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  1. 74 Kossut J.

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1
Publication available in full text mode
Content available

Low-Dimensional Structures of Diluted Magnetic (Semimagnetic) Semiconductors - a Subjective Review

100%
Kossut J.
Acta Physica Polonica A
|
2001
|
vol. 100
|
issue Supplement
111-138
EN
A review on selected areas of research on low-dimensional structures consisting of diluted magnetic (semimagnetic) semiconductors is attempted. In the introductory part, after short historical preliminaries, we describe some early ideas and speculations concerning properties of such structures and we confront them with real results obtained in the course of their experimental investigations. Several issues are addressed to but, in view of the richness of the field, the review is by no means not pretending to be an exhaustive one. We begin with early studies of metal-insulator-semiconductor structures and progress to structures grown by molecular beam epitaxy. We discuss, in particular, the idea of a spin superlattice, type-I-to-type-II transition, wave functions mapping, and detailed determination of the profiles of realistic quantum wells. We put special emphasis on digital growth and, specifically, parabolic and half-parabolic quantum wells, use of tunable g-factor in studies of trions, self-assembled quantum dots, spin injection and, finally, on hybrid structures of ferromagnets and diluted magnetic semiconductor structures. The focus of the present paper is on work done in the Institute of Physics of the Polish Academy of Sciences, mostly in the Laboratory of Growth and Physics of Low-Dimensional Crystals.
2
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Exchange Integrals in Hg_{0.95}Mn_{0.05}Se Obtained by the Wave Shape Analysis of Shubnikov-de Haas Oscillations

64%
Dybko K., Kossut J.
Acta Physica Polonica A
|
1993
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vol. 84
|
issue 4
697-700
EN
The Shubnikov-de Haas oscillations were measured in Hg_{0.95}Mn_{0.05}Se at liquid helium temperatures, in magnetic fields up to 13 tesla. The analysis of the experimental data was performed by means of the wave shape method. The dependence of effective g-factor of the conduction electrons and difference Dingle temperature on magnetic field was determined. Non-zero difference Dingle temperature is a clear evidence of spin dependent scattering in this material. The sp-d exchange constants consistent with extracted spin splitting and difference Dingle temperature, α = -0.39 ± 0.06 eV, β = 0.95 ± 0.07 eV, are in good agreement with the values obtained from magnetooptics.
3
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Exciton Binding Energy in Extremely Shallow Quantum Wells

64%
Kossut J., Furdyna J.
Acta Physica Polonica A
|
1995
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vol. 87
|
issue 2
528-532
EN
The usual approach to the problem of excitons in semiconductor quan­tum wells is to assume that both the electron or the hole are primarily localized in the potential well regions defined by the band offsets, i.e., that the quantum wells are deep. We re-examine the problem of the exciton in the presence of a very shallow square well potential due to the (small) conduc­tion and valence band offsets in a semiconducting heterostructure. We show that the combined effects of the shallow well and the Coulomb interaction between the electron and the hole are equivalent to an effective potential acting on the center-of-mass of a three-dimensional exciton. We calculate the shape of such a potential and show it to be satisfactorily approximated by the potential of a parabolic well.
4
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Dependence of Exciton Binding Energy on Magnetic Field in Parabolic Quantum Well Made of Diluted Magnetic Semiconductors

64%
Kyrychenko F., Kossut J.
Acta Physica Polonica A
|
1998
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vol. 94
|
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
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Content available

Optical Properties of a Semimagnetic Quantum Well in a Proximity of a Superconducting Film

52%
Lebecki K., Kłopotowski Ł., Kossut J.
Acta Physica Polonica A
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2005
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vol. 108
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issue 5
741-748
EN
We consider, via numerical calculations, a hybrid structure made of a semimagnetic Cd_{1-x}Mn_xTe quantum well deposited in a close proximity to superconducting niobium film. We simulate photoluminescence and the Faraday rotation spectra, modified by the presence of vortices in this type II superconductor. The magnitude of the evaluated effects is small - the vortex induced spectral line shape variation is of the order of 1% at 1 K and 0.1% at 3 K and is expected to occur mainly in the field range between 0.03 T and 0.05 T.
6
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Stimulated and Laser Emission in CdZnTe/CdMnTe Double Quantum Well Heterostructures

