High Mobility Samples Research Articles

Charge transfer and low-temperature electron mobility in a strained Si layer in relaxed Si1−xGex

Calculated results for charge transfer and low-temperature electron mobility in strained silicon grown epitaxially on relaxed Si1−xGex are presented versus the thickness of an undoped spacer layer and other structural and materials parameters. The indicated conduction band offset for Si on relaxed Si0.7Ge0.3 is 180±15 meV. Scattering by the remote doping impurities that supply the carriers is found to be the dominant scattering mechanism in high-mobility samples. Samples with enhanced interface scattering are expected to have a stronger temperature dependence of mobility.

Magnetization instabilities at tilted magnetic fields in the quantum Hall regime.

In high-mobility samples, we show that a magnetization instability occurs at filling factor \ensuremath{\nu}=2 in the quantum-Hall-effect regime under the application of a parallel magnetic field. The instability is caused by feedback from the screened exchange energy of the electrons. We perform calculations on ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As-InP heterostructures and show that a surprising jump in the magnetization occurs at experimentally accessible magnetic fields and sample mobilities. This phenomenon is experimentally observable and possible experiments are proposed.

InSb-based materials for detectors

The development of new epitaxial techniques has given rise to a variety of novel material combinations. Pseudomorphic combinations where the partners have lattice constants which differ by more than 1% are currently being extensively studied. The built-in strain can alter the symmetry and magnitudes of the bandgaps concerned. The growth and properties of narrow-gap semiconductor systems are reviewed together with their use as components for strained-layer structures. The materials discussed are InSb, InAs and the alloys of these two compounds. The alloy system InAs1-xSbx is prone to metallurgical problems such as ordering and phase separation in the mid-alloy range but high-mobility samples have been grown. Other alloy systems are also affected by similar problems. Spike-doped and n-i-p-i structures are studied in InSb and InAs. Minimal dopant diffusion is found. The control of the material properties achieved has enabled the fabrication of a number of prototype device structures including n-i-p-i photodetectors and resonant interband tunnelling structures with inversion barriers.

Development of the microwave Hall effect technique using an ESR spectrometer and a network analyser

An electron spin resonance (ESR) system was modified to conduct microwave Hall effect (MHE) measurements, but the ESR capability of the system was fully retained using interchangeable equipment. An X-band microwave frequency, a square-law detector, and phase-sensitive detection of the ESR signal were utilized during the modification. The ESR cavity was replaced with a bimodal MHE cavity and source modulation was used instead of field modulation. A cancelling channel was added to cancel out the non-ideal power at the secondary mode. A vector network analyser was used to tune the cavity and to measure the mobility of high mobility samples. A bimodal TE112 cylindrical cavity and a TE102 rectangular cavity were designed and constructed, and both gave good results. Silicon semiconductor samples, which had previously been well characterized, were used to calibrate the system and calculate the calibration constants.

Self-consistent Landau-level broadening by acoustic phonons in two-dimensional electron systems.

We have studied the contribution of electron-phonon scattering to the Landau-level broadening of a two-dimensional electron gas in a strong perpendicular magnetic field. The results depend qualitatively on the contribution to the Landau-level width from electron-disorder scattering and it is necessary to treat both contributions self-consistently. In high-mobility samples, electron-phonon scattering produces a peak in the density of states at the Fermi energy.

Novel semiconductor materials and structures produced by MBE and MOCVD

The development of new epitaxial techniques has given rise to a variety of new material combinations. Pseudomorphic combinations where the partners have lattice constants which differ by more than 1% are currently being extensively studied. The built-in strain can alter the symmetry of magnitudes of the band concerned. Interesting examples of systems currently being investigated are strained layer superlattices based on Si/Si1-xGex, GaAs/InAs and InSb/InSb1-xAsx.Two narrow-gap systems are being explored in separate MBE programmes at the London University Interdisciplinary Research Centre as components for strained layer structures. The first is based on InSb, InAs and alloys of these two compounds. The second concerns heterostructures of alpha tin and InSb. Highlights of these two programmes are the following: the first observation of RHEED oscillations in InSb, attainment of record mobilities in thick film InAs (⩾ 200,000 cm2/Vs), the control of silicon and beryllium doping to very high concentrations enabling the fabrication of spike-doped and n-i-p-i structures in both materials with minimal dopant diffusion and highly non-linear optical absorption and the observation of striking Quantum Hall Effect. One of the most interesting and surprising results concerns the observation of a strong Shubnikov-de Haas effect and mobilities close to the bulk value in ultrathin films of weakly doped InSb grown on mismatched GaAs substrates. This result demonstrates that the dislocations found in the interface region are only weakly charged and therefore do not scatter the carriers significantly.The alloy system InAsxSb1-x is prone to metallurgical problems such as ordering and phase separation in the mid alloy range but high mobility samples have been grown. Other alloy systems, e.g., Al1-xInxAs, are currently of great interest and are also prone to similar problems.The strain associated with the small but significant mismatch between α-Sn and InSb stabilises the alpha phase up to 100°C and opens up an energy gap of 0.2eV. The first observation of the Shubnikov-de Haas effect with this heterostructure system demonstrates the presence of a high density two-dimensional electron gas at the interface. The carrier density is too high to arise solely from the band offsets. A complicated cyclotron resonance spectrum is observed.

