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Characterization of the KrF laser-induced plasma plume created above an YBaCuO superconducting target and preparation of superconducting thin films
Applied
Surface
Science 46 (1990) 7X-83 North-Holland
Characterization of the KrF laser-induced plasma plume created above an YBaCuO superconducting target and preparation of superconducting thin films C. Girault, D. Damiani, and A. Catherinot
Received
29 May 1990: accepted
C. Champeaux,
for publication
P. Marchet,
C. Germain,
J.P. Mercurio,
J. Aubreton
26 June 1990
The laser-induced plasma plume created above an YBaCuO superconducting target irradiated by a KrF laser beam (24X nm) I\ investigated by time-resolved spectroscopy. High-resolution spectra are obtained and election velocities of ablated species arc deduced as a function of the ambient oxygen pressure. Preliminary results on the preparation of Y Ba,Cu ?O, I thin films on (100) SrTiO, substrates, by the excimer laser ablation of a stoichiometric 1 : 2 : 3 target. are presented.
1. Introduction The discovery of superconductivity in metallic copper oxides at temperatures higher than that of liquid nitrogen has generated unprecedented interest in the fabrication of thin films of the materials. Among the different successful techniques of preparation (electron beam evaporation [l], sputtering [2,3] and molecular beam [4]). pulsed laser ablation of superconducting targets [5,6] appears to be one of the most promising because of its relative simplicity and ability to provide films with a composition very close to the stoichiometry of the target. While most of the works are devoted to the optimization of the deposition parameters and the annealing conditions, few studies have been undertaken in order to understand the transport phenomena of the Y/Ba/Cu/O species from the target to the substrate. This paper deals both with the investigation of the plasma plume produced by the KrF excimer laser ablation of bulk YBa,Cu @_ , superconducting targets and with the preparation of thin films. A high-resolution study of the emission 0169-4332/90/$03.50
‘I: 1990
Elsevier Science Publishers
spectrum of the plasma plume is presented and ejection velocities of the ablated products are deduced, versus ambient oxygen pressure. from a temporal evolution of spatially resolved spectroscopic measurements. Finally. preliminary results on YBaCuO superconducting thin films prepared by the laser ablation process are presented.
2. Experimental
set-up
The experimental set-up is schematically shown in fig. 1. The experiments are performed in an ultra-high vacuum stainless-steel cell equipped with fused silica windows for spectroscopic investigation. The laser source is an excimer laser (Lambda Physik EMG 101) operating at 248 nm, at a repetition rate of 5 Hz, with a pulse duration of 14 ns and an energy per pulse ranging from 20 to 300 mJ. The laser beam which crosses a diaphragm to make it uniform is focused onto a rotating sintered YBa,Cu,O, , pellet located within the cell. According to the 35” angle 01 incidence. the laser spot is elliptical and the fluence
B.V. (North-Holland)
C. Girault et al. / Characterization
EXCIMEH
LASER
h=248nm
KrF
TO
19
of KrF laser-induced plasma plume above YBaCuO
LJ _
DELAY GENERATOR
DESKTOP BOXCAR _
-
TO+nT
COMPUTER
X.T
1 q
PLOTTER
SPECTROMETER
TEKTRONIX v
2440
L r
,
Fig. 1. Experimental
can be changed by varying the lens-to-target distance. The material evaporated perpendicularly to the pellet surface is deposited on a SrTiO, substrate at a distance of about 36 mm. The substrates can be heated up to 900” C by a halogen lamp. Oxygen can be added during the deposition and cooling processes. The chamber is evacuated by a turbomolecular pump to less than lo-’ mbar. The laser-induced plasma plume is imaged onto the entrance slit of a high-resolution spectrometer (THR 1000 Jobin Yvon, resolving power = 100000) with a magnification of 4, using a fused silica lens and a system of three mirrors allowing a spatial resolution of better than 0.1 mm. The spectrometer is equipped with an RCA 7265 photomultiplier tube (rise time < 2 ns) connected to a boxcar averager (EG&G PAR 4400, resolution 2
set-up.
ns) for spectrum recording, or to a Tektronix digitizer (500 MHz) for temporal evolution ies. The electronic devices are synchronized laser pulses using a delay generator.
