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Effects of transverse magnetic field on a laser-produced Zn plasma plume and ZnO films grown by pulsed laser deposition
Applied Surface Science 253 (2007) 8054–8058 www.elsevier.com/locate/apsusc
Effects of transverse magnetic field on a laser-produced Zn plasma plume and ZnO films grown by pulsed laser deposition Tae Hyun Kim, Sang Hwan Nam, Hye Sun Park, Jae Kyu Song *, Seung Min Park * Department of Chemistry, Kyunghee University, Seoul 130-701, Republic of Korea Available online 2 March 2007
Abstract By adopting a fast photography and time-resolved optical emission spectrometry, we have investigated the effects of transverse magnetic field on the expansion dynamics and enrichment of Zn atoms and Zn+ ions in a plume produced by laser ablation of a Zn target in oxygen atmosphere. Plume splitting due to the magnetic field was apparent but the splitting patterns of Zn and Zn+ were totally different. The surface morphology and photoluminescence characteristics also changed significantly. In particular, the growth rate increased by as much as 2.4–4.3 times compared to the conventional PLD method. # 2007 Elsevier B.V. All rights reserved. PACS : 79.20.Ds; 52.50.Jm Keywords: Laser ablation; Laser-induced plasma; Magnetic field
1. Introduction Pulsed laser deposition (PLD) has been widely employed as a straightforward method to grow thin films of superconductors, ferroelectrics, metals, and refractory materials [1]. In PLD, the quality of the deposited film is in essence determined by the characteristics of the laser-produced plasma plume formed via irradiation of a solid target using a focused pulsed laser beam. In this regard, it would be highly desirable to control the properties of the plume, which consists of electrons, atoms, ions, and molecules including clusters. Among these, energetic ions are, in general, most valuable species to produce films with high quality. Relative concentration and kinetic energy of ions in the plume can be controlled by changing parameters such as laser fluence and its wavelength. A more direct method is to apply electric field between the target and the substrate. Many researchers investigated the effects of electric field on the laser-produced plume and observed that kinetic energy of ions, optical emission from neutral and ionic species, and properties of the deposited films
* Corresponding authors. Tel.: +82 2 961 0226; fax: +82 2 957 4856. E-mail addresses: [email protected] (J.K. Song), [email protected] (S.M. Park). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.154
are strongly dependent on the polarity and strength of the electric field [2–4]. As well as electric field, magnetic field would certainly have significant effects on the formation and expansion of plume through interaction of current density with magnetic field. Recently, Kobayashi et al. [5,6] developed a new PLD method named ‘‘Aurora PLD’’ under application of axial magnetic field from the substrate side. This method has been developed with a goal to enhance the activation and/or ionization of the ablated species during transport from the target to substrate and turned out to be superior to the conventional PLD in producing high-quality films. Also PLD under a curved magnetic field proved to be useful in the growth of debris free films by confining and guiding the plume [7]. Here, we present experimental results related to the dramatic effect of transverse magnetic field on the expansion dynamics of the Zn plume in oxygen atmosphere as well as on the morphology and photoluminescence (PL) of the deposited films. Time-resolved spatial distributions of Zn* and Zn+* were separately monitored by coupling of an intensified charge coupled device (ICCD) with interference filters. In particular, under the transverse magnetic field, splitting of the plume into two lobes was clearly observed for Zn*, while more localized Zn+* ions was split axially and their emission became much more intense.
