Abstract
Titanium dioxide (TiO2) thin films were deposited on p-Si (100) and Corning glass substrates held at room temperature by DC magnetron sputtering at different oxygen partial pressures in the range Pa. The influence of oxygen partial pressure on the structural, electrical, and optical properties of the deposited films was systematically studied. XPS studies confirmed that the film formed at an oxygen partial pressure of Pa was nearly stoichiometric. TiO2 films formed at all oxygen partial pressures were X-ray amorphous. The optical transmittance gradually increased and the absorption edge shifted towards shorter wavelengths with the increase of oxygen partial pressure. Thin film capacitors with configuration of Al/TiO2/p-Si have been fabricated. The results showed that the leakage current density of films formed decreased with the increase of oxygen partial pressure to Pa owing to the decrease in the oxygen defects in the films thereafter it was increased. The current transport mechanism in the TiO2 thin films is shown to be Schottky effect and Fowler-Nordheim tunnelling currents.
1. Introduction
Titanium dioxide (TiO2) thin films are widely used because of their remarkable electrical and optical properties. TiO2 can be used as an alternative dielectric to silicon dioxide films in ultra-large-scale integration due to its much higher dielectric constant [1], lower leakage current, and higher breakdown strength than that of SiO2 [2]. TiO2 thin films have attracted considerable attention for applications in photocatalysts and solar cells [3]. These films also exhibit a large potential in the fields such as protective antireflection coatings [4], gas sensors [5], and air purification [6]. TiO2 films were prepared by numerous deposition techniques such as metal organic chemical vapour deposition, spray pyrolysis, sol-gel process, thermal oxidation, pulsed laser deposition, and reactive magnetron sputtering [7–10]. However, the electrical properties of the oxide layer are affected by the growth technique due to reactions at the oxide/Si substrate interface [11]. Among these techniques, magnetron sputtering method provides more advantage in controlling the microstructure and composition of the films. The structural and electrical properties are known to be easily affected by the deposition conditions such as the substrate temperature, substrate bias voltage, and oxygen partial pressure as well as the postdeposition annealing. The effect of substrate temperature [12] and substrate bias voltage [13] on the structural, electrical, and dielectric properties of magnetron sputtered TiO2 films was reported. In this investigation, the influence of oxygen partial pressure on the structural and electrical behavior of DC magnetron sputtered TiO2 thin films was studied.
2. Experimental
2.1. Thin Film Preparation
Thin films of TiO2 were deposited on RCA-cleaned and HF-treated, low-resistivity (0.01–0.02 Ωcm) p-type silicon (100) and Corning glass substrates by DC reactive magnetron sputtering technique. Pure titanium target of 100 mm diameter and 3 mm thick was used as sputter target. The vacuum pumping system employed for sputtering was a combination of diffusion and rotary pumps. Pure oxygen and argon were used as reactive and sputtering gases, respectively. After achieving ultimate pressure of 2 10−4 Pa, the fixed quantities of oxygen and argon gases were admitted into the sputter chamber through the flow controlled needle valves and their flow rates were monitored individually employing Aalborg mass flow controllers. The TiO2 films were deposited at different oxygen partial pressures by fixing the other process parameters kept constant. The deposition conditions maintained during the growth of the TiO2 films are given in Table 1. In order to fabricate metal-oxide-semiconductor structure, TiO2 layer was formed on the p-Si (100) wafers. Aluminium top contacts were formed by sputtering using shadow masks over the oxide layer with fixed area of 9 10−4 cm2. The back contacts of thick Al layer, which was deposited by sputtering. High doping in Si ensured that the back contacts were ohmic. The sample configurations for metal insulator-semiconductor for electrical measurements are shown in Figure 1.
