InAs Quantum Dot Structures Research Articles

Molecular beam epitaxial growths of high-optical-gain InAs quantum dots on GaAs for long-wavelength emission

Uniform InAs quantum dots (QDs) have been realized on GaAs by molecular beam epitaxy, which can increase maximum optical gain in laser diodes without sacrificing original high-density and high optical quality characteristics. This improvement enables laser operation under higher environment temperature. In order to improve QD uniformity, we searched for a proper growth condition and sequence for suppressing In out-diffusion during QD coverage. Although the same initial InAs QD structure on GaAs was used, PL linewidth was reduced after coverage under such improved conditions. In QD lasers, higher optical gain as large as 54cm−1 at room temperature which is about 10cm−1 larger than our previous data was realized. Those improved QDs are attractive for realizing novel optical devices.

Emission and HR-XRD study in InAs quantum dot structures prepared at different QD’s growth temperatures

Papers in the Appendix were published in electronic format as Volume 1534

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.

Long wavelength strain-engineered InAs five stacks quantum dots laser diode growth by molecular beam epitaxy

In this paper the molecular beam epitaxy (MBE) growth of GaAs based 1.5μm transmission window InAs quantum dots (QDs) was systematically studied. GaAs based InAs QDs with 1.5μm emission were fabricated successfully by multi-layer stacked InAs QD structures. Owing to a strong electronic coupling between the QD layers, the QDs and the quantum wells (QWs), strain-engineered QDs introducing antimony (Sb) with emission wavelength up to 1.5μm at room temperature were achieved. The QD laser diodes (LDs) were fabricated, with cavity lengths of 1000μm and stripe width of 100μm. The output performance was achieved with the threshold current densities of the device was as low as 164A/cm2, the continuous wave operating up to 22mW at room temperature (RT) was achieved.

Energy states and carrier transport processes in metamorphic InAs quantum dots

Photoluminescence excitation spectroscopy is used to probe energy states and carrier transport in InAs quantum dot structures grown on InGaAs metamorphic layers, designed for room temperature emission at 1.3, 1.4, or 1.5 μm. The dominant spectral feature is shown to arise from the partially relaxed InGaAs confining layer. In structures with a low indium composition or thin InGaAs layer, a clear wetting layer feature is observed which acts as the dominant reservoir for carriers thermally excited from the quantum dots. Structures with high indium composition and/or thick InGaAs lack a wetting layer and carriers escape directly to the InGaAs layers.

Correlating structure, strain, and morphology of self-assembled InAs quantum dots on GaAs

We report on the use of a direct x-ray phase retrieval method, coherent Bragg rod analysis, to characterize self-assembled InAs quantum dots (QDs) grown epitaxially on GaAs substrates. Electron density maps obtained close to the x-ray absorption edges of the constituent elements are compared to deconvolute composition and atomic spacing information. Our measurements show no evidence of a wetting layer and reveal bowing of the atomic layers throughout the QD, extending from the QD-substrate interface. This leads to a half-layer stacking shift which may act to partially decouple the QDs electronically from the substrate.

Electronic structure of InAs quantum dots with GaAsSb strain reducing layer: Localization of holes and its effect on the optical properties

The electronic structure of InAs quantum dots covered with the GaAs1−ySby strain reducing layer has been studied using the k⃗⋅p⃗ theory. We explain previous experimental observations of the red shift of the photoluminescence emission with increasing y and its blue shift with increasing excitation power. For y>0.19, type-II dots are formed with holes localized in the GaAsSb close to the dot base; two segments at the opposite sides of the dot, forming molecular-like states, result from the piezoelectric field. We also propose an experiment that could be used to identify the hole localization using a vertical electric field.

Optical gain of multi‐stacked InAs quantum dots grown on InP(311)B substrate by strain‐compensation technique

AbstractWe used a strain‐compensation technique to fabricate multi‐stacked InAs quantum dot (QD) structures on InP(311)B substrates. We stacked 60 layers of InAs QDs without degrading the crystal quality and achieved a structure with a total QD density of 4.9 × 1012 cm‐2. Then dependence of optical gain on the number of QD layers stacked was investigated, using variable stripe‐length (VSL) method. The optical gain increased with increase in the number of QD layers. We obtained an optical gain of 49.7 cm‐1 when 60 QD layers were stacked. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High‐density quantum dot superlattice for application to high‐efficiency solar cells

