- Email: [email protected]
Laser ablation of yttrium-containing oxides in various ambient gases studied by time-resolved emission spectroscopy
Chemical Physics Letters 424 (2006) 54–57 www.elsevier.com/locate/cplett
Laser ablation of yttrium-containing oxides in various ambient gases studied by time-resolved emission spectroscopy Takamichi Kobayashi *, Toshimori Sekine National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 24 February 2006; in final form 18 April 2006 Available online 27 April 2006
Abstract Temporal emission profiles of YO molecule produced by laser ablation of yttrium-containing oxides have been measured in various ambient gases. Regardless of the kind of the ambient gas, the YO emission intensity is decreased with decreasing gas pressure. However, observed temporal behaviors differ significantly depending on the ambient gas. For O2 and CO2, similar profiles have been observed, while for non-oxide gases (He and N2), observed profiles are quite different from each other and also from those for oxide gases. This kind of measurement is very simple and useful for probing the dynamics of the ablation and postablation processes. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Pulsed laser ablation for the deposition of high-quality thin films is a well established technique. The nature of the laser-generated plasma and the surrounding atmosphere significantly influence film characteristics. Information on the details of the gas-phase chemistry is desirable for optimization of plasma conditions during laser ablation [1–6]. Emission spectroscopy is one of the techniques that have been successfully used to probe the dynamics of the ablation and postablation processes [2–5]. It is well known that YO is easily produced by laser ablation of yttrium-containing oxides such as Y–Ba–Cu oxide and Y2O3 [5]. In oxygen atmosphere, it is reported that the ablation plume from Y–Ba–Cu oxide becomes red due to YO chemiluminescence [6] and the oxygen pressure is an important parameter controlling the quality of the deposited films since the formation of oxides (YO, BaO, and CuO) is essential for the production of high quality films [4]. The YO transitions in the visible region is well studied [7,8] and YO emission measurement was proven *
Corresponding author. Fax.: +81 29 851 2768. E-mail address: [email protected] (T. Kobayashi). 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.04.056
useful as a convenient diagnostic for optimization of plasma conditions [3,4]. YO formation in laser ablation of yttrium-containing oxides is fairly well studied. There are only two main ways for YO formation: (i) laser-ablated yttrium atoms react with oxygen-containing species to form YO, (ii) YO molecules are directly ejected from the sample surface as laser-ablated species. Which process is dominant appears to vary from experiment to experiment [1–6] and there are still some ambiguous points. In this letter, we report a time-resolved emission study of the plume generated from laser ablation of YAG (Y3Al5O12) crystal and other yttrium-containing oxides with various surrounding atmospheres. This kind of temporal profile measurements can provide essential information on the dynamics of the ablation and postablation processes. 2. Experimental The experimental set-up employed in this study was rather simple. The Nd:YAG laser (532 nm, 8 ns pulse, and 5–20 mJ/pulse) was incident normal and focused down to about 0.5 mm in diameter on the sample surface. When the laser focus point was too deep inside the sample, i.e. more than 2 mm, the YO emission disappeared. Emission from the vicinity of the irradiated point on the sample sur-
T. Kobayashi, T. Sekine / Chemical Physics Letters 424 (2006) 54–57
face was collected and introduced into a spectrometer. Dispersed light was then transmitted to a streak camera to obtain the time-varying emission image recorded by a CCD camera. Four different ambient gases were used, namely O2, CO2, He, and N2 with the gas pressure near one atmosphere in all the experiments. The oxygen contents in CO2, He, and N2 are expected to be less than 1%. Samples used were single crystals of YAG, YVO4, and YAlO3. 3. Results and discussion Representative streak images of emission from the laserablated YAG in different atmospheres and a typical emission spectrum are shown in Fig. 1. For (a)–(d), the ordinate
Fig. 1. Streak images of YO emission in different ambient gases (1 atm.). (a) In O2 ambient, (b) in CO2 ambient, (c) in He ambient, and (d) in N2 ambient. The ordinate represents time, which runs from top to bottom, with the full scale corresponding to 50.0 ls. The abscissa represents wavelength, which increases toward the right, with the full scale corresponding to 45.65 nm. Data accumulation was done over 5 laser pulses. (e) is a YO emission spectrum (A ! X) obtained by vertical integration of YO emission intensity. In addition to YO bands, some atomic lines are observed. They appear more brightly in (d) than other images partly because the relative intensity of YO emission is lower in nitrogen than other gases. Some of those lines correspond to transitions of yttrium atom and indicated with a star in (e).
