- Email: [email protected]
Co bilayer DMS thin films
Solid State Communications 150 (2010) 1587–1591
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Hydrogenation effect on electrical, optical and magnetic properties of ZnSe/Co bilayer DMS thin films S.P. Nehra ∗ , M. Singh Thin Films and Membrane Science Laboratory, Department of Physics, University of Rajasthan, Jaipur-302004, India
article
info
Article history: Received 18 May 2010 Accepted 23 May 2010 by A.H. MacDonald Available online 1 June 2010 Keywords: A. DMS thin films B. Hydrogenation C. I–V characteristics D. Uv–Vis spectroscopy
abstract This paper reports ZnSe/Co bilayer diluted magnetic semiconductor thin films have been prepared by using thermal evaporation technique. The bilayer DMS thin films were hydrogenated at different pressures (15–45 psi) for a constant time of 30 min. Before and after hydrogenations of these bilayer thin films the electrical, optical and magnetic properties have been investigated. Electrical resistivity and optical band gap were found to be increased with respect to hydrogenation pressure. X-ray diffraction (XRD) and magnetic measurements confirmed the formation of DMS ZnSe/Co bilayer DMS thin films. Raman spectra show the presence of hydrogen in these thin films. Surface topography study of as-grown, annealed and hydrogenated ZnSe/Co bilayer thin films indicates uniform deposition, mixing of layers and increment in roughness at the surface due to hydrogen passivation effect respectively. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Diluted magnetic semiconductors are compound of alloy semiconductors containing a large fraction of magnetic ions [1]. They have unusual magnetic characteristics due to the presence of isolated magnetic ions in the semiconducting lattice. The diluted magnetic semiconductor (DMS) is expected to play an important role in interdisciplinary materials science and future spintronics because charge and spin degrees of freedom are accommodated into single matter and their interplay is expected to explore novel physics and new devices [2–5]. Diluted magnetic semiconductors (DMS) are compound of alloy semiconductors containing a large fraction of magnetic ions (Mn+2 , Cr+2 , Fe+2 , Co+2 ) and are studied mainly on II–VI based materials such as CdTe and ZnSe etc. This is because such +2 magnetic ions are easily incorporated into the host II–VI crystals by replacing group II cations. In II–VI based DMS such as (CdMn) Se, magneto-optic properties have been extensively studied, and optical isolators were recently fabricated using their large Faraday effect [6,7]. Since the discovery of ferromagnetism in diluted magnetic semiconductors (DMS) such as Mn-doped InAs or GaAs, it has been intensively studied in order to fabricate a new functional semiconductor taking advantage of the spin degree of freedom in DMS. It is expected to establish new semiconductor spin electronics (spintronics) as a practical technology based on such new functional materials [8,9].
∗
Corresponding author. Tel.: +91 141 2702457; fax: +91 141 2711049. E-mail addresses: [email protected] (S.P. Nehra), [email protected] (M. Singh). 0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.05.038
The origin of ferromagnetism in the DMS is discussed by many researchers [10–17]. Pakhomov et al. prepared (Zn,Co)O:Al DMS superlattices, with the Curie temperature higher than room temperature, by alternating deposition of ZnO and Co as multilayers on the atomic scale. Over-doping with cobalt leads to a suppression of high-temperature ferromagnetism. At the same time, Co granules are formed at decreasing thickness of ZnO layers, leading both to superparamagnetism and a crossover of conduction to hopping between metal granules [18]. Hydrogen passivation is a good method for investigating the origin of ferromagnetism in DMS. Hydrogenation gave strong evidence which proved the mechanism of ferromagnetism [19–22]. Hydrogenation is a mature process in modern electronics to change the electrical properties of silicon films. It would also be useful for investigating the relation between ferromagnetism and carriers in the Si- based DMS. Preparations of DMSs are mainly done by (i) thin film growth e.g. (a) molecular beam epitaxy (MBE), (b) pulse laser depositions (PLD) and (c) chemical vapour depositions (CVD) and (ii) bulk crystal growth e.g. Bridgman method. In this article, we present the effect of hydrogen on the electrical, optical and magnetic properties of ZnSe/Co BLs DMS thin films, successfully deposited using thermal evaporation technique. 2. Experimental details Bilayer ZnSe/Co DMS thin films were deposited on glass substrates by thermal evaporation technique at a base pressure of 10−5 Torr. Compound zinc selenide powder (99.99% purity) and elemental Co powder (99.98% purity) procured from Alfa
S.P. Nehra, M. Singh / Solid State Communications 150 (2010) 1587–1591
3. Results and discussions 3.1. Structural characteristics The structural characteristics of as-grown and vacuum annealed at 333 K ZnSe/Co BLs diluted magnetic semiconductor thin films are shown in Fig. 1. The as-grown ZnSe/Co BLs thin film shows an amorphous nature while the vacuum annealed film shows a polycrystalline nature. There were small peaks at 27.27°, 45.10° and 60.50°, 51.44° observed corresponding to (111), (220) and (111), (200) planes of cubic ZnSe and Co respectively. Due to the
360
ZnSe 111
300
200 Co
ZnSe
320
111 Co
