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.

Effects of reactor pressure on GaN nucleation layers and subsequent GaN epilayers grown on sapphire substrate

The influence of reactor pressure on GaN nucleation layer (NL) and the quality of subsequent GaN on sapphire is studied. The layers were grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrates and investigated by in situ laser reflectometry, atomic force microscope, scanning electron microscope, X-ray diffraction and photoluminescence. With the increase of reactor pressure prior to high-temperature GaN growth, the size of GaN nuclei formed after annealing decreases, the spacing between nucleation sites increases and the coalescence of GaN nuclei is deferred. The optical and crystalline qualities of GaN epilayer were improved when NLs were deposited at high pressure. The elongated lateral overgrowth of GaN islands is responsible for the quality improvement. (C) 2003 Elsevier Science B.V. All rights reserved.

Optical investigation on the annealing effect and its mechanism of GaInAs/GaNAs quantum wells

Thermal annealing of GaInAs/GaNAs quantum wells (QWs) as well as other nitrogen- and indium-contained QW structures grown by molecular beam epitaxy and its effect on optical properties are investigated. The photoluminescence (PL) and photovoltaic (PV) spectra of annealed GaInAs/GaNAs QWs show that the luminescence properties become degraded due to the N diffusion from the GaNAs barrier layers to the GaInAs well layer. Meantime, the annealing-induced blueshift of the PL peak in this QW system is mainly induced by the change of In distribution, suggesting that the In reorganization is greatly assisted by the N-induced defects. The elucidation of annealing effect in GaInAs/GaNAs QW samples is helpful for a better understanding to the annealing effect in the GaInNAs/GaAs QWs. (C) 2003 Elsevier Science B.V. All rights reserved.

X-ray diffraction analysis of MOCVD grown GaN buffer layers on GaAs(001) substrates

In order to understand the growth feature of GaN on GaAs (0 0 1) substrates grown by metalorganic chemical vapor deposition (MOCVD), the crystallinity of GaN buffer layers with different thicknesses was investigated by using double crystal X-ray diffraction (DCXRD) measurements. The XRD results showed that the buffer layers consist of predominantly hexagonal GaN (h-GaN) and its content increases with buffer layer thickness. The nominal GaN (111) reflections with chi at 54.74degrees can be detected easily, while (0 0 2) reflections are rather weak. The integrated intensity of reflections from (111) planes is 4-6 times that of (0 0 2) reflections. Possible explanations are presented. (C) 2003 Elsevier Science B.V. All rights reserved.

Localized exciton dynamics in AlInGaN alloy

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved PL (TRPL) at various temperatures. The fast red-shift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(-t/,tau)13], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed QDs or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent beta on temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered QDs. Furthermore, the localized states are found to have 0D density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature. (C) 2003 Elsevier Science Ltd. All rights reserved.

The effect of inserting strain-compensated GaNAs layers on the luminescence properties of GaInNAs/GaAs quantum well

Photoluminescence (PL) properties of GaInNAs/GaAs quantum wells (QWs) with strain-compensated GaNAs layers grown by molecular beam epitaxy are investigated. The temperature-dependent PL spectra of GaInNAs/GaAs QW with and without GaNAs layers are compared and carefully studied. It is shown that the introduction of GaNAs layers between well and barrier can effectively extend the emission wavelength, mainly due to the reduction of the barrier potential. The PL peak position up to 1.41 mum is observed at the room temperature. After adding the GaNAs layers into QW structures, there is no essential deterioration of luminescence efficiency. N-induced localization states are also not remarkably influenced. It implies that with optimized growth condition, high-quality GaInNAs/GaAs QWs with strain-compensated GaNAs layers can be achieved. (C) 2003 Elsevier Science B.V. All rights reserved.

Method for measurement of lattice parameter of cubic GaN layers on GaAs (001)

An extended technique derived from triple-axis diffraction setup was proposed to measure lattice parameters of cubic GaN(c-GaN) films. The fully relaxed lattice parameters of c-GaN are determined to be 4.5036+0.0004 Angstrom, which is closer to the values of a hypothetical perfect crystal. The speculated zero setting correction (Deltatheta) is very slight and within the range of the accuracy of measurement. Additionally, we applied this method to analyze strain of four different kinds of c-GaN samples. It is found that in-plane strain caused by large lattice mismatch and thermal expansion coefficients mismatch directly influence the epilayer growth at high temperatures, indicating that the relaxation of tensile strain after thermal annealing helps to improve the crystalline quality of c-GaN films and optical properties. (C) 2003 Elsevier Science B.V. All rights reserved.

Influence of semi-insulating InP substrates on InAlAs epilayers grown by molecular beam epitaxy

Tensile-strained InAlAs layers have been grown by solid-source molecular beam epitaxy on as-grown Fe-doped semi-insulating (SI) InP substrates and undoped SI InP substrates obtained by annealing undoped conductive InP wafers (wafer-annealed InP). The effect of the two substrates on InAlAs epilayers and InAlAs/InP type II heterostructures has been studied by using a variety of characterization techniques. Our calculation data proved that the out-diffusion of Fe atoms in InP substrate may not take place due to their low diffusion, coefficient. Double-crystal X-ray diffraction measurements show that the lattice mismatch between the InAlAs layers and the two substrates is different, which is originated from their different Fe concentrations. Furthermore, photoluminescence results indicate that the type II heterostructure grown on the wafer-annealed InP substrate exhibits better optical and interface properties than that grown on the as-grown Fe-doped substrate. We have also given a physically coherent explanation on the basis of these investigations. (C) 2003 Elsevier Science B.V. All rights reserved.

