Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications

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Handbook of dielectric, piezoelectric and ferroelectric materials970

Zhu Y Y, Xiao R F, Fu J S (1997b), ‘Second-harmonic generation in quasi-periodically

domain-inverted Sr

0.6

0.4

optical superlattices’, Opt. Lett., 22, 1382

Zhu Y Y, Xiao R F, Fu J S (1998), ‘Third harmonic generation through coupled second-

order nonlinear optical parametric processes in quasi-periodically domain-inverted

0.6

0.4

optical superlattices’, Appl. Phys. Lett., 73, 432–434

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32.1 Introduction

One of the dreams of materials scientists is to make a material with an

artificially designed structure that will provide the desired properties. Artificial

superlattices, for which theoretical prediction was made by Esaki and coworkers

in 1970s,

1–3

were realized by the subsequent progress of thin film growth

technologies such as molecular beam epitaxy (MBE) and chemical vapor

deposition (CVD) techniques. Compound semiconductor superlattices, such

as GaAs/GaAlAs superlattices, have been formed by precise control of one

atomic layer deposition. These superlattices are used for some practical

devices. Artificial superlattices consisting of oxide compounds are current

research topics for material and ceramic scientists because oxides have various

physical properties, such as dielectric, piezoelectric, magnetic, optical and

electron-transfer. Oxide superlattices may provide unknown material properties

or significant enhancement of known properties. In particular, perovskite

oxides are a treasure-house of interesting properties because ferroelectricity,

piezoelectricity, high dielectricity, electron conductivity and other properties

are obtained by changing constituent elements in the same perovskite structure.

A pioneer work of artificial superlattices of perovskite oxide, BaTiO

(BTO)/SrTiO

(STO), was carried out by Iijima et al.

4,5

in the early 1990s.

They succeeded in the monolayer growth of BTO and STO using alternate

evaporation of Ba or Sr and Ti as well as co-evaporation of these elements.

The metal elements evaporated from electron-beam-heated or tungsten-wire-

heated sources were oxidized in a high-vacuum chamber by partially ozonized

oxygen. Their reactive evaporation process became the base of the MBE

process currently used for the deposition of oxide superlattices. Other pioneer

works of perovskite oxide artificial superlattice were reported by Koinuma

and coworkers

6–9

and Tabata et al.

in the 1990s. Koinuma et al. developed

a laser MBE system which enabled monolayer depositions of perovskite

oxides. They also developed a process to make atomically flat surface on

STO

single crystal substrates, which is one of the most important techniques

Dielectric and optical properties of perovskite

artificial superlattices

T TSURUMI and T HARIGAI, Tokyo Institute of

Technology, Japan

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Handbook of dielectric, piezoelectric and ferroelectric materials972

required for atomic layer epitaxy of perovskite oxides. The enhancement of

material properties in artificial superlattices was demonstrated by Tabata et

al.,

who found the enhancement of the dielectric permittivity in the BTO/

STO superlattice with a stacking periodicity of two unit cells/two unit cells.

We started perovskite superlattice work in 1994 and published the first report

on the formation of BTO/STO superlattices using computer-controlled atomic-

layer-depositions of BaO, SrO and TiO

After these works, perovskite artificial superlattices were prepared by several

research groups using laser MBE process,

12–21

reactive MBE process

22–24

and radiofrequency (RF)-sputtering process.

25–28

Kim et al.

12,13

demonstrated

the enhancement of dielectric properties in artificial BTO/STO superlattices

by manipulating lattice strains in the superlattices. They observed large

dielectric permittivity (ε

= 1230) and extremely large dielectric nonlinearity

in BTO/STO superlattice with a stacking periodicity of two unit cells/two

unit cells. Christen et al.

prepared SrZrO

(SZO)/STO and BTO/STO

superlattices using pulsed laser deposition (laser MBE) and indicated that

the electric field-dependence of the permittivity of both types of structure

had a different behavior from that observed in the corresponding solid solutions,

and, especially, the data of the SZO/STO superlattices were consistent with

the assumption that strain induced ferroelectricity in the STO films at room

temperature. A constrained ferroelectricity was also found in the (001)-textured

PbZrO

/BaZrO

superlattice films by Wu and Hung

and in the BTO/STO

superlattices by Shimuta et al.

