Thermoelectric effect in laser annealed printed nanocrystalline silicon layers

Nanocrystalline boron and phosphorus doped silicon particles were produced in a microwave reactor, collected, and dispersed in ethanol. Pulsed laser annealing of spin‐coated films of these particles resulted in p‐ and n‐type conductive layers on flexible substrates if a threshold laser energy density of 60 mJ/cm² was exceeded. The thermopower of the laser sintered layers exhibits a distinct maximum at a doping concen‐ tration around 10¹⁹ cm^–3 for both boron and phosphorus doping with an absolute value of the Seebeck coefficient of about 300 µV/K. Since the thermal conductivity of the layers is reduced by nearly the same factor compared to bulk crystalline silicon as the electrical conductivity, these results are promising for the application of such nanocrystalline layers in thin film thermoelectric devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ferromagnetic CrSb film fabricated by pulse-laser deposition and rapidly annealing

CrSb film was fabricated by pulse laser deposition (PLD) on Si (1 0 0) wafer. Strong ferromagnetism was observed in the CrSb film annealed at a high heating/cooling rate of 200 K/s, which can be attributed to the presence of ferromagnetic zinc blende (ZB) CrSb phase. The appearance of metastable ZB-CrSb results from the strong inner stress due to the precipitation of the monoclinic Sb.     

Effect of carbon content on structure and magnetic properties for (Fe,Ni)1−xCx by mechanical alloying

FCC (Fe₅₅Ni₄₅)_1−xC_x   supersaturated solid solution was prepared in a wide concentration range (0?x?0.9)$(0?x?0.9)$ by mechanical alloying of nanocrystalline Fe₅₅Ni₄₅ with graphite. The lattice constant of Fe₅₅Ni₄₅ increases linearly with increasing carbon content up to x=0.25$x=0.25$. At the same time, it is found that the magnetic moment per metal atom (Fe, Ni) decreases linearly with increasing carbon content for 0?x?0.25$0?x?0.25$ with a slope of 1.2 μ_B/at. For high carbon content, x?0.5$x?0.5$, it is observed that the decrease of lattice constant and increase of moment per metal atom (Fe, Ni) with increasing C content, indicates that the dissolution of carbon is hindered by the high-volume fraction of graphite in the initial powder mixture. The complete amorphization of x=0.5$x=0.5$ does not occur after the extended ball milling. The alloying effect of carbon on the magnetization is compared with other metalloid B, P, and Si in Fe- and Ni-based binary system.     

Preparation of crack‐free nano‐crystalline yttria‐stabilized zirconia

In recent research and development, yttria‐stabilized zirconia have become increasingly important for many industrial applications such as gas sensors and solid oxide fuel cells. In many of these applications, the suppression of uncontrolled gas transport due to porosity or cracks is essential. In this work, the influence of the sintering parameters on the crack formation in n‐ZrO₂·9.5 mol% Y₂O₃, manufactured by room‐temperature compaction of sputtered ZrY nanocrystallites with subsequent oxidation and sintering, was studied. A procedure that enables the production of nearly crack‐free fully dense samples is presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural characterization of SiGe nanoclusters formed by rapid thermal annealing

This work presents the structural characterization of nanoclusters formed from a-Si:H/Ge heterostructures processed by rapid thermal annealing (RTA) at 1000 °C for annealing times varying between 30 s and 70 s. The a-Si:H layers were grown on electron cyclotron resonance (ECR) using SiH₄ and Ar precursor gases. The Ge layer was grown in an e-beam evaporation system. The structural characterizations were performed by high-resolution X-ray diffractometer (HRXRD) on grazing incidence X-ray reflection mode (GIXRR) and micro-Raman measurements. The average grain size, Ge concentration (x_Ge) and strain were estimated from Lorentzian GIXRR peak fit. The average grain size varied from 3 nm to 7.5 nm and decreased with annealing time. The x_Ge increase with annealing time and varied from 8% to 19%, approximately. The strain calculated for (1 1 1), (2 2 0) and (3 1 1) peaks at 40 s, 50 s, 60 s and 70 s annealing time suggest the geometrical changes in nanoclusters according to process time.     

Structure and magnetic properties of nanocrystalline Fe75Si25 powders prepared by mechanical alloying

Nanocrystalline Fe₇₅Si₂₅ powders were prepared by mechanical alloying in a planetary ball mill. The evolution of the microstructure and magnetic properties during the milling process were studied by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer measurements. The evolution of non-equilibrium solid solution Fe (Si) during milling was accompanied by refinement of crystallite size down to 10 nm and the introduction of high density of dislocations of the order of 10¹⁷ m⁻². During the milling process, Fe sites get substituted by Si. This structural change and the resulting disorder are reflected in the lattice parameters and average magnetic moment of the powders milled for various time periods. A progressive increase of coercivity was also observed with increasing milling time. The increase of coercivity could be attributed to the introduction of dislocations and reduction of powder particle size as a function of milling time.     

