Search results

1

Coupling and Strain Effects in Vertically Stacked Double InAs/GaAs Quantum Dots: Tight-Binding Approach

100%

Jaskólski W., Zieliński M., Bryant G.

Acta Physica Polonica A

|

2004

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vol. 106

|

issue 2

193-205

EN

The empirical tight-binding approach is used to study atomic-scale effects on electronic coupling in vertically stacked, self-assembled InAs/GaAs quantum dots. A model with unstrained dots is first studied to isolate the atomistic coupling effects from the strain effects. The strain effects are next considered by means of the valence force field method. Electron levels in coupled quantum dots follow closely the simple analogy of coupled dots as artificial molecules. The electron ground state of double dot has always bonding-like character. The coupling of hole states is more complicated because the coupling depends both of the hole envelope function and the atomic character of the hole state. It is shown that the character of the hole ground state of double dot changes from antibonding to bonding-like, when the distance between the dots decreases. It reorders hole levels, changes state symmetries, and makes changes in optical spectra. The calculated red-shift of the lowest transition for closely-spaced dots agrees well with experimental data. We present also some preliminary results on strain effects in such nanocrystals.

2

Strain and Spin-Orbit Effects in Self-Assembled Quantum Dots

100%

Zieliński M., Jaskólski W., Aizpurua J., Bryant G.

Acta Physica Polonica A

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2005

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vol. 108

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issue 5

929-940

EN

The effects of strain and spin-orbit interaction in self-assembled lens-shaped InAs/GaAs quantum dots are investigated. Calculations are performed with empirical tight-binding theory supplemented by the valence force field method to account for effects of strain caused by lattice mismatch at the InAs-GaAs interface. It is shown that both effects influence strongly the electron and hole energy structure: splitting of the energy levels, the number of bound states, density distributions, and transition rates. We show that piezoelectric effects are almost negligible in quantum dots of the size investigated.

3

Influence of Semiconductor Bands Bending on Exciton Photoluminescence

63%

Konin A.

Acta Physica Polonica A

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2008

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vol. 113

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issue 3

1035-1038

EN

A theoretical model for calculation of the Wannier-Mott exciton distribution in semiconductor sample accounting for the energy bands bending near semiconductor surface is presented. It is shown that the exciton distribution essentially depends on the surface potential under certain sample surface and bulk parameters. Changing the surface potential value we can study the exciton photoluminescence from the illuminated surface and from a thin layer near this surface.

4

Dependence of Exciton Binding Energy on Magnetic Field in Parabolic Quantum Well Made of Diluted Magnetic Semiconductors

63%

Kyrychenko F., Kossut J.

Acta Physica Polonica A

|

1998

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vol. 94

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issue 3

406-410

EN

Influence of an external magnetic field on exciton binding in parabolic quantum wells composed of diluted magnetic semiconductors are studied variationally using separable trial wave functions. The effect is due to a change of shape of the confining potential induced by an external magnetic field. The validity of the results of the calculations is checked by solving numerically the corresponding one-dimensional Schrodinger equations. Further, we check the results by performing variational calculations in the case of the Ex11 exciton (i.e. with electron and hole in their ground subbands) with a non-separable trial wave function. It is shown that, contrary to the case of a rectangular quantum well, for the parabolic quantum well the magnetic field dependence of the binding energy is significant.

5

Shake-Up Processes in Photoluminescence of Two-Dimensional Holes in a High Magnetic Field

51%

Wójs A., Bryja L., Misiewicz J., Potemski M., Reuter D., Wieck A.

Acta Physica Polonica A

|

2006

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vol. 110

|

issue 3

429-435

EN

Recombination of excitons and positive trions is studied by two-beam photoluminescence of a two-dimensional hole gas in a high magnetic field. The singlet, dark-triplet and bright-triplet states of a free trion are resolved, and their binding energies are determined. Recombination of acceptor-bound trions is also detected, including a low-energy cyclotron replica, corresponding to a hole shake-up process. Identification of all these different transitions was possible by analysis of optical selection rules and the comparison of experimental spectra with realistic numerical calculations.

6

Motional Enhancement of Exciton Magnetic Moments

51%

Litvinov A., Kochereshko V., Cox R., Besombes L., Mariette H., Boukari H., Loginov D., Davies J., Smith L., Wolverson D.

Acta Physica Polonica A

|

2007

|

vol. 112

|

issue 2

161-165

EN

The phenomenon in which giant enhancement of exciton magnetic moments occurs due to translational motion was found for light hole excitons in ZnTe/ZnMgTe quantum well structures. Decreasing diamagnetic shifts as the number of the exciton quantized state increases were found for the first time.

