M.A. Pate

Tuning self-assembled InAs quantum dots by rapid thermal annealing

Blueshifts in the photoluminescence emission energies from an ensemble of self-assembled InAs quantum dots are observed as a result of postgrowth thermal annealing. Enhancement of the integrated photoluminescence emission and narrowing of the full width half-maxima (from 55 to 12 meV) occur together with blueshifts up to 300 meV at annealing temperatures up to 950 °C. Evidence that the structures remain as dots comes form the observation of level filling and photoluminescence excitation studies which reveal LO phonon peaks occurring at multiples of ∼30 meV from the detection energies.

Voltage enhancement in quantum well solar cells

It is known that quantum well solar cells (QWSCs) can enhance short circuit current and power conversion efficiency in comparison with similar, conventional solar cells made from the quantum well (QW) barrier material alone. In this article we report measurements of the dark‐current and open‐circuit voltage (Voc) of a number of quantum well cells in three different lattice‐matched material systems, namely, Al0.35Ga0.65As/GaAs, GaInP/GaAs, and InP/InGaAs. We also present the results obtained from comparable control cells without wells formed either from the material of the barriers or the well material alone. Our results clearly demonstrate in all three cases that, at fixed voltage, QWSC dark currents are systematically lower than would be expected from control cells with the same effective absorption edge. Measurements of Voc in a white‐light source show that the open‐circuit voltages of the QWSCs are higher than those of control cells formed from the well material. Furthermore, this enhancement is more t...

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.

Hybrid magneto-electric states in resonant tunnelling structures

Double barrier resonant tunnelling structures with wide undoped quantum wells are used to study quantum ballistic transport in the presence of a magnetic field B. The structures are based on n−GaAs/(AlGa)As with well widths of 60 and 120 nm. At B=0, the wider well structure (120 nm) shows as many as 70 resonances in I(V). With B applied in the plane of the barriers (B·J) these resonances evolve into hybrid magneto-electric states. At sufficiently large B, the electron orbits no longer extend to the second barrier and tunnelling occurs into cycloidal interface states which are localised near the emitter barrier. A theoretical model for the observed resonances based on the quantisation of the hybrid and cycloidal orbits is presented. Ballistic path lengths of at least 400 nm are observed.

All‐optical switching between modes of a GaAs/GaAlAs multiple quantum well waveguide

A strain‐induced multiple quantum well waveguide which can support two transverse modes in the plane normal to the quantum wells was excited in the TE mode by 350‐ns‐long probe pulses of low intensity from a dye laser tuned to a wavelength of 886 nm. A laser diode emitting at 850 nm was used in order to switch the 886 nm light between two positions at the exit facet of this overmoded waveguide. The switching mechanism has been shown to be non‐thermal in origin.

The application of quantum well solar cells to thermophotovoltaics

Abstract We discuss the advantages of quantum well solar cells (QWSCs) for thermophotovoltaic (TPV) applications and illustrate them with InP/InGaAs and GaInAsP/InGaAs QWSCs which were designed for other applications and have not been optimised for TPV. It is shown that an InP p-i-n solar cell with 15 lattice matched InGaAs quantum wells (QWs) in the i region has an increase in open circuit voltage ( V oc ) of (1.7 ± 0.1) times that of a control cell of InP with InGaAs in the i-region under an illuminating spectrum close to that expected from an ideal ytterbia emitter. Also, using an InGaAsP quaternary cell of band gap wavelength of 1.1 Am with 60 InGaAs QWs under the same illuminating spectrum the current density is increased by a factor of (2.4 ± 0.1) over that of the InP QWSC. The quaternary cell also absorbs longer wavelengths without any significant loss in V OC . Better temperature coefficients for the former quantum well solar cell than the control cell are observed in a spectrum approximating a black body at 3000 K. Further advantages of QWs for narrow band and broad band illuminating spectra are discussed.

Post Growth Fabrication of GaAs/AlGaAs Reflection Modulators via Impurity Free Disordering

Disordering of GaAs/AlGaAs multiple quantum well asymmetric Fabry-Perot modulators (AFPM) was carried out using impurity free vacancy diffusion (IFVD), involving the deposition of a SiO2 cap followed by rapid thermal annealing at 930°C. Blue shifts of up to 52 meV, while maintaining clearly resolved heavy and light hole excitons, were achieved. This enabled the production of both normally-off (contrast >10 dB for -5 V bias) and normally-on (reflection change >30% for -8V) AFPMs from the same wafer, thus displaying how IFVD can be used to tailor the optoelectronic properties after growth.

OPTICAL INVESTIGATION OF CHARGE ACCUMULATION AND BISTABILITY IN AN ASYMMETRIC DOUBLE BARRIER RESONANT TUNNELING HETEROSTRUCTURE

Abstract A photoluminescence study of an asymmetric double barrier resonant tunneling structure, exhibiting intrinsic bistability, is reported. The variation of PL linewidth with bias provides a direct signature of the charge build-up in the well which occurs during resonant tunneling. Accurate values of the charge density are deduced from the Landau level filling in PL spectra taken as a function of magnetic field.

INVERTED BISTABILITY IN THE CURRENT-VOLTAGE CHARACTERISTICS OF A RESONANT TUNNELING DEVICE

Abstract The current-voltage characteristics of an asymmetric double barrier resonant tunneling device show a butterfly-shaped hysteresis loop in which, for a range of voltage, the off-resonant current exceeds the resonant current. This “inverted bistability” is due to the effects of space charge buildup in the quantum well. Magnetoquantum oscillations in the tunnel current with B | J are used to investigate the distribution of charge within the device and the intersubband scattering processes which control the charge buildup.

Application of intermixing to p‐type GaAs/AlAs distributed Bragg reflectors for series resistance reduction in vertical cavity devices

In this paper we experimentally investigate the application of selective interdiffusion to p‐type (Zn doped) distributed Bragg reflectors, as employed within a range of vertical cavity devices, as a means of lowering the series resistance. Various rapid thermal anneal temperatures and times are studied, both with and without silica encapsulants. The degree of intermixing, and hence series resistance reduction, is found to be cap dependent and this is verified both by secondary ion mass spectrometry and electrical resistance measurements. Both these techniques suggest that the intermixing, due primarily to Zn enhanced interdiffusion, is increased when no encapsulant is used. In this case a series resistance reduction approaching 50% is achieved within the 14 period GaAs/AlAs Bragg reflector. In the silica encapsulated case Ga vacancies are injected into the structure and these suppress the movement of Zn. The resulting Ga diffusion coefficient in this case is found to decrease by a factor of 3 relative to ...