Stacked Quantum Dots Layers Research Articles

In-Well Ionization from Monolayer Quantum Dots for Non-Carrier-Injection Electroluminescence.

Quantum dot (QD) light-emitting devices operating in non-carrier-injection (NCI) mode have attracted intense interest. Revealing the source of carriers that support the periodic electroluminescence is important because there is no injection of carriers from the external electrode. Electrons/holes generated by well-to-well multiple ionization in adjacent QDs are generally recognized as the carrier source for electroluminescence, and the stacked QD layers are necessary. In this work, NCI electroluminescence (NCI-EL) from monolayer QDs is successfully demonstrated, which cannot be properly explained by the previously proposed mechanism of multiple ionization. A working mechanism related to periodic in-well ionization is proposed, in which electrons tunnel directly from the valence band of QDs to the conduction band to form free electrons and holes. The effects of driving voltage amplitude, frequency, and QD size on the NCI-EL performance are investigated. Finite element simulation is used to clarify the ionization process. We believe this work can extend the working mechanism model of NCI-EL from QDs and provide guidance for promoting QD-based light-emitting device performance.

E‐Band InAs/GaAs Trilayer Quantum Dot Lasers

The introduction of a closely stacked quantum dot (QD) structure is considered as one of the most effective approaches to extend the emission wavelength of InAs/GaAs QDs beyond 1.3 μm (1300 nm). Herein, a trilayer QD structure (three closely stacked QD layers) is proposed to further extend the emission wavelength of QDs. Room‐temperature (RT) emission at 1418 nm from InAs/GaAs trilayer QDs is demonstrated. Moreover, based on these results, an E‐band InAs/GaAs trilayer QD laser is fabricated on a GaAs substrate, achieving RT oscillation at 1370 nm with a low threshold current density of 96 A cm−2 under continuous‐wave mode operation. The results indicate that the trilayer QD structure is promising for realizing high‐performance QD lasers on GaAs substrate over 1300 nm without incorporating metamorphic buffer layers.

Stacked Self‐Assembled Cubic GaN Quantum Dots Grown by Molecular Beam Epitaxy

We have investigated the stacking of self‐assembled cubic GaN quantum dots (QDs) grown in Stranski–Krastanov (SK) growth mode. The number of stacked layers is varied to compare their optical properties. The growth is in situ controlled by reflection high energy electron diffraction to prove the SK QD growth. Atomic force and transmission electron microscopy show the existence of wetting layer and QDs with a diameter of about 10 nm and a height of about 2 nm. The QDs have a truncated pyramidal form and are vertically aligned in growth direction. Photoluminescence measurements show an increase of the intensity with increasing number of stacked QD layers. Furthermore, a systematic blue‐shift of 120 meV is observed with increasing number of stacked QD layers. This blueshift derives from a decrease in the QD height, because the QD height has also been the main confining dimension in our QDs.

Efficient Heterotransfer between Visible Quantum Dots

We fabricated green–red (GR) and blue–red (BR) bilayer stacked quantum dots (QDs) using electrospray deposition. Along with steady state and time-resolved photoluminescence (PL), subnanosecond donor PL decay and corresponding acceptor PL rise signals were observed, which are ascribed to the energy transfer between different visible QDs (heterotransfer). The heterotransfer rates were estimated as (0.57 ± 0.01 ns)−1 and (0.65 ± 0.02 ns)−1 for GR and BR systems, respectively, which agree well with theoretical calculations. Owing to their geometrical proximity, mixed QD layers with GR and BR showed qualitatively higher heterotransfer efficiencies of 64% and 81%, compared to stacked QD layers, which have efficiencies of 23% and 64%, respectively.

Effect of spacer layer thickness on structural and optical properties of multi-stack InAs/GaAsSb quantum dots

The structural and optical properties of ten-stack InAs/GaAsSb quantum dots (QDs) with different spacer layer thicknesses (ds = 2, 5, 10, and 15 nm) are reported. X-ray diffraction analysis reveals that the strain relaxation of the GaAsSb spacers increases linearly from 0% to 67% with larger ds due to higher elastic stress between the spacer and GaAs matrix. In addition, the dislocation density in the spacers with ds = 10 nm is lowest as a result of reduced residual strain. The photoluminescence peak energy from the QDs does not change monotonically with increasing ds due to the competing effects of decreased compressive strain and weak electronic coupling of stacked QD layers. The QD structure with ds = 10 nm is demonstrated to have improved luminescence properties and higher carrier thermal stability.

