Dot-in-well Research Articles

Investigation of type-II Ga(As)Sb/GaAs quantum dots embedded in an InGaAs quantum well for solar cell applications

We theoretically investigate the electronic and optical properties of charge carriers in type-II Ga(As)Sb/GaAs quantum dots (QDs) embedded in an InGaAs quantum well (QW), to determine their viability for use in intermediate band solar cell (IBSC) devices. This hybrid structure has the advantage of allowing a large flexibility to alter wave function distribution of the electron ground state, which is often considered as an intermediate band (IB) in QD-IBSC. The polarized transitions dipole moment (TDM), the polarized absorption coefficients and the radiative lifetime were studied as a function of QD height. Our theoretical results show an enhancement of inter-band absorption spectrum with a redshift of the peak corresponding to h0→IB transition and a blueshift of the continuum→IB peak, as the height of QD increases. The intra-band light absorption spectrum exhibits a redshift and its magnitude remains almost unchanged, as the height of QD increases. Furthermore this structure provides strong intra-band light absorption comparable to that of type-I QDs and equivalent to h0→IB transition strength. A long carrier lifetime in IB of about 30ns was found, but remains more than an order of magnitude less than the inter-band radiative time due to the low confinement potentials of the electron along the ρ-axis in this type-II dot-in-well (DWELL) structure.

InAs/GaAs quantum dot laser epitaxially grown on on-axis (001) GaAsOI substrate.

Quantum dot (QD) laser as a light source for silicon optical integration has attracted great research attention because of the strategic vision of optical interconnection. In this paper, the communication band InAs QD ridge waveguide lasers were fabricated on GaAs-on-insulator (GaAsOI) substrate by combining ion-slicing technique and molecular beam epitaxy (MBE) growth. On the foundation of optimizing surface treatment processes, the InAs/In0.13Ga0.87As/GaAs dot-in-well (DWELL) lasers monolithically grown on a GaAsOI substrate were realized under pulsed operation at 20 °C. The static device measurements reveal comparable performance in terms of threshold current density, slope efficiency and output power between the QD lasers on GaAsOI and GaAs substrates. This work shows great potential to fabricate highly integrated light source on Si for photonic integrated circuits.

Temperature Dependence of Quantum Dots-In-Well Infrared Photo Detectors (QDIPs) using Photoluminescence

A research of quantity dots-in-well infrared photo detectors (QDIPs) produces helpful outcomes for creating a twocolor QDIP. Quantum dot infrared photo detectors (QDIPs) have been shown to be a main technology in mid-and longwavelength infrared detection owing to their capacity for normal incidence operation and low dark current. This research explores infrared detectors based on intersubband transitions in a novel heterostructure of InAs / In0.15 Ga0.85 As / GaAs quantum dotsin-well (DWELL). The InAs quantum dots are also positioned in an In0.15 Ga0.85 in the DWELL framework, which in turn is well positioned with the In0.1Ga0.9As obstacle in GaAs quantum. Using fourier transform infrared spectroscopy, the optical characteristics of the sample were researched using photoluminescence and photocurrent. Spectrally adjustable reaction was noted at 6.2μm and 7.5μm with prejudice and lengthy wave IR reaction

Adiabatic two-step photoexcitation effects in intermediate-band solar cells with quantum dot-in-well structure

We studied the dynamics of electrons generated by two-step photoexcitation in an intermediate-band solar cell (IBSC) comprising InAs/GaAs/Al0.3Ga0.7As dot-in-well (DWELL) structure using time-resolved photocurrent (TRPC) measurement. The examined IBSC exhibited considerably slower photocurrent decay than a conventional InAs/GaAs quantum dot IBSC, which is due to the extraordinarily long-lived electrons in the DWELL. In order to retrieve the electron lifetime from the decay profile, we developed a model reproducing the observed decay and performed parameter fitting. The fitting results indicate that the electron lifetime in the DWELL is approximately 30 μs. In the two-colour excitation TRPC measurement, we found that an additional infrared (IR) light accelerates the photocurrent decay while the photocurrent increases by approximately 3%, because the additional IR light causes two-step photoexcitation of electrons in the DWELLs towards the conduction band. Furthermore, we demonstrated that the open-circuit voltage increases with increasing of the contribution of the second IR excitation process.

Dynamics of Broadband Lasing Cascade from a Single Dot-in-well InGaAs Microdisk

The development of a fast semiconductor laser is required for the realization of next-generation telecommunication applications. Since lasers operating on quantum dot ground state transitions exhibit only limited gain due to the saturation effect, we investigate lasing from excited states and compare its corresponding static and dynamic behavior to the one from the ground state. InAs quantum dots (QDs) grown in dot-in-well (DWELL) structures allowed to obtain light emission from ground and three excited states in a spectral range of 1.0–1.3 μm. This emission was coupled to whispering gallery modes (WGMs) of a 6 μm microdisk resonator and studied at room temperature by steady-state and time-resolved micro-photoluminescence. We demonstrate a cascade development of lasing arising from the ladder of quantum dot states, and compare the lasing behavior of ground and excited state emission. While the lasing threshold is being increased from the ground state to the highest excited state, the dynamic behavior is improved: turn-on times and lifetimes of WGMs become shorter paving the way towards high frequency direct driven microlasers.

