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Mechanism of ablation of CdS at laser wavelengths in the visible and in the UV
Applied Surface Science 253 (2007) 6339–6342 www.elsevier.com/locate/apsusc
Mechanism of ablation of CdS at laser wavelengths in the visible and in the UV Marı´a Jadraque, Jesu´s Alvarez, Rebeca de Nalda, Margarita Martin * Instituto de Quı´mica Fı´sica ‘‘Rocasolano’’, CSIC, Serrano 119, 28006 Madrid, Spain Available online 26 January 2007
Abstract The mechanisms of laser ablation of CdS targets at different laser wavelengths have been investigated. (CdS)n+ cluster formation is only observed upon 532 nm ablation. The time and energy distributions of neutral S, S2, Cd and CdS show significant dependence on laser wavelength. Bimodal distributions are observed at 266 and 308 nm. For the former, the average kinetic energy increases significantly with mass, taking values in the range of 0.3–1.7 eV. At 308 nm the slow component of the time distribution disappears at distances above the target larger than 1 cm. At this wavelength, the observed time distribution appears to reflect mainly the dynamics of the expansion. At 532 nm the time distribution is monomodal and the average kinetic energies are below 0.2 eV. Clear indications of the participation of thermal (at 532 nm) and non-thermal mechanisms (at 266 nm) have been found. It is tentatively concluded that the cluster formation observed upon ablation at 532 nm can be related to the thermal ablation mechanisms in which the low kinetic energy of the species in the plume and their similar velocities favor the aggregation processes. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser ablation; Clusters; Plume dynamics; CdS; PLD; TOF MS
1. Introduction PLD (pulsed laser deposition) of CdS targets at wavelengths of the ablating laser ranging from the near IR to the UV is an efficient way to produce cadmium sulfide thin films with properties as optical devices [1,2]. Ablation of CdS under vacuum and in the presence of He, at several laser wavelengths, has been shown to be a source for CdnSm+ free clusters [3,4]. At the wavelength of 337 nm, the mechanism of ablation of CdS targets has been investigated, by analysis of the dynamics of the atomic S and Cd and molecular S2 of the plume. The dynamics of the plume shows effects characteristics of an unsteady adiabatic expansion, with apparent temperatures dependent on the ejection polar angle [5]. Aiming at a better understanding of the laser/target interaction regime that favors cluster formation and can provide some control on the final deposition process, in this work we have investigated the mechanisms of ablation of CdS targets at several laser wavelengths. We have concentrated on the study of the composition and dynamics of the plume, obtained upon ablation at 532, 308 and 266 nm. The first
* Corresponding author. Tel.: +34 915619400; fax: +34 915642431. E-mail address: [email protected] (M. Martin). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.01.059
corresponds to energy regions below the energy gap of the target whereas the later is well over it. 2. Experimental Ablation of CdS targets was carried out under vacuum conditions better than 2 106 mbar. The target was placed in the extracting region of a linear time-of-flight mass spectrometer (TOF MS) and could be rotated and displaced at different distances with respect to the flight axis of the spectrometer. The ablating laser beam was limited by several apertures and focused by a quartz lens (f.l. of 50 cm), forming the focal point at a distance of 7 cm behind the position of the target; the area interacted by the laser beam at the target surface was of 4 103 cm2. Extraction/acceleration of the positive ions of the plume was performed by a set of five parallel plates; a total bias voltage (continuous or pulsed) of 2000 V was applied between the first and the last plate. The ablation plume was produced between the first and the second plate with the plume axis perpendicular to the axis of the TOF MS; an electric field typically in the range of 91–227 V cm1 deflected the ions along the TOF axis. A more detailed description of the TOF MS has been given elsewhere [6]. The neutral species were
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M. Jadraque et al. / Applied Surface Science 253 (2007) 6339–6342
