Emission Of InAs Quantum Dots Research Articles

Impact of composition of AlGaInAs confining barriers on emission of InAs quantum dots embedded in AlGaAs/GaAs dot-in-a-well heterostructures

The emission of InAs quantum dots (QDs) grown on Al0.30 Ga0.70As/GaAs heterostructures and integrated into additional capping/buffer quantum wells (QW), known as dot-in-a-well (DWELL) structures, has been investigated. Two different AlGaInAs confining barriers (CBs) and buffer layers (BLs) are compared. The first QD structure includes the Al0.30 Ga0.70As CB (#1) and the In0.15Ga0.85As BL. The second QD structure consists of the Al0.40Ga0.45In0.15As CB (#2) and the In0.25Ga0.75As BL. Comparison of photoluminescence (PL) spectra has revealed that the ground state (GS) emission band in the structure #2 with Al0.40Ga0.45In0.15As CB is characterized by the lower peak energy, smaller PL linewidth (more homogeneous QD sizes) and higher GS emission intensity compared to that parameters in the structure #1 with Al0.30 Ga0.70As CB. The smaller potential barriers at the Al0.40Ga0.45In0.15As CB/QD interfaces in #2 lead to faster thermal decrease in the GS PL intensity at high temperatures compared with that in #1. High-resolution X-ray diffraction (HR-XRD) method was used for the study of the QD structures with the aim of monitoring the sizes and compositions of QDs and QWs. Numerical simulation of HR-XRD scans has shown that the material compositions of QDs and QW in #2 with Al0.40Ga0.45In0.15As CB has not changed in the process of QD structure growth at high temperatures compared to those in #1. The obtained results are interesting for further improvement of InAs/GaAs QD structures for telecommunication technology and optoelectronic applications.

Optical properties of self-assembled InAs quantum dots based P–I–N structures grown on GaAs and Si substrates by Molecular Beam Epitaxy

Extensive work on InAs quantum dots grown on GaAs substrates has been reported in the literature. However, research in the use of different substrate materials such as silicon to achieve an ideal and full integration of photonic and electronic systems is still a challenge. In this work we have investigated the effect of the substrate material (Si and GaAs) and strain reducing layer on the optical properties of InAs quantum dots for possible applications in laser devices grown by Molecular Beam Epitaxy. Two InAs quantum dots structures with similar active regions grown on GaAs and Si substrates using strain reducing layer consisting of InAs QDs/6 nm In0.15Ga0.85As have been investigated. Atomic Force Microscopy, Transmission Electron Microscopy, and photoluminescence have been used for the characterization of the samples. We have observed a red shift of the InAs QD photoluminescence peak energy for the sample grown on Si substrate as compared to the sample grown on GaAs substrate, which was associated with residual biaxial strain from the Si/GaAs heterointerface. This red-shift of the photoluminescence peak energy is accompanied by a broadening of the photoluminescence spectrum from ∼31 meV to a value of ∼46 meV. This broadening is attributed to the quantum dots size inhomogeneity increase for samples grown on Si substrate. This result open new insights for the controlling the emission of InAs quantum dots for photonic devices integration using Si substrates.

Annealing Impact on Emission of InAs Quantum Dots in GaAs/Al0.30Ga0.70As Structures with Different Capping Layers

Annealing Impact on Emission of InAs Quantum Dots in GaAs/Al0.30Ga0.70As Structures with Different Capping Layers

Scalable in operando strain tuning in nanophotonic waveguides enabling three-quantum-dot superradiance.

The quest for an integrated quantum optics platform has motivated the field of semiconductor quantum dot research for two decades. Demonstrations of quantum light sources, single photon switches, transistors and spin-photon interfaces have become very advanced. Yet the fundamental problem that every quantum dot is different prevents integration and scaling beyond a few quantum dots. Here, we address this challenge by patterning strain via local phase transitions to selectively tune individual quantum dots that are embedded in a photonic architecture. The patterning is implemented with in operando laser crystallization of a thin HfO2 film 'sheath' on the surface of a GaAs waveguide. Using this approach, we tune InAs quantum dot emission energies over the full inhomogeneous distribution with a step size down to the homogeneous linewidth and a spatial resolution better than 1 µm. Using these capabilities, we tune multiple quantum dots into resonance within the same waveguide and demonstrate a quantum interaction via superradiant emission from three quantum dots.

