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Aluminum nitride thin films prepared by radical-assisted pulsed laser deposition
Vacuum 59 (2000) 649}656
Aluminum nitride thin "lms prepared by radical-assisted pulsed laser deposition M. Ishihara *, K. Yamamoto , F. Kokai, Y. Koga Department of Advanced Chemical Technology, National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Laser Laboratory, Institute of Research and Innovation, 1201 Takada, Kashiwa, Chiba 277-0861, Japan
Abstract A radical-assisted pulsed-laser deposition technique was used to deposit aluminum nitride thin "lms at low substrate temperature (473 K) with a sintered AlN target and the optical emission spectra were measured during the laser ablation. In a nitrogen gas atmosphere, the optical emission peaks of atomic aluminum and singly ionized Al> were observed from the ablation plume within the limit of detection. In a nitrogen radical atmosphere, that of atomic nitrogen and molecular nitrogen (N and N>) were observed from the ablation plumes. The deposited "lms prepared in nitrogen gas atmosphere contained aluminum above the stoichiometric concentration. By using nitrogen radicals to assist the pulsed-laser deposition, the concentration of nitrogen in the "lms increased to 50 at% and stoichiometric AlN "lms were prepared. It is very e!ective for the fabrication of high-quality AlN thin "lms to use the nitrogen radicals to assist the pulsed-laser deposition. 2000 Elsevier Science Ltd. All rights reserved. Keywords: AlN; Surface acoustic wave; PLD; Optical emission
1. Introduction Aluminum nitride (AlN) is a wide band gap (6.2 eV) III}V compound with a high thermal conductivity (320 W/mK at 300 K) [1], a high chemical stability (2500 K) [2] and a high electric resistance (10 ) cm) [3]. It is utilized as an electric insulation with heat radiation. Particularly because the surface acoustic wave (SAW) velocity of AlN along the c-axis is highest (6;10 m/s) [4] among those of piezoelectric materials, AlN thin "lms have attracted increasing interest for
* Corresponding author. 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 3 2 9 - 8
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application to SAW "lters operating at over 1 GHz in wireless communications technology. The AlN thin "lms have been prepared by several methods such as reactive magnetron sputtering [5], chemical-vapor deposition (CVD) [6], plasma-assisted CVD [7] and metalorganic CVD [8]. In the pulsed-laser deposition (PLD) method, the focused laser beam can evaporate the high melting point materials such as ceramics at low substrate temperature, and the contamination with impurities is avoidable in ultra-high vacuum. The species of the ablated particle depend on the wavelength and the #uence of the laser. Although it is said that the PLD method can transfer the complex molecular structure from targets onto substrates [9], it is necessary for the preparation of the stoichiometric AlN thin "lms to deposit in a nitrogen plasma ambient [10] or an ammonia gas ambient at high substrate temperature (1223 K) [11]. Each of these techniques has disadvantages in the degradation of substrate interfaces during deposition due to either plasma or thermal damage. Consequently, the growth of AlN "lms at low temperature and at low-plasma damages has become increasingly important and valuable. A nitrogen radical source can supply the active nitrogen without the plasma damage because the nitrogen plasma does not irradiate the substrate directly. In this study, the AlN thin "lms were deposited by radical-assisted PLD method at low substrate temperature, and we investigated the relationship between the composition of the "lms and the deposition conditions.
2. Experimental Fig. 1 shows a schematic diagram of the radical-assisted PLD apparatus. The vacuum chamber was evacuated to a base pressure of (1;10\ Pa by a turbomolecular pump. The fundamental (1064 nm), the second harmonics (532 nm) and the fourth harmonics (266 nm) of a Q-switched Nd : YAG laser beam (Continuum, PL8010) were used to irradiate a rotating target surface by focusing with a lens. The repetition rate of the Nd : YAG laser was 10 Hz and the pulse width was 3}5 ns. A sintered AlN plate with the purity of 99 wt% (Tokuyama, Shapal) was used as the target, which was located about 40 mm away from Si(1 0 0) substrates. The AlN thin "lms were deposited in the atmosphere of a nitrogen gas ambient (8;10\ Pa) and a nitrogen radical ambient
Fig. 1. Schematic diagram of the Nd : YAG laser deposition system.
