Energy level positions of the nickel 2+/1+ and cobalt 2+/3+ charge states have been used to estimate band edges for the valence and conduction bands of ZnSe-based alloys with cation (ZnCdSe) and anion (ZnSSe) substitution. Chemical trends in band offsets of heterostructures of Zn- or Mn-based II-VI compounds are analysed. Further on, the change of Ni^{2+}(3d^{8}) and Co^{2+}(3d^{7}) intra-d shell transition bands upon the alloying of host material is discussed.
Photoluminescence measurements have been carried out for Zn_{1-x} Cd_{x} Se:Co and ZnS_{x}Se_{1-x}:Co mixed crystals. Changes of recombination channels are observed in mixed crystals as compared to the emission of host binary compounds. Character of changes is slightly different for the alloys with cation and anion substitution. Photoluminescence kinetics of the L-line and of two other Co^{2+} intra-shell emission bands was measured to determine radiative decay rates.
The framework of the virtual crystal approximation and the molecular field approximation has been widely used in the description of the extended states affected by the presence of the p-d exchange interaction in diluted magnetic semiconductors. We review shortly the known examples of diluted magnetic semiconductors systems which are described beyond these approximations. The interesting case constitutes the nearly binding impurity potential in Cd_{1-x}Μn_{x}S. The lattice approaches developed earlier allowed to prove in this case the breakdown of the virtual crystal approximation for low concentrations of magnetic ions and to explain the apparent concentration dependence of the exchange integral. We take explicitly into account the substitutional disorder neglected in the lattice theories and we rederive these results within the dilute and the average t-matrix alloy approach. We generalize systematically this theory to include also the magnetic disorder of the dilute spin subsystem appearing at finite temperatures. When the off-diagonal exchange scattering terms are taken into account in our calculations, we get a good agreement with the experimental data on the asymmetric field dependent spin splitting and the zero-field concentration dependence of the energy gap.
We present Hall effect and resistivity measurements as a function of pressure performed on MBE-grown Cd_{1-x}Mn_{x}Te (with x = 0.14) layer (1 μm) doped with bromine. The experimental data were analysed using positive and negative U model of the Br centres. We found that both models could reproduce the experimental points, but in the case of positive U model - only under assumption that the sample was completely uncompensated.
In this paper we investigate electron emission/capture from/to the DX state of indium in CdTe by means of high pressure freeze-out cycle and steady-state photo-conductivity experiments. The results indicate that the DX state is occupied by two electrons. A comparison with deep level transient spectroscopy data shows that two-electron emission occurs at low temperatures, while one-electron emission takes place at high temperatures.
We explore the possibility of using electron paramagnetic resonance (EPR) of Mn^{++} for measuring uniaxial strain in II-VI superlattices. This work is motivated by the fact that the EPR spectrum of Mn^{++} is very strongly affected by crystalline fields. Changes in a crystalline field which arise from strain are thus automatically expected to have a profound effect on the EPR spectrum. Consistent with this expectation, we have observed giant crystal field splittings of Mn^{++} EPR lines in ZnTe/MnTe, CdTe/MnTe, and ZnTe/MnSe superlattices. The EPR spectra observed in these systems are ascribed to isolated Mn^{++} ions diffused into the ZnTe or the CdTe layers from the respective MnTe or MnSe layers. In addition to providing precise information oii the magnitude and the sign of strain produced by lattice mismatch between the superlattice constituents, we show that the EPR spectrum also provides a direct measure of strain fluctuations in the layered medium.
We present experimental evidence that at high pressures indium donors in CdTe localize electrons in spatially correlated manner. We have studied Hall mobility, μ_{H}, as a function of electron concentration, n_{H}, at T=77 K. Changes of n_{H} have been achieved by two methods. High pressure freeze-out of electrons onto localized states of In-donors leads to the mobility enhancement with respect to the situation when n_{H} has been modified by means of a subsequent annealing of the sample. As a result, depending on the degree of spatial correlations in the impurity charges arrangement, different values of μ_{H} correspond to the same value of n_{H}. The variation of mobility with electron concentration suggests that the localized state of In-donor represents likely negatively charged DX state.
Absorption and reflectivity measurements have been carried out for Zn_{1-x}Cd_{x}Se:Ni and ZnS_{x}Se_{1-x}:Ni solid solutions. Energy level positions of nickel 2+/1+ charge state have been used for estimation of band offsets for the valence and conduction bands of ZnCdSe/ZnSe and ZnSSe/ZnSe. Intra-shell transitions of Ni^{2+} were also studied.
Two types of samples were studied. In the material with higher donor concentration four electron traps labelled by us as E1 to E4 were found. For the traps E2 and E3 energies obtained from Arrhenius plots are equal to 0.24 eV and 0.36 eV, respectively. Electric field enhanced electron emission from the levels E1 and E4 was observed and described in terms of Frenkel-Poole mechanism. Capture process from the traps E2 was found to be thermally activated with energetic barriers equal to 0.20 eV for E2.
