Springer TAP vol98 ferroelectric thin films (springer 2005)

Topics in Applied Physics Volume 98 Topics in Applied Physics is part of the SpringerLink service For all customers with standing orders for Topics in Applied Physics we offer the full text in electronic form via SpringerLink free of charge Please contact your librarian who can receive a password for free access to the full articles by registration at: springerlink.com → Orders If you not have a standing order you can nevertheless browse through the table of contents of the volumes and the abstracts of each article at: springerlink.com → Browse Publications Topics in Applied Physics Topics in Applied Physics is a well-established series of review books, each of which presents a comprehensive survey of a selected topic within the broad area of applied physics Edited and written by leading research scientists in the field concerned, each volume contains review contributions covering the various aspects of the topic Together these provide an overview of the state of the art in the respective field, extending from an introduction to the subject right up to the frontiers of contemporary research Topics in Applied Physics is addressed to all scientists at universities and in industry who wish to obtain an overview and to keep abreast of advances in applied physics The series also provides easy but comprehensive access to the fields for newcomers starting research Contributions are specially commissioned The Managing Editors are open to any suggestions for topics coming from the community of applied physicists no matter what the field and encourage prospective editors to approach them with ideas Managing Editors Dr Claus E Ascheron Dr Hans J Koelsch Springer-Verlag GmbH Tiergartenstr 17 69121 Heidelberg Germany Email: claus.ascheron@springer-sbm.com Springer-Verlag New York, LLC 233, Spring Street New York, NY 10013 USA Email: hans.koelsch@springer-sbm.com Assistant Editor Adelheid H Duhm Springer-Verlag GmbH Tiergartenstr 17 69121 Heidelberg Germany Email: adelheid.duhm@springer-sbm.com Masanori Okuyama Yoshihiro Ishibashi (Eds.) Ferroelectric Thin Films Basic Properties and Device Physics for Memory Applications With 172 Figures 123 Professor Masanori Okuyama Osaka University Graduate School of Engineering Science Department of Systems Innovation 1-3 Machikaneyama-cho, Toyonaka 560-8531 Osaka, Japan okuyama@ee.es.osaka-u.ac.jp Professor Yoshihiro Ishibashi Aichi Shukutoku University Nakakute-cho 480-1197 Aichi, Japan yishi@asu.aasa.ac.jp Library of Congress Control Number: 2004117860 Physics and Astronomy Classification Scheme (PACS): 68.55.-a, 77.80.-e, 81.15.-z, 77.84.-s, 77.22.-d ISSN print edition: 0303-4216 ISSN electronic edition: 1437-0859 ISBN 3-540-24163-9 Springer Berlin Heidelberg New York This work is subject to copyright All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer Violations are liable for prosecution under the German Copyright Law Springer is a part of Springer Science+Business Media springeronline.com © Springer-Verlag Berlin Heidelberg 2005 Printed in Germany The use of general descriptive names, registered names, trademarks, etc in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use Typesetting: DA-TEX · Gerd Blumenstein · www.da-tex.de ockler GbR, Leipzig Production: LE-TEX Jelonek, Schmidt & Vă Cover design: design & production GmbH, Heidelberg Printed on acid-free paper SPIN: 11308157 57/3141/YL 543210 Preface The prominent properties of ferroelectric materials such as polarization hysteresis, large dielectric constant, and remarkable piezoelectric, pyroelectric and electro-optical effects can all be applied in electronic devices Especially in the form of thin films, ferroelectrics show excellent features when combined with Si active electronic devices such as nonvolatile memories, capacitors, surface acoustic wave (SAW) filters, ultrasonic and infrared sensors, optical modulators, and switches Among these, nonvolatile memories utilizing ferroelectric thin films have attracted special attention recently because of their low power dissipation and fast switching In order to realize the ultralarge scale integration of ferroelectric thin-film memory devices, which might be competitive with various current dynamic random access memories, comprehensive studies of ferroelectric thin films ranging from basic physics to device physics are indispensable A tremendous amount of research on ferroelectric