Field emission characteristics of CNTs synthesized by bias-assisted ICPHFCVD and ICPCVD techniques

Carbon nanotubes (CNTs) were vertically well-grown on Ni/Cr-deposited glass substrates at 580 °C by ICPCVD and bias-assisted ICPHFCVD techniques. The vertically well-aligned CNTs showed multi-walled type with hollow structure. The measured critical current density on CNTs grown by the ICPCVD technique was 1.0×10^–6 A cm^–2 at 5 V m^–1 of turn-on field and 7.7×10^–5 A cm^–2 at 7.8 V m^–1 of the critical field. On the other hand, the critical current density on CNTs grown by the bias-assisted ICPHFCVD technique was 3.7×10^–7 A cm^–2 at 3 V m of turn-on field and 3.3×10^–4 A cm^–2 at 6.8 V m^–1 of the critical field, respectively. On comparing the two processes, it can be concluded that CNTs grown by bias-assisted ICPHFCVD are more suitable than those grown by ICPCVD for the possible application of field emission displays (FEDs).     

Proton conducting polymer electrolyte: II poly ethylene oxide + NH4l system

A new proton conducting polymer electrolyte PEO + NH₄l system has been investigated. The solution-cast films of different stochiometric ratios have been prepared and characterized. Proton transport has been established using various experimental studies, namely optical microscopy, X-ray diffraction, differential thermal analysis, infrared, coulometry transient ionic current and electrical conductivity measurements at different temperatures and humidity. The maximum conductivity of the complexed material has been found to be 10^–5 S cm^–1. Both H⁺ ion and I^– anion movements are involved with respective transference numbers and mobilities ast _H+=0.74, ,t _l–=0.09, _H+=4.97 × 10^–6 cm² V^–1 s^–1 and _l–=7.65.     

Synthesis of (Pb,La) (Zr,Ti)O3 films using a diol based sol–gel route

Morphotropic phase boundary compositions of lead zirconate titanate (PZT) modified with 2, 5 and 10 mol% lanthanum (PLZT) have been prepared using a diol based sol–gel route. Thin films of these PLZT compositions were fabricated on platinized silicon substrates by a spin coating technique. The effects of firing temperature and lanthanum modifications were investigated with regard to phase development, microstructure, and ferroelectric and dielectric characteristics. A strong 1 1 1 orientation developed as the amount of lanthanum doping increased. The results indicate that the values of remanent polarization, P _r, and dielectric constant, _r, decrease, relative to unmodified PZT, for films modified with 2 and 5 mol% lanthanum. The 5 mol% La films for example had a P _r of 14 C cm^–2 and an _r value of 700 compared to 31 C cm^–2 and 1480 for undoped PZT films. At these La concentrations there was also an improvement in the leakage current density by two orders of magnitude compared with unmodified PZT. The 10 mol% La sample did not exhibit any switchable polarization behaviour.     

Film thickness dependence of critical current density for YBa2Cu3O7 films post-annealed at a low oxygen partial pressure

The variation of critical current density at 77 K as a function of film thickness was studied for YBa₂Cu₃O₇ films on (100) LaAlO₃ substrates. Film thicknesses were in the range 0.2–1.6m. The films were deposited by co-evaporation and post-annealed under conditions which have previously resulted in high-quality films (750°C and an oxygen partial pressure of 29 Pa). The critical current density at 77 K exceeds 1 MA cm^–2 for the thinner films, and decreases with increasing film thickness in excess of about 0.4m. The decrease is in rough agreement with a switch fromc-axis toa-axis growth at about this critical thickness. A good anticorrelation was found between room temperature resistivity and critical current density at 77 K. The results are compared to those obtained before by post-annealing at 850°C in 1 atm of oxygen.     

