双层堆垛长波长InAs/GaAs量子点发光性质研究

研究了双层堆垛InAs/GaAs/InAs自组织量子点的生长和光致发光(PL)的物理性质。通过优化InAs淀积量、中间GaAs层厚度以及InAs量子点生长温度等生长条件,获得了室温光致发光1391～1438nm的高质量InAs量子点。研究发现对量子点GaAs间隔层实施原位退火、采用Sb辅助生长InGaAs盖层等方法可以增强高密度(2×1010 cm-2)InAs量子点的发光强度,减小光谱线宽,改善均匀性和红移发光波长。     

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.     

Fine structure splitting and biexciton binding energy in single self-assembled InAs/AlAs quantum dots

We report on photoluminescence investigations of individual InAs quantum dots embedded in an AlAs matrix which emit in the visible region, in contrast to the more traditional InAs/GaAs system. Biexciton binding energies, considerably larger than for InAs/GaAs dots, up to 9 meV are observed. The biexciton binding energy decreases with decreasing dot size, reflecting a possible crossover to an antibinding regime. Exciton and biexciton emission consists of linearly cross polarized doublets due to a large fine structure splitting up to 0.3 meV of the bright exciton state. With increasing exciton transition energy the fine structure splitting decreases down to zero at about 1.63 eV. Differences with InAs/GaAs QDs may be attributed to major dot shape anisotropy and/or larger confinement due to higher AlAs barriers.     

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.     

A study of photosensitive InAs/GaAs heterostructures with quantum dots grown by ion-beam deposition

The possibility of obtaining ion-beam-deposited InAs/GaAs heterostructures with quantum dots for photovoltaic converters is shown. The surface morphology of the grown heterostructures is analyzed by scanning probe microscopy. Quantum dots and InAs nanoclusters with planar dimensions from 20 to 100 nm and a height from 5 to 80 nm are detected. The average surface density of quantum-dimensional InAs objects with a size below 35 nm is 10⁵ mm⁻². In the photoluminescence spectra (T = 300 K), a peak is revealed with a maximum at the wavelength λ = 1150 nm (hν ≈ 1.1 eV), which shows that the grown heterostructures contain InAs quantum dots of various sizes.     

Effects of growth conditions on the formation of self-assembled InAs quantum dots grown on (1 1 5)A GaAs substrate

Effects of growth conditions on the formation of InAs quantum dots (QDs) grown on GaAs (1 1 5)A substrate were investigated by using the reflection high-energy electron diffraction (RHEED) and photoluminescence spectroscopy (PL). An anomalous evolution of wetting layer was observed when increasing the As/In flux ratio. This is attributed to a change in the surface reconstruction. PL measurements show that QDs emission was strongly affected by the InAs deposited amount. No obvious signature of PL emission QDs appears for sample with 2.2 ML InAs coverage. Furthermore, carrier tunneling from the dots to the non-radiative centers via the inclination continuum band is found to be the dominant mechanism for the InAs amount deposition up to 4.2 MLs.     

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.     

The refractive nonlinearities of InAs/GaAs quantum dots above-bandgap energy

The third-order optical nonlinear refractive properties of InAs/GaAs quantum dots grown by molecular beam epitaxy have been measured using the reflection Z-scan technique at above-bandgap energy. The nonlinear refractive index and nonlinear absorption index of the InAs/GaAs quantum dots were determined for wavelengths from 740 to 777 nm. The measured results are compared with the nonlinear refractive response of several typical III-V group semiconductor materials. The corresponding mechanisms responsible for the large nonlinear response are discussed.     

液滴外延下生长参数对InAs纳米结构形貌的影响

利用液滴外延法在GaAs(001)衬底表面制备InAs量子点,通过控制变量分别研究沉积速率、沉积量对In液滴在GaAs表面生长过程中的影响.使用原子力显微镜(Atomic Force Microscope, AFM)表征InAs纳米结构形貌,得出结论：(1)沉积速率主要通过影响In液滴成核率来控制液滴的密度,即随着沉积速率的增大,In原子在衬底表面的成核率增加,InAs量子点密度增加,实验符合生长动力学经典成核理论.(2)沉积量的改变主要影响液滴的熟化过程,即随着沉积量的增大,可参与生长的活跃的In原子增加,促进了液滴熟化,使得扩散坍塌的原子数量增加,导致在InAs纳米结构中出现多量子点现象.     

Electron and hole levels of InAs quantum dots in a GaAs matrix

Photoluminescence, capacitance-voltage and transmission electron microscopy studies have been carried out on structures containing a sheet of a self-assembled InAs quantum dots formed in GaAs matrices after the deposition of a 1.7 ML of InAs at 480°C. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the capacitance-voltage technique. From analysis of photoluminescence and capacitance-voltage measurements it follows that the quantum dots have electron levels 80 meV below the bottom of the GaAs conduction band and two heavy-hole levels at 100 meV and 170 meV above the top of the GaAs valence band.     

Growth conditions effects on optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs (1 1 3)A substrate

We have investigated the optical properties of InAs/GaAs (1 1 3)A quantum dots grown by molecular beam epitaxy (MBE) with different growth rates by photoluminescence spectroscopy (PL) as a function of the excitation density and the sample temperature (10–300 K). Reflection high-energy electron diffraction (RHEED) is used to investigate the formation process of InAs quantum dots (QDs). A redshift of the InAs QDs PL band emission was observed when the growth rate was increased. This result was explained by the increase of the InAs quantum dot size with increasing growth rate. A significant redshift was observed when the arsenic flux was decreased. The evolution of the PL peak energy with increasing temperature has showed an S-shaped form due to the localization effects and is attributed to the efficient relaxation process of carriers in different InAs quantum dots and to the exciton transfer localized at the wetting layer.     

