Formation and Optical Characteristics of Type-II Strain-Relieved GaSb/GaAs Quantum Dots by Using an Interfacial Misfit Growth Mode

We report the formation and optical characteristics of GaSb/GaAs type-II quantum dots (QDs) by using an interfacial misfit (IMF) growth mode. A V/III ratio during the growth of GaSb QDs determines the selectivity of IMF and conventional Stranski–Krastanov (SK) growth modes. This transition between SK and optimized IMF QDs is rather abrupt and occurs within a factor-of-2 variations in V/III ratio. The IMF QDs emit at longer wavelength ($cong {1.1} mu$m) compared to the SK QD peak emission at $ cong {1.02} mu$m at low temperature (LT) (4 K) because of their strain-free nature of the IMF growth mode. A blueshift of the photoluminescence (PL) peak is observed with increased excitation densities due to the Coulomb interaction between physically separated electrons and holes characteristics of the type-II band alignment. LT time-resolved PL measurements show a long decay time of $ cong {20}$–40 ns from the transition between GaSb IMF QDs and GaAs 2-D electron gas, which is characteristic of the type-II band alignment.      

Electron emissions in InAs quantum dots containing a nitrogen incorporation induced defect state: the influence of thermal annealing

With the incorporation of nitrogen (N) into InAs quantum dots (QDs), the carrier distribution near the QD displays electron emissions from a localized N-induced defect state at 0.34?eV and a weak emission at 0.15?eV from the QD. This defect state causes drastic carrier depletion in the neighboring GaAs bottom layer near the QD, which can effectively suppress tunneling emission for the QD excited states. As a result, electrons escape from the QD ground state through thermal emission to near the GaAs conduction band, rather than through thermal emission to the QD first excited state and a subsequent tunneling to the GaAs conduction band, as observed in InAs QDs without N incorporation. Thermal annealing can weaken the defect emission and enhance the QD emission, suggesting a removal of the defect state and a recovery of carriers in the QD. Increasing annealing temperature can significantly decrease the emission time and energy of the QD emission, which is explained by a weakening of tunneling suppression due to the removal of the defect state.     

The Hybridization of CdSe/ZnS Quantum Dot on InGaN Light-Emitting Diodes for Color Conversion

We have demonstrated the fabrication and characterization of hybrid CdSe/ZnS quantum dot (QD)–InGaN blue LEDs. The chemically synthesized red light (${lambda}$ = 623 nm) QD solutions with different concentrations were dropped onto the blue InGaN LEDs with an emission peak of 453 nm and the turn-on voltage of 2.6 V. In this configuration, the CdSe/ZnS core/shell QDs played the role of a color-conversion center. It was clearly observed that the emission intensity from QDs was increased with increasing QD concentration. With a QD concentration of 10 mg/ml in toluene was incorporated, the ratio of emission intensity of QDs to that of InGaN quantum wells reached 0.17, whereas the Commission Internationale de l’Eclairage (CIE) chromaticity coordinates greatly shifted to (0.29, 0.14). From the spatial mapping of electroluminescence spectra, the decrease of the intensity of $E_{{rm QW}}$ seems to be faster than that of $E_{{rm QD}}$, which suggests that the QD film thickness may be thicker in the edge of the surface of InGaN chip. There will, therefore, convert higher proportion of blue light to red light. Also, the resin-encapsulated hybrid LEDs have a divergence angle (the full angle at $1/e^2$ intensity) of about 20 $^{circ}$ as the device is operated at 10 mA. Furthermore, under the injection current of 20 mA and room temperature, this device can be operated for more than 1000 h without any obvious degradation. From our results, it can be proven that the synthesized QDs are promising nanophosphors for color-conversion applications of solid-state LEDs. However, to more efficiently convert the blue light to red light, a denser QD solution with higher quantum yield must be utilized.      

Understanding and optimizing spin injection in self-assembled InAs/GaAs quantum-dot molecular structures

Semiconductor quantum-dot (QD) structures are promising for spintronic applications owing to their strong quenching of spin relaxation processes that are promoted by carrier and exciton motions. Unfortunately, the spin injection efficiency in such nanostructures is very low and the exact physical mechanism of the spin loss is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QDs and QD molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer that immediately surround the QDs and QMSs. These localized excitons in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We attribute the microscopic origin of the severe spin loss observed during spin injection to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We determined that the AEI of the injected excitons and, thus, the efficiency of the spin injection processes are correlated with the overall geometric symmetry of the QMSs. This symmetry largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier. These results pave the way for a better understanding of spin injection processes and the microscopic origin of spin loss in QD structures. Furthermore, they provide a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of QMSs and overcome a major challenge in spintronic device applications utilizing semiconductor QDs.

