Ground-state Exciton Binding Energy Research Articles

Optical Absorption of Excitons in Strained Quasi 2D GaN Quantum Dot

The influences of hydrostatic pressure combined to the size effect on the behavior of the exciton in 2D GaN/AlN quantum ultra‐thin disk, on the binding energy and optical absorption coefficient are investigated. Our approach is performed in the framework of effective mass theory and by using a variational method with a robust trial wave function and taking into account the dependence of the size, the dielectric constant and the effective masses on the pressure. Variations of the excitonic binding energy, optical transition energy are determined according to hydrostatic pressure. The results of our numerical calculations show that the applied pressure favors the electron–hole attraction leading to an increase of the ground state exciton binding energy, and causes a shift of the absorption coefficient toward high energies (blue shift).

Growth and optical property of PbS/ZnS nanocrystals

PbS quantum dots (QDs) were synthesized via colloidal hot-injection route, and then new PbS/ZnS nanoparticles (NCs) were prepared by two different routes. PbS/ZnS-A NCs were fabricated using the epitaxial growth method, and PbS/ZnS-B NCs were prepared by the cation substitution method. The structure and morphology of these NCs were characterized by XRD, TEM and vis-NIR absorption. The morphology can be controlled by different growth mechanisms. Epitaxial growth can result in the formation of nano-heterojunction of PbS-ZnS. Doping growth almost does not change the surface morphology of the PbS/ZnS NCs, but changes the relative growth rate of various crystal facets. For two kinds of PbS/ZnS NCs, the ground state exciton binding energy becomes stronger due to introducing of Zn, and there is an obvious blue shift in optical band gap with respect to the pure PbS QDs.

Effect of AlGaAs composition on the type-I to type-II transition in AlInAs/AlGaAs self-assembled quantum dots

We study the transition from type-I to type-II AlInAs/AlGaAs self-assembled quantum dots (QDs) which is induced by changing the Al composition of an AlGaAs matrix. We found theoretical evidence for type-II band alignment above the crossover Al concentration . For this purpose, we obtained the phase diagram for different QD radii, and we can identify the region where the electron states derived from the X-valley are lower than those derived from the Γ-point. The spatial distribution of electrons and holes is very sensitive to QD size variations. Furthermore, the effect of the QD type on the exciton binding energy is investigated. The ground-state exciton binding energy is always significantly smaller for type-II than for corresponding type-I systems, which originates from the redistribution of the electron and hole wave function.

Laser Field Effects on Optical Properties in the Zinc-Blende GaN/AlGaN Quantum Well.

Based on the effective mass approximation, the laser field effects on exciton states and optical properties in the zinc-blende (ZB) GaN/AlGaN quantum well (QW) are investigated by means of a variational method. Numerical results show that the ground-state exciton binding energy, the emission energy, the oscillator strength, and the linear optical susceptibility are highly dependent on the well width and laser field amplitude in the ZB GaN QW. The laser field decreases the exciton binding energy and the oscillator strength; however, it also increases the emission energy in the QW for any well width. In particular, our results also show that the laser field has remarkable effects on the exciton states and optical properties in the ZB GaN/AlGaN QW with the small well width case.

Intense laser field effects on exciton states in direct-gap Ge/SiGe quantum well

Based on the effective mass approximation, the laser field effects on exciton states and optical properties in the direct-gap Ge/SiGe quantum well (QW) are investigated by means of a variational method. Numerical results show that the ground-state exciton binding energy and the emission energy are dependent highly on the well width and laser field amplitude in the direct-gap Ge/SiGe QW. The laser field decreases the exciton binding energy, however, it also increases the emission energy in the QW for any well width. In particularly, our results also show that the laser field has remarkable effects on exciton states and optical properties in the direct-gap Ge/SiGe QW with the small well width case.

