Effects of rapid thermal annealing on the energy levels of InAs/InP self-assembled quantum dots

We have studied the effects of thermal annealing on the electrical properties of InAs/InP self-assembled quantum dots (QDs) using deep level transient spectroscopy (DLTS). It was found from the DLTS measurements that the activation energy of the QD signal varied from 0.47 to 0.60 eV and the emission cross section changed from 1.01×10⁻¹⁵ to 9.63×10⁻¹⁴ cm² when the annealing temperature increased up to 700 °C. As a result of the thermal annealing process at the temperature ranging from 500 to 600 °C, the higher activation energy and the larger emission cross section of the QD related signal were observed for the annealed samples compared to those for the as-grown sample. On the basis of the capture barrier height for the QDs structure being lowered from 0.24 to 0.06 eV at the annealing temperature of 700 °C, thermal damage was considered as the reason. The appropriate annealing process can provide a clue for the engineering of the energy levels in the self-assembled QD structures.     

Pyroelectric properties in three phases coexistence Pb[(Mn0.33Nb0.67)0.5(Mn0.33Sb0.67)0.5]0.08(ZrxTi1−x)0.92O3 lead ceramics

The bulk dense Pb[(Mn_0.33Nb_0.67)_0.5(Mn_0.33Sb_0.67)_0.5]_0.08(Zr_xTi_1−x)_0.92O₃ pyroelectric ceramics have been successfully prepared by the conventional solid method. The effect of three phases coexistence in the ceramics is studied. When x = 0.95 and 0.85 in the ceramics, the maximum pyroelectric coefficient peaks appear at 23 °C and 45 °C, and the maximum values are 26.5 × 10⁻⁴ C/m² °C and 25.5 × 10⁻⁴ C/m² °C, respectively. The maximum pyroelectric coefficient appears large while the peaks widths are small. When the two kinds of ceramic powders mixed with the mol ratio of 2:1, the pyroelectric coefficient of the ceramics is above 10.0 × 10⁻⁴ C/m² °C in a broad temperature range from 20 °C to 55 °C. The possible physical mechanism of the temperature broadened phenomenon is briefly discussed.     

Thermal considerations on the design and operation of lens ducts

In this paper, heat effects due to powerful pump beam in the lens duct delivery system is investigated and subsequent thermal effects are described. Temperature raise in the device, thermal loading, and material considerations for the lens ducts fabrication are reported. For an absorbed intensity of 10 W/cm² in a 4 cm-long lens duct, the temperature difference between the midpoint and its faces, for a lens duct made from borosilicate is 7.05°C, for sapphire 4.01°C, and for synthetic fused silica is the smallest value of 0.54°C. For a typical absorbed intensity of 1.4 W/cm² in a 4 cm-long lens duct, with a steady-state time constant of 120 s, the fractional thermal loading is 1.98% for BK-7, 1.89% for sapphire, and only 0.16% for synthetic fused silica. By considering different glass materials, the overall results show that synthetic fused silica is superior to some other materials for the fabrication of a lens duct. The reported analytical method and the results of parametric studies can be helpful in material selection and construction of an effective lens duct.     

Characterization of InN epilayers grown on Si(1 1 1) substrates at various temperatures by MBE

InN films have been grown by plasma-assisted molecular beam epitaxy (PAMBE) and characterized by various technologies. It was found that the structural, optical and electrical properties can be drastically improved by raising growth temperature from 440 to 525 °C. Grainy morphology was found in the grain size was found in atomic force microscope images. The large grain size was about 360 nm for a film grown at 525 °C. These films exhibited Wurtzite structure with a c/a ratio ranging from 1.59 to 1.609. The dislocation densities estimated by X-ray diffraction techniques closely agreed with those analyzed by plan-view transmission electron microscopy. Photoluminescence (PL) studies confirmed near band-to-band transitions and the narrowest low-temperature PL peak width was found to be 24 meV at 0.666 eV. Carrier concentrations decreased from 1.44×10¹⁹ to 1.66×10¹⁸ cm⁻³ and Hall mobility increased from 226 to 946 cm² V⁻¹ s⁻¹ as the growth temperature is progressively increased from 440 to 525 °C. Raman spectra also indicated improved crystal quality as the growth temperature was raised.     

