The emission wavelength tuning of InAs/InP quantum dots with thin GaAs, InGaAs, InP capping layers by MOCVD

InAs quantum dots (QDs) were grown on InP substrates by metalorganic chemical vapor deposition. The width and height of the dots were 50 and 5.8 nm, respectively on the average and an areal density of 3.0×10¹⁰ cm⁻² was observed by atomic force microscopy before the capping process. The influences of GaAs, In_0.53Ga_0.47As, and InP capping layers (5–10 ML thickness) on the InAs/InP QDs were studied. Insertion of a thin GaAs capping layer on the QDs led to a blue shift of up to 146 meV of the photoluminescence (PL) peak and an InGaAs capping layer on the QDs led to a red shift of 64 meV relative to the case when a conventional InP capping layer was used. We were able to tune the emission wavelength of the InAs QDs from 1.43 to 1.89 μm by using the GaAs and InGaAs capping layers. In addition, the full-width at half-maximum of the PL peak decreased from 79 to 26 meV by inserting a 7.5 ML GaAs layer. It is believed that this technique is useful in tailoring the optical properties of the InAs QDs at mid-infrared regime.     

Molecular beam epitaxy growth and characterization of self-assembled InAs quantum dots on (1 0 0) InAlAs/InP substrates

Self-assembled InAs quantum dots (QDs) on In_0.52Al_0.48As layer lattice matched to (1 0 0) InP substrates have been grown by molecular beam epitaxy (MBE) and evaluated by transmission electron microscopy (TEM) and photoluminescence (PL). TEM observations indicate that defect-free InAs QDs can be grown to obtain emissions over the technologically important 1.3–1.55 μm region. The PL peak positions for the QDs shift to low energy as the InAs coverage increases, corresponding to increase in QD size. The room temperature PL peak at 1.58 μm was observed from defect-free InAs QDs with average dot height of 3.6 nm.     

Strong photoluminescence and laser operation of InAs quantum dots covered by a GaAsSb strain-reducing layer

GaAsSb strain-reducing layers (SRLs) are applied to cover InAs quantum dots (QDs) grown on GaAs substrates. The compressive strain induced in InAs QDs is reduced due to the tensile strain induced by the GaAsSb SRL, resulting in a redshift of photoluminescence (PL) peaks of the InAs QDs. A strong PL signal around a wavelength of 1.3 μm was observed even at room temperature. A laser diode containing InAs QDs with GaAsSb SRLs in the active region was fabricated, which exhibits laser oscillation in pulsed operation at room temperature. These results indicate that GaAsSb SRLs have a high potential for fabricating high efficient InAs QDs laser diodes operating at long-wavelength regimes.     

Optical properties of InAs quantum dots in a Si matrix

We report on the optical properties of nanoscale InAs quantum dots in a Si matrix. At a growth temperature of 400°C, the deposition of 7 ML InAs leads to the formation of coherent islands with dimensions in the 2–4 nm range with a high sheet density. Samples with such InAs quantum dots show a luminescence band in the 1.3 μm region for temperatures up to 170 K. The PL shows a pronounced blue shift with increasing excitation density and decays with a time constant of 440 ns. The optical properties suggest an indirect type II transition for the InAs/Si quantum dots. The electronic structure of InAs/Si QDs is discussed in view of available band offset information.     

InAs/AlAs quantum dots with InGaAs insertion layer: dependence of the indium composition and the thickness

We have systematically studied the effect of an In_xGa_1−xAs insertion layer (IL) on the optical and structural properties of InAs quantum dot (QD) structures. A high density of 9.6×10¹⁰ cm⁻² of InAs QDs with an In_0.3Ga_0.7As IL has been achieved on a GaAs (1 0 0) substrate by metal organic chemical vapor deposition. A photoluminescence line width of 25 meV from these QDs has been obtained. We attribute the high density and high uniformity of these QDs to the use of the IL. Our results show that the InGaAs IL is useful for obtaining high-quality InAs QD structures for devices with a 1.3 μm operation.     

Long-wavelength luminescence from GaSb quantum dots grown on GaAs substrates

Self-assembled GaSb quantum dots (QDs) with a photoluminescence wavelength longer than 1.3 μm were successfully grown by suppressing the replacement of As and Sb on the surface of the GaSb QDs. This result means that GaSb can thus join InAs or GaInAs as a suitable material for QD lasers for optical communications.     

