Rajesh K. Katiyar

Photovoltaic effect in transition metal modified polycrystalline BiFeO3 thin films

We report photovoltaic (PV) effect in multiferroic Bi0.9Sm0.1Fe0.95Co0.05O3 (BSFCO) thin films. Transition metal modified polycrystalline BiFeO3 (BFO) thin films have been deposited on Pt/TiO2/SiO2/Si substrate successfully through pulsed laser deposition (PLD). PV response is observed under illumination both in sandwich and lateral electrode configurations. The open-circuit voltage (Voc) and the short-circuit current density (Jsc) of the films in sandwich electrode configuration under illumination are measured to be 0.9 V and −0.051 µA cm−2. Additionally, we report piezoresponse for BSFCO films, which confirms ferroelectric piezoelectric behaviour.

Photovoltaic effect in a wide-area semiconductor-ferroelectric device

Millimeter-diameter planar devices of glass/ZnO:Al/BiFeO3/La0.67Sr0.33CoO3 (LSCO) heterostructures were fabricated by pulsed laser deposition (PLD) techniques. Diode-like behavior with high short-circuit current (SSC ∼ 4 mA/cm2) and open-circuit voltage (OCV ∼ 0.22 V) was obtained under the illumination of about 1% of maximum solar energy. Impedance spectroscopy revealed that electrode/dielectric interface and grain-boundary conduction are mainly responsible for the photo-current. Electrode/dielectric interface, grain boundary impedance, and low-frequency ac conductivity change by almost three orders of magnitude under weak light. Relaxation time of the photo-carriers changes from 80 ms to 96 μs suggesting that with optimal collecting instruments, one should expect currents several orders higher.

Studies of the switchable photovoltaic effect in co-substituted BiFeO3 thin films

We report the photovoltaic properties of doped ferroelectric [Bi0.9La0.1][Fe0.97Ti0.02Zr0.01]O3 (BLFTZO) thin films. Polycrystalline BLFTZO films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition technique. Al-doped ZnO transparent top electrodes complete the ZnO:Al/BLFTZO/Pt metal-ferroelectric-metal capacitor structures. BLFTZO showed switchable photoresponse in both polarities. The open circuit voltage (VOC) and short circuit current (JSC) were found to be ∼0.022 V and ∼650 μA/cm2, respectively after positive poling, whereas significant difference in VOC ∼ 0.018V and JSC ∼ 700 μA/cm2 was observed after negative poling. The observed switchable photocurrent and photovoltage responses are explained on the basis of polarization flipping in BLFTZO due to the applied poling field.

Photovoltaic properties of Aurivillius phase Bi5FeTi3O15 thin films grown by pulsed laser deposition

We report a remarkable photovoltaic effect in pulsed laser deposited multiferroic aurivillius phase Bi5FeTi3O15 (BFTO) thin films sandwiched between ZnO:Al transparent conductive oxide top electrode and SrRuO3 bottom electrode fabricated on amorphous fused silica substrates. The structural and micro structural properties of these films were analysed by X-ray diffraction and atomic force microscopy techniques. The films were showing a photo sensitive ferroelectric behaviour with a notable apparent polarization in the range of 10–15 μC/cm2. These films also exhibited a switchable photo-response and this parameter was observed to be sensitive to polarisation field and the polarization direction. The device shows a large ON/OFF photo current ratio with an open circuit voltage of 0.14 V. The photo response at zero bias of this BFTO based heterostructures showed rapid increase to a saturation value of 6 μA at zero bias.

Switchable photovoltaic effect in bilayer graphene/BiFeO3/Pt heterostructures

We report the switchable photovoltaic effects in graphene/BiFeO3/Pt heterostructures. Pure phase polycrystalline BiFeO3 films were deposited on Pt/TiO2/SiO2/Si substrates by pulse laser deposition. A bilayer graphene was transferred onto the BiFeO3 film which serves as transparent conducting electrodes. The heterostructures showed switchable photovoltaic effect depending on ferroelectric polarization directions indicating depolarization field induced separation of photo-generated carriers. The open circuit voltage (VOC) and short circuit current density (JSC) were measured to be ∼110 mV, ∼92 μA/cm2 in positive polarity and similar values were obtained when the polarity was reversed. The JSC and VOC also showed rapid response (<100 ms) as a function of light exposure time.