thin films and their application to memory devices has been carried out This book gathers together remarkable research results relating to the basic physics of size effects, searches for new materials, the development of new preparation methods, microscopic and macroscopic characterization, and the fabrication and characterization of device structures In Part I, the phase transition of a ferroelectric thin film is analyzed in detail on the basis of the Tilley–Zeks model, and its characteristic features are clarified In Part II, preparation methods for ferroelectric thin films such as chemical solution, metal-organic chemical vapor deposition (MOCVD) and sputtering are described for the preparation of PZT and Bi-layer-structured ferroelectric thin films Island structures of nanometer size are observed in the initial nucleation stage and their ferroelectric behavior is discussed In Part II, a description is also given of the spatial polarization distribution observed by scanning nonlinear dielectric microscopy, which has ultrahigh spatial resolution, and the applicability of the polarization domains to disk memory with a size of the order of Tbits is proved In Part III, topics on relaxor ferroelectrics showing dispersive dielectric phenomena are described The colossal piezoelectric property is analyzed in the vicinity of the morphotropic phase boundary Domain structures in relaxor ferroelectrics are analyzed in detail, and the mechanism has been clarified by analyzing di- VI Preface electric properties of superlattice structures with various kinds of ordering periodicity In Part IV, metal–ferroelectric–insulator–semiconductor (MFIS) structures in ferroelectric-gate FETs are studied The stability of the MFIS structure is analyzed theoretically, considering the space charge distribution The memory retention of the MFIS structure has been analyzed, considering leakage current through the ferroelectric junction, and long retention has been achieved in structures using SrBi2 Ta2 O9 and YMnO3 thin films This book contains valuable information on both theoretical approaches and experimental efforts, and we hope that the book will offer some help not only to beginners but also to specialists in ferroelectric physics and engineering who would like to have an idea about the progress of research in the field of ferroelectric thin films and devices This work has been carried out under Grants-in-Aid for scientific research in the priority area “Control of Material Properties of Ferroelectric Thin Films and Their Application to Next-Generation Memory Devices”, sponsored by the Ministry of Education, Culture, Sports, Science and Technology, Japan, for 2000–2004 Last but not least, we would like to express our sincere thanks to Drs Kaoru Yamashita and Takeshi Kanashima for their tremendous efforts in arranging the manuscripts for this book and taking care of the research project Without their contribution, this book might not have come out in due time This publication was supported by Grant-in-Aid for Publication of Scientific Research Results 165284, 2004, sponsored by the Japan Society for the Promotion of Science (JSPS) Osaka, Nagoya, January 2005 Masanori Okuyama Yoshihiro Ishibashi Contents Part I Theoretical Aspects Theoretical Aspects of Phase Transitions in Ferroelectric Thin Films Yoshihiro Ishibashi Introduction The Tilley–Zeks Model Transition Temperature and Polarization Profile 3.1 The Case of Zero Extrapolation Length (δ+ = δ− = 0) 3.2 The Case of Positive Extrapolation Length (δ+ = δ− = δ > 0) 3.3 The Case of Negative Extrapolation Length (δ+ = δ− = δ < 0) Asymmetric Films 4.1 The Positive–Positive Case (δ+ > 0, δ− > 0) 4.2 The Negative–Negative Case (δ+ < 0, δ− < 0) 4.3 The Mixed Case 4.3.1 The Case of |δ− | < |δ+ | 4.3.2 The Case of |δ− | > |δ+ | Notes on Exact and Approximate Polarization Profiles Concluding Remarks References 3 12 14 15 15 15 16 16 16 19 20 Part II Preparation and Characterization of Ferroelectric Thin Films Theoretical Aspects of Phase Transitions in Ferroelectric Thin Films Shin-ichi Hirano, Takashi Hayashi, Wataru Sakamoto, Koichi Kikuta, Toshinobu Yogo Introduction 1.1 The Chemical Solution Deposition Process 1.2 Representative Ferroelectric Thin Films for Memory Devices 1.3 Layer-Structured Bi4 Ti3 O12 -Based Thin Films Rare-Earth-Ion-Modified Bi4 Ti3 O12 Thin Films 25 25 26 28 28 29 VIII Contents 2.1 Chemical Processing of (Bi,R)4 Ti3 O12 Precursor Solutions, Powders