Search

Layer-by-layer epitaxial growth of CdSe on ZnSe has been observed at temperatures as low as 170–240 °C. Beyond this range, Reflection high energy electron diffraction (RHEED) specular spot intensity oscillations disappear, signifying a multilayer growth mode. While conventional epitaxial growth of CdSe at ∼300 °C results in formation of a rough layer-like structure, a method combining the epitaxial growth of a quasi-two-dimensional (2D) CdSe layer at 230 °C and subsequent annealing at 310 °C induces the formation of discrete CdSe quantum dots (QDs). Here we show that the formation of QDs by this method relies essentially on the layer-by-layer growth of CdSe at 230 °C, which allows an induced roughening of the as-grown 2D layer, during the subsequent annealing step. The layer-by-layer growth mode results possibly due to the nucleation of small and irregular precursor 2D clusters during growth at low temperatures, at the kink sites of which potential barriers for downward inter-layer transport of adatoms is low/absent.

We present a detailed study of combined exciton-electron processes in modulation doped quantum well structures contain a 2DEG of low density. We observed resonance exciton-electron processes in magnetic fields reveal in a separate narrow spectral lines, as well as nonresonant processes reveal in the exciton line broadening. Combine processes with participate of the excitons have been observed as well as with participate of trions (negatively charged exciton-electron complexes).

A review of our recent experiments on the studies of spin-lattice relaxation in semimagnetic semiconductor quantum wells (QWs) is given. Three series of magnetic nanostructures are studied: undoped (Cd,Mn)Te/(Cd,Mg)Te QWs; n-doped (Cd,Mn)Te/(Cd,Mg)Te QWs with two-dimensional electron gas; p-doped (Cd,Mn)Te/(Cd,Mg,Zn)Te QWs with two-dimensional hole gas. In the experiments, we use a technique which combines an injection of nonequilibrium phonons and an optical detection of the induced changes via the exciton photoluminescence. We observe that doping induces a significant acceleration of the spin-lattice relaxation process, which is due to spin-flip transitions between Mn ions and free carriers. We demonstrate experimentally that it is possible to tune the value of spin-lattice relaxation time by changing the intensity of the laser excitation in the doped QWs.

Properties of a two-dimensional electron gas with a density of 5×10 11 cm −2 in a ZnSe/(Zn,Be,Mg)Se quantum well structure have been investigated by means of polarized magneto-luminescence in fields up to 47 T and by an optically detected resonance (ODR) technique. With increasing magnetic field, the optical spectra of the electronic system changes its appearance from Landau-level-like with a linear shift of lines, to excitonic-like with a diamagnetic shift. The transition occurs at filling factor 2. Pronounced oscillatory features at even filling factors are found for the luminescence intensity and polarization degree, the energy of optical transitions, and in ODR signal.

We measure the spin–lattice relaxation time of Mn ions in Cd1−xMnxTe/Cd0.66Mg0.27Zn0.07Te (x=0.0035) semimagnetic quantum wells in the presence of a two-dimensional hole gas (2DHG) with sheet densities up to 3×1010 cm−2. We show that the presence of a 2DHG reduces the spin–lattice relaxation time by more than one order of magnitude. This reduction is much stronger than a similar effect induced by the presence of a two-dimensional electron gas, which is due to the heavier hole mass and stronger p–d interaction for holes with Mn ions. We demonstrate experimentally the possibility to tune the spin–lattice relaxation time by varying the density of the 2DHG through above-barrier photoexcitation.

We report on cw and time-resolved photoluminescence (PL) studies of Cd(Mn)Se/Zn(Mn)Se diluted magnetic semiconductor nanostructures grown by molecular beam epitaxy. Excitonic PL intensity, decay time and Zeeman splitting have been studied systematically as a function of Cd(Mn)Se nominal thickness, Mn concentration and sample design. Wave function mapping has been performed, evidencing the formation of semi-magnetic quantum disk islands in the samples with thick enough Cd(Mn)Se insertions.

The in-plane motion of excitons (X) and negatively charged excitons (X−) in a doped CdTe/CdMgTe quantum well has been measured at 4.2 K in magnetic fields up to 12 T, using spatially resolved photoluminescence (PL) spectroscopy. The populations of X and X− have been found to be in local dynamical equilibrium. The measured PL patterns in real space could therefore be described by considering free and mobile excitons in equilibrium with localised charged excitons.

Photoluminescence, photoluminescence excitation and reflectivity spectra of CdTe-based modulation-doped quantum well structures with 2DEG have been studied in magnetic fields. Resonant lines corresponding to combined optical transitions in which the photo-generation of excitons as well as trions is accompanied by the excitation of an additional electron from states below the Fermi level to states above the Fermi level were observed and analyzed.