In0.25Ga0.75As growth on low-temperature thin buffer layers formed on GaAs (001) substrate

High-quality In0.25Ga0.75As films were grown on low-temperature (LT) ultra-thin GaAs buffer layers formed on GaAs (0 0 1) substrate by molecular beam epitaxy. The epilayers were studied by atomic force microscopy (AFM), photo luminescence (PL) and double crystal X-ray diffraction (DCXRD), All the measurements indicated that LT thin buffer layer technique is a simple but powerful growth technique for heteroepitaxy. (C) 2002 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.

Crystallographic tilt in GaN layers grown by epitaxial lateral overgrowth

The crystallographic tilt in GaN layers grown by epitaxial lateral overgrowth (ELO) on sapphire (0001) substrates was investigated by using double crystal X-ray diffraction (DC-XRD). It was found that ELO GaN stripes bent towards the SiNx mask in the direction perpendicular to seeding lines. Each side of GaN (0002) peak in DC-XRD rocking curves was a broad peak related with the crystallographic tilt. This broad peak split into two peaks (denoted as A and B), and peak B disappeared gradually when the mask began to be removed by selective etching. Only narrow peak A remained when the SiNx mask was removed completely. A model based on these results has been developed to show that there are two factors responsible for the crystallographic tilt: One is the non-uniformity elastic deformation caused by the interphase force between the ELO GaN layer and the SiNx mask. The other is the plastic deformation, which is attributed to the change of the threading dislocations (TDs)-from vertical in the window regions to the lateral in the regions over the mask.

SiO2/Ta interface reaction in magnetic multilayers and its influence on Ta buffer layers

Ta is often used as a buffer layer in magnetic multilayers. In this study, Ta/Ni81Fe19/Ta multilayers were deposited by magnetron sputtering on sing-crystal Si with a 300-nm-thick SiO2 film. The composition and chemical states at the interface region of SiO2/Ta were studied using the X-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that there is an 'inter-mixing layer" at the SiO2/Ta interface due to a thermodynamically favorable reaction: 15 SiO2 + 37 Ta = 6 Ta2O5 + 5 Ta5Si3. Therefore, the Ta buffer layer thickness used to induce NiFe (111) texture increases.

Structural characteristic of cubic GaN nucleation layers on GaAs(001) substrates by MOCVD

The structural characteristic of cubic GaN (C-GaN) nucleation layers on GaAs(0 0 1) substrates by metalorganic chemical vapor deposition was in detail investigated first by X-ray diffraction (XRD) measurements, using a Huber five-circle diffractometer and an intense synchrotron X-ray source. The XRD results indicate that the C-GaN nucleation layers are highly crystallized. Phi scans and pole figures of the (1 1 1) reflections give a convincing proof that the GaN nucleation layers show exactly cubic symmetrical structure. The GaN(1 1 1) reflections at 54.74degrees in chi are a measurable component, however (002) components parallel to the substrate surface are not detected. Possible explanations are suggested. The pole figures of {1 0 (1) over bar 0} reflections from H-GaN inclusions show that the parasitic H-GaN originates from the C-GaN nucleation layers. The coherence lengths along the close-packed [1 1 1] directions estimated from the (1 1 1) peaks are nanometer order of magnitude. (C) 2002 Elsevier Science B.V. All rights reserved.

A novel line-order of InAs quantum dots on GaAs

A novel line-order of InAs quantum dots (QDs) along the [1, 1, 0] direction on GaAs substrate has been prepared by self-organized growth. After 2.5 monolayer InAs deposition, QDs in the first layer of multi-layer samples started to gather in a line. Owing to the action of strong stress between layers, almost all the dots of the fourth layer gathered in lines. The dots lining up tightly are actually one-dimensional superlattice of QDs, of which the density of electronic states is different from that of isolated QDs or quantum wires. The photoluminescence spectra of our multi-layer QD sample exhibited a feature of very broad band so that it is suitable for the active medium of super luminescent diode. The reason of dots lining up is attributed to the hill-and-valley structure of the buffer, anisotropy and different diffusion rates in the different directions on the buffer and strong stress between QD layers. (C) 2002 Published by Elsevier Science B. V.

Fe-diffusion-induced defects in InP annealed in iron phosphide ambient

Photoluminescence (PL) and photo induced current transient spectroscopy (PICTS) have been used to study deep levels in semi-insulating (SI) InP prepared by annealing undoped InP in pure phosphorus (PP) and iron phosphide (IP) ambient. Defects are much fewer in IP SI-InP than in PP SI-InP. Deep-level-related PL emission could only be detected in IP SI-InP. The results indicate that Fe diffusion inhibits the thermal formation of a number of defects in annealed InP. A complex defect has been formed in the annealing process in the presence of Fe.