Power density and temperature dependent multi-excited states in InAs/GaAs quantum dots

Self-assembled InAs/GaAs (001) quantum dots (QDs) were grown by molecular beam epitaxy using ultra low-growth rate. A typical dot diameter of around 28 ± 2 nm and a typical height of 5 ± 1 nm are observed based on atomic force microscopy image. The photoluminescence (PL) spectra, their power and temperature dependences have been studied for ground (GS) and three excited states (1–3ES) in InAs QDs. By changing the excitation power density, we can significantly influence the distribution of excitons within the QD ensemble. The PL peak energy positions of GS and ES emissions bands depend on an excitation light power. With increasing excitation power, the GS emission energy was red-shifted, while the 1–3ES emission energies were blue-shifted. It is found that the full width at half maximum of the PL spectra has unusual relationship with increasing temperature from 9 to 300 K. The temperature dependence of QD PL spectra shown the existence of two stages of PL thermal quenching and two distinct activation energies corresponding to the temperature ranges I (9–100 K) and II (100–300 K).     

InAs/AlAs quantum dots with InGaAs insertion layer: dependence of the indium composition and the thickness

We have systematically studied the effect of an In_xGa_1−xAs insertion layer (IL) on the optical and structural properties of InAs quantum dot (QD) structures. A high density of 9.6×10¹⁰ cm⁻² of InAs QDs with an In_0.3Ga_0.7As IL has been achieved on a GaAs (1 0 0) substrate by metal organic chemical vapor deposition. A photoluminescence line width of 25 meV from these QDs has been obtained. We attribute the high density and high uniformity of these QDs to the use of the IL. Our results show that the InGaAs IL is useful for obtaining high-quality InAs QD structures for devices with a 1.3 μm operation.     

Resonant tunneling of electrons through single self-assembled InAs quantum dot studied by conductive atomic force microscopy

Resonant tunneling of electrons through a quantum level in single self-assembled InAs quantum dot (QD) embedded in thin AlAs barriers has been studied. The embedded InAs QDs are sandwiched by 1.7-nm-thick AlAs barriers, and surface InAs QDs, which are deposited on 8.3 nm-thick GaAs cap layer, are used as nano-scale electrodes. Since the surface InAs QD should be vertically aligned with a buried one, a current flowing via the buried QD can be measured with a conductive tip of an atomic force microscope (AFM) brought in contact with the surface QD-electrode. Negative differential resistance attributed to electron resonant tunneling through a quantized energy level in the buried QD is observed in the current–voltage characteristics at room temperature. The effect of Fermi level pinning around nano-scale QD-electrode on resonance voltage and the dependence of resonance voltage on the size of QD-electrodes are investigated, and it has been demonstrated that the distribution of the resonance voltages reflects the size variation of the embedded QDs.     

Improvement of the optical property and uniformity of self-assembled InAs/InGaAs quantum dots by layer-by-layer temperature and substrate rotation

The influence of layer-by-layer temperature and substrate rotation on the optical property and uniformity of self-assembled InAs/In_0.2Ga_0.8As/GaAs quantum dots (QDs) gown with an As₂ source was investigated. An improvement in the optical property of QDs was obtained by the precise control and optimization of growth temperature utilized for each layer, i.e., InAs QDs, InGaAs quantum wells, GaAs barriers and AlGaAs layers, respectively. By using a substrate rotation, the QD density increased from ∼1.4×10¹⁰ to ∼3.2×10¹⁰ cm⁻² and its size also slightly increased, indicating a good quality of QDs. It is found that the use of an appropriate substrate rotation during growth improves the room-temperature (RT) optical property and uniformity of QDs across the wafer. For the QD sample with a substrate rotation of 6 rpm, the RT photoluminescence (PL) intensity is much higher and the standard deviation of RT-PL full-width at half-maximum is decreased by 35% compared to that grown without substrate rotation.     

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.     

Narrow bandgap mid-infrared photodetectors based on InAsSbP quantum dots

The results of investigations of mid-infrared photodetectors based on InAsSbP quantum dot (QD) grown on InAs(100) substrate by modified liquid phase epitaxy are presented. The atomic force microscope measurements have shown that the surface density of grown QDs is (4–8) × 10⁹ cm^?2. Also, the morphology and crystalline quality of grown QDs are investigated by a scanning tunneling microscope. Photodetectors based on n-InAs(100) substrate with InAsSbP QDs on its surface were fabricated in the form of a photoconductor cell. The photoresponse spectrum extended up to 4 μm was observed. The optical properties of fabricated structures were investigated under He–Ne laser irradiation with wavelength of 1.15 μm. It was found that the relative surface conductance increases by 16% at power density of 0.15 W/cm². Capacitance hysteresis with maximal remnant capacitance of 2.17 nF at 10³ Hz was observed as well.     