Magnetoresistance Oscillations in a Two-Dimensional Electron Gas Subject to a Weak Lateral Superlattice Potential

Recent progress of our understanding of the novel magnetoresistance oscillations observed on two-dimensional electron gases with a weak lateral superlattice is reviewed. The discussion is based on a quantum mechanical single-electron picture, taking consistently into account the effect of the superlattice on the energy spectrum and the effect of randomly distributed impurities on collision broadening and transport scattering rate. This, first, allows to explain the original results of Weiss et al. on high-mobility AlxGa1-xAsGaAs heterostructures with an unidirectional modulation potential produced by holographic illumination: the large-amplitude oscillations of ρxx, measured when the current flows in the direction of the modulation, are atributed to a bandconductivity σyy, and the weaker antiphase oscillations of ρyy (∝ σxx, Current parallel to equipotentials) are shown to result from quantum oscillations of the scattering rate. Second, for the case of a modulation with square symmetry, where the energy spectrum is given by Hofstadter's famous "butterfly", it explains the dramatic suppression of the bandconductivity, which has recently been observed on high-mobility samples, as a consequence of the splitting of Landau levels into the Hofstadter-type subbands.

Low and High Field Quenching of the Hall Effect and Coulomb Blockade in Ballistic Junctions

The Hall voltage in narrow high-mobility samples at low magnetic fields depends strongly on the geometry of the junction used to measure it. It can be quenched, enhanced or even negative. At high fields and low temperatures, dips appear in quantum Hall plateaux when the junction is almost isolated. This can be explained as a magnetic diffraction effect. We report preliminary observations of large oscillations as a function of gate voltage when a junction is surrounded by tunnel barriers. These are probably caused by the Coulomb Blockade of single electron tunnelling.

Quantum magnetotransport in tilted magnetic fields: exact results for parabolic wells

The energy level structure of a quasi-two-dimensional electron system in a parabolic quantum well with the magnetic field tilted with respect to the sample plane was investigated. Based on this model and on the linear response theory the analytical expressions for the magnetoconductivity tensor were derived. Owing to the anisotropy induced by the tilted field, two diagonal components of conductivity are no longer equivalent. In high mobility samples, precisely quantized Hall plateau develop in the off-diagonal component of conductivity regardless of field orientation.

Observation of high mobility and cyclotron resonance in 20 Å silicon delta-doped GaAs grown by MBE at 480 °C

The authors report on temperature-dependent Hall effect measurements on Si delta-doped GaAs samples grown by MBE at 480 °C, 530 °C and 620 °C. In the best sample grown at 480 °C the mobility is 6760 cm2 V-1 s-1 at 4.2 K. To the authors' knowledge this is the highest mobility ever reported in a delta-doped structure. From subband population measurements the spreading of the donors in the samples grown at low temperature is determined to be 20 Å. On these high-mobility samples they were able to perform the first reported cyclotron resonance measurements. The electron effective mass is found to be considerably higher than that at the Gamma-conduction band minimum in GaAs.

Properties of Gainasp Alloys Investigated by Optically Detected Macnetic Resonance Techniques

The quaternary GaxIn1−xAsyP1−y semiconductor alloy system has considerable importance for present day optoelectronic and microwave device applications. For state of the art high mobility samples grown by metal organic chemical vapor deposition (MOVPE) there are few experimental techniques which both can asess band structure related properties (effective mass m*, g-values of free electrons) and impurity related properties (luminescence, mobility and lifetimes). In this paper we compare optical and transport properties of the quaternary compound GaxIn1−xAsyP1−y, (x=0.47,y-l; x=0.42,y=0.92; x=0.28,y=0.61; x=0.12,y=0.34) lattice matched to*{nP by optically detected magnetic resonance techniques.

Quantum Hall effect of two-dimensional electron gas in AlyGa1−yAs/Ga1−xInxAs/GaAs pseudomorphic structures

The quantum Hall effect of electrons in AlGaAs/Ga1−x Inx As/GaAs pseudomorphic strained heterostructures is reported for the first time. Despite the large strain induced by the lattice mismatch, clear quantum Hall plateaus and Shubnikov–de Haas oscillation are observed, indicating the presence of a well behaved two-dimensional electron gas. Although the mobility of the sample is fairly low, a pronounced asymmetry in the spin splitting of the magnetoresistivity appears at high magnetic fields in contrast to the previous results on AlGaAs/GaAs structures where asymmetry appears only in the high-mobility samples and disappears when the mobility is lower. This indicates that the shape of the state density is not influenced significantly by the presence of the high concentration of the short-range cluster-scattering centers.

Resonant conduction in ballistic quantum channels.

The conductance of a short, narrow ballistic constriction in a two-dimensional electron gas in a semiconductor heterostructure is calculated exactly for a simple but plausible model. A novel phenomenon is predicted: Such quantum channels should exhibit resonant conduction in highmobility samples at low temperatures. Their conductance should oscillate strongly as a function of Fermi level and gate voltage. This behavior can be used to measure directly the aspect ratio of the channels.

