Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of In_x Ga_1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the In_xGa_1−xN/GaN MQWs with high in compositions.     

Investigation of microstructure and V-defect formation in In<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N/GaN MQW grown using temperature-gradient metalorganic chemical vapor deposition

Temperature-gradient metalorganic chemical vapor deposition (MOCVD) was used to deposit In_xGa_1−xN/GaN multiple quantum well (MQW) structures with a concentration gradient of indium across the wafer. These MQW structures were deposited on low defect density (2×10⁸ cm⁻²) GaN template layers for investigation of microstructural properties and V-defect (pinhole) formation. Room temperature (RT) photoluminescence (PL) and photomodulated transmission (PT) were used for optical characterization, which show a systematic decrease in emission energy for a decrease in growth temperature. Triple-axis x-ray diffraction (XRD), scanning electron microscopy, and cross-sectional transmission electron microscopy were used to obtain microstructural properties of different regions across the wafer. Results show that there is a decrease in crystal quality and an increase in V-defect formation with increasing indium concentration. A direct correlation was found between V-defect density and growth temperature due to increased strain and indium segregation for increasing indium concentration.     

Molecular beam epitaxial growth and characterization of Cd-based II–VI wide-bandgap compounds on Si substrates

We have carried out a detailed study on the growth of Cd-based II–VI compounds on Si substrates using molecular beam epitaxy (MBE). CdTe, CdSe, CdSeTe, and CdZnSeTe layers were nucleated and grown on Si(211) substrates in order to study a broad range of semiconductor properties, such as crystal structure, fundamental bandgap, surface morphology, and defect and dislocation density as a function of the constituent elements. For structural characterization, we used transmission electron microscopy (TEM) and x-ray diffraction, which indicated that cubic Cd_1-xSe_xTe material had been grown on Si substrates throughout the entire composition range. Likewise, photoreflectance (PR) and photoluminescence (PL) were used to measure the optical response in the near bandgap (E_g) region. Results indicated nonrandom ordering of CdSeTe/Si material. Studies on quaternary CdZnSeTe material indicated that Zn and Se concentrations directly impact surface morphology with extremely smooth surfaces obtained as Zn content is decreased.     

Molecular beam epitaxy of CdSe and the derivative alloys Zn1−x Cd x Se and Cd1−x M x Se

We report the epitaxial growth of CdSe, Zn_1−x Cd _x Se (0 ≤x ≤ 1) and Cd_1−x Mn _x Se (0 ≤x ≤ 0.8) on (100) GaAs. X-ray diffraction (XRD), electron diffraction and transmission electron microscopy (TEM) indicate that all the epilayers have the cubic (zinc-blende) structure of the GaAs substrate. The energy gaps of these materials were measured using reflectivity measurements. We also report the growth of ZnSe/Zn_1−x Cd _x Se superlattices. TEM and XRD measurements show that high quality modulated structures with sharp interfaces are possible.     

Microcharacterization of composition modulations in epitaxial ZnSe1-xTex

Molecular beam epitaxial growth of the ZnSe_1-xTe_x (x=0.44-0.47) alloy on vicinal (001) GaAs substrates tilted four, six, and nine degree-[111]A or B results in partial phase separation of the alloy with a vertical modulation between different compositions. Transmission electron microscopy images of samples grown on four degree-tilted substrates showed superlattice-like structures, with periods in the range 13.4-28.9Â. Lattice images reveal diffuse interfaces between light and dark bands. Period variations were detected in isolated regions of some samples. We present evidence that the modulation develops at the growth surface, and remains stable in the bulk at temperatures up to 450°C. Satellite spot pairs with approximate indices (h k 1 + δ) were present near the zinc-blende spots in electron diffraction patterns and x-ray diffraction data, as expected from material with a sinusoidal composition profile. The orientation of the spots reveals that the modulation vector is parallel to the growth direction, rather than to [001]. The [111]A- and B-tilted samples showed significant modulation, while the five degree-[110] and on-axis material showed no detectable modulation. The modulation wavelength did not strongly depend on growth temperature in the range examined (285–335°C). Samples showing composition modulation did not exhibit significantly altered low-temperature luminescence spectra from material with no modulation.     