and Thin Films 2.2 Crystallization and Pyrolysis Behavior of (Bi,R)4 Ti3 O12 Precursors 2.3 Crystallization of (Bi,R)4 Ti3 O12 Thin Films 2.4 Surface Morphologies of (Bi,R)4 Ti3 O12 Films 2.5 Phase Transition and Ferroelectric Properties 2.6 Effect of Nd Content on Nd-Modified BIT (BNT) Thin Films 2.7 Effect of Processing Temperature on Nd-Modified BIT (BNT) Thin Films Ge-Doped (Bi,Nd)4 Ti3 O12 Thin Films 3.1 Fabrication of (Bi,Nd)4 (Ti,Ge)3 O12 Films 3.2 Microstructure and Electrical Properties of (Bi,Nd)4 (Ti,Ge)3 O12 Films UV Processing of (Bi,Nd)4 Ti3 O12 (BNT) Thin Films 4.1 Changes in the Chemical Bonding of Excimer-UV-Irradiated BNT Precursor Films 4.2 Effect of UV Light Irradiation on the Crystal Orientation of the Resultant Thin Films 4.3 Surface Morphology of UV-Light-Irradiated BNT Thin Films 4.4 Ferroelectric Properties of UV-Irradiated BNT Thin Films 4.5 Fatigue and Leakage Current Properties of UV-Irradiated BNT Thin Films References Pb-Based Ferroelectric Thin Films Prepared by MOCVD Masaru Shimizu, Hironori Fujisawa, Hirohiko Niu Introduction Experimental Procedure Microscopic Observation of the Initial Growth Stages of PbTiO3 and PZT Thin Films on Various Substrates 3.1 Growth Process of PbTiO3 and PZT Thin Films on Polycrystalline Pt/SiO2 /Si 3.2 Growth Process of PZT Thin Films on SrTiO3 Single Crystals 3.3 Growth Process of PZT Thin Films on Epitaxial SrRuO3 /SrTiO3 Epitaxial PZT Ultrathin Films 4.1 Preparation of PZT Ultrathin Films on SrRuO3 /SrTiO3 4.2 Ferroelectric Properties of PZT Ultrathin Films Self-Assembled PbTiO3 and PZT Nanostructures and Their Ferroelectric Properties 5.1 Preparation of Self-Assembled PbTiO3 and PZT Nanostructures on Various Substrates 5.2 Piezoelectric and Ferroelectric Properties of PbTiO3 Nanostructures 29 31 33 35 36 39 41 43 43 45 46 47 48 50 51 54 56 59 59 61 62 62 64 65 67 67 67 71 71 72 Contents IX References 74 Spontaneous Polarization and Crystal Orientation Control of MOCVD PZT and Bi4 Ti3 O12 -Based Films Hiroshi Funakubo Introduction Spontaneous Polarization 2.1 PZT Films 2.2 Bi4 Ti3 O12 -Based Films Remanent Polarization of Polycrystalline Ferroelectric Films Prepared on Si Substrates 3.1 PZT Films 3.2 Bi4 Ti3 O12 -Based Films 3.3 Low-Temperature Deposition 3.4 PZT Films 3.5 Bi4 Ti3 O12 -Based Films References Rhombohedral PZT Thin Films Prepared by Sputtering Masatoshi Adachi Introduction Experimental Procedures PZT Films on (Pb,La)TiO3 (PLT)/Pt/Ti/SiO2/Si and Ir/SiO2 /Si Rhombohedral PZT on (111) Ir/(111) SrTiO3 and (100) Ir/(100) SrTiO3 Substrates References Scanning Nonlinear Dielectric Microscopy Yasuo Cho Introduction Principle and Theory of SNDM 2.1 Nonlinear Dielectric Imaging with Subnanometer Resolution 2.2 Comparison between SNDM Imaging and Piezoresponse Imaging Higher-Order Nonlinear Dielectric Microscopy 3.1 Theory of Higher-Order Nonlinear Dielectric Microscopy 3.2 Experimental Details of Higher-Order Nonlinear Dielectric Microscopy Three-Dimensional Measurement Technique 4.1 Principle and Measurement System 4.2 Experimental Results Tb/in2 Ferroelectric Data Storage Based on SNDM References 77 77 77 78 80 83 83 85 86 87 87 88 91 91 92 93 101 103 105 105 106 107 111 112 112 113 115 116 117 118 123 X Contents Part III Relaxors Analysis of Ferroelectricity and Enhanced Piezoelectricity near the Morphotropic Phase Boundary Makoto Iwata, Yoshihiro Ishibashi Introduction Free Energy and Phase Diagram Dielectric Constants, Elastic Constants and Electromechanical Coupling Constants Polarization Reversal Enhanced Piezoelectricity Under an Oblique Field Magnetostrictive Alloys of Rare-Earth–Fe2 Compounds References Correlation Between Domain Structures and Dielectric Properties in Single Crystals of Ferroelectric Solid Solutions Naohiko Yasuda Introduction Single-Crystal Preparation 2.1 Flux Method 2.2 Solution Bridgman Method Measurement Domain Structures in the PIN–PT Solid Solution 4.1 Temperature Dependence of the Permittivity, Domain Structure and Birefringence 4.2 The Effect of a dc Bias Field on the Domain Structure Domain Structures in a (001) Plate of a PMN–PT Solid Solution References Relaxor Superlattices: Artificial Control of the Ordered– Disordered State of B-Site Ions in Perovskites Hitoshi Tabata Relaxor Behavior in Perovskite-Type Dielectric Compounds 1.1 Introduction 1.2 Experimental Procedure 1.3 Results and Discussion 1.4 Conclusions Artificial Control of the Ordered/Disordered State of B-Site Ions