Dot Size Uniformity Research Articles

High density and uniformity 1300 nm InAs/GaAs quantum dots grown on silicon substrate through molecular beam epitaxy

The integration of III/V materials into silicon-based microelectronics has been the momentum in the development progress of silicon photonics in the past few decades. In this paper, the growth of InAs/GaAs quantum dots with the high density of 6.5 × 1010/cm2 on silicon substrate is demonstrated. The influence of different deposition amount of indium on the density of quantum dots under the same arsenic flux pressure is discussed in detail, from 2.21 monolayer, 2.38 monolayer to 2.55 monolayer. Atomic force microscopy measurement and photoluminescence test are conducted to characterize the materials growth. The InAs/GaAs quantum dots exhibit the best dot density and size uniformity as well as the strongest intensity of photoluminescence at the deposition amount of 2.38 monolayer. This result provides stable foundation for the realization of III/V quantum dot materials as the photonic components into silicon-based lasers.

Vertical strain-induced dot size uniformity and thermal stability of InAs/GaAsN/GaAs coupled quantum dots

We report, for the first time, 10-layer InAs/GaAsN/GaAs quantum dots (QDs) by varying the GaAs layer thickness and investigate their optical properties. Due to propagation of strain field from bottom to top, multiple stacking of self-assembled QDs helps special ordering of QDs on surface resulting in dot size uniformity. InAs coupled QDs with GaAsN/GaAs spacers of 2/10 nm exhibited a bimodal distribution with narrow linewidths and low As-related deep level defect peak intensity, attributable to good optical quality of QDs. N-like local vibrational mode in Raman spectroscopy was found to be consisted of ten oscillation fringes, possibly because of ten periods of GaAsN layer used in QD-heterostructures. Higher strain coupling resulted in lesser blue-shift with increasing annealing temperature as compared to the lower coupling, leading to thermal stability of coupled QDs. Coupled QDs exhibiting narrow linewidths and thermal stability could be employed in fabrication of photodetectors.

Evidence of quantum dot size uniformity in strain-coupled multilayered In(Ga)As/GaAs QDs grown with constant overgrowth percentage

Strain coupled multi-layer quantum dot (QD) structures are limited due to their non-uniform dot size distribution, as-grown defects and dislocations. Even with these limitations, they are subject of interest because of high optical and device efficiency. In this work, we propose a modified growth strategy in which monolayer (ML) deposition of QDs is varied based on reflection high energy electron diffraction (RHEED) pattern to achieve defect free structure with uniform dot size distribution. The overgrowth percentage, i.e. monolayer deposition above critical thickness (minimum thickness required for transition from two-dimensional (2D) to three-dimensional (3D) confined structures) was kept constant in modified strategy. To verify this claim, we have compared it with conventional strain coupled multilayer structure and demonstrated improved optical performance and device characteristics. Photoluminescence spectroscopy exhibited monomodal QD distribution in strain-coupled multi-stacked heterostructure grown by the proposed strategy. Power dependent photoluminescence (PL) measurement confirms existence of single ground state peak. The proposed strategy yielded a defect free structure and better carrier capture rate compared to conventional multilayer QDs. The spectral response observed from proposed quantum dot infrared photodetector (QDIP) exhibited multiple peaks in mid-IR region (3.92µm and 4.54µm), which is one of the essential requirements for fabricating infrared detectors or focal plane arrays. Post-growth thermal annealing treatment at various temperatures (650–800°C) were performed to verify the change in activation energy, ground-state emission peak, and thermal stability. Negligible change in the ground-state PL emission peak (< 2%) and thermal activation energy (< 2%) were observed up to 750°C. The full width at half maximum (FWHM) values of the ground-state peak of the annealed samples exhibited a similar trend. High-temperature thermal stability was achieved probably due to high strain field hindering material interdiffusion and improved quantum confinement realized from uniform QD sizes. The modified growth strategy used in this paper is helpful not only for improved optical characteristics, but also for useful device performances, such as a night vision camera in defense areas, cancer detector in medical applications and fabrication of thermal imagers; focal plane arrays.

Growth of InAs Quantum Dots on GaAs (511)A Substrates: The Competition between Thermal Dynamics and Kinetics.

The growth process of InAs quantum dots grown on GaAs (511)A substrates has been studied by atomic force microscopy. According to the atomic force microscopy studies for quantum dots grown with varying InAs coverage, a noncoherent nucleation of quantum dots is observed. Moreover, due to the long migration length of In atoms, the Ostwald ripening process is aggravated, resulting in the bad uniformity of InAs quantum dots on GaAs (511)A. In order to improve the uniformity of nucleation, the growth rate is increased. By studying the effects of increased growth rates on the growth of InAs quantum dots, it is found that the uniformity of InAs quantum dots is greatly improved as the growth rates increase to 0.14 ML s(-1) . However, as the growth rates increase further, the uniformity of InAs quantum dots becomes dual-mode, which can be attributed to the competition between Ostwald ripening and strain relaxation processes. The results in this work provide insights regarding the competition between thermal dynamical barriers and the growth kinetics in the growth of InAs quantum dots, and give guidance to improve the size uniformity of InAs quantum dots on (N11)A substrates.