High-density 1.54 μm InAs/InGaAlAs/InP(100) based quantum dots with reduced size inhomogeneity

Self-assembled InAs quantum dots (QDs) were grown by solid source molecular beam epitaxy. The impact of the growth parameters like the growth temperature of the InGaAlAs nucleation layer, V/III ratio and growth rate during growth of QD layers were carefully investigated by using atomic force microscopy and photoluminescence spectroscopy. The excellent size uniformity of InAs QDs grown on InP substrates are verified by narrow photoluminescence line widths of 17meV for single QD layers and 26meV for stacked QD layers, respectivaly. Both values measured at 10K.

Current injected spectrum change in flat‐topped InAs/InP QD arrayed waveguide LED with different QD heights

AbstractWe have investigated the current injected spectrum of flat‐topped LED using self‐assembled Stranski‐Krastanov (S‐K) InAs/InP quantum dots (QDs) grown by selective area low pressure metalorganic vapor phase epitaxy and a double‐capping procedure. Selective area growth using an SiO2 mask with narrow stripes was carried out to tailor a wide emission range for the QDs in sixteen arrayed waveguides. Each waveguide core con‐tained three stacked QD layers with different QD heights and Ga content in the GaInAs buffer layer. We have measured the spectrum change of current injection, and have estimated the intensity variation in each QDs layer. By increasing the FCL thickness of three QDs layers, output intensity of each layer has become equal, and we have obtained more than 500 nm spectrum width with flat‐topped spectrum shape. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anisotropic optical properties of excitons in strain-controlled InAs quantum dots

We report the optical anisotropy of excitons in self-assembled InAs quantum dots (QDs), the strain of which is controlled by tuning the lattice constant of the spacer layers between stacked QD layers. The strain dependence of the energy structure for excitons is investigated using the four-wave mixing technique. The fine-structure beat and the exciton-biexciton beat in four-wave mixing measurements clearly show that the fine-structure splitting of exciton states increases with increasing strain in the growth plane. The strain also causes significant optical anisotropy, which is observed by the angular-dependent measurements of the collinear polarization of excitation pulses. The strain dependence of the optical anisotropy is accurately reproduced by a model calculation in which the valence band mixing between heavy- and light-hole bands is taken into account. We demonstrate that the optical properties of excitons in QD ensembles are successfully controlled by tuning the lattice constant of the spacer layers.

Optimisation of the physical properties of InAs/InGaAs/GaAs QDs heterostructures embedded p-i-n GaAs solar cell

In order to enhance absorption at infrared range for GaAs–based solar cell, we have grown multi–stacked InAs/InGaAs/GaAs quantum dots (QDs) heterostructure in the active region by Solid–Source Molecular Beam Epitaxy (SS–MBE). Two different families of dots were observed in the photoluminescence (PL) spectra. Temperature–dependent study was carried out at 10–240 K temperature range. Distinctive, asymmetric shape located in the high energy for the PL spectra of the QD solar cell sample can be deconvoluted in two sub–bands. From the temperature–dependent PL measurement, the two sub–bands are associated with the ground–state emission from the two families of InAs dots with different size. Besides, the spectral response of multi–stacked InAs/InGaAs QD solar cells extends the photo–absorption spectra towards a wavelength longer than the GaAs bandgap of 1280 nm. However, the QDs solar cell shows an enhanced short–circuits current density of 7.8 mA/cm² compared to the GaAs reference cell. The performance of the QD solar cells indicates that the InAs/InGaAs/GaAs QD heterostructures facilitate the fabrication of highly stacked QD layers that are suitable for solar cells devices requiring thick QD layers for sufficient light absorption.