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Thermal stability of the peak emission wavelength in multilayer InAs/GaAs QDs capped with a combination capping of InAlGaAs and GaAs

The effect of rapid thermal annealing (RTA) on the optical properties of a 10 layer stacked InAs/GaAs quantum dot (QD) heterostructure where the QDs are overgrown with a combination of quaternary InAlGaAs and GaAs capping have been investigated. TEM micrographs showed that the shape of the QDs is preserved for annealing temperatures up to 800 degrees C. The peak emission wavelength of the investigated heterostructures remains stable on annealing at temperatures upto 750 degrees C, which is unusual in QD samples. This phenomenon is attributed due to the suppression of the strain-enhanced intermixing in such structures. One of the reasons behind such suppression is the strain driven phase separation of Indium from the overgrown quaternary alloy, which maintains an In rich region across the QD periphery thereby checking the out-diffusion of Indium from the dots. The overlapping vertical strain from the under lying dot layers in the QD stack also maintains a strain relaxed state at the QD base, thereby preventing the material mixing at the base of the pyramidal QDs. This stability of wavelength is of paramount importance in optoelectronic devices where the design is based on the emission wavelength of the active region.     

An approach to suppress the blue-shift of photoluminescence peaks in coupled multilayer InAs/GaAs quantum dots by high temperature post-growth annealing

Effect of post-growth annealing on 10 layer stacked InAs/GaAs quantum dots (QDs) with InAlGaAs/GaAs combination capping layer grown by molecular beam epitaxy has been investigated. The QD heterostructure shows a low temperature (8 K) photoluminescence (PL) emission peak at 1267 nm. No frequency shift in the peak emission wavelength is seen even for annealing up to 700 °C which is desirable for laser devices requiring strict tolerances on operating wavelength. This is attributed to the simultaneous effect of the strain field, propagating from the seed layer to the active layer of the multilayer QD (MQD) and the indium atom gradient in the capping layer due to the presence of a quaternary InAlGaAs layer. Higher activation energy (of the order of ∼250 meV) even at 650 °C annealing temperature also signifies the stronger carrier confinement potential of the QDs. All these results demonstrate higher thermal stability of the emission peak of the devices using this QD structure.     

Metamorphic self-assembled quantum dot nanostructures

The light-emission energy E of self-assembled semiconductor quantum dots (QDs) is determined by the complex interplay of parameters such as compositions of QDs and confining layers (CLs), strain of QDs (imposed by the QD mismatch to CLs) and sizes and shapes of QDs. In order to have RT emission in the 1.55 μm photonic window from InAs QDs, the QD–CL lattice mismatch should be in the 4–5% range, values much lower than that of pseudomorphic InAs on GaAs (7%). We show that by: i) growing InAs QDs on virtual substrates consisting of metamorphic InGaAs buffers on GaAs and ii) using the thickness-dependent partial relaxation of buffers (acting also as lower CLs, LCLs) and suitable InGaAs compositions, the QD–CL mismatch can be tuned in the 5–7% range. Our experimental results on MBE-grown metamorphic InAs/In_xGa_1−xAs QD structures show that for x and LCL thicknesses d in the 0.09–0.35 and 20 nm–1000 nm ranges, respectively, the band-gap of the QD material and the band-discontinuities that confine carriers are such that the RT emission wavelengths range from 1.3 μm up to values that may exceed 1.55 μm. By using x and d as two degrees-of-freedom, not only that E can be selected but also the barrier energy for confined carriers' thermal escape can be maximised, in order to achieve efficient emission at RT.     

The research progress of quantum dot lasers and photodetectors in China

Self-assembled In(Ga)As/GaAs quantum dots (QDs), known as "artificial atoms" for their fully quantized electronic states, show unique physical properties and new prospects for applications. Based on the QDs, an increasing number of leading laboratories on the world engage in developing novel optoelectronic devices with special advantages over existing devices. This paper reviews the current research developments in self-assembled QD devices in China, covering QD lasers and inter sub-level QD photodetectors. QD devices in China are undergoing a rapid advance and have achieved the world's best results in terms of certain characteristics.     