2440 studwith
3. Results and discussion Typical time-integrated emission spectra emitted by the species present in the slice located 2 mm above an YBaCuO target are shown in fig. 2. The experiment has been performed at a pressure of 10m5 mbar (argon) with a laser fluence of 4 J/cm’ and a spectral resolution of 0.3 A. Assignment of the spectral lines [7] and emission bands [8] is made according to standard tabulation. In
80
C. Girnult et al. / Chrrracterizatmn
ofKrF
NANOMETERS (al
NANOMETERS (b)
NANOMETERS (ci Fig. 2. High-resolution spectrum (0.3 A) of the KrF laser-Induced plasma plume on an YBaCuO superconducting target. Observations are performed at d = 2 mm using a boxcar gate width of 40 ns located 180 ns after the beginning of the interaction.
Irr.rer-Induced
(639.25 and 640.15 nm) and the A’SX’Z system of BaO (603.96 nm) are observed. Moreover. transitions involving atomic oxygen are present at the wavelengths 777.2, 777.4 and 777.54 nm, as shown in fig. 2c. All these species have been observed in plasma slices located at various distances from the target. To determine the influence of oxygen atmosphere on the laser-induced plasma characteristics, the same experiment has been carried out with different oxygen pressure values in the chamber. From a qualitative point of view. we observe that an increase of the O2 pressure value leads to a color change of the plasma plume which from a green white colour becomes reddish. The emitted spectra are similar to those presented in fig. 2 but we notice. as reported previously by Wu et al. [9], that the emission intensities of almost all the detected lines and bands are significantly enhanced under oxygen atmosphere. Analysis of the temporal evolution of the line or band intensities emitted by a given slice of the plume has been performed for ail the detected species as a function of the distance d of the observed slice from the YBaCuO target surface. As examples, the temporal evolution of the Ba* line at 553.55 nm is shown in fig. 3 for several values of d and for an oxygen pressure of 5 x 10 ’ mbar. We observe that the relaxation duration increases with the value of d and depends on the detected species. For each ablated product, the relaxation is assumed to be well described by a monoexponential decay f(r)
the range 350-550 nm (fig. 2a) the spectrum points out the presence of all the elementary products (neutral and ionized) of the YBaCuO target. namely, Y * (404.76, 408.37, 410.24, 412.83, and 414.39 nm), Y+ * (377.4 nm), Ba* (553.55 and 577.77 nm), Baf * (455.4 nm), Cu* (578.21 nm), and Cut* (490.97 and 493.16 nm). Between 550 and 650 nm (fig. 2b) emission bands of diatomic molecules emanating from the A2n-X2Z system of YO (597.2, 598.77, 600.36 and 616.51 nm), the A’SX211 system of CuO
plustnrrplume uhoue YBuC‘uO
= I,, exp( -ct).
where r, is the characteristic decay constant of the species i [lo]. Moreover, it is clear from these kinetics that the time location of the maximum emission intensity is proportional to the distance d, leading to an estimation of the time-of-flight velocity from the target surface to the observed slice for each considered species. Thus, the velocity components V, of the detected species in a direction normal to the YBaCuO target surface have been studied versus the ambient oxygen pressure. As shown in fig. 4 for a laser fluence of 3 J/cm’. the ejection
C. Girault et al. / Characterization 160
of KrF laser-induced plasma plume above YBaCuO
n
1
I
r Pq
200
= 5.10-2
600
400
TIME
1
I
1
mhar
800
1000
(ns)
Fig. 3. Temporal evolution of the 553.55 nm Ba* spectral line intensity for different values of the distance d from the YBaCuO target and for an oxygen pressure of 5 X lo-* mbar. LP indicates the laser pulse temporal location.
velocities VL of the ablated products are very sensitive to the oxygen pressure in the chamber. At lo-” mbar the measured velocities VL are of the same order for all the detected species (1061.3 X lo6 cm/s). This indicates that the ejection of the different products during the interaction with the KrF laser beam, in our experimental conditions, mainly results from an ablative decomposias suggested by different tion phenomenon, authors [11,12] rather than from a thermal process. It appears that the velocities of atomic species (neutral and ionized) remain quite constant from vacuum up to an oxygen pressure of about lop2 mbar and decrease rapidly beyond, probably owing to hydrodynamical effects induced by the ambient gas. The decrease of the velocities with the oxygen pressure is more regular for the diatomic mole-
cules and in a first approximation the form
follows a law of
v, = k Log(f’02> for YO, BaO and 0: molecules. This spectroscopic study has been followed by the first attempts to fabricate in our laboratory superconducting thin layers. The films have been deposited on (100) SrTiO, substrates in an oxygen residual atmosphere, with a laser fluence of 2.5 J/cm’. After deposition, the heating was switched off and the film slowly cooled to room temperature under an oxygen pressure of 30 mbar. The film thickness ranges from 6000 to 7000 A. The composition of the films has been investigated by EDAX (electron diffraction X-ray analysis). With the above experimental conditions de-
duced from found are stoichiometry
the spectroscopic study. the values reproducibly close to the ideal (1 : 2 : 3). Resistance is determined
1
I
1
I
I
me-
Ba+
1.6
_
.Y+
lclJ+
‘-‘---------‘-A
-
12x9:
;
v
I)
ct -_-_-----__-__
____ 0
8.Kq
i
0
Fig.
