M. P. Grimshaw

Resonant tunneling between parallel, two‐dimensional electron gases: A new approach to device fabrication using in situ ion beam lithography and molecular beam epitaxy growth

Using the techniques of in situ focused ion beam lithography and molecular beam epitaxy regrowth high quality, patterned back gate, double quantum well devices have been fabricated. Independent ohmic contacts were made to the two two‐dimensional electron gases (2DEGs) using a ‘‘selective depletion’’ scheme, and using further gates the carrier densities in each well were controlled. Resonant tunneling between the two electron gases was observed as a function of carrier density in each 2DEG, and as a function of the bias applied between the two wells. Extremely large peak‐to‐valley ratios were observed, resulting from removal of unwanted parallel tunneling paths.

Magnetization studies of Landau level broadening in two-dimensional electron systems

We have used a torque magnetometer to measure de Haas - van Alphen oscillations in the magnetization of two-dimensional electrons in GaAs/AlGaAs heterostructures and multiple-quantum-well systems for temperatures ranging from 0.125 K to 4.2 K and in magnetic fields of up to 15 T. Our results indicate that for high magnetic fields the density of states can be described by a series of Lorentzian-broadened Landau levels with a broadening that is independent of the magnetic field, B, and Landau level index, n. However, at low magnetic fields the Lorentzian-broadened density of states becomes indistinguishable from a Gaussian one with a broadening that is proportional to . The high-field behaviour of the Landau level line-shape is shown to differ appreciably from the low-field case as reported by other workers using both magnetization and other experimental methods. The reliability of this and other experimental techniques is discussed.

Raman studies of plasmon modes in a drifting two‐dimensional electron gas

We present the first Raman‐scattering studies of the behavior of the intrasubband plasmon mode of a two‐dimensional electron gas which is undergoing lateral drift in an applied electric field. The data clearly show the expected Doppler shifts of the modes traveling up‐ and downstream, together with the expected dependence on the wave vector, but at higher drift velocities, the expected linear shift is distorted because of electron heating effects.

Aharonov–Bohm effect and one‐dimensional ballistic transport through two independent parallel channels

We have developed a new technique to contact a submicrometer metal dot gate independently of surrounding gates. With this we have made two ballistic channels in parallel on a GaAs‐AlGaAs heterostructure. Independent control over all the gates allows unprecedented flexibility to adjust both the size of the dot and the widths of the channels. In a perpendicular magnetic field we can obtain extremely large Aharonov–Bohm oscillations and a double‐frequency oscillation indicative of edge state charging in an open system. At low magnetic fields we see no evidence for any interaction between the one‐dimensional subbands in the two channels.

Surface decontamination of patterned GaAs substrates for molecular beam epitaxy regrowth using a hydrogen radical source

To investigate the efficiency of hydrogen radical surface cleaning of patterned GaAs substrates, resonant tunneling devices have been fabricated in which the regrowth interface lies between the collector and the double barrier structure. Secondary-ion-mass spectroscopy data collected at the patterned substrate/regrown layer interface show a significant reduction of contaminant species after radical cleaning at ∼450 °C for 2 h. The current–voltage characteristics of the resonant tunneling device further indicate a significant improvement in the quality of the regrowth interface in comparison to that achieved by standard thermal decontamination.

Variation of surface morphology with substrate temperature for molecular beam epitaxially grown GaSb(100) on GaAs(100)

Using molecular beam epitaxy (MBE) several layers of GaSb were grown on GaAs at substrate temperatures of 400, 475, and 550 °C, and the surface morphology was studied with an in situ ultra high vacuum scanning tunneling microscope (STM). We have observed spiral mound growth of different morphology originating from surface dislocations for the samples grown at 400 and 475 °C, however at 550 °C there is no spiral mound growth and neighboring dislocations are joined by a single step. The surfaces have different rms surface roughness and dislocation density which has important consequences with regard to heterointerface quality.

Electronic properties of a one‐dimensional channel field effect transistor formed by molecular beam epitaxial regrowth on patterned GaAs

A high mobility electron gas was grown by molecular beam epitaxy on patterned GaAs consisting of a p‐GaAs/n‐GaAs multilayered structure. By contacting to the p‐GaAs and n‐GaAs layers separately and applying the appropriate bias voltages, we were able to laterally modulate the electron gas to form narrow conduction channels. At 1.5 K we obtain an electron mobility of 5×105 cm2/V s and when operated in the field effect transistor mode a transconductance of 50 ms/mm for a device with a 50‐μm source/drain separation.

FABRICATION OF INDEPENDENT CONTACTS TO TWO CLOSELY SPACED TWO-DIMENSIONAL ELECTRON GASES USING MOLECULAR BEAM EPITAXY REGROWTH AND IN SITU FOCUSED ION BEAM LITHOGRAPHY

Lateral patterning of a buried GaAs epilayer can be achieved during molecular beam epitaxy growth by in situ implantation with a high energy focused ion beam. Applying this technique, 30 keV Ga ions have been used to form small, highly resistive, regions in the backgate layer of a double two‐dimensional electron gas (2DEG) structure. Independent contacts to the two 2DEGs were then achieved by selectively depleting out regions of the upper and lower 2DEGs with potentials on patterned front and backgates. In the resulting devices, the magnetoresistance of the two 2DEG layers was measured both together and separately. It was demonstrated that a bias of up to ±50 mV could be applied, across the 20 nm AlGaAs barrier separating the 2DEGs, with leakage currents of <0.01 nA at 4.2 K. Finally, resonant tunneling between the two 2DEGs was observed when the barrier thickness was reduced to 7 nm. These results have demonstrated the success of this novel in situ fabrication route.

Tunable electron-hole gases in gated InAs/GaSb/AlSb systems

Gated structures have been fabricated on the InAs/AlSb/GaSb system enabling the control of the relative electron and hole carrier densities in these type II crossed band gap structures. By using insulated gates, a wide gate voltage range, with very low leakage levels, is obtained, allowing the study of these systems from electron dominated transport through to hole dominated. Associated with this transition, a change from positive to negative transconductance is observed. Studies of the quantum Hall effect show features related to both electron and hole Landau level formation with the crossover from electron to hole dominated transport found to be a function of the perpendicular magnetic field.

Variation of the confinement potential of a quasi‐one‐dimensional electron gas by lateral p‐n junctions

We present the first study of a new system where the confinement potential of a high mobility quasi‐one‐dimensional electron gas on the (100) GaAs surface is varied using two, two‐dimensional hole gases produced on the adjacent (311)A surfaces. The structure consists of two lateral two‐dimensional p‐n junctions, placed back‐to‐back to form a p‐n‐p structure. The confinement potential of the narrow n‐type channel can thus be modulated by applying a bias to the adjacent p‐type regions. Magnetoresistance measurements of the narrow channel show magnetic depopulation of the one‐dimensional subbands [K.‐F. Berggren, T. J. Thornton, D. J. Newson, and M. Pepper, Phys. Rev. Lett. 57, 1769 (1986)], following the model of Berggren et al. [Phys. Rev. B 37, 10118 (1988)] widths and one‐dimensional carrier concentrations are extracted to fully characterize the dependence of the channel on the applied ‘‘hole‐gate’’ voltage.