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Layer-by-layer deposition of epitaxial TiN–CrN multilayers on MgO(0 0 1) by pulsed laser ablation
Applied Surface Science 235 (2004) 460–464
Layer-by-layer deposition of epitaxial TiN–CrN multilayers on MgO(0 0 1) by pulsed laser ablation Kei Inumaru*, Takayoshi Ohara, Kazuma Tanaka, Shoji Yamanaka Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Received in revised form 13 February 2004; accepted 3 March 2004 Available online 20 June 2004
Abstract Titanium nitride (TiN)–chromium nitride (CrN) multilayers were deposited epitaxially on MgO(0 0 1) by pulsed laser ablation combined with nitrogen radical irradiation. High quality TiN film was first deposited on MgO(0 0 1) substrate. During the alternate deposition of six TiN monolayers and three CrN monolayers, reflection high-energy electron diffraction (RHEED) intensity oscillations were observed continuously, showing layer-by-layer growth of a transition metal nitride multilayer structure. An X-ray diffraction peak corresponding to a multilayer periodicity of 2.1 nm was observed. The multilayer sample showed metallic conductivity and ferromagnetic behavior with a Curie temperature of ca. 70 K. The ferromagnetism may be attributable to the formation of a TiN–CrN mixed phase at the multilayer boundaries. # 2004 Elsevier B.V. All rights reserved. PACS: 68.55.a; 68.65.Cd; 75.70.i Keywords: Titanium nitride; Chromium nitride; Epitaxy; RHEED oscillation; Pulsed laser deposition; Superlattices
1. Introduction Recently, many experimental and theoretical studies that reflect the wide variety of physical properties of transition-metal nitrides have been reported [1–28]. For example, 3d transition metal nitrides show antiferromagnetism (MnN [1–4], CrN [5–8], FeN [9,10]), Pauli paramagnetism (CoN) [11], and superconductivity (TiN, VN) [29]. Thin films and superlattices of these compounds have also attracted much attention [1–5,10–27]. Discovery of the electron-doped layered hafnium nitride superconductor (b-HfNCl) with a Tc *
Corresponding author. Tel.: þ81-824-24-7741; fax: þ81-824-24-5494. E-mail address: [email protected] (K. Inumaru).
of as high as 25.5 K [30] stimulated interest in the properties of two-dimensional layers or superlattices based on group-IV metal nitrides such as HfN, ZrN, and TiN. A multilayer structure formed by alternate deposition of two different type of nitride layers such as superconductive TiN and semiconducting [8] and antiferromagnetic CrN would exhibit interesting properties at low temperatures. Among the group-IV metal nitrides, TiN has been studied most extensively as regards deposition as thin films, and these studies have used various methods such as reactive-magnetron sputtering [22,23] and pulsed laser deposition (PLD) [17,24–27]. The PLD method combined with RHEED intensity oscillation observation has been widely used as a useful method for the synthesis of metal oxide superlattices [31,32]. As
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.260
K. Inumaru et al. / Applied Surface Science 235 (2004) 460–464
regards transition-metal nitrides, however, there have been few studies on such controlled layer-by-layer growth. Timm et al. [25,26] reported the RHEED oscillation observations for TiN growth on a Si(0 0 1) substrate, but a considerable misfit between TiN and the substrate caused three-dimensional growth after deposition of a thickness of several two-dimensional layers, i.e. Stranski–Krastanov growth. Later, we successfully carried out layer-by-layer growth of TiN on MgO with monitoring by RHEED oscillation during deposition by a PLD method. This was reported as a brief communication [17]. In this study, TiN–CrN multilayered structures were grown epitaxially on MgO(0 0 1) by using a PLD method. We succeeded in monitoring the layer-bylayer growth of transition-metal nitride multilayers by using RHEED intensity oscillation. Electrical conductivity and magnetic properties of the nitride multilayers were measured.
