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Angular resolved EPMA analysis of thin films prepared by pulsed laser ablation of CdWO4(010)
s __
__ l!iB
ELSEVIER
applied
surface science Applied Surface Science lOO/lOl
(1996) 264-267
Angular resolved EPMA analysis of thin films prepared by pulsed laser ablation of CdWO,(O 10) Katsumi Tanaka *, Naruhiko Yokota, Natsuki Shirai, Quan Zhuang, Ryohei Nakata Department
ofElectronic Engineering, Uniwrsity of Electra-C~~mmunications, T&y Received
15 August
1995; accepted 25 October
182. Japan
1995
Abstract Thin films prepared by focused 266 nm pulsed laser ablation of CdWOJOlO) single crystals were analyzed with an electron probe micro analysis (EPMA) as a function of the desorbing angle 0 from the normal surface, which corresponds to the position of the films. The spatial distribution of the ablated particles and the chemical composition of the films were studied by focusing the effects of O2 gas pressure and the substrate-target distance. These results were interpreted by the
comparison with the size of the ablation plume determined by the photograph. The thin films with nearly the stoichiometric composition showed a photoluminescence similar to the single crystal.
1. Introduction
2. Experimental
Laser ablation has gathered much interest both from applicational and mechanistic point of view [I]. We recently reported the photochemical laser ablation of CdWO,(OlO> scintillator studied with timeof-flight (TOF) mass spectrometric methods [2]. CdWO, has been used as one of the X-ray scintillators with a photoluminescence peaked at 540 nm [3]. If the thin film can be prepared on Si, it can be directly applied to a device for detecting high energy beams. Ambient gas has been used to the thin film preparation by laser ablation, where the mass spectrometric TOF analysis cannot be applied. Therefore in this report we present results of EPMA analyses of the films prepared by the pulsed laser ablation of CdWO,(OlO) in the presence of OZ.
Experiments were carried out in an ICF 70 stainless cube, in which the pressure could be controlled from an ultra high vacuum region to an O? pressure up to 1 atm. A CdWO, single crystal was cleaved along (010) plane and the cut plate was placed parallel to a Sic1 11) substrate heated at 673 K. The CdWO, target temperature was RT. The fourth barmanic generation (266 nm) of an Nd:YAG pulsed laser (- 4 ns. 10 Hz) was focused with a quartz lens and was radiated to the CdWO, target through a sapphire viewport with an angle of 45”. The laser beam spot size was 3.36 mm’. Laser ablation was carried out for 3 min. (1800 shots) at laser fluences of 220 + 40 mJ/cm*. The Cd and W distributions in the thin film was analyzed with an EPMA (JCXA733, JEOL) using an analyzing program for oxides as a function of angle 8 from surface normal to an analyzed point of the film (center of the thin film corresponds to 8 = 0”).
* Corresponding + 81-424-803801;
author. Tel.: + 81-424-832161 ext. 3412; fax: e-mail: [email protected].
0169-4332/96/$15.00 Copyright PII SO169-4332(96)00301-7
0 1996 Elsevier Science B.V. All rights reserved.
K. Tanaka et al/Applied
265
Surjace Science lOO/ 101 (1996) 266267
3. Results and discussion The stoichiometric thin film preparation by the 266 nm laser ablation was successful in the presence of 0,. Fig. 1 shows Cd EPMA intensities in the films prepared by the laser ablation of CdWO,(OlO) in the presence of 0, as a function of 0. The Cd EPMA intensity at the center of the film is normalized. Oxygen pressures were 1, 10, 30 and 50 Torr. Here the target-substrate distance was kept constant (21 mm). These data indicate that the forward peaking of Cd species during laser ablation is enhanced with high pressure of OZ. Normally if surface species thermally evaporate, the spatial distribution should be widespread and the intensity obeys cos 0. Compared to a thermal evaporation, the spatial distribution of ablated species is strongly forward peaking and the behavior is summarized by n value of cos”0 intensity variation. The n values of Cd and W in the films when the 0, pressure and the substrate-target distance are changed are listed in Table 1. It is found that (a) the n value increases as 0, pressure is increased, (b) these n values are quite large compared to a thermal evaporation process, and (c) W species takes much stronger forward peaking than Cd species in the ablated films. Interestingly there is a maximum n value both of Cd and W in the films when the target-substrate distance changes from 8 mm to 21 mm (n value of Cd changes 28, 34 and 23
Desorbingangle 8 Fig. 1, The EPMA intensities of Cd in the thin films prepared by the laser ablation of CdWO, in the presence of 02. Here the 0, pressure was 1 Torr (o), 10 Torr (01, 30 Torr (A) and 50 Torr ( n ) and the target-substrate was 21 mm. The lines in the figure indicate cos”0 relations and these values are listed in Table 1.