M. Ishihara et al. / Vacuum 59 (2000) 649}656
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(8;10\ Pa) about 1 lm for 1 h. An active nitrogen plasma was generated by a RF plasma source (SVT ASSOCIATES, RF 4.53) located about 30 mm away from the substrate, which was inductively coupled by RF energy (13.56 MHz, 300 W) in a cylindrical pyrolytic boron nitride (pBN) discharge chamber supplied with pure N gas (2 sccm). The nitrogen plasma was introduced to the deposition chamber through a pBN aperture with a hole size of 2 mm . Although nitrogen radicals and nitrogen ions were produced in the discharge chamber, the nitrogen radicals occupied most of active nitrogen species at the substrate position [12]. The Si substrate was not heated and the substrate temperature (473 K) was only dependent on self-heating of the plasma. The deposited "lms were characterized by electron probe microanalysis (EPMA: JEOL, JXA-8800) and Fourier transform infrared spectroscopy (FTIR: SHIMADZU, FTIR-8700). FTIR spectroscopy in the range of 400}2000 cm\ was employed to examine the chemical bond of the "lms on the Si substrate. The optical emission spectra of the ablation plume from an aluminum target and the sintered AlN target were measured by a photonic multichannel analyzer (Hamamatsu, PMA-11) through a quartz view port of the PLD chamber. All time-integrated spectra, obtained with an exposure time of 2 s, were accumulated over 100 consecutive laser shots. The averaged spectra from the photonic multichannel analyzer, with a correction for the detector spectral response, were transferred to a personal computer for subsequent display and storage. The dark current signal, measured on the basis of the background spectrum with the ablation laser blocked, was below the noise level.
3. Results and discussion Fig. 2 shows the atomic concentrations of aluminum, nitrogen and oxygen in the "lms deposited in the nitrogen atmosphere, using the sintered AlN target with the change of the laser wavelength (#uence of 2 J/cm). The concentrations and the surface morphology were analyzed by EPMA and its CCD camera, respectively. At a wavelength of 266}1064 nm, the concentration of nitrogen in the
Fig. 2. E!ect of the laser wavelength on the atomic concentrations of Al, N and O in the "lms deposited in nitrogen atmosphere with the analyses of EPMA (laser #uence: 2 J/cm, Pressure: 8;10\ Pa, target: sintered AlN).
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"lms was less than in the stoichiometric AlN and decreased with increase in the laser wavelength. Although the stoichiometric AlN thin "lms were colorless and transparent, those of the above "lms were brown and semitransparent. The color of the sintered AlN target surface changed from white to silver during the ablation. The silver material was assumed to be metallic aluminum, which was formed as a result of thermal dissociation when the sintered AlN target was melted by the focused laser beam. Since the laser beam ablates the Al-rich surface of the sintered AlN target, the deposited "lms contain aluminum more than the stoichiometric sintered AlN target. Large particles were deposited on the "lms at the laser wavelength of 1064 nm particularly. These results suggest that the compositions of the "lms deposited in the nitrogen atmosphere were Al-rich and the stoichiometric AlN thin "lms were not prepared at the experimental conditions. Fig. 3 shows the emission spectra from the plasma plumes in the atmosphere of vacuum (1;10\ Pa). The targets of (a) aluminum and (b) AlN were ablated by the laser beam at the wavelength of 266 and 1064 nm and the #uence of 2 J/cm. The emission of 532 nm is due to the ablation laser. In case of aluminum target, strong emission peaks of atomic aluminum (309 and 396 nm) [13] were observed from the ablation plume at the laser wavelength of 266 nm (Fig. 3a).
Fig. 3. Emission spectra from the plasma plumes during laser ablation of (a) Al and (b) sintered AlN (laser #uence: 2 J/cm, pressure: 1;10\ Pa).