Influence of Mg Composition on Optical Properties of Exciton Confinement in Strained Wurtzite ZnO/MgxZn1−xO Cylindrical Quantum Dots

Based on the effective-mass approximation and variational approach, excitonic optical properties are investigated theoretically in strained wurtzite (WZ) ZnO/MgxZn1−xO cylindrical quantum dots (QDs) for four different Mg compositions: x = 0.08, 0.14, 0.25, and 0.33, with considering a three-dimensional carrier confinement in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The ground-state exciton binding energy, the interband emission wavelength, and the radiative lifetime as functions of the QD structural parameters (height and radius) are calculated in detail. The computations are performed in the case of finite band offset. Numerical results elucidate that Mg composition has a significant influence on the exciton states and optical properties of ZnO/MgxZn1−xO QDs. The ground-state exciton binding energy increases with increasing Mg composition and the increment tendency is more prominent for small height QDs. As Mg composition increases, the interband emission wavelength has a blue-shift if the dot height L < 3.5 nm, but the interband emission wavelength has a red-shift when L > 3.5 nm. Furthermore, the radiative lifetime increases rapidly with increasing Mg composition if the dot height L > 3 nm and the increment tendency is more prominent for large height QDs. The physical reason has been analyzed in depth.

Excitonic optical absorption in wurtzite InGaN/GaN quantum wells

Within the framework of the effective-mass and envelope function theory, exciton states and optical properties in wurtzite (WZ) InGaN/GaN quantum wells (QWs) are investigated theoretically considering the built-in electric field effects. Numerical results show that the built-in electric field, well width and in composition have obvious influences on exciton states and optical properties in WZ InGaN/GaN QWs. The built-in electric field caused by polarizations leads to a remarkable reduction of the ground-state exciton binding energy, the interband transition energy and the integrated absorption probability in WZ InGaN/GaN QWs with any well width and In composition. In particular, the integrated absorption probability is zero in WZ InGaN/GaN QWs with any In composition and well width L > 4 nm. In addition, the competition effects between quantum confinement and the built-in electric field (between quantum size and the built-in electric field) on exciton states and optical properties have also been investigated.

Finite barrier width effects on exciton states and optical properties in wurtzite InGaN/GaN quantum well

Exciton states and optical properties in wurtzite (WZ) InGaN/GaN quantum well (QW) are investigated theoretically, considering finite barrier width and built-in electric field effects. Numerical results show that when the barrier width increases, the ground-state exciton binding energy, the interband transition energy and the integrated absorption probability increase first and then they are insensitive to the variation of the barrier width. For any barrier width, the ground-state exciton binding energy and the integrated absorption probability have a maximum when the well width is 1 nm; moreover, the integrated absorption probability goes to zero when the well width is larger than 6 nm. In addition, the competition effects between the built-in electric field and quantum confinement are also investigated in the WZ InGaN/GaN QW.

Radiative life time of an exciton confined in a strained GaN/Ga1-xAlxN cylindrical dot: built-in electric field effects

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically. The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included. Numerical calculations are performed using a variational procedure within the single band effective mass approximation. Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed. The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton. It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased, and that the exciton binding energy, the oscillator strength and the radiative lifetime each as a function of structural parameters (height and radius) sensitively depend on the strong built-in electric field. The obtained results are useful for the design of some opto-photoelectronic devices.

Electric field effects on optical properties in zinc-blende InGaN/GaN quantum dot

The effect of electric field on exciton states and optical properties in zinc-blende (ZB) InGaN/GaN quantum dot (QD) are investigated theoretically in the framework of effective-mass envelop function theory. Numerical results show that the electric field leads to a remarkable reduction of the ground-state exciton binding energy, interband transition energy, oscillator strength and linear optical susceptibility in InGaN/GaN QD. It is also found that the electric field effects on exciton states and optical properties are much more obvious in QD with large size. Moreover, the ground-state exciton binding energy and oscillator strength are more sensitive to the variation of indium composition in InGaN/GaN QD with small indium composition. Some numerical results are in agreement with the experimental measurements.