55
represents time, which runs from top to bottom, with the full scale corresponding to 50.0 ls. The abscissa represents wavelength, which increases toward the right, with the full scale corresponding to 45.65 nm. The strong horizontal line at the top of each image corresponds to the laser pulse incident on the sample. It is seen that many parallel vertical lines are almost evenly spaced. This is a YO band system corresponding to the A2P3/2,1/2 ! X2R transition. Several atomic lines are also observed in the streak images. It should be noted that the brightness of the YO streak images do not reflect the actual emission intensity since the experimental conditions are not exactly the same and the contrast of the streak images have been manipulated. Fig. 1e is an emission spectrum for this transition obtained by temporal (vertical) integration of band intensities. Fig. 2 shows the temporal behavior of the YO emission for different atmospheres obtained by horizontal integration of
Fig. 2. Temporal emission profiles of YO measured in different ambient gases. The abscissa represents time where the incidence of ablation laser pulse is taken as time zero. Each profile was obtained by horizontally integrating the emission intensity of corresponding streak image (Fig. 1a– d). The laser fluence is 10 mJ/pulse. The emission intensities are roughly normalized but since the experimental conditions were slightly different for different measurements the emission intensities presented here may contain fairly large uncertainties up to about ±10%.
56
T. Kobayashi, T. Sekine / Chemical Physics Letters 424 (2006) 54–57
emission intensity plotted against time. We focused only on the YO emission in this study but there are many other emission lines observed all over the visible and ultraviolet regions, mostly due to laser-ablated atoms and atomic ions. For the other two samples, i.e. YVO4, and YAlO3, similar results were obtained as far as the YO emission is concerned and thus only the results of YAG crystal are presented in this report. Fig. 3 shows the effect of laser fluence on the time-varying YO emission for the cases where the ambient gas is He and O2. It is clearly seen in Figs. 1 and 2 that the temporal emission profiles of YO vary significantly depending on the ambient gas. The temporal profiles of O2 and CO2 appear similar and show relatively strong YO emission starting immediately after the incidence of the ablation laser pulse, while the emission intensity is initially weak for He atmosphere. For N2 atmosphere, a prolonged weak emission is
observed, which is in contrast to O2, CO2, or He cases where the YO emission fades away relatively quickly. The ablation laser fluence also affects the temporal profiles of the YO emission. Fig. 3 shows that the maximum emission point is slightly delayed as the laser fluence is increased but above 15 mJ/pulse further delay was not observed. It should be noted that the emission measurement here is basically line-integrated measurement but since the ambient gas pressure is high (1 atm) the luminous plume is considered to be well confined near the ablation point on the sample surface. When the ambient gas pressure is decreased, the YO emission is also decreased, indicating that the excited A state (the emitting state) of YO is produced mainly by collisions between laser-ablated species and ambient gas molecules or atoms. It is also presumed from this fact that only a small portion of YO molecules, if any, are in the excited A state when ejected from the sample surface as ablated species. Most of the laser-ablated YO molecules are considered to be in the ground electronic state when ejected and they are excited to the emitting state (A2P) by collisions with the ambient gas molecules or atoms. This process should be the main process for the formation of the excited A state of YO (YO(A)) when the ambient gas is He or N2 since there is no oxygen in the surrounding atmosphere. In addition to this process, another process has to be taken into account when the ambient gas is O2 or CO2, namely laser-ablated yttrium atoms react with oxygen-containing species (O, O2, or CO2) in the ambient gas to form YO(A), where oxygen atoms may be produced by collisions between laser-ablated energetic electrons and the ambient O2 or CO2 [4]. Both processes should contribute to the formation of YO(A) in O2 or CO2 atmosphere but the latter process could be a dominant one if we consider the following observations: (1) many laser ablation works have shown that, in an oxygen environment, YO is produced in postablation reactive collisions between ejected Y atoms and oxygen, (2) similar temporal emission profiles were observed for O2 and CO2 ambient (oxidant environment), which are quite different from those for He and N2 ambient (non-oxidant environment) where the dominant YO formation process is the direct ejection of YO as ablated-species. The latter process is considered responsible for the appearance of the common feature observed in the temporal emission profiles for O2 and CO2 atmospheres, namely the relatively strong emission starting immediately after the ablation laser incident (see Figs. 1 and 2). 4. Conclusions
Fig. 3. Temporal emission profiles of YO for various laser fluences measured in (a) He ambient and (b) O2 ambient.
