Dots-in-a-well Research Articles

Influence of bimodal distribution and excited state emission on photoluminescence spectra of InAs self-assembled quantum dots

Optical properties of self-assembled quantum dots (SAQDs) samples were investigated through photoluminescence (PL) with variation of temperature and excitation density. Two types of InAs SAQDs were studied: dots grown directly on GaAs, and dots grown inside an InGaAs quantum well, heterostructure known as dot-in-a-well (DWELL). The QDs structures were produced through conventional growth (with several deposition rates) as well as through pulsed growth mode, which consists in deposition cycles with growth interruption intervals. In the samples of QDs grown on GaAs, two PL peaks were identified and their behavior as a function of temperature can be explained by taking into account a bimodal distribution of dot sizes. For the DWELL sample only one PL peak was identified, which was assigned to the emission of the ground state of QDs belonging to a larger Gaussian distribution of dot sizes. SAQDs samples showed an anomalous behavior of the peak energy, integrated intensity and linewidth (FWHM) of the PL signal, as a function of temperature. To explain the observed results, a model was proposed that takes into account the size distribution of dots and the excited state emission of the family of larger dots. The QDs can be distributed in a bimodal size distribution, and the emission from the excited state can be observed, or not, depending on the growth conditions and on the potential profile of the dot. Such aspects can be combined to explain several peculiarities of the PL spectra observed in this and other published papers on SAQDs optical properties.

Continuous time-varying biasing approach for spectrally tunable infrared detectors

In a recently demonstrated algorithmic spectral-tuning technique by Jang et al. [Opt. Express 19, 19454-19472, (2011)], the reconstruction of an object's emissivity at an arbitrarily specified spectral window of interest in the long-wave infrared region was achieved. The technique relied upon forming a weighted superposition of a series of photocurrents from a quantum dots-in-a-well (DWELL) photodetector operated at discrete static biases that were applied serially. Here, the technique is generalized such that a continuously varying biasing voltage is employed over an extended acquisition time, in place using a series of fixed biases over each sub-acquisition time, which totally eliminates the need for the post-processing step comprising the weighted superposition of the discrete photocurrents. To enable this capability, an algorithm is developed for designing the time-varying bias for an arbitrary spectral-sensing window of interest. Since continuous-time biasing can be implemented within the readout circuit of a focal-plane array, this generalization would pave the way for the implementation of the algorithmic spectral tuning in focal-plane arrays within in each frame time without the need for on-sensor multiplications and additions. The technique is validated by means of simulations in the context of spectrometry and object classification while using experimental data for the DWELL under realistic signal-to-noise ratios.

Comparison of different concepts of InAs quantum dot growth on GaAs for 1.3-μm-range lasers

The DWELL (dots-in-a-well), DUWELL (dots-under-the-well), and GC-SRL (gradient-composition-strain-reducing-layer) concepts of InAs quantum dot epitaxial growth under identical conditions have been compared. The laser structures are designed for operation near 1.3 μm. An improved procedure is proposed for estimating the InAs growth rate. The dependence of the photoluminescence peak’s spectral position and intensity on the structure type and epitaxy conditions has been studied. A 10-stack DUWELL ridge waveguide laser diode with ground-state lasing at room temperature has been demonstrated.

Probing into hybrid organic-molecule and InAs quantum-dots nanosystem with multistacked dots-in-a-well units

Hybridizing air-stable organic-molecules with advanced III-V semiconductor quantum-dots (QDs) structures can be utilized to create a new generation of biochemical sensing devices. In order to enhance their optical performances, the active regions in these QDs structures commonly consist of multistacked dots-in-a-well (DWELL) units. The effects of grafted molecules on the performances of the QDs structures with multistacked DWELLs, however, still remain unclear. Here, we show the significant improvements in the optical properties of InAs QDs in a hybrid nanosystem obtained by grafting biocompatible diazonium salt compound (amine donor) atop InAs QDs structure. Since its interface between the QDs structure and molecular monolayer retains an uncontaminated and non-oxidized condition, the nanosystem is an ideal platform to study the intrinsic properties of charge-carrier transport inside the system. Because of the complexity of the energy-levels in the QDs structure due to the existing surface QDs and DWELLs, selective excitation wavelengths (400, 633, and 885 nm, respectively) with different photo-energies are used to exactly analyze the complete charging mechanism in these QDs. A clear view of charge-carrier transfer inside the nanosystem is revealed by employing photoluminescence technique under selective-wavelength excitations. The present work provides new quantitative evidences for exploiting inorganic QDs applications in complex biological systems.

