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Structural and optical analysis of nanocrystalline thin films of mixed rare earth oxides (Y1-xErx)2O3
Thin Solid Films 518 (2010) 4058–4065
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Structural and optical analysis of nanocrystalline thin films of mixed rare earth oxides (Y1-xErx)2O3 M. El-Hagary a,b,⁎, M. Emam-Ismail a,c, S.H. Mohamed a,d, A.S. Hamid a,b, S. Althoyaib a a
Physics department, College of Science, Qassim University, P. O. 6644, 5145 Buryadh, Kingdom of Saudi Arabia Physics Department, Faculty of Science, Helwan University, 11792 Helwan, Cairo, Egypt Physics Department, Faculty of Science, Ain Shams University, 11566 Abbassia, Cairo, Egypt d Physics Department, Faculty of Science, Sohag University, Sohag 82524, Egypt b c
a r t i c l e
i n f o
Article history: Received 4 June 2009 Received in revised form 26 January 2010 Accepted 9 February 2010 Available online 14 February 2010 Keywords: Rare earth oxides X-ray diffraction Thin films Optical properties
a b s t r a c t Nanocrystalline thin films of mixed rare earth oxides (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) were deposited by electron beam evaporation technique on polished fused silica glass at different substrate temperatures (200-500 °C). The effect of the substrate temperature as well as the mixing parameter (x) on the structural and optical properties of these films has been investigated by using X-ray diffraction (XRD), energy dispersive x-ray analysis and optical spectrophotometry. XRD investigation shows that mixed rare earth oxides film (Y1-xErx)2O3 grown at lower substrate temperature (Ts ≤ 300 °C) are poorly crystalline, whereas films grown at higher substrate temperatures (Ts ≥ 400 °C) tend to have better crystallinity. Furthermore, the mixing parameter (x) was found to stabilize the cubic phase over the entire of 0.1 ≤ x ≤ 1. The crystallite size of the films was found to vary in the range from 25 to 39 nm. Optical band gap of the films was deterimined by analysis of the absoprtion coeffifcient. For films deposited at different substrate temperatures direct and indirect transitions occur with energies varied from 5.29 to 5.94 eV and from 4.23 to 4.51 eV, respectively. However, films of different composition x, give optical band gap varied from 6.14 to 5.86 eV for direct transition and from 5.23 to 4.22 eV for indirect transitions. Consequently, one may conclude that it is possible to tune the energy band gap by relative fraction of constituent oxides. It was found that optical constants increase with increasing the substrate temperature. Nevertheless, the values of n and k decrease with increasing the mixing parameter, x. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Technological applications of thin dielectric films in microelectronic, nanoelectronic, spintronic and optoelectronic devices (thin film capacitors, thin film transistors, varicaps and insulating coatings; in optical coatings, in electroluminescent devices or gas sensors, as protective coatings, etc.) have stimulated great interest in preparation and physical examination of different thin film rare earth compounds, such as rare earth oxides [1–17]. Their interesting physical properties make them very promising materials for use in various devices. Rare earth oxide films are characterized by a good chemical, thermal and mechanical stability [4]. They exhibit also interesting optical properties such as; a good transparency from ultraviolet up to infrared, high refractive index [5,6] and good dielectric and insulating properties (relatively high dielectric constant and low dielectric losses as well as high dielectric breakdown field strength) [4,5].
⁎ Corresponding author. Physics Department, Faculty of Science, Helwan University, 11792 Helwan, Cairo, Egypt. Tel.: + 966 6 3800050 4061; fax: + 966 6 3801581. E-mail address: [email protected] (M. El-Hagary). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.02.024
Band gap and refractive index are crucial parameters in optical waveguide devices. The higher the value of refractive index, the more confined the optical transmission in the guide wave becomes, thus leading to more efficient pumping and amplification. In the recent years, pure rare earth oxide Y2O3 planar optical waveguiding thin films attracted much attention due to their potential application in optoelectronics [18,19]. Furthermore, Y2O3 is an important material for optical application (as a host matrix for metallic inclusions) because its ability to be a host material for rare earth atoms [20]. Rare earth oxides can be mixed with each other, such as in (La1-xYx)2O3 [21], resulting in enhances their advantages by upgrading their properties which may assist in tuning the lattice parameters and improve the layer epitaxy. One of the most important applications of the mixed rare earth oxides is their use as buffer layers for fabrication of epitaxically coated high temperature superconductor (HTSC) on different substrates. Lattice mismatch between the buffer layer and HTSC film can be greatly reduced or even cancelled by mixing rare earth oxides to give a lattice parameter which exactly, or nearly, matching that of HTSC, utilizing the fact that the ionic radii of rare earths elements decrease monotonically with increasing atomic number [22]. This lattice matching is very important for the fabrication of epitaxically grown high quality HTSC. Crystallographic
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investigations by X-ray diffraction measurements of mixed rare earth oxides have been studied extensively by Heiba and co-workers; see Refs e.g.,(Er1-xGdx)2O3 [23], (Eu1-xYbx)2O3 [24,25], (Gd1-xHox)2O3 and (Gd1-xYbx)2O3 [22] (Gd1-xYx)2O3 [26]. An extensive and explicit structural and optical characterization for mixed rare earth oxides (Y1-xErx)2O3 thin films have never been reported in the literature. In some papers, the photoluminescence spectra of Er doped with Ce3+ and Gd3+ is explicitly discussed [27,28]. In other paper, more focused on and electrical properties of Er doped with Gd3+ [29]. Therefore, the aim of the present work is to improve the optical waveguide parameters by investigating the variation of the optical (e.g. refractive index and optical band gap etc.) and structural properties of mixed rare earth oxides (Y1-xErx)2O3 (0 ≤ x ≤ 1) thin films deposited by electron beam evaporation from stoichiometric of the rare earth oxides Y2O3 and Er2O3 upon mixing parameter x and preparation conditions. 2. Experimental details 2.1. Preparation of the bulk material A series of polycrystalline sample of (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) were synthesized by a conventional solid state reaction method in air. Stoichiometric amounts of high-purity (99.999%) analytical grade Y2O3 and Er2O3 powders produced by MV Lab. INC., USA (99.999%) were mixed in a ball mortar. The mixed powders were pressed into a disk-shape pellet and calcined at 600 °C for about 24 hours with intermediate grindings and pelletizations. The disk samples were sintered at 1350 °C for 15 h in ambient atmosphere with intermediate grindings and pelletizations. The final disk-shape samples are obtained after a very slow cooling process from the sintering to room temperature. 2.2. Preparation of thin films The mixed rare earth oxides (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) thin film were prepared by electron beam evaporation using high vacuum coating unit type Edward Auto 306. The system was pumped to a pressure of 5 × 10-4 Pa. The pellets of (Y1-xErx)2O3 samples with 0.1 ≤ x ≤ 1 were heated in a vacuum chamber with the electron gun in order to degas the material before evaporation process. The films were deposited on amorphous glass and polished fused silica glass (25 mm × 25 mm) as substrates. The substrates were carefully cleaned by using acetone and distilled water. The conditions of evaporation were controlled by substrate temperature and rate of evaporation. The substrate temperature (Ts) was varied in the range 200-500 °C and the rate of evaporation was adjusted to be 2 nm/sec. The thickness of the films was kept to be constant at 150 nm (±8 nm). The substrates were rotated during the deposition and the source to the substrate distance was 20 cm. Film thickness and the rate of evaporation were monitored with a quartz crystal monitor attached to the vacuum system. The structure and phase purity of the samples and as-deposited films were checked at room temperature by means of X-ray powder diffraction (XRD) Shimadzu Diffractometer XRD 6000, Japan, with CuKα1 radiation (λ = 1.54056 Å). The data were collected by step-scan modes in a 2θ range between 10° and 80° with step-size of 0.02° and step time of 0.6 seconds. Pure Silicon∼ Si 99.9999% was used as an internal standard. The elemental composition of the films was analyzed by an energy dispersive X-ray spectrometer unit (EDXS) interfaced with a scanning electron microscope (Philips XL) operating an accelerating voltage of 30 kV. The relative error of determining the indicated elements does not exceed 5%. Optical characterization of the films has been carried out from spectral transmittance and reflectance, which were obtained through JASCO V-670 double beam
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spectrophotometer. The measurements have been performed in the wavelength range from 190 to 2500 nm. The transmittance and reflectance measurements were taken at normal incident.
