Optical Properties of Ga 1-x Mn x As

1. Optical Properties of Ga1-xMnxAs C. C. Chang, T. S. Lee, and Y. H. Chang Department of Physics, National Taiwan University Y. T. Liu and Y. S. Huang Department of Electronics, National Taiwan University of Science and Technology J. Furdyna Department of Physics, University of Notre Dame

2. Outlines: • Review on semiconductor spintroincs • Basic properties of III-Mn-V • Optical properties of GaMnAs • Experimental results and discussions • Summary

3. Review on semiconductor spintroincs • Classical device: use the electrical and particle properties of the electron. • Quantum device: use the wave properties of electron. • Spin-properties- non volatile memory, integration of memory and logic devices, spin-FET, quantum computing, etc.

4. Requirement for spintronic devices • 1. spin-injection • 2. spin-manipulation • 3.spin-detection • An example: spin-FET

5. Problem with the spin-injection • Conductance mismatch

6. Basic knowledge about magnetism • Paramagnetism: • Atoms have magnetic moments but the coupling between the magnetic moments is small and the magnetic moment of the atoms are randomly oriented.

7. Origin of ferromagnetism • Direct exchange interaction • Super-exchange interaction • Indirect exchange

8. Nature Choice for magnetic semiconductor II1-x-Mnx-VI (Diluted magnetic semiconductor) • 1. Anti-ferromagnetic at high x, paramagnetic at low x. • 2. large spin g-factor • 3. Spin polarized LED, Spin superlattice, etc.

9. MBE phase diagram of Ga1-xMnxAs

10. Some basic knowledge about the material properties of Ga1-xMnxAs • The samples were grown at low temperature with MBE, the quality of the material is usually very poor • Mn is an acceptor and in principle could donate a hole for electrical conduction. • Mn in GaMnAs is a substitutional acceptor? (yes, Soo et al. APL 80, 2654 (2002) • Metal-insulator transition and Anderson localization are essential ingredient of the problem

11. Basic Properties of Ferromagnetic Semiconductors • Magneto-transport property (Anomalous Hall Effect) • RH=(R0/d) B+(RM/d) M • Magnetic property

12. Carrier induced ferromagnetism? • Dependence of TC on x Metal-insulator transitions

13. Summary of optical studies • InMnAs (Hirakawa et al.,, Physica E10, 215 (2001)) • The conductivity could be well fitted with Drude model, indicating the holes are delocalizd. • Localization length of hole estimated to be 3-4 nm, close to the average inter-Mn distance. • Add figure from their paper

14. GaMnAs:(Hirakawa et al. PRB 65, 193312, (2002)) • Non-Drude-like FIR response observed. • Broad conductivity peak near 200meV observed. • Estimated mean free path of 0.5nm implies that even for the metallic sample the hole wavefunction is localized. RKKY?

15. GaMnAs (Singley et al. PRL 89, 097203 (2002)) • A broad band centered at 200 meV is observed. • From the sum rule analysis it was found that the charge carrier has a very heavy effective mass 0.7me< m*< 15me. for the x=.052 sample. It is suggested that the holes reside in the impurity band

16. GAMnAs (Yang et al., PRB 67, 04505 (2003)) • A non-perturbative self-consistent study which treat both disorder and interaction on equal footing. • The broad peak centered at 220 meV is present even in a one band approximation. • Non-Drude behavior could be accounted for if multiple scattering is taken in to consideration • A new feature at around 7000 cm-1, originated from the transition from heavy hole to split off band is predicted.

17. Meatal-insulator transitions in doped semiconductor • Impurity level broaden into an impurity band. • Impurity band merge with the valence band. • Where are position of the Fermi level and the position of the mobility edge?

18. Samples:

19. T-dependent magnetization

20. T-dependent RXX

21. Source Beam splitter Detector • NIR (13000~4000 cm-1) Tungsten Si/Ca InSb (LN2) • MIR(4000~400 cm-1) Globar KBr MCT (LN2) • FIR(400~10 cm-1) Hg Lamp Mylar 6μm Bolometer (LHe)

22. FIR transmission data • Flat response in the low energy region • Zero transmission for x=4.8% sample

23. Transmission data –IR and near IR Absorption dips for low x samp[learound 2000 cm -1. Peculiar behavior of x=4.8% sample: below opaque below about 1500cm-1 but become transparent above 1500 cm-1

24. Plasma frquency • ωp=(4πn e2/ m*) ½ • n=m* ωp2 / 4πn e2 • Take ћ ωp= 2000cm-1, m*= 0.5 me, we get • n=5* 10 19 cm-3

25. AB =- log TR • Three peaks could be identified: 1648 cm-1 for X=.14% sample, 1712 cm-1 for 2.4% sample and 1872 cm-1 for x=3.35% sample.

26. Absorption spectra in mid and near IR

27. Refletance spectra in FIR

28. Reflectance spectra in mid-IR and near IR

29. Real part of conductivity in the mid and near IR

32. Band filling effect for InP heavily doped with Se • P.M. Raccah et al., APL 39, 496 (1981) • High doping concentration > 10 20 cm-1 • Optical gap increases from 1.34 to 1.9 eV

33. Contactless electro-reflectance (CER) results

36. Above bandgap feature in the CER spectra: band filling effect?

37. EF= ћ 2kF2/2m* • kF=(3π2n)1/3 • n= (2m* EF/ ћ 2)3/2/3π2 • Take m*=0.5me , EF=30 meV, • We find n=0.3×10 20 cm-3

38. Summary and Conclusions: • The FIR response of the sample appears to be flat • Non-Drude-like optical conductivity behavior is observed in the mid-IR spectra • Clear Absorption peaks observed for sample with x<=3.3% • Metallic behavior obseved for samples with x>=4.8% • From the the transmission data, plasma frequency and carrier concentration could be obtained. • Indication of band filling effect observed for some samples with EF about 30 meV high than the valence band edge. • The carrier concentration obtained from plasma frequency are consistent with the carrier concentration obtained from the band filling effect.