Exploring Advantages of Pulsed Laser Deposition with the TJNAF Free Electron Laser

Transcription

1 Exploring Advantages of Pulsed Laser Deposition with the TJNAF Free Electron Laser Anne Reilly 1, Chris Allmond 1, Jason Gammon 1 Shannon Watson 1 and Jung Gi Kim 2 1 Department of Physics, College of William and Mary 2 Department of Physics, Hanyang University, South Korea Work supported by the Jeffress Memorial Trust and NSF grant DMR

2 Abstract The unique combination of parameters of the TJNAF-FEL: sub-picosecond pulses, high average power, high repetition rate and tunability allow us to explore new regimes for PLD. We demonstrate the growth of a high quality films with high deposition rates. JAP 93, 3098 (2003)

3 Why Pulsed Laser Ablation and Deposition with the FEL? Advantages of PLD: FLEXIBLE GREATER CONTROL OVER ENVIRONMENT COMPLEX ALLOYING (High Tc superconductors) CONTINUOUS, THIN FILMS WITH NEW PROPERTIES EPITAXY AT LOW TEMPERATURE laser Currently, PLD is limited due to lack of understanding of fundamental processes of laser-target and laser-plasma interactions. Also, laser sources have been limited.

4 TJNAF-FEL a combination of parameters unique to any laser: ULTRAFAST (650 fs pulse width) HIGH POWER (>2 kw) TUNABLE (currently IR) HIGH REPETITION RATE (18, 34, 74 MHz) CW or Pulsed Operation

5 Why PLD with the FEL? Many benefits! ULTRAFAST (picosecond pulses): lower ablation threshold, less plasma absorption, much less target damage, eliminates particulate problem. Gamaly et al., Physics of Plasmas, (2002) HIGH REPETITION RATE: high deposition rate, greater control over growth dynamics (multiple pulse effects). Gamaly et al., J. Appl. Phys., (1999) TUNABLE: Enhanced ablation/deposition with resonant absorption; use specific resonances to control growth processes. Demonstrated in calcite and polymers: Park and Haglund, Appl. Phys. A, (1997) D.M. Bubb et al., Appl. Phys. Lett., (2001)

6 FEL-PLD experimental setup FEL exit port FEL beam CaF 2 lens sapphire window Substrates or Faraday Target cup Turbopumped vacuum chamber on optical table 1 x 10-6 Torr FEL at 3.1 µm 2 target-substrate distance PVD Products window assembly Silicon substrates (no heating/cooling) Alignment mode, ablation/deposition remotely controlled. fiber CCD spectrometer

7 Optical Emission of FEL Plasma: Blackbody Radiation

8 FEL- PLD Plasma Plumes: Fe target, cw beam, ~ 0.1 J/cm 2 Nb target, cw beam, ~ 0.6 J/cm 2

9 Optical spectra shows significant blackbody emission: MHz 10 µj/pulse pulsed beam, 60 Hz 1 ms macropulse FEL harmonic Intensity (arb. units) MHz 20 µj/pulse 37.4 MHz 4 µj/pulse Uncorrected spectra Wavelength (nm)

10 Pulsed ablation, T ~ 2500 K (independent of rep. rate and pulse energy) CW ablation: T ~ 1700 K FEL harmonics Intensity (arb. units) MHz cw 18.7 MHz, 60 Hz pulsed (1 ms) Wavelength (nm)

11 Compared with amplified Ti:Saph (1 mj/pulse, 1 khz): 500 Ti emission lines (NIST database) 1000 Intensity (arb. units) Ti:Saph FEL FEL harmonics Intensity (arb. units) W avelength (nm) 0

12 Blackbody radiation with FEL ablation: Dense Plasma? G. Mehlman et al., J. Appl. Phys., (1993) Heating of nanoparticles? D. B. Geohegan et al., Appl. Phys. Lett., (1993)

13 Time-Resolved Optical, Electron and Ion emission: Contribution of micropulses

14 Time-resolved optical emission Building of emission over macropulse, with individual micropulse contributing: Oscilloscope reading (Intensity ) in Volts FEL TRIGGER time (microseconds) Oscilloscope reading (Intensity) in Volts Trigger on emission ns x time (ns) 250 µs macropulse 18 MHz Fresh target As groove is drilled FeMn target

15 Electron and Ion emission (FeMn) Electron and positive ion pulse - lasts over macropulse. Electron energy < 100 ev Oscilloscope output (arbitrary units) FEL trigger 50 µs macropulse, 37 MHz (+100 V bias) (-100 V bias) Time (Microseconds) Oscilloscope output (arbitrary units) Triggered on emission Time (ns) Vacuum: 6 x 10-6 Torr

16 Thin Film Quality (Magnetic Materials) Benefit of sub-picosecond pulses and high repetition rate: high quality films with high deposition rates Demonstrated in comparison to amplified Ti:Sapphire system (150 fs, 1 mj/pulse, 1 khz)

17 Amplified Ti:Sapphire versus FEL (NiFe) Exploring dependence on pulse power and repetition rate FEL 5 µj/37.4 MHz Amplified TS 0.7 mj/1 khz SEM AFM

18 Amplified Ti:Sapphire versus FEL (NiFe) X-ray diffraction reveals more crystalline order in FEL grown film Intensity silicon FeNi3 (111) Ti:Saph (200) Fe (200) (220) (311) FEL θ (degrees)

19 Amplified Ti:Sapphire versus FEL (NiFe) Large effect on magnetic properties Magnetization (emu) a)ti:saph b)fel Field (Gauss) Crystallized Fe? Crystalline orientation? Roughness?

20 Amplified Ti:Sapphire versus FEL (NiFe) Deposition Rates Amp. Ti:Saph 1 mj/pulse, 1 khz 1 Å/s 1x10-3 Å /pulse FEL 5 µj/pulse, 37.4 MHz 17 Å/s 5x10-7 Å /pulse Possibility of much higher rates with FEL: 200 Å/s for Nb M. Shinn, Proc. SPIE (2000)

21 Preliminary comparisons- Work needs to be done! What power and repetition rate are needed for desired film properties? Consider: Deposition rates Morphology and Structure Electronic and Magnetic Properties

22 Conclusions The TJNAF-FEL gives a unique combination of laser parameters Interesting opportunities to explore lasertarget and laser-plasma interactions FEL-PLD gives high quality films with very high deposition rates

23 Thank you Michelle Shinn and FEL staff Dennis Manos, Mool Gupta, Hani El-sayed Ali Amy Wilkerson Jim Greer and PVD Products Jim Horwitz (NRL), Jim Fitzgerald (UVA) and all those with whom I have had helpful discussions