Independent control of InAs quantum dot density and size on AlxGa1–xAs surfaces

Decoupling of InAs quantum dot (QD) size and density on Al_xGa_1?xAs surfaces (x = 0, 0.15, 0.30, and 0.45) is achieved by using a low growth rate and careful control of the temperature. The deposition rate of 0.01 μm/h, instead of 0.05 μm/h, allows the QDs to ripen with additional InAs deposition while the substrate temperature (490–520 °C) determines the QD density. On the GaAs surface, an increase of 10 °C results in an order of magnitude lower QD density. The increase of Al in the Al_xGa_1?xAs surfaces results in a higher dot density, lower dot size, and an increased size distribution. All surfaces show reduced QD density with increasing temperature and an identical zero dot density temperature at 523 °C. The GaAs surface shows increasing QD height with temperature while the Al_xGa_1?xAs surfaces show the opposite trend, but the InAs volume fraction in QDs for all surfaces decreases with increasing temperature, implying a more stable wetting layer. Increasing Al content also increases the InAs volume fraction in QDs, implying the wetting layer for all but the 520 °C samples is less than one monolayer. Photoluminescence samples demonstrate ground state QD energies above the GaAs bandedge.     

Selective Intermixing of InAs/InGaAs/InP Quantum Dot Structure With Large Energy Band Gap Tuning

Selective postgrowth band gap tuning of self-assembled InAs/InGaAs/InP quantum dot (QD) structures has been investigated. Very large band gap blueshift of over 158 meV of the InAs QD structure has been received through the intermixing by exposing the sample under argon plasma and followed by thermal annealing at 780 °C. Selective intermixing of the InAs QD structure has been studied by depositing a $hbox{SiO}_{2}$ mask layer on the sample for the intermixing. The largest selective band gap shift between the $hbox{SiO}_{2}$ covered and uncovered regions of the sample reaches 77 meV. This intermixing selectivity decreases when the annealing temperature is increased. This reduction in the intermixing selectivity is attributed to the enhanced QDs intermixing of the $hbox{SiO}_{2}$-masked samples because of the out diffusion of different elements from the InAs/InGaAs/InP QD structure into the $hbox{SiO}_{2}$ cover layer. Three different energy band gap shifts of an InAs/InGaAs/InP QD structure across the wafer have been received by this postgrowth selective intermixing. The selective band gap tuning paves a way for monolithic integration of passive and active optoelectronic devices in QD systems.      

1.55 $mu$m InAs/InAlGaAs Quantum Dot DFB Lasers

For the fabrication of quantum dot (QD) distributed feedback (DFB) lasers, self-assembled InAs/InAlGaAs QDs were grown on the InP/InGaAs grating structures by a molecular beam epitaxy. Ridge-waveguide QD DFB lasers with a stripe width of 3 mum were fabricated. Single-mode lasing operation around the wavelength of 1.56 mum was successfully achieved under the pulsed and continuous-wave (CW) modes at room temperature. The lasing operation was also observed up to 70 degC under pulsed mode, which is the first observation on the single-mode lasing around 1.55 mum. The characteristic temperatures calculated from the temperature dependence of the threshold current density were 121.8 K from room temperature to 45 degC, which was reduced to 74.2 K from 45 degC to 70 degC.     

Influences of silicon doping in quantum dot layers on optical characteristics of InAs/GaAs quantum dot infrared photodetector

We have investigated the effects of silicon doping concentration within thirty-period self-assembled quantum dot (QD) layers on quantum dot infrared photodetectors (QDIPs). The lens-shaped quantum dots with the dot density of 1 × 10¹¹ cm^− 2 were observed by atomic force microscope (AFM). From the high ratio of photoluminescence (PL) peak intensities from dot layer to that from wetting layer, we have concluded that high dot density caused the short diffusion length for carriers to be easily captured by QDs. Moreover, the Si-doped samples exhibited the multi-state transitions within the quantum dots, which were different to the single level transition of undoped sample. Besides, the dominant PL peaks of Si-doped samples were red-shifted by about 25 meV compared to that of the undoped sample. It should result from the dopant-induced lowest transition state and therefore, the energy difference should be equal to the binding energy of Si in InAs QDs.     

Thermal stability of the peak emission wavelength in multilayer InAs/GaAs QDs capped with a combination capping of InAlGaAs and GaAs

The effect of rapid thermal annealing (RTA) on the optical properties of a 10 layer stacked InAs/GaAs quantum dot (QD) heterostructure where the QDs are overgrown with a combination of quaternary InAlGaAs and GaAs capping have been investigated. TEM micrographs showed that the shape of the QDs is preserved for annealing temperatures up to 800 degrees C. The peak emission wavelength of the investigated heterostructures remains stable on annealing at temperatures upto 750 degrees C, which is unusual in QD samples. This phenomenon is attributed due to the suppression of the strain-enhanced intermixing in such structures. One of the reasons behind such suppression is the strain driven phase separation of Indium from the overgrown quaternary alloy, which maintains an In rich region across the QD periphery thereby checking the out-diffusion of Indium from the dots. The overlapping vertical strain from the under lying dot layers in the QD stack also maintains a strain relaxed state at the QD base, thereby preventing the material mixing at the base of the pyramidal QDs. This stability of wavelength is of paramount importance in optoelectronic devices where the design is based on the emission wavelength of the active region.     

