InAs/GaAs lasers with very thin active layer

InAs/GaAs laser structures based on atomically thin InAs strained quantum wells were prepared by metal–organic vapour phase epitaxy. The dependence of electroluminescence spectra on the thickness, as well as on the number of InAs quantum wells, was studied. The temperature dependence of the mode structure and optical output power were studied in the range from 25 to 100°C. The position of laser emission can be shifted by changing the thickness and the number of InAs active layers from 1.15 to 1.4 eV. Wavelength switching with increasing operating temperature and excitation current was observed.     

Molecular beam epitaxy growth methods of wavelength control for InAs/(In)GaAsN/GaAs heterostructures

We discuss the molecular beam epitaxy (MBE) growth methods of emission wavelength control and property investigations for different types of InAs/(In)GaAsN/GaAs heterostructures containing InGaAsN quantum-size layers: (1) InGaAsN quantum wells deposited by the conventional mode in a GaAs matrix, (2) InAs quantum dots deposited in a GaAsN matrix or covered by an InGaAs(N) layer, and (3) InAs/InGaAsN/GaAsN strain-compensated superlattices with quantum wells and quantum dots. The structures under investigation have demonstrated photoluminescence emission in a wavelength range of ～1.3-1.8?μm at room temperature without essential deterioration of the radiative properties.     

Quantum dots in InAs layers of subcritical thickness on GaAs(100)

Ensembles of InAs quantum dots formed at the GaAs(100) surface have been studied by the methods of reflection high-energy electron diffraction and photoluminescence. The amount of deposited InAs corresponds to a wetting layer thickness smaller than the critical value necessary for the transition from two-to three-dimensional growth. It is experimentally shown that, at a deposited film thickness of 1.5 and 1.6 monolayers, islands are formed after keeping the sample in a flow of As₄. The influence of the substrate temperature on the kinetic characteristics of the formation of InAs/GaAs islands has been studied.     

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.     

Photoluminescence studies of InAs/GaAs quantum dots covered by InGaAs layers

Photoluminescence (PL), PL excitation (PLE), and time-resolved PL were used to study effects of InGaAs layers on the optical properties of InAs/GaAs quantum dots (QDs). A rich fine structure in the excited states of confined excitons (up to n = 4 quantum states) was observed, providing useful information to study the quantum states in the InAs/GaAs QDs. A significant redshift of the PL peak energy for the QDs covered by InGaAs layers was observed, attributing to the decrease of the QD strain and the lowing of the quantum confinement.     

Growth of InAs Quantum Dots on GaAs (511)A Substrates: The Competition between Thermal Dynamics and Kinetics

The growth process of InAs quantum dots grown on GaAs (511)A substrates has been studied by atomic force microscopy. According to the atomic force microscopy studies for quantum dots grown with varying InAs coverage, a noncoherent nucleation of quantum dots is observed. Moreover, due to the long migration length of In atoms, the Ostwald ripening process is aggravated, resulting in the bad uniformity of InAs quantum dots on GaAs (511)A. In order to improve the uniformity of nucleation, the growth rate is increased. By studying the effects of increased growth rates on the growth of InAs quantum dots, it is found that the uniformity of InAs quantum dots is greatly improved as the growth rates increase to 0.14 ML s^?1. However, as the growth rates increase further, the uniformity of InAs quantum dots becomes dual‐mode, which can be attributed to the competition between Ostwald ripening and strain relaxation processes. The results in this work provide insights regarding the competition between thermal dynamical barriers and the growth kinetics in the growth of InAs quantum dots, and give guidance to improve the size uniformity of InAs quantum dots on (N11)A substrates.     

Self-assembling InAs and InP quantum dots for optoelectronic devices

Stranski–Krastanov growth in molecular beam epitaxy allows the preparation of self assembling InAs and InP quantum dots on GaAs and Ga_0.52In_0.48P buffer layers, respectively. InAs dots in GaAs prepared by slow growth rates and low temperature overgrowth provide intense photoluminescence at the technologically important wavelength of 1.3 μm at room temperature. Strain induced vertical alignment, size modification and material interdiffusion for stacked dot layers are studied. A blue shift of the ground state transition energy is observed for the slowly deposited stacked InAs dots. This is ascribed to enhanced strain driven intermixing in vertically aligned islands. For very small densely stacked InP and InAs dots the reduced confinement shift causes a red shift of the ground state emission. The InP quantum dots show intense and narrow photoluminescence at room temperature in the visible red spectral range. First InP/Ga_0.52In_0.48P quantum dot injection lasers are prepared using threefold stacked InP dots. We observe lasing at room temperature in the wavelength range between 690–705 nm depending on the size of the stacked InP dots.     