A parametric study of the deposition of the TiN thin films by laser reactive ablation of titanium targets in nitrogen: the roles of the total gas pressure and the contaminations with oxides

The multipulse excimer laser-reactive ablation of a titanium target in nitrogen has been found to result in a total pressure of the ambient gas in the range 7–70 bar, in the deposition on to a silicon collector surface of high-purity f c c TiN thin films. These films were hard and adherent to substrate. The deposition rate was 0.03–0.05 nm per pulse for an incident laser fluence of 5 J cm^–2. For a lower gas pressure of a few microbars the deposits were amorphized with an excess of titanium. For a nitrogen pressure larger than 100 bar, the layers were contaminated with oxides. The oxides became more abundant with further increase in the gas pressure, and the deposited layer consisted of oxides only when the pressure reached several millibars.     

Study of light absorption in n-type and p-type GaInAs and the possibility of making 1.55-μm GaInAs/InP Bragg mirrors

Effects of impurity doping on optical properties of epitaxially grown Ga_0.47In_0.53As semiconductors lattice-matched to InP have been studied. The absorption coefficient decreased and the photoluminescent peak energy increased at very high doping levels, due to a Burstein–Moss shift. Absorption in n-type Ga_0.47In_0.53As was negligible at a wavelength of 1.55 m when the donor (Si) concentration was higher than 7.7 × 10¹⁸cm^–3, but was higher than 4000 cm^–1 at = 1.3 m. Absorption in p-type Ga_0.47In_0.53As was 2600 cm^–1 and 7000 cm^–1 at 1.55 and 1.3 m, respectively, when the acceptor (Be) concentration was 3.4 × 10¹⁹cm^–3. Emission from n-type Ga_0.47In_0.53As experienced a maximum shift of 170 meV towards higher energies relative to emission from undoped Ga_0.47In_0.53As, while emission from p-type Ga_0.47In_0.53As shifted 45 meV. The Burstein–Moss effect was used in growth of 1.55-m distributed Bragg mirrors having 20 pairs of quarter-wave n–Ga_0.47In_0.53As/InP layers. Reflectivity greater than 95% and a stop-band width of 60 nm were demonstrated.     

Ion-implantation technology for improved GaAs MESFETs performance

Electrical properties of Si-implanted and co-implanted with Mg or Be in semi-insulating GaAs was studied. The Si-implanted MESFETs with and without buried p-layer (formed by Mg or Be) have been fabricated and characterized by their d.c. and r.f. performance. The experimental results showed that the device with a buried p-layer can effectively suppress the substrate leakage current (thus good pinch-off characteristic) and obtained higher gain linearity than these without a buried p-layer. For 1 m×100 m MESFETs device with co-implantation of Si (8×10¹² cm^–2) and Be (6×10¹¹ cm^–2) demonstrated uniform transconductance (g_m) of 115 mS mm^–1 with the gate voltage ranging from –1 to 1 V and reduced pinch-off voltage compared to those with co-implantation of Si and Mg (6×10¹¹ cm^–2). The measured f_T and f_max of a 1 m×25 m MESFET with co-implantation of Si and Be are 10 and 39 GHz, respectively. However, FETs with increased Mg dose (from 6×10¹¹ cm^–2 to 2×10¹² cm^–2) in a buried p-layer can obtain higher transconductance and saturation current.     

Translucent low-thermal expansion ceramics

The optical transmittance of pressureless sintered KZr₂P₃O₁₂ (KZP) ceramics was studied in the ultraviolet, visible light, and near infrared spectrum ranges. Thermal expansion, strength, and thermal shock resistance of the ceramics were also studied. KZP ceramics with different additives (0–8 wt% of MgO, ZnO or SrO) show a 30–70% transmittance from 0.4–2 m, a low thermal expansion coefficient and high thermal shock resistance. These polyfunctional ceramics might be useful as optical windows.     

Absorption Spectrum of ZnO Precipitates in ZnSe

The absorption spectrum of ZnO precipitates in ZnSe saturated with oxygen is studied in the spectral range 500–2000 cm^–1 and is shown to correlate with the transmission spectrum of single-crystal ZnO. Saturation of ZnSe with oxygen in the course of growth leads to ZnO precipitation during cooling. The precipitates give rise to narrow absorption bands in the range 5.8–7.1 m, which correlate with the components of the multiphonon absorption spectrum of ZnO, formed by combinations of LO and TO phonons. The 2LO mode defines the long-wavelength transmission edge of ZnO. In addition, the spectrum shows strong absorptions due to the LO + TO(10.2 m) and 2TO (11.3 and 12.7 m) modes. The possible role of SeO₂ is discussed. It is suggested that ZnO with a high carrier concentration may act as a gray filter in the spectral range 2–9 m.     