Superlattices of thin ferroelectric and non-

ferroelectric perovskite layers, e.g. short-period BTO/STO, have been reported

to exhibit enhancements of dielectric constant and remanent polarization.

However, it should be noted that the movement of space charges in the

superlattices can spuriously produce an apparent significant enhancement of

dielectric permittivity as pointed out by O’Neill et al.

The dielectric properties

of perovskite superlattices remain ambiguous.

As for the theoretical works on the perovskite superlattices, Neaton and

Rabe

performed first-principle calculations on the BTO/STO superlattices

and indicated that significant polarization enhancement could be achieved in

perovskite oxide superlattices. This enhancement arose from the combined

effects of strain, induced in the BTO layers by the epitaxial growth, and

internal electric fields, associated with the superlattice geometry. The STO

layers were found to be tetragonal and polar, possessing nearly the same

polarization as the BTO layers. The first-principles calculation of Kim et

al.

32–34

demonstrated that the BTO/STO superlattices under in-plane

compressive state showed enhanced stability of tetragonal phase while the

stability of monoclinic phase was enhanced under the in-plane tensile state.

Yang et al.

also performed first-principles calculations on the SZO/STO

superlattices and indicated that the lattice distortion from the lattice mismatch

in the superlattices led to the formation of spontaneous polarization in the

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Dielectric and optical properties of perovskite 973

superlattices, although neither SZO nor STO is ferroelectric. Lo and Jiang

employed Landau–Khatalnikov theory to demonstrate that enhanced dielectric

permittivity occurred when the stacking periodicity was reduced to a few

monolayers in the superlattices under in-plane interfacial strains. All of these

works pointed out that the lattice distortion due to the in-plane stress is a key

factor in explaining the enhancements of dielectric permittivity and remanent

polarization in perovskite superlattices.

In our research on perovskite artificial superlattices, a modern MBE system

was developed first and deposition process refined in 1990s. BTO/STO

and

SZO/STO artificial superlattices were prepared and their optical and dielectric

properties evaluated. It was found from a series of our works that BTO/STO

exhibited remarkable high dielectric permittivity above 30000 in the in-

plane direction and SZO/STO exhibited strain-induced ferroelectricity. The

enhancement of the permittivity in BTO/STO superlattices was recently

confirmed in the multilayered films prepared by the RF-sputtering process.

These results have provided experimental evidence for the theoretical works.

In this chapter, the deposition processes for the oxide artificial superlattices

are reviewed first. The lattice distortions induced by the lattice mismatch

between the film and substrate and between constituent layers in the

superlattices were analyzed using high-resolution X-ray diffraction (XRD)

equipment. The results obtained for the BTO/STO and SZO/STO superlattices

are discussed in Section 32.3. The optical properties of the superlattices

measured with a spectroscopic ellipsometer and the dielectric properties of

the superlattices measured with planer electrodes are reviewed in Sections

32.4 and 32.5.

32.2 Preparation of artificial superlattices

32.2.1 Laser MBE system

Since the works of Koinuma et al.

6–9

and Tabata et al.,

the laser MBE

system has been commonly used for the deposition of perovskite superlattices.

A schematic drawing of a typical laser MBE system is shown in Fig. 32.1.

It is composed of three main parts: (1) a laser ablation chamber (base pressure

of 1 × 10

–10

torr), (2) a KrF excimer laser (248 nm), and (3) in-situ reflection

high-energy electric diffraction (RHEED) (20kV) and X-ray photoelectron

spectrum (XPS) analyzer. A pulsed laser beam (typically, 10ns, 2–10Hz and

1 J/cm

) is focused onto a ceramic target to deposit the corresponding film on

a substrate heated with a Ta heater under flushing of O

gas at 10

–8

–10

–5

torr.

The in-situ RHEED pattern and its intensity at the specular beam spot and

XPS can be monitored during the film growth. The deposition rate is about

0.5nm/min for a laser frequency of 5Hz. For the growth of artificial

superlattices, the ceramic target is changed automatically with a computer-

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Handbook of dielectric, piezoelectric and ferroelectric materials974

controlled system. The thickness of one molecular layer deposition is controlled

by monitoring RHEED intensity oscillations or simply by the number of

laser pulse shots to the ceramic targets.

