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A comparative study of Co thin film deposited on GaAs (1 0 0) and glass substrates
Materials Science and Engineering B 130 (2006) 120–125
A comparative study of Co thin film deposited on GaAs (1 0 0) and glass substrates A. Sharma ∗ , R. Brajpuriya, S. Tripathi, D. Jain, R. Dubey, T. Shripathi, S.M. Chaudhari University Grant Commission, Department of Atomic Energy Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India Received 6 October 2005; received in revised form 16 February 2006; accepted 25 February 2006
Abstract The structural, magnetic and transport properties of Co/GaAs (1 0 0) and Co/glass thin films have been investigated. The structural measurements reveal the crystalline nature of Co thin film grown on GaAs, while microcrystalline nature in case of glass substrate. The film grown on GaAs shows higher coercivity (49.0 G), lower saturation magnetization (3.65 × 10−4 ) and resistivity (8 cm) values as compared to that on glass substrate (22 G, 4.77 × 10−4 and 18 cm). The grazing incidence X-ray reflectivity and photoemission spectroscopy results show the interaction between Co and GaAs at the interface, while the Co layer grown on glass remains unaffected. These observed results are discussed and interpreted in terms of different growth morphologies and structures of as grown Co thin film on both substrates. © 2006 Elsevier B.V. All rights reserved. Keywords: Co thin film; XRD; VSM; GIXRR; PES; AFM
1. Introduction The Co and Co-based thin film structures have attracted interest of several researchers in recent years due to their possible applications in many areas of technology, such as magnetic data storage, spin valve and microelectronic devices [1–3]. Recently, the area of growth of magnetic thin films on compound semiconductor substrates is being developed as being interesting from both the scientific and technological points of view in connection with spintronics [4–6]. In such structures, the combination of magnetism of ferromagnetic layer along with the electronic properties of underlying semiconductor substrate offers tremendous opportunity for designing futuristic innovative magnetotransport devices. In many giant magneto resistive (GMR) devices, Co plays an important role due to high spin polarization of carriers at Fermi level [7]. Therefore, an understanding of growth and micro-structural evolution of Co thin layers, their corresponding crystal structures and associated magnetic and transport properties are of great technological interest.
∗
Corresponding author. Tel.: +91 731 2463913; fax: +91 731 2462294. E-mail address: [email protected] (A. Sharma). Deceased.
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.02.065
There exist a number of studies in the literature regarding Co/GaAs system as it shows various growth morphologies under different conditions. Prinz have observed that Co film on GaAs substrate grows in non-equilibrium body centered cubic (bcc) structure, which is not found in bulk structure of this material [8]. Similarly, a recent study by Izquierdo et al. ˚ exists in shows that the film having thickness above 350 A, the hexagonal closed packed (hcp) structure [9]. In yet another study on Co/GaAs interface using conversion electron extended X-ray absorption spectroscopy technique, Idzerda et al. have found that Co film grows on GaAs surface with a metastable BCC phase [10]. Recently, Ludge et al. have studied the metallic nanostructures of Co on GaAs (4 × 2) surface using scanning tunneling microscopy (STM), surface X-ray photoelectron spectroscopy (SXPS), low energy electron diffraction (LEED) and reflection anisotropy spectroscopy (RAS) [11]. By SXPS they found that the deposition of 1 ML thick Co film on GaAs (0 0 1) forms CoAs and CoGa compounds, while RAS and LEED techniques revealed that at sub monolayer coverage, Co atoms incorporated on clean (4 × 2) surface as row like structure and authors have concluded that these rows act as onedimensional metallic nano wires. The strong chemical reaction and interface disruption were observed by Zhang et al. when ˚ Co on GaAs (0 0 1) they deposited [even low coverage (∼2 A)]
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[12]. Using photoemission spectroscopy, they found that Ga atoms interdiffuse into Co matrix and get fully diluted. However, ˚ Co is deposited on GaAs. a stable interface is formed when 10 A However, very few reports are available on Co growth on glass substrate. In this regard, an important study was carried out by Kharmouche et al., in which they studied structural and magnetic properties of Co films evaporated on glass substrate as a function of the Co thickness [13]. They observed that films were polycrystalline with (0 0 0 1) texture. It was found that surface and stress induced uniaxial magnetic anisotropy decreases as the film thickness increases. In another study, fcc Co (1 1 1) film grown on glass substrate by Gu and Wang using sputtering technique [14]. Thickness dependent study carried out by Morawe et al. shows that Co film growth on glass in both hcp (0 0 0 1) and fcc (1 1 1) orientations [15]. However, as per the best of our knowledge no report available in the literature correlates structural, magnetic and transport properties of Co thin films on GaAs and glass substrates. Therefore, in the present work, we have studied the structural, magnetic and transport properties of Co thin films deposited on these substrates at room temperature. The results obtained from different characterization techniques are analyzed and discussed. 2. Experimental The n-type GaAs (1 0 0) wafer used in present study was etched in the chemical solution of HCl:H2 O2 :H2 O (volume ratio 1:1:50) for 1 min. After dipping into de-ionized water for removing organic contamination, finally the substrate was ultrasonic ˚ cleaned along with float glass substrate. The 400 A-thick Co film was deposited simultaneously on both GaAs (1 0 0) (refereed as film-I) and float glass substrate (refereed as film-II) using ion beam sputtering deposition technique. Before deposition, the base pressure was better than 4 × 10−6 Torr. The structural characterization of these as deposited thin film samples was done using X-ray diffraction (XRD) technique, on Rigaku RINT 2000 diffractometer equipped with a rotating Cu ˚ (operated at 40 kV anode as the source of X-rays at λ = 1.54 A and 30 mA). The grazing incidence X-ray reflectivity (GIXRR) measurements have been carried out on Siemens D 5000 diffractometer. For this measurement, the vertical divergence of the incident beam was controlled by using a solar slit and corresponding horizontal divergence of the incident beam was limited by 0.5 mm slit. The associated magnetic properties were obtained using Vibrating Sample Magnetometer (VSM). The atomic force microscopic (AFM) images were recorded on Nanoscope E Digital Instruments model in contact mode. The resistivity measurements were done using the standard four-probe method. After deposition, samples were quickly transferred to the experimental chamber of a PES workstation installed on the toroidal grating monochromator (TGM) beamline on Indus-1 synchrotron radiation source. The details of this beamline are described elsewhere [16]. The experimental station is equipped with an angle-integrated spectrometer having 180◦ hemispherical electron energy analyzer (Omicron EA 125). All the measurements reported here were carried out at photon energy of
