Carl E. Norman

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.

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.

Surface-acoustic-wave-driven luminescence from a lateral p-n junction

The authors report surface-acoustic-wave-driven luminescence from a lateral p-n junction formed by molecular beam epitaxy regrowth of a modulation doped GaAs∕AlGaAs quantum well on a patterned GaAs substrate. Surface-acoustic-wave-driven transport is demonstrated by peaks in the electrical current and light emission from the GaAs quantum well at the resonant frequency of the transducer. This type of junction offers high carrier mobility and scalability. The demonstration of surface-acoustic-wave luminescence is a significant step towards single-photon applications in quantum computation and quantum cryptography.

Lateral n–p junction for acoustoelectric nanocircuits

We report the experimental realization of a device comprising a lateral n–p junction grown by focused-ion molecular-beam epitaxy and a transducer to generate a surface acoustic wave. Acoustic charge transport across the junction and the accompanying photon emission are demonstrated. This type of light-emitting diode is suitable for integration into acoustoelectric nanocircuits in which quasione-dimensional semiconductor channels serve as “wires” through which packets of charge are transported by surface acoustic waves. The diode provides a means by which to extend the functionality of acoustoelectric nanocircuits into the optical domain.

Comparison of spontaneous and piezoelectric polarization in GaN/Al0.65Ga0.35N multi-quantum-well structures

We report a luminescence study of GaN/Al0.65Ga0.35N multi-quantum-well structures. The surface of the samples exhibits microcracking allowing the same quantum well to be measured under two different strain conditions. We can accurately describe the emission energies in the two strain conditions by considering piezoelectric polarization alone in contrast to the theoretical prediction that spontaneous polarization effects should dominate.

Encoded and multiplexed surface plasmon resonance sensor platform.

We present a flexible new sensor system that combines the joint advantages of (i) discretely functionalized, code-bearing, microparticles and (ii) label-free detection using grating-coupled surface plasmon resonance. This system offers the possibility of simultaneously investigating the real-time binding kinetics of a variety of molecular interactions. One single multiplexed assay could employ a wide range of immobilization chemistries, surface preparation methods, and formats. Thus, the new system offers a very high level of assay conformability to the end user, particularly when compared to fixed microarrays.

The two-dimensional lateral injection in-plane laser

In this paper, a two-dimensional (2-D) p-n junction was used for population inversion in a GaAs quantum-well laser. The device, incorporating modulation doping within the core of a separate confinement heterostructure, was designed to exploit the amphoteric behavior of silicon in GaAs [doping p-type on [311]A facets and n-type on [100]]. It is believed to be the first lasing device to use an amphoterically doped junction for population inversion. In the first attempted design (described here), CW lasing was achieved at temperatures up to 90 K. The factors affecting the temperature dependence of threshold are discussed in the context of possible design improvements. The device may eventually show improved modulation bandwidth over conventional vertical injection lasers with bulk contacts, since its geometry and the 2-D nature of the injection offer reduced capacitance, HEMT integration, and an elimination of carrier capture problems.

Strong directional dependence of single-quantum-dot fine structure

By isolating quantum dots in microstructures with cleaved facets, we measure individual-quantum-dot photoluminescence emitted in the in-plane direction, in addition to the widely studied vertical direction. The emission is shown to be polarized in the plane, and the observed fine structure is found to be extremely directionally-dependent. These characteristics are attributed to exciton states with orthogonally aligned dipoles in the plane. The result suggests possibilities for single-quantum-dot devices, including side-emitting single-photon sources.

Optical spectroscopic study of electric field sharing effects in piezoelectric GaN/Al0.65Ga0.35N multi-quantum well structures

Abstract We have studied a series of GaN/Al 0.65 Ga 0.35 N samples of varying well and barrier widths by cathodoluminescence and photoluminescence spectroscopy. These samples consist of unstrained GaN quantum wells and strained Al 0.65 Ga 0.35 N barriers. Some of the samples exhibit micro-cracks on their surface around which the barriers are partially relaxed and the quantum wells strained. The observation of the quantum well emission under two different strain conditions allows the relative effects of spontaneous polarization and piezoelectric polarization on the emission energies to be compared. We are able to accurately model the emission energies for both strain configurations if we consider the large electric fields present within the quantum wells to be due almost entirely to piezoelectric and field sharing effects.

Optical control of the mobility of a MODFET with a layer of self-assembled quantum dots

Abstract We report an experimental study of GaAs/Al 0.33 Ga 0.67 As modulation doped field effect (MODFET) transistors, in which an InAs layer of self-assembled quantum dots is placed in one of the Al 0.33 Ga 0.67 As barrier layers close to the two-dimensional electron gas (2DEG). We find the source–drain resistance is bistable with the two states controlled by illumination and applied gate bias. Brief illumination induces a large, persistent drop in the resistance, which can be recovered by applying a positive gate bias. Magneto-transport measurements show that while illumination causes only a relatively small change in the 2DEG density, it can greatly enhance its mobility. We suggest this is because the 2DEG mobility is limited by percolation of the electrons through the rough electrostatic potential induced by the charged dots. Illumination reduces the negative charge trapped in the dots, thus smoothing the conduction band potential, which produces a large increase in the mobility.