Low temperature two-dimensional mobility of a GaAs heterolayer

The two-dimensional electron mobility for a GaAs single-interface heterolayer at low temperatures is computed, as a function of electron sheet density, in terms of the Fang-Howard-Stern model wave-function, for both deformation-coupled and piezoelectric-coupled scattering by acousticmode phonons. The temperature range of validity for this mobility proportional to 1/T is estimated. The unscreened mobilities are also given for comparison. The ion-scattering mobility, for various distances of the donor ion layer from the interface, is also computed, using the same model wave-function. It appears that, in the conditions of interest, lattice scattering will not dominate the overall mobility but can have a significant effect on it.     

First-principles mobility calculations and atomic-scale interface roughness in nanoscale structures

Calculations of mobilities have so far been carried out using approximate methods that suppress atomic-scale detail. Such approaches break down in nanoscale structures. Here we report the development of a method to calculate mobilities using atomic-scale models of the structures and density functional theory at various levels of sophistication and accuracy. The method is used to calculate the effect of atomic-scale roughness on electron mobilities in ultrathin double-gate silicon-on-insulator structures. The results elucidate the origin of the significant reduction in mobility observed in ultrathin structures at low electron densities.     

Electron mobilities in isomorphic In0.53Ga0.47As quantum wells on InP substrates

The influence of the doping level, illumination, and width of isomorphic In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As quantum wells grown on InP substrates on the electron mobility is studied. The persistent photoconductivity at low temperatures is found. Band diagrams are calculated and optimal parameters are found for obtaining the maximum electron mobility. The quantum and transport electron mobilities in dimensional quantization subbands are obtained from the Shubnikov-de Haas effect. The electron mobilities are calculated in dimensional quantization subbands upon scattering by ionized impurities taking intersubband transitions into account. Scattering by ionized impurities in samples studied is shown to be dominant at low temperatures.     

Experimental Determination of Carrier Mobilities in Organic Molecular Crystals

The measurement of carrier mobilities in organic molecular crystals enables one to understand the basic mechanisms of charge-transport in these systems. This, in turn, has direct bearing on electronic conduction in biologically important molecules such as 5-carotene, chlorophyll, nucleic acids, etc. However, because of many inherent difficulties (for example: low quantum–efficiency of carrier–generation; severe trapping effects; development of space-charge regions; electrode crystal contact and other barrier phenomena; etc.) progress toward better understanding remains limited. In view of these difficulties, it is not surprising that the first successful determination of drift mobilities in atypical molecular crystal, namely anthracene, was not completed until 1960 when LeBlanc' and Kepler independently measured mobilities using a method now known as the “pulse photocurrent technique”. Despite this apparent initial success, no consistent approach or the general application of such methods to organic materials has evolved. As a result, the list of organic molecular crystals for which reliable mobility data exists is quite small.     

Theoretical study of electron mobility for silicon-carbon alloys

Electron mobilities in strained Si_1−xC_x layers grown on a Si substrate and relaxed alloys are calculated as functions of carbon content, alloy scattering potential, and doping concentration at room temperature. The electron mobility model is backed by experimental data. In the case of doped strained Si_1−xC_x, the results of our electron mobility model indicates that for systems with a doping concentration greater than 10¹⁸ cm⁻³, there is no substantial decrease in the in-plane mobility with an increase in the carbon mole fraction. However, for low doping concentrations, the mobility decreases with a decrease in the carbon mole fraction.     

Über die Photoleitung von Anthrazen

The dependence of the photocurrent on voltage, light intensity and electrode separation and also the decay-time of the photocurrent is measured. All these measurements are largely influenced by the electrode material. The results obtained with Cu I- and Al-electrodes and also with insulating electrodes are given and the different behavior is discussed by means of a model. Hereby conclusions are made about the mean ranges and the mobility of the holes which are mainly responsible for the photocurrent.     

Photomagnetic effect under conditions of bipolar Hall-effect conductivity due to volume and surface excitation

A study was made concerning the Hall mobilities of electrons and holes as well as the photomagnetically measurable electron mobility in high-resistivity gallium arsenide doped with oxygen, over a wide range of intensities of radiation absorbed in the volume and at the surface. The feasibility of a reliable determination of electron and hole mobilities under conditions of strongly p- and n- bipolar Hall mobilities has been demonstrated experimentally.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp.21–25, February, 1976.     

Electron drift mobility measurements in amorphous AsxSe(1 − x) thin films

The electron mobilities for three different composition of a-As_xSe(_{1 ? x}) thin films x = 0.05, 0.10 and 0.17) were measured by the time-of-flight method. Increasing dispersion of transient current with increasing As concentration was found though the mobilities remain field-independent, suggesting Gaussian-type propagation of the mean of the charge sheet in the concentration range investigated. When compared with earlier data, the electron mobility values obtained in the present work show two kinds of decreasing exponential dependences on increasing As concentration, whereby with over 4 % As the effect is weaker.     

