- Email: [email protected]
Characteristics of single crystalline AlN films grown on Ru(0 0 0 1) substrates
ARTICLE IN PRESS
Journal of Crystal Growth 297 (2006) 317–320 www.elsevier.com/locate/jcrysgro
Characteristics of single crystalline AlN films grown on Ru(0 0 0 1) substrates S. Inouea, K. Okamotoa, T. Nakanob, H. Fujiokaa,b, a
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-ku, Tokyo 153-8505, Japan Kanagawa Academy of Science and Technology (KAST), 3-2-1 Sakado Takatsu-ku, Kawasaki 213-0012, Japan
b
Received 20 September 2006; received in revised form 29 September 2006; accepted 29 September 2006 Communicated by K.H. Ploog Available online 22 November 2006
Abstract We have grown AlN films on Ru(0 0 0 1) substrates using a low-temperature growth technique with pulsed laser deposition. We found that AlN(0 0 0 1) grows epitaxially on Ru(0 0 0 1) with an in-plane epitaxial relationship of AlN[1 1 2¯ 0]//Ru[1 1 2¯ 0]. Electron backscattering diffraction observations revealed that neither 301 rotational domains nor cubic phase domains were present in the AlN films and the full-width at half-maximum of the distribution in the AlN[0 0 0 1] crystalline orientation was 0.561. Spectroscopic ellipsometry measurements showed that the AlN/Ru hetero-interface was quite abrupt, which is important for fabrication of highfrequency film bulk acoustic resonators. r 2006 Elsevier B.V. All rights reserved. PACS: 61.14.Hg; 68.55.Jk; 79.60.Dp; 81.05.Bx; 81.05.Ea; 81.15.Fg Keywords: A1. Interfaces; A3. Laser epitaxy; B1. Nitrides; B3. Filters
1. Introduction To build the next generation of wireless mobile telecommunication systems with higher carrier frequencies, the development of high-frequency filters operating in the GHz-band range is very important. Film bulk acoustic resonators (FBARs) are promising candidates for this purpose because they have several conspicuous advantages over conventional filter devices, including low insertion loss, high-power operation, and high Q factor at high frequencies [1,2]. FBARs consist of a thin piezoelectric film sandwiched between metal electrodes. Poly crystalline AlN films are widely used as piezoelectric layers for FBARs because AlN possesses a high acoustic wave velocity, a reasonable piezoelectric coupling coefficient, and a good temperature coefficient [3–5]. Many researchers have also Corresponding author. Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-ku, Tokyo 153-8505, Japan. Tel.: +81 3 5452 6342; fax: +81 3 5452 6343. E-mail address: [email protected] (H. Fujioka).
0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.09.051
investigated the influence of the electrode material on the performance of the FBARs [6–12], and found that Mo is suitable for the bottom electrodes due to its low resistivity, high density, and large Young’s modulus [8,11–13]. Several other researchers have asserted that Ru is better than Mo for the electrodes because it has a greater density and larger Young’s modulus [14,15]; therefore, one can expect improved rejection levels at the pole of the FBAR characteristics by the use of Ru electrodes. It is quite natural to believe that the use of single crystalline AlN instead of poly-crystalline AlN films should lead to a dramatic enhancement in the performance of FBAR devices. However, it is known that the growth of single crystalline AlN on metal substrates using conventional epitaxial growth techniques such as metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) is very difficult due to the significant interfacial reactions between the AlN film and the metal electrodes at high growth temperatures above 800 1C. Recently, we found that the use of pulsed laser deposition (PLD) allows us to grow GaN and AlN films at low