Stacked layers of InAs self-assembled quantum dots

Carrier injection and subsequent radiative recombination in two vertically stacked (but electronically only weakly coupled) layers of InAs/GaAs self-assembled quantum dots (SADs) embedded in the intrinsic region of a double hetero p-i-n structure was investigated by electroluminescence (EL) spectroscopy in the temperature range from 20 to 300 K. In such structures the filling of the SADs by charge carriers strongly depends not only on the applied voltage, but also on the relative position of the SAD layers within the i-region and on the temperature. The experimental data provide evidence of the dominant role of hole dynamics in the recombination processes in the stacks of SADs. The difference of the electronic structure of the SADs in the top and bottom layers is reflected by independent contributions of the two quantum dot layers to the electroluminescence from the SADs. The possibility to tune the emission spectra by varying the thickness of the GaAs layer between neighbouring SAD layers and by using the indium flush technique is demonstrated.     

Dependence of the optical properties on the GaAs spacer thickness for vertically stacked InAs/GaAs quantum dots

Temperature-dependent photoluminescence (PL) measurements have been carried out to investigate the dependence of the optical properties on the GaAs spacer thickness for vertically stacked InAs/GaAs quantum dots (QDs). The PL spectra showed that the peak corresponding to the interband transitions form the ground electronic subband to the ground heavy-hole band (E₀-HH₁) of the InAs QDs shifted to a higher energy side with increasing the GaAs spacer thickness and that the full width at maximum (FWHM) of the (E₀-HH₁) peak rapidly decreased with increasing a spacer thickness. The position energy and the FWHM of the (E₀-HH₁) peak for the InAs/GaAs QDs at several temperatures were observed. The present observations can help improve understanding of the dependence of the optical properties on the GaAs spacer thickness for vertically stacked InAs/GaAs QDs.     

Optimizing the spacer layer thickness of vertically stacked InAs/GaAs quantum dots

Vertically stacked multilayers of self-organized InAs/GaAs quantum dots (QDs) structures with different GaAs intermediate layer thicknesses varying between 2.8 and 17 nm are grown by solid source molecular beam epitaxy (SSMBE) and investigated by photoluminescence spectroscopy (PL). For 17 nm thick GaAs spacer, the PL spectra show two well separated features attributed to the formation of two QDs family with a bimodal size distribution indicating no correlation between the dots in different layers. In the meanwhile, the structures having thinner spacer thickness demonstrate single PL peaks showing an enhancement of high energy side asymmetrical broadening when increasing the excitation power. The corresponding emission energies exhibit a red shift when the spacer layer thickness decreases and correlated with the enhancement of the vertical electronic coupling as well as the rise of the QD's size in the upper layers induced by the build up of the strain field along the columns. The spacer thickness of 8.5 nm is found to yield the best optical properties.     

Electrical characterization of InAs/GaAs quantum dot structures

The electrical properties of InAs quantum dots (QD) in InAs/GaAs structures have been investigated by space charge spectroscopy techniques, current–voltage and capacitance–voltage measurements. Au/GaAs/InAs(QD)/GaAs Schottky barriers as well as ohmic/GaAs/InAs(QD)/GaAs/ohmic structures have been prepared in order to analyze the apparent free carrier concentration profiles across the QD plane, the electronic levels around the QD and the electrical properties of the GaAs/InAs(QD)/GaAs heterojunction. Accumulation and/or depletion of free carriers at the QD plane have been observed by Capacitance–Voltage (C–V) measurements depending on the structure parameters and growth procedures. Similarly, quantum dot levels which exhibit distributions in energy have been detected by Deep Level Transient Spectroscopy (DLTS) and Admittance Spectroscopy (AS) measurements only on particular structures. Finally, the rectification properties of the InAs/GaAs heterojunction have been investigated and the influence of the related capacitance on the measured capacitance has been evidenced.     

Shape and size control of InAs/InP (113)B quantum dots by Sb deposition during the capping procedure

The role of Sb atoms present on the growth front during capping of InAs/InP (113)B quantum dots (QDs) is investigated by cross-sectional scanning tunnelling microscopy, atomic force microscopy, and photoluminescence spectroscopy. Direct capping of InAs QDs by InP results in partial disassembly of InAs QDs due to the As/P exchange occurring at the surface. However, when Sb atoms are supplied to the growth surface before InP capping layer overgrowth, the QDs preserve their uncapped shape, indicating that QD decomposition is suppressed. When GaAs(0.51)Sb(0.49) layers are deposited on the QDs, conformal growth is observed, despite the strain inhomogeneity existing at the growth front. This indicates that kinetics rather than the strain plays the major role during QD capping with Sb compounds. Thus Sb opens up a new way to control the shape of InAs QDs.     

Structural characterization of GaSb-capped InAs/GaAs quantum dots with a GaAs intermediate layer

GaSb incorporation to InAs/GaAs quantum dots is considered for improving the opto-electronic properties of the systems. In order to optimize these properties, the introduction of an intermediate GaAs layer is considered a good approach. In this work, we study the effect of the introduction of a GaAs intermediate layer between InAs quantum dots and a GaSb capping layer on structural and crystalline quality of these heterostructures. As the thickness of the GaAs intermediate layer increases, a reduction of defect density has been observed as well as changes of quantum dots sizes. This approach suggests a promising method to improve the incorporation of Sb to InAs heterostructures.     

