POLAR MATERIALS: FROM PHYSICAL PHENOMENA TO APPLICATIONS · POLAR MATERIALS: FROM PHYSICAL PHENOMENA TO APPLICATIONS Lucian Pintilie National Institute of Materials Physics, Atomistilor

POLAR MATERIALS: FROM

PHYSICAL PHENOMENA TO

APPLICATIONS

Lucian Pintilie

National Institute of Materials Physics, Atomistilor 405A, Magurele, Romania E-mail address: [email protected]

Conferinţa Diaspora în Cercetarea Ştiinţifică şi Invăţământul Superior din România

Eveniment aflat sub Înaltul Patronaj al

Preşedintelui României.

1 Timisoara, 25-28 Aprilie 2016

2 Timisoara, 25-28 Aprilie 2016

Outlines

Introduction

Interfaces in ferroelectrics

Self-doping in epitaxial ferroelectrics

Polar heterostructures

Light and polar materials

Next steps

Timisoara, 25-28 Aprilie 2016 3

Introduction

NTT Technical Review 8 (2010)

Polar materials (inorganic)

Lack of center of symmetry-polar axis-polarization-

pyroelectricity, ferroelectricity, piezoelectricity, non-linear

optical effects

Polar semiconductors

(e.g. GaN, AlN, ZnO) Ferroelectrics, especially those with

perovskite structure (e.g. BaTiO3,

Pb(Zr,Ti)O3, or multiferroics (BiFeO3)


The Nobel Prize in Physics 2014 - awarded jointly to Isamu Akasaki,

Hiroshi Amano and Shuji Nakamura "for the invention of efficient blue light-

emitting diodes which has enabled bright and energy-saving white light

sources“.

AlN – for high voltage, high power electronic

devices


Applications of ferroelectrics Non-volatile memories (exploiting the reversible polarization); ultrasound generation and reception, micro-motors, actuators, MEMS (exploiting the piezoelectriv effect); thermal imaging (exploiting the pyroelectric effect); optical switches; energy harvesting (solar reactor, photovoltaic).


Interfaces in ferroelectrics In many cases the sample is a capacitor-electrode interfaces

How interact polarization with the interface?

Multi-layers and super-lattices-internal interfaces

How do the interfaces affect the macroscopic properties?

Samples prepared by pulsed laser deposition-epitaxy (minimizing extrinsic effects)

PZT, BaTiO3, BiFeO3; single crystal substrate-SrTiO3; bottom SrRuO3 electrode



The electrode-ferroelectric interface

Polarization controlled properties of electrode interface

C-V characteristics obtained for different metals as top electrodes on: a) PZT grown on STO (001);

b) BTO grown on STO (001); c) PZT grown on STO (111); d) BTO grown on STO (111).


-6 -4 -2 0 2 4 6

-100

-80

-60

-40

-20

0

20

40

60

80

100

Po

lari

za

tio

n (C

/cm

2)

Voltage (V)

SRO

Pt

Cu

Au

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

Cu

rre

nt (m

A)

300 K

a)

-8 -6 -4 -2 0 2 4 6 8-100

-80

-60

-40

-20

0

20

40

60

80

100

Po

lari

za

tio

n (C

/cm

2)

Voltage (V)

SRO

Cu

b)

-80

-60

-40

-20

0

20

40

60

80

Cu

rre

nt (

A)

150 K

-6 -4 -2 0 2 4 6-100

-80

-60

-40

-20

0

20

40

60

80

100

Pola

rizatio

n (C

/cm

2)

Voltage (V)

SRO

Pt

Cu

Au

c)

300 K-0,4

-0,2

0,0

0,2

0,4

Cu

rren

t (m

A)

-6 -4 -2 0 2 4 6-50

-40

-30

-20

-10

0

10

20

30

40

50

SRO

Po

lari

za

tio

n (C

/cm

2)

Voltage (V)

150 Kd)

-80

-60

-40

-20

0

20

40

60

80

Cu

rre

nt (

A)

Hysteresis loops recorded for the PZT layer deposited on (001)STO (a), for the BTO layer

deposited on (001)STO (b), for PZT film deposited on (111)STO (c), and for BTO film deposited on

(111)STO.