2.2. Thin Film Characterization
TheX-ray photoelectron spectroscopic (XPS) studies were performed to analyze the core level binding energies by using SPECS GmbH spectrometer (Phoibos 100 MCD Energy Analyzer) with MgKα1 radiation (1253.6 eV). Thechemical binding configuration, structural properties ofTiO2 thin films were determined using Raman spectra recorded at room temperatureusing Jobin Yvon (Model HR 800 UV) Raman spectrometer in the wavenumber range 100–1000 cm−1. X-ray diffractometer (Seifert model 3003 TT) with CuKα1 radiation ( nm) was used to analyze the crystallographic structure of the films formed on silicon substrates. The physical thickness of the films was measured by using -step profilometer. The optical transmittance of the films formed on Corning glass substrates was recorded by using UV-Vis-NIR double beam spectrophotometer (Perkin Elmer model Lambda 950) in the wavelength range 300–1500 nm. The electrical properties of the TiO2 thin films were evaluated by current-voltage (I-V) measurements. The test samples with metal-insulator-semiconductor (MIS) capacitor structures were prepared on p-type Si (100) substrates. Aluminum electrodes were prepared by using DC reactive magnetron sputtering. The current-voltage characteristics of the Al/TiO2/p-Si capacitors were measured by using Hewlett Packard (model hp 4140 B) pA meter.
3. Results and Discussion
3.1. Structural Properties
Figure 2 shows the dependence of deposition rate of TiO2 films on the oxygen partial pressure. The deposition rate of the films formed at low oxygen partial pressure of 9 10−3 Pa was 4.6 nm/min. The deposition rate of the films is decreased to 1.9 nm/min at an oxygen partial pressure of 9 10−2 Pa. The high deposition rate at low oxygen partial pressure was due to the high sputtering yield of metallic titanium and insufficient oxygen present in the sputter chamber. The decrease of deposition rate at higher oxygen partial pressure was related to the chemical reaction between the target surface and the reactive gas of oxygen. This led to the formation of oxide layer on the target surface and in turn reduces in the deposition rate [14]. Jagadeesh Chandra et al. [15] also observed the decrease of films thickness with the increase of oxygen partial pressure in RF magnetron sputtered Ta2O5 films.
Figure 3 shows the XPS survey spectra of TiO2 thin films formed at different oxygen partial pressures. The survey spectra show the characteristic of titanium, oxygen, and carbon peaks. A carbon 1s peak was observed at a binding energy of 284.4 eV on all samples before ion bombardment. The presence of this peak is related to organic surface contamination, which corresponds to the fact that the samples are exposed to air before measurements. The carbon peak disappeared from the surface of TiO2 films after presputtering with argon ion bombardment.
Figure 4(a) shows the narrow scan spectra of Ti 2p in TiO2 films formed at different oxygen partial pressures in the range 9 10−3–6 10−2 Pa. The Ti 2p signal is associated with two peaks which are representative for Ti 2p3/2 to Ti 2p1/2 spin-orbital splitting. The films formed at 9 10−3 Pa showed the core level binding energies of Ti 2p3/2 and Ti 2p1/2 is 457.2 and 462.7 eV (energy separation of 5.5 eV). The core level binding energies of the films formed at 6 10−2 Pa shifted to 458.2 and 463.9 eV (energy separation of 5.7 eV) which indicated the presence of Ti4+ oxidation state in TiO2. Liu et al. [16] reported the core level binding energies of Ti 2p with two peaks located at 458.1 and 463.7 eV for Ti 2p3/2 and Ti 2p1/2 with energy separation of 5.6 eV. Figure 4(b) showed the narrow scan spectra of O 1s at different oxygen partial pressures. The core level binding energy of O 1s peak shifted from 529.7 to 530.5 eV with increase of oxygen partial pressure from 9 10−3 to 6 10−2 Pa, respectively. The chemical composition of the deposited films was determined from the area under the peak using the sensitivity factors (, , and O ). The films deposited at 6 10−2 Pa showed the ratio of oxygen to titanium, that is, , which indicated the growth of nearly stoichiometric films [17].