Novel magnetoresistance oscillations in a periodically modulated two-dimensional electron gas.

A novel type of magnetoresistance oscillations is observed in high-mobility AlGaAs/GaAs heterojunctions with a holographically induced lateral periodic modulation in one direction. Theoretically, the effect is shown to result from an oscillatory dependence of the bandwidth of the modulation-broadened Landau levels of the two-dimensional electron system on the band index, which leads in high-mobility samples to a strongly anisotropic oscillating contribution to the conductivity tensor. The oscillations, periodic in ${\mathrm{B}}^{\mathrm{\ensuremath{-}}1}$, reflect the commensurability of cyclotron diameter at the Fermi energy and modulation period.

Nonlocal quantum transport in narrow multibranched electron wave guide of GaAs-AlGaAs

Abstract Quantum transport of electrons through a narrow multibranched electron wave guide is investigated. The electron wave guides are fabricated from high and low mobility GaAsAlGaAs wafers. The background average resistance revealed quite different behavior in both samples. It is found in a high mobility (ballistic) sample that the resistance is negative and with applying magnetic field it approaches to zero at B∼0.2T, where the diameter of the cyclotron orbit is comparable to the channel width W∼0.3μm. In a low mobility (diffusive) sample, the average resistance is positive and non-zero in the entire field range. The positive resistance of the diffusive sample is comparable to a starting film resistivity, while the negative resistance originates from a bend of the current carrying channel and is attributed to the ballistic electron transmission.

High-mobility variable-density two-dimensional electron gas in inverted GaAs-AlGaAs heterojunctions

Inverted heterointerfaces (GaAs on AlGaAs), which are basic constituents of all quantum wells and superlattices, have been significantly improved using electron diffraction and a refined molecular beam epitaxy growth procedure. Utilizing them in a novel structure allowed the variation of the electron density over a wide range, with peak mobilities of 4×105 cm2/V s. The continuously variable electron density allowed comparison to a theoretical analysis of the low-temperature scattering mechanisms, and their relation to the growth process, establishing the importance of interface charges and roughness. High-mobility samples were used to observe the quantum Hall effect with varying carrier concentrations in a single structure.

Variation of thermopower of two-dimensional electrons in GaAs and InGaAs at low temperatures

Thermoelectric power of two-dimensional electron gas has been calculated by using the formula derived from the Boltzmann equation assuming constant density-of-states and relevant scattering mechanisms. The values increase linearly with temperature. The present theory is applicable to the high mobility samples, in which the additional contribution due to phonon drag may account for higher experimental values. For low mobility GaAs samples, experimental values are lower and in InGaAs the values decrease with temperature. We have considered the subband energy to be a Gaussian variable caused by long-range potential fluctuations and found that the calculated values come closer to the experimental data the larger the value of the broadening parameter is.

Cyclotron-resonance linewidth oscillations in the integer and fractional quantum Hall regimes.

The cyclotron-resonance linewidth was studied in GaAs/GaAlAs heterostructures as a function of filling factor, magnetic field, and frequency. Maxima in the cyclotron-resonance linewidth are found for integer and for fractional Landau-level fillings of (5/3, (4/3, and (2/3, giving evidence for a reduced screening due to density-of-states gaps. For partly filled levels a strong reduction of the linewidth due to resonant screening is found. The oscillations in the linewidth are highest for high-mobility and low-density samples.

A simple contactless method for evaluating the low-temperature parameters of a two-dimensional electron gas

A sample of two-dimensional electron-gas material is stuck, face upward, onto two flat, Corbino-configuration metal electrodes, thereby providing capacitive coupling (a few pF) to the gas through the high-dielectric-constant substrate material. At high magnetic fields and low temperatures, the minima in σxx produce a high resistance in series with the two coupling capacitors, and this resistance can be measured with an audio-frequency (10 kHz) capacitance bridge. The resistance peaks in high-mobility samples are very sharply defined, and their periodicity in 1/B gives directly the electron concentration. At low fields, prior to the appearance of the peaks, the resistance is proportional to μB2/n, thus relative mobilities may be determined.

Quantized Hall effect and weak localization effects in two-dimensional HgTe–CdTe superlattices

Electronic transport in the two-dimensional gas formed in superlattices of HgTe–CdTe has been investigated in both the weak-field (B<4 mT) and the intense-field regime (up to 29 T). We find that in high-mobility samples (53 000 cm2/V s) coherent backscattering effects (weak localization) are easily observable in the first regime while in the second regime quantized Hall steps are obtained in the same sample. The weak localization results have been exploited to obtain the temperature dependence of the electron dephasing rate and the spin–orbit scattering rate. Unlike AlGaAs–GaAs and Si–metal–oxide semiconductor field-effect transistor systems the spin–orbit scattering rate is very large (0.6 times the elastic scattering rate.) The quantized Hall-effect data are complicated by three factors which we discuss. These complications prevent an unambiguous interpretation at present, although differences from the AlGaAs–GaAs superlattice results are observed.