2. Experimental The PLD system used was equipped with a RHEED apparatus, a KrF excimer laser (COMPex 102, Lamda Physics, Goettingen, Germany) and an RF-plasma radical source (Model RF 4.5, SVTA Inc., MN, USA). The residual pressure of the chamber was less than 108 Torr. MgO(0 0 1) substrates (Advanced Film Technology Inc., Tokyo, Japan) were cleaned ultrasonically in methanol and then annealed under O2 flow. Since flatness of the substrate is essential for growth in a layer-by-layer mode, we used AFM to examine the surfaces of the substrates annealed at various temperatures. The following procedure was found to be suitable for the annealing: the temperature of the MgO substrates was raised to 973 K over 1 h, then heated up to 1223 K at a rate of 50 K h1, after which this temperature was held for 5 h. AFM measurements revealed that flat terraces as wide as ca. 0.5 mm were formed [17]. The annealed MgO exhibited a RHEED pattern (not shown) with clear spots on the zeroth Laue circle, which shows the presence of highly flat MgO surface. TiN epitaxial thin films and TiN–CrN superlattices were deposited using Ti and Cr metal disks as the targets. A polycrystalline TiN disk was also used for
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comparison with the Ti metal disk. The thin films were grown under nitrogen radical irradiation. During the deposition, the specular spot intensity in the RHEED pattern was recorded after every 20 laser pulses of irradiation. X-ray diffraction reciprocal space mapping was carried out by using an XPert MRD diffractometer (Philips, The Netherlands) equipped with a Ge(2 2 0) 4-crystals monochrometer. Magnetization of thin films was measured with a magnetometer (MPMS5S, Quantam Design, USA). Magnetic field was applied parallel to the thin films and MgO(0 0 1). Electronic conductivity of the samples was measured by using a 4-probe method in the temperature range of 2–100 K, where the current flowed parallel to the multilayers.
3. Results and discussion 3.1. Growth of TiN layers on MgO monitored by RHEED oscillation First of all, we tried to deposit TiN in a layer-bylayer mode. Fig. 1 shows an example of the RHEED oscillation obtained, which we reported in a previous brief communication [17]. Each cycle of the oscillation corresponds to the deposition of a TiN monolayer with a thickness of 0.212 nm. The deposition conditions were as follows: substrate temperature, 973 K; laser fluence, ca. 13 J cm2; RF power of the radical source, 350 W; N2 gas feeding rate, 1 cm3 min1 (STD); pressure during deposition, 2 105 Torr. The oscillation was found to be very sensitive to
Fig. 1. RHEED oscillation of TiN growth on MgO(0 0 1) detected at the specular spot in the [1 1 0] azimuth.
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the nitrogen feeding rate (i.e. the flux of nitrogen radical) and it was suppressed by increasing the N2 gas feed to 3 cm3 min1 (STD). Although the oscillation dumped after ca. 14 cycles, the Laue circle was still observed after deposition of a thickness of 30 nm, demonstrating that the surface remained highly flat. AFM observation of the TiN film revealed that the terraces were highly flat, with a small roughness of less than 0.4 nm. It was essential to use a metal Ti target for the observation of RHEED oscillation: When we used TiN as the target, we obtained crystallized TiN films, but were unable to observe RHEED oscillation. The RHEED oscillation was also sensitive to the N2 feeding rate (i.e. the flux of nitrogen radical). These findings can be interpreted in terms of the mobility of Ti species on the top surface during the deposition. On the thin film surface, Ti species from the metal target would have higher mobility than those from the TiN target, because the former species seems likely to be partially nitridized. It is expected that the flux of nitrogen radicals also influences the mobility of the surface species. Similar phenomena were reported for a homo-epitaxy of SrTiO3 by MBE, in which the conditions suitable for RHEED oscillations were determined by a balance between substrate temperature and feeding rate of oxygen [33]. A high-resolution X-ray diffraction reciprocal space map for TiN/MgO is shown in Fig. 2. The TiN 1 1 3 reflection was aligned vertically to that of the MgO 1 1 3. The very narrow width in the [1 1 0] direction of the spot of TiN demonstrates the excellent quality of the epitaxial film, with no relaxation and no mosaic disorder in this direction. The calculated position for the bulk TiN 1 1 3 (corresponding to a lattice a ¼ 0:4242 nm) is shown in Fig. 2. This demonstrates that the TiN lattice of the film shrunk along the
Fig. 2. High-resolution X-ray diffraction reciprocal space map of TiN/MgO(0 0 1). The contour lines represent the intensities of 7, 13, 21, 42, 100, 5 103 , and 2 104 cps. The scales are expressed with reference to the MgO 1 1 3 spot.
