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.     

GaAs间隔层厚度对InAs/GaAs量子点分子光学性质的影响

采用光致荧光发射谱(PL)和时间分辨荧光发射谱(TRPL)研究了GaAs间隔层厚度对自组装生长的双层InAs/GaAs量子点分子光学性质的影响.首先,测量低温下改变激发强度的PL谱,底层量子点和顶层量子点的PL强度比值随激发强度发生变化,表明两层量子点之间的耦合作用和层间载流子的转移随着间隔层厚度变大而变弱.接着测量改变温度的PL谱,量子点荧光光谱峰值位置(Emax)、半峰全宽及积分强度随温度发生变化,表明GaAs间隔层厚度直接影响到量子点内载流子的动力学过程和量子点发光的热淬灭过程.最后,TRPL测量发现60mL比40mL间隔层厚度样品的载流子隧穿时间有明显延长.     

Excitation transfer in novel self-organized quantum dot structures

We report on studies of excitation transfer processes in vertically self-organized pairs of unequal-sized quantum dots (QDs), created in InAs/GaAs bilayers having differing InAs deposition amounts in the first (seed) and subsequent layer. The former and latter enable independent control, respectively, of the density and the size distribution of the second layer QDs. This approach allows us to enhance the average volume and improve the uniformity of InAs QDs, resulting in low-temperature photoluminescence at 1.028 eV with a linewidth of 25 meV for 1.74 ML (seed)/3.00 ML InAs stacking. The optical properties of the formed pairs of unequal-sized QDs with clearly discernible ground-state transition energy depend on the spacer thickness and composition. Photoluminescence results provide evidence for nonresonant energy transfer from the smaller QDs in the seed layer to the larger QDs in the second layer in such asymmetric QD pairs. Transfer times down to 20 ps (36 ML GaAs spacer) are estimated, depending exponentially on the GaAs spacer thickness.     

Microstructural and optical properties of multiple closely stacked InAs/GaAs self-assembled quantum dot arrays

The microstructural and the optical properties of multiple closely stacked InAs/GaAs quantum dot (QD) arrays were investigated by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The AFM and the TEM images showed that high-quality vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The PL peak position corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E₁-HH₁) of the InAs/GaAs QDs shifted to higher energy with increasing GaAs spacer thickness. The activation energy of the electrons confined in the InAs QDs increased with decreasing with GaAs spacer thickness due to the coupling effect. The present results can help to improve the understanding of the microstructural and the optical in multiple closely stafcked InAs/GaAs QD arrays.     

Thermal activation and thermal transfer of localized excitons in InAs self-organized quantum dots

We have investigated the temperature dependence of photoluminescence (PL) properties of a number of InAs/GaAs heterostructures with InAs layer thickness ranging from 0.5 monolayer (ML) to 3 ML. The temperature dependence of the InAs exciton energy and linewidth was found to display a significant difference when the InAs layer thickness is smaller or larger than the critical thickness around 1.7 ML, indicating spontaneous formation of quantum dots (QDs). A model, involving exciton recombination and thermal activation and transfer, is proposed to explain the experimental data. In the PL thermal quenching study, the measured thermal activation energies of different samples demonstrate that the InAs wetting layer may act as a barrier for thermionic emission of carriers in high quality InAs multilayers, while in InAs monolayers and submonolayers the carriers are required to overcome the GaAs barrier to thermally escape from the localized states.     

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.     

Analysis of room temperature PL spectra of InAs/GaAs/InP and InAs/InP self-assembled QDs: A five-band study

In this paper, room temperature PL spectra of InAs self-assembled dots grown on GaAs/InP and InP substrate are presented. For analyzing different positions of the PL peaks, we examine the strain tensor in these quantum dots (QDs) using a valence force field model, and use a five-band k·p formalism to find the electronic spectra. We find that the GaAs tensile-stained layer affects the position of room temperature PL peak. The redshift of PL peak of InAs/GaAs/InP QDs compared to that of InAs/InP QDs is explained theoretically.     

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.     

Stress evolution during growth of bilayer self-assembled InAs/GaAs quantum dots

We investigated the stress evolution during molecular-beam epitaxy of bilayer InAs/GaAs(001) quantum dot (QD) structures in real time and with sub-monolayer precision using an in-situ cantilever beam setup. During growth of the InAs at 470 °C a stress of 5.1 GPa develops in the wetting layer, in good agreement with the theoretical misfit stress. At a critical thickness of 1.5 monolayers the strain is relieved by the QD formation. In the case of InAs/GaAs bilayer structures, the second InAs layer grows identical to the first for GaAs spacer thicknesses exceeding ∼13 nm. For thinner spacers the critical thickness for the 2D/3D transition in the second layer decreases. The stress of the second InAs layer does not reach the value of the first, indicating that InAs QDs grow on partially strained areas due to the strain field of the previous InAs layer. PACS 68.35.-p; 68.35.Gy; 68.65.Hb; 81.07.Ta; 81.10.Aj     

Effects of Si-doped GaAs layer on optical properties of InAs quantum dots

We have investigated the optical properties of InAs self-assembled quantum dots (SAQDs) with the Si-doped GaAs barrier layer. Two types of samples are fabricated according to the position of the Si-doped GaAs layer. For type A samples the Si-doped GaAs layer is grown below the QDs, whereas for type B samples the Si-doped GaAs layer is grown above the QDs. For both types of samples the excited-state emissions caused by state filling are observed in photoluminescence (PL) spectra at high excitation power densities. The bandgap renormalization of QDs can be found from the shift of the PL peak energy. Particularly, for type A samples the Si atoms act as nucleation centers during the growth of InAs QDs on the Si-doped GaAs layer and affect the density and the size of the QDs. The Si-doped GaAs layer in type A samples has more effects on the properties of QDs, such as state filling and bandgap renormalization than those of type B samples.     