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134 eV, using 50 eV pass energy giving a constant resolution of ∼0.65 eV. Energy calibration of the spectrometer was done using known core levels of standard samples, such as Au, Ag and Pt. The sample was properly grounded in order to avoid any charging effect. The depth profiling of the sample was done out using Ar+ ion gun attached at oblique incidence, with the argon ion energy and ion beam current kept at 1 keV and 1 A. Since photon flux at the sample is of the order of 5 × 1010 photons/s/mrad/0.1 bandwidth, measurements can be performed in a short time as compared to a laboratory source. During the time of measurement, we do not observe any appreciable change in beam current, which is of importance in the present analysis, since the intensity of the photoemission peaks is directly proportional to the beam current. The PES measurements were carried out in a pressure better than 5 × 10−10 Torr. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. XRD measurements Fig. 1 shows the XRD patterns of Co thin films deposited on both the substrates. The XRD pattern of film-I shows three wellresolved sharp peaks at around 2θ values of 31.56◦ , 43.96◦ and 65.94◦ due to reflections from GaAs (2 0 0), Co (0 0 2) and GaAs (4 0 0) planes as shown in Fig. 1(a). It is known from standard data for bulk Co that the lattice spacing for hcp (0 0 2) phase is ˚ and for fcc (1 1 1) phase it is 2.046 A ˚ [14]. However, 2.023 A ˚ in the present case, the lattice spacing is found to be 2.024 A, matching fairly well with the reported hcp Co value, indicating the absence of any strain between the layer and substrate. Further, it is well documented in literature that Co grows epitaxially on GaAs substrate and the lattice spacing of as grown Co becomes close to half that of the GaAs, which implies a 2 × 2 construction upon GaAs lattice. This arrangement modifies the crystalline structure of Co making it body centered cubic (bcc)
Fig. 1. (a and b) XRD patterns of Co/GaAs and Co/glass thin films.
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one, below the critical thickness of 2–15 nm [17,18]. It is also known that this bcc phase of Co film does not exist at Co/GaAs (1 0 0) interface when prepared at room temperature [19]. Similarly, in our case the Co film grown on GaAs substrate at room temperature also shows a hexagonal closed packed (hcp) structure. However, the XRD pattern of film-II shows a relatively broad hump around 44◦ indicating the microcrystalline nature of asdeposited film (see Fig. 1b). The absence of sharp crystalline peak may be due to the lower grain size of Co atoms grown on glass substrate. To better understand the difference, we have estimated the “out-of-plane” coherence length of both the films using Scherer formulism [20] and the values are found to be ˚ for Co/GaAs (film-I) and 38 A ˚ for Co/glass (film-II) sam166 A ples, respectively. 3.2. Grazing incidence X-ray reflectivity measurements In case of thin film, GIXRR measurements provide quantitative measure of various parameters, such as surface and interface roughness, thickness, electron density, etc. These parameters are obtained from the oscillations and decreasing pattern of reflectivity by fitting experimentally measured GIXRR patterns with the help of computer based Parratt’s formulism [21]. Fig. 2 shows such a reflectivity pattern as a function of momentum transfer vector Qz for both the films. It is clear that up to critical wave vector (Qc ), the reflectivity is maximum and suddenly decreases beyond Qc . This fall depends on the electron density of the layer, ˚ −1 ) in comparison and the higher value of Qc for film-I (0.055 A ˚ −1 ) can be correlated with AFM, with that for film-II (0.058 A which show connected island like structure (i.e. porous) in filmI, while layer structure in film-II. The corresponding electron density values shown in Table 1 also give an idea of the mass density of the system, as these quantities are proportional to each other. Beyond Qc , there is a noticeable change in the reflectivity pattern of film-I, which shows a larger number of well-defined oscillations. On the other hand, film-II shows less number of uneven oscillations with the drop in intensity being approxi-
Fig. 2. (a and b) Fitted GIXRR patterns of Co/GaAs and Co/glass thin films.
Table 1 Fitted GIXRR parameter of Co/GaAs and Co/glass thin films Sample
Parameters Thickness, ˚ d (A)
Roughness, ˚ σ (A)
Electron density, ˚ −2 ) ρ (A
Co/GaAs CoO Co Co–As GaAs
23.11 387.65 5.54 ∞
2.111 6.602 12.99 12.614
4.684E−5 6.865E−5 1.319E−4 3.851E−5
Co/Glass CoO Co SiO2
47.26 398.14 ∞
18.903 14.625 5.269
7.000E−5 7.248E−5 2.015E−5
mately one order of magnitude greater than film-I. In principle, these are related to the higher roughness of film-II as compared to film-I as also evident from the roughness values obtained from both films (see Table 1). While obtaining the parameter for thickness it was seen that in case of film-I, the pattern is not fitted by considering only Co and CoO layers. Therefore, on the basis of our PES results as well as the study carried out by Zhang et al. [12] in which they have shown that the PES technique has revealed the formation of CoAs at the interface of Co/GaAs system, we have obtained ˚ top CoO the best fitting with this possibility. Apart from 23.11 A ˚ layer, the thickness values are found to be 387.65 A for Co and ˚ for CoAs phase. The thickness of Co layer in film-II is 5.54 A ˚ top CoO layer. These values are found to be 398.14 with 47.26 A fairly matching with that expected after deposition. The discussion regarding the formation of CoAs phase at the interface is also discussed in the later part of this paper (PES measurements). 3.3. Magnetization measurements On comparing the hysteresis loops of both the thin film samples, one can clearly uncover the difference in magnetic properties of these thin films. Fig. 3(a and b) represents a comparison of such hysteresis loops for both the cases. For these
Fig. 3. (a and b) Hysteresis loops of Co/GaAs and Co/glass thin films.