CS

CF

RF

Rc

Schottky contacts


Potential barrier not correlated with work function

Large density of free carriers


PRB 75, 104103 2007

Polarization term 0.7-0.9 eV

P=1 C/m2

εop=6

εst=60 or 40

0 200 40020

40

60

80

100

120

140

160

Die

lectr

ic c

on

sta

nt

Thickness (nm)

Potential barrier around 1 eV

XPS results – 1.4-1.7 eV the potential barriers at electrode interfaces ACS Appl. Mater. Interfaces 6, 2929−2939 (2014)

J. Phys. D: Appl. Phys. 44, 255301 (2011)

The barrier is not the simple difference between metal

work function Φm

and the electron affinity of the

ferroelectric material χf – interface states?


S factor

2

0

0

11.01

1

S

eDS

JAP 36, 3212 (1965)

APL 74, 1168 (1999)

PRL 58, 1260 (1986)

S~0.3 for PZT

Dδ~1.6x1014 cm-2eV-1 (ε=40)

P~90 μC/cm

2 (E

g~3.5 eV)!

It can be that polarization acts as “interface states”, pinning the

Ferrmi level and controlling the height of the potential barriers


XPS investigation

Metal deposition-2 monolayers;

4 monolayers…10 nm

XPS spectra recorded after

each deposition

The shift of the binding energy

was monitored as function of

metal thickness

Band bending occurs at the

interface

0.2-0.5 eV for both PZT and

BTO


Tranzitie de faza indusa de grosime in super-

retele PZT-PZO (feroelectric-antiferoelectric).

Cu scaderea grosimii straturilor componente faza

antiferoelectrica devine feroelectrica.

Applied Physics Letters 91, 122915 (2007)


Interfacial polarization


Self-doping in epitaxial ferroelectrics

AFM/PFM studies revealing that upward polarization is dominant

in epitaxial PZT films


The density of the free

carriers increases with

decreasing the thickness of

the PZT films

The deduced empirical law is:

)(

102)(

273

nmdmn

x

Polarization induced self-

doping

Scientific Reports 5, 14974

(2015)

0 50 100 150 200 250 300

94

95

96

97

98

Po

lari

za

tio

n (

C/c

m2)

Thickness (nm)


There is a variation of stoichiometry with thickness, suggesting different

densities of cation and oxygen vacancies as the thickness increases


Suggested model-oxygen vacancies

pmv O

Free charge from

oxygen vacancies

Free charge from

the bottom SRO

electrode

Bound polarization

charges

0F

B

OF

0FO )(2

exp21)(21

deEgne

P

d

f

Tk

EEdeEgn

e

P

dn mm

v

v

d

PBAnVc

vOECO

)(3

)(

102)(

273

nmdmn

x

e

PdeEgnVfPB

A

mEC

2)()(

3

0F

Tk

EEf v

B

OFexp21

A= 3.09 ± 0.05

f~10

10 % of oxygen

vacancies are

ionized

Tk

EE

nn

v

v

v

B

OF

O

O

exp21


Polar heterostructures

150K

350K

450K

Pt-PZT-ZnO (MFS) structures

epitaxial columnar

policristalin

J. Appl. Phys. 112, 104103 (2012);

Appl. Phys. Lett. 96, 012903 (2010)

Orientation of the C-V hysteresis dependent on

structural quality and temperature


Artificial multiferroic structures-combination of

ferroelectric and ferrimagnetic materials.


Light and polar materials

Light – photovoltaic

pyroelectric

J. Mater. Chem. A, 2014, 2, 6027–6041 | 6029

dTdPp S /

)/( dtdTApip

A current is generated when there is a

temperature variation of the sample

Intrinsic internal electric field

to separate the charge

carriers


J. Appl. Phys. 110, 044105 (2011)

Short-circuit

photocurrent dependes

on the electrode

materials

Photovoltaic


Necache et al., Nature Photonics DOI: 10.1038/NPHOTON.2014.255

Efficiency 8.1 % by band-gap engineering in a multilayer!