5. Temperature YBaCuO
PRESSURE
I
1
,
,
!
(mbar 1
I
I
1
I
woe t
Ba +Y
/
j
the
resistlwty
film deposited on a SrTiO,
dependence
of
suhstratc.
for
AII
by the standard four-point probe technique. The layers thus obtained are superconducting without any post-deposition annealing. A typical example is given in fig. 5: the superconducting phase is obtained with an onset critical temperature of X8 K and a relatively sharp transition.
4. Conclusion 0 10-e
b 4
/
1
10-2
PRESSURE
I
1
I
104
1
1
(mbar)
1
I
1
,
6dO
10 10
*Vi-l
.GO “06
t\. ‘:I:, --___ i
0
0
lo-6
!
1
/
1
10-4
PRESSURE
;a___
:,
C
1
1
1 ~t%ar~
Fig. 4. Velocity component I’1 normal to the target surface as a function of the oxygen ambient pressure.
A systematic spectroscopic investigation of the plasma plume created by the interaction of a KrF laser beam and an YBaCuO superconducting target shows emissions of neutral and ionized atomic species and of diatomic oxides (YO, BaO and CuO). Time- and spatially resolved spectroscopic measurements lead to estimations of the velocity component V, normal to the target surface of the ablated species as a function of the ambient oxygen pressure, pointing out a strong decrease of V, for elemental species for PC,, > lo-’ mbar. First results on the preparation of superconducting YBa ?Cu ?O, , thin films are presented but these investigations are in an early stage. Further experiments are in progress in order to improve the characteristics of the resistive transition.
C. Girault et al. / Characterizurron of Krt;‘ laser-induced plasma plume above YBaCuO
Acknowledgment
This work is partly contract No. 88-084.
supported
by DRET
under
References
PI B. Oh, M. Naito,
P. Rosenthal, S. Amason, R. Barton. M.R. Beasley. T.H. Geballe. R.H. Hammond and A. Kapitulnik. Appl. Phys. Lett. 51 (1987) 852. PI M. Hong, S.H. Liou, J. Kwo and B.A. Davidson, Appl. Phys. Lett. 51 (1987) 694. [31 R.J. Lin, J.H. Kung and P.T. Wu. Physica C 153-155 (1988) 796. [41J. Kwo, T.C. Hsien, M. Hong, R.M. Fleming, S.H. Liou. B.A. Davidson and L.C. Feldman, MRS Symp. Proc. 99 (1987) 339. X.D. Wu. S.A. Shahem. N. [51 D. Dijkamp. T. Venkatesan.
83
Jisrawi, Y.H. Minke, W.L. McLean and M. Croft, Appl. Phys. Lett. 51 (1987) 619. [61 H.V. Habermeier and G. Mertena, Physica C 153 (1988) 1429. and N.S. Sventitskii. Tables of Spectral [71A.R. Striganov Lines of Neutral and Ionized Atoms (IFI-Plenum. New York, 1968). of PI R.W.B. Pearse and A.G. Gaydon, The Identification Molecular Spectra. 4th ed. (Chapman and Hall. London, 1976). [91 X.D. Wu, B. Dutta, MS. Hegde. A. Inam, T. Venkatesan. E.W. Chase, C.C. Chang and R. Howard, Appl. Phys. Lett. 54 (1989) 179. and A. Catherinot, [lOI C. Girault, D. Damiani. J. Aubreton Appl. Phys. Lett. 54 (1989) 2035. P. Mattocks. L. Shi. X.W. Wang, S. [Ill H.S. Kwok, Witanachchi, Q.Y. Ying, J.P. Zheng and D.T. Shaw. Appl. Phys. Lett. 52 (1988) 1825. X.D. Wu. A. Inam and J.B. Watchman. u21 T. Venkatesan, Appl. Phys. Lett. 53 (1988) 1193.
