- Email: [email protected]
The interaction of ambient background gas with a plume formed in pulsed laser deposition
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applied
surface science
ELSEVIER
Applied Surface Science
I I5 (1997) 279-284
The interaction of ambient background gas with a plume formed in pulsed laser deposition X.Y. Chen *, S.B. Xiong, Z.S. Sha, Z.G. Liu Nrrtionul
L.aboratop
of SolidState
Microstructures
Received
and Department
of Phy.Gcs. Nanjing
10 June 1996: accepted 2 December
Uniwmity.
Nmjin,y
-7IMF.3.
Chitul
1996
Abstract The propagation of KrF excimer laser-produced plasmas from Pb(Zr,,,,Ti,,,)O, has been studied with emphasis on topics relevant to the interaction of a plume with ambient gas in oxide film growth by pulsed laser deposition. A gated CCD was employed to investigate the overall shape and propagation of the laser plasma in different background gases. It is found that the plume image in both 20 Pa 0, and Ar is spherical. With increasing 0, pressure to 60 Pa, the plumes are prolonged along the target normal direction and suppressed in the direction parallel to the target surface. Above 60 Pa, the plumes are suppressed along both directions. When the ambient gas is Ar, the changes of the plumes are contradictious. The similarity of plume shape in 20 Pa 0, and Ar has been interpreted as that the interaction of ablated species with ambient gas is mainly elastic. The differences are thought to be due to the interaction of ablated species with ambient gas. which is reactive in 0, and elastic in Ar. It was proposed that the elastic scattering of ablated species by background gas leads to the nonstoichiometry of the deposited film, while the reactive collision of ablated species with ambient gas is favorable to reduce the nonstoichiometry and to enhance the oxygen incorporation in the film. It was proposed that the propagation of the ablated species can be divided into three regimes.
1. Introduction Pulsed growth
laser
ferroelectric (LSCO)
deposition
of high-T, oxides and
(PLD)
for
superconductors, such
Pb(Zr,Ti,
as YBCO,
_.,)O,
in situ
film
conductive
and
La,,5Sr,,,Co0,
(PZT),
requires
the
in order to obtain the high quality films. The ejected plume of the target material is scattered and attenuated by the background oxygen, forming oxides and possibly clusters which may aid oxygen incorporapresence
of background
. Corresponding author. [email protected].
oxygen
Fax:
0169~3332/97/$17.00 Copyright PI/ 80169~4332(96)01087-2
during
deposition
+ 86-25-3300535:
e-mail:
tion in the growing films [I ,Z]. The interaction of the ablation plume with ambient background gases is not only a complex hydrodynamic phenomenon but also a chemical process which is often ignored due to its complexity. So the complete understanding of plume dynamics should be based on both hydrodynamics and chemical dynamics. The chemical aspect will become to be crucial when the ablated material composes transition metal elements which have several relative stable chemical value states. The chemical value state in as grown film can be changed by changing the interaction dynamics condition of ablated species with the ambient gas during plume propagation. For example, V02(VJ+) has been de-
0 1997 Elsevier Science B.V. All rights reserved
280
X. Y. Chen et al. /Applied
Surface Science I15 (1997) 279-284
posited by ablation of V,0,(V3’) target [3]. It is believed that both elastic scattering and reactive collision of ablated species with ambient gas molecules can take place. What the elastic scattering and reactive collision bring out on the component and structure of film is unclear, though the hydrodynamic properties of the plume can be described quantitatively by using the drag model and shock wave model in agreement with experiments [4-61. For example, how the oxygen is incorporated in the film is not clear at present. In this paper, the propagation of a laser-induced plume will be studied with emphasis on topics relevant to elastic scattering and reactive collision of the plume with ambient gas. The effect of the interaction of the plume with ambient gas on the film component will be discussed.
2. Experimental The apparatus used in this work is shown in Fig. 1. A PZT target mounted on the rotating holder was ablated by a KrF excimer laser in a vacuum chamber containing oxygen or argon gas background, respectively. The target was prepared by the usual ceramic processing method and is 20 mm in diameter and 2 mm thick. Before ablation, the chamber was pumped to 10d4 Pa by a molecular pump and then filled with the desired gas to the needed pressure. The CCD camera controlled by a computer was used to take images of the plume in the form of time-integrated photographs of plume emission on the side-view. The laser fluence was about 15 J/(cm’ pulse).
