O. P. Thakur

Magneto-dielectric coupling and transport properties of the ferromagnetic-BaTiO3 composites

Ferromagnetic and large magnetoresistance (MR) nanocomposites of La0.67Sr0.33MnO3-BaTiO3 (LSMO-BTO) are synthesized via sol-gel route. The X-ray diffraction confirms the existence of two chemically separated phases in the composites. The maximum MR (35%) was achieved in LSMO-5% BTO (LB5). The coupling between the coexisting phases is observed from the dielectric anomaly at the ferromagnetic transition (Tc = 353 K) for LB5 composition. We observed maximum magnetodielectric effect at Tc of 1.18% in magnitude for LB5 and the effect of magnetic field on other composites was significant. These results are related to the large spin polarization within grains as well as at the grain boundaries and the evidence of variation in dielectric parameters with magnetic field reveal the magnetoelectric coupling in LSMO-BTO nanocomposites.

Modeling of capacitive sensor filled with elastic dielectrics and its advantages

Electrostriction phenomena and mathematical modeling in respect of capacitive sensor filled with elastic dielectric material have been described. A comparative theoretical analysis of the traditional air gap capacitive sensor with the electrostriction based solid-state capacitive sensor has been carried out thoroughly. Difficulties and limitations with air gap capacitive sensor are highlighted. Solid-state capacitive sensor with elastic dielectric material sandwiched between the compliant electrodes exhibits higher mechanical and electrical stability, sensitivity, higher tolerance with environment and withstands large load in comparison to the air gap capacitive sensor. An experimental setup with proper solution has been described for the measurement of relative capacitance variation. An alternative method of current-mode bridge has also been suggested for the purpose of sensing pressure and strain with high accuracy.

Effect of Fillers on Electromechanical Properties of Composites for Potential Sensing Applications

Among dielectrics, electrostrictive materials possess good electromechanical properties and are preferred in many applications, including tactile sensing. These properties can be further enhanced by addition of filler particles of materials like Al2O3, SiO2, TiO2, ZnO, and so forth. Improved properties of such composite materials result in enhancement of sensors response. The objective of this paper is to analyze and compare the improvement in properties of these composite dielectric materials. Graphs have been plotted for values of tan delta and relative real permittivity with respect to variation of TiO2 and ZnO filler particle concentration. It is found that in nanocomposite with ZnO filler particles, reduction in losses is more as compared with TiO2 filled nanocomposite, whereas TiO2 is a better filler material in comparison to ZnO for increasing permittivity of the nanocomposite. Further study has been done in respect of increase in interfacial volume fraction for various sizes and concentrations of filler particles. It is observed that when filler particle size is reduced from 5 to 0.5 nm, interfacial volume fraction increases by 900%. Another aspect studied is variation in Youngs modulus on increasing volume percentage of microfillers for four different sizes in range (0.15-0.92 μm). For all sizes of microfillers, Youngs modulus initially increases indicating enhancement in mechanical strength or toughness of composite, but later on, it starts decreasing. It is also observed that there is diverse opinion among researchers on the electromechanical properties of the composite and analysis has been made on the possible reasons for such diverse opinion.

Be ion irradiation induced p- to n-type conversion in HgCdTe

Be ion implantation/irradiation in HgCdTe has been explored from the point of view of fabricating n region on p-type structures. Be implantation was carried out in vacancy doped p-type bulk HgCdTe crystals at 200KeV ion energy and a fluence of 1×1014ions∕cm2. Implanted samples were annealed at 150°C under nitrogen ambient for different durations. Be appears to be settling as an isoelectronic impurity in HgCdTe after the implant annealing process. A n-type region with reasonably good electron mobility is formed as a consequence of the redistribution of irradiation induced defects.

Comprehensive magnetotransport characterization of two dimensional electron gas in AlGaN/GaN high electron mobility transistor structures leading to the assessment of interface roughness

Magnetotransport in two distinct AlGaN/GaN HEMT structures grown by Molecular Beam Epitaxy (MBE) on Fe-doped templates is investigated using Shubnikov de-Haas Oscillations in the temperature range of 1.8–6 K and multicarrier fitting in the temperature range of 1.8–300 K. The temperature dependence of the two dimensional electron gas mobility is extracted from simultaneous multicarrier fitting of transverse and longitudinal resistivity as a function of magnetic field and the data is utilized to estimate contribution of interface roughness to the mobility and the corresponding transport lifetime. The quantum scattering time obtained from the analysis of Shubnikov de Haas Oscillations in transverse magnetoresistance along with the transport lifetime time were used to estimate interface roughness amplitude and lateral correlation length. The results indicate that the insertion of AlN over layer deposited prior to the growth of GaN base layer on Fe doped GaN templates for forming HEMT structures reduced the parallel conduction but resulted in an increase in interface roughness.