T.H. Kim et al. / Applied Surface Science 253 (2007) 8054–8058
2. Experimental The schematic diagram of the experiment is shown in Fig. 1. A Zn target (Niraco, 99.999%) with size of 25.4 mm in diameter was used as purchased without further treatment. The target was mounted in between the poles which were apart by 1.0 cm. The magnetic field was minimum (0.19–0.29 T) at the center and maximum (0.30–0.32 T) near the poles along the field direction as shown in the inset of Fig. 1. A Q-switched Nd:YAG laser (l = 1064 nm, Minilite II, Continuum, pulse duration = 6 ns) operating at 10 Hz was loosely focused onto the Zn target (Nilaco, 99.999%) placed in a vacuum chamber using a lens (focal length 30 cm) with an angle of incidence of 08. The diameter of the focused laser spot was 1.7 mm and the laser fluence at the target surface is estimated to be 1.8 J/cm2. The Zn target was rotated by a standard rotary motion feedthrough to avoid a target aging effect. Si substrate with size of 5 mm 5 mm was installed between the magnets at a position 3.1 cm away from the target and PLD was performed for 30 min in oxygen atmosphere. Oxygen gas (99.999%) was fed to the chamber by a needle valve and the pressure was measured by a full range gauge (Balzers PKR250). In order to elucidate the effects of the magnetic field on the expansion of the plume and deposition of ZnO films, the magnets were replaced with Al blocks of the same size when the expansion was examined without field or the films were grown by conventional PLD. Optical emission studies on the laser-generated plume of the Zn target were done in oxygen atmosphere. Optical emission from the electronically excited states of Zn and Zn+ in a plume was collected using a lens of 5 cm focal length. The emission spectra were recorded at 8 mm away from the target surface, where the emission was intense enough in the pressure range chosen in our experiment and the continuum emission near the target surface was not collected. The optical emission at the
Fig. 1. The schematic diagram of the experiment. Temporal images of Zn plasma plume generated by irradiation of a Zn target with a focused 1064 nm laser pulse were obtain by an ICCD with magnetic field on and off. Two permanent magnets were mounted facing each other and the distance between them was 1.0 cm. The inset shows the spatial variation of the magnetic field. The laser fluence was 1.7 J/cm2. To get time-resolved optical emission spectra, ICCD was removed and the plume emission was fed to a monochromator coupled to an ICCD via an optical fiber bundle mounted on a three-dimensional translational stage.
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sampling position was fed to the monochromator coupled to an ICCD detector (Andor, DH 734) via an optical fiber bundle (Spex 700FB). The diameter of the entrance of optical fiber bundle was 0.8 mm. The gate width of the ICCD was fixed at 10 ns. Both spatially and temporally resolved measurements of the emission spectra were obtained by translating the receiving end of the optical fiber within the image of plasma. Also, timeresolved plume images were captured by using the ICCD. 3. Results and discussion Fig. 2 shows time-resolved optical emission spectra of the plume with the magnetic field on and off, recorded at 8 mm away from the target as a function of the delay time between the laser pulse and the gate of ICCD. The emission peak at 481.05 nm originates from Zn* and the two peaks at 491.16 nm and 492.40 nm represent Zn+*. Under magnetic field, the temperature of the plume is in general higher due to joule heating and electromagnetic compression of the plasma and thus electronically excited species are more enriched in the plasma. Neogi et al. [8] reported that the emission intensities from both carbon atoms and ions increased significantly with magnetic field. In our experiment, however, the optical emission intensity from Zn* decreased down to less than half while that from Zn+* increased by more than factor of 3 with application of the magnetic field. Since the ionization potential of Zn (9.39 eV) is lower than that of carbon (11.26 eV), Zn atoms (including Zn*) are more easily ionized via energetic collisions under the magnetic field and accordingly the emission intensity from Zn* atoms decreases.
Fig. 2. Time-resolved optical emission spectra representing Zn* and Zn+* at various delay times between the laser pulse and the gate pulse of ICCD with magnetic field off (a) and on (b). The spectra at 500 ns after laser ablation clearly indicate that the intensity of optical emission from Zn* decreases while that from Zn+* increases with field on. Optical emission was sampled at a position 8 mm away from the target. The oxygen pressure was 13 Pa.
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T.H. Kim et al. / Applied Surface Science 253 (2007) 8054–8058
Fig. 3. The time-resolved images of (a) Zn* with field off, (b) Zn* with field on, (c) Zn+* with field off, and (d) Zn+* with field on. The oxygen pressure was 13 Pa.
