Structural and Optical Properties of Pulsed Laser Deposited ZnO/TiO2 and TiO2/ZnO Thin Films

R. K. Jain, Praveen K. Jain

 
Abstract. ZnO/TiO2 and TiO2/ZnO thin films have been deposited on single crystal Si (100) substrate using pulsed laser deposition (PLD) technique in order to improve structural and optical properties of ZnO and TiO2 thin films. It was observed that the deposition of TiO2 film prior to ZnO, exhibited higher crystallinity along (002) diffraction peak, small compressive strain and stress and thereby rendering better optical properties as compared to ZnO films deposited directly on Si substrates. On the other hand, TiO2 thin film deposited on Si substrate exhibited pure anatase phase while the use of ZnO buffer was found to improve the crystallinity of TiO2 thin film. The photoluminescence spectra showed that TiO2 and ZnO buffer layers enhanced ultraviolet emissions of the ZnO and TiO2 thin films to a larger extent, respectively.
Keywords: ZnO, TiO2, Optical properties, Photoluminescence
PACS: 78.66.Hf, 78.55.Et, 68.37.Ps
Introduction
ZnO is suitable for the production of light emitting devices and a promising candidate for the next generation of electronic devicesdue to its wide band gap (3.37 eV) and large exciton binding energy (60 meV)[1]. ZnO thin films play an important role in solid-state display devices, solar cells and exciting acoustic waves at microwave frequencies[2]. Titanium dioxide (TiO2) is one of the most important semiconductors with high photocatalytic activity, non-toxicity, stability in aqueous solution, and is relatively inexpensive. The excellent photocatalytic property of TiO2 is due to its wide band gap and long lifetime of photogenerated holes and electrons [3-4]. It has been reported that the deposition of ZnO or TiO2 thin films on Si substrates at elevated temperature leads to increase in oxygen vacancies as the surface 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 following considerations: (a) Both are wide-band-gap materials, (b) both exhibit high chemical and thermal stability, (c) both have high refractive indices, high transmittance in the visible region and intense absorption in the ultraviolet band and (d) both are low cost material.

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EXPERIMENTAL DETAILs
ZnO 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) technique. The KrF excimer laser with wavelength of 248 nm was used for deposition. The pulse repetition rate was 10Hz with laser fluence of about 2–3Jcm−2. The target to substrate distance, working O2 pressure and deposition temperature were kept 35mm, 50 mTorr, and 500°C respectively. The thickness of the film and buffer layer was measured using cross section FE-SEM and found to be ~200 nm and 50 nm, respectively. The phase and orientation of as-grown thin films were characterized by X-ray diffractometry (Bruker AXS D-8 Advance Diffarctometer) using CuKα (λ=1.5407 Å) radiation. The surface morphology was examined using atomic force microscope (NTMDT: NTEGRA model). Absorption 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 DISCUSSION
XRD pattern reveled that ZnO thin film grown on Si (100) substrate was preferentially oriented along the c-axis with a hexagonal wurtzite structure and the use of TiO2 buffer layer increases crystallinity along (002) diffraction peak as shown in Figure 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 between substrate and TiO2 thin film. The improved crystallinity of thin film using buffer layer resulted from the mismatch in thermal expansion coefficient between ZnO and TiO2, which is smaller than that of between ZnO and Si or TiO2 and Si. The lattice 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 decrease of lattice mismatch is another reason for the improved crystallinity.
The crystallite size calculated using Scherer’s 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 decreased with the buffer layer. Figure 2 shows the AFM image of the deposited thin films. The grain size and average surface roughness increases when buffer layer is used due to enhancement 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 green–yellow emission band. The ultraviolet emission of ZnO films is generally considered to be resulted from recombination of free exciton, 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 slightly bigger than the TiO2band gap. The emission band from 390 to 450nm (corresponding to 3.2–2.75eV) 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 transfer
(FRET) between ZnO and TiO2. After the excitation of electron–hole 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 value 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 deposited 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].
CONCLUSISON
ZnO, 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. The band gap of ZnO and TiO2 thin film grown on buffer layer found to decrease due to improved crystallinity.
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