The effect of post-growth rapid thermal annealing on InAs/InGaAs dot-in-a-well structure monolithically grown on Si

The effect of post-growth annealing (PGA) on dot-in-well (DWELL) structures grown on Si substrates has been studied. The photoluminescence (PL) measurements showed that, compared to the DWELL structures directly grown on GaAs, the PGA process induces a distinct difference in the tuning of the emission properties. Then, transmission electron microscopy imaging of the samples revealed that PGA improved the uniformity of quantum dots (QDs) while the size of the QDs increased, in agreement with a corresponding red shift and a decrease of the full width at half maximum in the PL emission spectrum. Finally, energy-dispersive x-ray linescan provided a quantitative analysis of the composition change of DWELL grown on Si in the as-grown, 700 °C annealed, and 800 °C annealed samples. The change in the InL/GaK concentration ratio became gradual between the QDs and surrounding materials after 800 °C annealing. The analysis of the optical properties, morphology evolution, and compositional change of the QDs as a function of the annealing temperature showed good agreement.

Annealing effect of the InAs dot-in-well structure grown by MBE

We have demonstrated that in situ annealing effect has to be taken into account in order to realize the 1.31 μm InAs quantum dot (QD) lasers with the dot-in-well (DWELL) structure. The photoluminescence (PL) properties have been investigated for the InAs DWELL samples annealed at different temperatures in situ, simulating the annealing process during the growth of the top cladding AlGaAs layer in the laser structure. The QDs with large size in the DWELL structure are vulnerable to the annealing process at temperatures above 550 °C, revealed by the drastic change in the PL spectra. However, the DWELL structure is stable during the annealing process at 540 °C for three hours. The thermal stability of the QDs in the DWELL structure has to be considered in the growth of QD lasers for long wavelength operation.

Detailed Study of the Influence of InGaAs Matrix on the Strain Reduction in the InAs Dot-In-Well Structure.

InAs/InGaAs dot-in-well (DWELL) structures have been investigated with the systematically varied InGaAs thickness. Both the strained buffer layer (SBL) below the dot layer and the strain-reducing layer (SRL) above the dot layer were found to be responsible for the redshift in photoluminescence (PL) emission of the InAs/InGaAs DWELL structure. A linear followed by a saturation behavior of the emission redshift was observed as a function of the SBL and SRL thickness, respectively. The PL intensity is greatly enhanced by applying both of the SRL and SBL. Finite element analysis simulation and transmission electron microscopy (TEM) measurement were carried out to analyze the strain distribution in the InAs QD and the InGaAs SBL. The results clearly indicate the strain reduction in the QD induced by the SBL, which are likely the main cause for the emission redshift.

InAs/GaAs quantum dot and dots-in-well infrared photodetectors based on p-type valence-band intersublevel transitions

InAs/GaAs quantum dot (QD) and dots-in-well (DWELL) infrared photodetector (QDIP) based on p-typevalence-band intersublevel hole transitions are reported. Two response bands observed at 1.5–3 and 3–10μmare due to optical transitions from the heavy-hole to spin–orbit split-off QD level and from the heavy-hole to heavy-hole level, respectively. The p-typehole response displays a well-preserved spectral profile (independent of the applied bias) observed in both QD and DWELL detectors. At elevated temperatures between 100 and 130K, the DWELL detector exhibits a strong far-infrared responses up to 70μm. An external quantum efficiency of 17% is demonstrated. The studies show the promise of p-typeQDs for developing infrared photodetectors.

Postgrowth intermixing of strain engineered InAs/GaAs quantum dots

This paper reports on the evolution of InAs/GaAs quantum dots’ (QDs) intermixing process depending on the QD position with respect to the InGaAs strain reducing layer. The postgrowth intermixing has been ensured by rapid thermal annealing (RTA) at different temperatures and the processed samples were investigated by photoluminescence (PL) spectroscopy. The reference sample, containing InAs QDs in pure GaAs matrix, demonstrates the highest intermixing degree with an emission energy blueshift up to 300meV. The reduction of the strain around the QDs for dot-in-well (DWELL) structure has been found to slightly reduces the intermixing degree leading to less emission energy blue shift. However, for the QDs with InGaAs underlying layer, a relative prohibition of the intermixing has been found to occur testifying the existing of thermally stable extended defects.

