Seeded self-ordering of GaAs/AlGaAs quantum wires on non-planar substrates

Self-ordering of GaAs/AlGaAs quantum wires grown by organometallic chemical vapor deposition on grooved substrates was studied. The evolution of the surface profile at the corner between two quasi-{111}A planes was evaluated using cross-sectional transmission electron microscopy. The radius of curvature at the corner exhibits a reproducible, self-limiting value of 7.7 ± 0.7 nm, which increases linearly during subsequent growth of GaAs layers and decreases exponentially to its self-limiting value during further growth of AlGaAs layers. This provides the basis for the self-ordering of periodic, vertically stacked arrays of quantum wires with virtually identical shape, size and composition.     

Self-limiting OMCVD growth of GaAs on V-grooved substrates with application to InGaAs/GaAs quantum wires

We demonstrate that the formation of GaAs quantum wires on self-limiting AlGaAs grown on V grooves occurs via a transient increase of the growth rates in a set of different nanofacets. Upon growth of sufficiently thick layers on AlGaAs, the GaAs surface reaches a self-limiting profile as well, through an equalization of the relative growth rates on these facets. Atomic force microscopy studies show that the step density in the facets along the groove evolves with GaAs thickness in the same way as the facets extension, thus suggesting a role of the step distribution in the establishment of the self-limiting profiles. The self-limiting GaAs groove profile is much broader than the AlGaAs one at corresponding growth temperatures; however, it can be sharpened down to a radius of curvature of 5 nm for T = 550°C. Under these conditions, GaAs was successfully used as a barrier material for growing vertical arrays of self-ordered InGaAs wires.     

Self-organization of nanostructures on planar and patterned high-index semiconductor surfaces

In order to directly control the size and in particular the position of nanostructures naturally formed on high-index semiconductor surfaces during molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE), the growth on patterned high-index GaAs substrates is investigated. During MBE of (AlGa)As on patterned GaAs (311)A substrates, a new phenomenon in the selectivity of growth has been found to form a fast growing sidewall on one side of mesa stripes oriented along the [01-1] direction. Preferential migration of Ga adatoms from the mesa top as well as the mesa bottom toward the sidewall develops a smooth convex curved surface profile without facets. This unique growth mode that does not occur for the perpendicular stripe orientation nor on other patterned GaAs (n11)A and B substrates is stable for step heights down to the quantum size regime to produce lateral quantum wires on patterned GaAs (311)A substrates. Quantum confinement of excitons in the wires has been demonstrated by the transition from two-dimensional to magnetic confinement with increasing magnetic field. For device applications it is important that the wires can be stacked in the growth direction without any increase in interface roughness or wire size fluctuations, indicating a self-limiting lateral growth mechanism. Finally, in strained layer epitaxy, (InGa)As islands can be selectively placed on the mesa top and bottom leaving entirely free the curved part along the fast growing side-wall.     

Photo- and cathodoluminescence of AlGaAs single quantum wires on vicinal GaAs (110) surfaces

AlGaAs quantum wires are naturally formed by the compositional modulation in an AlGaAs layer on vicinal GaAs (110) surfaces with quasi-periodic giant growth steps by molecular beam epitaxy. We put an Al mask with a 0.3 μm wide slit on the sample surface to get the single quantum wire's photoluminescence (PL). Two sharp PL bands are observed through this slit and are concluded to come from the two single quantum wires. Cathodoluminescence (CL) spot spectra with a small excitation area show the fine structures with several sharp lines which originate from different quantum wires. The monochromatic CL images clearly show the individual single quantum wires.     

Studies on the structure of crescent-shaped GaAs quantum wires by combination of electron microscopy and photoluminescence spectroscopy

Crescent-shaped GaAs quantum wires were fabricated on a V-grooved GaAs(001) substrate using a flow rate modulation epitaxy technique in metalorganic chemical vapor phase deposition method. The microstructures of the quantum wires were investigated using electron microscopy. The optimum growth conditions for the quantum wire superlattice structures were discussed from the observed microstructures. The optical properties of the fabricated single quantum wires were also investigated using photoluminescence spectroscopy. The relationship between the microstructures of the quantum wires and the observed optical properties is discussed on the basis of computer simulations of the electronics structure.     