Self-assembled growth of GaAs anti quantum dots in InAs matrix by migration enhanced molecular beam epitaxy

Self-assembled GaAs anti quantum dots (AQDs) were grown in an InAs matrix via migration enhanced molecular beam epitaxy. The transmission electron microscopy image showed that the 2D to 3D transition thickness is below 1.5 monolayers (MLs) of GaAs coverage. The average diameter and height of the GaAs AQDs for 1.5 ML GaAs coverage taken from the atomic force microscopy image were approximately 29.0 nm and 1.4 nm, respectively. The density was approximately 6.0 x 10(10) cm(-2). The size of the AQDs was enlarged in the InAs matrix compared with that on the surface. These results indicate that the GaAs AQDs in the InAs matrix under tensile strain can be effectively formed with the assistance of the migration enhanced epitaxy method.     

Analysis of the Vertical and Lateral Interactions in a Multisheet Array of InAs/GaAs Quantum Dots

The vertical and lateral interactions in a multisheet array of InAs/GaAs quantum dots are analyzed by finite element method(FEM).It is shown that due to the effects of vertical interaction,nucleation prefers to happen above buried quantum dots(QDs).Meanwhile,the effects of lateral interaction adjust the spacing of lateral neighboring QDs.The vertical coupling becomes strong with deceasing GaAs spacer height and increasing number of buried layers,while the lateral coupling becomes strong with increasing InAs...     

自组织InAs/GaAs与InGaAs/GaAs量子点生长及退火情况的比较

本文采用MBE进行InAs/GaAs与InGaAs/GaAs量子点的生长,利用RHEED进行实时监测,并利用RHEED强度振荡测量生长速率。对生长的InAs/GaAs和InGaAs/GaAs两种量子点生长过程与退火情况进行对比,观察到当RHEED衍射图像由条纹状变为网格斑点时,InAs所需要的时间远小于InGaAs;高温退火下RHEED衍射图像恢复到条纹状所需要的时间InAs比InGaAs要长。     

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.     

张应变GaAs层引入使InAs/InP量子点有序化排列的机理研究

采用ＬＰ－ＭＯＶＰＥ技术,在（００１）ＩｎＰ衬底上生长的ＩｎＡｓ／ＩｎＰ自组装量子点是无序的。为了解决这个问题,在ＩｎＰ衬底上先生长张应变的ＧａＡｓ层,然后再生长ＩｎＡｓ层,可得到有序化排列的量子点。本文对张应变ＧａＡｓ层引入使量子点有序化排列的机理进行了分析,为生长有序化、高密度,均匀性好自组装量子点提供了依据。     

The growth of infrared antimonide-based semiconductor lasers by metal-organic chemical vapor deposition

We describe the metal-organic chemical vapor deposition (MOCVD) growth of InAsSb/InAs and GaAsSb/GaAs(P) multiple quantum well (MQW) and InAsSb/InAsP and InAsSb/InPSb strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. We also describe the growth and initial characterization of GaAsSbN/GaAs MQW structures. By changing the layer thickness and composition of the InAsSb SLSs and MQWs, we have prepared structures with low temperature (<20 K) photoluminescence wavelengths ranging from 3.2 to 6.0 m. We have made gain-guided, injection lasers using undoped, p-type AlAs_0.16Sb_0.84 for optical confinement and both strained InAsSb/InAs MQW and InAsSb/InAsP and InPSb SLS active regions. The lasers and LEDs utilize the semi-metal properties of a p-GaAsSb/n-InAs heterojunction as a source for electrons injected into the active regions. Cascaded, semi-metal, mid-infrared, injection lasers with pseudomorphic InAsSb multiple quantum well active region lasers and LEDs are reported. We also report on GaAsSb/GaAs(P) lasers and LEDs emitting at 1.1 to 1.2 m grown on GaAs substrates and using AlGaAs layers for confinement.     

Improved photoluminescence efficiency of patterned quantum dots incorporating a dots-in-the-well structure

InAs quantum dots embedded in InGaAs quantum well (DWELL: dots-in-the-well) structures grown on nanopatterned GaAs pyramids and planar GaAs(001) surface are comparatively investigated. Photoluminescence (PL) measurements demonstrate that the DWELL structure grown on the GaAs pyramids exhibits a broad quantum well PL band (full width at half-maximum ～ 90?meV) and a higher quantum dot emission efficiency than the DWELL structure grown on the planar GaAs(001) substrate. These properties are attributed to the InGaAs quantum well with distributed thickness profile on the faceted GaAs pyramids, which introduces a tapered energy band structure and enhances carrier capture into the quantum dots.