MECHANISM AND CHARACTERISTICS OF PHOTOVOLTAIC RESPONSES IN SANDWICHED FERROELECTRIC PLZT THIN FILM DEVICES QIN MENG (B. Eng., Zhejiang University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE (2009) i Acknowledgement I would like to express my heartfelt gratitude to my main supervisor Associate Professor Yung C. Liang and co-supervisor Senior Scientist Dr. Yao Kui, for giving me this precious opportunity to be a Ph.D. candidate of National University of Singapore (NUS) and to research work in the A*STAR Institute of Materials Research Engineering (IMRE) in this exciting field of ferroelectric thin film materials. I highly appreciate their patience, encouragement and support. I also would like to express my sincere gratitude to them for their academic guidance, constructive comments, and invaluable advice throughout the years. They managed to coach me through the whole Ph.D. research project. My Ph.D. project would not be possible without both of the supervisors. My scientific study would hardly be productive without the assistance from researchers at IMRE and the excellent research environment provided by IMRE. I would like to specially thank Mr. Lim Poh Chong, Ms. Lai Doreen, Ms. Shen Lu, Mr. Wang Weide and Mr. Chum Chan Choy for their technical assistance in the XRD, SEM, AFM, DC and RF sputtering experiments. I am also very grateful for all the fellow colleagues working in Dr. Yao Kui’s group, including Dr. Santiranja Shannigrahi, Ms. Gan Bee Keen, Dr. Tan Chin Yaw, Ms. Alicia Huang, Ms. Goh Poh Chin, Ms. Tan Sze Yu, Ms. Christina Tan, Mr. Chen Yifan, Mr. Luong Trung Dung, Mr. Ang Kai Yang, Mr. Chen Shuting, Ms. Li Xue and Mr. Ji Wei. I greatly appreciate the cooperation and discussion with them during my whole research work. ii In addition, I would like to greatly acknowledge the financial support from the postgraduate programme of National University Singapore during my Ph.D. study. Thanks also go to my parents for their encouragement, love, support and trust even though I am thousands of miles away from them. Finally, I would express my special thanks to my husband Liu Min, who is my everlasting source of happiness. None of this work would be possible without his endless support. Thus I dedicate this dissertation to my loving husband. iii Table of Contents CHAPTER INTRODUCTION . 1.1 1.2 1.2.1 1.2.2 1.2.3 1.3 1.4 FERROELECTRIC MATERIALS PHOTOVOLTAIC EFFECT IN FERROELECTRIC MATERIALS Interface-based and bulk-based photovoltaic effect . Photovoltaics in ferroelectric bulk ceramics Photovoltaics in ferroelectric PLZT-based thin films . 14 OBJECTIVES AND RESEARCH SCOPE 20 ORGANISATION OF THE THESIS . 22 CHAPTER SAMPLE FABRICATION AND CHARACTERISATION TECHNIQUES . 24 2.1 PREPARATION OF FERROELECTRIC THIN FILMS . 24 2.1.1 Chemical solution deposition . 24 2.1.2 DC/RF magnetron sputtering . 26 2.2 STRUCTURAL AND MICROSCOPIC CHARACTERISATIONS . 30 2.2.1 X-ray diffraction (XRD) 30 2.2.2 Field emission scanning electron microscope (SEM) . 34 2.2.3 Atomic force microscope (AFM) 37 2.3 ELECTRIC AND PHOTOVOLTAIC PROPERTY CHARACTERISATIONS 38 2.3.1 Dielectric property characterisation 38 2.3.2 Four point probe technique 40 2.3.3 Hall effect measurement . 42 2.3.4 Polarisation-electric field hysteresis loop characterisation . 44 2.3.5 Photovoltaic property characterisation 45 CHAPTER PHOTOVOLTAIC CHARACTERISTICS IN POLYCRYSTALLINE AND EPITAXIAL PLZT FERROELECTRIC THIN FILMS 47 3.1 3.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.4 INTRODUCTION 47 EXPERIMENTAL PROCEDURE 48 RESULTS AND DISCUSSION . 49 Structural and ferroelectric properties . 49 Characteristics of illuminated J-V curve and power conversion efficiency . 51 Effects of Schottky barrier and polarisation on photovoltaic responses 52 Effect of incident UV intensity on the photovoltaic responses 56 CONCLUSION 58 CHAPTER THICKNESS EFFECTS ON PHOTOCURRENT IN PLZT FERROELECTRIC THIN FILMS . 59 4.1 4.2 4.3 4.4 4.5 4.5.1 4.5.2 4.5.3 4.6 INTRODUCTION 59 EXPERIMENTAL PROCEDURE 61 MEASUREMENT RESULTS . 62 THEORETICAL MODEL 65 DISCUSSION . 