arXiv subjects

R. Ramaseshan

Publications and source records attributed to R. Ramaseshan.

8 recordsLinked to original sources

arXiv2025

Mitigating parasitic contributions in measured piezoresponse for accurate determination of piezoelectric coefficients in Sc-alloyed-AlN thin films using piezo-response force microscopy

We present a methodology to mitigate the effect of the parasitic electrostatic contribution usually present in piezoresponse force microscopy (PFM) measurement for quantitative characterization of polycrystalline piezoelectric thin films using a case study on a set of Al1-xScxN thin films. It involves minimizing the voltage sensitivity of the measured piezoresponse by optimizing the optical lever sensitivity using the laser positioning of the beam-bounce system. Additionally, applying a dc-voltage offset (determined through Kelvin probe force microscopy) during PFM scans and positioning the probe over the interior or edge portion of the specimen are explored to minimize the local and non-local electrostatic tip-sample interaction. The results shows that the effective piezoelectric coefficient (d33-eff) of our c-axis oriented wurtzite (wz)-Al1.0Sc0.0N thin film is 4.9 pm per Volt. The highest enhancement in the d33-eff value occurred in the wz-Al0.58Sc0.42N thin film. Above x = 0.42, the d33-eff reduces due to phase-mixing of the wz-Al1-xScxN phase with cubic-Sc3AlN phase till the piezoelectricity finally disappear at x = 0.51

cond-mat.mtrl-sci↗

arXiv2024

Structural, optical and mechanical properties of Cr doped \b{eta}-Ga2O3 single crystals

Undoped and Cr doped \b{eta}-Ga2O3 (100) single crystals are grown by optical floating zone method. The full width at half maximum of rocking curve is found to be 106 arcsec for undoped Ga2O3 crystals whereas the 100 and 200 ppm of Cr doped Ga2O3 crystals display multiple rocking curves with large peak widths indicating the presence of structural defects. Raman measurements reveal broadening in the vibrational mode of ~ 350 cm-1 with a shoulder peak indicating the Cr3+ dopants preferentially substitute for Ga3+ at the octahedral sites. Further, the Cr doped Ga2O3 crystals display strong optical absorption bands about 420 and 597 nm in the UV-Vis spectroscopy. Moreover, the observation of sharp characteristic photoluminescence emission lines at 690 and 697 nm also confirms the Cr substitution in the doped crystals. The indentation hardness increases nearly linear from 13.0 to 17.9 GPa whilst the indentation modulus decreases from 224.9 to 202.4 GPa upon Cr doping of 200 ppm in \b{eta}-Ga2O3. The structural defects caused by the Cr doping interrupt the movement of indentation induced dislocations that results in the increase of hardness of the Cr doped \b{eta}-Ga2O3 (100) single crystals.

cond-mat.mtrl-sci↗

arXiv2020

Structural, optical and mechanical properties of Y2Ti2O7 single crystal

We report on the growth of Y2Ti2O7 single crystals by optical floating zone technique. X-ray diffraction and Raman spectroscopy studies confirm the structural quality of the crystal. The UV-Vis optical studies reveal that the grown crystals have a high optical transparency with an optical band gap of 3.44 eV. The hardness of Y2Ti2O7 single crystal is measured for the first time using nanoindentation. The measured hardness, indentation and bulk modulus are found to be 16.4$\pm$0.4, 321.1$\pm$6.9 and 243.3$\pm$5.2 GPa respectively, which are higher than its polycrystalline counterpart and its constituent metal oxides, Y2O3 and TiO2.

cond-mat.mtrl-sci↗

arXiv2018

Spectroscopic Ellipsometry investigation on anisotropic optical properties of sputtered AlN films