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measurements, magnetic field was applied parallel to the sample surface and the hysteresis loops were recorded up to the saturation magnetization. One can see from observed results that the hysteresis loop of film-I is square in shape indicating that the distribution of crystalline anisotropy is rather sharp, which makes the domain magnetization switching beyond certain applied magnetic field as shown in Fig. 3a. Observed coercivity and saturation magnetization values are found to be 49.0 G and 3.65 × 10−4 emu, respectively. Large vertical jump with retentivity almost equal to the saturation magnetization (Ms ) and lower coercivity (Hc ) value indicate the soft magnetic behaviour with a strong anisotropy leading to in plane easy direction of magnetization. However, a drastic change is observed in the shape of hysteresis loop of film-II as shown in Fig. 3b. The observed Hc and Ms values in this case are found to be 22 G and 4.77 × 10−4 emu, respectively. The higher coercivity of film-I as compared to that of film-II can be understood in terms of different structures and growth mechanisms in each case. As we have already discussed earlier, the crystalline hcp Co phase is obtained for Co/GaAs system whereas microcrystalline nature is obtained for Co/glass system. It is well known that when the crystal grains are large, crystalline magnetic anisotropy is large and magnetization in each crystal grain orients in different directions, because easy axes are not parallel to each other [22]. Therefore, larger grain size should lead to a large coercivity in Co/GaAs than Co/glass thin films. On the other hand, the lower saturation magnetization of filmI as compared to film-II can be understood in terms of initial interaction of Co atoms with GaAs substrate. As discussed earlier, the GIXRR measurements also show the formation of CoAs phase at the interface. Hence, the initial formation of this CoAs phase may reduce the magnetic moment of Co film. 3.4. PES measurements In order to further support the above-mentioned magnetization measurements, we have investigated the interaction of Co with GaAs at room temperature using PES technique. To better understand the evolution and modification that has occurred in the sample at interface, the core level and valence band spectra were recorded on as-deposited thin film sample. Fig. 4 shows the core level spectra of Co 3p, Ga 3d and As 3d as a function of sputtering time. The core level spectra recorded before sputtering shows a small peak due to Co 3p at a binding energy (BE) position of 60.2 eV, which is shifted from its bulk position by 1.2 eV towards higher BE side. The shift is attributed to the oxidation of Co atoms at the surface during exposure to the atmosphere. Our GIXRR measurements also show presence of ˚ CoO layer on top of film-I. The absence of Ga and As 23.11 A peaks in the spectrum recorded before sputtering is expected due to lower escape depth of photoelectrons at this photon energy ˚ since the GaAs surface is covered by 400 A-thick ˚ (∼30 A) Co layer in the present case. The core level spectra recorded after 40 min sputtering shows shift in Co peak towards its bulk BE position with an enhancement in intensity due to removal of contaminations and still, no appearance of Ga and As atoms is seen. However, the spectrum recorded after 80 min sputtering
Fig. 4. Core level spectra of Co 3p, Ga 3d and As 3d as a function of sputtering time.
shows well-defined elemental peak of pure Co 3p at BE position 59.1 eV that matches well with the reported value of bulk Co. Still, no peaks corresponding to Ga and As are observed. Moreover, the core level spectra recorded after 120 min sputtering again shows shift in Co 3p peak position towards higher BE side by 0.4 eV with reduction in peak intensity along with the appearance of buried Ga 3d and As 3d peaks since the photoemission signals are now coming from Co/GaAs interface region. Both photoemission peaks are also shifted from their elemental BE position by 0.6 and 0.5 eV, respectively, towards lower BE side, indicating the participation of Co and GaAs in chemical reaction. These BE positions of Ga 3d and As 3d peaks are in good agreement with those reported for CoAs phase [12]. This confirms the chemical reaction and formation of CoAs phase in Co/GaAs thin film during deposition as also reflected from our GIXRR measurements. The spectrum shows major contributions from Ga and As, since the cross-section of Ga 3d (6.685) and As 3d (7.088) photoelectrons is larger as compared to Co 3p
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(0.8578) at this photon energy [23]. These Ga 3d and As 3d photoemission peaks also show asymmetric nature towards higher BE side, which is the characteristic of metallic CoAs phase. In order to further support these results, we have also recorded corresponding valence band spectra and Fig. 5 shows such valance band (VB) measurements carried out on film-I as a function of sputtering time. The valence band measurement recorded before sputtering shows peaks at BE of around 0.65 and ∼6 eV corresponding to Co 3d and O 2p. The VB spectrum recorded after 40 min sputtering shows reduction in oxygen peak intensity with a shift in Co 3d density of states towards its original bulk position. These observed shifts arise from the removal of oxygen from the sample surface. Still, the spectrum shows a very small presence of oxygen on the surface, which is removed by further sputtering. The spectra recorded after 80 min sputtering shows well-defined sharp elemental Co 3d density of states at BE position of 1.0 eV, that matches well with the reported one [24]. It also shows complete absence of oxygen on Co surface. After 120 min sputtering, the spectrum shows modifications in BE position and shape of the photoemission peak. The peaks become broader in nature and shifted towards higher BE side. The centroid of this band is located at 1.3 eV from Fermi energy.
Fig. 5. Valence band spectra of Co/GaAs thin films as a function of sputtering time.
Fig. 6. (a and b) Two- and three-dimensional AFM images of Co/GaAs and Co/glass thin films.