Controllable growth and optical properties of InP and InP/InAs nanostructures on the sidewalls of GaAs nanowires

The growth and optical properties of InP and InP/InAs nanostructures on GaAs nanowires are investigated. InP quantum well and quantum dots (QDs) are formed on the sidewalls of GaAs nanowires successively with increasing the deposition time of InP. The GaAs/InP nanowire heterostructure exhibits a type-II band alignment. The wavelength of the InP quantum well is in the range of 857–892 nm at 77 K, which means that the quantum well is nearly fully strained. The InP quantum dot, which has a bow-shaped cross section, exhibits dislocation-free pure zinc blende structure. Stranski-Krastanow InAs quantum dots are subsequently formed on the GaAs/InP nanowire core-shell structure. The InAs quantum dots are distributed over the middle part of the nanowire, indicating that the In atoms contributing to the quantum dots mainly come from the vapor rather than the substrate. The longest emission wavelength obtained from the InAs QDs is 1039 nm at 77 K. The linewidth is as narrow as 46.3 meV, which is much narrower than those on planar InP substrates and wurtzite InP nanowires, suggesting high-crystal-quality, phase-purity, and size-uniformity of quantum dots.

Capping InAs quantum dots with an InGaAsSb strain-reducing layer to improve optical properties and dot-size uniformity

The superior operation characteristics of quantum dot (QD) lasers with excited-state (ES) emission, namely high saturation gain and modulation bandwidth, are advantageous for applying lasers in optical-fiber communication. The InAs QDs in this study were covered with an InGaAsSb capping layer to reduce In–Ga intermixing, thereby increasing the QD density and dot-size uniformity. However, the extension of ES emission to 1310nm in InAs/GaAs QD heterostructures is difficult because of the internal compressive strain and In–Ga intermixing. This study used an InGaAsSb strain-reducing layer (SRL) to cap InAs QDs, elongating the ground-state and excited-state (ES) emission wavelengths to 1426 and 1310nm, respectively. The optical properties of the InAs/(In)GaAsSb QD structures were measured using power-dependent photoluminescence and time-resolved photoluminescence measurements. The energy band alignment of the InAs QD heterostructure changed from a type-II to Quasi-type I confinement (transition state from type-II to type-I) when the GaAsSb SRL was replaced with an InGaAsSb layer under a constant Sb flux during the epitaxy process. Therefore, an enhanced radiative recombination efficiency was observed in the InAs/InGaAsSb QDs at the long first ES emission at 1310nm.

Carrier Hopping and Relaxation in InAs/GaAs Quantum Dot Heterostructures

We investigate the effect of carrier dynamics on the temperature dependence of photoluminescence spectra from InAs/GaAs quantum dot heterostructures with different dot size uniformity. Intersublevel relaxation lifetimes and carrier transferring mechanisms are simulated using a model based on carriers relaxing and thermal emission of each discrete energy level in the quantum dot system. Calculated relaxation lifetimes are decreasing with temperature and have larger values for sample with lower dot size uniformity. In the quantitative discussion of carrier dynamics, the influence of thermal redistribution on carriers relaxing process of quantum dot system is demonstrated by our model.

MOVPE GROWTH OF THE InP BASED MID-IR EMISSION QUANTUM DOT STRUCTURES

In this paper, semiconductor quantum dot structures for mid-infrared emission were self-assembled on InP substrate by using metal–organic vapor phase epitaxy growth. The InAs quantum dots grown at different conditions have been investigated. To improve the grown quantum dot's shape, the dot density and the dot size uniformity, a two-step growth method has been used and investigated. By changing the composition of the In x Ga 1-x As matrix layer of the InAs / In x Ga 1-x As / InP quantum dot structure, emission wavelength of the InAs quantum dot structure has been extended to the longest &gt; 2.35 μm measured at 77 K. For the narrower bandgap semiconductor InAsSb quantum dots, the emission wavelength was measured at &gt; 2.8 μm.

Improving the characteristics of intermediate-band solar cell devices using a vertically aligned InAs/GaAsSb quantum dot structure

This study demonstrates the feasibility of improving the optical properties of a vertically aligned quantum dot (QD) structure and the performance of a quantum dot intermediate band solar cell (QD-IBSC) by capping a GaAsSb layer on the InAs QDs. Experimental results indicate that dot-size uniformity is significantly improved due to the strain modification in the evolution of the successive vertically aligned dot layer growth. A solar cell device with an InAs/GaAsSb columnar dot structure increases the short-circuit current density (Jsc) by 8.8%, compared to a GaAs reference cell. This dot structure also increases quantum efficiency by up to 1200nm through the absorption of lower-energy photons. The InAs/GaAsSb QD-IBSC also improves the open-circuit voltage (Voc), indicating a reduction in misfit defect density and recombination current density. The results of this study confirm the ability of a columnar InAs/GaAsSb QD structure to enhance the device performance.

Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots

The effects of multi-ligands on the formation and optical transitions of ZnSe quantum dots have been investigated. The dots are synthesized using 3-mercapto-1,2-propanediol and polyvinylpyrrolidone ligands, and have been characterized by X-ray diffraction, transmission electron microscopy (TEM), UV–visible absorption spectroscopy, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy. TEM reveals high monodispersion with an average size of 4nm. Polymer-stabilized, organic ligand-passivated ZnSe quantum dots exhibit strong UV emission at 326nm and strong quantum confinement in the UV–visible absorption spectrum. Uniform size and suppressed surface trap emission are observed when the polymer ligand is used. The possible growth mechanism is discussed.

Critical size of substrate mesa for single quantum dot growth

Achieving periodicity and size uniformity of quantum dots is significant in improving the performance of quantum dot based devices. An efficient way to control the size and arrangement of quantum dots is to employ a patterned substrate in quantum dot growth. In this paper, the composition, position and shape transition of quantum dots are considered to obtain the minimum energy of the system. The least-energy principle is used here to estimate the critical size of the patterned substrate for single quantum dot growth. Numerical results are in good agreement with experiment, which can help one to control the arrangement of quantum dots by projection of the patterned substrate.

Influencing factors on the size uniformity of self-assembled SiGe quantum rings grown bymolecular beam epitaxy

The size uniformity of self-assembled SiGe quantum rings, which are formed by cappingSiGe quantum dots with a thin Si layer, is found to be greatly influenced by the growthtemperature and the areal density of SiGe quantum dots. Higher growth temperaturebenefits the size uniformity of quantum dots, but results in low Ge concentration as well asasymmetric Ge distribution in the dots, which induces the subsequently formedquantum rings to be asymmetric in shape or even broken somewhere in the ridge ofrings. Low growth temperature degrades the size uniformity of quantum dots, andthus that of quantum rings. A high areal density results in the expansion andcoalescence of neighboring quantum dots to form a chain, rather than quantum rings.Uniform quantum rings with a size dispersion of 4.6% and an areal density of7.8×108 cm − 2 are obtained at the optimized growth temperature of640°C.

Bit patterned media based on block copolymer directed assembly with narrow magnetic switching field distribution

Electron-beam (E-beam) directed assembly, which combines the long-range phase and placement registration of e-beam lithography with the sharp dot size and spacing uniformity of block copolymer self assembly, is considered highly promising for fabricating templates that meet the tight magnetic specifications required for write synchronization in bit patterned media magnetic recording systems. In our study, we show that this approach also yields a narrower magnetic switching field distribution (SFD) than e-beam patterning or block copolymer self-assembly alone. We demonstrate that the pattern uniformity, i.e., island diameter and placement distributions are also important for achieving tight magnetic SFDs.

Extended optical properties beyond band-edge of GaAs by InAs quantum dots and quantum dot molecules

We have reported the optical properties including spectral response and photoluminescence (PL) measurements of conventional quantum dots (QDs) and high-density quantum dot molecules (HD-QDMs). For the extended spectral response beyond band-edge of GaAs, 15-stack conventional InAs QDs give peak centered at 907 nm while 3-stack InAs HD-QDMs give broad peak between 915 and 985 nm. Moreover, extended spectral response from HD-QDMs is also higher and broader than that from QDs. It is found from the PL result that the peak and full width at half maximum (FWHM) from HD-QDMs are also higher and broader than those of conventional QDs. The agreement of these two optical behaviors could be explained by lower dot size uniformity and higher dot density of HD-QDMs sample.

Improved quantum confinement of self-assembled high-density InAs quantum dot molecules in AlGaAs∕GaAs quantum well structures by molecular beam epitaxy

Self-assembled, multistack InAs quantum dot molecules (QDMs) were grown by a modified molecular beam epitaxial (MBE) technique, which involves multiple stacking and multiple cycling of the thin-capping-and-regrowth process, so as to obtain a large volume density of quantum dots on the sample. Furthermore, the high-density InAs QDMs were also grown sandwiched either between a double heterostructure (DHS) or between a quantum-well (QW) structure. It was found from microphotoluminescence (μ-PL) measurements that the QDMs sandwiched between these structures give broader PL spectra than those of the as-grown QDMs. The broadening of the PL spectra is associated with the poorer dot size uniformity, which arises from the long and complicated MBE growth processes. However, comparing between the QDMs in the DHS and in the QW structure, the latter give narrower PL spectra. The narrower PL spectra for the QDM-in-QW structure is attributed to the improved quantum confinement effect arising from the use of the QW.