3. Results and discussion 3.1. Composition and structural characterization EDXS analysis confirms an agreement between nominal concentrations (calculated) and analytical concentrations of the mixed rare earth oxides films. The XRD pattern of mixed rare earth oxides (Y0.8Er0.2)2O3 thin film deposited on fused silica glass substrate at ambient substrate temperature revealed (Fig. 1 (a)) single broad diffraction peak without any sign of the presence of well-defined peaks. Subsequently, this suggests that the film is amorphous. In contrast, Fig. 1 (b) shows the XRD patterns of (Y0.8Er0.2)2O3 deposited on fused silica glass substrate at various substrate temperatures Ts = 200, 300, 400 and 500 °C in vacuum. It is observed that the film deposited at Ts = 200 °C is still amorphous. While by increasing the substrate temperature some small and narrow diffraction peaks are observed. Such peaks are superimposed on large and broad background of the amorphous component of the films and the fused silica glass substrate. These peaks were contributed by crystallites in the films, and thus these films consist of microcrystallites embedded in an amorphous matrix. As can be seen in Fig. 1 (c) films grown at substrate temperature Ts = 300 °C have two broad diffraction peaks (222) and (400), which are indicative of poor crystallinity. But XRD patterns of film grown at Ts ≥ 400 °C (see Fig. 1 (d), (e)) have three diffraction peaks (222), (440) and (400). This reveals clearly that the intensity of (222) diffraction peak increases and their full width at half-maximum (FWHM) decreases with increasing the substrate temperature, which indicates that the film crystallinity has improved. Thus, the XRD patterns of mixed rare earth oxides film establish clearly that films grown at lower temperature are poorly crystalline, whereas films grown at higher temperatures tend to have better crystallinity. During deposition, the kinetics of atomic arrangement is
Fig. 1. XRD patterns of (Y0.8Er0.2)2O3 (choose as selected film) deposited on fused silica glass substrates at various substrate temperatures Ts = 200, 300, 400 and 500 °C.
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mainly determined by substrate temperature and energy of deposited atoms [30]. Therefore, at relatively high substrate temperature, adatoms on the surface have high mobility; they have enough time to move on surface to look for the lowest energy sites before covered by the next layer of atoms. Otherwise, low substrate temperature results in low ad-atom mobility, which limits the formation of lower energy structure [30]. The diffraction peaks occurred at 29.1°, 33.9°and 48.8° can be indexed on the basis of Powder Diffraction Standards Data of Er2O3 and Y2O3 (JCPDS; cards no. 79-1257 (for Y2O3) and 77-0461 (for Er2O3)). Accordingly, the XRD patterns are attributed at all temperature of growth to body centered cubic type structure (BCC-C type structure with space group Ia3̄) with preferred orientations in the (222), (440), and (400) planes. A similar results have been reported by different groups for Er2O3 [31,32] and Y2O3 [30,33], respectively. Cho et al. attributed preferred orientation dependence in Y2O3 films deposited on Si substrate to the competition between surface energy and strain energy [33]. Apparently, sufficient internal strain does not exist in our films structure. Hence, planes of high atomic density are preferred during film growth and this likely due to the minimization of the surface energy associated with high atomic density planes. Among the deposition conditions, the type of the substrate is one of the most important parameters to determine crystalline phases. Shown in Fig. 2 is the XRD patterns of (Y0.5Er0.5)2O3 thin films deposited on normal glass and fused silica glass substrates at Ts = 500 °C. The full width at half-maximum of the (222) peak is narrower for film grown on fused silica substrate than for glass substrate indicating better crystallization for cubic phase of the former than of the latter. Additionally, the crystallinity of the substrate and the deposition method can also play an important role for crystallinity quality of the thin film. In the present work, the substrates were amorphous, the substrate temperature was relatively low and the energies of the oxide components arriving on the substrate by electron beam evaporation technique use in this work is lower than those of other deposition techniques. Accordingly, the films are formed with a small degree of crystallinity. Fig. 3 represents the XRD patterns of the (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films deposited on fused silica glass substrates with thickness of about 150 nm at substrate temperature Ts = 500 °C. It is evident that all the relative peak intensities follow the pattern of the standard JCPDS data cards (see above) indicating that the films are polycrystalline in nature and the mixing parameter x was found to stabilize the cubic phase over the entire of 0.1 ≤ x ≤ 1. Moreover, we observed a slight shift of the main Bragg's peak (111) to larger angle, which indicates the changes in the lattice parameters due to strain in the host on incorporation of the dopant ions into the basic cell. The shifting of the XRD peaks towards larger diffraction angles with increase of the Er concentration in the (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) system may be attributed to the increase of the Er amount in the mixed oxide in the films.
Fig. 3. θ -2θ XRD patterns of the (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films deposited on fused silica glass substrates with thickness of about 150 nm at substrate temperatures Ts = 500 °C.
The lattice parameter a of the deposits for preferentially oriented reflections was estimated from the XRD patterns using the standard relation: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a = d h2 + k2 + l2
The lattice parameter a of (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films with their corresponding errors obtained from XRD data is plotted as a function of the mixing parameter in Fig. 4. It is clearly seen that the lattice parameter decreases linearly with increase the mixing parameter and cover the range from a = 1.06212(9) for Y2O3 film to a = 1.05503(7) for Er2O3 film. The lattice parameter of (Y1-xErx)2O3 thin films obeyed Vegard's law, which indicates that the solid solution of the mixed rare earth oxides can exist. The lattice parameter is in good agreement with those reported in the literature for the parents Y2O3 [33] and Er2O3 films [32,34]. The crystallite (grain) sizes (r) of the films were calculated from the most intense broadening of the (222), (440) and (400) peaks using scherrer's formula [35], r = k Vλ = B cosðθÞ
Fig. 2. XRD patterns of (Y0.5Er0.5)2O3 thin films deposited on normal glass and fused silica glass substrates at the substrate temperature Ts = 500 °C.