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Corresponding core level spectrum also shows appearance of buried Ga 3d and As 3d peak, thus we can conclude that the valence band spectrum now reflects the photoemission signals coming from Co/GaAs interface region. The observed results indicate that some chemical reaction has occurred at the interface between Co and GaAs, which modifies the shape and BE positions of the peaks also supporting our core level results. Also, the metallic Fermi edge suggests that the mixture of Co and GaAs is metallic in nature. This metallic nature of Co/GaAs interface was also reported by Ludge et al. [25]. These results can also be correlated with our magnetization measurements that show reduced magnetic moment of film-I in comparison with film-II. This reduction can also be understood theoretically in terms of As poisoning, since each As atom contributes with a moment −3.8 B to the net magnetic moment [26]. Our GIXRR measurements also show the formation of CoAs phase in film-I at the interface. The thickness of this intermixed region is cal˚ culated to be 5.54 A. All these observations on as deposited Co/GaAs thin film sample suggest that there is a slight intermixing and formation of CoAs phase at the interface during deposition. 3.5. Resistivity and AFM measurements In order to achieve a better understanding towards the growth mechanism of Co film on both the substrates, we have also carried out resistivity as well as AFM measurements on each sample. The calculated resistivity values are found to be 8 cm for film-I and 18 cm for film-II, respectively. These values are higher than the corresponding bulk Co (6 cm) [27] and are mainly attributed to the atomic rearrangement and structural changes taking place during deposition of these films. In this respect, AFM measurements provide a background for this and these images are also helpful in understanding the quantitative difference in resistivity values obtained for both the cases. Fig. 6(a and b) shows the two- and three-dimensional AFM images of Co thin films deposited on GaAs and glass substrates. The AFM images corresponding to film-I show island like growth where islands are connected to each other. However, film-II shows layer-by-layer growth. This is also reflected in GIXRR measurements, which show higher value of electron density in case of film-II. The difference in resistivity values can correlated with larger grain size of Co atoms on GaAs substrate than on glass substrate as also evident from AFM images. These resistivity results are also supported by XRD measurements, which show larger grain size in Co/GaAs than that in Co/glass thin film. 4. Conclusion The structural, magnetic and transport properties of Co/GaAs (1 0 0) and Co/glass thin films have been investigated. The structural measurements reveal the crystalline nature of Co thin film grown on GaAs, while microcrystalline nature in case of glass substrate. The film grown on GaAs shows higher coercivity, lower saturation magnetization and resistivity values as compared to that on glass substrate. The grazing incidence X-ray
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reflectivity and photoemission spectroscopy results show the interaction between Co and GaAs at the interface, while the Co layer grown on glass remains unaffected. These observed results are discussed and interpreted in terms of different growth morphologies and structures of as grown Co thin film on both substrates. Acknowledgements We are thankful to Dr. V. Ganeshan (AFM), Dr. N.P. Lalla (XRD), Mr. Avinash Wadikar (PES) and Dr. G. Chandrashekharan (VSM) for their help in the measurements. We are also thankful to Dr. V.R. Reddy and Prof. Ajay Gupta for fruitful discussions. This work is dedicated to the memory of Dr. S.M. Chaudhari who passed away untimely during the completion of this work. References [1] R.T. Heap, S.J. Greaves, J. Phys. D. Appl. Phys. 27 (1994) 1343. [2] T. Pan, G.W.D. Spratt, L. Tang, L.L. Lee, Y. Feng, D.E. Laughlin, J. Appl. Phys. 81 (1997) 3952. [3] S.P. Murarka, Silicide for VLSI Applications, Academic Press, New York, 1983. [4] G.A. Prinz, Phys. Today 48 (1995) 58. [5] D. Wang, R. Wu, A.J. Freeman, Phys. Rev. Lett. 70 (1993) 869. [6] S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665. [7] S.S.P. Parkin, Phys. Rev. Lett. 71 (1993) 1641. [8] G.A. Prinz, Phys. Rev. Lett. 54 (1985) 1051. [9] M. Izquierdo, M.E. Davila, C.M. Teodorescu, J. Chrost, H. Ascolani, J. Avila, M.C. Asensio, Appl. Surf. Sci. 234 (2004) 468. [10] Y.U. Idzerda, B.T. Jonker, W.T. Elam, G.A. Prinz, J. Vac. Sci. Technol. A 8 (1990) 1572. [11] K. Ludge, P. Vogt, W. Richter, B.O. Fimland, W. Braun, N. Esser, J. Vac. Sci. Technol. B 22 (2004) 2008. [12] F.P. Zhang, P.S. Xu, C.G. Zhu, E.D. Lu, H.Z. Guo, F.Q. Xu, X.Y. Zhang, J. Electron. Spectrosc. Relat. Phenom. 101–103 (1999) 485. [13] A. Kharmouche, S.M. Cherif, A. Bourzami, A. Layadi, G. Schmerber, J. Phys. D. Appl. Phys. 37 (2004) 2583. [14] B.X. Gu, H. Wang, J. Magn. Magn. Mater. 187 (1998) 47. [15] Ch. Morawe, A. Stierle, N. Metoki, K. Brohl, H. Zabel, J. Magn. Magn. Mater. 102 (1991) 223. [16] S.M. Chaudhari, D.M. Phase, A.D. Wadikar, B.A. Dasannacharya, G.S. Ramesh, M.S. Hegade, Curr. Sci. 82 (2002) 305. [17] M.A. Mangan, G. Spanos, T. Ambrose, G.A. Prinz, Appl. Phys. Lett. 75 (1999) 346. [18] Y.Z. Wu, H.F. Ding, C. Ding, D. Wu, G.S. Dong, X.F. Jin, K. Sun, S. Zhu, J. Magn. Magn. Mater. 198 (1999) 297. [19] Y.Z. Wu, H.F. Ding, C. Jing, D. Wu, G.L. Liu, V. Gordon, G.S. Dong, X.F. Jin, S. Zhu, K. Sun, Phys. Rev. B 57 (1998) 11935. [20] J.P. Eberhart, Analyse Structurale et Chimiques des Materiaux, Bordas, Paris, 1989. [21] L.G. Parratt, Phys. Rev. 95 (1954) 359. [22] B.D. Cullity, Introduction to Magnetic Materials, Addison Wesley, Massachusetts, 1972. [23] J.J. Yeh, Atomic calculation of Photoionization Cross-sections and Asymmetry Parameter, Gordon and Beach, NJ, USA, 1993. [24] V. Kinsinger, I. Dezsi, P. Steiner, G. Langouche, J. Phys. F: Met. Phys. 18 (1988) 1515. [25] K. Ludge, B.D. Schultz, P. Vogt, M.M.R. Evans, W. Braun, C.J. Palmstrom, W. Richter, N. Esser, J. Vac. Sci. Technol. B 20 (2002) 1591. [26] D. Singh, J. Appl. Phys. 71 (1992) 3431. [27] A. Sharma, R. Brajpuriya, S. Tripathi, S.M. Chaudhari, J. Vac. Sci. Technol. A 24 (2006) 74.
