Self-assembled InAs Quantum Dot Structures Research Articles

Role of segregation in InAs/GaAs quantum dot structures capped with a GaAsSb strain-reduction layer

We report a combined experimental and theoretical analysis of Sb and In segregation during the epitaxial growth of InAs self-assembled quantum dot structures covered with a GaSbAs strain-reducing capping layer. Cross-sectional scanning tunneling microscopy shows strong Sb and In segregation which extends through the GaAsSb and into the GaAs matrix. We compare various existing models used to describe the exchange of group III and V atoms in semiconductors and conclude that commonly used methods that only consider segregation between two adjacent monolayers are insufficient to describe the experimental observations. We show that a three-layer model originally proposed for the SiGe system [D. J. Godbey and M. G. Ancona, J. Vac. Sci. Technol. A 15, 976 (1997)] is instead capable of correctly describing the extended diffusion of both In and Sb atoms. Using atomistic modeling, we present strain maps of the quantum dot structures that show the propagation of the strain into the GaAs region is strongly affected by the shape and composition of the strain-reduction layer.

Mid-Infrared Emission From InAs Quantum Dots Grown by Metal–Organic Vapor Phase Epitaxy

We have demonstrated mid-infrared emission from the self-assembled InAs quantum dots grown on InP substrate by metal-organic vapor phase epitaxy using low toxic tertiarybutylarsine and tertiarybutylphosphine as group V sources in pure nitrogen ambient. Emission wavelength of the InAs quantum dots has been extended to mid-infrared region by embedding the InAs quantum dots in graded InxGa1-xAs matrix layers. When compared with that of the InAs quantum dots grown on lattice matched In0.53Ga0.47As/InP matrix, emission wavelength of the InAs quantum dots red shifted by up to 370 nm when embedded the InAs quantum dots in graded In0.53rarr0.8Ga0.47rarr0.2As barriers. The longest emission wavelength of >2.35 mum from the self-assembled InAs quantum dot structure has been measured at 77 K. The full-width at half-maximum of the photoluminescence emission spectrum of the InAs quantum dots is as narrow as 25.5 meV. The results achieved would be promising to high performance mid-infrared quantum dot lasers on InP substrate

Characterization of hydrogen passivated defects in strain-engineered semiconductor quantum dot structures

The effects of hydrogen incorporation on carrier relaxation and recombination efficiencies in a large series of InAs self-assembled quantum dot structures deposited on InGaAs lower confining layers with different thicknesses and compositions have been addressed. With increasing H dose we observe an improvement in the radiative efficiency. By comparing steady state and time resolved photoluminescence measurements, it is established that the H passivation does not enhance the relaxation and capture efficiencies, but instead directly improves the emission yield from carriers in the dots. We therefore conclude that the H-passivated defects are located nearby, or even inside, the dots.

Combined optical and electrical studies of the effects of annealing on the intrinsic states and deep levels in a self-assembled InAs quantum-dot structure

The effects of postgrowth rapid thermal annealing on the electronic states in a relatively long wavelength (∼1.3μm), self-assembled InAs∕GaAs quantum-dot structure are investigated. We combine optical and electrical experiments, i.e., photoluminescence (PL) and deep-level transient spectroscopy (DLTS) measurements, to identify the underlying physical processes responsible for the changes in the PL spectra at different annealing temperatures. Physical parameters of the intrinsic and deep-level states are quantitatively determined in the DLTS experiments. These include the thermal excitation energies, densities, and their changes with the annealing temperature. We observe that the densities of the deep levels that coexist in the quantum-dot layer decrease and a new deep level, about 0.62eV below the GaAs conduction band edge, is formed at elevated temperatures. Both effects explain the variations in the PL spectra. Moreover, beyond what can be revealed in the PL experiments, the DLTS spectra show a more complex electronic structure of both optically active and inactive states.

Room-temperature operations of memory devices based on self-assembled InAs quantum dot structures

Memory devices have been fabricated in high-electron-mobility transistors with embedded InAs quantum dots (QDs). We show that memory operations can be fully controlled by gate biases at room temperature, without the need for light excitations to erase memory states. Real-time measurements indicate a charge retention time of a few minutes. Neither such retention time nor the self-consistent simulations can justify the picture that the memory effect is due to charging/discharging of intrinsic QD states. Experiments at a series of gate biases point to the presence of deep levels coexisting in the QD layer(s), which are responsible for the memory effect.

