M. N. Kamalasanan

Sol-gel synthesis of ZnO thin films

Abstract Transparent and crackfree thin films of ZnO have been deposited on fused silica, soda glass, silicon wafers and KBr single crystals using the sol-gel technique. A sol has been prepared by reacting zinc acetate and ethylene glycol and dissolving the resultant transparent brittle solid in dry n-propanol. A proton acceptor like triethyl amine was added to assist hydrolysis of zinc acetate. The resulting solution was readily gelled on addition of water. Films were spin cast on polished substrates, gelled in humid air and pyrolysed at 450 °C to get polycrystalline ZnO thin films. The films were characterized using X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and UV-Vis absorption spectroscopy.

Structural and optical properties of sol‐gel‐processed BaTiO3 ferroelectric thin films

BaTiO3 thin films having a perovskite structure were deposited onto stainless steel and fused quartz substrates by sol‐gel processing. Crystalline, transparent, and crack‐free films of 5000 A thickness were fabricated by spinning and post‐deposition annealing at a temperature of 600 °C. Ferroelectric properties were confirmed by P‐E hysteresis loops. The dielectric constant and optical transmission were also measured.

Dielectric and ferroelectric properties of BaTiO3 thin films grown by the sol‐gel process

Transparent smooth and crack‐free BaTiO3 thin films were deposited on stainless steel, fused silica, platinum plates, and platinized silicon wafers (100) using the sol‐gel process. Barium 2‐ethyl hexanoate and titanium isopropoxide were used as precursors. Annealing of the films at 750 °C for 2 h was necessary to get polycrystalline films. The electrical properties of the films prepared on stainless‐steel substrates showed an electrode barrier effect whereas those prepared on platinum substrates were susceptible to ambient atmospheric humidity. However, films grown on platinum substrates and measured under dry conditions showed very good electrical properties. Ferroelectric hysteresis and C‐V characteristics were also studied on these films.

Structural and optical properties of ferroelectric Bi4Ti3O12 thin films by sol‐gel technique

Ferroelectric thin films of Bi4Ti3O12 were fabricated on several types of substrates, including quartz, steel plates, and indium‐tin‐oxide coated glass slides, by sol‐gel technique. Crystalline, transparent, and crack‐free films of 5000 A thickness were fabricated by spinning and post‐deposition annealing at a temperature of 550 °C. Ferroelectric properties were confirmed by P‐E hysteresis loops. The dielectric constant and optical transmission were also measured.

Optical and electrical properties of BaTiO3 thin films prepared by chemical solution deposition

Barium titanate sol was prepared using barium ethyl hexanoate and titanium isopropoxide. The sol was then spin coated on p-type single crystal silicon (100) wafers, stainless steel and fused silica substrates and annealed to give polycrystalline, transparent, and crack-free films. The surface morphology and structural properties of the films were studied using scanning electron microscopy and X-ray diffraction respectively. Crystalline phase could form only at an annealing temperature of 650°C and above. The effect of post deposition annealing on the optical and structural properties as well as on the band gap were analysed. Transmission spectra were recorded and from this, refractive index, extinction coefficient and thickness were calculated for films on fused silica annealed at different temperatures. The dispersion curve for the refractive index n of 650°C annealed film is fairly flat beyond 450 nm and rises sharply towards the shorter wavelength region, showing the typical shape of a dispersion curve near an interband transition. The present study indicates the validity of the DiDomenico model for the interband transition with a single electronic oscillator. The refractive indices lie in the range 1.75–2.5 for films annealed in the range 300–750°C. The electrical measurements were conducted on metal-ferroelectric-semiconductor (MFS) and metal-ferroelectric-metal (MFM) capacitors. The typical measured small signal dielectric constant was 66 and 140 at 1 MHz for the MFS and MFM capacitors respectively. Debye type dispersion was observed for films on stainless steel substrates with an activation energy of about 0.34 eV. The low field ac conduction was found primarily due to hopping of electrons through the trap centres. The I-V characteristics of the MFS capacitor were found to be ohmic at low fields and space charge limited at high fields. I-V characteristics of the MFM capacitor showed a strange behaviour, linear dependence of current on voltage up to 106 V/m and V5/2 dependence beyond 106 V/m.

