Hydrogen in Wide Gap Semiconductor Why many types of wide gap semiconductor have n type conductivity ? K.Shimomura (KEK- MSL)

Impurity in Semiconductor Shallow donor acceptor Bohr radious a=  ×(m e /m*)×a 0 ~20×a 0 Ionization Energy (13.6eV) × (m*/m e )/  2 ~50meV Hyperfine constant A ～ A 0 Unintentional impurity difficult to study(less than ｐｐｍ ). Origin of n type conductivity in wide gap semiconductors. More then 30 years old problem !

GaN 1.Direct Wide Band Gap Structure 3.4eV~365nm Blue LED etc. 2.exhibit strong n type conductivity

Production Method of GaN A lot of hydrogen ! Electric structure of hydrogen can be simulated by muonium !

 SR

Discovery of Shallow Muonium (CdS,1999 Gil et. al)

Origin of n type conductivity in ZnO Theoretical Study C.G.Van de Walle Hydrogen behaves as a shallow donor/ Phys.Rev.Lett.85,1012(2000) Experimental Study by  SR Discovery of Weakly Bounded Muonium S.F.Cox et. al Phys.Rev.Lett.86,1012(2001) K.Shimomura et. al Phys.Rev.Lett.89,25505(2002)

 SR result on ZnO in KEK- MSL

1.Two kinds of Muonium have been clearly observed. 2.Both Muonium has axial symmetry along to [0001] axis. 3.Hyperfine constants of the observed muoniums are times smaller than the muonium in va cuuum. These value is well correspond to the simple model calculation for shallow donor. 4.Ionization energy of these muoniums are also similar to the ionization energy of the un- intentional donor observed by Hall effect measurements. These results indicate hydrogen could behave as a shallow donor and might be an origin of n type conductivity in ZnO.

ZnO ENDOR

Shallow Muonium could be found in GaN ? Theory Hydrogen negative U Deep center J.Neugebaner, C.G.Van de Walle Phys.Rev.Lett,75, 4452(1995) C.G.Van de Walle, J.Neugebaner Nature 423, 626(2003) Experiment K.Shimomura et al PRL92, (2003)

Theoretical studies in GaN 、 ZnO

Results in GaN TRIUMF M15 12H ~600Mev.

Results in GaN

External field dependence of the ratio of satellite peaks are explained by muonium ’ s electron polarization in high magnetic field ( ~Tesla). Muonium have [0001] axis symmetry. Hyperfine parameter A // =+337(10) kHz, A ⊥ =-243(30)kHz Ionization Energy ~5meV

Hydrogen level in semiconductors (Theoretical Study by C.G.Van de Walle et al.)

Hydrogen level in oxides (Theoretical study )

Titanium dioxide (TiO 2 ) Widely used as photo catalizer Strong n type conductivity Wide Ban Gap 3.3eV

Results in TiO 2

A ～ 0.2 to 1.2MHz Ionization Energy ～ ３ meV Shallow Muonium !

Summary  SR is powerful tool for the stiudies of origin of n type conductivity in GaN, ZnO and TiO 2, which are the most promising material for optelectronics and photo catalysis. ＪＰＡＲＣ  SR with Ultra Slow muon Beam (Dilute Magnetic Semiconductor GaMnAs etc.)  SR with negative muon Beam (N in ZnO or TiO 2 etc) Ｎｅｕｔｒｉｎｏ Ｆａｃｔｏｒ y or Intense Muon Source (10 10 /s/cm 2 ) Creation of new type of semiconductor （ Co-doping method Osaka Univ.)