Table 1 N values of cos”8 dependencies on the and W species in the films prepared by of CdWO,(OlO) as a function of 0. The that of target-substrate distance both listed
EPMA intensity of the Cd the 266 nm laser ablation effect of 0, pressure and on the Cd/W ratios are
n of cos”8 Cd O2 pressure (Torr) a 1 17 10 b 23 30 30 47 50
Target-substrate 5 8 15 21 b
distance (mm) ’ 4 28 34 23
W
10 33 60 66
negative 29 46 33
’ Target-substrate distance 2 1 mm. b The same experiment. ’ O2 pressure 10 Torr.
at 8, 15 and 21 mm, while that of W changes 29,46 and 33, respectively). These results suggest that as the target-substrate distance increases, ablated species flying in the off-normal direction are suppressed probably because of losing their kinetic energies by collision with 0, molecules, however when the distance exceeds a threshold value, ablated species flying in the normal direction also lose their kinetic energies by collision with 0,. The former suggestion may be influenced by that kinetic energies of ablated species in off-normal direction will be lower than those in the normal direction. The negative IZ value at the target-substrate distance of 5 mm implies that the periphery is much thicker than the center of the thin film. This should be influenced by the reflection of ablated species on the Si substrate because of the target-substrate distance smaller than the plume size. We have no information on the thickness of the CdWO, ablated thin films. We assume that the thickness is quite small, probably below an EPMA detection limit, N km. In such a case the EPMA intensity of the thin films should reflect the thickness. The EPMA detection limit depends also on the absorption coefficient of the sample. CdWO, has a very large X-ray absorption coef-
266
K. Tanaka et al. /Applied Surface Science 100 / 101 C1996) 264-267
ficient. Therefore analysis of the thin film by EPMA must be carefully studied with consideration of these factors. An angular distribution of ablated W ionic species measured by Q-mass in vacuum took cos5”8 [2]. While in the presence of 1 Torr 0, a forward peaking of W species in the film took COS’~~ and II value increased as O2 pressure increased. However the laser fluences used and the laser spot sizes are quite different between the two experiments and these factors must be studied. We could see ‘plume’ in the laser ablation of CdWO, at the laser fluences used in the present study. The effects of the plume size on these data are so significant that we tried to determine the plume size by photographs. Fig. 2 shows an ablation plume observed in the presence of 1 Torr OZ. The measured size of the plume (length and width) was 21.4 and 8.3 mm, 9.5 and 2.6 mm. 7.7 and 4.8 mm, and 6.5 and 3.8 mm at O2 pressure of 1, 10. 30. and 50 Torr, respectively. The effect of 0, pressure on the Cd/W ratio in the films was examined at a constant target-substrate distance of 21 mm. while the effect of target-
Fig. 2. The photograph ml/cm”.
substrate distance on the ratio was examined at a constant O? pressure of 10 Torr. The atomic ratios of Cd/W in the thin films are shown in Fig. 3 as a function of 8. The ratio in the film prepared at O2 pressure of 1 Torr was smaller than 1.0 and decreased as the angle t3 increased, which is influenced by the n dependency shown in Table 1 (17 for Cd and 10 for W). The Cd/W ratio at the center of the films increased as the 0, pressure increased from 1 Torr to 50 Torr. While at O? pressures of 10, 30 and 50 Torr, the Cd/W ratios increased toward the periphery of the film. It is easily found that the most probable O? pressure should be present between 1 Torr and 10 Torr for the stoichiometric Cd/W ratio in the film. At the substrate-target distance of 5, 8, and 15 mm, the stoichiometric Cd/W ratio was almost obtained. However, when the distance increased to 21 mm, the Cd/W ratio in the film increased to about 1.7 and it increased as the angle 0 increased. As the length of the ablation plume was 9.5 mm at O2 pressure of 10 Torr, the almost stoichiometric Cd/W ratio can be obtained at the substrate-target distance near the value. This is based
of ‘plume’ in the pulsed laser ablation of CdWOJOlO)
in the presence of 1 Torr 0:
with a laser fluence of 260
K. Tanaka et al./Applied
Surface Science lOO/ 101 (19961264-267
267
determined. Although the crystal structure is unknown, photoluminescence was observed on the films with the almost stoichiometric composition. According to Ref. [3] CdWO, single crystal should show the photoluminescence peaked at 540 nm. However the CdWO, single crystal we used showed a photoluminescence peaked at around 460 nm. The films with the stoichiometric Cd and W composition in Fig. 3 showed photoluminescences peaked at 430 to 450 nm. The films prepared by the other conditions showed no photoluminescence.