Optically-induced charge storage in self-assembled InAs quantum dots

We present results on spectrally resolved photo-resistance studies of optically-induced charge storage effects in self-organized InAs quantum dots (QDs). The stored charge can be detected and erased electrically. The investigated structure designed for electron or hole storage in the QDs consists of a modulation doped two-dimensional channel which was grown on top of a layer of InAs QDs, separated by an asymmetric tunnel barrier. Our results show that optical QD charging with spectral resolution provides information on the charging dynamics and on the quantity and spectral dependence of stored charges in the QDs. This is a novel technique by which QD excitation spectra can be studied. Spectrally resolved storage effect measurements on electrons as well as on holes allowed to investigate thermal redistribution of carriers in the quantum dot layer. It was found that only at low temperatures carriers can be stored selectively over long time scales in the InAs QDs. The charge storage effect is observable for several hours at temperatures up to 170 K, for several seconds up to 250 K due to an increase in thermal emission of stored charges.     

Numerical simulation of electronic properties of coupled quantum dots on wetting layers

Self-assembled quantum dots are grown on wetting layers and frequently in an array-like assembly of many similar but not exactly equal dots. Nevertheless, most simulations disregard these structural conditions and restrict themselves to simulating a pure single quantum dot. For reasons of numerical efficiency we advocate the effective one-band Hamiltonian with energy-?and position-dependent effective mass approximation and a finite height hard-wall 3D confinement potential for computation of the energy levels of the electrons in the conduction band. Within this model we investigate the geometrical effects mentioned above on the electronic structure of a pyramidal InAs quantum dot embedded in a GaAs matrix. We find that the presence of a wetting layer may affect the electronic structure noticeably. Furthermore, we establish that, in spite of the large bandgap of the InAs/GaAs heterostructure, if the dots in a vertically aligned array are sufficiently close stacked there is considerable interaction between their eigenfunctions. Moreover, the eigenfunctions of such an array are quite sensitive to certain structural perturbations.     

Modeling InAs quantum-dot formation on the side surface of GaAs nanowires

We have theoretically studied the formation of InAs quantum dots (QDs) on the side surface of GaAs nanowires (NWs). The effective energies of formation of a thin InAs layer and QDs on the NW side surface are compared with allowance for elastic stresses at the radial heteroboundary of two materials with lattice mismatch. The concept of a critical thickness of the external (wetting) layer is introduced, at which the mechanical stresses stimulate three-dimensional growth of QDs. The dependence of the critical layer thickness on the NW diameter and elastic constants of the system is determined. The phenomenon of partial filling of the NW side surface by QDs is explained by a decrease in the thickness of a deposited InAs layer with increasing height. The results of modeling agree well with the available experimental data.     

InAs quantum dots capped by GaAs,In0.4Ga0.6As dots,and In0.2Ga0.8As well

We have fabricated and characterized three types of InAs quantum dots (QDs) with different InxGa1-xAs capping layers. Post-growth atomic force microscopy measurements show that the In0.2Ga0.8As/InAs structure has a smooth surface (dot-in-well structure), whereas the In0.4Ga0.6As/InAs structure revealed large QDs with a density similar to that underneath InAs QDs on GaAs (dot-in-dot). With increasing In mole fraction of the capping layer and increasing In0.4Ga0.6As thickness, the energy position of the room-temperature photoluminescence (PL) peak is red-shifted. The quantum dot-in-dot structure emits stronger room-temperature PL than does the quantum dot-in-well structure. With a spatially distributed strain in the InAs quantum dot, we have solved the three-dimensional Schr?dinger equation by the Green's function theory for the eigenvalues and eigen wave functions. It is concluded that the ground state increases its wave function penetration into the low-barrier InxGa1-xAs capping layer so that its energy position is red-shifted. The reduced PL peak intensity of the dot-in-well (compared with GaAs covered dots) is due to the reduced overlapping between the ground state and the extended states above the GaAs barrier. The overlapping reduction in the dot-in-dot is over compensated for by the reduced relaxation energy (full width at half-maximum), indicating the importance of the sample quality in determining the PL intensity.     

