Low-field Transport Research Articles

A full-band Monte Carlo model for hole transport in silicon

Hole transport in bulk silicon is explored using an efficient and accurate Monte Carlo (MC) tool based on the local pseudopotential band structure. Acoustic and optical phonon scattering, ionized impurity scattering, and impact ionization are the dominant scattering mechanisms that have been included. In the interest of computational efficiency, momentum relaxation times have been used to describe ionized impurity scattering and self-scattering rates have been computed in a dynamic fashion. The temperature and doping dependence of low-field hole mobility is obtained and good agreement with experimental data has been observed. MC extracted impact ionization coefficients are also shown to agree well with published experimental data. Momentum and energy relaxation times are obtained as a function of the average hole energy for use in moment based hydrodynamic simulators. The MC model is suitable for studying both low-field and high-field hole transport in silicon.

Microscopic Aspects of the Staebler-Wronski Effect

ABSTRACTThe microscopic origin and the creation mechanisms of metastable, light-induced defects in hydrogenated amorphous silicon are reviewed. Based on excitonic electron-hole pair recombination, a consistent quantitative description of defect creation kinetics can be obtained, including the experimentally observed differences between continuous wave and pulsed illumination as well as the effect of competing recombination pathways in compensated material. High resolution spin resonance spectra obtained by low-field spin-dependent transport are used to examine the interaction of metastable defects with hydrogen.

Observation of large low-field magnetoresistance in trilayer perpendicular transport devices made using doped manganate perovskites

We report on the fabrication of a new class of trilayer epitaxial thin film devices based on the doped perovskite manganates La–Ca–Mn–O and La–Sr–Mn–O. We show that large resistance changes, up to a factor of 2, can be induced by a moderate applied magnetic field below 200 Oe in these trilayers supporting current-perpendicular-to-plane transport. These results show that low-field spin-dependent transport in manganates can be accomplished, the magnitude of which is suitable for magnetoresistive field sensors.

Electron mobility in selectively doped GaAs/Inx Ga1−xAs multiple quantum well structures

The photoluminescence, the temperature dependence of the electrical conductivity (0.4<T<300 K), the magnetoresistance and the Hall effect were investigated in selectively doped GaAs/In xGa1−xAs multiple quantum well (MQW) structures. The dependencies of the electron mobility on the width of the quantum wells and on temperature were measured. It is shown that in narrow MQW structures the value of mobility is restricted by interface roughness scattering. In wider MQW structures, the alloy scattering has a detrimental effect on the low magnetic field transport properties. A negative magnetoresistance was observed in low magnetic fields. From a detailed comparison between theory of weak localisation and experiment the wave function relaxation time τϕ and its temperature dependence were evaluated. The quantum Hall effect was investigated in all samples in the temperature interval 0.4 K ÷ 4.2 K in magnetic fields up to 40 T.

Ultrahigh-magnetic-field cyclotron resonance in a two-dimensional electron gas at the grain boundary of Hg1-x-yCdxMnyTe.

The two-dimensional electron gas adjacent to the grain boundary in a bicrystal of the narrow-gap semiconductor p-type ${\mathrm{Hg}}_{0.79}$${\mathrm{Cd}}_{0.19}$${\mathrm{Mn}}_{0.02}$Te has been studied by means of cyclotron-resonance measurements in the mega-Gauss field range with ${\mathrm{CO}}_{2}$ and CO laser radiation sources. Under these conditions, the resonance energies are larger than both the depth of the confining potential well and the band-gap energy. For a photon energy of \ensuremath{\Elzxh}\ensuremath{\omega}=120 meV a pronounced single resonance line is observed at 23 T. The corresponding cyclotron effective mass exhibits a strong nonparabolicity---it increases about three times between 23 and 100 T, in agreement with the results of our self-consistent band-structure computations. Measurements in tilted magnetic fields reveal a lack of the angular dependence of the resonance field. The resonance linewidth at 23 T is larger by a factor of 7 than that evaluated from low-field transport data. This is interpreted as a result of the degeneracy of the first excited Landau level with the quasicontinuum of higher subband levels. In addition, the linewidth exhibits an abrupt increase in the magnetic-field range between 20 and 30 T. This is discussed in terms of two possible mechanisms: one is the magnetic-field-induced transfer of electrons into the ground-state subband, which is charcterized by efficient scattering of electrons by grain boundary defects; the other is the magnetic-field-induced localization of the two-dimensional electron gas at the grain boundary.

Temperature dependence of electron mobility in a free — Standing quantum well

We have studied theoretically the low-field electron transport in a free-standing quantum well where the deformation potential scattering of electrons by acoustic phonons is the major mechanism of the electron relaxation. The quantization of acoustic phonons, their multisubband spectrum, and the exact form of the dilatational acoustic modes are taken into account. We have numerically solved the kinetic equation for electrons in the low electric field limit. At low lattice temperature the obtained electron distribution function has several peaks associated with scattering by different dilatational phonons. The electron mobility has temperature dependence similar to that described by the Bloch-Grüneisen formula, however we have obtained T −3 dependence of mobility in the low temperature region.

