Subhas Chandra

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.

White organic LEDs and their recent advancements

White organic light emitting devices (WOLEDs) are being considered as substitutes for conventional white light sources. They are efficient solid-state lighting sources and their power efficiencies have surpassed those of the incandescent light sources, especially due to recent improvement in device architectures, molecular engineering in synthesis of new materials and the incorporation of electrophosphorescent emitters. This paper reviews the various approaches to achieve white light emission from organic light emitting diodes (OLEDs), their advantages, disadvantages and recent progress. The device architecture and problems related to various device designs have been discussed.

Application of conducting polyaniline as sensor material for ammonia

The change in resistance of polyaniline on exposure to aqueous ammonia has been utilized for the study of a prototype chemical sensor. This paper discusses various aspects of polyaniline and its suitability as a chemical sensor, particularly with reference to aqueous ammonia solution.

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.

Metal/semiconductive polymer Schottky device

Development of a metal/organic‐semiconductor Schottky junction as an alternative to the metal/inorganic‐semiconductor junction is reported. Metal/polypyrrole (PP) junctions have been prepared with electrochemically deposited doped PP films of different thickness and various metals (In, Sn, Ti, and Al) as electrodes. The electrical characteristics of the junction depend upon the work functions of PP and the metal. It has been possible to prepare Schottky barriers on the PP films with a metal electrode having a work function lower than that of the polymer. Various physical characteristics of the polymer, work function, Fermi level, and carrier concentration have been estimated.

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.

Low‐frequency ac conduction in lightly doped polypyrrole films

The total measured ac conductivity σ(ω)m of lightly doped polypyrrole films has been measured in the frequency range 100 Hz–10 MHz and in the temperature range 77–350 K. In the low‐temperature region the measured ac conductivity is almost independent of temperature but shows a strong dependence on frequency and can be described by a relation σ(ω) = Aωs where the exponent s has been observed to be less than unity. In the high‐temperature region the frequency dependence becomes weak at low frequencies but remains strongly frequency dependent at high frequencies. The weak frequency dependence is due to the contribution of dc conductivity to the measured ac conductivity. A clear Debye‐type loss peak is observed by substracting the contribution of dc conductivity. The frequency dependence of conductivity remains less than quadratic at low frequencies indicating thereby some distribution of relaxation times. This is confirmed by the measurement of dielectric constant as a function of frequency and temperature. ...

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.