A new method to fabricate InGaN quantum dots by metalorganic chemical vapor deposition

A new method to form nanoscale InGaN quantum dots using MOCVD is reported, This method is much different from a method. which uses surfactant or the Stranski-Krastannow growth mode. The dots were formed by increasing the energy barrier for adatoms, which are hopping by surface passivation, and by decreasing the growth temperature. Thus, the new method can be called as a passivation-low-temperature method. Regular high-temperature GaN films were grown first and were passivated. A low-temperature thin layer of GaN dot was then deposited on the surface that acted as the adjusting layer. At last the high-density InGaN dots could be fabricated on the adjusting layer. Atomic force microscopy measurement revealed that InGaN dots were small enough to expect zero-dimensional quantum effects: The islands were typically 80 nm wide and 5 nm high. Their density was about 6 x 10(10) cm(-2). Strong photoluminescence emission from the dots is observed at room temperature, which is much stronger than that of the homogeneous InGaN film with the same growth time. Furthermore, the PL emission of the GaN adjusting layer shows 21 meV blueshift compared with the band edge emission of the GaN due to quantum confine effect. (C) 2002 Elsevier Science B.V. All rights reserved.

Strain-induced morphological evolution and preferential interdiffusion in SiGe epitaxial film on Si(100) during high-temperature annealing

Strain relaxation in initially flat SiGe film on Si(1 0 0) during rapid thermal annealing is studied. The surface roughens after high-temperature annealing, which has been attributed to the intrinsic strain in the epilayers. It is interesting to find that high-temperature annealing also results in roughened interface, indicating the occurrence of preferential interdiffusion. It is suggested that the roughening at the surface makes the intrinsic strain in the epilayer as well as the substrate unequally distributed, causing preferential interdiffusion at the SiGe/Si interface during high-temperature annealing. (C) 1999 Elsevier Science B.V. All rights reserved.

Strain-induced morphological evolution and preferential interdiffusion in SiGe epitaxial film on Si(100) during high-temperature annealing

Strain relaxation in initially flat SiGe film on Si(1 0 0) during rapid thermal annealing is studied. The surface roughens after high-temperature annealing, which has been attributed to the intrinsic strain in the epilayers. It is interesting to find that high-temperature annealing also results in roughened interface, indicating the occurrence of preferential interdiffusion. It is suggested that the roughening at the surface makes the intrinsic strain in the epilayer as well as the substrate unequally distributed, causing preferential interdiffusion at the SiGe/Si interface during high-temperature annealing. (C) 1999 Elsevier Science B.V. All rights reserved.

Behavior of Pentacene Initial Nucleation on Various Dielectrics and Its Effect on Carrier Transport in Organic Field-Effect Transistor

The influence of dielectric surface energy on the initial nucleation and the growth of pentacene films as well as the electrical properties of the pentacene-based field-effect transistors are investigated. We have examined a range of organic and inorganic dielectrics with different surface energies, such as polycarbonate/SiO2, polystyrene/SiO2, and PMMA/SiO2 bi-layered dielectrics and also the bare SiO2 dielectric. Atomic force microscopy measurements of sub-monolayer and thick pentacene films indicated that the growth of pentacene film was in Stranski-Kranstanow growth mode on all the dielectrics. However, the initial nucleation density and the size of the first-layered pentacene islands deposited on different dielectrics are drastically influenced by the dielectric surface energy. With the increasing of the surface energy, the nucleation density increased and thus the average size of pentacene islands for the first mono-layer deposition decreased. The performance of fabricated pentacene-based thin film transistors was found to be highly related to nucleation density and the island size of deposited Pentacene film, and it had no relationship to the final particle size of the thick pentacene film. The field effect mobility of the thin film transistor could be achieved as high as 1.38 cm(2)/Vs with on/off ratio over 3 x 10(7) on the PS/SiO2 where the lowest surface energy existed among all the dielectrics. For comparison, the values of mobility and on/off ratio were 0.42 cm(2)/Vs and 1 x 10(6) for thin film transistor deposited directly on bare SiO2 having the highest surface energy.

