Temperature-dependent bimodal size evolution of InAs quantum dots on vicinal GaAs(100) substrates

The effects of growth temperature on the bimodal size distribution of InAs quantum dots on vicinal GaAs(100) substrates grown by metal organic chemical vapor deposition are studied. An abnormal trend of the bimodal size evolution on temperature is observed. With the increase of the growth temperature, while the density of the large dots decreases continually, that of the small dots first grows larger when temperature was below 520 degrees C, and then exhibits a sudden decrease at 535 degrees C. The trend is explained by taking into account the presence of multiatomic steps on the substrates. Photoluminescence (PL) studies show that quantum dots on vicinal substrates have a narrower PL linewidth, a longer emission wavelength, and a larger PL intensity than those of the dots with exact substrates. (c) 2006 American Institute of Physics.

Strain evolution in GaN layers grown on high-temperature AlN interlayers

The strain evolution of the GaN layer grown on a high-temperature AlN interlayer with GaN template by metal organic chemical vapor deposition is investigated. It is found that the layer is initially under compressive strain and then gradually relaxes and transforms to under tensile strain with increasing film thickness. The result of the in situ stress analysis is confirmed by x-ray diffraction measurements. Transmission electron microscopy analysis shows that the inclination of edge and mixed threading dislocations rather than the reduction of dislocation density mainly accounts for such a strain evolution. (c) 2006 American Institute of Physics.

Evolution of mosaic structure in InN grown by metalorganic chemical vapor deposition

Mosaic structure in InN layers grown by metalorganic chemical vapor deposition at various temperatures has been investigated by X-ray diffraction (XRD). With a combination of Williamson-Hall measurement and fitting of twist angles, it was found that variation of growth temperature from 450 to 550 degrees C leads to the variation of the lateral coherence length, vertical coherence length, tilt and twist of mosaic blocks in InN films in a, respectively, monotonic way. In particular, mosaic tilt increases whereas mosaic twist decreases with elevating temperature. Atomic force microscopy shows the morphological difference of the InN nucleation layers grown at 450 and 550 degrees C. Different coalescence thickness and temperature-dependent in-plane rotation of InN nuclei are considered to account for the XRD results. (c) 2006 Elsevier B.V. All rights reserved.

Evolution of the amount of InAs in wetting layers in an InAs/GaAs quantum-dot system studied by reflectance difference spectroscopy

The wetting layer (WL) in InAs/GaAs quantum-dot systems has been studied by reflectance difference spectroscopy (RDS). Two structures related to the heavy-hole (HH) and light-hole (LH) related transitions in the WL have been observed. On the basis of a calculation model that takes into account the segregation effect and exciton binding energies, the amount of InAs in the WL (t(WL)) and its segregation coefficient ( R) have been determined from the HH and LH transition energies. The evolutions of tWL and R exhibit a close relation to the growth modes. Before the formation of InAs dots, t(WL) increases linearly from similar to 1 to similar to 1.6 monolayer (ML), while R increases almost linearly from similar to 0.8 to similar to 0.85. After the onset of dot formation, t(WL) is saturated at similar to 1.6 ML and R decreases slightly from 0.85 to 0.825. The variation of tWL can be interpreted by using an equilibrium model. Different variations of in-plane optical anisotropy before and after dot formation have been observed.

Optical anisotropy and strain evolution of GaAs surfaces at the onset of the formation of InAs quantum dots

By using reflectance difference spectroscopy we have studied the in-plane optical anisotropy of GaAs surfaces covered by ultrathin InAs layers. The strain evolution of the GaAs surface with the InAs deposition thickness can be obtained. It is found that the optical anisotropy and the surface tensile strain attain maximum values at the onset of the formation of InAs quantum dots (QDs) and then decrease rapidly as more InAs QDs are formed with the increase of InAs deposition. The origin of the optical anisotropy has been discussed.

