Rajeswari Kolagani

Large photoinduced conductivity reduction in thin films of metallic ferromagnetic manganites

This paper reports on a study of photoinduced resistivity changes in thin films of ferromagnetic metallic (FMM) manganites. We have observed a significant increase of resistance in La0.7Ba0.3MnO3 thin film under continuous wave argon ion laser illumination presumably associated with photoinduced demagnetization. Strong dependence of resistance on thermal/illumination history is consistent with the coexistence of two phases: ferromagnetic metallic phase and photoinduced less conductive phase.

Thermal stability of SrTiO3/SiO2/Si Interfaces at Intermediate Oxygen Pressures

The thermal stability of epitaxial SrTiO3 thin films grown by molecular-beam epitaxy on Si (001) has been studied using x-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and thermodynamic calculations. Our studies focus on the stability of the SrTiO3/Si structures under the conditions typically employed in the pulse laser deposition (PLD) growth of complex metal oxide heteroepitaxy on Si. We observe additional Bragg peaks in thermally treated SrTiO3 buffered Si structures, corresponding to possibly TiSi2 and/or SrSiO3, reaction products which are consistent with the reaction schemes we propose. In addition, OM and SEM reveal microstructures that are not readily accounted for solely by the solid state reactions as put forth by previous workers but can be reasonably explained by our proposed reaction schemes. Using our observations and thermodynamic analysis, we argue that reactions involving the gaseous species SiO(g), the reactivity of which has not been previously cons...

Heteroepitaxy of Nd0.67Sr0.33MnO3 on silicon for bolometric x-ray detector application

We have recently reported the design concept and sensor fabrication for a novel bolometric x-ray detector based on a rare earth manganite material for application as a total energy monitor for the Linac Coherent Light Source (LCLS) free electron laser at the Stanford Linear Accelerator Center (SLAC). The detector employs epitaxial thin films of Nd(0.67)Sr(0.33)MnO(3) grown on Si by pulsed laser deposition. In this paper we report details of the fabrication of the actual detector, its response characteristics under photon illumination from LCLS, and improvements in the growth scheme of the sensor material on Si using a buffer/template layer scheme that employs yttria-stabilized zirconia, cerium oxide (CeO(2)), and bismuth titanate (Bi(4)Ti(3)O(12)). The thermal sensor response changes linearly with the energy of an optical calibration laser as expected, and the signals from optical and x-ray pulses at LCLS are very similar, thereby validating the design concept. To the best of our knowledge, the LCLS detector application reported here is the first practical use of colossal magnetoresistive manganite bolometers.

Charge order in photosensitive Bi0.4Ca0.6MnO3 films

We report the structural and electronic characterization of the charge order phase in Bi0.4Ca0.6MnO3 films, in which photoinduced resistivity changes have been observed at temperatures approaching room temperature. Lattice distortions associated with the charge order are observed in all films, and both the wavevectors and displacements of the distortions are in the plane of the film. Films under compressive and tensile strain are observed to have different resonant x-ray scattering characteristics—a result that may shed light on the mechanism responsible for the photosensitivity exhibited by this material.

Effects of oxygen depletion on photoinduced and transport properties of thin films of charge-ordered manganites

In this paper, we report a study of conductive and photoinduced properties of charge-ordered Bi0.4Ca0.6MnO3 thin films with different oxygen contents. The oxygen content of the films was varied following different annealing or deposition protocols. Change in film’s resistivity during annealing reflects oxygen dynamics at high temperatures. It was found that oxygen-deficient films exhibit a significant increase in the photoinduced resistivity changes and the lifetime of photoinduced conductive phase. Possible origin of this effect is discussed.

Fabrication of Cryogenic Manganite Bolometers to Measure the Total Energy at the LCLS Free Electron X-Ray Laser

We are developing cryogenic bolometers to measure the total energy of the Linac Coherent Light Source (LCLS) free electron X-ray laser that is currently being built at the Stanford Linear Accelerator Center. LCLS will produce ultrashort ~ 200 fs X-ray laser pulses with ~ 1013 photons at 0.8 keV up to ~ 1012 photons at 8 keV per pulse at a repeat interval as short as 8 ms, and will be accompanied by a halo of spontaneous undulator radiation. Our bolometer consists of a 375 mu m thick Si absorber and a Nd0.67 Sr0.33 MnO3 sensor operated at its metal-insulator transition. It will measure the total energy of each pulse with a precision of < 1%, and is designed to meet the conflicting requirements of radiation hardness, sensitivity, linearity over a dynamic range of three orders of magnitude, and readout speed compatible with the LCLS pulse rate. Here we discuss bolometer design and fabrication, and the photoresponse of prototype devices to pulsed optical lasers.

