The effect of deposition angle and thickness on structural and optical properties of manganese oxide thin films

Manganese oxide thin films were deposited on glass substrates by electron gun evaporation method under ultra-high vacuum condition. Thickness of the layers was measured 60 and 120 nm, by a quartz crystal method. Deposition conditions such as deposition rate, vacuum pressure, incidence of angle and substrate temperature were the same for all layers. After producing pure manganese oxide layers a post-annealing method was used in the presence of a uniform oxygen flow of 300 (sccm) and at 600 K annealing temperature. Optical reflectance and transmittance of the layers were measured in the wavelength of 350–850 nm by a spectrophotometer. Kramers–Kronig relations were used to calculate the optical constant. The influence of annealing temperature and oxygen flow on optical properties is investigated. It was found that film thickness and deposition angle plays an important role on the nanostructures as well as optical properties of layers and cause significant variations in behavior of thin manganese oxide films. The physical properties of materials were characterized by X-ray diffraction (XRD), FE-SEM, AFM, EDX, and UV–Vis techniques.


Introduction
Nanostructured transition metal oxide thin films have attracted the attention of researchers and industry due to their unique physical and chemical properties (Shaikh et al. 2019;Astinchap et al. 2019;Ahmadian et al. 2019;Jamali and Tehrani 2020). Their special properties are because of the finite size and high surface to volume ratio. Among these nanomaterials, manganese oxide is particularly interested due to its non-toxicity, low prices, and availability (Sharma et al. 2016). Thin film technology occupy a prominent place in basic research and the use of thin film semiconductors have attracted much interest in an expanding variety of applications in various electronic and optoelectronic devices due to their low production costs (Asogwa et al. 2010). The metal oxide thin films are an important group of the nanostructured materials. The nanomaterials of thin films can be synthesize and grown by different techniques. Thin films can be deposited upon a substrate by different common techniques such as pulsed laser deposition (Bea et al. 2012). Chemical vapor deposition (Medina-Valtierra et al. 2003;Naeem et al. 2015) reactive magnetron sputtering (Zhu et al. 2009), spray pyrolysis (Allah et al. 2007) atomic layer deposition (Nilsen et al. 2003), chemical bath deposition (Unuma et al. 2003), sol-gel method (Ching et al. 2004;Chen et al. 2009; and so on.Manganese oxides with various valence states and crystalline structures are currently under investigation for electrochemical,electronic, catalytic and other applications. Toupin et al. 2002;Jiang and Kucernak 2002;Raymundo-Pinero et al. 2005). Various approaches have been used to fabricate manganese dioxide, such as self-reacting micro emulsion (Xu et al. 2008), precipitation (Subramanian et al. 2008), room-temperature solid reaction (Yuan et al. 2009), so no chemical (Zolfaghari et al. 2007), and hydrothermal methods (Yan et al. 2009). Manganese oxide is a transition metal oxide. Cubic Mn 2 O 3 , tetragonal Mn 3 O 4 and cubic MNO structures could be obtained from MnO 2 by varying the post-annealing conditions. Among these oxides, MnO 2 is most stable. Such varieties in their structures and hence in physical and chemical properties make them attractive to study their fundamental physical properties and technological applications (Fau et al. 1994;Vaalletta and Pliskin 1967). The mean object of this research is to improve the knowledge of the relationships between the thin film deposition by physical vapor and the properties of the resulting MnO films. In this work, we discuss the effect of annealing and thickness on the properties of manganese oxide thin films, and its applications. These thin films were prepared by electron gun evaporation method. The film was characterized using UV-Vis spectrophotometer, (AFM), (XRD), FE-SEM.

