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ZnS nanoparticles were synthesized by hydrothermal and co-precipitation methods. The starting precursors were sodium thiosulfate and zinc chloride for the hydrothermal method; zinc acetate dihydrate and sodium polyphosphate for the co-precipitation method. Scanning electron microscopy (SEM) images of ZnS powder prepared by hydrothermal route show the co-existence of individual ZnS nanoparticles and microspheres formed by the nanoparticles. When acrylamide was added to the process, ZnS porous nanospheres were obtained. Structural properties of ZnS powder prepared by both methods were studied by means of X-ray diffraction (XRD) which revealed that ZnS nanoparticles has the cubic phase and the average size of ZnS nanocrystallites is in the range of 10 nm. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of ZnS powder synthesized by hydrothermal method indicated that the luminescence intensity of the ZnS nanoparticles was affected by reaction time and surfactant meanwhile a blueshift of about 8 nm compared to bulk ZnS was observed in PLE data of ZnS powder prepared by co-precipitation method, that indicates the quantum confinement effects. [DOI: 10.1380/ejssnt.2011.521]

A theory is given of the density of states (DOS) in the low-energy tail for the disordered two-dimensional electron gas (2D EG) in a semiconductor heterostructure subjected to Gaussian random fields of any origin (especially of short range). The calculation is carried out by means of a path-integral technique within the approximation based on a non-local harmonic modelled action and that of the ground-state contribution. A simple analytic expression for the 2D DOS and different variational equations for the curvature of the trial well are then obtained, which describe the DOS tail in explicit dependence on the potential correlator, i.e., on the disorder origin as well as the geometry of the realistic 2D EG. An interpolation scheme for obtaining the 2D DOS over the whole energy region is proposed. The DOS in the deep tail is found to roughly go as a decreasing exponential function whose exponent is proportional to |E| n with n varying from 1 to 2 for very short- and long-range potential correlations, respectively. The case of a Gaussian potential correlator is thoroughly examined.

A theory is developed of the density of states (DOS) of the two-dimensional electron gas (2D EG) in semiconductor heterostructures, taking into account the effect of disorder caused by some random field existing in the sample. For a smooth random field, the calculation is carried out within its Gaussian statistics and a semiclassical approach. A simple closed expression thus obtained includes the classical DOS and its quantum correction as well, which describe the DOS of the 2D EG in explicit dependence on the rms of the potential and of the force of the random field. The disorder effect is found to smear out the step-like singularity of the DOS at an unperturbed band edge of the ideal 2D EG into a tail deep below the band edge. A detailed treatment is given of the case when the disorder is due to remote ionized impurities, which are distributed randomly or correlated in the sample.

A variational approach is given for the effect from single-side modulation doping on low-temperature transport properties of the charge carriers confined in a square infinite quantum well (QW). We obtained analytic expressions which describe the doping effects on the carrier distribution in the well, their roughness-induced scattering in the in-plane and screening by them. The calculation of the transport lifetimes is performed for holes in a SiGe/Ge/SiGe square QW, and the result is found in quantitative agreement with recently measured dependence on experimental conditions such as channel width and carrier density.

We present a theory of the band-bending effects on two-dimensional 2D carriers confined in a modulationdoped square quantum well. We develop a tractable variational evaluation of several physical quantities that are important in the theory of 2D systems. Analytic expressions of the envelope wave function and the 2D screening form factor allow us to compute various electronic properties such as electrical mobility, density of states in the presence or in the absence of magnetic fields, and Landau level broadening. We prove that in the case of the interface roughness scattering, the band-bending effects lead to a peak in the channel-width dependence of the mobility and a minimum in the Landau level broadening. Our modeling explains recent measurements for a 2D hole gas.

A theory is given of the mobility of a two-dimensional hole gas (2DHG) at low temperature in narrow square $\mathrm{Si}∕{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}∕\mathrm{Si}$ quantum wells at high Ge content. Different from the previous treatment, we have carried out a proper calculation of the misfit deformation potential and 2DHG screening. As a result, the scattering mechanisms due to surface roughness, misfit deformation potential, and alloy disorder are found to dominate the 2DHG mobility. Our theory enables a very good quantitative description of recently measured data about the dependence of the $8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ mobility of holes in a $\mathrm{Si}∕{\mathrm{Si}}_{0.2}{\mathrm{Ge}}_{0.8}∕\mathrm{Si}$ quantum well on the channel width varying from $25--70\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Further, this provides evidence in favor of screening of short-range interactions such as alloy disorder.