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.     

Structural and optical properties of high-density (>10/cm) InAs QDs with varying Al(Ga)As matrix layer thickness

We have obtained high-density (>10¹¹/cm²) InAs quantum dot (QD) structures by using an Al(Ga)As matrix layer. With increase of the AlAs matrix layer thickness, the density of QDs increases a little and the luminescence intensity emitted from InAs QDs decreases. We have used a thin GaAs insertion layer (IL) for the reason of keeping InAs QDs from an aluminum intermixing towards QDs. As the thickness of GaAs IL increases, the density of QDs decreases slightly due to the reduction of the roughness of an AlAs matrix layer. However, the luminescence intensity increases with increase in the thickness of GaAs IL resulting from the efficient blocking of an aluminum intermixing towards QDs.     

Long-wavelength light emission from InAs quantum dots covered by GaAsSb grown on GaAs substrates

We fabricated InAs quantum dots (QDs) with a GaAsSb strain-reducing layer (SRL) on a GaAs(0 0 1) substrate. The wavelength of emission from InAs QD is shown to be controllable by changing the composition and thickness of the SRL. An increase in photoluminescence intensity with increasing compositions of Sb and thickness of the GaAsSb SRL is also seen. The efficiency of radiative recombination was improved under both conditions because the InAs/GaAsSb/GaAs hetero-interface band structure more effectively suppressed carrier escape from the InAs QDs.     

Growth of three-dimensional quantum dot crystals on patterned GaAs (0 0 1) substrates

The growth of a three-dimensional (3D) InAs quantum dot (QD) crystal on a patterned GaAs (0 0 1) substrate is demonstrated. The morphology of QDs grown on a surface patterned with shallow holes is studied as a function of the amount of deposited InAs. We observe that the QDs form in the patterned holes close to each other forming lateral QD bimolecules for InAs coverages below the commonly observed critical thickness of 1.6 monolayers. When the coverage increases, the QD bimolecules coalesce to form larger single QDs. The QDs in the holes are then capped with a Ga(Al)As spacer. The buried QD array serves as a strain template for controlling the formation site of the QDs in the second layer. By tuning the growth conditions for the second and subsequent layers, we achieve a 3D InAs QD crystal with a high degree of perfection. A detail investigation of the growth on hole patterns with different periodicities is presented.     

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.     

近红外波段InAs量子点结构与光学特性

采用分子束外延技术,分别在480,520℃的生长温度下,制备了淀积厚度2.7ML的InAs/GaAs量子点。用原子力显微镜对样品进行形貌测试和统计分布。结果表明,在相应的生长温度下,量子点密度分别为8.0×10¹⁰,5.0×10⁹cm^-2,提高生长温度有利于获得大尺寸的量子点,并且量子点按高度呈双模分布。结合光致发光谱的分析,在480℃的生长条件下,最近邻量子点之间的合并导致了量子点尺寸的双模分布;而在525℃的生长温度下,In偏析和InAs解析是形成双模分布的主要原因。     

Quantum dots formed by activated spinodal decomposition of InGa(Al)As alloy on InAs stressors

We have studied quantum dots (QDs) fabricated by activated spinodal decomposition (ASD) of an InGa(Al)As alloy deposited on top of self-organized InAs nanoscale stressors on GaAs substrate. Such a growth sequence results in a strong red shift of the PL emission down to 1.3 μm at 300 K. This red shift is caused by the formation of In-rich areas in the vicinity of the InAs islands, which increase the effective dot size. Beyond a certain critical InAs composition or nominal thickness of the InGa(Al)As layer the PL line shifts back towards higher energies. Adding Al to the alloy increases the red shift for a given In concentration. Room temperature lasing near 1.3 μm with threshold current densities of about 85 A/cm² was achieved for lasers based on three-fold stacked ASD-formed QDs, with a maximum cw output power of 2.7 W.     

自组织生长InAs/GaAs量子点发光动力学研究

介绍了最新发展的粒子数混合超快光谱测量技术,以及采用该技术对自组织生长InAs/GaAs量子点发光动力学的研究结果.实验发现,自组织InAs/GaAs量子点结构的发光寿命大约为1ns,与InAs层厚度关系不大;激子寿命与温度有一定的关系,但没有明显的实验证据表明与量子点的δ态密度有关;用粒子数混合技术,实验上可直接观察到量子点中载流子在激发态能级的态填充过程. 关键词：     

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.     

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.