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Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1−x(TiO2)x thin films
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Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films M. Chandra Sekhar a,b,n, N. Nanda Kumar Reddy b, V.K. Verma b, S. Uthanna c a
Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro Gyeongsan-si Gyeongsangbuk-do, 3854, Republic of Korea Department of Physics, Madanapalle Institute of Technology and Science, Madanapalle 517325, India c Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517502, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 August 2016 Received in revised form 5 September 2016 Accepted 5 September 2016
Tantalum pentoxide (Ta2O5) is promising as gate dielectric for the next generation memory devices. Titanium doped Ta2O5 thin films leads to reduction in the leakage current density of the device. Thin films of (Ta2O5)1 x(TiO2)x (x ¼0 to 0.18) were deposited on quartz and p-Si (100) substrates by DC reactive magnetron sputtering. X-ray diffraction studies revealed the presence of tantalum oxide, and the size of crystallite in the (Ta2O5)1 x(TiO2)x films was decreased with the increase of composition x. The surface stoichiometry of (Ta2O5)1 x(TiO2)x thin films (x ¼0.00 to 0.18) has been studied by X-ray photoelectron spectroscopy. The binding energies for various core level electrons of the different elements present in these films have been determined. FTIR study confirmed the presence of titanium oxide in the tantalum oxide films. The Optical band gap of the (Ta2O5)1 x(TiO2)x films formed on quartz substrate decreased from 4.50 to 4.32 eV with the increase in the content of titanium. An Al/(Ta2O5)1 x(TiO2)x/p-Si metal–insulator–semiconductor (MIS) device was fabricated whose leakage current density decreased from 2.10 10 5 to 3.6 10 8 A/cm2 with the increase of titanium, i.e., x r 0.15. The Schottky barrier height values were extracted from current-voltage (I-V) characteristics of Al/(Ta2O5)1 x(TiO2)x/p-Si (MIS) structure and are found to be ranging from 0.66 eV to 0.82 eV for different dopant compositions. The capacitance was measured for pure Ta2O5 films at 1 MHz and was found to be 4.79 10 9 F. Also, it was noticed that the capacitance was found to be increased in (Ta2O5)1 x(TiO2)x films to 7.10 10 9 F for the dopant composition x ¼ 0.15. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Tantalum oxide Titanium Sputtering Leakage currents Band gap Dielectric constant
1. Introduction High quality dielectric materials with high permittivity are required for the continuous miniaturization of microelectronic devices. Due to prominent device scaling in ultra-large-scale integrated circuits (ULSI), the conventional ultra-thin gate oxide SiO2 is not acceptable for various practical reasons. Many of these problems can be solved if the SiO2 layer is replaced with a high dielectric constant material (high-k). Several researchers tried for high-k metal oxides such as TiO2, ZrO2, HfO2 and Ta2O5 [1–5] have been extensively investigated as possible replacement of silicon dioxide. Tantalum oxide (Ta2O5) is extensively used as gate dielectric in MIS capacitor due to its high dielectric constant and a high degree of compatibility with the compound materials used in advanced microelectronic devices [6–9]. An extensive number of doped ions can be accommodated in the crystal structure of Ta2O5. n Corresponding author at: Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro Gyeongsan-si Gyeongsangbuk-do, 3854, Republic of Korea. E-mail address: [email protected] (M. Chandra Sekhar).
The doping of Ta2O5 with another metal oxide enhances the electrical and dielectric properties. It is assumed that these dopants will act as modifiers if Ta2O5 is doped with an oxide making the bonding with excess oxygen in metal oxides [10]. Hence, this process may reduce the leakage currents and trap density. The electrical properties of Ta2O5 mixed with TiO2, Y2O3 or WO3 improved when compared to the pure Ta2O5 [11,12]. The improvement in electrical properties may be due to compensation of preexisting partial oxygen vacancies in Ta2O5 by the dopant. The small quantity of TiO2 can remarkably increase the permittivity of Ta2O5 [9,11]. This enhancement in dielectric properties depends on the quantity of TiO2 in Ta2O5 and post-deposition annealing conditions. Cava et al. [9] observed that the dielectric constant of Ta2O5 can be increased from 35 to 126 through the addition of 8% TiO2. The enhancement of dielectric constant in Ta2O5-TiO2 may be correlated to the phase transformation existing at TiO2 content of 13% in Ta2O5 [13]. The electrical and dielectric property of the capacitors strongly depends on the type of gate material. The implementation of high-k dielectric materials in device fabrication requires the gate electrode as metal instead as poly-Si due to its incompatibility with most of high-k materials. The type of metal
http://dx.doi.org/10.1016/j.ceramint.2016.09.034 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: M. Chandra Sekhar, et al., Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.034i
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electrode influences the leakage current, hence, in the present investigation the selection has been made Al as metal gate for (Ta2O5)1 x(TiO2)x capacitors. Substantial consideration has been paid to the metal–semiconductor (MS) Schottky contacts due to their dynamic influence to the semiconductor devices and the integrated circuits [14–16]. Electronic properties of these Schottky contacts are obtained by its barrier height and ideality factor parameters [17]. The presence of an insulator layer (such as SiO2, Si3N4, Ta2O5, TiO2) between a metal and a semiconductor translates the metal–semiconductor (MS) structure into a metal–insulator–semiconductor (MIS) structure [18]. Karatas et al. [19] reported that the device performance, reliability and stability are significantly influenced by interfacial insulator layer. Thus, in determining the electrical characteristics of metal–insulator–semiconductor (MIS) structures, the interfacial insulator layer plays a significant role. Over last two decades, MIS structures have been widely investigated to determine the effects of the interfacial layer quality between Si and the thermally grown insulator/oxide layer on the device characteristics of these devices [19–22]. Previously, we reported the effect of sputter power on Schottky barrier characteristics of Al/TiO2/p-Si structure [21]. (Ta2O5)1 x(TiO2)x film on electrical and structural characteristics of Al/(Ta2O5)1 x(TiO2)x/p-Si structure. In this study, we extended our analysis on the effect of variation of (Ta2O5)1 x(TiO2)x thin films composition (with 0 r x r0.18) on p-Si by DC reactive magnetron sputtering and study their chemical composition, core level binding energies, structural, optical, electrical, dielectric properties and correlated the results with dopant content of titanium in tantalum oxide films.
2. Experimental 2.1. Thin film preparation The p-Si (100) substrates were first cleaned by deionized water and acetone to remove organic and other impurities. Then the Silicon substrates were dipped in hydrofluoric acid (HF) solution (HF: deionized water is in the ratio 1:40) in order to remove the silicon oxide (SiO2) on the surface of silicon substrate. DC reactive magnetron sputtering technique was employed to deposit the (Ta2O5)1 x(TiO2)x films with composition x (0 r x r0.18) on p-Si. The Mosaic target used in the present study comprises of 4" diameter circular disc of titanium (99.99% purity) and small tantalum pieces with different areas (99.99% purity). Argon gas (99.99% pure) and oxygen (99.999% pure) were used as sputtering and reactive gas, respectively. The sputtering chamber was evacuated to a base pressure of 2 10 4 Pa with diffusion pump and rotary pump combination. The partial pressures of the two gases were controlled with mass flow controllers. Prior to the deposition of mixed oxide films, the process parameters were optimized for deposition of tantalum oxide films at different current densities at a base pressure of 2 10 4 Pa, oxygen partial pressure 5 10 2 Pa and sputtering pressure of 0.5 Pa with substrate temperature 303 K. The as-deposited Ti-doped Ta2O5 films were annealed in air at 973 K for 1 h. After deposition of the Ti-doped Ta2O5 films, MOS (metal/oxide/semiconductor) capacitors were fabricated with the configuration of Al/(Ta2O5)1 x(TiO2)x/p-Si by deposition of aluminum as top electrode by photolithography technique whose area is 7 10 2 cm2. Metal oxide semiconductor (MOS) capacitor of mixed-oxide (Ta2O5)1 x(TiO2)x was prepared for studying the electrical and dielectric properties. 2.2. Thin film characterization The X-ray photo-electron spectroscopic studies are performed
to analyze the chemical binding energy states by using SPECS GmbH spectrometer (Phoibos 100MCD Energy Analyzer) with MgKα1 radiation (1253.6 eV). The structural properties are studied by the X-ray diffraction (Siefert X-ray diffractometer, model 3003 TT) using CuKα1 radiation (wavelength λ ¼0.15406 nm) source filtered by Nickel thin film at a scan speed of 0.03 °/sec in the 2θ, from 20 to 60 °. The peak positions were determined precisely using RAYFLEX-Analyze software. The chemical bonding configuration of the formed films on silicon substrates was obtained from the Fourier transform infrared spectrophotometer (Nicolet model 5700 FT-IR) in the wavenumber range 400 – 1600 cm 1. The thickness of the films is measured with α- step profilometer and it is found to be about 350 7 5 nm. The capacitance – voltage (C–V) characteristics of the (Ta2O5)1 x(TiO2)x capacitors are measured with MIOKI (model 3532-50) LCR meter. The current – voltage (I–V) characteristic of the capacitor is measured with the Hewlett Peckard (model hp 4140B) pA meter. UV–VIS–NIR double beam spectrophotometer (Perkin Elmer model Lambda 950) is used for measuring the optical transmittance of the films deposited on quartz substrates in the wavelength (λ) range 200 – 800 nm.
3. Results and discussions 3.1. Structural properties Fig. 1 shows the representative XPS survey scan spectrum of (Ta2O5)1 x(TiO2)x thin films formed at oxygen partial pressure of 5 10 2 Pa, are prepared in the composition range 0 r x r l and annealed at 973 K. The survey scan spectrum indicates the characteristic tantalum (Ta 4 f at about 25 28 eV, Ta 3d at about 230 – 240 eV), titanium, (Ti 2p at about 458 – 466 eV), oxygen (O 1 s about 530 eV) and carbon peaks. At the binding energy 284.4 eV, a carbon 1s peak was observed. The presence of this peak is related to the organic surface contamination, which corresponds to the fact that the samples are exposed to air before measurements. Carbon peak disappears from the surface of (Ta2O5)1 x(TiO2)x films after pre argon sputtering. Fig. 2(a) shows the XPS core level binding energy spectra of Ta 4f of the (Ta2O5)1 x(TiO2)x films with composition x ¼0, 0.04, 0.12 and 0.18. The core level binding energies are measured the binding energies from 25 to 28 eV exhibits two peaks corresponding to Ta 4f7/2 and Ta 4f5/2. The XPS spectrum of Ta2O5 revealed that the characteristic doublet of Ta 4f7/2 and Ta 4f5/2 peaks formed due to spin orbit splitting in Ta2O5. The core level binding energy of Ta 4f7/2 and Ta 4f5/2 were 25.95 and 27.87 eV indicated the Ta5 þ state in the (Ta2O5)1 x(TiO2)x films for composition x ¼0 [23]. The separation between Ta 4f7/2 and Ta 4f5/2 peaks was observed about 1.92 eV and is consistent with that reported in the literature for Ta2O5 [24,25]. The core level binding energies of Ta 4f7/2 and Ta 4f5/2 were 25.95 and 27.87 eV for the (Ta2O5)1 x(TiO2)x films with x¼ 0. The core level binding energy of Ta 4f7/2 and Ta 4f5/2 were 26.62–28.79 eV for the (Ta2O5)1 x(TiO2)x films with x ¼0.18. The shift in the core level binding energy for Ta 4f7/2 was found to be 0.67 eV. The intense peaks of Ta 4f7/2 and O 1s core level binding energies have been used to characterize the chemical state of Ta in the formed films. The atomic ratio for the formed films was estimated from the core level area ratio of O 1s and Ta 4f7/2 has been used to determine the chemical composition of these oxide films. The ratio of oxygen to tantalum (O/Ta) was estimated nearly 2.6 at composition x ¼ 0, which is indicated the growth of nearly stoichiometric Ta2O5 films. The increase in the core level binding energies generally indicates an increase in positive charge on the atom. The X-ray photoelectron spectroscopic measurements on sputtered films (Ta2O5)1 x(TiO2)x with x ¼0, 0.12, 0.18 and 1 are presented in
Please cite this article as: M. Chandra Sekhar, et al., Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.034i
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Fig. 1. XPS narrow scan spectra of (Ta2O5)1 x(TiO2)x thin films with variation of composition x.
