P. T. Coleridge

Inter-subband scattering in a 2D electron gas

A new technique, using the absolute amplitude of the low-field Shubnikov-de Haas oscillations, is presented which allows the direct measurement of inter-subband scattering in 2D heterojunction structures. When applied to a heterojunction with two subbands occupied the technique is used to show that inter-subband scattering is independent of temperature between 1 and 4 K. In agreement with previous reports frequency and amplitude intermodulation, which increases with temperature, is also observed. This is attributed to mixing, by thermal or impurity damping, between the two sets of oscillations in the Fermi energy. This explanation is supported by a model calculation which reproduces the features of the experimental results.

Metal-insulator transition atB=0inp-type SiGe

Observations are reported of a metal-insulator transition in a 2D hole gas in asymmetrically doped strained SiGe quantum wells. The metallic phase, which appears at low temperatures in these high mobility samples, is characterised by a resistivity that decreases exponentially with decreasing temperature. This behaviour, and the duality between resistivity and conductivity on the two sides of the transition, are very similar to that recently reported for high mobility Si-MOSFETs.

Microwave radiation induced magneto-oscillations in the longitudinal and transverse resistance of a two-dimensional electron gas

We confirm the existence of magneto-resistance oscillations in a microwave-irradiated two-dimensional electron gas, first reported in a series of papers by Zudov et al. [Phys. Rev. B 64 (2001) 201311] and Mani et al. [Nature (London) 420 (2002) 646]. In our experiments, on a sample with a moderate mobility, the microwave induced oscillations are observed not only in the longitudinal but also in the transverse-resistance (Hall resistance). The phase of the oscillations is such that the decrease (increase) in the longitudinal resistance is accompanied by an increase (decrease) in the absolute value of the Hall resistance. We believe that these new results provide valuable new information to better understand the origin of this interesting phenomenon.

Experimental study of weak antilocalization effects in a high-mobilityInxGa1−xAs/InPquantum well

The magnetoresistance associated with quantum interference corrections in a high mobility, gated

Growth of high mobility GaN and AlGaN/GaN high electron mobility transistor structures on 4H-SiC by ammonia molecular-beam epitaxy

{\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}s}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{P}

Physical and electrical investigation of ohmic contacts to AlGaAs/GaAs heterostructures

quantum well structure is studied as a function of temperature, gate voltage, and angle of the tilted magnetic field. Particular attention is paid to the experimental extraction of phase-breaking and spin-orbit scattering times when weak anti-localization effects are prominent. Compared with metals and low mobility semiconductors the characteristic magnetic field

Electron spin-orbit splitting in a InGaAs/InP quantum well studied by means of the weak-antilocalization and spin-zero effects in tilted magnetic fields

{B}_{\mathrm{tr}}=\ensuremath{\Elzxh}/4\mathrm{eD}\ensuremath{\tau}

The hall insulator in 2-dimensional sige hole gases

in high mobility samples is very small and the experimental dependencies of the interference effects extend to fields several hundreds of times larger. Fitting experimental results under these conditions therefore requires theories valid for arbitrary magnetic field. It was found, however, that such a theory was unable to fit the experimental data without introducing an extra, empirical, scale factor of about 2. Measurements in tilted magnetic fields and as a function of temperature established that both the weak localization and the weak antilocalization effects have the same, orbital origin. Fits to the data confirmed that the width of the low field feature, whether a weak localization or a weak antilocalization peak, is determined by the phase-breaking time and also established that the universal (negative) magnetoresistance observed in the high field limit is associated with a temperature independent spin-orbit scattering time.

Effective masses in high-mobility 2D electron gas structures

Ammonia molecular-beam epitaxy has been used to grow high-quality epilayers of GaN and AlGaN/GaN heterostructure field-effect transistor (HFET) structures on insulating 4H-SiC. The growth process, which used a magnetron sputter epitaxy deposited buffer layer of AlN, has been described previously. Ex situ pretreatment of the SiC substrate was found to be unnecessary. For a single 2.0 μm thick silicon doped epilayer, a room temperature (RT) electron mobility of 500 cm2/Vs was measured at a carrier density of 6.6×1016 cm−3. For the HFET structure, a room temperature mobility of 1300 cm2/Vs at a sheet carrier density of 3.3×1012 cm−2 was observed, increasing to 11 000 cm2/Vs at 77 K. The surface morphology of the layers indicated a coalesced mesa structure similar to what we observed for growth on sapphire, but with a lower overall defect density and correspondingly larger grain size. The observation of well-resolved Shubnikov de Haas oscillations at fields as low as 3 T indicated a relatively smooth interface.

Universality in an integer quantum Hall transition

The mechanism by which Ni‐Au‐Ge metallizations establish electrical contact to the two‐dimensional electron gas (2DEG) in modulation‐doped AlGaAs/GaAs heterostructures is investigated. Transmission electron microscopy was used to examine samples after electrical characterization by magnetoresistance measurements at cryogenic temperatures. We present a picture in which a 2DEG of reduced electron density exists under the deposited metallization. The success of the contacting procedure is described in terms of the magnitude of this density and the size, areal density, and penetration depth of a series of metallic spikes which establish the electrical link to the 2DEG. We suggest that the electrical behavior is not dominated by the current injection process at the spike/2DEG interface but is instead dictated by scattering from the array of antidots formed by the spikes and by a dependence of the 2DEG density on the size of the metallic pad. The implications of this picture for future nanostructure devices, fe...