Investigation of impurity levels in thin polycrystalline SmS films

Data obtained in the study of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films (thickness ～0.5–0.8 μm) performed in the temperature region 4.2–440 K have been used to correct the band structure model of this material. It has been shown that the main impurity levels in thin polycrystalline SmS films are levels corresponding to localized states close to the conduction band bottom, as well as the impurity donor levels E _i which belong to Sm ions filling vacancies in the S sublattice. The tail of localized states has been found to extend up to the energy of impurity donor levels.     

The mechanism of the appearance of an electromotive force on heating of SmS single crystals

We analyze the experimental variation of the concentration of conduction electrons in semiconducting SmS single crystals with increasing temperature within a shallow-impurity model. It is shown that the appearance of an electromotive force is due to accumulation of the critical concentration of free electrons, which results in screening of the Coulomb potential of Sm²⁺ impurity ions that are responsible for the creation of donor levels with an activation energy of 0.045 eV in the band gap of SmS single crystals.     

Study of the electrical properties of thin SmS films at high pressures

The pressure-induced shift of impurity levels under hydrostatic compression (?1.9 × 10^?2 meV/MPa) at T = 300 K has been derived from measurements of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films on glass substrates at different pressures. The difference between the pressure-induced shifts of impurity levels in thin films and single crystals has been attributed to the effect of elastic properties of the substrate material. It has been shown that the semiconductor-metal phase transition in SmS films does not occur at pressures of up to 1000 MPa, because the impurity levels triggering the mechanism of phase transition at such pressures are not in the conduction band.     

Electrical conductivity of SmS polycrystals

The electrical conductivity of SmS polycrystals has been studied in the temperature range 300?C870 K. It has been shown that, at 300 K ?? T ?? 700 K, the concentration of conduction electrons is determined by electron transfer from impurity donor levels, and at T > 700 K, by that from the samarium 4f levels.     

Defect structure in SmS

The experimental dependence of the concentration of conduction electrons on the size of x-ray coherent scattering regions is obtained. The dependence is analyzed within the previously developed concentration model of the energy spectrum of samarium sulfide SmS. It is shown that the impurity donor levels in the SmS compound correspond to the samarium defect ions located at the boundaries of coherent scattering regions.     

Heating-induced endothermal effect in semiconducting samarium sulfide

The effect of heat absorption is revealed in the temperature range where the thermovoltaic effect occurs in single-crystal samarium sulfide (SmS). It is shown that the heat absorption is caused by the collective injection of electrons from donor levels into the conduction band.     

Investigation of the temperature dependences of the electrical resistance of SmS single crystals at different pressures

The pressure-induced shift of impurity levels under hydrostatic pressure at T = 300 K (−9.6 meV/MPa) has been obtained from measurements of the temperature dependences of the electrical resistance of SmS single crystals at different pressures. The obtained value confirms the validity of the existing model of the semiconductor-metal phase transition in samarium monosulfide.     

Dielectric mismatch effect on the photoionization cross section and intersublevel transitions in GaAs nanodots

A theoretical study of the electronic states in a spherical quantum dot with and without a hydrogenic impurity is performed within the effective mass approximation taking into account the dielectric mismatch effect. By considering the joint action of the quantum confinement and polarization charges, the photoionization cross section for an on-center donor and intersublevel optical absorption are investigated. We found that: i) the subband energies increase while the 1s and 2p impurity levels decrease when the dielectric mismatch between the dot and its environment enhances; ii) the dielectric mismatch has a significant effect on the peak position and magnitude of the photoionization cross section so that the behavior of this quantity can indicate the material in contact with the nanostructure; iii) the absorption spectrum is less sensitive to the environment dielectric properties but it significantly depends on the dot radius as well as on the impurity presence. The possibility of tuning the resonant energies by using the combined effect of the quantum confinement and dielectric mismatch between the dot and the surrounding medium can be useful in designing new optoelectronic devices.     