A theory is presented of the quantum and transport lifetimes of the two-dimensional electron gas (2DEG) at low temperature in a uniformly doped wurtzite group-III-nitride single heterostructure. In the calculation, besides the well-known scattering mechanisms, we took into account the roughness-induced scatterings, viz. misfit piezoelectric charges and misfit deformation potential. We proved that these exhibit important scattering sources for the 2DEG in wurtzite group-III-nitride heterostructures, in particular, the piezoelectric charges and alloy disorder dominate the transport and quantum lifetimes in a high-density regime (e.g., ${n}_{s}=2\ifmmode\times\else\texttimes\fi{}{10}^{12}--10\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$). Further, it is found that because of uniform doping, the electron distribution may be remarkably shifted far away from the key scattering sources, thus increasing the lifetimes. Our theory is able to provide a good quantitative explanation of the recent experimental data about the 2DEG lifetimes in a background-doped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ heterostructure. The theory may reproduce not only the magnitude but also the nonmonotonic (bell-shaped) dependencies on the carrier density of both the transport and quantum lifetimes as well as their ratio, which could not be understood if starting merely from the scattering mechanisms known so far.

We present a theory of the low-temperature mobility of the two-dimensional electron gas 2DEG in wurtzite group-III-nitride heterostructures, e.g., AlGaN/ GaN, taking adequate account of the roughness-induced scattering mechanisms and the effect due to sheet polarization charges. The squeeze of the electron distribution in the quantum well by positive piezoelectric and spontaneous polarization-induced charges on the interface is calculated in an analytic form. Thus, we obtained simple expressions describing the squeeze-related enhancement of the 2DEG screening and the unscreened potentials for different scattering sources. Altogether, their screened potentials may be strongly enhanced, so that the 2DEG mobility may be remarkably reduced by sheet polarization charges. Moreover, we proved that the roughness-induced piezoelectric charges and the roughnessinduced deformation potential exhibit new important scattering mechanisms governing the 2DEG transport in wurtzite III-nitride heterostructures. The partial 2DEG mobilities limited by them may be of the order of or less than 10 3 cm 2 / V s. Our theory turns out to be successful in the quantitative explanation of recent experimental data about the high-density 2DEG mobility, e.g., its nonmonotonic dependence on carrier density and its enhancement in the double heterostructure, which have not been understood starting merely from the conventional scattering mechanisms.

We present a theoretical study of the disorder effect due to interface roughness on piezoelectricity in wurtzite group-III-nitride heterostructures, e.g., AlGaN/GaN. We have proved that interface roughness gives rise to random nonuniform fluctuations in the piezoelectric polarization. As a result, besides the uniform density of sheet piezoelectric and spontaneous polarization-induced charges on the interface, reported in the existing literature, there must exist fluctuating densities of bulk piezoelectric charges inside of both the strained and relaxed layers as well as a fluctuating density of sheet piezoelectric charges on the interface. The densities of these charges and their electric field were generally found to be high. The maximal rms density of roughnessinduced bulk charges may be so large as 10 21 e/cm 3 , while the rms density of roughness-induced sheet charges may be of the order of magnitude of the uniform density of sheet piezoelectric charges, up to 10 13 e/cm 2 . Thus, the effects of piezoelectric polarization on the conductivity in actual wurtzite group-III-nitride heterostructures turn out to be counteracting, namely as a source of making up the two-dimensional electron gas, but also as a source of their scattering.

A theoretical study is presented of effects from strain fluctuations in a surface quantum well (SQW), which is composed of a strained zinc-blende-structure layer grown on a (001) substrate. It is found that in analogy to a zinc-blende quantum well (QW) with buried interfaces there always exist in the SQW a large fluctuating density of bulk piezoelectric charges and a high random piezoelectric field. However, in contrast to the buried QW case, roughness at a free surface of the SQW causes random fluctuations in the dilatation, which give rise to perturbing deformation potentials. The piezoelectric field and deformation potentials supply strong disorder interactions as new important scattering mechanisms for confined charge carriers in SQW's. A comparison of their effects in a SQW and the QW under equal conditions is given.