postionized by an ArF excimer laser (l = 193 nm) which intersected the plume along a direction perpendicular to both the plume and TOF axis. The energy was reduced to 0.25– 1.5 mJ by inserting two variable apertures in the laser beam path. A lens (f.l. of 40 cm) focused the beam on the plume; the depth of the focus was estimated in several mm. Under the low fluence (of 0.015–0.1 J cm2) and mild focusing conditions, it can be assumed that the postionizing laser excites a column perpendicularly to the plume expansion axis and parallel to the extracting/accelerating plates. Under these conditions the MCP’s detects the ion signal integrated over the column excited by the laser. 3. Results and discussion 3.1. Plume composition At the ablation wavelength of 266 nm, only ions with low m/ z values were observed. The observed neutrals were S, S2, Cd, CdS and a weak feature tentatively assigned to Cd2. Under typical conditions, relatively broad peaks are obtained; however, at low density of the plume (probing the plume at 1 cm above the target surface and at a delay after the ablating pulse of 30 ms) the isotopic composition of Cd can be resolved as shown in Fig. 1. At 532 nm the ions observed were dependent on the extraction conditions. When the extracting voltage pulse is switched at delays shorter than 1.7 ms with respect to the ablation pulse, only peaks corresponding to low m/z values can
be observed. At delays between 1.7 and 3.1 ms, the mass spectra show intense features that can be assigned to Cd+ and Cd2+ together with cluster signal up to values of m/z near 500 amu. Peaks corresponding to Sn+, Cdn+, (CdS+)n (n = 1, 2) and nonstoichiometric CdSn+ (n = 2, 3) and Cd2Sn+ (n = 3–5) clusters were assigned. The neutral species are basically the same as observed upon 266 nm laser ablation. The spectrum obtained probing the plume at 4 mm above the target and at a delay of 8 ms with respect to the ablating laser is shown in Fig. 1. 3.2. Plume dynamics and kinetic energy distribution The time distributions, f(t), of the neutrals formed in the ablation, at a given distance above the surface target, were measured at the ablation wavelengths of 266, 308 and 532 nm. The time distribution was obtained recording the mass spectra obtained by ArF postionization, with the ArF laser beam probing the plume at the selected distance and at different time delays with respect to the ablating laser pulse. The ablation laser fluences were close to the minimum to observe a measurable signal. At the wavelength of 266 nm the time distributions of S, S2, Cd and CdS, measured at 5 mm above the surface and at a fluence of 0.1 J cm2, are bimodal. As shown in Table 1, in both components, the mass dependence of the experimental vmp (defined as L/tmp where L is the distance to the target surface and tmp is the time at the maximum of the experimental time distribution) is far from the dependence on m1/2 expected from a thermal mechanism. Although the number of experimental points is relatively small, in order to get information about the range of energy spanned by the distribution, the data were analyzed in terms of two Maxwell– Boltzmann (M-B) distribution functions as shown in Fig. 2. The parameters of the fitting are given in Table 1. For the slow component, the best fitting to the experimental data is obtained setting the flow velocity, u, to values different to zero. The fast component can be well fitted to a half range M-B. Aiming at a better characterization of the energy distribution, the experimental f(t) distribution was converted to kinetic energy distribution making use of the transformation f(E) / f(t)t3. The experimental probing conditions of the plume at which this transformation can be applied are discussed in Refs. [7,8]. As shown in Fig. 2, the representation of ln[f(E)] versus kinetic Table 1 Experimental vmp and parameters of the M-B functions best fitting the two components of the time distributions of neutral species in the plume, at 266 nm laser ablation and 0.5 cm above the target surface Species
Component
u (ms1)
T (K)
vmp (ms1)
S
Fast Slow Fast Slow Fast Slow
0 234 30 0 350 12 0 305 12 286 23
2034 353 124 20 4756 655 111 13 15900 3040 244 24 293 56
1220 440 1350 440 1754 425 411
S2 Fig. 1. Neutral composition of the ablation plume, observed by postionization at 193 nm. (a) Ablation wavelength is 266 nm; fluence is 0.1 J cm2. The plume is postionized at 1 cm above the target surface and at a delay of 30 ms after the ablating pulse. (b) Ablation wavelength is 532 nm and fluence is 0.3 J cm2, postionizing the plume at 4 mm above the target and at a delay of 8 ms.
Cd CdS
T (K) a
hEi (eV)
2359
0.3
4308
0.5
13393
1.7
Fluence is 0.1 J cm2. a Obtained form the linear fitting to the slope of ln[f(E)] versus E.