7

Binding of Biexcitons in GaAs/AlGaAs Superlattices

51%

Mizeikis V., Birkedal D., Langbein W., Lyssenko V. G., Hvam J. M.

Acta Physica Polonica A

|

1997

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vol. 92

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issue 5

923-928

EN

Binding of the heavy-hole excitons and biexcitons in GaAs/Al_{0.3}Ga_{0.7}As superlattices is studied using linear and nonlinear optical techniques. High biexciton binding energies characteristic of quasi two-dimensional biexcitons are observed in superlattices with considerable miniband dispersion.

8

Dielectric Function of Nitride Semiconductors: Recent Experimental Results

51%

Goldhahn R.

Acta Physica Polonica A

|

2003

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vol. 104

|

issue 2

123-147

EN

A review on the experimental determination of the dielectric function for hexagonal nitride semiconductors is presented. The peculiarities of nitride samples such as surface roughness and extended interface layers alter in comparison to an ideal film spectroscopic ellipsometry or reflectance spectra in a characteristic manner. It requires the application of multi-layer models for data analysis in order to determine reliable dielectric functions. Results of such an analysis for GaN covering a broad spectral range are given. Below the band gap, both ordinary and extraordinary components of the dielectric function tensor are determined for GaN as well as for AlN. The dielectric functions of MBE-grown InN characterised by a band gap of around 0.75 eV and a sputtered film exhibiting an absorption edge of around 1.9 eV are compared with results of first-principles calculations. Good agreement between theory and experiment is only found for the MBE-grown material providing further evidence that InN is a "narrow" band gap semiconductor. Finally, photocurrent measurements of a GaN Schottky-diode reveal the influence of electric fields on the shape of the excitonic absorption edge. The interpretation is supported by results of dielectric function calculations.

9

Characteristics of Optical and Photoconductive Properties in Bulk and Thin Film of TlS_2 Single Crystals

45%

Badr A.

Acta Physica Polonica A

|

2007

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vol. 112

|

issue 1

77-91

EN

The photoconductivity measurements were carried out for bulk single crystals of TlS_2 by using the steady state (dc) method in order to elucidate the nature of the dc photoconductivity in these crystals. The photoconductivity measurements were carried out in the temperature range of 77-300 K, excitation intensity range of 2150-5050 lx, applied voltage range of 10-25 V, and wavelength range of 400-915 nm. As a result of the dc photoconductivity, the temperature dependence of the energy gap width was described and the temperature coefficient of the band gap was determined. Reflectance and transmittance spectra of the TlS_2 thin films were measured in the incident photon energy range of 2.1-2.45 eV and in the temperature range of 77-300 K. With the aid of these spectra, the temperature dependence of optical transports and parameters were elucidated. In the low energy region of the studied incident photon energy range, the above-mentioned spectra were analyzed for describing the refractive index as a function of wavelength. As results of the refractive index-wavelength variations, both the oscillator and dispersion energies of the refractive index were thereafter estimated.

10

Ultrafast Processes in Semiconductor Structures

45%

Viña L.

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vol. 96

|

issue 5

573-592

EN

We review the dynamics of some of the more relevant optical processes in semiconductor quantum wells. We concentrate on the linear regime and study the time evolution of the light emission, using time-resolved photoluminescence spectroscopy. In intrinsic materials, excitonic effects determine their optical properties. Here we describe the formation and recombination of excitons, and the dependence of these processes on lattice temperature, exciton density, and energy of the excitation light pulses. We also describe the dynamics of the exciton's spin by optical orientation experiments. We discuss the principal mechanisms responsible for the spin flip of the excitons and clarify the role of the exciton localization. Finally, we will show that exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors. In doped quantum wells, we show that the two spin components of an optically created two-dimensional electron gas are well described by the Fermi-Dirac distributions with a common temperature but different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energy but accelerated by thermal spreading of the carriers.

11

Electro-Optical Properties of II-VI Superlattices

45%

Schillak P., Czajkowski G.

Acta Physica Polonica A

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2007

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vol. 112

|

issue 2

301-304

EN

We show how to compute electro-optical spectra of semiconductor superlattices in the region of interband electronic transitions. The method uses the microscopic calculation of eigenvalues and eigenfunctions and the macroscopic real density matrix approach to compute the electromagnetic fields and susceptibilities. The electron-hole screened Coulomb potential is adapted and the valence band structure is taken into account in the cylindrical approximation, thus separating light- and heavy-hole motions. We calculate the electro-optical functions, including the optical Stokes parameters and ellipsometric functions for the case of oblique incidence. Results are given for Zn_{1-x}Cd_xSe/ZnSe superlattices and a good agreement with experiments is obtained.

