Dots-in-a-well Research Articles

Effect of thermal annealing and strain engineering on the fine structure of quantum dot excitons

The fine structure splitting of bright exciton states is measured for a range of thermally annealed InGaAs quantum dot (QD) samples with differing degrees of $\mathrm{In}∕\mathrm{Ga}$ intermixing and also for a dot-in-a-well (DWELL) structure. Magnitudes of the fine structure splitting are determined in polarization-resolved differential transmission experiments from measurements of the period of quantum beats observed in QD exciton dynamics. The splitting is found to decrease in structures with weaker strain: both for $\mathrm{In}∕\mathrm{Ga}$ intermixed QD's and also in dots surrounded by strain-reducing layers (DWELL's). Our findings pave the way to the achievement of entangled two photon sources based on emission from individual QD's, currently prevented since the fine structure splitting is larger than the radiative linewidth.

Interdiffusion and structural change in an InGaAs dots-in-a-well structure by rapid thermal annealing

Post-growth rapid thermal annealing (RTA) has been used to investigate an interdiffusion and the structural change in an InGaAs dots-in-a-well (DWELL) structure grown by molecular beam epitaxy using an alternately supplying InAs and GaAs sources. In the case of the as-grown sample, which has a metastable quantum structure due to an intentional deficit of source materials, it is found that an InGaAs quantum well (QW) coexists with the premature quantum dots (QDs), and an intermediate layer exists between the QW and the QDs. Through the RTA process at 600 and 800°C for 30s, metastable structure changes into a normal DWELL structure composed of QDs and QW as a result of the intermixing of premature QDs and the intermediate layer.

Theoretical modeling and experimental characterization of InAs∕InGaAs quantum dots in a well detector

Theoretical modeling and experimental characterization of InGaAs∕GaAs quantum dots-in-a-well (DWELL) intersubband heterostructures, grown by molecular beam epitaxy are reported. In this heterostructure, the self-assembled dots are confined to the top half of a 110Å InGaAs well which in turn is placed in a GaAs matrix. Using transmission electron microscopy, the quantum dots are found to be pyramidal in shape with a base dimension of 110Å and height of 65Å. The band structure for the above mentioned DWELL heterostructure was theoretically modeled using a Bessel function expansion of the wave function. The energy levels of the three lowest states of the conduction band of the quantum dot are calculated as a function of the electric field. Intersubband n-i-n detectors were fabricated using a ten layer DWELL heterostructure. The spectral response of the detector is measured at a temperature between 30 and 50 K and compared with the prediction of our theoretical model.

Carrier dynamics in anInGaAsdots-in-a-well structure formed by atomic-layer epitaxy

We investigated the effects of carrier dynamics on the temperature dependence of the photoluminescence (PL) of an $\mathrm{InGaAs}$ dots-in-a-well (DWELL) structure. The quantum dots (QDs) were formed by the atomic layer epitaxy (ALE) technique alternately supplying $\mathrm{InAs}$ and $\mathrm{GaAs}$ sources. It was found from the PL measurements at various temperatures that the DWELL structure was accomplished through the generation process of the intermediate layer between the quantum well (QW) and the QDs during the formation of the QDs inside a QW. The temperature dependence of the PL was fitted well with the thermal quenching equations on the basis of the rate equation model. The rate equations can be explained by the carrier dynamics, which included in the radiative recombination, the carrier thermal escape and the carrier capture process occurring in these three layers, i.e., QW, QD, and the intermediate layer.

Normal-incidence InAs/In0.15Ga0.85As quantum dots-in-a-well detector operating in the long-wave infrared atmospheric window (8–12 μm)

Normal incidence InAs/In0.15Ga0.85As dots-in-a-well (DWELL) detectors are reported in which the peak operating wavelength was tailored from 7.2 to 11 μm using heterostructure engineering of the DWELL structure. Using an optimized design, a detector with a spectral response spanning the long-wave infrared atmospheric window (8–12 μm) is obtained. Spectral response peaks were observed at λp=10.3 μm and 11.3 μm under positive and negative bias, respectively. These peaks are attributed to bound-to-bound transitions from the InAs quantum dot to the InGaAs well.

Study of structural and optical properties of quantum dots-in-a-well heterostructures

Optical and structural characteristics of InAs/InGaAs quantum dots-in-a-well (DWELL) heterostructures were investigated. These structures, in which InAs quantum dots are placed in an InGaAs quantum well, which in turn is buried in a GaAs matrix, can be used to realize high performance midinfrared detectors. In this article, we report on the structural and optical properties of these DWELL heterostructures. Five DWELL samples with varying well widths were grown and their properties investigated using transmission electron microscopy (TEM), photoluminescence, normal incidence absorbance, and photocurrent measurement. From the TEM studies, it was observed that, as the InGaAs well thickness increased from 70 to 120 Å, the dots were confined to the upper half of the well. Moreover, a progressive shift was observed in the photocurrent response from 6.7 to 10 μm. Using this approach, a long wave infrared atmospheric window detector was realized.