M. Jadraque et al. / Applied Surface Science 253 (2007) 6339–6342
Fig. 2. Time and energy distribution of S, S2, Cd and CdS in the plume measured at the ablation wavelength of 266 nm. The plume is postionized at 0.5 cm above the target surface. Ablation fluence is 0.1 J cm2.
energy shows also evidence of a bimodal behavior. The set of experimental points corresponding to the fast component, in the range of energies where the overlap with the slow component is negligible, can be fitted to a straight line; a temperature can be obtained from its slope, allowing to make a rough estimation of the average kinetic energy carried out by the observed species; the estimated values are given in Table 1. Despite the good fitting of f(t) to a M-B function, the range and nearly linear dependence on mass of the average kinetic energy, support that the ablation mechanism does not follow a thermal behavior. The nature of this mechanism is however unclear. Non-thermal laser/target interactions have been reported to lead to photochemically desorbed molecules, characterized by a distribution resembling a M-B function [9]. In dielectric and semiconductors at low laser fluences near the ablation threshold, the plume dynamics and ion yield dependence on the ablation laser fluence indicate that an electronic mechanism involving ion emission plays a role in initiating the ablation [10]. Finally, regarding the low T values obtained by the M-B fitting of the slow component of the distribution, a possible explanation is that it reflects the dynamics of heavier species that undergo fragmentation in the plume leading to the observed smaller masses. The time distribution of S2, formed upon 308 nm ablation, is shown in Fig. 3. A bimodal time distribution is obtained when the plume is probed at a distance from the target surface smaller than 1 cm. However at larger distances the slower component disappears suggesting that the two components have different
6341
Fig. 3. Time distribution of S2 in the plume measured at the laser wavelength of 308 nm, at 0.7 and 1 cm above the target and at ablation fluences of 0.1 and 0.3 J cm2.
angular distribution. The time distributions could be fitted to one or two shifted M-B functions. At laser fluence of 0.1 J cm2, the experimental vmp of the slow component is close to that measured for the fast component obtained at 266 nm, measured under roughly similar conditions of fluence and distance. However, in terms of the M-B analysis, the main contribution to vmp at 308 nm comes from the development of a collisionally-induced flow velocity, u. The fitting parameters are given in Table 2. For the fast component of f(t), at the lowest fluence of 0.1 J cm2, u and T show little dependence on distance above the target, compatible with a regime of plume expansion in which terminal conditions had been reached. At the highest fluence the variation of u and T with distance is more compatible with a moderately accelerating and cooling plume expansion. S and Cd were also observed in the mass spectra; however, the low signal intensity precludes measurements to characterize the effect of mass. At the ablation wavelength of 532 nm the time distributions of S, S2, Cd and CdS are shown in Fig. 4, together with the single M-B curve best fitting the experimental distributions. The parameters of the fitting are given in Table 3. In Fig. 4 the kinetic energy distribution (obtained from the time distribution) is depicted. From the slope of the representation of ln[f(E)] versus E, the temperature and average kinetic energy, can be estimated. For Cd and CdS, it is observed that the energy distribution shifts towards slightly higher energy values, with respect to the energy distribution for S and S2; this is consistent with weak collisional effects in the plume
Table 2 Experimental vmp and parameters of the M-B functions best fitting the components of the time distributions of S2 in the plume, at 308 nm laser ablation Fluence (J cm2)
0.1 0.3 0.1 0.3
L (102 m)
0.7 0.7 1 1
Fast component
Slow component
u (ms1)
T (K)
u (ms1)
T (K)
2318 80 2220 230 2116 350 3275 160
3228 554 6890 1020 2637 227 1142 100
539 149 0
1404 437 1390 411
Fast, vmp (ms1)
Slow, vmp (ms1)
2800 3500 2500 3225
1043 740
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M. Jadraque et al. / Applied Surface Science 253 (2007) 6339–6342
average kinetic energy is consistent with a thermal ablation mechanism. 4. Summary and final conclusions
Fig. 4. Time distribution of S, S2, Cd and CdS in the plume at the laser wavelength of 532 nm, measured at 0.5 cm above the target and at 0.3 J cm2.