Optical properties of the InAs/InAlGaAs quantum dots subjected to thermal treatments

The inter-diffusion kinetics of group-III elements at the interface between self-assembled InAs quantum dots (QDs) and InAlGaAs barriers were investigated indirectly by post-growth annealing treatments and photoluminescence (PL) spectroscopy. The emission wavelength of the InAs/InAlGaAs QDs subjected to thermal annealing at 550 °C was 1444 nm at 10 K, which indicated a 57 nm red shift compared to the as-grown sample (1387 nm). The emission wavelength was blue-shifted with further increases in annealing temperature to 650 °C. Although there was a blue shift in the emission wavelength at an annealing temperature of 600 and 650 °C, the emission peak was still longer than that of the as-grown sample. These results were explained by the difference in inter-diffusion probability between group-III elements at the interface between the InAs QDs and InAlGaAs barrier.     

Thermal-induced intermixing effects on the optical properties of long wavelength low density InAs/GaAs quantum dots

The effect of post-growth rapid thermal annealing on the photoluminescence properties of long wavelength low density InAs/GaAs (001) quantum dots (QDs) with well defined electronic shells has been investigated. For an annealing temperature of 650 °C for 30 s, the emission wavelength and the intersublevel spacing energies remain unchanged while the integrated PL intensity increases. For higher annealing temperature, blue shift of the emission energy together with a decrease in the intersublevel spacing energies are shown to occur due to the thermal activated In–Ga interdiffusion. While, this behaviour is commonly explained as a consequence of the enrichment in Ga of the QDs, the appearance of an additional exited state for annealing temperatures higher than 650 °C suggests a variation of the intermixed QDs's volume/diameter ratio toward QDs's enlargement.     

The Control on Size and Density of InAs QDs by Droplet Epitaxy (April 2009)

We report on the ability to grow InAs quantum dots (QDs) by droplet epitaxy (DE) using solid-source molecular beam epitaxy (MBE). In particular, the control of the size and density of InAs QDs at near room temperatures are achieved as a function of substrate temperature and crystallization condition. For a typical range of QD density ( ~10⁹ to 10¹⁰ cm^-2), the growth window is revealed to be fairly narrow ( ~20degC). In droplets are extremely sensitive to surface diffusion and arsenic background pressure even at near room temperatures. As a result, a very careful fabrication procedure is required to crystallize In droplets in order to fabricate desired shape of InAs QDs. For this purpose, we developed a double-step crystallization process, in which As background recovery and high-temperature crystallization are introduced. In addition, the results by DE are compared with QDs fabricated by Stranski-Krastanow (S-K) growth approach in terms of size and density. The results can find applications in optoelectronics as the fabrication of QDs by DE approach has more flexibility over S-K approach, i.e., more freedom of size and density control.     

An approach to suppress the blue-shift of photoluminescence peaks in coupled multilayer InAs/GaAs quantum dots by high temperature post-growth annealing

Effect of post-growth annealing on 10 layer stacked InAs/GaAs quantum dots (QDs) with InAlGaAs/GaAs combination capping layer grown by molecular beam epitaxy has been investigated. The QD heterostructure shows a low temperature (8 K) photoluminescence (PL) emission peak at 1267 nm. No frequency shift in the peak emission wavelength is seen even for annealing up to 700 °C which is desirable for laser devices requiring strict tolerances on operating wavelength. This is attributed to the simultaneous effect of the strain field, propagating from the seed layer to the active layer of the multilayer QD (MQD) and the indium atom gradient in the capping layer due to the presence of a quaternary InAlGaAs layer. Higher activation energy (of the order of ∼250 meV) even at 650 °C annealing temperature also signifies the stronger carrier confinement potential of the QDs. All these results demonstrate higher thermal stability of the emission peak of the devices using this QD structure.     

Transformation of InAs islands to quantum ring structures by metalorganic vapor phase epitaxy

The transformation of InAs islands to quantum rings (QRs) by metalorganic vapor phase epitaxy is investigated. After covering the InAs islands with a thin GaAs partial capping layer and annealing under tertiarybutylarsine (TBAs) flow, ring-shaped nanostructures with a density of 10(7)-10(9)?cm(-2) are obtained at 500-600?°C. The effects of the growth temperature, annealing process and thickness of the partial capping layer are studied. Optimum values for the annealing time and the partial capping layer thickness were found to be 60-120?s and 0.5-2.0?nm, respectively. Low temperature photoluminescence (PL) emission peaks from islands and QRs grown at 500?°C are observed at 1.04?eV and 1.22?eV, respectively. The annealing temperature affected the QR evolution and the PL emission from the QRs due to the temperature dependence of the diffusion rate of indium atoms.     

