Sarita Thakoor

INFLUENCE OF SAMPLE THICKNESS ON THE PERFORMANCE OF PHOTOSTRICTIVE CERAMICS

Ferroelectric materials with high photovoltaic properties are of interest for new optical devices such as photostrictive ceramic actuators. However, fabrication of these devices requires development of materials exhibiting high photovoltage and photocurrent. In pursuit of these high performance photostrictive materials, the present research examines the influence of illumination intensity, degree of polarization, and sample thickness on the photostrictive response of WO3 doped lanthanum-modified lead zirconate titanate (Pb,u2002La)(Zr,u2002Ti)O3 ceramics prepared by oxide mixing process. A model for calculating the optimum sample thickness for maximum photostrictive response is proposed. This model agrees well with experimental results, and should be useful in designing photostrictive devices.

Solid‐state thin‐film memistor for electronic neural networks

We report on a tungsten‐oxide‐based, nonvolatile, electrically reprogrammable, variable resistance device as an analog synaptic memory connection for electronic neural networks. A voltage controlled, reversible injection of H+ ions in electrochromic thin films of WO3 is utilized to modulate its resistance. A hygroscopic thin film of Cr2 O3 is the source of H+ ions. The resistance of the device can be tailored and stabilized over a wide dynamic range (∼four orders of magnitude), and the programming speed is modulated by the control voltage. The suitability of such a device in terms of its response speed, reversibility, stability, and cyclability for its use in electronic neural networks is discussed.

High Tc superconducting NbN films deposited at room temperature

Thin films of niobium nitride with superconducting transition temperature (Tc ) of 15.7 K have been deposited on a variety of amorphous as well as crystalline substrates including glass, glazed ceramic, fused quartz, and sapphire, maintained at room temperature, by dc reactive magnetron sputtering in a mixture of Ar and N2 gases. The effects of the deposition conditions, particularly the carrier gas pressure and composition, on the film crystal structure, orientation, and resistivity have been studied in an effort to maximize the superconducting transition temperature. A study of the variation of nitrogen consumption with nitrogen injection pressures for constant background argon pressures is conducted and found to be an absolute indicator of the NbN formation systematics. Initially, the consumption increases linearly with the injection pressure but beyond a certain threshold, it shows a distinct drop. The desired high Tc u2009NbN with B1 crystal structure is formed in the vicinity of this turning point of th...

High speed, nondestructive readout from thin‐film ferroelectric memory

High speed, polarization direction dependent photoresponse from ferroelectric lead zirconate titanate (PbZr0.53Ti0.47O3) thin films, sandwiched between conducting electrodes to form a memory capacitor, is reported. Laser pulses with a full width at half maximum of ∼10 ns at 532 nm wavelength are utilized to readout the photoresponse signal from individual polarized elements. Such readout is repeated over a million times, with no detectable degradation in the photoresponse or the remanent polarization, suggesting its potential as a nondestructive readout (NDRO) of nonvolatile polarization state in thin‐film ferroelectric memories. In principle, both electronic as well as thermal mechanisms could be triggered by such photon exposure of ferroelectric thin films. A comparison of the photoresponse from capacitors with semitransparent and opaque top electrodes suggests that the observed NDRO signal is primarily due to thermally triggered mechanisms.

A lead‐on‐sapphire superconducting cavity of superior quality

A cavity consisting of a superconducting lead film on a sapphire substrate has been fabricated to obtain the enhanced frequency stability possible with this configuration. The cavity exhibits a quality value Q exceeding 2×109 in its TE011 mode with a resonant frequency of 2.689 GHz. Methods of fabrication and testing of the cavity are presented in this article. Since the interface between the film and substrate is exposed to the full value of the resonant magnetic field, our experiment is the most sensitive test to date for enhanced losses at the interface itself. We find no evidence of such losses. In fact, the measured values of the surface resistance match very well predictions for rf losses based on the BCS theory.

Optically addressed ferroelectric memory and its applications

Abstract Thin films of perovskite titanates can be composition-tailored to exhibit ferroelectric, pyroelectric, or piezoelectric properties, in varying degree of combinations, and thereby modulating their response when illuminated with light. This paper reviews the potential applications of photoresponse from lead zirconate titanate thin films. In ferroelectric materials, such as lead zirconate titanate (PZT), for example, the photoresponse shows a clear dependence on remanent polarization. The main highlight of the paper is a review of the concept of optically addressed ferroelectric memory and identification of its high impact applications. Incidence of energetic laser beam pulses gives rise to two different classes of phenomena: first, a thermally triggered piezoelectric/pyroelectric response and second, an optoelectronic response. Optimizing the device geometry and selecting the illumination characteristics, one can easily control the dominating mechanism in a device. The optoelectronic effect emerges...