Diffuse diffracted features and ordered domain structures in GalnP layers grown by organometallic vapor phase epitaxy

Transmission electron diffraction (TED) and transmission electron microscope (TEM) studies have been made of organometallic vapor phase epitaxial Ga_xIn_1−xP layers (x ≈ 0.5) grown at temperatures in the range 570–690°C to investigate ordering and ordered domain structures. TED and TEM examination shows that the size and morphology of ordered domains depend on the growth temperature. The ordered domains change from a fine rod-like shape to a plate-like shape as the growth temperature increases. The domains are of width 0.6∼2 nm and of length 1∼10 nm. Characteristic diffuse features observed in TED patterns are found to depend on the growth temperature. Extensive computer simulations show a direct correlation between the ordered domain structures and such diffuse features. A possible model is suggested to describe the temperature dependence of the ordered domain structure.     

Effect of Ni-doping concentration on structural,optical and magnetic properties of CdSe nanorods

Cd_1−xNi_xSe (x=0.00, 0.05, 0.10 and 0.15) nanorods have been synthesized using solvothermal route. X-ray diffraction (XRD) studies show that the nanorods possess wurtzite phase having hexagonal structure. Transmission electron microscopy (TEM) reveals the formation of nanorods having lengths 150–200 nm and diameters 10–15 nm. This has been further confirmed by high-resolution transmission electron microscopy (HRTEM) wherein lattice fringes corresponding to CdSe have been well observed. Reflectance measurements illustrate the red-shift in band gap with increase of Ni-doping concentration. Raman spectra display the shifting of longitudinal optical (LO) phonon modes, and modes activation due to doping in CdSe nanorods. Magnetic hysteresis (M–H) loops at room temperature reveal the ferromagnetism in nanorods. The saturation magnetization values have been observed to increase upto 10% due to carrier mediated exchange interactions, whereas a decrease has been observed for 15% Ni-doping concentration due to super-exchange interactions.     

Structural and optical properties of copper enriched ZnSe thin films prepared by closed space sublimation technique

Thin films of Zn_1?xCu_xSe (0.00≤x≤0.20) have been prepared by the closed space sublimation technique. Various structural and optical properties have been investigated through X-ray diffraction (XRD), atomic force microscopy (AFM), spectrophotometry, spectroscopic ellipsometry (SE) and Fourier transform infrared spectroscopy (FTIR). The effect of Cu concentration has been observed on the physical properties of Zn_1?xCu_xSe films for varying concentrations of copper. X-ray diffraction patterns show that the films are polycrystalline having preferential orientation along the (111) plane. Full width at half maximum (FWHM) values obtained by XRD show that FWHM decreases up to 10% copper concentration while an opposite trend has been observed beyond this concentration. RMS values calculated by AFM shows that the deposited films have smooth morphology; crystallinity improves with increasing Cu concentration and optimum results are shown with 10% Cu concentration. Various optical parameters i.e. absorption coefficient (α), extinction coefficient (k), reflectance (R), refractive index (n), optical conductivity (σ_op) and electrical conductivity (σ_el) have been determined using transmission spectra at different copper concentrations. From the reflection spectra it is observed that reflectance increases with the increase of copper concentration. The band gap energy has been determined using k spectra at various copper concentrations through spectroscopic ellipsometry. It is found that the band gap energy of the films decreases with the increase of copper concentration while dielectric constant increases. FTIR analysis revealed that the characteristic ZnSe bond stretching–vibrating mode occurs at 670.8 cm^?1.     

Characterization survey of Ga<Subscript>x</Subscript>In<Subscript>1−x</Subscript>As/InAs<Subscript>y</Subscript>P<Subscript>1−y</Subscript> double heterostructures and InAs<Subscript>y</Subscript>P<Subscript>1−y</Subscript> multilayers grown on InP

Low-bandgap, lattice-mismatched Ga_xIn_1−xAs (GaInAs) grown using InAs_yP_1−y (InAsP) compositional-step grades on InP is a primary choice for lightabsorbing, active layers in high-efficiency thermophotovoltaic (TPV) devices. The GaInAs/InAsP double heterostructures (DHs) show exceptional minority carrier lifetimes of up to several microseconds. We have performed a characterization survey of 0.4–0.6-eV GaInAs/InAsP DHs using a variety of techniques, including transmission electron microscopy (TEM). Dislocations are rarely observed to thread into the GaInAs active layers from the InAsP buffer layers that terminate the graded regions. Nearly complete strain relaxation occurs in buried regions of the InAsP grades. The buffer-layer strain prior to deposition of the active layer is virtually independent of the net misfit. Foreknowledge of this buffer-layer strain is essential to correctly lattice match the buffer to the GaInAs active layer.     