Fig. 1. Experimental apparatus laser produced plasma plume.
used for getting
CCD image of
3. Results and discussions In vacuum, the ablation of PZT results in a forward-directed plume, with a spatial distribution exhibiting cos’Y3 (n > 4) symmetry about the target normal [7], as shown in Fig. 2(b). The slightly introduced oxygen (and Ar) with 2 Pa pressure results in the increase of the plume length, as shown in Fig. 2(c). This surprising result suggests the oxygen with about 2 Pa pressure does not impede the plume expansion as a described drag model [5]. In order to understand the question, the formation and motion of plume plasma have to be considered. When an intense laser beam is irradiated on a target, the target is melted, vaporized, and transformed into plasma simultaneously. The plasma will constantly absorb the laser energy until the end of the laser pulse. So the temperature of the plasma rises continuously to a highest value T,, at the end of the laser pulse, depending on the absorbed laser energy. Secondly, the plasma expands continuously to a volume V, at the end of laser pulse. V, is less in ambient gas than in vacuum due to the confinement of ambient gas in the expansion during the laser pulse. So the density of the plasma is larger in ambient gas than in vacuum. Therefore, the plasma in ambient gas can absorb more laser energy. Thus, the T,, in ambient gas is higher than in vacuum. Singh et al. have proposed that the expansion of the plume can be clarified into two stages [8]. One is the isothermal expansion during the laser pulse, the other is the adiabatic expansion after the laser pulse. The plasma with higher T, can eject to further distance after the laser pulse. At such a low O2 or Ar pressure, the plasma expands as an ensemble [9]. So only the species on the plasma surface react with the ambient gas. The great majority of plume species are in the plasma and do not react with the ambient gas. Thus the interactions of plume with ambient gas are negligible. Increasing the ambient gas pressure to about 20 Pa, the shape of the plume changes gradually to be spherical from the highly forward-directed shape in vacuum. The plumes in 20 Pa Ar show to be quite similar to that in 20 Pa O,, as shown in Fig. 2(d) and (e>. Since the scattering of ablated species by ambient gas molecules in Ar is elastic because the Ar is inert, it can be inferred from the plume shape simi-
X. Y. Chen et al. /Applied
281
Surface Science I I.5 (1997) 270-284
(a’ KrF laser beam
relative to the ablated species. The later assumption is based on the fact that the speed of ablated species (about 105-lo6 cm/s) is much larger than that of the Ar atom at room temperature. Under these two assumptions, the mean free path A of ablated species is: A=
kT (I)
%-(r,,+r,)y
where the T and p are the temperature and pressure of ambient gas, respectively, rAr and I’~ are the effective radii of the Ar atom and ablated metal. respectively, and k is the Boltzmann constant. From Eq. (I), the mean free path of ablated metal atoms (Pb. Ti, Zr) in 20 Pa Ar at room temperature is about 0.6 cm. So the metal atoms are scattered 8 times averagely before they arrive on the substrate, if the substrate is 5 cm away from the target. In fact, the
-o-Fig. 2. (a) Geometry of ablation. (h-i) Pa 02,
CCD image
Win Ar
of plume produced by laser
plume images from PZT target in (b) vacuum. Cc) 7
Cd) 20 Pa Oz. (e) 20 Pa Ar. (f) 40 Pa Oz. (g) 40 Pa, (h)
IS0 Pa 0,.
nc ma
and (i) I50 Pa Ar. respectively.
. ---_-
larity that scattering of ablated species by oxygen molecular is mostly elastic at present pressure. With the further increase of the ambient gas pressure to above 40 Pa. the plume images in O1 and Ar showed a remarkable difference, as shown in Fig. 2(f, g) and (h, i). In 40 Pa Ar, the plume is slightly suppressed along the target normal direction and prolonged in the direction parallel to the target surface, compared with the plume in 20 Pa Ar, as shown in Fig. 3. This can be interpreted in terms of elastic scattering of ablated species with ambient gas molecules. It is assumed that the ablated species is in the form of atoms and the Ar atoms are static
-.b
30
I
_1t , , , , , , , , , , 0
50
100
150
200
250
Ambient Gas Pressure(Pa) Fig. 3. The
plume length
(15)and
width (W)
which is defined a.\
drawing in insert versus the gas pressure at given laser fluence in (at 0:
and (h)
Ar. The
curves (a. n ). (b. 0)
and width in O2 and Cc, 0 ), Cd, 0) in Ar. respectively. guide the eyes.
are the plume length
is the plume length and width
The solid and dashed curves are plotted to
282
X. Y. Chen et al. /Applied
(4
(b)
Surface Science I15
Cc)
Fig. 4. The change tendency which are expressed by an arrow head (A ) of a plume with increasing the ambient gas pressure in (a) 20-60 Pa 0,. (b) above 60 Pa 0,. and (c) above 20 Pa Ar, respectively. The dashed circles in (a) and (c) stand for the plume image both in 20 Pa Ar and 0,. (c) shows the plume expand to the flank side of the target surface and suppressed along the target normal direction, (b) and (c) show that the plume is suppressed in the direction parallel to the target surface with increasing the ambient gas pressure. The former means the more ablated species are scattered to flank side and the later is on the contrary with increasing the ambient gas pressure.
