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Modification of structural and transport properties in epitaxial YBa2CU3Ox films by pulsed laser irradiation
N. Chechenin, A. Chernysh, V. Korneev, E. Monakhov, B. Seleznev
To cite this version:
N. Chechenin, A. Chernysh, V. Korneev, E. Monakhov, B. Seleznev. Modification of structural and transport properties in epitaxial YBa2CU3Ox films by pulsed laser irradiation. Journal de Physique III, EDP Sciences, 1993, 3 (12), pp.2173-2188. �10.1051/jp3:1993268�. �jpa-00249075�
Classification Physic-s Absn.acts
61.808 74.65 74,75
Modification of structural and transport properties in epitaxial YBa~CU~O~ films by pulsed laser irradiation
N, G, Chechenin, A. V. Chemysh, V, V, Komeev, E. V, Monakhov and B, V, Seleznev
Institute of Nuclear Physics, Moscow State University, 119899 Moscow, Russia
(Received 22 October 1992, revised 3 June 1993, accepted 14 September 1993)
Rdsumd.-Les effects de pulses laser (~20ns) sur des films minces dpitaxids
d'YBa2Cu~O/SrTiO~ ont dt6 6tud16s par r6trodiffusion Rutherford et canalisation afin de caractdriser leur composition et leur structure et h l'aide de la technique des 4 points pour les
mesures 61ectriques. On a trouv6 que la fusion par pulse laser suivie par une trempe entraine une
transition d'un film monocristallin vers un film polycristallin, La formation de joints de grains s'accompagne d'une augmentation d'un facteur 10-40, de la rdsistivitd dlectrique h tempdrature ambiante, ddpendante des propridtds initiales du film et des conditions d'irradiation. Contrairement h l'irradiation corpusculaire (ions, neutrons) l'endommagement produit par la pulse laser
n'entraine pas la disparition de la superconductivitd h haute tempdrature (HTS), qui persiste jusqu'aux plus hautes doses utilisdes. On a trouvd qu'un modble thermique peut ddcrire de fagon consistante, la profondeur de la rugosit6 de surface et en considdrant un modble simple h deux couches on peut comprendre la ddpendance de la dose sur la rdsistivitd h tempdrature ambiante pour des films relativement dpais.
Abstract. Pulsed (~20ns) laser effects in epitaxial YBa~CU~O,/SrTiO~ thin films were
investigated, using RBS/channeling for compositional and structural characterization and 4-point technique for electrical measurements, It was found that laser pulse melting and following quenching lead to a transition from a single-crystalline to a polycrystalline state in films, The formation of grain boundaries causes a room temperature electrical resistivity increase by a factor 10-40, depending on the initial film properties and on the irradiation conditions. Unlike corpuscular (ions, neutrons j irradiation, the laser pulse induced structural damage did not lead to the disappearance of HTS, which persisted up to the highest laser fluences used. It was found, that
a thermal model can consistently describe the fluence dependence of disorder, depth of surface relief and helps, with a simple two-layer model, in understanding of fluence dependence of room
temperature resistivity in a relatively thick film.
1. Introduction.
To investigate the appearance and degradation of high-temperature superconductivity (HTS) it is important to study the correlation between structural and electrophysical properties of the
HTS materials in different conditions. Numerous investigations of electron, neutron, and other corpuscular irradiation effects have demonstrated that, besides a defect accumulation, a
smooth decrease of the critical temperature T~, an increase of the width AT~ of the
superconducting transition, and, in some range of fluences and magnetic field applied, an
increase of the critical current take place (see for example [1-4]).
In spite of interesting physics and impressive achievements in applications of lasers in
technologies of HTS film preparation [5], film pattering [6] and SQUID production [6], investigations of laser influence on the structure and transport properties of thin films are
surprisingly rare. At small laser energy, the voltage response to optical pulses has been studied [8-10]. At high energy of laser irradiation, the surface topography modification, damage and
evaporation of HTS thin films have been demonstrated [I1, 12].