(a)
(b)
Figure 5 shows the X-ray diffraction profiles of the TiO2 thin films deposited at different oxygen partial pressures. It can be seen that all the films have not shown any diffraction peaks indicating the films were amorphous in nature. The amorphous structure of TiO2 thin films can be attributed to low surface mobility of deposition particles [18].
Anatase has a space group (I41/amd) containing two formula units per primitive unit cell. From the group analysis, there are six Raman active modes, and three infrared active modes, . The Raman spectra for anatase single crystal were investigated by Ohsaka [19] who noticed six allowed bands in the first order Raman spectra were identified at 144 cm−1 (), 197 cm−1 (), 399 cm−1 (), 513 cm−1 (), 519 cm−1 (), and 639 cm−1 (). The Raman spectra for the as-prepared TiO2 thin films at different oxygen partial pressures were shown in Figure 6. Raman peaks seen at 153, 516, 659, and 810 cm−1 are assigned as (ν6), (), (ν1), and Ti–O modes related to the anatase phase [20]. It is noted that the Raman peaks at 302 cm−1 and 520 cm−1 appeared from Si-Si bonding mode of the Si substrate. The Raman peaks at 516 cm−1 were not present because the intensity of Si dominates the intensity of doublet peaks. The Raman peaks at 233, 430, and 620 cm−1 related to the rutile phase [21]. It is seen that the peak at 153 cm−1 ( mode) strengthens, with increase in oxygen partial pressure from 9 10−3 to 6 10−2 Pa, thereafter it weakens with the oxygen partial pressure up to 9 10−2 Pa due to the reduction of film thickness. The rutile peak intensity was also decreased with increasing oxygen flow ratio as well as film thickness.
3.2. Optical Properties
The optical transmittance spectra of the TiO2 films formed at different oxygen partial pressures are shown in Figure 7. Obviously, the optical transmittance is high in the visible region and it increased with the increase of oxygen partial pressure from 9 10−3 to 9 10−2 Pa, respectively. The optical transmittance of the films formed at low oxygen partial pressure of 9 10−3 Pa was low maybe due to the formation of nonstoichiometric TiO2 films. As the oxygen partial pressure increased to 6 10−2 Pa, the adequate oxygen can repair the oxygen vacancies and hence increase the transmittance. The relation between the absorption coefficient and concentration of free carriers can be written as [22] where is the absorption coefficient, N the concentration of free carriers, the refractive index, the effective mass of free carrier, and the absorption wavelength. Equation (1) shows that the absorption coefficient of TiO2 films is directly proportional to the concentration of free carrier, namely, twofold concentration of oxygen vacancies. Hence, the substoichiometry of the TiO2 films induced by insufficient oxygen content is the main cause of transmittance differences. The transmittance of TiO2 thin films deposited at 9 10−3 Pa is low, which indicated that the films have more oxygen vacancies. As the oxygen partial pressure increased to 6 10−2 Pa, the transmittance of the films increased since the oxygen ion vacancies tend to decrease and hence the composition of the films approached to the stoichiometric TiO2. Heo et al. [23] also observed the increase in optical transmittance with the increase of oxygen flow by RF magnetron sputtering method. The optical absorption edge of the films shifted markedly towards lower wavelengths side with the increase of oxygen partial pressure. The optical absorption coefficient (α) of the films was evaluated from the optical transmittance and reflectance data using the relation where is the thickness of the film. The optical band gap () of the films was estimated from the intercept of the plots of 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 where is the optical absorption edge width parameter. The optical band gap of the films increased from 3.45 to 3.60 eV with the increase of oxygen partial pressure from 9 10−3 to 9 10−2 Pa. The low value of optical band gap of the films formed at low oxygen partial pressure was due to the formation of nonstoichiometric films. In the present investigation, the stoichiometric films deposited at an oxygen partial pressure of 6 10−2 Pa showed an optical band gap of 3.50 eV.
