Two-dimensional Electron Transport Research Articles

Monte Carlo simulation of the low-temperature mobility of two-dimensional electrons in a silicon inversion layer

The transport of two-dimensional electrons in a silicon inversion layer is determined by Monte Carlo simulation in the temperature range 4.2–40 K, where the electrical quantum limit occurs and only the lowest subband is populated. Two scattering mechanisms are taken into account: scattering by distant ionized impurities and scattering by surface roughness elements, along with their dependence on the polarizability of the two-dimensional electron gas. The mobility and coefficient of warm electrons in weak electric fields are calculated. The data are compared with previously published results.

RF resonance in two-dimensional electron transport in the Wigner solid regime

We have measured the small-signal AC response (from 300 kHz to 3 MHz) in the presence of a DC current in the reentrant insulating phase around Landau level filling factor ν = 1 5 in a high quality two-dimensional electron system in GaAs AlGaAs and observed a broad resonance on top of a background that changes from being capacitive at low currents to inductive at high currents. The resonance frequency which shows a sublinear dependence on the DC current, is interpreted as that of the washboard oscillations of the Wigner crystal.

Two-dimensional electron transport in the presence of magnetic flux vortices.

We have considered the conductivity properties of a two-dimensional electron gas (2DEG) in two different kinds of inhomogeneous magnetic fields, i.e., a disordered distribution of magnetic flux vortices and a periodic array of magnetic flux vortices. The work falls in two parts. In the first part we show how the phase shifts for an electron scattering on an isolated vortex can be calculated analytically and related to the transport properties through a force balance equation. In the second part we present numerical results for the Hall conductivity of the 2DEG in a periodic array of flux vortices. We find characteristic peaks in the Hall conductance when plotted against the filling fraction. It is argued that the peaks can be interpreted in terms of topological charge piling up across local and global gaps in the energy spectrum.

Electronic dielectric response of composite materials: application to two-dimensional electronic transport

We derive the formal expression for the dielectric response of composite semiconducting materials and then show that the inverse dielectric function can be approximately deduced in the particular case of a two-dimensional electron gas (2DEG) embedded between two media. In given situations, these surrounding media contribute noticeably to the non-local dielectric response. This allows us to propose a general formulation for the actual scattering potentials acting against the motion of the 2DEG in transport experiments. The improvement brought about by our model is pointed out in the particular case of ionized impurity scattering.

Two-dimensional electron transport in selectively doped Ga0.25In0.75As0.5P0.5/InP heterostructure

The Hall mobility of the two-dimensional electron gas in selectively doped Ga0.25In0.75As0.5P0.5, lattice matched to InP, is calculated in the temperature range from 4.2 to 300 K. The pseudo-two-mode nature of the polar optic phonons is considered in addition to deformation potential acoustic, piezoelectric, alloy, ionized impurity (remote and background), and surface roughness scattering. The energy-band nonparabolicity and screening effects on the scattering probabilities are incorporated. The Boltzmann equation is solved by an iterative method and compared with the available experimental data. It is found that at low temperature, with a suitable choice of the alloy scattering potential, theory agrees with the experiment while there are discrepancies between the theory and experimental data on mobility as the temperature increases if one considers polar scattering by the longitudinal-optical vibrations. On the other hand, theoretical results for all temperatures can be fitted with the experiment if in addition to the above scattering mechanisms, interaction with the transverse-optical phonons is assumed to take place.

A discrete ordinates study of two-dimensional electron transport

The authors have developed a numerical algorithm to study electron transport in two spatial dimensions using the method of discrete ordinates. A spatial differencing scheme based on the method of characteristics has been used, in contrast to traditional methods that use finite differencing. In their multigroup approach to energy dependence, the within-group cross sections are modelled using elastic electron nuclear scattering while the outside-group cross sections are treated in the continuous slowing down approximation. Two cross section formalisms have been used for the elastic scattering part. Results of a two-dimensional calculation using two quadrature sets of order four and six, for the two scattering formalisms, are presented and compared with available results.