Cd_{1-x}Zn_{x}Se films with different zinc content were deposited by electron beam evaporation technique onto glass substrates for the application of solid-state photovoltaic devices. The structural, surface morphological and optical properties of Cd_{1-x}Zn_{x}Se films have been studied in the present work. The host material, Cd_{1-x}Zn_{x}Se, has been prepared by the physical vapor deposition method of electron beam evaporation technique under the pressure of 1 × 10^{-5} mbar. The X-ray diffractogram indicates that these alloy films are polycrystalline in nature, of hexagonal structure with strong preferential orientation of the crystallites along (002) direction. Linear variation of lattice constant with composition (x) is observed. Surface roughness measured by atomic force microscopy is used to estimate the interface roughness. The optical properties show that the band gap (E_{g}) values vary from 2.08 to 2.64 eV as zinc content varies from 0.2 to 0.8. The surface morphological studies show the very small, fine and hardly distinguishable grains smeared all over the surface. The material properties would be altered and excellently controlled by adiusting the system composition x.
An analysis of rare earth (RE) energy level positions in wide bandgap sulphides is presented. It is shown that the Jörgensen's refined spin-pairing energy theory (RESPET) predicts correctly the photo-ionization (PI) energy of Sm in ZnS.
The transport phenomena in (Zn,Fe)Se were studied. In order to obtain iron centers in Fe^{3+} charge state the crystals were doped by Ag what produces acceptors compensating Fe^{2+} donors. The results are explained in terms of thermally activated jumping of charges between Fe^{3+} and Fe^{2+} centers. The nature of activation energy is discussed. The polaron model seems to be not valid in our case. The Coulomb interaction between charged acceptors and "holes" on iron centers is considered as the origin of thermal activation of jumps. We suggest the deviation from random and mutually independent distributions of charged Ag acceptors and Fe^{3+} ions resulting from the electrostatic interactions between them at high temperatures.
Photoconductivity and optical quenching of the photoconductivity were measured. The photoneutralization of the Fe^{3+} centres has been found to occur for photon energies down to 0.8 eV (i.e. the threshold can not be at 1.1 eV, as often quoted). The conclusion from our previous paper that the lattice relaxation energy is small, has been confirmed. It has to be smaller than 0.1 eV.
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.
The photo-ESR and photoluminescence experiments have been performed on high-resistivity ZnSe:Eu crystals. We report the first evidence that the energy level of Eu^{2+} ground state is located within the ZnSe forbidden gap, approximately 2.1 eV below the bottom of the conduction band.
In this paper we present our observation of electric dipole spin resonance and current induced spin resonance in ZnO bulk crystals, both effects result from spin-orbit coupling. Electric dipole spin resonance originates from admixture of spin state due to spin-orbit interaction which leads to probability of spin-flip caused by electric component of microwave field. In current induced spin resonance phenomenon the electric component of microwave field induces electron motion leading to mean ac spin-orbit field that acts on electron spin. Spin-orbit interaction can be described in terms of the Rashba field, an effective magnetic field that induce spin resonance. In contrast to magnetic and electric dipole spin resonances the current induced resonance is characterized by dispersive-like line shape.
Deep-level transient spectroscopy was used to study the defect levels in p-type CdTe layers grown by the molecular-beam epitaxy technique on lattice-mismatched GaAs substrates. In our measurements we have observed five hole traps. Two of the traps, displaying exponential capture kinetics, have been assigned to native point defects, the Cd vacancy and a complex formed of Cd vacancy and Te antisite, produced in the CdTe layers during their growth. The other two traps have been attributed to electronic states of threading dislocations on the ground of their logarithmic capture kinetics. The last trap, which was observed only when the investigated space charge region was close to the metal-semiconductor interface, has been ascribed to surface states.
Ζn_{1-x}Mg_{x}Se mixed crystals with x ranging from 0 to 0.56 were obtained by high pressure Bridgman method. It has been found that a phase transition from sphalerite structure to wurtzite one occurs at x = 0.185 ± 0.03. The crystals exhibit blue-violet and yellow-green (depending on x) luminescence in the temperature range from 40 K to room temperature. An attempt has been also made to dope Ζn_{1-x}Μg_{x}Se crystals with Al. The incorporation of Al produces a strong green photoluminescence in the temperature range from 40 K to 300 K but almost completely quenches the near-band-edge emission.
We present a new model approach which goes beyond the virtual crystal approximation and the molecular field approximation in the description of the spin splitting of band states in diluted magnetic semiconductors. The theory is formulated within a one-band effective mass approximation and explicitly takes into account a chemical and magnetic disorder caused by substitutional ions. The results confirm the apparent dependence of the p-d exchange integral on x in Cd_{1-x}Mn_{x}S for small concentrations of Mn ions and we predict the asymmetric field and temperature dependence of the spin splitting in this compound.
We have obtained ZnSe single crystals by vertical Bridgman method. The surface was examined using scanning electron microscope (SEM). As usual crystals exhibited twins, which were observed on the (110) surface. On as-grown and n-type samples we observed long (of the order of millimetres) linear structures parallel to the (111) plane which correspond to the twin plane. Our observations and measurements enabled us to estimate the electronic potential as being of the order of a few volts and extending less than 1 μm in the direction perpendicular to the twin planes.
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