in Ba(Zr,Ti)O3 by a Superlattice Technique 2.1 Introduction 2.2 Experimental 2.3 Results and Discussion 127 127 128 131 134 140 144 145 147 147 148 148 149 150 150 150 152 156 158 161 161 161 162 163 167 167 167 168 170 Improvement of Memory Retention 227 Fig Capacitance retention for various values of proportionality factor of Jf (Ef , 0), B Fig Capacitance retention for various values of β to describe how much current through the ferroelectric layer was observed, such that Jf (Eve , 0) = BJf0 (Eve , 0) Jf0 (Eve , 0) was obtained experimentally and used in the analysis described in Sect [15] When B = 1, which means Jf (Eve = 35 kV/cm, 0) ≈ 10−8 A/cm2 , the time dependence of the capacitance for the modeled MFIS structure shows good agreement with the experimental results for the Al/SBT/SiO2 /Si capacitor Figure shows that the retention time for B > 10−3 decreases rapidly with an increase of B, while the current for B < 10−3 corresponds to almost the maximum, constant retention time of ×108 s This result implies that a value of Jf (Eve , 0) less than some given value (corresponding to B ≈ 10−3 ) does not significantly affect the retention time; however, if Jf (Eve , 0) becomes larger than that value, seriously degraded retention characteristics are observed 2.3.3 Effects of Absorption Current in Ferroelectric Layer The time dependence of the current through the ferroelectric layer in the MFIS structure was also studied The absorption current Jf (t) decays exponentially with time, as shown in (5) Retention characteristics for various values of β are shown in Fig A large β gives remarkably long retention times, even when the initial current at t = is constant The results mean that the total charge, which is the integral of the current, is the essential origin of the retention degradation of the MFIS structure assumed here 228 Masanori Okuyama and Minoru Noda 2.3.4 Discussion of Current Reduction From a comparison of Figs and 6, it can be seen that a reduction of Jf (Eve , 0) improves the retention characteristics more than a reduction of Ji (Ei ) does It is also preferable that the current through the ferroelectric decreases rapidly with time Therefore, it is desired that the current through the ferroelectric should be made as small as possible Advanced Structures to Improve Retention Time In order to enhance the memory retention time, some improvements to reduce the current through the metal–ferroelectric junction have been proposed and simulated 3.1 Enhancement of Barrier Height The most plausible cause of memory retention degradation is the current through the ferroelectric layer, which is attributed to Schottky conduction and changes drastically with the barrier height of Schottky junction In an MFIS structure, metals with a large work function are generally considered to contribute to increasing the barrier height for electrons and decreasing it for holes injected from the top metal electrode into the ferroelectric layer Since Jf (Eve , 0) is considered to depend exponentially on the effective φB , as described in (8), even a slight increase of φB (∆φB ) can result in a large decrease of Jf (Eve , 0) If the Schottky current through the ferroelectric layer is multiplied by a factor of 10−2 (Fig 8(a)), then the modeled MFIS structure shows a retention time of about 108 s (Fig 8(b)), which corresponds to a increase of 0.12 eV in the effective φB of the ferroelectric layer (Fig 8(c)) [15] Experimentally, however, optimizing the work function of the top metal electrode is not realistic, because pinning of the surface Fermi level in the ferroelectric layer can make the barrier height less dependent on the work function of the metal [20] 3.2 Insertion of Ultrathin Insulator Layer Between Metal and Ferroelectric Layers Insertion of an ultrathin insulator layer between the top metal electrode and the ferroelectric layer is expected to be effective in decreasing Jf (Eve , 0); this forms an MIFIS structure The retention characteristics of the MIFIS structure (Fig 9(a)) have been simulated by using a physical model similar to that for the MFIS structure Calculated retention characteristics of the MIFIS structure are given in Fig 9(b) The structures exhibit remarkably better retention characteristics than the original MFIS structure The insulator between the metal and the ferroelectric layers should be very thin, so as to Improvement of Memory Retention 229 Fig Calculated effects of decreasing