The absorption of the visible electromagnetic radiation in HTS materials is a result of the electron subsystem excitation, which in time of
~ 10~ ~~ s transfers into the phonon subsystem, Most of the laser beam effects are of a thermal nature, An exception is, probably, the effect of a
pulsed voltage response, where the direct break up of Cooper pairs by photons may have some contribution [8-10],
Our preliminary study of laser induced structural effects in epitaxial thin
YBa~CU~O~/SrTiO~(YBCO/STO) films showed that irradiation by pulses with an energy
density W
~ 0.3 J/cm~ leads to disappearance of ion channeling in the films [13]. In this paper
we investigate simultaneously structural, compositional and charge transport evolution in the YBCO/STO thin films under laser pulse influence. Irradiations were done in air and in
vacuum. Rutherford backscattering spectrometry and channeling (RBS/C) technics together
with electrical resistance measurements were applied, and some of the obtained data are
compared with thermal model calculations.
2. Experimental.
Deposition of the epitaxial YBCO films was performed by means of a KrF (A
=
248 nm) eximer laser with energy per pulse of 0.2 J [14]. The fluence on the target was 2-4 J/cm2. A
rotating fused silica lens insured the scanning of the laser spot over the target surface. Oxygen
pressure in the cell was between 0. I-I-o Torr during the deposition. A single-crystalline SrTi03 substrate with an orientation loo) was placed at a distance of 3 cm from the target and kept at a temperature of 700 °C. The rate of the YBCO film growth was about 1.5 ilpulse. The
films had an even, shiny surface, c-axis aligned with the normal to the surface, the
superconductive transition temperature T~=88-92K and the width of the transition AT~ ~ l K. In our experiments films with thicknesses of 0.2 ~Lm (« thin
» film) and 1.5 ~Lm («thick» film) were used. The normalized minimum yield, x~,~, (Xm,n
= H~/H~, where H~ and H~ are RBS yields in the channeling and random mode, respectively) varied from lo fli
to 20 9b, indicating a reasonable quality of the epitaxial films used in our experiments.
The irradiations of the samples were carried out both under vacuum and in air, using a ruby
(A = 694 nm laser with a pulse duration (FWHM) T
I 20 ns. Before getting on a target, the
laser beam went through a silica cylinder diffuser (Z 4 mm x 40 mm), which made the beam
intensity uniform in the cross-section. A sample was mounted l mm behind the back end of
the diffuser and was covered by a tantalum mask with a slit (m 2 mm wide), providing the
irradiation all over the sample width (~ 3 mm), but without influence on the electrical contacts.
The irradiation was carried out on the same surface area of the sample, usually, with an
increase of the energy density from pulse to pulse. After each pulse the samples were analysed by RBS/C technics, and the temperature dependence of the electrical resistance R(T) was
measured.
RBS/C spectra were measured using 2 MeV ~He+ ions and
a surface-barrier Si detector with
an energy resolution of 15 kev. The R(T) measurements were made by a standard four-
terminal method in a vacuum cell. Strip-like gold contacts were deposited on the sample
surface through a mask by thermal evaporation. Contact springs were also coated by a thin gold layer.
3. Results.
3.I THERMAL FIELD CALCULATION. Since the thermal equilibrium between the electron and
phonon subsystems sets in solids in the picosecond scale, one can assume that nanosecond laser pulses influence structural, compositional and electrophysical properties predominantly
i>ia the evolution of the thermal field in irradiated sample. The heat conduction equation
was numerically solved, where C, p, K are the specific heat, density and thermal conductivity, respectively. The heat source density, Q, is determined by optical parameters :
Q
= (I R(t )) J(t ) a (z, t exp I- ~ a (.x, t dx (2)
o
where R and « are the light reflection and absorption coefficients, respectively. The laser pulse shape function J(t) in (2) was approximated by a Gaussian with FWHM duration time
T = 20 ns. The one-dimensional form of the heat equation is justified by the fact that both, the laser spot size (Z 4 mm) and the width of the slit in the mask (q 2 mm), were much larger
than the absorption depth (1/« 0.I ~Lm).