Temporal images of Zn* and Zn+* with the magnetic field on and off are illustrated in Fig. 3. It is clear that localized Zn+* ions are distributed in the front part of the plume and much less localized Zn* atoms follow them, as shown in images with field off at a delay time of 500 ns. The velocities of Zn* and Zn+* as measured by the peak position of each emission profile of the ICCD image at 500 ns were 7.8 103 m/s and 1.5 104 m/s, respectively. With magnetic field on, the temporal images of both Zn* and Zn+* changed significantly. For Zn*, formation of two lobes near the poles was clearly observed as previously reported by Neogi et al. [8] for the carbon plume. According to them, the J B force, where J is the charge current density and B is the magnetic field, is induced by interaction of the magnetic field with the electrons flow in the outermost boundary of the plume. The plume is more accelerated near the poles where the field is higher and as a result the plume breaks into two lobes. On the other hand, there was no formation of such lobes for Zn+* ions. Instead, they were split axially into faster and slower ones, which are considered to stem from the accelerating J B force in the outermost boundary and the decelerating J B force inside the plume. The velocities of the faster and slower parts were 1.7 104 m/s and 1.1 104 m/s, respectively. One of the most promising applications of PLD in magnetic field is the deposition of high-quality films at room temperature as demonstrated by Tachiki et al. [9,10]. They could achieve epitaxial growth of NiO films by adopting the Aurora PLD while the films fabricated by the conventional PLD were polycrystalline. The Aurora PLD also turned out to be an attractive technique to improve crystallinity of ZnO films grown on quartz glass at room temperature. In particular, photoluminescence (PL) intensity was significantly enhanced by elevation of the substrate temperature just up to 100–200 8C [6], which is a quite low temperature
range in conventional PLD. Another feature of PLD in magnetic field is a substantial increase in the growth rate of films. According to Kobayashi et al. [5], the deposition rate of the Eclipse-Aurora PLD was approximately twice as high as that of the conventional PLD. They explained that the increase in the rate originates from the enhanced ionization and the forward movement of ions along with the magnetic flux line. In our transverse geometry, the increase in the growth rate of ZnO films grown on Si substrate at room temperature was even more distinct: it increased by a factor of 2.4–4.3 depending on the ambient oxygen pressure. This implies that the enrichment of energetic ions in the plasma is more important than the magnetic flux effect as far as the growth rate is concerned. The surface morphologies of ZnO films grown at different oxygen pressures with magnetic field on and off are shown in Fig. 4. With magnetic field applied, at pressures below 1.3 Pa, films consisted of grains with size up to several hundred nanometers, which are considered to have been formed presumably due to the fast growth rate of films under the magnetic field. The differences in surface morphologies with field on and off, however, were reduced with increase in the ambient pressure: at 66 Pa, films deposited with field on and off both consisted of nano-grains with size of 50 nm. The ZnO films deposited on Si at room temperature without magnetic field showed no PL, but as the magnetic field was applied PL signal in the ultraviolet region was apparent as depicted in Fig. 5. After rapid thermal annealing (RTA) at 800 8C for 3 min in air, the PL peak appeared for the film grown with magnetic field off, although the PL intensity in the visible range due to the defects was still high [11]. For the ZnO films under magnetic field, the PL intensity in the UV region was dominant and increased by as much as 40 times after RTA.
T.H. Kim et al. / Applied Surface Science 253 (2007) 8054–8058
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Fig. 4. The SEM images of ZnO films deposited at (a) 0.13 Pa with field off, (b) 0.13 Pa with field on, (c) 1.3 Pa with field off, (d) 1.3 Pa with field on, (f) 13 Pa with field off, (f) 13 Pa with field on, (g) 66 Pa with field off, and (h) 66 Pa with field on.
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T.H. Kim et al. / Applied Surface Science 253 (2007) 8054–8058
emission intensity of Zn+* increased at the expense of emission from Zn* since Zn atoms can be easily ionized via collisions with energized electrons produced in the magnetic field, which also contribute to a dramatic increase in the growth rate and PL intensity of ZnO films fabricated by PLD.
