Electrical and magnetic properties of mn doped si thin films

ARTICLE IN PRESS Physica B 404 (2009) 1686–1688 Contents lists available at ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb Electrical and magnetic properties of Mn-doped Si thin films T.T Lan Anh a, S.S Yu a, Y.E Ihm a,Ã, D.J Kim a, H.J Kim a, S.K Hong a, C.S Kim b a b School of Materials Engineering, Chungnam National University, 220 Gungdong, Yuseong-gu, Daejeon 305-764, Republic of Korea Korea Research Institute of Standards and Science, Daejeon 305-600, Republic of Korea a r t i c l e in f o a b s t r a c t Article history: Received 30 September 2008 Received in revised form 29 January 2009 Accepted February 2009 We have studied electrical and magnetic properties of x at% Mn-doped Si thin films with high Mn concentrations (x at% ¼ 7.5, 9.1, and 11.3), which were prepared by molecular beam epitaxy Our data reveals that the films are p-type semiconductors at room temperature, and their hole density is about 1020 cmÀ3 When temperature increases from to 300 K, the resistivity of 7.5 at% Mn film decreases and can be described by Mott’s variable-range-hopping model The resistivity of 9.1 at% Mn film does not change remarkably In contrast, the resistivity of 11.3 at% Mn film increases, indicating metallic characteristics at temperatures below 240 K Magnetic measurements reveal that the films exhibit the low-temperature ferromagnetic ordering, which is largely related to the presence of secondary phase & 2009 Elsevier B.V All rights reserved PACS: 75.50.Pp 75.50.Lk 75.70.Ài Keywords: Si–Mn spintronic materials Electrical and magnetic properties Introduction In recent years, diluted magnetic semiconductors (DMSs) doped with the transitional metals, such as Mn-doped semiconductors III–V [1–3], II–VI [4,5], and group IV [6,7], have been of much interest, because of applicability in spintronics devices Given the important role of Si in electronic devices, the inclusion of Si in DMSs would represent a promising approach Accordingly, many attempts have been made to fabricate Mn-doped Si ferromagnetic semiconductors The first study on Si-based DMSs has been carried out by Nakayama et al [8] It has been believed that Mn in the Si host locates in both interstitial and substitutional sites, in which the interstitial site is preferred This caused an anomalous Hall effect at around 70 K, possibly due to the presence of an internal magnetic field generated from Mn spins in the Si matrix Following this study, some works on dependences of electrical and magnetic properties on the substrate temperature and heat treatment for polycrystalline Si1ÀxMnx films were also performed [9,10] More recently, the ferromagnetic ordering above 400 K in Si1ÀxMnx has been reported [11,12] It has been believed that the nature of ferromagnetism in these thin films is related to hole-mediated magnetic interactions However, by studying ferromagnetic Si0.95Mn0.05 bulks with Curie temperature (TC) E75 K, Kwon et al [13] have shown that the origin of ferromagnetism is due to the interactions taking place between à Corresponding author Tel.: +82 42 821 6635; fax: +82 42 822 3206 E-mail address: yeihm@cnu.ac.kr (Y.E Ihm) 0921-4526/$ - see front matter & 2009 Elsevier B.V All rights reserved doi:10.1016/j.physb.2009.02.001 the Mn ions in the Si host lattice Thus, the nature of ferromagnetism in this material family is still an issue of debate In this work, we have studied electrical and magnetic behaviors of Mn-doped Si thin films (with high Mn concentrations) grown by molecular beam epitaxy (MBE) The combination of the structural analysis by a means of X-ray diffraction allows us to understand the magnetic nature of the samples Experiment Thin films of x at% Mn-doped Si were grown on Si(0 1) substrates in MBE chamber equipped with solid sources of Si and Mn Firstly, a native Si oxidation layer on the substrates was removed by heating them up to 1100 1C for 30 After that, the system was cooled slowly down to 200 1C at a maximum rate of 1C/s This temperature was maintained for 60 to deposit the films To make the films with different Mn concentrations, the Mn flux was controlled by changing Mn-effusion-cell temperature Finally, the fabricated films were measured the thickness by an Alpha-Step (500 Profiler) The Mn concentrations were measured by X-ray photoelectron spectroscopy (XPS), using a VG HB-100 multilab with a monochromatic Al Ka X-ray source (hn ¼ 1486.6 eV) The structure of the films was characterized by an X-ray diffractometer (XRD) Electrical properties and Hall effect were determined by means of a quantum design physical properties measurement system (PPMS-6000), where four probes were arranged according to the van-der Pauw