the current through the ferroelectric layer in an MFIS structure (a) Field dependence of the current through the ferroelectric layer, and (b) hold time dependence of the capacitance retention for the initial barrier height in the ferroelectric (∆φB = 0) and an increased barrier height (∆φB = 0.12 eV) (c) Band diagram for the increased ferroelectric barrier height in the MFIS structure Fig (a) Band diagram of MIFIS structure (b) Calculated capacitance retention of the MIFIS structure for various thicknesses of the inserted insulator layer: di0 = 0.1, 0.5, 1.0 and 5.0 nm enhance the voltage applied to the ferroelectric layer in the write operation The current through the insulator layer on the silicon substrate was assumed to correspond to A = as given in Fig 5, and the current through the ferroelectric layer was assumed to correspond to B = as given in Fig It was assumed that the thickness di0 of the additional SiO2 thin layer was 0.1, 0.5, 1.0 or 5.0 nm, and the current through the insulator layer was assumed to be due to the Schottky, Fowler–Nordheim and direct tunneling conduction The direct tunneling current, Jt through an additional thin insulator layer was assumed to be given by the following equation [21]: 230 Masanori Okuyama and Minoru Noda Fig 10 (a) Capacitance retention characteristics of MFIS structures for various insulator dielectric constants i up to i = 30 (b) Dependence of the ferroelectric polarization on the applied gate voltage in the MFIS structures Jt = αEi di0 exp − 2di0 √ ϕm , h ¯ (13) where h ¯ is Planck’s constant, ϕ is the work function difference for the M– I system and m is the carrier mass The parameter α was determined to make Ji for di0 = 0.1 nm comparable to the current through the ferroelectric layer, which is about 10−7 A/cm2 In this calculation, ϕ = 3.2 eV for the system Al/SiO2 , and the free-electron mass were used When di0 = 1.0 nm, the calculation indicates that the retention time will be over 10 years, as shown in Fig 9(b) 3.3 High-k Insulator Layer Instead of SiO2 Film Using a high-k insulator is expected to increase the reverse polarization in an MFIS structure and reduce the depolarization field High-k dielectrics have been studied intensively for application in gate oxide layers in MOS transistors with a small effective oxide thickness (EOT) and a small tunneling current Some dielectric constants εi that have been reported are 20–25 for HfO2 , 27 for 3.3 nm La2 O3 (0.48 nm EOT), 8.5 for 2.1 nm Al2 O3 (0.96 nm EOT) [22] and 31 for 12 nm Pr2 O3 (1.4 nm EOT) [23] Retention characteristics for various values of εi up to 30 have been simulated (Fig 10(a)) As εi increases, the ratio of the share of the voltage in the ferroelectric layer to the total Vg becomes larger and so the retention time becomes long The initial capacitance difference between the curves for positive and negative Vg becomes larger because the initial capacitance difference has been increased by making the ferroelectric hysteresis loop large (Fig 10(b)) The retention curves for negative Vg extend to longer hold times before convergence occurs than those for positive Vg , as shown in Fig 10(a) Improvement of Memory Retention 231 Experimental Improvement of Retention Time by O2 Annealing In the previous section, it was proposed that the current through the ferroelectric could be reduced by some improvements such as barrier height enhancement, insulator insertion and a high-k insulator layer The insertion of MgO and SiO2 films as insulators and the use of a high-k insulator PrOx film have been tried [13, 24], but a significant improvement was not obtained The barrier height enhancement by annealing the ferroelectric film in an O2 atmosphere has been successful in elongating the retention time through reduction of the current [25, 26] So, in this section, annealing and its effect on the retention are described [6, 8, 9, 12, 13, 26] 4.1 Effect of O2 Annealing on Physical Properties of SBT Thin Films on (111) Pt/Ti/SiO2 /Si Substrates SBT thin films on (111) Pt/Ti/SiO2 /Si were annealed in a atm O2 atmosphere at 600 ◦ C for 20 minutes From XRD analysis, it was found that the preferential (115) peak intensity of the O2 -annealed SBT thin film was much enhanced, although the crystalline orientation was almost the same as that of the as-deposited film [26] Therefore, it is considered that the O2 annealing improved the ferroelectricity of the Bi-layered SBT AFM images of SBT film surfaces