Annealing Effects on MgO Films Grown using e-beam Evaporation

Present work investigates the annealing effects on MgO thin films deposited using e-beam evaporation method. MgO thin films of thickness 5 and 50 nm were evaporated from MgO-pellet in ultra-high vacuum (2×10 Torr). As deposited thin films exhibit coordination similar to MgO bulk as envisaged from near edge X-ray absorption fine structure measurements. As deposited films were annealed at 300, 400 and 500C in open environment. Thickness of films remain unaltered with annealing within experimental error. Raman spectroscopic measurements further confirm the presence of bands associated with Mg-O bonding at such low thicknesses. Keywordse-beam evaporation, Annealing, Rutherford back scattering, Raman spectroscopy.


Introduction
In past few years, MgO has been material of great interest with the increasing demand over spintronic materials and observation of defect induced ferromagnetism (DIF). It has been seen that DIF depends upon the pre and post synthesis treatment as well as method of deposition Pathak et al., 2017). Recent reports show the presence of unusual magnetic properties in MgO, hence study of MgO is pronounced for the understanding of newly emerging field of d o ferromagnetism. This motivates number of researchers to investigate this phenomenon in this material under different treatment Cao et al., 2017;Qi et al., 2017).
It has also been shown experimentally that by decreasing the film thickness, enhancement/increase in magnetization can be achieved Pathak et al., 2017;Cao et al., 2017;Qi et al., 2017). Deposition methods play important role in determining the properties of thin films. The properties of thin films are determined by stoichiometric proportion, lattice mismatch (Hu et al., 2010;Gazquez et al., 2013;Loureiro et al., 2014), nature of growth (Wu et al., 2014), stress developed at film substrate interface (Rao et al., 2014). Hence, number of methods are developed to deposit thin films to get control over these parameters. These methods are e-beam evaporation, pulsed laser deposition, atomic layer deposition, radio frequency sputtering method and chemical vapor deposition (Kern and Schuegraf, 2001).
Near edge X-ray absorption fine structure (NEXAFS) of any material is an important tool to get information of local co-ordination, valence state of constituent ions (Singh et al., 2018a;Sharma et al., 2018). Thus, these measurements are suitable to characterize as-deposited films. Apart from deposition methods, post deposition treatment like annealing also affects the behavior of thin films. Rutherford Backscattering (RBS) spectroscopic measurements provide a way to understand the changes associated with compositional change and thickness (Singh et al., 2016). On the other hand, Raman spectroscopic measurements pave up the way to understand the changes associated with annealing on vibrational modes (Singh et al., 2011). Thus present work investigates annealing effects on e-beam evaporated MgO thin films using RBS and Raman spectroscopic measurements.

Experimental Details
Magnesium oxide thin films of thickness 5 and 50 nm were grown by e-beam evaporation method on silicon substrate. Thickness of these films was controlled using quartz crystal monitor. The substrate was kept at room temperature and deposition rate was 1 Å/s. The details of film deposition are published elsewhere (Singh et al., 2015). In order to investigate effect of annealing both films are annealed at 300, 400 and 500 o C.
NEXAFS measurements for as-deposited films were performed at 10D XAS-KIST beamline of Pohang Accelerator Laboratory, South Korea. RBS experiments were carried out using alpha particle beam from 1.7 MV Pelletron Accelerator at Inter University Accelerator Center (IUAC), New Delhi, India. RBS simulation was done using RUMP program (Singh et al., 2016). Raman spectroscopic measurements were performed using In Via Raman microscope from Renishaw UK installed at IUAC, New Delhi. The system consists of Ar ion laser with 514.5 nm wavelength and 50 mw power.

Nature of Deposited Film
To investigate annealing effect it is required to get information about the nature of as-deposited films. Hence, measured NEXAFS spectra of both the films are shown in Figure 1.
O K-edge NEXAFS spectra of both films exhibit spectral features at 538.2, 540.2, 546, 5, 553.6 and 556.9 eV in post edge region . The observed spectral features in post-edge region are characteristics of MgO and reported by number of researchers for MgO thin films and nanoparticles. For these films, pre-edge spectral features appears at 533.3 eV which is reported for ultrathin films (Luches et al., 2004;Linder et al., 2006;Singh et al., 2018b). Thus, NEXAFS measurements reveals formation of well-coordinated MgO thin films. To further reveal the annealing effects on these films, RBS and Raman spectroscopic measurements were performed and discussed in next section.  Table 1 shows the parameters used for simulation of this film. Table 1 shows composition and thickness of these films at various annealing temperature. It is clear that thickness of as-grown film is 5 nm and remain unaltered with increase of annealing temperature. Moreover, composition also remain same with annealing.  Figure 2 shows RBS spectra of film grown for film thickness of 5 nm. At a glance, it appears that there are no Mg 2+ and O 2on the surface of Si, however, O K-edge spectrum clearly reveals the presence of MgO (Figure 1). It is contemplated that Mg 2+ and O 2ions can't be detected by RBS measurements due to low thickness of the films.  Figure 2. Experimental (open circles) and simulated (red line) RBS spectra of as-deposited thin film of thickness 5 nm annealed at 300, 400 and 500 o C Figure 3 shows RBS spectra of as-grown film of thickness of 50 nm at various annealing temperature. RBS spectra of MgO thin films exhibit spectral features corresponding to Mg 2+ and O 2ions. Thickness of as-grown film is 102 nm as determined from RBS simulation. This value of thickness is almost double from that determined from quartz crystal monitor. Composition of as-grown thin film is Mg0.5O0.5. Composition modifies slightly with annealing (Table 2). Since, it is thick layer, hence, it is expected that a significant amount of Mg 2+ and O 2ions will diffuse into Si, hence, a diffused layer (Layer 2 in Table 2) is also considered during simulation. Composition of both MgO layer as well as diffused layer modifies significantly. Table 2 shows a variation of thickness of MgO film which is associated with depth resolution of RBS technique.  Figure 3. Experimental (open circles) and simulated (red line) RBS spectra of as-deposited thin film of thickness 50 nm annealed at 300, 400 and 500 o C.  Figure 4 shows the Raman spectra for as-grown and annealed thin films having thickness 50 nm.

Raman Spectroscopic Measurements
In the spectra of these films two small bands appears at 970 cm -1 and 1122 cm -1 apart from intense band at 518 cm -1 . The intense band at 518 cm -1 is due to Si substrate (Parker et al., 1967).
The band appearing at 970 cm -1 is associated with second order Raman band present in Si (Meier et al., 2006). In the spectrum of MgO, second order Raman bands appears around 1088 cm -1 (Meier et al., 2006). In our case, band around 1122 cm -1 may be associated with second order Raman band in MgO (Manson et al., 1971;Schlecht et al., 1973).

Conclusion
In conclusion, MgO thin films of thickness 5 and 50 nm were grown using e-beam evaporation method. O K-edge near edge X-ray absorption fine structure measurements revealed local electronic structure analogues to bulk MgO. RBS measurements reflect no change of thickness within experimental error after annealing. Raman measurements show occurrence of Raman bands at such low thickness.

Conflicts of Interests
The authors confirm that this article contents have no conflict of interest.