ablated species was composed of some clusters with larger radii. They are scattered more times. This means elastic scattering is an important interaction during the plume propagation. The highly forwarddirected species are scattered by the Ar atoms to deviate from the original target normal direction and flight to flank side. This results in increasing plume width (IV) and decreasing plume length CL). It can be expected that the plume length is reduced and the plume width is increased further with increasing Ar pressure, because the higher Ar pressure means the increased elastic scattering of ablated species with Ar atoms. This effect can be clearly seen by comparing Fig. 2(g) with (i), as well as curve (c) and (d) shown in Fig. 3. Fig. 3 shows the L slightly decreases and W increases monotonously with increasing Ar pressure, when the Ar pressure is larger than 20 Pa. Fig. 4(c) schematically shows the change tendency of plume with increasing of Ar pressure. The increasing plume width means that some of the ablated species are scattered to the flank aside of the target surface by the ambient gas. Thus, some of them cannot arrive on the substrate which is often situated straight ahead of the target. The scattering angle is different for different kind of species with different mass and radii. This leads to the nonstoichoimetry of the growing film. It is clear that the mass ratio of ablated species and ambient gas molecules plays an important role in deciding the scattering angle. So it is expected that the lighter the
f 1997)
279-284
ablated species are, the more the ablated species are lost during plume propagation. The typical examples are the Li loss in LiNbO, films and K loss in KNbO, and K(Ta, Nb , _ J 10, films prepared by PLD [lo- 121. These experiments proved that the K loss actually originated from the interaction of ablated species with 0, molecules. This elastic scattering even result in backscattering of ablated species, especially for those atoms which are lighter than oxygen molecules. This result will be described elsewhere. In 40 Pa Oz cases, the plumes are prolonged along the target normal direction and suppressed in the direction parallel to the target surface from 20 Pa to about 60 Pa. Above 60 Pa, the plume is suppressed along both directions, as shown in Figs. 2 and 3. The change tendency of plume with increasing 0, pressure is also schematically plotted in Fig. 4(a) and (b). It can also be seen clearly that the spherical shape of a plume at 20 Pa changes to a forward-directed shape at pressures above 40 Pa, as shown in Fig. 2(f) and (i). The forward-directed shape of a plume means less ablated species are scattered to the flank side of the target surface and so there is less loss of ablated species, as discussed above. From the scattering view point, if the collisions of ablated species with ambient gas are completely inelastic or chemically reactive, the ablated species and ambient gas will merge into a single monoxide or multioxide molecule. The as-formed heavier monoxides or multioxides will be scattered less to the flank side, compared to lighter species direct ablated from the target. These differences of plume image in 0, and Ar with same pressure, particularly the contrary change tendency of plume with increasing the ambient gas pressure, suggests that the interactions of plume with background gas are different in 0, and Ar. This indicates the interaction is inelastic or reactive in high pressure Oz. The effect that the reactive collisions only take place at relative high pressure can be interpreted by means of a shock (blast) wave model [13- 171. In PLD, the ejected material due to ablation of the target acts as a piston which pushes ahead the gas in front of it [ 13- 171. Behind the piston is a partial vacuum and ahead the piston the gas becomes increasingly compressed and eventually forms a shock wave. In order to form the shock wave the piston has to be accelerated many times of mean free path of
X. Y. Chen et al. /Applied Surface Science I15
ambient gas. Thus, with lower ambient gas pressure, the piston will need to be accelerated in a longer distance to obtain a high enough compressed gas, i.e. shock wave. If the ambient gas pressure is too low, the shock wave cannot be formed or is too weak. At given laser fluence, target and ambient gas. the shortest distance that shock waves can be formed is inversely proportional to the ambient gas pressure [ 131. So, an high enough ambient gas pressure is necessary in order to generate a strong enough shock wave between target and substrate. A strong shock wave means a highly compressed ambient gas layer with a quite high temperature as follows [ 181:
where T, is the temperature of ambient gas, M is the math number of the shock wave, being able to obtain 25 in early stages of the expansion of the plume, and y is the specific heat ratio of the ambient gas. From Eq. (2) it follows that the temperature of the shock heated oxygen will be up to 2 X 10” K. At this high temperature, the percentage (Y of 0, dissociated molecules can be written as follows [ 131:
a2
-ZZ 1-a
M3/5) 0
Mrr/~I,~Jk;T
7
1 . -go0 . _ . e-L’/hT goo, tzo.