directions parallel to the surface of the substrate by 0.7%, to fit the MgO(0 0 1) lattice. 3.2. Layer-by-layer growth of TiN–CrN multilayers monitored by RHEED oscillation Fig. 3 shows the RHEED oscillation during the growth of TiN–CrN multilayers. Ti and Cr metal targets were ablated alternately with 900 laser pulses. Clear oscillations were observed during the ablation of both Ti and Cr targets. The frequencies of the oscillations for the two targets differ from each other, but are almost constant for the same target. This is the first example, to our knowledge, of layer-by-layer growth of transition metal nitride superlattices monitored by RHEED oscillation. From the frequencies, the growth rates for TiN and CrN were estimated: the 900 pulses deposit an average of 6.3 monolayers for TiN (one monolayer ¼ 0.212 nm) and 2.8 monolayers for CrN (one monolayer ¼ 0.207 nm). Therefore, the
Fig. 3. RHEED oscillation during growth of TiN–CrN multilayers on MgO(0 0 1).
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Fig. 4. X-ray diffraction of TiN–CrN multilayers.
Fig. 5. Temperature dependence of electric resistivity of CrN–TiN multilayers.
periodicity of the superlattice was calculated to be 1.9 nm. As shown in Fig. 3, the oscillation intensity became gradually weaker, and almost disappeared after 6300 pulses. The deposition of TiN and CrN was repeated 15 times, and at the last stage TiN was deposited (900 pulses) as the top layer. Fig. 4 shows the X-ray diffraction peak of the multilayer. A weak peak was observed at d ¼ 2:1 nm (2y ¼ 4.158), which was consistent with the results obtained by RHEED oscillation. This finding can be interpreted in terms of the formation of the superlattice TiN–CrN multilayer. One reason for the weakness of the X-ray diffraction peak is that the X-ray scattering factors of Ti and Cr are very close. Another possible reason could be that the composition profile of the TiN– CrN boundary was not sharp. In other words, a TiN and CrN mixed phase might be formed at the boundaries. In fact, no satellite peaks due to the superlattice were observed. The RHEED oscillation in Fig. 3 clearly demonstrated that CrN layers were grown on TiN/ MgO in an epitaixial layer-by-layer mode, though the lattice mismatch between CrN and TiN/MgO is not very small (1.6%). There is a possibility that mixing of Cr and Ti at the boundaries compensated the mismatch to some extent and helped the epitaxy. The intense MgO 0 0 2 peak hid the 0 0 2 diffraction due to the NaCl crystal structure of TiN and CrN.
ductor. TiN is a metallic conductor, and semiconductor-like behavior of CrN has been reported [8]. Therefore, the metallic conductivity of the TiN–CrN multilayer would be attributable to the TiN layers, while the contribution of the TiN–CrN mixed region to the electronic conductivity is unknown. The temperature dependence of the magnetization of the TiN–CrN multilayer sample measured under a magnetic field of 10 Oe is shown in Fig. 6. Large magnetization was observed at temperatures below ca. 65 K. The inset of Fig. 6 shows a field-magnetization curve for the same sample at 12 K. Hysteresis was clearly observed, demonstrating that the sample was ferromagnetic. The magnetization shown in Fig. 6 has a maximum at around 30 K with decreasing temperature. The reason for this behavior is not clear at present. In a magnetic field of 100 Oe, the curve of the field cooling mode below 20 K became almost flat (data not shown). The ferromagnetism may be ascribed to the formation of TiN–CrN solid solution
3.3. Electrical conductivity and magnetic properties of TiN–CrN multilayers Fig. 5 shows the temperature dependence of the electrical resistivity of the TiN–CrN multilayered sample. The resistivity decreased as the sample was cooled, showing that the sample is a metallic con-
Fig. 6. Temperature dependence of magnetization of CrN–TiN multilayers. Area and thickness of the sample were 31.5 mm2 and 32 nm, respectively. Magnetic field was 10 Oe. The inset shows field-magnetization hysteresis loop for the same sample at 12 K.
