Effects of rapid thermal annealing on the optical properties of InAs/(In)GaAs quantum dots with different areal density

The effects of rapid thermal annealing on the optical properties of InAs/(In)GaAs quantum dots (QDs) with different areal density were investigated by photoluminescence (PL) measurement. The annealing results in PL peak energy blue-shift which strongly depends on QD areal density and capping layer. It is noticeable that low-density QDs and/or InGaAs-capped QDs are more sensitive to the annealing. We attribute the larger energy blue-shift from these samples to enhanced strain-driven diffusion and/or defect-assisted diffusion.     

Sb/As intermixing in self-assembled GaSb/GaAs type II quantum dot systems and control of their photoluminescence spectra

The intermixing of Sb and As atoms induced by rapid thermal annealing (RTA) was investigated for type II GaSb/GaAs self-assembled quantum dots (QD) formed by molecular beam epitaxy growth. Just as in InAs/GaAs QD systems, the intermixing induces a remarkable blueshift of the photoluminescence (PL) peak of QDs and reduces the inhomogeneous broadening of PL peaks for both QD ensemble and wetting layer (WL) as consequences of the weakening of quantum confinement. Contrary to InAs/GaAs QDs systems, however, the intermixing has led to a pronounced exponential increase in PL intensity for GaSb QDs with annealing temperature up to 875 °C. By analyzing the temperature dependence of PL for QDs annealed at 700, 750 and 800 °C, activation energies of PL quenching from QDs at high temperatures are 176.4, 146 and 73.9 meV. The decrease of QD activation energy with annealing temperatures indicates the reduction of hole localization energy in type II QDs due to the Sb/As intermixing. The activation energy for the WL PL was found to drastically decrease when annealed at 800 °C where the QD PL intensity surpassed WL.     

Luminescence properties of InAs quantum dots formed by a modified self-assembled method

The luminescence properties of self-assembled InAs quantum dots (QDs) on GaAs (1 0 0) substrates grown by molecular beam epitaxy have been investigated using temperature-dependent photoluminescence (PL) and time-resolved PL (TRPL). InAs QDs were grown using an In-interruption growth technique, in which the indium flux was periodically interrupted. InAs QDs grown using In-interruption showed reduced PL linewidth, redshifted PL emission energy, increased energy level spacing between the ground state and the first excited state, and reduced decay time, indicating an improvement in the size distribution and size/shape of QDs.     

Ordered InAs Quantum Dots with Controllable Periods Grown on Stripe-Patterned GaAs Substrates

GaAs （001） substrates are patterned by electron beam lithography and wet chemical etching to control the nucleation of lnAs quantum dots （QDs）. InAs dots are grown on the stripe-patterned substrates by solid source molecular beam epitaxy. A thick buffer layer is deposited on the strip pattern before the deposition of InAs. To enhance the surface diffusion length of the In atoms, InAs is deposited with low growth rate and low As pressure. The AFM images show that distinct one-dimensionally ordered InAs QDs with homogeneous size distribution are created, and the QDs preferentiMly nucleate along the trench. With the increasing amount of deposited InAs and the spacing of the trenches, a number of QDs are formed beside the trenches. The distribution of additional QDs is long-range ordered, always along the trenchs rather than across the spacing regions.     

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.     

Effects of thermal annealing on the interband transitions of single and vertically stacked InAs/GaAs self-assembled quantum dots

Photoluminescence (PL) measurements have been carried out to investigate the annealing effects in one-period and three-periods of InAs/GaAs self-assembled quantum dots (QDs) grown on GaAs substrates by using molecular beam epitaxy. After annealing, the PL spectra for the annealed InAs/GaAs QDs showed dramatic blue shifts and significant linewidth narrowing of the PL peaks compared with the as-grown samples. The variations in the PL peak position and the full width at half-maximum of the PL peak are attributed to changes in the composition of the InAs QDs resulting from the interdiffusion between the InAs QDs and the GaAs barrier and to the size homogeneity of the QDs. These results indicate that the optical properties and the crystal qualities of InAs/GaAs QDs are dramatically changed by thermal treatment.     

