Structural and optical properties of InAs quantum dots grown by molecular beam epitaxy

In this work, we have studied the dependence of the size and luminescence of self-assembled InAs quantum dots (SAQDs) on the growth conditions. The SAQDs were grown on GaAs (1 0 0) substrates by molecular beam epitaxy (MBE). Their structural and optical properties were studied by atomic force microscopy (AFM), and photoluminescence spectroscopy (PL). The growth of the InAs SAQDs was in situ monitored by reflection high-energy electron diffraction (RHEED). The shape and size of the InAs SAQDs were significantly affected by the growth temperature and the arsenic over-pressure. We observe a decrease of the SAQDS density and an increase in their height by increasing the growth temperature, and/or decreasing the arsenic over-pressure. This is accompanied by a remarkable red-shift of the PL emission energy from 1.3 to .     

Excitons in single and double GaAs/AlGaAs/ZnSe/Zn(Cd)MnSe heterovalent quantum wells

Exciton photoluminescence spectra, photoluminescence excitation spectra, and magnetophotoluminescence spectra of single (GaAs/AlGaAs/ZnMnSe) and double (GaAs/AlGaAs/ZnSe/ZnCdMnSe) heterovalent quantum wells formed by molecular beam epitaxy are studied. It is shown that the exciton absorption spectrum of such quantum wells mainly reproduces the resonant exciton spectrum expected for usual quantum wells with similar parameters, while the radiative exciton recombination have substantial distinctions, in particular the additional localization mechanism determined by defects generated by heterovalent interface exists. The nature of these localization centers is not currently clarified; their presence leads to broadening of photoluminescence lines and to an increase in the Stokes shift between the peaks of luminescence and absorption, as well as determining the variation in the magnetic g factor of bound exciton complexes.     

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.     

GaAs structures with InAs and As quantum dots produced in a single molecular beam epitaxy process

Epitaxial GaAs layers containing InAs semiconductor quantum dots and As metal quantum dots are grown by molecular beam epitaxy. The InAs quantum dots are formed by the Stranskii-Krastanow mechanism, whereas the As quantum dots are self-assembled in the GaAs layer grown at low temperature with a large As excess. The microstructure of the samples is studied by transmission electron microscopy. It is established that the As metal quantum dots formed in the immediate vicinity of the InAs semiconductor quantum dots are larger in size than the As quantum dots formed far from the InAs quantum dots. This is apparently due to the effect of strain fields of the InAs quantum dots upon the self-assembling of As quantum dots. Another phenomenon apparently associated with local strains around the InAs quantum dots is the formation of V-like defects (stacking faults) during the overgrowth of the InAs quantum dots with the GaAs layer by low-temperature molecular beam epitaxy. Such defects have a profound effect on the self-assembling of As quantum dots. Specifically, on high-temperature annealing needed for the formation of large-sized As quantum dots by Ostwald ripening, the V-like defects bring about the dissolution of the As quantum dots in the vicinity of the defects. In this case, excess arsenic most probably diffuses towards the open surface of the sample via the channels of accelerated diffusion in the planes of stacking faults.     

Structural, luminescent, and transport properties of hybrid AlAsSb/InAs/Cd(Mg)Se heterostructures grown by molecular beam epitaxy

The fabrication of new hybrid AlAsSb/InAs/Cd(Mg)Se heterostructures by molecular beam epitaxy and investigation of their structural, luminescent, and transport properties are reported for the first time. These structures show intense luminescence both in the infrared and in the visible regions of the spectrum. This factor, taken together with structural data, indicates a heterointerface of high quality between the III–V and II–VI layers. A theoretical estimate is made of the relative positions of energy bands in the proposed hybrid structures, indicating that the InAs/CdSe interface is a type-II heterojunction, whereas the InAs/Cd_0.85Mg_0.15Se interface is a type-I heterojunction with a large valence band offset ΔE _v≈1.6 eV. The data obtained on the longitudinal electron transport at the InAs/Cd(Mg)Se heterointerface are in good agreement with the theoretical estimate.     

Hybrid AlGaAs/GaAs/AlGaAs nanowires with a quantum dot grown by molecular beam epitaxy on silicon

Data on the growth features and physical properties of GaAs inserts embedded in AlGaAs nanowires grown on Si(111) substrates by Au-assisted molecular beam epitaxy are presented. It is shown that by varying the growth parameters it is possible to form structures like quantum dots emitting in a wide wavelength range for both active and barrier regions. The technology proposed opens up new possibilities for the integration of direct-band III–V materials on silicon.     