ð1Þ
ð2Þ
where r is the crystallite size, λ is the wavelength of the X-ray used, B is the FWHM of diffraction peak, θ is the corresponding Bragg angle and k' is a constant approximately equal 0.9. The crystallite size (r) of
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Fig. 4. The variation of lattice parameter as a function of the mixing parameter for (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films deposited on fused silica glass substrates with thickness ∼ 150 nm at substrate temperatures Ts = 500 °C.
mixed rare earth oxides thin film was observed to vary from 25 nm (Er2O3) to 39 nm (Y2O3) for different x concentration films. The variation of crystallite size as a function of substrate temperature for mixed rare earth oxides (Y0.8Er0.2)2O3 film grown on fused silica glass is presented in Fig. 5. The crystallite size increased initially with the increase of substrate temperature and reached a maximum value of 29.8 nm at 500 °C and became more or less constant afterwards. The increase in grain size with the growth temperature could be attributed to the enhanced reaction kinetics during the deposition as well as improvement in the ad-atom mobility on the substrate surface. Ghosh et al. explained the decreasing trend of grain size with the increase of strain due to the retarded crystal growth as the stretched lattice increases the lattice energy and diminishes the driving force for the grain growth in sol-gel grown ZnO films [36].
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material and their mixed compounds have shown some amount of variations in optical properties based to substrate temperature like most other similar reactive oxides [37]. Fig. 6 (a) shows the normal incident of the transmittance T(λ) of nanocrystalline (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures Ts = 300, 400 and 500 °C. It can be observed that the optical transmittance of the films was highly influenced by the deposition temperature. The optical transmittance of the films changed significantly from 60% to 88% with the increase of deposition temperature from 300 °C to 500 °C. The higher transmittance observed in the films at Ts = 500 °C was attributed to less scattering effects, structural homogeneity and better crystallinity [38]. Total spectral distribution of reflectance R(λ) and transmittance T (λ) of nanocrystalline (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C is shown in Fig. 6 (b). The transmittance data were corrected relative to the optically identical uncoated substrate. It can be seen that the films exhibit highly transparent (T (λ) ∼ 0.7 - 0.9) down to a wavelength of 700 nm. In the absorption region a non-sharp absorption edge is observed which might be attributed to the mixed nature of the films. It is further noticed that as Er content increases the transmission decreases and shifted towards higher wavelength. A remarkable shift of absorption edge of mixed rare earth oxides film (Y2O3/ Er2O3) towards lower wavelength is observed compared to the pure Er2O3, which is much more likely due to changes in the electronic structure of the film as their chemical composition change while its energy remains less than
3.2. Optical properties Mixed rare earth oxides (Y1-xErx)2O3 have not only exhibit interesting optical properties but also opened up possibility of utilizing this material for making metal dielectric filters and coatings on temperature sensitive substrates. Thin films of rare earth oxide
Fig. 5. The variation of crystallite size with substrate temperature for mixed rare earth oxide (Y0.8Er0.2)2O3 film grown on fused silica glass.
Fig. 6. (a) The normal incident of the transmittance T (λ) of (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures; (b) The normal incident of the reflectance R(λ) and transmittance T (λ) of (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C.
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that of Y2O3 oxide film. Consequently, it is possible to control the energy band gap by relative fraction of constituent oxides. The optical absorption coefficient, α, of (Y1-xErx)2O3 (0 ≤ x ≤ 1) films is evaluated from the experimental data of transmittance and reflectance through rare earth oxides film/fused silica layered structure. At photon energies where the high absorption coefficient permits interference effects to be neglected, but not multiple internal reflections, the optical absorption coefficient is given as [39]: αðλÞ =
1 ln ðA × BÞ d
A = ½ð1−R1 Þð1−R2 Þð1−R3 Þ = ½2T ð1−R2 R3 Þ;
ð3Þ ð4Þ
and n o 2 1=2 B = 1 + 1 + ½R1 ðR2 + R3 −2R2 R3 Þ= ð1−R2 R3 ÞA
ð5Þ
In Eqs. (4) and (5), T is the transmittance, R1, R2, R3 are the power reflection coefficients of air/rare earth oxides film, film/fused silica substrate and fused silica substrate/air interfaces, respectively and d is the thickness. Similarly, this method has been used for calculating the optical absorption coefficient of GaN films by Muth et al. [40]. The optical energy band gap, Eg, of the (Y1-xErx)2O3 (0 ≤ x ≤ 1) films can be determined from optical measurements by fitting the data to the Tauc's relation [41,42]: δ
αE = α0 ðE−Eg Þ
ð6Þ
3 where α0 is a constant (α0 = ðe2 = nch2 me* Þð2mr Þ =2 ) which depends on ⁎ effective (me ) and reduced masses (mr) of charge carries and the refractive index of the material (n). E ( = hν) is the incident photon energy and the δ is an exponent that depends on the type of the band transitions involved; e.g. δ=1/2 or 3/2 for allowed direct interband transitions and forbidden direct interband transitions, respectively and δ=2 or 3 for allowed indirect interband transitions and forbidden indirect interband transitions, respectively. The intercept of the linear extrapolated fit to the experimental data of a plot (αhν)2 versus photon energy, hν, gives the value of the direct optical band gap while the extrapolation of the linear part of a plot (αhν)1/2 versus photon energy, hν, gives the value of the indirect band gap. The direct band gap represents the onset of intrinsic absorption of the oxide, while the indirect energy gap represents the onset of absorption involving some defect states. Fig. 7 (a) shows the change of (αhν)2 with the photon energy, hν, for nanocrystalline (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures Ts = 300, 400 and 500 °C. From the obtained results, a direct electronic transition across the band gap of the films is observed. The extrapolation of the linear part of the curves to the photon energy axis would give the optical band gap that varied in the range, 5.29–5.94 eV with the increase of deposition temperature from 300 °C to 500 °C. Fig. 7 (b) exhibits the plot of (αhν)1/2 versus photon energy, hν, for nanocrystalline (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures Ts = 300, 400 and 500 °C. The obtained linearity in the photon energy ranges from 4.23-4.51 eV indicating allowed indirect band gaps. This increase of the corresponding direct and indirect band gaps of mixed rare earth oxides with Ts is probably due to the reduction of defects at the grain boundaries and a decrease of structural disorder in the films [43]. Moreover, the decrease of lattice strain in the films with growth temperature might also has contributed to the increase of energy band gap in the present study. The direct and indirect allowed band gaps of the nanocrystalline (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C are extracted from the variations of (αhν)2 with the photon energy, hν, (Fig. 8 (a)) and (αhν)1/2 versus hν (Fig. 8 (b)), respectively. The values of the energy band gap of (Y1-xErx)2O3 (0 ≤ x ≤ 1) films were estimated by linear
Fig. 7. (a) (αhν)2 with the photon energy, hν, for (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures. (b) (αhν)1/2 versus photon energy for (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures. A linear regression to the energy axis is used to determine the band gap.
extrapolation of the line to the photon energy axis. It is found that the values of the band gap vary from 6.14 eV (Y2O3) and 5.86 eV (Er2O3) for direct band gaps and 5.23 eV (Y2O3) and 4.22 eV (Er2O3) for indirect transitions, which are corresponding to the two ends of the solid solution. These values are very close to the reported values by several authors [32,44,45]. However, the energy gap values remain in between these two limits of the energy gaps which indicate that band gaps are shifted to the lower energy side as more and more Er incorporated. The decrease of the optical band gap with increasing mixed parameter, x (Er concentration) may be due to the increase in grain size, the reduction in the disorder and decrease in the density of the defect states. Note that, the slope of the absorption curve changes with respect to the composition of x. Different slopes of these spectra could be due to change in stoichiometry of formation of structurally disordered thin films (see Table 1). The values of direct and indirect band gaps for all films are given in Tables 2 and 3. The value of optical constants, refractive index, n and extinction coefficient, k, has been calculated using the theory of reflectivity of light, whereas the reflectance from a thin film can be expressed in terms of Fresnel's coefficient which is given by [46]: R = ⌊ðn−1Þ + k 2
2
⌋ = ⌊ðn + 1Þ2 + k2 ⌋
and α = 4πk/λ [47], where λ is the wavelength.
