Study on growth mechanism of low-temperature prepared microcrystalline Si thin films

Microcrystalline silicon thin films at different growth stages were prepared by hot wire chemical vapor deposition. Atomic force microscopy has been applied to investigate the evolution of surface topography of these films. According to the fractal analysis I it was found that, the growth of Si film deposited on glass substrate is the zero-diffused stochastic deposition; while for the film on Si substrate, it is the finite diffused deposition on the initial growth stage, and transforms to the zero-diffused stochastic deposition when the film thickness reaches a certain value. The film thickness dependence of island density shows that a maximum of island density appears at the critical film thickness for both substrates. The data of Raman spectra approve that, on the glass substrate, the a-Si: H/mu c-Si:H transition is related to the critical film thickness. Different substrate materials directly affect the surface diffusion ability of radicals, resulting in the difference of growth modes on the earlier growth stage.

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.

Luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates

In this study, we report comparative luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). High-density InGaN quantum dots (QDs) are formed on GaN templates by decreasing the growth temperature and increasing the adatom hopping-barrier through surface passivation. Atomic force microscopy (AFM) has been employed to estimate the size and height of these dots. Photoluminescence (PL) spectra recorded from (1120) InGaN QDs/(1102) sapphire show much stronger emission intensity compared to spectra recorded from (0001) InGaN QDs/(0001) sapphire. Due to the absence of strong spontaneous polarization and piezoelectric field, such (1150) InGaN QDs in the active layers would lead to high efficiency light emitting devices. (c) 2005 Elsevier B.V. All rights reserved.

Extremely low density InAs quantum dots realized in situ on (100) GaAs

Extremely low density self-assembled InAs quantum dots are grown by a combination technique of in situ annealing for 2 min and pause of substrate rotation during molecular beam epitaxy. The surface morphology and structural characteristics of the quantum dots are scrutinized by atomic force microscopy and photoluminescence spectra. It is found that the quantum dot size and density increase as the InAs deposition amount rises. Quantum dots with a density between 2.5 x 10(7) cm(-2) and 2.2 x 10(8) cm(-2) are 2-5 nm in height and 18-39 nm in diameter. It is believed that as-grown InAs nanodots may be of important value for future single quantum dot research.

MBE growth of very short period InAs/GaSb type-II superlattices on (001) GaAs substrates

First, GaSb epilayers were grown on (001) GaAs substrates by molecular beam epitaxy. We determined that the GaSb layers had very smooth surfaces using atomic force microscopy. Then, very short period InAs/ GaSb superlattices (SLs) were grown on the GaSb buffer layer. The optical and crystalline properties of the superlattices were studied by low-temperature photoluminescence spectra and high resolution transition electron microscopy. In order to determine the interface of SLs, the samples were tested by Raman-scattering spectra at room temperature. Results indicated that the peak wavelength of SLs with clear interfaces and integrated periods is between 2.0 and 2.6 mu m. The SL interface between InAs and GaSb is InSb-like.

Investigation of Ge-Si atomic interdiffusion in Ge nano-dots multilayer structure by double crystal X-ray diffraction

The fluctuations of the strained layer in a superlattice or quantum well can broaden the width of satellite peaks in double crystal X-ray diffraction (DCXRD) pattern. It is found that the width of the 0(th) peak is directly proportional to the fluctuation of the strained layer if the other related facts are ignored. By this method, the Ge-Si atomic interdiffusion in Ge nano-dots and wetting layers has been investigated by DCXRD. It is found that thermal annealing can activate Ge-Si atomic interdiffusion and the interdiffusion in the nano-dots area is much stronger than that in the wetting layer area. Therefore the fluctuation of the Ge layer decreases and the distribution of Ge atoms becomes homogeneous in the horizontal Ge (GeSi actually) layer, which make the width of the 0(th) peak narrow after annealing.

Atomic configurations of dislocation core and twin boundaries in 3C-SiC studied by high-resolution electron microscopy

The defects in 3C-SiC film grown on (001) plane of Si substrate were studied using a 200 kV high-resolution electron microscope with point resolution of 0.2 nm. A posterior image processing technique, the image deconvolution, was utilized in combination with the image contrast analysis to distinguish atoms of Si from C distant from each other by 0.109 nm in the [110] projected image. The principle of the image processing technique utilized and the related image contrast theory is briefly presented. The procedures of transforming an experimental image that does not reflect the crystal structure intuitively into the structure map and of identifying Si and C atoms from the map are described. The atomic configurations for a 30 degrees partial dislocation and a microtwin have been derived at atomic level. It has been determined that the 30 degrees partial dislocation terminates in C atom and the segment of microtwin is sandwiched between two 180 degrees rotation twins. The corresponding stacking sequences are derived and atomic models are constructed according to the restored structure maps for both the 30 degrees partial dislocation and microtwin. Images were simulated based on the two models to affirm the above-mentioned results.

