Quantum Hall Measurements Research Articles

Induced superconductivity in graphene

Graphene layers, prepared by mechanical exfoliation, were contacted by superconducting electrodes consisting of a titanium–aluminium bilayer. Quantum hall measurements in the normal state confirmed the single layer nature of the graphene samples. Proximity induced supercurrents were observed in all samples, below 1 K. Using a backgate, the Fermi energy could be swept from valence to conduction band via the Charge neutrality point, demonstrating supercurrents carried by holes and electrons, respectively. Interestingly, a finite supercurrent was also observed at the charge neutrality (or Dirac) point, where the density of carrier states vanishes. Our results demonstrate phase coherence in graphene.

AC longitudinal and contact resistance measurements of quantum Hall devices

A new coaxial measurement system has been developed to investigate the ac longitudinal resistance along the high-potential side of the quantum Hall resistance (QHR). A novel equivalent circuit of the QHR is used to analyse the ac measurements of the longitudinal resistances along both the low- and high-potential sides of the QHR sample. In addition, a bridge for the measurement of ac contact resistances of the sample is presented. For the first time, it is now possible to perform all the ac measurements whose dc equivalents are well established for reliable dc quantum Hall measurements. While the ac longitudinal resistances on the high- and low-potential sides of the sample are very similar, interesting differences have been observed at high resolution.

Structural and transport characterization of AlSb/InAs quantum-well structures grown by molecular-beam epitaxy with two growth interruptions

We have investigated the electron transport properties and the atomic morphology of AlSb/InAs/AlSb quantum wells (QW) grown by molecular-beam epitaxy. Different shutter sequences were used in producing an InSb-like interface. The highest mobility was obtained for a QW width of 15 nm and an InSb-like interface grown by two growth interruptions, one before and one after the deposition of one indium monolayer. For this shutter sequence, several samples with an InAs channel width from 6 to 25 nm were grown and characterized using high-resolution transmission electron microscopy, classical, and quantum Hall measurements. For a channel width less than 15 nm, the interface roughness becomes dominant, leading to a sharp decrease in the electron mobility. The electron effective mass determined by the temperature dependence of the Shubnikov–de Haas oscillazion amplitude is 0.0374m0. Transmission electron microscopy images show an atomically abrupt interface and disordered regions directly above the AlSb/InAs interface which can be as large as 2.4 nm.

AlSb/InAs HEMTs using modulation InAs(Si)-doping

AlSb/InAs HEMTs have been fabricated using Si-doping in a very thin (9 A) InAs layer located in the upper AlSb barrier. Quantum Hall measurements show that there is no parallel channel. Devices with a 0.5 µm gate length exhibit a drain current density of > 1 A/mm and an fTLg product of 30 GHzµm at VDS = 0.4 V after correction for the gate bonding pad capacitance.

Evidence of a Hybridization Gap in ``Semimetallic'' InAs/GaSb Systems

InAs/GaSb composite quantum wells sandwiched by AlSb are studied by using capacitance-voltage, quantum Hall, and three-terminal transfer measurements. Our data reveal a positive energy gap resulting from the hybridization of in-plane dispersions of electrons in InAs and holes in GaSb for conventionally recognized ``semimetallic'' InAs/GaSb heterostructures.

Methods for magnetotransport characterization of IR detector materials

Advanced techniques for the magnetotransport characterization of semiconductor IR detector materials are reviewed. Both conventional mixed-conduction and 'mobility spectrum' analyses of the resistivity tensor as a function of magnetic field and temperature are discussed, as well as a hybrid approach which exploits the advantages of both methods. Assuming that the data are sufficiently sensitive, one obtains concentrations and mobilities for each electron and hole species present in a given sample. This is particularly valuable for narrow-gap infrared detector materials such as Hg1-xCdxTe, since the coexistence of multiple species tends to be the rule rather than the exception, and 'anomalous' Hall data are easily misinterpreted if inferences are drawn from measurements at only a single magnetic field. In addition to bulk electron and hole densities and mobilities, one can determine inversion and accumulation layer properties from the anomalous Hall effect, acceptor binding energies and compensation ratios from the low-temperature freeze-out of free holes, and energy gaps from the temperature dependence of the intrinsic carrier concentration. Shubnikov-de Haas and quantum Hall measurements provide additional information about the spatial distribution of the carriers. Electron and hole mobilities in Hg1-xCdxTe detector materials will be briefly reviewed, and theoretical and experimental results compared.

Interactions between hot injected electrons and the cold electrons in the two-dimensional electron gas base of a vertical hot electron transistor

We introduce a vertical hot electron transistor structure in which the base is a two-dimensional electron gas (2DEG), upon which we can carry out both magnetoresistance and quantum Hall measurements as the transistor is turned on. Hot electrons are injected to have a controlled angle of incidence with cold electrons in a 2DEG and we examine the resulting effects on its magnetoresistance. We present new data on the various forms of electron heating that we are able to achieve in the base.

CuGeMn, a new material for high precision cryo-resistors

The electrical resistance of CuGeMn alloys was measured as a function of temperature. It shows maxima at temperatures between 1.5 K and 9.1 K. Since the temperature coefficient at the maxima is zero and since the resistivity is high, these alloys are very suitable for the construction of cryo-resistors which are operated in liquid helium and can be used to improve quantum Hall measurement systems. >

Quantum Hall measurements from 4 K to 20 mK

We report on precision measurements of the quantum Hall resistance (QHR) in the temperature range of 20 mK to 4 K. These include intercomparisons of the ohm maintained at the National Research Council (OHM NRC) and the International System ohm (OHM SI) and the QHR. The effects of nonzero Rxx minima, sample inhomogeneity, nonohmic contacts, and dissipation are summarized.

Four-terminal quantum hall and Shubnikov-de Haas measurements with pulsed electron fields

Quantum Hall and Shubnikov-de Haas resistances have been measured at 2.2 K on two-dimensional electron gases in both MOCVD- and MBE-grown GaAs-AlxGa1−x As heterojunctions. Four-terminal measurements in Hall bar geometries have been performed using pulsed electric fields with pulse duration down to 100ns. There are no indications that the Quantum Hall effect breaks down on this time scale.

Quantum Hall measurements with pulsed electric fields

Quantum Hall and Shubnikov-de Haas measurements were performed on a two-dimensional electron gas in MOCVD-grown GaAs-Al xGa 1−xAs heterojunctions at 2.2 K. We used pulsed electric fields with pulse lengths as short as 100 ns. There are no indications that the Quantum Hall effect breaks down on this time scale clearly demonstrating that the Quantum Hall effect is not solely a d.c. effect.

Coulomb correlations and the quantum Hall effect

We argue that at low temperatures and large magnetic fields, Coulomb correlations among the electrons in two-dimensional structures determine the recently observed temperature dependence of the quantum Hall measurements for the highest fields.