b
340
220
Aesar, Johnson Matthey Company, USA were used for deposition. The source to substrate distance was kept at 15 cm in all cases. The detailed conditions for thermal evaporation technique are described in Ref. [23]. The evaporation was done in maintained vacuum by slowly increasing the current to heat the tantalum boat until evaporation was reached. Deposition of BLs ZnSe/Co thin films have been performed by the stacked layer method. We have deposited ZnSe and Co respectively to obtain ZnSe/Co BLs thin film structure. The thickness of ZnSe/Co BLs DMS thin films was 250 nm (150 nm ZnSe/ 100 nm Co) measured by quartz crystal thickness monitor. BLs ZnSe/Co DMS thin films were vacuum annealed at a constant temperature of 60 °C for an hour in a vacuum of 10−5 Torr by using a vacuum coating unit for mixing to get the homogeneous structure and interdiffusion of bilayer thin films of ZnSe/Co. Hydrogenation of ZnSe/Co bilayer thin films was performed by keeping these samples in a hydrogenation cell, where hydrogen gas was introduced at different pressures for 30 min. Before hydrogen gas was introduced in the cell, it was connected to the vacuum coating unit to produce a vacuum of 10−3 Torr. This process was performed at a constant temperature of 300 K. The X-ray diffraction (XRD) patterns of as-grown and vacuum annealed DMS thin films were recorded with the help of PANalytical X’pert PRO MPD PW3040/60 X-ray diffractometer using Cu Kα radiation, as a radiation source of wavelength λ = 1.540598 Å. The tube was operated at 45 KV, 40 mA with a scanning speed of 0.090(2θ )/s. The XRD patterns of all films were taken from 12° to 80° (2θ ). The peaks of the XRD patterns were searched by computer programming using Powder X Software. Transverse I–V characteristics of as-grown hydrogenated and annealed hydrogenated samples were recorded using a Keithley238 high current source measuring unit. The applied voltage was within the range of −2.0 to +2.0 V with increasing step of 0.1 V. For I–V characteristics, electrode contacts were made using silver (Ag) paste on the thin films. I–V characteristics of bilayer thin films were monitored with the help of SMUSweep computer software. All the measurements were performed at room temperature. Magnetization measurements with temperature and magnetic field were performed using a SQUID (Quantum Design physical properties measurement system). For M–T curves the applied field was fixed at 1000 Oe. Magnetic moment dependence on the applied field was measured at 2 K. The transmission spectra of as deposited and annealed hydrogenated thin films were carried out in the wavelength range 250–800 nm with the help of a Spectrophotometer U-3300. The Raman spectra of as-grown and hydrogenated samples were recorded using a green laser beam of wavelength 532 nm. (Raman model −3000 system). All the measurements were performed at room temperature. The optical micrographs were observed with the help of a Labomed optical microscope at 10× magnification having resolution of the order of 1 µm and the microscope was kept in reflection mode. The micrographs were stored in computer through standard software (Pixel View). Recorded two-dimensional (2D) optical micrographs were converted into three-dimensional (3D) images with the help of the Scanning Probe Image Processor computer program.
Intensity (arb.)
1588
280 260 240
a
(a) As-grown ZnSe/Co BLs thin film. (b) Vacuum annealed at 333 K.
220 200 180 160 140 120 20
30
40
50
60
70
80
2θ
Fig. 1. XRD patterns for (a) as-grown and (b) vacuum annealed ZnSe/Co BLS thin films.
Fig. 2a. I–V characteristics of (a) as-grown and (b) annealed at 333 K ZnSe/Co BLs thin films.
annealing, crystallinity is found to be increased indicating grain growth. Co peaks are clear evidence of the presence of magnetic material in the thin films, which is also confirmed by magnetic measurements. 3.2. I–V characteristics As-grown and annealed at 333 K ZnSe/Co BLs thin films show a partially semiconducting nature as shown in Fig. 2a. Current–voltage characteristics indicate the mixing of BLs thin films due to annealing because current was found to be reduced in the case of annealed BLs thin film. Because the outermost layer was elemental Co and the contacts were made on top of the film therefore current was found to be increased. In the case of the annealed sample, due to interdiffusion of ZnSe and Co, current was found to be reduced. Similar results were carried out by Nehra et al. [23] in the case of I–V characteristics study of as-grown and annealed CdTe/Mn BLs thin films. Current–voltage characteristics of as-grown and hydrogenated ZnSe/Co BLs thin films are shown in Fig. 2b. In the case of hydrogenated samples current was found to be reduced with increasing hydrogen pressure. This may be attributed to hydrogen controlling the flow of charges by taking electrons from Co and hydrogen has passivated defects at the interface. The electronic passivation of host impurities induced by hydrogen in a semiconductor well agrees as reported by Pankove et al. [24]. 3.3. Optical properties Optical absorption spectra of as-grown, annealed and hydrogenated ZnSe/Co BLs thin films are shown in Fig. 3a. It may be
S.P. Nehra, M. Singh / Solid State Communications 150 (2010) 1587–1591
Fig. 2b. I–V characteristics of as-grown and hydrogenated ZnSe/Co thin films at different pressures.
1589
Fig. 3a. Wavelength versus absorption plots for ZnSe/Co BLs thin films.