A comparison between contactless electroreflectance and photoreflectance spectra from n-doped GaAs on a semi-insulating GaAs substrate

Contactless electroreflectance (CER) and photoreflectance (PR) measurements have been performed on samples with the structure of an n-doped GaAs epitaxial layer on a semi- insulating GaAs substrate. Modulated reflectance signals from the n-GaAs surface and those from the n-GaAs/SI-GaAs interface are superposed in PR spectra. For the case of CER measurement, however, Franz-Keldysh oscillations (FKOs) from the interface, which are observed in PR spectra, cannot be detected. This discrepancy is attributed to different modulation mechanisms of CER and PR. In CER experiments, the electric field modulation cannot be added to the interfacial electric field because of the effective screening by the fast response of carriers across the interface. FKOs from the interface without any perturbation by the surface signals are extracted by subtracting CER spectra from PR spectra.

Estimation of frequency response of directly modulated lasers from optical spectra

A simple method for estimating the frequency responses of directly modulated lasers from optical spectra is presented. The frequency-modulation index and intensity-modulation index of a distributed feedback laser can be obtained through the optical spectrum analyses. The main advantage is that the measurement setup is very simple. Only a microwave source and an optical spectrum analyser are needed and there is no need to use a calibrated broadband photodetector. Experiment shows that the proposed method is as accurate as the swept frequency method using a network analyzer and is applicable to a wide range of modulation powers.

Effect of oxidation on the optical and surface properties of AlGaN

The chemical properties of AlxGa1-xN surfaces exposed to air for different time periods are investigated by atomic force microscopy (AFM), photoluminescence (PL) measurement and X-ray photoelectron spectroscopy (XPS). PL and AFM results show that AlxGa1-xN samples exhibit different surface characteristics for different air-exposure times and Al contents. The XPS spectra of the Al 2p and Ga 2p core levels indicate that the peaks shifted slightly, from an Al-N to an Al-O bond and from a Ga-N to a Ga-O bond. All of these results show that the epilayer surface contains a large amount of Ga and Al oxides. (c) 2006 Elsevier B.V. All rights reserved.

Study on Raman spectra of GaMnAs

Raman spectra of diluted magnetic semiconductor GaMnAs alloy were reported. The coupled plamon-LO-phonon ( CPLP) mode has LO-like polarization properties. With increasing Mn concentration, the CPLP mode shifts to low frequency. The hole density in the alloy was determined from the ratio of the intensity of the CPLP mode to that of the unscreened LO mode in the depletion layer. The hole density increases with the increase of the Mn composition. The Raman spectra of GaMnAs alloy were measured at different temperature. It is confirmed that the hole density in the alloy increases with the increase of the temperature.

Raman evidence for atomic correlation between the two constituent tubes in double-walled carbon nanotubes

Four well-resolved peaks with very narrow linewidths were found in the D-band and G'-band features of double-walled carbon nanotubes (DWNTs). This fact implies the occurrence of additional van Hove singularities (vHSs) in the joint density of states (JDOS) of DWNTs, which is consistent with theoretical calculations. According to their peak frequencies and theoretical analysis, the two outer peaks can be deduced to originate from a strong coupling between the two constituent tubes of commensurate DWNTs and the two inner peaks were curvature-related and assigned to originate from the two tubes with a weak coupling. This observation and elucidation constitute the first Raman evidence for atomic correlation and the resulting electronic structure change of the two constituent tubes in DWNTs. This result opens the possibility of predicting and modifying the electronic properties of DWNTs for their electronic applications.

Silicon doping induced increment of quantum dot density

Low-indium-content self-assembled InGaAs/GaAs quantum dots (SAQD) were grown using solid-source molecular beam epitaxy (MBE) and investigated by atomic force microscopy and photoluminescence (PL) spectroscopy. Silicon, which was doped at different quantum dot (QD) growth stages, markedly increased the density of QD. We obtained high density In0.35Ga0.65As/GaAs(001) quantum dots of 10(11)/cm(2) at a growth temperature of 520degreesC. PL spectra and distribution statistics show the high quality and uniformity of our silicon-doped samples. The density increment can be explained using the lattice-hardening mechanism due to silicon doping.

Energy spectra of two-electron two-dimensional quantum dots confined by elliptical and bowl-like potentials

The laterally confining potential of quantum dots (QDs) fabricated in semiconductor heterostructures is approximated by an elliptical two-dimensional harmonic-oscillator well or a bowl-like circular well. The energy spectrum of two interacting electrons in these potentials is calculated in the effective-mass approximation as a function of dot size and characteristic frequency of the confining potential by the exact diagonalization method. Energy level crossover is displayed according to the ratio of the characteristic frequencies of the elliptical confinement potential along the y axis and that along the x axis. Investigating the rovibrational spectrum with pair-correlation function and conditional probability distribution, we could see the violation of circular symmetry. However, there are still some symmetries left in the elliptical QDs. When the QDs are confined by a "bowl-like" potential, the removal of the degeneracy in the energy levels of QDs is found. The distribution of energy levels is different for the different heights of the barriers. (C) 2003 American Institute of Physics.

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.