Table 3 Experimental vmp and parameters of the M-B function best fitting the time distributions of the neutral species in the plume, at 532 nm laser ablation and 0.5 cm above the target surface Species
u (ms1)
T (K)
vmp (ms1)
T (K)a
hEi (eV)
S S2 Cd CdS
0 0 470 14 439 12
557 23 1162 50 385 40 397 41
625 625 625 540
860 1200 1500 1400
0.1 0.15 0.2 0.2
Fluence is 0.3 J cm2. a Obtained form the linear fitting to the slope of ln[f(E)] versus E.
that lead to the development of a flow velocity, measurable at the distance probed by the postionization laser. This flow velocity causes the displacement of the energy distribution f(E) towards higher values of the energy, leading to some overestimation of the average energy obtained for Cd and CdS. We conclude that the range and mass dependence of the
The mechanisms of ablation of CdS targets at different laser wavelengths have been investigated. Comparison of the plume dynamics at the ablation wavelengths of 523, 308 and 266 nm shows clear indications of the participation of thermal (at 532 nm) and non-thermal mechanisms (at 266 nm). Work to measure the ion dynamics in the plume and the dependence on laser fluence, to further investigate the ablation mechanism at 266 nm, is planned. Cluster formation is only observed upon ablation at 532 nm. It is tentatively concluded that the cluster formation can be related to the thermal ablation mechanisms in which the low kinetic energy of the species in the plume and the similar velocities favor the aggregation processes. Acknowledgements Financial support by Spanish DGI, MCyT (BQU200308531-C02) is acknowledged. References [1] A. Erlacher, H. Miller, B. Ullrich, J. Appl. Phys. 95 (2004) 2927. [2] B. Ullrich, J.W. Tomm, N.M. Dushkina, Y. Tomm, H. Sakai, Y. Segawa, Solid State Commun. 116 (2000) 33. [3] E. Sanville, A. Burnin, J.J. BelBruno, J. Phys. Chem. A 110 (2006) 2378. [4] A. Burnin, E. Sanville, J.J. BelBruno, J. Phys. Chem. A 109 (2005) 5026. [5] A. Namiki, T. Kawai, K. Ichige, Surf. Sci. 166 (1986) 189. [6] R. Torres, M. Martin, Appl. Surf. Sci. 193 (2002) 149. [7] K.L. Saenger, J. Appl. Phys. 68 (1989) 4435. [8] L. Dı´az, M. Santos, J.A. Torresano, M. Castillejo, M. Jadraque, M. Martı´n, M. Oujja, E. Rebollar, Appl. Phys. A 85 (2006) 33. [9] F.M. Zimmermann, W. Ho, J. Chem. Phys. 100 (1994) 7700. [10] V. Marine, N.M. Bulgakova, L. Patrone, I. Ozerov, Appl. Phys. A 79 (2004) 771.
Mechanism of ablation of CdS at laser wavelengths in the visible and in the UV Marı´a Jadraque, Jesu´s Alvarez, Rebeca de Nalda, Margarita Martin * Instituto de Quı´mica Fı´sica ‘‘Rocasolano’’, CSIC, Serrano 119, 28006 Madrid, Spain Available online 26 January 2007
Abstract The mechanisms of laser ablation of CdS targets at different laser wavelengths have been investigated. (CdS)n+ cluster formation is only observed upon 532 nm ablation. The time and energy distributions of neutral S, S2, Cd and CdS show significant dependence on laser wavelength. Bimodal distributions are observed at 266 and 308 nm. For the former, the average kinetic energy increases significantly with mass, taking values in the range of 0.3–1.7 eV. At 308 nm the slow component of the time distribution disappears at distances above the target larger than 1 cm. At this wavelength, the observed time distribution appears to reflect mainly the dynamics of the expansion. At 532 nm the time distribution is monomodal and the average kinetic energies are below 0.2 eV. Clear indications of the participation of thermal (at 532 nm) and non-thermal mechanisms (at 266 nm) have been found. It is tentatively concluded that the cluster formation observed upon ablation at 532 nm can be related to the thermal ablation mechanisms in which the low kinetic energy of the species in the plume and their similar velocities favor the aggregation processes. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser ablation; Clusters; Plume dynamics; CdS; PLD; TOF MS
