Second-Harmonic Generation Studies of Silicon Interfaces

Transcription

1 Second-Harmonic Generation Studies of Silicon Interfaces Z. Marka 1, Y. D. Glinka 1, Y. Shirokaya 1, M. Barry 1, S. N. Rashkeev 1, W. Wang 1, R. D. Schrimpf 2,D. M. Fleetwood 2 and N. H. Tolk 1 1 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN Department of Electrical Engineering and Computer Science Vanderbilt University, Nashville, TN Supported by AFOSR-MURI and ONR

2 Outline Second Harmonic Generation (SHG) at Interfaces X-ray damage characterization with SHG SHG studies of Si/SiO 2 and Si/ZrSiO x interface Future Initiatives

3 Theory: Electric-Field-Induced-SH (EFISH) General Equation for SHG: P (2w) = c ( 2) d,b E( w)e( w) + c ( 2 ) d,s E( w)e( w) + c ( 2 ) q,b E( w) E( w) + c ( 3) d,b e( w ~ ) E( w)e( w) ( 3) cd,b e( w ~ ) E( w)e( w) : Electric - Field - Induced SH e( w ~ ) : Space Charge Field at the Interface E(w): Electric Field of Fundamental Beam

4 Si : System With Inversion Symmetry Under Electric Dipole Approximation P( 2w) = c ( 2) E( w)e( w) Inversion - P( 2w) = c ( 2) (-E( w))( -E( w)) c ( 2 ) = in Bulk But not at surfaces or with static electric field where symmetry is broken

5 Electron-Hole Dynamics at Si/SiO 2 Interface e - Electrons 8 9 ev 1.1 ev Si O ev E DC SHG Signal (a. u.) Laser Blocked SiO 2 Holes Time (Sec) 1. Electron-hole pair creation 2. Carrier injection (charge separation) E DC Field 3. Second-harmonic generation Electrons trapped at surface

6 Capabilities: Ultrafast High-Power Tunable Lasers IR and UV OPAs (Optical Parameteric Amplifier) Range:.6 6 ev Pulse Energy: 1 µj Rep. Rate: 1 KHz Pulse Width: 15 fs or 3.5 ps Free Electron Laser Range:.1 1 ev Pulse Energy: ~ 5 mj Rep. Rate: 3 Hz Pulse Width: 2-6 µs (macro), 1ps (micro) Ti: Sapphire Laser Range: ev Pulse Energy: ~ 1 nj Rep. Rate: 76 MHz Pulse Width: 15fs

7 X-ray Irradiation Effects Motivation Characterization of traps due to radiation damage: traditional electronic methods (e. g. C-V) less reliable for thin oxides optical methods: second-harmonic generation (SHG) noninvasive, contactless alternative very sensitive for thin oxides

8 Second Harmonic Generation What is Second Harmonic Generation? Lack of Inversion Symmetry e.g. Electric Field at Interface I 2w (t) c (2) + c (3) E DC (t) 2 8 nm w 2w E DC Contactless technique. Time-dependent SHG can give information on Trap Densities and Lifetimes multiphoton electron-hole injection and dynamics.

9 SHG Experimental Setup PMT Ti:Sapphire Laser l = 7-92 nm, t p = 1 fs, P av = 4 mw RG-Filter Polarizers Lenses Prism w UG-Filter Iris 3eV Sample Rotation Table I (2w) (t) c (2) + c (3) 2 E(t) I (w)2

10 Pronounced Radiation Effect 7 block laser light 6 SHG signal (a. u.) unblock laser light after X-ray irradiation (7.6 Mrad) SiO 2 Si no contacts H 2 annealed 6.5 nm 1 before X-ray irradiation Time (sec)

11 Time scales Two time scales t r : e-h recombination time D 1 /D 2 vs. blocking time (fast) t d : decay time of radiation effect SHG signal D 2 D 1 D 1 /D 2 vs. time after irradiation (slow) d blocking time Time SiO 2 Si O 2 Electrons Radiationinduced traps Holes

12 Charge Recombination Time (t r ) SHG signal (a. u.) / t r =1.3 min Blocking time (sec) Time (sec) for irradiated sample t r = 1.3 min for unirradiated sample t r = 82 min => faster electron transport across the irradiated oxide

13 Decay Time (t d ) of Radiation Effect SHG signal (a. u.) minutes after irradiation 3 hours 1 minutes after irradiation 8 hours 5 minutes after irradiation Time (sec) Decrease in radiation-induced effect with time: annealing / t d =13 min Time after irradiation (min) For samples without H 2 anneal treatment this timescale is in the order of days.

