CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
TRANSITION FROM EXPANSION TO SHOCK COMPRESSION
IN LASER IRRADIATED Si BY MULTIPLE SHOTS
Akio Yazaki, Hiroaki Kishimura, Yoichiro Hironaka, Fumikazu Saito,
Kazutaka G. Nakamura, and Ken-ichi Kondo
Materials and Structures Laboratory, Tokyo Institute of Technology,
Nagatsuta, Midori, Yokohama, 226-8503, Japan
Abstract Picosecond time-resolved X-ray diffraction measurements have been performed on laser
irradiated Si single crystal. Two types of experiments were performed in air. In one set of
experiments, single shot irradiation on the silicon (111) crystal was performed at an irradiance range
of 1 - 10 GW/cm2. The results of X-ray diffraction measurement showed both thermal expansion
and transient elastic shock compression. It is deduced that the ablation threshold falls in the range in
irradiance of 1 - 10 GW/cm2. The second set of experiments was performed with multiple shot
irradiation in the range in irradiance of GW/cm2. Lattice compression due to the multiple laser
irradiation was observed. This result indicates that multiple irradiation causes reduction of the
ablation threshold.
INTRODUCTION
irradiation using picosecond time-resolved X-ray
diffraction. Various numbers of multiple laser
irradiations and irradiation powers were examined.
The mechanism of shock generation was also
discussed.
In recent years, advances in the development of
picosecond sources of hard X-rays have opened the
possibility of the direct observation of the changes
in material structure in the nanosecond to
picosecond time scale.
Time resolved X-ray
diffraction studies have been reported in recent
years l"3. Laser heating was studied and lattice
expansion was reported by single laser irradiation
on a Si (111) at a few GW/cm 2 . 4 " 6 On the other
hand, Hironaka et al 1 studied multiple laser shots
on Si(lll) at the same power density (4 GW/cm2)
and observed lattce compression up to -1.05%. In
the present paper, we investigated transformation
from expansion to compression of Si(lll) by laser
EXPERIMENTAL
Time resolved X-ray diffraction with ultra-short
X-ray pulses (1.93 A, 10 ps, 10 Hz) is used to probe
structural dynamics in Si pumped by laser pulses
(780 nm, 300ps). Details of the experimental setup
are shown elsewhere 8. The 300 ps pulsed beam is
divided into two beams by a beam splitter. One is
used for sample excitation after passing through an
optical delay line, and another is compressed to
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about 50 fs and irradiated on an iron target at 1017
W/cm2 for X-ray generation. The pulse width of
the X-rays was measured to be about 10 ps. The
Fe Ka line was used for the diffraction experiment.
A 300 ps laser pulse was focused to a spot size of 1
X 4, 2 X 5 and 1 X 3 mm on a target in air with an
energy of 60 - 100 mJ/pulse. The axis of
irradiation was normal to the surface. Two types
of the experiments were performed under varying
conditions of laser irradiations. In one set of
experiments, single shot irradiations, varying power
density, were performed.
The other set of
experiments was performed with multiple laser
shots.
The samples used were n-type and
non-doped Si (111) wafers with the thickness of 860
and 525 jim, respectively. The X-ray pulse was
aimed at the center of the laser focal spot on the Si
(111) and the diffracted X-rays were detected with
an LN2-cooled CCD area image sensor (512X512
channels of 25 jam X 25 fim size). Data are
accumulated for 600 shots. We define the time
zero at the arrival of the peak of pump pulse and
X-ray producing pulse at the position of sample.
RESULTS AND DISCUSSION
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dffractedaTg(e(2B)
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362
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Diffracted angle (28|
FIGURE 1. Scattering spectra for the laser heated Silicon under
single laser shot irradiation. The conditions are (a) power density
of 5 GW/cm2 and the time delay of 250ps. (b) 10 GW/cm2, 350
ps. The circles are the experimental results, and the solid lines
are calculated X-ray diffraction profiles.