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at the boundaries. In fact, Cr0.25Ti0.75N thin film on MgO prepared by using Cr–Ti alloy as the target is ferromagnetic with a Curie temperature of 60 K [34]. This could also explain the weakness of the X-ray diffraction peak mentioned above.
4. Conclusions We were able to observe RHEED oscillations during the growth of CrN–TiN multilayers on MgO(0 0 1) after optimization of the pre-annealing condition of the MgO substrates. Usage of metal targets was essential for the oscillation. X-ray diffraction confirmed the formation of the multilayer structure, while mixing of TiN and CrN may have occurred at the boundaries. The multilayers showed metallic electroconductivity and ferromagnetic behavior with Tc ¼ ca. 70 K, where the ferromagnetism may be ascribed to the TiN–CrN mixed phase.
Acknowledgements This work was partially supported by a CREST project from JST, a Grant-in-Aid from the Japan Ministry of Education for Science, Culture, Sports and Technology (MEXT), and the Japan Sheet Glass Foundation. References [1] K. Suzuki, T. Kaneko, H. Yoshida, Y. Obi, H. Fujimori, H. Morita, J. Alloys Compd. 306 (2000) 66. [2] K. Suzuki, T. Suzuki, Y. Fujiwara, T. Kaneko, H. Yoshida, Y. Obi, S. Tomiyoshi, J. Alloys Compd. 360 (2003) 34. [3] H.Q. Yang, H. Al-Brithen, E. Trifan, D.C. Ingram, A.R. Smith, J. Appl. Phys. 91 (2002) 1053. [4] H.Q. Yang, H. Al-Brithen, A.R. Smith, J.A. Borchers, R.L. Cappelletti, M.D. Vaudin, Appl. Phys. Lett. 78 (2001) 3860. [5] K. Suzuki, T. Kaneko, H. Yoshida, Y. Obi, H. Fujimori, J. Alloys Compd. 280 (1998) 294. [6] A. Filippetti, N.A. Hill, Phys. Rev. Lett. 85 (2000) 5166. [7] A. Filippetti, W.E. Pickett, B.M. Klein, Phys. Rev. B 59 (1999) 7043.
[8] P. Subramanya Herle, M.S. Hedge, N.Y. Vasathacharya, S. Philip, J. Solid State Chem. 134 (1997) 120. [9] T. Hinomura, S. Nasu, Phys. B 237–238 (1997) 557. [10] K. Suzuki, Y. Yamaguchi, T. Kaneko, H. Yoshida, Y. Obi, H. Fujimori, H. Morita, J. Phys. Soc. Jpn. 70 (2001) 1084. [11] K. Suzuki, T. Kaneko, H. Yoshida, H. Morita, H. Fujimori, J. Alloys Compd. 224 (1995) 232. [12] N. Takeuchi, Phys. Rev. B 66 (2002) 153405. [13] A.R. Smith, H. Al-Brithen, D.C. Ingram, D. Gall, J. Appl. Phys. 90 (2001) 1809. [14] R.M. Frazier, J. Stapleton, G.T. Thaler, C.R. Abernathy, S.J. Pearton, R. Rairigh, J. Kelly, A.F. Hebard, M.L. Nakarmi, K.B. Nam, J.Y. Lin, H.X. Jiang, J.M. Zavada, R.G. Wilson, J. Appl. Phys. 94 (2003) 1592. [15] S.J. Pearton, Y.D. Park, C.R. Abernathy, M.E. Overberg, G.T. Thaler, J. Kim, F. Ren, J. Electron. Mater. 32 (2003) 288. [16] J.S. Lee, J.D. Lim, Z.G. Khim, Y.D. Park, S.J. Pearton, S. Chu, J. Appl. Phys. 93 (2003) 4512. [17] K. Inumaru, T. Ohara, S. Yamanaka, Appl. Surf. Sci. 158 (2000) 375. [18] K. Inumaru, H. Okamoto, S. Yamanaka, J. Cryst. Growth 237–239 (2002) 2050. [19] K. Inumaru, K. Sakamoto, H. Okamoto, S. Yamanaka, Phys. B 328 (2003) 123. [20] K. Sakamoto, K. Inumaru, S. Yamanaka, Appl. Surf. Sci. 199 (2002) 303. [21] K. Inumaru, Y. Kuroda, K. Sakamoto, M. Murashima, S. Yamanaka, J. Alloys Compd. 