TEM Imaging for Compositional Analysis of Self-Assembled InAs Quantum Dot Structures

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Dual-Bias Modulation Method for Scanning Tunneling Spectroscopy

We proposed a novel method for scanning tunneling spectroscopy, in which a difference in differential conductance (dI/dV) between two d.c. biases was extracted using a double lock-in technique. We investigated some experimental parameters to be optimized, and finally obtained the spectra of dI/dV differences on self-assembled InAs quantum dot structures. As a result, we observed different conductance gaps attributable to a difference between the band gaps of the quantum dot and the surrounding wetting layer, and observed some peaks in the spectra on the dot as well. In addition, reasonable current-voltage curves were numerically reproduced from the measured dI/dV spectra.

Structural and Optical Effects of Capping Layer Material and Growth Rate on the Properties of Self-Assembled InAs Quantum Dot Structures

ABSTRACTIn order to develop nanoengineering methods to control electronic spectrum of self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy, we have utilized atomic force microscopy (AFM), photoluminescence (PL) and TEM methods to investigate the effects of capping layer growth on the physical/chemical properties as well as the optical/electronic performance of QD device structures. Capping layer material choice (or its absence all together) has been found to directly influence QD dimensions (size, height), and subsequently, to affect QD emission wavelength. We report results of QD lateral size and height as well as densities of InAs QDs capped with 2ML (monolayers) of AlAs or GaAs grown at various rates. Our AFM results are complemented by PL measurements, where the optical properties of capped versus non-capped QDs have been explored and direct correspondence between structural differences induced by capping and the electronic/optical properties of QDs is demonstrated. Analysis of the data shows that the results can be explained by two competing surface processes. The first of which is the redistribution of indium between QDs on top of the 2D wetting layer, resulting in the increase of QD size with time. The second effect is the diffusion of indium out of the QDs and onto the top of the capping layer. TEM with multislice image simulation has supported our AFM and PL observations with the demonstration of “indium driven” alloy intermixing in the overlayer as well as significant alloying in the InAs wetting layer.

Microanalysis of Self-assembled InAs Quantum Dot Structures Grown for Infrared Detector Applications

ABSTRACTIn an effort to develop materials that are sensitive to mid and far infrared radiation, we examine InAs quantum dot/GaAs matrix multilayer structures grown by molecular beam epitaxy (MBE). Customized electrical and optical properties result from nanoscale-level manipulation of the dots' physical dimensions. The MBE growth temperature can be set to yield dots having the desired lateral dimension; however this leads to dots of insufficient vertical height. It is therefore necessary to grow the dots in a manner that allows independent control of the lateral and vertical dimensions. In this experiment, the vertical dimension is controlled by growing the dots in a multilayer structure with GaAs matrix layers. An initial layer of InAs quantum dots was grown on top of GaAs, followed by a few seconds short growth of GaAs, and then followed by the growth of another layer of InAs dots. The GaAs laterally surrounds, but does not bury, the InAs quantum dots. When the second layer of InAs dots is grown, they tend to self-organize directly on top of the exposed first layer of dots. We then grew a third layer of dots in the same manner. This effectively results in a pseudo-single layer of dots of the desired height which is then completely buried in GaAs. The goal is to develop structures that can be integrated into high operating temperature quantum dot infrared detectors (QDIPs) that have maximum sensitivity, robustness, and portability.

Abnormal temperature-dependent photoluminescence characteristics of stacked InAs self-assembled quantum dot structures grown by molecular beam epitaxy

We have investigated the temperature-dependent photoluminescence (PL) characteristics of stacked self-assembled InAs/GaAs quantum dot structures grown by molecular beam epitaxy (MBE). A step-like behavior of the peak energies of excitonic bands as a function of temperature is observed in two temperature ranges of 60–90 K and 120–150 K, while the envelope of the change of peak energies basically follows the Varshini law with InAs parameters up to 150 K. The thermal activation energy of the electron–hole emission through a GaAs barrier in the quantum dots was measured to be 76 meV. We observed an unusual increase of integrated photoluminescence intensity with temperature in the step regions, suggesting that the excitonic recombination in stacked quantum dots occurs in a condition where higher oscillator strength occurred with a certain lattice temperature.