(3) where MO is the mass of the 0 atom, v is the frequency of vibration of the 0, molecule, I,, is the rotation inertia of the 0, molecule, g,, and g,, are the statistical weights of the 0 atom and the 0: molecule in the ground state, ~1~. is the molecular number in volume units, U is the dissociation energy of a O? molecule. T is the temperature of the shock wave layer, and k is the Boltzmann constant. From Eqs. (2) and (3) it can be estimated that at least 95% of the 0, molecules in a shock wave layer are dissociated at early stages of the shock wave formation. Thus, the ablated metal species can react with 0 atoms that come from dissociation of 0, in a shock wave layer [ 191. If the 0, molecules cannot be dissociated into 0 atoms. it is will be impossible that the ablated species react with the ambient oxygen except for a small number of high energy species [ 191. It should be pointed out that the different metal species have a different chemical activity to react
C1997) 27%-784
283
with 0 atoms and so different effects on reaction collision of ablated metal species with an ambient gas. This is out of the present consideration. It has been shown that oxygen deficiency of the oxide film is critical even when a stoichiometric target is used. while the metal species can keep stoichiometry [20]. This indicates oxygen in the target is lost during plume propagation, while the transfer of metal species is complete. It is often found that the oxygen deficiency in oxide films is a common problem. especially in preparing nonstoichiometric oxides such as La,- ,Sr,CoO,_J, YBa-,Cu,O, ~, etc. A difficulty to achieve the high quality films is how to increase the oxygen content in the films. It has been reported that more than 40% of the oxygen in the film comes from the target and about of 60% from the ambient gas at deposition [21]. This means a great quantity of oxygen in films comes from the ambient gas. It can be inferred that the chemical reaction of ablated with the ambient gas as discussed above is the way to incorporate the ambient oxygen into the films. This oxygen incorporation mechanism may play a dominant role in making the as grown film have enough oxygen. According to the above discussions, the transportation of ablated species can be divided into three regimes under condition of strong shock wave formation. The first regime is from the target surface to the position where the oxygen dissociation due to blast wave begins to take place. In this regime, the interaction of ablated species with ambient gas is mainly elastic. The second regime is after the first regime to the position at which the shock wave begins to turn into the sound wave. In this regime, the reactive collision is especially important. The ablated metal species are partially oxidized into metal oxides through reactive collisions with the ambient gas. After the second regime is the last regime. In this regime. the materials which come directly from target or formed reactive collisions during the second regime began to diffuse freely under gravity. In the case of the elastic interaction of ablated species with the ambient gas. the motion of released material can be described quantitatively by drag model. In the case of shock wave formation. the motion of released material can be described by shock wave model. So there always exist a drag model regirne (elastic scattering regime) firstly and shock wave (inelastic
284
X.Y. Chen et al./Applied
Surface Scimce 115 (1997) 279-284
collision regime) secondly during the plume propagation, if the shock wave can be formed. This is in agreement with the Geohegan results [5]. It showed that there were two regimes. The first regime was described by the drag model and the second regime by the shock wave model. According to the forming conditions of shock waves, the high enough laser fluence and ambient gas pressure are both necessary. If one of them cannot be met, the shock cannot be formed or is too weak to dissociate the oxygen molecules. In these cases, there will only exist elastic collisions during the propagation of plume.
4. Conclusions This paper shows that the plume images from the PZT target in PLD are similar in O2 and Ar with pressures below 20 Pa, but remarkably different with pressures above 30 Pa. The similarities can be explained as that the main interaction of ablated species with the ambient gas is elastic. The differences are interpreted as that the interaction between ablated species with ambient gas is reactive in 0, and elastic in Ar. The changes of plume image in ambient gas with different pressures indicate that the elastic scattering results in the ablated species being scattered out of the substrate, and reactive collisions reduce this effect. So it is thought that elastic scattering leads to the non-stoichiometry of films, while the reactive collisions reduce the nonstoichiometry and enhance the oxygen incorporation. It is also proposed that the propagation of the ablated species in ambient gas is divided into three regimes under the condition of shock wave generation: (1) an elastic scattering regime which is described by drag model close to the target at first, and then (2) a reactive collision regime that is described by blast wave model, and finally (3) a sound wave regime (free diffusion model).
Acknowledgements This work was financially supported by the National Natural Science Foundation of China.
References ill D.B. Geohegan.