We review results of magnetooptical studies related to various methods of spin splitting engineering possible in quantum well (QW) structures made of II-VI diluted magnetic semiconductors (DMS). The experiments made use of unique structures, based on Cd 1-x-y Mn x Mg y Te, with a precise in-plane profiling of either QW width or n-type doping intensity and of structures with digital profiling of the composition of constituent materials in the growth direction. The most typical version of the spin splitting engineering, available also in bulk DMSs, namely the one via an adjustment of (homogeneous) Mn composition, or via the lattice temperature or/and a magnetic field, was used to study magnetooptical properties of QW containing electrons as a function of their g-factor (tuned from g * eff = - 1.46 to + 55!). We also demonstrate three further methods of modifying the spin splitting in low-dimensional structures made of DMSs. They involve: (i) atomically precise control over Mn ion spatial distribution in the growth direction, (ii) reduction of the s-d exchange constant with increasing electron confinement, (iii) modification of Mn spin temperature by the electron gas heated by photo-excited carriers.

We report magneto-optical studies of ZnSe/(Zn,Mg)(S,Se) quantum wells with n-type modulation doping. Polarized photoluminescence and spin-flip Raman scattering spectroscopy is used to investigate the energy and spin structure of neutral and negatively charged excitons. The binding energy of charged excitons changes very little in magnetic fields up to 15 T. An inverted population of neutral exciton spin sublevels is found in magnetic fields; it is associated with the spin-dependent formation of charged excitons.

The in-plane optical anisotropy of ZnSe/BeTe multi-layered heterostructures has been studied by analyzing the linear polarization of the spatially indirect photoluminescence. A pronounced quantum-confined Pockels effect is reported in ZnSe/BeTe double-barrier structures. A tight-binding model is proposed which consistently interprets the experimental findings and suggests a new mechanism of the Pockels effect realized in type-II heterostructures. The model takes into account that the ZnSe/BeTe heterosystem exhibits a type-II band alignment with large band offsets and that in the zinc-blende lattice the chemical bonds are oriented in 〈1 1 1〉-like directions and, when shifted along the [0 0 1] principal axis, alternatively change their orientation in the (0 0 1) plane from [1 1 0] to [1 1 0] and vice versa.

An alternative technological approach for the realization of (Cd,Mn)Te nanostructures is presented. Focused ion beam lithography and subsequent thermal annealing are used for a lateral modulation of the bandgap, allowing both, the realization of quantum wires and quantum dots with adjustable size and a well-defined lateral potential. Due to the high quantum efficiency of the buried nanostructures, photoluminescence spectroscopy even on single quantum dots becomes accessible, allowing an experimental insight on optical transitions in three-dimensionally confined semimagnetic structures.

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn1 − x − yBexMnySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.

The electric field-induced in-plane optical anisotropy (Pockels effect) of ZnSe/BeTe multi-layered heterostructures has been studied by analysing the linear polarization of the spatially indirect photoluminescence. A pronounced quantum-confined Pockels effect was found in ZnSe/BeTe double-barrier structures. A tight-binding model is proposed which consistently interprets the experimental findings and suggests a new mechanism for the effect as realized in type-II heterostructures. The model takes into account that the ZnSe/BeTe heterosystem exhibits a type-II band alignment with large band offsets and that in the zinc-blende lattice the chemical bonds are oriented in 〈111〉 directions and, when shifted along the [001] principal axis, alternatively change their orientation in the (001) plane from [1 1 0] to [110] and vice versa. A light-emitting diode based on a single ZnSe/BeTe interface is demonstrated.

We report on reflectivity studies of negatively charged excitons (trions) and exciton states in ZnSe/(Zn,Mg)(S,Se) quantum wells with a two-dimensional electron gas of low density ( n e ⩽1.2×10 11 cm −2 ). The radiative damping (i.e. oscillator strength) of the trion states grows linearly with increasing electron concentration. The exciton radiative damping is weakly sensitive to the electron concentration, but we found a strong contribution of exciton–electron scattering processes to the broadening of the exciton resonance. An optical method to determine the electron concentration of diluted 2DEG is developed.

We report on a field-dependent photoluminescence (PL) emission rate for the transitions between band states in modulation-doped CdTe/Cd1−xMgxTe single quantum wells in the integer quantum Hall region. The recombination time observed for the magneto-PL spectra varies in concomitance with the integer quantum Hall plateaus. Furthermore, different PL decay times were observed for the two circular polarizations, i.e. for the transitions between the Zeeman split subbands of the Landau levels. We analyzed the data in comparison with the experimentally determined spin polarization of the conduction electrons and the Zeeman splitting of the valence band. Furthermore, we discuss the relevance of the spin polarization of the conduction electrons, the electron–hole exchange interaction and the spin-flip processes of the hole states for the PL decay time.

