Properties of La0.7Sr0.3MnO3 thin films grown on gallium nitrides

PERGAMON

Solid State Communications 121 (2002) 631±634

www.elsevier.com/locate/ssc

Properties of La0.7Sr0.3MnO3 thin ®lms grown on gallium nitrides D.-W. Kim a, D.H. Kim a, T.W. Noh a,*, E. Oh b, H.C. Kim c, H.-C. Lee c a

School of Physics and Research Center for Oxide Electronics, Seoul National University, Seoul 151-747, South Korea b Brain Korea 21 Physics Research Division, Seoul National University, Seoul, South Korea c Material Science Laboratory, Korea Basic Science Institute, Taejon 305-333, South Korea Received 30 October 2001; accepted 10 December 2001 by C.N.R. Rao

Abstract Polycrystalline La0.7Sr0.3MnO3 (LSMO) thin ®lms were grown on GaN layers by pulsed laser deposition. High quality LSMO Ê thick LSMO/GaN ®lm ®lms could be prepared without the aid of buffer layers. Ferromagnetic transition temperature of a 500 A was as high as 330 K and coercive ®eld was less than 20 Oe at room temperature. Current±voltage measurements of LSMO/ GaN contacts showed a clear rectifying behavior, which might be caused by Schottky barrier formation at the interfaces. The LSMO/GaN hybrid can be a candidate for spin-injectors, which will be useful for room temperature and low ®eld applications. q 2002 Published by Elsevier Science Ltd. PACS: 73.40.Cg; 75.70.Pa; 81.15.Fg; 85.80.Jm Keywords: A. Magnetic ®lms and multilayers; A. Semiconductors; D. Electronic transport

Spin-polarized carrier injection into semiconductors has attracted much attention during the last few years for possible device applications such as magnetic storage in semiconductors and spin transistors [1]. Electrical spin injection into a non-magnetic semiconductor has been successfully demonstrated by making use of a ferromagnetic metal [2]. However, until now the spin polarization in such a hybrid remains within a few percent. Schmidt et al. attributed the limited ef®ciency to the large conductivity mismatch between the ferromagnetic metals and semiconductors [3]. Colossal magnetoresistance (CMR) materials can be alternative candidates for ef®cient spininjectors with their resistivities being comparable to those of semiconductors [4,5] and they are known to have nearly 100% spin polarizations [6]. However, there was no report related to the electrical properties of the CMR/semiconductor interface, as far as we know [7±9]. In the case of Si substrates, CMR thin ®lms have been grown on insulating buffer layers to suppress serious chemical reaction at the interfaces [7±10] and hence transport across the barrier is expected to be dif®cult. To fabricate CMR/semiconductor hybrids, La0.7Sr0.3MnO3 * Corresponding author. Tel.: 182-2-880-6616; fax: 182-2-8751222. E-mail address: [email protected] (T.W. Noh).

(LSMO) ®lms were prepared on GaN layers. Among the CMR materials, LSMO has relatively high ferromagnetic transition temperature (TC) of 370 K [5]. GaN has superior thermal stability compared to those of other semiconductors [11] and ferromagnetic ®lms grown on GaN-based two dimensional electron gas (2DEG) structures could be useful for future spin transistors [12]. Based on such expectations, we investigated the physical properties of LSMO/GaN heterostructures. GaN layers were grown on sapphire (Al2O3) substrates by metal±organic chemical vapor phase epitaxy. LSMO ®lms were grown on the GaN layers by the pulsed laser deposition technique using a KrF excimer laser …l ˆ 248 nm†: The substrate temperature and the oxygen pressure were 750 8C and 10 mTorr, respectively. After deposition, the ®lms were annealed in situ at 600 8C in 400 Torr of oxygen Ê ambient. X-ray diffraction (XRD) measurements of a 500 A thick LSMO ®lm grown on a GaN layer showed that the ®lm is polycrystalline. GaN has the wuÈrtzite structure with Ê ) and hexagonal symmetry (a ˆ 3:186 and c ˆ 5.178 A LSMO has the perovskite-type structure with cubic symÊ ). Under the same conditions, LSMO metry (a ˆ 3.87 A ®lm was also grown on a Al2O3 substrate, which has the corundum type structure with hexagonal symmetry Ê and c ˆ 12.99 A Ê ). The LSMO/Al2O3 ®lm was (a ˆ 4.76 A also polycrystalline and this implied that the crystalline