Fig. 1. These results show that the band intensity appears to be proportional to the amount of Ta in the film. Fig. 1 XPS spectra also describes the presence of Ta 4f, Ti 2p, Ta 3d and O 1s core levels, respectively, for films (Ta2O5)1 x(TiO2)x grown with x ¼0.12 and 0.18. The results obtained from XPS studies shows that the sputtered (Ta2O5)1 x(TiO2)x films have been grownup close to the nominal stoichiometry. Fig. 2(b) shows the core level binding energy spectra of Ti 2p of the (Ta2O5)1 x(TiO2)x films with composition x ¼0.04, 0.12, 0.18 and 1. The Ti 2p3/2 peak for Ti4 þ in TiO2 generally lies in the binding energy range 458.6 – 459.2 eV, while the Ti 2p3/2 peaks for Ti þ 2 and Ti0 lie in the binding energy ranges 454.9 – 455.2 eV and 453.7 – 454.2 eV respectively [26]. The core level binding energy of Ti 2p3/2 and Ti 2p1/2 were 458.13 and 465.80 eV for the (Ta2O5)1 x(TiO2)x films with composition x ¼0.04. The binding energy separation between the Ti 2p3/2 and Ti 2p1/2 peaks is 7.67 eV. The core level binding energy of Ti 2p3/2 was shifted from 458.1 to 458.9 eV for the (Ta2O5)1 x(TiO2)x films with a composition x from 0.04 to 1 [27]. The observed shift in the core level binding energy was 0.8 eV. Fig. 2(b) will shows the Ti 2p3/2 peak location is at 458.45 eV, demonstrating it corresponds to Ti4 þ . Additionally, Ti 2p1/2 peak location is found at 464.20 eV, in (Ta2O5)1 x(TiO2)x films with composition x ¼ 0.18, implying that a spin-orbit splitting of 5.75 eV, a value which lies much closer to the oxidation state of Ti4 þ (5.5 eV) [26]. The spin orbit splitting in between Ti 2p3/2 and Ti 2p1/2 peaks were about 5.75 eV and is consistent with the reported values in the literature for TiO2. The overall Ti 2p spectrum confirms the presence of Ti4 þ species in the formed films. The Ti 2p spectrum indicated the absence of peaks at lower energy side of the main peak, which is indicated that there
is no characteristic metallic titanium. The combined GaussianLorentzian functions do not indicate the presence of any of the phases such as Ti2O3, TiO and metallic titanium by curve fitting in Ti 2p spectra [28]. The ratio of oxygen to titanium (O/Ti) was estimated nearly 2 at composition x ¼1, which indicates the growth of nearly stoichiometric TiO2 films. Fig. 2(c) shows the core level binding energy spectra of O 1s for (Ta2O5)1 x(TiO2)x films. The O 1s peak was observed in the binding energy range 530 – 531 eV. There is no considerable shift in binding energies for O 1s peak with composition x. The Ta and Ti atoms in these films were fully oxidized since no metallic states could be observed in the spectra. The ratio of oxygen to tantalum (O/Ta) and oxygen to titanium (O/ Ti) in the formed films was estimated from the XPS peak area by using relative sensitivity factors. The Ti doped Ta2O5 films, which corresponds to the composition ratios of 0.04, 0.08, 0.12, 0.15 and 0.18 respectively. In Fig. 2, all three core levels in the TiO2 doped Ta2O5 films, i.e., Ta 4f, Ti 2p, and O 1s, shift to higher binding energy states when the Ti/(Ti þTa) ratio increases. This shifting was due to the existence of dissimilar metal atoms, i.e., Ti vs. Ta [29– 32]. Binding energies in these films were strongly influenced by the charge transfer mechanism that involved all three constituent elements, i.e., Ta, Ti and O. The electronegativity of Ta (1.50) is less than the dopant Ti (1.54). As a result, the charge transfer between Ta and O decreased and between Ti and O increased when the Ti dopant composition was increased. The decrease of the charge transfer increases the binding energies, which showed up in the XPS spectra. Fig. 3 compares the XRD patterns of the Ti-doped Ta2O5 films with composition x, (x ¼0 to 0.18). The films deposited at room temperature (303 K) were found to be X-ray amorphous. These
Please cite this article as: M. Chandra Sekhar, et al., Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.034i
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Fig. 2. XPS narrow scan spectra of (Ta2O5)1 x(TiO2)x thin films with different values of composition x for (a) Ta 4 f, (b) Ti 2p and (c) O 1 s.
films were annealed in air at 973 K for 1 h. It is observed that, the crystal structure of the doped films are not strongly influenced by the variation of Ti dopant composition. When the composition ratio was increased upto 0.18, the diffraction peaks obtained from (0 0 1), (1 11 0), (2 0 1), (0 0 2) and (3 11 1) planes were observed at 2θ of 22.9°, 28.3°, 37.1°, 46.8° and 56.3° respectively, indicated the growth of orthorhombic Ta2O5 [JCPDS card No. 25–0922] over tapping of tantalum and titanium [33,34]. The sensitivity of the XRD is not sufficient to detect it. Though a limited amount of the crystalline phase of TiO2 is present in these thin films. Among all other peaks, the intensity of (0 0 1) orientation had been prominent. The intensity of the strongest peak, i.e., (0 0 1) decreased when the composition x was increased. The composition x also affected the crystallite size in the doped films. The value of full width-at-half-maximum of the diffraction peak (0 0 1) is increased
by increasing the titanium composition. The crystallite size (L) of the films was calculated from the (0 0 1) peak using the Debye Scherrer's formula [35].
L = 0.9λ/β cos θ
(1)
where β is the full width at half maximum intensity. The crystallite size of the films decreased from 72 to 58 nm with the increase in dopant composition x (from x ¼0 to 0.18). This indicates that the higher dopant composition might reduce their crystallite size. The crystallite size reduction is confirmed the effectiveness of the Ti doping technique. It is well known that a grain boundary inside the crystallized high-k gate dielectrics increases the leakage current. By suppressing the film crystallization process, the current leakage is decreased making the film suitable for MOS transistor applications.
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Fig. 3. X-ray diffraction profiles of (Ta2O5)1 x(TiO2)x thin films in the composition range 0r x r0.18. Fig. 5. Optical transmittance spectra of (Ta2O5)1 x(TiO2)x thin films in the composition range 0r x r 0.18.
O–Ti stretching vibration mode of TiO2 [27]. As the composition ratio was increased to x¼ 0.18 the intensity of the absorption band at 436 cm 1 was increased. While the formation of the films by reactive magnetron sputtering with sputter power of 200 W, the oxygen radicals with high energy in the reactive sputtering interacts with silicon resulting the formation of SiO2 layer. After annealing process the peak observed at 1083 cm 1 was attributed to interstitial oxygen in the silicon due to the formation of SiO2 layer [40]. Thus, the leakage currents were significantly affected by the presence of interfacial SiO2 layer [41]. 3.2. Optical properties
Fig. 4. FTIR spectra of (Ta2O5)1 x(TiO2)x thin films in the composition range 0r x r 0.18.
Fig. 4 shows the Fourier transform infrared spectra for the (Ta2O5)1 x(TiO2)x thin films with variation of composition x, from x ¼0 to 0.18. These peaks represent the infrared transmittance spectra in the range 400–1600 cm 1 of the (Ta2O5)1 x(TiO2)x films annealed in oxygen ambient at 973 K. A significant peak identified at 522 cm 1 in all FTIR spectra confirmed the existence of Ta2O5. Also, the spectra exhibit the absorption bands at 630, 836 and 1083 cm 1 (Ta–O–Ta stretching and Si–O stretching) [36,37]. Franke [38] and Kaliwoh [39] et al. observed the strong peak found at approximately 522 cm 1 confirms the presence of Ta2O5 in polycrystalline form. With increasing the composition ratio, the peak intensity of the absorbance at 522 and 630 cm 1 decreases upto x¼ 0.08. At composition ratios, x 40.08, the positions of absorption band at 522 cm 1 were shifted towards the lower wavenumber side. The (Ta2O5)1 x(TiO2)x thin films for composition x ¼0 have no absorption bands related to titanium oxide. At x ¼0.08 an absorption band located at 436 cm 1 was related to Ti–
Fig. 5 shows the optical transmittance spectra of the (Ta2O5)1 x(TiO2)x films with the increase in composition from x ¼0 to 0.18. The optical transmittance of the films at composition x ¼0 showed 92% at wavelength 400 nm. The transmittance of the films increased to 96% with composition x (x ¼0.18). At about 300 nm, a sharp absorption edge was observed. From the Fig. 5, it is observed that the optical absorption edge is shifted towards the higher wavelength side with increase of composition x. The optical absorption coefficient (α) of the films is estimated from the optical transmittance (T) data in the wavelength range 200–1000 nm using the relation
α = ( 1/t) ln ⎡⎣ T⎤⎦
(2)
where, T is the transmittance and t is the thickness of the film. Near the fundamental absorption edge the optical absorption coefficient (α) of the formed films was found to be exponentially dependent on the photon energy (hν). The optical band gap (Eg) of the films is estimated from the intercept of the plots of (αhν)2 versus photon energy (hν) which assumes the direct transition between the top of the valance band and the bottom of the conduction band using the relation
(αhν) = A (hν–Eg)1/2
(3)
where, A is the optical absorption edge width parameter. By extrapolating the straight line portions of the (αhν)2 versus (hν) curves, the optical band gap for different film compositions was
Please cite this article as: M. Chandra Sekhar, et al., Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.034i
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Fig. 7. Leakage current density for Al/(Ta2O5)1 x(TiO2)x/Si capacitors (with 0 r x r0.18). Fig. 6. Variation of optical band gap for (Ta2O5)1 x(TiO2)x thin films (with 0r x r 0.18).
obtained. The optical band gap for the (Ta2O5)1 x(TiO2)x thin films (at x ¼ 0) for direct forbidden transitions is found to be 4.5 eV [42]. The optical transmission spectra of (Ta2O5)1 x(TiO2)x thin films exhibits clearly a shift in the fundamental absorption edge towards the higher wavelength side which is indicated the decrease in optical band gap. In our earlier investigation [43], we reported the optical band gap of 3.43 eV for the TiO2 films annealed at 973 K. Fig. 6 shows the variation of optical band gap with composition x. The estimated energies of the optical band gap from the plots of (αhν)2 versus photon energy (hν) were in the range 4.50 – 4.26 eV. The optical band gap of the (Ta2O5)1 x(TiO2)x film is 4.50 eV (at x ¼0) and it decreased to 4.26 eV with composition x (at x ¼ 0.18) [44]. This is readily understood that the optical band gap Ta2O5 is greater than TiO2. Due to decrease of the number of Ta5 þ by the substitution of Ti4 þ ions, the optical band gap values were observed to be decreasing with the increase in composition x. 3.3. Electrical properties Fig. 7 shows the current density – voltage (J - V) plots of the Al/ (Ta2O5)1 x(TiO2)x/p-Si capacitors with variation of composition x (x ¼0 to 0.18). It is seen from the figure that the J - V curves showed the asymmetric behavior for positive and negative voltages. It may be possible to the differences in the work functions between aluminum and p-Si and the conduction mechanisms through the Al/(Ta2O5)1 x(TiO2)x/p-Si stack. A leakage current density of 2.1 10 5 A/cm2 was obtained (at an electric field of 1.5 V) at x¼ 0, the titanium incorporation seems to suppress the leakage current it is decreased to 3.6 10 8 A/cm2 at the composition x ¼0.15, and it is increased to 5.5 10 7 A/cm2 with the value of x 4 0.18. The leakage current density is influenced by the grain size and hence in crystallinity. The grain boundary structure of the films is denser and more uniformly increasing with composition x. The crystallinity of the films is decreased with increase in composition x. From the optical properties discussed above, titanium oxide has a relatively small band gap 3.43 eV than the band gap for tantalum oxide 4.5 eV. The values from energy band diagram for the tantalum oxide, the band gap is wider and the conduction band minimum is higher than that of the TiO2. Thus, the Ta2O5 sample has a higher potential barrier across the oxide [45]. Therefore, the leakage current of the Ta2O5 is smaller than
that of the TiO2, regardless of the deterioration of the interface caused by the oxidation of the substrate. When Ti is doped in Ta2O5, the reaction of Ti and Ta2O5 should decrease the leakage currents. When leakage current of Al/(Ta2O5)1 x(TiO2)x/Si capacitors is plotted as a function of electric field, it is observed that Schottky emission takes place at low electric fields and Poole– Frenkel emission at higher electric fields. The typical semilog-current density-voltage (J-V) characteristics of the Al/(Ta2O5)1 x(TiO2)x/p-Si (with 0 r x r0.18) MIS structure are illustrated Fig. 7. The current density–voltage (J–V) relation for a barrier between metal-semiconductor (MS) or metalinsulator-semiconductor (MIS) structure is given by [46].