1.5 μm emission from InAs quantum dots with InGaAsSb strain-reducing layer grown on GaAs substrates

InGaAsSb strain-reducing layers (SRLs) are applied to cover InAs quantum dots (QDs) grown on GaAs substrates. The compressive strain induced in InAs QDs from the GaAs is reduced due to the tensile strain induced by the InGaAsSb SRL, because the lattice constant of InGaAsSb is closer to InAs lattice constant than that of GaAs, resulting in a significant red shift of photoluminescence peaks of the InAs QDs. The emission wavelength from InAs QDs can be controlled by changing the Sb composition of the InGaAsSb SRL. The 1.5 μm band emissions were achieved in the sample with an InGaAsSb SRL whose Sb compositions were above 0.3. The calculation of the electron and the hole wave functions using the transfer matrix method indicates that the electron and the hole were localized around InAs QDs and InGaAsSb SRL.     

Time-resolved photoluminescence spectroscopy of InAs quantum dots on InP with various InAlGaAs barrier thicknesses

The Optical characteristics of InAs quantum dots (QDs) embeded in InAlGaAs on InP have been investigated by photoluminescence (PL) spectroscopy and time-resolved PL. Four different QD samples are grown by using molecular beam epitaxy, and all the QD samples have five-stacked InAs quantum dot layers with a different InAlGaAs barrier thickness. The PL yield from InAs QDs was increased with an increase in the thickness of the InAlGaAs barrier, and the emission peak positions of all InAs QD samples were measured around 1.5 μm at room temperature. The decay time of the carrier in InAs QDs is decreased abruptly in the QD sample with the 5 nm InAlGaAs barrier. This feature is explained by the tunneling and coupling effect in the vertical direction and probably defect generation.     

Bandgap engineering of self-assembled InAs quantum dots with a thin AlAs barrier

We investigate the effects of a thin AlAs layer with different position and thickness on the optical properties of InAs quantum dots (QDs) by using transmission electron microscopy and photoluminescence (PL). The energy level shift of InAs QD samples is observed by introducing the thin AlAs layer without any significant loss of the QD qualities. The emission peak from InAs QDs directly grown on the 4 monolayer (ML) AlAs layer is blueshifted from that of reference sample by 219 meV with a little increase in FWHM from 42–47 meV for ground state. In contrast, InAs QDs grown under the 4 ML AlAs layer have PL peak a little redshifted to lower energy by 17 meV. This result is related to the interdiffusion of Al atom at the InAs QDs caused by the annealing effect during growing of InAs QDs on AlAs layer.     

Enhanced photoluminescence intensity of 1.3-μm multi-layer InAs/InGaAs dots-in-well structure using the high growth temperature spacer layer step

The effects of growth temperature of the GaAs spacer layers (SPLs) on the photoluminescence (PL) efficiency of multi-layer GaAs-based 1.3-μm InAs/InGaAs dots-in-well (DWELL) structures have been investigated. It is found that the PL intensity of DWELLs is enhanced by incorporating a high growth temperature step for GaAs SPLs. This improved PL efficiency could be understood in term of reducing the non-radiative recombination centers. An extremely low continuous-wave room-temperature threshold current density of 35 A/cm² is achieved for an as-cleaved 5-layer device with emission at 1.31 μm by using this growth technique.     

Continuous wavelength tuning of InAs quantum dots on InP (1 0 0) and (3 1 1)A substrates by chemical-beam epitaxy

The growth of InAs quantum dots (QDs) on InP (1 0 0) and (3 1 1)A substrates by chemical-beam epitaxy is studied. The InAs QDs are embedded in a GaInAsP layer lattice-matched to InP. We demonstrate an effective way to continuously tune the emission wavelength of InAs QDs grown on InP (1 0 0). With an ultra-thin GaAs layer inserted between the QD layer and the GaInAsP buffer, the peak wavelength from the InAs QDs can be continuously tuned from above 1.6 μm down to 1.5 μm at room temperature. The major role of the thin GaAs layer is to greatly suppress the As/P exchange during the deposition of InAs and subsequent growth interruption under arsenic flux, as well as to consume the segregated In layer floating on the GaInAsP buffer. Moreover, it is found that InP (3 1 1)A substrates are particularly promising for formation of uniform InAs QDs. The growth of InAs on InP (3 1 1)A consists of two stages: nanowire formation due to strain-driven growth instability and subsequent QD formation on top of the wires. The excellent size uniformity of the InAs QDs obtained on InP (3 1 1)A manifests itself in the narrow photoluminescence line width of 26 meV at 4.8 K.     