0038-1098/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0038-109 8(02)00033-9

632

D.-W. Kim et al. / Solid State Communications 121 (2002) 631±634

Fig. 1. Atomic force microscope images of (a) a GaN layer and (b) a LSMO/GaN ®lm. Rms roughnesses of a GaN layer and a LSMO/GaN ®lm Ê , respectively. were estimated to be 1.54 and 2.42 A

quality of the LSMO ®lms might be determined from the difference of the crystal symmetry. 1 Atomic force microscopy measurements showed that the ®lm surfaces were very ¯at as shown in Fig. 1. Root-meanÊ thick square (rms) roughnesses of a GaN layer and a 500 A Ê, LSMO/GaN ®lm were estimated to be 1.54 and 2.42 A respectively. Peak-to-valley height deviations are nearly Ê. comparable to the unit cell height of LSMO, i.e. 3.87 A In spite of polycrystalline nature of the LSMO layer, the surface was extremely ¯at. Fig. 2 shows temperature dependence of magnetization of the LSMO/GaN ®lm. The temperature dependence of dc magnetization was obtained using a superconducting quantum interference device (SQUID) magnetometer. TC was estimated to be about 330 K, which is somewhat lower than that of a single crystal (370 K) [5], but still higher than the room temperature. Saturation magnetization value is about 600 emu/cm 3, which is comparable to that of an epitaxial ®lm grown on a SrTiO3 substrate [13]. Fig. 3 shows the magnetic hysteresis loop of LSMO/GaN ®lm measured at room temperature. In this measurement, the magnetic ®eld was applied parallel to the substrate 1 Variations of thickness and growth temperature of the LSMO ®lms affect their crystalline quality. Growth with preferred orientation was observed for thicker LSMO/GaN ®lms and LSMO/Al2O3 ®lms grown above 800 8C.

surface. The coercive ®eld is less than 20 Oe, which is comparable to those of epitaxial ®lms [13]. As shown in the inset, the magnetization was nearly saturated at about 1 kOe. These results indicate that the LSMO/GaN ®lms can be used for low-®eld applications. Ê The resistivity as a function of temperature for the 500 A thick LSMO/GaN ®lm is shown in Fig. 4 by a solid curve. Transport measurements were performed using the conventional four-probe method. The resistivity is nearly two orders of magnitude larger than that of a single crystal [5]. A metal±insulator transition like feature is observed and the peak temperature (TP) is around 250 K, which is signi®cantly lower than TC ˆ 330 K: The large difference between TP and TC was previously reported for textured LSMO ®lms grown on Si substrates and it was explained by grain boundary effects [8,9]. In Fig. 4 the resistivity under the magnetic ®eld of 1 T applied in the plane of the substrate (dashed curve) and the corresponding MR ratio (solid circles) are shown. The MR ratio increases with decreasing temperature and reach up to 38% at 10 K. These results are consistent with the previous reports on polycrystalline CMR ®lms [8,9]. Since hopping probability through grain boundaries can be changed depending on temperatures, the transport properties of polycrystalline samples might show different temperature dependence to that of a single crystal [14]. The temperature dependence of the MR ratio con®rmed that the difference

D.-W. Kim et al. / Solid State Communications 121 (2002) 631±634

Fig. 2. Temperature of dependence magnetization of a LSMO/GaN ®lm. The magnetization was taken on warming in a ®eld of 5000 Oe after cooling in a 5000 Oe ®eld.

633

Fig. 4. Temperature dependent resitivities with and without magnetic ®eld of 1 T. Solid and dotted lines indicate the resistivities under 0 and 1 T, respectively. Magnetoresistance ratio was calculated as uR0T 2 R1T u=R0T £ 100%:

in TP and TC of the LSMO/GaN ®lm originates from the grain boundary effects. Fig. 5 shows a current±voltage (I±V) curve obtained for a LSMO/GaN:Si interface and schematic diagram of the geometry for a three-probe method. We grew patterned LSMO ®lms with a mask on Si-doped n-GaN layers, whose electron concentration, mobility, and resistivity being 1.3 £ 10 18 cm 23, 303 cm 2/V s, and 0.016 V cm, respectively. Au electrodes were evaporated on the LSMO and GaN layers by using a shadow mask with 0.2 mm diameter holes. The turn-on voltage is about 0.7 V and the rectifying behaviors were observed. This might be due to the Schottky barrier formation at the LSMO/GaN interface.

