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Current Applied Physics (2006) 545–548 www.elsevier.com/locate/cap www.kps.or.kr Magneto-transport properties of amorphous Ge1ÀxMnx thin films Sang Soo Yu a, Tran Thi Lan Anh a, Young Eon Ihm a,*, Dojin Kim a, Hyojin Kim a, Soon Ku Hong a, Sangjun Oh b, Chang Soo Kim c, Hwack Joo Lee c, Byung Chill Woo c a Chungnam National University, Daejeon 305-764, Republic of Korea Korea Basic Science Institute, Daejeon 305-333, Republic of Korea Korea Research Institute of Standards and Science, Daejeon 305-600, Republic of Korea b c Available online 18 January 2006 Abstract Amorphous Ge1ÀxMnx thin films were grown in order to expand the solubility limit of Mn The amorphous Ge1ÀxMnx thin films were grown on (1 0)Si substrate at 373 K by using a thermal evaporator The solubility of Mn in amorphous Ge1ÀxMnx thin films reaches up to 17 at.% The amorphous Ge1ÀxMnx thin films are ferromagnetic and the TC is $150 K The largest saturation magnetization of amorphous Ge1ÀxMnx thin films is $100 emu/cm3 for x = 0.118 at K The variation of electrical resistivity with respect to temperature reveals that the amorphous Ge1ÀxMnx thin films have semiconductor characteristics The in-field electrical resistivity of amorphous Ge1ÀxMnx thin films is lower than the zero-field electrical resistivity when T < TC, but the reverse is true when T > TC However, the in-field electrical resistivity of amorphous Ge1ÀxMnx thin films is always higher than the zero-field electrical resistivity when x > $12 at.% Magneto-transport characteristics of amorphous Ge1ÀxMnx thin films show anomalous Hall phenomenon and negative magnetoresistance when T < TC The results suggest that the Mn atoms in amorphous Ge1ÀxMnx thin films be related to spin dependent scattering depending on magnetization Ó 2005 Elsevier B.V All rights reserved PACS: 75.50.Pp; 75.60.Ej Keywords: Magnetic semiconductor; Spintronics materials; GeMn semiconductors; Amorphous semiconductor Introduction Recently magnetic element doped semiconductors, such as Mn-doped III–V, II–IV [1,2] and group IV semiconductors [3–12], have been studied extensively due to the application potentials in the spin-polarized injection Among them Si- or Ge-based magnetic semiconductors are very attractive since the well-established current semiconductor technologies can be applied to them directly But these materials have low solubility limit of Mn and thus low Curie temperature Recently, theoretical and experimental works indicate that the Curie temperature of Si- or Gebased magnetic semiconductor increases with Mn concen- * Corresponding author Tel.: +82 42 821 6635; fax: +82 42 822 3206 E-mail address: yeihm@cnu.ac.kr (Y.E Ihm) 1567-1739/$ - see front matter Ó 2005 Elsevier B.V All rights reserved doi:10.1016/j.cap.2005.11.057 tration [3,5] In the case of Ge1ÀxMnx, when Mn exceeds solubility limit of crystalline phase Ge, secondary phases, such as Ge3Mn5 and Ge8Mn11 [7,9,13,14], are precipitated and they cause fatal disadvantages on electronic properties However, amorphous Ge1ÀxMnx phase can contain Mn beyond the solubility limit of crystalline Ge1ÀxMnx without secondary phases In this work, amorphous Ge1ÀxMnx thin films were grown in order to increase Curie temperature, and the magneto-transport properties of the a-Ge1ÀxMnx thin films were studied Experimental Amorphous Ge1ÀxMnx (a-Ge1ÀxMnx) thin films were grown on thermally oxidized (1 0)Si wafer by using an evaporator Mn and Ge were co-evaporated at 373 K in the vacuum of 10À6 Torr Film thickness was measured 546 S.S Yu et al / Current Applied Physics (2006) 545–548 10 10 Resistivity(ohm-cm) by using alpha step, and was $500 nm The composition of a-Ge1ÀxMnx thin films was determined by using an energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy The microstructure of a-Ge1ÀxMnx thin films was analyzed by using conventional X-ray diffractometer (XRD) and transmission electron microscopy (TEM) Magnetization and magneto-transport were analyzed by using a magnetic properties measurement system, a physical property measurement system and a superconducting quantum interference device in the temperature range of 2–300 K with magnetic field sweep between À9 T and T Hall effect was measured at room temperature using 0.5 T electromagnet For Hall and resistivity measurements, Ohmic contacts using high purity In were made on the a-Ge1ÀxMnx thin films 10 -1 10 -2 10 -3 10 -4 10 10 20 30 40 Mn at% 10 Mobility(cm2/V.sec) p Resistivity(Ohm-cm) XRD patterns given in Fig show Ge1ÀxMnx thin films are amorphous when x < 0.173, and diffraction peaks of Mn appear when x > 0.310 Mn precipitates were examined closely by taking cross-sectional TEM images and diffraction patterns The electrical resistivities of Ge1ÀxMnx thin films were measured at room temperature and are plotted in Fig The electrical resistivity of Ge1ÀxMnx thin films decreases with increasing Mn concentration and is · 10À4– 102 X cm In order to analyze the variation of the electrical resistivity Hall measurement has been done at room temperature The Ge1ÀxMnx thin films are turned out to have p-type carriers, and