⎛ −qV ⎞⎤ ⎛ qV ⎞⎡ ⎟⎥ ⎟⎢ 1 − exp⎜ J = J0 exp⎜ ⎝ kT ⎠⎦ ⎝ nkT ⎠⎣
(4)
where Jo is the saturation current density and can be written as
⎛ −qΦb ⎞ ⎟ Jo = A**T2exp⎜ ⎝ kT ⎠
(5)
where, q is the electronic charge, Ann is the effective Richardson's constant equals to 32 A/cm2K2 for p-Si [21], A is the diode area, k is the Boltzmann's constant, Фb is the (Schottky barrier height) SBH, and n is the ideality factor. The ideality factor is extracted from the slope of the linear region of the forward-bias lnI versus V curves and can be obtained from Eq. (4) as
n=
q ⎛ dV ⎞ ⎟⎟ ⎜⎜ kT ⎝ d( ln J ) ⎠
(6)
The SBH (Фb) is determined from the extrapolated Jo and is expressed as
Φb =
kT ⎛ A**T2 ⎞ ⎟⎟ ln⎜⎜ q ⎝ J0 ⎠
(7)
The value of Jo is obtained from the intercept of the linear extrapolation of the forward J-V curve at zero voltage. The experimental values of n and Фb were determined from Eqs. (6) and (7), respectively. The estimated SBH of Al/(Ta2O5)1 x(TiO2)x/p-Si MIS structure with the dopant composition x¼ 0 is found to be 0.66 eV. The estimated SBH of the Al/(Ta2O5)1 x(TiO2)x/p-Si MIS structures are 0.74 eV for the dopant composition x ¼0.04, 0.76 eV for the dopant composition x ¼0.08, 0.69 eV for the dopant composition
Please cite this article as: M. Chandra Sekhar, et al., Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.034i
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Fig. 8. Plots ln(J) versus E1/2 (Schottky Conduction (Ta2O5)1 x(TiO2)x/p-Si capacitors (with 0r x r 0.18).
Mechanism)
of
Al/
x ¼0.12, 0.82 eV for the dopant composition x ¼0.15 and 0.71 eV for the dopant composition x ¼0.18. The SBHs extracted for Al/ (Ta2O5)1 x(TiO2)x/p-Si MIS structure are found to be in good agreement with those reported by Selçuk et al. [47] for Al/SiO2/pSi SDs. Moreover, the ideality factors for Al/(Ta2O5)1 x(TiO2)x/p-Si MIS structure are found to be varied from 4.21 to 1.93. It is clearly seen that the estimated ideality factor values are greater than unity. This high value of n for Al/(Ta2O5)1 x(TiO2)x/p-Si MIS structure is due to the effect of oxide layers, series resistances and inhomogeneities of barrier [48]. One of the main reasons of barrier inhomogeneities in MIS structures may be ascribed to the result of interface defects such as caused by grain boundaries, multiple phases, facets, defects and mixture of different phases [49]. Fig. 8 shows the plots ln(J) versus E1/2 of Al/(Ta2O5)1 x(TiO2)x/p-Si capacitors for positive bias voltages with variation of composition x. It is noticed that the leakage currents are nearly proportional to the electrical field. Schottky emission (SE) occurs at the interface between the (Ta2O5)1 x(TiO2)x film and the Al electrode where a Schottky barrier is formed. Schottky emission is a field-assisted thermionic emission of an electron over a potential energy barrier and it can be explained according to the following equation:
JSE = A*T2exp[ − q(ΦB − √qE/4Лε0kr )/kT ],
7
(8)
where A*¼(4Лqm*k2/h3), which is termed as the effective Richardson constant. JSE is the current density, m* is the effective mass of an electron mass in oxide. ΦB, kr, k and h are the Schottky barrier height, dynamic dielectric constant, Boltzmann's constant, and Planck's constant respectively. If the plots of ln(J) versus E1/2 be in the linear form, then the current conducting through an oxide that is followed by SE. The Schottky barrier height and kr values can be extracted from the intercept of the y-axis and the slope of the plot. Fig. 9 shows the plots ln (J/E) Vs E1/2 of Al/(Ta2O5)1 x(TiO2)x/pSi capacitors for positive bias voltages. From the Fig. 9, it is observed that the values increased with composition x, the linearity decreased which shows that the PF emission (thermal emission of the charge carriers) diminished due to the reduction of traps. The emission of trapped electrons into the conduction band of insulator leads to the Poole-Frenkel emission. This Poole-Frenkel emission is a field-assisted thermal de-trapping of a carrier from the bulk oxide into the conduction band. Therefore, it is a bulklimited conduction mechanism and extensively applicable according to the following equation: [50]
Fig. 9. Plots ln(J/E) versus E1/2 (Poole-Frenkel Conduction Mechanism) of Al/ (Ta2O5)1 x(TiO2)x/p-Si capacitors (with 0r x r 0.18).
JPF = qNc μ E exp⎡⎣ −q Φt − √qE/Лεokr /kT⎤⎦
(
)
(
)
(9)
where JPF is the current density attributed to Poole-Frenkel emission, Nc the density of states in the conduction band and Φt the trap energy level in the oxide, respectively. From the above equation the value of kr can be extracted from the slope of a ln(J/E) versus E1/2 plot. The capacitance–voltage (C–V) curves were obtained by sweeping the gate voltage was swept from positive to negative bias and back again (from þ3 to 3 V) at an operating frequency of 1 MHz of Al/(Ta2O5)1 x(TiO2)x/p-Si capacitors variation of composition x (x ¼ 0 to 0.18) are shown in Fig. 10. The Capacitance-Voltage (CV) curves show very low hysteresis which indicates the low trap densities. The characteristic curves are similar to normal capacitance – voltage dependence observed in 3 typical ranges, i.e. accumulation, depletion and inversion. For negatively biased structure, negative electron charge at the gate is balanced by positive hole charges accumulated near the surface of the semiconductor (p-type). Switching the bias voltage into positive direction makes the semiconductor surface depleted from the holes, so that the additional capacitance of space charge layer Cs, is serially connected to Cox. In strong inversion conditions, potential changes at the gate are determined by the presence of electrons at the semiconductor metal interface. In an MOS capacitor, the capacitance C measured in the accumulation state which is equal to oxide capacitance Cox ¼4.79 nF of the films with composition x ¼0.00. The capacitance of (Ta2O5)1 x(TiO2)x films increased from 4.79 to 7.1 nF for the variation of x ¼0 to 0.15, and it is measured at composition x ¼ 0.18 was 6.1 nF. From figure it is revealed that the accumulation capacitance is increased with the variation of x ¼0 to 0.15. It could be due to the reduction of structural defects in (Ta2O5)1 x(TiO2)x layer with increasing x. Apart from this, the depletion layer thickness is also reduced noticeably with variation of x, resulting in the improvement of interface quality at (Ta2O5)1 x(TiO2)x/Si interface. The possible explanation for this observation might be the reduction in the defects of dielectric layer and/or interfacial state density (Dit). While, there is noticeable frequency dispersion at accumulation region. The frequency dispersion in the C - V plots is not only due to the defects in high-k and high-k/Si stacks, but also strongly depends on series
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Fig. 10. Capacitance–voltage characteristics of Al/(Ta2O5)1 x(TiO2)x/Si capacitors in the composition range 0r x r0.18.
resistance. Hence, it is confirmation for the significant improvement in the SiO2 layer thickness. The oxygen vacancies in Ta2O5 films can be compensated by Ti4 þ ions as they are substitutionally incorporated into Ta5 þ sites [8,51]. Consider this assumption (Ta5 þ radius, 0.07 nm, is almost the same as that of Ti4 þ ion, 0.068 nm), the observed reduction in the positive charge after titanium doping can be a result of this compensation. With increasing titanium content the negative charge increases. It indicates that the charge is assigned to titanium related defects, but not that of tantalum. The trend of enhanced capacitance with increasing titanium amount is observed. Dielectric constant (k) of the (Ta2O5)1 x(TiO2)x films was calculated by using the following relation
C = k εoA/t
(10)
where C is the capacitance (F), d the thickness of (Ta2O5)1 x(TiO2)x thin films (mt), εo is the permittivity of free space (8.854 10–12 F/ m) and A the area of the dielectric film (mt2), respectively. The dielectric constant of (Ta2O5)1 x(TiO2)x films was 27 at composition x ¼0 and it is increased to 38 with the increase of composition x, i.e., x o 0.15. The dielectric constant measured at composition x ¼ 0.18 was found to be 33. The increase of dielectric constant with composition x from x ¼0 to 0.15 is due to Ti-doped Ta2O5 leads to increase in capacitance in the accumulation region, hence, increase in the dielectric constant of the (Ta2O5)1 x(TiO2)x films.
4. Conclusions Thin films of (Ta2O5)1 x(TiO2)x (with x ¼ 0 to 0.18) were
prepared by the DC reactive magnetron sputtering technique by mosaic target tantalum-titanium, in an oxygen partial pressure of 5 10 2 Pa onto p-Si and quartz substrates at 303 K. The formed films (Ta2O5)1 x(TiO2)x were annealed in air at 973 K for 1 h. The XPS, XRD and IR measurements revealed that the sputtered films were nearly stoichiometric (having the same oxidation state) with respect to the target material composition. The typical spin orbit splitting observed in between Ta 4 f7/2 and Ta 4 f5/2 peaks was about 1.92 eV and a separation of 5.75 eV between two peaks of Ti 2p3/2 and Ti 2p1/2 confirms the formation of Ta2O5 and TiO2. The optical band gap decreased with increasing Ti dopant in (Ta2O5)1 x(TiO2)x films. The optical band decreases from 4.50 eV to 4.26 eV with the increase of x (x ¼0 to 0.18). The electrical measurements exhibit a decrease in leakage current density from 2.1 10 5 A/cm2 (at an electric field of 1.5 V) to 3.6 10 8 A/cm2 with increasing composition x from 0 to 0.15. Moreover, the leakage current measured at composition x ¼0.18 was found to be 5.5 10 7 A/cm2. The Ti incorporation into Ta2O5 causes the generation of negative oxide charge which reduces the existing positive charge in Ta2O5. The positive oxide charge in Ta2O5 is addressed to oxygen a vacancy, the titanium doping is away to suppress oxygen vacancies, that results in the decrease in leakage current density. Two possible conduction mechanisms such as Schottky effect and Poole-Frenkel conduction mechanism were used to explain the origin of leakage currents in the sputtered (Ta2O5)1 x(TiO2)x thin films. The capacitance of the (Ta2O5)1 x(TiO2)x films was observed to be increased with the increase of dopant composition from x ¼0 to 0.15 may be ascribed to the structural improvements. Thus, it is concluded that the thin films of (Ta2O5)1 x(TiO2)x containing 0.15% TiO2 exhibited better electrical properties when compared to the pure Ta2O5 films.
Please cite this article as: M. Chandra Sekhar, et al., Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.034i
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Acknowledgments Authors are thankful to Dr. G. Mohan Rao, Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore for extending the X-ray photoelectron spectroscopic facilities and useful discussions.