Structural and electrical properties of the Y-Ba-Cu-O superconducting films prepared by spray pyrolysis

High-T _c , superconducting YBa₂Cu₃O_7– thin films have been grown on (100) MgO substrates by a chemical spray pyrolysis method. The crystal structure and surface morphology have been studied by X-ray diffraction and scanning electron microscopy, respectively. The assprayed films were amorphous and insulating, but upon annealing the films became superconducting and show a textured surface morphology with an average grain size of the order of 5–15m. The films were highly oriented with thec-axis being perpendicular to the substrate surface. Three different microstructures were recorded: long rod-shaped grains, platelet or rounded-shape grains, and a melting-like growth. Electrical measurements were carried out in a low-temperature cryostat using a standard d.c. four-probe technique. The onset transition temperature was around 83–86 K, and the completion of the transition to zero resistance was in the range 73–78 K. The magnitude of the measured critical current density was in the range 750–3750 A/cm² at 30 K. A correlation between the resistance of the tunnelling junctions and the critical current density was found from the theoretical models.     

Low-temperature PECVD deposition of highly conductive microcrystalline silicon thin films

In this paper, we present the characterization results of doped n-type microcrystalline hydrogenated-silicon (c-Si : H) films deposited in a plasma-enhanced chemical vapor deposition in the temperature range between 70 and 250 °C. The interest in these films arises from the fact that they combine the high optical absorption of amorphous silicon with the electronic behavior of the crystalline silicon, making them interesting for the production of large electronic devices such as solar cells, image sensors, and flat panels. It is shown that n-type c-Si : H films with high electrical conductivity can be obtained even at low temperature deposition, around 120 °C (=2.9 S cm^–1). The structural properties of the films have been studied by Raman and infrared spectroscopy that allowed for the determination of the crystalline fraction. Electrical measurements were performed by a.c. impedance spectroscopy, Hall effect, and dark conductivity. Characteristics suitable for application in electronic devices were obtained with the developed deposition parameters set-up; the best dark conductivity values were around 1 S cm^–1 for deposition temperatures within the 120–140 °C range. Some conclusions regarding the correlation between electrical and structural properties are presented for the considered temperature range.     

Preparation and characterization of copolymerized methylsilsesquioxane-benzylsilsesquioxane microparticles for electrophoretic sol-gel deposition

Copolymerized methylsilsesquioxane-benzylsilsesquioxane, (100 – x)MeSiO_3/2·xBnSiO_3/2 (mol%), microparticles of 0.3–0.4 m in average diameter have been prepared by the sol-gel method and applied to the formation of thick films on indium tin oxide (ITO)-coated glass substrates by the electrophoretic deposition. The microparticles obtained were considered to be hybrid containing both methyl and benzyl groups from thermal analyses and structural investigations. The microparticles became fusible at x 80 by a heat treatment. Thick film, ca. 5 m thick, consisting of electrophoretically deposited pure BnO_3/2 microparticles became transparent upon thermal sintering of the particles during heat treatment at temperatures higher than 100°C. On the other hand, the increase in optical transmittance during the heat treatment of the films consisting of 20MeSiO_3/2·80BnSiO_3/2 microparticles was as small as about 20% in visible range even after the heat treatment. The smaller increase in transmittance is probably due to the relatively high viscosity of the 20MeSiO_3/2·80BnSiO_3/2 upon sintering in comparison with pure BnO_3/2 for elimination of the voids in the films.     