Self-formation of hexagonal nanotemplates for growth of pyramidal quantum dots by metalorganic vapor phase epitaxy on patterned substrates

We demonstrate the self-formation of hexagonal nanotemplates on GaAs (111)B substrates patterned with arrays of inverted tetrahedral pyramids during metal-organic vapor phase epitaxy and its role in producing high-symmetry, site-controlled quantum dots (QDs). By combining atomic force microscopy measurements on progressively thicker GaAs epitaxial layers with kinetic Monte Carlo growth simulations, we demonstrate self-maintained symmetry elevation of the QD formation sites from three-fold to six-fold symmetry. This symmetry elevation stems from adatom fluxes directed towards the high-curvature sites of the template, resulting in the formation of a fully three-dimensional hexagonal template after the deposition of relatively thin GaAs layers. We identified the growth conditions for consistently achieving a hexagonal pyramid bottom, which are useful for producing high-symmetry QDs for efficient generation of entangled photons.

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Abnormal optical behaviour of InAsSb quantum dots grown on GaAs substrate by molecular beam epitaxy

InAs(Sb) quantum dots (QDs) samples were grown on GaAs (001) substrate by Molecular Beam Epitaxy (MBE). The structural characterization by Atomic Force Microscopy (AFM) of samples shows that InAsSb islands size increases strongly with antimony incorporation in InAs/GaAs QDs and decreases with reducing the growth temperature from 520 °C to 490 °C. Abnormal optical behaviour was observed in room temperature (RT) photoluminescence (PL) spectra of samples grown at high temperature (520 °C). Temperature dependent PL study was investigated and reveals an anomalous evolution of emission peak energy (EPE) of InAsSb islands, well-known as “S-inverted curve” and attributed to the release of confined carriers from the InAsSb QDs ground states to the InAsSb wetting layer (WL) states. With only decreasing the growth temperature, the S-inverted shape was suppressed indicating a fulfilled 3D-confinement of carriers in the InAsSb/GaAs QD sample.     

Site‐Controlled InGaAs Quantum Dots with Tunable Emission Energy

Semiconductor quantum‐dot (QD) systems offering perfect site control and tunable emission energy are essential for numerous nanophotonic device applications involving spatial and spectral matching of dots with optical cavities. Herein, the properties of ordered InGaAs/GaAs QDs grown by organometallic chemical vapor deposition on substrates patterned with pyramidal recesses are reported. The seeded growth of a single QD inside each pyramid results in near‐perfect (<10 nm) control of the QD position. Moreover, efficient and uniform photoluminescence (inhomogeneous broadening <10 meV) is observed from ordered arrays of such dots. The QD emission energy can be finely tuned by varying 1) the pyramid size and 2) its position within specific patterns. This tunability is brought about by the patterning of both the chemical properties and the surface curvature features of the substrate, which allows local control of the adatom fluxes that determine the QD thickness and composition.     

Stark effect in a multilayer system of coupled InAs/GaAs quantum dots

Electron emission in a system of vertically coupled quantum dots (VCQDs) in InAs/GaAs p-n-heterostructures obtained by molecular beam epitaxy has been studied by means of deep-level transient spectroscopy (DLTS) as a function of the number of quantum dot (QD) rows and the reverse bias voltage. For a GaAs spacer thickness of d _GaAs = 40 Å, the system occurs in a molecular state, irrespective of the number of QD rows. An increase in this number leads to a decrease in the Stark shift, which is probably related to a decrease in the lattice strain potential in the vicinity of VCQDs.     

Piezotronic effect on the luminescence of quantum dots for micro/nano-newton force measurement

The luminescence of semiconductor quantum dots (QDs) can be adjusted using the piezotronic effect. An external mechanical force applied on the QD generates a piezoelectric potential, which alters the luminescence of the QD. A small mechanical force may induce a significant change on the emission spectrum. In the case of InN QDs, it is demonstrated that the unforced emission wavelength is more than doubled by a force of 1 μN. The strategy of using the piezotronic effect to tune the color of the emission leads to promising noncontact forcemeasurement applications in biological and medical sensors and force-sensitive displays. Several piezoelectric semiconductor materials have been investigated in terms of the tunability of the emission wavelength in the presence of an external applied force. It is found that CdS and CdSe demonstrate much higher tunability δλ/δF, which makes them suitable for micro/nano-newton force measurement applications.

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Temperature and excitation density dependent photoluminescence of sputtering-induced GaAs/AlGaAs quantum dots

GaAs/AlGaAs quantum dots (QDs) are fabricated by low-energy ion beam sputtering and molecular beam epitaxy (MBE) re-growth. Temperature (6.5-78?K) and excitation power density (0.49-3.06?W?cm(-2)) dependent photoluminescence (PL) are presented and discussed in detail. The low-temperature PL emission at 720?nm is attributed to GaAs QDs with height of ～6.1?nm and base width of ～23?nm, calculated based on the quantum box model with infinite potential barrier. The calculated QD dimensions are in good agreement with those obtained from atomic force microscopy (AFM) analysis. Nonradiative recombination and Auger-assisted recombination are found to be the main PL quenching mechanisms at high temperature.     