Structural, optical, and dielectric properties of sol‐gel derived SrTiO3 thin films

Transparent and crackfree SrTiO3 thin films were deposited on silicon wafers, fused silica, and stainless‐steel substrates by sol‐gel technique. Strontium ethyl hexanoate and titanium isopropoxide were used as starting materials. The surface topology of the films were studied by electron micrography and the structural properties by x‐ray diffraction. The refractive index and band gap were measured by optical transmission and absorption spectroscopy. The films show very low leakage current and nearly temperature independent dielectric constant at high frequencies. The dielectric constant and loss factor at 1 kHz at room temperature were 131 and 0.022, respectively. The frequency dependent ac conductivity has been explained on the basis of potential barriers formed by the charge carriers trapped at intercrystalline regions.

Structural and microstructural evolution of barium titanate thin films deposited by the sol‐gel process

Barium titanate thin films were fabricated by the sol‐gel technique from two different precursors, one using barium 2‐ethyl haxanoate and other using barium hydroxide dissolved in methoxy ethanol as barium source. The films were examined at different curing temperatures using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy, optical transmission spectroscopy, and x‐ray diffraction. The films prepared from an ethyl hexanoate precursor were found to crystallize around 550 °C into tetragonal polycrystalline form, whereas those prepared from a barium hydroxide precursor were crystallized into cubic form around 600 °C. The films prepared from the ethyl hexanoate precursor showed larger shrinkage during heat treatment and were more porous than those made from the hydroxide precursor. The changes in thickness, refractive index, and band gap of the films annealed at different temperatures were correlated with chemical modifications of the precursor film as well as structural and micros...

Synthesis and characterization of 9,10-bis(2-phenyl-1,3,4-oxadiazole) derivatives of anthracene: Efficient n-type emitter for organic light-emitting diodes

With a general aim to make anthracene derivatives multifunctional (n-type emitter) and also study their suitability as electron transport layers for organic light emitting diodes (OLED), and with a more specific interest to understand the charge transport and packing pattern in the solid state due to the rotating side rings, we report the synthesis and characterization of six novel molecules (5–10) in which the 9 and 10 positions of anthracene have been directly substituted by phenyloxadiazole groups. We have carried out detailed studies of these molecules including photophysical, electrochemical, electroluminescent studies and solid state structure determination through crystallographic techniques. The electron affinity is very high, around 3.1–3.2 eV, and the ionization potential is around 5.9–6.0 eV, comparable to the more commonly used electron transport electroluminescent layer Alq3. The studies reveal that the new molecules being reported by us, in addition to the high thermal stability, are quite efficient in a two layer unoptimized device with the device structure ITO/α-NPD/5–10/LiF/Al and have an emission in pure green. They also show very high efficiency as electron transport layer in device structure ITO(120nm)/α-NPD(30nm)/Ir(ppy)3 doped CBP(35nm)/BCP(6nm)/5(28nm)/Al(150nm). From these studies we conclude that the anthracene derivatives also have considerable potential as multifunctional layers and as electron transport layers in OLED.

Metalorganic chemical vapor deposition technique for growing c‐axis oriented ZnO thin films in atmospheric pressure air

ZnO thin films have been grown in atmospheric pressure and ambient atmospheric air using zinc 2‐ethyl hexanoate as zinc source by metalorganic chemical vapor deposition technique. The films grown on glass above 350 °C showed c‐axis orientation as seen from x‐ray diffraction studies. The films were highly transparent and free from any visual defects. The growth rate and morphology of the film was found to depend on the substrate temperature. Auger electron spectroscopy shows the presence of carbon as an impurity.

Sensitization of photoconductivity in tetragonal lead monoxide

The thermal sensitization process for the enhancement of photoconductivity in tetragonal lead monoxide has been investigated. At a critical annealing temperature the photosensitivity has been found to have a maximum value (≅5×102) while the conductivity is observed to have a minimum value (≅10−10Ω cm). The transient response of the photocurrent exhibits an overshoot, the height of which is strongly dependent on the annealing temperature. These and other findings have been explained on the basis of sensitization due to the creation of two types of centres in the material.