Acknowledgment This research was supported by the Kyung Hee University Research Fund, 2004.
References
Fig. 5. The PL intensity of ZnO films grown at 1.3 Pa with magnetic field off and on. The films were grown at room temperature and annealed at 800 8C for 3 min in air. The inset shows the PL intensity of as-grown films.
4. Conclusions By using time-resolved imaging of Zn* and Zn+* in Zn plasma generated by laser ablation of a Zn target in an oxygen atmosphere, we found that a transverse magnetic field across the plume has significant influence on their expansion dynamics and enrichment of ions. The J B force is considered to play a role in the breakage of Zn* distribution into two lobes and the axial splitting of Zn+* distribution under the magnetic field. The
[1] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [2] H. Izymi, K. Ohata, T. Hase, K. Suzuki, T. Morishita, S. Tanaka, J. Appl. Phys. 68 (1990) 6331. [3] D. Fried, T. Kushida, G.P. Reck, E.W. Rothe, J. Appl. Phys. 72 (1992) 1113. [4] J. Maramoto, I. Sakamoto, Y. Nakata, T. Okada, M. Maeda, Appl. Phys. Lett. 75 (1999) 751. [5] T. Kobayashi, H. Akiyoshi, M. Tachiki, Appl. Surf. Sci. 197–198 (2002) 294. [6] S. Yata, Y. Nakashima, T. Kobayashi, Thin Solid Films 445 (2003) 259. [7] Y.Y. Tsui, D. Vick, R. Fedosejevs, Appl. Phys. Lett. 70 (1997) 1953. [8] A. Neogi, R.K. Thareja, J. Appl. Phys. 85 (1999) 1131. [9] M. Tachiki, T. Kobayashi, Jpn. J. Appl. Phys. 38 (1999) 3642. [10] M. Tachiki, T. Hosomi, T. Kobayashi, Jpn. J. Appl. Phys. 39 (2000) 1817. [11] J.S. Kang, H.S. Kang, S.S. Pang, E.S. Shim, S.Y. Lee, Thin Solid Films 443 (2003) 5.
Effects of transverse magnetic field on a laser-produced Zn plasma plume and ZnO films grown by pulsed laser deposition Tae Hyun Kim, Sang Hwan Nam, Hye Sun Park, Jae Kyu Song *, Seung Min Park * Department of Chemistry, Kyunghee University, Seoul 130-701, Republic of Korea Available online 2 March 2007
Abstract By adopting a fast photography and time-resolved optical emission spectrometry, we have investigated the effects of transverse magnetic field on the expansion dynamics and enrichment of Zn atoms and Zn+ ions in a plume produced by laser ablation of a Zn target in oxygen atmosphere. Plume splitting due to the magnetic field was apparent but the splitting patterns of Zn and Zn+ were totally different. The surface morphology and photoluminescence characteristics also changed significantly. In particular, the growth rate increased by as much as 2.4–4.3 times compared to the conventional PLD method. # 2007 Elsevier B.V. All rights reserved. PACS : 79.20.Ds; 52.50.Jm Keywords: Laser ablation; Laser-induced plasma; Magnetic field
1. Introduction Pulsed laser deposition (PLD) has been widely employed as a straightforward method to grow thin films of superconductors, ferroelectrics, metals, and refractory materials [1]. In PLD, the quality of the deposited film is in essence determined by the characteristics of the laser-produced plasma plume formed via irradiation of a solid target using a focused pulsed laser beam. In this regard, it would be highly desirable to control the properties of the plume, which consists of electrons, atoms, ions, and molecules including clusters. Among these, energetic ions are, in general, most valuable species to produce films with high quality. Relative concentration and kinetic energy of ions in the plume can be controlled by changing parameters such as laser fluence and its wavelength. A more direct method is to apply electric field between the target and the substrate. Many researchers investigated the effects of electric field on the laser-produced plume and observed that kinetic energy of ions, optical emission from neutral and ionic species, and properties of the deposited films
* Corresponding authors. Tel.: +82 2 961 0226; fax: +82 2 957 4856. E-mail addresses: [email protected] (J.K. Song), [email protected] (S.M. Park). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.154