InP/InAlAs/InGaAs quantum wires

Single InGaAs quantum wires and stacked InGaAs quantum wires with InAIAs barriers have been fabricated on V-grooved InP substrates by low pressure metal-organic chemical vapour deposition (MOCVD). We have found growth conditions where the InAIAs barrier exhibits a resharpening effect, similar to that of AlGaAs utilized for growth on GaAs substrates. The existence of structural and electronic quantum wires in the bottom of the grooves is proven.     

In-situ monitoring, structural, and optical properties of ultrathin GaSb/GaAs quantum wells grown by OMVPE

We present a study of the growth of strained ultrathin GaSb quantum well (QW) layers in a GaAs host crystal by organometallic vapor phase epitaxy (OMVPE). We report surface anisotropy features observed by reflectance difference spectroscopy (RDS) during exposure of the GaAs (001) surface to trimethylantimony (TMSb) and during subsequent growth interruption. We demonstrate the formation of a floating layer of Sb during growth of GaAs over GaSb quantum well layers. The periodic nature of the RDS signal during growth of multiple quantum well (MQW) structures allows us to construct time-resolved RDS spectra, detailing the evolution of the surface anisotropy. We show how x-ray diffraction (XRD) data may be used to determine the graded compositional profile resulting from Sb segregation at the GaAs/GaSb interface. Photoluminescence (PL) spectra at 2 K from MQW structures exhibit two peaks below the GaAs bandgap. The lower-energy peak, which we attribute to a type-II transition at the GaSb/GaAs interface, shifts logarithmically with excitation power density. The higher energy peak shows no shift with excitation power, and is attributed to a transition occurring within the graded barrier layers.     

GaAs（331）A衬底上分子束外延生长自组织InAs纳米结构形貌演化机制

采用分子束外延方法在GaAs(331)A高指数衬底上制备自对齐InAs量子线(QWR)或者三维(3D)岛状结构。InAs量子线(QWR)选择性生长在GaAs层的台阶边缘。通过原子力显微镜(AFM)仔细研究了InAs纳米微结构的表面形貌，发现不同的生长条件如衬底温度、生长速率和InAs层厚度等，对InAs表面形貌有很大的影响。低温更容易导致线状纳米微结构的形成，而高温更利于3D岛状结构形成。表面形貌的转变归结于表面能同应变能之间的竞争。     

Calculation of the surface characteristics and pressures of InAs quantum dots in a GaAs matrix

A theoretical model for calculating the energy characteristics of surfaces of InAs quantum dots in a GaAs(100) matrix is described. The model is based on notions of nonequilibrium thermodynamics and surface physics. The results of calculating the magnitudes of the surface energy and adhesion physical quantities as well as pressures in the vicinity of the edges of InAs quantum dots in a GaAs(100) matrix are presented. The causes of bending of the profile of the lower part of the quantum dot are presented using the Young relationship. These results can be used to asses the stress-relaxation mechanisms during the course of the selforganization of InAs quantum dots in a GaAs(100) matrix.     

Lateral ordering of quantum dots and wires in the (In,Ga)As/GaAs(100) multilayer structures

The surface morphology and optical properties of the (In,Ga)As/GaAs(100) multilayer structures with self-organized quantum dots and quantum wires, which were grown by molecular-beam epitaxy, are investigated. It is found that the ordered arrangement of quantum dots in the heterointerface plane starts to form during the growth of the first periods of the multilayer structure. As the number of periods increases, quantum dots line up in series and form wires along the \([0\bar 11]\) direction. An increase in the lateral ordering of the structures under consideration correlates with an increase in the optical emission anisotropy governed by relaxation anisotropy of elastic strains and by the shape of nano-objects. A possible mechanism of lateral ordering of quantum dots and wires in multilayer structures, which includes both anisotropy effects of the strain fields and adatom diffusion, as well as the elastic interaction of neighboring quantum dots, is discussed.     