attributed that absorption of annealed BLs thin films is lower than as-grown BLs thin films, indicating the mixing of BLs thin films due to the annealing. Due to annealing there is an increase in crystallinity or grain growth as confirmed by structural characteristics resulting reduction in absorption. Hydrogenated BLs thin films show higher absorption comparative to as-grown BLs thin films and this increases with increasing hydrogenation pressure. Therefore, it may be attributed that hydrogen has passivated defects at the surface or interface of BLs thin films resulting in an increase in absorption through hydrogenated BLs thin films. The nature of the transition can be investigated on the basis of the dependence of the absorption coefficient with the incident photon energy hυ . For direct and indirect allowed transitions, the theory of fundamental absorption leads to the following photon energy dependence near the absorption edge:
α hν
∝ (hν − Eg )m
where hυ and Eg are the photon and the band gap energy, respectively. In this relation, the values of m are 1/2 and 2 for direct allowed and indirect allowed transitions, respectively. The above relation is known as the Tauc relation [25]. The plot of (α hν)2 versus hν of as-grown, annealed and hydrogenated ZnSe/Co BLs thin films is shown in Fig. 3b. The optical band gap of annealed BLs thin film was found to be reduced due mixing of semiconductor and metal at the interface and increased in hydrogenated ZnSe/Co BLs thin films with increasing pressure of hydrogenation. The optical band gap of annealed BLs thin film was found to reduce from 3.21 to 3.20 eV due mixing of semiconductor and metal at the interface and increased from 3.35 to 3.50 eV in hydrogenated ZnTe/Co BLs thin films with increasing pressure (15 to 45 psi) of hydrogenation as shown in the inset of Fig. 3b. It is clearly seen that with the introduction of hydrogen, the band gap of the films is broadened dramatically from 3.34 to 3.38 eV with the increasing hydrogenation pressure 15 to 45 psi. The optical band gaps of asgrown and hydrogenated ZnTe/Co thin films were larger than that of the ZnTe thin films, which is due to a band-filling effect known as Burstein–Moss shift. Similar results are also observed by Tark et al. [26] and Hao et al. [27] in the case of effect of hydrogen in electrical and optical properties of Al-doped ZnO thin films. Similar results are observed in the case of CdTe/Mn BLs thin films [28]. 3.4. Magnetic properties The temperature dependence of magnetic moment is shown in Figs. 4a and 4b shows the magnetic field dependence of magnetic moment. Magnetic moment decreases with increasing temperature up to 100 K after this it becomes constant up to room temperature. ZnSe/Co BLs thin films show paramagnetic nature up to
Fig. 3b. Optical band gap of as-grown, annealed and hydrogenated ZnSe/Co BLs thin films at different pressures.
Fig. 4a. The temperature dependence on magnetization (in ZFC mode) under an applied field of 1000 Oe for as-grown and hydrogenated ZnSe/Co BLs thin films.
100 K. These ZnSe/Co BLs thin films behaves as DMS; that is they do not show any sign of blocking phenomena at very low temperatures, and their magnetization does not go to zero and remains constant in the temperature range of 100 to 300 K as suggested by Pakhomov et al. [18] in the case of magnetic study of transition from granular to diluted magnetic semiconductor multilayers in ion-deposited ZnO/Co. After hydrogenation these films’ moment was found to reduce with temperature as well as applied magnetic field. Similar results were also observed by Liu et al. [29].
1590
S.P. Nehra, M. Singh / Solid State Communications 150 (2010) 1587–1591
3.5. Raman spectra
Fig. 4b. M–H plots of (a) as-grown, (b) Hydrogenated at 15 psi and (c) Hydrogenated at 30 psi ZnSe/Co BLs thin films obtained at T = 2 K..
Hydrogen molecules at normal pressure are infrared (IR) inactive due to their lack of dipole moment, but they can be studied by Raman scattering. Molecular hydrogen was directly observed by Raman spectroscopy. Raman spectra of as-grown and hydrogenated ZnSe/Co thin films are shown in Fig. 5. Comparing the Raman spectra of as-grown and hydrogenated samples it may be concluded that due to hydrogenation the peak intensity and number of peaks increases clearly showing evidence of the hydrogenation process. Some new peaks were observed at 1656, 1920, 2800, 3551 and 3732 cm−1 . Due to hydrogenation the Raman peak intensity was found to be increased. Similar results are also observed by Nehra et al. [28] in the case of Raman spectroscopy of CdTe/Mn BLS thin films. This result indicates that the ZnSe/Co interface was improved due to hydrogenation. This improvement in heterointerface is due to the elimination of defects formed at the interface by hydrogenation. Similar results were observed by Kim [30] and Leitch et al. [31]. 3.6. Surface topography Surface topography of as-grown and annealed ZnSe/Co thin films is shown in Fig. 6a, indicating the uniform deposition and mixing of BLs respectively. Fig. 6b shows the surface topography of hydrogenated ZnSe/Co BLs thin films at different pressure, indicating that surface roughness increases with increasing pressure of hydrogenation. Similar results were also observed by Hao et al. [27] in the case of hydrogenated Al-doped ZnO semiconductor thin films. 4. Conclusion
Fig. 5. Room temperature Raman spectra of (a) as-grown and (b) hydrogenated ZnSe/Co BLs thin films.
They found that the saturation magnetization decreases after hydrogenation while the structural properties of the films do not change. It may be suggested that incorporation of hydrogen electrically passivates the Co acceptors and removes the hole essential to the roaming ferromagnetism. Thus hydrogenation allows us to control the ferromagnetic properties or the DMSs as suggested by Goennenwein et al. [19]. Hydrogen passivation is a good method for investigating the origin of ferromagnetism in DMS.
a
The study of structural and magnetic properties confirms the formation of ZnSe/Co bilayer DMS thin films. Electrical and optical properties show that resistivity and optical band gap increase with hydrogenation pressure due to hydrogen passivated defects. Raman spectra confirm the presence of hydrogen in these BLs thin films. Surface topography study of as-grown, annealed and hydrogenated ZnSe/Co BLs thin films indicate the uniform deposition, mixing of layers and increment in roughness at the surface due to hydrogen passivation effect respectively. Acknowledgements This work is financially supported by the University Grant Commission (UGC) major research project F. No. -33-4/2007 (SR), New Delhi. The authors are thankful to the Department of Physics, University of Rajasthan, Jaipur (India) for providing experimental facilities. The authors are also thankful to Mr. Neeraj Kumar, Tata Institute of Fundamental Research (TIFR) Mumbai for
b 100.0
100.0 75.0
75.0
50.0
50.0 25.0
25.0 Y[Pixels]
Y[Pixels]
0.0
0.0 –25.0
–25.0 –50.0
–50.0
10 µm
–75.0
Z[Arbitary] 10000.0 5000.0
–100.0 –100.0
0.0 X[Pixels]
100.0
10 µm
–75.0
Z[Arbitary] 15000.0 5000.0
–100.0 –100.0
0.0
100.0
X[Pixels]
Fig. 6a. Surface topography of (a) as grown and (b) vacuum annealed at 333 K ZnSe/Co BLs thin films.
