Transparent Electro-active Oxides and Nano-technology Hideo HOSONO Frontier 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 ・ 7Al 2 O 3 (I) encaging active anions (O , O 2  and H  ) and their functional properties August 29 Nanoporous Crystal 12CaO ・ 7Al 2 O 3 (II) RT-stable electride, their electronic properties ( metal-insulator transition, metal-superconductor transition) and device application August 30 Nano-maching of transparent dielectrics by femtosecond 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 Conduction Band Minimum Work Function

What is semiconductor E CBM – E F ~ kT for N-type E F - E VBM ~ kT for P-type W N carrier is controllable over several orders of magnitude by Intentionall doping For Insulator | E (band edge) –E F | >> kT

Electrical Conductivity Mobility (cm 2 (Vs) -1 ) Carrier Concentration (cm -3 ) m = t / m* Effective mass Carrier relaxation time ( inverse of mean free path) i.e., depends on quality of sample

Effective mass m* m*  dE 2 /dk 2  m* is an intrinsic material property.

SnO 2 : crystal structure Rutile-type structure

SnO 2 : band structure VBM CBM Density of States

Si:band structure VBM CBM

Carrier Mobility in various semicond/. Why is the electron mobility is larger than hole mobility,?

Constitution of Liquid crystal displays

Thin Film Transistor(TFT) Gate Electron path （ channel ） Dorain Souce source スイッチ・オフ スイッチ・オン Gate Voltage OffGate Voltage On Dorain Gate Semicond Insulator

LCD Pixel Circuit LC (signal line) (voltage line)

Thin Film Solar Cells P-type Si N-type Si Superstrate type glass TCO(SnO 2 ) Metal(Ag, Al) Active pure Si-layer TCO(ITO)

Comparison of TCO with metal

In 2 O 3 :crystal structure CaF 2

ITO(In 2 O 3 ): electronic structure Fan &Goodenough(1977) DOS(eV -1 ) Intensity Energy(eV)

In 2 O 3 ： Sn content and Carrier Conc. Sn content(Sn/In) (%) Carrier Conc(10 21 cm -3 )

Plasma Frequency w p = ne 2 eoeeoe ∞ m* 2 Typical metal and ITO Material electron density Collective oscillation

Absorption, reflection in TCOs Visible range Due to VB-CB transition Reflection due to Carrier electron Absorption due to free carrier Plasma frequency Wavelength(mm)

Resistivity and reflectance ＠８００ nm イオン不純物散乱 Carrier Conc.(cm -3 ) scattering due to Ionized impurity Resistivity Reflectance

Resistivity (Min) vs Year

Two types of carrier scattering Grain boundary Scattering ( m g ) Ionized impurity Scattering ( m i ) Carrier conc(cm -3 ) Hall mibility(cm 2 (Vs) -1

Material design for N-type TOS e - 2- Edge-sharing MO 6 Octahedron Chain ns 0 orbital M i+ : p -block heavy cation e.g. In,Ga

SnO 2 : crystal structure Rutile-type structure

SnO 2 : 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 Sn 4+ O 2- Oxygen vacncy Free electron Electron becomes mobile, =>candidate of transparent metals SnO 2

酸素が抜けた跡 Mg 2+ O 2- Trapped electron (color center) O-vacancy When electron is doped to insulator via oxygen vacancy formation