configuration Magnetic ARTICLE IN PRESS 1687 T.T Lan Anh et al / Physica B 404 (2009) 1686–1688 Results and discussion Si (004) The thickness of three samples of Si:Mn thin films with different Mn concentrations were measured using an Alpha-Step After deposited for 60 min, the films have a thickness of about 100 nm (i.e., the growth rate is $16 A˚/min) By a means of XPS, we obtained the Mn concentration in the samples to be 7.5, 9.1, and 11.3 at% and designated as the samples A, B, and C, respectively Fig shows XRD patterns of all the samples The samples A and B may be regarded as single phase as they exhibited XRD peaks (0 2) at 33.07o and (0 4) at 69.21o generated from the Si structure Moreover, in comparison with the standard spectra of silicon, Si(0 2) at 33.20o and Si(0 4) at 69.70o [JCPDS file No 80-0018], the positions of different peaks for the Mn-doped Si thin films in this work shift toward the lower angle, indicating the lattice expansion resulted from the incorporation of Mn in Si lattice However, the Mn concentration increase in the sample C led to the appearance of a new XRD peak (besides the peaks from Si) centered at about 44o, which was identified to be an index (2 0) of SiMn compound At room temperature, we measured the hole density of the films using on a Hall device Our results showed that the samples A, B, and C were p-type semiconductors, and their hole densities were 1.3  1020, 3.4  1020, and 5.87  1020 cmÀ3, respectively We found that the Mn-concentration increase in Mn-doped Si film resulted in the hole-density increase Similarly, Park et al [6] also found the hole-density increases progressively with enhancement in Mn concentration, in Mn-doped Ge materials Our data also demonstrated that the resistivity of the films decreases with increasing Mn concentrations, consistent with earlier reports [7,10] To further understand the dynamics of carriers in the films, we investigated the temperature dependence of resistivity, r(T), in the range of 5–300 K As shown in Fig the resistivity characteristics of the samples were significantly different With increasing Mn concentrations in Mn-doped Si films, resistivity of the films decreased, in good agreement with our hole density data At temperatures T4$200 K, as temperature increased, r(T) of the films decreased gradually, exhibiting conventional semiconductor characteristics However, the variation of r(T) becomes more complicated when temperature was below 200 K For the first film (sample A), r(T) increases which can be described by Mott’s variable range hopping model of r(T) ¼ r0 exp(T0/T)1/4, where T0 is a characteristic temperature and r0 is a constant [14] We used this model to fit two temperatures of 5–25 K and 25–250 K, (Fig 1(a) inset) For the first region, we obtained T0 ¼ 1.6 K and r0 ¼ 2.12 O cm, while in the second region T0 and r0 were 10.3 K and 2.15 O cm, respectively One can see that obtained r0 values in two regions are close to each other while their T0 values are quite different This could be due to the difference in carriers’ hopping mechanism in two regions For the sample B, its resistivity at 200 K is 2.7  10À4 O cm, and with the temperature decreases r(T) changes just about 1.0% This feature is very different compared to that of the sample A (and C as well) Clearly, the Mn doping into the Si host at an appropriate value ($9.1 at%) makes resistivity of the film quite stable versus temperature in the region To200 K The situation becomes more complicated as paying attention to r(T) of the sample C, where the Mn concentration is 11.3 at% With decreasing temperature from 240 K, r(T) decreases This reveals that the film C exhibits metallic behavior Based on the feature of r(T) variation, we could divide the r(T) curve into two parts The first one (30 KoTo240 K) corresponds to the r(T) value which decreases gradually, and the other (To30 K) corresponds to the r(T) value which decreases rapidly as lowering temperature (Fig 2(b)) At temperatures Mn-doped Si films 0.50 Sample A Sample B Sample C 0.45 Sample A SiMn (210) Sample B -7.6 ~25 K -7.8 -8.2 0.35 0.3 0.30 0.25 TC Sample C 0.21 40 50 60 70 80 Sample C 0.19 10 15 20 25 30 T (K) 30 0.4 0.5 0.6 T-1/4 (K-1/4) 0.20 0.20 20 Experiment - 25 K 25 - 250 K -8.0 ρ (mΩ.cm) Resistivity (mΩ.cm) Si (002) 0.40 Ln (ρ) (mΩ.cm) measurements were carried out at temperatures between and 300 K using a superconducting quantum interference device (SQUID) magnetometer, in which the external magnetic field was applied parallel to the film plane 40 80 120 160 200 240 280 T (K) 2θ (Degree) Fig XRD patterns of the samples A, B, and C corresponding to the Mn concentration of 7.5, 9.1, and 11.3 at%, respectively Fig Temperature dependences of electrical