Experimental and theoretical study of polarization-dependent optical transitions in InAs quantum dots at telecommunication-wavelengths (1300-1500 nm)

The design of some optical devices, such as semiconductor optical amplifiers for telecommunication applications, requires polarization-insensitive optical emission at long wavelengths (1300–1550 nm). Self-assembled InAs/GaAs quantum dots (QDs) typically exhibit ground state optical emissions at wavelengths shorter than 1300 nm with highly polarization-sensitive characteristics, although this can be modified by the use of low growth rates, the incorporation of strain-reducing capping layers, or the growth of closely-stacked QD layers. Exploiting the strain interactions between closely stacked QD layers also affords greater freedom in the choice of growth conditions for the upper layers, so that both a significant extension in their emission wavelength and an improved polarization response can be achieved due to modification of the QD size, strain, and composition. In this paper, we investigate the polarization behavior of single and stacked QD layers using room temperature sub-lasing-threshold electroluminescence and photovoltage measurements, as well as atomistic modeling with the NEMO 3-D simulator. A reduction is observed in the ratio of the transverse electric (TE) to transverse magnetic (TM) optical mode response for a GaAs-capped QD stack as compared to a single QD layer, but when the second layer of the two-layer stack is InGaAs-capped, an increase in the TE/TM ratio is observed, in contrast to recent reports for single QD layers.

Selective MOVPE growth of InAs QDs using double-cap procedure

Broadband spectrum emission was obtained for self-assembled Stranski–Krastanov (S–K) InAs/InP quantum dots (QDs) grown by selective area low pressure metalorganic vapor phase epitaxy (MOVPE) using the double-cap procedure. Selective area growth using a SiO2 narrow stripe mask array pattern was carried out to control and widen the emission wavelength range of the QDs in 16 stripe mask array waveguides. The height of the QDs in the multi stacked QD layers was changed using the double-cap procedure, and the strain was controlled by changing the composition of the GaInAs buffer layer under the QDs, which resulted in the broadband spectrum emission of this structure.

Multi-stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs

We report GaAs-based quantum dot (QD) solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a highly stacked and well-aligned InGaAs QD structure of over 30 layers without using a strain compensation technique by the intermittent deposition of InGaAs layers. Moreover, there was no degradation in crystal quality. The external quantum efficiency of multi-stacked InGaAs QD solar cells extends the photo-absorption spectra toward a wavelength longer than the GaAs band gap, and the quantum efficiency increases as the number of stacking layers increases. The performance of the QD solar cells indicates that the novel InGaAs QDs facilitate the fabrication of highly stacked QD layers that are suitable for solar cell devices requiring thick QD layers for sufficient light absorption.

High optical quality site-controlled quantum dots

GaAs substrates were pre-patterned by electron beam (e-beam) lithography and wet-chemically etched holes. The hole structures, presented here, enable growth of site-controlled InAs quantum dots with low dot density in the range of 10 8 cm −2 by molecular beam epitaxy (MBE). In order to reduce defect related non-radiative recombination processes at the etched surface, wet-chemical etching and a modified growth of stacked quantum dot layers are developed. The total buffer layer thickness could be increased to 55 nm GaAs by only one overgrowth step of a quantum dot seed layer in comparison to strain-coupled multi-stacked structure with typically 10 nm buffer layer thickness between each stack.

Strain effect in determining the geometric shape of self-assembled quantum dot

The geometric shape of a self-assembled quantum dot (QD) formed by the strain-induced Stranski–Krastanov mode has been studied as a function of strain energy by the short-range valence-force-field approach. It has been shown by dynamic bond relaxation through strain energy minimization that for the most commonly used InAs QD in GaAs and InP matrices and Ge QD in Si matrix, a pyramidal shape is energy favoured over an hemispherical shape when the QD is not capped due to the lattice relaxation at the QD surface. When the QD becomes totally embedded in the background material, the elastic strain energy of a hemispherical InAs QD is minimal. The results agree with experimental observations. We further studied the coupling of strain fields of QDs in adjacent QD layers which shows that QDs in multiply stacked QD layers can be aligned along the layer growth direction in order to minimize the strain energy.

Optical properties and modal gain of InGaN quantum dot stacks

AbstractWe present investigations of the optical properties of stacked InGaN quantum dot layers and demonstrate their advantage over single quantum dot layer structures. Measurements were performed on structures containing a single layer with quantum dots or threefold stacked quantum dot layers, respectively. A superlinear increase of the quantum dot related photoluminescence is detected with increasing number of quantum dot layers while other relevant GaN related spectral features are much less intensive when compared to the photoluminescence of a single quantum dot layer. The quantum dot character of the active material is verified by microphotoluminescence experiments at different temperatures. For the possible integration within optical devices in the future the threshold power density was investigated as well as the modal gain by using the variable stripe length method. As the threshold is 670 kW/cm2 at 13 K, the modal gain maximum is at 50 cm–1. In contrast to these limited total values, the modal gain per quantum dot is as high as 10–9cm–1, being comparable to the IIVI and III‐As compounds. These results are a promising first step towards bright low threshold InGaN quantum dot based light emitting devices in the near future (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Process‐free estimation of threshold current density of InAs quantum dot laser

