In incorporation efficiency and composition fluctuations in MOVPE grown GaInN/GaN hetero structures and quantum wells

GaInN layers play a key role in short wavelength optoelectronic devices for the visible spectrum. However, the epitaxial growth of In containing nitrides is more problematic than that of GaN and AlGaN. In order to increase the In incorporation efficiency, lower growth temperatures of around 700–800°C are needed. We have optimized the metalorganic vapor-phase epitaxial growth of GaInN by decreasing the H₂/N₂ ratio in the gas-phase and increasing the growth rate. However, the deposited films showed strong indications for compositional fluctuations. Besides a large miscibility gap predicted for GaInN, the mismatch induced strain for GaN may play a major role in these growth problems.     

Analysis of defect displacements in GaP film using convergent beam electron diffraction

Sensitivity of the convergent beam electron diffraction on three dimensional strain field was utilized to determine the Burgers Vector of 60° dislocations and the displacement vector of stacking faults in GaP crystal grown by molecular beam epitaxy. Analysis was accomplished from the observations of splitting of HOLZ lines in the convergent electron beam Bragg disc and from splitting of Kikuchi lines associated with the HOLZ Bragg reflection. A limited sucess of analysis was obtained by using the electron beam orientations other than [111]. The CBED method seems to be useful for the determination of non-ZOLZ fault vector components particularly in a specimen situated in a microscope with a limited tilt range.     

Systematic studies of impurities and nitrogen doping of photo-assisted MOVPE grown ZnSe using DESe, DMZn, TEN and TMSiN3

We present a study on the impurity incorporation and nitrogen doping of ZnSe epilayers grown by metalorganic vapour phase epitaxy (MOVPE) on (1 0 0)GaAs using dimethylzinc.triethylamine (DMZn.TEN), diethylselenium (DESe) and trimethylsilylazide (TMSiN3) as the Zn, Se and N precursors, respectively. Both pyrolytic and photoassisted MOVPE (PA-MOVPE) experiments have been carried out to identify the conditions for high purity growth. Characterization included both secondary ion mass spectrometry (SIMS) analysis to assess the incorporation of H, N and halogen impurities in the epilayers and 10 K photoluminescence (PL) measurements. SIMS elemental analysis of halogens in undoped ZnSe shows that the concentration of these impurities is of the order of 1×1015 cm-3, whilst the hydrogen concentration is about 2×1017 cm-3. In nominally undoped ZnSe epilayers an unexpectedly high level of nitrogen, ranging between 3×1016 cm-3 and 1×1018 cm-3, was found. The presence of N in undoped epilayers was confirmed by 10 K PL spectra, which are dominated by a N-related donor-acceptor-pair (DAP) band along with its LO-phonon replica and weaker bound exciton features in the near band-edge region. Finally, intentionally nitrogen doped ZnSe samples were grown by using TMSiN3 under PA-MOVPE conditions at 380 °C. SIMS analysis shows an efficient N incorporation up to 1×1020 cm-3 but increasing the N precursor partial pressure causes the growth rate to decrease together with an increase of the H content in the layers. 10 K PL spectra of doped ZnSe show a sharp nitrogen bound exciton peak in the near band-edge region along with dominant features ascribed to a free electron to acceptor transition at 2.710 eV and to a DAP band at around 2.695 eV, followed by their LO-phonon replica. © 1998 Chapman & Hall     

Surface characterization of GaN and AlGaN layers grown by MOVPE

Surface properties of GaN and Al_0.17Ga_0.83N materials grown by metal organic vapor phase epitaxy (MOVPE) were systematically investigated by X-ray photoelectron spectroscopy (XPS). Air-exposed samples showed highly non-stoichiometric surfaces, which included a large amount of natural oxides. Deposition of Al on the air-exposed GaN surface caused interfacial reactions, resulting in the formation of oxide layers including Al₂O₃ and Ga oxide at the interface. A natural oxide layer of AlGaN surface possessed a complicated composition distribution in depth where the Al-oxide component was dominant on the topmost layer. Such natural oxide layers were found to be removed from GaN and AlGaN surfaces after the treatment in an NH₄OH solution at 50°C for 10 min, resulting in oxide-free and well-ordered surfaces.     