We report on resonant tunneling in MBE-grown semimagnetic (Zn, Mn)Se/BeTe double-barrier structures. Manganese has been incorporated in the quantum-well region at a concentration of up to 15%. The structures have been investigated using current–voltage measurements at low temperatures with an applied magnetic field up to 7 T. In an external magnetic field, no indication of a giant Zeeman splitting of the subbands in the semimagnetic quantum well could be detected, which is very surprising. Photoluminescence spectra as a function of applied voltage show a variety of different features, including type II electron–hole recombination from the conduction band of the emitter or the quantum well to the valence band of the BeTe barriers, which is a special feature of the BeTe–ZnSe system used here.

We study the Landau level fan diagram for charged and uncharged states in n-type modulation doped quantum wells with photoluminescence (PL) and photoluminescence excitation (PLE) measurements. In magnetic fields, the adjacent peak distance is different for PL and PLE. The peak energy for PL oscillates periodically in inverse magnetic fields while the peak energy for PLE does not oscillate. Time resolved PL shows that the difference of adjacent peak distance in PL and PLE is due to the hole relaxation and breaking of the transition selection rule. Within the experimental resolution, we attribute the different oscillatory behavior of the PL and PLE peak energies to different self-energy corrections for charged and uncharged states.

We propose a vertical transport of the photo-excited carriers under the influence of the two dimensional carrier gas (2DCG) in GaAs Ga 1−x Al x As heterojunctions before excitonic recombination. The vertical transport is responsible for a long photoluminescence (PL) rese time of the excitonic transitions as well as for the PL intensity oscillations with magnetic field. The vertical transport is proved for the first time by resonant PL intensity variation at the cyclotron resonance condition of the 2DCG, by which the effective masses of two dimensional electrons as well as two dimensional holes in the n- and p-type heterojunctions are determined.

Excitons in Cd 1-x Mn x Te and Cd 1-x-y Mg y Mn y .Te parabolic and half-parabolic quantum wells are investigated by magnetooptical methods. The diagonal and nondiagonal (in the harmonic oscillator quantum numbers) transitions are observed. Apart from Is excitons we observed also 2s and 3s states in the presence of a magnetic field. The results are discussed in terms of a simple variational calculation of the exciton energy. It is shown that a parabolic confining potential very effectively reduces the dimensionality of the excitons leading to large values of their binding energy even in relatively wide quantum wells. The truly parabolic shape of the confining potential in the investigated structures is proved by an analysis of spin splitting of the exciton states.

We present an optical study of novel ZnSe/BeTe heterostructures with a type-II band alignment based on beryllium chalcogenides. A strong exciton transition involving a confined electron and a quasibound hole state (both in the ZnSe layer) is observed in photoluminescence, photoluminescence excitation and reflectivity spectra. This exciton state is drastically broadened by increasing temperature due to enhanced exciton—acoustic phonon interaction. Pronounced features related to the quasibound hole states in the ZnSe layer (upto n = 4) are detected.

The theory of resonant optical reflection from long-period multiple quantum-well structures has been extended to take into account the difference between the dielectric constant of the barrier, e b , and the nonresonant contribution, e a , to the dielectric function in the wells. The reflection spectra taken from CdTe/CdMgTe quasi-Bragg quantum wells reveal, in addition to sharp excitonic features, smooth interference fringes determined by the mismatch e a ≠ ϵ b . Implications arising due to the dielectric constant mismatch as well as due to detuning from the resonant Bragg condition are analyzed and discussed.

Reflectance, photoluminescence and photoluminescence excitation spectra have been studied in modulation-doped CdTe/(Cd,Mg)Te quantum well structures containing a 2DEG of low density. Optical transitions related to the creation of excitons, bound exciton-electron complexes (trions) and exciton-cyclotron resonance (ExCR) states have been found in these spectra. A modification of the high-energy tail on the exciton absorption line into several discrete ExCR lines was observed when a magnetic field was applied. The polarization of the exciton and ExCR reflectance lines has been analyzed.

An investigation of strongly polarized lines in the spectra of epitaxially grown ZnSe films is presented. From the observed dependence of the polarization with the sample thickness, 〈1 1 0〉 aligned extended defects are identified. For the thinnest ZnSe films the defects associated with the Y line form first in the [ 1 1 0] direction. From this observation, we suggest that these defects are directly correlated with misfit dislocations. However, an asymmetry in the residual strain is not detected in these samples. In 2 μm thick nitrogen-doped ZnSe films, a considerable polarization of all lines associated with the nitrogen acceptor is observed while the Y line is not polarized. Linearly polarized luminescence is thus a sensitive optical tool to characterize structural defects in ZnSe films.