The electron mobility transition in n-GaAs heavily doped channels

Low-field electron transport properties in heavily doped n-GaAs channels at 300 K are investigated by Monte Carlo simulation. The matrix element for electron-impurity scattering is obtained from the Fourier-transformed Coulomb potential accounting for the screening effects of the two-dimensional electron gas. The effects of the screening and the impurity profile are analysed. Electron mobility is calculated for several values of sheet electron density and n-GaAs thickness. For low sheet electron density, the calculated mobility increases with the sheet electron density and is almost independent of n-GaAs thickness. For high sheet electron density, the mobility approaches a constant value and becomes independent of sheet electron density. These findings are explained for the first time as a transition from a two- to three-dimensional nature and are well confirmed by our experiments.

Magnetotransport in antidot arrays

We study magnetotransport in lateral two-dimensional superlattices with periods on the order of 100 nm. Pronounced low-field transport anomalies reflect the interplay of two characteristic lengths of the system, the cyclotron radius R c and the period a. Measurements at very low temperatures reveal quantum oscillations, periodic in the magnetic field B, which are superimposed upon the commensurability anomalies in the magnetoresistance pxx. We show that the latter effect can be interpreted in terms of quantized periodic orbits which takes into account the chaotic nature of the classical phase space.

Some Low Field Transport Properties of LPE grown GaAs and AlAs Mixed Alloys and Saxena's Deep Donor

GaAs and AlAs mixed crystals offer the possibilities of their potential applications in various modern optical, electrical and microwave heterostructure devices to realize complete systems performing various operations for both civil and defence operations. For x >~ 0.25, a deep level, known as Saxena's deep donor dominated the electrical characteristics of Ga/sub 1-x/Al/sub x/As crystals and its activation energy increases with x, being 0.170 eV at x = 0.44, the energy of this level below the X minima decreases with x, reaching a value of 0.106 eV at x = 0.78. The results show that the lowest energy indirect minima is L in GaAs lying 0.280 eV above the T-L cross-over composition is x = 0.47. A long lifetime (T < 50 K) photoconductivity effect is also observed, which is associated with the indirect nature of the deep level. An increase in photo-Hall mobility at low temperatures over the dark mobility shows that this level has the properties of an acceptor-like centre. Shallow donor levels are also present in the crystals, but are heavily compensated. The Hall to drift mobility ratio peaks near the direct-indirect minima cross-over composition and has a peak value of ~ 3.8 at x = 0.42.

Self-consistent calculation of Hall mobilities in the valence band of Hg0.8Cd0.2Te.

The Kadanoff-Baym nonequilibrium Green-function formalism is used to calculate the heavy- and light-hole conductivity in weak electric and magnetic fields in narrow gap ${\mathrm{Hg}}_{0.8}$${\mathrm{Cd}}_{0.2}$Te. The polar-optical-phonon and charged-impurity scatterings are included in the self-consistent Born approximation. As compared with Boltzmann-equation calculations, unusually high values of light-hole Hall scattering factor are reported. The method appreciably changes the current picture of the low-field transport in this and similar materials. Comparison with experiment is made.

Monte Carlo calculations of base transit time in bipolar transistors

Previous analytical treatments of low-field electron transport in the base of bipolar transistors have all made the implicit assumption that the velocity distributions are Gaussian. Using material and scattering parameters appropriate to GaAs and a simple model for the transistor base in which the collector is replaced by a step potential, we have found that near the collector these distributions are distorted, leading to a diffusion velocity exceeding the thermal velocity. We also investigated the effects of the base-emitter boundary condition and found that standard injection techniques can force non-equilibrium distributions. This becomes critical as the base lengths become sub-micron and boundary effects dominate. With use of a dead region, where the electron distributions could relax before injection, to overcome this problem we found close agreement with the analytical results of Herbert.

Transport of quasi-two-dimensional electrons in heterojunction field effect transistors

High and low field transport properties of electrons confined to quasi-two-dimensional sheets in semiconductor heterostructures are discussed within the context of their importance in the operation of field effect transistors. Modulation-doped heterostructures, in which the two-dimensional electron gas is in undoped material, have extremely large mobilities; these mobilities are larger than what can actually be fully used by the transistor. Doped-channel quantum well structures have lower mobilities and velocities, but field effect transistors fabricated from such structures operate very well in part because of the larger electron concentration which is possible.

Magnetic field influence on the low and high electric field transport in the spin-density wave state of the organic conductor (TMTSF) 2NO 3

We have studied the influence of the magnetic field in the plane perpendicular to the highly conducting axis on the ohmic and non-ohmic electrical transport in the spin-density wave state of the organic conductor (TMTSF) 2NO 3. In particular, we report (1) the angular dependence of the magnetoresistance at different temperatures and in low electric fields and (2) magnetic field dependence of the threshold electric field for the non-ohmic conductivity. We discuss obtained results and compare them with the behaviour found in the spin-density wave state of the other (TMTSF) 2X compounds.

