Structural and Optical Properties of Pulsed Laser

Structural and ocular Properties of Pulsed LaserStructural and opthalmic Properties of Pulsed Laser Deposited ZnO/TiO2 and TiO2/ZnO Thin FilmsR. K. Jain, Praveen K. JainAbstract. ZnO/TiO2 and TiO2/ZnO repress films have been deposited on single crystal Si (100) substratum apply pulsed laser deposition (PLD) technique in order to alter morphological and visual properties of ZnO and TiO2 tighten films. It was observed that the deposition of TiO2 film prior to ZnO, exhibited high crystallinity along (002) diffraction peak, small compressive strain and stress and at that placeby rendering break off optical properties as compared to ZnO films deposited coordinately on Si substrates. On the otherwise hand, TiO2 lightly film deposited on Si substrate exhibited pure anatase arrange while the enforce of ZnO dampen zone zone was found to improve the crystallinity of TiO2 turn out film. The photoluminescence spectra tapeed that TiO2 and ZnO fender classs enhanced ultra violet radiation arcs of the ZnO and TiO2 repress films to a larger extent, respectively.Keywords ZnO, TiO2, Optical properties, PhotoluminescencePACS 78.66.Hf, 78.55.Et, 68.37.Ps approachZnO is suitable for the production of light emitting devices and a promising candidate for the succeeding(prenominal) generation of electronic devicesdue to its wide telephone break of serve (3.37 eV) and large exciton cover song postal code (60 meV)1. ZnO prune films play an important role in solid display devices, solar cells and exciting acoustic waves at microwave frequencies2. atomic number 22 dioxide (TiO2) is one of the most important semiconductors with high photocatalytic activity, non-toxicity, stability in sedimentary solution, and is relatively inexpensive. The excellent photocatalytic property of TiO2 is due to its wide plenty gap and long lifetime of photogenerated holes and electrons 3-4. It has been reported that the deposition of ZnO or TiO2 veer films on Si substrates a t elevated temperature leads to increase in oxygen vacancies as the wax Si atoms easily capture oxygen atom from ZnO or TiO2, which deteriorates the quality of the these films 5. So it is required to improve various properties of ZnO and TiO2 films for their potential applications. In the present study, a systematic investigation has been performed in order to improve the structural and optical properties of these films using buffer layers. ZnO and TiO2 are chosen as a buffer layer material on the basis of pastime considerations (a) Both are wide- circumstances-gap materials, (b) both exhibit high chemical and thermal stability, (c) both have high refractive indices, high transmittance in the perceptible region and intense absorption in the ultraviolet band and (d) both are low cost material.EXPERIMENTAL DETAILsZnO and TiO2 thin films have been deposited on Si (100) substrate by ablating high purity (99.9%) ZnO and TiO2 ceramic target using pulsed laser deposition (PLD) techni que. The KrF excimer laser with wavelength of 248 nm was practiced for deposition. The pulse repetition rate was 10Hz with laser fluence of about 23Jcm2. The target to substrate distance, working O2 pressure and deposition temperature were kept 35mm, 50 mTorr, and 500C respectively. The thickness of the film and buffer layer was deliberate using cross section FE-SEM and found to be 200 nm and 50 nm, respectively. The variant and orientation of as- magnanimous thin films were characterized by X-ray diffractometry (Bruker AXS D-8 emanation Diffarctometer) using CuK (=1.5407 ) radiation. The surface morphology was examined using atomic force microscope (NTMDT NTEGRA model). assiduity spectra have been taken using UV-VIS-NIR spectrophotometer (Varian Cary 5000) and PL study was performed using photoluminescence spectrometer (Perkin Almer LS-55).RESULTS AND wordXRD pattern reveled that ZnO thin film grown on Si (100) substrate was preferentially oriented along the c-axis with a hex agonal wurtzite structure and the use of TiO2 buffer layer increases crystallinity along (002) diffraction peak as shown in judge 1. On the other hand, TiO2 thin film exhibit pure anatase phase and crystallinity was improved along (004) plane by inserting the ZnO buffer layer surrounded by substrate and TiO2 thin film. The improved crystallinity of thin film using buffer layer topiced from the pair in thermal expansion coefficient between