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High-resolution X-ray di ractometer was used to study structural quality, lattice parameters and mis t strain in p-ZnTe/n-CdTe heterojunctions grown by the molecular-beam epitaxy technique on two di erent (001)-oriented substrates of GaAs and CdTe. The X-ray di ractometer results indicate that the CdTe layers, grown on lattice mismatched GaAs substrate, are partially relaxed, by the formation of mis t dislocations at the interface, and display residual vertical strain of the order of 10−4. The presence of threading dislocations in the layers e ectively limits the e ciency of solar energy conversion in the investigated heterojunctions. Homoepitaxially grown CdTe layers, of much better structural quality, display unexpected compressive strain in the layers and the relaxed lattice parameter larger than that of the substrate. Possible reasons for the formation of that unusual strain are discussed.

Systematic studies of amplitude and energy changes of excitonic lines in a strong excitation regime were carried out by a pump-probe method. The series of samples containing quantum wells with well width from 80 A to 140 A was investigated. One 80 A sample was n-doped with iodine, the rest of the quantum wells were intentionally undoped but contained a 2D gas of free holes. Its density could be varied by changing an intensity of additional illumination. The resonant creation of high population of e1hh1 excitons causes the energetic blue shift of the line due to interactions between excitons. The blue shift did not depend on the concentration of 2D gas of carriers whereas it did depend on the power of excitation beam. Model calculations of absorption show qualitative agreement with the experimental data.

Time-resolved correlation measurements were performed to examine the statistics of photon emission from a single quantum dot at high excitation levels. The range of strong powers was determined by the saturation of intensity of excitonic lines. The significant contribution of pulsed background luminescence forced us to sophisticated analysis of the photoluminescence spectra. For quantum dots grown in the Stranski–Krastanov mode, one-photon emission was found to a good approximation. Fluctuation type quantum dots exhibit a significant level of multiphoton emission.

Scanning tunneling spectroscopy was used to check the tunneling IiV characteristics of junctions formed by n-ZnO nanowires deposited on Si substrates with n- and p-type electrical conductivity (i.e. n-ZnO nanowire/ n-Si and n-ZnO nanowire/p-Si junctions, respectively). Simultaneously, several phenomena which in∞uence the measured IiV spectra were studied by atomic force microscopy. These in∞uencing factors are: the deposition density of the nanowires, the possibility of surface modiflcation by tip movement (difierence in attraction forces between nanowires and the p-Si and n-Si) and the aging of the surface. ZnO nanowires (ZnO NWs) are expected to play an important role as functional units in future nanoscale devices. Di‐culty in reproducible p-type doping of ZnO (nominally undoped ZnO is of n-type) demands a method of electrical conductivity type veriflcation which is cost efiective and easy to perform. Scanning probe microscopy (SPM) has high spatial resolution and can probe local electronic properties of the material. Several interesting applications of SPM techniques on ZnO NWs were reported (conducting-atomic force microscopy (AFM) [1{3], NW bending [4], patterning growth locations [5]). In this work scanning tunneling spectroscopy (STS) is used for the identiflcation of pin junction behavior of ZnO NWs on Si substrate. For n-ZnO NWs deposited on Si substrate, we expect | depending on the Si electrical conductivity type | the presence (for p-Si substrate) or the absence (for n-Si) of the tunneling IiV curves of diode slope indicating

