Grid-pattern aligned InAs self-assembled quantum dots grown on InP substrate

We present a method to control the nucleation sites of InAs self-assembled quantum dots (QDs). Tensile-strained material, such as GaAs used here, was grown on InP substrates before InAs deposition. This thin GaAs layer can provide a surface with grid-pattern trenches which have the same function as atomic-steps and are promising for the formation of QDs with controlled nucleation sites. Atom force microscopy (AFM) measurement was performed and the AFM images indicate that the InAs islands grown with our technique are grid-pattern aligned and have good homogeneity and low size fluctuation. In addition, another kind of three-dimensional structure with larger size would coexist with normal QDs if a 30nm thick GaAs layer was deposited. This revised version was published online in June 2006 with corrections to the Cover Date.     

Formation process for and strain effect in InAs quantum dots grown on GaAs substrates by using molecular beam epitaxy

Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) measurements were used to investigate the dependences of the formation process and the strain on the As/In ratio and the substrate temperature of InAs quantum dots (QDs) grown on GaAs substrates by using molecular beam epitaxy. The thickness of the InAs wetting layer and the shape and the size of the InAs QDs were significantly affected by the As/In ratio and the substrate temperature. The strains in the InAs layer and the GaAs substrate were studied by using RHEED patterns. The magnitude in strain of the InAs QDs formed at a low substrate temperature was larger than that in InAs QDs grown at high substrate temperature. The present results can help to improve the understanding of the formation process and the strain effect in InAs QDs.     

Self-assembled quantum dots: five years later

Self-assembled quantum dots (QDs) have been grown with good reproducibility by molecular beam epitaxy with up to five well-resolved zero-dimensional interband transitions measured by state-filling spectroscopy. The intersublevel energy spacing is shown to be readily tunable by adjusting the temperature of the substrate during the growth of the QDs and/or of the cap layer, or with post-growth annealing. The uniformity of InAs/GaAs QDs is optimized by studying the growth parameters affecting the equilibrium shape such as the amount of strain material deposited and the annealing time following the InAs deposition allowing the QDs ensemble to evolve. Such uniform QDs are also obtained for samples with multiple stacked layers. This allows us to study the effects of charged carriers, of tunneling between coupled QDs, of electrical injection, and of lasing in QDs with well-resolved excited states having adjustable intersublevel energy spacing.     

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.     

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.     

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.     

Dependence of bimodal size distribution on temperature and optical properties of InAs quantum dots grown on vicinal GaAs （100） substrates by using MOCVD

Self-assembled InAs quantum dots (QDs) are grown on vicinal GaAs (100) substrates by using metal-organic chemical vapour deposition (MOCVD). An abnormal temperature dependence of bimodal size distribution of InAs quantum dots is found. As the temperature increases, the density of the small dots grows larger while the density of the large dots turns smaller, which is contrary to the evolution of QDs on exact GaAs (100) substrates. This trend is explained by taking into account the presence of multiatomic steps on the substrates. The optical properties of InAs QDs on vicinal GaAs(100) substrates are also studied by photoluminescence (PL) . It is found that dots on a vicinal substrate have a longer emission wavelength, a narrower PL line width and a much larger PL intensity.     

Low Density Self-Assembled InAs/GaAs Quantum Dots Grown by Metal Organic Chemical Vapour Deposition

The self-assembled InAs quantum dots （QDs） on GaAs substrates with low density （5×10^8 cm^-2） are achieved using relatively higher growth temperature and low InAs coverage by low-pressure metal-organic chemical vapour deposition. The macro-PL spectra exhibit three emission peaks at 1361, 1280 and 1204nm, corresponding to the ground level （GS）, the first excited state （ES1） and the second excited state （ES2） of the QDs, respectively, which are obtained when the GaAs capping layer is grown using triethylgallium and tertiallybutylarsine. As a result of micro-PL, only a few peaks from individual dots have been observed. The exciton-biexciton behaviour was clearly observed at low temperature.     

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.     

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。     

Effect of GaAs(1 0 0) 2° surface misorientation on the formation and optical properties of MOCVD grown InAs quantum dots

The influence of GaAs(1 0 0) 2° substrate misorientation on the formation and optical properties of InAs quantum dots (QDs) has been studied in compare with dots on exact GaAs(1 0 0) substrates. It is shown that, while QDs on exact substrates have only one dominant size, dots on misoriented substrates are formed in lines with a clear bimodal size distribution. Room temperature photoluminescence measurements show that QDs on misoriented substrates have narrower FWHM, longer emission wavelength and much larger PL intensity relative to those of dots on exact substrates. However, our rapid thermal annealing (RTA) experiments indicate that annealing shows a stronger effect on dots with misoriented substrates by greatly accelerating the degradation of material quality.     

