Correlation Of Charges Research Articles

Effects of oxide-charge space correlation on electron mobility in inversion layers

The effect of the space correlation of oxide charges on the effective mobility of electrons in the channel of an NMOS transistor has been studied. Mobility has been obtained by a single-electron Monte Carlo simulation in which space correlation has been included simultaneously with other effects such as screening of point charges by mobile carriers, image charges, and phonon and surface-roughness scattering. Results have been obtained by assuming different degrees of space correlation for several temperature values and diverse oxide-charge concentrations and positions. The charged-centre space correlation is shown to have a noticeable effect on the electron mobility.

Peculiarities of transport properties in semiconductors with resonant impurities: HgSe : Fe versus PbTe : Cr

We compare the transport properties of two semiconducting systems containing resonant donors, HgSe : Fe and PbTe : Cr, with concentrations of dopants corresponding to the Fermi level being stabilized at the resonant state. It is known that under such conditions, at a low temperature, there arises a spatial correlation of electric charges localized on donor centers. The occurrence of the correlation is driven by the inter-donor Coulomb repulsive interaction. In the case of HgSe : Fe, the correlation leads to a sizable enhancement of the conduction electron mobility. In the case of PbTe : Cr, high values of the dielectric constant limit the temperature region in which the correlation is expected to occur, to temperatures below approximately 3.5 K. Therefore, no direct observation of the mobility enhancement was possible. On the other hand, a lack of a strong suppression of the mobility by the resonant scattering mechanism can be taken as an indirect indication that the positions of charged donor centers are correlated to such an extent that formation of the Coulomb gap (due to inter-donor Coulomb repulsive interaction) causes the resonant scattering to be very inefficient.

Inclusion of charge correlations in calculations of the energetics and electronic structure for random substitutional alloys.

Currently, the coherent-potential approximation (CPA) implemented via the multiple-scattering theory of Korringa, Kohn, and Rostoker (KKR) gives the best first-principles description of the electronic structure for random substitutional alloys. However, the total energy has an important component of electrostatic energy missing, namely, that arising from the correlation of charges with varying atomic environments. We develop a ``charge-correlated'' CPA method (cc-CPA) which includes (some) local environmental charge correlations within the KKR-CPA method. We investigate the cc-CPA energetics for several alloys and show that the formation energies are in better agreement with experimental results. These calculations show that the excess charge on a species is almost completely screened by the first-neighbor shell. We then derive a simplified scheme to include the vast majority of the omitted electrostatic energy from charge correlations which requires only a species-dependent shift of the potentials within the original KKR-CPA method. We also discuss the ramifications on the electronic structure.

Potential Fluctuations in HgSe:Fe,Ga - a Numerical Study

We study spatial correlation of donor charges in HgSe:Fe,Ga using a few simple quantities (mean electrostatic energy of various donor sites, fluctuations of the electrostatic potential). The study uses Monte Carlo simulation techniques. We are able to indicate general trends in the behavior of the correlation when proportion of correlating (Fe3+) and noncorrelating (Ga) charged centers in the sample is changed.

Iron doping: a tool to improve the electrical properties of Hg1-xZnxSe

The authors present results of measurements of electron concentration and mobility in mixed crystals of Hg1-xZnxSe (0<or=x<or=0.07) doped with resonant Fe donors (0<or=nFe<or=5e1019 cm-3) from liquid helium to room temperature. The data indicate that the presence of Fe considerably improves the electrical properties of the material. The analysis of the mobility in terms of the scattering from ionized centres (with possible spatial correlation of impurity charges taken into account) and alloy scattering leads to an agreement between the measured data and the calculated values.

Electron Scattering and the Band Structure of Mixed Crystals Hg1-xFexSe

HgSe:Fe is a suitable material for studies of the effects associated with the spatial correlation of charged centers [1, 3-5]. The correlation of the localized charges influences the electron concentration n e and the electron mobility μ at low temperatures. The theoretical description of the experimental results for the concentrations of iron nFe smaller than 2 x 10 19 cm-3 is satisfactory only when the correlations between Fe+ centers are taken into account [2, 3]. On the other hand, the mobility drop observed for Fe > 2 x 10 19 cm-3 has not yet been explained [6]. Also, the details of the composition dependence of n e on x are not completely understood, particularly in view of disagreements concerning the energy gap vs. x variation [7]. In ths paper we put emphasis on determination of the changes of the energetic positions of the band edges with composition of the crystal. As pointed out by Kossut [8], the same two parameters V and W which describe in the linear approximation the Γ6 and 8, respectively, band edges variation with x can also be used to estimate the scattering rate of the conduction electrons by chemical disorder (the alloy scattering mechanism). Therefore, the analysis of ne and μ can provide an independent information related to the band stucture of mixed crystals.