AbstractWe characterized the optical absorption and solar cell characteristics of high‐density InAs self‐assembled quantum dots (QDs) grown on GaAs (001) substrates by atomic hydrogen‐assisted molecular beam epitaxy. The GaNAs material can be used as strain‐compensating layer (SCL) thereby minimizing the net strain, and thus is advantageous for multi‐stacking of InAs QDs structures. As a result, dislocations and coalesced islands were not observed in 100 layer‐stacked QDs. For QD solar cell characterization, the short‐circuit current density of QDSC increases with increasing number of QD stacks, and reaches as high as 26.4 mA/cm2 for 50 layer‐stacked sample under air‐mass 1.5 condition. However, the light absorption by QD superlattice as determined by optical absorption measurements at is limited to ∼ 10% even for 100 layer‐stacked samples. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High optical property vertically aligned InAs quantum dot structures with GaAsSb overgrown layers

This study investigates the feasibility of growing high quality columnar InAs/GaAsSb quantum dots (QDs) on a GaAs (100) substrate using a molecular beam epitaxial system. Structural and photoluminescence (PL) studies are conducted on vertically aligned, ten-period InAs quantum dot (QD) stacks with two different overgrown layer designs. Experimental results indicate an increased dot density of 5×1010cm−2 with completely suppressed coalescences in vertically aligned InAs quantum dots capped by a GaAsSb layer. This finding demonstrates a columnar dot structure with an enhanced luminescent intensity, activation energy, and narrow spectral line width of 22meV.

Effects of a InGaAs Strained Layer on Structures and Photoluminescence Characteristics of InAs Quantum Dots

The influences of a thin InGaAs layer grown on GaAs(100) substrate before deposited InAs self-assembled quantum dots(SAQDs) were experimentally investigated. Scanning electronic microscope (SEM) measurements show that the InGaAs strained layer may release the strain between wetting layer and QDs, and then enlarge size of QDs. When the thickness of InAs layer is small, the QDs are chained. Temperature dependent photoluminescence (TDPL) measurements show that the PL peaks of InAs QDs with In0.1Ga0.9As show much more red shift compared with the QDs directly deposited on GaAs matrix, and PL integral intensity enhances as T rises from 50K to 90K. We attribute this enhancement to the small potential barrier between WL and QDs produced by the InGaAs stained layer.

Influence of strain reducing layers on electroluminescence and photoluminescence of InAs/GaAs QD structures

We present a comparison of photo- (PL) and electro-luminescence (EL) spectra of quantum dot (QD) structures with different strain reducing layers (SRL). Simple QD structures without SRL have negligible difference between the PL and EL maxima, which are near 1250 nm. InGaAs and GaAsSb SRLs were used to shift the luminescence maximum towards telecommunication wavelengths at 1.3 or 1.55 μm. We have found that MOVPE prepared QD structures with SRL exhibit an EL maximum at a considerably shorter wavelength than the PL maximum measured on similar samples without doping in the absence of built-in electric field. A mechanism to explain this phenomenon is proposed for both types of SRLs. The GaAsSb SRL is more suitable for long wavelength EL due to the higher confinement potential of electrons compared to InGaAs SRL. EL maximum at 1300 nm and PL maximum at 1520 nm were achieved on InAs QD structures with GaAs 0.87Sb 0.13 SRL (type I heterojunction).

Efficiency of spin injection in novel InAs quantum dot structures: exciton vs. free carrier injection

Unambiguous experimental evidence for a significant difference in efficiency of excitonic vs. free carrier spin injection is provided in novel laterally arranged self-assembled InAs/GaAs quantum dot structures, from optical orientation and tunable laser spectroscopy. A lower efficiency of exciton spin injection as compared to free carrier spin injection from wetting layers into QDs results in a distinct feature in luminescence polarization of the QDs as a function of excitation photon energy. It is shown that this difference is not related to carrier density and state-filling effects arising from the difference in optical absorption efficiency between the excitons and free carriers. Rather, it is a genuine property for exciton spin injection that suffers stronger spin relaxation due to Coulomb exchange interaction.

Energy level scheme ofInAs/InxGa1−xAs/GaAsquantum-dots-in-a-well infrared photodetector structures

A thorough investigation of quantum-dots-in-a-well structures for infrared photodetector applications has been performed employing different experimental techniques. The electronic structure of self-assembled InAs quantum dots embedded in an In0.15Ga0.85As/GaAs quantum well (QW) was deduced from photoluminescence (PL) and PL excitation (PLE) spectroscopy. From polarization-dependent PL it was revealed that the quantum dots hold two electron energy levels and two heavy-hole levels. Tunnel capacitance spectroscopy confirmed an electron energy level separation of about 50 meV, and additionally, that the conduction-band ground state and excited state of the dots are twofold and fourfold degenerates, respectively. Intersubband photocurrent spectroscopy, combined with simultaneous interband pumping of the dots, revealed a dominant transition at 150 meV (8.5 mu m) between the ground state of the quantum dots and the excited state of the QW. Results from detailed full three-dimensional calculations of the electronic structure, including effects of composition intermixing and interdot interactions, confirm the experimentally unravelled energy level scheme of the dots and well.