Controlled fabrication of Si nanostructures by high vacuum electron beam annealing

Silicon nanostructures, called Si nanowhiskers, have been successfully synthesized on Si(1 0 0) substrate by high vacuum electron beam annealing (EBA). Detailed analysis of the Si nanowhisker morphology depending on annealing temperature, duration and the temperature gradients applied in the annealing cycle is presented. A correlation was found between the variation in annealing temperature and the nanowhisker height and density. Annealing at 935 °C for 0 s, the density of nanowhiskers is about 0.2 μm⁻² with average height of 2.4 nm grow on a surface area of 5×5 μm, whereas more than 500 nanowhiskers (density up to 28 μm⁻²) with an important average height of 4.6 nm for field emission applications grow on the same surface area for a sample annealed at 970 °C for 0 s. At a cooling rate of −50 °C s⁻¹ during the annealing cycle, 10–12 nanowhiskers grew on a surface area of 5×5 μm, whereas close to 500 nanowhiskers grew on the same surface area for samples annealed at the cooling rate of −5 °C s⁻¹. An exponential dependence between the density of Si nanowhiskers and the cooling rate has been found. At 950 °C, the average height of Si nanowhiskers increased from 4.0 to 6.3 nm with an increase of annealing duration from 10 to 180 s. A linear dependence exists between the average height of Si nanowhiskers and annealing duration. Selected results are presented showing the possibility of controlling the density and the height of Si nanowhiskers for improved field emission properties by applying different annealing temperatures, durations and cooling rates.     

Transparent conductive ZnO:Ga films prepared by DC reactive magnetron sputtering at low temperature

Ga-doped ZnO (ZnO:Ga) transparent conductive films were deposited on glass substrates by DC reactive magnetron sputtering. The structural, electrical, and optical properties of ZnO:Ga films were investigated in a wide temperature range from room temperature up to 400 °C. The crystallinity and surface morphology of the films are strongly dependent on the growth temperatures, which in turn exert an influence on the electrical and optical properties of the ZnO:Ga films. The film deposited at 350 °C exhibited the relatively well crystallinity and the lowest resistivity of 3.4 × 10⁻⁴ Ω cm. More importantly, the low-resistance and high-transmittance ZnO:Ga films were also obtained at a low temperature of 150 °C by changing the sputtering powers, having acceptable properties for application as transparent conductive electrodes in LCDs and solar cells.     

Derivation of temperature and emittance from airborne multispectral thermal infrared scanner data

A new iterative algorithm is proposed to calculate surface temperature and emittance from data of an airborne scanner with reflective and thermal multispectral channels. For water areas the surface temperature can be determined from the thermal channels alone with an accuracy of 0.5–0.9°C, assuming a well-calibrated sensor and known atmospheric parameters. For non-water areas, information from the reflective bands of the airborne sensor is used for each image pixel to perform a pre-classification of the thermal band data by assigning a representative emittance value to one of the thermal bands. For a number of earth surface types this emittance value can be estimated with an accuracy of about±0.01, leading to a surface temperature accuracy of 0.5–1.5°C.     

Design of an optical fiber sensor for linear thermal expansion measurement

Design and operation of an optical fiber device for temperature sensing and thermal expansion measurement are reported. The modulated intensity has been measured by using a pair of 450 μm core fiber, one acting as the source and the other one as receiving fiber. In this design, the light intensity modulation is based on the relative motion of the optical fibers and a reflective coated lens. By using displacement calibration data for this sensor, the linear thermal expansion of the aluminum rod is determined. This sensor shows an average sensitivity of about 11.3 mV/°C for temperature detection and 7 μm/°C for thermal expansion detection. Device resolution for a linear expansion measurement is about 3 μm for a dynamic range of 600 μm corresponding to a temperature change of 100°C. The measured linear expansion results are checked against the expected theoretical ones and an agreement within ±2 μm is noticed. The operation of this sensor was also compared with other types and some advantages are observed, which verify the capability of this design for such precise measurements.     