References [1] The International Technology Roadmap for Semiconductors, 2002 Update, Semiconductor Industry Association, Austin, TX, International SEMATECH, 2001/2002. [2] M. Kadoshima, M. Hiratani, Y. Shimamoto, K. Torii, H. Miki, S. Kimura, T. Nabatame, Rutile-type TiO2 thin film for high-k gate insulator, Thin Solid Films 424 (2003) 224–228. [3] S. Ramanathan, C.M. Park, P.C. McIntyre, Electrical properties of thin film zirconia grown by ultraviolet ozone oxidation, J. Appl. Phys. 91 (2002) 4521–4527. [4] A. Callegari, E. Cartier, M. Gribelyuk, H.F. Okorn-Schmidt, T. Zabel, Physical and electrical characterization of Hafnium oxide and Hafnium silicate sputtered films, J. Appl. Phys. 90 (2001) 6466–6475. [5] G.B. Alers, D.J. Werder, Y. Chabal, H.C. Lu, E.P. Gusev, E. Garfunkel, T. Gustafsson, R. Urdahl, Intermixing at the tantalum oxide/silicon interface in gate dielectric structures, Appl. Phys. Lett. 73 (1998) 1517–1523. [6] K.W. Kwon, I.S. Park, D.H. Han, E.S. Kim, S.T. Ahn, M.Y. Lee, Ta/sub 2/O/sub 5/ capacitors for 1 Gbit DRAM and beyond, IEDM 94 (1994) 835–838. [7] Y. Takaishi, M. Sakao, S. Kamiyama, H. Suzuki, W. Watanable, Low-temperature integrated process below 500/spl deg/C for thin Ta/sub 2/O/sub 5/ capacitor for giga-bit DRAMs, IEDM 94 (1994) 839–842. [8] H. Fujikawa, Y. Taga, Effects of additive elements on electrical properties of tantalum oxide films, Appl. Phys. 75 (1994) 2538–2542. [9] R.F. Cava, W.F. Peck Jr., J.J. Krajewski, Enhancement of the dielectric constant of Ta2O5 through substitution with TiO2, Nature 377 (1995) 215–217. [10] L. Manchanda, M.D. Morris, M.L. Green, R.B. van Dover, F. Klemens, T. W. Sorsch, P.J. Silverman, G. Wilk, B. Busch, S. Aravamudhan, Multi-component high-K gate dielectrics for the silicon industry, Microelectron. Eng. 59 (2001) 351–359. [11] D.H. Triyoso, R.I. Hegde, S. Zollner, M.E. Ramon, S. Kalpat, R. Gregory, X. D. Wang, J. Jiang, M. Raymond, R. Rai, D. Werho, D. Roan, B.E. White Jr., P. J. Tobin, Impact of titanium addition on film characteristics of HfO2 gate dielectrics deposited by atomic layer deposition, J. Appl. Phys. 98 (2005) 1–8. [12] K.M.A. Salam, H. Fukuda, S. Nomura, Effects of additive elements on improvement of the dielectric properties of Ta2O5 films formed by metalorganic decomposition, J. Appl. Phys. 93 (2003) 1169–1175. [13] J.L. Waring, R.S. Roth, Effect of oxide additions on the polymorphism of tantalum pentoxide (system Ta2O5-TiO2), J. Res. Nat. Bur. Stand. A Phys. Chem. 72A (1968) 175–186. [14] M. Sze Simon, K.N.G. Kwok, Physics of Semiconductor Devices, Wiley, 2007. [15] C.H. Lin, C.W. Liu, Metal-insulator-semiconductor photodetectors, Sensors 10 (2010) 8797–8826. [16] N. Nanda Kumar Reddy, V. Rajagopal Reddy, Chel-Jong Choi, Influence of rapid thermal annealing effect on electrical and structural properties of Pd/Ru Schottky contacts to n-type GaN, Mater. Chem. Phys. 130 (2011) 1000–1006. [17] J.H. Evans-Freeman, M.M. El-Nahass, A.A.M. Farag, A. Elhaji, Current transport mechanisms and deep level transient spectroscopy of Au/n-Si Schottky barrier diodes, Microelectron. Eng. 88 (2011) 3353–3359. [18] M.M. Bulbul, Frequency and temperature dependent dielectric properties of Al/Si3N4/p- Si(1 0 0) MIS structure, Microelectron. Eng. 84 (2007) 124–128. [19] S. Karatas, F. Yakuphanoglu, F.M. Amanullah, Capacitance–voltage and conductance– voltage characteristics of Ag/n-CdO/p-Si MIS structure prepared by sol–gel method, J. Phys. Chem. Solids 73 (2012) 46–51. [20] E. Ayyildiz, A. Turut, H. Efeoglu, S. Tuzemen, M. Saglam, Y.K. Yogurtcu, Effect of series resistance on the forward current-voltage characteristics of Schottky diodes in the presence of interfacial layer, Solid-State Electron. 39 (1996) 83–87. [21] M. Chandra Sekhar, N. Nanda Kumar Reddy, B. Venkata Rao, G. Mohan Rao, S. Uthanna, Influence of sputter power on structural and electrical properties of TiO2 films for Al/TiO2/Si gate capacitors, Surf. Interface Anal. 46 (2014) 465–471. [22] H. Kim, M.D. Kumar, J. Kim, Highly-performing Ni/SiO2/Si MIS photodetector for NIR detecting applications, Sens. Actuators A 233 (2015) 290–294. [23] P. Shang, S. Xiong, L. Li, D. Tian, W. Ai, Investigation on thermal stability of Ta2O5, TiO2 and Al2O3 coatings for application at high temperature, Appl. Surf. Sci. 285B (2013) 713–720.
9
[24] E. Atanassova, D. Spassov, X-ray photoelectron spectroscopy of thermal thin Ta2O5 films on Si, Appl. Surf. Sci. 135 (1998) 71–82. [25] S.V. Jagadeesh Chandra, S. Uthanna, G. Mohan Rao, Effect of substrate temperature on the structural, optical and electrical properties of dc magnetron sputtered tantalum oxide films, Appl. Surf. Sci. 254 (2008) 1953–1960. [26] J. Moulder, W. Stickle, P. Sobol, K. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., 1995. [27] M. Chandra Sekhar, P. Kondaiah, S.V. Jagadeesh Chandra, G. Mohan Rao, S. Uthanna, Effect of substrate bias voltage on the structure, electric and dielectric properties of TiO2 thin films by DC magnetron sputtering, Appl. Surf. Sci. 258 (2011) 1789–1796. [28] B. Liu, L. Wen, X. Zhao, The photoluminescence spectroscopic study of anatase TiO2 prepared by magnetron sputtering, Mater. Chem. Phys. 106 (2007) 350–353. [29] G. Lucovsky, J.L. Whittern, Y. Zhang, A molecular orbital model for the electronic structure of transition metal atoms in silicate and aluminate alloys, Solid-State Electron. 46 (2002) 1687–1697. [30] G. Lucovsky, G.B. Raynor, Y. Zhang, C.C. Fulton, R.J. Nemanich, G. Appel, H. Ade, J.L. Whittern, Electronic structure of transition metal high-k dielectrics: interfacial band offset energies for microelectronic devices, Appl. Surf. Sci. 212– 213 (2003) 563–569. [31] M.D. Ulrich, J.G. Hong, J.E. Rowe, G. Lucovsky, A.S.Y. Chan, T.E. Madey, Soft x-ray photoelectron spectroscopy of (HfO2)x(SiO2)1 x high-k gate-dielectric structures, J. Vac. Sci. Technol. B 21 (2003) 1777–1782. [32] F. Giustino, A. Bongiorno, A. Pasquarello, Modeling of Si 2p core-level shifts at Si– (ZrO2)x(SiO2)1 x interfaces, Appl. Phys. Lett. 81 (2002) 4233–4235. [33] T.J. Bright, J.I. Watjen, Z.M. Zhang, C. Muratore, A.A. Voevodin, D.I. Koukis, D. B. Tanner, D.J. Arenas, Infrared optical properties of amorphous and nanocrystalline Ta2O5 thin films, J. Appl. Phys. 114 (2013), 083515-083515-10. [34] W. Miao, M.M. Zhu, Z.C. Li, Z.J. Zhang, Optical and Electric Properties of Aligned- Growing Ta2O5 Nanorods, Mater. Trans. 49 (2008) 2288–2291. [35] B.D. Cullity, Elements of X-ray Diffraction, 2nd edn., Addison Wesley, London, 1978. [36] C.H. An, K. Sugimoto, Ellipsometric Examination of Growth and Dissolution Rates of Ta2O5 Films Formed by Metalorganic Chemical Vapor Deposition, J. Electrochem. Soc. 139 (1992) 1956–1962. [37] J.Y. Zhang, Q. Fang, I.W. Boyd, Growth of tantalum pentoxide film by pulsed laser deposition, Appl. Surf. Sci. 138 (1999) 320–324. [38] E. Franke, C.L. Trimble, M.J. DeVries, J.A. Woollam, M. Schubert, F. Frost, Dielectric function of amorphous tantalum oxide from the far infrared to the deep ultraviolet spectral region measured by spectroscopic ellipsometry, J. Appl. Phys. 88 (2000) 5166–5174. [39] N. Kaliwoh, J.Y. Zhang, I.W. Boyd, Ultrathin silicon dioxide films grown by photo-oxidation of silicon using 172 nm excimer lamps, Appl. Surf. Sci. 168 (2000) 288–291. [40] S. Yoo, I. Kim, S. Kim, C.H. Hahn, C. Lee, S. Cho, Effects of annealing temperature and method on structural and optical properties of TiO2 films prepared by RF magnetron sputtering at room temperature, Appl. Surf. Sci. 253 (2007) 3888–3892. [41] Y. Nishioka, S. Kimura, H. Shinriki, K. Mukai, Dielectric characteristics of extremely thin Ta205/Si02 on Si, J. Electrochem. Soc. 134 (1987) 410–415. [42] F. Rubio, J. Denis, J.M. Albella, J.M. Martinez-Duart, Sputtered Ta2O5 antireflection coatings for silicon solar cells, Thin Solid Films 90 (1982) 405–408. [43] M. Chandra Sekhar, P. Kondaiah, G. Mohan Rao, S.V. Jagadeesh Chandra, S. Uthanna, Post-deposition annealing influenced structural and electrical properties of Al/TiO2/Si gate capacitors, Superlattices Microstruct. 62 (2013) 68–80. [44] N. Kaliwoh, J.Y. Zhang, I.W. Boyd, (Ta2O5)1 x(TiO2)x deposited by photo-induced CVD using 222 nm excimer lamps, Appl. Surf. Sci. 186 (2002) 246–250. [45] Q. Lu, Y. Mu, J.W. Roberts, M. Althobaiti, V.R. Dhanak, J. Wu, C. Zhao, C.Z. Zhao, Q. Zhang, L. Yang, I.Z. Mitrovic, S. Taylor, P.R. Chalker, Electrical properties and interfacial studies of HfxTi1–xO2 high permittivity gate insulators deposited on germanium substrates, Materials 8 (2015) 8169–8182. [46] E.H. Rhoderick, R.H. Williams, second ed,Metal-Semiconductor Contacts, 136, Clarendon Press, Oxford 1988, p. 121. [47] B. Selçuk, S. Bilge Ocak, S. Karadeniz, Gaussian distribution on electrical characteristics of Al/SiO2/p-Si structures, Am. J. Mater. Sci. 2 (4) (2012) 125–130. [48] M.A. Yeganeh, S. Rahmatollahpur, R. Sadighi-Bonabi, R. Mamedov, Dependency of barrier height and ideality factor on identically produced small Au/p-Si Schottky barrier diodes, Phys. B 405 (2010) 3253–3258. [49] E.H. Nicollian, J.R. Brews, MOS (metal oxide semiconductor) Physics and technology, Wiley-Interscience, 2003. [50] S.M. Sze, Physics of Semiconductor Devices, 2nd ed, Wiley, New York 1981, pp. 402–404. [51] K.M.A. Salam, H. Fukuda, S. Nomura, Effects of additive elements on improvement of the dielectric properties of Ta2O5 films formed by metalorganic decomposition, J. Appl. Phys. 93 (2003) 1169–1175.