Gigantic negative transconductance and mobility modulation in a double-quantum-well structure via gate-controlled resonant coupling

Transport of two-dimensional electrons in a novel double-quantum-well (DQW) field-effect transistor was systematically studied with emphasis on the effect of resonant interaction. By introducing ionized impurities appropriately into one of the QWs, wave-function-dependent scattering process was sensitively controlled by the gate voltage Vg. A prominent valley structure was observed in the channel conductance −Vg characteristics at resonance with the peak-to-valley ratio of 3 at 4.2 K. This nonlinear characteristic is caused by the deformation of electron wave functions in the DQW and is found to be well explained by the theoretical calculation. The DQW structure can be utilized for both negative transconductance and velocity modulation devices.

Influence of magnetic fields on the two-dimensional electron transport in weakly disordered fluorine-intercalated graphite fibers.

We report measurements of the temperature dependence of the resistivity in zero magnetic field and in magnetic fields up to 5 tesla for weakly disordered fluorine-intercalated graphite fibers (${\mathrm{C}}_{\mathit{x}}$F) with various fluorine concentrations (3.6x5.8). These results have been used to determine the importance of weak localization and carrier-carrier interaction in these two-dimensional (2D) systems. We found that both quantum phenomena contribute to the logarithmic increase of resistivity with decreasing temperature, though the interaction effect is generally dominant. Also, the magnitude of the resistivity increase is very sensitive to the fluorine concentration. The temperature dependence of the inelastic scattering rate 1/${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}$, as obtained from magnetoresistance measurements, is well fitted by a polynomial expression of the form ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}^{\mathrm{\ensuremath{-}}1}$=\ensuremath{\alpha}T+\ensuremath{\beta}${\mathit{T}}^{2}$. The linear term ${\mathrm{\ensuremath{\tau}}}_{\mathit{i}}^{\mathrm{\ensuremath{-}}1}$(T) exhibits a good quantitative agreement with the theoretical prediction for the expected disorder dependence of the carrier-carrier scattering in 2D disordered metals.

High-frequency transport of two-dimensional hot electrons in GaAs and In/sub 0.53/Ga/sub 0.47/As quantum wells at low temperatures

Small-signal AC mobility of hot electrons itinerant two dimensionally in GaAs and (In, Ga)As quantum wells at a lattice temperature of 4.2 K is calculated on the basis of a drifted Fermi-Dirac distribution function in the regime where energy loss occurs via deformation potential acoustic scattering and momentum loss predominantly via background impurity scattering for GaAs quantum wells and alloy scattering for (In, Ga)As quantum wells. Results are obtained for different well widths and sheet carrier concentrations. Cutoff frequencies around 15 and 100 GHz are indicated, respectively, for GaAs and (In, Ga)As quantum wells.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Two-dimensional electron transport studied by electroluminescence

We have studied electroluminescence related to transport in a two-dimensional electron gas located at an n-type GaAs GaAlAs interface, at sample temperatures from 10 to 300 K. At all temperatures, GaAs band-to-band recombination radiation is seen, and in addition, at 10 K is also observed exciton recombination related to GaAs, recombination of electrons in the two-dimensional layer with holes in GaAs, and conduction-band to carbon-acceptor transitions. However, no radiation which could be related to Si donors or to GaAlAs could be detected. Data sets showing photon yields and electrical current versus sample potentials are presented, and a qualitative discussion is given.

Two-dimensional electronic transport in In0.53Ga0.47As quantum Wells

Transport properties of the electrons itinerant two dimensionality in a square quantum well of In0.53Ga0.47As are studied in the framework of Fermi-Dirac statistics including the relevant scattering mechanisms. An iterative solution of the Boltzmann equation shows that the ohmic mobility is controlled by LO phonon scattering at room temperature, but below 130 K alloy scattering is predominant. The calculated mobilities with a suitable value of the alloy scattering potential agree with the experimental results over a range of lattice temperature. For lattice temperatures below 25 K where the carrier energy loss is governed by the deformation potential acoustic scattering, the warm electron coefficient is found to be negative. Its magnitude decreases with increasing lattice temperature and is greater for larger channel widths. Values of the small-signal AC mobility of hot electrons at a lattice temperature of 4.2 K are obtained for different sheet carrier densities and channel widths. Cut-off frequencies around 100 GHz are indicated.