Even though the LSMO/GaN contacts were non-Ohmic, carriers could be transferred from the ferromagnetic LSMO ®lms to the semiconducting GaN layers. As shown in Fig. 4, resistivity of the LSMO layer seemed to be higher than that of the GaN layer but it was determined by intergrain transport hindered by grain boundaries. The I±V characteristics could be explained based on intragrain transport properties of the LSMO layers rather than the intergrain transport. However, the intragrain properties could not be obtained for our polycrystalline samples. Therefore quantitative understanding of the obtained I±V curve seems to be dif®cult at present stage.

Fig. 3. Magnetic hysteresis of a LSMO/GaN ®lm at room temperature. Magnetic ®eld was applied parallel to the surface.

Fig. 5. A current±voltage curve of a LSMO/GaN:Si interface and schematic diagram of the geometry for the three-probe method.

634

D.-W. Kim et al. / Solid State Communications 121 (2002) 631±634

In summary, LSMO ®lms were grown directly on GaN/ Al2O3 substrates by the pulsed laser deposition. Due to the superior stability of GaN, high quality LSMO ®lms could be obtained without buffer layers. Current±voltage measurements for the LSMO/GaN interfaces might imply that the LSMO ®lms could be useful for spin-injectors into the semiconducting GaN layers. Acknowledgements This work was ®nancially supported by MOST through the Creative Research Initiative Program and National Program for Tera-level Nanodevices as one of the 21st century Frontier Programs. Works at Korea Basic Science Institute were supported by National Research Laboratory project of MOST. References [1] S. Datta, B. Das, Appl. Phys. Lett. 665 (1990) 56. [2] P.R. Hammar, B.R. Bennett, M.J. Yang, M. Johnson, Phys. Rev. Lett. 203 (1999) 83. [3] G. Schmidt, D. Ferrand, L.W. Molenkamp, A.T. Filip, B.J. can Wees, Phys. Rev. B R4790 (2000) 62.

[4] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnach, R. Ramesh, L.H. Chen, Science 413 (1994) 264. [5] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 14 (103) (1995) 51. [6] J.-H. Park, E. Vescovo, H.-J. Kim, C. Kwon, R. Ramesh, T. Venkatesan, Nature 794 (1998) 392. [7] Z. Trajanovic, C. Kwon, M.C. Robson, K.-C. Kim, M. Rajeswari, R. Ramesh, T. Venkatesan, S.E. Lo¯and, S.M. Bhagat, D.K. Fork, Appl. Phys. Lett. 1005 (1996) 69. [8] J.Y. Gu, C. Kwon, M.C. Robson, Z. Trajanovic, K. Ghosh, R.P. Sharma, R. Shreekala, M. Rajeswari, T. Venkatesan, R. Ramesh, T.W. Noh, Appl. Phys. Lett. 1763 (1997) 70. [9] R. Shreekala, M. Rajeswari, K. Ghosh, A. Goyal, J.Y. Gu, C. Kwon, Z. Trajanovic, T. Boettcher, R.L. Greene, R. Ramesh, T. Venkatesan, Appl. Phys. Lett. 282 (1997) 71. [10] D.B. Fenner, A.M. Viano, D.K. Fork, G.A.N. Connell, J.B. Boyce, F.A. Ponce, J.C. Tramontana, J. Appl. Phys. 2176 (1991) 69. [11] S.D. Wolter, B.P. Luther, D.L. Waltemyer, C. Oenneby, S.E. Mohney, R.J. Molnar, Appl. Phys. Lett. 2156 (1997) 70. [12] M.A. Khan, X. Hu, A. Tarakji, G. Simin, J. Yang, Appl. Phys. Lett. 1339 (2000) 77. [13] F. Tsui, M.C. Smoak, T.K. Nath, C.B. Eom, Appl. Phys. Lett. 2421 (2000) 76. [14] S. Lee, H.Y. Hwang, B.I. Shraiman, W.D. Ratcliff II, S-W. Cheong, Phys. Rev. Lett. 4508 (1999) 82.