4. Conclusions
o-o
10
0
Wangle
20 8
Fig. 3. The atomic ratio of Cd/W in the thin films as a function of 8. The ablation condition was in the presence of O? at a pressure of 1 Torr (0 1. 10 Torr (O), 30 Torr ( A ) and 50 Torr ( n 1, and at a target-substrate distance of 5 mm ( @ 1. 8 mm (0 1, 15 mm (0) and 21 mm (0).
on the idea that the stoichiometric Cd/W ratio is maintained in the plume. When the substrate-target distance exceeds the plume length, a Cd rich film should be obtained. The thin film, which was obtained by the pulsed laser ablation of CdWO, at O? pressure of 10 Torr and the substrate-target distance of 5 mm. showed a new X-ray diffraction peak at 28 = 35”. This peak can be assigned to CdW0,(021), CdWO,(OO2) and WO,(122). Therefore at the present moment the crystalline structure of the ablated film cannot be
The stoichiometric thin films can be prepared by the 266 nm laser ablation of CdWO,(OlO) with a laser fluence of 220 k 40 mJ/cm2 in the presence of 0, at a pressure between 1 and 10 Torr with the target-substrate distance near the plume length, and the thin film thus obtained by the laser ablation shows a photoluminescence similar to the CdWO,(OlO) target.
References [I] J.C. Miller and D.G. Geohegan. Laser Ablation: Mechanisms and Applications II, AIP Conf. Proc.. Vol. 288 (AIP, New York, 1993). [2] K. Tanaka, T. Miyajima, N. Shirai, Q, Zhuang and R. Nakata, J. Appl. Phys. 77 (19951 6581. [3] C.D. Greskovich, D. Cusano. D. Hoffman and R.J. Riedner, J. Am. Ceram. Sot. Bull. 71 (19921 1120.
__ l!iB
ELSEVIER
applied
surface science Applied Surface Science lOO/lOl
(1996) 264-267
Angular resolved EPMA analysis of thin films prepared by pulsed laser ablation of CdWO,(O 10) Katsumi Tanaka *, Naruhiko Yokota, Natsuki Shirai, Quan Zhuang, Ryohei Nakata Department
ofElectronic Engineering, Uniwrsity of Electra-C~~mmunications, T&y Received
15 August
1995; accepted 25 October
182. Japan
1995
Abstract Thin films prepared by focused 266 nm pulsed laser ablation of CdWOJOlO) single crystals were analyzed with an electron probe micro analysis (EPMA) as a function of the desorbing angle 0 from the normal surface, which corresponds to the position of the films. The spatial distribution of the ablated particles and the chemical composition of the films were studied by focusing the effects of O2 gas pressure and the substrate-target distance. These results were interpreted by the
comparison with the size of the ablation plume determined by the photograph. The thin films with nearly the stoichiometric composition showed a photoluminescence similar to the single crystal.
1. Introduction
2. Experimental
Laser ablation has gathered much interest both from applicational and mechanistic point of view [I]. We recently reported the photochemical laser ablation of CdWO,(OlO> scintillator studied with timeof-flight (TOF) mass spectrometric methods [2]. CdWO, has been used as one of the X-ray scintillators with a photoluminescence peaked at 540 nm [3]. If the thin film can be prepared on Si, it can be directly applied to a device for detecting high energy beams. Ambient gas has been used to the thin film preparation by laser ablation, where the mass spectrometric TOF analysis cannot be applied. Therefore in this report we present results of EPMA analyses of the films prepared by the pulsed laser ablation of CdWO,(OlO) in the presence of OZ.
