Transparent Electro-active Oxides and Nano-technology

Hideo HOSONOFrontier Collaborative Research Center &

Materials and Structures Laboratory,Tokyo Institute of Technology, Yokohama,

JAPAN

Schedule of lecture : Part (I) Transparent Oxide Semiconductors

August 8 Introduction with Grand Prix -awarded Movie of Transparent Electro-active Materials Project What is semiconductor / transparent oxides ?

August 9 N-type transparent Oxide Semiconductor.: electronic structure, application as TCOs, material designing for novel N-type TCO, and Nano-TCO and applications

August 10 P-type Transparent Oxide Semiconductor: material design concept , examples, and devices based on PN-junction

August 13 Comprehensive understanding of TOS viewed from band lineup

August 14 Thermoelectric oxides and performance enhancement utilizing artificial nanostructure (Dr.S.W;Kim of TIT), Exam (I)

Part II TAOS, C12A7, fs-laser

August 27 Transparent Amorphous Oxide Semiconductors(TAOS) and their application to TFTs

August 28 Nanoporous Crystal 12CaO ・ 7Al2O3 (I) 　 encaging active anions (O, O2

and H) and their functional properties

　　　　　　 August 29 Nanoporous Crystal 12CaO ・ 7Al2O3 (II) RT-stable electride, their electronic properties ( metal-insulator tr

ansition, metal-superconductor transition) and device application　　　　　 August 30 Nano-maching of transparent dielectrics by femtoseco

nd laser pulse 　　　　　　 August 31 Summary of the lecture and Examination (II)

Energy Diagram

Vacuum level)

Valence band

Conduction band

Ionization potential

Valence Band Maximum

Fermi Level

Band Gap

Electron Affinity

ConductionBand Minimum

Work Function

What is semiconductor

ECBM – EF ~ kT for N-type

EF- EVBM ~ kT for P-type

W Ncarrier is controllable over several orders of magnitude by Intentionall doping

For Insulator | E(band edge) –EF | >>kT

Electrical Conductivity

Mobility (cm2(Vs)-1) Carrier Concentration (cm-3)

= / m*Effective mass

Carrier relaxation time ( inverse of mean free path)

i.e., depends on quality of sample

Effective mass m*

m* dE2/dk2

m* is an intrinsicmaterial property.

SnO2 : crystal structure

Rutile-type structure

SnO2: band structure

VBM

CBM

Density of States

Si:band structure

VBMCBM

Carrier Mobility in various semicond/.

Why is the electron mobility is larger than hole mobility,?

source

Lighting tubeLED

backlitepolarizer polarizer Color filter

Liquid crystalTransparent electrode

switch（ TFT)

SWITCH（ TFT)

Constitution of Liquid crystal displays

Thin Film Transistor(TFT)Gate

Electron path（ channel）

Dorain

Souce

sourcesource

スイッチ・オフ スイッチ・オンGate Voltage Off Gate Voltage On

Dorain

Gate

Semicond

Insulator

LCD Pixel Circuit

LC

(signal line)

(voltage line)

Thin Film Solar Cells

P-type Si

N-type Si

hSuperstrate type

glassTCO(SnO2)

Metal(Ag, Al)

Active pure Si-layer

TCO(ITO)

Comparison of TCO with metal

In2O3 :crystal structure

CaF2

ITO(In2O3): electronic structure

Fan &Goodenough(1977)

DOS(eV-1)Intensity

Energy(eV

)

In2O3 ： Sn content and Carrier Conc.

Sn content(Sn/In) (%)

Car

rier

Con

c(10

21 c

m-3)

Plasma Frequency

p = ne2

o ∞m*

2

Typical metal and ITO

Material

elec

tron

de

nsi

ty Collective oscillation

Absorption, reflection in TCOs

Visiblerange

Due to VB-CBtransition

Reflection due to Carrier electron

Absorption due to free carrier

Plasma frequency

Wavelength(m)

Resistivity and reflectance ＠８００nm

イオン不純物散乱

Carrier Conc.(cm-3)

scattering due toIonized impurity

Res

istiv

ity

Ref

lect

ance

Resistivity (Min) vs Year

Two types of carrier scattering

Grain boundary Scattering ( g )

Ionized impurity Scattering ( i )

Carrier conc(cm-3)

Hal

l mib

ility

(cm

2 (V

s)-1

Material design for N-type TOS

e

-

2-

Edge-sharing MO6

Octahedron Chain

ns0 orbital

M i+ : p -block heavy cation

e.g. In,Ga

SnO2 : crystal structure

Rutile-type structure

SnO2: band structure

VBM

CBM

Density of States

Various TCOs

Nano TCOs

Ex. ZnO by Wang (Georgia Tech)

spring

ring

spiral

Nanowire arrays

Nano power generator

ZnO nanowire

Piezo electric

Wang (Science 2006)

Electron doping via oxygen vacancy formation

Sn4+ O2-

Oxygen vacncy

Free electron

Electron becomes mobile,=>candidate of transparent metals

Excess electrons are injected

position

Ele

ctro

n E

nerg

y

CB is due to metal’sOrbital

M2+ M2+ M2+

O2- O2- O2- O2-M2+ M2+ M2+

O2-

Oxy

gen

vacn

acy

O2- O2-

Excess electronscannot find stableSites.

M2+ M2+ M2+

O2- O2- O2-

When oxygens are removed -------・

Defect-free

SnO2

Mg2+

O2-

Trapped electron(color center)

Ele

ctro

n E

nert

gy

M2+ M2+ M2+

O2- O2- O2- O2-M2+ M2+ M2+

O2-

Oxy

gen

vaca

ncy

O2- O2-

When e- is removed………Defect-free state

M2+ M2+ M2+

O2- O2- O2-

e- is stabilized by deformingthe lattice

E- becomes immobile（ color center ）　 => remains insulating

position

Oxy

gen

vaca

ncy

O-vacancy

When electron is doped to insulator via oxygen vacancy formation