52%
Kowalczyk L., Karczewski G., Wojtowicz T., Kossut J.
Acta Physica Polonica A
|
1995
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vol. 88
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issue 4
787-790
EN
Stimulated emission by optical excitation has been investigated in CdZnTe/CdMnTe quantum well heterostructures. Laser action has been achieved at 4.2 K and at 77 K with relatively low threshold values of the excitation intensity. Photοluminescence excitation spectra of the stimulated emission were obtained indicating that the optical gain involves exciton-exci­ton inelastic scattering.
7
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Consequences of Spatial Fluctuation of Coulomb Potential in AlGaAs with DX Centers

52%
Wilamowski Z., Kossut J., Jantsch W., Ostermayer G.
Acta Physica Polonica A
|
1991
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vol. 80
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issue 3
385-388
EN
Electric charges on randomly distributed impurities in semiconductors produce a spatially fluctuating potential. When the impurities are partially filled, their occupancy is not random but there appears a spatial correlation of the impurity charges appearing due to the inter-impurity Coulomb interactions. We show that when these interactions are taken into account then (i) the activation energy of the electron concentration, (ii) thermal emission kinetics, (iii) capture kinetics, (iv) persistent photoconductivity kinetics and (v) the electron mobility (in a steady state as well as during transients) in GaAlAs:Si can be explained in a consistent way. The energy diagram con cerning the DX center levels with respect to minima of the conduction band as well as the capture and emission barriers (including the effect of the alloy splitting) is constructed within an approach making use of the notion of the impurity self-screening.
8
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Content available

Injection of Optically Generated Spins through Magnetic Nonmagnetic Heterointerface: Ruling out Possible Detection Artifacts

52%
Ghali M., Kossut J., Janik E.
Acta Physica Polonica A
|
2004
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vol. 106
|
issue 2
207-214
EN
We report on injection of optically created spin-polarized carriers into CdTe-based materials. The injected spins are initially aligned in a diluted magnetic semiconductor CdMnTe layer located on the top of CdMgTe layer in CdMnTe/CdMgTe spintronic generic model structures. A critical discussion of possible artifacts that may complicate the spin detection and its quantitative analysis is given. Although the spin injection efficiency, 80%, has been found by us to be basically independent of the thickness of the spin detecting layer, there is an essential difference between thin and wide detectors related to the strain-induced lifting of the valence band degeneracy in the former, when assessing the efficiency of the spin injection. Most importantly, we observe an effect of switching the spin injection process on and off by an external magnetic field variation within a relatively narrow field range. This effect can be achieved by a careful design of the interface between the diluted magnetic semiconductor and the non-magnetic semiconductor.
9
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Changing the Properties of the CdTe/ZnTe Quantum Dots by in situ Annealing during the Growth

52%
Wojnar P., Karczewski G., Wojtowicz T., Kossut J.
Acta Physica Polonica A
|
2007
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vol. 112
|
issue 2
283-288
EN
We present an attempt to control the properties of CdTe/ZnTe self-assembled quantum dots during their formation in the process of molecular beam epitaxy. Namely, the structures were in situ annealed at various temperatures and annealing times after the formation of quantum dots, before the deposition of a capping layer. Depending on the annealing parameters, the dots exhibit different optical properties which were studied by means of spatially resolved photoluminescence. From the analysis of these results, the information about relative changes of the average size and sheet density of quantum dots was extracted.
10
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Monte Carlo Simulations of Spatial Correlations of Charged Centers in δ-Doped Layers

52%
Sobkowicz P., Wilamowski Z., Kossut J.
Acta Physica Polonica A
|
1992
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vol. 82
|
issue 4
645-648
EN
Results of Monte Carlo simulations of a 2D system of charged donors are presented. They enable to study the effects related to a spatial correlation of donor charges located on a random donor matrix. A qualitative difference between DX^{+} and DX¯ models is observed. In the first case, strong temperature dependence of the correlations and a "freezing-like" behaviour is found. The origin of the freezing is traced to the random distribution of donor sites by comparing the system with a liquid-like model where the charges may assume arbitrary positions within a plane. In the second case only nearest-neighbour correlations are observed. The simulations have direct application in analysis of the behaviour of the DX centers in the GaAs planarly doped with Si.
11
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Weakly Diluted Magnetic CdTe/Cd_{1-x}Mn_{x}Te Semiconductor Structures Grown by MBE