Investigation of the interface properties of MOVPE grown AlGaN/GaN high electron mobility transistor (HEMT) structures on sapphire

We have developed a virtual GaN substrate on sapphire based on a two-step growth method. By optimizing the growth scheme for the virtual substrate we have improved crystal quality and reduced interface roughness. Our Al_0.22Ga_0.78N/GaN HEMT structure grown on the optimized semi-insulating GaN virtual substrate, exhibits Hall mobilities as high as 1720 and 7350 cm²/Vs and sheet carrier concentrations of 8.4 × 1012 and 10.0 × 1012 cm^− 2 at 300 K and 20 K, respectively. The presence of good AlGaN/GaN interface quality and surface morphology is also substantiated by X-Ray reflectivity and Atomic Force Microscopy measurements. A simplified transport model is used to fit the experimental Hall mobility.     

A study of transition regions in InAsPSb/InAs heterostructures grown by MOVPE

A study by secondary-ion mass spectrometry of InAs _x P _y Sb_1–x–y/InAs heterostructures (x > 0.55) grown by vapor-phase epitaxy for lattice-mismatched with substrates samples revealed a noticeable and extended (~800 nm) exponential variation of the As and P content (y up to 0.12) across the layer thickness. The lattice mismatch calculated from the experimentally determined distribution of the As and P components was the strongest at the interface between the epitaxial layer and the substrate and decreased away from the heterointerface into the epitaxial layer.     

MOVPE of structures with aluminum nanocluster layers in a GaAs matrix

Metalorganic vapor phase epitaxy (MOVPE) was used to grow semiconductor structures comprising a GaAs single crystal matrix with incorporated layers of aluminum nanoclusters (Al-NCs). A new regime of GaAs overgrowth on Al-NC layers is proposed, which ensures planarization of the semiconductor layer surface at a thickness comparable with the height of Al-NCs.     

MOVPE grown self-assembled and self-ordered InSb quantum dots in a GaSb matrix assessed by AFM, CTEM, HRTEM and PL

Self-assembled InSb quantum dots (QDs) were grown by metal-organic vapour phase epitaxy (MOVPE) in a GaSb matrix. Atomic force microscopy (AFM), conventional diffraction contrast transmission electron microscopy (CTEM), high resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) were used for the assessment of the QDs. Reductions in the ^III/_V ratios and growth rates resulted in a change of the morphology of the InSb islands from hillocks without facets, and a low level of order to dumbbell shaped islands with distinct facets and a higher level of order.     

Analysis of high-speed-deformation-induced defect structures using heterogeneity parameter of dislocation distribution

An appropriate parameter, named the heterogeneity parameter, is introduced for quantitatively representing the nature of distribution of deformation-induced dislocations. The parameter assumes values ranging from 0 to 1, the extremes corresponding to homogeneous and heterogeneous distribution, respectively. The parameter can represent distribution of intermediate nature, typified by stochastically random distribution which has a heterogeneity parameter of about 0.35. Heterogeneity parameter is used to analyze variation in dislocation structure in many kinds of metals for a wide range of deformation speed from 10⁻² to 10⁷ s⁻¹ in strain rate. The parameter has large values at slow-speed-deformation, and decreases with increasing strain rate, reaching the level of random distribution at a deformation speed of around 10⁴ s⁻¹. Above this strain rate, formation of vacancy clusters increases remarkably. On the basis of these results of analysis, occurrence of dislocation-free plastic deformation during high-speed-deformation is proposed.     

