Surama Malik

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.

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.

A photomodulated reflectance study of InAs/GaAs self-assembled quantum dots

Photomodulated reflectance (PR) spectra have been measured for self-assembled InAs/GaAs quantum dot(QD) structures consisting of a pair of QD layers, with a GaAs spacer either 50 or 100 A thick. The PR clearly reveals five confined-state QD transitions, at both 80 and 300 K, as well as features from the two-dimensional confining and GaAs layers. The measuredQD transition energies correlate well with photoluminescencespectra at 13 K, using high laser excitation powers to incur level filling. Annealing one of the samples produces a strong blueshift in the QD transitions.

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.

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.

Controlling growth of InAs/GaAs self-assembled quantum dots to give 1.3 μm room temperature emission

We have investigated the extent to which the emission wavelength of self-assembled InAs/GaAs quantum dots can be controlled by growth parameters using conventional solid source MBE. Changing from conventionally high growth rates to a very low growth rate (LGR) and a relatively high substrate temperature, tunes the photoluminescence (PL) emission from 1.1 μm to 1.3 μm at room temperature. Atomic force micrographs obtained from uncapped samples reveal that these LGRQDs are larger, lower in density and extremely uniform in size. The improved size uniformity is reflected in the reduction of the PL linewidth from 78 meV to 22 meV. Under conditions of high excitation, emission from the ground and two excited states each separated by ∼70 meV is observed. This implies a parabolic confining potential. Time resolved photoluminescence (TRPL) measurements of dots grown under the various growth conditions yield radiative lifetimes which reflect the depth of the confining potential. A comparison of the decay times measured for the excited states show that the relaxation of carriers within the dots cannot be ascribed to phonon effects.