Magnetism and luminescence evolution due to nitrogen doping in manganese-gallium oxide nanowires

We report a new method for large-scale production of GaMnN nanowires, by annealing manganese-gallium oxide nanowires in flowing ammonia at high temperature. Microstructure analysis indicates that the GaMnN nanowires have wurtzite GaN structure without Mn precipitates or Mn-related second phases. Magnetism evolution due to nitrogen doping in manganese-gallium oxide nanowires was evaluated by magnetic measurements. Magnetic measurement reveals that the magnetization increases with the increase of nitrogen concentration. Ferromagnetic ordering exists in the GaMnN nanowires, whose Curie temperature is above room temperature. Luminescence evolution was investigated by the cathodoluminesence measurement for a single nanowire and photoluminescence measurement in a temperature range between 10 and 300 K. Experimental results indicate that optical properties can be modulated by nitrogen doping in manganese-gallium oxide nanowires. (c) 2005 Elsevier B.V. All rights reserved.

Evolution of wetting layer of InAs/GaAs quantum dots studied by reflectance difference spectroscopy

For the InAs/GaAs quantum-dot system, the evolution of the wetting layer (WL) with the InAs deposition thickness has been studied by reflectance difference spectroscopy (RDS) in combination with atomic force microscopy and photoluminescence. One transition related to the light hole in the WL has been observed clearly in RDS, from which its transition energy and in-plane optical anisotropy (OA) are determined. The evolution of WL with the InAs dot formation and ripening has been discussed. In addition, the remarkable changes in OA at the onsets of the dot formation and ripening have been observed, implying the mode transitions of atom transport between the WL and the dots.

Size evolution and optical properties of self-assembled InAs quantum dots on different matrix

InAs quantum dots have been grown by solid source molecular beam epitaxy on different matrix to investigate the effect on the structure and optical properties. High density of 1.02 x 10(11) cm(-2) of InAs islands on In0.15Ga0.85As and In0.15Al0.85As underlying layer has been achieved. Atomic force microscopy and photoluminescence spectra show the size evolution of InAs islands on In0.15Ga0.85As underlying layer. A strong 1.3 mum photoluminescence from InAs islands on In0.15Ga0.85As underlying layer and with InGaAs strain-reduced layer has been obtained. Single-mirror light emitting diode structures with InAs quantum dots capped by InGaAs grown on InGaAs layer as active layer were fabricated and the corresponding radiative efficiency was deduced to be as high as 20.5%. Our results provide important information for optimizing the epitaxial structures of 1.3 mum wavelength quantum dots devices. (C) 2003 Elsevier B.V. All rights reserved.

Influences of reactor pressure of GaN buffer layers on morphological evolution of GaN grown by MOCVD

The morphological evolution of GaN thin films grown on sapphire by metalorganic chemical vapor deposition was demonstrated to depend strongly on the growth pressure of GaN nucleation layer (NL). For the commonly used two-step growth process, a change in deposition pressure of NL greatly influences the growth mode and morphological evolution of the following GaN epitaxy. By means of atomic force microscopy and scanning electron microscope, it is shown that the initial density and the spacing of nucleation sites on the NL and subsequently the growth mode of FIT GaN epilayer may be directly controlled by tailoring the initial low temperature NL growth pressure. A mode is proposed to explain the TD reduction for NL grown at relatively high reactor pressure. (C) 2003 Elsevier B.V. All rights reserved.

The evolution of InAs/InAlAs/InGaAlAs quantum dots after rapid thermal annealing

We have studied how the optical properties of InAs self-assembled quantum dots (QDs) grown on GaAs substrate are affected when depositing an InAlAs/InGaAs combination overgrowth layer directly on it by rapid thermal annealing (RTA). The photoluminescence measurement demonstrated that the InAs QDs experiences an abnormal variation during the course of RTA. The model of transformation of InAs-InAlAs-InGaAlAs could be used to well explain the phenomena. (C) 2003 Elsevier Science B.V. All rights reserved.

Mechanisms of the sidewall facet evolution in lateral epitaxial overgrowth of GaN by MOCVD

The lateral epitaxial overgrowth of GaN was carried out by low-pressure metalorganic chemical vapor deposition, and the cross section shape of the stripes was characterized by scanning electron microscopy. Inclined {11-2n} facets (n approximate to 1-2.5) were observed in the initial growth, and they changed gradually into the vertical {11-20} sidewalls in accordance with the process of the lateral overgrowth. A model was proposed utilizing diffusion equations and boundary conditions to simulate the concentration of the Ga species constituent throughout the concentration boundary layer. Solutions to these equations are found using the two-dimensional, finite element method. We suggest that the observed evolution of sidewall facets results from the variation of the local V/III ratio during the process of lateral overgrowth induced by the lateral supply of the Ga species from the SiNx mask regions to the growing GaN regions.