Independent control of InAs quantum dot density and size on AlxGa1–xAs surfaces

Decoupling of InAs quantum dot (QD) size and density on Al_xGa_1?xAs surfaces (x = 0, 0.15, 0.30, and 0.45) is achieved by using a low growth rate and careful control of the temperature. The deposition rate of 0.01 μm/h, instead of 0.05 μm/h, allows the QDs to ripen with additional InAs deposition while the substrate temperature (490–520 °C) determines the QD density. On the GaAs surface, an increase of 10 °C results in an order of magnitude lower QD density. The increase of Al in the Al_xGa_1?xAs surfaces results in a higher dot density, lower dot size, and an increased size distribution. All surfaces show reduced QD density with increasing temperature and an identical zero dot density temperature at 523 °C. The GaAs surface shows increasing QD height with temperature while the Al_xGa_1?xAs surfaces show the opposite trend, but the InAs volume fraction in QDs for all surfaces decreases with increasing temperature, implying a more stable wetting layer. Increasing Al content also increases the InAs volume fraction in QDs, implying the wetting layer for all but the 520 °C samples is less than one monolayer. Photoluminescence samples demonstrate ground state QD energies above the GaAs bandedge.     

Lasing in ultra-narrow emission from GaAs quantum dots coupled with a two-dimensional layer

We report electrically injected lasing in GaAs quantum dots (QDs) grown on GaAs(001) by droplet epitaxy. High-quality GaAs QDs with superior uniformity are formed using improved growth techniques involving the insertion of a two-dimensional layer, control of the As flux for GaAs crystallization, and thin AlGaAs layer capping with high-temperature annealing. The QDs show ultra-narrow luminescence with a linewidth of 20?meV. Ground-state lasing from a laser diode containing fivefold-stacked QD layers is observed at low temperature under pulsed operation.     

A perspective on functionalizing colloidal quantum dots with DNA

This perspective provides an overview of the techniques that have been developed for the conjugation of DNA to colloidal quantum dots (QDs), or semiconductor nanocrystals. Methods described include: ligand exchange at the QD surface, covalent conjugation of DNA to the QD surface ligands, and one-step DNA functionalization on core QDs or during core/shell QD synthesis in aqueous solution, with an emphasis on the most recent progress in our lab. We will also discuss emerging trends in DNA-functionalized QDs for potential applications.      

Capacitance transient analysis of different-sized InAs/GaAs quantum dot structures

The energy states of InAs/GaAs self-assembled quantum dots (QDs) were analyzed by comparing between two QD systems with different QD sizes. The electrical properties of the QD systems were investigated via capacitance-voltage measurements and capacitance transient spectroscopy (also known as deep-level transient spectroscopy) with selective carrier injection and extraction which can be achieved with very small pulse amplitude under bias variation. For the large QDs, several energy states were found with the use of selective carrier injection and extraction. The thermal-activation energies obtained from the capacitance transient spectra of the large QDs were distributed from 70 to 600 meV. This energy distribution was originated from the quantized states of the individual QDs and the size distribution of the QDs. The spectra of the small QDs showed a well-defined energy state of E(c) - 132 meV. From these results, it was estimated that two to four electrons fill a single QD under the proper measurement bias of 0.2 V pulse.     

Polarization dependence of the stark shift in the absorption edge of InGaAs/GaAs quantum dot heterostructures

The Stark effect has been studied in multilayer InGaAs/GaAs laser structures with self-assembled quantum dots (QDs). A shift in the absorption edge depending on the reverse bias voltage has been measured in a two-section laser diode. The QD absorption edge shifts toward longer wavelengths with increasing electric field strength. It is established that the QD absorption depends on the polarization of light. The intensity at which TE-polarized luminescence in laser structures is studied is more than ten times higher than that of the TE-polarized emission component, which is explained by higher amplification of the TE mode.     

Anomalous dynamic characteristics of semiconductor quantum-dot lasers generating on two quantum states

Dynamics of the emission spectra of semiconductor quantum dot (QD) lasers generating on two quantum states has been experimentally studied. Being pumped with 30-ns current pulses, a QD laser ceased to generate 2–5 ns after switch-on and exhibited a pause up to 10 ns or longer, depending on the pumping pulse amplitude. After the subsequent switch-on, the laser generated short (200–300 ps) pulses of emission from the excited state of QDs followed by minima of comparable duration (dark pulses) corresponding to the ground-state emission. This behavior is explained in terms of the laser Q-switching due to the charge carrier density redistribution between the excited and ground states.