in: Laser Ablation of Electronic Materials: Basic Mechanics and Applications, Eds. E. Fogarassy and S. Lazare (Elsevier, Amsterdam, 1992) p. 73. l21 R.A. Lindley, R.M. Gilenbach, C.H. Ching and J.S. Lash, J. Appl. Phys. 76 (1994) 5457. 131 D.H. Kim and H.S. Kwok, Appl. Phys. Lett. 65 (1994) 3188. [41 W.K.A. Kumuduni, Y. Nakayama, Y. Nakata. T. Okada and M. Maeda. J. Appl. Phys. 74 (1993) 7510. [51 D.B. Geohegan, Thin Solid Films 220 (1992) 138. and Appli[61 D.B. Geohegan, in: Laser Ablation-Mechanism cations (Springer. Berlin. 199 I) p. 28. [71 T. Venkatesan et al., Appl. Phys. Len. 53 (1988) 1431. [81 R.K. Singh, O.W. Holland and J. Narayan, J. Appl. Phys. 68 (1990) 233. 191 J. Gozano. F. Vega and C. Afonso. J. Appl. Phys. 77 1995) 6588. 1101 Y. Shibata. K. Kaya, K. Akashi. M. Kanai, T. Kawai and S. Kawai. Appl. Phys. Lett. 61 (1992) 1000. [III C. Zaldo. D.S. Gill, R.W. Eason. J. Mendioda and P.J. Chandler, Appl. Phys. Lett. 65 ( 1994) 502. Appl. [I21 S. Yilmaz, T. Ventaken and R. Gerhard-Multhaupt, Phys. Lett. 58 (1991) 2479. 1131 Y.B. Zel’dovich and Y.P. Raizer, Physics of Shock Waves and High-Temperature Hydrodynamics Phenomena (Academic Press. New York. 1967). [141 P.E. Dyer and J. Sidu. J. Appl. Phys. 64 (1988) 4657. 1151 B. Steverding, J. Appl. Phys. 45 (1974) 3507. ll61 R.B. Hall. J. Appl. Phys. 40 (1969) 1941. [171 D.A. Freiwald and R.A. Axford. J. Appl. Phys. 46 (1975) 1171. lts1 P.E. Dyer, A. Issa and P.H. Key, Appl. Surf. Sci. 46 (1990) 89. t191 R.W. Dreyfus, Appl. Surf. Sci. 86 (1995) 29. DOI M. Hirdtani. K. Imagawa and K. Takgi, J. Appl. Phys. 78 (1995) 4258. A.N. Smal, B.A. Kdesov and V.P. Dll M.R. Predtechensky, Ivanov, Appl. Supercond. 1 (1993) 2005.
c!SJ
..
..I. 5.: ;:ii’,:~~:~:i,:ii,:~~.~ ,.,. ,., ,. _,,.,:,:; ..,. ->_..‘::.?‘x+.:.::j/ .A.. .. . .. . .,.,.,., ,. ,~ ;:?@yy
. .. . ..
..,)....)
.. . . . .A:.:
,.>
applied
surface science
ELSEVIER
Applied Surface Science
I I5 (1997) 279-284
The interaction of ambient background gas with a plume formed in pulsed laser deposition X.Y. Chen *, S.B. Xiong, Z.S. Sha, Z.G. Liu Nrrtionul
L.aboratop
of SolidState
Microstructures
Received
and Department
of Phy.Gcs. Nanjing
10 June 1996: accepted 2 December
Uniwmity.
Nmjin,y
-7IMF.3.
Chitul
1996
Abstract The propagation of KrF excimer laser-produced plasmas from Pb(Zr,,,,Ti,,,)O, has been studied with emphasis on topics relevant to the interaction of a plume with ambient gas in oxide film growth by pulsed laser deposition. A gated CCD was employed to investigate the overall shape and propagation of the laser plasma in different background gases. It is found that the plume image in both 20 Pa 0, and Ar is spherical. With increasing 0, pressure to 60 Pa, the plumes are prolonged along the target normal direction and suppressed in the direction parallel to the target surface. Above 60 Pa, the plumes are suppressed along both directions. When the ambient gas is Ar, the changes of the plumes are contradictious. The similarity of plume shape in 20 Pa 0, and Ar has been interpreted as that the interaction of ablated species with ambient gas is mainly elastic. The differences are thought to be due to the interaction of ablated species with ambient gas. which is reactive in 0, and elastic in Ar. It was proposed that the elastic scattering of ablated species by background gas leads to the nonstoichiometry of the deposited film, while the reactive collision of ablated species with ambient gas is favorable to reduce the nonstoichiometry and to enhance the oxygen incorporation in the film. It was proposed that the propagation of the ablated species can be divided into three regimes.
1. Introduction Pulsed growth
laser
ferroelectric (LSCO)
deposition
of high-T, oxides and
(PLD)
for
superconductors, such
Pb(Zr,Ti,
as YBCO,
_.,)O,
in situ
film
conductive
and
La,,5Sr,,,Co0,
(PZT),
requires
the
in order to obtain the high quality films. The ejected plume of the target material is scattered and attenuated by the background oxygen, forming oxides and possibly clusters which may aid oxygen incorporapresence
of background
. Corresponding author. [email protected].
oxygen
Fax:
0169~3332/97/$17.00 Copyright PI/ 80169~4332(96)01087-2
during
deposition
+ 86-25-3300535:
e-mail:
tion in the growing films [I ,Z]. The interaction of the ablation plume with ambient background gases is not only a complex hydrodynamic phenomenon but also a chemical process which is often ignored due to its complexity. So the complete understanding of plume dynamics should be based on both hydrodynamics and chemical dynamics. The chemical aspect will become to be crucial when the ablated material composes transition metal elements which have several relative stable chemical value states. The chemical value state in as grown film can be changed by changing the interaction dynamics condition of ablated species with the ambient gas during plume propagation. For example, V02(VJ+) has been de-
0 1997 Elsevier Science B.V. All rights reserved
280
X. Y. Chen et al. /Applied
Surface Science I15 (1997) 279-284
posited by ablation of V,0,(V3’) target [3]. It is believed that both elastic scattering and reactive collision of ablated species with ambient gas molecules can take place. What the elastic scattering and reactive collision bring out on the component and structure of film is unclear, though the hydrodynamic properties of the plume can be described quantitatively by using the drag model and shock wave model in agreement with experiments [4-61. For example, how the oxygen is incorporated in the film is not clear at present. In this paper, the propagation of a laser-induced plume will be studied with emphasis on topics relevant to elastic scattering and reactive collision of the plume with ambient gas. The effect of the interaction of the plume with ambient gas on the film component will be discussed.