12

Optical Anisotropy of Quantum Disks in the External Static Magnetic Field

45%

Schillak P., Czajkowski G.

Acta Physica Polonica A

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2008

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vol. 114

|

issue 5

1349-1354

EN

We show how to compute the optical functions (the complex magneto-susceptibility, dielectric function, magneto-reflection and ellipsometric spectra) for semiconductor quantum disks exposed to a uniform magnetic field in the growth direction, including the excitonic effects. The optical response is calculated for an oblique incidence of the propagating electromagnetic wave and for input waves with different polarization. The method uses the microscopic calculation of nanostructure excitonic wave functions and energy levels, and the macroscopic real density matrix approach to compute the electromagnetic fields and susceptibilities. The electron-hole screened Coulomb potential is adapted and the valence band structure is taken into account in the cylindrical approximation, thus separating light- and heavy-hole motions. The novelty of our approach is that the solution is obtained in terms of known one-particle electron and hole eigenfunctions, since, in the considered nanostructure due to confinement effects accompanied by the e-h Coulomb interaction, the separation of the relative- and center-of-mass motion is not possible. We obtain both the eigenvalues and the eigenfunctions. The convergence of the proposed method is examined. We calculate the magnetooptical functions, including the optical Stokes parameters and ellipsometric functions for the case of oblique incidence. Numerical calculations were performed for InAs (disk)/ GaAs (barrier) disks. A good agreement with experiments was obtained.

13

Magnetic Ions in Studies of Semiconductor Quantum Well Structures

45%

Kossacki P.

Acta Physica Polonica A

|

1998

|

vol. 94

|

issue 2

147-154

EN

A few aspects of a relation between magnetic properties of Mn ion system and electronic states confined in CdTe based quantum wells are discussed. It is shown that the influence of the magnetic fluctuations on the states confined in the quantum well results in the reduction of the relative valence band offset with temperature. Also the decrease in the effective Zeeman splitting is observed by a modulation technique at high temperatures and is explained in terms of the modification of the selection rules for an exciton localised in thermal fluctuations. For the p-doped samples the giant Zeeman splitting is used to tune the polarization of a hole gas by a small magnetic field. The "competition-like" relation between the oscillator strength of exciton (X) and positive trion (X^{+}) is proved experimentally.

14

Optical properties of Quantum Disks: Real density matrix approach

32%

Czajkowski G., Silvestri L.

Open Physics

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2006

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vol. 4

|

issue 2

254-269

EN

We show how to compute the optical response of a Quantum Disk (QDisk) to an electromagnetic wave as a function of the incident wave polarization, in the energetic region of interband transitions. Both the TM and TE polarization in guided-wave geometry are analyzed. The method uses the microscopic calculation of Quantum Disk eigenfunctions and the macroscopic real density matrix approach to compute the effective QDisk susceptibility, taking into account the valence band structure of the QDisk material and the Coulomb interaction between the electron and the hole. Analytical expressions for the QDisk susceptibility are obtained for a certain model electron - hole potential. Using these expressions, all optical functions can be computed. Results for the absorption coefficient are computed for InAs/GaAs QDisks. Fair agreement with experiments is obtained.

Refine search results

Coupling and Strain Effects in Vertically Stacked Double InAs/GaAs Quantum Dots: Tight-Binding Approach

Strain and Spin-Orbit Effects in Self-Assembled Quantum Dots

Influence of Semiconductor Bands Bending on Exciton Photoluminescence

Dependence of Exciton Binding Energy on Magnetic Field in Parabolic Quantum Well Made of Diluted Magnetic Semiconductors

Shake-Up Processes in Photoluminescence of Two-Dimensional Holes in a High Magnetic Field

Motional Enhancement of Exciton Magnetic Moments

Binding of Biexcitons in GaAs/AlGaAs Superlattices

Dielectric Function of Nitride Semiconductors: Recent Experimental Results

Characteristics of Optical and Photoconductive Properties in Bulk and Thin Film of TlS_2 Single Crystals

Ultrafast Processes in Semiconductor Structures

Electro-Optical Properties of II-VI Superlattices

Optical Anisotropy of Quantum Disks in the External Static Magnetic Field

Magnetic Ions in Studies of Semiconductor Quantum Well Structures

Optical properties of Quantum Disks: Real density matrix approach