Many works on the mechanism of YO formation during the initial laser ablation and the postablation events have been reported. Ablation conditions are not exactly the same and the conclusions on the dominant YO formation mechanism vary from experiment to experiment [1–6]. It was shown, in this study, that the temporal emission behavior of YO changes dramatically when the ambient gas is
T. Kobayashi, T. Sekine / Chemical Physics Letters 424 (2006) 54–57
changed. YO molecules were found to be formed by reaction between laser-ablated yttrium atoms and oxygen-containing ambient species and also YO molecules are directly ejected from the sample surface as ablated species and they emit light by collisional excitation with ambient gas species. The former process is observed in O2 and CO2 atmospheres and the latter process is considered to take place for all the ambient gases. The YO emission observed immediately after the ablation laser incident may be attributed to YO(A) molecules produced by the former process. Our experimental conditions were somewhat different from those of the previous works, the main difference being the high ambient pressure ( one atmospheric pressure), thus a simple comparison between our results and previously reported results may not be appropriate. Time-resolved emission measurement technique presented in this report is fairly simple and can be used as a convenient tool to
57
probe the dynamics of the ablation and postablation processes for diagnosis and optimization of plasma conditions.
References [1] X. Yang, Y. Tang, X. Liu, Q. Qin, Appl. Spec. 53 (1999) 278. [2] R.C. Dye, R.E. Muenchausen, N.S. Nogar, Chem. Phys. Lett. 181 (1991) 531. [3] H.F. Sakeek, T. Morrow, W.G. Graham, D.G. Walmsley, J. Appl. Phys. 75 (1994) 1138. [4] X.D. Wu, B. Dutta, M.S. Hegde, A. Inam, T. Venkatesan, E.W. Chase, C.C. Chang, R. Howard, Appl. Phys. Lett. 54 (1989) 179. [5] D. Fried, T. Kushida, G.P. Reck, E.W. Rothe, J. Appl. Phys. 73 (1993) 7810. [6] C.E. Otis, P.M. Goodwin, J. Appl. Phys. 73 (1993) 1957. [7] A. Bernard, R. Bacis, P. Luc, Astrophys. J. 227 (1979) 338. [8] C.L. Chalek, J.L. Gole, J. Chem. Phys. 65 (1976) 2845.
Laser ablation of yttrium-containing oxides in various ambient gases studied by time-resolved emission spectroscopy Takamichi Kobayashi *, Toshimori Sekine National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received 24 February 2006; in final form 18 April 2006 Available online 27 April 2006
Abstract Temporal emission profiles of YO molecule produced by laser ablation of yttrium-containing oxides have been measured in various ambient gases. Regardless of the kind of the ambient gas, the YO emission intensity is decreased with decreasing gas pressure. However, observed temporal behaviors differ significantly depending on the ambient gas. For O2 and CO2, similar profiles have been observed, while for non-oxide gases (He and N2), observed profiles are quite different from each other and also from those for oxide gases. This kind of measurement is very simple and useful for probing the dynamics of the ablation and postablation processes. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Pulsed laser ablation for the deposition of high-quality thin films is a well established technique. The nature of the laser-generated plasma and the surrounding atmosphere significantly influence film characteristics. Information on the details of the gas-phase chemistry is desirable for optimization of plasma conditions during laser ablation [1–6]. Emission spectroscopy is one of the techniques that have been successfully used to probe the dynamics of the ablation and postablation processes [2–5]. It is well known that YO is easily produced by laser ablation of yttrium-containing oxides such as Y–Ba–Cu oxide and Y2O3 [5]. In oxygen atmosphere, it is reported that the ablation plume from Y–Ba–Cu oxide becomes red due to YO chemiluminescence [6] and the oxygen pressure is an important parameter controlling the quality of the deposited films since the formation of oxides (YO, BaO, and CuO) is essential for the production of high quality films [4]. The YO transitions in the visible region is well studied [7,8] and YO emission measurement was proven *
Corresponding author. Fax.: +81 29 851 2768. E-mail address: [email protected] (T. Kobayashi). 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.04.056
