Two-dimensional electron gas separated from Coulomb centers by a spacer is considered. Electrons are treated semi classically in limit of weak fluctuation and T = 0. Two screening models, Thomas-Fermi and RPA, are in dependently used. The second moment of the potential energy distribution is calculated analytically as a function of the spacer width. The qualitative arguments for the Gaussian character of the distribution are given and the density of states for such a distribution is calculated.
An influence of static external electric and magnetic fields on donor states in a quantum well is studied by variational means. The energies of the 1s and 2p_{±} hydrogenic states were calculated for different structure parameters (well depth and width) and different fields applied. The calculated 1s-2p_{±} transition energies agree well with experimental data.
The general goal of this work is to investigate the defects formed on the surface of the Cz-Si wafers subjected to helium implantation, vacuum annealing and nitrogen plasma treatment. The performed scanning electron microscopy study has shown that in the general case two types of surface defects can be formed: cone-shaped inclusions with the base diameter of 0.2-2 μm and the ratio of diameter to height of approximately 1:1, as well as crystallographically oriented line defects with the length equal to 0.2-2 μm. The concentration of these defects depends on the conditions of implantation and plasma treatment.
The problem of a hydrogenic donor in a semiconductor double quantum well under a homogeneous electric field is studied by variational means. The energy levels corresponding to the quantum states 1s, 2s, 2p have been calculated with the help of the many-element variational basis consisting of the exponential functions. The influence of the electric field on the donor binding energy is determined for different impurity positions. The 1s-2p transition energies have been calculated for a possible comparison with experiment.
Direct observation of the suppression of the Auger effect on shallow donors by the magnetic field in the luminescence of manganese ions in semiconducting CdF_{2}:Mn crystals is presented. The magnetic field decreases the probability of the Auger effect, which is spin-dependent energy transfer from the manganese ions to the electrons occupying shallow donors. This results in the increase in the decay times of the luminescence.
We discuss transport properties of graphene related to the resonant scattering from impurities and defects. Two different models describing defects in the bulk of graphene or at the graphene surface are used for the calculation of self energy of electrons scattered from short-range impurities or defects. The results of numerical calculations demonstrate a resonant character of resistance. In the case of neutral impurities or defects the scattering also leads to a resonant decrease of the spin relaxation time.
Far infrared magnetospectroscopic studies of negative donor ions (D^{-}), and donors in the presence of many excess electrons in high magnetic fields in GaAs/AlGaAs quantum wells are reviewed. Both singlet and triplet transitions of well-center D^{-}ions were observed and are in good agreement with recent theoretical calculations. For off-well-center D^{-}ions evidence for a predicted magnetic-field-induced "unbinding" of the second electron was found. In the presence of many excess electrons the D^{-}singlet and -triplet transitions are blue-shifted substantially and evolve into bound magnetoplasmon excitations. Cusps are observed at integral and fractional Landau-level filling factors (ν) in a plot of normalized blue-shift of the D^{-}singlet-like bound magnetoplasmon transition vs. ν. For ν<1, the singlet-like bound magnetoplasmon transition continuously approaches the isolated D^{-}singlet transition with increasing magnetic field, while the triplet-like transition loses strength, irrespective of the electron density. Exact diagonalization studies of a donor ion with a few electrons in a parabolic lateral confining potential show the importance of electron-electron interactions and localization due to the long-range fluctuating potential in explaining this behavior. High pressure studies in a specially designed diamond anvil cell exhibit a continuous evolution from bound magnetoplasmon transitions to isolated D^{-}transitions to neutral donor transitions in a single sample as the pressure is increased and the electron density in the wells is decreased.
Lattice-mismatch-induced defects were studied by means of deep-level transient spectroscopy in high-purity GaAs_{1-x}Sb_{x} layers (x = O to 3%) grown by liquid phase epitaxy on GaAs substrates. Microscopic nature and formation mechanism of two electron traps and two hole traps, which appeared in the layers as a result of Sb incorporation into the crystal lattice, are briefly discussed.
In the framework of the effective-mass approximation and the modified Lee-Low-Pines variational method, we present a theoretical study of the effect of the confined longitudinal-optical phonon and two types of surface-optical phonon (top and side mode) on the binding energy of shallow donor in cylindrical quantum dot. The effect of quantum confinement is described by an infinitely deep potential well. The impact of these different phonon modes is important and depends on the dimension of the quantum dot.
The electrical properties of the CdTe/ZnTe quantum dot system have been analyzed to identify deep-level defects related with the presence of quantum dots. The capacitance-voltage (C-V) and deep level transient spectroscopy measurements were used to investigate the samples. A reference ZnTe sample (without dots) was also studied for comparison. Both samples were grown by molecular beam epitaxy technique on the n-type GaAs substrate. The quantum dots were formed by a Zn-induced reorganization of a thin CdTe layer. The presence of quantum dot formation was confirmed by micro-photoluminescence measurements. The deep level transient spectra for both samples are complex. In order to characterize individual contributions to the deep level transient spectra the latter have been simulated by separated Gaussian components [1]. The results of the deep level transient spectroscopy measurements yield the conclusion that the same defects are present in both materials but there is an increased concentration of the defects in the quantum dot structures. No deep level associated directly with the quantum dot confinement has been identified.
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.