372 (2004) L1. [22] B.O. Johansson, J.E. Sundgren, J.E. Greene, A. Rockett, A. Barnett, J. Vac. Sci. Technol. A 3 (1985) 303. [23] N. Hirashita, J.E. Greene, U. Helmersson, J. Birch, J.-E. Sundgren, J. Appl. Phys. 70 (1991) 4963. [24] A. Kumar, H.L. Chan, U. Ekanayake, A. Wierzbicki, N.B. Dahotre, J. Mat. Eng. Perform. 6 (1997) 577. [25] R. Timm, R.R. Willmot, J.R. Huber, Appl. Phys. Lett. 71 (1997) 1966. [26] P.R. Willmott, R. Timm, J.R. Huber, Appl. Surf. Sci. 127–129 (1998) 105. [27] M.B. Lee, M. Kawasaki, M. Yoshimoto, M. Kumagai, H. Koinuma, Jpn. J. Appl. Phys. 33 (1994) 6308. [28] L. Mancera, J.A. Rodriguez, N. Takeuchi, J. Phys. Cond. Mat. 15 (2003) 2625. YN. [29] H.F. George, H.K. John, Phys. Rev. 93 (1954) 1004. [30] S. Yamanaka, K. Hotehama, H. Kawaji, Nature 392 (1998) 580. [31] M. Kawasaki, K. Takahashi, T. Maeda, R. Tsuchiya, M. Shinohara, O. Ishiyama, T. Yonezawa, M. Yoshimoto, H. Koinuma, Science 266 (1994) 1540. [32] K. Ueda, H. Tabata, T. Kawai, Science 280 (1998) 1064. [33] M. Naito, H. Yamamoto, H. Sato, Phys. C 305 (1998) 233. [34] K. Inumaru, Y. Miyaki, K. Tanaka, S. Yamanaka, in preparation.
Layer-by-layer deposition of epitaxial TiN–CrN multilayers on MgO(0 0 1) by pulsed laser ablation Kei Inumaru*, Takayoshi Ohara, Kazuma Tanaka, Shoji Yamanaka Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Received in revised form 13 February 2004; accepted 3 March 2004 Available online 20 June 2004
Abstract Titanium nitride (TiN)–chromium nitride (CrN) multilayers were deposited epitaxially on MgO(0 0 1) by pulsed laser ablation combined with nitrogen radical irradiation. High quality TiN film was first deposited on MgO(0 0 1) substrate. During the alternate deposition of six TiN monolayers and three CrN monolayers, reflection high-energy electron diffraction (RHEED) intensity oscillations were observed continuously, showing layer-by-layer growth of a transition metal nitride multilayer structure. An X-ray diffraction peak corresponding to a multilayer periodicity of 2.1 nm was observed. The multilayer sample showed metallic conductivity and ferromagnetic behavior with a Curie temperature of ca. 70 K. The ferromagnetism may be attributable to the formation of a TiN–CrN mixed phase at the multilayer boundaries. # 2004 Elsevier B.V. All rights reserved. PACS: 68.55.a; 68.65.Cd; 75.70.i Keywords: Titanium nitride; Chromium nitride; Epitaxy; RHEED oscillation; Pulsed laser deposition; Superlattices
1. Introduction Recently, many experimental and theoretical studies that reflect the wide variety of physical properties of transition-metal nitrides have been reported [1–28]. For example, 3d transition metal nitrides show antiferromagnetism (MnN [1–4], CrN [5–8], FeN [9,10]), Pauli paramagnetism (CoN) [11], and superconductivity (TiN, VN) [29]. Thin films and superlattices of these compounds have also attracted much attention [1–5,10–27]. Discovery of the electron-doped layered hafnium nitride superconductor (b-HfNCl) with a Tc *
Corresponding author. Tel.: þ81-824-24-7741; fax: þ81-824-24-5494. E-mail address: [email protected] (K. Inumaru).
