Ferroelectric and Photovoltaic Properties of Transition Metal doped Pb(Zr0.14Ti0.56Ni0.30)O3-delta Thin Films

We report nearly single phase Pb(Zr0.14Ti0.56Ni0.30)O3-delta(PZTNi30) ferroelectric having large remanent polarization (15-30 {\mu}C/cm2), 0.3-0.4 V open circuit voltage (VOC), reduced band gap (direct 3.4 eV, and indirect 2.9 eV), large ON and OFF photo current ratio, and the fast decay time. Reasonably good photo current density (1-5 {\mu}A/cm2) was obtained without gate bias voltage which significantly increased with large bias field. Ferroelectric polarization dictates the polarity of VOC and direction of short circuit current (ISC), a step forward towards the realization of noncentrosymmetric ferroelectric material sensitive to visible light.


Introduction:
Ferroelectric semiconductors and their bandgap engineering represent the most fascinating research area since the discovery of ferroelectricity and related phenomenon. [1,2] Recently it has been experimentally observed that solid-solution of potassium-niobate and barium-nickel-niobate perovskite ferroelectrics possess immense potential for photovoltaic (PV) applications with reduced band gap and moderate quantum efficiency [3]. Generally, ferroelectric materials are high bandgap insulators with low leakage current system. A longstanding challenge in solid state physics is the tailoring of bandgap of ferroelectric host matrix with transition metal ions at B-site, which in turn keeps the polarization intact with an enhancement of the bulk photovoltaic (PV) effect. [4,5,6,7] As we know in general, organic and other semiconconductor based PV cells require p-n junctions for the creation of photo-induced charge carriers, and hence the limitation of these devices is that it cannot produce open circuit voltage (V OC ) above the band gap of the materials. In these systems p-n junction is a property of the interface and not of a bulk property of materials. However, ferroelectric photovoltaic systems have unique natural properties, including granularity and non-centrosymmetry, and hence these do not need any p-n junction for photo currents. They can also produce exceptionally large V OC far above their bandgap for in-plane configuration with domains and domains walls manipulation. [8] Recently, Yang et al. [9] have shown the above band gap V OC by tailoring the in-plane domains and domain walls in BiFeO 3 (BFO) thin films, which is relatively small bandgap (E g ~ 2.6-2.9 eV) ferroelectric semiconductor. The basic mechanism of the domainwall-based ferroelectric PV is quite different from that of inversion center-symmetry absence in the bulk ferroelectric. [8,10] 3 Bennett et al. [11,12] had utilized first principle density functional theory (DFT) calculations on the solid solution of PbTiO 3 (PTO) and Ba(Ti 1-x Ce x )O 3 (BTCO) with partial substitution of different transition metal cations, and they predicted that removal of 50 % Ti ions or more can lower the band gap below 1 eV with remanent polarization comparable to that of pure PTO and BTCO. This particular prediction can lead the design and discovery of new lowbandgap semiconductor ferroelectrics. However, in reality it would be difficult to produce single phase complex systems of lead titanate with transition metal cations. Many other ferroelectric materials, such as Pb(Zr 1-x Ti x )O 3 [13,14], LiNbO 3 [15] and BaTiO 3 [16] also exhibit photoelectric and photovoltaic effects under illumination of visible and near ultraviolet light; but the magnitude of photo current and voltage obtained for the device application are far below the photo-electronics requirements. In this respect ferroelectric BFO with its very high polarization ~ 90 μC/cm 2 [17] and a direct band gap ~2.67eV [18] had shown tremendous potential for such optoelectronic applications. [5,19,20] Pintilie et al. [3] reported band gap in Pb(Zr 1-x Ti x )O 3 system that increased with Zr content from 3.9 eV to 4.4 eV. A lower bandgap value (3.9 eV) and larger photocurrent signal The orientation and phase purity of these films were examined at room temperature by xray diffraction systems (Siemens D5000 and Rigaku Ultima III) using CuK  radiation with wavelength of  = 1.5405 Å. Room-temperature topography and domain images of these thin films were recorded by Piezo force microscopy (PFM) (Veeco) operated in contact mode and using an ultra-sharp silicon tip with a resonance frequency of about 25 kHz. The film thickness was determined using an X-P-200 profilometer and filmetrics. To investigate the electrical properties square capacitors were fabricated by dc sputtering with semi transparent Pt top 5 electrodes with area of ~10 -4 cm 2 utilizing a shadow mask. Frequency dependence of the dielectric and ferroelectric properties were measured using an HP4294A impedance analyzer and Radiant tester respectively at room temperature. Photovoltaic current was measured using solar simulator and Keithley-2401 at room temperature.  Table 1. In both cases, PZTNi30 films have a tetragonal crystal structure with a reduction of the tetragonality (c/a), i.e. c/a for polycrystalline and oriented films was ~1.003, comparatively smaller than the host matrix (c/a = 1.041).