Experiments were carried out in an ICF 70 stainless cube, in which the pressure could be controlled from an ultra high vacuum region to an O? pressure up to 1 atm. A CdWO, single crystal was cleaved along (010) plane and the cut plate was placed parallel to a Sic1 11) substrate heated at 673 K. The CdWO, target temperature was RT. The fourth barmanic generation (266 nm) of an Nd:YAG pulsed laser (- 4 ns. 10 Hz) was focused with a quartz lens and was radiated to the CdWO, target through a sapphire viewport with an angle of 45”. The laser beam spot size was 3.36 mm’. Laser ablation was carried out for 3 min. (1800 shots) at laser fluences of 220 + 40 mJ/cm*. The Cd and W distributions in the thin film was analyzed with an EPMA (JCXA733, JEOL) using an analyzing program for oxides as a function of angle 8 from surface normal to an analyzed point of the film (center of the thin film corresponds to 8 = 0”).
* Corresponding + 81-424-803801;
author. Tel.: + 81-424-832161 ext. 3412; fax: e-mail: [email protected].
0169-4332/96/$15.00 Copyright PII SO169-4332(96)00301-7
0 1996 Elsevier Science B.V. All rights reserved.
K. Tanaka et al/Applied
265
Surjace Science lOO/ 101 (1996) 266267
3. Results and discussion The stoichiometric thin film preparation by the 266 nm laser ablation was successful in the presence of 0,. Fig. 1 shows Cd EPMA intensities in the films prepared by the laser ablation of CdWO,(OlO) in the presence of 0, as a function of 0. The Cd EPMA intensity at the center of the film is normalized. Oxygen pressures were 1, 10, 30 and 50 Torr. Here the target-substrate distance was kept constant (21 mm). These data indicate that the forward peaking of Cd species during laser ablation is enhanced with high pressure of OZ. Normally if surface species thermally evaporate, the spatial distribution should be widespread and the intensity obeys cos 0. Compared to a thermal evaporation, the spatial distribution of ablated species is strongly forward peaking and the behavior is summarized by n value of cos”0 intensity variation. The n values of Cd and W in the films when the 0, pressure and the substrate-target distance are changed are listed in Table 1. It is found that (a) the n value increases as 0, pressure is increased, (b) these n values are quite large compared to a thermal evaporation process, and (c) W species takes much stronger forward peaking than Cd species in the ablated films. Interestingly there is a maximum n value both of Cd and W in the films when the target-substrate distance changes from 8 mm to 21 mm (n value of Cd changes 28, 34 and 23
Desorbingangle 8 Fig. 1, The EPMA intensities of Cd in the thin films prepared by the laser ablation of CdWO, in the presence of 02. Here the 0, pressure was 1 Torr (o), 10 Torr (01, 30 Torr (A) and 50 Torr ( n ) and the target-substrate was 21 mm. The lines in the figure indicate cos”0 relations and these values are listed in Table 1.
Table 1 N values of cos”8 dependencies on the and W species in the films prepared by of CdWO,(OlO) as a function of 0. The that of target-substrate distance both listed
EPMA intensity of the Cd the 266 nm laser ablation effect of 0, pressure and on the Cd/W ratios are
n of cos”8 Cd O2 pressure (Torr) a 1 17 10 b 23 30 30 47 50
Target-substrate 5 8 15 21 b
distance (mm) ’ 4 28 34 23
W
10 33 60 66
negative 29 46 33
’ Target-substrate distance 2 1 mm. b The same experiment. ’ O2 pressure 10 Torr.
at 8, 15 and 21 mm, while that of W changes 29,46 and 33, respectively). These results suggest that as the target-substrate distance increases, ablated species flying in the off-normal direction are suppressed probably because of losing their kinetic energies by collision with 0, molecules, however when the distance exceeds a threshold value, ablated species flying in the normal direction also lose their kinetic energies by collision with 0,. The former suggestion may be influenced by that kinetic energies of ablated species in off-normal direction will be lower than those in the normal direction. The negative IZ value at the target-substrate distance of 5 mm implies that the periphery is much thicker than the center of the thin film. This should be influenced by the reflection of ablated species on the Si substrate because of the target-substrate distance smaller than the plume size. We have no information on the thickness of the CdWO, ablated thin films. We assume that the thickness is quite small, probably below an EPMA detection limit, N km. In such a case the EPMA intensity of the thin films should reflect the thickness. The EPMA detection limit depends also on the absorption coefficient of the sample. CdWO, has a very large X-ray absorption coef-
266
K. Tanaka et al. /Applied Surface Science 100 / 101 C1996) 264-267
ficient. Therefore analysis of the thin film by EPMA must be carefully studied with consideration of these factors. An angular distribution of ablated W ionic species measured by Q-mass in vacuum took cos5”8 [2]. While in the presence of 1 Torr 0, a forward peaking of W species in the film took COS’~~ and II value increased as O2 pressure increased. However the laser fluences used and the laser spot sizes are quite different between the two experiments and these factors must be studied. We could see ‘plume’ in the laser ablation of CdWO, at the laser fluences used in the present study. The effects of the plume size on these data are so significant that we tried to determine the plume size by photographs. Fig. 2 shows an ablation plume observed in the presence of 1 Torr OZ. The measured size of the plume (length and width) was 21.4 and 8.3 mm, 9.5 and 2.6 mm. 7.7 and 4.8 mm, and 6.5 and 3.8 mm at O2 pressure of 1, 10. 30. and 50 Torr, respectively. The effect of 0, pressure on the Cd/W ratio in the films was examined at a constant target-substrate distance of 21 mm. while the effect of target-
Fig. 2. The photograph ml/cm”.