2. Experimental The apparatus used in this work is shown in Fig. 1. A PZT target mounted on the rotating holder was ablated by a KrF excimer laser in a vacuum chamber containing oxygen or argon gas background, respectively. The target was prepared by the usual ceramic processing method and is 20 mm in diameter and 2 mm thick. Before ablation, the chamber was pumped to 10d4 Pa by a molecular pump and then filled with the desired gas to the needed pressure. The CCD camera controlled by a computer was used to take images of the plume in the form of time-integrated photographs of plume emission on the side-view. The laser fluence was about 15 J/(cm’ pulse).
Fig. 1. Experimental apparatus laser produced plasma plume.
used for getting
CCD image of
3. Results and discussions In vacuum, the ablation of PZT results in a forward-directed plume, with a spatial distribution exhibiting cos’Y3 (n > 4) symmetry about the target normal [7], as shown in Fig. 2(b). The slightly introduced oxygen (and Ar) with 2 Pa pressure results in the increase of the plume length, as shown in Fig. 2(c). This surprising result suggests the oxygen with about 2 Pa pressure does not impede the plume expansion as a described drag model [5]. In order to understand the question, the formation and motion of plume plasma have to be considered. When an intense laser beam is irradiated on a target, the target is melted, vaporized, and transformed into plasma simultaneously. The plasma will constantly absorb the laser energy until the end of the laser pulse. So the temperature of the plasma rises continuously to a highest value T,, at the end of the laser pulse, depending on the absorbed laser energy. Secondly, the plasma expands continuously to a volume V, at the end of laser pulse. V, is less in ambient gas than in vacuum due to the confinement of ambient gas in the expansion during the laser pulse. So the density of the plasma is larger in ambient gas than in vacuum. Therefore, the plasma in ambient gas can absorb more laser energy. Thus, the T,, in ambient gas is higher than in vacuum. Singh et al. have proposed that the expansion of the plume can be clarified into two stages [8]. One is the isothermal expansion during the laser pulse, the other is the adiabatic expansion after the laser pulse. The plasma with higher T, can eject to further distance after the laser pulse. At such a low O2 or Ar pressure, the plasma expands as an ensemble [9]. So only the species on the plasma surface react with the ambient gas. The great majority of plume species are in the plasma and do not react with the ambient gas. Thus the interactions of plume with ambient gas are negligible. Increasing the ambient gas pressure to about 20 Pa, the shape of the plume changes gradually to be spherical from the highly forward-directed shape in vacuum. The plumes in 20 Pa Ar show to be quite similar to that in 20 Pa O,, as shown in Fig. 2(d) and (e>. Since the scattering of ablated species by ambient gas molecules in Ar is elastic because the Ar is inert, it can be inferred from the plume shape simi-
X. Y. Chen et al. /Applied
281
Surface Science I I.5 (1997) 270-284
(a’ KrF laser beam
relative to the ablated species. The later assumption is based on the fact that the speed of ablated species (about 105-lo6 cm/s) is much larger than that of the Ar atom at room temperature. Under these two assumptions, the mean free path A of ablated species is: A=
kT (I)
%-(r,,+r,)y
where the T and p are the temperature and pressure of ambient gas, respectively, rAr and I’~ are the effective radii of the Ar atom and ablated metal. respectively, and k is the Boltzmann constant. From Eq. (I), the mean free path of ablated metal atoms (Pb. Ti, Zr) in 20 Pa Ar at room temperature is about 0.6 cm. So the metal atoms are scattered 8 times averagely before they arrive on the substrate, if the substrate is 5 cm away from the target. In fact, the
-o-Fig. 2. (a) Geometry of ablation. (h-i) Pa 02,
CCD image
Win Ar
of plume produced by laser
plume images from PZT target in (b) vacuum. Cc) 7
Cd) 20 Pa Oz. (e) 20 Pa Ar. (f) 40 Pa Oz. (g) 40 Pa, (h)
IS0 Pa 0,.
nc ma
and (i) I50 Pa Ar. respectively.