ARTICLE IN PRESS S. Inoue et al. / Journal of Crystal Growth 297 (2006) 317–320
substrate temperatures, thereby suppressing the interfacial reactions [16–18]. In fact, successful epitaxial growth of group III nitrides was demonstrated even at room temperature [19,20]. In this paper, we report on the PLD low-temperature epitaxial growth of AlN on Ru(0 0 0 1) substrates, which is an important step in improving the performance of FBARs. 2. Experimental procedure We chose the basal plane of Ru as the substrate for the epitaxial growth of AlN since the Ru(0 0 0 1) plane and AlN(0 0 0 1) plane share threefold rotational symmetry. Commercially available as-cut single crystalline Ru(0 0 0 1) substrates (99.999%) were mechanically polished and degreased in organic solvents. These processes render the mirror-polished Ru surface suitable for the epitaxial growth of AlN. The as-polished substrates were loaded into an ultra-high vacuum (UHV) PLD chamber with a background pressure of 6 1010 Torr and annealed at 840 1C. 120 nm thick AlN films were deposited at 400–800 1C by ablating a sintered AlN target (99.99%) in a nitrogen ambient at a pressure of 1.0 102 Torr using a KrF excimer laser (l ¼ 248 nm, t ¼ 20 ns). The energy density and the pulse repetition rate of the laser were set at 3.0 J/cm2 and 30 Hz, respectively. The surface of the AlN during growth was monitored by RHEED. After growing the films, the samples were investigated by various ex-situ characterization techniques, including X-ray photoelectron spectroscopy (XPS) with a monochromatic Al Ka (1486.7 eV) X-ray source, spectroscopic ellipsometry (SE) with the light incident at an angle of 751, atomic force microscopy (AFM), and electron backscattering diffraction (EBSD). 3. Results and discussion In order to produce flat and clean Ru(0 0 0 1) surfaces suitable for epitaxial growth, annealing was performed in UHV conditions. In-situ XPS measurements were performed to check the removal of contaminants during the annealing process. Fig. 1(a) shows the O1s XPS spectra for the Ru(0 0 0 1) surface during annealing. O1s peaks are clearly observed at around 532 eV on the as-polished substrate that stem from the surface oxide layers and contaminants. These peaks become smaller with increasing annealing temperature and disappear at 600 1C. This result is consistent with the published data that report that ruthenium oxides are removed in the form of various RuOx fragments during the annealing process at around this temperature [21,22]. Unfortunately, removal of carbon by the annealing process could not be monitored by XPS due to the superimposition of the C1s and C KLL peaks on the Ru3d and Ru MNN peaks. Although the as-polished substrates exhibit a blurred RHEED pattern, the structure annealed at 840 1C has a sharp streaky 1 1 pattern, as seen in Fig. 1(b). AFM observations revealed that the root-
O1s 800°C 700°C
Intensity (a.u.)
318
600°C 500°C 400°C Aspolished
540
530 Binding Energy (eV)
b
Fig. 1. (a) O1s XPS spectra for Ru(0 0 0 1) during annealing at various temperatures and (b) RHEED pattern of a Ru substrate after annealing at 840 1C. The incidence of the electron beam is parallel to Ru[1 1 2¯ 0].