75 Thickness-dependent photocurrent . 75 The effect of thickness-dependent depolarisation field on photocurrent 76 The effects of internal field and polarisation on photocurrent . 78 CONCLUSION 80 CHAPTER IMPROVED PHOTOVOLTAIC EFFICIENCY IN NANO-SCALED FERROELECTRIC THIN FILMS . 82 5.1 INTRODUCTION 82 5.2 EXPERIMENTAL PROCEDURE 83 5.3 RESULTS AND DISCUSSION . 85 5.3.1 Photovoltaic efficiency in sol-gel-derived polycrystalline and epitaxial films . 85 iv 5.3.2 Improved efficiency in sputtered epitaxial films . 91 5.3.3 Simulated high efficiency in nano-scaled ferroelectric thin films . 93 5.4 CONCLUSION 95 CHAPTER STABILITY OF PHOTOVOLTAGE AND TRAP OF LIGHT-INDUCED CHARGES IN FERROELECTRIC THIN FILMS . 97 6.1 6.2 6.3 6.4 6.4.1 6.4.2 6.5 INTRODUCTION 97 EXPERIMENTAL PROCEDURE 98 RESULTS 99 DISCUSSION . 102 The asymmetric photovoltage in electrodes-sandwiched thin film configuration . 102 Stability of photovoltage and trap of light-induced charges 106 CONCLUSION 111 CHAPTER PHOTOVOLTAIC MECHANISMS IN FERROELECTRIC THIN FILMS WITH SCREENING EFFECT 113 7.1 7.2 7.3 7.3.1 7.3.2 7.3.3 7.3.4 7.4 INTRODUCTION 113 THEORETICAL MODEL 115 DISCUSSION . 121 Photocurrent for PLZT thin films sandwiched between different electrode pairs 121 Effects from crystalline structure, polarisation and conductivity of electrodes . 123 Screening effect on electrode charge distribution and photocurrent . 124 Photovoltaic output in the ideal case: Ohmic contact and no screening effect 128 CONCLUSION 130 CHAPTER CONCLUSIONS 131 8.1 MAJOR FINDINGS 131 8.1.1 Schottky effect in photovoltaics of ferroelectric thin films 132 8.1.2 Thickness effect in photovoltaics of ferroelectric thin films . 132 8.1.3 Screening effect in photovoltaics of ferroelectric thin films . 133 8.1.4 Stability of photovoltage under multi-cycle UV illumination . 133 8.1.5 Improved photovoltaic efficiency in ferroelectric thin films . 134 8.2 CONTRIBUTIONS AND IMPLICATIONS 134 8.3 RECOMMENDATIONS FOR FUTURE WORK . 136 BIBLIOGRAPHY 140 APPENDIX (PUBLICATIONS) . 153 JOURNAL PAPERS 153 CONFERENCE PRESENTATIONS . 153 v List of Tables Table 2-1 Sputtering conditions for PLZT thin film, LSMO electrode and Au electrodes. 30 Table 2-2. Angle settings for XRD (111) plane φ-scan in cubic and quasi cubic perovskite crystals 33 Table 2-3. Correction factor for measurement using four point probe technique. s is probe distance. d is diameter of the circle or the side of a rectangle which is perpendicular to the probe line. a refers to second edge of rectangle 41 Table 3-1. Work function and interfacial Schottky barrier data of the polycrystalline and epitaxial PLZT thin films on different substrates. 55 Table 4-1 Parameters used for curve fitting of polycrystalline Au/PLZT/Pt thin film. 74 Table 4-2 Parameters obtained from the curve fitting for the photocurrent in PLZT thin films under different light intensities 74 Table 5-1. Linear fitting data for thickness-dependent photovoltage in sol-gel-derived polycrystalline and epitaxial PLZT thin films .87 Table 6-1 Linear fitting slope b, calculated Neff and ΔV according to Fig. 6-9, experimental ΔV data of the positively and negatively poled Au/PLWZT/Pt thin film. 109 Table 7-1 Parameters and data for the numerical simulations for PLZT thin films .122 vi List of Figures Fig. 1-1. PZT unit cell: (1) Perovskite-type lead zirconate titanate (PZT) unit cell in the symmetric cubic state above the Curie temperature. (2) Tetragonally distorted unit cell below the Curie temperature. Fig. 1-2. Ferroelectric polarisation-electric field (P–E) hysteresis loop. Circles with arrows represent the polarisation state of the material at the indicated fields. The symbols are explained in the text. (Data source: Ref. [1]) .4 Fig. 1-3. Schematic illustration of physical mechanism of photovoltaic effect in ferroelectrics. .7 Fig. 1-4. Schematic illustration of physical mechanism of conventional interfacebased photovoltaic effect, wherein the internal field E only exists in a very thin depletion layer at the junction but not the entire bulk region of the material Fig. 2-1. Flow chart for the preparation of the precursor solutions of 0.5 mol% WO3 doped (Pb0.97La0.03)(Zr0.53Ti0.48)O3 thin films. .26 Fig. 2-2. Illustration of DC sputtering system. Target (cathode) and substrate (anode) are placed on two parallel electrodes inside a chamber filled with inert gas (Ar) [81]. 