Effects of contact space charge on the performance of quantum intersubband photodetectors

Highly non-uniform electric field exists in the active region of quantum intersubband devices, primarily due to the presence of PN junctions forming between heavily doped contact regions and non-intentionally doped barriers. Using a combination of experiments and theoretical simulations, we investigate the effect of this non-uniform internal electric field on the photodetector operation. Three quantum dots-in-a-well (DWELL) photodetectors have been fabricated with top spacer, bottom spacer, and no spacer around the active region, respectively, to demonstrate the effect of the non-uniform field. Drift-diffusion based calculations of the electric field provide further insight into the device operation.

Effect of Deposition Temperature on the Opto-Electronic Properties of Molecular Beam Epitaxy Grown InAs Quantum Dot Devices for Broadband Applications

The effect of the quantum dot (QD) deposition temperature is discussed for dot-in-a-well (DWELL) structures with a view to their optimization for broadband applications. Atomic force microscopy (AFM) analysis allows the measurement of the quantum dot and the defective island density. The reduced QD growth temperature results in broad emission spectrum and increased defective island density. Reduced electroluminescence efficiency, higher reverse leakage currents, and lower reverse breakdown voltage could be correlated to the presence of the defective island density. Maximal output power is obtained for devices with a QD growth temperature of 500 °C, whilst the preferred spectral shape and QD density is obtained at the lowest temperature, 470 °C. To benefit from broad emission bandwidth, the growth conditions need to be further optimized to avoid, or at least reduce, the defective island density.

Effect of Asymmetric Well Quantum Dots-in-a-well Infrared Photodetectors on Density of States Using NEGF Formalism

Local Density of State (LDOS) and hence Density of State (DOS) at different energy levels of asymmetric conical shaped InAs/InGaAs quantum dots-in-a-well (DWELL) infrared photo-detector has been calculated through Non-Equilibrium Green's Function (NEGF) approach to explain the tri-band operation. Method of finite difference is used to solve the kinetic equation of Green's function and Self-energy is calculated in a recursive manner. Monotonic red shift of spectral response with incremental quantum well width is observed in terms of energy level difference, which is found in good agreement with previously reported experimental data, and the wavelengths corresponding to the peaks of responsivity is estimated from the DOS profile. Relative effect of changing the bottom quantum well width on ground and other higher state energy levels are also reported.

All-optical Switches based-on GaAs/AlAs Quantum Dots Vertical Cavities

In this paper, we present an all-optical switch based on self-assembled GaAs/AlAs quantum dots (QDs) within a vertical cavity. Two essential aspects of this novel device have been investigated, which includes the QD/cavity nonlinearity with appropriately designed mirrors and the intersubband carrier dynamics inside QDs. Verticalreflection- type switches have been investigated with an asymmetric cavity that consists of 12 periods of GaAs/Al0.8Ga0.2As for the front mirror and 25 periods for the back mirror. The thicknesses of the GaAs and AlGaAs layers are chosen to be 89 and 102nm, respectively. To give a dot-in-a-well (DWELL) structure, 65nm dimension of Si was deposited within an 20nm AlAs QW. All-optical switching via the QD excited states has been achieved with a time constant down to 750 fs, wavelength tunability over 29.5nm. These results demonstrate that QDs within a vertical cavity have great advantages to realize low-power consumption polarization-insensitive micrometer-sized switching devices for the future optical communication and signal processing systems

Carrier dynamics and activation energy of CdTe quantum dots in a CdxZn1−xTe quantum well

We investigate the optical properties of CdTe quantum dots (QDs) in a Cd0.3Zn0.7Te quantum well (QW) grown on GaAs (100) substrates. Carrier dynamics of CdTe/ZnTe QDs and quantum dots-in-a-well (DWELL) structure is studied using time-resolved photoluminescence (PL) measurements, which show the longer exciton lifetime of the DWELL structure. The activation energy of the electrons confined in the DWELL structure, as obtained from the temperature-dependent PL spectra, was also higher than that of electrons confined in the CdTe/ZnTe QDs. This behavior is attributed to the better capture of carriers into QDs within the surrounding QW.