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Table 2 Direct and indirect band gap values and refractive index Cauchy fitting parameters for the nanocrystalline (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C. x
Direct band gap (eV)
Indirect band gap (eV)
0 0.3 0.5 0.7 1
6.14 6.10 5.94 5.90 5.86
5.23 4.89 4.51 4.41 4.22
Cauchy fitting parameters for refractive index a
b
c
1.92 1.82 1.76 1.67 1.59
1.25 × 104 -1.85 × 104 -6.02 × 102 -3.76 × 103 3.69 × 102
1.71 × 109 2.60 × 109 2.28 × 109 2.10 × 109 2.00 × 109
based on a theory of light propagation. An example of an extracted n (λ) and it's accompanying Cauchy fit are shown in Fig. 9 (a). It should be noted that refractive index value of the present Er2O3 thin film (1.61 at 589.3 nm) is comparable to the refractive index of that value reported in literature for thin film [32,49,50] and slightly lower than that value reported for bulk crystal (1.96 at 589.3 nm) by Medenbach et al. [51]. In the case of Y2O3, our value (1.973 at 543 nm) may be compared to the value of the thin film reported in Ref. 45 and for bulk crystal of Y2O3 (1.936 at 543.5 nm) [52]. Our values for k are in agreement with those reported in Ref. [53] for Er2O3 thin film. Fig. 10 (a), (b) display the spectral variation of refractive index (n) and extinction coefficient (k) of (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures Ts = 300, 400 and 500 °C. It is clear that both optical constants increase with increasing the substrate temperature. Harris et al. [54] reported that the film density increases as the substrate temperature increases. Consequently, from the Lorentz–Lorenz law, the refractive index also increases. The increase of the density of a film is accompanied by a decrease in the porosity. The porosity of the films can be evaluated from the relation [55]:
Fig. 8. The variation of (αhν)2 with the photon energy, hν, (a) and (αhν)1/2 versus hν (b), of (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films deposited on fused silica glass substrates with thickness ∼ 150 nm at substrate temperatures Ts = 500 °C.
The spectral variations of refractive index and extinction coefficient of the mixed rare earth nanocrystalline oxides (Y1-xErx)2O3 (0 ≤ x ≤ 1) thin film of thickness 150 nm grown on fused silica glass at Ts = 500 °C are shown in the inset of Fig. 9 (a) and Fig. 9 (b), respectively. Obviously, the values of both n and k decrease with increasing the wavelength. Nevertheless, the values of optical constants n and k decrease with increasing the mixing parameter, x. For all films studied, the index of refraction was found to fit well to a second order Cauchy dispersion relation of the form [48]: 2
4
n ðλÞ = a + ðb = λ Þ + ðc = λ Þ
ð8Þ
P=
! n2 −1 1− 2 × 100 nb −1
ð9Þ
where nb (∼1.948 at 580 nm [52]) is the refractive index of the bulk material, and n is the refractive index obtained from Fig. 10(a) at corresponding wavelength (n = 1.82 (T s = 500 °C), n = 1.61 (Ts = 400 °C) and n = 1.38 (Ts = 300 °C) Thus, the porosity of the films at different substrate temperatures is calculated as P = 17% (Ts = 500 °C), 43% (Ts = 400 °C) and 67% (Ts = 300 °C). This suggests that the films deposited at higher substrate temperature were compact, whereas the films deposited at lower substrate temperature were more porous. Accordingly, it can be concluded that the films deposited at higher substrate temperature have better optical quality since they were more compact. The increase of k with substrate temperature is also explained by the increase in film density.
where a, b and c are fitting parameters depending on material (summarized in Table 1). This empirical fit was derived by Cauchy
Table 1 Compositions analysis by EDXS of nanocrystalline (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C. x
formula
Y2O3 (at. %)
Er2O3 (at. %)
0 0.3 0.5 0.7 1
Y2O3 Y1.4Er0.6O3 YErO3 Y0.6Er1.4O3 Er2O3
99.1 72.7 51.5 30.8 -
27.2 48.4 69.2 99.7
Table 3 Direct and indirect band gap values and refractive index Cauchy fitting parameters for the nanocrystalline for (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures. Ts (°C)
Direct band gap (eV)
Indirect band gap (eV)
300 400 500
5.29 5.80 5.94
4.23 4.31 4.51
Cauchy fitting parameters for refractive index a
b
c
1.20 1.54 1.76
4.23 × 104 1.25 × 104 -6.02 × 102
1.13 × 108 1.71 × 109 2.28 × 109
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Fig. 9. Spectral variation of optical constants of (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films grown on fused silica glass with thickness ∼150 nm at substrate temperatures Ts = 500 °C. (a) The fitting curves of the dispersion relation curve of refractive index (n) obtained by second order Cauchy dispersion formula; the inset represents the refractive index data versus λ extracted from the experimental data of R and T (b) The variation of extinction coefficient (k) with wavelength.
Fig. 10. Spectral variation of optical constants of (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures. (a) The fitting curves of the dispersion relation curve of refractive index (n) obtained by second order Cauchy dispersion formula; the inset represents the refractive index data versus λ extracted from the experimental data of R. and T (b) The variation of extinction coefficient (k) with wavelength.
4. Conclusions
one may conclude that it is possible to control the energy band gap by relative fraction of constituent oxides. Finally, a relatively high refractive index is observed for the mixed rare earth oxides (Y1-xErx)2O3 film grown at higher substrate temperature.