The Application of Hartree Approximation in Exciton Recombination Energy for Conical InAs/GaAs Quantum Dots

Under the framework of the Hartree approximation, the ground state exciton binding energy and the interband transition energy are calculated by solving the coupled Schrodinger equations taking into account the coulomb interaction. The self-consistent effective confining potentials are obtained using a fast three-dimensional Fourier transform in every step of the reduced Schrodinger equations. The exciton binding energy increases at first, and then goes through the process of the decline with the increment of the size of conical InAs/GaAs quantum dot. The ground-state exciton oscillator strength becomes larger when the size of the quantum dots increases. It indicates that the radiative lifetime of the exciton will become shorter. The temperature will affect the interband transition energy, but the exciton binding energy is almost temperature-independent.

EXCITON STATES AND INTERBAND TRANSITIONS IN THE DIRECT-GAP Ge/SiGe QUANTUM DOT

The ground-state exciton binding energy and interband emission wavelength in the direct-gap Ge / SiGe quantum dot (QD) are investigated by means of a variational approach, within the framework of effective-mass approximation. Numerical results show that the ground-state exciton binding energy has a maximum value with increasing quantum size of the direct-gap Ge / SiGe QD. The interband emission wavelength is increased when the QD size is increased. Our results indicate the direct-gap Ge / SiGe QD can be applied for long wavelength optoelectronic devices.

Exciton states in zinc-blende GaN/AlGaN quantum dot: Effects of electric field and hydrostatic pressure

Abstract Based on the effective-mass approximation, the effects of the electric field and hydrostatic pressure on exciton states in a cylindrical zinc-blende (ZB) GaN/AlGaN quantum dot (QD) are investigated variationally. Numerical results show that the electric field leads to a remarkable reduction of the ground-state exciton binding energy and interband transition energy in the case of any hydrostatic pressures. However, the hydrostatic pressure increases the exciton binding energy and interband transition energy in the case of any electric fields. In particular, the electric field has a remarkable influence on the exciton binding energy in the QD with large dot size and small hydrostatic pressure; moreover, the hydrostatic pressure obviously affects the exciton binding energy in the QD with small dot size and weak electric field.

Donor bound excitons confined in a [formula omitted] quantum dot

Exciton states confined in a GaN/Ga 1− x Al x N Quantum dot are investigated within the framework of single band effective mass approximation. Charged donor bound exciton, D + X energy is computed as a function of dot radius for different Al concentration and the height of cylindrical dot. The interband optical transition of GaN/Ga 1− x Al x N dot is calculated with the various structural parameters. The valence band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. Binding energy of the ground state of exciton is calculated, with the inclusion of 2D Hartree dielectric screening function. Our results show that quantum size has a considerable influence on exciton states and interband optical transitions of the narrow dots. The ground state exciton binding energy and the interband emission energy are increased when the cylindrical quantum dot height or radius is decreased and the effect of exciton has influence on the interband emission energy. The donor bound exciton binding energy increases monotonically as Al concentration increases. Our results are compared with the existing available literature.

Curved nanowire structures and exciton binding energies

Growth of quantum-confined semiconductor structures is a complicated process thatmay lead to imperfect and complex shapes as well as geometrical nonuniformitieswhen comparing a large number of intended identical structures. On the otherhand, the possibility of tuning the shape and size of nanostructures allows forextra optimization degrees when considering electronic and optical propertiesin various applications. This calls for a better understanding of size and shapeeffects. In the present work, we express the one-band Schrödinger equation in curvedcoordinates convenient for determining eigenstates of curved quantum-wire andquantum-dash structures with large aspect ratios. Firstly, we use this formulation tosolve the problem of single-electron and single-hole states in curved nanowires.Secondly, exciton states for the curved quantum-wire Hamiltonian problem are foundby expanding exciton eigenstates on a product of single-particle eigenstates. Asimple result is found for the Coulomb matrix elements of an arbitrarily curvedstructure as long as the radius-of-curvature is much larger than the cross-sectionaldimensions. We use this general result to compute the groundstate exciton bindingenergy of a bent nanowire as a function of the bending radius-of-curvature. Itis demonstrated that the groundstate exciton binding energy increases by40 meV as the radius-of-curvature changes from20 to2 nm while keeping the total length (and volume) of the nanowire constant.

