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Magnetoresistance in dilute magnetic semiconductors is studied in the hopping transport regime. Measurements performed on Cl-doped Zn1-xMnxSe with x<8% are compared with simulation results obtained by a hopping transport model. The energy levels of the Cl donors are affected by the magnetization of Mn atoms in their vicinity via the s-d exchange interaction. Compositional disorder, in particular, the random distribution of magnetic atoms, leads to a magnetic-field induced broadening of the donor energy distribution. As the energy distribution broadens, the electron transport is hindered and a large positive contribution to the magnetoresistance arises. This broadening of the donor energy distribution is largely sufficient to account for the experimentally observed magnetoresistance effects in n-type (Zn,Mn)Se with donor concentrations below the metal-insulator transition. [ABSTRACT FROM AUTHOR]

InAs quantum dot (QD) layers grown by molecular-beam epitaxy were investigated by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. To achieve the highest possible In concentration in the QDs, InGaAs (instead of GaAs) cap layers with different In concentrations were deposited after the growth of the InAs QD layer. We combine different TEM techniques to determine the shape, size, and composition of the QDs. By applying a post-processing procedure, we are able to reconstruct the In concentration in the QDs which is measured too low in TEM due to the embedding of the QDs in material with lower In concentration and averaging along the finite TEM sample thickness. The determination of the composition of the layers on an atomic scale shows that the In concentration in the QDs increases in growth direction and reaches values up to 90%. Redistribution of indium during the InGaAs cap layer growth leads to a decrease of the In concentration in the cap layer with respect to the nominal In concentration. The observed redshift of the PL peak with increasing In concentration in the cap layer is attributed to the enlargement of island size and the change of the strain in the QD layers. [ABSTRACT FROM AUTHOR]

The influence of the conditions during growth of InAs/GaAs quantum-dot structures on GaAs(001) by molecular-beam epitaxy was investigated systematically with respect to achieving quantum-dot photoluminescence in the 1 eV range. The growth temperature, As flux, growth rate, InAs deposit, and growth interruption time before cap layer growth were varied. Photoluminescence spectroscopy and transmission electron microscopy were used to study the optical and structural properties. Large InAs quantum dots with photoluminescence in the 1 eV range were obtained at a low growth rate of 0.0056 ML/s. Analyzing in particular the low-growth-rate regime, we found that an InAs deposition of at least 2.4 ML and a growth temperature of 500-510 °C were crucial to obtain large quantum dots with a high size uniformity. Composition analyses by transmission electron microscopy revealed a significantly higher In concentration in the quantum dots grown at low growth rate compared to high-growth-rate samples. [ABSTRACT FROM AUTHOR]

Our work is concerned with the occurrence and distribution of the sphalerite, wurtzite, and rocksalt phases, which can be present in the ZnMnSe system, and the analysis of structural defects. For this purpose, ZnMnSe layers with thicknesses between 700 and 1000 nm and Mn concentrations of 0%, 4%, 11%, 14%, 17%, 29%, 31%, 43%, 50%, 70%, 85%, and 100% were deposited by molecular-beam epitaxy on GaAs (001) substrates. The structure analyses were performed by transmission electron microscopy. A high density of stacking faults exceeding 109 cm-2 is already present for a Mn concentration of 14% suggesting that lower Mn concentrations should be used for spin-aligning layers. A significant volume fraction of twinned regions is contained in the Zn0.69Mn0.31Se layer. ZnMnSe layers with Mn concentrations of 43%, 50%, and less than 30% consist exclusively of the sphalerite phase. The sphalerite and a small volume fraction of the wurtzite phase are contained in the sample with 31% of manganese. A mixture of the sphalerite, wurtzite, and rocksalt phases is observed for Mn concentrations of 70% and 85%. The presence of the wurtzite phase is correlated with regions, which contain particularly high densities of stacking faults. Only the rocksalt and the sphalerite phases remain for pure MnSe. [ABSTRACT FROM AUTHOR]

We use room-temperature photoreflectance spectroscopy to investigate the influence of indium on the electronic structure of Ga[sub 1-x]In[sub x]N[sub y]As[sub 1-y]/GaAs multiple quantum wells. To fit our experimental data, a semiempirical theoretical model based on the band anticrossing Hamiltonian is successfully applied. Thus, we can extract some information about the Hamiltonian, in particular, the dependence of the coupling parameter C[sub NM] on the In concentration in GaInNAs. C[sub NM] is shown to decrease with increasing indium mole fraction, confirming theoretical predictions. [ABSTRACT FROM AUTHOR]

The optical properties of GaInNAs/GaAs multi-quantum wells were investigated by photoluminescence excitation (PLE) spectroscopy, as well as by photoluminescence (PL), under various excitation intensities and at various temperatures. The PLE spectra demonstrated pronounced excitonic features and the corresponding transitions were identified. At low temperatures the PL spectra were sensitive to the excitation intensity. Under fixed excitation intensity, both the peak energy and the linewidth of photoluminescence showed anomalous temperature dependence, specifically an S-shaped temperature dependence of the peak energy and a N-shaped temperature dependence of linewidth in the PL spectra. The observed results are explained consistently in terms of the exciton localization effect due to the local fluctuations of nitrogen concentration. [ABSTRACT FROM AUTHOR]

A study was conducted, by using the photoluminescence excitation (PLE) spectroscopy and photoluminescence (PL), to investigate the optical properties of GaInNAs/GaAs multi-quantum wells under various excitation intensities and at various temperatures. The results are explained in terms of the exciton localization effect due to the local fluctuations of nitrogen concentration.

In n-doped (Zn,Mn)Se, the (s,d) exchange influences the ground state level of the donor-bound electrons in an external B-field, depending on their local Mn environment. Due to the statistical Mn arrangement, this should induce a broadening of the donor energy distribution, inducing a magneto-resistance in transport, and also a broadening of the energy distribution of donor-bound excitons ( D0 ,X). We have investigated this broadening for Zn0.94Mn0.06Se:Cl (n= 4.5×1017 cm-3) by electronic spin flip Raman spectroscopy (ESFRS) on the donor electrons. Utilizing the resonant enhancement of the ESFRS intensity upon resonance of the laser light with ( D0 ,X), we actually observe a significant broadening of the ESFRS resonance profiles with increasing B-field, up to 11.5 meV (FWHM) for B = 5 T. Furthermore we detect a second resonance profile contribution, whose FWHM shows a much weaker B-field dependence. We assign this contribution to a resonance mediated by the free-exciton ( X0). Our Raman results agree very well with magneto-transport results at the same sample. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]