Observation of a Bloch-Grüneisen regime in two-dimensional electron transport.

We have observed a rapid decrease in the rate by which two-dimensional electrons in very high-mobility (\ensuremath{\mu}\ensuremath{\approxeq}${10}^{7}$ ${\mathrm{cm}}^{2}$/V sec) GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterostructures are scattered by acoustic phonons as the temperature approaches T=0. This precipitous drop in scattering rate is caused by strong phase-space restrictions for electron-phonon scattering. The characteristic temperature for this transition is not the Debye temperature of the GaAs host material, but the temperature at which phonons of twice the Fermi wave vector cease to be thermally excited.

Hot-electron transport in selectively doped n-type AlGaAs/GaAs heterojunctions

We investigate the high-field transport of two-dimensional (2D) electrons in selectively doped AlGaAs/GaAs heterojunctions. The dependencies of mobilities on external applied electric fields are systematically studied by pulsed Hall measurements up to 1.5 kV/cm on various samples with mobilities ranging from 4×104 to 1.3×106 cm2/V s. Measured dependencies of mobilities on electric fields are compared with theories. It was found that the simple theoretical approach based on the electron temperature model is insufficient, and that the measured mobilities are in excellent agreement with the recent theoretical results of Lei et al. [Phys. Rev. B 33, 4382 (1986)]. The high-field descreening effect is found to be more responsible for the reduction of 2D electron mobilities than the electron heating. At very high fields (several kV/cm), drift velocities of electrons determined by two-terminal I-V measurements are found to saturate gradually with increasing electric fields. Measured saturation velocities do not depend sensitively on electron densities and low-field mobilities, and are (2.0±0.2)×107 cm/s at both 4.2 and 77 K, and (1.2±0.1)×107 cm/s at 295 K. The lattice-temperature dependence of saturation velocities is compared with a crude estimation which takes into account the polar optical phonon scattering, and is found to be in rather good agreement. Consequently, it is clarified that high electron mobility transistors are superior to Si metal-oxide-semiconductor field-effect transistors in the ultimate switching speed at least by a factor of 2.5 at 77 K and 1.8 at 300 K.

A Self-Consistent Monte Carlo Simulation for Two-Dimensional Electron Transport in MOS Inversion Layer

Hot electron transport in a MOS inversion layer on a (100) silicon surface was analyzed by using a rigorous Monte Carlo method while taking into account changes in subband structures due to electron repopulation. An iterative procedure of the method consisted of a self-consistent calculation of the Schrödinger and Poisson's equations as well as a Monte Carlo calculation for electron scattering. The calculated relative electron population in each subband significantly differed from the results of a conventional Monte Carlo calculation in a tangential electric field higher than 10 kV/cm, above which carrier heating effects greatly influence the subband structures. The calculated electron drift velocity is in reasonable agreement with the experimental data.

Two-dimensional electron transport in InP surface layers

An Ensemble Monte Carlo (EMC) approach to simulate electron transport in InP surface layers. The EMC includes scattering caused by surface roughness, ionized impurities, defects, and polar optic and acoustic phonons. The EMC has been used to analyze two different ranges experiment mobility measurements reported in the literature. Low mobility samples with weak temperature dependence are dominated by surface roughness and ionized impurity scattering. Polar optic scattering at moderate electrical fields is shown to have a strong influence on high mobility samples.