Study of valence-band intersublevel transitions in InAs/GaAs quantum dots-in-well infrared photodetectors

The n-type quantum dot (QD) and dots-in-well (DWELL) infrared photodetectors, in general, display bias-dependent multiple-band response as a result of optical transitions between different quantum levels. Here, we present a unique characteristic of the p-type hole response, a well-preserved spectral profile, due to the much reduced tunneling probability of holes compared to electrons. This feature remains in a DWELL detector, with the dominant transition contributing to the response occurring between the QD ground state and the quantum-well states. The bias-independent response will benefit applications where single-color detection is desired and also allows achieving optimum performance by optimizing the bias.

Avalanche quantum dot-in-well long wavelength infrared photodetectors: Linear and Geiger mode operation

Employment of additional multiplication region in the structure of quantum dots-in-well (DWELL) intersubband photodetectors can be a useful alternative for amplification of the photocurrent and giving higher gain to detected optical signal. For such detector the photocurrent of DWELL detector is increased by avalanche multiplication to achieve higher responsivity and resonant tunneling barriers are used in absorption region to reduce the dark current. In this paper the linear and Geiger mode operation of proposed detector is studied for operation at λ=11μm. In linear operation, the specific detectivity (D*) is increased about one order of magnitude in comparison to experimentally reported conventional DWELL detectors. We also report the single photon detection around mid and long infrared wavelength by gated mode operation of this detector with single photon quantum efficiency of about 3%.

High temperature operation In(Ga)As quantum dot infrared photodetector focal plane arrays passivated with 6.5 nm-thick Al<SUB align="right">2O<SUB align="right">3 layer

Quantum dot infrared photodetectors (QDIPs) have been shown to be a key technology in broad infrared wavelength (3–12 µm) detection due to their potential for normal incidence operation, low dark current density and higher temperature operation. In our research, we have been investigating infrared detectors based on intersubband transitions in a novel InAs/In 0.15 Ga 0.85 As quantum dots-in-well (DWELL) and normal GaAs-capped InAs quantum dot heterostructures. The study is to demonstrate a dual-band infrared image based on two stacked InGaAs and GaAs- capped InAs QDIP structure, with the use of nano-scale Al 2 O 3 surface passivated layer by atomic layer deposition (ALD) system deposited to decrease the device shot noise and boost the operation temperature of the thermal imaging FPA to 180 K. It is worth to mention the specific detectivities (D*) of test device passivated with Al 2 O 3 measured under 80 K and Vb = –3.3 V for mid-wavelength detection is as high as 2.1 × 10 11 cmHz 1/2 /W. By a two-step selective backside etching process, the uncorrected non-uniformity of NEDT is lowered to 6.7%. It reveals the high uniformity of infrared image taken from QWIP FPA with this proposed process is comparable to the conventional one with the mixture of wet- and dry-etching ICP-RIE.

Abnormal temperature dependencies of photoluminescence and carrier transfer in InAs QDs and DWELL structures grown on GaAs (1 1 5)A emitting near 1.3 µm wavelength

In this work, an optical comparative study of InAs quantum dots (QDs) and InAs/In0.25Ga0.75As dot-in-well (DWELL) grown by molecular beam epitaxy on GaAs (115)A substrate has been carried out. Emission peak at 1.3µm has been achieved from the DWELL sample. It is found that a monotonical decrease of the integrated PL intensity and the FWHM has taken place, with an increasing temperature up to 270K for the QDs sample. However, we observe an unusual increase of the FWHM and an invariant integrated PL intensity in the range of temperature (60–150K). We have attributed this result to two mechanisms: the acoustic–phonon interaction and the tunnel escaping carriers among the inhomogeneous dots size via the piezoelectric field. Theoretical modeling procedure has been applied to analyse the ground state PL peak position evolution as a function of temperature using three models (Varshni, “Vina, Logothetidis and Cardona”, and Passler). The comparison between theoretical and experimental data revealed that the PL contribution of the inhomogeneous size of quantum dots takes place only for temperatures higher than 160K. These results can help improve our understanding of some fundamental properties of DWELL and QDs grown on the GaAs high index substrates.

Hybrid Quantum Well/Quantum Dot Structure for Broad Spectral Bandwidth Emitters

We report a hybrid quantum well (QW)/quantum dot active element for an application in broadband sources. These structures consist of an InGaAs QW and six InAs dot-in-well (DWELL) layers. The single QW is designed to emit at a wavelength coincident with the second excited state of the quantum dot. We compare two hybrid QW/quantum dot samples where the QW position is changed, and show that carrier transport effects make QW placement very important through current-voltage, capacitance-voltage, photocurrent, and temperature-dependent spontaneous emission measurements. Using the optimal structure, due to the combined effects of quantum dot ground states, first excited state, and QW emission, a positive modal gain spanning ~300 nm is achieved for the segmented contact device. The values for modal gain are further confirmed by simultaneous three-state lasing, which is studied spectroscopically. Finally, a hybrid QW/quantum dot superluminescent diode (SLD) is reported; the device exhibits a 3 dB emission spectrum of 213 nm, centered at 1230 nm with a corresponding output power of 1.1 mW. The hybrid SLD is then assessed for an application in an optical coherence tomography system; an axial resolution of ~4 μm is predicted.