In summary, nanocrystalline mixed rare earth oxides (Y1-xErx)2O3 thin film were grown by electron beam evaporation technique on fused silica glass substrate at different substrate temperatures in the range 200-500 °C. The influence of substrate temperature and mixing parameter (x) on the structural and optical properties was studied. The X-ray diffraction patterns of the films showed that, films deposited at lower substrate temperature are amorphous while films grown at higher temperature are crystalline with preferred orientations in the (111), (110), and (100) planes. These peaks are attributed to body centered cubic type structure (BCC-C type structure). The spectrophotometry investigations for mixed rare earth oxides show that the energy gaps and optical constants are depending on the substrate temperature and the mixing parameter as well. In the wavelength range 200-2500 nm films show direct and indirect transitions which are associated with the optical band gap. Precise evaluation of the optical band gap revealed direct transition varied from 5.29-5.94 eV and indirect transitions at 4.23-4.51 eV as substrate temperature increases. However, the change of optical band gap upon of mixing parameter (x) is recorded, Eg = 6.14-5.86 eV for direct transition and Eg = 5.23-4.22 eV for indirect transition. Thus,
Acknowledgements This work was supported by the Deanship of the Scientific Research at Qassim University, Kingdom of Saudi Arabia under grant No. SR-D-008-60. References [1] G. Adaschi, N. Imanaga, Chem. Rev. 98 (1998) 1479. [2] M. Hong, J. Kwo, J.P. Mannaerts, A.M. Sergent, Science 283 (1999) 1897. [3] L.O. Eyring, Nonstoichiometric Oxides, Toft Sørensen, Editor, Acad. Press, NewYork, 1981 p. 337. [4] M. Gasgnier, Phys. Status Solidi, A 114 (1989) 11. [5] M.K. Jayaraj, C.P.G. Vallabhan, Thin Solid Films 177 (1989) 59. [6] B. Elidrissi, M. Addou, M. Regragui, C. Monty, A. Bougrine, A. Kachouane, Thin Solid Films 379 (2000) 23. [7] O.V. Fursenko, T.V. Semikina, A.N. Shmyrjeva, Solid State Phenom. 63/64 (1998) 341. [8] M. Li, Z. Wang, G. Xiong, S. Fan, Q. Zhao, K. Lin, Thin Solid Films 375 (2000) 271.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Structural and optical analysis of nanocrystalline thin films of mixed rare earth oxides (Y1-xErx)2O3 M. El-Hagary a,b,⁎, M. Emam-Ismail a,c, S.H. Mohamed a,d, A.S. Hamid a,b, S. Althoyaib a a
Physics department, College of Science, Qassim University, P. O. 6644, 5145 Buryadh, Kingdom of Saudi Arabia Physics Department, Faculty of Science, Helwan University, 11792 Helwan, Cairo, Egypt Physics Department, Faculty of Science, Ain Shams University, 11566 Abbassia, Cairo, Egypt d Physics Department, Faculty of Science, Sohag University, Sohag 82524, Egypt b c
a r t i c l e
i n f o
Article history: Received 4 June 2009 Received in revised form 26 January 2010 Accepted 9 February 2010 Available online 14 February 2010 Keywords: Rare earth oxides X-ray diffraction Thin films Optical properties
a b s t r a c t Nanocrystalline thin films of mixed rare earth oxides (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) were deposited by electron beam evaporation technique on polished fused silica glass at different substrate temperatures (200-500 °C). The effect of the substrate temperature as well as the mixing parameter (x) on the structural and optical properties of these films has been investigated by using X-ray diffraction (XRD), energy dispersive x-ray analysis and optical spectrophotometry. XRD investigation shows that mixed rare earth oxides film (Y1-xErx)2O3 grown at lower substrate temperature (Ts ≤ 300 °C) are poorly crystalline, whereas films grown at higher substrate temperatures (Ts ≥ 400 °C) tend to have better crystallinity. Furthermore, the mixing parameter (x) was found to stabilize the cubic phase over the entire of 0.1 ≤ x ≤ 1. The crystallite size of the films was found to vary in the range from 25 to 39 nm. Optical band gap of the films was deterimined by analysis of the absoprtion coeffifcient. For films deposited at different substrate temperatures direct and indirect transitions occur with energies varied from 5.29 to 5.94 eV and from 4.23 to 4.51 eV, respectively. However, films of different composition x, give optical band gap varied from 6.14 to 5.86 eV for direct transition and from 5.23 to 4.22 eV for indirect transitions. Consequently, one may conclude that it is possible to tune the energy band gap by relative fraction of constituent oxides. It was found that optical constants increase with increasing the substrate temperature. Nevertheless, the values of n and k decrease with increasing the mixing parameter, x. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Technological applications of thin dielectric films in microelectronic, nanoelectronic, spintronic and optoelectronic devices (thin film capacitors, thin film transistors, varicaps and insulating coatings; in optical coatings, in electroluminescent devices or gas sensors, as protective coatings, etc.) have stimulated great interest in preparation and physical examination of different thin film rare earth compounds, such as rare earth oxides [1–17]. Their interesting physical properties make them very promising materials for use in various devices. Rare earth oxide films are characterized by a good chemical, thermal and mechanical stability [4]. They exhibit also interesting optical properties such as; a good transparency from ultraviolet up to infrared, high refractive index [5,6] and good dielectric and insulating properties (relatively high dielectric constant and low dielectric losses as well as high dielectric breakdown field strength) [4,5].
⁎ Corresponding author. Physics Department, Faculty of Science, Helwan University, 11792 Helwan, Cairo, Egypt. Tel.: + 966 6 3800050 4061; fax: + 966 6 3801581. E-mail address: [email protected] (M. El-Hagary). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.02.024
Band gap and refractive index are crucial parameters in optical waveguide devices. The higher the value of refractive index, the more confined the optical transmission in the guide wave becomes, thus leading to more efficient pumping and amplification. In the recent years, pure rare earth oxide Y2O3 planar optical waveguiding thin films attracted much attention due to their potential application in optoelectronics [18,19]. Furthermore, Y2O3 is an important material for optical application (as a host matrix for metallic inclusions) because its ability to be a host material for rare earth atoms [20]. Rare earth oxides can be mixed with each other, such as in (La1-xYx)2O3 [21], resulting in enhances their advantages by upgrading their properties which may assist in tuning the lattice parameters and improve the layer epitaxy. One of the most important applications of the mixed rare earth oxides is their use as buffer layers for fabrication of epitaxically coated high temperature superconductor (HTSC) on different substrates. Lattice mismatch between the buffer layer and HTSC film can be greatly reduced or even cancelled by mixing rare earth oxides to give a lattice parameter which exactly, or nearly, matching that of HTSC, utilizing the fact that the ionic radii of rare earths elements decrease monotonically with increasing atomic number [22]. This lattice matching is very important for the fabrication of epitaxically grown high quality HTSC. Crystallographic
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investigations by X-ray diffraction measurements of mixed rare earth oxides have been studied extensively by Heiba and co-workers; see Refs e.g.,(Er1-xGdx)2O3 [23], (Eu1-xYbx)2O3 [24,25], (Gd1-xHox)2O3 and (Gd1-xYbx)2O3 [22] (Gd1-xYx)2O3 [26]. An extensive and explicit structural and optical characterization for mixed rare earth oxides (Y1-xErx)2O3 thin films have never been reported in the literature. In some papers, the photoluminescence spectra of Er doped with Ce3+ and Gd3+ is explicitly discussed [27,28]. In other paper, more focused on and electrical properties of Er doped with Gd3+ [29]. Therefore, the aim of the present work is to improve the optical waveguide parameters by investigating the variation of the optical (e.g. refractive index and optical band gap etc.) and structural properties of mixed rare earth oxides (Y1-xErx)2O3 (0 ≤ x ≤ 1) thin films deposited by electron beam evaporation from stoichiometric of the rare earth oxides Y2O3 and Er2O3 upon mixing parameter x and preparation conditions. 2. Experimental details 2.1. Preparation of the bulk material A series of polycrystalline sample of (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) were synthesized by a conventional solid state reaction method in air. Stoichiometric amounts of high-purity (99.999%) analytical grade Y2O3 and Er2O3 powders produced by MV Lab. INC., USA (99.999%) were mixed in a ball mortar. The mixed powders were pressed into a disk-shape pellet and calcined at 600 °C for about 24 hours with intermediate grindings and pelletizations. The disk samples were sintered at 1350 °C for 15 h in ambient atmosphere with intermediate grindings and pelletizations. The final disk-shape samples are obtained after a very slow cooling process from the sintering to room temperature. 2.2. Preparation of thin films The mixed rare earth oxides (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) thin film were prepared by electron beam evaporation using high vacuum coating unit type Edward Auto 306. The system was pumped to a pressure of 5 × 10-4 Pa. The pellets of (Y1-xErx)2O3 samples with 0.1 ≤ x ≤ 1 were heated in a vacuum chamber with the electron gun in order to degas the material before evaporation process. The films were deposited on amorphous glass and polished fused silica glass (25 mm × 25 mm) as substrates. The substrates were carefully cleaned by using acetone and distilled water. The conditions of evaporation were controlled by substrate temperature and rate of evaporation. The substrate temperature (Ts) was varied in the range 200-500 °C and the rate of evaporation was adjusted to be 2 nm/sec. The thickness of the films was kept to be constant at 150 nm (±8 nm). The substrates were rotated during the deposition and the source to the substrate distance was 20 cm. Film thickness and the rate of evaporation were monitored with a quartz crystal monitor attached to the vacuum system. The structure and phase purity of the samples and as-deposited films were checked at room temperature by means of X-ray powder diffraction (XRD) Shimadzu Diffractometer XRD 6000, Japan, with CuKα1 radiation (λ = 1.54056 Å). The data were collected by step-scan modes in a 2θ range between 10° and 80° with step-size of 0.02° and step time of 0.6 seconds. Pure Silicon∼ Si 99.9999% was used as an internal standard. The elemental composition of the films was analyzed by an energy dispersive X-ray spectrometer unit (EDXS) interfaced with a scanning electron microscope (Philips XL) operating an accelerating voltage of 30 kV. The relative error of determining the indicated elements does not exceed 5%. Optical characterization of the films has been carried out from spectral transmittance and reflectance, which were obtained through JASCO V-670 double beam
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spectrophotometer. The measurements have been performed in the wavelength range from 190 to 2500 nm. The transmittance and reflectance measurements were taken at normal incident.