have also been studied before and after annealing [26] The asdeposited SBT showed some large hillocks on a flat, fine surface (Fig 11(a)), while the surface of the annealed SBT was dense, with grains (Fig 11(b)) but not with large hillocks The O2 annealing enlarged the SBT grain sizes on the flat terrace and removed the hillocks, which implies that the surface roughness of the SBT, which enhances the current, was improved by the O2 -annealing treatment 4.2 Polarization Retention Characteristics of Pt/SBT/Pt Capacitors The polarization retention characteristics of Pt/SBT/Pt capacitors were analyzed [26] by applying a pulsed triangular voltage to measure the C–V characteristics of an MFIS capacitor held under a DC bias The definition of the retained-polarization ratio is the ratio of the remanent polarization after a hold time to the maximum remanent polarization immediately after the device had been polarized As shown in Fig 12(a), a capacitor consisting of as-deposited SBT/Pt/Ti/SiO2/Si with a top Pt electrode showed a polarization retention degradation which was severely affected by the hold DC bias voltage On the other hand, a capacitor consisting of O2 -annealed SBT/Pt/Ti/SiO2/Si with a top Pt electrode showed much more retained polarization, which was independent of the hold DC bias voltage (Fig 12(b)) 232 Masanori Okuyama and Minoru Noda Fig 11 AFM images of (a) as-deposited and (b) O2 -annealed SBT thin films [26] Fig 12 Polarization retention characteristics of (a) as-deposited and (b) O2 annealed SBT thin films [26] The polarization retention characteristics were improved by the O2 annealing, possibly because the annealing improved the crystallinity of the SBT 4.3 Current Conduction in SBT Films The current density through as-deposited and O2 -annealed SBT thin films were studied by using Pt/SBT/Pt diodes [26] As shown in Fig 13(a), O2 annealing succeeded in decreasing the current density through the SBT thin film The current density through the as-deposited and the O2 -annealed SBT film was analyzed into two contributions, from the Schottky and the Frenkel– Poole conduction The Schottky current JSk was obtained from (8) and the Frenkel–Poole current was expressed as ⎫ ⎧ ⎨ −q φB − qEf /πεf ⎬ JFP ∼ E exp (14) ⎭ ⎩ kT As shown in Fig 13(b), both the Schottky and the Frenkel–Poole conduction are decreased by the O2 annealing The Schottky conduction has a larger Improvement of Memory Retention 233 Fig 13 (a) Current density through as-deposited and O2 -annealed SBT thin films (b) The current densities in (a), analyzed into combinations of Schottky and Frenkel–Poole conduction [26] Fig 14 Capacitance retention characteristics of two MFIS structures, consisting of as-deposited SBT/SiON/Si with Al electrodes and O2 -annealed SBT/SiON/Si with Pt electrodes value than the Frenkel–Poole conduction, similarly to what was found in Sect 2.2, and consists of carrier transport brought about by thermionic emission across the metal–ferroelectric interface, whereas the Frenkel–Poole conduction is brought about by field-enhanced thermal excitation of trapped carriers into the band Therefore, the decreased contributions from both the Schottky and the Frenkel–Poole conduction to the current density shown in Fig 13(b) imply that the O2 annealing has increased the barrier height of the ferroelectric layer and decreased the trap density in the ferroelectric layer 4.4 Retention Improvement of MFIS Structures by O2 Annealing The capacitance retention characteristics of two MFIS structures consisting of an as-deposited SBT/SiON/Si structure with Al electrodes and an O2 - 234 Masanori Okuyama and Minoru Noda Fig 15 Flowchart of effects of O2 annealing on retention characteristics of MFIS structures using ferroelectric PLD-SBT thin films annealed SBT/SiON/Si structure with Pt electrodes were studied, as shown in Fig 14 [26] Hold DC bias voltages were applied to compensate the flatband shifts shown in the capacitance retention characteristics of the two MFIS structures, which are considered to be caused by the work function differences between the metal and the SBT film and by fixed charge distributions in the ferroelectric and insulator layers The O2 -annealed SBT/SiON/Si structure with Pt electrodes showed much more retained capacitance than did the asdeposited SBT/SiON/Si structure with Al electrodes This result indicates that O2 annealing can be expected to improve the capacitance retention characteristics