Kinetic Monte Carlo simulation of spatially ordered growth of quantum dots on patterned substrate

We report the growth of well-ordered InAs QD chains by molecular beam epitaxy system. In order to analyze and extend the results of our experiment, a detailed kinetic Monte Carlo simulation is developed to investigate the effects of different growth conditions to the selective growth of InAs quantum dots (QDs). We find that growth temperature plays a more important role than growth rate in the spatial ordering of the QDs. We also investigate the effect of periodic stress on the shape of QDs in simulation. The simulation results are in good qualitative agreement with our experiment.     

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.     

Extremely Low Density InAs Quantum Dots with No Wetting Layer

Extremely low density InAs quantum dots （QDs） are grown by molecular beam droplet epitaxy. The gallium deposition amount is optimized to saturate exactly the excess arsenic atoms present on the GaAs substrate surface during growth, and low density InAs/GaAs QDs （4× 10^6 cm^-2） are formed by depositing 0.65 monolayers （MLs） of indium. This is much less than the critical deposition thickness （1.7 ML）, which is necessary to form InAs/GaAs QDs with the conventional Stranski-Krastanov growth mode. The narrow photoluminescence linewidth of about 24 meV is insensitive to cryostat temperatures from IO K to 250K. All measurements indicate that there is no wetting layer connecting the QDs.     

带有InGaAs覆盖层的InAs量子点红外探测器材料的发光与光电响应

利用分子束外延技术(MBE),在GaAs(001)衬底上自组织生长了不同结构的InAs量子点样品,并制备了量子点红外探测器件。利用原子力显微镜(AFM)和光致发光(PL)光谱研究了量子点的表面结构、形貌和光学性质。渐变InGaAs层的插入有效地释放了InAs量子点所受的应力,抑制了量子点中In组分的偏析,提高了外延层的生长质量,降低了势垒高度,使InAs量子点荧光波长红移。伏安特性曲线和光电流(PC)谱结果表明,生长条件的优化提高了器件的红外响应,具有组分渐变的InGaAs层的探测器响应波长发生明显红移。     

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.     

等离子体增强InAs单量子点荧光辐射的研究

通过测量光致发光(PL)谱、PL时间分辨光谱及不同激发功率下PL发光强度,研究了低温(5 K)下等离子体对InAs单量子点PL光谱的增强效应.采用电子束蒸发镀膜技术在InAs量子点样品表面淀积了5 nm厚度的金膜,形成纳米金岛膜结构.实验发现,金岛膜有利于量子点样品发光强度的增加,最大PL强度增加了约5倍,其主要物理机理是金岛膜纳米结构提高了量子点PL光谱的收集效率.     

Formation and ordering of epitaxial quantum dots

Single quantum dots (QDs) have great potential as building blocks for quantum information processing devices. However, one of the major difficulties in the fabrication of such devices is the placement of a single dot at a pre-determined position in the device structure, for example, in the centre of a photonic cavity. In this article we review some recent investigations in the site-controlled growth of InAs QDs on GaAs by molecular beam epitaxy. The method we use is ex-situ patterning of the GaAs substrate by electron beam lithography and conventional wet or dry etching techniques to form shallow pits in the surface which then determine the nucleation site of an InAs dot. This method is easily scalable and can be incorporated with marker structures to enable simple post-growth lithographic alignment of devices to each site-controlled dot. We demonstrate good site-control for arrays with up to 10 micron spacing between patterned sites, with no dots nucleating between the sites. We discuss the mechanism and the effect of pattern size, InAs deposition amount and growth conditions on this site-control method. Finally we discuss the photoluminescence from these dots and highlight the remaining challenges for this technique. To cite this article: P. Atkinson et al., C. R. Physique 9 (2008).     

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.     

Suppressing the multimodal size distribution of InAs/GaAs quantum dots through flattening the surface fluctuation

A method of suppressing the multimodal size distribution of InAs/GaAs quantum dots(QDs) using molecular beam epitaxy through flattening the substrate surface is reported in this work.It is found that the surface roughness plays an important role in the growth of QDs through continuous surface evolution(SEQDs).SEQDs are the main components of small QD ensemble in QDs with multimodal size distribution.It is suggested that most of the SEQDs are very likely to nucleate during the growth interruption rather than...