ZnO and TiO2, which is little than that of between ZnO and Si or TiO2 and Si. The fretwork mismatch between ZnO and Si (1 0 0) are 40%, whereas for their counterparts i.e. between ZnO and anatase-structured TiO2 are 14% 6. Therefore, the abate of radiator grille mismatch is another reason for the improved crystallinity.The crystallite coat measured using Scherers formula is shown in Table 1. The strain along the c axis, zz is given by the following equation 7 (1)where c is the lattice parameter of the strained ZnO films calculated from x- ray diffraction data and c0 is the unstrained lattice parameter of ZnO. The lattice mismatch between film and substrates can result in varying degrees of stress during the growth process of thin films. The results show that the compressive strain is present in all fabricated ZnO and TiO2 films, which is derived from lattice mismatch between substrates and films owing to increase in crystallite size, and the stress is diminishd with the buffer layer. Figure 2 shows the AFM image of the deposited thin films. The grain size and come surface roughness increases when buffer layer is used due to sweetener in crystallinity.Figure 3 shows the room temperature PL spectra of ZnO and TiO2 thin films grown on Si substrate with and without buffer layer. The ZnO film deposited on Si (100) substrate exhibits strong ultraviolet emission peak along with weak commonsyellow emission band. The ultraviolet emission of ZnO films is generally considered to be resulted from recombination of idle excito n, whereas the green emission is mainly resulting from oxygen vacancies 8. The PL spectra of TiO2 thin film deposited on Si (100) substrate shows a broad emission band from 390 to 450nm and there are two emission peaks superimposed on the broad emission band. The peak before 350nm (3.5eV) is ascribed to direct electron-hole recombination which should be equal to or jolly bigger than the TiO2band gap. The emission band from 390 to 450nm (corresponding to 3.22.75eV) arises from indirect band gap and surface recombination processes. Further observation indicates that there are two small peaks at the wavelength range from 460 to 500 nm. These PL signals are attributed to excitonic PL, which mainly result from surface oxygen vacancies and defects of the films. It is observed that ZnO thin film deposited on the TiO2 buffer layer shows stronger ultraviolet emission, as compared to ZnO thin film grown without buffer layer, with no visible emission. The absence of visible emission shows the defect free formation of film. Similarly, the use of ZnO buffer layer also removes the oxygen defects emission peak of TiO2 thin film.The enhanced ultraviolet emission from ZnO thin films grown on TiO2 buffer layer is also probably connected with fluorescence resonance energy transmit(FRET) between ZnO and TiO2. After the excitation of electronhole pairs in TiO2 layer, the energy is easily transferred to ZnO films due to resonance effect 9 as a result, the band gap emission of ZnO is enhanced.From optical absorption spectra of ZnO and TiO2 thin films, It is observed that ultraviolet absorption edge of ZnO and TiO2 film with buffer layer has a red-shift, compared with ZnO and TiO2 thin film grown on bare Si (100) substrate. The rate of direct band gap was found to be 3.29 and 3.24 eV for ZnO thin films grown on Si substrate without and with TiO2 buffer layer, respectively. On the other hand, the value of indirect band gap was found to be 3.24 and 3.19 eV for TiO2 thin films deposi ted on Si (100) substrate without and with ZnO buffer layer. The decrease in optical band gap of the films could be related to the enhancement in crystallite (grain) size leading to a smaller number of grain boundaries. On the other hand the compressed lattice will provide a wider band gap because of the increased repulsion between the oxygen 2p and the zinc 4s bands 10.CONCLUSISONZnO, TiO2, ZnO/TiO2 and TiO2/ZnO thin films on Si (100) substrate were prepared by pulsed laser deposition technique. XRD and AFM result demonstrate that the crystallinity of ZnO and TiO2 thin films are considerably improved by using TiO2 and ZnO buffer layer, respectively. Compared with PL of ZnO thin film, UV intensity of ZnO grown on TiO2 buffer layer has increased about two fold. Similarly, the ZnO buffer layer improved the UV emission of TiO2 thin film. 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