of MFIS structures The mechanisms of the retention improvement of MFIS structures by O2 annealing have been considered, as shown in the flowchart in Fig 15 The O2 annealing improves the crystallinity of the PLD-SBT and decreases the Schottky conduction in the PLD-SBT The Frenkel–Poole and absorption currents through the PLD-SBT are believed to be decreased by decreasing the trap density The polarization retention characteristics are also improved As a result, we believe that an improvement in the retention of an MFIS structure can be achieved by O2 annealing [25, 26] 4.5 More Improvement by Rapid Thermal Annealing It is expected that rapid thermal annealing (RTA) for a short time at a high temperature will improve the crystallinity and thus the ferroelectricity in a film without sacrificing the excellent interface properties of the MFIS structure From the viewpoint of ultra-LSI (ULSI) processes, it is adequate to anneal the ferroelectric layer in an MFIS gate stack simultaneously forming Improvement of Memory Retention 235 Fig 16 (a) Retention curves of capacitance of MFIS structures formed using SBT films treated by RTA (b) Extrapolated retention characteristics of MFIS structures annealed at 1000 ◦ C for 30 s the source–drain high-density dopant regions of shallow junctions in MOSFETs, by RTA at around 1000 ◦C for 10 s The initial heating rate was about 50 ◦ C/s from room temperature to 800 ◦ C, and then about 10 ◦ C/s from 800 ◦C to 1050 ◦C Several MFIS samples were annealed at 600 ◦ C to 1000 ◦C in an O2 atmosphere for 30 s and Figure 16(a) shows the retention characteristics for annealing conditions of 900 ◦ C for 30 s, 900 ◦C for and 1000 ◦C for 30 s The MFIS sample annealed at 1000 ◦C for 30 s shows a large difference between the on and off capacitances even after ×105 s (i.e., d) In Fig 16(b), the extrapolated retention time is estimated to be 3.3 ×107 s (i.e., year) It is especially important to note that the case of 1000 ◦ C for 30 s showed a very long retention time, even though the initial on/off capacitance ratio was small compared with the cases of annealing at 900 ◦C, with a memory window of about 0.5 V It is suggested in this case that the crystallinity of the ferroelectric dominates the retention capability, rather than the insulator–Si interface conduction Photoyield Spectroscopic Studies on SBT Thin Films 5.1 Principle of Photoyield Spectroscopy of SBT Films As investigated in Sect 4, an O2 -annealing treatment is an effective way to reduce leakage currents through SBT films deposited by pulsed laser deposition and to improve the retention characteristics In this section, we describe the effects of O2 annealing on the electronic properties of the SBT film, studied by the photoemission spectroscopy technique of photoyield ultraviolet spectroscopy (UV-PYS) UV-PYS gives information about the total number of photoelectrons which are excited from all the occupied states and Fermi levels, as shown in Fig 17 Our UV-PYS system gives very precise data, with 236 Masanori Okuyama and Minoru Noda Fig 17 Relation between UV-PYS spectra and energy states a resolution of about ±0.05 eV X-ray photoelectron spectroscopy (XPS) also gives precise information about electronic characteristics such as the valence bands and core levels in SBT films, but its spectral resolution depends on the spectral width of the X-ray source and is much worse than that of UV-PYS These features indicate that UV-PYS is preferable for estimating the starting point of a photoemission spectrum, such as a Fermi level UV-PYS spectra for an SBT film before and after an O2 -annealing treatment were studied If the SBT was an ideal insulator, the threshold energy for photoemission from the valence band would be about 7.7 eV [27], which is larger than the value obtained in the present work Instead, the photoemission from the SBT surface, as shown in Fig 18, seems to have a threshold energy which is much closer to the SBT work function of 5.4 eV This result indicates that the threshold energy detected by UV-PYS is the excitation energy from the Fermi level to the vacuum level, because the maximum energy of the occupied states is indicated by the band tail in the valence band, and can be considered to be the Fermi level 5.2 Effects of O2 Annealing on SBT Thin Films Studied by UV-PYS The UV-PYS spectra obtained for the SBT film before and after O2 -annealing treatment were analyzed The photoemission yield from the SBT, Y , is described by the following equation, which has the form of an indirect-opticalexcitation equation [28]: 5/2 Y ∼ (¯ hω − Eth ) , (15) where h ¯ ω is the excitation photon energy and Eth is the photoemission threshold energy Figure 18 shows Y 2/5 vs photon energy The experimental data can be fitted well with straight lines The as-deposited SBT film exhibits a Fermi energy of 5.90 eV, while the O2 -annealed film exhibits a value of 5.56 eV These results indicate that the O2 -annealing treatment has increased the Fermi level of the film surface by 0.34 eV Improvement of Memory Retention 237 Fig 18 UV-PYS spectra of PLDSBT thin films before and after O2 annealing Fig 19 Band diagrams considered for the PLD-SBT surface before and after O2 annealing If an electron affinity of 3.5 eV is assumed for the SBT [27], the energy difference between the Fermi level and the conduction band minimum can be considered to be the barrier height for electrons of the metal–ferroelectric interface, and is estimated to be 2.40 eV for the as-deposited film and 2.06 eV for the O2 -annealed film, as shown in Fig 19 On the other hand, the hole barrier height is estimated to be 1.80 eV for the as-deposited film and 2.14 eV for the O2 -annealed film if a band gap of 4.2 eV is assumed [27] The Fermi-level differences discussed above suggest that the as-deposited SBT film shows ptype thermionic conduction, with its Fermi level of 0.30 eV being lower than the intrinsic value obtained after O2 annealing; however, the film shows much less conduction when the Fermi level is close to the intrinsic level This idea qualitatively supports the experimental studies on the effects of O2 annealing described in Sect 4, where the O2 annealing was considered to decrease the Schottky current through the SBT film and successfully improved the memory retention characteristics of the MFIS structure 238 Masanori Okuyama and Minoru Noda Acknowledgement The authors would like to thank Dr Mitsue Takahashi for helping with the work References [1] E Fujisaki, T Kijima, H Ishiwara: Appl Phys Lett 78, 1285 (2001) 220 [2] K Sakamaki, S Migita, S Xio, H Ota, S Sakai, Y Tarui: Extended abstracts of 48th 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K Makihara, K Tseng: Appl Phys Lett 60, 2126 (1992) 224 Improvement of Memory Retention 239 [19] K Kobayashi, A Teramoto, M Hirayama: in Proceedings of the 33rd IEEE International Symposium on Reliability Physics (Las Vegas 1995) p 168 225 [20] S Sze: Physics of Semiconductor Devices, 2nd ed (Wiley, New York 1981) Chap 5, pp 270–279 228 [21] E Burstein, S Lundqvist: Tunneling Phenomena in Solids (Plenum, New York 1969) Chap 3, p 23 229 [22] A Chin: Abstracts of MRS High-k Gate Dielectric Workshop (New Orleans 2000) p 13 230 [23] H Osten, J Liu, P Gaworzewski, E Bugiel, P Zaumseil: Tech Dig 2000 IEEE International Electron Device Meeting (2000) p 653 230 [24] M Noda, K Kodama, S Kitai, M Takahashi, T Kanashima, M Okuyama: J Appl Phys 93, 4137–4143 (2003) 231 [25] S B Xiong, S Sakai: Appl Phys Lett 75, 1613 (1999) 231, 234 [26] K Kodama, M Takahashi, D Ricinschi, A Lerescu, M Noda, M Okuyama: Jpn J Appl Phys 41, 2639 (2002) 231, 232, 233, 234 [27] J Scott: Jpn J Appl Phys 38, 2272 (1999) 236, 237 [28] E Kane: Phys Rev 127, 131 (1962) 236 Index absorption current, 225, 234 MFIS structure, 220 band diagram, 222, 224 barrier height, 220, 223, 228 oxygen annealing, 220 depolarization field, 219–221 energy band diagram, 222, 224 ferroelectric-gate FET, 220 Frenkel–Poole current, see asoPoole– Frenkel emission232 high-k insulator, 230, 231 memory retention, 219, 220, 228, 237 metal–ferroelectric–insulator– semiconductor structure, see MFIS structure polarization relaxation, 220, 221 polarization retention, 219, 234 PrOx , 231 pulsed laser deposition, 235 rapid thermal annealing, 234 RTA, 234 SBT, 220 Schottky current, 226, 228 SiON, 233 ultraviolet photoyield spectroscopy, 220 UV-PYS, 220 ... 1.2 Representative Ferroelectric Thin Films for Memory Devices 1.3 Layer-Structured Bi4 Ti3 O12 -Based Thin Films Rare-Earth-Ion-Modified Bi4 Ti3 O12 Thin Films ... 16 19 20 Part II Preparation and Characterization of Ferroelectric Thin Films Theoretical Aspects of Phase Transitions in Ferroelectric Thin Films Shin-ichi Hirano, Takashi Hayashi, Wataru Sakamoto,... Content on Nd-Modified BIT (BNT) Thin Films 2.7 Effect of Processing Temperature on Nd-Modified BIT (BNT) Thin Films Ge-Doped (Bi,Nd)4 Ti3 O12 Thin Films