Mobility Of Excess Electrons Research Articles

Mobility of excess electrons in methylamine vapor and electron scattering processes in dense polar vapors

Mobility of excess electrons in methylamine vapor and electron scattering processes in dense polar vapors

Properties of silicon-doped AlxGa1-xAs grown by molecular beam epitaxy

The incorporation of silicon in AlxGa1-xAs grown by molecular beam epitaxy was investigated. In Al0.25Ga0.75As and Al0.35Ga0.65As doping levels were proportional to the incident silicon flux for effusion cell temperatures below 1125°C. At 1125°C the electron concentration saturated at about 2×1018 cm-3 and then decreased slightly. For cell temperatures above 1150°C silicon precipitation observable with an optical microscope took place. Silicon incorporation decreased with increasing growth temperature above 650°C with an activation energy of 2.1 eV. Electron mobilities in excess of 1000 cm2V-1 s-1 were obtained for doping levels above 1×1018 cm-3. Donor activation energies were typically between 5 and 20 meV and increased slightly as the silicon flux was increased.

Mobility enhancement in modulation-doped GaAs/AlAs heterostructures grown by molecular beam epitaxy

Modulation-doped GaAs/AlAs heterojunctions have been successfully grown for the first time. Liquid nitrogen electron mobility in excess of 100 000 cm2 V−1 s−1 has been obtained at a sheet electron concentration of 3.27×1011 cm−2. The interface properties studied by reflection electron diffraction are consistent with results of electrical measurement. The advantages of modulation-doped structures with high aluminum concentrations are discussed.

Correlation of positronium yield with excess Electron Mobility in Liquid Neopentane

The positron lifetime technique has been used to study the positronium (Ps) yield as a function of temperature in liquid neopentane. Positron lifetime spectra have been recorded in the temperature range: 23-190°C; the critical temperature of neopentane is T c = 160.3°C. The results display a clear correlation between the measured Ps yield and the excess electron mobility, i.e. the Ps yield increases (decreases) with increasing (decreasing) excess electron mobility. The ortho-Ps yield, I 3, ( 3 4 of the total Ps yield) increases from 52% at 23°C to a maximum of 58.5% at 120°C. Above 120°C I 3 decreases to a minimum of 57% at the critical temperature. Beyond the critical point, I 3 increases with rising temperature. In the present paper we give a rather detailed discussion of the influence of the properties of secondary/excess electrons on the measured Ps yield. Furthermore, we give a short discussion of the state of Ps in liquid neopentane.

Migration of excess electrons in high density supercritical ammonia

Excess electron mobility measurements in supercritical ammonia, which show a transition from the quasifree to the localized electron state in the density range from about 1×1020 to 3×1021 molecules/cm3 have been extended to densities comparable to that of liquid ammonia (n≲1.5×1022 molecules/cm3; 410⩽T⩽460 K). The drift mobility reaches a minimum of μ = 3.2×10−2 cm2 V−1 s−1 at about n≃4.7×1021 molecules/cm3. By further increasing the fluid density, the mobility increases again by a factor of ∼ 5 approaching a plateau (maximum) with μ≃1.65×10−1 cm2 V−1 s−1 at n≃1.5×1022 molecules/cm3 (T = 410 K). The temperature dependence of the electron mobility is strongest at the density where the minimum occurs. At higher densities, the temperature dependence is drastically reduced. From these facts and other available experimental results, it is concluded that a ’’trap-controlled hopping’’ transport of the electron dominates at ammonia vapor densities n<nc (nc = 8.23×1021 molecules/cm3 is the critical density of ammonia). At higher densities (n≳nc), the observed mobilities can be interpreted to be due to a weakly activated small-polaron hopping which is influenced by the molecular dynamics in supercritical ammonia. An ionlike motion of the localized (solvated) electron as often supposed can be excluded to be an essential transport channel in supercritical ammonia.

Electrical, optical, and magnetic properties of Hg1−xMnxTe

We discuss the band structure and the optical, electrical, and magnetic properties of the compound semiconductor Hg1−xMnxTe, formed by substitution of the magnetic Mn ions for Hg in the HgTe lattice. In the absence of a magnetic field, this alloy is very similar to its sister compound Hg1−xCdxTe. Although Mn is not a group II element, Hg1−xMnxTe crystallizes in the zincblende structure, forming good quality crystals up to x≊0.35. Its energy gap and related band parameters vary with x, but at a rate about twice as fast as in Hg1−xCdxTe. The electrical properties of Hg1−xMnxTe are again similar to those of Hg1−xCdxTe, exhibiting, e.g., electron mobilities in excess of 106cm2/V⋅s. Because Mn is a magnetic ion, Hg1−xMnxTe differs from Hg1−xCdxTe in its magnetic properties, as well as in the behavior of its electrical and optical properties in the presence of a magnetic field. For example, for x≳0.17 Hg1−xMnxTe exhibits a transition to the spin glass phase at low temperatures. Furthermore, the presence of Mn ions leads to an exchange interaction between the localized magnetic moments and the band electrons, which in turn affects the band parameters and leads to new and spectacular effects in the transport and optical properties.