Structure and properties of samarium overlayer and Sm/Rh surface alloy on Rh(100)

The structure and properties of Sm overlayer and Sm/Rh surface alloy have been investigated with Auger electron spectroscopy (AES), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption spectroscopy (TDS). The growth of Sm on Rh(100) at room temperature (RT) appears following the Stranski-Krastanov growth mode and only the trivalent state Sm is observed from XPS results. Thermal treatment of the Sm film at 900 K leads to the formation of ordered surface alloy which shows the c(5 root2 x root2)R45 degrees and c(2 x 2) LEED patterns. Annealing the Sm film at temperature above 400 K makes the binding energy (B.E.) of Sm 3d(5/2) shift to higher energy by 0.7 eV, which indicates charge transfer from Sm to Rh(100) substrate, causing the increase of CO desorption temperature.

Experiments and their analysis for self-assembly growth of GaN quantum dots via MOCVD

In recent years, growth of GaN-based materials-related quantum dots has become a hot topic in semiconductor materials research. Considerable efforts have been devoted to growth of self-assembled quantum dots of GaN-based materials via MOCVD (Metal Organic Chemical Vapor Deposition) and there are a lot of relevant literatures. There is, however, few review papers for the topic. In this paper, different experimental methods for fabrication of quantum dots of GaN-based materials via MOCVD are critically reviewed and the experimental conditions and parameters, which may affect growth of the quantum dots, are analyzed, with an aim at providing some critical reference for the related future experiment research.

Quantum-dot superluminescent diode: A proposal for an ultra-wide output spectrum

We propose a novel superluminescent diode (SLD) with a quantum dot (QD) active layer, which should give a wider output spectrum than a conventional quantum well SLD. The device makes use of inhomogeneous broadness of gain spectrum resulting from size inhomogeneity of self-assembled quantum dots grown by Stranski-Krastanow mode. Taking a design made out in the InxGa1-xAs/GaAs system for example, the spectrum characteristics of the device are simulated realistically, 100-200 nm full width of half maximum of output spectrum can be obtained. The dependence of the output spectrum on In composition, size distribution and injection current of the dots active region is also elaborated.

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

The wetting layers (WL) in InAs/GaAs quantum-dot system have been studied by reflectance difference spectroscopy (RDS), in which two structures related to the heavy-hole (HH) and light-hole (LH) transitions in the WL have been observed. The evolution and segregation behaviors of WL during Stranski-Krastanow (SK) growth mode have been studied from the analysis of the WL-related optical transition energies. It has been found that the segregation coefficient of Indium atoms varies linearly with the InAs amount in WL. In addition, the effect of the growth temperature on the critical thickness for InAs island formation has also been studied. The critical thickness defined by the appearance of InAs dots, which is determined by AFM, shows a complex variation with the growth temperature. However, the critical thickness determined by RDS is almost constant in the range of 510-540 degrees C.

High power continuous-wave operation of self-organized In(Ga)As/GaAs quantum dot lasers

Quantum dot (QD) lasers are expected to have superior properties over conventional quantum well lasers due to a delta-function like density of states resulting from three dimensional quantum confinements. QD lasers can only be realized till significant improvements in uniformity of QDs with free of defects and increasing QD density as well in recent years. In this paper, we first briefly give a review on the techniques for preparing QDs, and emphasis on strain induced self-organized quantum dot growth. Secondly, self-organized In(Ga)As/GaAs, InAlAs/GaAlAs and InAs/InAlAs Qds grown on both GaAs and InP substrates with different orientations by using MBE and the Stranski-Krastanow (SK) growth mode at our labs are presented. Under optimizing the growth conditions such as growth temperature, V/III ratio, the amount of InAs, InxGa1-xAs, InxAl1-xAs coverage, the composition x etc., controlling the thickness of the strained layers, for example, just slightly larger than the critical thickness and choosing the substrate orientation or patterned substrates as well, the sheet density of ODs can reach as high as 10(11) cm(-2), and the dot size distribution is controlled to be less than 10% (see Fig. 1). Those are very important to obtain the lower threshold current density (J(th)) of the QD Laser. How to improve the dot lateral ordering and the dot vertical alignment for realizing lasing from the ground states of the QDs and further reducing the Jth Of the QD lasers are also described in detail. Thirdly based on the optimization of the band engineering design for QD laser and the structure geometry and growth conditions of QDs, a 1W continuous-wave (cw) laser operation of a single composite sheet or vertically coupled In(Ga)As quantum dots in a GaAs matrix (see Fig. 2) and a larger than 10W semiconductor laser module consisted nineteen QD laser diodes are demonstrated. The lifetime of the QD laser with an emitting wavelength around 960nm and 0.613W cw operation at room temperature is over than 3000 hrs, at this point the output power was only reduced to 0.83db. This is the best result as we know at moment. Finally the future trends and perspectives of the QD laser are also discussed.