A comparative study of Co thin film deposited on GaAs (1 0 0) and glass substrates A. Sharma ∗ , R. Brajpuriya, S. Tripathi, D. Jain, R. Dubey, T. Shripathi, S.M. Chaudhari University Grant Commission, Department of Atomic Energy Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India Received 6 October 2005; received in revised form 16 February 2006; accepted 25 February 2006
Abstract The structural, magnetic and transport properties of Co/GaAs (1 0 0) and Co/glass thin films have been investigated. The structural measurements reveal the crystalline nature of Co thin film grown on GaAs, while microcrystalline nature in case of glass substrate. The film grown on GaAs shows higher coercivity (49.0 G), lower saturation magnetization (3.65 × 10−4 ) and resistivity (8 cm) values as compared to that on glass substrate (22 G, 4.77 × 10−4 and 18 cm). The grazing incidence X-ray reflectivity and photoemission spectroscopy results show the interaction between Co and GaAs at the interface, while the Co layer grown on glass remains unaffected. These observed results are discussed and interpreted in terms of different growth morphologies and structures of as grown Co thin film on both substrates. © 2006 Elsevier B.V. All rights reserved. Keywords: Co thin film; XRD; VSM; GIXRR; PES; AFM
1. Introduction The Co and Co-based thin film structures have attracted interest of several researchers in recent years due to their possible applications in many areas of technology, such as magnetic data storage, spin valve and microelectronic devices [1–3]. Recently, the area of growth of magnetic thin films on compound semiconductor substrates is being developed as being interesting from both the scientific and technological points of view in connection with spintronics [4–6]. In such structures, the combination of magnetism of ferromagnetic layer along with the electronic properties of underlying semiconductor substrate offers tremendous opportunity for designing futuristic innovative magnetotransport devices. In many giant magneto resistive (GMR) devices, Co plays an important role due to high spin polarization of carriers at Fermi level [7]. Therefore, an understanding of growth and micro-structural evolution of Co thin layers, their corresponding crystal structures and associated magnetic and transport properties are of great technological interest.
∗
Corresponding author. Tel.: +91 731 2463913; fax: +91 731 2462294. E-mail address: [email protected] (A. Sharma). Deceased.
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.02.065
There exist a number of studies in the literature regarding Co/GaAs system as it shows various growth morphologies under different conditions. Prinz have observed that Co film on GaAs substrate grows in non-equilibrium body centered cubic (bcc) structure, which is not found in bulk structure of this material [8]. Similarly, a recent study by Izquierdo et al. ˚ exists in shows that the film having thickness above 350 A, the hexagonal closed packed (hcp) structure [9]. In yet another study on Co/GaAs interface using conversion electron extended X-ray absorption spectroscopy technique, Idzerda et al. have found that Co film grows on GaAs surface with a metastable BCC phase [10]. Recently, Ludge et al. have studied the metallic nanostructures of Co on GaAs (4 × 2) surface using scanning tunneling microscopy (STM), surface X-ray photoelectron spectroscopy (SXPS), low energy electron diffraction (LEED) and reflection anisotropy spectroscopy (RAS) [11]. By SXPS they found that the deposition of 1 ML thick Co film on GaAs (0 0 1) forms CoAs and CoGa compounds, while RAS and LEED techniques revealed that at sub monolayer coverage, Co atoms incorporated on clean (4 × 2) surface as row like structure and authors have concluded that these rows act as onedimensional metallic nano wires. The strong chemical reaction and interface disruption were observed by Zhang et al. when ˚ Co on GaAs (0 0 1) they deposited [even low coverage (∼2 A)]
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[12]. Using photoemission spectroscopy, they found that Ga atoms interdiffuse into Co matrix and get fully diluted. However, ˚ Co is deposited on GaAs. a stable interface is formed when 10 A However, very few reports are available on Co growth on glass substrate. In this regard, an important study was carried out by Kharmouche et al., in which they studied structural and magnetic properties of Co films evaporated on glass substrate as a function of the Co thickness [13]. They observed that films were polycrystalline with (0 0 0 1) texture. It was found that surface and stress induced uniaxial magnetic anisotropy decreases as the film thickness increases. In another study, fcc Co (1 1 1) film grown on glass substrate by Gu and Wang using sputtering technique [14]. Thickness dependent study carried out by Morawe et al. shows that Co film growth on glass in both hcp (0 0 0 1) and fcc (1 1 1) orientations [15]. However, as per the best of our knowledge no report available in the literature correlates structural, magnetic and transport properties of Co thin films on GaAs and glass substrates. Therefore, in the present work, we have studied the structural, magnetic and transport properties of Co thin films deposited on these substrates at room temperature. The results obtained from different characterization techniques are analyzed and discussed. 2. Experimental The n-type GaAs (1 0 0) wafer used in present study was etched in the chemical solution of HCl:H2 O2 :H2 O (volume ratio 1:1:50) for 1 min. After dipping into de-ionized water for removing organic contamination, finally the substrate was ultrasonic ˚ cleaned along with float glass substrate. The 400 A-thick Co film was deposited simultaneously on both GaAs (1 0 0) (refereed as film-I) and float glass substrate (refereed as film-II) using ion beam sputtering deposition technique. Before deposition, the base pressure was better than 4 × 10−6 Torr. The structural characterization of these as deposited thin film samples was done using X-ray diffraction (XRD) technique, on Rigaku RINT 2000 diffractometer equipped with a rotating Cu ˚ (operated at 40 kV anode as the source of X-rays at λ = 1.54 A and 30 mA). The grazing incidence X-ray reflectivity (GIXRR) measurements have been carried out on Siemens D 5000 diffractometer. For this measurement, the vertical divergence of the incident beam was controlled by using a solar slit and corresponding horizontal divergence of the incident beam was limited by 0.5 mm slit. The associated magnetic properties were obtained using Vibrating Sample Magnetometer (VSM). The atomic force microscopic (AFM) images were recorded on Nanoscope E Digital Instruments model in contact mode. The resistivity measurements were done using the standard four-probe method. After deposition, samples were quickly transferred to the experimental chamber of a PES workstation installed on the toroidal grating monochromator (TGM) beamline on Indus-1 synchrotron radiation source. The details of this beamline are described elsewhere [16]. The experimental station is equipped with an angle-integrated spectrometer having 180◦ hemispherical electron energy analyzer (Omicron EA 125). All the measurements reported here were carried out at photon energy of