3. Results and discussion 3.1. Composition and structural characterization EDXS analysis confirms an agreement between nominal concentrations (calculated) and analytical concentrations of the mixed rare earth oxides films. The XRD pattern of mixed rare earth oxides (Y0.8Er0.2)2O3 thin film deposited on fused silica glass substrate at ambient substrate temperature revealed (Fig. 1 (a)) single broad diffraction peak without any sign of the presence of well-defined peaks. Subsequently, this suggests that the film is amorphous. In contrast, Fig. 1 (b) shows the XRD patterns of (Y0.8Er0.2)2O3 deposited on fused silica glass substrate at various substrate temperatures Ts = 200, 300, 400 and 500 °C in vacuum. It is observed that the film deposited at Ts = 200 °C is still amorphous. While by increasing the substrate temperature some small and narrow diffraction peaks are observed. Such peaks are superimposed on large and broad background of the amorphous component of the films and the fused silica glass substrate. These peaks were contributed by crystallites in the films, and thus these films consist of microcrystallites embedded in an amorphous matrix. As can be seen in Fig. 1 (c) films grown at substrate temperature Ts = 300 °C have two broad diffraction peaks (222) and (400), which are indicative of poor crystallinity. But XRD patterns of film grown at Ts ≥ 400 °C (see Fig. 1 (d), (e)) have three diffraction peaks (222), (440) and (400). This reveals clearly that the intensity of (222) diffraction peak increases and their full width at half-maximum (FWHM) decreases with increasing the substrate temperature, which indicates that the film crystallinity has improved. Thus, the XRD patterns of mixed rare earth oxides film establish clearly that films grown at lower temperature are poorly crystalline, whereas films grown at higher temperatures tend to have better crystallinity. During deposition, the kinetics of atomic arrangement is
Fig. 1. XRD patterns of (Y0.8Er0.2)2O3 (choose as selected film) deposited on fused silica glass substrates at various substrate temperatures Ts = 200, 300, 400 and 500 °C.
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mainly determined by substrate temperature and energy of deposited atoms [30]. Therefore, at relatively high substrate temperature, adatoms on the surface have high mobility; they have enough time to move on surface to look for the lowest energy sites before covered by the next layer of atoms. Otherwise, low substrate temperature results in low ad-atom mobility, which limits the formation of lower energy structure [30]. The diffraction peaks occurred at 29.1°, 33.9°and 48.8° can be indexed on the basis of Powder Diffraction Standards Data of Er2O3 and Y2O3 (JCPDS; cards no. 79-1257 (for Y2O3) and 77-0461 (for Er2O3)). Accordingly, the XRD patterns are attributed at all temperature of growth to body centered cubic type structure (BCC-C type structure with space group Ia3̄) with preferred orientations in the (222), (440), and (400) planes. A similar results have been reported by different groups for Er2O3 [31,32] and Y2O3 [30,33], respectively. Cho et al. attributed preferred orientation dependence in Y2O3 films deposited on Si substrate to the competition between surface energy and strain energy [33]. Apparently, sufficient internal strain does not exist in our films structure. Hence, planes of high atomic density are preferred during film growth and this likely due to the minimization of the surface energy associated with high atomic density planes. Among the deposition conditions, the type of the substrate is one of the most important parameters to determine crystalline phases. Shown in Fig. 2 is the XRD patterns of (Y0.5Er0.5)2O3 thin films deposited on normal glass and fused silica glass substrates at Ts = 500 °C. The full width at half-maximum of the (222) peak is narrower for film grown on fused silica substrate than for glass substrate indicating better crystallization for cubic phase of the former than of the latter. Additionally, the crystallinity of the substrate and the deposition method can also play an important role for crystallinity quality of the thin film. In the present work, the substrates were amorphous, the substrate temperature was relatively low and the energies of the oxide components arriving on the substrate by electron beam evaporation technique use in this work is lower than those of other deposition techniques. Accordingly, the films are formed with a small degree of crystallinity. Fig. 3 represents the XRD patterns of the (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films deposited on fused silica glass substrates with thickness of about 150 nm at substrate temperature Ts = 500 °C. It is evident that all the relative peak intensities follow the pattern of the standard JCPDS data cards (see above) indicating that the films are polycrystalline in nature and the mixing parameter x was found to stabilize the cubic phase over the entire of 0.1 ≤ x ≤ 1. Moreover, we observed a slight shift of the main Bragg's peak (111) to larger angle, which indicates the changes in the lattice parameters due to strain in the host on incorporation of the dopant ions into the basic cell. The shifting of the XRD peaks towards larger diffraction angles with increase of the Er concentration in the (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) system may be attributed to the increase of the Er amount in the mixed oxide in the films.
Fig. 3. θ -2θ XRD patterns of the (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films deposited on fused silica glass substrates with thickness of about 150 nm at substrate temperatures Ts = 500 °C.
The lattice parameter a of the deposits for preferentially oriented reflections was estimated from the XRD patterns using the standard relation: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a = d h2 + k2 + l2
The lattice parameter a of (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films with their corresponding errors obtained from XRD data is plotted as a function of the mixing parameter in Fig. 4. It is clearly seen that the lattice parameter decreases linearly with increase the mixing parameter and cover the range from a = 1.06212(9) for Y2O3 film to a = 1.05503(7) for Er2O3 film. The lattice parameter of (Y1-xErx)2O3 thin films obeyed Vegard's law, which indicates that the solid solution of the mixed rare earth oxides can exist. The lattice parameter is in good agreement with those reported in the literature for the parents Y2O3 [33] and Er2O3 films [32,34]. The crystallite (grain) sizes (r) of the films were calculated from the most intense broadening of the (222), (440) and (400) peaks using scherrer's formula [35], r = k Vλ = B cosðθÞ
Fig. 2. XRD patterns of (Y0.5Er0.5)2O3 thin films deposited on normal glass and fused silica glass substrates at the substrate temperature Ts = 500 °C.
