transparent electro-active oxides and nano-technology hideo hosono frontier collaborative research...
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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