Evaluation of electronic transport properties and conduction band offsets of asymmetric InAs/InxGa1−x As/GaAs dot-in-well structures

The charge carrier distribution profile, carrier dynamics, barrier height and conduction band offset for asymmetric InAs/InxGa1−x As/GaAs dot-in-well (DWELL) structures are evaluated from temperature-dependent electronic transport and spectroscopic measurements. The charge carrier leakages via defects lead to less charge carrier accumulation in the DWELL architecture in comparison with the associated QWs. Furthermore, the differential capacitance–voltage measurements show the resonance of charge carrier escape rate with the ground and excited states of the DWELL structure. The peak of the charge carrier escape rate related to the excited state is almost independent of the temperature while that of the ground state changes significantly with heating. It is explained on the basis of changes in the accumulated carrier density due to carrier localization and nonradiative recombination processes, via defects/dislocations/Auger recombination, which are dominant at moderate and high temperatures, respectively. Furthermore, the obtained values of barrier height and conduction band offsets for asymmetric InAs/InxGa1−x As/GaAs (x = 0.18) DWELL structures are 194 ± 10 meV and 287 ± 10 meV, respectively. Downward transitions from both ground and excited states of quantum dots (QDs) are observed in photoluminescence while upward transitions only to the excited state energy levels of QDs are observed in photoreflectance spectroscopy. This is due to the filling of the ground state energy levels of QDs, which is confirmed by the electron transport measurements. Hence, it is concluded that interband absorption occurs to the excited states, whereas intersubband absorption predominantly takes place from the ground states to the continuum of the QD ensemble.

MOCVD Grown Quantum Dot-in-a-Well Solar Cells

This paper reports the experimental work on the characterization of quantum dot-in-well (DWELL) solar cell grown by metal-organic chemical vapor deposition (MOCVD) without employing any post-growing optimization like antireflection coating and metal grid. The structure of the 10-layer DWELL solar cells is studied by cross-sectional transmission electron microscopy (TEM). Room temperature photoluminescence (PL) spectra show strong quantization at 1178.5 nm with a linewidth of 79.9 nm. External quantum efficiency spectra show enhancement in the spectral response of the photocurrent with respect to the reference quantum dot cell (without DWELL structure). In spite of the reduction in conversion efficiency due to poor collection of current in external circuit compared to reference quantum dot cell it show the improvement in open circuit voltage.

Correlation between defect density and current leakage in InAs∕GaAs quantum dot-in-well structures

We present a study of InAs∕GaAs quantum dot-in-well (DWELL) material using transmission electron microscopy and leakage current-voltage measurements. The spacer layers between the DWELL layers have a variety of annealing and growth temperatures. We show that there is a strong correlation between spacer layer, annealing temperature, defect density, and these leakage currents, with the most defective sample having 30 times more defects and a leakage current several orders of magnitude above that of the least defective. Cross section transmission electron microscope (TEM) shows that surface roughness above defective dots is responsible for the high defect densities. However, even in the best sample the reverse bias leakage current is several orders of magnitude above that typically seen in quantum well materials and a measurable density of defective dots are observed in planar view TEM.

Design, growth, fabrication, and characterization of InAs∕GaAs 1.3μm quantum dot broadband superluminescent light emitting diode

In this paper we discuss a technique for broadening the emission and gain spectra of 1.3μm quantum dot superluminescent light emitting diodes (SLEDs). By incorporating different amounts of indium in different wells of a multi-dot-in-well stack we are able to tailor the emission and gain spectra of the devices. This technique allows us to overlap the ground state of one dot-in-well (DWELL) with the excited state of another to achieve broader and flatter emission spectra compared to a SLED design comprising DWELL layers of constant indium composition. Due to the low internal loss of these structures, this broadening is achieved without a significant reduction in the output power of the devices.

Broad-Band Superluminescent Light Emitting Diodes Incorporating Quantum Dots in Compositionally Modulated Quantum Wells

We demonstrate a technique for tailoring the emission bandwidth of ∼1.3 µm quantum dot superluminescent light emitting diodes. A broadening of the emission is achieved by incorporating the InAs quantum dot layers in InGaAs quantum wells of different indium compositions. These structures exhibit a broader and flatter emission compared to a simple dot-in well structure comprised of wells of identical indium composition.