We report on the molecular beam epitaxy and the properties of Be-chalcogenides. Especially the interface between ZnSe and BeTe and BeTe/ZnSe superlattices have been studied by means of HRXRD (high resolution X-ray diffraction), HRTEM (high resolution transmission-electron-microscopy), RHEED (reflection-high-energy-electron-diffraction), photo-electron-spectroscopy (PES) and photo-luminescence (PL). A strong dependence of the structural and electronic properties of these superlattices (SL) on the interface configuration has been found. A high valence band offset of about 1 eV between BeTe and ZnSe has been determined by means of photo-luminescence and photo-electron-spectroscopy.

We have grown Be-chalcogenides for the first time by molecular beam epitaxy (MBE) on GaAs substrates. The lattice constants of both BeTe and BeSe are smaller than that of GaAs, therefore Be containing compounds such as (BeMg)Te or (BeZn)Se and (BeMgZn)Se are ideal candidates to replace ZnTe or Zn(SSe) and (ZnMg)(SSe) ternaries and quaternaries, respectively. In this contribution we present first results on the p-type doping of such layers using a modified nitrogen plasma source. The p-type dopability of BeTe as determined by van-der-Pauw measurements is comparable to that of ZnTe, free-hole concentrations of more than 1 × 1020cm−3 have been reached. Heavily p-doped ternary (BeMg)Te or (BeMn)Te layers are obtained easily which are fully lattice-matched to GaAs. On the other hand, (BeZn)Se and (BeMgZn)Se ternaries and quaternaries seem to behave similar to their sulphur-containing counterparts, an increase of the band gap leads to a decrease of the free-hole concentration. Based on these results, we propose an alternative for the p-type doping of wide gap II–VI semiconductors. The small lattice mismatch of BeTe to GaAs allows to grow BeTe ZnSe superlattices with excellent structural quality. We have calculated possible superlattice structures which should have an effective band gap comparable to or higher than ZnSe and at the same time are highly p-doped and almost lattice-matched to the GaAs substrate.

Beryllium chalcogenides are semiconductors with a large band gap. They have a higher-degree of covalent bonding than all of the other II–VI compounds. The bond energies of BeTe, BeSe and BeS are comparable to those of GaN. Due to their band gaps and their lattice constants they can be incorporated in quaternary mixed crystals which are lattice matched to GaAs. The hardness of II–VI materials containing beryllium offers new possibilities to enlarge the lifetime of laser diodes, emitting in the green and blue spectral range. First light-emitting diodes (LEDs) with beryllium as a constituent have been produced on GaAs substrates by molecular beam epitaxy (MBE). Double heterostructure devices were realized with quantum wells either of BeZnSeTe, ZnSe or BeZnSe. Special ohmic contacts consisting of BeTeZnSe graded superlattices were employed. Although for the first devices the growth conditions were not optimized, the lifetime at typical operating currents was very promising.

Chemical surface modification of porous silicon layers with porosity P =75% was performed by placing the layers into a tetraethoxysilane (TEOS)/ethanol/water solution. The duration of this treatment t T was varied from 3 h to 232 h. Infrared (IR) reflectance spectra show a decrease of Si–H vibrations and a strong increase of Si–O vibrations with increasing t T . Additionally, the mass variation of these layers was determined. Photoluminescence (PL) spectra of the modified samples show a continuous increase in the integral PL intensity (up to sixfold) and a slight red-shift (up to 0.07 eV) with increasing t T . Time-resolved PL measurements were performed in order to study the effect of tetraethoxysilane (TEOS) treatment on non-radiative luminescence mechanisms within the silicon crystallites. TEOS treatment was also applied on highly porous layers with P =80, 85 and 88% resulting in non-destructive subcritical drying and bright luminescence of these layers. © 1997 Elsevier Science S.A.

On the basis of low temperature photoluminescence structural defects in molecular beam epitaxy (MBE) grown ZnSe are identified. We investigate a series of ZnSe samples, in which the layer thickness has been varied from 50 up to 4300 nm. The photoluminescence of the Y0 line at an energy of 2.61 eV shows a degree of linear polarization up to 80% with respect to an 〈110〉 axis indicating a strong internal distortion preferentially aligned collinear to one of the four 〈110〉 axes. Excitation of the line is possible for different photon energies below the band gap. The photoluminescence excitation spectra is dependent sensitively on the detection wavelength of the monochromator. The very narrow Y0 line exhibits a fine structure of five subcomponents at zero magnetic field. The Zeeman-effect of the Y0 line is investigated. A strong anisotropy of the Zeeman-splitting is observed. In Faraday-configuration ( B → ∥ 〈 001 〉 ) the effective g-factor is geff = 0.7 .2 and the two magnetic subcomponents are shifted towards lower energies. The Y0 line is interpreted as an excitonic recombination associated with dislocations.