52%
Wojtowicz Τ., Karczewski G., Kossut J.
Acta Physica Polonica A
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1995
|
vol. 88
|
issue 4
631-642
EN
In this paper we review the results of our effort to grow layers and low-dimensional structures containing Cd_{1-x}Μn_{x}Te diluted magnetic semi­conductor with relatively high values of Mn molar fraction x. A high quality of the structures grown so far is demonstrated by making use of results of several selected experiments. In the case of the epilayers having bulk-like thickness with x ≥ 0.7 we discuss, in particular, the magnetic phase dia­gram as well as we report on collective spin excitation (magnons) observed in Raman scattering experiments. The discussion of the growth of different quantum wells, including rectangular, digital, parabolic and wedge quantum wells, is accompanied by a brief overview of their optical and magnetoop­tical properties. These results include first measurements concerning mag­netic polarons in quantum wells embedded in Cd_{1-x}Mn_{x}Te barriers with 0.4 ≤ x ≤ 0.8. Finally, we report on the present status of the search for dimensional effects in the spin-glass phase performed with the use of our specially designed superlattice structures.
12
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Coulomb versus Madelung Gap: Ordering in a System of Point Charges

52%
Sobkowicz P., Wilamowski Z., Kossut J.
Acta Physica Polonica A
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1993
|
vol. 84
|
issue 3
515-518
EN
Spatial correlations of impurity charges in a mixed valence regime are studied with the use of Monte Carlo simulations. The influence of various kinds of disorder on the one-particle density of states is determined. A continuous transitions from a "soft" Coulomb gap (density of states vanishing only at the chemical potential) to a "hard" gap (with a finite range of energies with vanishing density of states) is found, driven by decreasing amount of built-in disorder in the system. The "hard" Coulomb gap resembles the Madelung gap, found in crystalline arrangements of charges. The similarity reflects the fact that both the Coulomb and Madelung gaps are manifestations of the same phenomenon, resulting from ordering of the positions of point charges, the only difference being the range of correlations.
13
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Potential Fluctuations in HgSe:Fe,Ga - a Numerical Study

52%
Sobkowicz P., Wilamowski Z., Kossut J.
Acta Physica Polonica A
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1993
|
vol. 84
|
issue 4
645-648
EN
We study spatial correlation of donor charges in HgSe:Fe,Ga using a few simple quantities (mean electrostatic energy of various donor sites, fluctuations of the electrostatic potential). The study uses Monte Carlo simulation techniques. We are able to indicate general trends in the behavior of the correlation when proportion of correlating (Fe^{3+}) and noncorrelating (Ga) charged centers in the sample is changed.
14
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Graded Quantum Well Structures Made of Diluted Magnetic Semiconductors

52%
Wojtowicz T., Kutrowski M., Karczewski G., Kossut J.
Acta Physica Polonica A
|
1998
|
vol. 94
|
issue 2
199-217
EN
In this paper we review results of studies of two types of spatially graded quantum well structures containing various layers of diluted magnetic semiconductors Cd_{1-x}Mn_{x}Te or Cd_{1-x-y}Mn_{x}Mg_{y}Te. The design of the structures has been recently proposed by us and suitable samples have been grown by a modified molecular beam epitaxy method. In the structures of the first type a digital profiling of the composition of the constituent material in the growth direction allowed to produce quantum wells with a specifically required shape of the confining potential (including parabolic, half-parabolic, triangular, and trapezoidal). Such samples were used for (i) determination of the conduction and valence band offsets in MnTe/CdTe and MgTe/CdTe systems, (ii) for the demonstration of an enhanced exciton binding in a parabolic confining potential as well as for (iii) demonstration of the possibility of "spin-splitting engineering" in diluted magnetic semiconductors quantum structures. In the second type of the structures, a precise in-plane profiling of either quantum well width or the barrier width or n-type doping intensity was realized. These structures were subsequently used for studies of the evolution of optical spectra with an increase in the concentration of confined two-dimensional gas of conduction electrons.
15
Content available remote

Spatial Correlation of Impurity Charges Induced by Coulomb Interactions - Application to DX Centers in GaAs