Zinc oxide (ZnO) is a wurtzite-type II-VI compound semiconductor with a wide, direct band-gap (3.37 eV) and has an exciton binding energy of 60 meV, larger than thermal energy at room temperature (RT) of kBT ~ 25 meV. By alloying with CdO, which has a cubic NaCltype structure and a narrower direct band gap of 2.3 eV, the band gap of ZnO can be redshifted to a blue or even a green spectral range. Therefore, ZnO [1] and ZnCdO [2] nanowires have attracted significant attention as promising “building blocks” for nanometerscale optoelectronic devices operating in the green to UV spectral region. We report on the photoluminescence (PL) studies of ZnO and Zn1-xCdxO (x~0.02-0.05) nanowires grown at different temperatures (435, 535, 570°C) by non-catalyst solid-vapor technique on SiO2/Si substrates. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show that the structure of ZnO and Zn1-xCdxO nanowires is of wurtzite-type with good crystal perfection. Surface morphology of samples was determined by scanning electron microscopy (SEM). The PL spectra of as-grown nanowires, nanowires extracted from the substrate and deposited onto Si wafer, and nanowires dispersed in ethanol solution by sonication were collected using 325 nm (He-Cd) or 351 nm (Ar + ) laser lines. The measurements were performed at temperatures from 10 K to 300 K. The temperature dependence of photoluminescence emitted by ZnO and Zn1-xCdxO nanowires was measured and analyzed. The low-temperature PL spectra for ZnO (Zn1-xCdxO) as-grown nanowires are dominated by the emission peak at 3.359 eV (3.361 eV) originating from the radiative recombination of donor-bound excitons (DX). At temperatures 80-100 K, a small peak at 3.369 eV (3.337 eV) assigned to the free-exciton (FX) recombination starts to appear. The FX peak redshifts with increasing temperature, its intensity increases and finally FX line dominates the spectrum at RT. Additionally at low temperature we observed the peak at 3.311 eV (3.328 eV) which most probably is due to a two-electron satellite (TES). In spite of the narrowing of band gap in Zn1-xCdxO, a blueshift (of about 2 meV) of the near-band edge (NBE) emission was observed in the PL spectra. It might be caused by a spinodal decomposition. In order to study PL from a small number of individual nanowires we either dispersed them in ethanol solution by sonication or extracted them from the substrate and deposited onto Si wafer. We present the dependence of PL spectra of Zn1-xCdxO nanowires in ethanol solution on excitation power. Also we measured polarization properties of emitted light. The comparison at RT of μ-PL measurements performed on several (~10) Zn1-xCdxO nanowires resting on Si wafer with the macro-PL of as-grown nanowires shows also a blueshift of the NBE emission. This effect is probably a consequence of selective transfer of nanowires, having smaller diameter, onto the silicon substrate. The research was partially supported by the Ministry of Science and Higher Education (Poland) through grant N515 015 32/0997 and by the Network "New materials and sensors for optoelectronics, information technology, energetic applications and medicine".

The problem of multiple-electron complexes is one of the most fundamental problems of physics. One of the examples of such complex is a negatively charged exciton (trion), which consists of three particles: two electrons and a hole. Trion states have been under investigation for more than a decade and are now well-studied objects. Still some problems remain unresolved. There are arguments why are triplet trion states, which are known to be dark, observed. To get a better understanding of this one should consider the formation mechanisms of ∗corresponding author

Time Resolved Photoluminescence Study of the Wide (Cd,Mn)Te/(Cd,Mg)Te Quantum Well J. Kutrowska, P. Bugajny, M. Baranowski, L. Bryja, M. Sypereka,∗, A. Wojs, J. Misiewicz, M. Wiater, G. Karczewski and T. Wojtowicz Institute of Physics, Wrocaaw University of Technology, Wybrze»e Wyspia«skiego 27, 50-370 Wrocaaw, Poland Institute of Physics, Polish Academy of Sciences, al. Lotnikow 32/46, 02-668 Warsaw, Poland

Fast spin relaxation of Mn 2 + ions in a magnetic quantum well of CdMnTe with 1% Mn fraction is related to a very efficient spin-flip interaction between Mn ions and free carriers. This mechanism of spin relaxation becomes dominant at increased excitation densities. The observed response of the photoluminescence bands to the Mn 2 + magnetic resonance indicates that free carriers are heated at the magnetic resonance conditions. A decrease in formation/recombination rates of free and trion excitons is observed. Donor bound exciton photoluminescence is enhanced, which we relate to delocalization of free excitons, caused by interaction with microwave heated free carriers.