AlGaInAs capping layer impact on emission and structure of AlGaAs/GaAs quantum wells with InAs quantum dots

Three types of GaAs/Al0.30Ga0.70As quantum wells (QWs) with InAs quantum dots (QDs) cavered by the different capping layers: GaAs (#1), Al0.30Ga0.70As (#2) and Al0.1Ga0.75In0.15As (#3), have been investigated. The photoluminescence (PL), its temperature dependence and high resolution X-ray diffraction (HR-XRD) methods were applied. It is revealed that QD emission in the structure #3 is characterized by the lower PL peak energy, highest PL intensity and smaller half widths of PL bands, in comparison with the QD emissions in #1 and #2. PL temperature dependences have been studied that revealing the QD material composition in #3 is closer to InAs than those in #1 and #2.HR-XRD scan fitting permits to understand the process of strain relaxation in studied structures at high QW growth temperatures. This process in the structures #1 and #2 was connected with material composition varying the QDs and capping layers due to Ga/In atom intermixing that leads to the QD emission shift into the higher energy range and PL intensity decreasing. Meanwhile in the structure #3 with Al0.1Ga0.75In0.15As capping, the strain relaxation manifests itself by InAs QD height decreasing without changing the InAs QD material composition. The advantages of Al0.1Ga0.75In0.15As capping layer used and its impact on the emission of InAs QDs have been discussed.

Precision tuning of InAs quantum dot emission wavelength by iterative laser annealing

Controlling the emission wavelength of quantum dots (QDs) over large surface area wafers is challenging to achieve directly through epitaxial growth methods. We have investigated an innovative post growth laser-based tuning procedure of the emission of self-assembled InAs QDs grown epitaxially on InP (001). A targeted blue shift of the emission is achieved with a series of iterative steps, with photoluminescence diagnostics employed between the steps to monitor the result of intermixing. We demonstrate tuning of the emission wavelength of ensembles of QDs to within approximately ±1 nm, while potentially better precision should be achievable for tuning the emission of individual QDs.

InAs quantum dot emission and annealing impact in quantum wells with strain reduced InAlGaAs layers

InAs quantum dots (QDs) embedded in Al0.30Ga0.70As/InGaAs/InGaAlAs/Al0.30Ga0.70As quantum wells (QWs) have been investigated in the as grown state and after thermal annealing at 640°C or 710°C for two hours. Two types of QD structures with different InGaAlAs capping layers (In0.15Ga0.85As (#1) and In0.15Al0.4Ga0.45As (#2)) are compared and studied by means of photoluminescence (PL), X-ray diffraction (XRD) and high resolution HR-XRD methods. XRD and HR-XRD techniques are used with the aim to control the quality of crystal structures, as well as the variation of material compositions and elastic strains in QW layers at thermal annealing.The higher PL intensity and lower energy of ground state (GS) emission are detected in the structure with the In0.15Al0.4Ga0.45As capping layer. Thermal annealing leads to the shift of PL spectra into the higher energy range and the value of this shift is more relevant in the structure with the In0.15Ga0.85As capping layer. The variation of GS emission peak versus temperature has been monitored within the range of 10–500K for as grown and annealed states and is compared with the temperature shrinkage of energy band gap in the InAs and GaAs bulk crystals. It is revealed that the QD composition in #2 is InAs and the efficiency of Ga/Al/In intermixing at thermal annealing in #2 is less than in #1.Thermal decay of the integrated PL intensities has been investigated in studied structures as well. The PL thermal decay 10-fold is revealed for the range of 10–300K in #1 with the In0.15Ga0.85As capping layer compared with the 5-fold thermal decay in #2 for the In0.15Al0.4Ga0.45As capping layer. Finally the reasons for PL spectrum transformation at annealing, the mechanism of PL thermal decay, and the advantages of QD structures with the In0.15Al0.4Ga0.45As capping layer have been analyzed and discussed.