Preparation of large-area double-walled carbon nanotube films and application as film heater

Large-area (larger than 30×30 cm²) double-walled carbon nanotube (DWCNT) films are prepared and application as a heating element for film heaters is demonstrated. A high heating efficiency is observed. Measurements indicate that the use of the DWCNT film heater would save energy consumption up to 20–30% when compared with a commercial film-like metal-based heater. Morphological analysis reveals that the special surface structure, appropriate electric and high thermal conductivities of the film formed by the network of entangled nanotube bundles may lead to the high heating performance. Considering large-area, shape flexibility, negligible weight and easy manipulation, the film exhibits promising potential applications as a film heater for thermal control in aircrafts, medical equipment, home appliances and other industrial fields at low temperature (below 400 °C).     

The effect of cerium oxide argon-annealed coatings on the high temperature oxidation of a FeCrAl alloy

Ceria coatings were applied in order to improve the adherence of alumina scales developed on a model Fe–20Cr–5Al alloy during oxidation at high temperature. These coatings were performed by argon annealing of a ceria sol–gel coating at temperatures ranging between 600 and 1000 °C. The influence of these coatings on the alloy oxidation behaviour was studied at 1100 °C. In situ X-ray diffraction (XRD) was performed to characterize the coating crystallographic nature after annealing and during the oxidation process. The alumina scale morphologies were studied by means of scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The present work shows that the alumina scale morphology observed on cerium sol–gel coated alloy was very convoluted. On the cerium sol–gel coated alloy, argon annealing results in an increase of the oxidation rate in air, at 1100 °C. The 600 °C argon annealing temperature results in a good alumina scale adherence under thermal cycling conditions at 1100 °C.     

Oxide ion conductivity and relaxation in CaREZrNbO7 (RE = La, Nd, Sm, Gd, and Y) system

CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ca²⁺–RE³⁺)_av/r(Zr⁴⁺–Nb⁵⁺)_av were larger than 1.34. Thus, the La, Nd, and Sm compounds have a cubic P-type structure and the Gd and Y ones have a defect F-type structure. The electrical conductivity was measured using complex-plane impedance analysis over a wide temperature (300–750 °C) and frequency (1 Hz–1 MHz) ranges. The conductivity relaxation phenomenon was observed in these compounds and the relaxation frequencies were found to show Arrhenius-type behavior and activation energies were in good agreement with those obtained from high temperature conductivity plots. These results support the idea that the relaxation process and the conductivity have the same origin. The ionic conductivity of CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system showed the maximum at the phase boundary between the F-type and P-type phases. On the other hand, the activation energy for the conduction decreased in the F-type phase and increased in the P-type phase with increasing ionic radius ratio. Among the prepared compounds, CaGdZrNbO₇ showed the highest ionic conductivity of 9.47 × 10^− 3 S/cm at 750 °C which was about twice as high as that observed in Gd₂Zr₂O₇ (4.2 × 10^− 3 S/cm at 800 °C). The grain morphology observation by scanning electron microscope (SEM) showed well-sintered grains. AC impedance measurements in various atmospheres further indicated that they are predominantly oxide ion conductors at elevated temperatures (> 700 °C).     

Epitaxial growth of highly transparent and conducting Sc-doped ZnO films on c-plane sapphire by sol–gel process without buffer

Highly transparent and conductive scandium doped zinc oxide (ZnO:Sc) films were deposited on c-plane sapphire substrates by sol–gel technique using zinc acetate dihydrate [Zn(CH₃COO)₂·2H₂O] as precursor, 2-methoxyethanol as solvent and monoethanolamine as a stabilizer. The doping with scandium is achieved by adding 0.5 wt% of scandium nitrate hexahydrate [(ScNO₃·6H₂O)] in the solution. The influence of annealing temperature (300–550 °C) on the structural, optical and electrical properties was investigated. X-ray Diffraction study revealed that highly c-axis oriented films with full-width half maximum of 0.16° are obtained at an annealing temperature of 400 °C. The surface morphology of the films was judged by SEM and AFM images which indicated formation of grains. The average transmittance was found to be above 92% in the visible region. ZnO:Sc film, annealed at 400 °C exhibited minimum resistivity of 1.91 × 10⁻⁴ Ω cm. Room-temperature photoluminescence measurements of the ZnO:Sc films annealed at 400 °C showed ultraviolet peak at 3.31eV with a FWHM of 11.2 meV, which are comparable to those found in high-quality ZnO films. Reflection high-energy electron diffraction pattern confirmed the epitaxial nature of the films even without introducing any buffer layer.     