Pulsed laser induced absorption (one–four) dependence of electron mobility in CdI2 and ZnS crystals

The photoconductivity and electron mobility of CdI₂ and ZnS crystals have been studied using N₂-laser, fundamental and frequency doubled Nd:YAG laser. Low values of the electron mobilities obtained in the present case have been attributed to laser-induced-absorption. It is low in one photon excitation and increases with the order of absorption.     

In-situ multi-information measurement system for preparing gallium nitride photocathode

We introduce the first domestic in-situ multi-information measurement system for a gallium nitride (GaN) photo- cathode. This system can successfully fulfill heat cleaning and activation for GaN in an ultrahigh vacuum environment and produce a GaN photocathode with a negative electron affinity (NEA) status. Information including the heat clean- ing temperature, vacuum degree, photocurrent, electric current of cesium source, oxygen source, and the most important information about the spectral response, or equivalently, the quantum efficiency (QE) can be obtained during prepa- ration. The preparation of a GaN photocathode with this system indicates that the optimal heating temperature in a vacuum is about 700 C. We also develop a method of quickly evaluating the atomically clean surface with the vacuum degree versus wavelength curve to prevent possible secondary contamination when the atomic level cleaning surface is tested with X-ray photoelectron spectroscopy. The photocurrent shows a quick enhancement when the current ratio between the cesium source and oxygen source is 1.025. The spectral response of the GaN photocathode is flat in a wavelength range from 240 nm to 365 nm, and an abrupt decline is observed at 365 nm, which demonstrates that with the in-situ multi-information measurement system the NEA GaN photocathode can be successfully prepared.     

Temperature-dependent subband mobility characteristics in n-doped silicon junctionless nanowire transistor

We have investigated the temperature-dependent effective mobility characteristics in impurity band and conduction subbands of n-doped silicon junctionless nanowire transistors. It is found that the electron effective mobility of the first subband in 2-fold valleys is higher than that of the second subband in 4-fold valleys. There exists a maximum value for the effective subband mobilities at low temperatures, which is attributed to the increase of thermally activated electrons from the ionized donors in the impurity band. The experimental results indicate that the effective subband mobility is temperature-dependent on the electron interactions by thermal activation, impurity scattering, and intersubband scattering.     

On effective mobilities in the prediction of structure-borne sound transmission between a source structure and a receiving structure,part II: Procedures for the estimation of mobilities

As is discussed in Part I, the structure-borne sound power transmission between multi-point, coupled structures can theoretically be described by effective mobility. The results from full scale measurements of transfer and point mobilities of compound structures show that in some cases the effective point mobility can be approximated by the ordinary point mobility. Estimation procedures for the ordinary point mobilities containing manageable expressions for engineering applications have been developed and some examples are presented. The basic reasoning behind these procedures are described. Promising agreement has been obtained with measurement results. Also, correction factors for the ordinary point mobility to obtain an approximate point mobility and their approximate frequency bounds are given. Comparison with measured effective point mobility shows good agreement.     

Mobility-Ordered 2D NMR Spectroscopy for the Analysis of Ionic Mixtures

Improvements in the 2D electrophoretic NMR experiment are presented. This experiment now displays the full range of positive and negative values of electrophoretic mobilities and achieves higher resolution in the mobility dimension. The electrophoresis chamber consists of concentric cylindrical tubes with the solution under study in the inner tube and a salt solution in the annulus between the tubes. Among the advantages of this design are (a) reduced effective path length for current and (b) unidirectional flow for ions of interest. Techniques based on the reverse-precession method are used to recover signs of the mobilities, and improved resolution is obtained in the mobility dimension by replacing Fourier transformation of truncated data sets with respect to the current by a linear-prediction analysis.     

Estimation of various scattering parameters and 2-DEG mobilities from electron mobility calculations in the three conduction bands Γ, L and X of gallium arsenide

The electron drift mobility in Γ conduction band of GaAs has been calculated before, but for the first time, we have made attempts to estimate the electron mobilities in higher energy L and X minima. We have also calculated the value of mobility of two-dimensional electron gas needed to predict hetero-structure device characteristics using GaAs. Best scattering parameters have been derived by close comparison between experimental and theoretical mobilities. Room temperature electron mobilities in Γ, L and X valleys are found to be nearly 9094, 945 and 247 cm²/V-s respectively. For the above valleys, the electron masses, deformation potentials and polar phonon temperatures have been determined to be (0.067, 0.22, 0.39m ₀), (8.5, 9.5, 6.5 eV), and (416, 382, 542 K) as best values, respectively. The 2-DEG electron mobility in Γ minimum increases to 1.54 × 10⁶ from 1.59 × 10⁵ cm²/V-s (for impurity concentration of 10¹⁴ cm⁻³) at 10 K. Similarly, the 2-DEG electron mobility values in L and X minima are estimated to be 2.28 × 10⁵ and 1.44 × 10⁵ cm²/V-s at 10 K, which are about ∼4.5 and ∼3.9 times higher than normal value with impurity scattering present.      