M. Ishihara et al. / Vacuum 59 (2000) 649}656
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However, an Al> emission peak (359 nm) [13] was very small. Although ultraviolet laser was more e!ective to ionize ablation species than the near-infrared laser, aluminum targets were hard to be ionized at the low laser #uence. The intensities of atomic aluminum emissions at the laser wavelength of 1064 nm were weaker than that of 266 nm. Since the aluminum surface is easy to re#ect the near-infrared light at 1064 nm [14], it is considered that the aluminum is hardly evaporated by the laser beam of 1064 nm. In case of sintered AlN target, the atomic aluminum emission was observed strongly. However, the emission peaks of active nitrogen dissociated from AlN such as atomic nitrogen (745 nm) [15] and molecular N ("rst positive system at 540}800 nm and second positive system at 329 and 337 nm) [16] and the emission peaks of molecular AlN (508 nm) [17] were not observed within the limit of detection (Fig. 3b). At the laser wavelength of 266 nm, the broad emission at 350}450 nm is an unknown emission. Though the sintered AlN target was ablated in a nitrogen gas ambient (8;10\ Pa), the emissions of nitrogen did not appear. It seemed that the active nitrogen such as nitrogen atoms was less than the active aluminum for the ablation of sintered AlN target in the atmosphere of a vacuum and a nitrogen gas ambient. At the higher laser #uence (&13 J/cm) with a wavelength of 266 nm, the emission peaks of aluminum atoms and Al> ions were observed clearly for the aluminum and sintered AlN targets, but the emission peaks of atomic nitrogen were observed to be very weak. The relative intensity of the Al> emission increased linearly with increase in the laser #uence in case of the aluminum target, however, the relative intensities of the Al> emission and the atomic nitrogen emission were almost equivalent in the case of the sintered AlN targets. The number of ablation particles with large size increased at the high laser #uence and the surface of the "lms became rough. It is necessary for the fabrication of high-quality AlN thin "lms to ablate the sintered AlN target at the low #uence. Fig. 4 shows the atomic concentrations of aluminum, nitrogen and oxygen in the "lms deposited in the nitrogen radical ambient, using the sintered AlN target with the change of the laser wavelength (#uence of 2 J/cm). In the nitrogen radical ambient, the concentration of nitrogen
Fig. 4. E!ect of the laser wavelength on the atomic concentrations of Al, N and O in the "lms deposited in nitrogen radical atmosphere with the analyses of EPMA (laser #uence: 2 J/cm, RF power: 300 W, N #ow rate: 2 sccm, pressure: 8;10\ Pa, target: sintered AlN).
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Fig. 5. FTIR spectra measured for "lms deposited (a) in the nitrogen gas atmosphere and (b) in the nitrogen radical atmosphere at laser wavelength of 266 nm (laser #uence: 2 J/cm, N #ow rate: 2 sccm, pressure: 8;10\ Pa, target: sintered AlN).