Extraction efficiency enhancement of single InAs quantum dot emission through light scattering on the Au nanoparticles

Single semiconductor quantum dots (QDs) have been considered as the promising solid-state single photon sources. To obtain bright quantum sources, the key issue is to enhance extraction efficiency of the QD emission, which is challenging since QDs normally emit isotropically in a high refractive index material. In this article, we investigate the influence of Au nanoparticles on the QD photoluminescence (PL) extraction efficiency based on the techniques of optically positioned QDs and single QD emission detection. The InAs QD samples studied are grown using the molecular beam epitaxy on a (001) GaAs substrate. The sample consists of, in sequence, a 200 nm GaAs buffer layer, a 100 nm AlAs sacrificed layer, a 30 nm GaAs, a QD layer, and a 100 nm GaAs cap layer. The QD sample is mounted in a cryostat cooled down to 5 K, and excited by illumination of a 640 nm diode laser (CW or pulsed with a repetition frequency of 80 MHz). Excitation laser beam is focused to an approximately 2 μm spot on the sample using a microscope objective (NA : 0.5) which is mounted on a nanocube XYZ piezo nanopositioning stage with a scanning range of 100×100×100 μm3. The QD PL is collected using the same objective and measured using a 0.5 m focal length monochromator equipped with a silicon charge-coupled device (CCD). The PL decay measurements are performed using a silicon avalanche photodiode (APD) and a time-correlated single-photon counting (TCSPC) board.#br#In order to study the influence of different environments surrounding the QDs on the spontaneous emission rate and the extraction efficiency, the same QD emissions are measured under the conditions that: (1) A typical QD is at first chosen and optically positioned and then its emission is measured. (2) A GaAs layer containing the QDs is lifted off from the as-grown sample by an AlAs sacrificed layer and placed on the Au film with or without Au nanoparticles. (3) Optical measurements are carried out to obtain the QD emission intensity. This technique enables us to compare the same QD emission intensity for the as-grown QD sample, which is placed on the Au film or on the Au nanoparticles.#br#In summary, it is found that the measured QD emission intensity increases up to 6 times that of the original for the QD placed on the Au nanoparticles, otherwise it is only doubled for the QD placed on the Au film. The time-resolved PL measurements show that the QDs have nearly the same decay time for the QDs in different environments, implying that the QD spontaneous emission rate has not been changed. Therefore, the enhanced PL is due to the increase of extraction efficiency. The physical mechanism underlying the Au nanoparticles-induced PL enhancement is attributed to the trapped QD emission light within the sample and scattered again by Au nanoparticles and collected by the microscopy objective.

Impact of capping layer type on emission of InAs quantum dots embedded in InGaAs/InxAlyGazAs/GaAs quantum wells

The optical and structural properties of In0.15Ga0.85As/InxAlyGazAs/GaAs quantum wells with embedded InAs quantum dots (QDs) were investigated by the photoluminescence (PL), its temperature dependence, X-ray diffraction (XRD), and high resolution (HR-XRD) methods in dependence on the composition of capping InxAlyGazAs layers. Three types of capping layers (Al0.3Ga0.7As, Al0.10Ga0.75In0.15As, and Al0.40Ga0.45In0.15As) have been used and their impact on PL parameters has been compared. Temperature dependences of PL peak positions in QDs have been analyzed in the range of 10–500 K and to compare with the temperature shrinkage of band gap in the bulk InAs crystal. This permits to investigate the QD material composition and the efficiency of Ga(Al)/In inter diffusion in dependence on the type of InxAlyGazAs capping layers. XRD and HR-XRD used to control the composition of quantum well layers. It is shown that QD material composition is closer to InAs in the structure with the Al0.40Ga0.45In0.15As capping layer and for this structure the emission 1.3 μm is detected at 300 K. The thermal decay of the integrated PL intensity has been studied as well. It is revealed the fast 102-fold thermal decay of the integrated PL intensity in the structure with the Al0.10Ga0.75In0.15As capping layer in comparison with 10-fold decay in other structures. Finally, the reasons of PL spectrum transformation and the mechanism of PL thermal decay for different capping layers have been analyzed and discussed.