52%
Kossut J., Wilamowski Z., Dietl T., Świątek K.
Acta Physica Polonica A
|
1991
|
vol. 79
|
issue 1
49-58
EN
The electron mobility enhancement observed in heavily doped GaAs under hydrostatic pressure is interpreted in terms of spatial correlation between the donor charges within partially occupied system of impurities induced by strong inter-donor Coulomb interaction. A simple analytic theory is given for both DX^{0} and DX^{-} models of the impurity state. The mobility is shown to increase together with pressure in both models. Estimates of the energy of the DX level are strongly perturbed by the inter-donor Coulomb interactions.
16
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Improvement of Electrical Properties of Hg_{1-x}Zn_{x}Se upon Doping with Fe

52%
Dobrowolski W., Grodzicka E., Kossut J., Witkowska B.
|
EN
Results of measurements of electron concentration and mobility in mixed crystals of Hg_{1-x}Zn_{x}Se (0 ≤ x ≤ 0.07) doped with resonant Fe donors (0 ≤ n_{Fe} ≤ 5 × 10^{19} cm^{-3}) at liquid helium temperatures are presented. The data show that there is a considerable improvement of the electrical properties of the material when Fe impurities are present. The analysis of the mobility in terms of the scattering from ionized centers (accounting for possible spatial correlation of impurity charges) and the alloy scattering is in agreement with the measured data.
17
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δ-Doped CdTe/Cd_{1-x}Mn_{x}Te Multiple Quantum Wells Investigated by Photoreflectance Spectroscopy

46%
Sitarek P., Misiewicz J., Karczewski G., Wojtowicz T., Kossut J.
Acta Physica Polonica A
|
1995
|
vol. 88
|
issue 5
901-904
EN
Room temperature photoreflectance spectroscopy was used to investigate CdTe/Cd_{1-x}Mn_{x}Te multiple quantum wells grown by MBE. Structures were indium δ-doped into the well or into the barrier. The value of heavy and light hole subbands splitting was measured and compared to the calculated ones. The influence of the position of δ-doping on the measured spectra was shown.
18
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Epitaxial Growths of II-VI Compounds on (110) Substrates

46%
Cywiński G., Wojtowicz T., Kopalko K., Karczewski G., Kossut J.
Acta Physica Polonica A
|
1998
|
vol. 94
|
issue 2
281-284
EN
We studied epitaxial growth conditions of II-VI semiconductors on (110) substrates, which is indispensable for fabrication of T-shaped quantum wire structures. We experimented with different types of (110)-oriented substrates and monitored the surface quality of deposited layers in situ by reflection high energy electron diffraction and ex situ by photoluminescence. The aim of this work is to find optimum growth conditions of II-VI compounds on a cleaved edge of a superlattice as required by the overgrowth method.
19
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Search for T-Shaped Quantum Wires in CdTe/CdMg(Mn)Te System

46%
Cywiński G., Wojtowicz T., Karczewski G., Kossut J., Gębicki W.
Acta Physica Polonica A
|
1998
|
vol. 94
|
issue 2
277-280
EN
We attempted to produce and to investigate T-shaped wire structures of II-VI compounds. Our samples were grown on GaAs hybrid substrates in a two-stage growth process. The photoluminescence measurements resulted into two different possible polarization behaviors of recorder signal. We interpret one of these behaviors as due to quantum wire formation.
20
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Local Electric In-Plane Potential Fluctuations in the CdTe/CdMgTe Based Multiple Quantum Wells

46%
Nogajewski K., Karpierz K., Łusakowski J., Grynberg M., Kossut J.
Acta Physica Polonica A
|
2008
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vol. 114
|
issue 5
1259-1265
EN
Unusual features in the magnetophotoconductivity spectra registered under far infrared illumination of the CdTe/CdMgTe based multiple quantum wells, uniformly n-doped are presented. It is shown that each spectrum exhibits one or two peaks of non-symmetrical shape, with position of their maxima dependent on the voltage applied to the sample. The peaks, observed in the configuration of the crossed electric and magnetic fields, are strongly shifted by a relatively weak in-plane electric field - of the order of 10-50 V/cm. Two different approaches to explain the observed influence are presented. Both are based on a two-step process leading to the photoconductivity signal. The first approach assumes that only the process of photon absorption is influenced by the external fields, the second one assumes that only the process of phonon assisted electron transfer from the excited donor state into the conduction band is influenced by the external fields.
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