Improvement of the field emission characteristics of an oxidized porous polysilicon using annealed Pt/Ti surface emitter electrode

The field emission characteristics of an oxidized porous polysilicon were investigated with different annealing temperatures. Pt/Ti, Ir, and Au/NiCr were used as surface emitter electrodes, and Pt/Ti emitter showed highly efficient and stable electron emission characteristic compared with the conventional Au/NiCr electrode. Thin Ti layer played an important role in promotion of adhesion of Pt to SiO₂ surface and uniform distribution of electric field on the OPPS surface. Additionally, the Ti layer efficiently blocked the diffusion of emitter metal, which resulted in more reliable emission characteristics. Pt/Ti emitter annealed at 350 °C/1 h showed the highest efficiency of 3.36% at V_ps=16 V, which resulted from the improvement of interfacial contact characteristics of thin emitter metal to an oxidized porous polysilicon. Annealing above 400 °C showed that Pt/Ti and Ir emitter electrode were thermally more stable than Au/NiCr emitter.     

Amphiphilic Poly(p‐phenylene)s for Self‐Organized Porous Blue‐Light‐Emitting Thin Films

Micro‐ and nanostructuring of conjugated polymers are of critical importance in the fabrication of molecular electronic devices as well as photonic and bandgap materials. The present report delineates the single‐step self‐organization of highly ordered structures of functionalized poly(p‐phenylene)s without the aid of either a controlled environment or expensive fabrication methodologies. Microporous films of these polymers, with a honeycomb pattern, were prepared by direct spreading of the dilute polymer solution on various substrates, such as glass, quartz, silicon wafer, indium tin oxide, gold‐coated mica, and water, under ambient conditions. The polymeric film obtained from C₁₂PPPOH comprises highly periodic, defect‐free structures with blue‐light‐emitting properties. It is expected that such microstructured, conjugated polymeric films will have interesting applications in photonic and optoelectronic devices. The ability of the polymer to template the facile micropatterning of nanomaterials gives rise to hybrid films with very good spatial dispersion of the carbon nanotubes.     

Study of metal barrier deposition-induced damage to porous low-k materials

A unique test structure based on a metal-insulator-semiconductor planar capacitor (Pcap) design was used to investigate several aspects of metal barrier-induced low-k damage. A special term called Effective Damage Thickness was introduced to describe the degree of damage. Ta(N) barrier was deposited on various dielectric films with porosity up to 32%. It has been found that the Effective Damage Thickness increases as the porosity increases. The damage is influenced more by the porosity of low-k films than the film density. Furthermore, the damage was modulated by Ta(N) deposition conditions. More damage was observed when higher target and/or substrate bias power was used, suggesting that the ion energy of the barrier material plays an important role in the low-k damage mechanism. A same degree of damage was observed for Ta barrier as for Ta(N), suggesting that Ta(N) deposition-induced low-k damage was primarily caused by Ta ions not nitrogen. Impact of Ru(Ta) and Cu(Mn) self forming barrier on low-k damage was also investigated. Among all the barriers studied in this work, the Ta-based barriers caused the most damage while the Cu(Mn) self forming barrier had the least damage to the low-k. The atomic masses for Ta, Ru, and Cu are 181, 101, and 64, respectively, corresponding with the observed degree of damage in the low-k material.     

Ferroelectric Materials: Probing Local and Global Ferroelectric Phase Stability and Polarization Switching in Ordered Macroporous PZT (Adv. Funct. Mater. 5/2011)

We describe the characterization, ferroelectric phase stability and polarization switching in strain‐free assemblies of PbZr_0.3Ti_0.7O₃ (PZT) nanostructures. The 3‐dimensionally ordered macroporous structures present uniquely large areas and volumes of PZT where the microstructure is spatially modulated and the composition is homogeneous. Variable temperature powder X‐ray diffraction (XRD) studies show that the global structure is crystalline and tetragonal at room temperature and undergoes a reversible tetragonal to cubic phase transition on heating/cooling. The measured phase‐transition temperature is 50–60 °C lower than bulk PZT of the same composition. The local ferroelectric properties were assessed using piezoresponse force spectroscopy that reveal an enhanced piezoresponse from the nanostructured films and demonstrate that the switching polarization can be spatially mapped across these structures. An enhanced piezoresponse is observed in the nanostructured films which we attribute to the formation of strain free films, thus for the first time we are able to assess the effects of crystallite‐size independently of internal stress. Corresponding polarization distributions have been calculated for the bulk and nanostructured materials using a direct variational method and Landau‐Ginzburg‐Devonshire (LGD) theory. By correlating local and global characterization techniques we have for the first time unambiguously demonstrated the formation of tetragonal and ferroelectric PZT in large volume nanostructured architectures. With the wide range of materials available that can be formed into such controlled architectures we conclude that this study opens a pathway for the effective studies of nanoscale ferroelectrics in uniquely large volumes.     

Surface morphology dependent tungsten oxide thin films as toxic gas sensor

Tungsten oxide (WO₃) films are of great importance in gas sensing technology due to its selectivity towards toxic gases. In this paper, structural, morphological and compositional properties of spray deposited and chemical vapor deposited WO₃ thin films were investigated using XRD, TEM, SEM, EDAX and Raman spectroscopy. These films have monoclinic crystal structure; and a filamentous network surface for spray deposited films whereas small flake-shaped microstructure was observed on the surface of chemical vapor deposited films. These films were studied for their gas sensing ability towards toxic gases like ammonia (NH₃) and sulphur dioxide (SO₂) as a function of temperature and concentration. Response-recovery characteristics were studied by varying gas concentration. The spray deposited films displayed higher gas response than the chemical vapor deposited films whereas the later exhibited lower optimum operating temperature as well as faster response and recovery. A correlation between the morphological, compositional, electrical and gas sensing properties of these films is also established.     

Luminescent chemically-etched porous poly-crystalline silicon on insulating substrates

Polycrystalline silicon deposited on insulating substrates has been chemically-etched to form thin films of porous material exhibiting room temperature visible photoluminescence with emission wavelengths of around 650 nm. Material of 4000 ? thickness was quickly converted to porous silicon within 15 s of etching, with an etch rate of 1–1.5 μm/h. In contrast to anodization, chemical-etching parameters have little effect on modulating the resultant peak wavelength. Peak photoluminescence intensity was achieved 8–12 s of etching in 1:3:5 parts HF:HNO₃:H₂O at room temperature with ambient lighting. The chemical etching process and its etch characteristics have been discussed in relation to its suitability for large area thin film devices.     

Innovative wick design for multi-source, flat plate heat pipes

We present a novel micro-heat pipe wick design and fabrication technique to significantly boost the effective thermal conductivity of the heat pipe relative to the monolithic substrate material. Extensive porous flow modeling of the process has provided critical information on the key parameters and the resulting anisotropic wick designs have shown robust performance improvements. A methanol charged copper device reported in this paper showed a maximum thermal conductivity of 760 W/m K prior to dry out. This represents a 1.9× increase over the conductivity of solid copper.     

Porous silicon as an intermediate layer for thin-film solar cell

The potential of porous silicon (PS) with dual porosity structure as an intermediate layer for ultra-thin film solar cells is described. It is shown that a double-layered PS with a porosity of % allows to grow epitaxial Si film at medium temperature (725°–800°C) and at the same time serves as a gettering/diffusion barrier for impurities from potentially contaminated low-cost substrate. A 3.5 μm thin-film cell with reasonable efficiency is realized using such a PS intermediate layer.