are strongly dependent on the polarity and strength of the electric field [2–4]. As well as electric field, magnetic field would certainly have significant effects on the formation and expansion of plume through interaction of current density with magnetic field. Recently, Kobayashi et al. [5,6] developed a new PLD method named ‘‘Aurora PLD’’ under application of axial magnetic field from the substrate side. This method has been developed with a goal to enhance the activation and/or ionization of the ablated species during transport from the target to substrate and turned out to be superior to the conventional PLD in producing high-quality films. Also PLD under a curved magnetic field proved to be useful in the growth of debris free films by confining and guiding the plume [7]. Here, we present experimental results related to the dramatic effect of transverse magnetic field on the expansion dynamics of the Zn plume in oxygen atmosphere as well as on the morphology and photoluminescence (PL) of the deposited films. Time-resolved spatial distributions of Zn* and Zn+* were separately monitored by coupling of an intensified charge coupled device (ICCD) with interference filters. In particular, under the transverse magnetic field, splitting of the plume into two lobes was clearly observed for Zn*, while more localized Zn+* ions was split axially and their emission became much more intense.
T.H. Kim et al. / Applied Surface Science 253 (2007) 8054–8058
2. Experimental The schematic diagram of the experiment is shown in Fig. 1. A Zn target (Niraco, 99.999%) with size of 25.4 mm in diameter was used as purchased without further treatment. The target was mounted in between the poles which were apart by 1.0 cm. The magnetic field was minimum (0.19–0.29 T) at the center and maximum (0.30–0.32 T) near the poles along the field direction as shown in the inset of Fig. 1. A Q-switched Nd:YAG laser (l = 1064 nm, Minilite II, Continuum, pulse duration = 6 ns) operating at 10 Hz was loosely focused onto the Zn target (Nilaco, 99.999%) placed in a vacuum chamber using a lens (focal length 30 cm) with an angle of incidence of 08. The diameter of the focused laser spot was 1.7 mm and the laser fluence at the target surface is estimated to be 1.8 J/cm2. The Zn target was rotated by a standard rotary motion feedthrough to avoid a target aging effect. Si substrate with size of 5 mm 5 mm was installed between the magnets at a position 3.1 cm away from the target and PLD was performed for 30 min in oxygen atmosphere. Oxygen gas (99.999%) was fed to the chamber by a needle valve and the pressure was measured by a full range gauge (Balzers PKR250). In order to elucidate the effects of the magnetic field on the expansion of the plume and deposition of ZnO films, the magnets were replaced with Al blocks of the same size when the expansion was examined without field or the films were grown by conventional PLD. Optical emission studies on the laser-generated plume of the Zn target were done in oxygen atmosphere. Optical emission from the electronically excited states of Zn and Zn+ in a plume was collected using a lens of 5 cm focal length. The emission spectra were recorded at 8 mm away from the target surface, where the emission was intense enough in the pressure range chosen in our experiment and the continuum emission near the target surface was not collected. The optical emission at the
Fig. 1. The schematic diagram of the experiment. Temporal images of Zn plasma plume generated by irradiation of a Zn target with a focused 1064 nm laser pulse were obtain by an ICCD with magnetic field on and off. Two permanent magnets were mounted facing each other and the distance between them was 1.0 cm. The inset shows the spatial variation of the magnetic field. The laser fluence was 1.7 J/cm2. To get time-resolved optical emission spectra, ICCD was removed and the plume emission was fed to a monochromator coupled to an ICCD via an optical fiber bundle mounted on a three-dimensional translational stage.