1. Introduction PLD (pulsed laser deposition) of CdS targets at wavelengths of the ablating laser ranging from the near IR to the UV is an efficient way to produce cadmium sulfide thin films with properties as optical devices [1,2]. Ablation of CdS under vacuum and in the presence of He, at several laser wavelengths, has been shown to be a source for CdnSm+ free clusters [3,4]. At the wavelength of 337 nm, the mechanism of ablation of CdS targets has been investigated, by analysis of the dynamics of the atomic S and Cd and molecular S2 of the plume. The dynamics of the plume shows effects characteristics of an unsteady adiabatic expansion, with apparent temperatures dependent on the ejection polar angle [5]. Aiming at a better understanding of the laser/target interaction regime that favors cluster formation and can provide some control on the final deposition process, in this work we have investigated the mechanisms of ablation of CdS targets at several laser wavelengths. We have concentrated on the study of the composition and dynamics of the plume, obtained upon ablation at 532, 308 and 266 nm. The first
* Corresponding author. Tel.: +34 915619400; fax: +34 915642431. E-mail address: [email protected] (M. Martin). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.01.059
corresponds to energy regions below the energy gap of the target whereas the later is well over it. 2. Experimental Ablation of CdS targets was carried out under vacuum conditions better than 2 106 mbar. The target was placed in the extracting region of a linear time-of-flight mass spectrometer (TOF MS) and could be rotated and displaced at different distances with respect to the flight axis of the spectrometer. The ablating laser beam was limited by several apertures and focused by a quartz lens (f.l. of 50 cm), forming the focal point at a distance of 7 cm behind the position of the target; the area interacted by the laser beam at the target surface was of 4 103 cm2. Extraction/acceleration of the positive ions of the plume was performed by a set of five parallel plates; a total bias voltage (continuous or pulsed) of 2000 V was applied between the first and the last plate. The ablation plume was produced between the first and the second plate with the plume axis perpendicular to the axis of the TOF MS; an electric field typically in the range of 91–227 V cm1 deflected the ions along the TOF axis. A more detailed description of the TOF MS has been given elsewhere [6]. The neutral species were
6340
M. Jadraque et al. / Applied Surface Science 253 (2007) 6339–6342
postionized by an ArF excimer laser (l = 193 nm) which intersected the plume along a direction perpendicular to both the plume and TOF axis. The energy was reduced to 0.25– 1.5 mJ by inserting two variable apertures in the laser beam path. A lens (f.l. of 40 cm) focused the beam on the plume; the depth of the focus was estimated in several mm. Under the low fluence (of 0.015–0.1 J cm2) and mild focusing conditions, it can be assumed that the postionizing laser excites a column perpendicularly to the plume expansion axis and parallel to the extracting/accelerating plates. Under these conditions the MCP’s detects the ion signal integrated over the column excited by the laser. 3. Results and discussion 3.1. Plume composition At the ablation wavelength of 266 nm, only ions with low m/ z values were observed. The observed neutrals were S, S2, Cd, CdS and a weak feature tentatively assigned to Cd2. Under typical conditions, relatively broad peaks are obtained; however, at low density of the plume (probing the plume at 1 cm above the target surface and at a delay after the ablating pulse of 30 ms) the isotopic composition of Cd can be resolved as shown in Fig. 1. At 532 nm the ions observed were dependent on the extraction conditions. When the extracting voltage pulse is switched at delays shorter than 1.7 ms with respect to the ablation pulse, only peaks corresponding to low m/z values can
be observed. At delays between 1.7 and 3.1 ms, the mass spectra show intense features that can be assigned to Cd+ and Cd2+ together with cluster signal up to values of m/z near 500 amu. Peaks corresponding to Sn+, Cdn+, (CdS+)n (n = 1, 2) and nonstoichiometric CdSn+ (n = 2, 3) and Cd2Sn+ (n = 3–5) clusters were assigned. The neutral species are basically the same as observed upon 266 nm laser ablation. The spectrum obtained probing the plume at 4 mm above the target and at a delay of 8 ms with respect to the ablating laser is shown in Fig. 1. 