Surface and Interface Properties of Metal-Organic Chemical Vapor Deposition Grown a -Plane Mg x Zn1– x O (0?≤ x ≤?0.3) Films

The a-plane Mg _x Zn_1−x O (0 ≤ x ≤ 0.3) films were grown on r-plane () sapphire substrates using metal-organic chemical vapor deposition (MOCVD). Growth was done at temperatures from 450°C to 500°C, with a typical growth rate of ∼500 nm/h. Field emission scanning electron microscopy (FESEM) images show that the films are smooth and dense. X-ray diffraction (XRD) scans confirm good crystallinity of the films. The interface of Mg _x Zn_1−x O films with r-sapphire was found to be semicoherent as characterized by high-resolution transmission electron microscopy (HRTEM). The Mg _x Zn_1−x O surfaces were characterized using scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). Low-energy electron diffraction (LEED) shows well-ordered and single-crystalline surfaces. The films have a characteristic wavelike surface morphology with needle-shaped domains running predominantly along the crystallographic c-direction. Photoluminescence (PL) measurements show a strong near-band-edge emission without observable deep level emission, indicating a low defect concentration. In-plane optical anisotropic transmission was observed by polarized transmission measurements.     

A comparative study of metamorphic InP/InGaAs double heterojunction bipolar transistors with InP and InAIP buffer layers grown by molecular beam epitaxy

We present a comparison of material quality and device performance of metamorphic InGaAs/InP heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy (MBE) on GaAs substrates with two different types of buffer layers (direct InP and graded InAlP buffers). The results show that the active layer of InP-MHBT has more than one order of magnitude more defects than that of the InAlP-MHBT. The InAlP-MHBTs show excellent direct current (DC) performance. Low DC current gain and a high base junction ideality factor from the InP-MHBT are possibly due to a large number of electrically active dislocations in the HBT active layers, which is consistent with a large number of defects observed by cross-sectional transmission electron microscopy (TEM) and rough surface morphology observed by atomic force microscopy (AFM).     

Carbon nanomaterial studied by atomic-force and electron microscopies

It is suggested to use the atomic-force microscopy (AFM) and transmission electron microscopy (TEM) to study carbon material synthesized by catalytic pyrolysis of ethanol. It is shown how AFM and TEM can be employed to determine the geometric parameters of carbon nanofibers and nanotubes, examine their mechanical and adhesion characteristics, and analyze their structure.     

离子束减薄对金属玻璃电镜样品的非均匀刻蚀

本文利用透射电子显微镜和原子力显微镜研究了金属玻璃样品在离子束作用后形成的厚度起伏特征。采用高分辨透射电镜和能谱分析结果表明金属玻璃样品中没有发生明显的结构和成分的变化。利用离子束与样品表面作用过程的粗化机理解释了出现这种特征厚度起伏的原因。这种厚度起伏是由材料本身性质和实验条件共同决定的。通过研究离子束与样品的作用方式,有利于确定合适的透射电镜样品制备方法,有助于利用透射电镜研究非晶样品的微观结构。     

Relaxed silicon-germanium on insulator substrate by layer transfer

The fabrication of 4 in, relaxed Si_1−xGe_x-on-insulator (SGOI) substrates by layer transfer was demonstrated. A high-quality relaxed Si_1−xGe_x layer was grown using ultrahigh vacuum chemical vapor deposition (UHVCVD) on 4 in. Si donor wafers. Thin Si_−xGe_x film (x=0.2 or 0.25) was then transferred onto an oxidized Si handle wafer by bonding and wafer splitting using hydrogen implantation. The resulting relaxed SGOI structures were characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM).     

Structural and chemical characterization of as-deposited microcrystalline indium oxide films prepared by dc reactive magnetron sputtering

Microcrystalline indium oxide (InO_x) films with thickness of 120–1600 nm were prepared by dc reactive magnetron sputtering in various mixtures of oxygen in argon at room temperature. The depositions were carried out onto Corning 7059 glass and silicon substrates. The conductivity of the as-deposited films can change in a controllable and fully reversible manner by about six orders of magnitude by alternately exposing the films to ultraviolet (UV) light (hv≥3.5eV) in vacuum and reoxidizing them in ozone. The microstructure of the films was investigated using transmission electron microscopy (TEM) and electron diffraction. For this purpose, films with a thickness of about 100 nm were deposited onto NaCl substrates. The surface and depth composition of the films were examined using Auger electron spectroscopy (AES) combined with depth profiling analysis. The depth profiles showed that all the films exhibit an extremely good in-depth uniformity, all the way to the interface with the glass substrate, regardless of their thickness. Quantitative Auger and energy dispersive x-ray (EDX) analyses were employed to determine the stoichiometry of the films. An oxygen deficiency of 2–5% has been observed with respect to the stoichiometric composition. The effects of film thickness and oxygen content in the sputtering gas on the stoichiometry were examined. Both AES and EDX analyses confirmed that the stoichiometry is invariant for these parameters.     