High speed optoelectronic response from the edges of lead zirconate titanate thin film capacitors

This letter reports on a high speed (∼10 ns), nondestructive, polarization‐dependent photoresponse from lead zirconate titanate (PZT) thin films, when illuminated with λ=532 nm, at ∼mW/μm2 of incident optical power. This photoresponse emerges primarily from the edges of the sandwich ferroelectric capacitors, consisting of PZT films with predominant c‐axis orientation. This response, in contrast with the thermally triggered effect, is unipolar in nature and occurs at an order of magnitude lower power level, and therefore offers a greater application potential. Its dependence on the crystal orientation and remanent polarization in the PZT film, and therefore its ability to track the built‐in E fields within the film, may offer such photoresponse as a high speed nondestructive evaluation tool.

TdC20. High speed opto-electronic non-destructive readout prom ferroelectric thin film capacitors

Abstract Polarization dependent photoresponse from ferroelectric lead zirconate titanate (PbZr0. 53Ti0. 47O3) thin films sandwiched between metal electrodes in a capacitor configuration is reported. This phenomenon has potential application as a non-destructive readout(NDRO) of nonvolatile polarization state of thin film ferroelectric memories. High speed readout using laser pulses with full width at half maximum of ∼10ns, at 532 nm wavelength is demonstrated. The polarization direction of the ferroelectric capacitor is reflected in the direction of the photocurrent response. The rise time of the photocurrent response is as fast as 25 ns and the relaxation time is fraction of a microsecond. The readout signal from individual polarized elements is repeated over a million times with no detectable degradation in the photoresponse or the remanent polarization as verified independently by the conventional destructive readout technique. In principle, both electronic as well as thermal mechanisms could be trigge...

Optically addressed ferroelectric memory with nondestructive readout

We present a review of the emerging optically addressed ferroelectric memory with nondestructive readout as a nonvolatile memory technology, identify its high-impact applications, and project on some novel device designs and architectures that will enable its realization. Based on the high-speed bidirectional polarization-dependent photoresponse, simulation of a readout circuit for a 16-kbit VLSI ferromemory chip yields read-access times of ~20 ns and read-cycle times of ~30 ns (~34 ns and ~44 ns, respectively, within a framework of a radiation-hard environment), easily surpassing those of the conventional electrical destructive readout. Extension of the simulation for a 64-kbit memory shows that the read-access and -cycle times are only marginally increased to ~21 ns and ~31 ns, respectively (~38 ns and ~48 ns, with a radiation-hard readout circuitry). Commercial realization of the optical nondestructive readout, however, would require a reduction in the incident (optical) power by roughly an order of magnitude for the readout or an enhancement in the delivered power-to-size ratio of semiconductor lasers for compact implementation. We present a new two-capacitor memory-cell configuration that provides an enhanced bipolar optoelectronic response from the edges of the capacitor at incident power as low as ~ 2 mW/µm(2). A novel device design based on lead zirconate titanate with the c axis parallel to the substrate is suggested to reduce the requirement of incident optical power further by orders of magnitude.

An optical probe for ferroelectric thin film memory capacitors

Abstract An optical probing of sol-gel derived polycrystalline thin films of lead zirconate titanate (PZT), sandwiched between two metal electrodes to form a memory capacitor, is described. In principle, both electronic as well as thermal mechanisms could be triggered by photon exposure on to ferroelectric thin films. Choice of the illumination intensity and wavelength, combined with the nature of the top electrode of the device determine the predominant phenomenon observed. The effects of two distinct optical probes: first, a non-coherent (300 nm to 600 nm), relatively weak intensity source and second, an energetic laser pulse are described. The photoresponse to the non-coherent illumination source is observed to be a probe of the space charge distribution within the ferroelectric thin film. On the other hand, utilizing laser pulses, with a full width at half maximum of ∼10 ns at 532 nm wavelength, a high speed nondestructive probing of the polarization state of the memory is demonstrated. The polarizati...