resistivity, r(T) for the samples (a) A, (b) B, and C; the insets show the r(T) data fitted to resistivity models as described in the text ARTICLE IN PRESS 1688 T.T Lan Anh et al / Physica B 404 (2009) 1686–1688 H = 795.8 kA/m Sample A Sample C M (kA/m) 50 M (kA/m) 40 30 40 T=5K 20 -20 -40 -4 20 -2 H (kOe) properties but also in their resistivity characteristics For the film C, with lowering temperature a strong decrease in r(T) starts taking place at TC, as can be seen in Fig 2(b) At temperatures below 30 K, the film exhibits ferromagnetic metal characteristics, which could be fitted well to a power law T2.1 as mentioned above For the film A, the amount of secondary phase is probably low, and thus a uniform phase of Mn-doped Si could be responsible for its electrical and magnetic behaviors However, an open question of where the secondary phase existing in this film is still an issue for our consideration Conclusions 10 0 50 100 150 T (K) 200 250 300 Fig The temperature dependences of magnetization for the samples A and C under an applied field of 795.8 kA/m; the inset shows the hysteresis loops of these two samples measured at K below 30 K, we found that r(T) could be fitted well by a power law Ta with a ¼ 2.1 (Fig 2(b) inset) The a value is close to 2.0, which is commonly observed in ferromagnetic metals [15] As seen later, this film has the ferromagnetic ordering at temperatures below 30 K In the range 30 KoTo240 K, the temperature increase from 30 K leads to the increase in resistivity, suggesting that the film C still exhibits metal behavior Notably, r(T) of the film C is always smaller than that of A and B (Fig 2) Fig shows the temperature dependences of magnetization for the films A and C under an external magnetic field of 795.8 kA/m, in which the field applied is parallel to the thin film plane One can see that with decreasing temperature from 300 K, magnetization of the films increases, particularly at temperatures below 200 K We measured hysteresis loops of these films at K (see the inset of Fig 3) and the values of saturation magnetization (MS) of A and C are 53 and 42 kA/m, respectively, while their coercive force (HC) is about 19.9 kA/m These results reflect that the films exhibit the ferromagnetic ordering at low temperatures Based on the M–T data, the TC, which are estimated from a Curie–Weiss fit to the data as the magnetization approaches zero, of the films A and C obtained are about 50 and 30 K, respectively Clearly, the increase in Mn-doping concentration in MnxSi1Àx films results in reduction of MS and TC values This is probably related to the limitation of Mn solubility in the Si semiconductor matrix At high Mn concentrations, the short-range antiferromagnetic interaction between Mn ions could occur, resulting in the competition for a long-range ferromagnetic interaction This situation leads to the reduction of magnetization and TC [3,16] It is also suggested that at high Mn concentrations Mn–Si related secondary phases could be formed, which dominate magnetic feature of the thin films For Si–Mn binary systems, the TC value of SiMn is about 30 K [17] while that of Si7Mn4 is about 45 K [18] Among them, TC of SiMn is close to that of the film C (with TCE31 K) This evidences SiMn existed in our sample C, in good agreement with XRD data shown in Fig Accordingly, the increasing Mn concentration in MnxSi1Àx films results in not only changes in their magnetic The electrical and magnetic properties of the thin films A, B, and C have been investigated At room temperature, all the films are p-type semiconductors with the hole density is of about 1020 cmÀ3 As increasing temperature, the characteristics of resistivity of the film depend strongly on Mn concentrations For the film A, resistivity decreases gradually that could be described by Mott’s variable-range-hopping model In contrast, resistivity of the film C increases, revealing metallic characteristics at temperatures below 240 K Particularly, for the film B, its resistivity is quite stable at temperatures below 200 K Magnetic measurements reveal that the films have the ferromagnetic feature at low temperatures Acknowledgments This work was supported by the Brain Korea 21 Program (BK21, the Ministry of Education & Human Resource Development, Korea) References [1] H Munekata, H Ohno, S von Molnar, A Segmuller, L.L Chang, L Esaki, Phys Rev Lett 63 (1989) 1849 [2] H Ohno, J Magn Magn Mater 200 (1999) 110 [3] T Dietl, H Ohno, F Matsukura, Phys Rev B 63 (2001) 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Physica B 404 (2009) 1686–1688 Results and discussion Si (004) The thickness of three samples of Si :Mn thin films with different Mn concentrations were measured using an Alpha-Step After deposited