Low and high electric field transport in CdIn2Te4

Low and high electric field transport in the CdIn2Te4 electro-optical semiconductor were investigated. The conductivity, the carrier concentration, and the mobility were measured versus the temperature using Hall techniques. Nonlinear current-voltage characteristics in high fields are interpreted in terms of thermal effects, not in terms of space-charge-limited currents as usually encountered. The nonreversibility of I-V measurements is also discussed, diffusion of species are proposed to explain the change of activation energy of the conductivity.

Monte Carlo simulations of low-field hole transport in strained InGaAs quantum wells

The authors report on Monte Carlo simulations of low-field hole transport at 77 K in InGaAs-AlGaAs quantum wells of different widths and alloy compositions. The valence subband structure is obtained using a k.p method within the infinite well approximation, which accounts for mixing between heavy and light hole states. The effect of alloy, impurity and phonon scattering are included in the transport simulations. Although the infinite well approximation is only expected to be reliable for barriers with an aluminium fraction greater than about 0.4, for which the heavy hole well is sufficiently deep, the results show good agreement with experimental measurements for a finite 90 AA In0.18Ga0.82As-GaAs quantum well. A study of hole transport in 90 AA InxGa1-xAs wells (0.10<or=x<or=0.25) predicts a mobility which increases with indium concentration since the reduction in the effective mass of the highest HH1 subband due to strain more than compensates for the greater alloy scattering rate. An analysis of wells with 18% indium content and widths in the range 50-150 AA indicates a general increase in hole mobility with well width but with a local minimum around 90 AA due to intersubband scattering from the HH1 subband to the heavier HH2 subband.

Mobility of strained and dislocated In/sub x/Ga/sub 1-x/As semiconductor material

A theoretical investigation of low-field electron transport properties in thick layers of partially strain-relieved lattice-mismatched In/sub x/Ga/sub 1-x/As on GaAs semiconductor material is performed. The results indicate that room-temperature improvements in low-field mobility with increasing indium concentration are possible, but only occur if the density of misfit dislocations can be held below a critical value. >

Current Researches on Electronic Transport in Semiconducting Compounds: An Overview

Recent advances in the researches on electronic transport in compound semiconductors are reviewed. Low-field transport In the bulk material leads to better estimates of material parameters, but there are problems with compensated semiconductors. Hot electron effects have gained a new dimension in submicron devices, transient dynamic response yielding a high drift velocity. Fine geometry control to within atomic dimensions in hetero-structures gives a two-dimensional electron gas with especially high low-temperature mobilities. Studies of electron transport here give a wealth of new information and provide opportunities for fast devices.

Well width dependence of electron transport in molecular-beam epitaxially grown InAs/AlSb quantum wells

We present the results of a systematic study of the low-field electron transport as a function of the well width for InAs/AlSb quantum wells grown on nominally [100]-oriented, GaAs semi-insulating substrates. Both room-temperature and low-temperature mobilities are significantly reduced in narrow wells due to the dominance of interface roughness scattering. The mobility peaks for well widths around 125 Å, and then decays again, most probably due to the onset of scattering by misfit dislocations nucleated as the quantum well width exceeds the critical layer thickness imposed by the 1.3% lattice mismatch between InAs and AlSb.

Perpendicular transport ina-Si:H/a-SiNx:H single- and double-barrier structures

Current bumps in the current-voltage characteristics of amorphous semiconductor double-barrier structures have previously been associated with a resonant-tunneling process through quantized levels in the well region of the structure. We investigate the perpendicular transport through single-(SB) and double-barrier (DB) structures of hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (a-${\mathrm{SiN}}_{\mathit{x}}$:H) grown by glow discharge. The current-voltage characteristics of these structures are studied at 295 and 77 K. Current bumps are observed in the I-V characteristics of both SB and DB structures, which suggests a different transport mechanism than previously proposed. We propose that the first current bump is simply a transition from a low-field (space-charge-limited) transport mechanism to a high-field (multiple-hopping) transport mechanism. The additional bumps are associated with the energy dependence of the density of localized states in the a-${\mathrm{SiN}}_{\mathit{x}}$:H and a-Si:H.

Weak localization in the narrow gap bulk semiconductors Hg 1- xCd xTe and InSb

Our investigation of electronic transport in Hg 1− x Cd x Te and InSb shows a linear-in-temperature correction to the resistance for T < 10 K, Δ R R ∼ 10 −3 , which vanishes with the applied magnetic field as — B 1 2 , characteristic of weak localization in 3-D systems. The data have been fit to 3D weak localization theory in order to extract the inelastic relaxation time, τ i , vs T. The results show that τ i varies ∼ T −2 for InSb and Hg 1− x Cd x Te, and confirm the similarity of the low field transport properties in these narrow gap systems.