increased to 50 at% and the concentration of oxygen decreased to (2 at% at the laser wavelength of 266}1064 nm. The deposit "lms were colorless and transparent. These results suggest that the nitrogen radical source is e!ective for the supply of active nitrogen to the substrate surface. Fig. 5 shows the FTIR spectra measured for the "lms deposited by using the sintered AlN target in the atmosphere of (a) nitrogen gas ambient and (b) nitrogen radical ambient. The laser wavelength was 266 nm and the #uence was 2 J/cm. The deposited "lms in the nitrogen gas ambient were brown and semitransparent and the absorption band between 500 and 900 cm\ was very weak in Fig. 5(a). However, the deposited "lms in the nitrogen radical ambient have a strong absorption band, which is a useful indication that the main chemical composition of the "lms is the strongly polar AlN [18] in Fig. 5(b). The modes coupled to infrared radiation are LO at 737 cm\, TO at 665 cm\ and TO at 630 cm\. Other weak peaks in the spectrum can be assigned to water and air in the spectrophotometer path. Since the Al}O band at 460 cm\ was not detected at the measurement condition, it is considered that the impurity of Al O is low. From these results, the stoichiometric AlN thin "lms were prepared by nitrogen radical-assisted PLD method. The AlN thin "lms prepared in the nitrogen radical atmosphere are more excellent than those prepared in the nitrogen gas atmosphere. Fig. 6 shows the emission spectrum near the substrate during the ablation of sintered AlN targets by the 266 nm laser beam with the #uence of 2 J/cm in the nitrogen radical ambient. The emission peak of atomic nitrogen was dominant compared to the emission peaks of atomic aluminum, molecular N and molecular N> (391 nm) [12]. When the sintered AlN target was ablated by the ablation laser in the nitrogen gas ambient without the nitrogen radical, the emission peaks of molecular N and molecular N> were not observed near the substrate. Although the Al-rich thin "lms were deposited in the nitrogen gas ambient, the stoichiometric AlN thin "lms were deposited in the nitrogen radical ambient. It is suggested that the active nitrogen was supplied su$ciently by the radical source to the substrate during the AlN deposition. Thus the AlN thin "lms deposited by using a radical-assisted PLD method were of stoichiometric concentration.
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Fig. 6. Emission spectrum near the substrate during the radical-assisted PLD (laser #uence: 2 J/cm, RF power: 300 W, N #ow rate: 2 sccm, pressure: 8;10\ Pa, target: sintered AlN).
4. Conclusion We succeeded in the growth of stoichiometric AlN thin "lms by nitrogen radical-assisted PLD method from a sintered AlN target at low substrate temperature (473 K). The concentration of nitrogen in the "lms was found to depend on the ablation laser wavelength and the activity of assisted nitrogen. In the nitrogen gas ambient, the concentration of nitrogen in the "lms decreased with increase in the laser wavelength, and was less than the stoichiometric sintered AlN target. However, by the assistance of nitrogen radicals during the "lm growth, the concentration of nitrogen corresponded to that of the stoichiometric AlN. The stoichiometric AlN thin "lms were prepared at the laser wavelength of 266 nm (2 J/cm) and in the nitrogen radical ambient (8;10\ Pa). It is very e!ective for the fabrication of high-quality AlN thin "lms to assist the active species of nitrogen.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Slack GA, Tanzilli RA, Pohl RO, Vandersande JW. J Phys Chem Solids 1987;48:641. Mroz TJ. Am Ceram Soc Bull 1992;71:782. Sheng TY, Yu ZQ, Collins GJ. Appl Phys Lett 1988;52:576. Du!y MT, Wang CC, O'Clock GD, MacFarlance III Jr. SH, Zanzucchi PJ. J Electron Mater 1973;2:359. Okano H, Tanaka N, Takahashi Y, Tanaka T, Shibata K, Nakano S. Appl Phys Lett 1994;64:166. Mogab CJ, Lugujjs M. J Appl Phys 1976;47:1302. Zhang W, Someno Y, Sasaki M, Hirai T. J Crystal Growth 1993;130:308. Sasaki T, Matsuoka T. J Appl Phys 1995;77:192. Cohen A, Allenspacher P, Brieger MM, Jeuck I, Opower H. Appl Phys Lett 1991;59:2186. Ogawa T, Okamoto M, Yap YK, Mori Y, Hatta A, Ito T, Sasaki T, Hiraki A. Diamond Relat Mater 1997;6:1015. Vispute RD, Talyansky V, Sharma RP, Choopun S, Downes M, Venkatesan T, Li YX, Salamanca-Riba LG, Iladis AA, Jones KA, McGarrity J. Appl Surf Sci 1998;127:431.
