Structural and electrical properties of high-quality 0.41 μm-thick InSb films grown on GaAs (1 0 0) substrate with InxAl1−xSb continuously graded buffer

High-quality InSb was grown on a GaAs (1 0 0) substrate with an InAlSb continuously graded buffer (CGB). The temperatures of In, Al K-cells and substrate were modified during the growth of InAlSb CGB. The cross-section TEM image reveals that the defects due to lattice-mismatch disappear near lateral structures in CGB. The measured electron mobility of 0.41 μm-thick InSb was 46,300 cm²/Vs at 300 K. These data surpass the electron mobility of state-of-the-art InSb grown by other methods with similar thickness of InSb.     

Photoluminescence plasmonic enhancement in InAs quantum dots coupled to gold nanoparticles

Photoluminescence enhancement due to dipole field from gold nanoparticles was observed at 77 K for GaAs capped InAs quantum dots. The gold nanoparticles were coupled to the surface of the cap layer by using dithiol ligands. The enhancement was investigated as a function of the GaAs capped layer thickness. An order of magnitude enhancement in the emission was observed in samples with a cap thickness of 12 nm. This enhancement however is drastically decreased in samples with a cap thickness of 200 nm. The observed enhancement is interpreted in terms of photon scattering from the large dipole scattering cross section.     

Effect of Al-doped ZnO film thickness on periodic GaAs subwavelength grating structures for photovoltaic device applications

We investigated the effect of Al-doped zinc oxide (AZO) films with different thicknesses deposited onto periodic cone-shaped GaAs subwavelength grating (SWG) structures on their physical properties. As the AZO deposition time was increased, the surface morphology of AZO deposited GaAs SWGs was changed. These structures exhibited the surface reflection of <～6.8% at 300–1200 nm because of their effective graded index distribution between air and the GaAs substrate via the AZO deposited GaAs SWGs, producing a lowest average reflectance of ～2.1% at 40 min of deposition time. With increasing the deposition time, the crystallinity of the AZO films deposited on GaAs SWGs was enhanced, which leaded to the decrease of the effective resistivity up to ～1.55 × 10^?3 Ω-cm at 100 min. The wetting behavior of a water droplet on the surface of samples was also studied.     

Ultralow-concentration Sm codoping in CsI:Tl scintillator: A case of little things can make a big difference

CsI:Tl scintillators were hindered from computer tomography and high-speed imaging applications by a serious afterglow problem. In this study, the effects of ultralow-concentration Sm codoping on the scintillation characteristics of CsI:Tl were investigated. Pulsed X-ray excited afterglow after 50 ms in 0.005 mol% Sm-codoped CsI:Tl was lowered by over one order of magnitude in comparison with Sm-free one. The beneficial effects of ultralow-concentration Sm codoping also appeared to be maintaining the light yield and energy resolution. The light yield and energy resolution after 0.005 mol% Sm codoping were 71,700 ± 6000 photons/MeV and 6.9% at 662 keV, respectively.     

Effects of SiC interphase by chemical vapor deposition on the properties of C/ZrC composite prepared via precursor infiltration and pyrolysis route

Silicon carbide (SiC) interphase was introduced by chemical vapor deposition (CVD) process to prevent carbon fiber degradation and improve fiber–matrix interface bonding of C/ZrC composite prepared via precursor infiltration and pyrolysis (PIP) process. Moderate thickness of SiC interphase in fiber bundles could increase the density of the composite, but when the thickness of SiC interphase was over 0.5 μm, more close pores formed and the density of the composite decreased. The SiC interphase could protect carbon fiber effectively from carbo-thermal reduction, but could not enhance the mechanical properties of C/ZrC composite. The flexural strength and fracture toughness of C/ZrC composites with 0.05 μm thickness SiC layer were 252 MPa and 13.6 MPa m^1/2, and for those with 0.5 μm thickness SiC layer 240 MPa and 12.8 MPa m^1/2, both close to the value of the composite without SiC interphase (254 MPa and 14.5 MPa m^1/2), while those with 0.7 μm thickness SiC layer were only 191 MPa and 10.8 MPa m^1/2, respectively. Moderate content of SiC interphase could improve the ablation property of C/ZrC composites; however excessive content of SiC interphase would decrease the ablation property.     