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Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films M. Chandra Sekhar a,b,n, N. Nanda Kumar Reddy b, V.K. Verma b, S. Uthanna c a
Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro Gyeongsan-si Gyeongsangbuk-do, 3854, Republic of Korea Department of Physics, Madanapalle Institute of Technology and Science, Madanapalle 517325, India c Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Tirupati 517502, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 August 2016 Received in revised form 5 September 2016 Accepted 5 September 2016
Tantalum pentoxide (Ta2O5) is promising as gate dielectric for the next generation memory devices. Titanium doped Ta2O5 thin films leads to reduction in the leakage current density of the device. Thin films of (Ta2O5)1 x(TiO2)x (x ¼0 to 0.18) were deposited on quartz and p-Si (100) substrates by DC reactive magnetron sputtering. X-ray diffraction studies revealed the presence of tantalum oxide, and the size of crystallite in the (Ta2O5)1 x(TiO2)x films was decreased with the increase of composition x. The surface stoichiometry of (Ta2O5)1 x(TiO2)x thin films (x ¼0.00 to 0.18) has been studied by X-ray photoelectron spectroscopy. The binding energies for various core level electrons of the different elements present in these films have been determined. FTIR study confirmed the presence of titanium oxide in the tantalum oxide films. The Optical band gap of the (Ta2O5)1 x(TiO2)x films formed on quartz substrate decreased from 4.50 to 4.32 eV with the increase in the content of titanium. An Al/(Ta2O5)1 x(TiO2)x/p-Si metal–insulator–semiconductor (MIS) device was fabricated whose leakage current density decreased from 2.10 10 5 to 3.6 10 8 A/cm2 with the increase of titanium, i.e., x r 0.15. The Schottky barrier height values were extracted from current-voltage (I-V) characteristics of Al/(Ta2O5)1 x(TiO2)x/p-Si (MIS) structure and are found to be ranging from 0.66 eV to 0.82 eV for different dopant compositions. The capacitance was measured for pure Ta2O5 films at 1 MHz and was found to be 4.79 10 9 F. Also, it was noticed that the capacitance was found to be increased in (Ta2O5)1 x(TiO2)x films to 7.10 10 9 F for the dopant composition x ¼ 0.15. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Tantalum oxide Titanium Sputtering Leakage currents Band gap Dielectric constant
1. Introduction High quality dielectric materials with high permittivity are required for the continuous miniaturization of microelectronic devices. Due to prominent device scaling in ultra-large-scale integrated circuits (ULSI), the conventional ultra-thin gate oxide SiO2 is not acceptable for various practical reasons. Many of these problems can be solved if the SiO2 layer is replaced with a high dielectric constant material (high-k). Several researchers tried for high-k metal oxides such as TiO2, ZrO2, HfO2 and Ta2O5 [1–5] have been extensively investigated as possible replacement of silicon dioxide. Tantalum oxide (Ta2O5) is extensively used as gate dielectric in MIS capacitor due to its high dielectric constant and a high degree of compatibility with the compound materials used in advanced microelectronic devices [6–9]. An extensive number of doped ions can be accommodated in the crystal structure of Ta2O5. n Corresponding author at: Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro Gyeongsan-si Gyeongsangbuk-do, 3854, Republic of Korea. E-mail address: [email protected] (M. Chandra Sekhar).
The doping of Ta2O5 with another metal oxide enhances the electrical and dielectric properties. It is assumed that these dopants will act as modifiers if Ta2O5 is doped with an oxide making the bonding with excess oxygen in metal oxides [10]. Hence, this process may reduce the leakage currents and trap density. The electrical properties of Ta2O5 mixed with TiO2, Y2O3 or WO3 improved when compared to the pure Ta2O5 [11,12]. The improvement in electrical properties may be due to compensation of preexisting partial oxygen vacancies in Ta2O5 by the dopant. The small quantity of TiO2 can remarkably increase the permittivity of Ta2O5 [9,11]. This enhancement in dielectric properties depends on the quantity of TiO2 in Ta2O5 and post-deposition annealing conditions. Cava et al. [9] observed that the dielectric constant of Ta2O5 can be increased from 35 to 126 through the addition of 8% TiO2. The enhancement of dielectric constant in Ta2O5-TiO2 may be correlated to the phase transformation existing at TiO2 content of 13% in Ta2O5 [13]. The electrical and dielectric property of the capacitors strongly depends on the type of gate material. The implementation of high-k dielectric materials in device fabrication requires the gate electrode as metal instead as poly-Si due to its incompatibility with most of high-k materials. The type of metal
http://dx.doi.org/10.1016/j.ceramint.2016.09.034 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: M. Chandra Sekhar, et al., Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.034i
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2
electrode influences the leakage current, hence, in the present investigation the selection has been made Al as metal gate for (Ta2O5)1 x(TiO2)x capacitors. Substantial consideration has been paid to the metal–semiconductor (MS) Schottky contacts due to their dynamic influence to the semiconductor devices and the integrated circuits [14–16]. Electronic properties of these Schottky contacts are obtained by its barrier height and ideality factor parameters [17]. The presence of an insulator layer (such as SiO2, Si3N4, Ta2O5, TiO2) between a metal and a semiconductor translates the metal–semiconductor (MS) structure into a metal–insulator–semiconductor (MIS) structure [18]. Karatas et al. [19] reported that the device performance, reliability and stability are significantly influenced by interfacial insulator layer. Thus, in determining the electrical characteristics of metal–insulator–semiconductor (MIS) structures, the interfacial insulator layer plays a significant role. Over last two decades, MIS structures have been widely investigated to determine the effects of the interfacial layer quality between Si and the thermally grown insulator/oxide layer on the device characteristics of these devices [19–22]. Previously, we reported the effect of sputter power on Schottky barrier characteristics of Al/TiO2/p-Si structure [21]. (Ta2O5)1 x(TiO2)x film on electrical and structural characteristics of Al/(Ta2O5)1 x(TiO2)x/p-Si structure. In this study, we extended our analysis on the effect of variation of (Ta2O5)1 x(TiO2)x thin films composition (with 0 r x r0.18) on p-Si by DC reactive magnetron sputtering and study their chemical composition, core level binding energies, structural, optical, electrical, dielectric properties and correlated the results with dopant content of titanium in tantalum oxide films.
2. Experimental 2.1. Thin film preparation The p-Si (100) substrates were first cleaned by deionized water and acetone to remove organic and other impurities. Then the Silicon substrates were dipped in hydrofluoric acid (HF) solution (HF: deionized water is in the ratio 1:40) in order to remove the silicon oxide (SiO2) on the surface of silicon substrate. DC reactive magnetron sputtering technique was employed to deposit the (Ta2O5)1 x(TiO2)x films with composition x (0 r x r0.18) on p-Si. The Mosaic target used in the present study comprises of 4" diameter circular disc of titanium (99.99% purity) and small tantalum pieces with different areas (99.99% purity). Argon gas (99.99% pure) and oxygen (99.999% pure) were used as sputtering and reactive gas, respectively. The sputtering chamber was evacuated to a base pressure of 2 10 4 Pa with diffusion pump and rotary pump combination. The partial pressures of the two gases were controlled with mass flow controllers. Prior to the deposition of mixed oxide films, the process parameters were optimized for deposition of tantalum oxide films at different current densities at a base pressure of 2 10 4 Pa, oxygen partial pressure 5 10 2 Pa and sputtering pressure of 0.5 Pa with substrate temperature 303 K. The as-deposited Ti-doped Ta2O5 films were annealed in air at 973 K for 1 h. After deposition of the Ti-doped Ta2O5 films, MOS (metal/oxide/semiconductor) capacitors were fabricated with the configuration of Al/(Ta2O5)1 x(TiO2)x/p-Si by deposition of aluminum as top electrode by photolithography technique whose area is 7 10 2 cm2. Metal oxide semiconductor (MOS) capacitor of mixed-oxide (Ta2O5)1 x(TiO2)x was prepared for studying the electrical and dielectric properties. 2.2. Thin film characterization The X-ray photo-electron spectroscopic studies are performed
to analyze the chemical binding energy states by using SPECS GmbH spectrometer (Phoibos 100MCD Energy Analyzer) with MgKα1 radiation (1253.6 eV). The structural properties are studied by the X-ray diffraction (Siefert X-ray diffractometer, model 3003 TT) using CuKα1 radiation (wavelength λ ¼0.15406 nm) source filtered by Nickel thin film at a scan speed of 0.03 °/sec in the 2θ, from 20 to 60 °. The peak positions were determined precisely using RAYFLEX-Analyze software. The chemical bonding configuration of the formed films on silicon substrates was obtained from the Fourier transform infrared spectrophotometer (Nicolet model 5700 FT-IR) in the wavenumber range 400 – 1600 cm 1. The thickness of the films is measured with α- step profilometer and it is found to be about 350 7 5 nm. The capacitance – voltage (C–V) characteristics of the (Ta2O5)1 x(TiO2)x capacitors are measured with MIOKI (model 3532-50) LCR meter. The current – voltage (I–V) characteristic of the capacitor is measured with the Hewlett Peckard (model hp 4140B) pA meter. UV–VIS–NIR double beam spectrophotometer (Perkin Elmer model Lambda 950) is used for measuring the optical transmittance of the films deposited on quartz substrates in the wavelength (λ) range 200 – 800 nm.
3. Results and discussions 3.1. Structural properties Fig. 1 shows the representative XPS survey scan spectrum of (Ta2O5)1 x(TiO2)x thin films formed at oxygen partial pressure of 5 10 2 Pa, are prepared in the composition range 0 r x r l and annealed at 973 K. The survey scan spectrum indicates the characteristic tantalum (Ta 4 f at about 25 28 eV, Ta 3d at about 230 – 240 eV), titanium, (Ti 2p at about 458 – 466 eV), oxygen (O 1 s about 530 eV) and carbon peaks. At the binding energy 284.4 eV, a carbon 1s peak was observed. The presence of this peak is related to the organic surface contamination, which corresponds to the fact that the samples are exposed to air before measurements. Carbon peak disappears from the surface of (Ta2O5)1 x(TiO2)x films after pre argon sputtering. Fig. 2(a) shows the XPS core level binding energy spectra of Ta 4f of the (Ta2O5)1 x(TiO2)x films with composition x ¼0, 0.04, 0.12 and 0.18. The core level binding energies are measured the binding energies from 25 to 28 eV exhibits two peaks corresponding to Ta 4f7/2 and Ta 4f5/2. The XPS spectrum of Ta2O5 revealed that the characteristic doublet of Ta 4f7/2 and Ta 4f5/2 peaks formed due to spin orbit splitting in Ta2O5. The core level binding energy of Ta 4f7/2 and Ta 4f5/2 were 25.95 and 27.87 eV indicated the Ta5 þ state in the (Ta2O5)1 x(TiO2)x films for composition x ¼0 [23]. The separation between Ta 4f7/2 and Ta 4f5/2 peaks was observed about 1.92 eV and is consistent with that reported in the literature for Ta2O5 [24,25]. The core level binding energies of Ta 4f7/2 and Ta 4f5/2 were 25.95 and 27.87 eV for the (Ta2O5)1 x(TiO2)x films with x¼ 0. The core level binding energy of Ta 4f7/2 and Ta 4f5/2 were 26.62–28.79 eV for the (Ta2O5)1 x(TiO2)x films with x ¼0.18. The shift in the core level binding energy for Ta 4f7/2 was found to be 0.67 eV. The intense peaks of Ta 4f7/2 and O 1s core level binding energies have been used to characterize the chemical state of Ta in the formed films. The atomic ratio for the formed films was estimated from the core level area ratio of O 1s and Ta 4f7/2 has been used to determine the chemical composition of these oxide films. The ratio of oxygen to tantalum (O/Ta) was estimated nearly 2.6 at composition x ¼ 0, which is indicated the growth of nearly stoichiometric Ta2O5 films. The increase in the core level binding energies generally indicates an increase in positive charge on the atom. The X-ray photoelectron spectroscopic measurements on sputtered films (Ta2O5)1 x(TiO2)x with x ¼0, 0.12, 0.18 and 1 are presented in
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Fig. 1. XPS narrow scan spectra of (Ta2O5)1 x(TiO2)x thin films with variation of composition x.