Tuning of long-wavelength emission in InxGa1−xAs quantum dot structures

This work explores the conditions to obtain the extension of the PL emission beyond 1.3 μm in InGaAs quantum dot (QD) structures growth by MOCVD. We found that, by controlling the In incorporation in the barrier embedding the QDs, the wavelength emission can be continuously tuned from 1.25 μm up to 1.4 μm at room temperature. However, the increase in the overall strain of the structures limits the possibility to increase the maximum gain in the QD active device, where an optical density as high as possible is required. By exploring the kinetics of QD surface reconstruction during the GaAs overgrowth, we are able to obtain, for the first time, emission beyond 1.3 μm from InGaAs QDs grown on GaAs matrix. The wavelength is tuned from 1.26 μm up to 1.33 μm and significant improvements in terms of line shape narrowing and room temperature efficiency are obtained. The temperature-dependent quenching of the emission efficiency is reduced down to a factor of 3, the best value ever reported for QD structures emitting at 1.3 μm.     

Tuning vertically stacked InAs/GaAs quantum dot properties under spacer thickness effects for 1.3 μm emission

Coherent InAs islands separated by GaAs spacer (d) layers are shown to exhibit self-organized growth along the vertical direction. A vertically stacked layer structure is useful for controlling the size distribution of quantum dots. The thickness of the GaAs spacer has been varied to study its influence on the structural and optical properties. The structural and optical properties of multilayer InAs/GaAs quantum dots (QDs) have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The PL full width at half maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 monolayers (ML). For the optimized structure, the TEM image shows that multilayer QDs align vertically in stacks with no observation of apparent structural defects. Furthermore, AFM images showed an improvement of the size uniformity of the QDs in the last layer of QDs with respect to the first one. The effect of growth interruption on the optical properties of the optimized sample (E30) was investigated by PL. The observed red shift is attributed to the evolution of the InAs islands during the growth interruption. We show the possibility of increasing the size of the QDs approaching the strategically important 1.3 m wavelength range (at room temperature) with growth interruption after InAs QD deposition.     

Structural and optical properties of In0.5Ga0.5As/GaAs quantum dots in an In0.1Ga0.9As well using repeated depositions of InAs/GaAs short-period superlattices for the application of optical communication

We report structural and optical properties of In_0.5Ga_0.5As/GaAs quantum dots (QDs) in a 100 Å-thick In_0.1Ga_0.9As well grown by repeated depositions of InAs/GaAs short-period superlattices with atomic force microscope, transmission electron microscope (TEM) and photoluminescence (PL) measurement. The QDs in an InGaAs well grown at 510 °C were studied as a function of n repeated deposition of 1 monolayer thick InAs and 1 monolayer thick GaAs for n=5–10. The heights, widths and densities of dots are in the range of 6–22.0 nm, 40–85 nm, and 1.6–1.1×10¹⁰/cm², respectively, as n changes from 5 to 10 with strong alignment along [1 −1 0] direction. Flat and pan-cake-like shape of the QDs in a well is found in TEM images. The bottoms of the QDs are located lower than the center of the InGaAs well. This reveals that there was intermixing—interdiffusion—of group III materials between the InGaAs QD and the InGaAs well during growth. All reported dots show strong 300 K-PL spectrum, and 1.276 μm (FWHM: 32.3 meV) of 300 K-PL peak was obtained in case of 7 periods of the QDs in a well, which is useful for the application to optical communications.     

Photoreflectance study of energy level structure of self-assembled InAs/GaAs quantum dots emitting at 1.3 μm

Photoreflectance and photoluminescence measurements were performed on the ensemble of self assembled InAs/GaAs quantum dots designed to emit at 1.3 μm. As many as six QDs-related optical transitions were observed in PR spectra, the energies of which were confirmed by high-excitation PL results. Numerical calculations allowed estimating the average size of the dots, which is larger than for standard InAs/GaAs QDs. This result is in agreement with structural data. Additionally, the energy level structure for such QDs was derived and compared with the electronic structure of standard InAs/GaAs dots. It was shown that the energy level structure of such large dots qualifies them for the active region of a laser emitting at 1.3 μm.     

