Post annealing effect on thin film composed ZnO nano-particles on porous silicon

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1 ARTICLE NANO BULLETIN Vol. 2, No. 2, , Jul 2013 Post annealing effect on thin film composed ZnO nano-particles on porous silicon Kevin Alvin Eswar 1,2,a, Aziz Azlinda 1,2,b, H Fadzilah Husairi 1,2,c, Mohamad Rusop 1,3,d, and Saifollah Abdullah 1,2,e 1 NANO-SciTech Centre (NST), Institute of Science, Shah Alam, Selangor, Malaysia 2 Faculty of Applied Sciences, Universiti Teknologi Mara (UiTM), Shah Alam, Selangor, Malaysia 3 NANO-Electric Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi Mara (UiTM), Shah Alam, Selangor, Malaysia a kevinalvin86@hotmail.com, b azlindazz@gmail.com, c mhusairifadzilah@yahoo.com, d rusop@salam.uitm.edu.my, e saifollah@salam.uitm.edu.my Received 28 Mar 2013; revised 22 Jun 2013; accepted 28 Jun 2013; published online 02 Jul ABSTRACT: Zinc acetate dihydrate as a starting material was used in precursor preparation. In order to prepare porous silicon (Psi) substrates, electrochemical etching was employed to modify the silicon surface. ZnO thin films were successfully deposited on porous silicon substrate by spin-coating technique. Post annealing temperatures were varied in a range of 300 C to 700 C to investigate its effect on morphologies and photoluminescence properties of the ZnO thin films. Field Emission Scanning Electron Microscopy (FESEM) results revealed that the thin films are composed of ZnO nano-particles and distributed uniformly on Psi surface. ZnO nano-particle sizes are varied from 21.3 nm to 25.9 nm when annealed in different temperatures. Atomic Force Microscopic (AFM) analysis was carried out to study the surface morphology. It was found that the lowest average surface roughness of ZnO thin films is obtained at an annealing temperature of 500 C, which is 1.46 nm. Photoluminescence (PL) spectroscopy was conducted in the range of 350 nm to 900 nm. The result shows three distinct peaks at 410 to 420 nm, 525 to 600 nm and 625 to 725 nm, which are attributed to nano-structured ZnO, ZnO defects and porous silicon respectively. Global Scientific Publishers KEYWORDS: ZnO thin film, post annealing, ZnO nano-particle, spin coating, AFM, photoluminescence. 1. Introduction Developments of semiconductor industries are among the fastest and are important in optoelectronic devices applications. Relating to this, ZnO is a promising semiconductor material, owing to its unique properties of wide band gap energy of ~3.37 ev and large binding energy of 60 mev [1]. These properties make it widely used in optoelectronic devices such as solar cells, gas sensors, UV sensors, light emitting device (LED) applications and so on [1-5]. In order to synthesis ZnO, various methods were applied such as laser deposition [1], RF magnetron sputtering [3, 4, 6], mist-atomization [7] and sol-gel deposition [2, 5, 8, 9]. High quality ZnO nanostructures are obtained by very expensive and complicated equipment [10]. However, expensive and complicated equipment for ZnO synthesization are not applicable in large scale productions. In this work, ZnO thin film was deposited by sol-gel spin coating. This simple method, using less expensive equipment, is also easy to handle when compared with other techniques. Therefore, this low-cost approach becomes quite appealing in a bulk production scenario. Substrate selection is very important to ZnO thin film properties. Physical, optical and electrical properties of ZnO will be different depending on the different substrate being used. P. Suresh et al. have studied the deposition of ZnO nanostructure on indium tin oxide (ITO), glass and poly-ethylene terephthalate polymer (PET) substrates [11]. They found that different substrates will produce a different type of ZnO nanostructure. Besides, substrate selections are also based on the device application. Hafiz et al. have fabricated UV photoconductive sensors on glass substrate [12] and Gao et al. have deposited ZnO films on silicon wafer in order to study the optical properties for solar cell applications. Furthermore, Yang et al. suggested that white light emitting diodes could be produced by depositing ZnO films on porous silicon substrate [4]. In this work, porous silicon substrate was prepared by modifying the silicon wafer surface using an electrochemical etching method. M. S. Kim et al. suggested that modification of the silicon surface can reduce the effect of large lattice mismatch between a single crystalline silicon wafer and ZnO [13, 14]. Investigation on various parameters such as heat treatment, molarity of precursor and stabilizer and aging time were conducted. The results show that the growth of ZnO thin films was influenced by these parameters, including the optical and electrical properties [15-18]. In ISSN /130212/06/$ Global Scientific Publishers