Properties of LT-AlGaN films and HT-GaN films using LT-AlGaN buffer layers grown on (0001) sapphire substrates

Low-temperature (LT) Al_xGa_1–xN (0.1 < x < 0.8) films, 0.4 m in thickness, were prepared on (0001) sapphire substrates at 500,C by alternate supply of Ga and Al alkyls and ammonia (NH₃). Al composition in the solid phase was identified based on the shift of the (0002) Bragg angle of X-ray diffraction. A series of high temperature (HT) GaN films, 1.0 m in thickness, were also grown at 1000,C on the LT-Al_xGa_1–xN coated (0001) sapphire substrates with buffer layer thickness ranging from 7.5 to 20 nm. It was found that the optimized LT-Al_xGa_1–xN buffer layer thickness decreases linearly with the Al-content. As-grown HT-GaN films having LT-LT-Al_0.43Ga_0.57N buffer layers show smooth surface based on optical microscopic (OM) observations. Transmission electron microscopy (TEM) confirms the mono-crystalline nature of the HT-GaN films. The quenched near band-edge photoluminescence (PL) emissions and an apparent yellow luminescence of the HT-GaN films are attributed to the LT-Al_xGa_1–xN buffer layer induced mosaic microstructure and bonding defects in the films.     

Preparation and properties of electroless Ni–Zn–P alloy films

Electroless deposition of Ni–Zn–P layers was studied on steel electrodes by varying the bath temperature (40–90°C), pH and chemical composition. The deposition parameters were optimized. Alloys containing 70–86 wt % Ni, 6–20 wt % Zn and 6–10 wt % P are obtained at 20 m h^–1 and 85°C. Corrosion measurements were performed in aerated 5% sodium chloride solution, the corrosion potential and current density are, respectively, –0.49 V/SCE and 2.6 A cm^–2.     

High pressure consolidation of B6O-diamond mixtures

Sintered composites in the B₆O-xdiamond (x= 0–80 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1400–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available diamond powder with various grain sizes (<0.25, 0.5–3, and 5–10 m). Relationship among the formed phases, microstructures, and mechanical properties of the sintered composites was investigated as a function of sintering conditions, added diamond content, and grain size of diamond. Sintered composites were obtained as the B₆O-diamond mixed phases when using diamond with grain sizes greater than 0.5 m, while the partial formation of the diamond-like carbon was observed when using diamond grain sizes less than 0.25 m. Microhardness of the sintered composite was found to increase with treatment temperature and pressure, and the fracture toughness slightly decreased. A maximum microhardness of H _v57 GPa was measured in the B₆O-60 vol% diamond (grain size < 0.25 m) sintered composite under the sintering conditions of 5 GPa, 1700°C and 20 min.     

Dielectric and pyroelectric properties of KNO3 thin-layers

A series of thin monocrystalline KNO₃ layers has been grown from the melt by a special technique. The changes of the dielectric constant, d.c. resistivity, activation energy and pyro-electric current behaviour with the layer thickness and temperature have been investigated. Also, the effect of the a.c. frequency on the dielectric constant is checked in the frequency range 1 to 10 kHz. A sharp drop in dielectric constant could be recorded up to 6 kHz. The layers exhibit essentially the same dielectric and pyroelectric properties as bulk KNO₃ single crystals. The various measured parameters of these layers were found to be thickness-dependent. TheI-V characteristics of the layers (5m thickness) reveal the presence of breakdown voltage at 10.95V. Maximum pyrocurrent was obtained at positive d.c. biasing voltage. Also, the layers show a high pyroelectric coefficient, e.g. for layer thickness 8.9m, = 60C cm^–2 K^–1 at 180° C. The study proved that thin layers of potassium nitrate have good pyroelectric properties and should be useful as pyroelectric thermal detector material.     

Ferroelectric Lead Zirconate Titanate Films Prepared by Spray Pyrolysis of Carboxylate Solutions

Ferroelectric PbTi_0.6Zr_0.4O₃films 0.5–1.5 m in thickness were produced on platinum substrates by spray pyrolysis of carboxylate solutions. The optimized compositions of the precursor solutions, containing methacrylic acid and ethylene glycol, are stable under normal conditions, allow the annealing temperature to be reduced, and lead to higher quality film surfaces and large grains. The film exhibit the following electrical properties: T _C= 360–460°C, _max= 1750 at T _C, tan = 0.02–0.1 at 1 kHz and room temperature, P _{s max} = 18 C/cm², P _{r max} = 15 C/cm²at 50 Hz, and E _c= 42–120 kV/cm.     