Effects of environmental and exciton screening in single-walled carbon nanotubes

The ground-state exciton binding energy for single-walled carbon nanotubes (SWCNTs) in vacuum calculated ignoring the screening of Coulomb interaction appears to be much greater than the corresponding band gap. The most essential contributions to the screening of electron-hole (e-h) interaction potential in semiconducting SWCNTs, which return the ground-state exciton binding energy into the energy gap, are considered. Our estimates on the screening efiects and exciton binding energies are in satisfactory agreement with the corresponding experimental data for concrete nanotubes.

Exciton states in wurtzite and zinc-blende InGaN/GaN coupled quantum dots

Based on the effective-mass approximation, exciton states in wurtzite (WZ) and zinc-blende (ZB) InGaN/GaN coupled quantum dots (QDs) are studied by means of a variational method. Numerical results show clearly that both the sizes and In content of QDs have a significant influence on exciton states in WZ and ZB InGaN/GaN coupled QDs. Moreover, the ground-state exciton binding energy decreases when the interdot barrier layer thickness increases in the WZ InGaN/GaN coupled QDs. However, the ground-state exciton binding energy has a minimum if the interdot barrier layer thickness increases in the ZB InGaN/GaN coupled QDs.

Exciton states in zinc-blende InGaN/GaN quantum dot

Abstract Within the framework of effective-mass approximation, exciton states confined in zinc-blende(ZB) InGaN/GaN quantum dot(QD) are investigated by means of a variational approach, considering finite band offsets. The ground-state exciton binding energy and the interband emission energy are investigated as functions of QD structural parameters in detail. Numerical results show clearly that both the QD size and In content of InGaN have a significant influence on the exciton states and interband optical transitions in the ZB InGaN/GaN QD.

Optical determination of the electron effective mass of strain compensated In0.4Ga0.6As0.995N0.005∕GaAs single quantum well

A detailed line shape analysis of the temperature dependent photoluminescence spectra of In0.4Ga0.6As1−yNy∕GaAs quantum well (QW) (y=0,0.005) is carried out and the relative contribution of free excitons and free carriers to the radiative recombination at different temperature is quantitatively assessed. The analysis extracts the binding energy of the e1-hh1 ground-state exciton which equals 9.72±1.24 and 17.5±0.9meV for InGaAs and InGaAsN (N=0.5%) single QW sample, respectively. By using a fractional dimension exciton binding energy model, an electron effective mass of me*=(0.11±0.015)m0 is determined for the highly strained dilute nitride single QW.

Near-field magnetoabsorption of quantum dots

We investigate the effect of an external magnetic field of variable orientation and magnitude (up to $20\phantom{\rule{0.3em}{0ex}}\mathrm{T}$) on the linear near-field optical absorption spectra of single and coupled III-V semiconductor quantum dots. We focus on the spatial as well as on the magnetic confinement, varying the dimensions of the quantum dots and the magnetic field. We show that the ground-state exciton binding energy can be manipulated utilizing the spatial and magnetic confinement. The effect of the magnetic field on the absorption spectra, increasing the near-field illumination spot, is also investigated. The zero-magnetic-field ``structural'' symmetry can be destroyed varying the magnetic field orientation and this affects the near-field spectra. The asymmetry induced (except for specific orientations along symmetry axes) by the magnetic field can be revealed in the near-field but not in the far-field spectra. We predict that near-field magnetoabsorption experiments, of realistic spatial resolution, will be in the position to bring to light the quantum dot symmetry. This exceptional symmetry-resolving power of the near-field magnetoabsorption is lost in the far field. The influence of the Coulomb interactions on the absorption spectra is also discussed. Finally, we show that certain modifications of the magnetoexcitonic structure can be uncovered using a realistically acute near-field probe of $\ensuremath{\approx}20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.