Selective growth of stacked InAs quantum dots by using the templates formed by the Nano-Jet Probe

We have demonstrated the selective area growth of stacked self-assembled InAs quantum dot (QD) arrays in the desired regions on a substrate and confirmed the photoluminescence (PL) emission exhibited by them at room temperature. These InAs QDs are fabricated by the use of a specially designed atomic force microscope cantilever referred to as the Nano-Jet Probe (NJP). By using the NJP, two-dimensional arrays with ordered In nano-dots are fabricated in the desired square regions on a GaAs substrate and directly converted into InAs QD arrays through the subsequent annealing by the irradiation of As flux. By using the converted QD arrays as strain templates, self-organized InAs QDs are stacked. These stacked QDs exhibit the PL emission peak at a wavelength of 1.02 μm.     

Optical properties of 1.3 μm room temperature emitting InAs quantum dots covered by In0.4Ga0.6As/GaAs hetero-capping layer

Room temperature 1.3 μm emitting InAs quantum dots (QDs) covered by an In_0.4Ga_0.6As/GaAs strain reducing layer (SRL) have been fabricated by solid source molecular beam epitaxy (SSMBE) using the Stranski–Krastanov growth mode. The sample used has been investigated by temperature and excitation power dependent photoluminescence (PL), photoluminescence excitation (PLE), and time resolved photoluminescence (TRPL) experiments. Three emission peaks are apparent in the low temperature PL spectrum. We have found, through PLE measurement, a single quantum dot ground state and the corresponding first excited state with relatively large energy spacing. This attribute has been confirmed by TRPL measurements which allow comparison of the dynamics of the ground state with that of the excited states. Optical transitions related to the InGaAs quantum well have been also identified. Over the whole temperature range, the PL intensity is found to exhibit an anomalous increase with increasing temperatures up to 100 K and then followed by a drop by three orders of magnitude. Carrier’s activation energy out of the quantum dots is found to be close to the energy difference between each two subsequent transition energies. PACS 68.65.Ac; 68.65.Hb; 78.67.Hc     

InAs/GaAs量子点1.3 μm单光子发射特性

采用双层耦合量子点的分子束外延生长技术生长了InAs/GaAs量子点样品,把量子点的发光波长成功地拓展到1.3 μm.采用光刻的工艺制备了直径为3 μm的柱状微腔,提高了量子点荧光的提取效率.在低温5 K下,测量得到量子点激子的荧光寿命约为1 ns;单量子点荧光二阶关联函数为0.015,显示单量子点荧光具有非常好的单光子特性;利用迈克耳孙干涉装置测量得到单光子的相干时间为22 ps,对应的谱线半高全宽度为30 μeV,且荧光谱线的线型为非均匀展宽的高斯线型.     

Formation of InAs quantum dots in silicon by sequential ion implantation and flash lamp annealing

InAs quantum dots (QDs) were successfully formed in single-crystalline Si by sequential ion implantation and subsequent milliseconds range flash lamp annealing (FLA). Samples were characterized by μ-Raman spectroscopy, Rutherford Backscattering Spectrometry (RBS) high-resolution transmission electron microscopy (HRTEM) and low temperature photoluminescence (PL). The Raman spectrum shows two peaks at 215 and 235 cm^?1 corresponding to the transverse optical (TO) and longitudinal optical (LO) InAs phonon modes, respectively. The PL band at around 1.3 μm originates from the InAs QDs with an average diameter 7.5±0.5 nm and corresponds to the increased band gap energy due to the strong quantum confinement size effect. The FLA of 20 ms is sufficient for InAs QDs formation. It also prevents the out-diffusion of implanted elements. Moreover, the silicon layer amorphized during ion implantation is recrystallized by solid-phase epitaxial regrowth during FLA.     

Phonon-assisted polar exciton–transitions in self-organized InAs/GaAs quantum dots

Phonon-assisted exciton transitions are investigated for self-organized InAs/GaAs quantum dots (QDs) using selectively excited photoluminescence (PL) and PL excitation spectroscopy. The results unambiguously demonstrate intrinsic recombination in the coherent InAs/GaAs QDs and the absence of a Stokes shift between ground state absorption and emission. Phonon-sidebands corresponding to a phonon energy of 34 meV are resolved and Huang–Rhys parameters of 0.015 and 0.08 are found for phonon-assisted emission and absorption, respectively, which are about one order of magnitude larger than in bulk InAs. Calculations of the exciton–LO–phonon interaction based on an adiabatic approximation and realistic wave functions for ideal pyramidal InAs/GaAs QDs show this enhanced polar coupling to result from the particular confinement and the strain-induced piezoelectric potential in such strained low-symmetry QDs.