Solar cells with (MA)PbI3

SEM image of the stack

Standard structure

FTO TiO2

perovskit

HTM-spiro-OMeTAD

Au electrodes, maximum area of 5x5 mm2

12 %-best performance ever obtained in Romania for any type of solar cells studied up to now-possibility to improve by optimizing

the process and component materials.

(MA)PbI3 molecules (blue-yellow) have electric dipole moments- tend to be

correlated resulting in a net polarization of the cell


Pyroelectric

Epitaxial PZT films grown by PLD on SRO/STO substrates and with top SRO contacts

Polycrystalline films grown by spin coating on Pt/Si substrates and with top Pt contacts

18 20 22 24 26

101

103

105

-50 0 50 100 150 200 250 300 35010

0

101

102

103

104

105

Inte

ns

ity

(a

rb.

un

it.)

Phi (0)

STO

SRO

PZT

PZ

T

Inte

nsity (

cps)

2 Theta (degrees)

SR

O

ST

O PZT

Structural characterization of the PZT films: XRD (left); PFM (middle); TEM (right)

AlN and ZnO thin films grown by RF-sputtering on Si substrates and with top Cu

electrodes

ZnO AlN


a) the frequency dependence of the

pyroelectric signal obtained from the

Pt/PZT/SRO/STO active element);

b) the dependence of the pyroelectric signal

on 1/ω, evidencing the frequency ω0

where the heating conditions for the

sample are changing from relatively

uniform to non-uniform;

c) the PFM image of the free surface of the

PZT layer, evidencing the presence of

the 90 domains.

Applied Physics Letters 103, 232902 (2013)

Phys. Rev. Lett. 109, 257602 (2012).

p~19x10-4 C/m2K


Optical amplification of the pyroelectric signal through the photogeneration of

free carriers by the continuous UV illumination


10 100

10

100

Pyro

ele

ctr

ic s

ignal (

V)

Frequency (Hz)

AlN ZnO

p=12.4 μCm-2K-1 for AlN

p~3 μCm-2K-1 for ZnO

Inferior to ferroelectrics but may work at

elevated temperatures, where ferroelectrics

are no longer pyroelectrically active


Next steps

strain engineering-induction of ferroelectric

phase in nominally non-ferroelectric materials

rectifying and ohmic contacts on ferroelectric-

ferrotronics?

doping-manipulating the concentration of free

carriers in close relation with polarization

band-gap tuning-adjusting band-gap for

optimal light absorption in ferroelectric solar

cells

pyro-photo combination-novel concepts for

light sensors (optical amplification and filtering)


Collaborators Samples

Dr. Chirila

Cristina

Dr. Luminita

Hrib

Dr. Stancu

Viorica

Structure

Dr. Pasuk

Iuliana

Drd. Raluca

Negrea

Dr. Corneliu

Ghica

XPS, PFM

Dr. Cristian Mihail

Teodorescu

Dr. Apostol

Nicoleta

Drd. Laura

Abramiuc

Dr. Trupina

Lucian

Electric, magnetic charac.

Drd. Andra Georgia

Boni

Drd.Mihaela Botea

Dr. Iuga Alin

Drd. Simona

Greculeasa

Dr. Pintilie Ioana

Dr. Kuncser Victor

Dr. George

Stan

Photovoltaic

Dr. Cristina

Besleaga


Funding

Funding Program/Project:

IFA-CEA

INTERFACING OXIDES (IFOX)

Project 8 SEE/2014 (EEA-JRP-RO-NO-2013-1)

Complex Idea Project “Effect of interfaces on charge

transport in ferroic/multiferroic heterostructures”

Project NOPYDET


THANK YOU FOR

YOUR ATTENTION!

Documents

POLAR MATERIALS: FROM PHYSICAL PHENOMENA TO APPLICATIONS · POLAR MATERIALS: FROM PHYSICAL PHENOMENA TO APPLICATIONS Lucian Pintilie National Institute of Materials Physics, Atomistilor