Structural and photoluminescence properties of CdO doped TiO2 thin films prepared by pulsed laser deposition

TiO2 thin films have been deposited at different concentration ofCdO of (x= 0.0, 0.05, 0.1, 0.15 and 0.2) Wt. % onto glass substratesby pulsed laser deposition technique (PLD) using Nd-YAG laserwith λ=1064nm, energy=800mJ and number of shots=500. Thethickness of the film was 200nm. The films were annealed todifferent annealing (423 and 523) k. The effect of annealingtemperatures and concentration of CdO on the structural andphotoluminescence (PL) properties were investigated. X-raydiffraction (XRD) results reveals that the deposited TiO2(1-x)CdOxthin films were polycrystalline with tetragonal structure and manypeaks were appeared at (110), (101), (111) and (211) planes withpreferred orientation along 2Ɵ around 27.30. The results ofphotoluminescence (PL) emission show that there are two peakspositioned are around 320 nm and 400 nm for predominated peakand 620 nm and 680 nm for the small peaks.


Introduction
Titanium dioxide has been one of the most extensively studied oxides because of its remarkable optical and electronic properties [1][2][3]. TiO 2 films have attracted attention for use in fabricating capacitors in microelectronics devices due to their unusually high dielectric constant [4,5]. The thin films of (TiO 2 ) have high band energy gap about (3.2 -3.29) eV, (3.69-3.78) eV for allowed and forbidden direct transition respectively [6]. Crystalline TiO 2 film exist in three phases: rutile (tetragonal with a=0.4594 nm, c=0.2958 nm), anatase (tetragonal with a=0.3785 nm, c=0.9514 nm.), and brookite (orthorhombic with a=0.9184 nm, b=0.5447 nm, c=0.5145 nm.), rutile being the most stable of the three, and the formation of its phase depending on the starting material, deposition method and temperature treatment. In particular, TiO 2 thin films can transform from amorphous phase into crystalline anatase and from anatase into rutile by changing temperature [7,8]. Rutile is usually the dominant phase in TiO 2 films, but in some recent work anatase-rich films have been synthesized. Many deposition methods can be used to prepare titanium oxides film: thermal [9] or anodic [10] oxidation of titanium, electron beam evaporation [11], chemical vapor deposition [12], plasma-enhanced chemical vapor deposition [13], solgel method [14,15] and reactive sputtering methods [16][17][18][19] Recently there are many applications of laser one of these applications in a thin film preparation field that called pulsed Laser deposition (PLD). With the pulsed laser deposition method, thin films are prepared by the ablation of one or more targets illuminated by a focused pulsed-laser beam. This technique was first used by smith and Turner in 1965 [20] for the preparation of semiconductor and dielectric thin films and was established due to the work of Dijkkamp and coworkers [21] on high-temperature superconductors in 1987. Cadmium oxide (CdO) has high electrical conductivity and high optical transmittance with a moderate refractive index in the visible region of the solar spectrum. In recent years it has found various applications in transparent electrodes, solar cells, photo transistors, photodiodes, gas sensors, etc. [22,23]. CdO films are wide, direct band-gap semiconductors with an optical energy gap of about 2.4 eV at room temperature. CdO, with its cubic structure, is also a II-VI n-type semiconductor with donor defects, such as Cd interstitials and oxygen vacancies [24].

Experimental details 1. Sample preparation
Titanium dioxide from Nano shell Company with a purity 99.99% and cadmium oxide with purity 99.99% were mixed at different concentration cadmium oxide of (x= 0.0, 0.05, 0.1, 0.15, 0.2) Wt. %. The powder of precursor was mixed together using agate mortar, the mixture was then pressed into pellets (1.5 cm) in diameter and (0.2 cm) thick, using hydraulic type (SPECAC), under pressure of 5 tons. The pellets were sintered in air at temperature (773 K) for 3 h.

Thin films preparation of TiO 2(1-x) CdO x by PLD
TiO 2(1-x) CdO x films were deposited on glass slides substrates of (10×10 mm) at room temperature and different concentration of CdO. The glass substrate was cleaned with dilated water using ultrasonic process for 15 minute to deposit the films at room temperature then annealing treatment at (423 and 523) K by furan (Precision Model 19 Vacuum Oven made in west Germany) under vacuum (8*10 -2 mbar). Finally, deposited thin films from TiO 2 :CdO by PLD technique using Nd:YAG with (λ= 1064 nm) SHG Q-switching laser beam at 800 mJ, repetition frequency (6Hz) for 500 laser pulse is incident on the target surface making an angle of 45° with it as shown in Fig. 1. The distance between the target and the laser was set to (10 cm), and between the target and the substrate was (1.5 cm), under vacuum of (10 −3 mbar). The thickness of TiO 2(1-x) CdO x thin film was measured using an optical interferometer method employing He-Ne laser 0.632 µm with incident angle 45°. This method depends on the interference of the laser beam reflected from thin film surface and then substrate, the films thickness t was determined using the following formula [25]: where x is fringe width, Δx is the distance between two fringes and λ is wavelength of laser He -Ne (632.8nm).