Magnetron Sputtering Growth of InAs0.3Sb0.7 Films on (100) GaAs Substrates: Strong Effect of Growth Conditions on Film Structure

The growth of highly lattice-mismatched InAs0.3Sb0.7 films on (100) GaAs Substrates by magnetron Sputtering has been investigated and even epitaxial lnAs(0.3)Sb(0.7) films have been successfully obtained. A strong effect of the growth conditions on the film structure was observed, revealing that there was a growth mechanism transition from three-dimensional nucleation growth to epitaxial layer-by-layer growth mode when increasing the substrate temperature. A qualitative explanation for that transition was proposed and the critical conditions for the epitaxial layer-by-layer growth mode were also discussed.

Nonpolar growth and characterization of InN overlayers on vertically oriented GaN nanorods

Nanostructured hexagonal InN overlayers were heteroepitaxially deposited on vertically oriented c-axis GaN nanorods by metal-organic chemical vapor deposition. InN overlayers grown in radial directions are featured by a nonpolar heteroepitaxial growth mode on GaN nanorods, showing a great difference from the conventional InN growth on (0001) c-plane GaN template. The surface of InN overlayers is mainly composed of several specific facets with lower crystallographic indices. The orientation relationship between InN and GaN lattices is found to be [0001](InN) parallel to [0001](GaN) and [1100](InN)parallel to[1100](GaN). A strong photoluminescence of InN nanostructures is observed. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3177347]

Epitaxial Growth of AlInGaN Quaternary Alloys by RF-MBE

AlInGaN quaternary alloys were successfully grown on sapphire substrate by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). AlInGaN quaternary alloys with different compositions were acquired by changing the Al cell's temperature. The streaky RHEED patterns were observed during AlInGaN quaternary alloys growth. Scanning Electron Microscope (SEM), Rutherford back-scattering spectrometry (RBS), X-Ray diffraction (XRD) and Cathodoluminescence (CL) were used to characterize the structural and optical properties of the AlInGaN alloys. The experimental results show that the AlInGaN quaternary alloys grow on the GaN buffer in the layer-by-layer growth mode. When the Al cell's temperature is 920 degrees C, the Al/In ratio in the AlInGaN quaternary alloys is about 4.7, and the AlInGaN can acquire better crystal and optical quality. The X-ray and CL full-width at half-maximum (FWHM) of the AlInGaN are 5arcmin and 25nm, respectively.

Structural and optical properties of 3D growth multilayer InGaN/GaN quantum dots by metalorganic chemical vapor deposition

Multilayer InGaN/GaN quantum dots (QDs) were grown on sapphire substrates through a three-dimensional growth mode, which was initiated by a special passivation processing introduced into the normal growth procedure. Surface morphology and photoluminescence properties of QDs with different stacking periods (from one to four) were investigated. The temperature dependences of the PL peak energies were found to show a great difference between two-layer and three-layer QDs. The fast redshift and the reversed sigmoidal temperature dependences of the PL energies for the former were attributed to the thermally activated carrier transfer from small to large dots. However, the increase of both the dot size and the spatial space among dots with the growing stacking periods reduced the carrier escape and retrapping. (C) 2004 Elsevier B.V. All rights reserved.

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photo luminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system. (c) 2004 Elsevier Ltd. All rights reserved.

Monte Carlo simulation of the modulated effect induced by the dislocation to the quantum dot growth

Performing an event-based continuous kinetic Monte Carlo simulation, we investigate the modulated effect induced by the dislocation on the substrate to the growth of semiconductor quantum dots (QDs). The relative positions between the QDs and the dislocations are studied. The stress effects to the growth of the QDs are considered in simulation. The simulation results are compared with the experiment and the agreement between them indicates that this simulation is useful to study the growth mode and the atomic kinetics during the growth of the semiconductor QDs. (c) 2006 Elsevier Ltd. All rights reserved.