S.P. Nehra, M. Singh / Solid State Communications 150 (2010) 1587–1591
a
1591
b 100.0 75.0
100.0
50.0 25.0 0.0
50.0 Y[Pixels] Y[Pixels]
–25.0
0.0
–50.0
10 µm
Z[Arbitary] 10000.0 5000.0
Z[Arbitary]
–50.0
–75.0
10 µm
–100.0
15000.0
–100.0 5000.0
–100.0
0.0
100.0
–100.0
0.0
100.0
X[Pixels]
c 100.0 50.0 Y[Pixels]
0.0
10 µm
–50.0
Z[Arbitary] 10000.0
–100.0 5000.0 –100.0
0.0
100.0
Fig. 6b. Surface topography of hydrogenated at pressure of hydrogen (a) 15 psi, (b) 30 psi and (c) 45 psi ZnSe/Co BLs thin films.
Magnetic measurements and M.P. Sharma, Department of Physics, University of Rajasthan, Jaipur for fruitful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15]
T. Adhikari, S. Basu, Japanese Journal of Applied Physics 33 (1994) 4581–4582. T. Fukurana, Applied Surface Science 223 (2004) 62–67. J.K. Furdyna, Journal of Applied Physics 64 (1988) R29. H. Ohno, Science 281 (1998) 951. K. Onodera, T. Masumoto, M. Kimura, Electronics Letters 30 (1994) 1954. M. Tanaka, Journal of Vacuum Science and Technology B16 (1998) 2267. S.K. Kamila, S. Basu, Bulletin Materials Science 25 (2002) 541–543. H. Ohno, Science 291 (2001) 840–841. A.F. OrlovO, L.A. Balagurov, A.S. Konstantinov, N.S. Perov, D.G. Yarkin, Journal of Magnetism and Magnetic Materials 320 (2008) 895–897. K. Sato, H.K. Yoshida, Semiconductor Science and Technology 17 (2002) 367–376. S.G. Anderson, F. Xu, M. Vos, J.H. Weaver, Physical Review B 39 (1989) 5079–5090. Zheng-Bin Gu, Chang-Sheng Yuan, Ming-Hui Lu, Jing Wang, Di Wu, ShanTao Zhang, Shi-Ning Zhu, Yong-Yuan Zhu, Yan-Feng Chen, Journal of Applied Physics 98 (2005) 053908-1-4. P. Yingzi, T. Liew, Y. Zhizhen, Z. Yinzhu, Chinese Science Bulletin 52 (2007) 2742–2746. K. Ueada, H. Tabata, T. Kawai, Applied Physics Letters 79 (2001) 988. Z. Jin, T. Fukumura, M. Kawasaki, K. Ando, H. Saito, T. Sekiguchi, Y.Z. Yoo, M. Murakami, Y. Matsumoto, T. Hasegawa, H. Koinuma, Applied Physics Letters 79 (2001) 3824.
[16] C. Sudakar, P. Kharel, G. Lawes, R. Suryanarayanan, R. Naik, V.M. Naik, Journal of Physics: Condensed Matter 19 (2007) 026212-1-9. [17] H.S. Hsu, J.C.A. Huang, S.F. Chen, C.P. Liu, Applied Physics Letters 90 (2007) 102506-1-3. [18] A.B. Pakhomov, B.K. Roberts, K.M. Krishnan, Applied Physics Letter 83 (2003) 4357–4359. [19] S.T.B. Goennenwein, T.A. Wassner, H. Huebl, M.S. Brandt, J.B. Philipp, M. Opel, R. Gross, A. Koeder, W. Schoch, A. Waag, Physical Review Letters 92 (2004) 227202-1-4. [20] H.J. Lee, C.H. Park, C.H. Jeong, K.J. Yee, C.R. Cho, Applied Physics Letters 88 (2006) 062504. [21] Y. Wang, L. Sun, L.G. Kong, J.F. Kang, X. Zhang, R.Q. Han, Journal of Alloys and Compounds (2006) 423 256. [22] S.Y. Park, S.W. Shin, P.J. Kim, J.-H. Kang, T.H. Kim, Y.P. Lee, Journal of Magnetism and Magnetic Materials 310 (2007) e708–e710. [23] S.P. Nehra, M.K. Jangid, Subodh Srivastava, Anil Kumar, Balram Tripathi, M. Singh, Y.K. Vijay, International Journal of Hydrogen Energy 34 (2009) 7306–7310. [24] J.I. Pankov, D.E. Carlson, J.E. Berkeyheiser, O. Wance, Physical Review Letters 51 (1983) 2224–2225. [25] J. Tauc, Plenum Press, New York, 1974 pp. 159. [26] S.J. Tark, Y.W. Ok, M.G. Kang, H.J. Lim, W.M. Kim, D. Kim, Journal of Electroceramics 23 (2009) 548–553. [27] X.T. Hao, F.R. Zhu, K.S. Ong, L.W. Tan, Semiconductor Science and Technology 21 (2006) 48–54. [28] S.P. Nehra, M. Singh, Journal of Alloys and Compounds 488 (2009) 356–359. [29] X.C. Liu, Y.B. Lin, J.F. Wang, Z.H. Lu, Z.L. Lu, J.P. Xu, L.Y. Lv, F.M. Zhang, Y.W. Du, Journal of Applied Physics 102 (2007) 033902-4. [30] M.D. Kim, H.S. Park, Journal of Applied Physics 84 (1998) 3125–3128. [31] A.W.R. Leitch, V. Alex, J. Weber, Physical Review Letters 81 (1998) 421–424.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Hydrogenation effect on electrical, optical and magnetic properties of ZnSe/Co bilayer DMS thin films S.P. Nehra ∗ , M. Singh Thin Films and Membrane Science Laboratory, Department of Physics, University of Rajasthan, Jaipur-302004, India
article
info
Article history: Received 18 May 2010 Accepted 23 May 2010 by A.H. MacDonald Available online 1 June 2010 Keywords: A. DMS thin films B. Hydrogenation C. I–V characteristics D. Uv–Vis spectroscopy
