第四章--1--铁电存储

Perovskite Structures
Temperature dependence of Perovskite crystal structure
Atomic Arrangement and Ferroelectricity
• The arrangement of the atoms in all ferroelectric crystals result in an equally stable state but with reoriented Ps. • A simple example is BaTiO3 for which the prototype is cubic. • The paraelectric to ferroelectric transformation at Tc may be viewed in terms of a low-frequency temperature-dependent mode of the crystal lattice, observable by optical or neutron spectroscopy.
Ferroelectric material
• A ferroelectric material falls into a group of dielectrics that exhibit spontaneous polarization. • Ferroelectrics have extremely high dielectric constants at low applied field frequencies. • For example, the dielectric constant for barium titanate at room temperature can be as high as 5000.
4, 4mm
Trigonal
__ 3, 3, 32, 3m, 3m
__ 3, 3m
3, 32, 3m
3, 3m
Hexagonal
__ 6, 6, 6/m, 622, 6mm, 6m2, (6/m)mm
6/m, (6/m)mm
__ 6, 6, 622, 6mm, 6m2
6, 6mm
Cubic
_ 23, m3, 432, 43m, m3m
Absolute Sense of Spontaneous Polarization
• The relative sense of Ps in a crystal is given by the charge developed on the polar faces as a single domain crystal is cooled below Tc. • This sense can be related to the atomic arrangement by making use of the anomalous scattering in an x-ray diffraction experiment.
Triclinic
_ 1, 1
_ 1
1
1
Monoclinic
2, m, 2/m
2/m
2, m
2, m
Orthorhombic
222, mm2, mmm
mmm
222, mm2
mm2,
Tetragonal
__ 4, 4, 4/m, 422, 4mm, 42m, (4/m)mm
4/m, (4/m)mm
__ 4, 4, 422, 4mm, 42m
S=？
max(P E) P E ( 0)

铁电相
顺电相
T TC
铁电体介电常数与温度的关系
Examples: Barium Strontium Titanate (BST)
B.位移型铁电体
•结构特点：钙钛矿结构
通式：ABO3，其中A为一价或二价金属，B为四价 或五价金属
The T dependence of Ps for BaTiO3
Ferroelectric Domains and Hysteresis Loop
• Ferroelectric crystals possess regions with uniform polarization called ferroelectric domains. • Polarization vs. Electric Field (P-E) hysteresis loop for a typical ferroelectric crystal is shown on the right.
I. Introduction
• Ferroelectricity is a phenomena which was discovered in 1921.
• Ferroelectricity has also been called Seignette electricity, as Seignette or Rochelle Salt (RS) was the first material found to show ferroelectric properties. • A huge leap in the research on ferroelectric materials came in the 1950's, leading to the widespread use of barium titanate (BaTiO3) based ceramics in capacitor applications and piezoelectric transducer devices .
2001 MRQW Dec. 11-12, 2001 Pasadena, CA
Ferroelectric Memory Reliability and Qualification
Gary F. Derbenwick and Stephen C. Philpy Celis Semiconductor Corporation Colorado Springs, Colorado 80918 (719) 262-5144 gary@
一、铁电体 的电畴结构和特性
具有宏观偶极矩的区域称为“电畴”。注意薄膜 的电畴
在初始状态，就铁电体整体而言，对 外界将不呈现电荷和极化状态（相当 与回线的O点）。
2. 无应力的晶体
G U T S P E U 内能 S熵 T 绝对温度

S不变时
稳定系统 G最小
Theory for Ferroelectric(cont.)
• • • • • Magnitude of Spontaneous Polarization Atomic Arrangement and Ferroelectricity Absolute Sense of Spontaneous Polarization Dimensionality of Ferroelectric Crystals Changes in Properties Near the Curie Temperature • Antiferroelectric Materials • Representative Ferroelectric Materials • Material Preparation and Poling
A O B
ABO3立方钙钛矿结构
Ba
O
Ps
立方晶系
四角晶系
Ps 正交晶系
Ps 三角晶系
BaTiO3晶体的晶格参数与温度的关系
四角相BaTiO3的自发极化强度与温度的关系

BaTiO3单晶的介电常数与温度的关系

III.Theory for Ferroelectric
• • • • • • • Ferroelectric material Crystal Symmetry Spontaneous Polarization and Pyroelectric Effect Ferroelectric Domains and Hysteresis Loop Curie Point and Phase Transitions Perovskite Structures Dielectric Hysteresis
m3, m3m
_ 23, 43m
------
பைடு நூலகம்
Spontaneous Polarization and Pyroelectric Effect
• The spontaneous polarization is given by the value of the dipole moment per unit volume or by the value of the charge per unit area on the surface perpendicular to the axis of spontaneous polarization. • change in the spontaneous polarization vector ΔPs is given by: ΔPs = π ΔT
Barium Titanate and its tetragonal crystal structure
Crystal Symmetry
Crystal Structure Point Groups CentroSymmetric Non-centrosymmetric
Piezoelectric Pyroelectric
P
常见的铁电体有：
自发极化Ps
矫顽电场Ec o
•酒石酸钾钠 4H2O） E （NaKC4H4O6·