Excess electron mobility in ethane. Density, temperature, and electric field effects

The excess electron mobility in liquid ethane was measured under orthobaric conditions as a function of temperature and electric field strength up to the critical temperature at 305.33 K. The low field mobility was found to rise strongly with temperature and exhibits a maximum value of 44 cm/sup 2/ V/sup -1/ s/sup -1/ at 2/sup 0/ below the critical temperature. At temperatures above 260 K the electron drift velocity shows a sublinear field dependence at high values of the electric field strength. These observations lead to the supposition that in liquid ethane a transition from transport via localized states to transport in extended states occurs. Measurements were also performed in fluid ethane at densities from 2.4 to 12.45 mol L/sup -1/ and temperatures from 290 to 340 K. On isochores in the vicinity of the critical density, an increase of the low field mobility with temperature was observed. This effect was found to disappear both at low (rho = 2.4 mol L/sup -1/) and high densities (rho greater than or equal to 9.2 mol L/sup -1/). In this density range, a sublinear field dependence of the drift velocities at high field strengths was noted. The critical velocity associated with themore » appearance of hot electrons was observed to decrease with higher densities indicating a smaller fractional energy transfer in electron molecule collisions. A compilation of electron mobilities in gaseous and liquid ethane shows that, up to densitiesof rho = 9.5 mol L/sup -1/, ..mu.. proportional to n/sup -1/ is fulfilled if temperature effects are ignored. At intermediate densities, 9 mol L/sup -1/ < rho < 16 mol L/sup -1/, a density dependence of ..mu.. proportional to rho/sup -5/ is found followed by a stronger mobility decrease toward the triple point. Positive ion mobilities measured under orthobaric conditions followed Walden's rule.« less

Theory of excess-electron mobility in compressed argon

The purpose of this paper is to calculate the mobility of excess electrons in compressed argon (20-100 atm) both in single-scattering and multiple-scattering approximations. Calculations in the single-scattering approximation are carried out with the aid of the Cohen-Lekner theory. A new formula for the mobility of excess electrons in the multiple-scattering approximation is derived from the Kubo-Greenwood relation for electric conductivity. Numerical results inferred from this formula and from the Cohen-Lekner theory are compared with the experimental data reported in the literature. The results obtained in this paper indicate that the Cohen-Lekner theory is applicable only at very low densities of gaseous argon and that inclusion of the long-range part of the electron-atom potential gives a significant improvement in the theoretical results. The multiple-scattering corrections have been estimated to be about (10-25)% of the value of mobility calculated in the single-scattering approximation.

Deformation-potential theory for the mobility of excess electrons in liquid argon

We investigate the motion of a slow electron in a simple classical fluid in the context of low-field mobility measurements of excess electrons in liquid argon, reported by Jahnke, Meyer, and Rice (JMR). The electron is described in terms of an effective Hamiltonian which is a generalization of the effective-mass Hamiltonian of an electron in a crystal. It is proposed that the primary source of electron scattering is a deformation potential produced by long-wavelength fluctuations of the fluid density. A semiphenomenological theory of the mobility is constructed on this basis. The dependence of the leading deformation-potential coefficient on the fluid density is obtained by fitting the data of JMR to the theory. A calculation of this coefficient based on a previous analysis by Jahnke, Holzworth, and Rice agrees well with that emerging from the data fitting.

Excess electron mobility in liquid ethane as a function of temperature between −50°C and +26°C

The excess electron mobility in liquid ethane was measured as a function of temperature between −50°C and +26°C. The mobility increased strongly with temperature reaching 40 cm 2V −1s −1. The dependence of the drift velocity on the electric field strength exhibited a sublinear relationship at higher values of the field strength. The magnitude of the mobility and the field dependence suggest that the electron transport proceeds in extended states.

Reply to a Comment on ’’Mobility of localized and quasifree excess electrons in liquid hydrocarbons’’

The comments of Freeman [J.Chem.Phys. 71,564(1979] on a previous paper by the authors [(J.Chem.Phys. 69,2401(1978)] are considered in this reply. It is contended that the ’semihydrodynamic’ expression in Eq.(2) of the former reference gives the mobility of localized electrons within a factor of two. (AIP)

Mobility of localized and quasifree excess electrons in liquid hydrocarbons

A recent model of electron transport in liquid hydrocarbons is compared with results of others. The use of input parameter values beyond their limits of applicability may have given rise to inconsistencies within values of other parameters derived from the model.

