T. J. Thornton

Single-electron effects in a point contact using side-gating in delta-doped layers

Side‐gated point‐contact structures in delta (δ)‐doped layers have been used to form single‐electron tunnel junctions with variable resistance. Clear Coulomb‐blockade effects have been observed in the current‐voltage characteristics. The measured characteristics are described in terms of a series of single‐electron transistors formed by microsegments within the point contact. The effective tunnel capacitance and side‐gate capacitance are estimated to be 10 and 1 aF, respectively, which are both one order of magnitude smaller than the reported capacitance of tunnel junctions made from Al or GaAs/AlGaAs heterostructures.

Si/SiGe quantum wells grown on vicinal Si(001) substrates: Morphology, dislocation dynamics, and transport properties

Compositionally graded, strain relaxed Si0.72Ge0.28 buffers were grown on vicinal Si(001) substrates by gas source molecular beam epitaxy. Misfit dislocations are shown to run along intersections of the {111} glide planes with the (11n) interface. X-ray diffraction studies demonstrate a relative tilt of the epilayer to the substrate in a direction which depends on the interplay between substrate orientation related preferential dislocation nucleation rates and surface contamination induced heterogeneous nucleation. Atomic force microscopy (AFM) images reveal an anisotropy in surface roughness on the μm scale related to reduced growth rates on vicinal surfaces. Transport properties at 0.4 K in two dimensional electron gases grown on these relaxed SiGe buffers show anisotropic scattering times similar to interface roughness scattering which can be correlated to terrace configurations in the nm range determined by AFM.

SI:SIGE QUANTUM WELLS GROWN ON (118) SUBSTRATES : SURFACE MORPHOLOGY AND TRANSPORT PROPERTIES

We have grown strained Si quantum wells on relaxed Si0.7Ge0.3 buffer layers using vicinal (118) silicon substrates. Compared to conventional (001) substrates the surface is tilted by 10° towards the [110] direction resulting in terraces with step edges which run parallel to [110]. The surface morphology of the layers shows “cross-hatching” characteristic of relaxed SiGe films grown on Si substrates. However, the cross-hatching is not orthogonal but aligns along directions in which (111) planes intersect the (118) surface. We have measured the low temperature transport properties of the two-dimensional electron gas confined within the strained Si channel. When measured with current flowing parallel to the step edges the electron mobility is approximately four times larger than that measured in a perpendicular direction showing the strong elastic scattering associated with the step edges. In contrast the single particle relaxation time is almost identical for the two different orientations.

Side gating in δ‐doped quantum wires

We have used a side gating technique to vary the width of narrow wires made from δ‐doped GaAs. The wires show pronounced quantum interference effects which can be used to determine the wire width and electron phase coherence length as a function of gate voltage. At zero gate bias the depletion from the etched surfaces is very small because of the high electron density and the electrical width of the wires is therefore only slightly smaller than the physical width.

Leakage currents in virtual substrates: measurements and device implications

Leakage currents originating in the virtual substrates which are required in many Si heterostructure systems have been measured. Both ohmic (AuSb) and Schottky (Pt) contacts to a modulation-doped Si:SiGe heterostructure show significant leakage when the contacts cover deep pits originating from growth defects and contaminants. Shallower pits emerging later in the growth process do not contribute to extra conduction. These pits appear after growth of the graded layer which leads us to conclude that carrier transport from the contact along the dense network of dislocations formed in the graded buffer layer is responsible for the leakage found in Si:SiGe systems making use of virtual substrates.

Characterization of n-channel Si/SiGe modulation doped structures grown by gas source molecular beam epitaxy

We have grown n-channel Si/SiGe modulation doped structures by gas source molecular beam epitaxy using arsine as the n-type dopant source. The structures were characterized by transmission electron microscopy, secondary ion mass spectroscopy, electrochemical capacitance voltage analysis and x-ray diffraction. Arsenic and free electron concentrations in excess of 1019 cm-3 could be obtained with substantial surface segregation. Several different structures have been grown and their transport properties investigated. Low-temperature electron mobilities of up to 60000 cm2 V-1 s-1 in the dark (75800 cm2 V-1 s-1 after illumination) were obtained with a sheet density range (4-7)*1011 cm-2. Parallel conduction is discussed in terms of the effect of illumination.

Negative magnetoresistance and electron-electron interaction in Si:SiGe quantum wells

Abstract We have measured the magnetoresistance of n-channel Si:SiGe quantum wells in the temperature range 0.4–6.6 K. For magnetic fields less than 1 T and before the onset of Shubnikov-de Haas oscillations, there is a broad negative magnetoresistance. The change in resistance follows a B2 dependence and can be explained in terms of the 2D electron-electron interaction.

Electron focusing with a double grid in AlGaAs/GaAs heterostructures

Electron focusing has been observed in single‐ and double‐grid structures. The magnetoresistance shows strong maxima and minima with values both above and below the zero field resistance. These are interpreted in terms of electron trajectories which are either backscattered from or transmitted through the grids, with a good match between the measured and calculated magnetic field values for the position of these peaks. The difference in magnetoresistance between the two structures is explained by an internal scattering mechanism which is present only for the double‐grid system.

Superconducting quantum wells for the detection of submillimeter wave electromagnetic radiation

We propose the use of superconducting quantum well structures for use as detectors of submillimeter wave radiation. The energy levels formed due to quantum confinement by superconducting barriers provide a useful system for radiation sensing. The well width could be readily altered by varying the temperature or the strength of an applied magnetic field so that the levels can be shifted in energy. This provides a means of tuning the detector over a range of frequencies in the terahertz range, enabling both photometric and spectroscopic observations. Also, the structure of the device should permit use of cofabricated antennas allowing efficient radiation coupling into the active region of the device. The geometry also allows the possibility of implementing one- and two-dimensional arrays.

Anomalous magnetoresistance at a mesoscopic bend

We have carried out magnetoresistance measurements on a mesoscopic cross formed from two split gates on a GaAs/AlGaAs 2DEG and have observed anomalous peaks in the bend resistance before the electron collimation is finally destroyed by the magnetic field. We have considered two possible explanations for this effect, and have rejected the possibility of quantum mechanical effects in favor of a classical one in which we postulate the presence of an impurity close to the center of the active region.