Surface morphology of ErP layers on InP and Ga0.52In0.48P

We have grown ErP on Ga_0.52In_0.48P (001) lattice-matched to GaAs and on InP (001) by low-pressure organometallic vapor phase epitaxy and investigated the surface morphology of ErP layers. Lattice-mismatch in ErP/Ga_0.52In_0.48P/GaAs heterostructures (Δa / a = − 0.8%) is much less than that of ErP/InP heterostructures (− 4.5%). Extended X-ray absorption fine structure measurement revealed that Er exists in the form of ErP rock-salt structure in both samples. The estimated growth rate of ErP on Ga_0.52In_0.48P is 1.8 ML/h. Our results demonstrate that ErP on Ga_0.52In_0.48P has smaller surface roughness than ErP on InP. Moreover, an ErP layer exists underneath the surface about 2 ML and the surface roughness does not depend on the ErP thickness in the range of our experiments (2.2–13.7 ML).     

Piezoelectric field in highly stressed GaN-based LED on Si (1 1 1) substrate

Stress states in GaN epilayers grown on Si (1 1 1) and c-plane sapphire, and their effects on built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) were investigated using the electroreflectance (ER) spectroscopic technique. Relatively large tensile stress is observed in GaN epilayers grown on Si (1 1 1), while a small compressive stress appears in the film grown on c-plane sapphire. The InGaN/GaN MQWs of LED on c-plane sapphire substrate has a higher piezoelectric field than the MQWs of LEDs on Si (1 1 1) substrate by about 1.04 MV/cm. The large tensile stress due to lattice mismatch with Si (1 1 1) substrate is regarded as external stress. The external tensile stress from the Si substrate effectively compensates for the compressive stress developed in the active region of the InGaN/GaN MQWs, thus reducing the quantum-confined Stark effect (QCSE) by attenuating the piezoelectric polarization from the InGaN layer.     

Structural analysis of distributed Bragg reflector mirrors

Quantum cascade (QC) lasers and vertical-cavity surface-emitting lasers (VCSELs) are of great interest due to their potential importance for a variety of device applications. Both kinds of lasers call for very highly reflective mirrors. Usually distributed Bragg reflector (DBR) mirrors, which consist of periodic quarter wavelength stacks of high and low refractive index compound semiconductors are used. These stacks are superlattices containing more than 40 individual layers. To obtain very high reflectivity DBRs alternating GaAs and AlAs layers are used for both the high and low index mirrors.GaAs/AlAs DBR structures containing 15 periods were characterized by the complementary use of RBS/channeling, TEM and HRXRD. Since the total thickness of a DBR exceeds 2 μm the RBS analysis was performed at two He-ion energies: 1.7 MeV and 3.82 MeV. After some stopping power corrections TEM and RBS provided similar results. Discrepancies with HRXRD data were attributed to the lateral inhomogeneity of produced superlattice. Virtues and pitfalls of complementary use of these techniques were discussed.     

Low-temperature annealed ohmic contacts to Si-doped GaAs and contact formation mechanisms

Using nonferromagnetic contact materials, Au(x nm)/Ge(y nm)/Pd(z nm) structures (where x, y, and z are the thicknesses of Au, Ge and Pd layers, respectively) are fabricated on Si-doped GaAs and studied as a function of x, y and z and n-type substrate doping density and annealing temperature to characterise them as ohmic contacts. The study shows that the structure with x = 100, y = 40 and z = 10, annealed at 180 °C for 1 h, contacts n-type GaAs more reliably with the low contact resistance. Using Rutherford backscattering spectrometry, contact formation mechanisms are also studied.     

