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TIME RESOLVED X-RAY ABSORPTION MEASUREMENTS ON PULSED LASER
IRRADIATED THIN SILICON FOILS AND SILICON PLASMAS
H. van Brug, F. Bijkerk, H. Gerritsen, K. Murakami, M.J. van der Wiel
To cite this version:
H. van Brug, F. Bijkerk, H. Gerritsen, K. Murakami, M.J. van der Wiel. TIME RESOLVED X-
RAY ABSORPTION MEASUREMENTS ON PULSED LASER IRRADIATED THIN SILICON
FOILS AND SILICON PLASMAS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-193-C8-
198. �10.1051/jphyscol:1986836�. �jpa-00226158�
TIME RESOLVED X-RAY ABSORPTION MEASUREMENTS ON PULSED LASER IRRADIATED THIN SILICON FOILS AND SILICON PLASMAS
H. VAN BRUG, F. B I J K E R K , H.C. GERRITSEN, K. MURAKAMI and M.J. VAN DER WIEL
FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
Nous presentons des mesures d'absorption de rayons X en resolution temporelle par des couches minces de silicium amorphe, sournises a un rayonnement laser pulse. Nous avons compare ces rbultats a des mesures d'absorption sur des clusters et des plasmas de silicium, produits par irradiation d'une plaque de silicium par un laser puls6. Notre appareil est constitue d'un plasma laser utilise comme source B rayons X, d'une chambre B kchantillon, d'un polychromateur et d'un systhme de dbtection multicanal.
ABSTRACT
W e present time resolved x-ray absorption measurements (90-300eV) on amorphous silicon foils, that are pulsed laser irradiated. These results are compared with absorption measurements on silicon clusters and silicon plasmas, produced by pulsed laser irradiation of bulk silicon. Our apparatus consists of: a laser plasma x-ray source, a sample stage, a polychromator and a multichannel detector system.
INTRODUCTION
We report time resolved x-ray absorption measurements on Si foils and Si clusters and plasmas.
These measurements were performed in order to get some information on the annealing and the cluster formation process.
First we discuss measurements on 600
A
Si foils on a 400A
C backing. These foils are irradiated and probed by an x-ray beam. These measurements were performed to get some information on the annealing process of Si. This yielded the first observation of x-ray absorption spectra of liquid Si, and of a plasma above the surface at high irradiation densities.Secondly, we discuss measurements on p'uticles emitted from a piece of bulk Si after irradiation.
This particle cloud is probed by an x-ray beam at variable heights above the emitting surface. Thus we obtain absorption spectra of Si clusters and Si plasmas, in various stages of ionization.
These subjects have the attention of many groups, see ref. [1,2,3,4,5].
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986836
JOURNAL DE PHYSIQUE
EXPERIMENTAL
Figure 1 shows an outline of our experimental system. X-ray absorption measurements were carried out by transmitting a broad continuum of x-rays through the absorbing sample, i.e. foil or cloud of particles, and dispersing the radiation afterwards. As an x-ray source we used a plasma that was created by focusing the output of a frequency doubled Nd-Yaglglass laser (7 J, 15 ns, 532 nm) on a Ta target to a spotsize of 65 pm. The hot Ta plasma emits an intense pulse of x-ray photons (1015 in one pulse of 15 ns) with a smooth spectral distribution, so that we obtain absorption spectra in one shot averaged over a period of 15 ns. Our measuring system allows us to record an absorption spectrum in one single laser shot with good statistics. The energy resolution was approximately 4 eV.
For a more detailed description see ref. [6,7,8]
As an annealing (pump) beam a fraction of the laser output was used to irradiate the foil or the piece of bulk Si in a vacuum of Tom, as shown in Fig. 1. The laser pump beam was focused to 0.5-5 rnm, depending on the energy density desired, while the diameter of the x-ray probe beam on the sample was approximately 0.2 mm. The laser beam has a spatial profile close to TEMOo and E M o l , and the remaifiing intensity variations are believed to occur on a scale larger than the probe area. By performing some of the measurements using a diffuser in the pump beam, it was also clear that possible intensity inhomogeneities have no significant effect on the x-ray absorption spectra.
For the foil absorption measurements the probe beam was passing through the foil, whereas it was passing above the particle emitting surface in the case of the measurements on clusters and plasmas. The irradiating energy density was varied from 0.1 to 3.6 ~ l c m ~ for the foil measurements and from 3.5 to 14.0 J/cm2 for the cluster and plasma measurements. To do time resolved measurements, the pump pulse on the absorbing sample was followed by the x-ray probe pulse with a variable time delay zd of 12,30 and 60 ns, as shown in Fig. 1.
