Effects of screening on the rate of energy loss in degenerate surface layers of compound semiconductors at low lattice temperatures

The energy loss rate of an electron in degenerate surface layers of a compound semiconductor for inelastic interaction with deformation and piezoelectric acoustic phonons is calculated with due account of the screening of the perturbing potential under the condition of low lattice temperature when the approximations of the well known traditional theory is not valid. The numerical results obtained for GaAs and CdS exhibits interesting features, significantly different from what follows if one either makes the traditional approximation of negligible phonon energy or disregards the screening of the scattering potential.     

Piezoelectric scattering limited mobility characteristics of a degenerate surface layer in compound semiconductors at low lattice temperatures

The theory is developed for piezoelectric scattering rate of carriers in a degenerate surface layer under the condition of low temperature when the approximations of the well-known traditional theory are not valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics for the surface layers under similar condition of low temperature. The results for the surface layers in GaAs and ZnO show that when one takes into account either the degeneracy of the carrier ensemble or the finite energy of the phonons or both, the energy dependence of the scattering rates changes significantly from what follows for a non-degenerate ensemble or from the traditional theory, where one makes use of the high-temperature approximation and thus assumes equipartition law for the phonon distribution, and neglects the phonon energy in the energy balance equation of the electron–phonon system. It is observed that the zero-field mobility characteristics that follow from these scattering rates are interesting in that they are quite different from what turns out either for a non-degenerate ensemble or in the high-temperature approximations.     

Effect of electrostatic screening due to non-equilibrium carriers on the lattice scattering rate at low temperatures

The intravalley acoustic scattering rate has been calculated here taking the screening by non-equilibrium electrons into account under the condition when the lifetime of the electrons is controlled by shallow attractive traps at low lattice temperature. The scattering rates now turn out to be field dependent and the characteristics are significantly different from what follows when the electron ensemble is in equilibrium with the lattice. The results indicate the possibility of interesting non-ohmic transport characteristics under these conditions. Numerical results are obtained for high purity samples of Si.     

On the mobility of very pure semiconductors at very low temperatures

The mobility μ of a very pure semiconductor at very low temperatures is investigated in terms of a model where electrons are scattered by charged impurities distributed uniformly in space, and the electron-electron interaction is taken into account by the Debye-Hueckel screening in the interaction potential. The equation for the current relaxation rate Γ, derived previously by the proper connected diagram expansion, incorporates the quasi-particle effect in a self-consistent manner. The solution of this equation at high carrier concentrations n yields the so-called Brooks-Herring formula. At lower concentrations, the solution deviates significantly from the latter. The solution is in general smaller than the standard expression for the rate based on the Boltzmann equation; and this is consistent with the existing conductivity data available. At the very low concentrations e.g. n = n₃ = 10¹³cm^?3 or lower for Ge, the mobility calculated is inversely proportional to the square-root of the impurity concentration n_s, and has a $\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$-dependence (T: temperature).

$\text{μ = 0.3597\&z.xl;h}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{k(k}{}_{\mathrm{B}}\text{T)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{(ze)}{}^{\mathrm{?1}}\text{n}{}_{\mathrm{s}}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}\text{m}{}^1{}^{\mathrm{<mtext>?3</mtext><mtext>4</mtext>}}$

, where k is the dielectric constant. The conductivity data directly comparable with this formula are not available at present. However, the quasi-particle effect which led to this peculiar concentration-dependence should also show itself in the cyclotron resonance width; there, experiment and theory both show the ${}_{\mathrm{n}}{}_{\mathrm{s}}$-dependence for very pure semiconductors.     

Rate of energy loss to intravalley acoustic modes in a degenerate surface layer at low lattice temperatures

The rate of loss of the energy of non-equilibrium electrons due to inelastic interaction with intravalley acoustic phonons in a degenerate surface layer is calculated for low temperatures when the approximations of the well-known traditional theory are not valid. The loss characteristics for GaAs and Si seem to be significantly different compared to what follows from the traditional approximations.     