A cluster-like approach for the valence transition in SmS compounds

The extended Falicov-Kimball model is applied to a cluster composed of three spinless ions. By one of these ions we simulate an interstitial impurity situation which often occurs in semiconducting SmS. The remaining two ions are taken as next nearest neighbours sitting on regular Sm lattice sites. For a proper choice of parameters these two ions may undergo a valence transition due to the presence of the impurity. The situation corresponding to an anionic impurity can be simulated in a similar way.     

Study of electrical resistivity of semiconductor SmS in the absence of a metallic phase on the surface

The temperature dependences of the electrical resistivity of samarium monosulfide single-crystal samples subjected to chemical treatment to remove a metallic phase from their surfaces have been measured in the range of 1.5–400 K at atmospheric pressure and at a pressure of 0.3 GPa. The temperature dependences of the activation energy of conduction electrons at these pressures and the piezoresistance coefficient of uniform compression have been calculated. It has been shown that the known model of the structure of the impurity-level spectrum in SmS remains partially valid at temperatures higher than 15 K. At lower temperatures, the existence of shallow donor centers in SmS and the hopping conduction over them should be taken into account.     

Electronic states of coupled quantum dot-ring structure under lateral electric field with and without a hydrogenic donor impurity

A detailed investigation of the lateral electric field effect on single electron states in coupled quantum dot-ring structure has been systematically studied for cases with and without an on-center hydrogenic donor impurity. The single electron energy spectrum has been found using the effective mass approximation and an exact diagonalization technique. The electron ground state's probability density has been examined for different values of the confinement energies and depth of dot confinement relative to the bottom of the quantum ring and barrier thickness. The energy level's dependence on the electric field strength has been studied considering the effects of mentioned parameters of the structure and hydrogenic donor impurity.     

Size effect on different impurity levels in semiconductors

The different impurity levels in crystals are investigated basing on a deformation potential model which is related to the impurity size effect. The donor levels such as E_d(O) and E_d(Se) in lightly doped GaP are found to be equal to 898.55 and 103.00 meV, respectively, in perfect agreement with the experimental results of Vink et al.     

Effect of the size of x-ray coherent-scattering regions on the electrical parameters of semiconducting SmS

The relation of the electrical properties of semiconducting samarium sulfide (SmS) to the size of coherent scattering regions for x-rays is considered. The dependence of the carrier concentration on the size of coherent scattering regions in single-crystal and polycrystalline SmS samples, as well as in thin samarium sulfide films, is established experimentally. A satisfactory agreement of the calculated curves with the experiment suggests that the size of coherent scattering regions has a dominant effect on the concentrations of charge carriers and defect samarium ions in SmS. The effect of thermal shocks on the size of coherent scattering regions in SmS single crystals and polycrystals, as well as in polycrystalline Sm_0.4Eu_0.6S samples, is investigated.     

Temperature dependence of the SmS lattice parameter

The behavior of the lattice parameter of single-crystal SmS with temperature was studied by x-ray diffractometry in the range 100–700 K. The observed features are assigned to a temperature-induced variation in the filling of the Sm²⁺f-shell ground-state multiplet levels. The temperature dependence of the thermal expansion coefficient of SmS was measured. It is shown that the lattice constant behavior in samples that exhibit a pronounced emf generation effect under heating is influenced by the transition of defect samarium ions from the divalent to trivalent state and that the effect itself derives from phase transitions in SmS.     

Screening influence on the Stark effect of impurity states in strained wurtzite GaN/AlxGa1-xN heterojunctions under pressure

The screening effect of the random-phase-approximation on the states of shallow donor impurities in free strained wurtzite GaN/Al x Ga 1 x N heterojunctions under hydrostatic pressure and an external electric field is investigated by using a variational method and a simplified coherent potential approximation.The variations of Stark energy shift with electric field,impurity position,Al component and areal electron density are discussed.Our results show that the screening dramatically reduces both the blue and red shifts as well as the binding energies of impurity states.For a given impurity position,the change in binding energy is more sensitive to the increase in hydrostatic pressure in the presence of the screening effect than that in the absence of the screening effect.The weakening of the blue and red shifts,induced by the screening effect,strengthens gradually with the increase of electric field.Furthermore,the screening effect weakens the mixture crystal effect,thereby influencing the Stark effect.The screening effect strengthens the influence of energy band bending on binding energy due to the areal electron density.     