. ---_-
larity that scattering of ablated species by oxygen molecular is mostly elastic at present pressure. With the further increase of the ambient gas pressure to above 40 Pa. the plume images in O1 and Ar showed a remarkable difference, as shown in Fig. 2(f, g) and (h, i). In 40 Pa Ar, the plume is slightly suppressed along the target normal direction and prolonged in the direction parallel to the target surface, compared with the plume in 20 Pa Ar, as shown in Fig. 3. This can be interpreted in terms of elastic scattering of ablated species with ambient gas molecules. It is assumed that the ablated species is in the form of atoms and the Ar atoms are static
-.b
30
I
_1t , , , , , , , , , , 0
50
100
150
200
250
Ambient Gas Pressure(Pa) Fig. 3. The
plume length
(15)and
width (W)
which is defined a.\
drawing in insert versus the gas pressure at given laser fluence in (at 0:
and (h)
Ar. The
curves (a. n ). (b. 0)
and width in O2 and Cc, 0 ), Cd, 0) in Ar. respectively. guide the eyes.
are the plume length
is the plume length and width
The solid and dashed curves are plotted to
282
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(b)
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Cc)
Fig. 4. The change tendency which are expressed by an arrow head (A ) of a plume with increasing the ambient gas pressure in (a) 20-60 Pa 0,. (b) above 60 Pa 0,. and (c) above 20 Pa Ar, respectively. The dashed circles in (a) and (c) stand for the plume image both in 20 Pa Ar and 0,. (c) shows the plume expand to the flank side of the target surface and suppressed along the target normal direction, (b) and (c) show that the plume is suppressed in the direction parallel to the target surface with increasing the ambient gas pressure. The former means the more ablated species are scattered to flank side and the later is on the contrary with increasing the ambient gas pressure.
ablated species was composed of some clusters with larger radii. They are scattered more times. This means elastic scattering is an important interaction during the plume propagation. The highly forwarddirected species are scattered by the Ar atoms to deviate from the original target normal direction and flight to flank side. This results in increasing plume width (IV) and decreasing plume length CL). It can be expected that the plume length is reduced and the plume width is increased further with increasing Ar pressure, because the higher Ar pressure means the increased elastic scattering of ablated species with Ar atoms. This effect can be clearly seen by comparing Fig. 2(g) with (i), as well as curve (c) and (d) shown in Fig. 3. Fig. 3 shows the L slightly decreases and W increases monotonously with increasing Ar pressure, when the Ar pressure is larger than 20 Pa. Fig. 4(c) schematically shows the change tendency of plume with increasing of Ar pressure. The increasing plume width means that some of the ablated species are scattered to the flank aside of the target surface by the ambient gas. Thus, some of them cannot arrive on the substrate which is often situated straight ahead of the target. The scattering angle is different for different kind of species with different mass and radii. This leads to the nonstoichoimetry of the growing film. It is clear that the mass ratio of ablated species and ambient gas molecules plays an important role in deciding the scattering angle. So it is expected that the lighter the
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ablated species are, the more the ablated species are lost during plume propagation. The typical examples are the Li loss in LiNbO, films and K loss in KNbO, and K(Ta, Nb , _ J 10, films prepared by PLD [lo- 121. These experiments proved that the K loss actually originated from the interaction of ablated species with 0, molecules. This elastic scattering even result in backscattering of ablated species, especially for those atoms which are lighter than oxygen molecules. This result will be described elsewhere. In 40 Pa Oz cases, the plumes are prolonged along the target normal direction and suppressed in the direction parallel to the target surface from 20 Pa to about 60 Pa. Above 60 Pa, the plume is suppressed along both directions, as shown in Figs. 2 and 3. The change tendency of plume with increasing 0, pressure is also schematically plotted in Fig. 4(a) and (b). It can also be seen clearly that the spherical shape of a plume at 20 Pa changes to a forward-directed shape at pressures above 40 Pa, as shown in Fig. 2(f) and (i). The forward-directed shape of a plume means less ablated species are scattered to the flank side of the target surface and so there is less loss of ablated species, as discussed above. From the scattering view point, if the collisions of ablated species with ambient gas are completely inelastic or chemically reactive, the ablated species and ambient gas will merge into a single monoxide or multioxide molecule. The as-formed heavier monoxides or multioxides will be scattered less to the flank side, compared to lighter species direct ablated from the target. These differences of plume image in 0, and Ar with same pressure, particularly the contrary change tendency of plume with increasing the ambient gas pressure, suggests that the interactions of plume with background gas are different in 0, and Ar. This indicates the interaction is inelastic or reactive in high pressure Oz. The effect that the reactive collisions only take place at relative high pressure can be interpreted by means of a shock (blast) wave model [13- 171. In PLD, the ejected material due to ablation of the target acts as a piston which pushes ahead the gas in front of it [ 13- 171. Behind the piston is a partial vacuum and ahead the piston the gas becomes increasingly compressed and eventually forms a shock wave. In order to form the shock wave the piston has to be accelerated many times of mean free path of
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ambient gas. Thus, with lower ambient gas pressure, the piston will need to be accelerated in a longer distance to obtain a high enough compressed gas, i.e. shock wave. If the ambient gas pressure is too low, the shock wave cannot be formed or is too weak. At given laser fluence, target and ambient gas. the shortest distance that shock waves can be formed is inversely proportional to the ambient gas pressure [ 131. So, an high enough ambient gas pressure is necessary in order to generate a strong enough shock wave between target and substrate. A strong shock wave means a highly compressed ambient gas layer with a quite high temperature as follows [ 181:
where T, is the temperature of ambient gas, M is the math number of the shock wave, being able to obtain 25 in early stages of the expansion of the plume, and y is the specific heat ratio of the ambient gas. From Eq. (2) it follows that the temperature of the shock heated oxygen will be up to 2 X 10” K. At this high temperature, the percentage (Y of 0, dissociated molecules can be written as follows [ 131:
a2
-ZZ 1-a
M3/5) 0
Mrr/~I,~Jk;T
7
1 . -go0 . _ . e-L’/hT goo, tzo.
