Search Results - (Author, Cooperation:M. Y. Simmons)

2018-08-28

American Physical Society (APS)

2160-3308

Physics

2018-03-07

Nature Publishing Group (NPG)

2041-1723

Biology

Chemistry and Pharmacology

Natural Sciences in General

Physics

2018-11-27

American Physical Society (APS)

2160-3308

Physics

2012-01-10

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-12-08

American Association for the Advancement of Science (AAAS)

2375-2548

Natural Sciences in General

2018-07-14

American Association for the Advancement of Science (AAAS)

2375-2548

Natural Sciences in General

1089-7550

AIP Digital Archive

Physics

We demonstrate how two two-dimensional electron gases can be independently contacted using only surface gates defined by electron-beam lithography: macroscopic bar gates to contact the lower layer, and mesoscopic split gates to contact the upper layer. When the technique is applied to a sample consisting of two 150 Å GaAs wells separated by 300 Å Al0.33Ga0.67As barrier, there is more than 30 MΩ isolation at 5 K for interlayer voltages up to ±30 mV; the interlayer isolation operates up to 30 K. The independent contacts are used for a preliminary investigation of two parallel one-dimensional quantum wires. © 2000 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We show that small structures can be defined in high mobility two-dimensional electron gases formed at a depth of 2770 A(ring) below the surface in GaAs/Al0.33Ga0.67As heterostructures. The differential conductance of one-dimensional constrictions defined by split gates in such deep electron gases showed more than 20 quantised plateaus. The absence of resonant structures on the plateaus demonstrates the absence of potential fluctuations in the constrictions. By applying a dc source-drain bias we have measured the energy spacings of the first 18 subbands, and the effect of a small perpendicular magnetic field on the energy spacings has been investigated. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We describe the fabrication and transport properties of clean long one-dimensional quantum wires formed in high-quality modulation-doped GaAs/AlGaAs heterostructures. In a 3 μm wire, we observe 25 conductance steps with no superimposed resonant features. With increasing split-gate length, we observe a crossover from ballistic towards diffusive transport, and in 5 and 6 μm wires, a reduction of conductance plateau values as large as 8% and 25% is observed, respectively. © 1999 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We have developed a technique for the fabrication of high-mobility electron gases formed in undoped GaAs/AlGaAs heterostructures. The use of an insulated gate allows independent control over the carrier density in the Hall bar and ohmic contact regions of the device. This unique design eliminates difficulties in obtaining reliable ohmic contacts, particularly in the low carrier density regime. In the absence of remote ionized impurity scattering, extremely high transport mobilities are obtained at low carrier densities (1×106 cm2 V−1 s−1 at 1×1010 cm−2). This design has been adapted to the formation of undoped one-dimensional electron gases that show clean and reproducible conductance plateau at 1.5 K. © 1999 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We investigate transport through a two-dimensional electron gas (2DEG) underneath a submicron periodic array of small Ge-Ag alloyed ohmic contacts and compare the results with the resistance of the small contacts. We find that as the contacts form, the effective mobility of the 2DEG underneath the array decreases and that the carrier concentration between the small contacts is reduced. From the low-field magnetoresistance we are able to identify ballistic cyclotron orbits commensurate with the period of the array. We find that reducing the diameter of the ohmic metallization to 0.2 μm severely inhibits ohmic contact formation. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Double layer structures have been used to measure the compressibility of both two-dimensional electron (2DEG) and two-dimensional hole gases (2DHG). This has been achieved by making independent contacts to the lower layer and using it to detect the compressibility of the upper layer. Both positive and negative compressibilities are observed, with the negative compressibility enhanced for hole gases, where the changes of the interaction energy are larger than those of the kinetic energy. The negative compressibility of the top layer leads to a charge transfer between the layers; this has been modelled and gives good agreement with the experiments when exchange and correlation is included. Due to disorder in the layers, however, the model deviates from the experimental results close to depletion. The dependence of the charge transfer upon distance between the electron gases has also been studied, with the charge transfer decreasing linearly with increasing spacing between the 2DEGs. This suggests that inter-layer interactions are insignificant. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Using an n-type (311)A GaAs substrate we have fabricated in situ back-gated GaAs/(Al,Ga)As hole gases with mobilities of μ=1.1×106 cm2 V−1 s−1 at 30 mK. We have investigated both experimentally and theoretically the scattering mechanisms that limit the mobility in both the [2¯33] and [011¯] directions. Using a combination of front and back gates to keep the carrier density constant, we can distinguish between scattering mechanisms which are primarily dependent on the carrier density and those that are sensitive to the shape of the hole wave function. This approach also eliminates complications arising from the variations of the Fermi surface anisotropy with carrier density. Our data confirms that anisotropic interface roughness scattering, arising from the nature of the (311)A GaAs surface, is the dominant scattering mechanism at carrier densities down to ps=5.0×1010 cm−2. © 1997 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

We describe a detailed device fabrication technique for the formation of a lateral quantum dot using a multilayered gated design. In our versatile system, a quantum dot is electrostatically defined by a split gate and two overlaying finger gates which introduce entrance and exit barriers to the dot. Periodic and continuous conductance oscillations arising from Coulomb charging effects are clearly observed in the transport properties at low temperatures. © 2000 American Institute of Physics.

Electronic Resource

1089-7623

AIP Digital Archive

Physics

Electrical Engineering, Measurement and Control Technology

A noninvasive voltage probe, consisting of a one-dimensional channel, has been fabricated within a modulation doped GaAs/AlGaAs heterostructure. With precision piezoelectric scanning equipment this probe has been brought to within 14 nm of a semiconductor device which includes surface gates and a 300 nm deep two-dimensional electron gas. Measurements of voltages applied to these conducting layers have been made at room temperature and at 4.2 K using the sensitive conductance of the one-dimensional channel. A voltage resolution of 0.48 mV has been observed at 4.2 K. Probe conductance measurements have also been made as a function of probe–sample separation. The conductance–separation data were fitted using a simple parallel plate capacitor model and a height resolution of 4 nm was calculated. Images of the sample layers have been obtained and a lateral resolution of 760 nm observed. © 2001 American Institute of Physics.

Electronic Resource