S. S. N. Bharadwaja

Dielectric and piezoelectric properties of lead-free (Bi,Na)TiO3-based thin films

Dielectric and piezoelectric properties of morphotropic phase boundary (Bi,Na)TiO3–(Bi,K)TiO3–BaTiO3 epitaxial thin films deposited on SrRuO3 coated SrTiO3 substrates were reported. Thin films of 350 nm thickness exhibited small signal dielectric permittivity and loss tangent values of 750 and 0.15, respectively, at 1 kHz. Ferroelectric hysteresis measurements indicated a remanent polarization value of 30 μC/cm2 with a coercive field of 85–100 kV/cm. The thin film transverse piezoelectric coefficient (e31,f) of these films after poling at 600 kV/cm was found to be −2.2 C/m2. The results indicate that these BNT-based thin films are a potential candidate for lead-free piezoelectric devices.

Low temperature charge carrier hopping transport mechanism in vanadium oxide thin films grown using pulsed dc sputtering

Low temperature charge transport in vanadium oxide (VOx) thin films processed using pulsed dc sputtering is investigated to understand the correlation between the processing conditions and electrical properties. It is identified that the temperature dependent resistivity ρ(T) of the VOx thin films is dominated by a Efros–Shklovskii variable range hopping mechanism [Efros and Shklovskii, J. Phys. C 8, L49 (1975)]. A detailed analysis in terms of charge hopping parameters in the low temperature regime is used to correlate film properties with the pulsed dc sputtering conditions.

Designing piezoelectric films for micro electromechanical systems

Piezoelectric thin films are of increasing interest in low-voltage micro electromechanical systems for sensing, actuation, and energy harvesting. They also serve as model systems to study fundamental behavior in piezoelectrics. Next-generation technologies such as ultrasound pill cameras, flexible ultrasound arrays, and energy harvesting systems for unattended wireless sensors will all benefit from improvements in the piezoelectric properties of the films. This paper describes tailoring the composition, microstructure, orientation of thin films, and substrate choice to optimize the response. It is shown that increases in the grain size of lead-based perovskite films from 75 to 300 nm results in 40 and 20% increases in the permittivity and piezoelectric coefficients, respectively. This is accompanied by an increase in the nonlinearity in the response. Band excitation piezoresponse force microscopy was used to interrogate the nonlinearity locally. It was found that chemical solution-derived PbZr0.52Ti0.48O3 thin films show clusters of larger nonlinear response embedded in a more weakly nonlinear matrix. The scale of the clusters significantly exceeds that of the grain size, suggesting that collective motion of many domain walls contributes to the observed Rayleigh behavior in these films. Finally, it is shown that it is possible to increase the energy-harvesting figure of merit through appropriate materials choice, strong imprint, and composite connectivity patterns.

Vanadium oxide thin films for bolometric applications deposited by reactive pulsed dc sputtering

Vanadium oxide (VOx) thin films were deposited by reactive pulse dc magnetron sputtering process using a pure vanadium metal target. The structural, microstructure, and electrical properties were correlated as a function of processing parameters such as substrate temperature, Ar:O partial pressures ratios, and pulsed dc power to fabricate these films. The VOx films deposited at various substrate temperatures between 30 and 300°C using a range of oxygen to argon partial pressure ratios exhibited huge variation in their microstructure even though most of them are amorphous to x-ray diffraction technique. In addition, the electrical properties such as temperature coefficient of resistance (TCR), resistivity, and noise levels were influenced by film microstructure. The TCRs of the VOx films were in the range of −1.1%to−2.4%K−1 having resistivity values of 0.1–100Ωcm. In particular, films grown at lower substrate temperatures with higher oxygen partial pressures have shown finer columnar grain structure and ex...

Highly textured laser annealed Pb(Zr0.52Ti0.48)O3 thin films

RF sputtered amorphous Pb(Zr0.52Ti0.48)O3 (PZT) films (∼300–350 nm in thickness) on {111}Pt/Ti/SiO2/Si or {001}PbTiO3/Pt/Ti/SiO2/Si substrates were laser crystallized to obtain highly textured {111} and {001} PZT thin films. The measured remanent polarizations and coercive fields were 31 µC/cm2 and 86 kV/cm for {001} films and 24 µC/cm2 and 64 kV/cm for {111} oriented PZT films, respectively. The maximum e31,f piezoelectric charge coefficients are ∼ −11 C/m2 for {001} and ∼ −9 C/m2 for {111} PZT thin films respectively.