Thermally-activated effects on photoluminescence line shape of InAs/GaAs quantum dot heterosystems

We investigate the temperature dependence of photoluminescence (PL) properties of InAs/GaAs quantum dot (QD) heterostructures. The temperature dependence of the InAs exciton energy and linewidth is found to display a significant difference due to the different dot size distribution. We propose a model, which takes into account the dot size distribution, state filling effect, carrier thermal escaping and retrapping, and electron-phonon scattering, to explain the carriers transferring mechanisms in the QD system. The experimental results are reproduced well by our model. Sample with lower dot size uniformity reveals evident thermal redistribution effect among dots, but the effect is weak for sample with higher dot size uniformity and is overcome by the electron-phonon scattering, which is consistent with the observed monotonically increasing linewidth of PL spectra with temperature.

Size uniformity of InAs dots on mesa-structure templates on (001) InP substrates grown by droplet metal-organic vapor phase epitaxy method

In order to improve size uniformity of InAs quantum dots (QDs) on (001) InP substrates using droplet heteroepitaxy by metal-organic vapor phase epitaxy, the authors carried out the growth of InAs QDs on size-controlled linear mesa-structure templates fabricated by using selective-area growth technique and surrounded by facet planes. InAs QDs preferentially formed on the (001) top surface facet. The maximum size of QD was limited by the width of the top surface. On the other hand, the minimum size was defined by the practically critical size. These facts imply that the narrow width of the top surface of templates controls the dispersion of size distribution. The authors obtained the narrow photoluminescence peak from the size-controlled InAs QDs with full width at half maximum of 85meV at around 1.5μm wavelength.

MOVPE growth of InAs quantum dots for mid-IR applications

InAs quantum dots (QDs) grown on In x Ga 1- x As/InP matrix by low pressure metal organic vapor phase epitaxy (LP-MOVPE) in nitrogen ambient were studied. Formation of the InAs QDs with different growth conditions was investigated. To improve the dot size uniformity, a two-step growth method was used and investigated. It is found that morphology of the InAs QDs formed on such In x Ga 1- x As/InP matrix is very sensitive to the growth conditions. InAs QDs with high density of 1.3 × 10 10 cm −2 are grown by using S-K growth method with fast growth rate. Using the two-step growth method, the InAs QDs size uniformity improves by 63% and 110% compared that of the dots grown by ordinary S-K method and ALE method, respectively. Narrow photoluminescence (PL) emission spectrum of the QDs grown by using the two-step growth method, is received. FWHM of the PL curve is measured at 26 meV and the peak emission wavelength is larger than 2.3 μm at 77 K.

A new method of two-step growth of InAs/GaAs quantum dots with higher density andmore size uniformity

A new two-step growth method for self-assembly growth of InAs quantum dots (QDs) withhigher dot density and more size uniformity has been investigated. Using this method, InAsQDs have been grown on GaAs(001) substrates by low pressure metal–organicchemical vapour deposition technology. The samples have been characterized byatomic force microscopy and low temperature photoluminescence measurement.The dot size, dot density and the size uniformity were found to be dependenton the interruption time between the two growth steps and the growth rate ofstep 2 growth in the two-step growth method. Under similar growth conditions,the dot density and size uniformity of the QDs formed by using this two-stepgrowth method have been improved by 80% and 50%, respectively, when comparedto those for QDs grown by the ordinary Stranski–Krastanov growth method.

Control of the nucleation density of Si quantum dots by remote hydrogen plasma treatment

The influence of surface pretreatments by remote hydrogen and/or argon plasma on the silicon nucleation density on thermally grown SiO 2 in the early stages of low pressure chemical vapor deposition (LPCVD) using monosilane (SiH 4) has been studied to control the areal density of Si quantum dots (Si-QDs) on SiO 2. In the remote H 2 (or D 2) plasma treatments prior to SiH 4-LPCVD, the formation of Si OH (Si OD) surface bonds, which act as reactive sites, was confirmed from no changes in the SiO 2 surface microroughness and in the thickness. In fact, by a H 2 plasma pretreatment, the areal density of Si-QDs as high as 7 × 10 10 cm −2 was obtained in the LPCVD condition of the dot density with as low as 6 × 10 8 cm −2 on as-grown SiO 2. By changing the pressure and the substrate temperature in the H 2 plasma pretreatment, the dot density was controlled in the range of 4 × 10 9 to 7 × 10 10 cm −2. Further increase in the dot density and improvement of the dot size uniformity was also demonstrated by the pretreatment of Ar plasma followed by H 2 plasma, presumably because of improved uniformity and integrity in SiO 2 surface coverage with OH bonds just before LPCVD.