We report the uniaxial anisotropic optical constants of Wurtzite type AlN films deposited on Si (100) substrate using DC reactive magnetron sputtering as a function of growth temperature (Ts, 35 to 600 C). Evolution of optical properties with Ts is investigated by Spectroscopic Ellipsometry (SE) technique. Thickness and roughness of these films are determined from the regression analysis of SE data, which are corroborated using TEM and AFM technique. Highly a-axis oriented AlN film grown at 400 C, exhibits high n and low k at 210 nm (deep-UV region) with a small birefringence and dichroism near to band edge, which can be used in isotropic deep UV optoelectronic device applications. All these AlN films exhibit transparent nature from near-infrared (NIR) to 354 nm, where optical band gap energies vary between 5.7 to 6.1 eV.

cond-mat.mtrl-sci↗

arXiv2016

Reduction of residual stress in AlN thin Films synthesized by magnetron sputtering technique

We report the reduction in residual stress of AlN thin films and also the crystal structure, surface morphology and nanomechanical properties of magnetron sputtered as a function of substrate temperature (Ts, 35 - 600 ?C). The residual stress of these films was calculated by sin2 technique and found that they are varying from tensile to compression with temperature (Ts). Evolution of crystalline growth of AlN films was studied by GIXRD and transmission electron microscopy (TEM) and a preferred a-axis orientation was observed at 400 ?C. The cross-sectional TEM micrograph and selected area electron difraction (SAED) of this film exhibited a high degree of orientation as well as a columnar structure. Hardness (H) measured by Nanoindentation technique on these films ranged between 12.8 - 19 GPa.

cond-mat.mtrl-sci↗

arXiv2014

Preferentially orientated E-beam TiN thin films using focused jet of nitrogen gas

A modified electron beam evaporator has been used judiciously to synthesize TiN thin films with (111) preferred orientation. This new design involved in creating local plasma by accelerating the secondary electrons emitted from the evaporating ingot by a positively biased semi-cylindrical anode plate kept in the vicinity and a jet of N2 gas has been focused towards the substrate as a reactive gas. We have observed a preferred orientation (111) with 25 degree angle to the surface normal and this was confirmed by pole figure analysis. The phenomenon of preferred orientation (111) has been explained based on the rate of evaporation. The residual stress by the classical sin2psi technique did not yield any tangible result due to the preferred orientation. The hardness and modulus measured by nanoindentation technique was around 19.5 GPa and 214 GPa. The continuous multicycle indentation test on these films exhibited a stress relaxation.

cond-mat.mtrl-sci↗

arXiv2013

Temperature dependence of dielectric constants in Titanium Nitride

The temperature dependence of optical constants of titanium nitride thin film is investigated using spectroscopic ellipsometry between 1.4 to 5 eV in the temperature range 300 K to 650 K in steps of 50 K. The real and imaginary parts of the dielectric functions "1(E) and "2(E) increase marginally with increase in temperature. A Drude Lorentz dielectric analysis based on free electron and oscillator model are carried out to describe the temperature behavior. The parameters of the Lorentz oscillator model also showed that the relaxation time decreased with temperature while the oscillator energies increased. This study shows that owing to marginal change in the refractive index with temperature, titanium nitride can be employed for surface plasmon sensor applications even in environments where rise in temperature is imminent.

cond-mat.mtrl-sci↗

arXiv2012

Optical properties and hardness of highly a-axis oriented AlN films

This paper reports optical and nanomechanical properties of seldom studied highly a-axis oriented AlN thin films for the first time. These films were deposited by reactive DC magnetron sputtering technique at an optimal target to substrate distance of 180 mm. Bragg-Brentano geometry X-ray and rocking curve (FWHM = 52 arcsec) studies confirmed the preferred orientation. Spectroscopic ellipsometry revealed that these films exhibit a refractive index of 1.93 at a wavelength of 546 nm. The hardness and elastic modulus of these films were 17 GPa and 190 GPa, respectively. The mechanical properties obtained here are much higher than the earlier reported and therefore can be useful as protective coating in thermo printing devices, piezoelectric films in bulk acoustic wave resonators.

cond-mat.mtrl-sci↗