Light-emitting diodes with BeMgZnSe cladding layers and an active region formed by a ZnSe, BeZnSe or BeZnSeTe single quantum well are characterized by means of current-voltage measurements and electroluminescence at room temperature. A longterm stability exceeding 4000 h has been estimated in first experiments at low current densities (15 A cm−2). The emission wavelength was tuned between 452 and 518 nm by varying the composition of the quantum well material. Additionally, first results on the n-type doping of BeMgZnSe during molecular beam epitaxy have been obtained. The properties of ohmic contacts to n-type epilayers are discussed.

In this contribution, we present first results on the p-type doping of beryllium-containing II–VI compounds such as BeTe, (BeMg)Te, (BeZn)Se and (BeMgZn)Se grown by molecular beam epitaxy (MBE) using a nitrogen plasma source. These materials are a very promising alternative to ZnSSe and MgZnSSe ternaries and quaternaries, respectively. The p-type dopability of (BeMg)Te:N as determined by means of van-der-Pauw measurements and far-infrared reflectivity (FIR) spectroscopy almost exceeds that of ZnTe, and free hole concentrations NA - ND of more than 1 × 1020 cm−3 have been reached. Heavily p-doped ternary (BeMg)Te:N layers are obtained easily which are lattice matched to GaAs and have a high structural quality. The sheet and contact resistance of such layers have been below 5 × 10−3 Ωcm and 7 × 10−3 Ωcm2, respectively. Therefore, this material is the ideal replacement for ZnTe:N, which is used nowadays as top contact layer in ZnSe-based blue-green laser devices. (BeZn)Se and (BeMgZn)Se ternaries and quaternaries seem to behave similarly to their sulfur-containing counterparts; an increase of the band gap leads to a decrease of the free hole concentration. Several (BeMgZn)Se/(BeZn)Se single quantum well (SQW) and multiple quantum well (MQW) light-emitting devices (LEDs) have been fabricated which all show a bright, deep blue emission at low turn-on voltages. One reason for the low operating voltage is that the valence band of BeTe is aligned rather well with that of GaAs, and together with the high p-type dopability of BeTe, a smooth transition from the p-GaAs substrate to p-ZnSe via a BeTe/ZnSe pseudograding is possible.

Beryllium-containing ZnSe-based compound semiconductors introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures. An overview of the advantages of beryllium chalcogenides is given here. A variety of BeTe-ZnSe and BeMgZnSe-ZnSe structures has been fabricated by molecular beam epitaxy, and their structural, optical and electrical properties have been investigated. The main aspects are the lattice matching of BeTe with its high lying valence band and high p-type dopability, the possibility to grade the valence band of GaAs down to the one of ZnSe by using a lattice matched ZnSe-BeTe pseudograding, the use of beryllium instead of sulphur for high band gap claddings, as well as the expected strengthening of beryllium compounds as compared to the standard materials used today on the basis of ZnMgSSe.

We have investigated g-factor oscillations of electrons as well as of photoexcited holes in a quasi-two dimensional electron gas (2DEG) of CdTe/(CdMg)Te modulation-doped single quantum wells by means of photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. The oscillation amplitude is found to be related to Landau level numbers and filling factors as, predicted by the theory. The exchange constants A NN , of the g-factor enlargement have been obtained experimentally for the first time. The observed enlargement of the g factor in CdTe/(CdMnMgTe) heterostructures, where the sign of the intrinsic g factor is reversed by the sp-d exchange interaction, confirms the results obtained for nonmagnetic CdTe/(Cdmg)Te heterostructures.

The origin of a compensation mechanism in highly halogen doped CdTe grown by molecular beam epitaxy is investigated by means of electrical transport, photoluminescence and positron annihilation measurements. From the data one can identify vacancies to play an important part in the compensation mechanism at high doping levels. The behaviour of highly doped compensated CdTe layers in the electrical transport measurements as well as in photoluminescence turned out to be dominated by potential fluctuations, which are probably created due to the random distribution of donor impurities and compensating defects.