Process-structure-property correlations in pulsed dc reactive magnetron sputtered vanadium oxide thin films

Cathode hysteresis in the reactive pulsed dc sputtering of a vanadium metal target was investigated to correlate the structural and electrical properties of the resultant vanadium oxide thin films within the framework of Berg’s model [Berg et al., J. Vac. Sci. Technol. A 5, 202 (1987)]. The process hysteresis during reactive pulsed dc sputtering of a vanadium metal target was monitored by measuring the cathode (target) current under different total gas flow rates and oxygen-to-argon ratios for a power density of ∼6.6.W/cm2. Approximately 20%–25% hysteretic change in the cathode current was noticed between the metallic and oxidized states of the V-metal target. The extent of the hysteresis varied with changes in the mass flow of oxygen as predicted by Berg’s model. The corresponding microstructure of the films changed from columnar to equiaxed grain structure with increased oxygen flow rates. Micro-Raman spectroscopy indicates subtle changes in the film structure as a function of processing conditions. The...

In situ laser annealing during growth of Pb(Zr0.52Ti0.48)O3 thin films

A pulsed laser deposition system with in situ laser annealing was utilized to grow Pb(Zr0.52Ti0.48)O3 thin films on a laser crystallized Pb(Zr0.20Ti0.80)O3 seed layer, at a temperature of ∼370 °C. Polycrystalline 1.1 μm thick films exhibited columnar grains with small grain sizes (∼30 nm). The films showed well-saturated hysteresis loops (with ∼25 μC/cm2 remanent polarization, ∼50 kV/cm coercive field) and exhibited loss tangents <2.5% with a permittivity of ∼730. Film orientation could be controlled via the substrate choice; {111} Pb(Zr0.52Ti0.48)O3 films were grown on oriented (111) Pb(Zr0.30Ti0.70)O3 sol-gel seed layers, while {001} films were prepared on (100) SrTiO3 single crystals.

Enhanced electrical and noise properties of nanocomposite vanadium oxide thin films by reactive pulsed-dc magnetron sputtering

Thin films of VOx (1.3 ≤ x ≤ 2) were deposited by reactive pulsed-dc magnetron sputtering of a vanadium metal target while RF-biasing the substrate. Rutherford back scattering, glancing angle x-ray, and cross-sectional transmission electron microscopy measurements revealed the formation of nanocolumns with nanotwins within VOx samples. The resistivity of nanotwinned VOx films ranged from 4 mΩ·cm to 0.6 Ω·cm and corresponding temperature coefficient of resistance between −0.1% and −2.6% per K, respectively. The 1/f electrical noise was analyzed in these VOx samples using the Hooge-Vandamme relation. These VOx films are comparable or surpass commercial VOx films deposited by ion beam sputtering.

Ultrafast crystallization kinetics in (Pb,La)(Zr 0.30 Ti 0.70 )O 3 thin films by pulsed excimer laser annealing

The crystallization kinetics of laser-annealed La-modified Pb(Zr,Ti)O3 (PLZT) thin films on LaNiO3-coated silicon substrates were investigated for substrate temperatures below 400°C. A KrF excimer laser having a ~20 ns pulse width and an energy density ~40 mJ/cm2 was used to crystallize the films. The perovskite phase developed with cumulative laser pulse exposures; it was found that ~380 to 400 nm thick films could be fully crystallized for a total exposure time of 0.1 to 1 ms. Laser-crystallized films exhibited comparable dielectric and ferroelectric properties to those prepared by rapid thermal annealing at 650°C for 1 min. The evolution of the dielectric properties as a function of the number of laser strikes suggests that once nuclei are present, they rapidly grow through the depth of the film. This is consistent with the electron microscopy results, which did not show a well-defined planar growth front that proceeds from the top to the bottom of the film. The resulting films showed comparatively large lateral grain sizes (on the order of 250 to 300 nm), with high defect concentrations. The nucleation and growth mechanisms were modeled using Avrami kinetics under rate-dependent and nonisothermal conditions. These results indicate that PLZT crystallization via laser annealing is nucleation-limited.

Extensions of molecular ruler technology for nanoscale patterning

By combining optical lithography and chemical self-assembly, the authors circumvent the limitations of photolithography and provide a parallel, low-cost alternative to fabricate sub-50nm features. Self-assembled multilayers, composed of alternating layers of α,ω-mercaptoalkanoic acids and copper (II) ions (“molecular rulers”), are used as an organic sidewall spacer resist on initial lithographic structures enabling the precise, proximal placement of a secondary structure via lift-off. Here, the authors implemented a positive-tone bilayer resist for improved line-edge characteristics of the secondary structure and evaluated the lithographic and electrical performance of nanostructures fabricated using this approach. Additionally, they describe extensions of this technique by which planar nanojunctions were created, and the generated nanometer-scale pattern was transferred to the underlying substrate.