1220–1280-nm Optically Pumped InAs Quantum Dot-Based Vertical External-Cavity Surface-Emitting Laser

We report an InAs quantum dot-based optically pumped vertical external-cavity surface-emitting laser (VECSEL), with a continuously variable emission wavelength from 1220 to 1280 nm through the use of epitaxial gradient across the wafer. We demonstrate the performance of two designs of this VECSEL. The first design, 4 × 3, makes use of a resonant periodic gain structure where three dot-in-a-well (DWELL) layers are located at the antinode of the E-field standing wave, with a total of four sets being used. The second design, 12 × 1, makes use of a single DWELL layer per antinode which is repeated 12 times for the same total of 12 DWELL layers. We demonstrate the lasing performance of the two structures as a function of distance from the center of the wafer and use known radial composition gradients in the molecular beam epitaxy reactor to achieve 50 nm continuous wavelength variation in the 4 × 3 structure and 60 nm in the 12 × 1 structure. We also demonstrate that the performance of the 12 × 1 structure is significantly improved compared to the 4 × 3 structure for all measured laser parameters, possibly due to increased pump absorption in the former structure's thicker barriers.

Data compressive paradigm for multispectral sensing using tunable DWELL mid-infrared detectors

While quantum dots-in-a-well (DWELL) infrared photodetectors have the feature that their spectral responses can be shifted continuously by varying the applied bias, the width of the spectral response at any applied bias is not sufficiently narrow for use in multispectral sensing without the aid of spectral filters. To achieve higher spectral resolutions without using physical spectral filters, algorithms have been developed for post-processing the DWELL's bias-dependent photocurrents resulting from probing an object of interest repeatedly over a wide range of applied biases. At the heart of these algorithms is the ability to approximate an arbitrary spectral filter, which we desire the DWELL-algorithm combination to mimic, by forming a weighted superposition of the DWELL's non-orthogonal spectral responses over a range of applied biases. However, these algorithms assume availability of abundant DWELL data over a large number of applied biases (>30), leading to large overall acquisition times in proportion with the number of biases. This paper reports a new multispectral sensing algorithm to substantially compress the number of necessary bias values subject to a prescribed performance level across multiple sensing applications. The algorithm identifies a minimal set of biases to be used in sensing only the relevant spectral information for remote-sensing applications of interest. Experimental results on target spectrometry and classification demonstrate a reduction in the number of required biases by a factor of 7 (e.g., from 30 to 4). The tradeoff between performance and bias compression is thoroughly investigated.

COMPARISON OF MONOLITHIC PASSIVELY MODE-LOCKED LASERS USING In(Ga)As QUANTUM DOT OR QUANTUM WELL MATERIALS GROWN ON GaAs SUBSTRATES

In this paper, a technology comparison between monolithic passively mode-locked lasers (MLLs) fabricated from 1.24 μm InAs dots-in-a-Well (DWELL) and 1.25 μm InGaAs single quantum well (SQW) structures grown using elemental source molecular beam epitaxy (MBE) is presented. 5 GHz optical pulses with sub-picosecond RMS jitter, high pulse peak power (1W) and narrow pulse width (< 10 ps) are typical of these monolithic two-section InAs DWELL passive MLLs. An InGaAs single quantum well MLL with the 42% indium is shown to exhibit a superior high-temperature performance. Compared with the typical operating range of the InAs DWELL devices (<60°C), the operation is in excess of 100 °C and is particularly attractive for clocking applications in next generation microprocessors. Based on an 8-band k . p analysis the reduction in the band edge density of states for such a quantum well is discussed.