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.     

Controlling the self-organization of InAs quantum dots upon growth by means of vapor-phase epitaxy on an antimony δ-doped GaAs buffer layer

This work examines the possibility of controlling the parameters of InAs/GaAs quantum dot arrays obtained by metal-organic chemical vapor deposition (MOCVD) at atmospheric pressure with using antimony as a surfactant. The possibility of controlling the parameters and optical properties of InAs quantum dot arrays by varying the surface concentration of Sb atoms in a GaAs buffer layer surface is demonstrated. A model of quantum dot array formation in the presence of Sb atoms is proposed.     

Optimization of InAs/GaAs quantum-dot structures and application to 1.3-μm mode-locked laser diodes

The self-assembled growth of InAs/GaAs quantum dots by molecular beam epitaxy is conducted by optimizing several growth parameters, using a one-step interruption method after island formation. The dependence of photoluminescence on areal quantum-dot density is systematically investigated as a function of InAs deposition, growth temperature and arsenic pressure. The results of this investigation along with time-resolved photoluminescence measurements show that the com- bination of a growth temperature of 490℃, with a deposition rate of 0.02 ML/s, under an arsenic pressure of 1×10^-6 Torr （1 Torr = 1.33322×10^2 Pa）, provides the best compromise between high density and the photoluminescence of quantum dot structure, with a radiative lifetime of 780 ps. The applicability of this 5-layer quantum dot structure to high-repetition-rate pulsed lasers is demonstrated with the fabrication and characterization of a monolithic InAs/GaAs quantum-dot passively mode-locked laser operating at nearly 1300 nm. Picosecond pulse generation is achieved from a two-section laser, with a 19.7-GHz repetition rate.     

Continuous-wave high-power (320 mW) single mode operation of electronic vertically coupled InAs/GaAs quantum dot narrow-ridge-waveguide lasers

High-power 3 μm-wide narrow-ridge-waveguide lasers with ten stacks of electronic vertically coupled InAs/GaAs quantum dots in the active region were demonstrated. Unlike that from conventional uncoupled InAs quantum dot lasers, a narrow lasing spectrum was observed because the carriers tunneled in the vertical direction. Continuous-wave operation in single lateral mode yielded a kink-free output power of 320 mW with an efficiency of 0.46 W/A , and a sensitivity of lasing wavelength to temperature of 0.28 nm/K. PACS 42.55.Px; 42.60.Jf; 78.55.Cr     

不同盖层对InAs/GaAs量子点结构和光学性质的影响

利用金属有机化合物气相沉积设备生长了不同盖层结构的InAs/GaAs量子点,采用原子力显微镜和光致发光光谱仪对量子点的结构和光学性质进行了研究.量子点层之间的盖层由一个低温层和一个高温层组成.对不同材料结构的低温盖层的对比研究表明,In组分渐变的InGaAs低温盖层有利于改善量子点均匀性、减少结合岛数目、提高光致发光强度;当组分渐变InGaAs低温盖层厚度由6.8 nm增加到12 nm,发光波长从1256.0 nm红移到1314.4 nm.另外,还对不同材料结构的高温盖层进行了对比分析,发现高温盖层采用In组分渐变的InGaAs材料有利于光致发光谱强度的提高. 关键词： 半导体量子点 盖层 组分渐变     

Sb-mediated growth of high-density InAs quantum dots and GaAsSb embedding growth by MBE

Self-assembled InAs quantum dots (QDs) with high-density were grown on GaAs(0 0 1) substrates by antimony (Sb)-mediated molecular beam epitaxy technique using GaAsSb/GaAs buffer layer and InAsSb wetting layer (WL). In this Sb-mediated growth, many two-dimensional (2D) small islands were formed on those WL surfaces. These 2D islands provide high step density and suppress surface migration. As the results, high-density InAs QDs were achieved, and photoluminescence (PL) intensity increased. Furthermore, by introducing GaAsSb capping layer (CL), higher PL intensity at room temperature was obtained as compared with that InGaAs CL.     

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.     

Formation and atomic structure of GaSb nanostructures in GaAs studied by cross-sectional scanning tunneling microscopy

GaSb nanostructures in GaAs, grown by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Three different samples were examined, containing a thin quantum well, a quantum well near the critical thickness for dot formation, and finally self-organized quantum dots with base lengths of 5–8 nm and heights of about 2 nm. The dots are intermixed with a GaSb content between 60% and 100%. Also small 3D and 2D islands were observed, possibly representing quantum dots in an early growth stage and quantum dot precursors. All GaSb layers exhibit gaps, which are indications of an island-like growth mode during epitaxy.     

The improvement of InAs/GaAs quantum dot properties capped by Graphene

InAs self‐assembled quantum dots (QDs) were grown by molecular beam epitaxy on (001) GaAs substrate. Uncapped and capped QDs with GaAs and graphene layers were studied using atomic force microscopy and Raman spectroscopy. Graphene multi‐layer was grown by chemical vapor deposition and transferred on InAs/GaAs QDs. It is well known that the presence of a cap layer modifies the size, shape, and density of the QDs. According to the atomic force microscopy study, in contrast to the GaAs capped sample, which induce a dramatic decrease of the density and height of dots, graphene cap layer sample presents a slight influence on the surface morphology and the density of the islands compared with the uncapped one. The difference shown in the Raman spectra of the samples is due to change of strain and alloy disorder effects on the QDs. Residuals strain and the relaxation coefficients have been investigated. All results confirm the best crystalline quality of the graphene cap layer dots sample relative to the GaAs capped one. So graphene can be used to replace GaAs in capping InAs/GaAs dots. To our knowledge, such study has not been carried out until now. Copyright © 2013 John Wiley & Sons, Ltd.