The internal site symmetry of exciton binding traps and the local structure are investigated revealing cadmium vacancy related defect complexes in aluminum-, chlorine-, iodine- and indium-doped CdTe films. The Zeeman experiments on the low temperature photoluminescence are discussed in terms of a modified effective Hamiltonian. The corresponding g-factor of the electron and the hole parameters K and L have been determined. From the studies of line intensities associated with the defect complexes as a function of the MBE growth conditions, cadmium vacancies are identified as constituents of these complex defects. The proposed microscopic models are compared with the observed properties of the bound exciton recombinations. Pronounced acoustical phonon replica and luminescence polarized parallel to a [110] axis are obviously evidences for a preferential alignment of the Cd vacancy related complex centers in the cubic CdTe lattice. A degradation of the luminescence intensity of complex associated lines with time has been detected.

By using a novel RF plasma based cracking source for group V dopants like As, we have been able to show that atomic As occurs in the plasma discharge of a combined Ar/As plasma cell. As atoms are expected to have a much higher sticking coefficient during molecular-beam epitaxy (MBE) of II-VI semiconductors as compared to the tetramers and dimers generated by standard effusion cells. CdTe films doped with plasma cracked As have carrier concentrations of about p = 2 x 10 16 cm -3 . Furthermore, we have investigated the dilution of a N-plasma with Ar to achieve a better control over the p-type doping of CdTe. At a dilution ratio of 10 (Ar :N) the net hole concentration remains the same (p = 2 x 10 17 cm -3 ) compared to pure N while the photoluminescence (PL) properties of the layers are improved. With the same dilution ratio we have reached p = 5 x 10 17 cm -3 in a ZnSe layer.

The anisotropy of the Zeeman splitting of bound excitons is studied as a function of the magnetic field direction in CdTe films doped with different dopants. The observed internal distortion is associated with defect complexes consisting of n-type dopants chlorine, iodine and indium compensated by nearby Cd vacancies. The Zeeman splittings observed in luminescence experiments at low temperatures are discussed in terms of a modified Hamiltonian taking into consideration the reduced site symmetry of the traps. The corresponding g-factor of the electron, the hole parameters K and L and an additional distortion parameter ratio W/D have been determined for the prominent A-, A'- and C-line. The diamagnetic shifts are discussed revealing the reduced mass of the bound excitons. After several months of storage at room temperature distinct changes of the photoluminescence are observed indicating that formation of defects takes place with time. Proposed microscopic models are discussed and compared with our experimental findings.

We have realized a spin superlattice structure based on a Cd 0.9 Mn 0.1 Te Cd 0.9 Mg 0.1 Te heterostructure, where the valence and the conduction band align with zero band offset. Due to the large spin-splitting of the conduction- and valence band-edges in the semimagnetic (CdMn)Te layer and the vanishing spin-splitting in the nonmagnetic (CdMg)Te layer a significant spatial separation of spin-up and spin-down carriers is achieved with increasing magnetic field strength. The formation of the spin superlattice is proved by the Zeeman pattern of the excitonic states. The Zeeman pattern directly reflects the energy shift of spin-up states localized in the nonmagnetic and spin-down states in the semimagnetic layers. In addition, the exciton magnetic polaron is used as a sensitive probe for the small confinement potentials controlled by the externally applied field, demonstrating the exciton localisation in the spin dependent quantum wells of a spin superlattice.

ZnTe and CdSe with lattice constants of 6.10 and 6.09 A are nearly lattice matched to InAs with a lattice parameter of 6.06 A. InAs is available as a high quality substrate material for molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED) and X-ray diffraction studies have been performed to investigate the nature of the MBE growth on InAs substrates with and without the epitaxial growth of an InAs buffer layer. For the quaternary Zn 1 − x Mg x Se y Te 1 − y (ZnMgSeTe), we could tune the band gap through the whole visible range. Lattice matched to the InAs substrate, rocking curve widths as low as 38 arcsec for ZnMgSeTe could be obtained. We will present results on structural and optical investigations of these layers and related quantum well structures. A pronounced curvature in the dependence of the band gap on composition could be found not only for ZnSeTe and MgSeTe, but also for ZnMgSe and ZnMgTe. A type-II band alignment between ZnTe and ZnMgSeTe allows us to measure band offsets directly via photoluminescence measurements in particular single quantum well structures.

Different kinds of oscillatory behaviour have been observed in CdTe based modulation doped single quantum wells (MDSQWs). Besides the oscillations of the Landau level half width, the Landau level energy and the electron g-factor, we have found that the g-factor of photo-excited holes oscillates too. This oscillatory property of the g-factor of photo-excited holes in a two-dimensional electron gas (2DEG) is also reflected in the oscillation of the integrated intensity of σ+ and σ− polarised luminescence emission.