Electron mobility of Sr1-xLaxTiO3 ceramics between 600 °C and 1300 °C

1-x La_xO₃ ceramics (0.003<x<0.40) were measured between 600 °C and 1300 °C. The density of electrons in the conduction band was determined from Hall and thermopower measurements. A chemical approach (Ti³⁺-titration) confirmed these results. The electron mobility was calculated by combining charge carrier density and conductivity data. The temperature dependence of the mobility obeys a power law. Its exponent varies from -1.5 for slightly doped samples (x=0.003) to -2.74 for lanthanum-rich samples (x=0.4) indicating a phonon scattering controlled transport behavior. The mobility data obtained from slightly doped ceramic samples agree very well with Hall mobilities found in undoped SrTiO₃ single crystals. Received: 22 January 1997/Accepted: 1 April 1997     

First-principles hybrid functional study of the electronic structure and charge carrier mobility in perovskite CH_3NH_3SnI_3

We calculate the electronic properties and carrier mobility of perovskite CH_3NH_3SnI_3 as a solar cell absorber by using the hybrid functional method. The calculated result shows that the electron and hole mobilities have anisotropies with a large magnitude of 1.4 × 10~4cm~2·V~(-1)·s~(-1) along the y direction. In view of the huge difference between hole and electron mobilities, the perovskite CH3NH3 Sn I3can be considered as a p-type semiconductor. We also discover a relationship between the effective mass anisotropy and electronic occupation anisotropy. The above results can provide reliable guidance for its experimental applications in electronics and optoelectronics.     

Low-temperature electron transport in GaAs

For temperatures between 5 and 80 K, electron mobilities in n-type GaAs have been calculated using ionized impurity cross sections derived from the partial wave phase shift method. The mobilities obtained differ significantly from those calculated previously using Brooks-Herring theory. Although agreement with data from a number of experimental samples is good above 30 K, the theoretical mobilities tend to overestimate experiment at lower temperatures. In order to improve the agreement it may be necessary to incorporate the effects of multi-ion scattering and collision-broadening, and to generalize the screened Coulomb scattering potential.     

Steady-state and transient electronic transport properties of β-(AlxGa1-x)2O3/Ga2O3 heterostructures: An ensemble Monte Carlo simulation

The steady-state and transient electron transport properties of $\beta $-(Al$_{x}$Ga$_{1-x}$)$_{2}$O$_{3}$/Ga$_{2}$O$_{3}$ heterostructures were investigated by Monte Carlo simulation with the classic three-valley model. In particular, the electronic band structures were acquired by first-principles calculations, which could provide precise parameters for calculating the transport properties of the two-dimensional electron gas (2DEG), and the quantization effect was considered in the $\varGamma $ valley with the five lowest subbands. Wave functions and energy eigenvalues were obtained by iteration of the Schrödinger-Poisson equations to calculate the 2DEG scattering rates with five main scattering mechanisms considered. The simulated low-field electron mobilities agree well with the experimental results, thus confirming the effectiveness of our models. The results show that the room temperature electron mobility of the $\beta $-(Al$_{0.188}$Ga$_{0.812}$)$_{2}$O$_{3}$/Ga$_{2}$O$_{3}$ heterostructure at 10 kV$ \cdot$cm$^{-1}$ is approximately 153.669 cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$, and polar optical phonon scattering would have a significant impact on the mobility properties at this time. The region of negative differential mobility, overshoot of the transient electron velocity and negative diffusion coefficients are also observed when the electric field increases to the corresponding threshold value or even exceeds it. This work offers significant parameters for the $\beta$-(Al$_{x}$Ga$_{1-x}$)$_{2}$O$_{3}$/Ga$_{2}$O$_{3}$ heterostructure that may benefit the design of high-performance $\beta$-(Al$_{x}$Ga$_{1-x}$)$_{2}$O$_{3}$/Ga$_{2}$O$_{3}$ heterostructure-based devices.     

Direct measurements of conductivity and mobility in millimeter-sized single-crystalline graphene via van der Pauw geometry

We report the direct measurements of conductivity and mobility in millimeter-sized single-crystalline graphene on SiO_2/Si via van der Pauw geometry by using a home-designed four-probe scanning tunneling microscope(4P-STM). The gate-tunable conductivity and mobility are extracted from standard van der Pauw resistance measurements where the four STM probes contact the four peripheries of hexagonal graphene flakes, respectively. The high homogeneity of transport properties of the single-crystalline graphene flake is confirmed by comparing the extracted conductivities and mobilities from three setups with different geometry factors. Our studies provide a reliable solution for directly evaluating the entire electrical properties of graphene in a non-invasive way and could be extended to characterizing other two-dimensional materials.