AbstractWe have associated threshold current density of 1.3 μm InAs quantum dot (QD) laser diodes with the photoluminescence (PL) peak intensity of as‐grown wafers. These InAs QD laser wafers with stacking structure of 5 and 10 QD layers were grown by molecular beam epitaxy. PL measurements have carried out by high power excitation at room temperature. The PL peak intensity from ground state of InAs QDs has been inversely proportional to the threshold current density. Additionally, enhancement of PL peak intensity by increase in stacked QD layers has been corresponded to increase in maximum modal gain of QD laser. These results indicate that PL characterization by high power excitation is useful to estimate quality of as‐grown wafers without fabrication of QD laser diode. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization control of gain of stacked InAs∕InP (100) quantum dots at 1.55μm: Interplay between ground and excited state transitions

The linear polarization of the optical gain of closely stacked InAs∕InP (100) quantum dots (QDs) grown by metal-organic vapor-phase epitaxy with emission wavelength tuned into the 1.55μm region is controlled by the number of stacked QD layers and the injection current. Increasing the number of stacked QD layers to five rotates the linear polarization of the cleaved-side photoluminescence and QD ground state (GS) gain, determined from the amplified spontaneous emission (ASE) of a Fabry–Pérot ridge-waveguide laser, from transverse electric (TE) to transverse magnetic due to vertical electronic coupling. When the QD GS ASE and gain saturate with an increase of the injection current and the excited state ASE and gain become dominant, the linear polarization of ASE and gain changes back to TE. This limits the polarization insensitive operation of QD-based semiconductor optical amplifiers, however, opening routes to novel functionalities.

Optical and structural properties ofIn0.5Ga0.5As quantum dots with different numbers of stacks grown by atomic layer molecular beamepitaxy: vertical realignment of weakly coupled quantum dots

We have investigated optical and structural properties of variousIn0.5Ga0.5As quantum dot (QD) structures as a function of stacking number, grown by atomic layer molecularbeam epitaxy (ALE). We found that the excitation power and temperature dependence ofwell-separated two emissions from 10 stacked InGaAs QD samples are different from those ofother QDs with the stacking numbers of 1, 3 and 5. Although the GaAs spacer thickness is∼35 nm at which the strain field penetration can be ignored for Stranski–Krastanovmode-grown InAs QDs, our ALE-grown QDs are influenced by the strain field penetrationdue to the larger size of ALE QDs. From transmission electron microscopic images, weobserved that the ten stacked QD sample has a complex QD size distributionpredominantly due to vertical size variation with stacking and that upper stacked (5–10)QD layers are vertically realigned due to the merged strain field penetration betweenlaterally coupled QDs.

High efficiency and high modal gain InAs/InGaAs/GaAs quantum dot lasers emitting at 1300 nm

High performance 1300 nm lasers based on self-organized InAs/InGaAs quantum dots (QDs) are reported. By optimizing the QD growth parameters and decreasing the waveguide thickness, a high modal gain and a low transparency current density of 32 cm−1 and 35 A cm−2, respectively, were obtained from a device containing five stacked QD layers. The internal quantum efficiency is as high as 90%.

Temperature dependence of surface photovoltage spectroscopy in vertically coupled self-organized InAs/GaAs quantum dots

The surface photovoltage (SPV) spectra of a series of vertically stacked self-organized InAs/GaAs quantum dot (QD)-based laser structures with different spacer layer (SL) thickness were obtained as a function of temperature (77 K </= T </= 300 K). A decrease of the compressive stress for thinner SL samples arising from coherent relaxation enables us to designate the effect of material intermixing as the most probable mechanism of the energetic blueshift of the observed structures. The turnaround characteristic of the temperature-dependent spectral intensity shows that the reduced SPV signal at higher temperature is limited by the carrier scattering and at lower temperature it is governed by the magnitude of built-in electric field and the escape efficiency of the photogenerated carriers. The dot states to be blueshifted by material intermixing are expected to have higher escape rate for carriers out of QDs, thus resulting in lower measurable temperature for the detected SPV signal. The relatively higher signal at low temperature for the 10 nm SL sample provides a direct evidence of the tunneling process of carriers in the stacked QD layers.