Atomic hydrogen assisted molecular beam epitaxy on patterned GaAs (311)A substrates: Formation of highly uniform quantum-dot arrays

The idea of combining self-organized growth with growth on patterned substrates to produce new types of nanostructures in a controlled manner is realized in atomic hydrogen assisted molecular beam epitaxy (MBE) on patterned GaAs (311)A substrates. In conventional MBE on patterned substrates mesa stripes along [01 ] develop a fast growing sidewall to form quasi-planar lateral quantum wires having a smooth, convex curved surface profile. In atomic hydrogen assisted MBE, the surface naturally develops quasiperiodic one-dimenional step arrays by step bunching along [ 33], i.e., perpendicular to the wire direction with a lateral periodicity around 40 nm. The step array is maintained over the curved sidewall without displacement. Thus, a dense array of dotlike nanostructures is realized with precise control of the position on the substrate surface. High uniformity of the dot array is revealed in micro-photoluminescence spectroscopy with the emission dominated by one single sharp line.     

Molecular beam epitaxy of quantum wires and quantum dots on patterned high-index substrates

Using a novel growth mechanism on patterned high-index GaAs (311)A substrates we have developed a new concept to fabricate quantum wires and quantum dots as well as coupled quantum wire-dot arrays by molecular beam epitaxy. The combination of self-organized growth with lithographic patterning and the assistance of atomic hydrogen produces these quasi-planar lateral nanostructure arrays with unprecedented uniformity in size and composition and with controlled positioning on the wafer. The sought for one- and zero-dimensional nature of these quantum wire and quantum dot arrays manifests itself in the superior optical properties. To functionalize our lateral semiconductor quantum wire and quantum dot arrays with the properties of magnetic thin films, epitaxial Fe layers have been grown on GaAs (311)A. Defect free Fe layers are obtained on As-saturated GaAs surfaces. The large electrical conductivity of thin Fe layers indicates reduced Fe-GaAs interface compound formation. An unusual in-plane spontaneous Hall-effect is observed in these epitaxial Fe layers of reduced symmetry.     

Magnetotransport phenomena in single AlGaAs/GaAs quantum wires grown on laterally patterned substrates

Single, sub-micrometer wide quantum wires have been fabricated using molecular beam epitaxy on mesa-etched GaAs substrates, where the GaAs wire is embedded in AlGaAs. By using a Hall bar pattern, potential probes were directly attached to the wires while grown. To avoid problems associated with anisotropic etching and regrowth, the wire structures were oriented along the 100 crystallographic directions. From this fabrication technique, described in detail by Shitara et al., Appl. Phys. Lett. 66, 2385 (1995), one can expect the formation of high quality “self-aligned” quantum wires with a confinement potential determined by the conduction band discontinuity of AlGaAs and GaAs. Here we study the four-point magnetoresistance of 50 μm long single quantum wires with widths between 250 and 700 nm from 1.3 to 21 K. A distinct weak localization peak and universal conductance fluctuations dominate the low magnetic field regime and are used to estimate the phase-coherence length of the electrons. Pronounced 1/B periodic quantum oscillations at magnetic fields above 1 T are consistent with the picture of wires with a square-well shaped confining potential.     

3D-to-1D carrier scattering in GaAs V-groove quantum wires

Epitaxial growth on nonplanar substrates is an attractive method for producing high quality quantum wire structures for applications in low-threshold lasers. However, a crucial factor in this application is the transfer of carriers between extended (3D) three-dimensional and confined (1D) one-dimensional states. In this paper we present a joint theoretical and experimental investigation of 3D-to-1D scattering in GaAs V-groove quantum wires confined either, within GaAs/AlAs short-period superlattice or bulk GaAlAs barriers. The scattering rates were computed using accurate wavefunctions for both the one-dimensional and the extended three-dimensional states. The rates are in good agreement with time-resolved luminescence measurements.     