14 Studies on 33 nm Thick Oxides 45 3.E-9 4 SHG signal (a. u.) unirradiated sample Capacitance (F) 2.E-9 1.E-9.E+ unirradiated sample 7.6 Mrad irradiation Voltage (V) Mrad irradiation Time (sec) Both SHG and C-V indicates the presence of positive trap charge in the oxide due to X-ray irradiation.

15 Conclusion second-harmonic generation can be used for characterization of radiation damage in Si/SiO 2 very promising method for ultrathin oxides for 6.5 nm oxide - t r = 1.3 min (electron transport across the oxide) - t d = 13 min (annealing of traps) - H 2 annealing makes a difference - E d center??

16 Carrier Dynamics at the Si/(ZrO 2 ) x (SiO 2 ) 1-x Interface Motivation Moore s Law Alternatives for gate oxides Comparison with Si/SiO 2 Interface Multiphoton Electron and Hole Injection

17 7 6 5 (a) Si/(ZrO 2 ) x (SiO 2 ) 1-x Photon energy 1.56 ev 1 1 Slope ~ 2. SHG vs a function of time for Si/(ZrO 2 ) x (SiO 2 ) 1-x 4 1 Laser power (mw) SHG intensity (arb. units) Slope ~ mw (b) Si/SiO 2 Photon energy 1.56 ev Increasing laser power Electron and hole injection SHG vs a function of time for Si/SiO Laser power (mw) 56 mw Increasing laser power Only electron injection Time (sec)

18 SHG intensity (arb. units) Constant (a) power 28 mw 1.45 ev 28 mw 1.47 ev 28 mw 1.51 ev 28 mw 1.53 ev 28 mw 1.56 ev 28 mw 1.61 ev Time (sec) Constant (b) hω 2 mw 1.56 ev 25 mw 1.56 ev 3 mw 1.56 ev 35 mw 1.56 ev 4 mw 1.56 ev 425 mw 1.56 ev SHG vs Time for Si/(ZrO 2 ) x (SiO 2 ) 1-x Dependence on photon energy at constant power - no threshold found Dependence on intensity at constant photon energy - threshold-like behavior - at low intensities electron injection dominates (2-photon process), at higher intensities hole injection (3-photon process) becomes significant

19 1 as a function of hω 7 Si/(ZrO 2 ) x (SiO 2 ) 1-x Laser power 28 mw SHG intensity (arb. units) Possible explanation: resonance due to interface states Two-photon energy (ev)

20 Schematic Representation of SHG Signal as a function of Time Conduction Band Offset Direct Bandgap 3.1 ev 2.5 ev SHG Hole injection Electron injection Indirect Bandgap 1.1 ev Si 4.3 ev Initial electric field compensation Valence Band Offset Time (ZrO 2 ) x (SiO 2 ) 1-x

21 Current and Future Initiatives Time-dependent pump (injection) probe (SHG) measurements Band offset determination

22 A New Approach to Time-Dependent Measurements Pump beam: carrier injection Probe beam Block and unblock Detect SHG from probe Below laser intensity for injection Advantages: Separation of injection from second-harmonic generation Can follow directly carrier movement while injection beam is blocked

23 Two thresholds in the SHG signal intensity are observed at time= χ 3 Isotropic (a.u.) threshold for SHG threshold for injection Laser Power (mw)

24 Preliminary results SHG intensity (a.u.) probe pump + probe 17 Ansgstrom SiO 2 on Si Pump: 8 nm (~35 mw) Probe: 8 nm (~18 mw) Time (s) probe

25 Proposed Two Color Experiment Si oxide E? Tunable UV-OPA: Provides charge separation By tuning pump laser wavelength we can observ thresholds for electron/hole injection Band-offset determination 1.1 ev? The created DC field is monitored separately by IR-OPA at energy below the band-gap or with Ti:Sapphire below injection threshold intensity.