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sampling position was fed to the monochromator coupled to an ICCD detector (Andor, DH 734) via an optical fiber bundle (Spex 700FB). The diameter of the entrance of optical fiber bundle was 0.8 mm. The gate width of the ICCD was fixed at 10 ns. Both spatially and temporally resolved measurements of the emission spectra were obtained by translating the receiving end of the optical fiber within the image of plasma. Also, timeresolved plume images were captured by using the ICCD. 3. Results and discussion Fig. 2 shows time-resolved optical emission spectra of the plume with the magnetic field on and off, recorded at 8 mm away from the target as a function of the delay time between the laser pulse and the gate of ICCD. The emission peak at 481.05 nm originates from Zn* and the two peaks at 491.16 nm and 492.40 nm represent Zn+*. Under magnetic field, the temperature of the plume is in general higher due to joule heating and electromagnetic compression of the plasma and thus electronically excited species are more enriched in the plasma. Neogi et al. [8] reported that the emission intensities from both carbon atoms and ions increased significantly with magnetic field. In our experiment, however, the optical emission intensity from Zn* decreased down to less than half while that from Zn+* increased by more than factor of 3 with application of the magnetic field. Since the ionization potential of Zn (9.39 eV) is lower than that of carbon (11.26 eV), Zn atoms (including Zn*) are more easily ionized via energetic collisions under the magnetic field and accordingly the emission intensity from Zn* atoms decreases.
Fig. 2. Time-resolved optical emission spectra representing Zn* and Zn+* at various delay times between the laser pulse and the gate pulse of ICCD with magnetic field off (a) and on (b). The spectra at 500 ns after laser ablation clearly indicate that the intensity of optical emission from Zn* decreases while that from Zn+* increases with field on. Optical emission was sampled at a position 8 mm away from the target. The oxygen pressure was 13 Pa.
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Fig. 3. The time-resolved images of (a) Zn* with field off, (b) Zn* with field on, (c) Zn+* with field off, and (d) Zn+* with field on. The oxygen pressure was 13 Pa.
Temporal images of Zn* and Zn+* with the magnetic field on and off are illustrated in Fig. 3. It is clear that localized Zn+* ions are distributed in the front part of the plume and much less localized Zn* atoms follow them, as shown in images with field off at a delay time of 500 ns. The velocities of Zn* and Zn+* as measured by the peak position of each emission profile of the ICCD image at 500 ns were 7.8 103 m/s and 1.5 104 m/s, respectively. With magnetic field on, the temporal images of both Zn* and Zn+* changed significantly. For Zn*, formation of two lobes near the poles was clearly observed as previously reported by Neogi et al. [8] for the carbon plume. According to them, the J B force, where J is the charge current density and B is the magnetic field, is induced by interaction of the magnetic field with the electrons flow in the outermost boundary of the plume. The plume is more accelerated near the poles where the field is higher and as a result the plume breaks into two lobes. On the other hand, there was no formation of such lobes for Zn+* ions. Instead, they were split axially into faster and slower ones, which are considered to stem from the accelerating J B force in the outermost boundary and the decelerating J B force inside the plume. The velocities of the faster and slower parts were 1.7 104 m/s and 1.1 104 m/s, respectively. One of the most promising applications of PLD in magnetic field is the deposition of high-quality films at room temperature as demonstrated by Tachiki et al. [9,10]. They could achieve epitaxial growth of NiO films by adopting the Aurora PLD while the films fabricated by the conventional PLD were polycrystalline. The Aurora PLD also turned out to be an attractive technique to improve crystallinity of ZnO films grown on quartz glass at room temperature. In particular, photoluminescence (PL) intensity was significantly enhanced by elevation of the substrate temperature just up to 100–200 8C [6], which is a quite low temperature
range in conventional PLD. Another feature of PLD in magnetic field is a substantial increase in the growth rate of films. According to Kobayashi et al. [5], the deposition rate of the Eclipse-Aurora PLD was approximately twice as high as that of the conventional PLD. They explained that the increase in the rate originates from the enhanced ionization and the forward movement of ions along with the magnetic flux line. In our transverse geometry, the increase in the growth rate of ZnO films grown on Si substrate at room temperature was even more distinct: it increased by a factor of 2.4–4.3 depending on the ambient oxygen pressure. This implies that the enrichment of energetic ions in the plasma is more important than the magnetic flux effect as far as the growth rate is concerned. The surface morphologies of ZnO films grown at different oxygen pressures with magnetic field on and off are shown in Fig. 4. With magnetic field applied, at pressures below 1.3 Pa, films consisted of grains with size up to several hundred nanometers, which are considered to have been formed presumably due to the fast growth rate of films under the magnetic field. The differences in surface morphologies with field on and off, however, were reduced with increase in the ambient pressure: at 66 Pa, films deposited with field on and off both consisted of nano-grains with size of 50 nm. The ZnO films deposited on Si at room temperature without magnetic field showed no PL, but as the magnetic field was applied PL signal in the ultraviolet region was apparent as depicted in Fig. 5. After rapid thermal annealing (RTA) at 800 8C for 3 min in air, the PL peak appeared for the film grown with magnetic field off, although the PL intensity in the visible range due to the defects was still high [11]. For the ZnO films under magnetic field, the PL intensity in the UV region was dominant and increased by as much as 40 times after RTA.