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134 eV, using 50 eV pass energy giving a constant resolution of ∼0.65 eV. Energy calibration of the spectrometer was done using known core levels of standard samples, such as Au, Ag and Pt. The sample was properly grounded in order to avoid any charging effect. The depth profiling of the sample was done out using Ar+ ion gun attached at oblique incidence, with the argon ion energy and ion beam current kept at 1 keV and 1 A. Since photon flux at the sample is of the order of 5 × 1010 photons/s/mrad/0.1 bandwidth, measurements can be performed in a short time as compared to a laboratory source. During the time of measurement, we do not observe any appreciable change in beam current, which is of importance in the present analysis, since the intensity of the photoemission peaks is directly proportional to the beam current. The PES measurements were carried out in a pressure better than 5 × 10−10 Torr. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. XRD measurements Fig. 1 shows the XRD patterns of Co thin films deposited on both the substrates. The XRD pattern of film-I shows three wellresolved sharp peaks at around 2θ values of 31.56◦ , 43.96◦ and 65.94◦ due to reflections from GaAs (2 0 0), Co (0 0 2) and GaAs (4 0 0) planes as shown in Fig. 1(a). It is known from standard data for bulk Co that the lattice spacing for hcp (0 0 2) phase is ˚ and for fcc (1 1 1) phase it is 2.046 A ˚ [14]. However, 2.023 A ˚ in the present case, the lattice spacing is found to be 2.024 A, matching fairly well with the reported hcp Co value, indicating the absence of any strain between the layer and substrate. Further, it is well documented in literature that Co grows epitaxially on GaAs substrate and the lattice spacing of as grown Co becomes close to half that of the GaAs, which implies a 2 × 2 construction upon GaAs lattice. This arrangement modifies the crystalline structure of Co making it body centered cubic (bcc)
Fig. 1. (a and b) XRD patterns of Co/GaAs and Co/glass thin films.
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one, below the critical thickness of 2–15 nm [17,18]. It is also known that this bcc phase of Co film does not exist at Co/GaAs (1 0 0) interface when prepared at room temperature [19]. Similarly, in our case the Co film grown on GaAs substrate at room temperature also shows a hexagonal closed packed (hcp) structure. However, the XRD pattern of film-II shows a relatively broad hump around 44◦ indicating the microcrystalline nature of asdeposited film (see Fig. 1b). The absence of sharp crystalline peak may be due to the lower grain size of Co atoms grown on glass substrate. To better understand the difference, we have estimated the “out-of-plane” coherence length of both the films using Scherer formulism [20] and the values are found to be ˚ for Co/GaAs (film-I) and 38 A ˚ for Co/glass (film-II) sam166 A ples, respectively. 3.2. Grazing incidence X-ray reflectivity measurements In case of thin film, GIXRR measurements provide quantitative measure of various parameters, such as surface and interface roughness, thickness, electron density, etc. These parameters are obtained from the oscillations and decreasing pattern of reflectivity by fitting experimentally measured GIXRR patterns with the help of computer based Parratt’s formulism [21]. Fig. 2 shows such a reflectivity pattern as a function of momentum transfer vector Qz for both the films. It is clear that up to critical wave vector (Qc ), the reflectivity is maximum and suddenly decreases beyond Qc . This fall depends on the electron density of the layer, ˚ −1 ) in comparison and the higher value of Qc for film-I (0.055 A ˚ −1 ) can be correlated with AFM, with that for film-II (0.058 A which show connected island like structure (i.e. porous) in filmI, while layer structure in film-II. The corresponding electron density values shown in Table 1 also give an idea of the mass density of the system, as these quantities are proportional to each other. Beyond Qc , there is a noticeable change in the reflectivity pattern of film-I, which shows a larger number of well-defined oscillations. On the other hand, film-II shows less number of uneven oscillations with the drop in intensity being approxi-
Fig. 2. (a and b) Fitted GIXRR patterns of Co/GaAs and Co/glass thin films.
Table 1 Fitted GIXRR parameter of Co/GaAs and Co/glass thin films Sample
Parameters Thickness, ˚ d (A)
Roughness, ˚ σ (A)
Electron density, ˚ −2 ) ρ (A
Co/GaAs CoO Co Co–As GaAs
23.11 387.65 5.54 ∞
2.111 6.602 12.99 12.614
4.684E−5 6.865E−5 1.319E−4 3.851E−5
Co/Glass CoO Co SiO2
47.26 398.14 ∞
18.903 14.625 5.269
7.000E−5 7.248E−5 2.015E−5
mately one order of magnitude greater than film-I. In principle, these are related to the higher roughness of film-II as compared to film-I as also evident from the roughness values obtained from both films (see Table 1). While obtaining the parameter for thickness it was seen that in case of film-I, the pattern is not fitted by considering only Co and CoO layers. Therefore, on the basis of our PES results as well as the study carried out by Zhang et al. [12] in which they have shown that the PES technique has revealed the formation of CoAs at the interface of Co/GaAs system, we have obtained ˚ top CoO the best fitting with this possibility. Apart from 23.11 A ˚ layer, the thickness values are found to be 387.65 A for Co and ˚ for CoAs phase. The thickness of Co layer in film-II is 5.54 A ˚ top CoO layer. These values are found to be 398.14 with 47.26 A fairly matching with that expected after deposition. The discussion regarding the formation of CoAs phase at the interface is also discussed in the later part of this paper (PES measurements). 3.3. Magnetization measurements On comparing the hysteresis loops of both the thin film samples, one can clearly uncover the difference in magnetic properties of these thin films. Fig. 3(a and b) represents a comparison of such hysteresis loops for both the cases. For these
Fig. 3. (a and b) Hysteresis loops of Co/GaAs and Co/glass thin films.