Luminescence properties of InAs/GaAs quantum dots prepared by submonolayer migration-stimulated epitaxy

The results of an investigation of the luminescence properties of an ensemble of InAs quantum dots, obtained by submonolayer migration-stimulated epitaxy on singular and vicinal GaAs(100) surfaces, are reported. The largest width at half-height of the photoluminescence line is observed in samples with a 3° disorientation, indicating that the size-variance of the quantum dots is largest in this case. Quasiequilibrium quantum dots are formed either with a long sample holding time in an arsenic flow or with a larger quantity of deposited indium. Fiz. Tekh. Poluprovodn. 31, 912–915 (August 1997)     

Molecular beam epitaxial growth of site-controlled InAs quantum dots on pre-patterned GaAs substrates

Ex situ electron-beam lithography followed by conventional wet etching has been used to pattern small holes 60–150 nm wide, 13 nm deep in GaAs substrates. These holes act as preferential nucleation sites for InAs dot growth during subsequent overgrowth. By varying either the InAs deposition amount or the thickness of a GaAs buffer layer, the occupancy over the patterned sites can be controlled. Comparison with growth on a planar substrate shows that preferential nucleation occurs due to a reduction in the apparent critical thickness above the pattern site; the magnitude of this reduction is dependent on the dimensions of the initial pattern.     

(In,Mn)As quantum dots: Molecular-beam epitaxy and optical properties

Self-assembled (In,Mn)As quantum dots are synthesized by molecular-beam epitaxy on GaAs (001) substrates. The experimental results obtained by transmission electron microscopy show that doping of the central part of the quantum dots with Mn does not bring about the formation of structural defects. The optical properties of the samples, including those in external magnetic fields, are studied.     

Transport parameters and optical properties of selectively doped Ga(Al)As/Zn(Mn)Se heterovalent structures with a two-dimensional hole channel

The growth of III–V/II–VI:Mn heterostructures with a high hole concentration in the AlGaAs:Be/GaAs/AlGaAs 2D channel situated in the immediate vicinity of the AlGaAs/Zn(Mn)Se heterovalent interface by molecular-beam epitaxy is reported. Despite the decrease in the hole concentration in the GaAs channel upon a decrease in the distance between the channel and the heterovalent interface, the hole concentration reaches a value of 1.5 × 10¹³ cm^?2 at a temperature of 300 K even at the minimum distance of 1.2 nm. Deep profiling by dynamic secondary-ion mass spectrometry confirmed the back diffusion of Mn from ZnMnSe into the III–V part. High hole concentration and the presence of magnetic manganese ions in the GaAs conduction channel determine the interest in the structures as possible objects in which the effect of magnetic ordering in heterogeneous semiconductor systems can be studied.     

Self-organization of quantum dots in multilayer InAs/GaAs and InGaAs/GaAs structures in submonolayer epitaxy

The experimental results of RHEED and scanning tunneling microscopy investigations of multilayer structures of InGaAs/GaAs quantum dots, obtained by submonolayer epitaxy on singular and vicinal GaAS (100) substrates, are reported. The results presented show that spatial ordering of nano-objects exists in multilayer structures for InAs and heteroepitaxial InGaAs layers. Fiz. Tekh. Poluprovodn. 33, 733–737 (June 1999)     

The effect of misorientation of the GaAs substrate on the properties of InAs quantum dots grown by low-temperature molecular beam epitaxy

The structural and optical properties of arrays of InAs quantum dots grown on GaAs substrates at relatively low temperatures (250 and 350°C) and with various degrees of misorientation of the surface are studied. It is shown that low-temperature growth is accompanied by the formation of quantum dot clusters along the dislocation loops on the singular surface and along the steps caused by the surface vicinality on the misoriented surface. The formation of quantum dot clusters leads to the appearance of a new long-wavelength band in the exciton photoluminescence (PL) spectra. It is found that the degrees of polarization of the PL spectral band for clusters of various shapes are different.     

Photoelectric properties of GaAs/InAs heterostructures with quantum dots

The capacitive photovoltage and photoconductivity spectra of GaAs/InAs heterostructures with quantum dots is discussed. For these structures, which were fabricated by metallorganic gas-phase epitaxy, the photosensitivity spectrum has a sawtoothed shape in the wavelength range where absorption by the quantum dots takes place, which is characteristic of a δ-function-like density of states function. The spectra also exhibit photosensitivity bands associated with the formation of single-layer InAs quantum wells in the structure. An expression is obtained for the absorption coefficient of an ensemble of quantum dots with a prespecified size distribution. It is shown that the energy distribution of the joint density of states, the surface density of quantum dots, and the effective cross section for trapping a photon can all be determined by analyzing the photosensitivity spectrum based on this assumption. Fiz. Tekh. Poluprovodn. 31, 1100–1105 (September 1997)     

Self-ordered CdSe quantum dots in ZnSe and (Zn, Mn)Se Matrices Assessed by transmission electron microscopy and photoluminescence spectroscopy

Single and multilayer sheets of self-assembled CdSe [quantum dots (QDs)] were grown by means of molecular beam epitaxy in both ZnSe and (Zn_0.9Mn_0.1)Se matrices. Both types of structure were assessed by means of transmission electron microscopy in the scanning, high-resolution, and diffraction-contrast modes. Complementary results from wider sample areas were obtained by means of photoluminescence spectroscopy. In one of the samples analyzed, a fractional monolayer of MnSe was deposited immediately before the CdSe deposition. A second structure grown under identical conditions, but without the MnSe fractional monolayer, was also analyzed. This comparison provides direct evidence for an enhanced size and shape homogeneity of 3D QDs caused by the presence of a tiny amount of MnSe at the interface. In the multilayer structure, we observed the co-existence of highly strained quasi-2D QDs and CdSe rich aggregates with compositional modulations on certain crystallographic planes in close proximity.     