Influence of silane flow on MOVPE grown GaN on sapphire substrate by an in situ SiN treatment

In this paper, the influence of silane flow on metalorganic vapour phase epitaxy (MOVPE) grown GaN on sapphire substrate by an in situ SiN treatment has been investigated. A flow of 10 sccm with treatment duration of 120 s appears to be the optimal value and improves the crystal quality. The dislocation density, determined by atomic force microscopic (AFM), is as low as 5×10⁸ cm⁻². A reduction of I₂ full width at half maximum (FWHM) to 4 meV and an increase of both BE/YL and BE/DAP intensity ratio are also obtained.     

Large-area, high-speed patterning of carbon nanotubes using material-assisted excimer laser photoablation

Large-area patterning of carbon nanotubes (CNTs) using a nonlithographic process is demonstrated. Projection imaging with deep ultraviolet radiation from 248 nm KrF excimer laser and material-assisted photoablation were used to pattern the CNTs. A matrix of CNTs dissolved in a DMF solution was deposited on a silicon wafer by spin coating, followed by coating of photodefinable polyimide on the CNTs. The CNTs and the polyimide layer were simultaneously patterned by the excimer laser projection photoablation process. Even though CNTs cannot be directly photoablated by low-fluence excimer laser radiation, simultaneous patterning of the illuminated CNT-polyimide combination region occurred due to the physical force of dissociated fragments of polyimide layer. We have demonstrated clean, large-area patterning of CNTs on 100 mm diameter Si wafers. Additionally, this patterning process is economical and provides higher throughput compared with conventional methods.     

Atomistic defect structures of Co3Ti containing boron,carbon and beryllium

The lattice properties of the L1₂-type Co₃Ti alloys doped with boron, carbon and beryllium were investigated by the X-ray diffraction analysis. The solubility limits of boron, carbon and beryllium into the Co₃Ti were shown to be 3.0, 0.2, and 0.6 at%, respectively. It was shown that the doping of boron enhanced both of the lattice parameter and the atomic ordering of the alloy, while the doping of carbon or beryllium reduced both of them. Based on these experimental results and the consideration for size (difference) and energetics between the constituent atoms and the additive atoms, it was proposed that boron occupied on the octahedral interstitial site, and carbon and beryllium occupied on the substitutional sites of cobalt and titanium respectively.     

Comparison of static response of structures using convex models and interval analysis method

In this paper, by combining the finite element analysis and non‐probabilistic convex models, we present the numerical algorithm of non‐probabilistic convex models and interval analysis method for the static displacement of structures with uncertain‐but‐bounded parameters. Under the condition of the box or interval vector determined from the ellipsoid of the uncertain‐but‐bounded structural parameter vector, by comparing the numerical algorithm of non‐probabilistic convex models and the interval analysis method in the mathematical proof and the numerical example, we can see that the width of the maximum or upper and minimum or lower bounds on the static displacement yielded by the numerical algorithm of non‐probabilistic convex models is tighter than those produced by the interval analysis method. Copyright © 2003 John Wiley & Sons, Ltd.     

Simulation of defect complex migration in ordered intermetallic structures

We examine how the interaction between different defects influences the atom diffusivity in intermetallic systems. To solve this problem, a new model was developed which allows one to simulate the effect of the interaction of defects on their diffusivities. Based on this model, we have developed a computer program to model diffusion processes in ordered structures. For this aim we used two methods: the Monte Carlo method and the method of static relaxation. Very competitive results are obtained. These results allow us to speak with confidence about the existence of a new diffusion mechanism, which is called the dynamic pair (DP) mechanism. This mechanism is typical for the ordered structures.     