High Operating Temperature Quantum-Dot Infrared Photodetector Using Advanced Capping Techniques

We demonstrate an improvement in the operating temperature of a quantum dot-in-a-well (DWELL)-based infrared photodetector with spectral response observable till 250 K. This improvement was achieved through engineering the dot geometry and the quantum confinement via postgrowth capping of the quantum dots (QDs) by selecting overlying materials under various growth conditions. The effect of the capping procedures was determined by examining the optical properties of the QDs. These were then introduced into the active region of a DWELL IR photodetector. Using this approach, the dark current density is as low as 6.3 × 10-7 A/cm2 (Vb = 7 V) at 77 K; the highest operating temperature is increased to 250 K with the λp = 3.2 μm. The peak detectivity is found to be 1 × 109 cm·Hz1/2 /W at 77 K and 7.2 × 107 cm·Hz1/2/W at 250 K.

Comparison of Quantum Dots-in-a-Double-Well and Quantum Dots-in-a-Well Focal Plane Arrays in the Long-Wave Infrared

Our previous research has reported on the development of the first generation of quantum dots-in-a-well (DWELL) focal plane arrays (FPAs), which are based on InAs quantum dots (QDs) embedded in an InGaAs well having GaAs barriers, which have demonstrated spectral tunability via an externally applied bias voltage. More recently, technologies in DWELL devices have been further advanced by embedding InAs QDs in InGaAs and GaAs double wells with AlGaAs barriers, leading to a less strained InAs/InGaAs/GaAs/AlGaAs heterostructure. These lower strain quantum dots-in-a-double-well devices exhibit lower dark current than the previous generation DWELL devices while still demonstrating spectral tunability. This paper compares two different configurations of double DWELL (DDWELL) FPAs to a previous generation DWELL detector and to a commercially available quantum well infrared photodetector (QWIP). All four devices are 320 × 256 pixel FPAs that have been fabricated and hybridized with an Indigo 9705 read-out integrated circuit. Radiometric characterization, average array responsivity, array uniformity and measured noise equivalent temperature difference for all four devices is computed and compared at 60 K. Overall, the DDWELL devices had lower noise equivalent temperature difference and higher uniformity than the first-generation DWELL devices, although the commercially available QWIP has demonstrated the best performance.

ALL-OPTICAL SWITCHES BASED ON GaAs/AlGaAs QUANTUM DOTS VERTICAL CAVITY

In this paper, an all-optical switch based on self-assembled GaAs/AlAs quantum dots (QDs) within a vertical cavity is designed and proposed. Two essential aspects of this novel device have been investigated, which include the QD/cavity nonlinearity with appropriately designed mirrors and the intersubband carrier dynamics inside QDs. The vertical-reflection-type switches have been investigated with an asymmetric cavity that consists of 12 periods of GaAs/Al0.8Ga0.2As and 25 periods for the front and back mirrors, respectively. The thicknesses of the GaAs and AlGaAs layers are chosen to be 89 and 102 nm, respectively. To give a dot-in-a-well (DWELL) structure, the 65 nm dimension of Si was recommended to deposit within a 20 nm AlAs QW. Results obtained have shown that all-optical switching via the QD excited states has been achieved with a time constant down to 275-fs and over 29.5 nm tunable wavelengths. These results demonstrated that QDs within a vertical cavity have great potential to realize low-power, consumption polarization-insensitive and micrometer-sized switching devices for future optical communication and signal processing systems.

Noise, Gain, and Responsivity in Low-Strain Quantum Dot Infrared Photodetectors With up to 80 Dot-in-a-Well Periods

We present a systematic study of noise, gain, responsivity, and specific detectivity D* , in a series of low-strain dot-in-a-well (DWELL) quantum dot infrared photodetectors (QDIPs). The lattice-matched GaAs quantum wells and AlGaAs barriers in these devices prevent the accumulation of excessive strain and allow the growth of up to 80 DWELL periods. We show that the photoconductive gain in these QDIPs is inversely proportional to the number of periods, while the total quantum efficiency is proportional to the number of periods, meaning that the responsivity remains constant at a given mean electric field as the number of periods is varied. The dark current in each QDIP was also found to be constant at a given mean electric field.