27 Fig. 2-3. The magnetron sputtering system. Magnets are mounted behind the target with North pole in the central part and South pole in the outer ring. The magnetic field lines point from the North pole to the South pole [81]. .28 Fig. 2-4. An unbalanced magnetron system, the outer magnet North poles are stronger than the inner magnet South poles therefore the field lines stretch further into the vacuum chamber [87]. .29 Fig. 2-5. XRD gonio scan measurements come down to measuring distances between planes with plane X-ray waves (wavelength of a few tenths of nanometer). When the Bragg condition nλ=2dsinθ is satisfied, a peak will be measured [91] 31 Fig. 2-6. Illustration of XRD setup for φ-scan and pole-figure measurements. The Xray source and detector are fixed, and the sample rotates around φ angle from 0° to 360° in both measurements. ψ is fixed in the φ-scan but rotates from 0° to 90° in the pole figure measurement 32 vii Fig. 2-7. Illustration of the angle ψ between (100) plane and (111) plane in a tetragonal lattice. The in-plane and out-of-plane lattice parameter is a and c respectively. .33 Fig. 2-8. Illustration of XRD rocking curve scan (ω-scan). ki and kf is the incident and diffracted x-ray vector respectively, and ∆k = ki - kf. The magnitude and orientation of both ki and kf are fixed, i.e. vary the orientation of ∆k relative to sample normal while maintaining its magnitude. The sample is rocked over a very small angular range during the ω-scan .34 Fig. 2-9. Schematic diagram of SEM .36 Fig. 2-10. Excitation volume and escape zone of various SEM signals in a material surface struck by incident electron beam .36 Fig. 2-11. Schematic diagram of AFM. .37 Fig. 2-12. Schematic of four point probe configuration. .41 Fig. 2-13 Configuration of (a) resistivity and (b) Hall effect measurement. .43 Fig. 2-14. Sawyer-Tower circuit for measurement of ferroelectric polarisation. The circuit includes an oscilloscope, a signal generator, a reference capacitor and the sample of ferroelectric capacitor 45 Fig. 2-15. Experimental setup for photovoltaic measurements. 46 Fig. 3-1. Gonio-scan XRD patterns of chemical-solution-derived (a) polycrystalline PLZT 3/52/48 film on Pt/Ti/SiO2/Si substrate and (b) epitaxial PLZT 3/52/48 film on Nb:STO substrate. The inset in (b) is the 3D (111)-plane pole figure of the epitaxial PLZT film. .50 Fig. 3-2. Ferroelectric polarisation-electric field (P-E) hysteresis loops for the chemical-solution-derived (a) polycrystalline and (b) epitaxial PLZT thin films. 50 Fig. 3-3. Experimental results of (a) illuminated J-V curves and (b) corresponding terminal voltage dependence of light-to-electricity power conversion efficiency for a 196-nm-thick polycrystalline PLZT thin film in different polarisation states .52 Fig. 3-4. Experimental results of (a) illuminated J-V curves and (b) corresponding terminal voltage dependence of light-to-electricity power conversion efficiency for a 180-nm-thick epitaxial PLZT thin film in different polarisation states .53 viii Fig. 3-5. (a) Illuminated J-V curves of a positively poled 45-nm-thick epitaxial PLZT thin film on Nb:STO under different incident UV intensities; (b) Light-intensity dependence of short circuit photocurrent; (c) Light-intensity dependence of maximum light-to-electricity conversion efficiency .57 Fig. 4-1. XRD gonio scan (θ-2θ scan) pattern of the sol-gel-derived Au/PLZT/Pt thin film annealed at 700 °C for 10 63 Fig. 4-2. Dielectric constant and dielectric loss of a sol-gel-derived polycrystalline Au/PLZT/Pt thin film annealed at 700 °C for 10 min. 63 Fig. 4-3. P-E hysteresis loop of a sol-gel-derived polycrystalline Au/PLZT/Pt thin film annealed at 700°C for 10 .64 Fig. 4-4. Experimental results of short circuit photocurrent vs. light intensity for solgel derived polycrystalline Au/PLZT/Pt films with different thicknesses (0.26, 0.54, 1.05, and 1.50 μm, respectively). Short circuit photocurrent was found to be linear with the incident light intensity for each different film thickness. 