R. Murray

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.

Strain and electronic interactions in InAs/GaAs quantum dot multilayers for 1300 nm emission

We have investigated the emission properties of low-growth-rate InAs/GaAs self-assembled quantum-dot multilayer samples with spacer layers of different thicknesses. For two layers with the same InAs coverage and a spacer of 40 nm, emission from the two layers is shown to be at different wavelengths. This is discussed in terms of local strain and surface undulation caused by the first layer influencing the growth of the second layer. We show that by annealing the surface before the growth of each subsequent layer this effect can be avoided for spacers of 40 nm and above. Furthermore, it is shown by collecting photoluminescence over a limited area of an N-layer sample, grown with surface annealing, that this sample exhibits a maximum modal gain N times that of a single layer. For spacers below 10 nm, only one emission peak is observed. However, we show that the observation of a single peak is due, in this case, to tunneling between adjacent layers, and that the maximum modal gain at 1300 nm of such electron...

Quantitative compositional analysis of InAs/GaAs quantum dots by scanning transmission electron microscopy

We present a quantitative technique for the direct compositional analysis of quantum dots (QDs), in which scanning transmission electron microscopy is applied to a capped InAs/GaAs QD layer in a structure also containing InxGa1−xAs/GaAs quantum well (QW) layers to provide an internal calibration of the In content. By obtaining energy dispersive x-ray analysis line scans through both QWs and QDs, the composition of the QDs can be determined by reference to the known composition of the QWs. In this article the method is described and demonstrated using two InAs/GaAs structures in which the QDs are nominally identical, but with different In composition in the calibration QW layers. We find that the QDs in both structures have an In composition of 65%–67% and the associated wetting layers contain approximately 12% In.

Competition between strain-induced and temperature-controlled nucleation of InAs/GaAs quantum dots

Atomic force microscopy and photoluminescence spectroscopy (PL) have been used to study asymmetric bilayer InAs quantum dot (QD) structures grown by molecular-beam epitaxy on GaAs(001) substrates. The two QD layers were separated by a GaAs spacer layer (SL) of varying thickness and were grown at different substrate temperatures. Grown independently, these two layers would exhibit a widely different QD number density, and this technique therefore enables us to assess the influence of the strain fields created by the dots in the first layer on the second-layer QD nucleation and characteristics. For very large SLs (>40 nm), total strain relief causes the QD nucleation to be controlled exclusively by the substrate temperature, which influences the migration of In adatoms. In this case, the optical and morphological properties of the second QD layer are identical to a structure with a single QD layer grown at the same temperature. In structures with a much smaller SL, strain effects dominate over the effect of...

New light from hybrid inorganic-organic emitters

We present the highlights of a research programme on hybrid inorganic?organic light emitters. These devices combine recent developments in III?V nitride technology (including UV emitting micro-arrays and specifically tailored quantum wells) with conjugated polymers to access the entire visible spectrum. Two types of devices are studied, those based on down conversion of the quantum well emission by radiative transfer and those based on non-radiative resonant energy transfer. The spectral and operating characteristics of the devices are described in detail. Selectable colour micro-arrays and bar emitters are demonstrated. The nature of the non-radiative energy transfer process has also been studied and we find transfer efficiencies of up to 43% at 15?K, with a 1/R2 dependence on the distance between quantum well and polymer layer, suggesting a plane?plane interaction. The relative importance of the non-radiative resonant energy transfer process increases with temperature to be up to 20 times more efficient, at 300?K, than the radiative transfer process.

Luminescence enhancement from hydrogen-passivated self-assembled quantum dots

We have measured a large increase (by a factor of up to 50) in the room-temperature emission of InAs/GaAs self-assembled quantum dots subjected to a hydrogen-passivation treatment. Smaller enhancements were measured at low temperatures. We tentatively attribute the improved optical signal to passivation of defects within the GaAs matrix and wetting layer adjacent to the dots. Annealing studies show that these benefits are lost following annealing at temperatures above 600 °C for 5 min.

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.

Quantum pillar structures on n+ gallium arsenide fabricated using ‘‘natural’’ lithography

Random arrays of CsCl hemispherically shaped islands with average diameters as small as 500 A have been made on n+GaAs substrates. The CsCl behaves as a resist of high selectivity when the GaAs is reactively ion etched in a BCl3 plasma. The resulting structure is a set of pillars all the same height, but with varying diameters, typically ±15% of the average value, 〈D〉. Typical pillar packing density, S, is 20%. Photoluminescence (PL) studies were made at 10 K on n+ and semi‐insulating GaAs using 514.5 nm exciting radiation. PL spectra from n+GaAs structures of 〈D〉=520±78 A, h=800 A, S=20%; 〈D〉=610±97 A, h=450 A, S=12%; 〈D〉=1215±210 A, h=600 A, S=20%, as well as a plane surface, are reported. There is a shift in the peak value of the PL curves with respect to the planar structure of 26, 13, and 2.5 meV, respectively. It may be possible to understand these shifts in terms of quantum confinement effects if the effective pillar size is reduced by surface space charge effects.