Fig. 1: Outline of the experimental system for time-resolved x-ray absorption measurements.
SHG: second harmonic generator, BS: beam splitter.
Foil measurements:
Typical x-ray absorption spectra for the foil measurements at zd of 12 ns are shown in Fig. 2 for various irradiating energy densities. The spectra are shown in the x-ray energy range from 90 to 300 eV. From our results the pump energy density can be divided into three ranges. Range I is below approximately 0.17 Ucm2, at which energy density we observe the first significant changes in the absorption spectra. This value is therefore thought to correspond to the annealing threshold Eth Range 11 is from 0.17 to approximately 1.0 J/cm2, where short lived liquid Si is produced such that annealing takes place. Range III is above 1.0 Jicm2 at which density further changes in the spectra are observed. This corresponds to the damage threshold Ed
.
It is found that spectra in the anneal range (11) show an edge shift of approximately 5 eV to higher energies and a smaller absorption height than an unannealed spectrum. This spectrum height reduction is explained by droplet formation, yielding a smaller effective p d (absorption coefficient times absorption length). The edge shift is due to the metallic-like electronic structure of molten Si. In the damage range (III), additional edge structures appear above the Ln,III-edge between 110 and 150 eV. These can be assigned to absorption lines of si4+ ions.In figure 3, the time evolution of the x-ray absorption spectra are shown. In 3a for spectra of the anneal range (11), and 3b for spectra of the damage range (111). In 3a it can be seen that resolidification takes place, i.e. reappearance of the LII,III- edge at 98 eV. In 3b the recombination of the plasma can be seen. Because the spectrum resembles those of range II very clearly, the ions are thought to recombine and subsequently form droplets. For more details about the foil measurements see ref. [9].
I
Fig. 2: Typical x-ray absorption spectra at Td of 12 ns for pump beam energy densities from 0 to 3.6
--
-. -.__- -
.-- ~lcm'. The edge at 280 eV is due to the carbonfoil (CK absorption). The vertical axis indicates the ratio of the incident x-rays I. to that of the transmitted x-rays I.
-. --. - -
, ,--- .---__.__-'-
-
t0 ,
100 200 300
ENERGY IeVl
JOURNAL DE PHYSIQUE
100 125 150 175
ENERGY C e V 1
l o - ' " ' ' ' ' ' , -
0-
100 200 300
ENERGY C e V l
Fig. 3: Time evolution of the x-ray absorption spectra for Si foils irradiated with an energy density of: (a) 0.3 and (b) 3.6 f/cm2.
Cluster and dasma measurements
In figures 4 and 5 we show spectra for the particle cloud absorption in the x-ray energy range from 90 to 190 eV. In Fig. 4a we present spectra measured at a pump energy density of 3.5 .Tlcm2 and a delay time zd of 12 ns. In Fig. 4b a spectrum of range I1 from the foil measurements is presented for comparison. Here zd is also 12 ns. The different spectra in 4a are recorded at different heights of the probe beam above the emitting surface. The spectra in Figs. 5a and 5b were recorded at a pump energy density of 14.0 .T/cm2 and a zd of 12 ns in 5a and 60 ns in 5b. Figure 5c is a foil absorption measurement of range III, with a zd of 12 ns.
From the measurement at 3.5 .T/cm2 it has been found that the particles emitted at this energy density have the same absorption spectrum as short lived liquid Si, and therefore we conclude that these particles are droplets (large sized Si clusters). The edge is indicated with "I-Si".
A full assignment of thestructures in Fig. 5 will be given elsewhere.[lO]. Here we only want to state that the edge for droplets, "1-Si" is not present, and that all structures as have been found in 5c are also present in 5a. This indicates that in the foil measurements parts of the foil are evaporated and probed while floating above the foil. In 5b it is clear that the ions recombine on a timescale of 60 ns.
From the plasma measurements it has been found that large sized clusters can be produced at relatively low energy densities, while at high energy densities small sized clusters occur.
We calculated the emitted layer thickness as a function of the pump energy density. The result is shown in Fig. 6. In this figure we see an onset for the layer removal of about 2.5 ~ / c m ~ , followed by a plateau and then a non linear rise with energy density.
The particle velocity was determined as 5 . 1 0 ~ mfs, where the highly charged ions move faster than those with a low charge number.