Features of molecular-beam epitaxy and structural properties of AlInSb-based heterostructures

AlInSb layers are grown on highly lattice-mismatched GaAs (100) substrates by molecular-beam epitaxy (MBE) and studied in situ by reflection high-energy electron diffraction and ex situ by scanning and transmission electron microscopy (SEM and TEM). It is shown that one feature of AlInSb/GaAs heterostructure features is a high probability of forming microtwins; methods for decreasing their concentration are proposed. To initiate AlInSb growth on GaAs substrates under high lattice-mismatch (∼14.5%) conditions and to stimulate the transition to 2D growth, the GaAs layer surface was preliminarily exposed to an antimony flux followed by deposition of an intermediate AlSb buffer layer. The optimization of initial MBE growth stages of Sb-containing layers on the GaAs surface allows a decrease in the defect density in the GaAs/AlInSb heterostructures more than by two orders of magnitude, including a drastic decrease in the microtwin density. Optimal MBE growth conditions for Al _x Al_{1 − x} Sb are determined in a wide composition range (0 < x < 0.3). The TEM and SEM studies confirm the high structural quality of grown GaAs/AlInSb heterostructures. Hall-effect measurements showed the dependence of the carrier mobility and concentration on the aluminum content in AlInSb layers and allowed preliminary conclusions on scattering mechanisms.     

Investigations on the nanostructures of GaN,InN and InxGa1−xN

A controllable approach to the formation of III- nitride nanocrystalline structures using hydrothermal assisted method is presented. The structural and morphological properties of the prepared nanostructures are analyzed using X-ray diffraction, Fast Fourier Transformation and transmission electron microscope techniques. The temperature dependent structural formation of nitride nanostructures have been systematically investigated using X-ray diffraction. Raman spectra of the samples grown at optimized condition exhibited different phonon modes of the respective nitrides (GaN, InN and In_xGa_1−xN). Nanoparticles and nanorods formation of the indium nitride and indium gallium nitride are observed in the TEM micrographs. FFT analysis revealed that the synthesized III-nitride nanostructures are of good crystalline quality. Nanorods of these nitrides showed better crystalline quality than the nanoparticles in the FFT reflections.     

Investigation of the degradation of smooth SiGe epitaxial layer on Si substrate

The degradation of smooth SiGe epitaxial layer was investigated by transmission electron microscopy (TEM), X-ray reflectivity (XRR) and atomic force microscopy (AFM). It was shown from AFM results that the crosshatch was formed with increasing annealing temperature, which indicated the degradation of smooth surface. The surface degradation was caused by the internal dislocations, which were observed by plan-view TEM (PTEM) and cross-sectional TEM (XTEM). From XTEM, the sharp interface between SiGe top layer and Si substrate was broadened and there were a lot of 60° dislocations formed in SiGe top layer, which resulted in the crosshatch on the surface. The crosshatch was also verified by PTEM.     

Dislocation configurations in Si/SixGe1-x strained layer heterostructures

The details of dislocation node structures in (100) Si/Si_xGe_1-x heterostructures are shown through weak-beam transmission electron microscopy studies. It is shown that the resulting configurations are due to dislocation glide on {100} and {111} planes, as well as the interaction between dissociated dislocations. It is shown that similar observations have been made in bulk deformed silicon. The implications of the observations on the issue of strain relaxation in strained layer heterostructures is also discussed.     

TaN-TiN binary alloys and superlattices as diffusion barriers for copper interconnects

Binary alloys and superlattices of TaN-TiN thin films were grown on Si(100) substrates with a TiN buffer layer using pulsed laser deposition. A special target assembly was used to manipulate the concentrations of these binary component films. The 60% TaN resulted in a TaN (3 nm)/TiN (2 nm) superlattice, while 30% and 70% TaN generated uniform Ta_xTi_1−xN alloys. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) confirmed the single-crystalline nature of these films. Four-point probe resistivity measurements suggest that these alloy and superlattice films have a lower resistivity than pure single-crystalline TaN films. The Cu-diffusion characteristic studies showed that these materials would have the potential as high-temperature diffusion barriers for Cu in ultra-large-scale integration technology.