Y. N. Mohapatra

Fatigue resistance in lead zirconate titanate thin ferroelectric films: Effect of cerium doping and frequency dependence

We demonstrate improvement in fatigue resistance and other ferroelectric properties through cerium doping in sol–gel derived lead zirconate titanate thin films. We have studied frequency dependence of fatigue behavior and show that the loss of polarization due to fatigue follows a universal scaling behavior with N/f2, where N is the number of switching cycles and f the frequency. The origin of the scaling is attributed to the drift of oxygen vacancies, which is the rate limiting process in the growth of the interface layer responsible for fatigue. Empirical fits for both undoped and cerium-doped samples show that switchable polarization follows stretched exponential decay with time or N/f. Cerium doping is shown to improve fatigue resistance by impeding the motion of oxygen vacancies.

White organic light-emitting diodes based on spectral broadening in electroluminescence due to formation of interfacial exciplexes

We demonstrate white organic light-emitting diodes (OLEDs) having spectral width of approximately 260 nm in electroluminescence (EL) in a simple bilayer structure, consisting of TPD and zinc benzothiazole, without taking recourse to complex strategies such as blending and doping. The EL is broader than the corresponding photoluminescence (PL) of both component materials and their structures. A deconvolution of PL and EL spectra shows that as large as 60% of the broad EL emission originates from multiple exciplexes formed at the interface during electrical excitation.

Mobility determination using frequency dependence of imaginary part of impedance (Im Z) for organic and polymeric thin films

In view of a growing need for purely electrical techniques of determining carrier mobility under actual device conditions in organic semiconductors, we demonstrate that instead of difference susceptance, the frequency dependence of imaginary part of impedance is a simpler, more convenient, and powerful method of determination of mobility. We illustrate it by using the method to determine field dependence of mobility, and hence Poole–Frenkel coefficient, for the case of small molecule m-MTDATA and polymeric MEH-PPV. We also determine the correct numerical factors required to obtain true transit times and compare that with electroluminescence transient. We also show that these measurements are robust in presence of dispersive transport avoiding interpretative difficulties in determination of mobility.

Leakage mechanism of Ba0.8Sr0.2TiO3/ZrO2 multilayer thin films

The temperature and field dependence of the I-V characteristics of Ba0.8Sr0.2TiO3 thin films and Ba0.8Sr0.2TiO3/ZrO2 multilayer thin films on Pt/Ti/SiO2/Si substrates are studied in the temperature range from 310 to 410 K. Leakage current behaviors of the film is analyzed in the light of various models. The bulk limited Poole–Frenkel mechanism is observed to dominate the leakage current in the temperature range of 310–410 K in the high field region. The energy of the trap levels calculated from the Poole–Frenkel model is in the range of 0.2–1.31 eV for different structures. In the low electric field region the conduction is Ohmic where as space charge limited current conduction is the dominant mechanism in the intermediate electric field region for all the temperatures.

Changes in the leakage currents in Ba0.8Sr0.2TiO3∕ZrO2 multilayers due to modulations in oxygen concentration

Use of multilayer dielectric thin films provides opportunities to optimize properties for different applications. In this work, the changes in the leakage current in Ba0.8Sr0.2TiO3 thin films upon introduction of ZrO2 layers of different thicknesses are studied. Unusual changes in the leakage current and the transition field with variation in the number and thickness of the ZrO2 layers are observed. Profile of the oxygen concentration across the sample, as determined by x-ray photoelectron spectroscopy, shows that the oxygen diffusion length controls the modulation in the concentration of the charged oxygen vacancies, and hence the depletion layer thicknesses, at the interfaces. A qualitative model for the observed behavior is provided.

Optical and microstructural characterization of sol/gel derived cerium-doped PZT thin films

Optical properties of cerium-doped PZT thin films on sapphire prepared by a sol-gel technique are investigated using both transmission and reflection spectra in the wavelength range 200 to 900 nm. The refractive index, extinction coefficient and thickness of the film are determined from the measured transmission spectra. The packing density of the film is calculated from its refractive index using the effective medium approximation (EMA), and average oscillator strength and wavelength are estimated using a Sellmeir-type dispersion equation. Absorption coefficient (α) and the band gap energy (Eg) of each film composition are also calculated. Possible correlations of microstructure and phase formation behaviour with changes in band gap energy and other optical properties are discussed.

TEMPERATURE-TIME DUALITY AND DEEP LEVEL SPECTROSCOPIES

Relaxation of deep levels in semiconductors is studied through capacitance transients. We explore the temperature‐time duality relationship which is inherent in such thermal relaxation processes. Using duality considerations we show the existence of four distinct spectroscopies. We demonstrate that the techniques for spectroscopic evaluation of capacitance transients are based on differential operators and provide a novel interpretation to spectroscopy. We extend this approach to higher order spectroscopy. Two families of higher order spectroscopy are analyzed using the formalism of temperature‐time duality and differential operators. From duality considerations we have suggested a novel deep level spectroscopy as well as various improvements in line shapes and spectroscopic quality of existing techniques.

Electrical characterization of MeV heavy-ion-induced damage in silicon: Evidence for defect migration and clustering

We have studied electrical activity of defects created by high-dose MeV heavy-ion implantation in n-silicon. Heavy damage induced by Ar+ and Au+ ions is embedded within depletion layers of Schottky diodes. The defects are characterized using capacitance–voltage (C–V), current–voltage (I–V), deep-level transient spectroscopy (DLTS) and time analyzed transient spectroscopy techniques. Large concentration of defects in the depletion layer of as-implanted device lead to unusual features in C–V and I–V characteristics. The damage layer is found to extend several microns beyond the ion range or the damage profile predicted by standard Monte Carlo simulation packages. The dominance of a single trap in the damaged region is established from hysteresis effect in C–V, space-charge-limited conduction in forward I–V and DLTS spectrum. With annealing in the temperature range of 400–600 °C, the observed changes in the defect profile indicate that the effective electrical interface between damaged and undamaged layer mo...

Improved dielectric properties and their temperature insensitivity in multilayered Ba0.8Sr0.2TiO3/ZrO2 thin films

The electrical and dielectric properties of Ba0.8Sr0.2TiO3 thin films and Ba0.8Sr0.2TiO3/ZrO2 multilayer thin films deposited on Pt/Ti/SiO2/Si substrates by sol-gel method are studied. The temperature dependence of the dielectric properties for pure Ba0.8Sr0.2TiO3 film and Ba0.8Sr0.2TiO3/ZrO2 multilayer films has been studied in the temperature range from 90 to 500 K. Both dielectric constant and dielectric loss exhibit minimal dispersion as a function of temperature in this range. It is observed that dielectric constant, dielectric loss, and tunability are reduced for multilayer thin films. Additionally, the ferroelectricity disappears in multilayer thin films. Our results show that the multilayered Ba0.8Sr0.2TiO3/ZrO2 design has excellent dielectric properties and they are stable over a broad temperature range (90–500 K), thereby making them excellent candidates for the next generation of enhanced performance temperature stable microwave devices.

Room‐temperature synthesis of copper germanide phase by ion beam mixing

This letter reports room‐temperature synthesis by ion beam mixing of the e1‐Cu3Ge phase which is a promising candidate for interconnect and contact material in very large scale integrated circuit technology. The resistivity of the mixed sample was found to be nearly the same as the one obtained from thermally prepared films. We briefly discuss the likely mechanisms of phase formation and conclude that reaction kinetics dominates over thermodynamic forces during phase formation. The sequence of phase formation is explained by effective heat of formation rule.