Structural, morphological and electronic properties of pulsed laser grown Eu2O3 thin films Sandeep Kumar, Ram Prakash, R. J. Choudhary, and D. M. Phase
Citation: AIP Conference Proceedings 1953, 100012 (2018); doi: 10.1063/1.5032948 View online: https://doi.org/10.1063/1.5032948 View Table of Contents: http://aip.scitation.org/toc/apc/1953/1 Published by the American Institute of Physics
Structural, Morphological And Electronic Properties Of Pulsed Laser Grown Eu2O3 Thin Films Sandeep Kumar1, a), Ram Prakash1, b), R. J. Choudhary2 and D. M. Phase2 1
Department of Physics, Shri Mata Vaishno Devi University, Katra-182320 (J&K) India 2 UGC DAE Consortium for Scientific Research, Indore-452001, India a)
Corresponding author:
[email protected] b)
[email protected];
[email protected]
Abstract. Herein, we report the growth, structural, morphological and electronic properties of Europium sesquioxide (Eu2O3) thin films on Si [1 0 0] substrate using pulsed laser deposition technique. The films were deposited at ~750 °C substrate temperature while the oxygen partial pressure (OPP) was varied (vacuum, ~ 1 mTorr, ~ 10 mTorr and ~ 300 mTorr). X-ray diffraction results confirm the single phase cubic structure of the film grown at ~300 mTorr. The XRD results are also supported by the Raman’s spectroscop y results. Eu-3d XPS core level spectra confirms the dominant contributions from the “3+” states of Eu in the film. Keywords: PLD; X-ray diffraction, XPS.
INTRODUCTION Rare earth oxides (REO’s) are worldwide focused because they have been extensively used as high performance magnets, catalysts, luminescent devices, and labels in biology based on their optical, electronic, and chemical characteristics. Amongst these compounds, europium sesquioxide (Eu2O3) thin films have potential applications in numerous modern devices, such as microelectronics, telecommunications, optoelectronic and optical devices [1-4]. Europium exists in three valence states in europium oxide i.e. europium monoxide (EuO, rock salt structure with space group ), Eu2O3 (monoclinic or cubic structure, space groups C2/m or , respectively) and in Eu3O4 (orthorhombic structure, space group Pnma) [5]. Eu3O4 being a mixed valence state. Most of the above properties depend upon the valence states of Eu such as valence states of Eu behave differently in luminescence processes. The Eu (3+) states shows strong emission band in the red region of the visible spectrum whereas Eu with (2+) states exhibits strong UV broad band luminescence [6]. Thus it is very important to grow mono phasic films to obtained good results. In this context we have grown the Eu2O3 films on Si [100] substrates. The reason behind choosing Si as substrate for deposition is due to the dependence of present semiconductor technology on Si. Thus, the compatibility of Eu2O3 thin films on Si substrate is highly desirable for device applications viewpoint.
EXPERIMENTAL Sample Preparation And Characterization Thin films of undoped Eu2O3 were grown on chemically cleaned (5 min each in acetone and methanol) silicon [100] substrate by pulsed laser deposition (PLD) technique using KrF (λ = 248 nm) excimer laser source. The pellet used for target was kept in furnace at 900 ºC for 12 hours for sintering. Before the start of deposition, a base vacuum ~10-6 Torr was achieved. During deposition, substrate temperature was fixed at ~750 ºC and oxygen partial pressure (OPP) was varied (vacuum, ~ 1 mTorr, ~ 10 mTorr and ~ 300 mTorr), while laser energy density was kept at 2 J/cm2. The laser pulse rate was kept at 10 Hz. The substrate to target distance was fixed at 5 cm. The deposition was
2nd International Conference on Condensed Matter and Applied Physics (ICC 2017) AIP Conf. Proc. 1953, 100012-1–100012-4; https://doi.org/10.1063/1.5032948 Published by AIP Publishing. 978-0-7354-1648-2/$30.00
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done for 20 minutes. After the deposition, the film was cooled to room temperature in same OPP as used during the deposition. Thickness of films was measured using Stylus Profilometer and found to be in the range of ~150 nm. The films were characterized using various techniques. The X-ray diffraction measurement of the films were examined using a standard diffractometer (Bruker D8 Advance) in the θ–2θ geometry with Cu Kα radiation (λ=0.15414 nm). The room temperature Raman spectrum were collected using a LabRam HR–800 micro-Raman spectrometer equipped with an optical microscope, CCD detector, and 488 nm excitation laser source. Digital Instrument Nanoscope III with Si3N4 was used for studying the surface morphology of the films. The X-ray photoelectron spectroscopy (XPS) measurement were performed using Omicron energy analyzer (EA–125) with Al Kα (1486.6 eV) X-ray source. The background vacuum in the analyzer chamber was of the order of 10-10 Torr during the XPS measurement.