T.H. Kim et al. / Applied Surface Science 253 (2007) 8054–8058
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Fig. 4. The SEM images of ZnO films deposited at (a) 0.13 Pa with field off, (b) 0.13 Pa with field on, (c) 1.3 Pa with field off, (d) 1.3 Pa with field on, (f) 13 Pa with field off, (f) 13 Pa with field on, (g) 66 Pa with field off, and (h) 66 Pa with field on.
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T.H. Kim et al. / Applied Surface Science 253 (2007) 8054–8058
emission intensity of Zn+* increased at the expense of emission from Zn* since Zn atoms can be easily ionized via collisions with energized electrons produced in the magnetic field, which also contribute to a dramatic increase in the growth rate and PL intensity of ZnO films fabricated by PLD.
Acknowledgment This research was supported by the Kyung Hee University Research Fund, 2004.
References
Fig. 5. The PL intensity of ZnO films grown at 1.3 Pa with magnetic field off and on. The films were grown at room temperature and annealed at 800 8C for 3 min in air. The inset shows the PL intensity of as-grown films.
4. Conclusions By using time-resolved imaging of Zn* and Zn+* in Zn plasma generated by laser ablation of a Zn target in an oxygen atmosphere, we found that a transverse magnetic field across the plume has significant influence on their expansion dynamics and enrichment of ions. The J B force is considered to play a role in the breakage of Zn* distribution into two lobes and the axial splitting of Zn+* distribution under the magnetic field. The
[1] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [2] H. Izymi, K. Ohata, T. Hase, K. Suzuki, T. Morishita, S. Tanaka, J. Appl. Phys. 68 (1990) 6331. [3] D. Fried, T. Kushida, G.P. Reck, E.W. Rothe, J. Appl. Phys. 72 (1992) 1113. [4] J. Maramoto, I. Sakamoto, Y. Nakata, T. Okada, M. Maeda, Appl. Phys. Lett. 75 (1999) 751. [5] T. Kobayashi, H. Akiyoshi, M. Tachiki, Appl. Surf. Sci. 197–198 (2002) 294. [6] S. Yata, Y. Nakashima, T. Kobayashi, Thin Solid Films 445 (2003) 259. [7] Y.Y. Tsui, D. Vick, R. Fedosejevs, Appl. Phys. Lett. 70 (1997) 1953. [8] A. Neogi, R.K. Thareja, J. Appl. Phys. 85 (1999) 1131. [9] M. Tachiki, T. Kobayashi, Jpn. J. Appl. Phys. 38 (1999) 3642. [10] M. Tachiki, T. Hosomi, T. Kobayashi, Jpn. J. Appl. Phys. 39 (2000) 1817. [11] J.S. Kang, H.S. Kang, S.S. Pang, E.S. Shim, S.Y. Lee, Thin Solid Films 443 (2003) 5.