High responsivity, LWIR dots-in-a-well quantum dot infrared photodetectors

In this paper we report studies on normal incidence, InAs/In 0.15Ga 0.85As quantum dot infrared photodetectors (QDIPs) in the dots-in-a-well (DWELL) configuration. Three QDIP structures with similar dot and well dimensions were grown and devices were fabricated from each wafer. Of the three devices studied, the first served as the control, the second was grown with an additional 400 Å AlGaAs blocking layer, and the third was grown on a GaAs n+ substrate with the intention of testing a single pass geometry. Spectral measurements on all three devices show one main peak in the long-wave IR (≈8 μm). The absorption was attributed to the bound-to-bound transition between the ground state of the InAs quantum dot and the ground state of the In 0.15Ga 0.85As well. Calibrated peak responsivity and peak detectivity measurements were performed on each device at 40, 60, and 80 K. For the same temperatures, frequency response measurements from ∼20 Hz to 4 kHz at a bias of V b=−1 V were also performed. The addition of the blocking layer was shown to slightly enhance responsivity, which peaked at ∼2.4 A/W at 77 K, V b=−1 V and responsivity was observed to be significantly reduced in the single pass (n+ substrate) sample. The rolloff of the frequency response was observed to be heavily dependent on temperature, bias, and irradiance. The results from the characterization of each sample are reported and discussed.

Optimizing the growth of 1.3 μm InAs/InGaAs dots-in-a-well structure

The structural and optical properties of GaAs-based 1.3 μm InAs/InGaAs dots-in-a-well (DWELL) structures have been optimized in terms of different InGaAs and GaAs growth rates, the amount of InAs deposited, and In composition of the InGaAs quantum well (QW). An improvement in the optical efficiency is obtained by increasing the growth rate of the InGaAs and GaAs layers. A transition from small quantum dots (QDs), with a high density (∼5.3×1010 cm−2) and broad size distribution, to larger quantum dots with a low dot density (∼3.6×1010 cm−2) and narrow size distribution, occurs as the InAs coverage is increased from 2.6 to 2.9 monolayers. The room-temperature optical properties also improve with increased InAs coverage. A strong dependence of the QD density and the QD emission wavelength on the In composition of InGaAs well has been observed. By investigating the dependence of the dot density and the high-to-width ratio of InAs islands on the matrix of InGaAs strained buffer layer (SBL), we show that the increasing additional material from wetting layer and InGaAs layer into dots and the decreasing repulsive strain field between neighboring islands within substrate are responsible for improving QD density with increasing In composition in InGaAs SBL. The optical efficiency is sharply degraded when the InGaAs QW In composition is increased from 0.15 to 0.2. These results suggest that the optimum QW composition for 1.3 μm applications is ∼15%. Our optimum structure exhibits a room temperature emission of 1.32 μm with a linewidth of 27 meV.

Characterization of rapid-thermal-annealed InAs/In0.15Ga0.85As dots-in-well heterostructure using double crystal x-ray diffraction and photoluminescence

The effect of rapid thermal annealing on a 10-layer InAs/In0.15Ga0.85As dots-in-a-well (DWELL) heterostructure was studied using double crystal x-ray diffraction (DCXRD) and photoluminescence (PL). From the x-ray rocking curves obtained for symmetric (004) and asymmetric (224) scans, the change in the in-plane and out-of-plane lattice constant and average composition in the DWELL structure were calculated. Thermally induced strain relaxation, which leads to an enhanced In/Ga interdiffusion preferentially along the growth direction, is believed to be the main mechanism for the changes in the structural and optical properties of the sample. Excellent correlation was observed between the PL and the DCXRD measurements.

The influence of quantum-well composition on the performance of quantum dot lasers using InAs-InGaAs dots-in-a-well (DWELL) structures

The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency /spl eta//sub i/, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In/sub 0.15/Ga/sub 0.85/As strained QW laser are also made. A threshold current density as low as 16 A/cm/sup 2/ is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that /spl eta//sub i/ and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T/sub 0/ and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL laser's /spl eta//sub i/ for the dot densities studied.

Low-threshold current density 1.3-μm InAs quantum-dot lasers with the dots-in-a-well (DWELL) structure

The wavelength of InAs quantum dots in an In/sub 0.15/Ga/sub 0.85/As quantum-well (DWELL) lasers grown on a GaAs substrate has been extended to 1.3-/spl mu/m. The quantum dot lasing wavelength is sensitive to growth conditions and sample thermal history resulting in blue shifts as much as 73 nm. The room temperature threshold current density is 42.6 A cm/sup -2/ for 7.8-mm cavity length cleaved facet lasers under pulsed operation.

Tunable grating-coupled laser oscillation and spectral hole burning in an InAs quantum-dot laser diode

Emission spectra are investigated of a low-threshold InAs quantum-dot laser of the dots-in-a-well (DWELL) type operating near 1230 nm. An external dispersion cavity with a diffraction grating is coupled to the laser diode to suppress the subsidiary modes and to tune the central wavelength. A wavelength-dependent competition between the grating-coupled mode and the internal Fabry-Perot modes of the laser suggests that a hole burning in the spectral density of a DWELL laser occurs with a characteristic spectral half width of /spl sim/13 nm (10.5 meV). Simple models of spectral flattening and spectral hole burning are presented to explain the broad free-running and grating-coupled lasing spectra of the DWELL device.