3.2. Plume dynamics and kinetic energy distribution The time distributions, f(t), of the neutrals formed in the ablation, at a given distance above the surface target, were measured at the ablation wavelengths of 266, 308 and 532 nm. The time distribution was obtained recording the mass spectra obtained by ArF postionization, with the ArF laser beam probing the plume at the selected distance and at different time delays with respect to the ablating laser pulse. The ablation laser fluences were close to the minimum to observe a measurable signal. At the wavelength of 266 nm the time distributions of S, S2, Cd and CdS, measured at 5 mm above the surface and at a fluence of 0.1 J cm2, are bimodal. As shown in Table 1, in both components, the mass dependence of the experimental vmp (defined as L/tmp where L is the distance to the target surface and tmp is the time at the maximum of the experimental time distribution) is far from the dependence on m1/2 expected from a thermal mechanism. Although the number of experimental points is relatively small, in order to get information about the range of energy spanned by the distribution, the data were analyzed in terms of two Maxwell– Boltzmann (M-B) distribution functions as shown in Fig. 2. The parameters of the fitting are given in Table 1. For the slow component, the best fitting to the experimental data is obtained setting the flow velocity, u, to values different to zero. The fast component can be well fitted to a half range M-B. Aiming at a better characterization of the energy distribution, the experimental f(t) distribution was converted to kinetic energy distribution making use of the transformation f(E) / f(t)t3. The experimental probing conditions of the plume at which this transformation can be applied are discussed in Refs. [7,8]. As shown in Fig. 2, the representation of ln[f(E)] versus kinetic Table 1 Experimental vmp and parameters of the M-B functions best fitting the two components of the time distributions of neutral species in the plume, at 266 nm laser ablation and 0.5 cm above the target surface Species
Component
u (ms1)
T (K)
vmp (ms1)
S
Fast Slow Fast Slow Fast Slow
0 234 30 0 350 12 0 305 12 286 23
2034 353 124 20 4756 655 111 13 15900 3040 244 24 293 56
1220 440 1350 440 1754 425 411
S2 Fig. 1. Neutral composition of the ablation plume, observed by postionization at 193 nm. (a) Ablation wavelength is 266 nm; fluence is 0.1 J cm2. The plume is postionized at 1 cm above the target surface and at a delay of 30 ms after the ablating pulse. (b) Ablation wavelength is 532 nm and fluence is 0.3 J cm2, postionizing the plume at 4 mm above the target and at a delay of 8 ms.
Cd CdS
T (K) a
hEi (eV)
2359
0.3
4308
0.5
13393
1.7
Fluence is 0.1 J cm2. a Obtained form the linear fitting to the slope of ln[f(E)] versus E.
M. Jadraque et al. / Applied Surface Science 253 (2007) 6339–6342
Fig. 2. Time and energy distribution of S, S2, Cd and CdS in the plume measured at the ablation wavelength of 266 nm. The plume is postionized at 0.5 cm above the target surface. Ablation fluence is 0.1 J cm2.
energy shows also evidence of a bimodal behavior. The set of experimental points corresponding to the fast component, in the range of energies where the overlap with the slow component is negligible, can be fitted to a straight line; a temperature can be obtained from its slope, allowing to make a rough estimation of the average kinetic energy carried out by the observed species; the estimated values are given in Table 1. Despite the good fitting of f(t) to a M-B function, the range and nearly linear dependence on mass of the average kinetic energy, support that the ablation mechanism does not follow a thermal behavior. The nature of this mechanism is however unclear. Non-thermal laser/target interactions have been reported to lead to photochemically desorbed molecules, characterized by a distribution resembling a M-B function [9]. In dielectric and semiconductors at low laser fluences near the ablation threshold, the plume dynamics and ion yield dependence on the ablation laser fluence indicate that an electronic mechanism involving ion emission plays a role in initiating the ablation [10]. Finally, regarding the low T values obtained by the M-B fitting of the slow component of the distribution, a possible explanation is that it reflects the dynamics of heavier species that undergo fragmentation in the plume leading to the observed smaller masses. The time distribution of S2, formed upon 308 nm ablation, is shown in Fig. 3. A bimodal time distribution is obtained when the plume is probed at a distance from the target surface smaller than 1 cm. However at larger distances the slower component disappears suggesting that the two components have different
6341
Fig. 3. Time distribution of S2 in the plume measured at the laser wavelength of 308 nm, at 0.7 and 1 cm above the target and at ablation fluences of 0.1 and 0.3 J cm2.