656 [12] [13] [14] [15] [16] [17] [18]
M. Ishihara et al. / Vacuum 59 (2000) 649}656 Vaudo RP, Yu Z, Cook JW, Schetzina JF. Opt Lett 1993;18:1848. Sawyer RA, Paschen F. Ann Phys 1972;84:1. Shiles E, Sasaki T, Inokuti M, Smith DY. Phys Rev B 1980;22:1612. Liu H, Frenkel AC, Kim JG, Park RM. J Appl Phys 1993;74:6124. Clay KJ, Speakman SP, Amaratunga GA, Silva SRP. J Appl Phys 1996;79:7227. Simmons JD, McDonald JK. J Mol Spectrosc 1972;41:584. Li Xin, Tansley TL. J Appl Phys 1990;68:5369.
Aluminum nitride thin "lms prepared by radical-assisted pulsed laser deposition M. Ishihara *, K. Yamamoto , F. Kokai, Y. Koga Department of Advanced Chemical Technology, National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Laser Laboratory, Institute of Research and Innovation, 1201 Takada, Kashiwa, Chiba 277-0861, Japan
Abstract A radical-assisted pulsed-laser deposition technique was used to deposit aluminum nitride thin "lms at low substrate temperature (473 K) with a sintered AlN target and the optical emission spectra were measured during the laser ablation. In a nitrogen gas atmosphere, the optical emission peaks of atomic aluminum and singly ionized Al> were observed from the ablation plume within the limit of detection. In a nitrogen radical atmosphere, that of atomic nitrogen and molecular nitrogen (N and N>) were observed from the ablation plumes. The deposited "lms prepared in nitrogen gas atmosphere contained aluminum above the stoichiometric concentration. By using nitrogen radicals to assist the pulsed-laser deposition, the concentration of nitrogen in the "lms increased to 50 at% and stoichiometric AlN "lms were prepared. It is very e!ective for the fabrication of high-quality AlN thin "lms to use the nitrogen radicals to assist the pulsed-laser deposition. 2000 Elsevier Science Ltd. All rights reserved. Keywords: AlN; Surface acoustic wave; PLD; Optical emission
1. Introduction Aluminum nitride (AlN) is a wide band gap (6.2 eV) III}V compound with a high thermal conductivity (320 W/mK at 300 K) [1], a high chemical stability (2500 K) [2] and a high electric resistance (10 ) cm) [3]. It is utilized as an electric insulation with heat radiation. Particularly because the surface acoustic wave (SAW) velocity of AlN along the c-axis is highest (6;10 m/s) [4] among those of piezoelectric materials, AlN thin "lms have attracted increasing interest for
* Corresponding author. 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 3 2 9 - 8
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M. Ishihara et al. / Vacuum 59 (2000) 649}656
application to SAW "lters operating at over 1 GHz in wireless communications technology. The AlN thin "lms have been prepared by several methods such as reactive magnetron sputtering [5], chemical-vapor deposition (CVD) [6], plasma-assisted CVD [7] and metalorganic CVD [8]. In the pulsed-laser deposition (PLD) method, the focused laser beam can evaporate the high melting point materials such as ceramics at low substrate temperature, and the contamination with impurities is avoidable in ultra-high vacuum. The species of the ablated particle depend on the wavelength and the #uence of the laser. Although it is said that the PLD method can transfer the complex molecular structure from targets onto substrates [9], it is necessary for the preparation of the stoichiometric AlN thin "lms to deposit in a nitrogen plasma ambient [10] or an ammonia gas ambient at high substrate temperature (1223 K) [11]. Each of these techniques has disadvantages in the degradation of substrate interfaces during deposition due to either plasma or thermal damage. Consequently, the growth of AlN "lms at low temperature and at low-plasma damages has become increasingly important and valuable. A nitrogen radical source can supply the active nitrogen without the plasma damage because the nitrogen plasma does not irradiate the substrate directly. In this study, the AlN thin "lms were deposited by radical-assisted PLD method at low substrate temperature, and we investigated the relationship between the composition of the "lms and the deposition conditions.