Surface
Science 46 (1990) 7X-83 North-Holland
Characterization of the KrF laser-induced plasma plume created above an YBaCuO superconducting target and preparation of superconducting thin films C. Girault, D. Damiani, and A. Catherinot
Received
29 May 1990: accepted
C. Champeaux,
for publication
P. Marchet,
C. Germain,
J.P. Mercurio,
J. Aubreton
26 June 1990
The laser-induced plasma plume created above an YBaCuO superconducting target irradiated by a KrF laser beam (24X nm) I\ investigated by time-resolved spectroscopy. High-resolution spectra are obtained and election velocities of ablated species arc deduced as a function of the ambient oxygen pressure. Preliminary results on the preparation of Y Ba,Cu ?O, I thin films on (100) SrTiO, substrates, by the excimer laser ablation of a stoichiometric 1 : 2 : 3 target. are presented.
1. Introduction The discovery of superconductivity in metallic copper oxides at temperatures higher than that of liquid nitrogen has generated unprecedented interest in the fabrication of thin films of the materials. Among the different successful techniques of preparation (electron beam evaporation [l], sputtering [2,3] and molecular beam [4]). pulsed laser ablation of superconducting targets [5,6] appears to be one of the most promising because of its relative simplicity and ability to provide films with a composition very close to the stoichiometry of the target. While most of the works are devoted to the optimization of the deposition parameters and the annealing conditions, few studies have been undertaken in order to understand the transport phenomena of the Y/Ba/Cu/O species from the target to the substrate. This paper deals both with the investigation of the plasma plume produced by the KrF excimer laser ablation of bulk YBa,Cu @_ , superconducting targets and with the preparation of thin films. A high-resolution study of the emission 0169-4332/90/$03.50
‘I: 1990
Elsevier Science Publishers
spectrum of the plasma plume is presented and ejection velocities of the ablated products are deduced, versus ambient oxygen pressure. from a temporal evolution of spatially resolved spectroscopic measurements. Finally. preliminary results on YBaCuO superconducting thin films prepared by the laser ablation process are presented.
2. Experimental
set-up
The experimental set-up is schematically shown in fig. 1. The experiments are performed in an ultra-high vacuum stainless-steel cell equipped with fused silica windows for spectroscopic investigation. The laser source is an excimer laser (Lambda Physik EMG 101) operating at 248 nm, at a repetition rate of 5 Hz, with a pulse duration of 14 ns and an energy per pulse ranging from 20 to 300 mJ. The laser beam which crosses a diaphragm to make it uniform is focused onto a rotating sintered YBa,Cu,O, , pellet located within the cell. According to the 35” angle 01 incidence. the laser spot is elliptical and the fluence
B.V. (North-Holland)
C. Girault et al. / Characterization
EXCIMEH
LASER
h=248nm
KrF
TO
19
of KrF laser-induced plasma plume above YBaCuO
LJ _
DELAY GENERATOR
DESKTOP BOXCAR _
-
TO+nT
COMPUTER
X.T
1 q
PLOTTER
SPECTROMETER
TEKTRONIX v
2440
L r
,
Fig. 1. Experimental
can be changed by varying the lens-to-target distance. The material evaporated perpendicularly to the pellet surface is deposited on a SrTiO, substrate at a distance of about 36 mm. The substrates can be heated up to 900” C by a halogen lamp. Oxygen can be added during the deposition and cooling processes. The chamber is evacuated by a turbomolecular pump to less than lo-’ mbar. The laser-induced plasma plume is imaged onto the entrance slit of a high-resolution spectrometer (THR 1000 Jobin Yvon, resolving power = 100000) with a magnification of 4, using a fused silica lens and a system of three mirrors allowing a spatial resolution of better than 0.1 mm. The spectrometer is equipped with an RCA 7265 photomultiplier tube (rise time < 2 ns) connected to a boxcar averager (EG&G PAR 4400, resolution 2
set-up.
ns) for spectrum recording, or to a Tektronix digitizer (500 MHz) for temporal evolution ies. The electronic devices are synchronized laser pulses using a delay generator.