mean-square (RMS) value of the Ru surface after annealing is less than 1.0 nm, which is consistent with the sharp streaky RHEED pattern. These results indicate that the optimized annealing process makes it possible to obtain a flat and clean Ru(0 0 0 1) surface suitable for epitaxial growth of AlN films. After annealing Ru(0 0 0 1) at 840 1C, the substrate temperature was decreased to 800 1C and growth of an AlN film was performed. As shown in Fig. 2(a), there are rings in the RHEED image for AlN, which indicates that polycrystalline AlN films grow on the substrates at this temperature. AFM observations show that the film grown at 800 1C has quite a rough surface with a RMS value of 22.9 nm as seen in Fig. 2(b). To investigate the reason for the poor crystallinity in this film, SE measurements and theoretical fitting of the data were performed using a multilayer model, consisting of a Cauchy layer for the AlN and interfacial layers with the effective medium approximation. We found that an interfacial layer with a thickness of 20 nm exists at the hetero-interface between the AlN and the Ru. We concluded that interfacial reactions between the two materials caused degradation in the crystalline quality and surface morphology. In order to suppress these interfacial reactions, we grew AlN at a reduced substrate temperature taking advantage of the use of PLD. In the case of growth at 720 1C, we observed the coexistence of several domains with various crystal orientations at a film thickness of several nm, but the (0 0 0 1) domain becomes dominant at a film thickness
ARTICLE IN PRESS S. Inoue et al. / Journal of Crystal Growth 297 (2006) 317–320
319
Interfacial layer thickness (nm)
20
15
10
5
0 Fig. 2. (a) RHEED pattern of an AlN film grown at 800 1C. The incidence of the electron beam is parallel to AlN[1 1 2¯ 0]. (b) A 5 5 mm2 AFM image for the surface of the AlN film grown at 800 1C. The vertical full scale is 144 nm. (c) RHEED pattern of an AlN film grown at 500 1C. The incidence of the electron beam is parallel to AlN[1 1 2¯ 0]. (d) A 1 1 mm2 AFM image for the surface of the AlN film grown at 500 1C. The vertical full scale is 12.4 nm.
a
b
Intensity (a.u.)
AlN[0001]
120°
0.56˚
0° 240° -1
0 1 Polar angle (deg.)
Fig. 3. (a) An AlN[1 1 2¯ 4] EBSD pole figure for the sample grown at 500 1C and (b) distribution of the AlN[0 0 0 1] EBSD crystal orientation.
of around 100 nm. Further reduction in growth temperature down to 600 1C led to successful epitaxial growth of single-domain AlN from the initial stage of film growth. Fig. 2(c) shows RHEED patterns of AlN films grown at 500 1C. The clear spotty patterns with in-plane six fold rotational symmetry indicates that AlN(0 0 0 1) grows epitaxially on Ru(0 0 0 1), which forms a striking contrast to the case of films grown at 800 1C. Careful interpretation of the RHEED patterns led us to conclude that the inplane epitaxial relationship is AlN[1 1 2¯ 0]//Ru[1 1 2¯ 0], which is quite reasonable because this alignment minimizes the lattice mismatch (15.0%). Fig. 2(d) shows an AFM image of the surface of AlN grown at 500 1C. We found that the surface morphology dramatically improved and the RMS value for this surface is just 1.5 nm, which is quite
400
500
600
700
800
Growth temperature (°C) Fig. 4. Growth temperature dependence of the interfacial layer thickness evaluated using SE.