3.3. Electrical Properties
Figure 8 shows the leakage current density versus applied voltage characteristics of the Al/TiO2/p-Si capacitors. The leakage current density measured at applied voltage V was decreased from 6.1 10−5 to 1.3 10−6 A/cm2 with the increase of oxygen partial pressure from 9 10−3 to 6 10−2 Pa. At higher oxygen partial pressure of 9 10−2 Pa, the leakage current density was increased to 5.5 10−6 A/cm2 [15, 24]. The increase in leakage currents at higher oxygen partial pressures was due to the decrease in film thickness. To validate the current transport mechanism through the Al/TiO2/p-Si capacitors, two main conduction mechanisms were examined. At low electric fields, the current density was proportional to the square root of the applied electric field (Schottky emission), while at higher electric fields it may be the possibility of Fowler-Nordheim conduction mechanism that was observed. The Schottky emission is given by the relation [17] where is the effective Richardson constant, the Schottky barrier height, the electronic charge, the Boltzmann constant, the electric field, and the Planck’s constant. The constant is given by the relation where is the permittivity of free space and the dielectric constant of the insulator. At higher bias voltages, the Fowler-Nordheim tunnelling of electrons through the triangular barrier to the conduction band of oxide will occur. The Fowler-Nordheim tunnelling current can be expressed as a function of the electric field in the gate oxide by the relation where = and are constants depending on and barrier height. , the electron effective mass in dielectric layer which is where is the free-electron mass [25].
Figure 9 shows ln versus plots of the Al/TiO2/p-Si capacitors formed at different oxygen partial pressures in the range 9 10−3–9 10−2 Pa with the applied positive bias voltage from 0 V to +10 V. Schottky emission follows at the interface between the TiO2 films and the aluminum electrode where a Schottky barrier is formed. This effect was attributed to the absorption current due to the dielectric relaxation phenomenon of the TiO2 capacitors. The plot ln versus showed a linear relationship with the applied electric field. This result indicates the I-V characteristics of TiO2 capacitors can be explained by Schottky emission in lower electric field region.
Figure 10 shows ln versus (Fowler-Nordheim) plots of Al/TiO2/p-Si capacitors at positive substrate bias. It is noted that there are two significant regimes shown in the Fowler-Nordheim plot. At low electric fields, Schottky emission is the dominant current transport mechanism, as shown by the leakage current density versus applied gate voltage. At higher electric fields, Fowler-Nordheim tunnelling was observed.
4. Conclusions
Titanium oxide thin films were formed on glass and p-silicon (100) substrates deposited by DC magnetron sputtering method under various oxygen partial pressures in the range 9 10−3–9 10−2 Pa. The deposition rate of the films was correlated with the oxygen partial pressure. From XPS studies it was confirmed that the films formed at an oxygen partial pressure of 6 10−2 Pa were nearly stoichiometric. X-ray diffraction studies indicated the films deposited at all oxygen partial pressure were amorphous. The optical transmittance of the films was increased with the increase of oxygen partial pressure; the transmittance edge shifted towards the lower wavelength side. This may be ascribed to the decrease of oxygen vacancies as the increase of oxygen partial pressure. The optical band gap of the films increased from 3.45 to 3.60 eV with the increase of oxygen partial pressure from 9 10−3 to 9 10−2 Pa, respectively. Thin film capacitors with configuration of Al/TiO2/p-Si have been fabricated. The leakage current density of the films formed at low oxygen partial pressure of 9 10−3 Pa was 6.1 10−5 A/cm2 and it was decreased to 1.3 10−6 A/cm2 with the increase of oxygen partial pressure to 6 10−2 Pa and thereafter it was increased to 5.5 10−6 A/cm2. The leakage current is found to be dominated by the Schottky emission at low electric field, whereas Fowler-Nordheim tunnelling currents appear at higher electric fields.
Acknowledgment
This work was carried out with the financial support of University Grant Commission, New Delhi, through a sanctioned Major Research Project: F. No. 34-36/2008 (SR) dated 30-12- 2008. One of the authors, M. Sekhar, is thankful to the University Grants Commission, New Delhi, for the award of UGC-RFSMS fellowship.