2. Experimental Fig. 1 shows a schematic diagram of the radical-assisted PLD apparatus. The vacuum chamber was evacuated to a base pressure of (1;10\ Pa by a turbomolecular pump. The fundamental (1064 nm), the second harmonics (532 nm) and the fourth harmonics (266 nm) of a Q-switched Nd : YAG laser beam (Continuum, PL8010) were used to irradiate a rotating target surface by focusing with a lens. The repetition rate of the Nd : YAG laser was 10 Hz and the pulse width was 3}5 ns. A sintered AlN plate with the purity of 99 wt% (Tokuyama, Shapal) was used as the target, which was located about 40 mm away from Si(1 0 0) substrates. The AlN thin "lms were deposited in the atmosphere of a nitrogen gas ambient (8;10\ Pa) and a nitrogen radical ambient
Fig. 1. Schematic diagram of the Nd : YAG laser deposition system.
M. Ishihara et al. / Vacuum 59 (2000) 649}656
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(8;10\ Pa) about 1 lm for 1 h. An active nitrogen plasma was generated by a RF plasma source (SVT ASSOCIATES, RF 4.53) located about 30 mm away from the substrate, which was inductively coupled by RF energy (13.56 MHz, 300 W) in a cylindrical pyrolytic boron nitride (pBN) discharge chamber supplied with pure N gas (2 sccm). The nitrogen plasma was introduced to the deposition chamber through a pBN aperture with a hole size of 2 mm . Although nitrogen radicals and nitrogen ions were produced in the discharge chamber, the nitrogen radicals occupied most of active nitrogen species at the substrate position [12]. The Si substrate was not heated and the substrate temperature (473 K) was only dependent on self-heating of the plasma. The deposited "lms were characterized by electron probe microanalysis (EPMA: JEOL, JXA-8800) and Fourier transform infrared spectroscopy (FTIR: SHIMADZU, FTIR-8700). FTIR spectroscopy in the range of 400}2000 cm\ was employed to examine the chemical bond of the "lms on the Si substrate. The optical emission spectra of the ablation plume from an aluminum target and the sintered AlN target were measured by a photonic multichannel analyzer (Hamamatsu, PMA-11) through a quartz view port of the PLD chamber. All time-integrated spectra, obtained with an exposure time of 2 s, were accumulated over 100 consecutive laser shots. The averaged spectra from the photonic multichannel analyzer, with a correction for the detector spectral response, were transferred to a personal computer for subsequent display and storage. The dark current signal, measured on the basis of the background spectrum with the ablation laser blocked, was below the noise level.
3. Results and discussion Fig. 2 shows the atomic concentrations of aluminum, nitrogen and oxygen in the "lms deposited in the nitrogen atmosphere, using the sintered AlN target with the change of the laser wavelength (#uence of 2 J/cm). The concentrations and the surface morphology were analyzed by EPMA and its CCD camera, respectively. At a wavelength of 266}1064 nm, the concentration of nitrogen in the
Fig. 2. E!ect of the laser wavelength on the atomic concentrations of Al, N and O in the "lms deposited in nitrogen atmosphere with the analyses of EPMA (laser #uence: 2 J/cm, Pressure: 8;10\ Pa, target: sintered AlN).
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"lms was less than in the stoichiometric AlN and decreased with increase in the laser wavelength. Although the stoichiometric AlN thin "lms were colorless and transparent, those of the above "lms were brown and semitransparent. The color of the sintered AlN target surface changed from white to silver during the ablation. The silver material was assumed to be metallic aluminum, which was formed as a result of thermal dissociation when the sintered AlN target was melted by the focused laser beam. Since the laser beam ablates the Al-rich surface of the sintered AlN target, the deposited "lms contain aluminum more than the stoichiometric sintered AlN target. Large particles were deposited on the "lms at the laser wavelength of 1064 nm particularly. These results suggest that the compositions of the "lms deposited in the nitrogen atmosphere were Al-rich and the stoichiometric AlN thin "lms were not prepared at the experimental conditions. Fig. 3 shows the emission spectra from the plasma plumes in the atmosphere of vacuum (1;10\ Pa). The targets of (a) aluminum and (b) AlN were ablated by the laser beam at the wavelength of 266 and 1064 nm and the #uence of 2 J/cm. The emission of 532 nm is due to the ablation laser. In case of aluminum target, strong emission peaks of atomic aluminum (309 and 396 nm) [13] were observed from the ablation plume at the laser wavelength of 266 nm (Fig. 3a).