of as high as 25.5 K [30] stimulated interest in the properties of two-dimensional layers or superlattices based on group-IV metal nitrides such as HfN, ZrN, and TiN. A multilayer structure formed by alternate deposition of two different type of nitride layers such as superconductive TiN and semiconducting [8] and antiferromagnetic CrN would exhibit interesting properties at low temperatures. Among the group-IV metal nitrides, TiN has been studied most extensively as regards deposition as thin films, and these studies have used various methods such as reactive-magnetron sputtering [22,23] and pulsed laser deposition (PLD) [17,24–27]. The PLD method combined with RHEED intensity oscillation observation has been widely used as a useful method for the synthesis of metal oxide superlattices [31,32]. As
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.260
K. Inumaru et al. / Applied Surface Science 235 (2004) 460–464
regards transition-metal nitrides, however, there have been few studies on such controlled layer-by-layer growth. Timm et al. [25,26] reported the RHEED oscillation observations for TiN growth on a Si(0 0 1) substrate, but a considerable misfit between TiN and the substrate caused three-dimensional growth after deposition of a thickness of several two-dimensional layers, i.e. Stranski–Krastanov growth. Later, we successfully carried out layer-by-layer growth of TiN on MgO with monitoring by RHEED oscillation during deposition by a PLD method. This was reported as a brief communication [17]. In this study, TiN–CrN multilayered structures were grown epitaxially on MgO(0 0 1) by using a PLD method. We succeeded in monitoring the layer-bylayer growth of transition-metal nitride multilayers by using RHEED intensity oscillation. Electrical conductivity and magnetic properties of the nitride multilayers were measured.
2. Experimental The PLD system used was equipped with a RHEED apparatus, a KrF excimer laser (COMPex 102, Lamda Physics, Goettingen, Germany) and an RF-plasma radical source (Model RF 4.5, SVTA Inc., MN, USA). The residual pressure of the chamber was less than 108 Torr. MgO(0 0 1) substrates (Advanced Film Technology Inc., Tokyo, Japan) were cleaned ultrasonically in methanol and then annealed under O2 flow. Since flatness of the substrate is essential for growth in a layer-by-layer mode, we used AFM to examine the surfaces of the substrates annealed at various temperatures. The following procedure was found to be suitable for the annealing: the temperature of the MgO substrates was raised to 973 K over 1 h, then heated up to 1223 K at a rate of 50 K h1, after which this temperature was held for 5 h. AFM measurements revealed that flat terraces as wide as ca. 0.5 mm were formed [17]. The annealed MgO exhibited a RHEED pattern (not shown) with clear spots on the zeroth Laue circle, which shows the presence of highly flat MgO surface. TiN epitaxial thin films and TiN–CrN superlattices were deposited using Ti and Cr metal disks as the targets. A polycrystalline TiN disk was also used for
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comparison with the Ti metal disk. The thin films were grown under nitrogen radical irradiation. During the deposition, the specular spot intensity in the RHEED pattern was recorded after every 20 laser pulses of irradiation. X-ray diffraction reciprocal space mapping was carried out by using an XPert MRD diffractometer (Philips, The Netherlands) equipped with a Ge(2 2 0) 4-crystals monochrometer. Magnetization of thin films was measured with a magnetometer (MPMS5S, Quantam Design, USA). Magnetic field was applied parallel to the thin films and MgO(0 0 1). Electronic conductivity of the samples was measured by using a 4-probe method in the temperature range of 2–100 K, where the current flowed parallel to the multilayers.