important for the fabrication of film bulk filter devices. We also performed XPS measurements on the surface of AlN grown at 500 1C and found that Ru3p3/2 peaks were not observed, indicating that no significant diffusion of Ru atoms into the AlN film had occurred. Fig. 3(a) shows an AlN[1 1 2¯ 4] EBSD pole figure. Six peaks are clearly seen in this figure, which indicates that neither 301 rotational domains nor cubic phase domains exist in the AlN films. Fig. 3(b) shows the distribution of the crystal orientation for the EBSD AlN[0 0 0 1] pole figure grown at 600 1C. This reveals that the FWHM of the distribution of the [0 0 0 1] crystalline orientation is as small as 0.561. These results indicate that a high-quality epitaxial AlN film grows, even at low substrate temperatures. Fig. 4 shows the growth temperature dependence of the interfacial layer thickness for the AlN/Ru heterostructures estimated by SE measurements. One can see that the interfacial reactions are well suppressed by the reduction in growth temperature and that the interfacial layer thickness between the AlN film and Ru(0 0 0 1) substrate is as thin as 0.9 nm at 600 1C. This result is quite consistent with the improved crystalline quality obtained by the reduction in growth temperature. 4. Conclusions We have succeeded in the epitaxial growth of AlN films on Ru(0 0 0 1) substrates using a low-temperature growth technique with pulsed laser deposition. We have found that AlN(0 0 0 1) grows on Ru(0 0 0 1) with an in-plane epitaxial relationship of AlN[1 1 2¯ 0]//Ru[1 1 2¯ 0], which equates to an in-plane lattice mismatch minimum (15.0%). EBSD observations revealed that neither 301 rotational domains nor cubic phase domains are present in the AlN films and the FWHM in the distribution of the AlN[0 0 0 1]
ARTICLE IN PRESS 320
S. Inoue et al. / Journal of Crystal Growth 297 (2006) 317–320
crystalline orientation is 0.561. SE measurements showed that the AlN/Ru hetero-interface is quite abrupt, which is important for fabrication of high-frequency FBARs. References [1] T.W. Grudkowski, J.F. Black, T.M. Reeder, D.E. Cullen, R.A. Wangner, Appl. Phys. Lett. 37 (1980) 993. [2] K.M. Lakin, J.S. Wang, Appl. Phys. Lett. 38 (1981) 125. [3] M.-A. Dubois, P. Muralt, Appl. Phys. Lett. 74 (1999) 3032. [4] K.M. Lakin, J. Belsick, J.F. McDonald, K.T. McCarron, in: Proceedings of the IEEE Ultrasonic Symposium 2001, p. 827. [5] T. Nishihara, T. Yokoyama, T. Miyashita, Y. Satoh, in: Proceedings of the IEEE Ultrasonic Symposium 2002, p. 969. [6] J.-B. Lee, J.-P. Jung, M.-H. Lee, J.-S. Park, Thin Solid Films 447–448 (2004) 610. [7] K.-W. Tay, C.-L. Huang, L. Wu, J. Vac. Sci. Technol. B 23 (2005) 1474. [8] H.-C. Lee, J.-Y. Park, K.-H. Lee, J.-U. Bu, J. Vac. Sci. Technol. B 22 (2004) 1127. [9] G.F. Iriarte, J. Bjurstro¨m, J. Westlinder, F. Engelmark, I.V. Katardjiev, IEEE Trans. Ultrason. Ferroelectron. Freq. Control 52 (2005) 1170.
[10] M.-A. Dubois, P. Muralt, J. Appl. Phys. 89 (2001) 6389. [11] M. Akiyama, K. Nagao, N. Ueno, H. Tateyama, T. Yamada, Vacuum 74 (2004) 699. [12] A. Sanz-Herva´sa, L. Vergara, J. Olivares, E. Iborra, Y. Morilla, J. Garcı´ a-Lo´pez, M. Clement, J. Sangrador, M.A. Respaldiz, Diamond Related Mater. 14 (2005) 1198. [13] S.-H. Lee, J.-K. Lee, J. Vac. Sci. Technol. A 21 (2003) 1. [14] W.-T. Lim, B.-K. Son, D.-H. Kang, C.-H. Lee, Thin Solid Films 382 (2001) 56. [15] Y. Satoh, T. Nishihara, T. Yokoyama, M. Ueda, T. Miyashita, Jpn. J. Appl. Phys. 44 (2005) 2883. [16] J. Ohta, H. Fujioka, M. Oshima, Appl. Phys. Lett. 83 (2003) 3060. [17] K. Okamoto, S. Inoue, N. Matsuki, T.-W. Kim, H. Fujioka, M. Oshima, Phys. Stat. Sol. A 202 (2005) R149. [18] S. Inoue, K. Okamoto, N. Matsuki, T.-W. Kim, H. Fujioka, Appl. Phys. Lett. 88 (2006) 261910. [19] M.H. Kim, M. Oshima, H. Kinoshita, Y. Shirakura, K. Miyamura, J. Ohta, A. Kobayashi, H. Fujioka, Appl. Phys. Lett. 89 (2006) 031916. [20] A. Kobayashi, Y. Kawaguchi, J. Ohta, H. Fujioka, K. Fujiwara, A. Ishii, Appl. Phys. Lett. 88 (2006) 181907. [21] I.J. Malik, J. Hrbek, J. Phys. Chem. 95 (1991) 10188. [22] A. Bo¨ttcher, H. Conrad, H. Niehus, Surf. Sci. 452 (2000) 125.