Fig. 1. These results show that the band intensity appears to be proportional to the amount of Ta in the film. Fig. 1 XPS spectra also describes the presence of Ta 4f, Ti 2p, Ta 3d and O 1s core levels, respectively, for films (Ta2O5)1 x(TiO2)x grown with x ¼0.12 and 0.18. The results obtained from XPS studies shows that the sputtered (Ta2O5)1 x(TiO2)x films have been grownup close to the nominal stoichiometry. Fig. 2(b) shows the core level binding energy spectra of Ti 2p of the (Ta2O5)1 x(TiO2)x films with composition x ¼0.04, 0.12, 0.18 and 1. The Ti 2p3/2 peak for Ti4 þ in TiO2 generally lies in the binding energy range 458.6 – 459.2 eV, while the Ti 2p3/2 peaks for Ti þ 2 and Ti0 lie in the binding energy ranges 454.9 – 455.2 eV and 453.7 – 454.2 eV respectively [26]. The core level binding energy of Ti 2p3/2 and Ti 2p1/2 were 458.13 and 465.80 eV for the (Ta2O5)1 x(TiO2)x films with composition x ¼0.04. The binding energy separation between the Ti 2p3/2 and Ti 2p1/2 peaks is 7.67 eV. The core level binding energy of Ti 2p3/2 was shifted from 458.1 to 458.9 eV for the (Ta2O5)1 x(TiO2)x films with a composition x from 0.04 to 1 [27]. The observed shift in the core level binding energy was 0.8 eV. Fig. 2(b) will shows the Ti 2p3/2 peak location is at 458.45 eV, demonstrating it corresponds to Ti4 þ . Additionally, Ti 2p1/2 peak location is found at 464.20 eV, in (Ta2O5)1 x(TiO2)x films with composition x ¼ 0.18, implying that a spin-orbit splitting of 5.75 eV, a value which lies much closer to the oxidation state of Ti4 þ (5.5 eV) [26]. The spin orbit splitting in between Ti 2p3/2 and Ti 2p1/2 peaks were about 5.75 eV and is consistent with the reported values in the literature for TiO2. The overall Ti 2p spectrum confirms the presence of Ti4 þ species in the formed films. The Ti 2p spectrum indicated the absence of peaks at lower energy side of the main peak, which is indicated that there
is no characteristic metallic titanium. The combined GaussianLorentzian functions do not indicate the presence of any of the phases such as Ti2O3, TiO and metallic titanium by curve fitting in Ti 2p spectra [28]. The ratio of oxygen to titanium (O/Ti) was estimated nearly 2 at composition x ¼1, which indicates the growth of nearly stoichiometric TiO2 films. Fig. 2(c) shows the core level binding energy spectra of O 1s for (Ta2O5)1 x(TiO2)x films. The O 1s peak was observed in the binding energy range 530 – 531 eV. There is no considerable shift in binding energies for O 1s peak with composition x. The Ta and Ti atoms in these films were fully oxidized since no metallic states could be observed in the spectra. The ratio of oxygen to tantalum (O/Ta) and oxygen to titanium (O/ Ti) in the formed films was estimated from the XPS peak area by using relative sensitivity factors. The Ti doped Ta2O5 films, which corresponds to the composition ratios of 0.04, 0.08, 0.12, 0.15 and 0.18 respectively. In Fig. 2, all three core levels in the TiO2 doped Ta2O5 films, i.e., Ta 4f, Ti 2p, and O 1s, shift to higher binding energy states when the Ti/(Ti þTa) ratio increases. This shifting was due to the existence of dissimilar metal atoms, i.e., Ti vs. Ta [29– 32]. Binding energies in these films were strongly influenced by the charge transfer mechanism that involved all three constituent elements, i.e., Ta, Ti and O. The electronegativity of Ta (1.50) is less than the dopant Ti (1.54). As a result, the charge transfer between Ta and O decreased and between Ti and O increased when the Ti dopant composition was increased. The decrease of the charge transfer increases the binding energies, which showed up in the XPS spectra. Fig. 3 compares the XRD patterns of the Ti-doped Ta2O5 films with composition x, (x ¼0 to 0.18). The films deposited at room temperature (303 K) were found to be X-ray amorphous. These
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Fig. 2. XPS narrow scan spectra of (Ta2O5)1 x(TiO2)x thin films with different values of composition x for (a) Ta 4 f, (b) Ti 2p and (c) O 1 s.
films were annealed in air at 973 K for 1 h. It is observed that, the crystal structure of the doped films are not strongly influenced by the variation of Ti dopant composition. When the composition ratio was increased upto 0.18, the diffraction peaks obtained from (0 0 1), (1 11 0), (2 0 1), (0 0 2) and (3 11 1) planes were observed at 2θ of 22.9°, 28.3°, 37.1°, 46.8° and 56.3° respectively, indicated the growth of orthorhombic Ta2O5 [JCPDS card No. 25–0922] over tapping of tantalum and titanium [33,34]. The sensitivity of the XRD is not sufficient to detect it. Though a limited amount of the crystalline phase of TiO2 is present in these thin films. Among all other peaks, the intensity of (0 0 1) orientation had been prominent. The intensity of the strongest peak, i.e., (0 0 1) decreased when the composition x was increased. The composition x also affected the crystallite size in the doped films. The value of full width-at-half-maximum of the diffraction peak (0 0 1) is increased
by increasing the titanium composition. The crystallite size (L) of the films was calculated from the (0 0 1) peak using the Debye Scherrer's formula [35].
L = 0.9λ/β cos θ
(1)
where β is the full width at half maximum intensity. The crystallite size of the films decreased from 72 to 58 nm with the increase in dopant composition x (from x ¼0 to 0.18). This indicates that the higher dopant composition might reduce their crystallite size. The crystallite size reduction is confirmed the effectiveness of the Ti doping technique. It is well known that a grain boundary inside the crystallized high-k gate dielectrics increases the leakage current. By suppressing the film crystallization process, the current leakage is decreased making the film suitable for MOS transistor applications.
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Fig. 3. X-ray diffraction profiles of (Ta2O5)1 x(TiO2)x thin films in the composition range 0r x r0.18. Fig. 5. Optical transmittance spectra of (Ta2O5)1 x(TiO2)x thin films in the composition range 0r x r 0.18.
O–Ti stretching vibration mode of TiO2 [27]. As the composition ratio was increased to x¼ 0.18 the intensity of the absorption band at 436 cm 1 was increased. While the formation of the films by reactive magnetron sputtering with sputter power of 200 W, the oxygen radicals with high energy in the reactive sputtering interacts with silicon resulting the formation of SiO2 layer. After annealing process the peak observed at 1083 cm 1 was attributed to interstitial oxygen in the silicon due to the formation of SiO2 layer [40]. Thus, the leakage currents were significantly affected by the presence of interfacial SiO2 layer [41]. 3.2. Optical properties
Fig. 4. FTIR spectra of (Ta2O5)1 x(TiO2)x thin films in the composition range 0r x r 0.18.
Fig. 4 shows the Fourier transform infrared spectra for the (Ta2O5)1 x(TiO2)x thin films with variation of composition x, from x ¼0 to 0.18. These peaks represent the infrared transmittance spectra in the range 400–1600 cm 1 of the (Ta2O5)1 x(TiO2)x films annealed in oxygen ambient at 973 K. A significant peak identified at 522 cm 1 in all FTIR spectra confirmed the existence of Ta2O5. Also, the spectra exhibit the absorption bands at 630, 836 and 1083 cm 1 (Ta–O–Ta stretching and Si–O stretching) [36,37]. Franke [38] and Kaliwoh [39] et al. observed the strong peak found at approximately 522 cm 1 confirms the presence of Ta2O5 in polycrystalline form. With increasing the composition ratio, the peak intensity of the absorbance at 522 and 630 cm 1 decreases upto x¼ 0.08. At composition ratios, x 40.08, the positions of absorption band at 522 cm 1 were shifted towards the lower wavenumber side. The (Ta2O5)1 x(TiO2)x thin films for composition x ¼0 have no absorption bands related to titanium oxide. At x ¼0.08 an absorption band located at 436 cm 1 was related to Ti–
Fig. 5 shows the optical transmittance spectra of the (Ta2O5)1 x(TiO2)x films with the increase in composition from x ¼0 to 0.18. The optical transmittance of the films at composition x ¼0 showed 92% at wavelength 400 nm. The transmittance of the films increased to 96% with composition x (x ¼0.18). At about 300 nm, a sharp absorption edge was observed. From the Fig. 5, it is observed that the optical absorption edge is shifted towards the higher wavelength side with increase of composition x. The optical absorption coefficient (α) of the films is estimated from the optical transmittance (T) data in the wavelength range 200–1000 nm using the relation
α = ( 1/t) ln ⎡⎣ T⎤⎦
(2)
where, T is the transmittance and t is the thickness of the film. Near the fundamental absorption edge the optical absorption coefficient (α) of the formed films was found to be exponentially dependent on the photon energy (hν). The optical band gap (Eg) of the films is estimated from the intercept of the plots of (αhν)2 versus photon energy (hν) which assumes the direct transition between the top of the valance band and the bottom of the conduction band using the relation
(αhν) = A (hν–Eg)1/2
(3)
where, A is the optical absorption edge width parameter. By extrapolating the straight line portions of the (αhν)2 versus (hν) curves, the optical band gap for different film compositions was
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Fig. 7. Leakage current density for Al/(Ta2O5)1 x(TiO2)x/Si capacitors (with 0 r x r0.18). Fig. 6. Variation of optical band gap for (Ta2O5)1 x(TiO2)x thin films (with 0r x r 0.18).
obtained. The optical band gap for the (Ta2O5)1 x(TiO2)x thin films (at x ¼ 0) for direct forbidden transitions is found to be 4.5 eV [42]. The optical transmission spectra of (Ta2O5)1 x(TiO2)x thin films exhibits clearly a shift in the fundamental absorption edge towards the higher wavelength side which is indicated the decrease in optical band gap. In our earlier investigation [43], we reported the optical band gap of 3.43 eV for the TiO2 films annealed at 973 K. Fig. 6 shows the variation of optical band gap with composition x. The estimated energies of the optical band gap from the plots of (αhν)2 versus photon energy (hν) were in the range 4.50 – 4.26 eV. The optical band gap of the (Ta2O5)1 x(TiO2)x film is 4.50 eV (at x ¼0) and it decreased to 4.26 eV with composition x (at x ¼ 0.18) [44]. This is readily understood that the optical band gap Ta2O5 is greater than TiO2. Due to decrease of the number of Ta5 þ by the substitution of Ti4 þ ions, the optical band gap values were observed to be decreasing with the increase in composition x. 3.3. Electrical properties Fig. 7 shows the current density – voltage (J - V) plots of the Al/ (Ta2O5)1 x(TiO2)x/p-Si capacitors with variation of composition x (x ¼0 to 0.18). It is seen from the figure that the J - V curves showed the asymmetric behavior for positive and negative voltages. It may be possible to the differences in the work functions between aluminum and p-Si and the conduction mechanisms through the Al/(Ta2O5)1 x(TiO2)x/p-Si stack. A leakage current density of 2.1 10 5 A/cm2 was obtained (at an electric field of 1.5 V) at x¼ 0, the titanium incorporation seems to suppress the leakage current it is decreased to 3.6 10 8 A/cm2 at the composition x ¼0.15, and it is increased to 5.5 10 7 A/cm2 with the value of x 4 0.18. The leakage current density is influenced by the grain size and hence in crystallinity. The grain boundary structure of the films is denser and more uniformly increasing with composition x. The crystallinity of the films is decreased with increase in composition x. From the optical properties discussed above, titanium oxide has a relatively small band gap 3.43 eV than the band gap for tantalum oxide 4.5 eV. The values from energy band diagram for the tantalum oxide, the band gap is wider and the conduction band minimum is higher than that of the TiO2. Thus, the Ta2O5 sample has a higher potential barrier across the oxide [45]. Therefore, the leakage current of the Ta2O5 is smaller than
that of the TiO2, regardless of the deterioration of the interface caused by the oxidation of the substrate. When Ti is doped in Ta2O5, the reaction of Ti and Ta2O5 should decrease the leakage currents. When leakage current of Al/(Ta2O5)1 x(TiO2)x/Si capacitors is plotted as a function of electric field, it is observed that Schottky emission takes place at low electric fields and Poole– Frenkel emission at higher electric fields. The typical semilog-current density-voltage (J-V) characteristics of the Al/(Ta2O5)1 x(TiO2)x/p-Si (with 0 r x r0.18) MIS structure are illustrated Fig. 7. The current density–voltage (J–V) relation for a barrier between metal-semiconductor (MS) or metalinsulator-semiconductor (MIS) structure is given by [46].