64 Fig. 4-5. The structure of the PLZT thin film sandwiched between the top and bottom electrodes and the mechanism of the photocurrent generation 66 Fig. 4-6. In short circuit steady state, electron and hole concentrations along depth in the polycrystalline Au/PLZT/Pt film under different light intensities .73 Fig. 4-7. Experimental data and fitting curves for thickness dependence of short circuit photocurrent under different light intensities for the polycrystalline Au/PLZT/Pt thin films. 73 Fig. 4-8. Experimental and simulation results of the thickness dependence of short circuit photocurrent Jsc epitaxial Au/PLZT/Nb:STO thin films (the epitaxial film was prepared using chemical solution deposition as described in Chapter 3). .74 Fig. 4-9. Fitting curves of thickness-dependent photocurrent in PLZT thin films in consideration of a constant depolarisation field and a thickness-dependant depolarisation field in Eq. (4.23). 77 Fig. 4-10. The relationship between short circuit photocurrent and internal electric field at different film thicknesses under UV illumination (0.60 mW/cm2) predicted by Eq. (4.21). The data used are listed in Table 4-1 and Table 4-2. The inset figure is the enlarged part of the curves at very low field region. .79 Fig. 4-11. The relationship between short circuit photocurrent and remnant polarisation at different film thicknesses under UV illumination (0.60 mW/cm2) ix predicted by Eq. (4.21). The data used are listed in Table 4-1 and Table 4-2. The inset figure is the enlarged part of the curves in very low polarisation region. .80 Fig. 5-1. Schematic illustration of physical mechanism of photovoltaic effect in a ferroelectric. .83 Fig. 5-2. XRD Gonio-scan pattern of the epitaxial PLZT thin film grown on single crystal Nb:STO substrate; the inset figure is the rocking curve of the PLZT (200) peak and the 3D XRD pole figure of PLZT (111) plane 85 Fig. 5-3. Experimental and linear fitting results of the thickness-dependent open circuit photovoltage Voc for sol-gel-derived (a) polycrystalline and (b) epitaxial PLZT thin films in different polarisation states. 87 Fig. 5-4. Experimental and simulation results of the thickness dependence of short circuit photocurrent Jsc in the sol-gel-derived (a) polycrystalline and (b) epitaxial PLZT thin films in different polarisation states .88 Fig. 5-5. Experimental and calculated results of the thickness dependence of maximum power conversion efficiency ηmax for the sol-gel-derived (a) polycrystalline and (b) epitaxial PLZT thin films in different polarisation states 89 Fig. 5-6. Experimental results of (a) illuminated J-V curves and (b) terminal voltage dependences of power conversion efficiencies at different incident UV intensities for a 68-nm-thick sputtered epitaxial PLZT thin film sandwiched between top LSMO and bottom Nb:STO electrodes. .92 Fig. 5-7. Simulation results of the thickness-dependent short circuit photocurrent and maximum power conversion efficiency for the epitaxial PLZT film in the nanoscale thickness range ~ 100 nm (using the ferroelectric parameters P ~ 30 µC cm-2, quantum efficiency β ~ 90%, top and bottom interfacial space charge density Neff1 ~ 2×1020 cm-3 and Neff2 ~ 1×1020 cm-3, carrier mobility µ ~ 100 cm2 V-1s-1, and carrier lifetime τ ~ 200 ps). After replacing the ferroelectric mobility and lifetime data with the Si parameters (carrier mobility µ ~1500 cm2 V-1s-1 and lifetime τ ~ 10 µs), the corresponding simulation results are also shown for reference. 