Time-resolved photoluminescence spectroscopy of InAs quantum dots on InP with various InAlGaAs barrier thicknesses

The Optical characteristics of InAs quantum dots (QDs) embeded in InAlGaAs on InP have been investigated by photoluminescence (PL) spectroscopy and time-resolved PL. Four different QD samples are grown by using molecular beam epitaxy, and all the QD samples have five-stacked InAs quantum dot layers with a different InAlGaAs barrier thickness. The PL yield from InAs QDs was increased with an increase in the thickness of the InAlGaAs barrier, and the emission peak positions of all InAs QD samples were measured around 1.5 μm at room temperature. The decay time of the carrier in InAs QDs is decreased abruptly in the QD sample with the 5 nm InAlGaAs barrier. This feature is explained by the tunneling and coupling effect in the vertical direction and probably defect generation.     

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.     

Study of InAs/GaAs quantum dots grown by MOVPE under the safer growth conditions

InAs quantum dots (QDs) have been formed on GaAs (001) substrate by metal-organic vapor phase epitaxy (MOVPE) under the safer growth conditions: using tertiarybutylarsine (TBA) to replace AsH₃ as the arsenic source and replacing hydrogen by pure nitrogen as the carrier gas. Effects of growth conditions on the QD formation have been investigated. It is observed that the wetting layer is stabilized with some material being transferred to form the QDs due to the strain relaxation process during the QD formation. Dot size dispersion becomes broader when the post-growth interruption is more than 20 s. Compared with normal one-step grown QDs, dot density increases greatly by 213% after employing two-step deposition for QD growth. This is explained by considering the indium-flux-dependent nucleation density at step 1 and kinetically self-limiting growth at step 2. The two photoluminescence (PL) emission peaks, 1.203 μm and 1.094 μm, from the two-step grown QDs are attributed to E1–HH1 and E1–LH1 transitions of the QDs, respectively. The measured results agree well with those received by an 8 k·p theoretical calculation. The narrow PL linewidth of ~50 nm shows high quality of the QDs. This paves the way to develop safer MOVPE process, using TBA/N₂ instead of AsH₃/H₂, to grow QDs for device application.     

InAs Quantum Dots On (001) GaAs Substrate with two Groups of Different Sizes Under Arsenic Shutter Closed Condition

The effect of temperature on the self-assembled InAs quantum dots (QDs) grown on GaAs substrate under arsenic shutter closed condition has been studied. From atomic force microscopy (AFM), it was found that the size of InAs dots exhibited a transition from single-sized uniformly distributed quantum dot (QD) at a growth temperature of 490°C to two groups of different sizes QDs at 510°C. Since the desorption rate of In atoms from the substrate surface is very high at 510°C, a growth model is proposed that attributes the larger sized QDs to the enhanced capture of desorbed In atoms by a local random protrusion which initiates a regenerative capture and growth process and leads to explosive growth.     

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.     

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.     

Temperature dependence of optical properties of InAs quantum dots grown on GaAs(113)A and (115)A substrates

We have investigated the temperature dependence of photoluminescence (PL) peak position of InAs self-assembled quantum dots (QDs) grown on GaAs(11N)A (N = 3, 5) substrates. The interband transition energy is calculated by the resolution of the 3D Schrödinger equation for a parallelepipedic InAs QD, with a width of about 8 nm and a height around 3 nm. Experimentally, it was found that the PL spectra quenches at about 160 K. In addition, the full width at half maximum (FWHM) has an abnormal evolution with varying temperature. The latter effect maybe due to the carrier repopulation between QDs. The disorientation of the GaAs substrate and the low width of terraces which was presented in the high index surfaces have an important contribution in the PL spectra. Despite the non-realist chosen shape of QD and the simplest adopted model, theoretical and experimental results revealed a clear agreement.     