Time-resolved photoluminescence from Si-in-SiNx/Si-in-SiC quantum well-dot structures

Si-in-SiN_x/Si-in-SiC quantum well-dot structures, with Si quantum dots slightly larger than 1.0 nm embedded in both amorphous SiN_x and SiC sublayers, were grown at nearly room temperature by using PECVD. Time-resolved photoluminescence for three samples in a period of 100/30 nm, 60/10 nm and 20/10 nm, respectively, has been measured at emission lengths ranging from 430 nm to 490 nm, and fitted with a stretched-exponential function. Typical decay time was at the order of one nanosecond, which could be attributed to the core-state emission. The matrix materials forming the well provide a non-uniform potential background which induces a modulation to the carrier diffusion process, thus resulting in an emission-wavelength dependent decay time. When confinement effect from the well comes into play as in the sample of smaller well width, the decay time can be below 1.0 ns and indifferent to the varied emission wavelength, and the carrier diffusion is dominated by hopping. These quantum well-dot systems of strong and fast decaying light emission in blue–violet colors might find potential utilization in GHz optical connection and other photoelectronic devices.     

Surface aluminizing on Ti–6Al–4V alloy via a novel multi-pass friction-stir lap welding method: Preparation process,oxidation behavior and interlayer evolution

Surface aluminizing on Ti–6Al–4V alloy was successfully performed via a novel solid-state method of multi-pass friction-stir lap welding (FSLW). The process principles are elucidated. The aluminized coating was tailored as ∼500 μm in thickness after preparation and milling treatment to remove the redundant pure Al. No annealing treatment was performed after the FSLW preparation. The as-processed Ti/Al interlayer was Ti-rich in chemical composition, more than 60 μm in thickness, and had an interval banding structure, due to the FSLW thermal–mechanical effects. Oxidation tests for bare TC4 and aluminized specimens were conducted at 700 °C. Surface morphologies, phases and interlayer evolutions of the oxidized specimens were investigated. The diffusion and/or possible reaction of the Ti/Al interlayer occurred simultaneity corresponding to the oxidation behaviors in the near-surface layer. The oxidation resistant roles of the aluminide interlayer and the upper abundant Al coating produced via the method were discussed. Abundant Al content in the coating of significant thickness benefited the anti-oxidation performance and forming of Ti/Al interlayer. The aluminide interlayer of considerable thickness, with composition gradients and good density at Ti/Al interface location, played a main protection role against oxidation to Ti-substrate.     

In vitro evaluation of osteoconductivity and cellular response of zirconia and alumina based ceramics

Developed ceria/yttria stabilized zirconia and ceria/yttria stabilized zirconia toughened alumina supported formation of apatite layer when immersed in simulated body fluid without any prior surface treatment. The formed mineral layer was confirmed as hydroxyapatite through X-ray diffraction patterns. The calcium/phosphate atomic ratio obtained from energy dispersive X-ray spectroscopy was found to be little less (Ca/P = 1.5) than that of pure hydroxyapatite (Ca/P = 1.7) which indicates the probability of mixed type calcium-phosphate compound formation. The achieved thickness of apatite layer was estimated through a surface profilometer and as high as ~ 17 μm thickness was found after 28 days of soaking. The biocompatibility of the developed materials was ensured through in vitro human osteoblast like cell (MG63) culture on ceramic discs. The morphology of attached cells was characterized through scanning electron microscopy and fluorescent microscopy which show multilayered interconnected cell growth within 8 days of culture period. Moreover, differentiation of MG63 cells was evaluated through MTT assay, total protein content and alkaline phosphatase activity.     

Microwave absorbing behavior of metal dispersed TiO2 nanocomposites

Metal dispersed TiO₂ nanocomposites were prepared by milling process. The microwave absorbing characteristics of the prepared nanocomposites with epoxy were studied in the 8.2–12.4 GHz frequency range for the microwave absorption application. The measured relative complex permittivity of metal dispersed nanocomposite-epoxy indicates higher values in comparison to the pure TiO₂-epoxy nanocomposite. The Reflection loss (RL) values were calculated for thickness from 0.1 to 2.2 mm with an interval of 0.1 mm and the maximum value of RL found for TiO₂-epoxy nanocomposite was −4.96 dB at 10.21 GHz frequency for 2.0 mm thickness. Whereas, RL value is improved to a maximum value of −13.67 dB at 10.13 GHz with Al dispersion (1.8 mm thickness) and −7.24 dB at 10.38 GHz with Ni dispersion (1.3 mm thickness). This study suggests the effectiveness metal particles dispersion for the development of thin microwave absorbers as well as increasing the level of RL.     