94 Fig. 6-1. Schematic illustration of the PLWZT thin films in the (a) sandwich electrode configuration with inter-electrode distance of 0.706 µm and (b) in-plane electrode configuration with inter-electrode distance of 10 µm 98 Fig. 6-2. XRD patterns of sol-gel derived PLWZT thin film on (a) Pt/Ti/SiO2/Si substrate and (b) YSZ/Si3N4/SiO2/Si substrate. .100 Fig. 6-3. Photovoltage response in the multi-cycle UV illumination before poling, and after positive and negative poling for the Au/PLWZT/Pt thin film electrode- x films promising for another type of photovoltaic applications – optical image storage or optical memory. In the optical memory, each bit of information of an optical image is stored as polarisation state in each element of the ferroelectric thin film; and then the stored information is read out as the polarisation-induced photovoltage or photocurrent signal under UV light illumination. It is worth trying to develop the UV sensor/detector, or optical memory devices for applications in the future work if high quality epitaxial PLZT thin films can be obtained at a lower cost and photovoltaic performance can be further improved. 139 Bibliography [1] D. 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Liang, and Santiranjan Shannigrahi, Thickness effects on photoinduced current in ferroelectric (Pb0.97La0.03)(Zr0.52Ti0.48)O3 thin films, Journal of Applied Physics, Vol. 101, pp. 014104/1-014104/8, 2007. 2. Meng Qin, Kui Yao, and Yung C. Liang, Photo induced current in (Pb0.97La0.03)(Zr0.52Ti0.48)O3 thin films of different thicknesses, Integrated Ferroelectrics, Vol. 88, pp. 58-67, 2007. 3. Meng Qin, Kui Yao, Yung C. Liang, and Bee Keen Gan, Stability of photovoltage and trap of light-induced charges in ferroelectric WO3-doped (Pb0.97La0.03)(Zr0.52Ti0.48)O3 thin films, Applied Physics Letters, Vol. 91, pp. 092904/1-092904/3, 2007. 4. Meng Qin, Kui Yao, Yung C. Liang, and Bee Keen Gan, Stability and magnitude of photovoltage in ferroelectric (Pb0.97La0.03)(Zr0.52Ti0.48)O3 thin films in multicycle UV light illumination, Integrated Ferroelectrics, Vol. 95, pp. 105-116, 2007. 5. Meng Qin, Kui Yao, and Yung C. Liang, High efficient photovoltaics in nanoscale ferroelectric thin films, Applied Physics Letters, Vol. 93, pp. 122904/1122904/3, 2008. 6. Meng Qin, Kui Yao, and Yung C. Liang, Photovoltaic characteristics in polycrystalline and epitaxial (Pb0.97La0.03)(Zr0.52Ti0.48)O3 ferroelectric thin films sandwiched between different top and bottom electrodes, Journal of Applied Physics, Vol. 105, pp. 061624/1-061624/8, 2009. 7. Meng Qin, Kui Yao, and Yung C. Liang, Photovoltaic mechanisms in ferroelectric thin films with the effects of the electrodes and interfaces, Applied Physics Letters, Vol. 95, pp. 022912/1-022912/3, 2009. Conference presentations 1. Meng Qin, Kui Yao, and Yung C. Liang, Thickness dependence of photo induced current in (Pb0.97La0.03)(Zr0.52Ti0.48)O3 thin films, 18th International Symposium on Integrated Ferroelectrics (ISIF 2006), Hawaii, USA, 2006. 2. Meng Qin, Kui Yao, Yung C. Liang, and Bee Keen Gan, Stability of photovoltage in PLZT thin films under multi-cycle UV illumination, 19th International Symposium on Integrated Ferroelectrics (ISIF 2007), Bordeaux, France, 2007. 153 3. Meng Qin, Kui Yao, and Yung C. Liang, Sol-gel derived highly-oriented (Pb0.97La0.03)(Zr0.52Ti0.48)O3 ferroelectric thin films, 20th International Symposium on Integrated Ferroelectrics (ISIF 2008), Singapore, 2008. 4. Meng Qin, Kui Yao, and Yung C. Liang, Photovoltaic responses in sol-gelderived epitaxial (Pb0.97La0.03)(Zr0.52Ti0.48)O3 ferroelectric thin films with different film thickness, 20th International Symposium on Integrated Ferroelectrics (ISIF 2008), Singapore, 2008. 154 [...]... 