useful as a convenient diagnostic for optimization of plasma conditions [3,4]. YO formation in laser ablation of yttrium-containing oxides is fairly well studied. There are only two main ways for YO formation: (i) laser-ablated yttrium atoms react with oxygen-containing species to form YO, (ii) YO molecules are directly ejected from the sample surface as laser-ablated species. Which process is dominant appears to vary from experiment to experiment [1–6] and there are still some ambiguous points. In this letter, we report a time-resolved emission study of the plume generated from laser ablation of YAG (Y3Al5O12) crystal and other yttrium-containing oxides with various surrounding atmospheres. This kind of temporal profile measurements can provide essential information on the dynamics of the ablation and postablation processes. 2. Experimental The experimental set-up employed in this study was rather simple. The Nd:YAG laser (532 nm, 8 ns pulse, and 5–20 mJ/pulse) was incident normal and focused down to about 0.5 mm in diameter on the sample surface. When the laser focus point was too deep inside the sample, i.e. more than 2 mm, the YO emission disappeared. Emission from the vicinity of the irradiated point on the sample sur-
T. Kobayashi, T. Sekine / Chemical Physics Letters 424 (2006) 54–57
face was collected and introduced into a spectrometer. Dispersed light was then transmitted to a streak camera to obtain the time-varying emission image recorded by a CCD camera. Four different ambient gases were used, namely O2, CO2, He, and N2 with the gas pressure near one atmosphere in all the experiments. The oxygen contents in CO2, He, and N2 are expected to be less than 1%. Samples used were single crystals of YAG, YVO4, and YAlO3. 3. Results and discussion Representative streak images of emission from the laserablated YAG in different atmospheres and a typical emission spectrum are shown in Fig. 1. For (a)–(d), the ordinate
Fig. 1. Streak images of YO emission in different ambient gases (1 atm.). (a) In O2 ambient, (b) in CO2 ambient, (c) in He ambient, and (d) in N2 ambient. The ordinate represents time, which runs from top to bottom, with the full scale corresponding to 50.0 ls. The abscissa represents wavelength, which increases toward the right, with the full scale corresponding to 45.65 nm. Data accumulation was done over 5 laser pulses. (e) is a YO emission spectrum (A ! X) obtained by vertical integration of YO emission intensity. In addition to YO bands, some atomic lines are observed. They appear more brightly in (d) than other images partly because the relative intensity of YO emission is lower in nitrogen than other gases. Some of those lines correspond to transitions of yttrium atom and indicated with a star in (e).
55
represents time, which runs from top to bottom, with the full scale corresponding to 50.0 ls. The abscissa represents wavelength, which increases toward the right, with the full scale corresponding to 45.65 nm. The strong horizontal line at the top of each image corresponds to the laser pulse incident on the sample. It is seen that many parallel vertical lines are almost evenly spaced. This is a YO band system corresponding to the A2P3/2,1/2 ! X2R transition. Several atomic lines are also observed in the streak images. It should be noted that the brightness of the YO streak images do not reflect the actual emission intensity since the experimental conditions are not exactly the same and the contrast of the streak images have been manipulated. Fig. 1e is an emission spectrum for this transition obtained by temporal (vertical) integration of band intensities. Fig. 2 shows the temporal behavior of the YO emission for different atmospheres obtained by horizontal integration of
Fig. 2. Temporal emission profiles of YO measured in different ambient gases. The abscissa represents time where the incidence of ablation laser pulse is taken as time zero. Each profile was obtained by horizontally integrating the emission intensity of corresponding streak image (Fig. 1a– d). The laser fluence is 10 mJ/pulse. The emission intensities are roughly normalized but since the experimental conditions were slightly different for different measurements the emission intensities presented here may contain fairly large uncertainties up to about ±10%.
