Hydrogen in Wide Gap Semiconductor     

Why many types of wide gap semiconductor have n type conductivity ?

K.Shimomura (KEK-MSL)

Impurity in SemiconductorShallow donor acceptor Bohr radious

a=×(me/m*)×a0

~20×a0 Ionization Energy

(13.6eV) × (m*/me)/ 2

~50meV Hyperfine constant

A ～ 10-4A0

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 ZnOTheoretical 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

SR result on ZnO in KEK-MSL

SR result on ZnO in KEK-MSL1.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 10-4 times smaller than the muonium in va cuuum. The

se 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 PRL92,135505 (2003)

Theoretical studies in GaN 、 ZnO

Results in GaNTRIUMF M15 12H ~600Mev.

Results in GaN

Results in GaN

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 (TiO2)

Widely used as photo catalizerStrong n type conductivityWide Ban Gap 3.3eV

Results in TiO2

Results in TiO2

Results in TiO2

Results in TiO2

Angular Dependence

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

0 10 20 30 40 50 60 70 80 90

Angle (100) vs. External Field

Freq

uenc

y Sp

littin

g

satelite1

satelite2

Results in TiO2

A ～ 0.2 to 1.2MHzIonization Energy ～ ３ meVShallow Muonium !

SummarySR is powerful tool for the stiudies of origin of n type conductivity in GaN, ZnO and TiO2, 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 TiO2 etc)Ｎｅｕｔｒｉｎｏ　Ｆａｃｔｏｒ y or Intense Muon Source (1010/s/cm2)　 Creation of new type of semiconductor　（ Co-doping method H.Yoshida @ Osaka Univ.)