Experimental details
Prior to physical vapor deposition, substrate cleaning is a necessary step. Prior to deposition, all glass substrates were ultrasonically cleaned in heated acetone then ethanol.The substrate holder was a disk of 35 cm in diameter with adjustable height up to 50 cm and also adjustable kippers for placing any kind of substrates. The distance between the center of the evaporation boat and the center of the substrate was 33 cm. Manganese oxide layers on the glass substrates in different thicknesses and deposition angles were made of silvergold manganese metal target by electron gun method. The purity of manganese oxide metal was 99.97%. Manganese oxide thin films in 60 and 120 nm thicknesses were deposited on glass substrates (20 × 20 × 1 mm 3 cut from microscope slide) by electron gun evaporation from tungsten boats at room temperature. After producing pure manganese oxide layers we used the post-annealing method in the presence of a uniform Oxygen flow 300 cm 3 /s and at 600 k annealing temperatures. Using an EDWARDS E19 A3 vacuum evaporation system with a pressure 3 × 10 -7 torr. The layers were deposited in ultra-high vacuum condition, using an electron gun evaporation method with the deposition rate of 0.8 A/s. Crystal and phase structure of the deposited manganese oxide layers were identified an using X-ray Pattern radiation (Cu K α radiation, λ = 0.15406 nm), a complete 2Ө scan was made between 20 to 85. The thickness of the manganese oxide thin films have been specified by quartz crystal technique. The studies were performed in the wavelength range from 350 to 850 nm, with the application of the quartz crystal device manufactured by (SIGMA INSTRUMENTS SQM-160-USA) Co. The measurements of the film thickness have been performing for two deposition angles, namely: 0°and 40º. To view the surface image and cross section of the samples were investigated by FE-SEM (S-4100, Hitachi, Japan). Surface physical morphology was obtained by means of AFM. Reflectance and Transmittance of the layers was determined with UV-VIS spectrophotometer (STELLER-USA) instrument. Other deposition conditions such as deposition rate, vacuum pressure, incidence of angle and substrate temperature were same for other layers. The spectra of layers were in the range of 350-850 nm wave length. Kramers-Kronig relations were derived to calculate optical properties as R, T, n, k, ε 1 , ε 2 , and optical band gap energy. Figure 1 shows the XRD patterns of the present samples recorded between 20° and 85° in different thicknesses (60 and 120 nm) and deposition angles of (0° and 40°). The results show that the samples annealed at 600 K has diffraction peaks at different angles. In Fig. 1a the peak of Mn2O3 at an angle of 2ϴ = 32.851 with miller's index (222) having cubic structure. This result is in good accordance with JCPDS card number (00-041-1442) and phase Mn3O4 at angles of 2ϴ = 50.211 and 2ϴ = 58.445 with miller's indexes (105) and (321) having tetragonal structures respectively (JCPDS: 00-008-0017). By increasing the deposition angle to 40° in the thickness of (60 nm) Fig. 1b the crystalline phase Mn3O4 at an angle of 2ϴ = 32.495 with miller's index (103) having tetragonal structure and good accordance with JCPDS Card number (00-008-0017).

X-Ray Diffraction analysis
By increasing the thickness to 120 nm in Fig. 1c and d the crystalline phases of manganese oxide on the layer have been increased. In Fig. 1c crystalline phases Mn 2 O 3 at  (222) having cubic structure respectively. These results are in good accordance with JCPDS Card number (01-071-1177). By increasing the deposition angle in Fig. 1d crystalline phases of Mn3O4 at angles of 2ϴ = 28.810, 2ϴ = 32.306 with, 2ϴ = 39.841, 2ϴ = 50.682, 2ϴ = 58.501with miller's indexes (112), (103), (004), (105), (321) having tetragonal structures respectively. These results are in good accordance with JCPDS Card number (00-008-0017). In general, it can be concluded that in Fig. 1a-d, increasing the thickness caused more crystallization of the layers and with enhancement the accumulation angle caused phases transition in the layers. Figure 2 shows EDAX diagrams of manganese oxide layers made on glass substrate at different deposition angles (vertical and 40°) and different thicknesses (60 and 120 nm) with annealing temperatures of 600 K.

Energy dispersive X-ray spectroscopy
As we can see in Fig. 2, in general, by increasing the thickness plus the annealing temperature, the absorption rate of manganese atoms on the layer is a significant increase. Annealing temperature is one of the most important parameters for layer growth, which increases activation energy and increases the depth of oxygen penetration into the layer. By increasing the thickness under the same accumulation conditions in Fig. 2a and b the amount and fraction of manganese atoms in the layer increased and itself absorbed more oxygen atoms in thicker layers, by increasing the deposition angle to 40° and deviation from the optimum deposition conditions in addition to less absorption of manganese atoms on the layer, the amount of oxygen impurities due to the increase of cavities and the absorption of impurities (oxygen) increased completely adapt to AFM images. As we see in general between 4 layers, in lower thicknesses, the deposition angle has less effect on growth, but with increasing thickness, the fraction of manganese atoms increases and the effect of deposition angle increases. Also, with increasing the angle of accumulation, the oxygen in the layer increases, which itself acts as an impurity factor in the growth of grains and causes the grains to become smaller.