Emission of InAs quantum dots embedded in InGaAs/InAlGaAs/GaAs quantum wells

Photoluminescence (PL), its temperature dependences and X-ray diffraction (XRD) have been investigated in MBE grown GaAs/Al0.3Ga0.7As/In0.15Ga0.85As/InAlGaAs/GaAs quantum wells (QWs) with embedded InAs quantum dots (QDs). Studied QD structures are the important part of low-threshold, high-power lasers for optical fiber communication systems. Three types of QD structures with different capping layers (GaAs, Al0.3Ga0.7As and Al0.1Ga0.75In0.15As) were investigated. The comparison of the impact of capping layer compositions on PL parameters of InAs QDs and the process of Ga/Al/In intermixing has been obtained.QD emission is analyzed in the temperature range of 10–500K and the QD emission shift has been compared with the temperature shrinkage of band gaps in the bulk InAs and GaAs crystals. Different changes of QD compositions owing to Ga/Al/In intermixing have been revealed in studied structures. The XRD study is used to control a composition of QW layers and to confirm additionally that the efficiency of Ga/Al/In intermixing depends on capping layer types.The thermal decay of integrated PL intensities has been investigated as well. The PL thermal decay 557-fold in the range 10–300K is revealed in the structure with GaAs capping layer in comparison with the 6- and 20-fold PL decays in structures with Al0.3Ga0.7As and Al0.1Ga0.75In0.15As capping layers, respectively. The reason of PL spectrum transformation and the mechanism of PL thermal decay in QD structures with different capping layer compositions have been analyzed and discussed.

Ground-state exciton emission of InAs quantum dots produced by focused-ion-beam-directed nucleation

We report on scanning micro-photoluminescence measurements on InAs quantum dots that were patterned using an in vacuo focused ion beam. The focused ion beam produced a square array of holes with an array spacing of 2μm, upon which multiple layers of InAs and GaAs were deposited. Two-dimensional mapping of the luminescence spatially and spectrally resolves the emission from individual dots. At some positions, multiple emission peaks are observed, which could have originated from (i) different exciton transitions of the same quantum dot, or (ii) different quantum dots. Power dependence measurements reveal that the emission increases linearly with power, and maps of the emission show that the positions of each peak are spatially separated, suggesting that the peaks are ground-state exciton emission from different dots.

Influence of thermal annealing on the electron emission of InAs quantum dots containing a misfit defect state

We have investigated the effect of postgrowth thermal annealing on the electron emission from InAs quantum dots (QDs) containing a misfit-related defect state induced by strain relaxation. Additional carrier depletion in the GaAs bottom layer near the QD, caused by the defect state, can effectively suppress electron tunneling from the QD, leading to the observation of a thermal emission from the QD electron ground state to the GaAs conduction band with a large emission energy of 213 meV, in contrast to defect-free nonrelaxed QDs in which an emission of 58 meV from the QD electron ground state to first excited state is observed. The emission energy is reduced to 193 meV and to 164 meV after annealing at 650 and 700 °C for 1 min, respectively. This emission energy reduction is correlated with the photoluminescence blueshift which is attributed to the interdiffusion of atoms across the QD interface. The electron emission from the QD first excited and ground states is found to be a thermal emission at high temperatures and a tunneling emission at low temperatures. The tunneling energy barrier is found to be comparable to the thermal emission energy, supporting a thermal emission to the GaAs conduction band. This study illustrates a significant effect of a defect state on the electron-emission process in the QDs, suggesting the possibility of modifying the electron emission time of the QDs by purposely introducing a deep defect state.

Quantum dot strain engineering of InAs∕InGaAs nanostructures

We present a complete study both by experiments and by model calculations of quantum dot strain engineering, by which a few optical properties of quantum dot nanostructures can be tailored using the strain of quantum dots as a parameter. This approach can be used to redshift beyond 1.31μm and, possibly, towards 1.55μm the room-temperature light emission of InAs quantum dots embedded in InGaAs confining layers grown on GaAs substrates. We show that by controlling simultaneously the lower confining layer thickness and the confining layers’ composition, the energy gap of the quantum dot material and the band discontinuities in the quantum dot nanostructure can be predetermined and then the light emission can be tuned in the spectral region of interest. The availability of two degrees of freedom allows for the control of two parameters, which are the emission energy and the emission efficiency at room temperature. The InAs∕InGaAs structures were grown by the combined use of molecular beam epitaxy and atomic layer molecular beam epitaxy; their properties were studied by photoluminescence and photoreflectance spectroscopies and by atomic force microscopy; in particular, by means of photoreflectance not only the spectral features related to quantum dots were studied but also those of confining and wetting layers. The proposed approach has been used to redshift the room-temperature light emission wavelength up to 1.44μm. The optical results were analyzed by a simple effective-mass model that also offers a rationale for engineering the properties of structures for efficient long-wavelength operation.