(3) where MO is the mass of the 0 atom, v is the frequency of vibration of the 0, molecule, I,, is the rotation inertia of the 0, molecule, g,, and g,, are the statistical weights of the 0 atom and the 0: molecule in the ground state, ~1~. is the molecular number in volume units, U is the dissociation energy of a O? molecule. T is the temperature of the shock wave layer, and k is the Boltzmann constant. From Eqs. (2) and (3) it can be estimated that at least 95% of the 0, molecules in a shock wave layer are dissociated at early stages of the shock wave formation. Thus, the ablated metal species can react with 0 atoms that come from dissociation of 0, in a shock wave layer [ 191. If the 0, molecules cannot be dissociated into 0 atoms. it is will be impossible that the ablated species react with the ambient oxygen except for a small number of high energy species [ 191. It should be pointed out that the different metal species have a different chemical activity to react
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with 0 atoms and so different effects on reaction collision of ablated metal species with an ambient gas. This is out of the present consideration. It has been shown that oxygen deficiency of the oxide film is critical even when a stoichiometric target is used. while the metal species can keep stoichiometry [20]. This indicates oxygen in the target is lost during plume propagation, while the transfer of metal species is complete. It is often found that the oxygen deficiency in oxide films is a common problem. especially in preparing nonstoichiometric oxides such as La,- ,Sr,CoO,_J, YBa-,Cu,O, ~, etc. A difficulty to achieve the high quality films is how to increase the oxygen content in the films. It has been reported that more than 40% of the oxygen in the film comes from the target and about of 60% from the ambient gas at deposition [21]. This means a great quantity of oxygen in films comes from the ambient gas. It can be inferred that the chemical reaction of ablated with the ambient gas as discussed above is the way to incorporate the ambient oxygen into the films. This oxygen incorporation mechanism may play a dominant role in making the as grown film have enough oxygen. According to the above discussions, the transportation of ablated species can be divided into three regimes under condition of strong shock wave formation. The first regime is from the target surface to the position where the oxygen dissociation due to blast wave begins to take place. In this regime, the interaction of ablated species with ambient gas is mainly elastic. The second regime is after the first regime to the position at which the shock wave begins to turn into the sound wave. In this regime, the reactive collision is especially important. The ablated metal species are partially oxidized into metal oxides through reactive collisions with the ambient gas. After the second regime is the last regime. In this regime. the materials which come directly from target or formed reactive collisions during the second regime began to diffuse freely under gravity. In the case of the elastic interaction of ablated species with the ambient gas. the motion of released material can be described quantitatively by drag model. In the case of shock wave formation. the motion of released material can be described by shock wave model. So there always exist a drag model regirne (elastic scattering regime) firstly and shock wave (inelastic
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collision regime) secondly during the plume propagation, if the shock wave can be formed. This is in agreement with the Geohegan results [5]. It showed that there were two regimes. The first regime was described by the drag model and the second regime by the shock wave model. According to the forming conditions of shock waves, the high enough laser fluence and ambient gas pressure are both necessary. If one of them cannot be met, the shock cannot be formed or is too weak to dissociate the oxygen molecules. In these cases, there will only exist elastic collisions during the propagation of plume.
4. Conclusions This paper shows that the plume images from the PZT target in PLD are similar in O2 and Ar with pressures below 20 Pa, but remarkably different with pressures above 30 Pa. The similarities can be explained as that the main interaction of ablated species with the ambient gas is elastic. The differences are interpreted as that the interaction between ablated species with ambient gas is reactive in 0, and elastic in Ar. The changes of plume image in ambient gas with different pressures indicate that the elastic scattering results in the ablated species being scattered out of the substrate, and reactive collisions reduce this effect. So it is thought that elastic scattering leads to the non-stoichiometry of films, while the reactive collisions reduce the nonstoichiometry and enhance the oxygen incorporation. It is also proposed that the propagation of the ablated species in ambient gas is divided into three regimes under the condition of shock wave generation: (1) an elastic scattering regime which is described by drag model close to the target at first, and then (2) a reactive collision regime that is described by blast wave model, and finally (3) a sound wave regime (free diffusion model).
Acknowledgements This work was financially supported by the National Natural Science Foundation of China.
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