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measurements, magnetic field was applied parallel to the sample surface and the hysteresis loops were recorded up to the saturation magnetization. One can see from observed results that the hysteresis loop of film-I is square in shape indicating that the distribution of crystalline anisotropy is rather sharp, which makes the domain magnetization switching beyond certain applied magnetic field as shown in Fig. 3a. Observed coercivity and saturation magnetization values are found to be 49.0 G and 3.65 × 10−4 emu, respectively. Large vertical jump with retentivity almost equal to the saturation magnetization (Ms ) and lower coercivity (Hc ) value indicate the soft magnetic behaviour with a strong anisotropy leading to in plane easy direction of magnetization. However, a drastic change is observed in the shape of hysteresis loop of film-II as shown in Fig. 3b. The observed Hc and Ms values in this case are found to be 22 G and 4.77 × 10−4 emu, respectively. The higher coercivity of film-I as compared to that of film-II can be understood in terms of different structures and growth mechanisms in each case. As we have already discussed earlier, the crystalline hcp Co phase is obtained for Co/GaAs system whereas microcrystalline nature is obtained for Co/glass system. It is well known that when the crystal grains are large, crystalline magnetic anisotropy is large and magnetization in each crystal grain orients in different directions, because easy axes are not parallel to each other [22]. Therefore, larger grain size should lead to a large coercivity in Co/GaAs than Co/glass thin films. On the other hand, the lower saturation magnetization of filmI as compared to film-II can be understood in terms of initial interaction of Co atoms with GaAs substrate. As discussed earlier, the GIXRR measurements also show the formation of CoAs phase at the interface. Hence, the initial formation of this CoAs phase may reduce the magnetic moment of Co film. 3.4. PES measurements In order to further support the above-mentioned magnetization measurements, we have investigated the interaction of Co with GaAs at room temperature using PES technique. To better understand the evolution and modification that has occurred in the sample at interface, the core level and valence band spectra were recorded on as-deposited thin film sample. Fig. 4 shows the core level spectra of Co 3p, Ga 3d and As 3d as a function of sputtering time. The core level spectra recorded before sputtering shows a small peak due to Co 3p at a binding energy (BE) position of 60.2 eV, which is shifted from its bulk position by 1.2 eV towards higher BE side. The shift is attributed to the oxidation of Co atoms at the surface during exposure to the atmosphere. Our GIXRR measurements also show presence of ˚ CoO layer on top of film-I. The absence of Ga and As 23.11 A peaks in the spectrum recorded before sputtering is expected due to lower escape depth of photoelectrons at this photon energy ˚ since the GaAs surface is covered by 400 A-thick ˚ (∼30 A) Co layer in the present case. The core level spectra recorded after 40 min sputtering shows shift in Co peak towards its bulk BE position with an enhancement in intensity due to removal of contaminations and still, no appearance of Ga and As atoms is seen. However, the spectrum recorded after 80 min sputtering
Fig. 4. Core level spectra of Co 3p, Ga 3d and As 3d as a function of sputtering time.
shows well-defined elemental peak of pure Co 3p at BE position 59.1 eV that matches well with the reported value of bulk Co. Still, no peaks corresponding to Ga and As are observed. Moreover, the core level spectra recorded after 120 min sputtering again shows shift in Co 3p peak position towards higher BE side by 0.4 eV with reduction in peak intensity along with the appearance of buried Ga 3d and As 3d peaks since the photoemission signals are now coming from Co/GaAs interface region. Both photoemission peaks are also shifted from their elemental BE position by 0.6 and 0.5 eV, respectively, towards lower BE side, indicating the participation of Co and GaAs in chemical reaction. These BE positions of Ga 3d and As 3d peaks are in good agreement with those reported for CoAs phase [12]. This confirms the chemical reaction and formation of CoAs phase in Co/GaAs thin film during deposition as also reflected from our GIXRR measurements. The spectrum shows major contributions from Ga and As, since the cross-section of Ga 3d (6.685) and As 3d (7.088) photoelectrons is larger as compared to Co 3p
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(0.8578) at this photon energy [23]. These Ga 3d and As 3d photoemission peaks also show asymmetric nature towards higher BE side, which is the characteristic of metallic CoAs phase. In order to further support these results, we have also recorded corresponding valence band spectra and Fig. 5 shows such valance band (VB) measurements carried out on film-I as a function of sputtering time. The valence band measurement recorded before sputtering shows peaks at BE of around 0.65 and ∼6 eV corresponding to Co 3d and O 2p. The VB spectrum recorded after 40 min sputtering shows reduction in oxygen peak intensity with a shift in Co 3d density of states towards its original bulk position. These observed shifts arise from the removal of oxygen from the sample surface. Still, the spectrum shows a very small presence of oxygen on the surface, which is removed by further sputtering. The spectra recorded after 80 min sputtering shows well-defined sharp elemental Co 3d density of states at BE position of 1.0 eV, that matches well with the reported one [24]. It also shows complete absence of oxygen on Co surface. After 120 min sputtering, the spectrum shows modifications in BE position and shape of the photoemission peak. The peaks become broader in nature and shifted towards higher BE side. The centroid of this band is located at 1.3 eV from Fermi energy.
Fig. 5. Valence band spectra of Co/GaAs thin films as a function of sputtering time.
Fig. 6. (a and b) Two- and three-dimensional AFM images of Co/GaAs and Co/glass thin films.
