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.
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
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.
Experimental setup
ablation pulse – 2
off-axis parabolic (OAP)
mirror
post machining pulse
gas-jet nozzle
substrate
holder
with heater
shielding
box
target
carousel
post-machining pulse
wavelength: 532 nm
energy: 1.4 J
pulse duration: 10 ns (FWHM)
focal spot: 2 mm (FWHM)
beam splitter
ablation pulse – 1
wavelength: 810 nm
energy: 230 mJ
pulse duration: 40 fs (FWHM)
focal spot: 5~2000 μm (FWHM)
ablation pulse – 1
relayed imaging system
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.
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.
Resolving the transient dynamics of ultrafast magnetic switching
photo diode – a
(Ia)
pump pulse
(circular polarization)
photo diode – b
(Ib)
polarizer
target
probe pulse
(linear polarization)
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
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/cm 2
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%).
57FeBO
3
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
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
Suppression and restoration originate from
drastic changes of the nuclear states and of the
interference within the nuclear transitions.
(c) perturbed
t’ = 8 ns
t’’ = 81 ns
(d) perturbed
t’ = 8 ns
t’’ = 188 ns
(e) perturbed
t’ = 8 ns
t’’ = 390 ns
Polycrystalline thin film of FeBO3 on glass
PLD parameters
ablation beam fluence: 10 J/cm 2
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.
Single-crystalline thin film of FeBO3 on 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.
Single-crystalline thin film of FeBO3 on CaCO3 (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.
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.
Thanks for your attention!