RESULTS AND DISCUSSION X-ray Diffraction and Raman’s Spectroscopy The phase confirmation of Eu2O3 thin films were investigated by the X-ray diffraction (XRD) technique. Figure 1 show the XRD patterns of Eu2O3 thin films grown at different oxygen partial pressure (OPP) i.e. vacuum, 1, 10 and 300 mTorr. The diffraction pattern of the Si [100] substrate is also plotted along with that of the film XRD patterns, in order to distinguish between the film and substrate XRD peaks. In Figure 1 the film peaks are marked as ‘F’ while substrate peaks are marked as ‘S’. These patterns are plotted in log scale so that these patterns would clearly indicate the presence of any type of impurity phase or co-occurrence of other europium oxide phases (EuO or Eu3O4). The observed XRD peaks of Eu2O3 film are indexed with the cubic phase of Eu 2O3 (JCPDS No. 43–1008).
FIGURE 1. XRD patterns of Eu2O3 thin films on Si [100] substrate deposited at various oxygen partial pressures (vacuum, 1, 10 and 300 mTorr).
FIGURE 2. Raman spectra of Eu2O3 thin film grown at different OPP. In both figures (1 and 2) ‘F’ corresponds to the film peaks while ‘S’ stands for Si substrate peaks.
The grain size (D) of the film is calculated using Debye-Scherrer formula [7].
where λ is the wavelength of X-ray source used, β (in radians) is the full width at half maxima (FWHM) of an individual peak at 2θ (where θ is the Bragg angle). The lattice strain (S) in the film also causes broadening of diffraction peak, which is represented by the relationship [8]. Lattice parameter, grain size and strain of the films were calculated using the (2 2 2) Bragg’s peak and the results are shown in Table 1. In the present study, it is observed that the films grown at vacuum, 1 mT and 10 mT OPP have some other peaks marked with “*” symbol besides film peaks. These peaks correspond to the other phases of europium oxides. Whereas an (1 1 1) oriented growth is observed for the film grown at 300 mT OPP, suggesting
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that this OPP condition is best for the growth of Eu2O3 on Si [1 0 0] substrate. Moreover, the lattice parameter of the film grown at 300 mT OPP, are more in consistent with the powder XRD card for Eu 2O3 (JCPDS file 43–1008, a = 10.86 Å). Thus XRD patterns confirm the single phase cubic structure of the Eu2O3 thin film grown at 300 mTorr. With increasing OPP a decrement in the grain size and increment in the strain is observed. TABLE 1. Lattice parameter, grain size and strain of the Eu2O3 films grown at different oxygen partial pressure OPP
Lattice Parameter (Å)
Grain Size (nm)
Strain
Vac 1 mT 10 mT 300 mT
10.990 10.878 10.867 10.855
47 45 42 37
0.00136 0.00141 0.00148 0.00168
For further insight into the nature of the pulsed laser deposited Eu2O3 films and their phase purity, Raman’s spectroscopic investigations were carried out at room temperature. Figure 2 (a)-(d) shows the room temperature Raman’s spectra of Eu2O3 thin films grown at altered OPP i.e. vacuum, 1, 10 and 300 mTorr in the spectral range of 100 to 500 cm−1. From group theory, a C-type cubic Eu2O3 is expected to have twenty two (4Ag, 4Eg, and 14Fg) Raman active modes with most prominent mode at ~340 cm−1 [9, 10]. The prominent peak at around ~340 cm−1 is commonly assigned to the symmetric Ag and degenerated Fg modes. From Figure 2(a) and 2(b) it is observed that this prominent mode is absent in the films grown at vacuum and 1 mTorr OPP. However, some Raman active modes in the lower spectral range are observed for these films. In Figure 2 (c) the modes are observed at 107, 118, 130, 142, 341 and 422 cm−1. Beside these modes, a feebly intense mode close to 390 cm−1 is also observed for the film grown at 10 mTorr. From the literature it is concluded that this mode belongs to the some other Eu based compounds [11, 12]. From Figure 2(d) the six Raman modes are observed at 108, 119, 132, 142, 341 and 428 cm−1 for the film grown at 300 mTorr. These modes correspond to the cubic phase of Eu 2O3 and also Raman vibration modes at ~390 cm−1 is not observed that is usually found in the mixed phase suggesting that the Eu2O3 thin film grown at 300 mTorr is of high purity. Thus both XRD and Raman’s spectroscopic studies confirm that the film grown at 300 mTorr belongs to the cubic phase of Eu2O3. Taking all these points in consideration, this best grown film is chosen for further characterizations. The surface morphology of the optimized film is examined by the atomic force microscopy (AFM). Figure 3 (a) and (b) shows the AFM micrographs of the film in 2D and 3D. From these micrographs, it is understood that film is highly dense with smooth and uniform surface grains. A spherical type shape is observed from the micrographs. Moreover it is also witnessed that film is free from voids and defects. The roughness of the film was estimated using the WSxM software and is found to be 4.1 nm [13].
a
b
FIGURE 3. AFM image of undoped Eu2O3 thin film (a) deposited at 300 mTorr OPP in 2D.