InAs/GaAs单量子点中电子/空穴自旋弛豫

利用分子束外延制备了三种类型量子点样品,它们分别是:未掺杂样品、n型Si调制掺杂样品和p型Be调制掺杂样品。在5 K温度下,采用共聚焦显微镜系统,测量了单量子点的光致发光谱和时间分辨光谱, 研究了单量子点中三种类型激子(本征激子、负电荷激子和正电荷激子)的电子/空穴自旋翻转时间。它们的自旋翻转时间常数分别为: 本征激子的自旋翻转时间约16 ns, 正电荷激子中电子的自旋翻转时间约2 ns, 负电荷激子中空穴的自旋翻转时间约50 ps。     

Reasons of emission inhomogeneity in InAs quantum dot structures

The photoluminescence (PL) inhomogeneity has been studied in InAs quantum dots (QDs) embedded in the symmetric In_0.15Ga_0.85As/GaAs quantum wells (QWs) with QDs grown at different temperatures. It was shown that three reasons are responsible for the emission inhomogeneity in studied QD structures: (i) the high concentration of nonradiative recombination centers in the capping In_0.15Ga_0.85As layer at low QD growth temperatures, (ii) the QD density and size distributions for the structure with QD grown at 510 ^°C, (iii) the high concentration of nonradiative recombination centers in the GaAs barrier at higher QD growth temperatures.     

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...     

A comprehensive study of the effect of <Emphasis Type="Italic">in situ</Emphasis> annealing at high growth temperature on the morphological and optical properties of self-assembled InAs/GaAs QDs

We investigate the effect of in situ annealing during growth pause on the morphological and optical properties of self-assembled InAs/GaAs quantum dots (QDs). The islands were grown at different growth rates and having different monolayer coverage. The results were explained on the basis of atomic force microscopy (AFM) and photo-luminescence (PL) measurements. The studies show the occurrence of ripening-like phenomenon, observed in strained semiconductor system. Agglomeration of the self-assembled QDs takes place during dot pause leading to an equilibrium size distribution. The PL properties of the QDs are affected by the Indium desorption from the surface of the QDs during dot pause annealing at high growth temperature (520°C) subsiding the effect of the narrowing of the dot size distribution with growth pause. The samples having high monolayer coverage (3.4 ML) and grown at a slower growth rate (0.032 ML s⁻¹) manifested two different QD families. Among the islands the smaller are coherent defect-free in nature, whereas the larger dots are plastically relaxed and hence optically inactive. Indium desorption from the island surface during the in situ annealing and inhomogeneous morphology as the dots agglomerate during the growth pause, also affects the PL emission from these dot assemblies.     

Temperature behavior of Franz-Keldysh oscillation and evaluation of interface electric fields attributed to strain effect of InAs/GaAs quantum dot

In this work, the electric field-induced Franz-Keldysh effect was used to investigate the localized electric fields in GaAs interfaces attributed to strain effect of InAs/GaAs quantum dots (QD). The electric fields were investigated by photoreflectance spectroscopy (PR). PR spectra of the InAs/GaAs QDs showed complex Franz-Keldysh oscillations (FKOs) with various temperatures. It is suggested that the FKOs originated from the interface electric fields predominately caused by the strain-induced polarization at GaAs interface near the InAs QDs. The InAs/GaAs QDs have a broad range of interface electric fields from ～10⁴ V/cm to ～2х10⁵ V/cm. Temperature behavior of FKO amplitude distribution is explained by temperature dependent carrier confinement effect.     

Emission and strain in InGaAs/GaAs quantum wells with InAs quantum dots obtained at different temperatures

The photoluminescence (PL), its temperature dependence and X ray diffraction (XRD) have been studied in the symmetric In_0.15Ga_0.85As/GaAs quantum wells (QWs) with embedded InAs quantum dots (QDs), obtained with the variation of QD growth temperatures (470–535 °C). The increase of QD growth temperatures is accompanied by the enlargement of QD lateral sizes (from 12 up to 28 nm) and by the shift non monotonously of PL peak positions. The fitting procedure has been applied for the analysis of the temperature dependence of PL peaks. The obtained fitting parameters testify that in studied QD structures the process of In/Ga interdiffusion between QDs and capping/buffer layers takes place partially. However this process cannot explain the difference in PL peak positions.     

Efficient two-photon absorption of CdSe-CdS/ZnS core-multishell quantum dots under the excitation of near-infrared femtosecond pulsed laser

We report that two-photon absorption (TPA) properties of semiconductor CdSe-core CdS/ZnS-multishell quantum dots (QDs) in toluene under excitation of femtosecond laser at 800 nm. The results show efficient TPA process and large TPA cross section of three types of size QDs, which is 1900, 5710, and 16060 GM (1 GM = 10⁻⁵⁰ cm⁴ s photon⁻¹), respectively. TPA cross section dramatically increases with increased core size, showing a strong size-dependence effect. Furthermore, two-photon excitation (TPE) fluorescence intensity not only depends on TPA capacity, but also relies on improved quantum yield resulting from passivation of QD surface by different coated monolayers (MLs). These facts in combination with the narrow fluorescence bandwidth make these QDs as promising probes for multicolor two-photon microscopy.