Fig. 3. Emission spectra from the plasma plumes during laser ablation of (a) Al and (b) sintered AlN (laser #uence: 2 J/cm, pressure: 1;10\ Pa).
M. Ishihara et al. / Vacuum 59 (2000) 649}656
653
However, an Al> emission peak (359 nm) [13] was very small. Although ultraviolet laser was more e!ective to ionize ablation species than the near-infrared laser, aluminum targets were hard to be ionized at the low laser #uence. The intensities of atomic aluminum emissions at the laser wavelength of 1064 nm were weaker than that of 266 nm. Since the aluminum surface is easy to re#ect the near-infrared light at 1064 nm [14], it is considered that the aluminum is hardly evaporated by the laser beam of 1064 nm. In case of sintered AlN target, the atomic aluminum emission was observed strongly. However, the emission peaks of active nitrogen dissociated from AlN such as atomic nitrogen (745 nm) [15] and molecular N ("rst positive system at 540}800 nm and second positive system at 329 and 337 nm) [16] and the emission peaks of molecular AlN (508 nm) [17] were not observed within the limit of detection (Fig. 3b). At the laser wavelength of 266 nm, the broad emission at 350}450 nm is an unknown emission. Though the sintered AlN target was ablated in a nitrogen gas ambient (8;10\ Pa), the emissions of nitrogen did not appear. It seemed that the active nitrogen such as nitrogen atoms was less than the active aluminum for the ablation of sintered AlN target in the atmosphere of a vacuum and a nitrogen gas ambient. At the higher laser #uence (&13 J/cm) with a wavelength of 266 nm, the emission peaks of aluminum atoms and Al> ions were observed clearly for the aluminum and sintered AlN targets, but the emission peaks of atomic nitrogen were observed to be very weak. The relative intensity of the Al> emission increased linearly with increase in the laser #uence in case of the aluminum target, however, the relative intensities of the Al> emission and the atomic nitrogen emission were almost equivalent in the case of the sintered AlN targets. The number of ablation particles with large size increased at the high laser #uence and the surface of the "lms became rough. It is necessary for the fabrication of high-quality AlN thin "lms to ablate the sintered AlN target at the low #uence. Fig. 4 shows the atomic concentrations of aluminum, nitrogen and oxygen in the "lms deposited in the nitrogen radical ambient, using the sintered AlN target with the change of the laser wavelength (#uence of 2 J/cm). In the nitrogen radical ambient, the concentration of nitrogen
Fig. 4. E!ect of the laser wavelength on the atomic concentrations of Al, N and O in the "lms deposited in nitrogen radical atmosphere with the analyses of EPMA (laser #uence: 2 J/cm, RF power: 300 W, N #ow rate: 2 sccm, pressure: 8;10\ Pa, target: sintered AlN).
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Fig. 5. FTIR spectra measured for "lms deposited (a) in the nitrogen gas atmosphere and (b) in the nitrogen radical atmosphere at laser wavelength of 266 nm (laser #uence: 2 J/cm, N #ow rate: 2 sccm, pressure: 8;10\ Pa, target: sintered AlN).