Ferroelectric photovoltaic properties in doubly substituted (Bi0.9La0.1)(Fe0.97Ta0.03)O3 thin films

Doubly substituted [Bi0.9La0.1][Fe0.97Ta0.03]O3 (BLFTO) films were fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition. The ferroelectric photovoltaic properties of ZnO:Al/BLFTO/Pt thin film capacitor structures were evaluated under white light illumination. The open circuit voltage and short circuit current density were observed to be ∼0.20 V and ∼1.35 mA/cm2, respectively. The band gap of the films was determined to be ∼2.66 eV, slightly less than that of pure BiFeO3 (2.67 eV). The PV properties of BLFTO thin films were also studied for various pairs of planar electrodes in different directions in polycrystalline thin films.

Photovoltaic effect and enhanced magnetization in 0.9(BiFeO3)–0.1(YCrO3) composite thin film fabricated using sequential pulsed laser deposition

We report on the photovoltaic effect and multiferroic properties of a 0.9(BiFeO3)–0.1(YCrO3) composite thin film deposited on a Pt/TiO2/SiO2/Si substrate by sequential ablation of BiFeO3 and YCrO3 ceramic targets using pulsed laser deposition. The desired composition of the composite was achieved by controlling the ablation time of respective targets. As confirmed by the x-ray diffraction pattern the resultant film was found to be polycrystalline in nature and composed of a mixture of both rhombohedral BiFeO3 and orthorhombic YCrO3 phases. Interesting multiferroic properties in terms of an enhanced saturation magnetization of ~14 emu cm−3 and the remnant polarization of ~4.5 µC cm−2 were observed where the enhancement in magnetization as compared to pristine BiFeO3 could be attributed to the super-exchange interaction between Fe and Cr-ions. The photovoltaic properties of the composite thin film were studied under white light illumination in both top–bottom and lateral electrode configurations. Short circuit current densities (JSC) = 1.48 µA cm−2 and 0.44 µA cm−2, and open circuit voltages (VOC) = 0.51 V and 0.32 V were observed in top–bottom and lateral electrode configurations, respectively.

Graphene/semiconductor silicon modified BiFeO3/indium tin oxide ferroelectric photovoltaic device for transparent self-powered windows

We report photovoltaic response of highly transparent graphene/BiFe0.95Si0.05O3 (BFSiO)/ITO/glass derived from bottom-up spin coating technique. The device exhibits short-circuit-current (ISC 0.75 mA) with 1000 fold upsurge and open-circuit-voltage (VOC ∼ 0.45 V) under standard AM 1.5 illumination through graphene. In combination, ISC of 0.63 mA and VOC of 0.35 V for same illumination through ITO, reveals the prospects of harvesting indoor light. Also, crystallographic structure, red shift in band gap, leakage behavior, and ferroelectric characteristics of BFSiO thin films are reported. Reproducible transient response of ISC and VOC with quick switching (<100 ms) for 20 consecutive cycles and stability (95%) over test period of 16 weeks signifies high endurance and retentivity, promising for building integrated self-powered windows.

Unipolar resistive switching in planar Pt/BiFeO3/Pt structure

We report unipolar resistive switching suitable for nonvolatile memory applications in polycrystalline BiFeO3 thin films in planar electrode configuration with non-overlapping Set and Reset voltages, On/Off resistance ratio of ∼104 and good data retention (verified for up to 3,000 s). We have also observed photovoltaic response in both high and low resistance states, where the photocurrent density was about three orders of magnitude higher in the low resistance state as compared to the high resistance state at an illumination power density of ∼100 mW/cm2. Resistive switching mechanisms in both resistance states of the planar device can be explained by using the conduction filament (thermo-chemical) model.

Absence of magnetism in Cr-doped In2O3: a case study of phase separation versus phase formation

Bulk ceramic Cr-doped In2O3 have been prepared using a solid-state reaction method at two different annealing temperatures. Room-temperature ferromagnetic behaviour is observed for all the compositions prepared at relatively low temperature. But from the Raman study the ferromagnetic oxide phase of CrO2 was identified. On the other hand, samples prepared at relatively high temperature show no detectable secondary phases as confirmed by x-ray diffraction and Raman study, but also no ferromagnetism. With increase in Cr content the paramagnetism state is merely enhanced. This is a strong indication that reports of ferromagnetism in Cr : In2O3 are due to a second phase, probably CrO2.