56
T. Kobayashi, T. Sekine / Chemical Physics Letters 424 (2006) 54–57
emission intensity plotted against time. We focused only on the YO emission in this study but there are many other emission lines observed all over the visible and ultraviolet regions, mostly due to laser-ablated atoms and atomic ions. For the other two samples, i.e. YVO4, and YAlO3, similar results were obtained as far as the YO emission is concerned and thus only the results of YAG crystal are presented in this report. Fig. 3 shows the effect of laser fluence on the time-varying YO emission for the cases where the ambient gas is He and O2. It is clearly seen in Figs. 1 and 2 that the temporal emission profiles of YO vary significantly depending on the ambient gas. The temporal profiles of O2 and CO2 appear similar and show relatively strong YO emission starting immediately after the incidence of the ablation laser pulse, while the emission intensity is initially weak for He atmosphere. For N2 atmosphere, a prolonged weak emission is
observed, which is in contrast to O2, CO2, or He cases where the YO emission fades away relatively quickly. The ablation laser fluence also affects the temporal profiles of the YO emission. Fig. 3 shows that the maximum emission point is slightly delayed as the laser fluence is increased but above 15 mJ/pulse further delay was not observed. It should be noted that the emission measurement here is basically line-integrated measurement but since the ambient gas pressure is high (1 atm) the luminous plume is considered to be well confined near the ablation point on the sample surface. When the ambient gas pressure is decreased, the YO emission is also decreased, indicating that the excited A state (the emitting state) of YO is produced mainly by collisions between laser-ablated species and ambient gas molecules or atoms. It is also presumed from this fact that only a small portion of YO molecules, if any, are in the excited A state when ejected from the sample surface as ablated species. Most of the laser-ablated YO molecules are considered to be in the ground electronic state when ejected and they are excited to the emitting state (A2P) by collisions with the ambient gas molecules or atoms. This process should be the main process for the formation of the excited A state of YO (YO(A)) when the ambient gas is He or N2 since there is no oxygen in the surrounding atmosphere. In addition to this process, another process has to be taken into account when the ambient gas is O2 or CO2, namely laser-ablated yttrium atoms react with oxygen-containing species (O, O2, or CO2) in the ambient gas to form YO(A), where oxygen atoms may be produced by collisions between laser-ablated energetic electrons and the ambient O2 or CO2 [4]. Both processes should contribute to the formation of YO(A) in O2 or CO2 atmosphere but the latter process could be a dominant one if we consider the following observations: (1) many laser ablation works have shown that, in an oxygen environment, YO is produced in postablation reactive collisions between ejected Y atoms and oxygen, (2) similar temporal emission profiles were observed for O2 and CO2 ambient (oxidant environment), which are quite different from those for He and N2 ambient (non-oxidant environment) where the dominant YO formation process is the direct ejection of YO as ablated-species. The latter process is considered responsible for the appearance of the common feature observed in the temporal emission profiles for O2 and CO2 atmospheres, namely the relatively strong emission starting immediately after the ablation laser incident (see Figs. 1 and 2). 4. Conclusions
Fig. 3. Temporal emission profiles of YO for various laser fluences measured in (a) He ambient and (b) O2 ambient.
Many works on the mechanism of YO formation during the initial laser ablation and the postablation events have been reported. Ablation conditions are not exactly the same and the conclusions on the dominant YO formation mechanism vary from experiment to experiment [1–6]. It was shown, in this study, that the temporal emission behavior of YO changes dramatically when the ambient gas is
T. Kobayashi, T. Sekine / Chemical Physics Letters 424 (2006) 54–57
changed. YO molecules were found to be formed by reaction between laser-ablated yttrium atoms and oxygen-containing ambient species and also YO molecules are directly ejected from the sample surface as ablated species and they emit light by collisional excitation with ambient gas species. The former process is observed in O2 and CO2 atmospheres and the latter process is considered to take place for all the ambient gases. The YO emission observed immediately after the ablation laser incident may be attributed to YO(A) molecules produced by the former process. Our experimental conditions were somewhat different from those of the previous works, the main difference being the high ambient pressure ( one atmospheric pressure), thus a simple comparison between our results and previously reported results may not be appropriate. Time-resolved emission measurement technique presented in this report is fairly simple and can be used as a convenient tool to
57
probe the dynamics of the ablation and postablation processes for diagnosis and optimization of plasma conditions.
References [1] X. Yang, Y. Tang, X. Liu, Q. Qin, Appl. Spec. 53 (1999) 278. [2] R.C. Dye, R.E. Muenchausen, N.S. Nogar, Chem. Phys. Lett. 181 (1991) 531. [3] H.F. Sakeek, T. Morrow, W.G. Graham, D.G. Walmsley, J. Appl. Phys. 75 (1994) 1138. [4] X.D. Wu, B. Dutta, M.S. Hegde, A. Inam, T. Venkatesan, E.W. Chase, C.C. Chang, R. Howard, Appl. Phys. Lett. 54 (1989) 179. [5] D. Fried, T. Kushida, G.P. Reck, E.W. Rothe, J. Appl. Phys. 73 (1993) 7810. [6] C.E. Otis, P.M. Goodwin, J. Appl. Phys. 73 (1993) 1957. [7] A. Bernard, R. Bacis, P. Luc, Astrophys. J. 227 (1979) 338. [8] C.L. Chalek, J.L. Gole, J. Chem. Phys. 65 (1976) 2845.