Effect of growth interruption on photoluminescence of self-assembled InAs quantum dot structures grown on (0 0 1) InP substrate by MOCVD

In this paper, we present the effects of growth interruption on self-assembled InAs quantum dots (QDs), which were grown on a (0 0 1) InP substrate by low-pressure metalorganic chemical vapor deposition (MOCVD). Large red shifts in the photoluminescence (PL) emission from the InAs QDs together with narrowing of the full-width at half-maximum (from 107 to 70 meV) of the PL are observed as a result of the growth interruption. Besides adatom migration, As/P exchange reaction during the interruption is demonstrated to play an important role in changing the optical properties of InAs/InP QDs.

Carrier dynamics of self-assembled InAs quantum dots on InP (311)B substrates

A study of the carrier dynamics of self-assembled InAs quantum dot structures on InP (311)B substrates is presented. By time-resolved photoluminescence spectroscopy, an efficient carrier capture from the wetting layer into the quantum dots is observed under high incident excitation condition. This behavior can be attributed to carrier relaxation assisted by the Auger effect. Moreover, first excited states which have a fast decay time of ∼60 ps are observed under the same condition. These results demonstrate the possibility of the realization of performance-improved injection lasers at 1.55 μm for optical telecommunication.

The influence of inter-diffusion on electron states in quantum dots

We calculate the variation of the potential and confinement energy due to inter-diffusion of a spherical In x Ga 1− x As quantum dot embedded in GaAs. The potential drops off more quickly with increasing diffusion length about a quantum dot than is the case for a quantum well. This results in a far more rapid variation of confinement energy for a dot than for a well, until eventually no electron states are confined in the diffused dot. We show that the spread in confinement energies due to variation in dot size and composition decreases with the diffusion-induced decrease in confinement energy. Our results indicate that the recently observed narrowing of photoluminescence line-width, that occurs as a result of rapid thermal annealing of self-assembled InAs quantum dot structures, can be explained in terms of normal inter-diffusion processes between the dots and surrounding matrix material.

Effect of Size Fluctuations on the Photoluminescence Spectral Linewidth of Closely Stacked InAs Self-Assembled Quantum Dot Structures

We calculated the photoluminescence (PL) spectra of InAs (or InGaAs) self-assembled quantum dots for single-layer, 2-layer stacked and 3-layer stacked structures employing three models via the effective mass approximation method to clarify the effect of size fluctuations on the PL spectral linewidth. We quantitatively confirmed that the PL spectral linewidth is determined much more by height fluctuations than by diameter fluctuations. We examined the dependence of the full-width at half maximum (FWHM) on the intermediate layer thickness d for 2-layer stacked structures from d=2 nm to 6 nm to clarify the effect of vertical coupling. The FWHM increases as d decreases in regions where the potential barrier between the upper and lower islands is so thin that its fluctuations are striking, while the FWHM decreases as d decreases in regions where there is an increase in the number of directly connected islands. We explained an anomalously narrow PL spectrum of 31 meV for a 3-layer stacked structure by considering extremely reduced island height fluctuations as well as vertical coupling.

Charge Separation Between Strain Coupled Quantum Dots in a Self-Assembled InAs Quantum Dot Structure

ABSTRACTWe present an InAs QDs structure designed to separate and store photo-generated electron-hole pairs. Charge separation in the structure is demonstrated using power dependent photoluminescence and biased photoluminescence. Preliminary data from time resolved photoluminescence suggest storage times in the device in the μsec range.

Light emission from individual InAs/GaAs self-assembled quantum dots excited by tunneling current injection

Emission from individual InAs quantum dots by tunneling current injection using a scanning tunneling microscope (STM) is reported. By scanning the STM tip above the self-assembled InAs quantum dot structures, a spatially resolved scanning tunneling luminescence (STL) image was measured, which contained some peaks originating from InAs quantum dots. The width of these peaks are of the same size as the average dot size, suggesting that the resolution of this method is much higher than 20 nm.