Magnetic semiconductors

Transcription

1 Magnetic semiconductors Classical magnetic semiconductors Diluted magnetic semiconductors Ferromagnetic semiconductors New materials, nanostructures, interfaces

2 Magnetic semiconductors Classical magnetic semiconductors

3 History of magnetic semiconductors 1st generation 1960 s- Gd 2 S 3, EuO, CdCr 2 Se 4,. ferromagnetic, semiconducting 2nd generation 1980 s Cd 1-x Mn x Te, (Mn in II-VI) antiferromagnetic, insulating applied for magneto-optical devices 3rd generation 1990 s- Ga 1-x Mn x As, (Mn in III-V), oxide DMS ferromagnetic, metallic spin-controlled semiconductor devices

4 MCD of spinel-type CdCr 2 X 4 and CuCr 2 X 4 metallic Cr 3.5+ metallic Cr 3.5+ sum rule <L Z > = 0 semicond Cr 3+ No change in XAS, MCD holes enter p orbitals A. Kimura et al., PRB 01

5 Magnetic circular dichroism (MCD) in core-level absorption

6 Magnetic semiconductors Diluted magnetic semiconductors

7 Diluted magnetic semiconductors A 1-x Mn x B II-VI, III-V semiconductor host + substitutional Mn ions Mn

8 Resonant photoemission from Cd 1-x Mn x Te Photoemission BIS On resonance. Band theory Off resonance. Difference = Mn 3d DOS Cluster calc. satellite main = 2 ev U = 4 ev Exp: Ley, Taniguchi, Fujimori,... PRB 87 Calc: Mizokawa, Fujimori, PRB 96 cf. Gunnarsson and Jepsen, PRB 88 near E F dominated by p character Nβ ~ -1 ev ( - ) Strong mageneto-optical effects

9 Resonant photoemission discrete level continuous level

10 Cluster model for transition-metal impurity in semiconductor d electron Nβ p hole U,, Tpd : adjustable parameters p-d exchange constant Nβ ~ - T pd2 /(U- )

11 Configuration-interaction cluster-model analysis of d-electron photoemission spectra Ground state Final states Photoemission satellite Ground state main Intensities

12 Configuration-interaction cluster(anderson)- model picture of TM impurity in semiconductor N N-1 N+1 T. Mizokawa and A.F., PRB 93

13 Intra-atomic d-d transitions for TM impurity in ZnS T. Mizokawa and A.F., PRB 93

14 Configuration-interaction cluster(anderson)- model picture of TM impurity in semiconductor N N-1 N+1 T. Mizokawa and A.F., PRB 93

15 Effects of ligand atom for Mn impurity in II-VI semiconductors d-d optical transitions CI calc. Expt. T. Mizokawa and A.F., PRB 93

16 Configuration-interaction cluster(anderson)- model picture of TM impurity in semiconductor N N-1 N+1 T. Mizokawa and A.F., PRB 93

17 Donor and acceptor levels for Mn impurity in II-VI semiconductor T. Mizokawa and A.F., PRB 93

18 Donor and acceptor levels for TM impurity in II-VI semiconductors T. Mizokawa and A.F., PRB 93

19 p-d exchange constant Nβ for TM impurity in II-VI semiconductors - T. Mizokawa and A.F., PRB 97

20 Magnetic semiconductors Ferromagnetic semiconductors

21 Ferromagnetism in MBE-grown Ga 1-x Mn x As Growth phase diagram Curie temperature T. Hayashi, M. Tanaka, J. Cryst. Growth, 97 H. Ohno et al., JMMM, 99

22 Transport and magneto-transport properties of Ga 1-x Mn x As Mn concentration dependence of electrical resistivity Magneto-resistance T C ~ magnetization F. Matsukura et al. PRB 98

23 Photon Factory surface interface beamline BL-18A CDM + ADES 400, CLAM 3

24 Comparison with band-structure calculation and cluster-model calculation Mn 3+ Mn 2+ + hole hole main satellite main = 1.5 ev U = 3.5 ev satellite p-d exchange Nβ = -1 ev

25 Mechanism for carrier-induced ferromagnetism Double exchange no (holes have p character) RKKY interaction no (Fermi energy << Nβ ) p-d exchange most likely p hole Nβ -1 ev

26 ARPES form 3D material

27 Band structures of GaAs and Ga 1-x Mn x As GaAs GaAs Ga Mn Mn As As 0 0 Binding Energy (ev) Binding Energy (ev) X Γ X Γ Okabayashi et al. PRB 2001