The electronic properties of the film were studied using the x-ray photoelectron spectroscopy. The XPS survey scan (not shown here) of the film neglects the presence of any type of impurity elements in the film. To confirm the valence state of Eu in the film the Eu–3d core level XPS measurement have been performed and the spectrum is depicted in Figure 4. The spectrum shows two main features at ~1133.7 eV (Eu–3d5/2) and ~1163.6 eV (Eu–3d3/2). These prominent features correspond to the 3+ states of Eu in the film while features belonging to the 2+ states (feebly intense) are observed at binding energy of ~1124.5 and ~1154.5 eV [14]. The prominent peaks are
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labeled as 3+ due to the trivalent 3d4f6 configurations (from bulk) while the less intense peaks appearing at lower binding energy than the main photoelectron peak as 2+, due to the divalent 3d4f7 configuration (from surface). Besides the 3+ and 2+ features of Eu, a photoemission peak is observed at ~1142.25 eV that is the satellite feature of Eu 3+ states [14].
FIGURE. 4. Eu-3d XPS core level spectrum of the Eu2O3 thin film.
CONCLUSION In conclusion, we have deposited Eu2O3 thin films at different OPP on Si [100] substrate by pulsed laser deposition technique. Structural properties of the films are studied by XRD and Raman’s spectroscopic measurements. XRD results confirms a highly (111) oriented growth of thin film deposited at 300 mTorr OPP and belongs to the cubic phase of Eu2O3 while the films grown at other OPP shows some impurity peaks related to the secondary phases of europium oxide. Raman’s results are also in good agreement with the XRD results. AFM micrographs (2D and 3D) results show a highly dense, smooth and uniform surface for the optimized film. Eu–4d XPS core level spectra shows the dominant signatures of 3+ states of Eu in the film with a very minor contribution from 2+ states at the surface.
ACKNOWLEDGMENTS SK heartfully thanks UGC-DAE CSR, Indore, India for fellowship under CRS project (No. CSR-IC-BL-12/CRS-1092014-15/1205).
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
T. T. Yan, D. S. Zhang, L. Y. Shi, and H. R. Li, J. Alloys Compd. 487, 483–488 (2009). M. J. Bierman, K. M. Van Heuvelen, D. SchmeiBer, T. C. Brunold, and S. Jin, Adv. Mater. 19, 2677–2681 (2007). J. Feng, G. M. Shan, A. Maquieira, M. E. Koivunen, B. Guo, B. D. Hammock, and I. M. Kennedy, Anal. Chem. 75, 5282– 5286 (2003). S. Kumar, R. Prakash, and V. K. Singh,, Rev. Adv. Sci. Eng. 4, 247–257 (2015). S. Kumar, R. Prakash, R.J. Choudhary, D.M. Phase, J. Appl. Phys. 120, 125309 (2016). N. Rakov and G. S. Maciel, J. Lumin. 127, 703–706 (2007). B. D. Cullity, Elements of X-Ray Diffraction (Addison-Wesley, Reading, MA, 1972). S. Tiwari, R. J. Choudhary, R. Prakash, and D. M. Phase, J. Phys.: Condens. Matter 19, 176002 (2007). W. B. White and V. G. Keramidas, Spectrochimica Acta Part A: Molecular Spectroscopy 28(3), 501–509 (1972). N. Dilawar, D. Varandani, V.P. Pandey, M. Kumar, S.M. Shivaprasad, P.K. Sharma, A.K. Bandyopadhyay, J. Nanosci. Nanotechnol. 6, 105–113 (2006). M. Yang, D. Huang, P. Hao, F. Zhang, X. Hou and X. Wang, J. Appl. Phys. 75(1), 651–653 (1994). J. G. Kang, Y. Jung, B. K. Min and Y. Sohn, Appl. Surf. Sci. 314, 158–165 (2014). I. Horcas, R. Fernández, J.M. Gomez-Rodriguez, J. Colchero, J. W. S. X. M. Gómez-Herrero, and A. M. Baro, Rev. Sci. Instrum. 78(1), 013705 (2007). E. J. Cho and S. J. Oh, Phys. Rev. B 59, 15613–15616 (1999).
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