increased to 50 at% and the concentration of oxygen decreased to (2 at% at the laser wavelength of 266}1064 nm. The deposit "lms were colorless and transparent. These results suggest that the nitrogen radical source is e!ective for the supply of active nitrogen to the substrate surface. Fig. 5 shows the FTIR spectra measured for the "lms deposited by using the sintered AlN target in the atmosphere of (a) nitrogen gas ambient and (b) nitrogen radical ambient. The laser wavelength was 266 nm and the #uence was 2 J/cm. The deposited "lms in the nitrogen gas ambient were brown and semitransparent and the absorption band between 500 and 900 cm\ was very weak in Fig. 5(a). However, the deposited "lms in the nitrogen radical ambient have a strong absorption band, which is a useful indication that the main chemical composition of the "lms is the strongly polar AlN [18] in Fig. 5(b). The modes coupled to infrared radiation are LO at 737 cm\, TO at 665 cm\ and TO at 630 cm\. Other weak peaks in the spectrum can be assigned to water and air in the spectrophotometer path. Since the Al}O band at 460 cm\ was not detected at the measurement condition, it is considered that the impurity of Al O is low. From these results, the stoichiometric AlN thin "lms were prepared by nitrogen radical-assisted PLD method. The AlN thin "lms prepared in the nitrogen radical atmosphere are more excellent than those prepared in the nitrogen gas atmosphere. Fig. 6 shows the emission spectrum near the substrate during the ablation of sintered AlN targets by the 266 nm laser beam with the #uence of 2 J/cm in the nitrogen radical ambient. The emission peak of atomic nitrogen was dominant compared to the emission peaks of atomic aluminum, molecular N and molecular N> (391 nm) [12]. When the sintered AlN target was ablated by the ablation laser in the nitrogen gas ambient without the nitrogen radical, the emission peaks of molecular N and molecular N> were not observed near the substrate. Although the Al-rich thin "lms were deposited in the nitrogen gas ambient, the stoichiometric AlN thin "lms were deposited in the nitrogen radical ambient. It is suggested that the active nitrogen was supplied su$ciently by the radical source to the substrate during the AlN deposition. Thus the AlN thin "lms deposited by using a radical-assisted PLD method were of stoichiometric concentration.
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655
Fig. 6. Emission spectrum near the substrate during the radical-assisted PLD (laser #uence: 2 J/cm, RF power: 300 W, N #ow rate: 2 sccm, pressure: 8;10\ Pa, target: sintered AlN).
4. Conclusion We succeeded in the growth of stoichiometric AlN thin "lms by nitrogen radical-assisted PLD method from a sintered AlN target at low substrate temperature (473 K). The concentration of nitrogen in the "lms was found to depend on the ablation laser wavelength and the activity of assisted nitrogen. In the nitrogen gas ambient, the concentration of nitrogen in the "lms decreased with increase in the laser wavelength, and was less than the stoichiometric sintered AlN target. However, by the assistance of nitrogen radicals during the "lm growth, the concentration of nitrogen corresponded to that of the stoichiometric AlN. The stoichiometric AlN thin "lms were prepared at the laser wavelength of 266 nm (2 J/cm) and in the nitrogen radical ambient (8;10\ Pa). It is very e!ective for the fabrication of high-quality AlN thin "lms to assist the active species of nitrogen.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
Slack GA, Tanzilli RA, Pohl RO, Vandersande JW. J Phys Chem Solids 1987;48:641. Mroz TJ. Am Ceram Soc Bull 1992;71:782. Sheng TY, Yu ZQ, Collins GJ. Appl Phys Lett 1988;52:576. Du!y MT, Wang CC, O'Clock GD, MacFarlance III Jr. SH, Zanzucchi PJ. J Electron Mater 1973;2:359. Okano H, Tanaka N, Takahashi Y, Tanaka T, Shibata K, Nakano S. Appl Phys Lett 1994;64:166. Mogab CJ, Lugujjs M. J Appl Phys 1976;47:1302. Zhang W, Someno Y, Sasaki M, Hirai T. J Crystal Growth 1993;130:308. Sasaki T, Matsuoka T. J Appl Phys 1995;77:192. Cohen A, Allenspacher P, Brieger MM, Jeuck I, Opower H. Appl Phys Lett 1991;59:2186. Ogawa T, Okamoto M, Yap YK, Mori Y, Hatta A, Ito T, Sasaki T, Hiraki A. Diamond Relat Mater 1997;6:1015. Vispute RD, Talyansky V, Sharma RP, Choopun S, Downes M, Venkatesan T, Li YX, Salamanca-Riba LG, Iladis AA, Jones KA, McGarrity J. Appl Surf Sci 1998;127:431.
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