Magnetic size growth in nanocrystalline copper ferrite

When nanocrystalline copper ferrite (average grain size D≈6 nm) is subjected to high-energy-milling in air over different periods up to 12 h, we observe both a progressive enhancement of the ferrite's magnetic response and a shifting of its superparamagnetic limit. These are revealed by the shift to higher values of the Mössbauer blocking temperature, the maximum of the zero-field cooled magnetization and the start of the irreversibility between the zero-field and field-cooled magnetization curves, while the saturation magnetization and the mean magnetic moment per particle increase. The X-ray diffraction data show that the spinel improves its crystallinity with the milling, by increasing the grain size up to ≈13 nm and, also, reducing its micro-strain level. After 10 h of milling the copper ferrite stabilizes in its cubic metastable phase.     

Age-induced phase transitions on mechanically alloyed amorphous GaSe

After aging it for four years at room temperature, a mechanically alloyed amorphous GaSe powder was transformed to a multi-phase crystalline alloy, where major phase is the trigonal Se one. The structural, thermal and optical properties of this aged amorphous GaSe were investigated through systematic X-ray diffraction, differential scanning calorimetry and Raman scattering measurements. The X-ray diffraction results on the aged GaSe powder suggest the presence of oxides, and X-ray absorption spectroscopy was employed to further investigate it.     

Improving pattern precision of chromium based black matrix by annealing

Black matrix is a major component of color filter used for blocking light in flat panel industry. Films made of chromium (Cr) and its two oxide/nitride combination layers are commonly used in black matrix for its high optical density and material stability. Each single Cr based layer film of the three multilayer combinations was produced on glass and then annealed in low pressure hydrogen environment. Etching, transmittance of visible light, and microstructure of these films without and with annealing were compared. It was found that annealing can mend the interference between the incident and reflected light for Cr based black matrix. Annealing can also improve the undercut defect and thus pattern precision. The lateral etching rates of Cr based films were found to be much lower and close to one another after annealing. It can be explained the film density and adhesion increase caused by the annealing process. This article provides a potential method in color filter fabrication to improve contrast and color interference issues if Cr based black matrix is used in display application.     

Enhanced infrared response of Si base p-n diode with self-assembled Ge quantum dots by thermal annealing

The effects of thermal annealing in Si base p-n diode with self-assembled Ge dots stacked in eight layers structure are investigated. The effects of annealing are discussed based on the photovoltage spectra, the PL spectra and the Raman spectra. Three main effects occur after thermal annealing: the reduction of point defects, the intermixing of Si-Ge and the strain relaxation. The experimental result shows that 800 °C might be a suitable annealing temperature for photovoltaic applications.     

Adsorption and photocatalytic degradation of methylene blue on Zn1−xCuxS nanoparticles prepared by a simple green method

Nanoparticles of Zn_1−xCu_xS with various dopant contents (0 ≤ x ≤ 0.15) were prepared in water by refluxing for 90 min at about 95 °C. Powder X-ray diffraction (XRD) patterns of the nanoparticles demonstrate that loading of Cu²⁺ ions does not change the crystal structure of ZnS. Scanning electron microscopy (SEM) images demonstrate that size of the nanoparticles decreases with increasing Cu²⁺ ions. UV-Vis diffuse reflectance spectra (DRS) of the nanoparticles show significant absorption in visible light region. Adsorption capacity of the nanoparticles for methylene blue (MB) increases with mole fraction of copper ions. Photocatalytic activity of the nanoparticles toward photodegradation of MB was evaluated under visible light irradiation. The results indicate that Zn_0.85Cu_0.15S nanoparticles exhibit highest photocatalytic activity among the prepared samples. Moreover, effects of refluxing time applied for preparation of the nanoparticles and calcination temperature were investigated.     

Synthesis and characterization of nanocrystalline TaN

Tantalum nitride (TaN) nanocrystals have been successfully synthesized at 650 °C through a solid-state reaction in an autoclave. The X-ray powder diffraction pattern indicates that the product is a mixture of hexagonal and metastable cubic TaN. Transmission electron microscopy images and selected area electron diffraction patterns show that the hexagonal TaN crystallites consist of nanorod with a typical size of about 50×1000 nm and the cubic TaN crystallites are composed of uniform particles with an average size of about 30 nm.     

Nd: YAG laser annealing of gallium-implanted silicon

A Q-switched Nd: YAG laser with a pulse duration of 20 ns was used to investigate effects of laser annealing in gallium implanted silicon. Rutherford backscattering and Hall-effect measurements were performed to evaluate the annealed layer. Differential Hall-effect measurements were carried out to obtain carrier concentration profiles after annealing. It was found that a maximum sheet carrier concentration of 8×10¹⁵ cm⁻² can be obtained for a gallium implantation of 10¹⁶ cm⁻² by laser annealing with an energy density of more than 1.0 J cm⁻². Although the peak carrier concentration was found to be 8.0×10²⁰ cm⁻³, the annealed layer showed polycrystalline structures even after annealing with an energy density up to 4J cm⁻². The annealing took place in the solid phase in this energy density range.     