Electric field effect on the donor impurity states in zinc-blende symmetric InGaN/GaN coupled quantum dots

Based on the effective-mass approximation, the donor binding energy in a cylindrical zinc-blende (ZB) symmetric InGaN/GaN coupled quantum dots (QDs) is investigated variationally in the presence of an applied electric field. Numerical results show that the ground-state donor binding energy is highly dependent on the impurity positions, coupled QDs structure parameters and applied electric field. The applied electric field induces an asymmetric distribution of the donor binding energy with respect to the center of the coupled QDs. When the impurity is located at the center of the right dot, the donor binding energy has a maximum value with increasing the dot height. Moreover, the donor binding energy is the largest and insensitive to the large applied electric field (F?400 kV/cm) when the impurity is located at the center of the right dot in ZB symmetric In_0.1Ga_0.9N/GaN coupled QDs. In addition, if the impurity is located inside the right dot, the donor binding energy is insensitive to large middle barrier width (Lmb?2.5 nm) of ZB symmetric In_0.1Ga_0.9N/GaN coupled QDs.     

Influence of the image charge effect on the hydrogen-like impurity-bound polaron in a spherical quantum dot in the presence of an electric field

We have investigated the influence of an external electric field on the binding energies and polaronic shifts of the ground and some first few excited states of a hydrogenic impurity in a spherical quantum dot by taking into account the image charge effect. By using Landau–Pekar variational method the general analytical expression is obtained for the impurity bound-polaron energies. It has been numerically identified the conditions (electric field, nominal radius of quantum dot, etc.) in which the bound-polaron states can be existence in GaAs quantum dot. We have shown that the polaronic shifts in the binding energy of 1s-like state are the same in cases with and without image charge effect while they for 2s-like state are not coincide and have different monotonic behavior versus confinement potential. Electron–phonon interaction lifts the degeneracy of the 2px-, 2py-, and 2pz-like states of a donor impurity and reduces their binding energies.     

Photoionization cross section of a donor impurity in a two-dimensional anisotropic quantum dot

A system of an electron with a hydrogenic impurity confined in a two-dimensional anisotropic quantum dot has been investigated. We report a calculation for the binding energy of a donor impurity. The important feature of a donor impurity in a two-dimensional anisotropic quantum dot is obtained via an analysis of the binding energy. The photoionization cross section associated with intersubband transitions has been calculated. The results are presented as a function of the incident photon energy. The results show that the photoionization cross section of a donor impurity in a two-dimensional anisotropic quantum dot is strongly affected by the degree of anisotropy and the size of the quantum dot.     

Hydrostatic pressure effect on the donor impurity states in asymmetric multiple quantum wells

Based on the effective-mass approximation, hydrostatic pressure effect on the donor binding energy in zinc blende (ZB) InGaN/GaN asymmetric multiple quantum wells (AMQWs) is investigated variationally. Numerical results show that the hydrostatic pressure increases the donor binding energy for any impurity position. Moreover, the hydrostatic pressure effect is more noticeable if the impurity is localized inside the wide well of the AMQWs. For any hydrostatic pressure, the donor binding energy is distributed asymmetrically with respect to the center of the AMQWs. In particular, the donor binding energy of impurity located at the center of the wide well of the AMQWs is insensitive to the increment of the inter-well barrier width if the inter-well barrier width is large.     

The structure of ion-acoustic waves in a low-frequency three-component electron–ion space plasma with two-electron populations

In the present paper, we have studied the binding energy of the shallow donor hydrogenic impurity, which is confined in an inhomogeneous cylindrical quantum dot (CQD) of \(\hbox {GaAs-Al}_{x}\hbox {Ga}_{1-x}\hbox {As}\). Perturbation method is used to calculate the binding energy within the framework of effective mass approximation and taking into account the effect of dielectric mismatch between the dot and the barrier material. The ground-state binding energy of the donor is computed as a function of dot size for finite confinement. The result shows that the ground-state binding energy decreases with the increase in dot size. The result is compared with infinite dielectric mismatch as a limiting case. The binding energy of the hydrogenic impurity is maximum for an on-axis donor impurity.