Journal of Crystal Growth 297 (2006) 317–320 www.elsevier.com/locate/jcrysgro
Characteristics of single crystalline AlN films grown on Ru(0 0 0 1) substrates S. Inouea, K. Okamotoa, T. Nakanob, H. Fujiokaa,b, a
Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-ku, Tokyo 153-8505, Japan Kanagawa Academy of Science and Technology (KAST), 3-2-1 Sakado Takatsu-ku, Kawasaki 213-0012, Japan
b
Received 20 September 2006; received in revised form 29 September 2006; accepted 29 September 2006 Communicated by K.H. Ploog Available online 22 November 2006
Abstract We have grown AlN films on Ru(0 0 0 1) substrates using a low-temperature growth technique with pulsed laser deposition. We found that AlN(0 0 0 1) grows epitaxially on Ru(0 0 0 1) with an in-plane epitaxial relationship of AlN[1 1 2¯ 0]//Ru[1 1 2¯ 0]. Electron backscattering diffraction observations revealed that neither 301 rotational domains nor cubic phase domains were present in the AlN films and the full-width at half-maximum of the distribution in the AlN[0 0 0 1] crystalline orientation was 0.561. Spectroscopic ellipsometry measurements showed that the AlN/Ru hetero-interface was quite abrupt, which is important for fabrication of highfrequency film bulk acoustic resonators. r 2006 Elsevier B.V. All rights reserved. PACS: 61.14.Hg; 68.55.Jk; 79.60.Dp; 81.05.Bx; 81.05.Ea; 81.15.Fg Keywords: A1. Interfaces; A3. Laser epitaxy; B1. Nitrides; B3. Filters
1. Introduction To build the next generation of wireless mobile telecommunication systems with higher carrier frequencies, the development of high-frequency filters operating in the GHz-band range is very important. Film bulk acoustic resonators (FBARs) are promising candidates for this purpose because they have several conspicuous advantages over conventional filter devices, including low insertion loss, high-power operation, and high Q factor at high frequencies [1,2]. FBARs consist of a thin piezoelectric film sandwiched between metal electrodes. Poly crystalline AlN films are widely used as piezoelectric layers for FBARs because AlN possesses a high acoustic wave velocity, a reasonable piezoelectric coupling coefficient, and a good temperature coefficient [3–5]. Many researchers have also Corresponding author. Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-ku, Tokyo 153-8505, Japan. Tel.: +81 3 5452 6342; fax: +81 3 5452 6343. E-mail address: [email protected] (H. Fujioka).
0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.09.051
investigated the influence of the electrode material on the performance of the FBARs [6–12], and found that Mo is suitable for the bottom electrodes due to its low resistivity, high density, and large Young’s modulus [8,11–13]. Several other researchers have asserted that Ru is better than Mo for the electrodes because it has a greater density and larger Young’s modulus [14,15]; therefore, one can expect improved rejection levels at the pole of the FBAR characteristics by the use of Ru electrodes. It is quite natural to believe that the use of single crystalline AlN instead of poly-crystalline AlN films should lead to a dramatic enhancement in the performance of FBAR devices. However, it is known that the growth of single crystalline AlN on metal substrates using conventional epitaxial growth techniques such as metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) is very difficult due to the significant interfacial reactions between the AlN film and the metal electrodes at high growth temperatures above 800 1C. Recently, we found that the use of pulsed laser deposition (PLD) allows us to grow GaN and AlN films at low