Tunable Anisotropic Absorption of Ag-Embedded SiO2 Thin Films by Oblique Angle Deposition

Ag-embedded SiO2 thin films are prepared by oblique angle deposition. Through field emission scanning electron microscopy （SEM）, an orientated slanted columnar structure is observed. Energy-dispersive x-ray （EDX） analysis shows the Ag concentration is about 3% in the anisotropic SiO2 matrix. Anisotropic surface plasma resonance （SPR） absorption is observed in the Ag-embedded SiO2 thin films, which is dependent on polarization state and incidence angle of two orthogonal polarized lights and the deposition angle. This means that optical properties and anisotropic SPR absorption can be tunable in Ag-embedded SiO2 thin films. Broadband polarization splitting is also observed and the transmission ratio Tp/Ts between p- and s-polarized lights is up to 2.7 for thin films deposited at a = 70°, which means that Ag-embedded SiO2 thin films are a promising candidate for thin film polarizers.     

Fabrication of multilayered Ge nanocrystals embedded in SiOxGeNy films

Multilayered Ge nanocrystals embedded in SiO_xGeN_y films have been fabricated on Si substrate by a (Ge + SiO₂)/SiO_xGeN_y superlattice approach, using a rf magnetron sputtering technique with a Ge + SiO₂ composite target and subsequent thermal annealing in N₂ ambient at 750 °C for 30 min. X-ray diffraction (XRD) measurement indicated the formation of Ge nanocrystals with an average size estimated to be 5.4 nm. Raman scattering spectra showed a peak of the Ge-Ge vibrational mode downward shifted to 299.4 cm⁻¹, which was caused by quantum confinement of phonons in the Ge nanocrystals. Transmission electron microscopy (TEM) revealed that Ge nanocrystals were confined in (Ge + SiO₂) layers. This superlattice approach significantly improved both the size uniformity of Ge nanocrystals and their uniformity of spacing on the ‘Z’ growth direction.     

CdSe &; ZnS core/shell nanoparticles generated by laser ablation of microparticles

Laser Ablation of Microparticles (LAM) is a process of nanoparticle formation in which microparticles in a flowing aerosol are continuously ablated by high-power laser pulses. For the first time, we have produced CdSe/ZnS core/shell nanoparticles using a double ablation apparatus, designed to undergo a two-step LAM process. This process can be inverted to produce ZnS/CdSe core/shell nanoparticles. The present work focuses on the range around ∼15 nm radius heterostructures and uses high-resolution transmission electron microscopy (HRTEM) to image core and shells. For smaller particles, core shell structures have been detected with energy dispersive spectroscopy (EDS) 5 nm spot size beam and fast Fourier transform (FFT) spectra. Differences in the ablation behavior were measured between the two IIB–VIA type semiconductors.     

On the thermal stability of Co2Z hexagonal ferrites for low-temperature ceramic cofiring technologies

Co₂Z hexaferrite Ba₃Co₂Fe₂₄O₄₁ was prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD studies show that at T<1300 °C different ferrite phases coexist with the M-type hexaferrite as majority phase between 1000 and 1100 °C and the Y-type ferrite at 1230 °C. The Z-type material has its stability interval between 1300 and 1350 °C. Both synthesis routes result in almost single-phase Z-type ferrites after calcination at 1330 °C, intermediate grinding and sintering at 1330 °C. The permeability of Co₂Z-type ferrite of about μ=20 is stable up to several 100 MHz, with maximum losses μ′′ around 700 MHz. Addition of 3 wt% Bi₂O₃ as sintering aid shifts the temperature of maximum shrinkage down to 950 °C and enables sintering of Z-type ferrite powders at 950 °C. However, the permeability is reduced to μ=3. It is shown here for the first time that Co₂Z ferrite is not stable under these conditions; partial thermal decomposition into other hexagonal ferrites is found by XRD studies. This is accompanied by a significant decrease of permeability. This shows that Co₂Z hexagonal ferrite is not suitable for the fabrication of multilayer inductors for high-frequency applications via the low-temperature ceramic cofiring technology since the material is not compatible with the typical process cofiring temperature of 950 °C.     

Low-temperature electronic transport behaviour of powder-metallurgical SiGe alloys

Electronic transport properties of mechanically alloyed phosphorous-doped SiGe alloys at low temperatures were examined. We found that in this granular medium hopping-processes show an influence on the electronic conductivity. In addition, Hall-measurements revealed that the electron mobility reflects the band-structure of this alloy concerning intervalley-scattering and alloy-scattering. Mobility reaches a minimum at an alloy composition of roughly 25 at.% silicon.