Neglecting the heat loss through the surface, one can write the boundary and initial conditions as follows :
~~ (t, 0
= 0, T(t, L
= To, T(0, z
= To. (3)
The second condition sets up a depth limit, L, for the temperature variation in the numerical solution. Further details of the thermal field calculations have been discussed in numerous
publications, in particular, on laser induced effects in semiconductors (for example, Ref. [15]).
The reflection and absorption coefficients, listed in table I, were estimated for two values of the oxygen content parameter x = 6.5 and x
=
7.0, using experimental data on the dielectric function s [16], and well-known relations for the real and imaginary terms in E [17]. The
temperature dependence of the optical coefficients was neglected in the present calculations.
The specific heat coefficient is determined by the phonon contribution at high temperatures.
Expanding the Debye formula
~~ ~ ~~~ i ~ o~ ei~ l
)~
~~ ~~~
over a small parameter x~ = T~/T at TN To, where T~ is Debye temperature, N is atomic
density, t~ is Boltzmann's constant, one can get the temperature dependence of C~(T) in the form
C~ = Co exp(-a/(T+b)). (5)
Table I. List of the parameters used in the thermal model calculations.
YBa2Cu30
SrTiO~
x = 6.5 x
=
7.0
Density, g/cm3 5.5 5.5 5.13
Absorption, «, ~Lm-' 12.5 15.7 0.5
Reflectivity, R 0.1 0. 14
Co, J/(g K) 0.5 0.5 0.7
a, K 37 37 79
b, K 79 79 24
Ko, x 10-~ W/(cm K) I-o 2.0 o-o
Y, W/cm o-o o-o 36
p, K loo
Latent heat, J/g 500 500
Melting temperature 000 °C 000 °C
In our calculations, the dependence C~(T) was approximated by the expression
C~ = Co exp (- TD/T) (6)
which is close to equation (5j, both in the form and numerically, but is more flexible in fitting
to-the existing experimental data. Parameters Co, a and b, listed in table I, were obtained from
fitting to the data [18, 19] for YBCO and STO.
The thermal conductivity was approximated by
K= Y/(T+p)+Ko, (7)
taking into account, that the phonon contribution is proportional to T- ', while the electron one becomes constant at high temperatures. The values of parameters Ko, y, p in equation (7), obtained from a best fit to the data references [19, 20], are given in table I. It should be noted
that we consider the heat transfer along the c-axis of YBCO crystal, while the thermal
conductivity along the a and b axes is 3-5 times higher. The value 500 J/cm3 was taken for the latent heat of YBCO melting as a typical one for compounds. A similar value has been used in
reference [21]. The melting temperature of the compound was taken to be equal to 000 °C
[22]. The simulation was carried out assuming a constant oxygen content during the temperature evolution of the irradiated sample.
In figure I some results of the calculations are shown for the pulsed laser irradiation of the
&-
a 1,2 "~'~"
~~
a) ~~
l,O ""
,"
,
' 'f
x_,,.;"
- j~
j
0.6 5
~ o1
, #
$
/ "
~
," ~ ~~ °~~
O.3 ~ ~~~°~~ ~~~~~~
l'
o in vacuum
~ damaged layer
~'i05
O.15 O.25 O.35 ~'i.O
I.O 2.O 3.O 4 5
Fluence (J/cm~) Fjuence (J/cm~)
aj b)
Fig. I. Melting depth i,emus energy density of laser pulses irradiating YBa~CU~O,/SrTiO~ films with thickness of 0.2 ~m (aj and 1.5 ~m (bj for the oxygen formula parameter x 6.5 (dashed curve) and
,r 7.0 (solid curve). For the comparison. experimental data are shown on the disordered layer thickness for the irradiation in air (squares) and in a vacuum (circles j, as well as the data on the surface relief depth, zc, in 1.5 ~m film (crossesj.
films with two thicknesses, used in the experiment. One of the thicknesses (0.2 ~Lm) is close to the radiation absorption depth (lla ), another (1.5 ~Lm) is essentially larger than Ila. One can
see that the energy density W
=
0.I J/cm~ is high enough to heat the surface layer up to the
melting temperature. The dependence of the maximum melt depth on the laser fluence is shown for two formula parameters of the oxygen concentration in the « thin
» film (Fig. la).
