S. Fafard

Size and shape engineering of vertically stacked self-assembled quantum dots

A new procedure for the growth of stacked self-assembled quantum dot layers is described. The main effect of the procedure is to convert the quantum dot population into a population of quantum disks of approximately equal height. Proposed model of the overgrowth process for highly strained 3D islands invokes mechanisms which may lead to a variety of dot shape modification and opens up new ways of influencing the dot population by deliberate control of the overgrowth process. Demonstrated very good performance of test devices indicate that the proposed procedure may have positive impact on the further development of quantum dot lasers.

Red-Emitting Semiconductor Quantum Dot Lasers

Visible-stimulated emission in a semiconductor quantum dot (QD) laser structure has been demonstrated. Red-emitting, self-assembled QDs of highly strained InAlAs have been grown by molecular beam epitaxy on a GaAs substrate. Carriers injected electrically from the doped regions of a separate confinement heterostructure thermalized efficiently into the zero-dimensional QD states, and stimulated emission at ∼707 nanometers was observed at 77 kelvin with a threshold current of 175 milliamperes for a 60-micrometer by 400-micrometer broad area laser. An external efficiency of ∼8.5 percent at low temperature and a peak power greater than 200 milliwatts demonstrate the good size distribution and high gain in these high-quality QDs.

InAs self‐assembled quantum dots on InP by molecular beam epitaxy

We present results of room temperature photoluminescence (PL) emission from a 0‐dimensional system in the ∼1.4 to ∼1.7 μm spectral region. Molecular beam epitaxy was used to grow InAs self‐assembled quantum dots in AlInAs on an InP substrate. Preliminary characterizations have been performed using PL and transmission electron microscopy. The low temperatures PL spectra also display excited state emission and state filling as the excitation intensity is increased.We present results of room temperature photoluminescence (PL) emission from a 0‐dimensional system in the ∼1.4 to ∼1.7 μm spectral region. Molecular beam epitaxy was used to grow InAs self‐assembled quantum dots in AlInAs on an InP substrate. Preliminary characterizations have been performed using PL and transmission electron microscopy. The low temperatures PL spectra also display excited state emission and state filling as the excitation intensity is increased.

MOLECULAR-BEAM EPITAXY GROWTH OF QUANTUM DOTS FROM STRAINED COHERENT UNIFORM ISLANDS OF INGAAS ON GAAS

The two‐ (2D) to three‐dimensional (3D) growth mode transition during the initial stages of growth of highly strained InxGa1−xAs on GaAs is used to obtain quantum dot structures. Transmission electron micrographs (TEM) reveal that when the growth of InxGa1−xAs is interrupted exactly at the onset of this 2D–3D transition, dislocation‐free islands (dots) of InGaAs result. Size distribution measurements from TEM images indicate that these dots are less than 300 A in diameter and remarkably uniform. A phase diagram is constructed, showing the growth conditions under which these strained coherent uniform dots form. Photoluminescence from layers containing these dots is observed and correlated with growth conditions and with structural data obtained from TEM images. We observe that the photoluminescence emitted from the dots and an underlying reference quantum well are nearly equal, indicating a high quantum efficiency for these quantum dots.

INTERMIXING IN QUANTUM-DOT ENSEMBLES WITH SHARP ADJUSTABLE SHELLS

State-filling spectroscopy is used to study the effects of alloy intermixing in quantum-dot (QD) ensembles having well-defined electronic shells. Rapid thermal annealing is performed on samples of self-assembled QDs grown with different intersublevel energy spacings. For InAs/GaAs QDs, the intersublevel is tuned between ∼90 and 25 meV. The intense and sharp shell structures observed in photoluminescence indicate unambiguously that the QDs retained their zero-dimensional density of states after the diffusion of the potential, which also causes strong blueshifts (up to ∼200 meV) and a pronounced narrowing of the inhomogeneously broadened emission (down to ∼12 meV).

Structural and optical properties of self‐assembled InGaAs quantum dots

A one step method for the production of quantum dots is presented. The method exploits the mismatch strain of molecular beam epitaxy (MBE) deposited InGaAs on GaAs to induce a transition from the two‐dimensional growth mode to the three‐dimensional (Stranski–Krastanow) growth mode. The cluster size is limited to quantum dimensions by precisely controlling the amount of InGaAs that is deposited in order to cause the growth mode transition. Very narrow lateral size distributions with standard deviations of 14% on the dot area have been obtained. Smooth MBE growth of GaAs over these clusters produces a layer of quantum dots, whose high quality and uniformity has been observed with transmission electron microscopy, atomic force microscopy, and photoluminescence (PL). The quantum dot PL intensity is enhanced compared to a reference quantum well. Resonances in photoluminescence excitation (PLE) spectra suggest that the density of states in these dots has minima close to zero between the quantum states, as expec...

Carrier energy relaxation by means of Auger processes in InAs/GaAs self-assembled quantum dots

Carrier relaxation processes are investigated in self-assembled InAs/GaAs quantum dots using time-resolved photoluminescence spectroscopy. The quantum-dot photoluminescence rise time has been measured as functions of carrier excitation density and excitation wavelengths. The measured relaxation time is about 32 ps at low excitation density and decreases by 1 over the excitation density from about 3 W/cm2, under nonresonant laser excitation. The threshold of this density-dependent regime occurs at a slightly higher density as the excitation wavelength increases and it disappears when the photon pumping energy is below the wetting layer barrier energy. These results clearly establish the regime where Auger processes become the dominant carrier relaxation mechanism in these self-assembled quantum dots.

Temperature effects on the radiative recombination in self-assembled quantum dots

Abstract Several ensembles of self-assembled quantum dots (QDs) based on the AlInAsAl/GaAs and InGaAs/GaAs material systems have been investigated using photoluminescence (PL), PL excitation (PLE) and time-resolved PL (TRPL). The influence of the temperature is measured by monitoring sharp spectral features (as narrow as ∼ 90 μeV) obtained when probing the PL of small QD ensembles (few hundreds QDs). Thermionic emission of the photocarriers out of the QD potential is found to be the dominant mechanism leading to the thermal quenching of the PL and temperature-independent linewidths are observed up to the onset of the PL quenching.

InAs self-assembled quantum-dot lasers grown on (100) InP

Five stacked layers of InAs quantum dots (QDs) embedded in quaternary InGaAsP are grown on (100) InP substrate to form a laser diode. The QD ensemble has a density of 1.5×1010 cm−2 and emits light at ∼1.6 μm at 77 K. Lasing wavelength and threshold current density can be shifted by changing the cavity length of the laser diode and the latter reaches a value as low as 49 A/cm2 at 77 K for a gate size of 2000 μm×150 μm. Temperature dependence of the threshold current is observed implying the presence of thermionic emission increasing with temperature.

Quantum dot infrared photodetectors

Abstract We discuss key issues related to quantum dot infrared photodetectors. These are the normal incidence response, the dark current, and the responsivity and detectivity. It is argued that the present devices have not fully demonstrated the potential advantages. The dominant infrared response in devices so far is polarized in the growth direction. The observed dark currents are several orders of magnitude higher than those for quantum well photodetectors; while ideally they should be lower. The areas that need improvements are pointed out.