Monte Carlo Simulations of Spatial Correlations of Charged Centers in δ-Doped Layers

Results of Monte Carlo simulations of a 2D system of charged donors are presented. They enable to study the effects related to a spatial correlation of donor charges located on a random donor matrix. A qualitative difference between DX+ and DX — models is observed. In the first case, strong temperature dependence of the correlations and a freezing-like behaviour is found. The origin of the freezing is traced to the randk distribution of donor sites by comparing the system with a liquid-like model where the charges may assume arbitrary positions within a plane. In the second case only nearest-neighbour correlations are observed. The simulations have direct application in analysis of the behaviour of the DX centers in the GaAs planarly doped with Si.

Monte Carlo simulations of spatial correlation effects of charged centres in delta -doping layers

This paper presents results of Monte Carlo simulations of a system of charged donors in delta -doping layers, aiming at elucidating the effects related to a spatial correlation of donor charges in a 2D configuration. The simulations yielded several quantities describing the system, among them the pair correlation functions, gain in the energy of the system due to correlation of charge locations and density of impurity states, all as functions of temperature. The calculations have direct application in analysis of the behaviour of the DX centres in GaAs planarly doped with Si.

Appearance and destruction of spatial correlation of DX charges in GaAs

Measurements of the Hall coefficient and the conductivity in n-GaAs heavily doped with silicon were performed at 4.2 K under hydrostatic pressure applied at an elevated temperature. The procedure ensures that a metastable occupation of the DX centres by electrons is induced. As shown earlier, electric charges on DX centres form, under these conditions, a spatially correlated system of charges. The correlation marks its presence as an enhancement of the electron mobility. Here the authors study the destruction of the spatial correlation of donor charges by illumination with an LED. A calculation within the short-range correlation model accounts well for the decrease of the mobility seen after illumination.

Consequences of Spatial Fluctuation of Coulomb Potential in AlGaAs with DX Centers

Electric charges on randomly distributed impurities in semiconductors produce a spatially fluctuating potential. When the impurities are partially filled, their occupancy is not random but there appears a spatial correlation of the impurity charges appearing due to the inter-impurity Coulomb interactions. We show that when these interactions are taken into account then (i) the activation energy of the electron concentration, (ii) thermal emission kinetics, (iii) capture kinetics, (iv) persistent photoconductivity kinetics and (v) the electron mobility (in a steady state as well as during transients) in GaA1As:Si can be explained in a consistent way. The energy diagram concerning the DX center 1evels with respect to minima of the conduction band as well as the capture and emission barriers (including the effect of the alloy splitting) is constructed within an approach making use of the notion of the impurity self-screening.

Spatial Correlation of Impurity Charges Induced by Coulomb Interactions - Application to DX Centers in GaAs

The electron mobility enhancement observed in heavily doped GaAs under hydrostatic pressure is interpreted in terms of spatial correlation between the donor charges within partially occupied system of impurities induced by strong inter-donor Coulomb interaction. A simple analytic theory is given for both DX 0 and DXmodels of the impurity state. The mobility is shown to increase together with pressure in both models. Estimates of the energy of the DX level are strongly perturbed by the inter-donor Coulomb interactions.

Resonant states in semiconductors: A quantitative study of HgSe:Fe

When the inter-impurity Coulomb interactions dominate over the hybridization between band and resonant impurity states, the spatial correlation of impurity charges may set in. The correlations lead to a reduction of conduction electron scattering and to a modification of the Fermi energy. We show by analytic and Monte-Carlo methods that such a picture enables quantitative description of the electron concentration and mobility dependences on the concentration of Fe impurities as well as on the hydrostatic pressure in HgSe: Fe.

Volumetric Pricing of Agricultural Water Supplies: A Case Study

Models of water consumption by rice producers are conceptualized and then estimated using cross‐sectional time series data obtained from 16 Texas canal operators for the years 1977–1982. Two alternative econometric models demonstrate that both volumetric and flat rate water charges are strongly and inversely related to agricultural water consumption. Nonprice conservation incentives accompanying flat rates are hypothesized to explain the negative correlation of flat rate charges and water consumption. Application of these results suggests that water supply organizations in the sample population converting to volumetric pricing will generally reduce water consumption.

Theory of Scattering of Electrons in a Nondegenerate-Semiconductor-Surface Inversion Layer by Surface-Oxide Charges

We present a systematic study of the effects of (i) the screening by mobile carriers, (ii) the correlation of the oxide charges, (iii) the distribution of the surface-oxide charges in the oxide, and (iv) the distribution of the electrons in the surface channel on the electron mobility in a nondegenerate-semiconductor-surface inversion layer covered with an insulator or oxide. It is found that the Debye-Huckel screening parameter for a two-dimensional classical electron gas is not valid for most ranges of temperature and electron concentration of practical interest. In these temperature and electron concentration ranges, the formula overestimates the screening effect. The effect of position correlation of the surface-oxide charges on the surface mobility is discussed in detail in the hard-sphere model. Correlation effect is important, especially at low temperatures where the mean electron wavelength is greater than the mean separation among the surface-oxide charges. Both the distribution of the surface-oxide charges in the oxide layer and the distribution of the electrons in the surface channel have similar effects in reducing scattering. Evaluation of experimental surface-conductivity mobility of electrons in a nondegenerate Si-Si${\mathrm{O}}_{2}$ surface channel due to scattering by surface-oxide charges is made.