Morphology and optical properties of single- and multi-layer InAs quantum dots

An understanding of the structural and optical properties of quantum dots (QDs) is critical for their use in optical communication devices. In this study, single- and multi-layer self-organized InAs QDs grown on (001) GaAs substrates by molecular beam epitaxy (MBE) were investigated. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the lateral size of multi-layer InAs QDs are larger and flatter than single-layer InAs QDs, which are oval-shaped. The change in shape and size may be attributed to the presence of InGaAs spacer layers in multi-layer InAs QDs. Reciprocal spacer mapping and fast Fourier transformation images clearly show that InGaAs spacer layers present in the multi-layer InAs QDs structures help to release the strain originally existing in the QDs. In addition, the photoluminescence peak of the multi-layer InAs QDs is broader than QD in the single-layer one, which implies that the multi-layer InAs QDs size variation is more random than the single-layer one and this corresponds with the HAADF-STEM images. These results prove that spacer layers release strain influencing the morphology and optical properties of the QDs.

Modulation spectroscopy on metamorphic InAs quantum dots

The modulation spectroscopy on 1.45 μm metamorphic InAs quantum dots (QDs) with In 0.3Ga 0.7As capping layer grown on GaAs substrate by molecular beam epitaxy (MBE) has been investigated by differential absorption (Δ α), electro-reflectance (ER), and photo-reflectance (PR) spectra at different reverse bias. The optical transitions of the ground state, excited states, and wetting layers were identified and discussed. The micro-structure characterization was also analyzed by TEM and AFM. The variation of refractive index spectra (Δ n) by calculating Δ α spectra through Kramers–Kronig transform is obtained to study the electro-absorption behaviors. Additionally, a simple physical model is proposed to explain the experimental values between the Δ n and Δ R spectra performed by two different modulation spectroscopies (Δ α and ER). The built-in electric field of metamorphic InAs QDs structure was determined to analyze the Franz–Keldysh Oscillation (FKO) extreme in PR spectra with different bias.

Reasons of emission inhomogeneity in InAs quantum dot structures

The photoluminescence (PL) inhomogeneity has been studied in InAs quantum dots (QDs) embedded in the symmetric In 0.15Ga 0.85As/GaAs quantum wells (QWs) with QDs grown at different temperatures. It was shown that three reasons are responsible for the emission inhomogeneity in studied QD structures: (i) the high concentration of nonradiative recombination centers in the capping In 0.15Ga 0.85As layer at low QD growth temperatures, (ii) the QD density and size distributions for the structure with QD grown at 510 ∘C, (iii) the high concentration of nonradiative recombination centers in the GaAs barrier at higher QD growth temperatures.

InAs/GaAs quantum dot structures emitting in the 1.55 μm band

We investigated different capping layers covering InAs quantum dot structures grown on GaAs substrates by metalorganic vapor phase epitaxy in order to receive strong photoluminescence at 1.55 μm. Atomic force microscopy showed that uncovered InAs quantum dots are 4–5 nm high lenses with a broad luminescence peaking at 1.43 μm. The GaAs capping improves the quantum dot homogeneity but quantum dot dissolution causes blue shift of emitted light. On the other hand, proper engineering of InGaAs strain reducing layer allows to shift the photoluminescence maximum to 1.55 μm. Analysis of photoluminescence and microscopy data supported by calculation of quantum dot electron states shows that this is caused both by the change of the electronic-barrier structure and by the increase of the height of the overgrown quantum dots.

Control of unpolarized emission in closely stacked InAs quantum dot structure

In this work, we studied the effect of some growth parameters on the polarization behavior of InAs/GaAs closely stacked quantum dot (CSQDs). In particular, we focused on the surface reconstruction time of GaAs spacer, its thickness and the number of QD layers. We found that the most effective parameter to enhance the TM/TE intensity ratio is the surface reconstruction time of the GaAs spacer before the subsequent QD deposition. By varying this parameter between 20 s and 120 s, a TM/TE ratio as high as 0.86 has been achieved. A further fine tuning of GaAs spacer thickness and QD layer number increased this ratio up to a value of 0.92 in structures containing only 3 QD layers.

Superposition of Positive and Negative Contributions to the Photocurrent Spectrum of InAs/InAlGaAs/InP Quantum Dot Infrared Photodetectors

InAs quantum dot structures grown on InGaAlAs lattice matched to InP were investigated by Fourier Transform Infrared Photocurrent Spectroscopy. The measured spectra shows an interesting anomalous feature for low bias voltage conditions. This feature is attributed to the superposition of two photocurrents with opposite directions. The origin of the observed effect is addressed.