Evaluation of B+-implanted n+-n-p HgCdTe structures using transient microwave reflectance

Transient microwave reflectance (TMWR) has been used to investigate the n+-n-p HgCdTe structures formed by B+ implantation and moderate annealing. Vacancy-doped p-type bulk HgCdTe (x∼0.285) crystals were used in this work. 130 keV B+ ions were implanted at a dose of 1×1014 cm−2. Samples were successively thinned through etching in 0.5% Br2 in CH3OH solutions with a standardized procedure for differential Hall/resistivity measurements. At every differential etch step, the TMWR signal was also recorded and theoretically analyzed for estimating bulk lifetime and surface recombination velocity. Results for as-implanted as well as moderately annealed samples have been presented. The study indicated the usefulness of TMWR analysis in evaluating implant anneal process for the formation of n+-n-p HgCdTe structures for photodetector fabrication.

Convexity of internal energy of cubic crystal deformed to orthorhombic structure

A generalized set of strain variablesqrN, has been defined to develop the expression for a generalized set of second order and third-order elastic moduliCrsN andCrstN for a cubic crystal deformed to orthorhombic structure. The HessainCrsNδqrδqs andCrstNδqrδqsδqt (r=1, 2……6; summation convention) are calculated in the new variables and compared withG-strength andS-strength, for both positive and negative loading environment.The convexity of the internal energy relative to various choice of strain measure is examined considering up to third degree terms in the internal energy expression. The computational results forbcc iron is presented according to the new moduli. The stable ranges thus obtained for iron under hydrostatic compressive and tensile stresses is found to generate the classical stable range, green-stable range and stretch-stable range as the specific cases. However,bcc iron does not seem to follow any conventional stable ranges under hydrostatic compression, where the present generalized stable range is found satisfactory.

Large negative magnetoresistance induced by interplay between smooth disorder and antidots in AlGaN/GaN HEMT structures

Large low temperature negative magnetoresistance (NMR) experimentally observed in AlGaN/GaN high electron mobility transistors (HEMT) structures grown by metalorganic chemical vapour deposition on sapphire substrate has been reported. A linear B-1 ln B dependence of magnetoresistance observed in our samples indicates the presence of random antidot array together with smooth disorder. It is proposed that the antidots are linked with high bandgap AlN rich regions formed due to possible Al-Ga segregation at the interface during growth and the smooth random disorder is due to interface roughness. The antidot density is estimated to be of similar to 7 to 8 x 10(10) cm(-2) in our samples. The magnitude ofNMRis also correlated with the extent of interface roughness indicated by x-ray reflectivity. It is also proposed that the formation of antidots is related with the lattice mismatch between substrate and epitaxial heterostructures. The NMR in AlGaN/GaN HEMT structures grown on SiC substrates having relatively lower lattice mismatch has been shown to have a usual B-2 and ln T dependences indicating only electron-electron interaction and absence of antidot-like scatterers.

Modelling of Sensing Performance of Electrostrictive Capacitive Sensors

Electrostriction is the quadratic coupling between the strain developed in a material and the electric field applied, while piezoelectricity is a linear coupling mechanism existing in a material without center of symmetry. Electrostrictive capacitive sensors have many advantages over vacuum or air gap capacitive sensors. Electrostrictive materials show reproducible, non-hysteric and tunable strain response. Electrostrictive dielectric material sandwiched between two electrodes in a capacitive sensor plays a very important role in performance of the sensor. The dielectric material to be selected is required to possess good electromechanical properties like high strain, high permittivity, good breakdown strength etc.

Observation of Low Mobility Electron in Vacancy Doped LPE Grown HgCdTe

Variable magnetic field Hall and resistivity data at different temperatures for vacancy doped LPE grown Hg0.71Cd0.29Te samples have been analyzed using multicarrier fitting. Samples grown from Te-rich melts by Horizontal Slider techniques have been investigated. Measurements were carried out at temperatures from 20 to 300 K using magnetic fields in 0–8 Tesla range. In addition to heavy hole and light hole an electron with low mobility (77 K value of ~812 cm2V−1s−1) was observed at temperatures below 150 K. Its presence has been attributed to interface as confirmed by Hall measurements of the interfacial layer (~4 µm above CdZnTe substrate) and is reported here for HgCdTe for the first time.