Growth and characterization of DyP/GaAs and DyAs/GaAs-based heterostructures and superlattices

Details of the structural and electrical properties of epitaxial DyP/GaAs and DyAs/GaAs is reported. DyP is lattice matched to GaAs, with a room temperature mismatch of less than 0.01%. DyAs, on the other hand, has a mismatch of nearly 2.4%. Both DyP and DyAs have been grown by solid source MBE using custom designed group V thermal cracker cells and group III high-temperature effusion cells. High-quality DyP and DyAs epilayers, as determined by XRD, TEM, and AFM analysis, were obtained for growth temperatures ranging from 500°C to 600°C with growth rates between 0.5 and 0.7 μm/h. The DyP epilayers are n-type with measured electron concentrations of the order of 3×10²⁰ to 4×10²⁰ cm⁻³, with room temperature mobilities of 250–300 cm²/V s, and with a barrier height of 0.75 eV to GaAs. The DyAs epilayers are also n-type with concentration of 1×10²¹ to 2×10²¹ cm⁻³, with mobilities between 25 and 40 cm²/V s. DyP is stable in air with no apparent oxidation taking place, even after months of ambient exposure to untreated air.     

Surface structures of in (√3 × √3 )R30° and (5√3 × 2) phases studied by surface extended X-ray absorption fine structure spectroscopy

The local surface structures of in the ( √3 × √3) R30° and (5√3 × 2) phases have been investigated by means of polarization-dependent sulfur K-edge surface EXAFS. In the (√3 × √3 ) R30° phase, sulfur adatoms are found to occupy threefold hollow sites with a S---Ni distance of 2.13 Å and an inclination angle ω of the Sz.sbnd;Ni bonds at 44° from the surface plane. In contrast, in the (5√3 × 2) phase, it is determined that the Sz.sbnd;Ni bond is longer, 2.18 Å, more inclined, ω = 31°, and that the coordination number is not 3 but 4. These results strongly support a picture involving reconstruction of the top nickel layer to form a rectangular structure. Consideration of several models proposed for the (5√3 × 2) phase leads to one which is compatible with both the present results and results recently reported using STM.     

Effects of deposition temperature on the surface roughness and the nonlinear optical susceptibility of sprayed deposited ZnO:Zr thin films

Zirconium doped zinc oxide thin films were deposited by reactive chemical pulverization spray pyrolysis technique on heated glass substrates at 400 °C, 450 °C and 500 °C using zinc and zirconium chlorides as precursors. The effect of zirconium dopant and surface roughness on the nonlinear optical properties was investigated using atomic force microscopy (AFM) and third harmonic generation (THG). The best value of susceptibility χ⁽³⁾ was obtained from the doped films with less roughness. A strong third order nonlinear optical susceptibility χ⁽³⁾ = 20.49 × 10⁻¹² (esu) of the studied films was found for the 5% doped sample at 450 °C.     

Tetra-σ bonding adsorption of pyridine on Si(1 0 0)-2 × 1

We studied adsorption of pyridine on Si(1 0 0) at room temperature using high resolution photoemission spectroscopy (PES) and near edge X-ray adsorption fine structure (NEXAFS) in the partial electron yield (PEY) mode. The Si 2p, C 1s, N 1s spectra of pyridine on Si(1 0 0) showed that pyridine is chemisorbed on Si(1 0 0)-2 × 1 through the formation of the tetra-σ-bonded structure with the N atom and three C atoms. NEXAFS was conducted to characterize the adsorption geometry of pyridine on Si(1 0 0). The π* orbital of CC bond showed a good angle dependence in C K-edge NEXAFS spectra, and we were able to estimate the adsorption angle between chemisorbed pyridine of CC bond and the Si(1 0 0) surface using an analytical solution of NEXAFS intensity. We find the coexistence of two different tight bridges with the adsorption angles 42 ± 2° and 45 ± 2° with almost equal abundance.     