In this paper the electronic states of self-organized CdTe quantum dots embedded in ZnTe matrix are studied by means of capacitance–voltage (C–V ) characteristics within the temperature range of 180–300 K. A reference diode of the same layer structure but without quantum dots is studied also for comparison. The C–V characteristics measured for the reference diode exhibit bulk behaviour in contrast to the quantum dots sample for which a characteristic step corresponding to discharging of quantum dots is clearly visible within broad range of temperatures. A quasistatic model based on the self-consistent solution of the Poisson equations is used to simulate the capacitance. By comparison the calculated C–V curve with experimental curve the apparent thermal activation energy for hole emission from the quantum dots to the ZnTe matrix is found to be equal to (0.12±0.03) eV.

In this study the capacitance‐voltage (C‐V ) and deep level transient spectroscopy measurements have been performed on ZnTe (p-type)‐Ti/Al Schottky diodes containing a layer of CdTe self-assembled quantum dots and on the reference diodes without dots for comparison. Both kinds of investigated samples were grown by molecular beam epitaxy technique. The dots were formed during the Stransky‐Krastanov growth mode. Comparison of the C‐V and deep level transient spectroscopy results obtained for both samples allows us to conclude that the 0.26 eV trap observed exclusively for the QD sample can be assigned to some defects in a wetting layer or CdTe/ZnTe interface.

Systematic studies of neutral heavy-hole excitonic line energy changes in a strong excitation regime were carried out by means of a pump-probe method for quantum wells containing a 2D gas of free holes. Energy shift of Xe1hh1 line was analyzed for different excitation energies at fixed delay between pump and probe pulses, also under external magnetic field. It was observed that this shift depends not only on the density of created excitons but also directly on the pump energy. In co-polarization configuration for excitation energy below an absorption resonance the energetic blue shift rises linearly with the elevated exciton density (localized excitons are created). For energies slightly above the resonance, the blue shift diminishes dramatically in spite of high exciton density present (delocalized excitons are created). Model absorption calculations are in qualitative agreement with the experimental data.

In this paper we present Raman scattering and photoluminescence spectra measured on CdTe/ZnTe self assembled quantum dots. The photoluminescence spectrum has shown two main emission peaks, both connected with existing of quantum dots. One presents direct deexcitation to ground state and the other is optical phonon (ω = 207.3 cm−1)-assisted deexcitation. The registered multiphonon emission process depends on temperature. At low temperature phonon spectra shows line at 450 cm−1 (TA+2LO in ZnTe) and 595 cm−1 (2LO+TO(Γ ) in ZnTe).

The spin-glass transition in Cd1-xMnxTe epitaxial 1ayers and bulk samples with 0.24 < x < 0.43 and in quantum well structures on the basis of Cd1-xMnxΤe were investigated by means of optical spectroscopy. Reduction of dimensionality of Cd1-xMnxTe 1ayers down to the quasi-two-dimensional case realized in Cd1-x Mnx Te/Cd1y Mn y Τe heterostructures frustrates the spin-glass formation, which is in agreement with theoretical predictions. The spin-glass formation is also frustrated in the vicinity of interfaces between semimagnetic and nonmagnetic semiconductors in CdTe/Cd1xMnx Te quantum wells. .