Effect of thermal annealing in the microstructural and the optical properties of uncapped InAs quantum dots grown on GaAs buffer layers

The effect of thermal annealing on self-assembled uncapped InAs/GaAs quantum dots (QDs) has been investigated using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images showed that the lateral sizes and densities of the InAs QDs were not changed significantly up to 650 °C. When the InAs/GaAs QDs were annealed at 700 °C, while the lateral size of the InAs QDs increased, their density decreased. The InAs QDs disappeared at 800 °C. PL spectra showed that the peaks corresponding to the interband transitions of the InAs QDs shifted slightly toward the high-energy side, and the PL intensity decreased with increasing annealing temperature. These results indicate that the microstructural and the optical properties of self-assembled uncapped InAs/GaAs can be modified due to postgrowth thermal annealing.     

Anomalous photoluminescence of InAs quantum dots implanted by Mn ions

The photoluminescence (PL) of Mn-implanted quantum dot (QD) samples after rapid annealing is studied. It is found that the blue shift of the PL peak of the QDs, introduced by the rapid annealing, decreases abnormally as the implantation dose increases. This anomaly is probably related to the migration of Mn atoms to the InAs QDs during annealing, which leads to strain relaxation when Mn atoms enter InAs QDs or to the suppression of the inter-diffusion of In and Ga atoms when Mn atoms surround QDs. Both effects will suppress the blue shift of the QD PL peaks. The temperature dependence of the PL intensity of the heavily implanted QDs confirms the existence of defect traps around the QDs.     

Mn-including InAs quantum dots fabricated by Mn implantation

Mn-including InAs quantum dots (QDs) were fabricated by Mn-ion implantation and subsequent annealing. The optical, compositional, and structural properties of the treated samples were analyzed by photoluminescence (PL) and microscopy. Energy dispersive X-ray (EDX) results indicate that Mn ions diffused from the bulk GaAs into the InAs QDs during annealing, and the diffusion appears to be driven by the strain in the InAs QDs. The temperature dependence of the PL of Mn-including InAs QD samples exhibits QDs PL characteristics. At the same time, the heavy Mn-including InAs QD samples have ferromagnetic properties and high T_c.     

Photoluminescence and lasing properties of InAs/GaAs quantum dots grown by metal-organic chemical vapour deposition

Photoluminescence （PL） and lasing properties of InAs/GaAs quantum dots （QDs） with different growth procedures prepared by metalorganic chemical vapour deposition are studied. PL measurements show that the low growth rate QD sample has a larger PL intensity and a narrower PL line width than the high growth rate sample. During rapid thermal annealing, however, the low growth rate sample shows a greater blueshift of PL peak wavelength. This is caused by the larger InAs layer thickness which results from the larger 2-3 dimensional transition critical layer thickness for the QDs in the low-growth-rate sample. A growth technique including growth interruption and in-situ annealing, named indium flush method, is used during the growth of GaAs cap layer, which can flatten the GaAs surface effectively. Though the method results in a blueshift of PL peak wavelength and a broadening of PL line width, it is essential for the fabrication of room temperature working QD lasers.     

Effects of size and shape on electronic states of quantum dots

A strain-modified, single-band, constant-potential three-dimensional model was applied to study the dependence of electronic states of InAs/GaAs quantum dots (QDs) of different shapes and sizes. The energy trend was found to decrease monotonically with increasing QD size (i.e.E ~ size ^?γ ) but exhibited minimum value at aspect ratio of approximately 0.5. The ground state energy for broad tip was found to be always lower than that of narrow tip. Thus, effort to alter the QD shape instead of the aspect ratio is proposed for longer wavelength emission with InAs/GaAs QDs. The energy dependency γ for volume was found to be approximately three times smaller than that for base length and height. A method was proposed to exploit this large difference for growth experimentalists to verify if the capped InAs QDs follow similar increase as the uncapped InAs QDs upon growth parameter variation.     

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.     