Thermal annealing effects on I–V–T characteristics of sputtered Cr/n-GaAs diodes

Sputtered Cr/n-GaAs Schottky diodes have been prepared and annealed at 200 and 400 °C. The current–voltage (I–V) characteristics of the as-deposited and annealed diodes have been measured in the temperature range of 60–320 K with steps of 20 K. The effect of thermal annealing on the temperature-dependent I–V characteristics of the diodes has been investigated experimentally. The ideality factor and barrier height (BH) values for 400 °C annealed diode approximately remain unchanged from 120 to 320 K, and those of the as-deposited sample from 160 to 320 K. The departures from ideality at low temperatures have been ascribed to the lateral fluctuations of the BH. The BH values of 0.61 and 0.74 eV for the as-deposited and 400 °C annealed diodes were obtained at room temperature, respectively. A Richardson constant value of 9.83 A cm⁻² K⁻² for 400 °C annealed Schottky diode, which is in close agreement with the known value of 8.16 A cm⁻² K⁻² for n-type GaAs. Furthermore, T₀ anomaly values of 15.52, 10.68 and 5.35 for the as-deposited and 200 and 400 °C annealed diodes were obtained from the nT versus T plots. Thus, it has been seen that the interface structure and quality improve by the thermal annealing at 400 °C.     

Design of a hybrid diffractive/refractive achromatized telecentric f·θ lens

The design of a hybrid diffractive/refractive achromatized telecentric f·θ lens with a field of view (FOV) 50° and an effective focal length of 750 mm is presented. The optical stop is placed at the front focal plane so that it is a telecentric system. The design is based on a traditional refractive counterpart, and the designed system consists of a hybrid diffractive/refractive lens and four refractive lenses. The designed f·θ lens shows a considerable reduction in weight with a simplified structure, and exhibits superior performance compared to the refractive system. It is emphasized that the designed f·θ lens can be applied to modern color scanning systems that operate in the visible wavelength range with high performance. It can also be applied to high-energy scanning systems. When applying the designed hybrid diffractive/refractive f·θ lens to the high-energy scanning system, a big laser operating in one longitudinal mode can be replaced by a small multi-mode laser, and the scanning system can be simplified greatly with the accuracy improved.     

Mechanical and electrical characteristics of silver stabilizer layer prepared by using nano silver paste for coated conductor

Mechanical and electrical properties of silver stabilizer layer of coated conductor, which was prepared using nano silver paste as starting materials, have been investigated. Nano silver paste was coated on YBCO (Y₁Ba₂Cu₃O_7−δ) film by a dip coating method with a speed of 25 mm/min. Coated film was dried in air and heat treated at 400–700 °C in a flowing oxygen atmosphere. Adhesion strength between YBCO and silver layer was measured by Tape test (ASTM D 3359). The hardness and electrical conductivity of the sample were measured by pencil hardness test (ASTM D 3363). Surface and volume resistance were measured by using LORESTA-GP (MITSUBISHI). The sample heat treated at 500 °C showed poor adhesiveness of 1B but it is clearly enhanced to 5B when samples were heat treated at higher than 600 °C. The silver layer heat treated at 700 °C showed a high hardness value of higher than 9H and a volume resistance of 1.417 × 10⁻⁷ Ω mm at room temperature. SEM observations showed that a dense silver layer was formed with a thickness of about 2 μm. Dip coated silver layer prepared by using nano silver paste showed superior electrical and mechanical characteristics which is comparable to those that sputter deposited Ag layer.     

Characteristics of ZnO:In thin films prepared by RF magnetron sputtering

In-doped ZnO (ZnO:In) transparent conductive thin films were deposited on glass substrates by RF magnetron sputtering. The effect of substrate temperature on the structural, electrical and optical properties of the ZnO:In thin films was investigated. It was found that higher temperature improves the crystallinity of the films and promotes In substitution easily. ZnO:In thin films with the best crystal quality were fabricated at 300 °C, which exhibit a larger grain size of 29 nm and small tensile strain of 0.9%. The transmittance of all the films was revealed to be over 85% in the visible range independence of the substrate temperatures and the lowest resistivity of ZnO:In thin films is 2.4×10⁻³ Ω cm.