Mobility as controlled by the transverse component of phonon wave vector in quantum layers at low temperatures

Without resorting to either the Kawaji’s simplified model of interaction with only two-dimensional phonons or to the equipartition approximation for the phonon distribution, the characteristics of the momentum relaxation time of the conduction electrons in a quantized surface layer for interaction with intravalley acoustic phonons have been analysed under the condition of low temperature. The scattering and the mobility characteristics thus obtained for an n-channel (1 0 0)-oriented Si inversion layer are apparently quite different from what follows in the traditional framework.     

Lattice-controlled electron transport characteristics in quantized surface layers at low temperature

A theory of intravalley acoustic scattering of the carriers in a non-degenerate two-dimensional electron gas is developed here under the condition of low lattice temperature when the assumptions of the well-known traditional theory are not valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics in an n-channel Si inversion layer. It is found that the finite energy of the phonons makes the energy and lattice temperature dependence of the scattering rate, and consequently the lattice temperature dependence of the mobility, significantly different from what follows, in the light of traditional theory which assumes equipartition law for the phonon distribution and neglects the phonon energy in comparison to the carrier energy.     

Studies of electron screening effects on the electron mobility in silicon surface inversion layers

Theoretical calculations of the effects of electron screening of the randomly distributed oxide charges on the electron mobility in surface inversion layers at 4.2 K show that screening substantially enhances the mobility even in weakly inverted surface layer. Surface conductances are measured at 4.2 and 77 K on n-channel MOS transistors with 4 × 10¹¹ ions/cm². Threshold voltages are obtained by matching the inversion layer conductance versus gate voltage data to the theory. The observed mobility magnitude at 4.2 K is about 2000 cm²/V-s which is in excellent agreement with the theory including screening but without adjustable parameters. A conductance tail of several volts wide below the theoretical threshold voltage and a large positive threshold voltage shift are observed at low temperatures which are attributed to an areal inhomogeneous distribution of fast surface states near the silicon conduction band edge.     

Determination of the diffusivity-mobility ratio in highly degenerate semiconductors at low temperatures from measurements of wavelength shift with cavityQ in junction lasers

The diffusivity-mobility ratio of the electrons in highly degenerate semiconductors in which prominent band tails are formed, at low temperatures when the tail states are mostly occupied, is expressed in terms of the characteristic energy of the tail. A method is then shown for directly determining the ratio from measurements of wavelength shift with cavityQ in junction lasers.The authors are indebted to Professor J. N.Bhar for his keen interest in the work.     

Tunneling-states model and the propagation characteristics of surface phonons in amorphous solids at low temperatures

The propagation characteristics of the surface phonon in an amorphous film are theoretically investigated at low temperatures based on the tunneling-states model. It is shown that the attenuation rate of the surface phonon is proportional to ω tanh ?ω/2k_BT and the relative velocity variation to ln (T/T₀) for the thickness of the amorphous film comparable to or larger than the surface phonon wavelength. Below 3K, our result can well account for recent measurements of the relative velocity variation by Hartemann et al.     

Growth of TiO2 coating on wood surface using controlled hydrothermal method at low temperatures

This paper examines the growth of anatase TiO₂ coating on a wood surface through the hydrolysis of tetrabutyl orthotitanate (TBOT) in different conditions, using a controlled hydrothermal method at low temperatures. Energy disperse X-ray analysis and Fourier transform infrared spectroscopy analysis confirm that the growth of TiO₂ coating on a wood surface is bonded to hydrocarbon chains. Several reaction factors that influence the morphologies and amount of TiO₂ present on wood surface were also investigated. As observed from the scanning electron microscopy images, the morphology and content of TiO₂ grown on a wood surface could be controlled under appropriate reaction conditions. Approximately 32.6% TiO₂ content on a wood surface could be obtained when specific conditions are applied.     

Hall effect in the accumulation layers on the surface of organic semiconductors

We have observed the Hall effect in the field-induced accumulation layer on the surface of single-crystal samples of a small-molecule organic semiconductor rubrene. The Hall mobility muH increases with decreasing temperature in both the intrinsic (high-temperature) and trap-dominated (low-temperature) conduction regimes. In the intrinsic regime, the density of mobile field-induced charge carriers extracted from the Hall measurements, nH, coincides with the density n calculated using the gate-channel capacitance and becomes smaller than n in the trap-dominated regime. The Hall data are consistent with the diffusive bandlike motion of field-induced charge carriers between trapping events.     