Circular polarized photoluminescence induced by magnetic fields in the Faraday geometry has been studied in CdTe Cd 1−x Mn x Te superlattices. An inversion of the sign of the magnetic hysteresis has been observed under excitation by nonpolarized light in the presence of magnetic fields. This effect is due to a nonequilibrium magnetization of Mn-ions induced by the spin orientation of photocarriers on Zeeman sublevels and is explained in terms of the dynamic polarization via a specific Overhauser process. This nonequilibrium magnetization has a long relaxation time of about 103 s at temperatures of 1.6 K caused by the slow thermo-activated spin relaxation of antiferromagnetic clusters of Mn-ions located in the barrier layers.

We report on optical studies of exciton magnetic polarons in CdTe/Cd 1−x Mn x Te quantum well structures with Mn contents 0.4 ≤ x ≤ 0.8. The magnetic polaron energies and formation times are measured by cw- and time-resolved photoluminescence under selective excitation. We find an overall increase of the polaron energy with increasing Mn-concentration in the whole range of Mn contents studied and attribute it to diffusion induced changes of the Mn profile at the interfaces between nonmagnetic and semimagnetic layers. In addition, we find that in these quantum structures the formation time of the polaron is predominantly determined by the Mn content and not by quantum confinement effects. The large band gap discontinuity (up to 1.2 eV) causes large splittings between heavy- and light-hole-states and leads to a strong anisotropy in the suppression of the magnetic polaron formation by magnetic fields applied parallel and perpendicular to the structure growth axis. For both orientations of magnetic field the polaron suppression is qualitatively described by model calculations.

We have compared the optical properties of CdTe/(CdMg)Te- and CdTe/(CdMn)Te-heterostructures by cw and time-resolved photoluminescence spectroscopy in the temperature range from 1,7 K up to room temperature, CdTe/(CdMg)Te-heterostructures show brighter luminescence intensities and longer luminescence decay times at room temperature compared to CdTe/(CdMn)Te-heterostructures. These findings are caused by a more efficient transport of carriers from the barrier into the well and by a reduced importance of non-radiative recombination processes in CdTe/(CdMg)Te quantum wells. Consequently (CdMg)Te is a promising material for opto-electronic devices.

We present a comparative study of two different spectroscopic techniques in order to determine the valence band offset in CdTe/(CdMn)Te quantum wells. On the one hand the energy difference between heavy- and light-hole excitons as a function of the heavy-hole transition energy is employed. We determine a valence band offset of Q v =0.30 for CdTe/(CdMn)Te, which is valid in the whole range of investigated Mn-contents up to x=0.27. An alternative determination of the valence band potential height is based on the tuning of potential heights due to external magnetic fields in semimagnetic quantum wells. Analysing the Zeeman splitting of the heavy-hole exciton in CdTe/(CdMn)Te quantum wells with different Mn-contents, we demonstrate a significant decrease of the valence band offset with increasing Mn-content in the barrier. From the comparison of the two different spectroscopic methodes including published data, we conclude that the valence band offset derived from the Zeeman splitting in CdTe/(CdMn)Te quantum wells with high Mn-content is strongly underestimated.

Circular polarized photoluminescence induced by magnetic fields in Faraday geometry has been used for the study of spin relaxation processes in CdTe/Cd 1− x Mn x Te short-period superlattices. The spin relaxation has been found to be effective for the free carriers with kinetic energy and to be suppressed for carriers localized at fluctuations of magnetization. The effect of the inversion of sign of the magnetic hysteresis has been observed under excitation by nonpolarized light in the presence of magnetic field. The effect is due to a nonequilibrium magnetization of the Mn-ions which is induced by spin orientation of carriers on Zeeman sublevels in magnetic fields. This magnetization has a long relaxation time of about 10 3 s at temperatures of 1.6 K caused by the “frozen” magnetic moments of spin-glass clusters situated in barrier layers.