Enhanced normal incidence photocurrent in quantum dot infrared photodetectors

The authors report an enhancement in the photocurrent caused by normal incidence (s-polarization) radiation in a quantum dot-in-a-well (DWELL) based infrared photodetector. The s-to-p polarization ratio was increased to 50%, compared to the 20% in conventional quantum dot (QD) detectors. This improvement was achieved through engineering the dot geometry and the quantum confinement via postgrowth capping materials of the QDs. The effect of the capping procedures was determined by examining the dot geometry using transmission electron microscopy (TEM) and s-to-p ratio of the polarized photocurrent in the DWELL infrared photodetector. The TEM image shows a quantum dot with a reduced base of 12 nm and an increased height of 8 nm. The infrared photodetector fabricated from this material shows peak photodetectivities of 1×109 cm Hz1/2/W at 77 K for a peak wavelength of 4.8 μm and 1×107 cm Hz1/2/W at 300 K for a peak wavelength of 3.2 μm. The dark current density is as low as 2×10−4 A/cm2 and the photoconductive gain is 100 at the optimal operating bias.

Very long wavelength quantum dot infrared photodetector using a modified dots-in-a-well structure with AlGaAs insertion layers

We report a modified dots-in-a-well (DWELL) infrared photodetector by inserting some very thin GaAs or AlGaAs layers into the InAs dots. The photoluminescence (PL) measurements indicate that the modified DWELL structure with the insertion layers (ILs) of GaAs has a larger peak intensity and a narrower PL linewidth than that without the ILs. For the modified DWELL detector with AlGaAs ILs, the peak detection wavelength reaches very long infrared window of 14.1 μm. The peak detectivity D∗ is 1.1×108 cm Hz1/2/W at 77 K under normal incidence infrared irradiation.

High temperature operation of quantum dots-in-a-well infrared photodetectors

Systematic study of various types of intersubband transitions in the quantum dots-in-a-well (DWELL) infrared photodetectors has been presented. By changing the thickness of the quantum well, the excited state energy can be tuned with respect to the barrier, without altering the quantum dot ground state. Bound to continuum transitions offer very high extraction probability for photoexcited electrons but poor absorption coefficient, while the bound to bound transitions have higher absorption but poorer extraction probability. Bound to quasi-bound transition is optimum for intermediate values of electric fields with superior signal to noise ratio. The bound to quasi-bound device has the detectivity of 4 × 10 11 cm Hz 1/2 W −1 (+3 V, f/2 optics) at 77 K and 7.4 × 10 8 cm Hz 1/2 W −1 at 200 K, which is highest reported detectivity at 200 K for detector with long wave cutoff wavelength. High performance focal plane arrays have been fabricated with noise equivalent temperature difference of 44 mK at 80 K for 6.1 μm peak wavelength.

Comparative Study on InAs/InGaAs Dots-in-a-Well Structure Grown on GaAs(311) B and (100) Substrates

The InAs/InGaAs dots-in-a-well (DWELL) structures were grown on both GaAs(311) B and (100) substrates by molecular beam epitaxy. Quantum dots (QDs) grown on GaAs(311) B substrate are of higher density and more uniform size distribution, yet QDs grown on GaAs(100) substrate demonstrate a bimodal size distribution. The growth mechanism of these surface morphologies was briefly discussed. We found that the photoluminescence (PL) linewidth of DWELL grown on GaAs(311) B substrate was much narrower than the linewidth of DWELL grown on GaAs(100), which suggests a promising advantage in many QD based devices. We also found the temperature had a stronger impact on the PL intensity of DWELL grown on GaAs(311) B, which was explained by the lower thermal active energy and higher density of interfacial point defects of DWELL grown on GaAs(311) B. These results provide people a rather comprehensive insight into the advantages and disadvantages of DWELL grown on GaAs(311) B substrates.