Structural and optical properties of heterostructures with InAs quantum dots in an InGaAsN quantum well grown by molecular-beam epitaxy

Results obtained in a study of the structural and optical properties of GaAs-based heterostructures with InAs quantum dot layers overgrown with InGaAsN quantum wells are presented. Transmission electron microscopy has been applied to analyze how the thickness of the InGaAsN layer and the content and distribution of nitrogen in this layer affect the size of nanoinclusions and the nature and density of structural defects. It is shown that the size of InAs nanodomains and the magnitude of the lattice mismatch in structures containing nitrogen exceed those in nitrogen-free structures. A correlation between the luminescence wavelength and the size and composition of nanodomains is demonstrated. Furthermore, a correlation between the emission intensity and defect density in the structure is revealed.     

Molecular beam epitaxy of CdSe and the derivative alloys Zn1−x Cd x Se and Cd1−x M x Se

We report the epitaxial growth of CdSe, Zn_1−x Cd _x Se (0 ≤x ≤ 1) and Cd_1−x Mn _x Se (0 ≤x ≤ 0.8) on (100) GaAs. X-ray diffraction (XRD), electron diffraction and transmission electron microscopy (TEM) indicate that all the epilayers have the cubic (zinc-blende) structure of the GaAs substrate. The energy gaps of these materials were measured using reflectivity measurements. We also report the growth of ZnSe/Zn_1−x Cd _x Se superlattices. TEM and XRD measurements show that high quality modulated structures with sharp interfaces are possible.     

Arrays of strained InAs quantum dots in an (In Ga)As matrix, grown on InP substrates by molecular-beam epitaxy

Arrays of strained InAs islands in an (In, Ga)As matrix on an InP(100) substrate are synthesized by molecular-beam epitaxy, and their structural and optical properties are investigated. According to transmission electron microscope and high-energy electron diffraction data, the critical thickness corresponding to the onset of island growth is 3 monolayers. The resulting InAs islands are coherently strained, and their base diameter varies from 20 nm to 90 nm. The formation of islands produces in the photoluminescence spectra a dominant long-wavelength line, which shifts toward lower energies as the effective thickness of the InAs increases. The radiation emitted by the InAs islands spans a wavelength range of 1.65–2 μm. Fiz. Tekh. Poluprovodn. 31, 1256–1260 (October 1997)     

The control of size and areal density of InAs self-assembled quantum dots in selective area molecular beam epitaxy on GaAs (0 0 1) surface

The growth of InAs quantum dots (QDs) on GaAs (0 0 1) substrates by selective area molecular beam epitaxy (SA-MBE) with dielectric mask is investigated. The GaAs polycrystals on the mask, which is formed during growth due to low GaAs selectivity between dielectric mask and epitaxial region in MBE, strongly affect the distribution of InAs QDs on the neighbouring epitaxial regions. It is found that the GaAs polycrystalline regions strongly absorb indium during QD growth, confirmed by microscopic and optical studies. GaAs polycrystalline deposit can be reduced under low growth rate and high-temperature growth conditions. Almost no reduction in QD areal density is observed when there is minimal polycrystalline coverage of the mask.     

Lateral photoconductivity of AlGaAs/InGaAs structures with quantum wells and self-organized quantum dots under interband illumination

The results of studying the special features of the dependence of lateral photoconductivity in AlGaAs/InGaAs structures with quantum dots and quantum wells on the intensity of interband light at low temperatures are reported. It is found that there is a threshold for the increase in photoconductivity. Oscillations of photoconductivity are observed at relatively high pulling fields. The effects of the pulling field and temperature on the photoconductivity are studied. The results are analyzed in terms of the theory of percolation of nonequilibrium charge carriers over localized states, taking into account the relaxation of stresses around quantum dots.     

A high-gain GaAs/AlGaAs n-p-n heterojunction bipolar transistor on(100) Si grown by molecular beam epitaxy

High-gain GaAs/AlGaAs n-p-n heterojunction bipolar transistors (HBT's) on Si substrates grown by molecular beam epitaxy (MBE) have been fabricated and tested. In this structure, an n⁺-InAs emitter cap layer was grown in order to achieve a nonalloyed ohmic contact. Typical devices with an emitter dimension of 50×50 μm² exhibited a current gain as high as 45 at a collector current density of 2×10³ A/cm² with an ideality factor of 1.4. This is the highest current gain reported for HBT's grown on Si substrates. Breakdown voltages as high as 10 and 15 V were observed for the emitter-base and collector-base junctions respectively. The investigation on devices with varying emitter dimensions demonstrates that much higher current gains can be expected