Optical properties of a series of CdTe/CdMnTe multi quantum weJl structures grown with MBE and ALE (CdTe quantum wells only) methods are compared. Based on the results of the photoluminescence experiments we conclude that the ALE growth leads to a different lateral scale of quantum welI width fluctuations, which results in different exciton properties in two multi quantum well systems studied. In the wells grown with ALE method excitons are less localized. They can migrate in a quantum well plane between quantum well regions varying in thickness by 1 monolayer. PACS numbers: 71.35.+z, 78.47.+p , 78.55.Et

Results of previous studies [1] on magnetic properties of CdMnTe epilayers grown by molecular beam epitaxy (MBE) on InSb substrates [2] revealed that magnetization measurements may serve as an effective tool for investigation of Mn distribution within the epilayer. Namely, we found systematically greater values of the high temperature magnetic susceptibility in MBE layers than in their bulk counterparts. Since spin—spin interactions in Cd 1-x Mn xTe are antiferromagnetic it was concluded in [1] that a smaller clusterization occurred during the non-equilibrium growth. However, this process did not lead to a change of magnetic properties at low temperatures, where transitions to either spin-glass (x < 0.60) or antiferromagnetic (x = 0.70) phases were observed to occur at the same temperatures as in bulk samples. The latter fact suggests that the growth technique does not affect these properties of the distribution function of magnetic atoms which is important for the long range magnetic order. Also, an enhanced paramagnetic contribution to the magnetic susceptibility at low temperatures has been found in CdTe/CdMnTe superlattice. The latter effect was assigned to the presence of isolated paramagnetic ions in the interface regions as a result of an intermixing of Mn and Cd ions and/or a smaller number of magnetic bonds at the interface. In this paper we report results of magnetic measurements performed on both thick (Cd0.60 Mn0.40Τe and MnTe) epilayer and CdTe/Cd 0.50Mn0.50Te superlattices grown by MBE on GaAs substrates [3]. In order to reduce effects of

Growth of cubic MnTe isonly possible by non-equilibrium techniques such as molecular beam epitaxy (MBE) [1-4]. Here, zinc-blende MnTe epilayers were grown by MBE in EΡI-620 system on (111) oriented BaF2. The bars of 5 x 5 or 15 x 15 mm2 were cleaved in 'order to provide slices of 0.5-1.0 mm thickness. Surfaces of such substrates had almost some of cleavage steps. The substrates were soldered to molybdenum blocks with gallium. MnTe epilayers were grown from Mn and Te sources, at Te rich conditions, either directly on the substrate or after a deposition of a very thin , (10-15 monolayers) CdTe buffer. The growth was monitored by in situ RHEED which showed diffraction streaks corresponding to a six-fold symmetry; no evidence of twinning was observed during all stages of the growth process. The growth was realized at about 300 or 4000C while changing the beam equivalent pressure (BEP) of Mn and Te fluxes in order to establish optimum growth conditions (BEP Te/Mn ratio was varied between 5 and 20). The layer thickness, determined by scanning electron microscopy, ranged from 0.4-1.1 μτμ, corresponding to the growth rate of about 0.2 μm/h. MnTe layers deposited on (111)-oriented BaF2 were characterized by (a) Nomarski contrast microscopy, (b) scanning electron microscopy, (c) X-ray analysis and (d) optical transmission measurements at 12 and 300 K. In the first stage of X-ray diffraction studies we determined the crystal phase composition of the layers using a conventional powder diffractometer. In all inves' tigated layers three crystallographic phases were detected: hexagonal MnTe, cubic MnTe and traces of cubic MnΤe2. The relative abundance of hexagonal to cubic MnTe changes from sample to sample without any evident correlation to the growth parameters (Tsubst , BEP). In majority of examined samples the main phase was cubic MnTe in the form of a single crystal layer with (111) crystallographic planes parallel to BaF2 (111) substrate. The interplanar spacing d, measured in

Magnetization dynamics in bulk (Cd,Mn)Te crystals has been studied by many authors since the eighties. It was demonstrated that in magnetic field of the order of 1 T, the magnetization relaxation is mainly caused by the spin–lattice interaction [1, 2]. Its characteristic time is of the order of microseconds for samples with Mn content equal to a few percent. Our present work is devoted to two-dimensional structures — quantum wells (QWs). In this paper we present results of magnetization relaxation studies of a (Cd,Mn)Te QW in a pulsed magnetic field.