⎛ −qV ⎞⎤ ⎛ qV ⎞⎡ ⎟⎥ ⎟⎢ 1 − exp⎜ J = J0 exp⎜ ⎝ kT ⎠⎦ ⎝ nkT ⎠⎣
(4)
where Jo is the saturation current density and can be written as
⎛ −qΦb ⎞ ⎟ Jo = A**T2exp⎜ ⎝ kT ⎠
(5)
where, q is the electronic charge, Ann is the effective Richardson's constant equals to 32 A/cm2K2 for p-Si [21], A is the diode area, k is the Boltzmann's constant, Фb is the (Schottky barrier height) SBH, and n is the ideality factor. The ideality factor is extracted from the slope of the linear region of the forward-bias lnI versus V curves and can be obtained from Eq. (4) as
n=
q ⎛ dV ⎞ ⎟⎟ ⎜⎜ kT ⎝ d( ln J ) ⎠
(6)
The SBH (Фb) is determined from the extrapolated Jo and is expressed as
Φb =
kT ⎛ A**T2 ⎞ ⎟⎟ ln⎜⎜ q ⎝ J0 ⎠
(7)
The value of Jo is obtained from the intercept of the linear extrapolation of the forward J-V curve at zero voltage. The experimental values of n and Фb were determined from Eqs. (6) and (7), respectively. The estimated SBH of Al/(Ta2O5)1 x(TiO2)x/p-Si MIS structure with the dopant composition x¼ 0 is found to be 0.66 eV. The estimated SBH of the Al/(Ta2O5)1 x(TiO2)x/p-Si MIS structures are 0.74 eV for the dopant composition x ¼0.04, 0.76 eV for the dopant composition x ¼0.08, 0.69 eV for the dopant composition
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Fig. 8. Plots ln(J) versus E1/2 (Schottky Conduction (Ta2O5)1 x(TiO2)x/p-Si capacitors (with 0r x r 0.18).
Mechanism)
of
Al/
x ¼0.12, 0.82 eV for the dopant composition x ¼0.15 and 0.71 eV for the dopant composition x ¼0.18. The SBHs extracted for Al/ (Ta2O5)1 x(TiO2)x/p-Si MIS structure are found to be in good agreement with those reported by Selçuk et al. [47] for Al/SiO2/pSi SDs. Moreover, the ideality factors for Al/(Ta2O5)1 x(TiO2)x/p-Si MIS structure are found to be varied from 4.21 to 1.93. It is clearly seen that the estimated ideality factor values are greater than unity. This high value of n for Al/(Ta2O5)1 x(TiO2)x/p-Si MIS structure is due to the effect of oxide layers, series resistances and inhomogeneities of barrier [48]. One of the main reasons of barrier inhomogeneities in MIS structures may be ascribed to the result of interface defects such as caused by grain boundaries, multiple phases, facets, defects and mixture of different phases [49]. Fig. 8 shows the plots ln(J) versus E1/2 of Al/(Ta2O5)1 x(TiO2)x/p-Si capacitors for positive bias voltages with variation of composition x. It is noticed that the leakage currents are nearly proportional to the electrical field. Schottky emission (SE) occurs at the interface between the (Ta2O5)1 x(TiO2)x film and the Al electrode where a Schottky barrier is formed. Schottky emission is a field-assisted thermionic emission of an electron over a potential energy barrier and it can be explained according to the following equation:
JSE = A*T2exp[ − q(ΦB − √qE/4Лε0kr )/kT ],
7
(8)
where A*¼(4Лqm*k2/h3), which is termed as the effective Richardson constant. JSE is the current density, m* is the effective mass of an electron mass in oxide. ΦB, kr, k and h are the Schottky barrier height, dynamic dielectric constant, Boltzmann's constant, and Planck's constant respectively. If the plots of ln(J) versus E1/2 be in the linear form, then the current conducting through an oxide that is followed by SE. The Schottky barrier height and kr values can be extracted from the intercept of the y-axis and the slope of the plot. Fig. 9 shows the plots ln (J/E) Vs E1/2 of Al/(Ta2O5)1 x(TiO2)x/pSi capacitors for positive bias voltages. From the Fig. 9, it is observed that the values increased with composition x, the linearity decreased which shows that the PF emission (thermal emission of the charge carriers) diminished due to the reduction of traps. The emission of trapped electrons into the conduction band of insulator leads to the Poole-Frenkel emission. This Poole-Frenkel emission is a field-assisted thermal de-trapping of a carrier from the bulk oxide into the conduction band. Therefore, it is a bulklimited conduction mechanism and extensively applicable according to the following equation: [50]
Fig. 9. Plots ln(J/E) versus E1/2 (Poole-Frenkel Conduction Mechanism) of Al/ (Ta2O5)1 x(TiO2)x/p-Si capacitors (with 0r x r 0.18).
JPF = qNc μ E exp⎡⎣ −q Φt − √qE/Лεokr /kT⎤⎦
(
)
(
)
(9)
where JPF is the current density attributed to Poole-Frenkel emission, Nc the density of states in the conduction band and Φt the trap energy level in the oxide, respectively. From the above equation the value of kr can be extracted from the slope of a ln(J/E) versus E1/2 plot. The capacitance–voltage (C–V) curves were obtained by sweeping the gate voltage was swept from positive to negative bias and back again (from þ3 to 3 V) at an operating frequency of 1 MHz of Al/(Ta2O5)1 x(TiO2)x/p-Si capacitors variation of composition x (x ¼ 0 to 0.18) are shown in Fig. 10. The Capacitance-Voltage (CV) curves show very low hysteresis which indicates the low trap densities. The characteristic curves are similar to normal capacitance – voltage dependence observed in 3 typical ranges, i.e. accumulation, depletion and inversion. For negatively biased structure, negative electron charge at the gate is balanced by positive hole charges accumulated near the surface of the semiconductor (p-type). Switching the bias voltage into positive direction makes the semiconductor surface depleted from the holes, so that the additional capacitance of space charge layer Cs, is serially connected to Cox. In strong inversion conditions, potential changes at the gate are determined by the presence of electrons at the semiconductor metal interface. In an MOS capacitor, the capacitance C measured in the accumulation state which is equal to oxide capacitance Cox ¼4.79 nF of the films with composition x ¼0.00. The capacitance of (Ta2O5)1 x(TiO2)x films increased from 4.79 to 7.1 nF for the variation of x ¼0 to 0.15, and it is measured at composition x ¼ 0.18 was 6.1 nF. From figure it is revealed that the accumulation capacitance is increased with the variation of x ¼0 to 0.15. It could be due to the reduction of structural defects in (Ta2O5)1 x(TiO2)x layer with increasing x. Apart from this, the depletion layer thickness is also reduced noticeably with variation of x, resulting in the improvement of interface quality at (Ta2O5)1 x(TiO2)x/Si interface. The possible explanation for this observation might be the reduction in the defects of dielectric layer and/or interfacial state density (Dit). While, there is noticeable frequency dispersion at accumulation region. The frequency dispersion in the C - V plots is not only due to the defects in high-k and high-k/Si stacks, but also strongly depends on series
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8
Fig. 10. Capacitance–voltage characteristics of Al/(Ta2O5)1 x(TiO2)x/Si capacitors in the composition range 0r x r0.18.
resistance. Hence, it is confirmation for the significant improvement in the SiO2 layer thickness. The oxygen vacancies in Ta2O5 films can be compensated by Ti4 þ ions as they are substitutionally incorporated into Ta5 þ sites [8,51]. Consider this assumption (Ta5 þ radius, 0.07 nm, is almost the same as that of Ti4 þ ion, 0.068 nm), the observed reduction in the positive charge after titanium doping can be a result of this compensation. With increasing titanium content the negative charge increases. It indicates that the charge is assigned to titanium related defects, but not that of tantalum. The trend of enhanced capacitance with increasing titanium amount is observed. Dielectric constant (k) of the (Ta2O5)1 x(TiO2)x films was calculated by using the following relation
C = k εoA/t
(10)
where C is the capacitance (F), d the thickness of (Ta2O5)1 x(TiO2)x thin films (mt), εo is the permittivity of free space (8.854 10–12 F/ m) and A the area of the dielectric film (mt2), respectively. The dielectric constant of (Ta2O5)1 x(TiO2)x films was 27 at composition x ¼0 and it is increased to 38 with the increase of composition x, i.e., x o 0.15. The dielectric constant measured at composition x ¼ 0.18 was found to be 33. The increase of dielectric constant with composition x from x ¼0 to 0.15 is due to Ti-doped Ta2O5 leads to increase in capacitance in the accumulation region, hence, increase in the dielectric constant of the (Ta2O5)1 x(TiO2)x films.
4. Conclusions Thin films of (Ta2O5)1 x(TiO2)x (with x ¼ 0 to 0.18) were
prepared by the DC reactive magnetron sputtering technique by mosaic target tantalum-titanium, in an oxygen partial pressure of 5 10 2 Pa onto p-Si and quartz substrates at 303 K. The formed films (Ta2O5)1 x(TiO2)x were annealed in air at 973 K for 1 h. The XPS, XRD and IR measurements revealed that the sputtered films were nearly stoichiometric (having the same oxidation state) with respect to the target material composition. The typical spin orbit splitting observed in between Ta 4 f7/2 and Ta 4 f5/2 peaks was about 1.92 eV and a separation of 5.75 eV between two peaks of Ti 2p3/2 and Ti 2p1/2 confirms the formation of Ta2O5 and TiO2. The optical band gap decreased with increasing Ti dopant in (Ta2O5)1 x(TiO2)x films. The optical band decreases from 4.50 eV to 4.26 eV with the increase of x (x ¼0 to 0.18). The electrical measurements exhibit a decrease in leakage current density from 2.1 10 5 A/cm2 (at an electric field of 1.5 V) to 3.6 10 8 A/cm2 with increasing composition x from 0 to 0.15. Moreover, the leakage current measured at composition x ¼0.18 was found to be 5.5 10 7 A/cm2. The Ti incorporation into Ta2O5 causes the generation of negative oxide charge which reduces the existing positive charge in Ta2O5. The positive oxide charge in Ta2O5 is addressed to oxygen a vacancy, the titanium doping is away to suppress oxygen vacancies, that results in the decrease in leakage current density. Two possible conduction mechanisms such as Schottky effect and Poole-Frenkel conduction mechanism were used to explain the origin of leakage currents in the sputtered (Ta2O5)1 x(TiO2)x thin films. The capacitance of the (Ta2O5)1 x(TiO2)x films was observed to be increased with the increase of dopant composition from x ¼0 to 0.15 may be ascribed to the structural improvements. Thus, it is concluded that the thin films of (Ta2O5)1 x(TiO2)x containing 0.15% TiO2 exhibited better electrical properties when compared to the pure Ta2O5 films.
Please cite this article as: M. Chandra Sekhar, et al., Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.034i
M. Chandra Sekhar et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Acknowledgments Authors are thankful to Dr. G. Mohan Rao, Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore for extending the X-ray photoelectron spectroscopic facilities and useful discussions.
