Mark Leadbeater

Single Photon Detection with a Quantum Dot Transistor

We propose and demonstrate a type of GaAs/AlGaAs modulation-doped field effect transistor (FET) which is sensitive to single photons. The FET contains a layer of InAs quantum dots formed using an in-situ, self-organising method, adjacent to the channel and separated from it by a thin AlGaAs barrier. Capture of a single photo-excited carrier by a quantum dot leads to a sizeable change in the source-drain current through the transistor, allowing the detection of a single photon. We show this is because the mobility of the electron channel is extremely sensitive to the charge trapped in the dots. This discovery may allow a new type of single photon detector to be developed which does not rely upon avalanche processes.

Quantum magnetic confinement in a curved two-dimensional electron gas

The ability to produce deliberately shaped or curved two-dimensional electron gases in semiconductors using recent developments in technology, for example regrowth of III-V semiconductors on patterned or etched substrates, opens the possibility of investigating not only the behaviour of electrons in a curved quasi-two-dimensional space, and the effects of varying that curvature, but also presents a novel way of investigating electron transport properties in a non-uniform transverse high magnetic field. It is shown that a semi-infinite two-dimensional electron gas subjected to a non-uniform magnetic field has, in addition to current-carrying edge states, one-dimensional states which lie within the interior of the gas, which also have a finite dispersion, an effect which may be used to create quantum wires or other structures. It is also shown that, in the absence of a magnetic field, curvature of the two-dimensional electron gas gives rise to a potential variation which is inversely proportional to the square of the radius of curvature, an effect which may also be used to confine the electronic motion to one dimension.

Electron transport in a non-uniform magnetic field

We have used MBE regrowth technology to produce a non-planar 2DEG at a GaAs/AlGaAs heterojunction grown over an etched facet. By applying a uniform magnetic field to this structure we obtain a spatially varying field component normal to the 2DEG. When the magnetic field is applied in the plane of the substrate, the resistance measured from one side of the facet to the other is found to be quantized at approximately the quantum Hall plateaux. Rotating the plane of the sample with respect to the magnetic field allows us to investigate edge state propagation and reflection in the different regions of the sample. By making the appropriate four-terminal resistance measurement we can directly determine the filling factor on the facet. In particular we study the novel situation where the transverse field component changes sign on the facet and the cyclotron orbits rotate in the opposite sense to those on the planar region.

Mobility (106 cm2 V−1 s−1) of 2DEGs, 30 nm from ex situ patterned GaAs regrowth interfaces

Abstract We have regrown, on ex situ patterned GaAs substrates, using hydrogen radical decontamination, two-dimensional electron gases (2DEGs) confined in 15 nm quantum wells (QWs) with the inverted AlGaAs/GaAs interface 30 nm from the regrowth interface (RI), which conduct before illumination at 1.5 K. Mobilities in excess of 1×10 6 cm 2 V −1 s −1 have been achieved. We have not observed a degradation in the mobility for a constant carrier concentration between 2DEGs, 200 and 30 nm from the RI. A control structure with the 2DEG 30 nm from the RI was also grown on an un-patterned GaAs substrate that was only thermally cleaned, which had a maximum mobility of 3×10 4 cm 2 V −1 s −1 after illumination at 1.5 K.

Ultrashort FETs formed by GaAs/AlGaAs MBE regrowth on a patterned δ doped GaAs layer

Abstract We report on the successful molecular beam epitaxial (MBE) regrowth of a modulation doped GaAs/AlGaAs two-dimensional electron gas (2DEG) only 160xa0A from a patterned GaAs mesa with a (1.5xa0K) mobility, without illumination, of 1×10 5 xa0cm 2 xa0V −1 xa0s −1 at a carrier concentration of 5×10 11 xa0cm −2 . At a regrowth interface/2DEG separation of 300xa0A, mobilities in excess of 1×10 6 xa0cm 2 xa0V −1 xa0s −1 have been realised. By incorporating a Si delta doped layer in the first GaAs growth and using this to modulate the potential within a regrown 2DEG, we have demonstrated the growth of an ultra short FET.

One-dimensional electron transport in devices fabricated by MBE regrowth over a patterned δ-doped backgate

Abstract MBE regrowth and hydrogen radical decontamination of an ex situ patterned GaAs wafer have been successfully used to create a one-dimensional constriction which exhibits quantised ballistic conductance. Deviations from exact quantisation are discussed in terms of electron reflection at the entrance to the constriction.

Double two‐dimensional electron gas structure formed by molecular beam epitaxy regrowth on an ex situ patterned n+‐GaAs back gate

We have regrown two two‐dimensional electron gases (2DEGs) in a wide GaAs quantum well on a large area ex situ patterned n+‐GaAs back gate. The transport in these channels is controlled by this gate and a surface front gate. We present results showing the control that the patterned back gate has over the carrier concentration in the low mobility back 2 DEG and the very low leakage currents that are observed from the back gate to the source‐drain channel at 1.5 K. Using four terminal resistance and magnetoresistance data the transition from two conducting channels to conduction in the low mobility back 2DEG is shown. The implications of these results for the fabrication of velocity modulated transistors are discussed.

Low-dimensional devices fabricated by molecular beam epitaxy regrowth over patterned δ-doped backgates

Abstract Hydrogen radical decontamination of ex situ patterned GaAs wafers was successfully used to grow a two-dimensional electron gas (2DEG) only 160xa0A from the regrowth interface. Interface roughness scattering limits the mobility of this 2DEG to 330u2008000xa0cm 2 /Vs. An initial growth incorporating a delta-doping layer is etched to expose a {411} facet intersecting this back gate and then a 2DEG is regrown over the patterned wafer. We can modulate the potential in the 2DEG on a length-scale given by the width of the doped layer projected onto the facet (