Physical,mechanical and morphological properties of polymer composites manufactured from carbon nanotubes and wood flour

The objective of this investigation was to evaluate physical, mechanical and morphological properties of experimental polymer type panels made from single-wall carbon nanotube (SWCNT) and wood flour. The composites with different SWCNTs (0, 1, 2, 3 phc) and maleic anhydride grafted polyethylene (MAPE) (0 and 3 phc) contents were mixed by melt compounding in an internal mixer and then the composites manufactured by injection molding method. The mass ratio of the wood flour to LDPE was 50/50 (w/w) in all compounds. Water absorption, thickness swelling, bending characteristics, impact strength and morphological properties of the manufactured composites were evaluated. Based on the findings in this work the water absorption and thickness swelling of the nanocomposites decreased with increasing with amount of the SWCNTs (from 1 to 3 phc) and MAPE (3 phc) in the panels. The mechanical properties of LDPE/wood-flour composites could be significantly enhanced with increased percentage of MAPE and SWCNTs content. Panels having 2 phc SWCNTs and 3 phc MAPE exhibited the highest impact strength value. Also Scanning Electron Microscope (SEM) micrographs showed that carbon nanotubes can fill the voids of wood plastic composites as well as addition of MAPE and SWCNTs enhanced interaction between the components.     

Experimental study on high strain rate behavior of high strength 600–1000 MPa dual phase steels and 1200 MPa fully martensitic steels

As one of high grade advanced high strength steels (AHSSs), dual phase (DP) steel sheets and fully martensitic (MS) steel sheets have been successfully used in automotive crash-resistance components for its great benefit in reducing vehicle weight while improving car safety as well as their advantage in cost saving through cold forming instead of hot forming. The strain rate sensitivity of 600/800/1000 MPa DP and 1200 MPa MS were studied in this paper through a split Hopkinson tensile bar (SHTB) setup and compared with each other. The experiments showed that all dual phase (DP) AHSS ranging from 600 MPa to 1000 MPa are of positive strain rate sensitivity. While for the tested 1200 MPa MS, negative strain rate sensitivity has been found. Possible reason for the difference has been investigated through metallographical observation and their microstructures.     

Laser surface modification of 316 L stainless steel with bioactive hydroxyapatite

Laser-engineered net shaping (LENS?), a commercial additive manufacturing process, was used to modify the surfaces of 316 L stainless steel with bioactive hydroxyapatite (HAP). The modified surfaces were characterized in terms of their microstructure, hardness and apatite forming ability. The results showed that with increase in laser energy input from 32 J/mm² to 59 J/mm² the thickness of the modified surface increased from 222 ± 12 μm to 355 ± 6 μm, while the average surface hardness decreased marginally from 403 ± 18 HV_0.3 to 372 ± 8 HV_0.3. Microstructural studies showed that the modified surface consisted of austenite dendrites with HAP and some reaction products primarily occurring in the inter-dendritic regions. Finally, the surface-modified 316 L samples immersed in simulated body fluids showed significantly higher apatite precipitation compared to unmodified 316 L samples.     