The advantage of the laser MBE process is relatively easy deposition of

high-quality films without precise control of chemical composition. The

target composition is copied to the film under optimum deposition conditions.

Because of this advantage, various artificial superlattices of different material

systems can be prepared without consuming time and manpower for optimizing

deposition conditions. Another advantage of the laser MBE is a wide variation

of deposition conditions. In particular, this process does not choose the

oxygen partial pressure in the film deposition, which enables the deposition

of oxide films at relatively high oxygen partial pressures to improve the

properties of oxide films. On the other hand, the biggest disadvantage of the

laser MBE is the restriction of substrate area. The deposition on a large area

is not realistic as far as a normal system is used, which severely restricts this

process for use as an industrial process. Moreover, the deposition of droplets,

which is frequently observed in a film growth, considerably reduces the film

quality. The laser MBE process is a quite useful process in basic research

aimed at the discovery of useful and interesting material systems as well as

their superlattice structures.

32.2.2 Reactive MBE system

Figure 32.2 shows the schematic illustration of an MBE system developed in

our laboratory.

11,37–39

The MBE chamber (Ulvac Co., base pressure of

1 × 10

–9

torr) is equipped with three Knudsen cells (K-cells), two electron-

32.1

Laser MBE system with

in-situ

RHEED and XPS apparatus.

Video

tape

recorder

CCD

camera

Quadrupole mass

spectrometer

Image

processor

Printer

Monitor

Targets

K-cell

Plume

Substrate

Ta heater

Gas

Variable leak valve

XPS

analyzer

ArF or KrF eximer

laser beam

Lens

Quartz window

RHEED gun

Cryopump

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Dielectric and optical properties of perovskite 975

beam-heated guns (EB guns), an RHEED gun and an electron cyclotron

resonance gun (ECR gun). The vacuum pumping system of the chamber

consists of a rotary pump, a turbo molecular pump and a titanium sublimation

pump. The K-cells are used for the evaporation of Ba and Sr metals while Ti

and Zr metals with relatively high melting temperatures are evaporated with

the EB guns. The deposition process is controlled with three computers; the

first and main computer reads the frequency of a quartz oscillator placed

near the substrate, calculates the film thickness from the frequency change

of the oscillator and outputs a signal to a deposition controller (Inficon, IC/

5) which operates shutters of each evaporation gun. For the frequency stability

of the quartz oscillator, distilled water at a constant temperature flows into

the quartz oscillator holder. The second computer is used for stabilizing

evaporation rate of the EB guns using proportional-integral-derivative (PID)

feedback control. The third computer is monitoring the RHEED patterns and

its intensity at the specular beam spot. These three computers are connected

with a local area network (LAN) to enable synchronized controls. One of the

most important points in the oxide film deposition with MBE system is the

oxidation of metals at the film surface in a high vacuum chamber. We employed

MBE

automatic control

system

LAN

Monitor

RHEED computer

LAN

GP-IB

K/S deposition

controller

MBE deposition

control computer

GP-IB

EB-gun emission

control computer

EB gun power supply

Isolation amp

DMM

EB gun

K-cell

ECR

plasma-gun

RHEED gun

Substrate

CCD

camera

Thickness

monitor

32.2

Schematic illustration of an MBE system.

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Handbook of dielectric, piezoelectric and ferroelectric materials976

an ECR gun for this purpose. The oxygen radicals and ions generated in the

ECR gun are continuously irradiated to the film surface during the film

deposition. The irradiation of oxygen ions (O

2–

) severely reduces the film

quality. To avoid the O

2–

irradiation to the film surface, a positive voltage is

applied to the plasma electric discharge room of the ECR gun (acceleration

voltage) and a negative voltage is applied to the substrate holder (bias voltage).

Perovskite superlattices are deposited by an alternate deposition process.

The deposition sequence of a BTO/STO superlattice is illustrated in Fig.