The influence of hydrostatic pressure up to 8 kbar on the barrier height of epitaxially MBE-grown Al on AlGaAs metal-semiconductor junctions is reported. The pressure change of the Schottky barrier on n-type AlGaAs is the same as that of the energy gap (for both direct and indirect-gap AlGaAs compositions), while for p-type AlGaAs it is negligible. This result is in direct conflict with a class of models of the Schottky barrier formation based on a concept of a semiconductor neutrality level alignment with the metal Fermi level.
By thermally stimulated currents we have investigated carrier transport and trapping in [poly-(2-methoxyl, 5-(3,77dimethyloctyloxy)] paraphenylenevinylene (MDMO-PPV). To assure selective excitation of the defect states the spectral width of the exciting light was varied from 1.77 eV up to 3.1 eV. The thermally stimulated current curves were shown to be a superposition of carrier generation from trapping states and thermally stimulated mobility growth. The extrinsic excitation resulted in 0.16 eV photoconductivity effective activation energy values, which decreased down to 0.05 eV for the intrinsic excitation. The deeper states with activation energies of 0.28-0.3 eV and 0.8-0.85 eV were identified, too. The results are direct indication of distributed in energy trapping and transport states with the standard deviation of the density of states of about 0.015 eV.
Using the photofield emission method, surface states and bulk excitations from clean and residual gas adsorbed (111) face of tungsten were measured. Residual gas shaded the surface states and diminished photoemission from the bulk excitations. Gas of the artificially increased background pressure 10^{-8} Pa adsorbed during the measurement deforms the photocurrent-voltage characteristic; the optical excitations observed for clean surfaces are exhibited, diminished or lost. For the adsorbed (111) face the state related to the metal-gas surface has been observed.
The changes of dopant vaporization enthalpy in GaAs:Si grown by molecular beam epitaxy revealed the presence of residual donors related to group VI elements. This has been confirmed by deep level transient spectroscopy studies of AlGaAs:Si layers grown in the same MBE system. It is argued that a commonly observed deep trap labelled E2 is probably related to Te, Se or S. The measurements have been performed on near-ideal Al Schottky barriers grown in situ by MBE.
Theoretical study of the binding energies of an off-center donor hydrogenic impurity in a cylindrical quantum well wires semiconductor is presented. Calculations are performed in the framework of the effective mass approximation using the variational approach. We describe the effect of the quantum confinement by an infinitely deep potential well and we take into consideration the interaction between the charge carrier (electron and ion) and the optical phonons (confined longitudinal optical and surface optical). Our results show that the impurity binding energy depends strongly on the spatial confinement, the impurity position and the polaronic corrections.
In this paper we report our experimental and theoretical studies on the effect of Gd impurity on the physical properties of the Heusler half-metallic ferromagnet Co_2MnSi. The analysis of the band structures of the doped alloy shows that the half-metallic properties are completely conserved if Gd substitutes Mn atoms. This effect is not determined by the spin-orbit interaction, but through the coupling between the R(4f) spin with the Mn(3d) itinerant electron spins. We evaluate the strength of such a coupling by calculating, in an ab initio fashion, the total energy of Co_{16}GdMn_7Si_8 compound for a parallel and antiparallel f-d coupling. The obtained magnetic moments of Co or Mn sites are in good agreement with the experimental ones.
Low-temperature scanning tunneling spectroscopy measurements on semiconductor surface are described. We consider both surfaces which do not possess surface states within the bulk bandgap, such as GaAs(110), and surfaces which do have states within the gap, such as Ge(111) 2×1 and Ge(111)c(2×8). Band bending in the semiconductor due to the electric field in the vacuum penetrating the semiconductor is found to be a substantial effect in the former case. Transport limitations in the semiconductor give rise to additional voltage drops, which can be observed by making measurements over a wide range of tunnel current magnitudes.
We present results of deep-level transient spectroscopy investigations of defects in a GaN-based heterostructure of a blue-violet laser diode, grown by plasma-assisted molecular beam epitaxy on a bulk GaN substrate. Three majority-carrier traps, T1 at E_C - 0.28 eV, T2 at E_C - 0.60 eV, and T3 at E_V + 0.33 eV, were revealed in deep-level transient spectra measured under reverse-bias conditions. On the other hand, deep-level transient spectroscopy measurements performed under injection conditions, revealed one minority-carrier trap, T4, with the activation energy of 0.20 eV. The three majority-carrier traps were revealed in the spectra measured under different reverse-bias conditions, suggesting that they are present in various parts of the laser-diode heterostructure. In addition, these traps represent different charge-carrier capture behaviours. The T1 trap, which exhibits logarithmic capture kinetics, is tentatively attributed to electron states of dislocations in the n-type wave-guiding layer of the structure. In contrast, the T2, T3, and T4 traps display exponential capture kinetics and are assigned to point defects.
A model for an artificial hydrogen molecule consisting of two positive on-axis Coulombic centers and two electrons coupled to them inside a double concentric quantum ring is considered. Such a nanostructure is assumed to be under the influence of external probes like hydrostatic pressure and magnetic field. By using the adiabatic approximation, the ground state energy is calculated as a function of the outer center line radius and the impurity Coulombic center separation, for different values of the hydrostatic pressure and magnetic field strength. In contrast to the single properties imposed by nature on the actual hydrogen molecule, our model allows us to explore a great variety of properties of the artificial hydrogen molecule by changing the ring dimensions. The artificial hydrogen molecule energy structure may be tuned by changing the external field strengths.
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