Characteristics of sol-gel derived PZT thin films with lead oxide cover layers and lead titanate interlayers

The sol-gel derived PbZr_0.53Ti_0.47O₃ (PZT) films were fabricated on the bare Pt/Ti/SiO₂/Si substrates or the same substrates coated by the PbTiO₃ (PT) interlayers. The post-deposition annealing temperature and time were optimized when the PbO cover layers and PbO vapour-containing atmosphere were compared with each other and adopted as the method to diminish the lead-loss problem during the high-temperature post-deposition annealing. The X-ray diffraction patterns, microstructures, and electrical properties such as relative permittivity, _r, remanent polarization, P _r, and coercive electrical field, E _c, were investigated in relation to the annealing conditions. The PZT films deposited on the bare Pt/Ti/SiO₂/Si substrates under the PbO vapour-containing atmosphere showed better electrical properties. This indicates that the PbO vapour-containing atmosphere may be the better method of lead-loss-prevention to process the lead-containing films rather than the PbO cover layer method. The electrical characteristics of the PZT films, _r=1150, a dissipation factor of 0.039, P _r=26 C cm^–2, and E _c=40.5 kV cm^–1 were measured at 1 kHz. When PZT films were deposited on substrates coated by the PT layers, PZT-PT films with single perovskite phase were derived by post-deposition annealing at 500 °C for 1 h. However, the relative electrical properties are very poor, i.e. E _r=160, P _r=2.0 C cm^–2 and E _c=75 kVcm^–1. The optimum combination for preparing PZT-PT films is a 40 nm PbTiO₃ interlayer and annealing conditions of 6 h at 550 °C in a PbO vapour-containing atmosphere; the derived films exhibit electrical properties of E _r=885, P _r=21.5 C cm^–2 and E _c=64 kV cm^–1. The combination of inserting a PT interlayer and annealing in a PbO vapour-containing atmosphere can prevent the formation of electrical short paths. In this case, nearly pin-hole-free PZT films can be grown on the PT (interlayer) /Pt/Ti/SiO₂/Si substrates. It is believed that it is possible to prepare the PZT films with nano-scale uniformity, reproducible quality, which may be worth considering for commercial applications.     

Tearing of thin polyimide films

Films of BTDA-ODA polyimide up to 58m thick were torn at constant cross-head speed in trousers tests. In common with previous results on polyolefins, the work of tearing was found to increase markedly with specimen thickness. A model for the increase, based on the volume of plastically deformed material adjacent to the crack plane, was found to be only qualitatively valid. The experimental slope of a plot of tearing energy (per unit area) against thickness was 70 MJ m^–3. Optical and scanning electron micrographs of torn films are discussed in regard to the modes of failure.     

Characterizations of InzGa1?z As1?x?yN xSbyP-i-N structures grown on GaAs by molecular beam epitaxy

The GaAs-based double-heterojunction P-i-N structures using In_zGa_1–zAs_1–x–yN_xSb_y as the i-layer is investigated for the first time using solid source molecular beam epitaxy. High quality coherent bulk InGaAsN_3.45%Sb (0.5 m) with a lattice-mismatch of 2.6 × 10^–3 can be achieved due to the surfactant properties of antimony, while the bulk InGaAsN_2% at 0.5 m with 1.06× 10^–3 mismatch is fully relaxed. Rapid thermal annealing (RTA) resulted in 25 meV low temperature (LT) photoluminescence (PL) full-width half-maximums (FWHM) for the bulk InGaAsNSb layers. InGaAsNSb experiences 30% less blueshifting with subjected under the same annealing conditions as InGaAsN with similar N content. Room temperature (RT) absorption measurements are in the range of 0.9–1.4 eV. The absorption edge of 1.41 m is achieved for sample D4.