Ultrashort-pulse laser deposition of thin films

1. Ultrashort-pulse laser deposition of thin films Y.-M. Chena,c(陳彥穆), Y.-C. Tsoua,c(鄒昀晉), C.-Y. Yeha,c(葉啟宇), C.-J. Chena,b,c(陳俊嘉), C.-S. Wua,b,c(吳強生), M.-J. Jianga,c,d(江銘哲), H.-H. Chuc(朱旭新), J.-Y. Lind(林俊元), J. Wanga,b,c(汪治平), and S.-Y. Chena,c(陳賜原) a. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan. b. Department of Physics, National Taiwan University, Taipei 106, Taiwan. c. Department of Physics, National Central University, Jhongli 320, Taiwan d. Department of Physics, National Chung Cheng University, Chia-Yi 621, Taiwan.

2. Outline The features of ultrashort-pulse laser deposition (UPLD) Experimental setup of UPLD Gd22Fe74.6Co3.4 amorphous thin film FeBO3 single-crystalline and polycrystalline films

3. The features of ultrashort pulse laser deposition (UPLD) PLD is of great success in depositing chemically complex materials due to its ability to nonthermally and congruently transfer material from a multicomponent target to a substrate. PLD is advantageous for the cases in which a lower substrate temperature is preferred, because the mobility of the atoms/ions can be provided by the momentum of the ions in the impinging plasma plume. The problem of ejection of macroscopic particles from the target in PLD can be greatly alleviated by using ultrashort (< 1 ps) laser pulse. The remaining particles can be decomposed by using another laser beam. The intensity of ultrashort pulse laser is high enough to ionize the target through optical-field ionization, so UPLD is adequate for all kinds of targets whether the target is transparent at the laser wavelength or not.

4. Experimental setup ablation pulse – 2 post-machining pulse wavelength: 532 nm energy: 1.4 J pulse duration: 10 ns (FWHM) focal spot: 2 mm (FWHM) off-axis parabolic (OAP) mirror post machining pulse gas-jet nozzle beam splitter substrate holder with heater target carousel ablation pulse – 1 wavelength: 810 nm energy: 230 mJ pulse duration: 40 fs (FWHM) focal spot: 5~2000 μm (FWHM) shielding box ablation pulse – 1 relayed imaging system

5. Eliminating particulates by using a post-machining pulse ablation pulse fluence: 3 J/cm2 post-machining pulse fluence: 3 J/cm2 substrate material: glass Surface images (SEM) 0 μs 100 μs 200 μs 300 μs 500 μs various time delays between ablation and post-machining pulses Particulates formed due to ejection of macroscopic particles or condensation in the plasma plume degrade the quality of the thin film. The SEM images show that the surface quality of the film can be improved significantly by using a post-machining pulse with appropriate time delay to decompose the particles in the plume or heat up the cooler part of the plume.

6. Ultrafast magnetic switching Ref: C. D. Stanciu et al., Phys. Rev. Lett. 99, 047601 (2007) Staciu et al. experimentally demonstrate that the magnetization of Gd22Fe74.6Co3.4 can be reversed in a reproducible manner by a single 40-fs circularly-polarized laser pulse, without any applied magnetic field. The direction of this opto-magnetic switching is determined only by the helicity of light. This finding reveals an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.

7. Resolving the transient dynamics of ultrafast magnetic switching photo diode – a (Ia) pump pulse (circular polarization) photo diode – b (Ib) probe pulse (linear polarization) target polarizer pump-probe scheme parameters pump pulse fluence: 5 mJ/cm2 pump pulse duration: 500 fs pump pulse wavelength: 800 nm probe pulse wavelength: 650 nm probe pulse duration: 40 fs

8. Ultrafast magnetic switching of Gd22Fe74.6Co3.4 Ref: J. Hohlfeld et al., Appl. Phys. Lett. 94, 152504 (2009) PLD parameters ablation beam fluence: 5 J/cm2 substrate material: glass target material: Gd22Fe74.6Co3.4 film thickness: 27 nm capping layer: SiO2 Magnetic thin films of Gd22Fe74.6Co3.4 are successfully produced by using UPLD. For ultrafast magnetic switching the transition time is measured to be 780 fs, which is as good as that reported by Hohlfeld et al., but with a fidelity (reliability) of 100% which is better than that reported by Hohlfeld et al. (75%).

9. 57FeBO3 single crystal for the experiment of storage of nuclear excitation energy through magnetic switching Ref: Yu. V. Shvyd’ko et al., Phys. Rev. Lett. 77, 3232 (1996) scheme of switching the hyperfine field directions in FeBO3 single crystal time spectra of the nuclear forward scattering in FeBO3 (a) unperturbed (b) perturbed t’ = 16 ns t’’ = 308 ns (111) surface (c) perturbed t’ = 8 ns t’’ = 81 ns e0: incident radiation (14.4 keV) Hc = 20 G: external field for initial setting Hp = 58 G: switching field perpendicular to Hc t’: timing of turning on Hp t’’: timing of turning off Hp (d) perturbed t’ = 8 ns t’’ = 188 ns (e) perturbed Suppression and restoration originate from drastic changes of the nuclear states and of the interference within the nuclear transitions. t’ = 8 ns t’’ = 390 ns

10. Polycrystalline thin film of FeBO3on glass PLD parameters ablation beam fluence: 10 J/cm2 substrate material: glass target material: FeBO3 film thickness: 400 nm substrate temperature: 25 ℃ The XRD of this film shows that the grown film is polycrystal of FeBO3.

11. Single-crystalline thin film of FeBO3on SiO2 (111) substrate Simulated FeBO3 XRD spectrum PLD parameters ablation beam fluence: 10 J/cm2 substrate material: SiO2 (111) target material: FeBO3 film thickness: 400 nm substrate temperature:550℃ Condition for constructive diffraction (n = integer) -h + k + l = 3n -1 + 1 +1 = 1 destructive The XRD of this FeBO3 film shows no diffraction peak except for the strongest diffraction peak of the SiO2 substrate, which is attenuated by the FeBO3 film. The disappearance of all the polycrystalline diffraction peaks support the transition into a single-crystalline film, and the nonexistence of the (111) peak is expected from the selection rule for hexagonal system.

12. Single-crystalline thin film of FeBO3onCaCO3 (104) substrate PLD parameters ablation beam fluence: 10 J/cm2 substrate material: CaCO3 (104) target material: FeBO3 film thickness: 400 nm substrate temperature: 550℃ effect of lattice mismatch Compared to the XRD of the substrate the XRD of this sample shows an additional peak at about 33 degrees, verifying that the grown film is FeBO3 single crystal with the normal in the (104) direction. The asymmetric broadening of the (104) XRD peak towards larger angle can be ascribed to the effect of lattice mismatch between the film and the substrate.

13. Summary Eliminating particulates in the deposition process is attained by using a post-machining pulse with appropriate time delay. Amorphous magnetic thin films of Gd22Fe74.6Co3.4 are successfully produced by using ultrashort-pulse laser deposition. Using these films ultrafast magnetic switching with a transition time of 780 fs and 100% fidelity is achieved. Polycrystalline FeBO3 film and single-crystalline FeBO3 films with (104) and (111) orientations are successfully produced by using pulsed laser deposition for the first time.

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