3. Results and discussion 3.1. Growth of TiN layers on MgO monitored by RHEED oscillation First of all, we tried to deposit TiN in a layer-bylayer mode. Fig. 1 shows an example of the RHEED oscillation obtained, which we reported in a previous brief communication [17]. Each cycle of the oscillation corresponds to the deposition of a TiN monolayer with a thickness of 0.212 nm. The deposition conditions were as follows: substrate temperature, 973 K; laser fluence, ca. 13 J cm2; RF power of the radical source, 350 W; N2 gas feeding rate, 1 cm3 min1 (STD); pressure during deposition, 2 105 Torr. The oscillation was found to be very sensitive to
Fig. 1. RHEED oscillation of TiN growth on MgO(0 0 1) detected at the specular spot in the [1 1 0] azimuth.
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the nitrogen feeding rate (i.e. the flux of nitrogen radical) and it was suppressed by increasing the N2 gas feed to 3 cm3 min1 (STD). Although the oscillation dumped after ca. 14 cycles, the Laue circle was still observed after deposition of a thickness of 30 nm, demonstrating that the surface remained highly flat. AFM observation of the TiN film revealed that the terraces were highly flat, with a small roughness of less than 0.4 nm. It was essential to use a metal Ti target for the observation of RHEED oscillation: When we used TiN as the target, we obtained crystallized TiN films, but were unable to observe RHEED oscillation. The RHEED oscillation was also sensitive to the N2 feeding rate (i.e. the flux of nitrogen radical). These findings can be interpreted in terms of the mobility of Ti species on the top surface during the deposition. On the thin film surface, Ti species from the metal target would have higher mobility than those from the TiN target, because the former species seems likely to be partially nitridized. It is expected that the flux of nitrogen radicals also influences the mobility of the surface species. Similar phenomena were reported for a homo-epitaxy of SrTiO3 by MBE, in which the conditions suitable for RHEED oscillations were determined by a balance between substrate temperature and feeding rate of oxygen [33]. A high-resolution X-ray diffraction reciprocal space map for TiN/MgO is shown in Fig. 2. The TiN 1 1 3 reflection was aligned vertically to that of the MgO 1 1 3. The very narrow width in the [1 1 0] direction of the spot of TiN demonstrates the excellent quality of the epitaxial film, with no relaxation and no mosaic disorder in this direction. The calculated position for the bulk TiN 1 1 3 (corresponding to a lattice a ¼ 0:4242 nm) is shown in Fig. 2. This demonstrates that the TiN lattice of the film shrunk along the
Fig. 2. High-resolution X-ray diffraction reciprocal space map of TiN/MgO(0 0 1). The contour lines represent the intensities of 7, 13, 21, 42, 100, 5 103 , and 2 104 cps. The scales are expressed with reference to the MgO 1 1 3 spot.
directions parallel to the surface of the substrate by 0.7%, to fit the MgO(0 0 1) lattice. 3.2. Layer-by-layer growth of TiN–CrN multilayers monitored by RHEED oscillation Fig. 3 shows the RHEED oscillation during the growth of TiN–CrN multilayers. Ti and Cr metal targets were ablated alternately with 900 laser pulses. Clear oscillations were observed during the ablation of both Ti and Cr targets. The frequencies of the oscillations for the two targets differ from each other, but are almost constant for the same target. This is the first example, to our knowledge, of layer-by-layer growth of transition metal nitride superlattices monitored by RHEED oscillation. From the frequencies, the growth rates for TiN and CrN were estimated: the 900 pulses deposit an average of 6.3 monolayers for TiN (one monolayer ¼ 0.212 nm) and 2.8 monolayers for CrN (one monolayer ¼ 0.207 nm). Therefore, the
Fig. 3. RHEED oscillation during growth of TiN–CrN multilayers on MgO(0 0 1).