abstract This paper reports ZnSe/Co bilayer diluted magnetic semiconductor thin films have been prepared by using thermal evaporation technique. The bilayer DMS thin films were hydrogenated at different pressures (15–45 psi) for a constant time of 30 min. Before and after hydrogenations of these bilayer thin films the electrical, optical and magnetic properties have been investigated. Electrical resistivity and optical band gap were found to be increased with respect to hydrogenation pressure. X-ray diffraction (XRD) and magnetic measurements confirmed the formation of DMS ZnSe/Co bilayer DMS thin films. Raman spectra show the presence of hydrogen in these thin films. Surface topography study of as-grown, annealed and hydrogenated ZnSe/Co bilayer thin films indicates uniform deposition, mixing of layers and increment in roughness at the surface due to hydrogen passivation effect respectively. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Diluted magnetic semiconductors are compound of alloy semiconductors containing a large fraction of magnetic ions [1]. They have unusual magnetic characteristics due to the presence of isolated magnetic ions in the semiconducting lattice. The diluted magnetic semiconductor (DMS) is expected to play an important role in interdisciplinary materials science and future spintronics because charge and spin degrees of freedom are accommodated into single matter and their interplay is expected to explore novel physics and new devices [2–5]. Diluted magnetic semiconductors (DMS) are compound of alloy semiconductors containing a large fraction of magnetic ions (Mn+2 , Cr+2 , Fe+2 , Co+2 ) and are studied mainly on II–VI based materials such as CdTe and ZnSe etc. This is because such +2 magnetic ions are easily incorporated into the host II–VI crystals by replacing group II cations. In II–VI based DMS such as (CdMn) Se, magneto-optic properties have been extensively studied, and optical isolators were recently fabricated using their large Faraday effect [6,7]. Since the discovery of ferromagnetism in diluted magnetic semiconductors (DMS) such as Mn-doped InAs or GaAs, it has been intensively studied in order to fabricate a new functional semiconductor taking advantage of the spin degree of freedom in DMS. It is expected to establish new semiconductor spin electronics (spintronics) as a practical technology based on such new functional materials [8,9].
∗
Corresponding author. Tel.: +91 141 2702457; fax: +91 141 2711049. E-mail addresses: [email protected] (S.P. Nehra), [email protected] (M. Singh). 0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.05.038
The origin of ferromagnetism in the DMS is discussed by many researchers [10–17]. Pakhomov et al. prepared (Zn,Co)O:Al DMS superlattices, with the Curie temperature higher than room temperature, by alternating deposition of ZnO and Co as multilayers on the atomic scale. Over-doping with cobalt leads to a suppression of high-temperature ferromagnetism. At the same time, Co granules are formed at decreasing thickness of ZnO layers, leading both to superparamagnetism and a crossover of conduction to hopping between metal granules [18]. Hydrogen passivation is a good method for investigating the origin of ferromagnetism in DMS. Hydrogenation gave strong evidence which proved the mechanism of ferromagnetism [19–22]. Hydrogenation is a mature process in modern electronics to change the electrical properties of silicon films. It would also be useful for investigating the relation between ferromagnetism and carriers in the Si- based DMS. Preparations of DMSs are mainly done by (i) thin film growth e.g. (a) molecular beam epitaxy (MBE), (b) pulse laser depositions (PLD) and (c) chemical vapour depositions (CVD) and (ii) bulk crystal growth e.g. Bridgman method. In this article, we present the effect of hydrogen on the electrical, optical and magnetic properties of ZnSe/Co BLs DMS thin films, successfully deposited using thermal evaporation technique. 2. Experimental details Bilayer ZnSe/Co DMS thin films were deposited on glass substrates by thermal evaporation technique at a base pressure of 10−5 Torr. Compound zinc selenide powder (99.99% purity) and elemental Co powder (99.98% purity) procured from Alfa
S.P. Nehra, M. Singh / Solid State Communications 150 (2010) 1587–1591
3. Results and discussions 3.1. Structural characteristics The structural characteristics of as-grown and vacuum annealed at 333 K ZnSe/Co BLs diluted magnetic semiconductor thin films are shown in Fig. 1. The as-grown ZnSe/Co BLs thin film shows an amorphous nature while the vacuum annealed film shows a polycrystalline nature. There were small peaks at 27.27°, 45.10° and 60.50°, 51.44° observed corresponding to (111), (220) and (111), (200) planes of cubic ZnSe and Co respectively. Due to the
360
ZnSe 111
300
200 Co
ZnSe
320
111 Co
b
340
220