Structure determination of low-dimensional conductor sodium molybdenum purple bronze Na 0.9Mo 6O 17

Structure determination of the molybdenum purple bronze Na 0.9Mo 6O 17 is carried out by single-crystal X-ray diffraction. The crystal is monoclinic with space group A2 and the lattice constants are a = 12.983(2), b = 5.518(1), c = 9.591(2) Å, β = 89.94(1)°, Z = 2. Full-matrix least-squares refinement gives the final values of R(F) = 0.028 and R w(F) = 0.040 for 1484 independent reflections, in which the occupancy factor of the sodium atom becomes 0.899(12). The present structure is built up of the linkage of the MoO 4 and MoO 6 polyhedra. There are slabs which consist of four layers of distorted MoO 6 octahedra sharing corners. Both the structure and the molybdenum valence distribution estimated from the MoO bond lengths are considered to lead to the two-dimensional electronic transport. This structure is compared with those of other members of molybdenum purple bronzes, K 0.9Mo 6O 17 and Li 0.9Mo 6O 17. The difference of the electronic properties among these compounds can be well understood on the basis of their structural characteristics.

Gate-controlled transport in narrow GaAs/AlxGa1-xAs heterostructures.

The transport of two-dimensional electrons in the narrow channel, realized by the split gate of a GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterojunction field-effect transistor, is investigated from 4.2 to 1.3 K as a function of the gate voltage ${V}_{g}$. Results from Shubnikov--de Haas effect and low-field magnetoresistance measurements show that the channel width W and the electron density ${n}_{s}$ decrease linearly with ${V}_{g}$ and that the mobility \ensuremath{\mu} decreases with ${n}_{s}$ to the power (3/2). The phase-breaking length ${l}_{i}$ in the one-dimensional weak-localization regime is studied as a function of ${V}_{g}$. We find that the functional dependence of ${l}_{i}$ on the channel conductance is in good agreement with theory.

Hybrid Two-Dimensional Electron Transport in Self-Consistent Electromagnetic Fields

The paper outlines features of the implicit hybrid simulation code ANTHEM, which uniquely provides histories for the transport and deposition of suprathermal and thermal electrons in laserproduced plasmas. The code models two-dimensional electron transport through steep density gradients and across contiguous collisional and collisionless target regions with the plasma dynamics dominated by self-consistent E and B fields. ANTHEM employs separate Eulerian fluid ion and thermal electron treatments and models suprathermal electrons as either a third fluid or as a set of collisional particle-in-cell (PIC) particles. We outline new techniques required to obtain implicit electromagnetic fields in two spatial dimensions permitting time steps well in excess of the local plasma period. A new implicit scattering model is discussed. The utility of our approach is demonstrated with sample applications to collisional surface transport on foil targets.

Two-dimensional electron transport and hot electron effects in InP/n-AlInAs heterostructures

Abstract We report two-dimensional electron gas (2DEG) transport in InP/n-AlInAs Heterostructures. This is the first demonstration of forming 2DEG in an InP with semiconductor heterojunctions. Temperature and electron density dependences of the 2DEG mobility are presented. This 2DEG system can be utilized for high speed and high power FETs. Hot electron gas transport in an InP is simulated with including electron-electron scattering and screening by high density electrons.

Hot electron transport in very short semiconductors

Abstract Experimental and theoretical investigations of hot electron transport in very short semiconductors, such as short n + n n + GaAs and Si MOSFETS are summarized placing main emphasis on drift velocity overshoot and hot electron effect of two dimensional system. Monte Carlo simulation reveals that the electron drift is very anisotropic in the overshoot region. Quasi-static analysis is made to obtain velocity distribution in short n + n n + GaAs. Ballistic transport is examined in sandwich type n + n n + GaAs with 300A channel length and Monte Carlo analysis shows an importance of potential drop not only in n region but also in n + region. Analysis of very short channel MOSFETs indicates that the normal electric field dependence of the mobility plays an important role in their characteristics. Hot electron transport of two-dimensional electrons in MOS inversion layers is examined experimentally and theoretically. Monte Carlo simulation shows that the first-order intervalley scattering results in velocity saturation at high fields.