Single shot experiments
Results of single laser shot irradiation experiments
are shown in Fig 1. Figure 1. (a) shows scattering
spectrum on the condition for laser irradiation with
power density of 5 GW/cm2 at time delay 250ps.
The X-ray diffraction peak was shifted to lower
Bragg angles relative to the spectrum from the
unirradiated crystal. Figure 1. (b) shows the result
of X-ray diffraction under irradiation with power
density of 10 GW/cm2. The new signal appeared at a
higher diffracted angle. The strain profile inside
the crystal can be estimated from the X-ray
diffraction data, (a) In the case of irradiance of 5
GW/cm2, apparent diffraction signal at a lower
angle corresponds to lattice expansion, (b) while
new signals at higher angles observed in irradiance
of 10 GW/cm2 provides evidence that lattice
compression had occurred. X-ray diffraction was
analyzed by a code based on dynamical diffraction
theory and obtained strain distributions are shown in
Fig 2.
Figure 2. (a) indicates that thermal
expansion due to laser heating occurred at this range
in irradiance and delay time, and the maximum
change in the strain and the surface temperature
were estimated to be -0.5% and 1500 K,
respectively.
It was also suggested that the
absorption coefficient of silicon is increasing to 1.2
X 104 cm"1, ten times as large as that to 780 nm light
at 300 K. Similar laser-induced lattice expansion
has been observed in Si. Larson et al4 studied
time resolved X-ray diffraction of 15 ns laser
irradiated Si at 0.1 GW/cm2 to find the lattice
temperature in Si and observed thermal expansion
due to the heating to reach the melting point.
On the other hand, at a power density of 10
GW/cm2, the X-ray diffraction shows lattice
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Multiple shots experiments
0.5
Figure 3. shows the X-ray diffracted spectrum from
the silicon (111) surface under multiple laser
irradiations (300 shots) with a power density of 4
GW/cm2.
Multiple shot experiments were
performed for several sets of shots (20 - 300 shots)
with the same power density at delay time 350 ps.
In all experiments, the shifts of the diffracted
spectrum to higher Bragg angle due to laser shocked
compression were observed and strains were
estimated to be ~ -1.4 %. The generation of shock
compression was observed even at 20-shots as
reduction of ablation threshold was induced by the
surface damage in laser irradiated area.
(a)
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I 02
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I
1
2
3
4
Depth from the surface (urn)
0
1
2
3
4
0.0
-0.5
(b)
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FIGURE 2. Strain Distributions under the surface of single shot
irradiated Silicon. The conditions are (a) with power density of 5
GW/cm2 and the time delay 250ps. (b) 10 GW/cm2, 350 ps.
0.5
0.0 ———
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compression. The maximum compression of 2.1 %
was estimated from the calculation. To account for
the phenomena observed in the present experiments,
the absorption coefficient was estimated using the
values of laser energy density deposited in the
sample, enthalpy changes, and latent heat of silicon.
The observed shock compression in this experiment
is due to laser ablation. The absorption depth of
780 nm light was estimated to be several hundreds
of nanometers (~ 0.4 |xm). The ablation occurred
in the thin surface layer (several hundreds of
nanometer) at the power density of 10 GW/cm2 (2
J/cm2 for 300 ps pulsed laser). Pronko et al 9
reported that the pulsed laser (780nm, 300ps)
induced dielectric breakdown threshold of silicon
was ~3 J/cm2, which provides further evidence of
laser ablation occurring in this range of laser
irradiation.
36.0
36.2
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Diffracted angle (2e)
FIGURE 3. Typical X-ray diffraction spectrum on the Si (111)
under multiple laser irradiation (300 shots) with power density of
4 GW/cm2. The time delay of the X-ray probe was 350ps.
ACKNOWLEDGEMENT
We thank Y. Okano, H. Kawano, and M. Hasegawa
for their help on experiments and valuable
discussions. This work has been supported by
CREST (Core Research for Evolutional Science and
Technology) program organized by Japan Science
and Technology Corporation (JST).
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