Aesar, Johnson Matthey Company, USA were used for deposition. The source to substrate distance was kept at 15 cm in all cases. The detailed conditions for thermal evaporation technique are described in Ref. [23]. The evaporation was done in maintained vacuum by slowly increasing the current to heat the tantalum boat until evaporation was reached. Deposition of BLs ZnSe/Co thin films have been performed by the stacked layer method. We have deposited ZnSe and Co respectively to obtain ZnSe/Co BLs thin film structure. The thickness of ZnSe/Co BLs DMS thin films was 250 nm (150 nm ZnSe/ 100 nm Co) measured by quartz crystal thickness monitor. BLs ZnSe/Co DMS thin films were vacuum annealed at a constant temperature of 60 °C for an hour in a vacuum of 10−5 Torr by using a vacuum coating unit for mixing to get the homogeneous structure and interdiffusion of bilayer thin films of ZnSe/Co. Hydrogenation of ZnSe/Co bilayer thin films was performed by keeping these samples in a hydrogenation cell, where hydrogen gas was introduced at different pressures for 30 min. Before hydrogen gas was introduced in the cell, it was connected to the vacuum coating unit to produce a vacuum of 10−3 Torr. This process was performed at a constant temperature of 300 K. The X-ray diffraction (XRD) patterns of as-grown and vacuum annealed DMS thin films were recorded with the help of PANalytical X’pert PRO MPD PW3040/60 X-ray diffractometer using Cu Kα radiation, as a radiation source of wavelength λ = 1.540598 Å. The tube was operated at 45 KV, 40 mA with a scanning speed of 0.090(2θ )/s. The XRD patterns of all films were taken from 12° to 80° (2θ ). The peaks of the XRD patterns were searched by computer programming using Powder X Software. Transverse I–V characteristics of as-grown hydrogenated and annealed hydrogenated samples were recorded using a Keithley238 high current source measuring unit. The applied voltage was within the range of −2.0 to +2.0 V with increasing step of 0.1 V. For I–V characteristics, electrode contacts were made using silver (Ag) paste on the thin films. I–V characteristics of bilayer thin films were monitored with the help of SMUSweep computer software. All the measurements were performed at room temperature. Magnetization measurements with temperature and magnetic field were performed using a SQUID (Quantum Design physical properties measurement system). For M–T curves the applied field was fixed at 1000 Oe. Magnetic moment dependence on the applied field was measured at 2 K. The transmission spectra of as deposited and annealed hydrogenated thin films were carried out in the wavelength range 250–800 nm with the help of a Spectrophotometer U-3300. The Raman spectra of as-grown and hydrogenated samples were recorded using a green laser beam of wavelength 532 nm. (Raman model −3000 system). All the measurements were performed at room temperature. The optical micrographs were observed with the help of a Labomed optical microscope at 10× magnification having resolution of the order of 1 µm and the microscope was kept in reflection mode. The micrographs were stored in computer through standard software (Pixel View). Recorded two-dimensional (2D) optical micrographs were converted into three-dimensional (3D) images with the help of the Scanning Probe Image Processor computer program.
Intensity (arb.)
1588
280 260 240
a
(a) As-grown ZnSe/Co BLs thin film. (b) Vacuum annealed at 333 K.
220 200 180 160 140 120 20
30
40
50
60
70
80
2θ
Fig. 1. XRD patterns for (a) as-grown and (b) vacuum annealed ZnSe/Co BLS thin films.
Fig. 2a. I–V characteristics of (a) as-grown and (b) annealed at 333 K ZnSe/Co BLs thin films.
annealing, crystallinity is found to be increased indicating grain growth. Co peaks are clear evidence of the presence of magnetic material in the thin films, which is also confirmed by magnetic measurements. 3.2. I–V characteristics As-grown and annealed at 333 K ZnSe/Co BLs thin films show a partially semiconducting nature as shown in Fig. 2a. Current–voltage characteristics indicate the mixing of BLs thin films due to annealing because current was found to be reduced in the case of annealed BLs thin film. Because the outermost layer was elemental Co and the contacts were made on top of the film therefore current was found to be increased. In the case of the annealed sample, due to interdiffusion of ZnSe and Co, current was found to be reduced. Similar results were carried out by Nehra et al. [23] in the case of I–V characteristics study of as-grown and annealed CdTe/Mn BLs thin films. Current–voltage characteristics of as-grown and hydrogenated ZnSe/Co BLs thin films are shown in Fig. 2b. In the case of hydrogenated samples current was found to be reduced with increasing hydrogen pressure. This may be attributed to hydrogen controlling the flow of charges by taking electrons from Co and hydrogen has passivated defects at the interface. The electronic passivation of host impurities induced by hydrogen in a semiconductor well agrees as reported by Pankove et al. [24]. 3.3. Optical properties Optical absorption spectra of as-grown, annealed and hydrogenated ZnSe/Co BLs thin films are shown in Fig. 3a. It may be
S.P. Nehra, M. Singh / Solid State Communications 150 (2010) 1587–1591
Fig. 2b. I–V characteristics of as-grown and hydrogenated ZnSe/Co thin films at different pressures.
1589
Fig. 3a. Wavelength versus absorption plots for ZnSe/Co BLs thin films.