ð1Þ
ð2Þ
where r is the crystallite size, λ is the wavelength of the X-ray used, B is the FWHM of diffraction peak, θ is the corresponding Bragg angle and k' is a constant approximately equal 0.9. The crystallite size (r) of
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Fig. 4. The variation of lattice parameter as a function of the mixing parameter for (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films deposited on fused silica glass substrates with thickness ∼ 150 nm at substrate temperatures Ts = 500 °C.
mixed rare earth oxides thin film was observed to vary from 25 nm (Er2O3) to 39 nm (Y2O3) for different x concentration films. The variation of crystallite size as a function of substrate temperature for mixed rare earth oxides (Y0.8Er0.2)2O3 film grown on fused silica glass is presented in Fig. 5. The crystallite size increased initially with the increase of substrate temperature and reached a maximum value of 29.8 nm at 500 °C and became more or less constant afterwards. The increase in grain size with the growth temperature could be attributed to the enhanced reaction kinetics during the deposition as well as improvement in the ad-atom mobility on the substrate surface. Ghosh et al. explained the decreasing trend of grain size with the increase of strain due to the retarded crystal growth as the stretched lattice increases the lattice energy and diminishes the driving force for the grain growth in sol-gel grown ZnO films [36].
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material and their mixed compounds have shown some amount of variations in optical properties based to substrate temperature like most other similar reactive oxides [37]. Fig. 6 (a) shows the normal incident of the transmittance T(λ) of nanocrystalline (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures Ts = 300, 400 and 500 °C. It can be observed that the optical transmittance of the films was highly influenced by the deposition temperature. The optical transmittance of the films changed significantly from 60% to 88% with the increase of deposition temperature from 300 °C to 500 °C. The higher transmittance observed in the films at Ts = 500 °C was attributed to less scattering effects, structural homogeneity and better crystallinity [38]. Total spectral distribution of reflectance R(λ) and transmittance T (λ) of nanocrystalline (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C is shown in Fig. 6 (b). The transmittance data were corrected relative to the optically identical uncoated substrate. It can be seen that the films exhibit highly transparent (T (λ) ∼ 0.7 - 0.9) down to a wavelength of 700 nm. In the absorption region a non-sharp absorption edge is observed which might be attributed to the mixed nature of the films. It is further noticed that as Er content increases the transmission decreases and shifted towards higher wavelength. A remarkable shift of absorption edge of mixed rare earth oxides film (Y2O3/ Er2O3) towards lower wavelength is observed compared to the pure Er2O3, which is much more likely due to changes in the electronic structure of the film as their chemical composition change while its energy remains less than
3.2. Optical properties Mixed rare earth oxides (Y1-xErx)2O3 have not only exhibit interesting optical properties but also opened up possibility of utilizing this material for making metal dielectric filters and coatings on temperature sensitive substrates. Thin films of rare earth oxide
Fig. 5. The variation of crystallite size with substrate temperature for mixed rare earth oxide (Y0.8Er0.2)2O3 film grown on fused silica glass.
Fig. 6. (a) The normal incident of the transmittance T (λ) of (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures; (b) The normal incident of the reflectance R(λ) and transmittance T (λ) of (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C.
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that of Y2O3 oxide film. Consequently, it is possible to control the energy band gap by relative fraction of constituent oxides. The optical absorption coefficient, α, of (Y1-xErx)2O3 (0 ≤ x ≤ 1) films is evaluated from the experimental data of transmittance and reflectance through rare earth oxides film/fused silica layered structure. At photon energies where the high absorption coefficient permits interference effects to be neglected, but not multiple internal reflections, the optical absorption coefficient is given as [39]: αðλÞ =
1 ln ðA × BÞ d
A = ½ð1−R1 Þð1−R2 Þð1−R3 Þ = ½2T ð1−R2 R3 Þ;
ð3Þ ð4Þ
and n o 2 1=2 B = 1 + 1 + ½R1 ðR2 + R3 −2R2 R3 Þ= ð1−R2 R3 ÞA
ð5Þ
In Eqs. (4) and (5), T is the transmittance, R1, R2, R3 are the power reflection coefficients of air/rare earth oxides film, film/fused silica substrate and fused silica substrate/air interfaces, respectively and d is the thickness. Similarly, this method has been used for calculating the optical absorption coefficient of GaN films by Muth et al. [40]. The optical energy band gap, Eg, of the (Y1-xErx)2O3 (0 ≤ x ≤ 1) films can be determined from optical measurements by fitting the data to the Tauc's relation [41,42]: δ
αE = α0 ðE−Eg Þ
ð6Þ
3 where α0 is a constant (α0 = ðe2 = nch2 me* Þð2mr Þ =2 ) which depends on ⁎ effective (me ) and reduced masses (mr) of charge carries and the refractive index of the material (n). E ( = hν) is the incident photon energy and the δ is an exponent that depends on the type of the band transitions involved; e.g. δ=1/2 or 3/2 for allowed direct interband transitions and forbidden direct interband transitions, respectively and δ=2 or 3 for allowed indirect interband transitions and forbidden indirect interband transitions, respectively. The intercept of the linear extrapolated fit to the experimental data of a plot (αhν)2 versus photon energy, hν, gives the value of the direct optical band gap while the extrapolation of the linear part of a plot (αhν)1/2 versus photon energy, hν, gives the value of the indirect band gap. The direct band gap represents the onset of intrinsic absorption of the oxide, while the indirect energy gap represents the onset of absorption involving some defect states. Fig. 7 (a) shows the change of (αhν)2 with the photon energy, hν, for nanocrystalline (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures Ts = 300, 400 and 500 °C. From the obtained results, a direct electronic transition across the band gap of the films is observed. The extrapolation of the linear part of the curves to the photon energy axis would give the optical band gap that varied in the range, 5.29–5.94 eV with the increase of deposition temperature from 300 °C to 500 °C. Fig. 7 (b) exhibits the plot of (αhν)1/2 versus photon energy, hν, for nanocrystalline (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures Ts = 300, 400 and 500 °C. The obtained linearity in the photon energy ranges from 4.23-4.51 eV indicating allowed indirect band gaps. This increase of the corresponding direct and indirect band gaps of mixed rare earth oxides with Ts is probably due to the reduction of defects at the grain boundaries and a decrease of structural disorder in the films [43]. Moreover, the decrease of lattice strain in the films with growth temperature might also has contributed to the increase of energy band gap in the present study. The direct and indirect allowed band gaps of the nanocrystalline (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C are extracted from the variations of (αhν)2 with the photon energy, hν, (Fig. 8 (a)) and (αhν)1/2 versus hν (Fig. 8 (b)), respectively. The values of the energy band gap of (Y1-xErx)2O3 (0 ≤ x ≤ 1) films were estimated by linear
Fig. 7. (a) (αhν)2 with the photon energy, hν, for (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures. (b) (αhν)1/2 versus photon energy for (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures. A linear regression to the energy axis is used to determine the band gap.
extrapolation of the line to the photon energy axis. It is found that the values of the band gap vary from 6.14 eV (Y2O3) and 5.86 eV (Er2O3) for direct band gaps and 5.23 eV (Y2O3) and 4.22 eV (Er2O3) for indirect transitions, which are corresponding to the two ends of the solid solution. These values are very close to the reported values by several authors [32,44,45]. However, the energy gap values remain in between these two limits of the energy gaps which indicate that band gaps are shifted to the lower energy side as more and more Er incorporated. The decrease of the optical band gap with increasing mixed parameter, x (Er concentration) may be due to the increase in grain size, the reduction in the disorder and decrease in the density of the defect states. Note that, the slope of the absorption curve changes with respect to the composition of x. Different slopes of these spectra could be due to change in stoichiometry of formation of structurally disordered thin films (see Table 1). The values of direct and indirect band gaps for all films are given in Tables 2 and 3. The value of optical constants, refractive index, n and extinction coefficient, k, has been calculated using the theory of reflectivity of light, whereas the reflectance from a thin film can be expressed in terms of Fresnel's coefficient which is given by [46]: R = ⌊ðn−1Þ + k 2
2
⌋ = ⌊ðn + 1Þ2 + k2 ⌋
and α = 4πk/λ [47], where λ is the wavelength.