angular distribution. The time distributions could be fitted to one or two shifted M-B functions. At laser fluence of 0.1 J cm2, the experimental vmp of the slow component is close to that measured for the fast component obtained at 266 nm, measured under roughly similar conditions of fluence and distance. However, in terms of the M-B analysis, the main contribution to vmp at 308 nm comes from the development of a collisionally-induced flow velocity, u. The fitting parameters are given in Table 2. For the fast component of f(t), at the lowest fluence of 0.1 J cm2, u and T show little dependence on distance above the target, compatible with a regime of plume expansion in which terminal conditions had been reached. At the highest fluence the variation of u and T with distance is more compatible with a moderately accelerating and cooling plume expansion. S and Cd were also observed in the mass spectra; however, the low signal intensity precludes measurements to characterize the effect of mass. At the ablation wavelength of 532 nm the time distributions of S, S2, Cd and CdS are shown in Fig. 4, together with the single M-B curve best fitting the experimental distributions. The parameters of the fitting are given in Table 3. In Fig. 4 the kinetic energy distribution (obtained from the time distribution) is depicted. From the slope of the representation of ln[f(E)] versus E, the temperature and average kinetic energy, can be estimated. For Cd and CdS, it is observed that the energy distribution shifts towards slightly higher energy values, with respect to the energy distribution for S and S2; this is consistent with weak collisional effects in the plume
Table 2 Experimental vmp and parameters of the M-B functions best fitting the components of the time distributions of S2 in the plume, at 308 nm laser ablation Fluence (J cm2)
0.1 0.3 0.1 0.3
L (102 m)
0.7 0.7 1 1
Fast component
Slow component
u (ms1)
T (K)
u (ms1)
T (K)
2318 80 2220 230 2116 350 3275 160
3228 554 6890 1020 2637 227 1142 100
539 149 0
1404 437 1390 411
Fast, vmp (ms1)
Slow, vmp (ms1)
2800 3500 2500 3225
1043 740
6342
M. Jadraque et al. / Applied Surface Science 253 (2007) 6339–6342
average kinetic energy is consistent with a thermal ablation mechanism. 4. Summary and final conclusions
Fig. 4. Time distribution of S, S2, Cd and CdS in the plume at the laser wavelength of 532 nm, measured at 0.5 cm above the target and at 0.3 J cm2.
Table 3 Experimental vmp and parameters of the M-B function best fitting the time distributions of the neutral species in the plume, at 532 nm laser ablation and 0.5 cm above the target surface Species
u (ms1)
T (K)
vmp (ms1)
T (K)a
hEi (eV)
S S2 Cd CdS
0 0 470 14 439 12
557 23 1162 50 385 40 397 41
625 625 625 540
860 1200 1500 1400
0.1 0.15 0.2 0.2
Fluence is 0.3 J cm2. a Obtained form the linear fitting to the slope of ln[f(E)] versus E.
that lead to the development of a flow velocity, measurable at the distance probed by the postionization laser. This flow velocity causes the displacement of the energy distribution f(E) towards higher values of the energy, leading to some overestimation of the average energy obtained for Cd and CdS. We conclude that the range and mass dependence of the
The mechanisms of ablation of CdS targets at different laser wavelengths have been investigated. Comparison of the plume dynamics at the ablation wavelengths of 523, 308 and 266 nm shows clear indications of the participation of thermal (at 532 nm) and non-thermal mechanisms (at 266 nm). Work to measure the ion dynamics in the plume and the dependence on laser fluence, to further investigate the ablation mechanism at 266 nm, is planned. Cluster formation is only observed upon ablation at 532 nm. It is tentatively concluded that the cluster formation can be related to the thermal ablation mechanisms in which the low kinetic energy of the species in the plume and the similar velocities favor the aggregation processes. Acknowledgements Financial support by Spanish DGI, MCyT (BQU200308531-C02) is acknowledged. References [1] A. Erlacher, H. Miller, B. Ullrich, J. Appl. Phys. 95 (2004) 2927. [2] B. Ullrich, J.W. Tomm, N.M. Dushkina, Y. Tomm, H. Sakai, Y. Segawa, Solid State Commun. 116 (2000) 33. [3] E. Sanville, A. Burnin, J.J. BelBruno, J. Phys. Chem. A 110 (2006) 2378. [4] A. Burnin, E. Sanville, J.J. BelBruno, J. Phys. Chem. A 109 (2005) 5026. [5] A. Namiki, T. Kawai, K. Ichige, Surf. Sci. 166 (1986) 189. [6] R. Torres, M. Martin, Appl. Surf. Sci. 193 (2002) 149. [7] K.L. Saenger, J. Appl. Phys. 68 (1989) 4435. [8] L. Dı´az, M. Santos, J.A. Torresano, M. Castillejo, M. Jadraque, M. Martı´n, M. Oujja, E. Rebollar, Appl. Phys. A 85 (2006) 33. [9] F.M. Zimmermann, W. Ho, J. Chem. Phys. 100 (1994) 7700. [10] V. Marine, N.M. Bulgakova, L. Patrone, I. Ozerov, Appl. Phys. A 79 (2004) 771.