2440 studwith
3. Results and discussion Typical time-integrated emission spectra emitted by the species present in the slice located 2 mm above an YBaCuO target are shown in fig. 2. The experiment has been performed at a pressure of 10m5 mbar (argon) with a laser fluence of 4 J/cm’ and a spectral resolution of 0.3 A. Assignment of the spectral lines [7] and emission bands [8] is made according to standard tabulation. In
80
C. Girnult et al. / Chrrracterizatmn
ofKrF
NANOMETERS (al
NANOMETERS (b)
NANOMETERS (ci Fig. 2. High-resolution spectrum (0.3 A) of the KrF laser-Induced plasma plume on an YBaCuO superconducting target. Observations are performed at d = 2 mm using a boxcar gate width of 40 ns located 180 ns after the beginning of the interaction.
Irr.rer-Induced
(639.25 and 640.15 nm) and the A’SX’Z system of BaO (603.96 nm) are observed. Moreover. transitions involving atomic oxygen are present at the wavelengths 777.2, 777.4 and 777.54 nm, as shown in fig. 2c. All these species have been observed in plasma slices located at various distances from the target. To determine the influence of oxygen atmosphere on the laser-induced plasma characteristics, the same experiment has been carried out with different oxygen pressure values in the chamber. From a qualitative point of view. we observe that an increase of the O2 pressure value leads to a color change of the plasma plume which from a green white colour becomes reddish. The emitted spectra are similar to those presented in fig. 2 but we notice. as reported previously by Wu et al. [9], that the emission intensities of almost all the detected lines and bands are significantly enhanced under oxygen atmosphere. Analysis of the temporal evolution of the line or band intensities emitted by a given slice of the plume has been performed for ail the detected species as a function of the distance d of the observed slice from the YBaCuO target surface. As examples, the temporal evolution of the Ba* line at 553.55 nm is shown in fig. 3 for several values of d and for an oxygen pressure of 5 x 10 ’ mbar. We observe that the relaxation duration increases with the value of d and depends on the detected species. For each ablated product, the relaxation is assumed to be well described by a monoexponential decay f(r)
the range 350-550 nm (fig. 2a) the spectrum points out the presence of all the elementary products (neutral and ionized) of the YBaCuO target. namely, Y * (404.76, 408.37, 410.24, 412.83, and 414.39 nm), Y+ * (377.4 nm), Ba* (553.55 and 577.77 nm), Baf * (455.4 nm), Cu* (578.21 nm), and Cut* (490.97 and 493.16 nm). Between 550 and 650 nm (fig. 2b) emission bands of diatomic molecules emanating from the A2n-X2Z system of YO (597.2, 598.77, 600.36 and 616.51 nm), the A’SX211 system of CuO
plustnrrplume uhoue YBuC‘uO
= I,, exp( -ct).
where r, is the characteristic decay constant of the species i [lo]. Moreover, it is clear from these kinetics that the time location of the maximum emission intensity is proportional to the distance d, leading to an estimation of the time-of-flight velocity from the target surface to the observed slice for each considered species. Thus, the velocity components V, of the detected species in a direction normal to the YBaCuO target surface have been studied versus the ambient oxygen pressure. As shown in fig. 4 for a laser fluence of 3 J/cm’. the ejection
C. Girault et al. / Characterization 160
of KrF laser-induced plasma plume above YBaCuO
n
1
I
r Pq
200
= 5.10-2
600
400
TIME
1
I
1
mhar
800
1000
(ns)
Fig. 3. Temporal evolution of the 553.55 nm Ba* spectral line intensity for different values of the distance d from the YBaCuO target and for an oxygen pressure of 5 X lo-* mbar. LP indicates the laser pulse temporal location.
velocities VL of the ablated products are very sensitive to the oxygen pressure in the chamber. At lo-” mbar the measured velocities VL are of the same order for all the detected species (1061.3 X lo6 cm/s). This indicates that the ejection of the different products during the interaction with the KrF laser beam, in our experimental conditions, mainly results from an ablative decomposias suggested by different tion phenomenon, authors [11,12] rather than from a thermal process. It appears that the velocities of atomic species (neutral and ionized) remain quite constant from vacuum up to an oxygen pressure of about lop2 mbar and decrease rapidly beyond, probably owing to hydrodynamical effects induced by the ambient gas. The decrease of the velocities with the oxygen pressure is more regular for the diatomic mole-
cules and in a first approximation the form
follows a law of
v, = k Log(f’02> for YO, BaO and 0: molecules. This spectroscopic study has been followed by the first attempts to fabricate in our laboratory superconducting thin layers. The films have been deposited on (100) SrTiO, substrates in an oxygen residual atmosphere, with a laser fluence of 2.5 J/cm’. After deposition, the heating was switched off and the film slowly cooled to room temperature under an oxygen pressure of 30 mbar. The film thickness ranges from 6000 to 7000 A. The composition of the films has been investigated by EDAX (electron diffraction X-ray analysis). With the above experimental conditions de-
duced from found are stoichiometry
the spectroscopic study. the values reproducibly close to the ideal (1 : 2 : 3). Resistance is determined
1
I
1
I
I
me-
Ba+
1.6
_
.Y+
lclJ+
‘-‘---------‘-A
-
12x9:
;
v
I)
ct -_-_-----__-__
____ 0
8.Kq
i
0
Fig.
