Optical Properties of Ga 1-x Mn x As 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

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

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.

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

Problem with the spin-injection Conductance mismatch

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.

Origin of ferromagnetism Direct exchange interaction Super-exchange interaction Indirect exchange

Nature Choice for magnetic semiconductor II 1-x -Mn x -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.

MBE phase diagram of Ga 1-x Mn x As

Some basic knowledge about the material properties of Ga 1-x Mn x As 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

Basic Properties of Ferromagnetic Semiconductors Magnetic property Magneto-transport property (Anomalous Hall Effect) R H =(R 0 /d) B+(R M /d) M

Carrier induced ferromagnetism? Dependence of T C on xMetal-insulator transitions

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

GaMnAs:(Hirakawa et al. PRB 65, , (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?

GaMnAs (Singley et al. PRL 89, (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 0.7m e < m*< 15m e. for the x=.052 sample. It is suggested that the holes reside in the impurity band

GAMnAs (Yang et al., PRB 67, (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.

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?

Samples: SampleMn concentrationStructure (Bottom>Top) Growth timeThickness 10529AX=1.4%GaAs LT-GaAs LT-GaMnAs 8 min 24 min 100 nm 300 nm 01016AX=2.4%GaAs LT-GaAs LT-GaMnAs 8 min 24 min 100 nm 300 nm 30422AX=3.3% GaAs LT-GaAs LT-GaMnAs 30min 11sec 800 sec 400 nm 2nm 200 nm 11127AX=4.8% GaAs LT-GaAs LT-GaMnAs 30min 11sec 900 sec 400 nm 2 nm 210 nm 21028GX=6.2% GaAs LT-GaAs LT-GaMnAs 20min 12sec 500 sec 300 nm 2.6 nm 120nm

T-dependent magnetization

T-dependent R XX

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)

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

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

Plasma frquency ω p =(4πn e 2 / m*) ½ n=m* ω p 2 / 4πn e 2 Take ћ ω p = 2000cm -1, m*= 0.5 m e, we get n=5* cm -3

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.

Absorption spectra in mid and near IR

Refletance spectra in FIR

Reflectance spectra in mid-IR and near IR

Real part of conductivity in the mid and near IR

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

Contactless electro-reflectance (CER) results

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

E F = ћ 2 k F 2 /2m* k F =(3π 2 n) 1/3 n= (2m* E F / ћ 2 ) 3/2 /3π 2 Take m*=0.5m e, E F =30 meV, We find n=0.3×10 20 cm -3

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 E F 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.