Studies of the Spin Dynamics of Charge Carriers in Semiconductors and their Interfaces S. K. Singh, T. V. Shahbazyan, I. E. Perakis and N. H. Tolk Department of Physics and Astronomy Vanderbilt University, Nashville, TN 37235 Buffalo Consortium Meeting 11/4/00 Funded in part by DARPA/ONR
Objectives Measure Important Parameters in Magnetic Semiconductors: Band-Offset : Using Internal Photo-Emission (IPE) and SHG Techniques. Ultrafast Spin Dynamics: Time-Resolved Photoluminescense and Reflectivity, Faraday Rotation. Probing Electron-Hole Dynamics: Time-Dependent Second Harmonic Generation (SHG). Develop Theoretical Models of Ultrafast Spin Dynamics.
Vanderbilt Capabilities Series of Ultrafast, High-Power, Tunable Lasers: FIR and UV OPAs (Optical Parameteric Amplifier) Range: 0.06 6 ev Pulse Energy: 10 µj Rep. Rate: 1 KHz Pulse Width: 150 fs or 3.5 ps Free Electron Laser Range: 0.1 1 ev Pulse Energy: ~ 50 mj Rep. Rate: 30 Hz Pulse Width: 4-6 µs Several Ti:Sapphire and Other Lasers 9 T Superconducting Magnet Having Four Optical Windows. Fast Electronics, Optics, etc.
Band-Offset Measurements in Si/Si 0.45 Ge 0.5 Photo Signal (mv) 300 250 200 150 100 50 0 0.95 ev 1.2 1.25 1.3 1.35 1.4 1.45 Wavelength (microns) 1.12 ev Si V Photons Si 0.45 Ge 0.55 (100 nm) hν Si Si 0.45 Ge 0.55 IPE is a sensitive technique to measure band offset (within 10 mev). Can be used to measure band offsets in Magnetic Semiconductors (e.g. InMnAs/GaMnSb)
Proposed Band-Offset Measurements in III 1-X Mn X V /III-V based Semiconductor Vanderbilt University CB 0.87 ev Fermi Level 0.8 ev 0.15 ev Holes Electrons 0.4 ev VB 0.47 ev In 1-X Mn X As GaSb Band offsets can be measured as a function of Magnetic field.
Proposed Lifetime Measurements of Spin-Polarized Carriers in Magnetic Semiconductors (e.g. In 1-X Mn X As/GaSb) CB GaSb VB σ + ~ 0.5 ev GaSb σ 0.8 ev K Create Photo-Excited Spin- Polarized Carriers in InAs Quantum Well. K Probe Spin-Scattering Mechanisms: Time-Resolved Photoluminescense in Magnetic Field. K Faraday Rotation In 1-X Mn X As K Study Role of Mn ion in Spin Scattering
Theory: Electric-Field-Induced-SH (EFISH) General Equation for SHG: P (2ω) = χ ( 2 ) d,b E( ω)e( ω) + χ ( 2 ) d,s E( ω)e( ω) + χ ( 2) q,b E( ω) E( ω) + χ ( 3) d,b ε( ω ~ 0) E( ω)e( ω) ( 3) χ ε ( ω ~ 0) d,b E( ω)e( ω) : Electric Field Induced SH ε ( ω ~ 0) : Space Charge Field at the Interface E(ω): Electric Field of Fundamental Beam
System With Inversion Symmetry Under Electric Dipole Approximation P( 2ω) = χ ( 2) E( ω)e( ω) Inversion P( 2ω) = χ ( 2) ( E( ω))( E( ω)) χ ( 2 ) = 0 in Bulk But not at surfaces or with static electric field where symmetry is broken
Experimental Setup Ti:Sapphire Laser λ =700-920nm, τ p =100fs, P av =400mW RG-Filter Polarizers Lenses Prism Sample Rotation Table PMT I (2 ω) (t) χ (2) +χ (3) E(t) 2 I ( ω) 2 ω UG-Filter Iris 2ω 3eV
Electron-Hole Dynamics at Si/SiO 2 Interface 9 ev SiO 2 1.1 ev Si e - O 2 1.5 ev Electrons E DC Holes SHG Signal (Counts/100 msec) 8000 6000 4000 2000 0 Laser Blocked 0 100 200 300 400 500 600 Time (Sec) E DC Field Charge Separation ( Carrier Injection) Electrons trapped at Surface New Charged Traps in Oxide due to Radiation
Comparison of SiO 2 and ZrSiO x Oxides SHG from Si-Dielectrics 1200 1000 SHG Intensity (a.u.) 800 600 400 Si (100)-SiO 2 Qualitative difference in the shape of the 200 215 mw curves in SiO 2 and 0 0 200 400 600 800 ZrSiO x : SHG Intensity (a.u.) 1400 1200 1000 800 600 400 Si (100)-ZrSiO x Difference in electron and hole injection rate? 200 137 mw 0 0 200 400 6 00 8 00 Time(second)
SHG measurements to probe electron-hole Dynamics in Magnetic Semiconductor Electrons hν hν In 0.94 Mn 0.06 As GaSb GaAs GaAs (100) 12 nm 500 nm 300 nm CB hν VB In 1-x Mn x As GaSb E F Holes Koshihara et al., PRL 78, 4617 (1997) K SHG measurements can probe dynamic electric field due to electron-hole migration, induced by photons. Understand photo-induced Ferromagnetism.
Summary Band-Offset : Internal Photo-Emission (IPE) and SHG Techniques. Lifetime of Photo-Excited Spin-Polarized Carriers: Faraday Rotation, Time-Resolved Photoluminescense and Reflectivity. Probing Electron-Hole Dynamics: Time-Dependent Second Harmonic Generation (SHG). Develop Theoretical Models of Ultrafast Spin Dynamics.
Samples InAs/GaMnSb, InMnAs/GaMnSb : BO InAs/GaSb : BO, SHG InMnAs/GaSb : SHG GaSb/InMnAs/GaSb QW : LT BO: Band Offset LT: Life Time SHG: Second Harmonic Generation