Growth mode and strain evolution during InN growth on GaN(0001) by molecular-beam epitaxy

Epitaxial growth of InN on GaN(0001) by plasma-assisted molecular-beam epitaxy is investigated over a range of growth parameters including source flux and substrate temperature. Combining reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM), we establish a relationship between film growth mode and the deposition condition. Both two-dimensional (2D) and three-dimensional (3D) growth modes of the film are observed. For 2D growth, sustained RHEED intensity oscillations are recorded while STM reveals 2D nucleation islands. For 3D growth, less than three oscillation periods are observed indicating the Stranski-Krastanov (SK) growth mode of the film. Simultaneous measurements of (reciprocal) lattice constant by RHEED suggest a gradual relaxation of the strain in film, which commences during the first bilayer (BL) deposition and almost completes after 2-4 BLs. For SK growth, 3D islanding initiates after the strain has mostly been relieved, presumably by dislocations, so the islands are likely strain free. (C) 2002 American Institute of Physics.

Size and shape evolution of self-assembled coherent InAs/GaAs quantum dots influenced by seed layer

The size and shape Evolution of self-assembled InAs quantum dots (QDs) influenced by 2.0-ML InAs seed layer has been systematically investigated for 2.0, 2.5, and 2.9-ML deposition on GaAs(1 0 0) substrate. Based on comparisons with the evolution of InAs islands on single layer samples at late growth stage, the bimodal size distribution of InAs islands at 2.5-ML InAs coverage and the formation of larger InAs quantum dots at 2.9-ML deposition have been observed on the second InAs layer. The further cross-sectional transmission electron microscopy measurement indicates the larger InAs QDs: at 2.9-ML deposition on the second layer are free of dislocation. In addition, the interpretations for the size and shape evolution of InAs/GaAs QDs on the second layer will be presented. (C) 2001 Elsevier Science B.V. All lights reserved.

Shape evolution of Ge/Si(001) islands induced by strain-driven alloying

The shape evolution of Ge/Si(001) islands grown by ultrahigh vacuum chemical vapor deposition were investigated by atomic force microscopy at different deposition rates. We find that, at low deposition rates, the evolution of islands follows the conventional pathway by which the islands form the pyramid islands, evolve into dome islands, and dislocate at a superdome shape with increasing coverage. While at a high deposition rate of 3 monolayers per minute, the dome islands evolve towards the pyramids by a reduction of the contact angle. The presence of the atomic intermixing between the Ge islands and Si substrate at high deposition rate is responsible for the reverse evolution. (C) 2001 American Institute of Physics.

Annealing-induced evolution of defects in low-temperature-grown GaAs-related materials

Infrared absorption spectroscopy, optical transient current spectroscopy (OTCS), and photoluminescence (PL) spectroscopy are used to investigate the annealing induced evolution of defects in low-temperature (LT)-grown GaAs-related materials. Two LT samples of bulk GaAs (sample A) and GaAs/AlxGa1-xAs multiple-quantum-well. (MQW) structure (sample B) were grown at 220 and 320 degreesC on (001) GaAs substrates, respectively. A strong defect-related absorption band has been observed in both as-grown samples A and B. It becomes weaker in samples annealed at temperatures above 600 degreesC. In sample A, annealed in the range of 600-800 degreesC, a large negative decay signal of the optical transient current (OTC) is observed in a certain range of temperature, which distorts deep-level spectra measured by OTCS, making it difficult to identify any deep levels. At annealing temperatures of 600 and 700 degreesC, both As-Ga antisite and small As cluster-related deep levels are identified in sample B. It is found that compared to the As cluster, the As-Ga antisite has a larger activation energy and carrier capture rate. At an annealing temperature of 800 degreesC, the large negative decay signal of the OTC is also observed in sample B. It is argued that this negative decay signal of the OTC is related to large arsenic clusters. For sample B, transient PL spectra have also been measured to study the influence of the, defect evolution on optical properties of LT GaAs/AlxGa1-xAs MQW structures. Our results clearly identify a defect evolution from AS(Ga) antisites to arsenic clusters after annealing.