Field Emission Scanning Electron Microscopy analysis
Images of field emission scanning electron microscopy were shown in Fig. 3a-d.As seen on, big seeds were appeared on the substrate by increasing thickness. Nucleation was seen in a smaller form in thicknesses of (60 nm) in Fig. 3a. When the thickness Enhancements to 120 nm in Fig. 3c, growth, and re-nucleation as well accession and combination is observed which is visible as interconnected islands. In Fig. 3b the nucleation is observed in a smaller form, by increasing the deposition angle in thicknesses (60 nm). As well in high thicknesses of (120 nm), growth, annexation and integration and re-nucleation with smaller grain sizes have happened that smaller grains are formed due to the further penetration of oxygen gas into the grains in accordance with atomic force microscope images. Also, by increasing deposition angle, the oxygen in the layer increases, which itself acts as an impurity factor in the growth of grains and causes the grains to become smaller.

Atomic Force Microscopy analysis
In general, Fig. 4 shows the topography of the manganese oxide layers made on the glass substrate at different thickness (60 and 120 nm) and deposition angle (0 and 40°) and annealing temperature a 600 K by electron gun method. Figure 4a, shows two-dimensional image of the atomic force microscope of the layer with a thickness of 60 nm and deposition angle of 40° and an annealing temperature of 600 K by electron gun method. As we can see, the size of the grains has increased due to the increase in thickness and the increase in the annealing temperature in this layer, coarse grains and interconnected tiny grains are quite evident. As we know, increasing the thickness cause to increases the size of the grains and increases the annealing temperature, cause to increases the activation energy of the grains and therefore causes to the surface and volumetric distribution of grains and their accession and integration. The grain size in this layer is 0.213 μm. Figure 4b shows the topography of the manganese oxide layer made on the glass substrate with a thickness of 60 nm and deposition Angle of 40 o and an annealing temperature of 600 K by electron gun method. As is evident in Fig. 4b of the two-dimensional image of the atomic force microscope of this layer, by increasing the deposition angle, the empty spaces between the grains are increased, and in addition, the grains are smaller and in some places larger. The grain size in this layer is 0.193 μm. Figure 4c shows two-dimensional image of the atomic force microscope of the layer with thickness of 120 nm and with vertical deposition angle and an annealing temperature of 600° Kelvin by electron gun method. As shown in Fig. 4c with increasing thickness, the grains grow and the surface is filled with coarse grains and due to the annealing temperature of 600° Kelvin, the accession and integration of the grains can be seen. Empty spaces are visible in the layer. The grain size in this layer is 0.230 μm. Figure 4d shows two-dimensional image of the atomic force microscope of the manganese oxide layer made on the glass substrate with a thickness of 120 nm and deposition angle of 40° and an annealing temperature of 600 K by electron gun method. As can be seen, the surface is filled with coarse grains, but compared to Fig. 4c it is smaller and the empty spaces have increased. The grain size in this layer is 0.225 μm. Increasing the accumulation angle, causing the seeds became smaller and increasing the holes and empty spaces in the layer. The average hardness for samples in thickness of 60 nm with deposition angle 0°, Fig. 4 2D AFM images of manganese oxide layers produced by PVD method at different deposition angles (0 and 40°) and different thickness (60 and 120 nm) with annealing temperature of 600 K thickness of 60 nm with deposition angle 40°, thickness of 120 nm with deposition angle 0°, thickness of 120 nm with deposition angle 40° are 6.01 nm, 6.2 nm, 11.71 nm, 6.34 nm respectively.
where E denotes the photon energy, E 2 the asymptotic limitation of the free-electron energy, and R (E) the reflectance. Hence, the θ (E) can be calculated. Then the real and imaginary parts of the refractive index were calculated, from which other parameters were obtained. The calculation of rctive index by using Kramers-Kroning relations the following formulas and graphs related to each are drawn.
The results of physical quantities are as follows: Reflectance and transmittance diagrams for the samples at different accumulation angle and thickness were shown in Fig. 5a-b. The maximum and minimum changes in reflection, are the result of differences in film thickness. As the thickness increased in the vertical deposition angle, Reflection was increased due to the completion of the layers and the disappearance of cavities. As well, due to these reasons transmittance decreases. At an accumulation angle of 40° with enhancement thickness due to the increase of depositional grains on the substrate, the reflectance has increased and due to the reduction of cavities, the transmittance has decreased. The maximum reflectance drop across the spectrum is detected at a thickness of 60 nm. By increasing the deposition angle to 40° in general, according to the atomic force microscopy images, the grains become more and more broken and tiny and this is due to the penetration of oxygen into the surface and depth of the seeds, thus, the layer was filled with tiny grains, which increases the reflection and reduces the transmittance (due to the reduction of cavities). The real part of the refractive index (n) (1) of manganese oxide thin films at accumulation angle of 0° and 40° with different thicknesses were shown in shape 5(c).
In general, at the vertical accumulation angle, the real part of the refractive index increases due to the denser formation of the layers. By increasing the deposition angle to 40° and forming fractal layers and forming non-homogeneous layers, the real part of the refractive index has increased. in Fig. 5d displays the extinction coefficient (k) of the layers produced in this work. In general, at vertical deposition angle, with increasing thickness and formation of complete layers by manganese oxide grains and of the much disappearance of the cavities, the imaginary part of the refractive index decreases. In addition, due to increasing the deposition angle and creating more cavities on the layer and in fact the formation of non-homogeneous layers, the imaginary part of the refractive index increases. Also, the real and imaginary parts of the refractive index via the down formulas were obtained.
The real and imaginary part of a dielectric constant of manganese oxide thin films in different accumulation angle and thicknesses were shown in Fig. 6a-b. In Fig. 6a, by increasing the thickness in the vertical deposition angle, real part of the dielectric have been increased and this means that the dielectric property is increased.in addition, with enhancement of the accumulation angle to 40° the real part of the dielectric constant increases,which is due to the effect of cavities.As can be seen in Fig. 6b, the imaginary part of the dielectric constant at the vertical deposition angle with increasing the thickness and the elimination of cavities due to formation of complete layers is reduced. The imaginary part of the dielectric constant increases by increasing the deposition angle due to formation more cavities on the layers has generally also increased. These are the spectral relationships of T and R ant those are presented.
so that the optical band gap (E g ) of the layerss can be calculated with the following relationship (Singh 1994): where hν is photon energy and Experimental absorption coefficient is given as: where c is the velocity of light and k(E) is the imaginary part of refractive index.
In this work, in Fig. 7, the band gap energy for manganese oxide layers was plotted. With increasing thickness, the band gap energy increases and according to previous analyzes, it is fully compatible with the dielectric properties of the layers. Also, by increasing the deposition angle and forming more cavities on the substrate and the effect of annealing temperature and the amount of impurities such as oxygen, manganese atoms absorb more oxygen (due to the receptivity of manganese atoms) and cause more semiconductor layers. Table 1 shows the values of band energy for the layers produced in this work. The calculated band gap energy from optical data in this study is in good agreement with the value of other works (A.K.M. Farid Ul Islam, R. Islam, K.A. Khan, Renew. En. 2289;Valletta et al. 1966;K. Wang Joo Kim 2004;Park et al. 2000;Chambouleyron et al. 1997;JCPDS 1442;Paiva-Santos et al. 2002;Al-Kuhaili 2006).