We work out a theory of piezoelectricity in an actual semiconductor heterostructure which is composed of a lattice-mismatched zinc-blende layer grown on a [001]-oriented substrate. In contrast to earlier theories, we predict a large density of fixed bulk piezoelectric charges, which are induced by strain fluctuations connected with interface roughness. The piezoelectric charges create a high electric field. The random piezoelectric field presents a conceptually new important scattering mechanism. The system of charge carriers in such a heterostructure becomes strongly disordered and includes generally both free electron-hole pairs near the interface and excitons far from it.

A theory is given of the electronic properties of modulation-doped quantum wires which undergo a thermal treatment, taking into account the Coulomb interaction among ionized impurities in the sample preparation. It is pointed out that the correlation among impurities weakens their field and is enhanced when elevating the doping level, lowering the freezing temperature for impurity diffusion, and reducing the size of the impurity system. The screening of the ionic correlation by charge carriers in the sample growth is of minor importance. In the limiting case of a one-dimensional impurity system, the correlation may totally suppress the random field at any doping level, so that a finite electron mobility is governed by other scattering mechanisms than impurity doping, e.g., interface roughness and alloying. It is found that the ionic correlation changes the electron mobility of quantum wires as regards not only its magnitude but its dependence on the doping conditions as well. For impurity systems of a small size, the mobility may be increased by up to more than one order of magnitude at a doping level of ${10}^{6}{\mathrm{cm}}^{\ensuremath{-}1}.$

We present a theoretical study of the effect from doping of quantum wells (QWs) on enhancement of the mobility limited by one-interface roughness scattering. Within the variational approach, we introduce the enhancement factor defined by the ratio of the overall mobility in symmetric two-side doped square QWs to that in the asymmetric one-side counterpart under the same doping and interface profiles. The enhancement is fixed by the sample parameters such as well width, sheet carrier density, and correlation length. So, we propose two-side doping as an efficient way to upgrade the quality of QWs. The two-interface roughness scattering is also incorporated to make comparison.

We present the theory of an ad hoc scattering mechanism for carriers confined in a heterostructure (HS) made of polar materials, such as zinc blends, nitrides, and oxides. We show that the carriers in an actual polar HS must be laterally scattered extra from both piezoelectric and spontaneous polarization charges bound on a rough interface of the system. This is due to roughness-induced fluctuations in the position of interface polarization charges, so referred to as polarization roughness (PR) scattering. The new scattering is combined with the normal surface roughness (SR) scattering, giving rise to an effective roughness-related process, which is referred to as polarization surface roughness (PSR) scattering. The PSR scattering is found to be more important for nearly forward events and at small sheet carrier densities, and it is one of the key mechanisms governing transport in polar HSs. This enables a successful explanation of the mobility data on polar HSs made, e.g., of AlGaN/GaN, which has not bee...

We presented a theoretical study of the effects from two-side (2S) doing on low-temperature lateral transport in square quantum wells (QWs). Within a variational approach, we obtained analytic expressions for the carrier distribution, screening function, and autocorrelation functions for various scattering mechanisms. We found that the mobility of a 2S-doped square QW is larger than that of the one-side (1S) doped counter part for scattering from both interfaces or from the top interface. However, the former is smaller than the latter for scattering from the bottom (substrate-side) interface. The mobility of a 2S-doped square QW exhibits a well-width evolution slower than the power-of-six law characteristic of the undoped QW. The mobility may be enhanced by 2S doping. We examine the dependence of the enhancement factor on QW parameters for optimization of the structure. This factor may achieve an order of magnitude, which is much larger than that provided by earlier methods. Our theory is able to reproduc...

We present a theory of the low-temperature transport of holes confined in the Ge strained channel of single-side modulation-doped SiGe/Ge/SiGe square quantum wells (QWs). Besides the well-known scattering mechanisms such as remote impurities and surface roughness, the theory includes misfit deformation potential. We prove that due to the effect from doping-induced band bending, the surface roughness and misfit deformation potential scatterings are considerably strengthened. Accordingly, these are found to be the key scattering mechanisms in the SiGe/Ge/SiGe system, which are still a subject under debate. Our theory can explain all recent experimental data about the transport properties of interest, namely, the carrier-density dependences of the hole mobility and the ratio of the transport to quantum lifetimes. Further, the calculated hole mobility in Ge strained QWs exhibits a special channel-width dependence with a sharp peak, which was observed but has not been explained so far.