Parity and differentiability restrictions on the electric field dependence of the mobility of charged particles in gases and liquids

The electric field dependences of mobility of excess electrons in liquid neopentane, given by Mozumder and Carmichael2 is examined from the point of view of parity invarance and differentiability.(AIP)

Mobility of localized and quasifree excess electrons in liquid hydrocarbons

The dependences of excess electron mobility on temperature and conduction state energy in saturated liquid hydrocarbons are measured in order to evaluate the mobilities in the localized and in the quasifree state and the energies of the localized state. These quantities appeared earlier as adjustable parameters of a two-state electron model. Localized electron mobility is described in terms of the motion of a microscopic bubble. The transport of the quasifree electrons is regarded as that of a plane wave scattered by density fluctuations of the liquid. This model, combined with the mobility equation of Cohen and Lekner, finds reasonable agreement with experimental values which vary between 34 and about 400 cm2 V−1 sec−1.

Excess electron mobility in liquid ethane at room temperature

The drift mobility of excess electrons in liquid ethane at 21°C has been measured as a function of the electric field strength up to 115 kV/cm. The low field mobility was determined to be 40 ± 5 cm2 V−1 s−1, which indicates that the electron motion proceeds in the extended state.

Excess electrons in ice

CONSIDERABLE evidence has accumulated for a highly structrued, ice-like nature of the aqueous medium in certain cellular systems, particularly in the vicinity of macromolecules such as DNA (refs 1–7). A consequence of this ordering of water molecules is that the dielectric relaxation time of the medium becomes considerably longer than the 10 ps or so found for normal water8. A correlation has been found in recent experiments between the time required for solvation of electrons in polar liquids9–13 and the dielectric relaxation time. On the basis of this a longer lifetime for unsolvated (‘dry’14) electrons in biological systems, than the few picoseconds15,16 found in normal aqueous media, would be expected. An extended lifetime for dry electrons, particularly if coupled with a high electron mobility, would increase the probability of this species playing a role in cellular radiation damage, as has recently been suggested17, and also in intra- and inter-molecular charge transport processes in biological systems. To obtain an insight into the properties of electrons in a structured aqueous medium, we have studied the conductivity change resulting from nanosecond pulse ionisation (irradiation) of ice. We outline here some conclusions we have reached concerning the yield and mobility of excess electrons as well as the processes of electron trapping and ion recombination in this medium.

Zero-field mobility of excess electrons in dense methane gas

Measurements of the low-field mobility of electrons are reported for dense methane gas above the critical temperature. Use of a thick-walled stainless steel cell permitted measurements to 200 atm or a density of 0.3 g/cc at -77°C. At low densities the mobility follows a (density)−1 dependence and increases with increasing temperature. The mobility goes through a maximum of about 1000 cm2/V sec at a density of 1×1022 molecules/cc. The variation in mobility at high densities depends primarily on fluid density and less on either fluid structure or temperature. The position of the mobility maximum is expected to correlate with a minimum in the energy of the lowest electronic eigenstate.

Comment on the application of the Cohen-Lekner theory to excess electron mobility in liquid krypton

The Cohen-Lekner theory of excess electron mobility in liquid rare gases is applied to liquid krypton at the triple point. A serious disagreement between the theory and experiment is pointed out and discussed.

Electron mobility in nonpolar liquids: the effect of molecular structure, temperature, and electric field

A survey is given on the mobility of excess electrons in liquid hydrocarbons and related compounds. It was found that the mobility is strongly influenced by the molecular structure of the liquid, by the temperature, and by the electric field strength. The mobility in hydrocarbons increases as the shape of the molecule approaches a sphere. The temperature coefficient is positive in most liquids over a limited temperature although exceptions have been observed in liquid methane. The field dependence of the mobility in high mobility liquids (&gt;10 cm2V−1s−1) showed a decrease of the mobility at higher field strengths while in low mobility liquids (&lt;1 cm2V−1s−1) it showed an increase. These results are discussed on the basis of the extended and the localized electron models. The predictions of these theories are compared with the experimental results and conclusions on the validity of the underlying assumptions are drawn.

The influence of nonelectronegative molecules on the mobility of excess electrons in liquefied rare gases and tetramethylsilane

Addition of nonelectronegative molecules (n-alkanes, alkenes, CO, CO2) to liquid argon, krypton, and xenon influences the drift velocity of excess electrons in an electric field. At high field strengths (104–105 V cm−1), where the electrons have mean energies exceeding kT, inelastic collisions with solute molecules lead to an increase of the drift velocity above the value of the pure solvent. Analysis of this effect yields the energy dependent product of collision cross section and mean fractional energy loss per collision.At low field strengths a decrease of the low field mobility with increasing solute concentration is observed from which the cross section for momentum transfer could be deduced. The influence of solutes on the low field mobility was also found in tetramethylsilane.