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Fig. 4. X-ray diffraction of TiN–CrN multilayers.
Fig. 5. Temperature dependence of electric resistivity of CrN–TiN multilayers.
periodicity of the superlattice was calculated to be 1.9 nm. As shown in Fig. 3, the oscillation intensity became gradually weaker, and almost disappeared after 6300 pulses. The deposition of TiN and CrN was repeated 15 times, and at the last stage TiN was deposited (900 pulses) as the top layer. Fig. 4 shows the X-ray diffraction peak of the multilayer. A weak peak was observed at d ¼ 2:1 nm (2y ¼ 4.158), which was consistent with the results obtained by RHEED oscillation. This finding can be interpreted in terms of the formation of the superlattice TiN–CrN multilayer. One reason for the weakness of the X-ray diffraction peak is that the X-ray scattering factors of Ti and Cr are very close. Another possible reason could be that the composition profile of the TiN– CrN boundary was not sharp. In other words, a TiN and CrN mixed phase might be formed at the boundaries. In fact, no satellite peaks due to the superlattice were observed. The RHEED oscillation in Fig. 3 clearly demonstrated that CrN layers were grown on TiN/ MgO in an epitaixial layer-by-layer mode, though the lattice mismatch between CrN and TiN/MgO is not very small (1.6%). There is a possibility that mixing of Cr and Ti at the boundaries compensated the mismatch to some extent and helped the epitaxy. The intense MgO 0 0 2 peak hid the 0 0 2 diffraction due to the NaCl crystal structure of TiN and CrN.
ductor. TiN is a metallic conductor, and semiconductor-like behavior of CrN has been reported [8]. Therefore, the metallic conductivity of the TiN–CrN multilayer would be attributable to the TiN layers, while the contribution of the TiN–CrN mixed region to the electronic conductivity is unknown. The temperature dependence of the magnetization of the TiN–CrN multilayer sample measured under a magnetic field of 10 Oe is shown in Fig. 6. Large magnetization was observed at temperatures below ca. 65 K. The inset of Fig. 6 shows a field-magnetization curve for the same sample at 12 K. Hysteresis was clearly observed, demonstrating that the sample was ferromagnetic. The magnetization shown in Fig. 6 has a maximum at around 30 K with decreasing temperature. The reason for this behavior is not clear at present. In a magnetic field of 100 Oe, the curve of the field cooling mode below 20 K became almost flat (data not shown). The ferromagnetism may be ascribed to the formation of TiN–CrN solid solution
3.3. Electrical conductivity and magnetic properties of TiN–CrN multilayers Fig. 5 shows the temperature dependence of the electrical resistivity of the TiN–CrN multilayered sample. The resistivity decreased as the sample was cooled, showing that the sample is a metallic con-
Fig. 6. Temperature dependence of magnetization of CrN–TiN multilayers. Area and thickness of the sample were 31.5 mm2 and 32 nm, respectively. Magnetic field was 10 Oe. The inset shows field-magnetization hysteresis loop for the same sample at 12 K.
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at the boundaries. In fact, Cr0.25Ti0.75N thin film on MgO prepared by using Cr–Ti alloy as the target is ferromagnetic with a Curie temperature of 60 K [34]. This could also explain the weakness of the X-ray diffraction peak mentioned above.
4. Conclusions We were able to observe RHEED oscillations during the growth of CrN–TiN multilayers on MgO(0 0 1) after optimization of the pre-annealing condition of the MgO substrates. Usage of metal targets was essential for the oscillation. X-ray diffraction confirmed the formation of the multilayer structure, while mixing of TiN and CrN may have occurred at the boundaries. The multilayers showed metallic electroconductivity and ferromagnetic behavior with Tc ¼ ca. 70 K, where the ferromagnetism may be ascribed to the TiN–CrN mixed phase.
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