ARTICLE IN PRESS S. Inoue et al. / Journal of Crystal Growth 297 (2006) 317–320
substrate temperatures, thereby suppressing the interfacial reactions [16–18]. In fact, successful epitaxial growth of group III nitrides was demonstrated even at room temperature [19,20]. In this paper, we report on the PLD low-temperature epitaxial growth of AlN on Ru(0 0 0 1) substrates, which is an important step in improving the performance of FBARs. 2. Experimental procedure We chose the basal plane of Ru as the substrate for the epitaxial growth of AlN since the Ru(0 0 0 1) plane and AlN(0 0 0 1) plane share threefold rotational symmetry. Commercially available as-cut single crystalline Ru(0 0 0 1) substrates (99.999%) were mechanically polished and degreased in organic solvents. These processes render the mirror-polished Ru surface suitable for the epitaxial growth of AlN. The as-polished substrates were loaded into an ultra-high vacuum (UHV) PLD chamber with a background pressure of 6 1010 Torr and annealed at 840 1C. 120 nm thick AlN films were deposited at 400–800 1C by ablating a sintered AlN target (99.99%) in a nitrogen ambient at a pressure of 1.0 102 Torr using a KrF excimer laser (l ¼ 248 nm, t ¼ 20 ns). The energy density and the pulse repetition rate of the laser were set at 3.0 J/cm2 and 30 Hz, respectively. The surface of the AlN during growth was monitored by RHEED. After growing the films, the samples were investigated by various ex-situ characterization techniques, including X-ray photoelectron spectroscopy (XPS) with a monochromatic Al Ka (1486.7 eV) X-ray source, spectroscopic ellipsometry (SE) with the light incident at an angle of 751, atomic force microscopy (AFM), and electron backscattering diffraction (EBSD). 3. Results and discussion In order to produce flat and clean Ru(0 0 0 1) surfaces suitable for epitaxial growth, annealing was performed in UHV conditions. In-situ XPS measurements were performed to check the removal of contaminants during the annealing process. Fig. 1(a) shows the O1s XPS spectra for the Ru(0 0 0 1) surface during annealing. O1s peaks are clearly observed at around 532 eV on the as-polished substrate that stem from the surface oxide layers and contaminants. These peaks become smaller with increasing annealing temperature and disappear at 600 1C. This result is consistent with the published data that report that ruthenium oxides are removed in the form of various RuOx fragments during the annealing process at around this temperature [21,22]. Unfortunately, removal of carbon by the annealing process could not be monitored by XPS due to the superimposition of the C1s and C KLL peaks on the Ru3d and Ru MNN peaks. Although the as-polished substrates exhibit a blurred RHEED pattern, the structure annealed at 840 1C has a sharp streaky 1 1 pattern, as seen in Fig. 1(b). AFM observations revealed that the root-
O1s 800°C 700°C
Intensity (a.u.)
318
600°C 500°C 400°C Aspolished
540
530 Binding Energy (eV)
b
Fig. 1. (a) O1s XPS spectra for Ru(0 0 0 1) during annealing at various temperatures and (b) RHEED pattern of a Ru substrate after annealing at 840 1C. The incidence of the electron beam is parallel to Ru[1 1 2¯ 0].
