M. L. Leadbeater

Detection of single photons using a field-effect transistor gated by a layer of quantum dots

We demonstrate that the conductance of a field-effect transistor (FET) gated by a layer of nanometer-sized quantum dots is sensitive to the absorption of single photons. Rather than relying upon an avalanche process, as in conventional semiconductor single-photon detectors, the gain in this device derives from the fact that the conductivity of the FET channel is very sensitive to the photoexcited charge trapped in the dots. This phenomenon may allow a type of three-terminal single-photon detector to be developed based upon FET technology.

Epitaxial ferromagnetic τ‐MnAl films on GaAs

We report the growth of epitaxial τ‐MnAl ferromagnetic films on GaAs substrates by molecular beam epitaxy (MBE). Reflection high‐energy electron diffraction and x‐ray diffraction show that the τ‐phase films grow with the c axis of the tetragonal unit cell normal to the {100}GaAs substrate surface. In the bulk, τ‐MnAl is a metastable ferromagnetic phase with uniaxial magnetocrystalline anisotropy. The large hysteresis observed in the Hall resistance versus applied magnetic field suggests that the easy magnetization direction is indeed parallel to the c axis in the MBE‐grown films. The growth of these ferromagnetic films with perpendicular magnetization on compound semiconductor substrates creates the possibility of novel devices that combine magnetic memory and magneto‐optic functions with semiconductor electronics and photonics.

Sequential tunneling due to intersubband scattering in double‐barrier resonant tunneling devices

Magnetoquantum oscillations in the tunnel current of double‐barrier n‐GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi‐bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.

High-frequency acousto-electric single-photon source

We propose a single optical photon source for quantum cryptography based on the acousto-electric effect. Surface acoustic waves (SAWs) propagating through a quasi-one-dimensional channel have been shown to produce packets of electrons which reside in the SAW minima and travel at the velocity of sound. In our scheme these electron packets are injected into a p-type region, resulting in photon emission. Since the number of electrons in each packet can be controlled down to a single electron, a stream of single (or N) photon states, with a creation time strongly correlated with the driving acoustic field, should be generated.

Magnetic field studies of negative differential conductivity in double barrier resonant tunnelling structures based on n-InP/(InGa)As

Abstract Negative differential conductivity (NDC) with a peak/valley ratio of 4.5:1 (4 K) and 2:1 (150 K) is observed in double barrier resonant tunnelling devices based on n-InP/(InGa)As. A transverse magnetic field applied in the plane of the tunnelling barriers ( J ¦ B ) significantly changes the current-voltage characteristics and eliminates the NDC for fields above −10 T. This behaviour is explained qualitatively in terms of the effect of the magnetic vector potential on the tunnelling electrons. The magneto-oscillations in the tunnelling current for J ‖ B are discussed in terms of a simple model of resonant tunnelling.

Galvanomagnetic properties of epitaxial MnAl films on GaAs

Single‐crystal films of τ‐MnAl are grown by molecular‐beam epitaxy on GaAs substrates. The extraordinary Hall effect shows a large hysteresis loop, and the magnetization is found to have a substantial component perpendicular to the plane of the film.

UNIVERSAL CONDUCTANCE FLUCTUATIONS IN THE MAGNETORESISTANCE OF SUBMICRON-SIZE N+-GAAS WIRES AND LATERALLY CONFINED N--GAAS/(ALGA) AS HETEROSTRUCTURES

Abstract Universal conductance fluctuations (UCF) in the magnetoresistance of electron beam lithographed n+-GaAs wires of length 10 μm and widths down to 90 nm are examined over the temperature range from 2 to 110 K and at magnetic fields up to 11 T. The data are interpreted in terms of a simple physical model based on Fourier transforms which indicates an intimate connection between the UCF and weak-localisation. Numerical values obtained for the phase-breaking scattering rate which contains a significant temperature-independent contribution. In the transverse magnetoresistance of laterally confined n−-GaAs/(AlGa)As heterostructures, both the UCF and the negative magnetoresistance, arising from localisation and interaction effects, vary as B cos θ where θ is the angle between B and the 2DEG plane.

Electroluminescence and impact ionization phenomena in a double‐barrier resonant tunneling structure

Electroluminescence (EL) due to impact ionization in the high field region of a double‐barrier resonant tunneling structure is reported. Knowledge of the charge distribution in the structure enables a detailed analysis to be made of the impact ionization rate as a function of electric field. Large peak‐to‐valley ratios of 15:1 in the EL emission intensity from the quantum well active region are observed.

Novel strained InP/InAsxP1−x quantum‐well modulation‐doped heterostructures

We report the first investigation of transport properties of two‐dimensional electron systems in strained InAsxP1−x quantum‐well channels. The InP/InAsxP1−x modulation‐doped heterostructures were grown by the low‐pressure OMCVD technique. The dependence of the transport parameters on the arsenic composition (x) has been studied using Hall, Shubnikov de Haas, and cyclotron resonance measurements. Hall mobilities of 6100 and 52 700 cm2/V s, with carrier concentrations of 2.3×1012 and 1.5×1012 cm−2 at 300 and 77 K, respectively, were measured for x=0.6.

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.