Christine Roberts

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.

Time resolved study of self‐assembled InAs quantum dots

We have investigated the exciton kinetics in self‐assembled InAs quantum dots and ultrathin quantum wells grown by molecular beam epitaxy on (001) oriented GaAs substrates. At low temperatures, the photoluminescence decay time of the quantum wells increases almost linearly while the decay time of the quantum dot system is independent of temperature. However, above 50 K there is a linear increase in the decay time of the dots which may be due to electrons escaping into the wetting layer or occupation of nonradiative exciton states. Under the conditions of high injection, relaxation from the excited states has a time constant of about 500 ps.

1.3 µm Room Temperature Emission from InAs/GaAs Self-Assembled Quantum Dots

We have investigated the growth conditions necessary to achieve strong room temperature emission at 1.3 µm for InAs/GaAs self-assembled quantum dots (QDs) using conventional solid source molecular beam epitaxy (MBE). A relatively high substrate temperature and very low growth rate (LGR) result in long wavelength emission with a small linewidth of only 24 meV. Atomic Force Micrographs obtained from uncapped samples reveal several differences between the LGRQDs and those grown at higher growth rates. The former are larger, more uniform in size and their density is lower by a factor of about 4. LGRQDs have been incorporated in p-i-n structures and strong room temperature electroluminescence detected. The light output of the QD p-i-n diodes is found to be significantly higher than a quantum well (QW) sample at least for current densities up to 0.5 kAcm-2.

Influence of indium segregation on the emission from InGaAs/GaAs quantum wells

Indium segregation in InxGa1−xAs/GaAs (0.05<x<0.25) quantum wells grown by molecular beam epitaxy has been investigated using low temperature photoluminescence. Additional features at low energy are evident in some of the spectra that are consistent with trapping of free excitons by In‐rich islands at the top interface, which occurs as the result of In segregation.

Simulating multiple quantum well solar cells

The quantum well solar cell (QWSC) has been proposed as a route to higher efficiency than that attainable by homojunction devices. Previous studies have established that carriers escape the quantum wells with high efficiency in forward bias and contribute to the photocurrent. Progress in resolving the efficiency limits of these cells has been dogged by the lack of a theoretical model reproducing both the enhanced carrier generation and enhanced recombination due to the quantum wells. Here we present a model which calculates the incremental generation and recombination due to the QWs and is verified by modelling the experimental light and dark current-voltage characteristics of a range of III-V quantum well structures. We find that predicted dark currents are significantly greater than experiment if we use lifetimes derived from homostructure devices. Successful simulation of light and dark currents can be obtained only by introducing a parameter which represents a reduction in the quasi-Fermi level separation.

Scanning Transmission Electron Microscopy (STEM) Study of InAs/GaAs Quantum Dots

Scanning transmission electron microscopy (STEM) and energy dispersive X-ray analysis (EDX) have been used to investigate the size and composition of InAs/GaAs quantum dot (QDs). It is shown that the QD exist within the wetting layer and not on it. In QD bilayers where the dots are uncorrelated along the growth direction a comparison of the indium EDX signals from the wetting layer (WL) and a dot allow us to estimate the compositions of these regions as In0.07Ga0.93As and In0.31Ga0.69As respectively. We have used the STEM technique to investigate the effects of annealing QDs in order to modify the emission energy. EDX measurements show that the dots increase in size by a factor of 2 for the longest anneals and there is a concomitant decrease in the indium concentration resulting in blue shifts up to 300 meV and a narrowing of the linewidth to ~12 meV.

3 m InAs resonant-cavity-enhanced photodetector

This paper presents the design, fabrication and optoelectronic characterization of an InAs resonant-cavity-enhanced photodetector, intended for the detection of methane gas, incorporating a 10 period GaAs/AlAs distributed-Bragg reflector. The spectrally narrowed responsivity was resonantly enhanced to a value of 34.7 A W−1, at an applied voltage of 3.0 V. This high resonantly enhanced peak in the responsivity was at λ = 3.14 µm, which is only 5% away from the target value.

1.3 μm InAs/GaAs quantum dot led

We have determined the growth conditions which result in a narrow linewidth and room temperature emission at 1.3pm from InAs/GaAs Quantum dots (QDs). The QDs formed under these conditions are extremely uniform in size and exhibit an emission linewidth of only 25meV. Single QD layers have been incorporated into p-i-n diodes which exhibit strong electroluminescence. We have compared the efficiency of these devices with a nominally identical quantum well device. The QD based device exhibits a higher electroluminescence efficiency, especially at low current densities. At higher current densities there is a loss of efficiency due to recombination from excited states.

Experimental and theoretical studies of a novel hetero-nipi reflection modulator

A novel GaAs/Al/sub 0.35/Ga/sub 0.65/As hetero-nipi all-optical reflection modulator has been investigated, both theoretically and experimentally. The modulation is found to be dominated by the refractive index changes that occur in the quantum wells as a result of optically induced electric field changes via the quantum-confined Stark effect. Good agreement is found between the theoretical and experimental behavior, with a maximum absolute experimental modulation of 11% and a maximum contrast ratio of 4.4:1 being observed at room temperature at optical pump densities of only 1.9 mW/cm/sup 2/. A numerical model of the device is developed, which predicts large reflectivity modulations of greater than 53% with a contrast ratio of 25:1 in optimized device structures. >

Voltage performance of quantum well solar cells in the Al/sub x/Ga/sub 1-x/As/GaAs and the GaAs/In/sub y/Ga/sub 1-y/As material systems

The open circuit voltage V/sub oc/ and reference voltage V/sub ref/ defined as a measure of the dark current quality, have been studied for a large number of quantum well (QW) solar cells and homogenous control cells. Samples were grown in the Al/sub x/Ga/sub 1-x/As/GaAs and GaAs/In/sub y/Ga/sub 1-y/As material systems. For both combinations, QW solar cells show a better voltage performance in V/sub oc/ and V/sub ref/ than one would expect from a single bandgap solar cell with the same effective absorption bandgap E/sub a/. For the AlGaAs/GaAs cells, V/sub oc/ is related to structural parameters of the QW cells such as the well width L/sub W/ and the Al fraction x. For the strained GaAs/InGaAs cells a relationship is found between V/sub ref/ and the barrier width L/sub B/, which is a dominant parameter in determining strain relaxation and defect formation at a fixed In fraction.