5. Temperature YBaCuO
PRESSURE
I
1
,
,
!
(mbar 1
I
I
1
I
woe t
Ba +Y
/
j
the
resistlwty
film deposited on a SrTiO,
dependence
of
suhstratc.
for
AII
by the standard four-point probe technique. The layers thus obtained are superconducting without any post-deposition annealing. A typical example is given in fig. 5: the superconducting phase is obtained with an onset critical temperature of X8 K and a relatively sharp transition.
4. Conclusion 0 10-e
b 4
/
1
10-2
PRESSURE
I
1
I
104
1
1
(mbar)
1
I
1
,
6dO
10 10
*Vi-l
.GO “06
t\. ‘:I:, --___ i
0
0
lo-6
!
1
/
1
10-4
PRESSURE
;a___
:,
C
1
1
1 ~t%ar~
Fig. 4. Velocity component I’1 normal to the target surface as a function of the oxygen ambient pressure.
A systematic spectroscopic investigation of the plasma plume created by the interaction of a KrF laser beam and an YBaCuO superconducting target shows emissions of neutral and ionized atomic species and of diatomic oxides (YO, BaO and CuO). Time- and spatially resolved spectroscopic measurements lead to estimations of the velocity component V, normal to the target surface of the ablated species as a function of the ambient oxygen pressure, pointing out a strong decrease of V, for elemental species for PC,, > lo-’ mbar. First results on the preparation of superconducting YBa ?Cu ?O, , thin films are presented but these investigations are in an early stage. Further experiments are in progress in order to improve the characteristics of the resistive transition.
C. Girault et al. / Characterizurron of Krt;‘ laser-induced plasma plume above YBaCuO
Acknowledgment
This work is partly contract No. 88-084.
supported
by DRET
under
References
PI B. Oh, M. Naito,
P. Rosenthal, S. Amason, R. Barton. M.R. Beasley. T.H. Geballe. R.H. Hammond and A. Kapitulnik. Appl. Phys. Lett. 51 (1987) 852. PI M. Hong, S.H. Liou, J. Kwo and B.A. Davidson, Appl. Phys. Lett. 51 (1987) 694. [31 R.J. Lin, J.H. Kung and P.T. Wu. Physica C 153-155 (1988) 796. [41J. Kwo, T.C. Hsien, M. Hong, R.M. Fleming, S.H. Liou. B.A. Davidson and L.C. Feldman, MRS Symp. Proc. 99 (1987) 339. X.D. Wu. S.A. Shahem. N. [51 D. Dijkamp. T. Venkatesan.
83
Jisrawi, Y.H. Minke, W.L. McLean and M. Croft, Appl. Phys. Lett. 51 (1987) 619. [61 H.V. Habermeier and G. Mertena, Physica C 153 (1988) 1429. and N.S. Sventitskii. Tables of Spectral [71A.R. Striganov Lines of Neutral and Ionized Atoms (IFI-Plenum. New York, 1968). of PI R.W.B. Pearse and A.G. Gaydon, The Identification Molecular Spectra. 4th ed. (Chapman and Hall. London, 1976). [91 X.D. Wu, B. Dutta, MS. Hegde. A. Inam, T. Venkatesan. E.W. Chase, C.C. Chang and R. Howard, Appl. Phys. Lett. 54 (1989) 179. and A. Catherinot, [lOI C. Girault, D. Damiani. J. Aubreton Appl. Phys. Lett. 54 (1989) 2035. P. Mattocks. L. Shi. X.W. Wang, S. [Ill H.S. Kwok, Witanachchi, Q.Y. Ying, J.P. Zheng and D.T. Shaw. Appl. Phys. Lett. 52 (1988) 1825. X.D. Wu. A. Inam and J.B. Watchman. u21 T. Venkatesan, Appl. Phys. Lett. 53 (1988) 1193.