attributed that absorption of annealed BLs thin films is lower than as-grown BLs thin films, indicating the mixing of BLs thin films due to the annealing. Due to annealing there is an increase in crystallinity or grain growth as confirmed by structural characteristics resulting reduction in absorption. Hydrogenated BLs thin films show higher absorption comparative to as-grown BLs thin films and this increases with increasing hydrogenation pressure. Therefore, it may be attributed that hydrogen has passivated defects at the surface or interface of BLs thin films resulting in an increase in absorption through hydrogenated BLs thin films. The nature of the transition can be investigated on the basis of the dependence of the absorption coefficient with the incident photon energy hυ . For direct and indirect allowed transitions, the theory of fundamental absorption leads to the following photon energy dependence near the absorption edge:
α hν
∝ (hν − Eg )m
where hυ and Eg are the photon and the band gap energy, respectively. In this relation, the values of m are 1/2 and 2 for direct allowed and indirect allowed transitions, respectively. The above relation is known as the Tauc relation [25]. The plot of (α hν)2 versus hν of as-grown, annealed and hydrogenated ZnSe/Co BLs thin films is shown in Fig. 3b. The optical band gap of annealed BLs thin film was found to be reduced due mixing of semiconductor and metal at the interface and increased in hydrogenated ZnSe/Co BLs thin films with increasing pressure of hydrogenation. The optical band gap of annealed BLs thin film was found to reduce from 3.21 to 3.20 eV due mixing of semiconductor and metal at the interface and increased from 3.35 to 3.50 eV in hydrogenated ZnTe/Co BLs thin films with increasing pressure (15 to 45 psi) of hydrogenation as shown in the inset of Fig. 3b. It is clearly seen that with the introduction of hydrogen, the band gap of the films is broadened dramatically from 3.34 to 3.38 eV with the increasing hydrogenation pressure 15 to 45 psi. The optical band gaps of asgrown and hydrogenated ZnTe/Co thin films were larger than that of the ZnTe thin films, which is due to a band-filling effect known as Burstein–Moss shift. Similar results are also observed by Tark et al. [26] and Hao et al. [27] in the case of effect of hydrogen in electrical and optical properties of Al-doped ZnO thin films. Similar results are observed in the case of CdTe/Mn BLs thin films [28]. 3.4. Magnetic properties The temperature dependence of magnetic moment is shown in Figs. 4a and 4b shows the magnetic field dependence of magnetic moment. Magnetic moment decreases with increasing temperature up to 100 K after this it becomes constant up to room temperature. ZnSe/Co BLs thin films show paramagnetic nature up to
Fig. 3b. Optical band gap of as-grown, annealed and hydrogenated ZnSe/Co BLs thin films at different pressures.
Fig. 4a. The temperature dependence on magnetization (in ZFC mode) under an applied field of 1000 Oe for as-grown and hydrogenated ZnSe/Co BLs thin films.
100 K. These ZnSe/Co BLs thin films behaves as DMS; that is they do not show any sign of blocking phenomena at very low temperatures, and their magnetization does not go to zero and remains constant in the temperature range of 100 to 300 K as suggested by Pakhomov et al. [18] in the case of magnetic study of transition from granular to diluted magnetic semiconductor multilayers in ion-deposited ZnO/Co. After hydrogenation these films’ moment was found to reduce with temperature as well as applied magnetic field. Similar results were also observed by Liu et al. [29].
1590
S.P. Nehra, M. Singh / Solid State Communications 150 (2010) 1587–1591
3.5. Raman spectra
Fig. 4b. M–H plots of (a) as-grown, (b) Hydrogenated at 15 psi and (c) Hydrogenated at 30 psi ZnSe/Co BLs thin films obtained at T = 2 K..
Hydrogen molecules at normal pressure are infrared (IR) inactive due to their lack of dipole moment, but they can be studied by Raman scattering. Molecular hydrogen was directly observed by Raman spectroscopy. Raman spectra of as-grown and hydrogenated ZnSe/Co thin films are shown in Fig. 5. Comparing the Raman spectra of as-grown and hydrogenated samples it may be concluded that due to hydrogenation the peak intensity and number of peaks increases clearly showing evidence of the hydrogenation process. Some new peaks were observed at 1656, 1920, 2800, 3551 and 3732 cm−1 . Due to hydrogenation the Raman peak intensity was found to be increased. Similar results are also observed by Nehra et al. [28] in the case of Raman spectroscopy of CdTe/Mn BLS thin films. This result indicates that the ZnSe/Co interface was improved due to hydrogenation. This improvement in heterointerface is due to the elimination of defects formed at the interface by hydrogenation. Similar results were observed by Kim [30] and Leitch et al. [31]. 3.6. Surface topography Surface topography of as-grown and annealed ZnSe/Co thin films is shown in Fig. 6a, indicating the uniform deposition and mixing of BLs respectively. Fig. 6b shows the surface topography of hydrogenated ZnSe/Co BLs thin films at different pressure, indicating that surface roughness increases with increasing pressure of hydrogenation. Similar results were also observed by Hao et al. [27] in the case of hydrogenated Al-doped ZnO semiconductor thin films. 4. Conclusion
Fig. 5. Room temperature Raman spectra of (a) as-grown and (b) hydrogenated ZnSe/Co BLs thin films.
They found that the saturation magnetization decreases after hydrogenation while the structural properties of the films do not change. It may be suggested that incorporation of hydrogen electrically passivates the Co acceptors and removes the hole essential to the roaming ferromagnetism. Thus hydrogenation allows us to control the ferromagnetic properties or the DMSs as suggested by Goennenwein et al. [19]. Hydrogen passivation is a good method for investigating the origin of ferromagnetism in DMS.
a
The study of structural and magnetic properties confirms the formation of ZnSe/Co bilayer DMS thin films. Electrical and optical properties show that resistivity and optical band gap increase with hydrogenation pressure due to hydrogen passivated defects. Raman spectra confirm the presence of hydrogen in these BLs thin films. Surface topography study of as-grown, annealed and hydrogenated ZnSe/Co BLs thin films indicate the uniform deposition, mixing of layers and increment in roughness at the surface due to hydrogen passivation effect respectively. Acknowledgements This work is financially supported by the University Grant Commission (UGC) major research project F. No. -33-4/2007 (SR), New Delhi. The authors are thankful to the Department of Physics, University of Rajasthan, Jaipur (India) for providing experimental facilities. The authors are also thankful to Mr. Neeraj Kumar, Tata Institute of Fundamental Research (TIFR) Mumbai for
b 100.0
100.0 75.0
75.0
50.0
50.0 25.0
25.0 Y[Pixels]
Y[Pixels]
0.0
0.0 –25.0
–25.0 –50.0
–50.0
10 µm
–75.0
Z[Arbitary] 10000.0 5000.0
–100.0 –100.0
0.0 X[Pixels]
100.0
10 µm
–75.0
Z[Arbitary] 15000.0 5000.0
–100.0 –100.0
0.0
100.0
X[Pixels]
Fig. 6a. Surface topography of (a) as grown and (b) vacuum annealed at 333 K ZnSe/Co BLs thin films.