mean-square (RMS) value of the Ru surface after annealing is less than 1.0 nm, which is consistent with the sharp streaky RHEED pattern. These results indicate that the optimized annealing process makes it possible to obtain a flat and clean Ru(0 0 0 1) surface suitable for epitaxial growth of AlN films. After annealing Ru(0 0 0 1) at 840 1C, the substrate temperature was decreased to 800 1C and growth of an AlN film was performed. As shown in Fig. 2(a), there are rings in the RHEED image for AlN, which indicates that polycrystalline AlN films grow on the substrates at this temperature. AFM observations show that the film grown at 800 1C has quite a rough surface with a RMS value of 22.9 nm as seen in Fig. 2(b). To investigate the reason for the poor crystallinity in this film, SE measurements and theoretical fitting of the data were performed using a multilayer model, consisting of a Cauchy layer for the AlN and interfacial layers with the effective medium approximation. We found that an interfacial layer with a thickness of 20 nm exists at the hetero-interface between the AlN and the Ru. We concluded that interfacial reactions between the two materials caused degradation in the crystalline quality and surface morphology. In order to suppress these interfacial reactions, we grew AlN at a reduced substrate temperature taking advantage of the use of PLD. In the case of growth at 720 1C, we observed the coexistence of several domains with various crystal orientations at a film thickness of several nm, but the (0 0 0 1) domain becomes dominant at a film thickness
ARTICLE IN PRESS S. Inoue et al. / Journal of Crystal Growth 297 (2006) 317–320
319
Interfacial layer thickness (nm)
20
15
10
5
0 Fig. 2. (a) RHEED pattern of an AlN film grown at 800 1C. The incidence of the electron beam is parallel to AlN[1 1 2¯ 0]. (b) A 5 5 mm2 AFM image for the surface of the AlN film grown at 800 1C. The vertical full scale is 144 nm. (c) RHEED pattern of an AlN film grown at 500 1C. The incidence of the electron beam is parallel to AlN[1 1 2¯ 0]. (d) A 1 1 mm2 AFM image for the surface of the AlN film grown at 500 1C. The vertical full scale is 12.4 nm.
a
b
Intensity (a.u.)
AlN[0001]
120°
0.56˚
0° 240° -1
0 1 Polar angle (deg.)
Fig. 3. (a) An AlN[1 1 2¯ 4] EBSD pole figure for the sample grown at 500 1C and (b) distribution of the AlN[0 0 0 1] EBSD crystal orientation.
of around 100 nm. Further reduction in growth temperature down to 600 1C led to successful epitaxial growth of single-domain AlN from the initial stage of film growth. Fig. 2(c) shows RHEED patterns of AlN films grown at 500 1C. The clear spotty patterns with in-plane six fold rotational symmetry indicates that AlN(0 0 0 1) grows epitaxially on Ru(0 0 0 1), which forms a striking contrast to the case of films grown at 800 1C. Careful interpretation of the RHEED patterns led us to conclude that the inplane epitaxial relationship is AlN[1 1 2¯ 0]//Ru[1 1 2¯ 0], which is quite reasonable because this alignment minimizes the lattice mismatch (15.0%). Fig. 2(d) shows an AFM image of the surface of AlN grown at 500 1C. We found that the surface morphology dramatically improved and the RMS value for this surface is just 1.5 nm, which is quite
400
500
600
700
800
Growth temperature (°C) Fig. 4. Growth temperature dependence of the interfacial layer thickness evaluated using SE.
