vicinity of the dot. This results in the appearance of localized electronic states that lie above the threshold of the 3D continuum. Such resonances are ``strain-induced localized states'' (SILSs) appearing in strained systems, whereas in strain-free systems the dot resonances in the continuum are the usual ``virtual bound states'' (VBSs). The SILSs were found to occur regardless of the thickness of the wetting layer and even in interdiffused dots, provided that the interdiffusion length is small compared to the QD size. Thus, the SILSs are well isolated from the environment by the protective potential wings, whereas the VBSs are strongly interacting. These features are seen in our calculated intraband as well as interband absorption spectra. Furthermore, we show that the local barrier created around the dot by these potential wings suppresses the 0D-2D (dot-wetting layer) hybridization of the electron states. Consequently, in contrast to findings of simple model calculations of envelope function, 0D-to-2D ``crossed transitions'' (bound hole-to-wetting layer electron) are practically absent because of their spatially indirect character. On the other hand, since no such barrier exists in the hole confining potential, a strong 0D-2D hybridization is present for the hole states. We show this to be the source for the strong 2D-to-0D crossed transitions determined experimentally.

Anisotropic enhancement of piezoelectricity in the optical properties of laterally coupled InAs/GaAs self-assembled quantum dots

We investigated the electronic and optical properties of self-assembled laterally double InAs/GaAs quantum dots coupled along the [110] and $[1\overline{1}0]$ directions with varying interdot distances. The coupling region provides a stronger confinement for both electrons and holes than in the center of each dot due to the weak compressive strain and positive biaxial strain in the region. The lateral coupling along the [110] $([1\overline{1}0])$ direction enhances the negative (positive) piezoelectric potential in the coupling region and lowers (raises) the potential in the middle of each dot. As a result, the piezoelectric potential decreases the splitting between the two transitions from the bonding $s$ orbitals to the two coupled $p$ orbitals in quantum dots coupled along the $[1\overline{1}0]$ direction and increases the splitting in the quantum dots coupled along the [110] direction. The direction of coupling is clearly distinguishable by the polarization of intraband transition since most transitions are polarized along the axis passing through the two dots. Lateral coupling enhances the polarization anisotropy of interband absorption spectra. In the presence of the piezoelectric potential, quantum dots coupled along the [110] direction exhibit larger redshift of the lowest exciton energies when the distance between the two dots is sufficiently close and have smaller exciton binding energies than quantum dots coupled along the $[1\overline{1}0]$ direction.

Interband and intraband optical transitions in InAs nanocrystal quantum dots: A pseudopotential approach

An atomistic pseudopotential method is used to investigate the electronic and optical properties of spherical InAs nanocrystals. Our calculated interband (valence-to-conduction) absorption spectra reproduce the features observed experimentally both qualitatively and quantitatively. The results relative to intraband (valence-to-valence and conduction-to-conduction) absorption successfully reproduce the recently measured photoinduced absorption spectra, which had so far been addressed only qualitatively. They exclude the hypothesis of a thermal activation process between dot-interior-delocalized hole states to explain the temperature dependence observed experimentally. Furthermore, based on the agreement of our data with the experimental valence intersublevel transitions and the almost complete overlap of the latter with scanning tunneling microscopic (STM) measurements, we question the simplistic attribution of the observed STM peaks obtained for negative bias.

Exciton photoluminescence, photoconductivity and absorption in GaSe 0.9Te 0.1 alloy crystals

The photoluminescence, photoconductivity and absorption in GaSe 0.9Te 0.1 alloy crystals have been investigated as a function of temperature and external electric field. It has been observed that the exciton peaks shift to lower energy in GaSe 0.9Te 0.1 alloy crystals compared to GaSe crystals. The long wavelength tails of interband photoluminescence, photoconductivity and absorption spectra are determined by the free exciton states and show an Urbach–Martienssen-type dependence to the photon energy. The maxima of the extrinsic photoluminescence and photoconductivity spectra were found to be determined by the acceptor centers with an energy of E A= E V+0.19 eV formed by the polytypism and defects complexes that include Se and Te anions.

Fundamental band edge optical absorption in Bi0.5Sb1.5Te3

Spectra of optical absorption in Bi 0.5 Sb 1.5 Te 3 films grown on mica and KBr substrates have been investigated for T = 145 and 300 K. The data obtained have been analyzed together with the data of investigations on the fundamental absorption edge for Bi 2 Te 3 available in the scientific literature. It has been revealed that the interband absorption spectra for both Bi 0.5 Sb 1.5 Te 3 and Bi 2 Te 3 represent a superposition of two components corresponding to direct and indirect allowed optical transitions. In this case, the least energy gap separating the valence band and the conduction band is direct for Bi 2−x Sb x Te 3 (x ≤ 1.5, T = 300 K). For Bi 0.5 Sb 1.5 Te 3 the temperature variation rates have been estimated for the thresholds of direct and indirect interband transitions. It has been shown that this solid solution is direct gap solution at T ≥ 145 K.