We always saw an onset of transition to superconductivity in the films used for this study,
therefore x
=
6.5 was taken as a lower limit for the oxygen formula parameter in the
calculations, as for x
~ 6.5 HTS starts to disappear [23]. Due to outgasing, one can expect the
oxygen concentration to be x
~ 7.0 in surface layers of the films used in our experiment, while inner layers of the films must have an oxygen concentration closer to
x = 7.0. In the case of the
« thick » film the contribution of the surface layer is small, and therefore the dependence for
x =
7.0 is only shown in figure 16. The accuracy of the thermal field calculations decreases
further at fluences, essentially exceeding the surface layer melting threshold, let say
W ~ 0.3 J/cm~, where the extrapolation of optical and thermal parameters is, of course, very uncertain.
3.2 RBS/CHANNELING DATA.
3.2. I Random spectra evolution. Figures 2-4 show RBS/channeling spectra for « thin » and
« thick » films irradiated with laser pulses with different fluences in air (Figs. 2, 4) and under
vacuum (Fig. 3). The arrows in figures 2, 3 point to maximum energies of the ions after
backscattering by surface atoms of Ba, Y, Cu. A significant decrease of the width of the
signal, corresponding to the scattering in the films was observed at large fluences (Fig. 2c), indicating a decrease of the film thickness (by about 0.I ~Lm for W
=
3 J/cm~) due to laser
induced sputtering. The RBS spectra for the « thick » film (Fig. 4) can be roughly divided into three parts the A-region represents the scattering inside the film, the B-region reflects the
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I ] .I : :
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~i ~ -~/ ). " ' '
m .i o.2/Jm
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o
iso 200 200 200
Channel Number
Fig. 2. RBS and channeling spectra for the 0.2 ~m film irradiated in air a) RBS (I ) and channeling (21 spectra before the irradiation and channeling spectrum after a fluence W 0.17 J/cm~(3) ; b) RBS (I)
and channeling (2j spectra after W 0.35 J/cm~ c) RBS spectra before the irradiation (I) and after W 3.0 J/cm2 (2).
a) b)
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,
' 1' ~~'
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~ " ' £' "~
~ 'O
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150 200 150 200
Channel Number
Fig. 3. -RBS and channeling spectra of the 0.2 ~m film irradiated in-vacuum a) RBS (I) and channeling (2) spectra of the initial sample, channeling spectra after fluences W 0,I J/cm~ j3j and W 0? J/cm~ (4) ; b) RBS jl) and channeling (2) spectra after W 0.~ J/cm~.
scattering in the substrate, and the C-part corresponds to the intermediate region. In the initial film the C-region is narrowest and reflects mostly a difference in the kinematic scattering factors for heavy and light atoms [24] at the film/substrate interface (Fig. 4a). The C-region
grows at the expense of the A-region when increasing the fluence (Figs. 4b, cl, displaying an
essential nonuniformity in the film thickness appeared at high fluences (W
~ l J/cm~). RBS
spectra analysis showed that the material composition in the A-region does not deviate from the stoichiometric 1-2-3 one for the heavy elements Y-Ba-Cu within
q 5 9b accuracy even for
as high fluences as 2.8 J/cm~.