References [1] The International Technology Roadmap for Semiconductors, 2002 Update, Semiconductor Industry Association, Austin, TX, International SEMATECH, 2001/2002. [2] M. Kadoshima, M. Hiratani, Y. Shimamoto, K. Torii, H. Miki, S. Kimura, T. Nabatame, Rutile-type TiO2 thin film for high-k gate insulator, Thin Solid Films 424 (2003) 224–228. [3] S. Ramanathan, C.M. Park, P.C. McIntyre, Electrical properties of thin film zirconia grown by ultraviolet ozone oxidation, J. Appl. Phys. 91 (2002) 4521–4527. [4] A. Callegari, E. Cartier, M. Gribelyuk, H.F. Okorn-Schmidt, T. Zabel, Physical and electrical characterization of Hafnium oxide and Hafnium silicate sputtered films, J. Appl. Phys. 90 (2001) 6466–6475. [5] G.B. Alers, D.J. Werder, Y. Chabal, H.C. Lu, E.P. Gusev, E. Garfunkel, T. Gustafsson, R. Urdahl, Intermixing at the tantalum oxide/silicon interface in gate dielectric structures, Appl. Phys. Lett. 73 (1998) 1517–1523. [6] K.W. Kwon, I.S. Park, D.H. Han, E.S. Kim, S.T. Ahn, M.Y. Lee, Ta/sub 2/O/sub 5/ capacitors for 1 Gbit DRAM and beyond, IEDM 94 (1994) 835–838. [7] Y. Takaishi, M. Sakao, S. Kamiyama, H. Suzuki, W. Watanable, Low-temperature integrated process below 500/spl deg/C for thin Ta/sub 2/O/sub 5/ capacitor for giga-bit DRAMs, IEDM 94 (1994) 839–842. [8] H. Fujikawa, Y. Taga, Effects of additive elements on electrical properties of tantalum oxide films, Appl. Phys. 75 (1994) 2538–2542. [9] R.F. Cava, W.F. Peck Jr., J.J. Krajewski, Enhancement of the dielectric constant of Ta2O5 through substitution with TiO2, Nature 377 (1995) 215–217. [10] L. Manchanda, M.D. Morris, M.L. Green, R.B. van Dover, F. Klemens, T. W. Sorsch, P.J. Silverman, G. Wilk, B. Busch, S. Aravamudhan, Multi-component high-K gate dielectrics for the silicon industry, Microelectron. Eng. 59 (2001) 351–359. [11] D.H. Triyoso, R.I. Hegde, S. Zollner, M.E. Ramon, S. Kalpat, R. Gregory, X. D. Wang, J. Jiang, M. Raymond, R. Rai, D. Werho, D. Roan, B.E. White Jr., P. J. Tobin, Impact of titanium addition on film characteristics of HfO2 gate dielectrics deposited by atomic layer deposition, J. Appl. Phys. 98 (2005) 1–8. [12] K.M.A. Salam, H. Fukuda, S. Nomura, Effects of additive elements on improvement of the dielectric properties of Ta2O5 films formed by metalorganic decomposition, J. Appl. Phys. 93 (2003) 1169–1175. [13] J.L. Waring, R.S. Roth, Effect of oxide additions on the polymorphism of tantalum pentoxide (system Ta2O5-TiO2), J. Res. Nat. Bur. Stand. A Phys. Chem. 72A (1968) 175–186. [14] M. Sze Simon, K.N.G. Kwok, Physics of Semiconductor Devices, Wiley, 2007. [15] C.H. Lin, C.W. Liu, Metal-insulator-semiconductor photodetectors, Sensors 10 (2010) 8797–8826. [16] N. Nanda Kumar Reddy, V. Rajagopal Reddy, Chel-Jong Choi, Influence of rapid thermal annealing effect on electrical and structural properties of Pd/Ru Schottky contacts to n-type GaN, Mater. Chem. Phys. 130 (2011) 1000–1006. [17] J.H. Evans-Freeman, M.M. El-Nahass, A.A.M. Farag, A. Elhaji, Current transport mechanisms and deep level transient spectroscopy of Au/n-Si Schottky barrier diodes, Microelectron. Eng. 88 (2011) 3353–3359. [18] M.M. Bulbul, Frequency and temperature dependent dielectric properties of Al/Si3N4/p- Si(1 0 0) MIS structure, Microelectron. Eng. 84 (2007) 124–128. [19] S. Karatas, F. Yakuphanoglu, F.M. Amanullah, Capacitance–voltage and conductance– voltage characteristics of Ag/n-CdO/p-Si MIS structure prepared by sol–gel method, J. Phys. Chem. Solids 73 (2012) 46–51. [20] E. Ayyildiz, A. Turut, H. Efeoglu, S. Tuzemen, M. Saglam, Y.K. Yogurtcu, Effect of series resistance on the forward current-voltage characteristics of Schottky diodes in the presence of interfacial layer, Solid-State Electron. 39 (1996) 83–87. [21] M. Chandra Sekhar, N. Nanda Kumar Reddy, B. Venkata Rao, G. Mohan Rao, S. Uthanna, Influence of sputter power on structural and electrical properties of TiO2 films for Al/TiO2/Si gate capacitors, Surf. Interface Anal. 46 (2014) 465–471. [22] H. Kim, M.D. Kumar, J. Kim, Highly-performing Ni/SiO2/Si MIS photodetector for NIR detecting applications, Sens. Actuators A 233 (2015) 290–294. [23] P. Shang, S. Xiong, L. Li, D. Tian, W. Ai, Investigation on thermal stability of Ta2O5, TiO2 and Al2O3 coatings for application at high temperature, Appl. Surf. Sci. 285B (2013) 713–720.
9
[24] E. Atanassova, D. Spassov, X-ray photoelectron spectroscopy of thermal thin Ta2O5 films on Si, Appl. Surf. Sci. 135 (1998) 71–82. [25] S.V. Jagadeesh Chandra, S. Uthanna, G. Mohan Rao, Effect of substrate temperature on the structural, optical and electrical properties of dc magnetron sputtered tantalum oxide films, Appl. Surf. Sci. 254 (2008) 1953–1960. [26] J. Moulder, W. Stickle, P. Sobol, K. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, Inc., 1995. [27] M. Chandra Sekhar, P. Kondaiah, S.V. Jagadeesh Chandra, G. Mohan Rao, S. Uthanna, Effect of substrate bias voltage on the structure, electric and dielectric properties of TiO2 thin films by DC magnetron sputtering, Appl. Surf. Sci. 258 (2011) 1789–1796. [28] B. Liu, L. Wen, X. Zhao, The photoluminescence spectroscopic study of anatase TiO2 prepared by magnetron sputtering, Mater. Chem. Phys. 106 (2007) 350–353. [29] G. Lucovsky, J.L. Whittern, Y. Zhang, A molecular orbital model for the electronic structure of transition metal atoms in silicate and aluminate alloys, Solid-State Electron. 46 (2002) 1687–1697. [30] G. Lucovsky, G.B. Raynor, Y. Zhang, C.C. Fulton, R.J. Nemanich, G. Appel, H. Ade, J.L. Whittern, Electronic structure of transition metal high-k dielectrics: interfacial band offset energies for microelectronic devices, Appl. Surf. Sci. 212– 213 (2003) 563–569. [31] M.D. Ulrich, J.G. Hong, J.E. Rowe, G. Lucovsky, A.S.Y. Chan, T.E. Madey, Soft x-ray photoelectron spectroscopy of (HfO2)x(SiO2)1 x high-k gate-dielectric structures, J. Vac. Sci. Technol. B 21 (2003) 1777–1782. [32] F. Giustino, A. Bongiorno, A. Pasquarello, Modeling of Si 2p core-level shifts at Si– (ZrO2)x(SiO2)1 x interfaces, Appl. Phys. Lett. 81 (2002) 4233–4235. [33] T.J. Bright, J.I. Watjen, Z.M. Zhang, C. Muratore, A.A. Voevodin, D.I. Koukis, D. B. Tanner, D.J. Arenas, Infrared optical properties of amorphous and nanocrystalline Ta2O5 thin films, J. Appl. Phys. 114 (2013), 083515-083515-10. [34] W. Miao, M.M. Zhu, Z.C. Li, Z.J. Zhang, Optical and Electric Properties of Aligned- Growing Ta2O5 Nanorods, Mater. Trans. 49 (2008) 2288–2291. [35] B.D. Cullity, Elements of X-ray Diffraction, 2nd edn., Addison Wesley, London, 1978. [36] C.H. An, K. Sugimoto, Ellipsometric Examination of Growth and Dissolution Rates of Ta2O5 Films Formed by Metalorganic Chemical Vapor Deposition, J. Electrochem. Soc. 139 (1992) 1956–1962. [37] J.Y. Zhang, Q. Fang, I.W. Boyd, Growth of tantalum pentoxide film by pulsed laser deposition, Appl. Surf. Sci. 138 (1999) 320–324. [38] E. Franke, C.L. Trimble, M.J. DeVries, J.A. Woollam, M. Schubert, F. Frost, Dielectric function of amorphous tantalum oxide from the far infrared to the deep ultraviolet spectral region measured by spectroscopic ellipsometry, J. Appl. Phys. 88 (2000) 5166–5174. [39] N. Kaliwoh, J.Y. Zhang, I.W. Boyd, Ultrathin silicon dioxide films grown by photo-oxidation of silicon using 172 nm excimer lamps, Appl. Surf. Sci. 168 (2000) 288–291. [40] S. Yoo, I. Kim, S. Kim, C.H. Hahn, C. Lee, S. Cho, Effects of annealing temperature and method on structural and optical properties of TiO2 films prepared by RF magnetron sputtering at room temperature, Appl. Surf. Sci. 253 (2007) 3888–3892. [41] Y. Nishioka, S. Kimura, H. Shinriki, K. Mukai, Dielectric characteristics of extremely thin Ta205/Si02 on Si, J. Electrochem. Soc. 134 (1987) 410–415. [42] F. Rubio, J. Denis, J.M. Albella, J.M. Martinez-Duart, Sputtered Ta2O5 antireflection coatings for silicon solar cells, Thin Solid Films 90 (1982) 405–408. [43] M. Chandra Sekhar, P. Kondaiah, G. Mohan Rao, S.V. Jagadeesh Chandra, S. Uthanna, Post-deposition annealing influenced structural and electrical properties of Al/TiO2/Si gate capacitors, Superlattices Microstruct. 62 (2013) 68–80. [44] N. Kaliwoh, J.Y. Zhang, I.W. Boyd, (Ta2O5)1 x(TiO2)x deposited by photo-induced CVD using 222 nm excimer lamps, Appl. Surf. Sci. 186 (2002) 246–250. [45] Q. Lu, Y. Mu, J.W. Roberts, M. Althobaiti, V.R. Dhanak, J. Wu, C. Zhao, C.Z. Zhao, Q. Zhang, L. Yang, I.Z. Mitrovic, S. Taylor, P.R. Chalker, Electrical properties and interfacial studies of HfxTi1–xO2 high permittivity gate insulators deposited on germanium substrates, Materials 8 (2015) 8169–8182. [46] E.H. Rhoderick, R.H. Williams, second ed,Metal-Semiconductor Contacts, 136, Clarendon Press, Oxford 1988, p. 121. [47] B. Selçuk, S. Bilge Ocak, S. Karadeniz, Gaussian distribution on electrical characteristics of Al/SiO2/p-Si structures, Am. J. Mater. Sci. 2 (4) (2012) 125–130. [48] M.A. Yeganeh, S. Rahmatollahpur, R. Sadighi-Bonabi, R. Mamedov, Dependency of barrier height and ideality factor on identically produced small Au/p-Si Schottky barrier diodes, Phys. B 405 (2010) 3253–3258. [49] E.H. Nicollian, J.R. Brews, MOS (metal oxide semiconductor) Physics and technology, Wiley-Interscience, 2003. [50] S.M. Sze, Physics of Semiconductor Devices, 2nd ed, Wiley, New York 1981, pp. 402–404. [51] K.M.A. Salam, H. Fukuda, S. Nomura, Effects of additive elements on improvement of the dielectric properties of Ta2O5 films formed by metalorganic decomposition, J. Appl. Phys. 93 (2003) 1169–1175.
Please cite this article as: M. Chandra Sekhar, et al., Structural, optical and electrical properties of DC reactive magnetron sputtered (Ta2O5)1 x(TiO2)x thin films, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.034i