Room-temperature 1.6μm light emission from InAs∕GaAs quantum dots with a thin GaAsSb cap layer

It is demonstrated that the emission of InAs quantum dots (QDs) capped with GaAsSb can be extended from 1.28to1.6μm by increasing the Sb composition of the capping layer from 14% to 26%. Photoluminescence excitation spectroscopy is applied to investigate the nature of this large redshift. The dominant mechanism is shown to be the formation of a type-II transition between an electron state in the InAs QDs and a hole state in the GaAsSb capping layer. The prospects for using these structures to fabricate 1.55μm injection lasers are discussed.

Spontaneous Emission Enhancement of Quantum Dots in a Photonic Crystal Wire

Photonic wires are the simplest extended low-dimensional systems. Photonic crystal confinement confers them a divergent density of states at zero-group-velocity points, which leads to enhancement of spontaneous emission rates [D. Kleppner, Phys. Rev. Lett. 47, 233 (1981)10.1103/Phys. Rev. Lett. 47.233]. We experimentally evidence, for the first time, the spectral signature of these Purcell factor singularities, using the out-of-plane emission of InAs quantum dots buried in GaAs/AlGaAs based photonic crystal based wire. Additionally, in-plane collection at the wire exit shows large enhancements of the signal at some of the density of states singularities.

Control of InAs Quantum Dot Emission Wavelengths in Narrow Regions by Selective Formation of GaInAs Covered Layers Grown with in situ Mask

We have succeeded in controlling the emission wavelength of a self-assembled InAs quantum dot (QD) structure in a narrow region. The emission wavelength of the QDs was varied locally by a covered GaInAs layer grown with an in situ mask, which can be fitted to the sample holder and removed in an ultra-high-vacuum environment. This mask enables the selective growth of high-quality self-assembled QDs with the desired emission wavelengths ranging from 1.23 µm to 1.32 µm. This technique has potential applications in the integration of microstructures with QDs into optoelectronic functional devices.

Charge-state dependence of InAs quantum-dot emission energies

The emission of electrons from self-assembled InAs quantum dots (SAQDs) is probed with transient capacitance spectroscopy (DLTS). The SAQDs are epitaxially grown on (100) GaAs and embedded in a slightly doped Schottky diode. Unprecedented resolution in our measurements enables us to determine different energies for emission from the s shell and the p shell of the quantum dots and to observe a strong field dependence of the activation energies derived from our DLTS spectra. Furthermore, we resolve different DLTS peaks for the emission from singly and doubly occupied s shells. The analysis of our data reveals that both the electric field as well as charge-state dependence of our spectra can be explained by a model in which phonon-assisted tunneling plays a crucial role for emission from the quantum-dot s state.

Growth interruption to tune the emission of InAs quantum dots embedded in InGaAs matrix in the long wavelength region

We investigate the effects of combining growth interruption (GI) and InGaAs strain reducing matrix at low In content on the optical properties of Stanski–Krastanow InAs QDs grown by molecular beam epitaxy. Atomic force microscopy and Photoluminescence (PL) measurements were carried out on three series of uncapped and capped samples grown in different conditions: (a) low arsenic pressure (As BEP=4×10 −6 Torr) and relatively low InAs growth temperature ( T InAs=530°C); (b) low As BEP (4×10 −6 Torr) and high T InAs (545°C); (c) high As BEP (7×10 −6 Torr) and low T InAs (530°C). Our studies clearly show that in the three growth regimes, GI causes very different effects on the morphological and optical properties of InAs QDs embedded in InGaAs matrix (In=0.12), ruled by the In diffusion and desorption processes. In particular, for samples grown at 530°C and As BeP=4×10 −6 Torr (series a), the arrangment of GI and QD embedding in an InGaAs matrix at low In content is an effcient method to tune the InAs QD PL emission and to improve their optical quality. In this case, indeed, the increase of the interruption time up to 300 s introduces a strong redshift (∼110 meV) in the PL peak with a decreasing of the linewidths from 52 to 26 meV, obtaining room temperature emission characterized by a narrow peak close to 1.3 μm.