important for the fabrication of film bulk filter devices. We also performed XPS measurements on the surface of AlN grown at 500 1C and found that Ru3p3/2 peaks were not observed, indicating that no significant diffusion of Ru atoms into the AlN film had occurred. Fig. 3(a) shows an AlN[1 1 2¯ 4] EBSD pole figure. Six peaks are clearly seen in this figure, which indicates that neither 301 rotational domains nor cubic phase domains exist in the AlN films. Fig. 3(b) shows the distribution of the crystal orientation for the EBSD AlN[0 0 0 1] pole figure grown at 600 1C. This reveals that the FWHM of the distribution of the [0 0 0 1] crystalline orientation is as small as 0.561. These results indicate that a high-quality epitaxial AlN film grows, even at low substrate temperatures. Fig. 4 shows the growth temperature dependence of the interfacial layer thickness for the AlN/Ru heterostructures estimated by SE measurements. One can see that the interfacial reactions are well suppressed by the reduction in growth temperature and that the interfacial layer thickness between the AlN film and Ru(0 0 0 1) substrate is as thin as 0.9 nm at 600 1C. This result is quite consistent with the improved crystalline quality obtained by the reduction in growth temperature. 4. Conclusions We have succeeded in the epitaxial growth of AlN films on Ru(0 0 0 1) substrates using a low-temperature growth technique with pulsed laser deposition. We have found that AlN(0 0 0 1) grows on Ru(0 0 0 1) with an in-plane epitaxial relationship of AlN[1 1 2¯ 0]//Ru[1 1 2¯ 0], which equates to an in-plane lattice mismatch minimum (15.0%). EBSD observations revealed that neither 301 rotational domains nor cubic phase domains are present in the AlN films and the FWHM in the distribution of the AlN[0 0 0 1]
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crystalline orientation is 0.561. SE measurements showed that the AlN/Ru hetero-interface is quite abrupt, which is important for fabrication of high-frequency FBARs. References [1] T.W. Grudkowski, J.F. Black, T.M. Reeder, D.E. Cullen, R.A. Wangner, Appl. Phys. Lett. 37 (1980) 993. [2] K.M. Lakin, J.S. Wang, Appl. Phys. Lett. 38 (1981) 125. [3] M.-A. Dubois, P. Muralt, Appl. Phys. Lett. 74 (1999) 3032. [4] K.M. Lakin, J. Belsick, J.F. McDonald, K.T. McCarron, in: Proceedings of the IEEE Ultrasonic Symposium 2001, p. 827. [5] T. Nishihara, T. Yokoyama, T. Miyashita, Y. Satoh, in: Proceedings of the IEEE Ultrasonic Symposium 2002, p. 969. [6] J.-B. Lee, J.-P. Jung, M.-H. Lee, J.-S. Park, Thin Solid Films 447–448 (2004) 610. [7] K.-W. Tay, C.-L. Huang, L. Wu, J. Vac. Sci. Technol. B 23 (2005) 1474. [8] H.-C. Lee, J.-Y. Park, K.-H. Lee, J.-U. Bu, J. Vac. Sci. Technol. B 22 (2004) 1127. [9] G.F. Iriarte, J. Bjurstro¨m, J. Westlinder, F. Engelmark, I.V. Katardjiev, IEEE Trans. Ultrason. Ferroelectron. Freq. Control 52 (2005) 1170.
[10] M.-A. Dubois, P. Muralt, J. Appl. Phys. 89 (2001) 6389. [11] M. Akiyama, K. Nagao, N. Ueno, H. Tateyama, T. Yamada, Vacuum 74 (2004) 699. [12] A. Sanz-Herva´sa, L. Vergara, J. Olivares, E. Iborra, Y. Morilla, J. Garcı´ a-Lo´pez, M. Clement, J. Sangrador, M.A. Respaldiz, Diamond Related Mater. 14 (2005) 1198. [13] S.-H. Lee, J.-K. Lee, J. Vac. Sci. Technol. A 21 (2003) 1. [14] W.-T. Lim, B.-K. Son, D.-H. Kang, C.-H. Lee, Thin Solid Films 382 (2001) 56. [15] Y. Satoh, T. Nishihara, T. Yokoyama, M. Ueda, T. Miyashita, Jpn. J. Appl. Phys. 44 (2005) 2883. [16] J. Ohta, H. Fujioka, M. Oshima, Appl. Phys. Lett. 83 (2003) 3060. [17] K. Okamoto, S. Inoue, N. Matsuki, T.-W. Kim, H. Fujioka, M. Oshima, Phys. Stat. Sol. A 202 (2005) R149. [18] S. Inoue, K. Okamoto, N. Matsuki, T.-W. Kim, H. Fujioka, Appl. Phys. Lett. 88 (2006) 261910. [19] M.H. Kim, M. Oshima, H. Kinoshita, Y. Shirakura, K. Miyamura, J. Ohta, A. Kobayashi, H. Fujioka, Appl. Phys. Lett. 89 (2006) 031916. [20] A. Kobayashi, Y. Kawaguchi, J. Ohta, H. Fujioka, K. Fujiwara, A. Ishii, Appl. Phys. Lett. 88 (2006) 181907. [21] I.J. Malik, J. Hrbek, J. Phys. Chem. 95 (1991) 10188. [22] A. Bo¨ttcher, H. Conrad, H. Niehus, Surf. Sci. 452 (2000) 125.