We have grown the ternary alloy Cd 1- x Mg x Te as well as Cd 1- x Mg x Te/CdTe quantum well structures by molecular beam epitaxy - to our knowledge for the first time. Cd 1- x Mg x Te exhibits some very interesting features: The band gap has been determined as a function of Mg concentration, and a band gap of 3.0 eV was found for zincblende MgTe at room temperature. Cd 1- x Mg x Te thin films with Mg concentrations of up to 0.75 were fabricated, which corresponds to a band gap of 2.8 eV at low temperatures. Therefore, the whole visible band gap range (at room temperature) can be covered with Mg concentrations between 0.30 (red) and 0.75 (blue). Bulk MgTe crystallizes in the wurtzite structure, but zincblende MgTe could be grown on (100) oriented CdTe substrates up to a layer thickness of approximately 500 nm. The lattice mismatch between zincblende MgTe and CdTe was found to be as small as 1.0%. The growth of cubic MgTe could be followed by reflection high energy electron diffraction (RHEED) oscillations. In general, excellent structural quality could be reached, which is demonstrated by the FWHM of 22 arc sec for a Cd 1- x Mg x Te thin film with 0.44 Mg concentration on a Cd 1- x Zn x Te nearly lattice matched substrate. The Poisson number of Cd 1- x Mg x Te has been determined by X-ray diffraction as a function of Mg concentration. Cd 1- x Mg x Te/CdTe single quantum well structures have been fabricated with a large confinement energy of up to 0.8 eV. The photoluminescence spectra show exciton lines with very narrow linewidths. We are able to observe excited exciton states, and from the energetic difference between 1s and 2s heavy hole exciton lines we deduce exciton binding energies. Very bright luminescence could be seen even at room temperature, which is an indication of a large exciton binding energy and an effective radiative recombination.

We report on the optical study of exciton magnetic polarons in structures with semimagnetic Cd 1− y Mn y Te quantum wells sandwiched between nonmagnetic Cd 1− x Mg x Te or semimagnetic Cd 1− x Mn x Te barriers. We have found magnetic polarons with energy of 9 meV formed from quasi-two-dimensional excitons in Cd 0.9 Mn 0.1 Te/Cd 0.7 Mg 0.3 Te quantum wells with thicknesses varying from 18 to 100 A, but no polaron formation has been found for quasi-three-dimensional excitons situated in quantum wells thicker than 300 A. The experimentally established effect of the dimensionality of the system on the stability of exciton magnetic polarons is in qualitative agreement with theoretical predictions. In Cd 1− y Mn y Te/Cd 1− x Mn x Te structures contributions of the carrier exchange with Mn spins located in the well and in the barrier layers are responsible for the process of magnetic polaron formation. Relation of these contributions has been demonstrated to be controlled by the quantum well width.

We have performed magneto-optical studies on a series of thin CdTe/(CdMn)Te single quantum wells at low temperatures and in external magnetic fields up to 7.5 T. As a consequence of the high manganese content of the barrier layers ( x 20%) the antiferromagnetic exchange interaction between the manganese spins exhibit spin-glass behaviour. By comparison of the Zeeman splitting of barrier and quantum well excitons, we found a reduced antiferromagnetic coupling of Mn-spins near the heterointerface. We assume that the missing of nearest-neighbour manganese spins adjacent to the transition from magnetic to nonmagnetic materials results in a reduced probability of finding antiferromagnetically coupled Mn-Mn clusters. The mechanism of interface-related frustrated antiferromagnetism leads to the astonishing result of comparable or even larger Zeeman splittings of quantum well excitons compared with those observed in barrier layers. If we introduce a second interface for the magnetic layer as can be realized with ultrathin superlattices, missing nearest neighbours produce a more pronounced frustration of antiferromagnetic coupling due to the quasi two-dimensional character of the magnetic layer, which is correlated with a higher percolation threshold.

Chlorine has been used successfully for the first time for n-type doping of CdTe epitaxial layers (epilayers) grown by photo-assisted molecular beam epitaxy. Similar to n-type doping of ZnSe layers, ZnCl 2 has been used as source material. The free-carrier concentration can be varied over more than three orders of magnitude by changing the ZnCl 2 oven temperature. Peak mobilities are 4700 cm 2 V −1 s −1 for an on concentration of 2×10 16 cm −3 and 525 cm 2 V −1 s −1 for 2×10 18 cm −3 . The electrical transport data obtained by Van der Pauw configuration and Hall structure measurements are consistent with each other, indicating a good uniformity of the epilayers. In photoluminescence the donor-bound-exciton emission dominates for all chlorine concentrations. This contasts significantly with results obtained for indium doping, commonly used for obtaining n-type CdTe epilayers. The superiority of chlorine over indium doping and the influence of growth parameters on the behaviour of CdTe:Cl layers will be discussed on the basis of transport, luminescence, secondary ion mass spectroscopy and X-ray photoelectron spectroscopy data.

We report the first investigation of two-dimensional magnetic polarons formed from excitons localized in very thin nonmagnetic CdTe quantum wells confined by Cd1−xMnxTe diluted-magnetic-semiconductor barriers. Analysis of the Stokes shift of the exciton luminescence peak under resonance excitation, which is a dramatically decreasing function of temperature and magnetic field, allows us to determine the magnetic polaron energy. The polaron energy measured as a function of quantum well width is found to be 25 meV in a 6 A thick quantum well and to vanish in wells thicker than 30 A.