28 Impurity band near E F in Ga 1-x Mn x As Ga Mn As - GaAs difference GaMnAs GaAs impurity band Okabayashi et al. PRB 2001

29 Difference between Ga 1-x Mn x As and In 1-x Mn x As InMnAs: Tc max ~ 50 K GaMnAs: Tc max ~ 100 K Drude peak Optical absorption Lattice constants InMnAs no Drude peak GaMnAs H. Ohno et al., APL 96 K. Hirakawa et al., Physica E, 2001 K. Hirakawa et al., PRB, submitted

30 Resonant photoemission from Ga 1-x Mn x As and In 1-x Mn x As On resonance Off resonance Mn 3d DOS J. Okabayashi et al. PRB 02

31 Cluster model analyses for Ga 1-x Mn x As and In 1-x Mn x As InMnAs: =1.0 ev, (pdσ)=0.8ev--> Nβ= -0.7 ev GaMnAs: =1.5 ev, (pdσ)=1ev--> Nβ= -1.0 ev

32 impurity states in Ga 1-x Mn x As and In 1-x Mn x As Ga Mn As (In,Mn)As Γ Intensity (arb. units) GaMnAs GaAs 18 ev Intensity (arb. units) InMnAs InAs Binding Energy (ev) X Binding Energy (ev)

33 Difference between Ga 1-x Mn x As and In 1-x Mn x As Ga 1-x Mn x As In 1-x Mn x As strong p-d hybridization weak large Nβ small high Tc low yes split-off states/ impurity band no quasi-bound holes free holes non-drude Drude J. Okabayashi et al. PRB 02

34 Curie temperatures for Mn-doped p-type semiconductors if hole-doped! T. Dietl et al, Science (2000)

35 Magnetic semiconductors New materials, nanostructures, interfaces

36 n-type Zn 1-x Mn x O Magnetic susceptibility Optical absorption T. Fukumura et al. APL 99

37 T. Mizokawa et al. PRB 02 Mn 3d DOS of Zn 1-x Mn x O and cluster-model analysis Resonant photoemission Cluster model analysis = 6.5 ev U = 5.2 ev Nβ= -2.7eV On resonance O resonance Mn 3d DOS 3 ev 4 ev 2 ev 4 ev 1.5 ev 4 ev 1.5 ev 3.5 ev -1.3eV -1.0eV -1.0eV -0.9eV

38 Electronic structure of n-type and p-type Zn 1-x Mn x

39 Ferromagnetism in n-type Zn 1-x V x oxygen deficiency ferromagnetic H. Saeki et al., Solid State Commun. 2001

40 Photoemission spectra of n-type Zn 1-x V x Zn 3d Zn 1-x V x O n-zno O 2p Paramagnetic V 3d Ferromagnetic Y. Ishida et al., unpublished

41 Room-temperature ferromagnetism? in Ga Mn x N Ga 1-x Mn S. Sonoda et al., JJAP 01

42 Nano-scale MnAs dots on S-passivated GaAs hypothetical zinc-blende-type MnAs theoretically predicted ferromagnet (Shirai et al.) K.Ono et al., JAP 02

43 Photoemission spectra of MnAs dots zinc-blende type most likely zinc-blende type hexagonal NiAs type

44 Mn 2p x-ray absorption of MnAs dots Expt. at SRRC J. Okabayashi et al., unpublished

45 Room temperature ferromagnetism in CdGeP 2, ZnGeP based DMS CdGeP 2 :Mn ZnGeP 2 :Mn CdSnP 2 :Mn Mn deposition thermal treatment at 400 Chalcopyrite structure Zn Ge P G.A. Medvedkin et al., JJAP 00

46 Room temperature ferromagnetism in bulk Zn 1-x Mn x GeP 2x B.J. Kim et al., PRL 02

47 Valence-band photoemission form ZnGeP 2 : Mn ZnGeP2:Mn hv=70ev d(å)=250 Intensity (arb.units) Strong Mn Auger (like MnX compounds) Zn3d Binding energy (ev) Mn 3d resonance No Mn Auger (like II-VI, II-V DMS) Y. Ishida et al., unpublished

48 Core-level intensities for ZnGeP 2 : Mn 3.0 Mn deposition series Core level intensity (arb.units) Ge 3d Mn 2p P 2p sputtering series Zn 2p3/ Nominal Mn thickness (A) Y. Ishida et al., unpublished