Fabrication of GaAs quantum wire structure using metal organic molecular beam epitaxy

GaAs quantum wires (100*20 nm/sup 2/) buried in AlAs layers have been successfully fabricated using metal organic molecular beam epitaxy (MOMBE) for the first time. The underlying growth mechanism is that, under appropriate As/sub 4/ pressure in MOMBE, GaAs preferentially grows only on the sidewalls of the patterned     

Influence of segregation effects on the electroluminescence spectra of InGaAs/GaAs quantum well heterostructures grown by H-MOVPE

Surface segregation during epitaxial growth of stressed InGaAs/GaAs quantum-well heterostructures significantly distorts the nominal concentration profile of quantum wells. The consideration of the effect for growth conditions and elastic stresses appearing during epitaxy on segregation made it possible to simulate the concentration profile with a high accuracy and to calculate the electroluminescence wavelength of actual InGaAs/GaAs heterostructures with quantum wells. It was shown that the observed effect of the long-wave-length shift for the interband transition wavelength in the important case of heterostructures with two neighboring quantum wells is caused by the influence of elastic stresses during growth.     

In segregation effects during quantum dot and quantum ring formation on GaAs(001)

In segregation during InAs growth on GaAs(001) is studied using a real time, in situ technique capable of measuring sample accumulated stress. A 50% surface In segregation of liquid-like stress free matter is deduced. A picture of growth below critical thickness for quantum dot formation is discussed on the basis of the equilibrium between pseudomorphic InAs and liquid In dominated by the stress energy. Quantum rings are produced when large (>10 nm height) quantum dots are covered with 2 nm of GaAs cap. A formation mechanism of the rings is presented. The possibility of tailoring photoluminescence emission through control over size and shape is demonstrated.     

Self-organized GaAs patterns on misoriented GaAs (1 1 1)B substrates using dilute nitrides by molecular beam epitaxy

Recently, the growth of patterned surfaces is being used to demonstrate the site control of the three-dimensional nanostructures, and in particular quantum dots. Nevertheless the pre-patterning techniques show some disadvantages. In this work, we report a novel in situ hole-patterning technique which consists of growing by molecular beam epitaxy a dilute nitride GaAsN layer on 1° and 2° towards [2¯ 1 1] misoriented GaAs(1 1 1)B substrates. Later, we carry out a regrowth of GaAs layers on this patterned surfaces in order to improve the surface quality and the homogeneity of the characteristics of holes (size, depth, etc.). Consecutively, we use these patterned surfaces to grow InAs quantum dots, whose growth on these misorientations results in a greater difficulty. A structural characterization of the resulting samples, both hole-patterns and quantum dots, has been performed. Besides, we have realized studies of the dependence of the surface morphology on some important parameters (including substrate misorientation, thicknesses of the GaAsN and GaAs layers grown and growth conditions).     

Electrical characteristics of single-gate interference transistors based on various semiconductor materials

The current-voltage and high-frequency characteristics of single-gate interference T transistors based on quantum wires made of various semiconductors, specifically, Si, Ge, GaAs, InAs, GaSb, InSb, GaP, and InP, were studied theoretically. Two scattering mechanisms were taken into account in the T-transistor model in order to assess their effect on the electrical characteristics of devices. The adequacy of the suggested model was verified by comparing the results of simulation with experimental data. The calculations were performed using the QW-NANODEV subsystem for simulating the devices based on quantum wires.     

Study of the Structural and Luminescence Properties of InAs/GaAs Heterostructures with Bi-Doped Potential Barriers

The influence of Bi in GaAs barrier layers on the structural and optical properties of InAs/GaAs quantum-dot heterostructures is studied. By atomic-force microscopy and Raman spectroscopy, it is established that the introduction of Bi into GaAs to a content of up to 5 at % results in a decrease in the density of InAs quantum dots from 1.58 × 10¹⁰ to 0.93 × 10¹⁰ cm^–2. The effect is defined by a decrease in the mismatch between the crystal-lattice parameters at the InAs/GaAsBi heterointerface. In this case, an increase in the height of InAs quantum dots is detected. This increase is apparently due to intensification of the surface diffusion of In during growth at the GaAsBi surface. Analysis of the luminescence properties shows that the doping of GaAs potential barriers with Bi is accompanied by a red shift of the emission peak related to InAs quantum dots and by a decrease in the width of this peak.