Characterization
XRD analysis using SHIMADZU 6000 X-ray diffractometer system was employed in order to obtained the crystal quality and phase structure of the films. The optical properties of TiO 2(1-x) CdO x thin films were investigated by PL spectroscopy using UV light excitation SL-174 (ELICO) Spectro Fluorometer, 150 watt Xenon Arc lamp, (EX and Em) from (200-900) nm, at photo excitation 350nm.

Results and discussion 1. Structural properties
The crystalline structure for TiO 2(1-x) CdO x recognized by study the phase of XRD for that material.  Fig. 2 it can be observed that the preferred orientation was along (110) direction for Rutile. In the x-ray patterns, it is cleared that the peaks intensities increase with increasing of the doping ratio from 5 to 20%. Also, it was noticed that the all film quality improves with the increasing of annealing temperature, and appear a new peak at concentration (0.15 and 0.2) Wt. % which recognized to CdO structure that corresponding to the reflection plane of (111).

Fig. 2: The X-Ray diffraction for TiO 2 : CdO thin films: a) at RT, b) at annealing 423K, c) at annealing 523K.
Table (1-a, b, c) gives the interplaner distance d, FWHM (Deg.), and grain size for the prepared samples in comparison with the standard value as in ASTM card. The structure of the TiO 2 : CdO film has been investigated by using XRD to ensure the stoichiometry of our material. We can observe that the values of d and 2θ are nearly similar to that in the ASTM cards as listed in Table 1. The mean grain size of thin film calculated using the Scherer's equation [26]: where G is the average crystalline grain size, λ is the wavelength, β represents the full-width at half maximum (FWHM) in radian and θ is the Bragg diffraction angle in degree. The grain sizes have been calculated by using Eqs. (1) and tabulated in Table 1. It is cleared from the table that d hkl and grain size increases with increasing of concentration of x. This implies that Cd partially substituted for Ti in TiO 2 structure.  Fig. (3-a, b, c) shows the PL spectrum of the TiO 2(1-x) CdO x films at room temperature and annealed to different annealing temperatures and different concentration of CdO. Typical luminescence behavior with two emission peaks, UV PL characteristics of TiO 2(1-x) CdO x films showed strong relation to the temperature. The first peak in PL spectra between (320-400) nm corresponds to the direct recombination between electrons in the conduction band and holes in the valence band [27]. In all the samples (pure and dopant) a broad peak was also observed at a lower energy or visible region (second peak). The intensity of the two peaks increases markedly with the increase of concentration, due to the large exciton bending energy of TiO 2(1-x) CdO x . Higher energy (shorter wavelength) excitation photons cause more phonons to be emitted before luminescence occurs. If the excitation energy is less than the energy difference between the ground state and the first excited state, then no optical absorption will occur, resulting in no PL. The PL emission might have close relation with the luminescence of the recombination of photo induced electrons and holes, the free and self-trapped electron-hole pair or excitons, which possibly resulted from the nointegrality of nanosized TiO 2 crystallite such as the lattice distortion and surface oxygen deficiencies. However, in thin films, the broad band visible emission at (620-680) (nm) this luminescence could be due to the self-trapped excitons of the charge transfer process. The Table 2 shows the peak values and the energy of the luminescence spectrum of all samples. It is observed from this table The value of the optical energy gap increases with increasing of Ta for all samples, this is due to the growth of grain size and the decrease in defect states near the bands and these in turn increase the value of E g . The optical energy gap decreases with increasing concentration this is due to the increase of the density of localized states in the E g which cause a shift to lower values.

Conclusion
Pure TiO 2 and TiO 2(1-x) CdO x thin films were deposited by PLD technique on glass substrates with different concentration of CdO at RT and annealed to different annealing temperatures (423 and 523) K. The resulting of TiO 2(1-x) CdO x films were characterized by XRD measurement and PL properties. X-ray diffraction (XRD) results reveals that the deposited TiO 2(1-x) CdO x thin films were polycrystalline with tetragonal structure of Phase classification Rutile Annealing the films in vacuum for one hour increases the grain size and noticed that the all film quality was improved and appear a new peak at content of (0.15 and 0.2) Wt. % which recognized to CdO structure. From PL analysis it is observed two emission peaks, UV PL characteristics of for pure TiO 2 and doped TiO 2 with CdO and the intensity gradually increases by increasing the doping.