国产金属有机化合物TMGa,TMAl,TMIn和TMSb的MOVPE鉴定

利用我们研制的常压ＭＯＶＰＥ设备对国产ＴＭＧａ、ＴＭＡｌ、ＴＭＩｎ和ＴＭＳｂ进行了鉴定，为此分别生长了ＧａＡｓ、ＡｌＧａＡｓ、ＩｎＰ、ＧａＳｂ外延层和ＧａＡｓ／ＡｌＡｓ、ＧａＳｂ／ＩｎＧａＳｂ超晶格和ＧａＡｓ／ＡｌＧａＡｓ量子阱结构。表征材料纯度的７７Ｋ载流予迁移率分别达到ＧａＡｓ：μ＿ｎ＝５６６００ｃｍ￣２／Ｖ·ｓ，Ａｌ＿（０．２５）Ｇａ＿（０．７５）Ａｓ：μ＿ｎ＝５１６０ｃｍ￣２／Ｖ·ｓ，ＩｎＰ：μ＿ｎ＝６５３００ｃｍ￣２／Ｖ·ｓ，ＧａＳｂ：μ＿ｐ＝５０７６ｃｍ￣２／Ｖ·ｓ。由１０个周期的ＧａＡｓ／ＡｌＡｓ超晶格结构组成的可见光区布拉格反射器已观测到很好的反射光谱和双晶Ｘ射线回摆曲线上高达±２０级的卫星峰。ＧａＡｓ／Ａｌ＿（０．３５）Ｇａ＿（０．６５）Ａｓ量子阱最小阱宽为１０，在ｌｉＫ下由量子尺寸效应导致的光致发光峰能量移动为３９０ｍｅＶ，其线宽为１２ｍｅＶ。这些结果表明上述金属有机化合物已达到较高质量。     

High-frequency homogenization for checkerboard structures: defect modes, ultrarefraction, and all-angle negative refraction

The counterintuitive properties of photonic crystals, such as all-angle negative refraction (AANR) [J. Mod. Opt.34, 1589 (1987)] and high-directivity via ultrarefraction [Phys. Rev. Lett.89, 213902 (2002)], as well as localized defect modes, are known to be associated with anomalous dispersion near the edge of stop bands. We explore the implications of an asymptotic approach to uncover the underlying structure behind these phenomena. Conventional homogenization is widely assumed to be ineffective for modeling photonic crystals as it is limited to low frequencies when the wavelength is long relative to the microstructural length scales. Here a recently developed high-frequency homogenization (HFH) theory [Proc. R. Soc. Lond. A466, 2341 (2010)] is used to generate effective partial differential equations on a macroscale, which have the microscale embedded within them through averaged quantities, for checkerboard media. For physical applications, ultrarefraction is well described by an equivalent homogeneous medium with an effective refractive index given by the HFH procedure, the decay behavior of localized defect modes is characterized completely, and frequencies at which AANR occurs are all determined analytically. We illustrate our findings numerically with a finite-size checkerboard using finite elements, and we emphasize that conventional effective medium theory cannot handle such high frequencies. Finally, we look at light confinement effects in finite-size checkerboards behaving as open resonators when the condition for AANR is met [J. Phys. Condens. Matter 15, 6345 (2003)].     

A TEM investigation of Zn3As2 grown on (001) and (111) InP by MOVPE

A TEM investigation of MOVPE grown Zn₃As₂ revealed the presence of a thin epitaxial Zn₃P₂ intermediate layer between the InP substrate and the Zn₃As₂ overgrowth. A model of the orientation relationships between Zn₃As₂, Zn₃P₂ and InP is presented. The origin of the Zn₃P₂ layer was ascribed to the diffusion of Zn atoms into the InP substrate, aided simultaneously by P diffusion from the substrate. A simulation of the Zn₃As₂ lattice image is presented for the first time, together with a high resolution transmission electron microscopy (HRTEM) image of the Zn₃As₂/Zn₃P₂ interface.     

Design and fabrication of polymeric micro-optical components using excimer laser ablation

Abstract

Numerical simulation was used to predict the profile of a three-dimensional aspherical microlens and a microprism array. Based on the simulated results, the desired micro-optical lens profile was obtained using excimer laser ablated polyimide. The simulation method applied to excimer laser ablation can significantly reduce the quantity of microablation experiments. Ablated microstructures with surface roughness R _a < 20 nm were successfully achieved for micro-optical components. The excimer laser ablation parameters included laser fluence, shot number, workstation velocity, and repetition rate. Numerical simulation can be applied to predict various profiles of microlens and microprism arrays with different dimensions, the desired geometries being formed by laser ablation.