18 of understanding and systematic characterisation for photovoltaic outputs in ferroelectric thin films (including polycrystalline and epitaxial thin films) In addition, the mechanisms for the Schottky effect, thickness effect and screening effect have not been clarified yet for the photovoltaics in ferroelectric thin films up to now Moreover, as a consequence of interfacial effects, the stability of. .. useful information for the choice of film dimension and/ or electrodes in the photovoltaic ferroelectric device design In addition, the investigation of stability issue of photovoltaic response in the multi-cycle UV illumination should enhance the understanding of interfacial effect in ferroelectric thin films It may also come to be useful for photovoltaic stability improvement in ferroelectric thin film. .. better understanding of photovoltaic effect in PLZT ferroelectric thin films in the aspect of various interfacial effects – thickness effect, Schottky effect and screening effect The proposed theoretical models that take interfacial effects into account should be useful for predicting photovoltaic outputs in PLZT ferroelectric thin films with different film thicknesses and electrode materials, and they... ceramics to thin films in the 1990s Among all the ferroelectric thin films, PZT is the most promising candidate among ferroelectric materials for photovoltaic 14 applications because of its outstanding ferroelectric and photovoltaic properties Photovoltaic studies to date on ferroelectric thin films have mainly focused on the PZT family Some photovoltaic properties in PZT bulk ceramics and thin films were... Photovoltaics in ferroelectric PLZT- based thin films With the advancement in the processing of complex ferroelectric thin films and the technology to integrate them onto silicon wafers (i.e the development of chemical deposition and physical deposition methods for ferroelectric thin films) in the 1980s [53], the research attention on ferroelectric- based photovoltaics gradually shifted from bulk ceramics to thin. .. illuminated I-V characteristic and light-to-electricity power conversion efficiency) in PLZT ferroelectric thin films To study the stability of photovoltage response under multi-cycle UV light illumination, and examine how the interfacial effect influences the stability performance of photovoltage in ferroelectric thin films To improve photovoltaic power conversion efficiency in PLZT ferroelectric thin. .. wireless energy transfer in microelectromechanical systems (MEMS) [33-36] So far, photovoltaics in ferroelectrics, especially in thin films, have still been undergoing investigation, and the underlying physical mechanism of ferroelectric- based photovoltaics is the main focusing point The physical mechanism of photovoltaic effect in ferroelectrics is still uncertain at present In the early years, a few... effects bring much difficulty to the photovoltaic study in ferroelectric thin films, because it means that the phenomenological and analytical theories developed for bulk ferroelectric photovoltaics in the early years are not applicable to ferroelectric thin films In ferroelectric thin films, interfacial effects play important roles in determining the photovoltaic output However, with regard to the interfacial... of the thin film, interfacial effect significantly influences the stability of the photovoltaic output Nevertheless, the interface-effect-induced stability issue in ferroelectric thin films has not been given much attention Obviously, addressing the stability issue and clarifying the underlying physical mechanisms are also critical tasks for the photovoltaic applications in ferroelectric thin films... existence of interfacial effects (including Schottky effect, thickness/size effect and screening effect) makes photovoltaics in ferroelectric thin films more complicated than those in bulk ceramics Compared with the small dimension of the bulk region of the film, the ferroelectric- electrode interface or an interfacial layer also occupies a considerable volume in the whole volume of ferroelectric thin film . effect in photovoltaics of ferroelectric thin films 132 8.1.3 Screening effect in photovoltaics of ferroelectric thin films 133 8.1.4 Stability of photovoltage under multi-cycle UV illumination. i MECHANISM AND CHARACTERISTICS OF PHOTOVOLTAIC RESPONSES IN SANDWICHED FERROELECTRIC PLZT THIN FILM DEVICES QIN MENG (B. Eng., Zhejiang University). photovoltage in electrodes -sandwiched thin film configuration 102 6.4.2 Stability of photovoltage and trap of light-induced charges 106 6.5 CONCLUSION 111 7 CHAPTER 7 PHOTOVOLTAIC MECHANISMS IN FERROELECTRIC