0
- 100 120 140 160 180
ENERGY [eVl
pump and probe beam and a pump beam energy density of 3.5 ~ / c m ~ . The different spectra are recorded at different distances between emitting surface and probe beam. The distances are, from top to bottom: 100, 200 and 300 pm. (b) X-ray absorption spectrum from the foil measurements, recorded at a delay time of 12 ns and a energy density of 0.7 k m 2 .
Fig. 6: Emitted Si layer thickness as function of the irradiance for 15 ns laser pulses of 532 nm light.
o ~ 9 ~ ~ ~ c T l ~ ~ ~
100 120 140 160 180
ENERGY [eVI
- - -
5
220-
u - u
> -
4 :
100-
> -
0 -
5 -
u -Fig. 5: (a) X-ray absorption spectra of the emitted Si particles at a delay time of 12 ns between pump and probe beam and a pump beam energy density of 14.0 .I/cm2. The distance are, from top to bottom: 100,200, 300,400, 500, 600 and 800 pm. (b) Same as (a), except for the delay time and the distances. The delay time is 60 ns, and the distances are, from top to bottom: 100, 200, 400, 600, 1000, 1200, 1500, 2000
0 5 10 15
ENERGY DENSITY
[J/cm21
, ' " . ' * . . ' ' . . s '
d
/
/ / /
,
/ /, /
x - - - - * - ,
I I I I
+
.
, ~. .
.. . . . . .
. - rand 3000 pm. (c) X-ray absorption spectrum from the foil measurements, recorded at a delay time of 12 ns and an energy density of 3.6 k m 2 .
C8-198 JOURNAL DE PHYSIQUE
CONCLUSION
In summary, we have reported a large change in the x-ray absorption spectra of Si foils under intense pulsed-laser irradiation. The energy density range between 0.17 ~ / c m ~ and 1.0 J/cm2 has been found to be the anneal range. Below this range the foil is only heated, while above this range damaging takes place. The observed edge height reduction for spectra under annealing conditions is explained by the formation of droplets. The damage range has been examined by probing in a cloud of particles emitted from a piece of bulk Si under pulsed-laser irradiation.
The onset for particle emission has been found to be at an energy density of approximately 3.5
~ / c r n ~ . The velocity of the emitted particles can be as high as 5-lo4 m/s. At low energy densities, below 6.0 ~ l c m ~ , mainly large sized clusters are formed. At higher energy density the production of small sized clusters, neutral Si and Si ions takes place. The level of ionization is dependent on the energy density used, e.g. at 14.0 ~ / c m ~ ions up to si4+ are formed. The emitted layer thickness as a function of the energy density is given; a typical value is 80
A
at 6.0 ~ l c m ~ for 15 ns laser pulses.ACKNOWLEDGEMENT
The authors wish to thank Rob Kemper for his technical assistance. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (Foundation for Research on Matter) and was made possible by financial support from the Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (Netherlands Organization for the Advancement of Pure Research).
REFERENCES
[I] B. C. Larson, C. W. White, T. S.Noggle, J. F. Barhorst, D. M. Mills; Appl. Phys. Lett.
a
282 (1983)
[2] K. Murakami, K. Masuda; Semiconductors probed by Ultrafast Laser Spectroscopy, vol 11, ed. by R. R. Alfano, Academic Press, N. Y. (1984), pp 171-195
[3] J. M. Lui, R. Yen, H. Kurz, N. Bloembergen; Appl. Phys. Lett. (9), 755 (1981) [4] L. A. Bloomfield, R. R. Freeman, W. L. Brown; Phys. Rev. Lett.
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(20), 2246 (1985) 151 D. Lubben, S. A. Barnet, K. Suzuki, S. Gorbatkin, J. E. Green; J. Vac. Sci. Techn. B3 (4),968 (1985)
[6] H. C. Gerritsen, H. van Brug, F. Bijkerk, M. J. van der Wiel; 3. Appl. Phys. 3 , 2 3 3 7 (1986)
[7] H. C. Gemtsen, H. van Bmg, M. Beerlage and M. J. van der Wiel; Nucl. Inst. and Meth. in Phys. Res. A238,546 (1985)
[a] H. C. Gerritsen, H. van Brug, F. Bijkerk and M. J. van der Wiel; submitted for publication in J. Phys. E
[9] H. C. Gemtsen, H. van Brug, F. Bijkerk, K. Murakami and M. J. van der Wiel; submitted for publication in J. Appl. Phys.
[lo] H. van Brug, K. Murakami, F. Bijkerk and M. J. van der Wiel; submitted for publication in J.
Appl. Phys.