Results and Discussion:
Surface topography and domains switching of the films were investigated by the conducting mode atomic force microscopy (AFM) and piezo force microscopy (PFM) respectively. AFM images revealed that average size of grains for PZTNi30/LSMO/LAO(100) heterostructures are less than 500 nm with average surface roughness ~ 5.5 nm (see Fig. 2  techniques such as capacitance-voltage and polarization-voltage. This may be due to detection limits of displacement current and leakage current by two different apparatus; however, further studies needed to clarify this minor discrepancy. 7 Direct and indirect bandgaps of PZTNi30 were determined from the UV-visible transmission data. The direct band gap, E g , was estimated from the modified square law using (αhν) 2 versus hν plots derived from the Tauc's relation [25,26], Where, absorption coefficient α is defined: Where, d is the film thickness, %T is the percentage of transmission, h photon energy, and E g band gap. ATauc (Tauc parameter) is the slope of the linear region in a plot of (αhν) 2 vs. h, whose extrapolation to (h) 2 = 0 would give the value of the direct bandgap.
On the other hand, data from indirect bandgaps meet usually the Tauc's law: Where, BTauc (Tauc parameter) is the slope of the linear region in a plot of (αhν) 1/2 vs. h, whose extrapolation to (h) 1/2 = 0 would give the value of the indirect band gap. It should be noted that these relationships are valid only for parabolic bands.
where Imax and Vmax describe the bias current and voltage points where the photogenerated power reaches the maximum, P in is the power density of the incident light and FF is the fill factor. The PCE of the PZTNi30 based heterostructure is obtained from Fig. 5. It is about ~ 0.006 (+/-0.004) % depending upon bias voltages and heterostructure configuration and its FF is 0.31 which is comparable to those obtained for perovskite oxides. [3,6] Polycrystalline samples showed exceptionally good switching of V OC under opposite polarity poling (±5 V) (see Fig. 5(b)); however, highly grain-oriented film had some in-built current even in dark without applying any voltage (see Fig. 5(a)) because it had some inbuilt polarization. The short circuit current density is much better for highly oriented films than polycrystalline films. These finding are comparable to ferroelectric BFO in MFM geometry, under similar growth and characterization conditions. [33] The strength of photocurrents, persistency over a period of time, and transient behavior under ON and OFF illumination of light were examined in PZTNi30/LSMO/LAO(100) hetorostructure over different periods of time with 0 and ±10 V bias E-field (see Fig. 6). Sudden interruption and illumination of light allow the decay and growth of photo charge carriers over time. Growth and decay of photocurrent for ON and OFF states at different switching times were carried out under 0, and +/-10 V bias E-field for short/long period of time (30/150 s) as can be seen in Fig. 6(a & b). Under these bias conditions, high photo-current density (0.1-0.5 mA/cm 2 ) and 1:4 to 1:5 ON and OFF current ratio were obtained with one second time period (experimental limit). An interesting feature in the transient currents can be seen in Fig. 6(c) and 6(d) during ON and OFF states, which exponentially increase or decrease with time, depending 10 on the biasing conditions. This may be due to development of displacement current along or opposite to photo-charge carriers under bias E-field condition. These results indicate that the domain orientation and flipping with bias voltage are important factors for the bulk ferroelectrics photocurrent. These results are also suitable for opto-memory applications. Ferroelectric oxides have very slow charge carriers compare to the Si-based or organic photovoltaic devices. [34] Under bias E-field, charge carriers in polar oxides took long time to grow and decay with long saturation time. In this regards, present investigation illustrates sharp growth and decay of photocharge carriers within the experimental limitations.

Conclusions:
In summary, we have successfully grown PZTNi30 single phase bulk photovoltaic ferroelectrics with switchable domains and photocurrents at nano/micro-scale. Substitutional modification by transition metal at Ti/Zr-site of PZT leads a decrease in direct and indirect band gaps without loss of its ferroelectric polarization. Experimentally, we showed that the cation modification of oxygen octahedra significantly reduces the indirect bandgap compared to direct bandgap. Photovoltaic effects are observed with significant amount of V OC (0.3-0.4 V) and good I SC (1-5 μA/cm 2 ); effect of poling and domains switching can be seen in the photocurrent and V OC performance. Thus, our investigations lead to the opportunities for more successful modification of ferroelectric materials for bulk photovoltaic effects and may be useful for optomemory and energy applications.