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Corresponding core level spectrum also shows appearance of buried Ga 3d and As 3d peak, thus we can conclude that the valence band spectrum now reflects the photoemission signals coming from Co/GaAs interface region. The observed results indicate that some chemical reaction has occurred at the interface between Co and GaAs, which modifies the shape and BE positions of the peaks also supporting our core level results. Also, the metallic Fermi edge suggests that the mixture of Co and GaAs is metallic in nature. This metallic nature of Co/GaAs interface was also reported by Ludge et al. [25]. These results can also be correlated with our magnetization measurements that show reduced magnetic moment of film-I in comparison with film-II. This reduction can also be understood theoretically in terms of As poisoning, since each As atom contributes with a moment −3.8 B to the net magnetic moment [26]. Our GIXRR measurements also show the formation of CoAs phase in film-I at the interface. The thickness of this intermixed region is cal˚ culated to be 5.54 A. All these observations on as deposited Co/GaAs thin film sample suggest that there is a slight intermixing and formation of CoAs phase at the interface during deposition. 3.5. Resistivity and AFM measurements In order to achieve a better understanding towards the growth mechanism of Co film on both the substrates, we have also carried out resistivity as well as AFM measurements on each sample. The calculated resistivity values are found to be 8 cm for film-I and 18 cm for film-II, respectively. These values are higher than the corresponding bulk Co (6 cm) [27] and are mainly attributed to the atomic rearrangement and structural changes taking place during deposition of these films. In this respect, AFM measurements provide a background for this and these images are also helpful in understanding the quantitative difference in resistivity values obtained for both the cases. Fig. 6(a and b) shows the two- and three-dimensional AFM images of Co thin films deposited on GaAs and glass substrates. The AFM images corresponding to film-I show island like growth where islands are connected to each other. However, film-II shows layer-by-layer growth. This is also reflected in GIXRR measurements, which show higher value of electron density in case of film-II. The difference in resistivity values can correlated with larger grain size of Co atoms on GaAs substrate than on glass substrate as also evident from AFM images. These resistivity results are also supported by XRD measurements, which show larger grain size in Co/GaAs than that in Co/glass thin film. 4. Conclusion The structural, magnetic and transport properties of Co/GaAs (1 0 0) and Co/glass thin films have been investigated. The structural measurements reveal the crystalline nature of Co thin film grown on GaAs, while microcrystalline nature in case of glass substrate. The film grown on GaAs shows higher coercivity, lower saturation magnetization and resistivity values as compared to that on glass substrate. The grazing incidence X-ray
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reflectivity and photoemission spectroscopy results show the interaction between Co and GaAs at the interface, while the Co layer grown on glass remains unaffected. These observed results are discussed and interpreted in terms of different growth morphologies and structures of as grown Co thin film on both substrates. Acknowledgements We are thankful to Dr. V. Ganeshan (AFM), Dr. N.P. Lalla (XRD), Mr. Avinash Wadikar (PES) and Dr. G. Chandrashekharan (VSM) for their help in the measurements. We are also thankful to Dr. V.R. Reddy and Prof. Ajay Gupta for fruitful discussions. This work is dedicated to the memory of Dr. S.M. Chaudhari who passed away untimely during the completion of this work. References [1] R.T. Heap, S.J. Greaves, J. Phys. D. Appl. Phys. 27 (1994) 1343. [2] T. Pan, G.W.D. Spratt, L. Tang, L.L. Lee, Y. Feng, D.E. Laughlin, J. Appl. Phys. 81 (1997) 3952. [3] S.P. Murarka, Silicide for VLSI Applications, Academic Press, New York, 1983. [4] G.A. Prinz, Phys. Today 48 (1995) 58. [5] D. Wang, R. Wu, A.J. Freeman, Phys. Rev. Lett. 70 (1993) 869. [6] S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665. [7] S.S.P. Parkin, Phys. Rev. Lett. 71 (1993) 1641. [8] G.A. Prinz, Phys. Rev. Lett. 54 (1985) 1051. [9] M. Izquierdo, M.E. Davila, C.M. Teodorescu, J. Chrost, H. Ascolani, J. Avila, M.C. Asensio, Appl. Surf. Sci. 234 (2004) 468. [10] Y.U. Idzerda, B.T. Jonker, W.T. Elam, G.A. Prinz, J. Vac. Sci. Technol. A 8 (1990) 1572. [11] K. Ludge, P. Vogt, W. Richter, B.O. Fimland, W. Braun, N. Esser, J. Vac. Sci. Technol. B 22 (2004) 2008. [12] F.P. Zhang, P.S. Xu, C.G. Zhu, E.D. Lu, H.Z. Guo, F.Q. Xu, X.Y. Zhang, J. Electron. Spectrosc. Relat. Phenom. 101–103 (1999) 485. [13] A. Kharmouche, S.M. Cherif, A. Bourzami, A. Layadi, G. Schmerber, J. Phys. D. Appl. Phys. 37 (2004) 2583. [14] B.X. Gu, H. Wang, J. Magn. Magn. Mater. 187 (1998) 47. [15] Ch. Morawe, A. Stierle, N. Metoki, K. Brohl, H. Zabel, J. Magn. Magn. Mater. 102 (1991) 223. [16] S.M. Chaudhari, D.M. Phase, A.D. Wadikar, B.A. Dasannacharya, G.S. Ramesh, M.S. Hegade, Curr. Sci. 82 (2002) 305. [17] M.A. Mangan, G. Spanos, T. Ambrose, G.A. Prinz, Appl. Phys. Lett. 75 (1999) 346. [18] Y.Z. Wu, H.F. Ding, C. Ding, D. Wu, G.S. Dong, X.F. Jin, K. Sun, S. Zhu, J. Magn. Magn. Mater. 198 (1999) 297. [19] Y.Z. Wu, H.F. Ding, C. Jing, D. Wu, G.L. Liu, V. Gordon, G.S. Dong, X.F. Jin, S. Zhu, K. Sun, Phys. Rev. B 57 (1998) 11935. [20] J.P. Eberhart, Analyse Structurale et Chimiques des Materiaux, Bordas, Paris, 1989. [21] L.G. Parratt, Phys. Rev. 95 (1954) 359. [22] B.D. Cullity, Introduction to Magnetic Materials, Addison Wesley, Massachusetts, 1972. [23] J.J. Yeh, Atomic calculation of Photoionization Cross-sections and Asymmetry Parameter, Gordon and Beach, NJ, USA, 1993. [24] V. Kinsinger, I. Dezsi, P. Steiner, G. Langouche, J. Phys. F: Met. Phys. 18 (1988) 1515. [25] K. Ludge, B.D. Schultz, P. Vogt, M.M.R. Evans, W. Braun, C.J. Palmstrom, W. Richter, N. Esser, J. Vac. Sci. Technol. B 20 (2002) 1591. [26] D. Singh, J. Appl. Phys. 71 (1992) 3431. [27] A. Sharma, R. Brajpuriya, S. Tripathi, S.M. Chaudhari, J. Vac. Sci. Technol. A 24 (2006) 74.