Effect of residual compressive stress on near-ultraviolet InGaN/GaN multi-quantum well light-emitting diodes

Thinning was investigated to reduce the residual compressive stress in GaN-based near-ultraviolet light-emitting diode (NUV-LED) substrates. This stress has a knock-on effect of reducing piezoelectric fields in the LED structure. As the sapphire substrate thickness is reduced, the compressive stress in the GaN layer is released, resulting in wafer bowing. The wafer bowing-induced mechanical stress alters the piezoelectric fields, which in turn reduces the quantum-confined Stark effect in the InGaN/GaN active region of the LED. The electroluminescence spectral peak wavelength was blue-shifted, and the internal quantum efficiency was improved by about 15% at an injection current of 50 mA. The LED with a 45-μm-thick sapphire substrate exhibited the highest light output power of ∼29 mW at an injection current of 50 mA, an improvement by about 39% compared to that of a 150-μm-thick sapphire substrate without increasing the operating voltage. The simulation results confirm that the relaxation of the compressive strain in the InGaN/GaN MQW structure results in the reduction of the piezoelectric field and improves the overlap of electron and hole wave functions with a corresponding increase in IQE.     

An innovative process to fabricate copper/diamond composite films for thermal management applications

Diamond dispersed copper matrix (Cu/D) composite films with strong interfacial bonding were produced by tape casting and hot pressing without carbide forming additives. The tape casting process offers an original solution to obtain laminated materials with accurate thickness control, smooth surface finish, material net-shaping, scalability, and low cost. This study presents an innovative process of copper submicronic particles deposition onto diamond reinforcements prior to densification by hot pressing. Copper particles act as chemical bonding agents between the copper matrix and the diamond reinforcements during hot pressing, thus offering an alternative solution to traditionnal carbide-forming materials in order to get efficient interfacial bonding and heat-transfer in Cu/D composites. It allows high thermal performances with low content of diamond, thus enhancing the cost-effectiveness of the materials. Microstructural study of composites by scanning electron microscopy (SEM) was correlated with thermal conductivity and thermal expansion coefficient measurements. The as-fabricated films exhibit a thermal conductivity of 455 W m^?1 K^?1 associated to a coefficient of thermal expansion of 12 × 10^?6 °C^?1 and a density of 6.6 g cm^?3 with a diamond volume fraction of 40%, which represents a strong enhancement relative to pure copper properties (λ_Cu = 400 W m^?1 K^?1, α_Cu = 17 × 10^?6 °C^?1, ρ_Cu = 8.95 g cm^?3). The as-fabricated composite films might be useful as heat-spreading layers for thermal management of power electronic modules.     

Morphology and thermal conductivity of polyacrylate composites containing aluminum/multi-walled carbon nanotubes

Polyacrylate composites with various fillers such as multi-walled carbon nanotube (CNT), aluminum flake (Al-flake), aluminum powders and Al–CNT were prepared by a ball milling. The thermal decomposition temperature increased by as much as 64 °C for polyacrylate/Al-flake 70 wt% composite compared to polyacrylate. The thermal conductivity of polyacrylate/Al–CNT composites increased from 0.50 to 1.67 W/m K as the Al–CNT content increases from 50 to 80 wt%. The thermal conductivity of the composite sheet increases with the sheet thickness. At the given filler concentration (90 wt%), the composite filled with aluminum powder of 13 μm has a higher thermal conductivity than the one filled 3 μm powder, and the composite filled with mixture of two powders showed a synergistic effect on the thermal conductivity. The morphology indicates that the dispersion of CNT in the polyacrylate/Al-flake + CNT composite is not perfect, and agglomeration of CNTs was observed.     

The influence of binder tow density on the mechanical properties of spatially reinforced composites. Part 1 – Impact resistance

This paper examines the influence of binder tow stitch density on the impact performance of advanced composite structures. Spatially reinforced composite reinforcements with multi-axis, multi-layer structures were woven on a specially developed loom. The binder tow stitch density, which was used to consolidate the structure, was varied in the range of 1–4 binder tow stitches/cm² (10 × 10 mm to 5 × 5 mm binder tow stitch spacing). A drop weight impact test (6.7 J/mm of composite thickness) was used to damage the samples. Both the depth of penetration and the damage area were measured after impact. The analysis of the results has shown that as the binder tow stitch density was increased the extent of damage decreased. The weave architecture, in terms of the relative position of the ±45° tows, was also shown to be a significant factor, the nearer the off-axis tows are to the impact surface the greater was the damage area.