substrate distance on the ratio was examined at a constant O? pressure of 10 Torr. The atomic ratios of Cd/W in the thin films are shown in Fig. 3 as a function of 8. The ratio in the film prepared at O2 pressure of 1 Torr was smaller than 1.0 and decreased as the angle t3 increased, which is influenced by the n dependency shown in Table 1 (17 for Cd and 10 for W). The Cd/W ratio at the center of the films increased as the 0, pressure increased from 1 Torr to 50 Torr. While at O? pressures of 10, 30 and 50 Torr, the Cd/W ratios increased toward the periphery of the film. It is easily found that the most probable O? pressure should be present between 1 Torr and 10 Torr for the stoichiometric Cd/W ratio in the film. At the substrate-target distance of 5, 8, and 15 mm, the stoichiometric Cd/W ratio was almost obtained. However, when the distance increased to 21 mm, the Cd/W ratio in the film increased to about 1.7 and it increased as the angle 0 increased. As the length of the ablation plume was 9.5 mm at O2 pressure of 10 Torr, the almost stoichiometric Cd/W ratio can be obtained at the substrate-target distance near the value. This is based
of ‘plume’ in the pulsed laser ablation of CdWOJOlO)
in the presence of 1 Torr 0:
with a laser fluence of 260
K. Tanaka et al./Applied
Surface Science lOO/ 101 (19961264-267
267
determined. Although the crystal structure is unknown, photoluminescence was observed on the films with the almost stoichiometric composition. According to Ref. [3] CdWO, single crystal should show the photoluminescence peaked at 540 nm. However the CdWO, single crystal we used showed a photoluminescence peaked at around 460 nm. The films with the stoichiometric Cd and W composition in Fig. 3 showed photoluminescences peaked at 430 to 450 nm. The films prepared by the other conditions showed no photoluminescence.
4. Conclusions
o-o
10
0
Wangle
20 8
Fig. 3. The atomic ratio of Cd/W in the thin films as a function of 8. The ablation condition was in the presence of O? at a pressure of 1 Torr (0 1. 10 Torr (O), 30 Torr ( A ) and 50 Torr ( n 1, and at a target-substrate distance of 5 mm ( @ 1. 8 mm (0 1, 15 mm (0) and 21 mm (0).
on the idea that the stoichiometric Cd/W ratio is maintained in the plume. When the substrate-target distance exceeds the plume length, a Cd rich film should be obtained. The thin film, which was obtained by the pulsed laser ablation of CdWO, at O? pressure of 10 Torr and the substrate-target distance of 5 mm. showed a new X-ray diffraction peak at 28 = 35”. This peak can be assigned to CdW0,(021), CdWO,(OO2) and WO,(122). Therefore at the present moment the crystalline structure of the ablated film cannot be
The stoichiometric thin films can be prepared by the 266 nm laser ablation of CdWO,(OlO) with a laser fluence of 220 k 40 mJ/cm2 in the presence of 0, at a pressure between 1 and 10 Torr with the target-substrate distance near the plume length, and the thin film thus obtained by the laser ablation shows a photoluminescence similar to the CdWO,(OlO) target.
References [I] J.C. Miller and D.G. Geohegan. Laser Ablation: Mechanisms and Applications II, AIP Conf. Proc.. Vol. 288 (AIP, New York, 1993). [2] K. Tanaka, T. Miyajima, N. Shirai, Q, Zhuang and R. Nakata, J. Appl. Phys. 77 (19951 6581. [3] C.D. Greskovich, D. Cusano. D. Hoffman and R.J. Riedner, J. Am. Ceram. Sot. Bull. 71 (19921 1120.