carrier concentration is · 1017–2 · 1022 cmÀ3 As seen in Fig 3, Hall analysis reveals that, as increasing Mn concentration, the hole concentration increases predominantly over the hole mobility which decreases, so -1 10 -2 10 µ 10 10 23 10 22 10 21 10 20 10 19 10 18 10 17 -3 10 ρ Number of carrier(cm-3) Fig Sheet resistivities of Ge1ÀxMnx semiconductor thin films measured at room temperature using four-point probe Results and discussion 0.1 -4 10 12 16 20 24 Mn at% Fig Hall data of Ge1ÀxMnx thin films measured at room temperature Si(002) Mn(330) Mn(332) x=51.2at% x=31.0at% x=23.0at% x=17.3at% x=11.8at% x=8.0at% x=5.8at% x=3.6at% 25 30 35 40 45 50 2θ(degree) Fig X-ray diffraction patterns of Ge1ÀxMnx films 55 that electrical resistivity decreases with increasing Mn concentration Fig is the magnetizations of Ge1ÀxMnx thin films measured using a SQUID Ge1ÀxMnx thin films are ferromagnetic at low temperature and gradually change into paramagnetic as temperature increases The Curie temperature of Ge1ÀxMnx thin films varies with Mn concentration and is approximately 150 K The saturation magnetization also varies with Mn concentration The largest saturation magnetization is $100 emu/cm3 at K for x = 0.118 Fig is the magnetic field effect of electrical resistivities of a-Ge1ÀxMnx thin films as a function of temperature Fig reveals typical semiconductor characteristics of electrical resistivity The in-field electrical resistivity of aGe1ÀxMnx thin films is lower than the zero-field electrical resistivity when T < TC, but the reverse is true when T > TC However, the electrical resistivity of a-Ge1ÀxMnx thin films is higher than the zero-field electrical resistivity in all range of temperature when Mn concentration is lar- 547 S.S Yu et al / Current Applied Physics (2006) 545–548 0.6 3.6at% 5.8at% 8.0at% 11.8at% 17.3t% 31.0at% 80 60 Hall resistivity(ohm-cm) Magnetization(emu/cc) 100 40 0.4 0.2 0.0 2K -0.2 110K -0.4 150K 180K 20 -0.6 -90,000 -60,000 -30,000 0 50 100 150 200 250 Fig Low temperature magnetizations of Ge1ÀxMnx thin films Applied magnetic field was T 30,000 60,000 90,000 Magnetic field(Oe) 300 Temperature(K) Fig Hall resistivity of a-Ge1ÀxMnx (x = 0.08) thin films as a function of temperature In order to investigate the details of magneto-transport characteristics the temperature dependence of anomalous Hall effect and magnetoresistance of a-Ge1ÀxMnx thin films have been measured Fig shows the Hall resistivity of a-Ge1ÀxMnx thin films in the magnetic field of ±9 T at 4.0x10 -4 3.5x10 -4 3.0x10 -4 0.0 270K 2.5x10 -4 -0.2 180K 2.0x10 -4 zero-field 6T MR ratio(%) Resistivity(Ohm-cm) 20K (a) 50 100 150 200 250 300 150K -0.4 70K -0.6 Temperature(K) 20K -4 5.0x10 -0.8 -4 2K -90,000 -60,000 -30,000 4.5x10 30,000 60,000 90,000 150K 6T -4 4.0x10 Magnetic field(Oe) zero-field 0.0 110K -4 3.5x10 -0.5 70K -4 MR ratio(%) Resistivity(Ohm-cm) (a) 3.0x10 -4 (b) 2.5x10 50 100 150 200 250 300 -1.0 20K -1.5 -2.0 2K Temperature(K) Fig Electrical resistivities of a-Ge1ÀxMnx thin films as a function of temperature: (a) x = 0.058 and (b) x = 0.147 -2.5 (b) -90,000 -60,000 -30,000 ger than $12 at.% The critical behavior of electrical resistivity near TC is known due to the scattering of carriers by magnetic spin fluctuation via exchange interaction [15,16] 30,000 60,000 90,000 Magnetic field(Oe) Fig Magnetoresistance of a-Ge1ÀxMnx thin films as a function of temperature: (a) x = 0.027 and (b) x = 0.047 548 S.S Yu et al / Current Applied Physics (2006) 545–548 various temperatures The Hall resistivity shows a hysteresis loop at low temperature As shown in Figs and 7, the amplitudes of the hysteresis of the Hall resistivities and the magnetoresistance ratio are seen to increase continually with decreasing temperature below TC The anomalous Hall resistivity is believed to originate from magnetic behaviour of a-Ge1ÀxMnx thin films Moreover, the negative magnetoresistance of $2.5% is shown in the temperature regime of K $ TC where the anomalous Hall effect also occurs However, when T > TC, negative magnetoresistance disappears because of the absence of magnetic moment Thus, the existence of the anomalous Hall effect and negative magnetoresistance indicates that Mn atoms are incorporated in the Ge host and lead to spin dependent scattering depending on magnetization of a-Ge1ÀxMnx films like general DMS Acknowledgements This work was supported by the Research Center for Advanced Magnetic Materials (RECAMM, Chungnam National University, Korea) and the Brain Korea 21 Program (BK21, the Ministry of Education & Human Resource Development, Korea) References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] H Ohno, Science 281 (1998) 951 T Dietl, H Ohno, Science 287 (2000) 1019 Y.D Park et al., Science 295 (2002) 651 S Cho et al., Phys Rev B 66 (2002) 033303 Y.-J Zhao, T 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function of temperature... Fig is the magnetic field effect of electrical resistivities of a-Ge1ÀxMnx thin films as a function of temperature Fig reveals typical semiconductor characteristics of electrical resistivity The
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