S.P. Nehra, M. Singh / Solid State Communications 150 (2010) 1587–1591
a
1591
b 100.0 75.0
100.0
50.0 25.0 0.0
50.0 Y[Pixels] Y[Pixels]
–25.0
0.0
–50.0
10 µm
Z[Arbitary] 10000.0 5000.0
Z[Arbitary]
–50.0
–75.0
10 µm
–100.0
15000.0
–100.0 5000.0
–100.0
0.0
100.0
–100.0
0.0
100.0
X[Pixels]
c 100.0 50.0 Y[Pixels]
0.0
10 µm
–50.0
Z[Arbitary] 10000.0
–100.0 5000.0 –100.0
0.0
100.0
Fig. 6b. Surface topography of hydrogenated at pressure of hydrogen (a) 15 psi, (b) 30 psi and (c) 45 psi ZnSe/Co BLs thin films.
Magnetic measurements and M.P. Sharma, Department of Physics, University of Rajasthan, Jaipur for fruitful discussions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15]
T. Adhikari, S. Basu, Japanese Journal of Applied Physics 33 (1994) 4581–4582. T. Fukurana, Applied Surface Science 223 (2004) 62–67. J.K. Furdyna, Journal of Applied Physics 64 (1988) R29. H. Ohno, Science 281 (1998) 951. K. Onodera, T. Masumoto, M. Kimura, Electronics Letters 30 (1994) 1954. M. Tanaka, Journal of Vacuum Science and Technology B16 (1998) 2267. S.K. Kamila, S. Basu, Bulletin Materials Science 25 (2002) 541–543. H. Ohno, Science 291 (2001) 840–841. A.F. OrlovO, L.A. Balagurov, A.S. Konstantinov, N.S. Perov, D.G. Yarkin, Journal of Magnetism and Magnetic Materials 320 (2008) 895–897. K. Sato, H.K. Yoshida, Semiconductor Science and Technology 17 (2002) 367–376. S.G. Anderson, F. Xu, M. Vos, J.H. Weaver, Physical Review B 39 (1989) 5079–5090. Zheng-Bin Gu, Chang-Sheng Yuan, Ming-Hui Lu, Jing Wang, Di Wu, ShanTao Zhang, Shi-Ning Zhu, Yong-Yuan Zhu, Yan-Feng Chen, Journal of Applied Physics 98 (2005) 053908-1-4. P. Yingzi, T. Liew, Y. Zhizhen, Z. Yinzhu, Chinese Science Bulletin 52 (2007) 2742–2746. K. Ueada, H. Tabata, T. Kawai, Applied Physics Letters 79 (2001) 988. Z. Jin, T. Fukumura, M. Kawasaki, K. Ando, H. Saito, T. Sekiguchi, Y.Z. Yoo, M. Murakami, Y. Matsumoto, T. Hasegawa, H. Koinuma, Applied Physics Letters 79 (2001) 3824.
[16] C. Sudakar, P. Kharel, G. Lawes, R. Suryanarayanan, R. Naik, V.M. Naik, Journal of Physics: Condensed Matter 19 (2007) 026212-1-9. [17] H.S. Hsu, J.C.A. Huang, S.F. Chen, C.P. Liu, Applied Physics Letters 90 (2007) 102506-1-3. [18] A.B. Pakhomov, B.K. Roberts, K.M. Krishnan, Applied Physics Letter 83 (2003) 4357–4359. [19] S.T.B. Goennenwein, T.A. Wassner, H. Huebl, M.S. Brandt, J.B. Philipp, M. Opel, R. Gross, A. Koeder, W. Schoch, A. Waag, Physical Review Letters 92 (2004) 227202-1-4. [20] H.J. Lee, C.H. Park, C.H. Jeong, K.J. Yee, C.R. Cho, Applied Physics Letters 88 (2006) 062504. [21] Y. Wang, L. Sun, L.G. Kong, J.F. Kang, X. Zhang, R.Q. Han, Journal of Alloys and Compounds (2006) 423 256. [22] S.Y. Park, S.W. Shin, P.J. Kim, J.-H. Kang, T.H. Kim, Y.P. Lee, Journal of Magnetism and Magnetic Materials 310 (2007) e708–e710. [23] S.P. Nehra, M.K. Jangid, Subodh Srivastava, Anil Kumar, Balram Tripathi, M. Singh, Y.K. Vijay, International Journal of Hydrogen Energy 34 (2009) 7306–7310. [24] J.I. Pankov, D.E. Carlson, J.E. Berkeyheiser, O. Wance, Physical Review Letters 51 (1983) 2224–2225. [25] J. Tauc, Plenum Press, New York, 1974 pp. 159. [26] S.J. Tark, Y.W. Ok, M.G. Kang, H.J. Lim, W.M. Kim, D. Kim, Journal of Electroceramics 23 (2009) 548–553. [27] X.T. Hao, F.R. Zhu, K.S. Ong, L.W. Tan, Semiconductor Science and Technology 21 (2006) 48–54. [28] S.P. Nehra, M. Singh, Journal of Alloys and Compounds 488 (2009) 356–359. [29] X.C. Liu, Y.B. Lin, J.F. Wang, Z.H. Lu, Z.L. Lu, J.P. Xu, L.Y. Lv, F.M. Zhang, Y.W. Du, Journal of Applied Physics 102 (2007) 033902-4. [30] M.D. Kim, H.S. Park, Journal of Applied Physics 84 (1998) 3125–3128. [31] A.W.R. Leitch, V. Alex, J. Weber, Physical Review Letters 81 (1998) 421–424.