ð7Þ
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4063
Table 2 Direct and indirect band gap values and refractive index Cauchy fitting parameters for the nanocrystalline (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C. x
Direct band gap (eV)
Indirect band gap (eV)
0 0.3 0.5 0.7 1
6.14 6.10 5.94 5.90 5.86
5.23 4.89 4.51 4.41 4.22
Cauchy fitting parameters for refractive index a
b
c
1.92 1.82 1.76 1.67 1.59
1.25 × 104 -1.85 × 104 -6.02 × 102 -3.76 × 103 3.69 × 102
1.71 × 109 2.60 × 109 2.28 × 109 2.10 × 109 2.00 × 109
based on a theory of light propagation. An example of an extracted n (λ) and it's accompanying Cauchy fit are shown in Fig. 9 (a). It should be noted that refractive index value of the present Er2O3 thin film (1.61 at 589.3 nm) is comparable to the refractive index of that value reported in literature for thin film [32,49,50] and slightly lower than that value reported for bulk crystal (1.96 at 589.3 nm) by Medenbach et al. [51]. In the case of Y2O3, our value (1.973 at 543 nm) may be compared to the value of the thin film reported in Ref. 45 and for bulk crystal of Y2O3 (1.936 at 543.5 nm) [52]. Our values for k are in agreement with those reported in Ref. [53] for Er2O3 thin film. Fig. 10 (a), (b) display the spectral variation of refractive index (n) and extinction coefficient (k) of (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures Ts = 300, 400 and 500 °C. It is clear that both optical constants increase with increasing the substrate temperature. Harris et al. [54] reported that the film density increases as the substrate temperature increases. Consequently, from the Lorentz–Lorenz law, the refractive index also increases. The increase of the density of a film is accompanied by a decrease in the porosity. The porosity of the films can be evaluated from the relation [55]:
Fig. 8. The variation of (αhν)2 with the photon energy, hν, (a) and (αhν)1/2 versus hν (b), of (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films deposited on fused silica glass substrates with thickness ∼ 150 nm at substrate temperatures Ts = 500 °C.
The spectral variations of refractive index and extinction coefficient of the mixed rare earth nanocrystalline oxides (Y1-xErx)2O3 (0 ≤ x ≤ 1) thin film of thickness 150 nm grown on fused silica glass at Ts = 500 °C are shown in the inset of Fig. 9 (a) and Fig. 9 (b), respectively. Obviously, the values of both n and k decrease with increasing the wavelength. Nevertheless, the values of optical constants n and k decrease with increasing the mixing parameter, x. For all films studied, the index of refraction was found to fit well to a second order Cauchy dispersion relation of the form [48]: 2
4
n ðλÞ = a + ðb = λ Þ + ðc = λ Þ
ð8Þ
P=
! n2 −1 1− 2 × 100 nb −1
ð9Þ
where nb (∼1.948 at 580 nm [52]) is the refractive index of the bulk material, and n is the refractive index obtained from Fig. 10(a) at corresponding wavelength (n = 1.82 (T s = 500 °C), n = 1.61 (Ts = 400 °C) and n = 1.38 (Ts = 300 °C) Thus, the porosity of the films at different substrate temperatures is calculated as P = 17% (Ts = 500 °C), 43% (Ts = 400 °C) and 67% (Ts = 300 °C). This suggests that the films deposited at higher substrate temperature were compact, whereas the films deposited at lower substrate temperature were more porous. Accordingly, it can be concluded that the films deposited at higher substrate temperature have better optical quality since they were more compact. The increase of k with substrate temperature is also explained by the increase in film density.
where a, b and c are fitting parameters depending on material (summarized in Table 1). This empirical fit was derived by Cauchy
Table 1 Compositions analysis by EDXS of nanocrystalline (Y1-xErx)2O3 (0 ≤ x ≤ 1) films of thickness 150 nm grown on fused silica glass at Ts = 500 °C. x
formula
Y2O3 (at. %)
Er2O3 (at. %)
0 0.3 0.5 0.7 1
Y2O3 Y1.4Er0.6O3 YErO3 Y0.6Er1.4O3 Er2O3
99.1 72.7 51.5 30.8 -
27.2 48.4 69.2 99.7
Table 3 Direct and indirect band gap values and refractive index Cauchy fitting parameters for the nanocrystalline for (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures. Ts (°C)
Direct band gap (eV)
Indirect band gap (eV)
300 400 500
5.29 5.80 5.94
4.23 4.31 4.51
Cauchy fitting parameters for refractive index a
b
c
1.20 1.54 1.76
4.23 × 104 1.25 × 104 -6.02 × 102
1.13 × 108 1.71 × 109 2.28 × 109
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Fig. 9. Spectral variation of optical constants of (Y1-xErx)2O3 (0.1 ≤ x ≤ 1) films grown on fused silica glass with thickness ∼150 nm at substrate temperatures Ts = 500 °C. (a) The fitting curves of the dispersion relation curve of refractive index (n) obtained by second order Cauchy dispersion formula; the inset represents the refractive index data versus λ extracted from the experimental data of R and T (b) The variation of extinction coefficient (k) with wavelength.
Fig. 10. Spectral variation of optical constants of (Y0.5Er0.5)2O3 film grown on fused silica glass at different substrate temperatures. (a) The fitting curves of the dispersion relation curve of refractive index (n) obtained by second order Cauchy dispersion formula; the inset represents the refractive index data versus λ extracted from the experimental data of R. and T (b) The variation of extinction coefficient (k) with wavelength.
4. Conclusions
one may conclude that it is possible to control the energy band gap by relative fraction of constituent oxides. Finally, a relatively high refractive index is observed for the mixed rare earth oxides (Y1-xErx)2O3 film grown at higher substrate temperature.
In summary, nanocrystalline mixed rare earth oxides (Y1-xErx)2O3 thin film were grown by electron beam evaporation technique on fused silica glass substrate at different substrate temperatures in the range 200-500 °C. The influence of substrate temperature and mixing parameter (x) on the structural and optical properties was studied. The X-ray diffraction patterns of the films showed that, films deposited at lower substrate temperature are amorphous while films grown at higher temperature are crystalline with preferred orientations in the (111), (110), and (100) planes. These peaks are attributed to body centered cubic type structure (BCC-C type structure). The spectrophotometry investigations for mixed rare earth oxides show that the energy gaps and optical constants are depending on the substrate temperature and the mixing parameter as well. In the wavelength range 200-2500 nm films show direct and indirect transitions which are associated with the optical band gap. Precise evaluation of the optical band gap revealed direct transition varied from 5.29-5.94 eV and indirect transitions at 4.23-4.51 eV as substrate temperature increases. However, the change of optical band gap upon of mixing parameter (x) is recorded, Eg = 6.14-5.86 eV for direct transition and Eg = 5.23-4.22 eV for indirect transition. Thus,
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