Electron emissions in InAs quantum dots containing a nitrogen incorporation induced defect state: the influence of thermal annealing

With the incorporation of nitrogen (N) into InAs quantum dots (QDs), the carrier distribution near the QD displays electron emissions from a localized N-induced defect state at 0.34?eV and a weak emission at 0.15?eV from the QD. This defect state causes drastic carrier depletion in the neighboring GaAs bottom layer near the QD, which can effectively suppress tunneling emission for the QD excited states. As a result, electrons escape from the QD ground state through thermal emission to near the GaAs conduction band, rather than through thermal emission to the QD first excited state and a subsequent tunneling to the GaAs conduction band, as observed in InAs QDs without N incorporation. Thermal annealing can weaken the defect emission and enhance the QD emission, suggesting a removal of the defect state and a recovery of carriers in the QD. Increasing annealing temperature can significantly decrease the emission time and energy of the QD emission, which is explained by a weakening of tunneling suppression due to the removal of the defect state.     

First-step nucleation growth dependence of InAs/InGaAs/InP quantum dot formation in two-step growth

First-step nucleation growth has an important impact on the two-step growth of high-quality mid-infrared emissive InAs/InGaAs/InP quantum dots (QDs). It has been found that an optimized growth rate for first-step nucleation is critical for forming QDs with narrow size distribution, high dot density and high crystal quality. High growth temperature has an advantage in removing defects in the QDs formed, but the dot density will be reduced. Contrasting behavior in forming InAs QDs using metal-organic vapor phase epitaxy (MOVPE) by varying the input flux ratio of group-V versus group-III source (V/III ratio) in the first-step nucleation growth has been observed and investigated. High-density, 2.5 × 10(10)?cm(-2), InAs QDs emitting at>2.15?μm have been formed with narrow size distribution, ～1?nm standard deviation, by reducing the V/III ratio to zero in first-step nucleation growth.     

Explanation of Unusual Photoluminescence Behavior from InAs Quantum Dots with InAlAs Capping

The effect of different kinds of cap layers on optical property of InAs quantum dots (QDs) on GaAs (100) substrate was studied. Temperature dependent photoluminescence (PL) indicates that the PL integrated intensity from the ground state of InAs QDs capped with an intermediate InAIAs layer drops very little as compared to QDs capped with a thin InGaAs or GaAs cap layer from 15 K up to room temperature. PL integrated intensity ratio of the first excited to ground states for InAs QDs capped with an intermediate InAIAs layer is unexpectedly decreased with increasing temperature, which are attributed to phonon bottleneck effect. A virtual barrier is proposed to describe this physics process and shows good agreement with experimental results when fitting the curve with the value of the virtual barrier 30 meV.     

Abnormal optical behaviour of InAsSb quantum dots grown on GaAs substrate by molecular beam epitaxy

InAs(Sb) quantum dots (QDs) samples were grown on GaAs (001) substrate by Molecular Beam Epitaxy (MBE). The structural characterization by Atomic Force Microscopy (AFM) of samples shows that InAsSb islands size increases strongly with antimony incorporation in InAs/GaAs QDs and decreases with reducing the growth temperature from 520 °C to 490 °C. Abnormal optical behaviour was observed in room temperature (RT) photoluminescence (PL) spectra of samples grown at high temperature (520 °C). Temperature dependent PL study was investigated and reveals an anomalous evolution of emission peak energy (EPE) of InAsSb islands, well-known as “S-inverted curve” and attributed to the release of confined carriers from the InAsSb QDs ground states to the InAsSb wetting layer (WL) states. With only decreasing the growth temperature, the S-inverted shape was suppressed indicating a fulfilled 3D-confinement of carriers in the InAsSb/GaAs QD sample.     

The Hanle effect and electron spin polarization in InAs/GaAs quantum dots up to room temperature

The Hanle effect in InAs/GaAs quantum dots (QDs) is studied under optical orientation as a function of temperature over the range of 150-300 K, with the aim of understanding the physical mechanism responsible for the observed sharp increase of electron spin polarization with increasing temperature. The deduced spin lifetime T(s) of positive trions in the QDs is found to be independent of temperature, and is also insensitive to excitation energy and density. It is argued that the measured T(s) is mainly determined by the longitudinal spin-flip time (T(1)) and the spin dephasing time (T(2)*) of the studied QD ensemble, of which both are temperature independent over the studied temperature range and the latter makes a larger contribution. The observed sharply rising QD spin polarization degree with increasing temperature, on the other hand, is shown to be induced by an increase in spin injection efficiency from the barrier/wetting layer and also by a moderate increase in spin detection efficiency of the QD.     

InAs自组织量子点(线)的制备和表征

在InP(001)基衬底上用分子束外延方法生长了InAs纳米结构材料，通过改变生长方式，得到了InAs量子点和量子线。根据扫描电镜和透射电镜观测结果的分析，认为衬底旋转时浸润层三角形状的台阶为InAs量子线的成核提供了优先条件，停止衬底旋转时InAlAs缓冲层沿[11^-0]方向分布的台阶促使InAs优先形成量子点。讨论了量子点和量子线的形成机理。