Photoreflectance characterization of InAs/GaAs self-assembled quantum dots grown by ALMBE

We report on a photoreflectance investigation in the 0.8-1.5 eV photon energy range and at temperatures from 80 to 300 K on stacked layers of InAs/GaAs self-assembled quantum dots (QDs) grown by Atomic-Layer Molecular Beam Epitaxy. We observed clear and well-resolved structures, which we attribute to the optical response of different QD families. The dependence of the ground state transition energy on the number of stacked QD layers is investigated and discussed considering vertical coupling between dots of the same column. It is shown that Coulomb interaction can account for the observed optical response of QD families with different morphology coexisting in the same sample. Received 17 November 1999     

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.     

脉冲激光原位辐照对InAs/GaAs(001)量子点生长的影响

在InAs/GaAs(001)量子点生长过程中, 当InAs沉积量为0.9 ML时, 利用紫外纳秒脉冲激光辐照浸润层表面, 由于高温下In原子的不稳定性, 激光诱导的原子脱附效应被放大, 样品表面出现了原子层移除和纳米孔. 原子力显微镜测试表明纳米孔呈现以[110]方向为长轴(尺寸: 20-50 nm)、[110]方向为短轴(尺寸: 15-40 nm)的表面椭圆开口形状, 孔的深度为0.5-3 nm. 纳米孔的密度与脉冲激光的能量密度正相关. 脉冲激光的辐照对量子点生长产生了显著的影响: 一方面由于纳米孔的表面自由能低, 沉积的InAs优先迁移到孔内, 纳米孔成为量子点优先成核的位置; 另一方面, 孔外的区域因为In原子的脱附, 量子点的成核被抑制. 由于带有纳米孔的浸润层表面具有类似于传统微纳加工技术制备的图形衬底对量子点选择性生长的功能, 该研究为量子点的可控生长提供了一种新的思路.     

The microstructure of vertically coupled quantum dots ensembles by EXAFS spectroscopy

The microstructure of samples containing Ge/Si, GaN/AlN, and InAs/AlAs (Ge, GaN, and InAs quantum dots (QDs) in the Si, AlN, and AlAs matrices, respectively) multilayer heterostructures has been investigated by EXAFS spectroscopy. The effect of the effective thicknesses of Si (or AlN) barrier layers, number of Ge (or GaN) layers in a sandwich, and annealing temperature on the size and shape of Ge (or GaN) clusters, interfacial diffusion in these systems, and formation of three-dimensional ordered ensembles of QDs has been established.     

Quantum rings formed in InAs QDs annealing process

InAs quantum dots (QDs) were grown by molecular beam epitaxy in the Stranski-Krastanow growth mode. The samples were placed between two undoped GaAs slices and annealed in nitrogen ambient at different temperature. Effect of annealing temperature on the evolution of QDs morphology is investigated by the AFM. This behavior can be attributed to the mechanisms of QDs ripening, intermixing and segregation in the annealing process. A number of QDs have evoluted into the uniform distribution quantum rings (QRs) when the sample was annealed at the temperature of 800 °C. The results indicated that high density and uniform QRs can be obtained by the post-growth technique.     

InAs SELF-ASSEMBLED NANOSTRUCTURES GROWN ON InP(001)

The 6-period stacked layers of self-assembled InAs quasi-quantum wires(qQWRs) and quantum dots(QDs) embedded into InAlAs on InP(001) substrates have been prepared by solid molecular beam epitaxy. The structures are characterized by atomic force microscopy(AFM) and transmission electron microscopy(TEM). From AFM we have observed for the first time that InAs qQWRs and QDs coexist, and we explained this phenomenon from the view of the energy related to the islands. Cross-sectional TEM shows that InAs qQWRs are vertically aligned every other layer along the growth direction [001], which disagrees with conventional vertical self-alignment of InAs QDs on GaAs substrate.     

Extremely low density self-assembled InAs/GaAs quantum dots

The self-assembled InAs/GaAs quantum dots (QDs) with extremely low density of 8×106 cm-2 are achieved using higher growth temperature and lower InAs coverage by low-pressure metal-organic chemical vapour deposition (MOVCD). As a result of micro-photoluminescence (micro-PL), for extremely low density of 8×106 cm-2 InAs QDs in the micro-PL measurements at 10 K, only one emission peak has been achieved. It is believed that the InAs QDs have a good potential to realize single photon sources.     

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.