Characterization of surface modification in atomic force microscope-induced nanolithography of oxygen deficient La0.67Ba0.33MnO3−δ thin films

We report our studies of the nanolithographic surface modifications induced by an Atomic Force Microscope (AFM) in epitaxial thin films of oxygen deficient Lanthanum Barium Manganese Oxide (La0.67Ba0.33MnO3−δ). The pattern characteristics depend on the tip voltage, tip polarity, voltage duration, tip force, and humidity. We have used Electron Energy Dispersive X-Ray Spectroscopy (EDS) to analyze the chemical changes associated with the surface modifications produced with a negatively biased AFM tip. A significant increase in the oxygen stoichiometry for the patterned regions relative to the pristine film surface is observed. The results also indicate changes in the cation stoichiometry, specifically a decrease in the Lanthanum and Manganese concentrations and an increase in the Barium concentration in the patterned regions.

Epitaxial integration of photoresponsive Bi0.4Ca0.6MnO3 with Si(001)

Previously it has been shown that the resistivity of Bi0.4Ca0.6MnO3 epitaxial thin films on oxide substrates decreases significantly upon illumination with visible light. The resistivity decrease is observed over a wide temperature range and is understood as arising due to the destruction of charge ordering. The light responsivity makes Bi0.4Ca0.6MnO3 thin films attractive for photonic and optoelectronic device applications. In this paper, we report the heteroepitaxy of Bi0.4Ca0.6MnO3 thin films on (001) Si which is relevant for the potential integration of the optoelectronic/photonic functionality of Bi0.4Ca0.6MnO3 with semiconductor electronics. As in the case of other perovskite oxides, heteroepitaxy of Bi0.4Ca0.6MnO3 on Si requires the use of buffer layers to circumvent the problems associated with the presence of an amorphous native silicon dioxide layer and the reactivity of perovskite oxides with Si at high temperatures. We demonstrate that high quality epitaxial thin films of Bi0.4Ca0.6MnO3 can be...

Nonbolometric photoresponse in (La,Pr)0.67Ca0.33MnO3 thin films

We have studied light-induced resistance changes (photoresponse) in the colossal magnetoresistive manganite material (La,Pr)0.67Ca0.33MnO3. The metal-insulator transition in this material is understood to be driven by the percolation of the metallic channel in an inhomogeneous matrix of insulating and metallic components. Our experiments reveal a nonthermal (nonbolometric) component of the light-induced resistance change, in addition to the expected resistance change related to heating (bolometric effect). This nonthermal component is seen only in the metal-insulator transition region. Our results suggest that this component may be associated with the light-induced resistance decrease in the insulating regions through an electronic mechanism.

A New Field Emission Scanning Electron Microscopy Facility with STEM and EDS Capabilities for Interdisciplinary Research and Education at Towson University, Fisher College of Science and Mathematics

Recently, Towson University (TU) was successful in acquiring a new state-of-the-art Thermo Fisher Scientific APREO LoVac Schottky Field Emission Gun (FEG) Scanning Electron Microscope (SEM). This microscope is a high-performance SEM that combines highand low-voltage ultra-high resolution (~1 nm) capabilities with an electrostatic lens design. In addition, it features beam deceleration and unique in-lens detection providing unprecedented contrast and versatility of samples for research, education, and training activities across multiple disciplines, in the Fisher College of Science and Mathematics (FCSM). The instrument uses an entirely oil-free vacuum system and is standardly equipped with a five-axes motorized x-y-z-tilt-rotate stage and features an Everhart-Thornley highvacuum secondary electron detector (SED) optimized for use across the available kV, current, and pressure range. It bears a Trinity detection system, comprised of a lower in-lens detector (T1), upper inlens detector (T2), and in-column detector (T3). The microscope includes a dedicated low-vacuum (LV) SED to provide charge-free topographic contrast imaging of non-conductive samples and offers an immersion lens, providing an additional magnetic objective lens. This lens, when combined with the electrostatic column, creates a compound final lens, which boosts the low-kV resolution performance to 1.0 nm at 1 kV without requiring beam deceleration. The Directional Back-Scatter (DBS) detector enables separate detection of electrons emitted at different angles. This instrument comes with dispersive X-ray Spectroscopy (EDS) and Scanning Transmission Electron Microscopy (S-TEM) capabilities as well.