Conclusions
In conclusion, the results of the study are summarized as follows: In this work, manganese oxide thin film was grown on the glass substrates by physical vapor deposition method and was annealed at 600 K. We studied variations in the optical and structural properties of these thin films that resulted from changing the thickness and deposition angle. In general, in atomic force microscope analysis, increasing thickness causes the layers to crystallize as much as possible and increasing the deposition angle causes phases transfers in the layers. As we can see in general between 4 layers, in lower thicknesses, the deposition angle has less effect on growth, but with increasing thickness, the fraction of manganese atoms increases and the effect of deposition angle increases. In FE-SEM analysis we can see that by increasing the deposition angle in accordance with atomic force microscope images, smaller grains are formed due to the greater penetration of oxygen gas into the grains. The optical reflectance and transmittance of the thin films at angles of deposition of (0° and 40°) and annealing temperature of 600 k, with increasing thickness, the reflection increases and the transmittance decreases. Depending on the thickness, in vertical deposition angle, extinction coefficient (k) decreased and refractive index (n) increased. On the other hand, extinction coefficient (k) and refractive index (n) increased with increasing deposition angle. By increasing the thickness and deposition angle, the real part of the dielectric constant have been increased and this means that the dielectric property is increased. The imaginary part of the dielectric constant with increasing the thickness and the elimination of cavities due to formation of complete layers is reduced. The imaginary part of the dielectric constant increases by increasing the deposition angle due to formation more cavities on the layers has generally also increased. The band gap energy is generally enhanced by increasing thickness, which is also in full compliance with the dielectric properties of the layers according to previous analyses.
Funding The authors have not disclosed any funding.

Conflict of interest
The authors have not disclosed any competing interests.