The influence of screened electron-phonon interaction on the mobility of carriers in semiconductors

The paper shows on a simple model that the screening of electron-phonon interaction by conduction electrons may substantially influence their mobility at sufficiently low temperatures.The author would like to thank Dr. J. R. Sandercock and Dr. M. A. Kinch for their interest and the information they provided on their hitherto unpublished work.     

Carrier concentration dependence and temperature dependence of mobility in silicon (100) N-channel inversion layers at low temperatures

The carrier concentration (N_s) dependence of electron mobility in Si (100) inversion layers has been measured at temperatures T = 1.5?70K for high- and low-mobility MOSFETs. An extrinsic term is observed in the T-dependent part of the scattering probability, τ^?1_T. At T = 4.4 K, τ^?1_T depends on N_s as N^?1.9_s in low mobility samples. In high-mobility samples, τ^?1_T increases with increasing N_s in high N_s region while τ^?1_T ∝ N^?1.6_s in low N_s region. The N_s-dependence of τ^?1_T becomes weaker with increasing T in both of low- and high-mobility samples. At N_s = 3 × 10¹² cm^?2, τ^?1_T depends on T as T^1.8 in low-mobility samples and τ^?1_T ∝ T^2.0 in high- mobility samples at T 5 K.     

A systematic study of the electron mobility in V-shaped quantum wires at low temperatures

We present a systematic study of the electron mobility in V-shaped AlGaAs/GaAs quantum wires taking into account the impurity (background, remote and interface) and the acoustic-phonon scattering. The electron scattering rates are calculated for wires with electron concentrations up to 10⁶ cm⁻¹ and temperatures up to 40 K by using Fermi’s golden rule. The effects of the interface roughness scattering and the alloy scattering are also discussed. The energy eigenstates and eigenvalues of the system under study are calculated using a finite difference method. We analyze the importance of each scattering mechanism on the mobility of several quantum wires of different qualities as a function of the electron concentration and the temperature.     

Effects of textural and surface characteristics of microporous activated carbons on the methane adsorption capacity at high pressures

The objective of this study is to relate textural and surface characteristics of selected microporous activated carbons to their methane storage capacity. In this work, a magnetic suspension balance (Rubotherm, Germany) was used to measure methane adsorption isotherms of several activated carbon samples. Textural characteristics were assessed by nitrogen adsorption on a regular surface area analyzer (Autosorb-MP, by Quantachrome, USA). N₂ adsorption was analysed by conventional models (BET, DR, HK) and by Monte Carlo molecular simulations. Elemental and surface analyses were performed by X-ray photoelectronic spectroscopy (XPS) for the selected samples. A comparative analysis was then carried out with the purpose of defining some correlation among the variables under study. For the system under study, pore size distribution and micropore volume seem to be a determining factor as long as the solid surface is perfectly hydrophobic. It was concluded that the textural parameters per se do not unequivocally determine natural gas storage capacities. Surface chemistry and methane adsorption equilibria must be taken into account in the decision-making process of choosing an adsorbent for gas storage.     

Diffusion coefficient of krypton atoms in helium gas at low and moderate temperatures

In the present work, using the Chapman–Enskog method for dilute gases, the diffusion coefficients of ground krypton atoms in a very weakly ionized helium buffer gas are revisited. The calculations are carried out quantum mechanically in the range of low and moderate temperatures. The ¹ Σ⁺ potential-energy curve via which Kr approaches He is constructed from the most recent ab initio energy points. The reliable data points used in the construction are smoothly connected to adequate long- and short-range forms. The calculations of the classical second virial coefficients and the Boyle temperature of the helium–krypton mixture are also discussed. These coefficients and their variations in terms of temperature are analysed by adopting the constructed HeKr potential and the Lennard–Jones form that fits it. The diffusion and elastic cross sections are also explored and the resonance features they exhibit are closely examined. The variation law of the diffusion coefficients with temperature is determined for typical values of density and pressure. The coefficients show excellent agreement with the available experimental data; the discrepancies do not exceed 5%.