3.2.2 Damage biiild-lip. The laser irradiation caused a strong damage build-up in the films,
as demonstrated in figures 2-4. Thus the in-air irradiation of 0.2 ~Lm film with the fluence W
= 0.17 J/cm~ let to an increase of the surface peak width as well as to an enlargement of the
minimum yield up to Xm,n = 37 fG in the channeling spectrum 2, as compared with the
spectrum 3 for unirradiated sample (Fig. 2a). After the laser irradiation with the pulse energy
~~ oFSz
~, l~ °":.
a)
~ .'~
', '<Z,
' ? oo
~o
lo,,
To ~'ihoo r<w
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'z'~ V o'o° W.%, ~
"z? R '
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100 150 00
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Fig. 4. RBS (I) and channeling (2) spectra of the 1.5 ~m film before (a) and after in-air irradiation W
= 0.35 J/cm~ (b) and W
=
2.8 J/cm~ (c).
density of 0.35 J/cm2, no channeling was observed in the film (Fig. 2b), which is then considered to be completely disordered.
In-vacuum irradiation of the 0.2 ~Lm YBCO film with the pulse energy density W
=
0. I J/cm~ caused the increase of
Xm~n from 12 fb up to 30 fG and a peak in the high energy part
of the aligned spectrum (3) in figure 2a, indicating an increased disorder in the film surface layer with a thickness of about 50 nm. Further irradiation with a fluence of 0.2 J/cm2 led to a
more significant disorder in the film (spectrum 4) : a depth of the damaged layer became about 120 nm, and the yield in the peak increased up to the random level, Xm,n increased to 58 9b. A
subsequent irradiation with a fluence of 0.3 J/cm2 destroyed channeling in the film (Fig. 3b),
so the film became disordered. The variation of the minimum yield on the laser fluence is
plotted in figure 5.
An analysis of the spectra presented in figures 2, 3 suggests that the structure of the single crystalline substrate remains practically unchanged. An increase of the relative channeling yield in the substrate (region below 150 channel) with an increase of the laser pulse energy is
accounted for enhanced multiple scattering in the disordered film.
Similar to the case of 0.2 ~Lm films, the irradiation of the 1.5 ~Lm film with an energy density
of 0.35 J/cm2 led to an appearance of a completely disordered surface layer with a thickness 0.3 ~Lm (channels 205-245 in Fig. 4b). A coincidence of the aligned and random spectra at W
=
2.8 J/cm~ indicates
a loss of the epitaxial structure all over the film thickness.
3.2.3 Temperature dependence of the film resistivity. Laser beam effects strongly the
resistivity of YBCO/STO film. Figure 6 shows the room temperature resistance Ro normalized
1.O ~H -Q-
/ / / /
.f /
m /
ll'/
O.5 ~~'
,
,' , D in air
1' G9
,/ O in vacuum
/
~'i.O
O-I O.2 O.3
W(J/cm~)
Fig. 5. The minimum yield, Xm,n, versus pulse energy density, W, for films with thickness of 0.2 ~m irradiated in a vacuum and in air.
~~s
§ o-z
Mm
IO
fi
O ~ /~~~~~~w~
~ ~,/
°,,f~'i~
Mm film
0 2 3 4 5
W (J/cm~)
Fig. 6. Dependence of the electrical resistance at room temperature on the laser pulse fluence for
« thin » and
« thick » films. By the cross a change in the resistance after the irradiation in a vacuum is shown. The resistance is normalized to that before influence. The solid line connects the in-air 0.2 ~m film data points, the dashed curve shows a prediction of the two-layer model (Eqs. (10-12)).
by the resistance of a given unirradiated film, Ro (W = 0), versus the fluence W. One can see that the rise of R (W is much stronger for the « thin » film than for the
« thick » one. Also the
figure shows that the irradiation of a « thin » sample in vacuum with W 0.3 J/cm~ gives
an
increase of R similar to that in air.
Figure 7 shows R(T)-curves for the « thick » film (1.5 ~Lm) normalized by the electrical resistance at room temperature, Ro, for different fluences. In spite of a complete disorder in a surface layer and of a large defect concentration in deeper layers of this YBCO film (Fig. 4b),