32.3. As the topmost atomic plane of STO single crystal substrates is a TiO

layer,

the deposition starts from one molecular layer (ML) of BaO followed

by one molecular layer of TiO

to form one unit cell of BTO lattice. For the

deposition of the BTO/STO and SZO/STO superlattices, BaO and TiO

layers

are respectively replaced with SrO and ZrO

layers according to the design

of superlattices. Typical deposition conditions of the BTO/STO and SZO/

STO superlattices are as follows:

BaO

SrO

TiO

Step 1 Step 2

Step 4 Step 3

Step 5 Step 6

Step 8 Step 7

SrTiO

1ML

BaTiO

1ML

BaTiO

1ML

BaO 1ML

TiO

1ML

TiO

1ML

SrO 1ML

32.3

Deposition sequence for BTO/STO artificial superlattice.

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Dielectric and optical properties of perovskite 977

• Substrates: (001) STO single crystal (Shinkosha Co.)

(001) Nb-doped STO single crystal (Shinkosha Co.)

• Substrate temperature: 600°C

• Oxygen partial pressure: 3.0 × 10

–6

–5.0 × 10

–6

torr

• Temperature of K-cell (Ba): 540–580°C

• Temperature of K-cell (Sr): 415–430°C

• Emission current of EB gun (Ti): 14–20mA

• Emission current of EB gun (Zr): 70–130mA

• Acceleration voltage: +50V

• Bias voltage: – 100V

A typical RHEED pattern and its oscillations are shown in Fig. 32.4. The

streak pattern of a deposited film and the RHEED oscillations indicates that

the two-dimensional atomic-layer-epitaxy is achieved with the MBE system.

Each oscillation of RHEED intensity corresponds to the deposition of one

molecular layer. It should be noted that the RHEED intensity oscillations

observed in the alternate deposition process are due to the difference of

electron reflectivity of each molecular layer, which is different from that

observed in a co-evaporation process such as the laser MBE, where the

oscillation corresponds to the step growth of one molecular layer of perovskite

lattice.

The advantages of the MBE process over other processes are film quality

over a large area and controllability of the atomic layer deposition. Schlom

et al.

succeeded in preparing almost perfect perovskite superlattices using

adsorption-controlled growth with a reactive MBE system and purified ozone

for the oxidation. The controlled synthesis of atomic-layered oxide

heterostructures offers great potential for tailoring the dielectric and ferroelectric

properties of materials. We have even made compositional-gradient artificial

modulated structure using the MBE system, as shown in Fig. 32.2.

However,

the deposition control and oxidation of metal sources in high vacuum still

need a state-of-art technique and the deposition rate of the MBE process

should be improved for industrial use of this process.

32.2.3 Sputtering system

Perovskite artificial superlattices can be formed using sputtering system. We

have constructed two sputtering systems, an ion beam sputtering (IBS) and

a conventional Rf-magnetron sputtering, for the growth of superlattices.

Schematic drawings of the two systems are shown in Fig. 32.5. The IBS

system uses a Kaufman-type ion beam gun to irradiate Ar ions to ceramic

targets fixed at 100 mm from the gun. Oxygen radicals and ions generated

with an ECR-gun are irradiated to the film surface to improve the crystallinity

of oxide films. The film thickness is monitored with the quartz oscillator and

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Handbook of dielectric, piezoelectric and ferroelectric materials978

RHEED intensity oscillations. We have succeeded in making NiO/ZnO artificial

superlattices using this system.

The IBS system with an oxidation assisting

radical source is suitable to prepare multilayer films and even superlattices

of simple oxide but the compositional deviation from the target restricts the

application of this system to perovskite oxides. At present, it is difficult to

find any advantage of the IBS process over the laser MBE system as far as

the perovskite superlattices are concerned.

(a)

BTO STO BTO STO BTO STO BTO STO

(b)

[110 azimuth

32.4

(a) RHEED intensity oscillation observed during the deposition

of [BTO

/STO

]

artificial superlattice and (b) an RHEED pattern of

deposited film.

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Dielectric and optical properties of perovskite 979

32.5

Schematic illustrations of (a) an ion beam sputtering system

and (b) an RF-magnetron sputtering system used for the depositions

of oxide superlattices.

(b)

Computer

Turbo

pump

RF-power

unit

RF-power

unit

Air

Substrate

Target

Relay unit

Air valve

Air actuator

(a)

Shutter

Sample

Quartz oscillating sensor

RHEED gun

ECR gun

Computer

RS232-C

Relay unit

Power source

Computer

VTR

Monitor

CCD

Motor

Motor driver

Target

Ion gun

Turbo

molecular pump

Screen

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