2 order to study the post heat treatment, annealing temperatures were varied in a range of 300 C to 700 C, while other parameters were maintained. 2. Experiment Zinc acetate was used as starting material along with diethanolamine and isopropyl as a stabilizer and solvent respectively. P-type silicon wafer was used in porous silicon preparation. Firstly, the silicon wafer was cleaned by acetone, followed by methanol and diluted hydrofluoric 40% acid in an ultrasonic bath. Next, the silicon was dried using nitrogen gas before being etched by electrochemical etching. Hydrofluoric acid 48% was mixed with absolute ethanol (HF48%: absolute ethanol = ratio of 1:1) as an electrolyte. Current densities and time were maintained at 20 ma/cm 2 and 20 minutes during the etching process. A ZnO precursor was prepared by dissolving in 0.15 M of zinc acetate into isopropyl and stirred. Diethanolamine was then slowly added into the solution and heated at 60 C for one hour to yield a clear and homogenous solution. Following this, the solution was continually stirred and aged at room temperature for 24 hours. The ratio of zinc acetate to diethanolamine was fixed at 1.0. A spin-coating method was used to deposit a ZnO thin film on the porous silicon surface. Rotation per minute was maintained at 3000 per 60 seconds. Then, 10 drops of ZnO precursor were dropped on the surface of the rotated porous silicon. Thereafter, the ZnO thin film wa dried at 150 C for 10 minutes to remove any volatile components. This process was repeated 10 times to increase the thickness of the ZnO thin film. Finally, ZnO thin films were annealed in various temperatures in the range of 300 C to 700 C for an hour each time. (PL) spectra were studied by HORIBA Jobin Yvon's LabRAM 800HR with 325 nm He-Cd laser as a source. 3. Results and discussions The p-type silicon wafer surface was successfully modified to porous after being etched at current densities of 20mA/cm 2 for 20 minutes. Figure 1 shows the FESEM image of porous silicon. As seen in fig. 1, irregular pores with sizes in the range of ~11.9 nm to ~30.6 nm were distributed over the porous silicon surface. The twodimensional and 3-dimensional AFM images are shown in fig. 2. The surface roughness of porous silicon is 1.95 nm. The reaction between silicon wafer and hydrofluoric acid with presence of absolute ethanol is suggested in Eq. (1) and Eq. (2) [19]. Si(OH) 4 SiO 2 + 2H 2 O (1) SiO 2 + 6HF SiF H H 2 O (2) (a) (b) Figure 1. FESEM image of porous silicon. Scale bar 100 nm. The prepared ZnO thin films were next studied by a Field Emission Scanning Electron Microscope, model JEOL JSM-J600F. An Atomic Force Microscopic (AFM), model XE-100 of Park Systems was employed to study the roughness of the ZnO thin films. Photoluminescenece Figure 2. Two-dimensional (a) and three-dimensional (b) AFM image of porous silicon. The size of (a) is 2 by 2 µm, (b) is 2 by 2 µm. As reported by Sailor, crystalline silicon wafer is thermodynamically unstable in air or water. It will react spontaneously to form oxide layer, SiO 2 [19]. In this work, hydroxide ions were supplied by absolute ethanol. ISSN /130212/06/$ Global Scientific Publishers

3 Table 1. Particle size of ZnO nano-particles and surface roughness in different annealing temperatures. Annealing Temperatures ( C) Particles Size (nm) Surface Roughness (nm) Then, in the presence of HF, SiO 2 is easily dissolved to form SiF The silicon hexafluoride ion is a stable dianion that is highly soluble in water. The ZnO thin film deposited on the porous silicon is shown in fig. 3. The left hand side shows the FESEM images while the threedimensional AFM image is presented on the right hand side. Based on the FESEM images, the thin films are composed by ZnO nanoparticles. The pores can also be clearly seen on the surface when the thin films were annealed at 500 C. In ZnO sol-gel synthesis, hydrolysis and condensation are reactions that involved in sample preparation [20]. ZnO particles are products of the reaction between zinc acetate dehydrate and isopropyl which stabilized by diethanolamine as reported by Minrui et al [21]. As can be seen in Eq. 3, [CH 3 COOZn] + ion, [CH 3 COO] - ion, H + ion and (OH) - ion are produced from hydrolysis of zinc acetate in isopropyl. Then, when diethanolamine was added to the solution, Zn(C 2 H 4 O) 2 NH was produced as described in Eq. 4, suggesting that ZnO is resulted from the decomposition of Zn(C 2 H 4 O) 2 NH [22]. Zn(CH3COO) 2 2(H 2 O) [CH 3 COOZn] + + 2H + + 2(OH) - (3) [CH 3 COOZn] + + HN(C 2 H 4 OH) 2 CH 3 COOH + Zn(C 2 H 4 O) 2 NH + H + (4) As can be seen from the fig 3, the average particle size depends on the post annealing temperature. Table 1 shows the average particle sizes of ZnO nano-particles at different annealing temperatures. The particle size of 24.7 nm increased to 25.6 nm when the annealing temperature was increased from 300 C to 400 C. This phenomena has also been observed by other researcher [15].This can be explained by thermal expansion where the kinetic energy of an atom increases due to the increase in annealing temperatures. However, the average particle size decreased to 24.4 nm at an annealing temperature of 500 C and 21.3 nm at 600 C. This can be explained where the atom has sufficient thermal energy and can move to any space within the particle & crystalline arrangement. Therefore, the size of the particle decreases due to the ability to fill the space within the crystalline structure. It is believed that the crystalline quality has also increased because the atom will move to the most favorable position [23]. In addition, the atoms will move to adjacent particles at 700 C, therefore, they will merges together and subsequently form a larger particle. The average surface roughness of ZnO thin film surface is 1.50 nm at annealing temperature of 300 C and increases to 1.64 nm at 400 C. However, the average surface roughness decreases to 1.45 nm at 500 C. Then it becomes rougher once more when increasing the annealing temperatures. The average surface roughness of thin film is increased at 400 C due to increases in the particle size. When annealed at 500 C, the crystalline quality increases due to the atom arrangements being in favorable position and therefore making the average surface roughness decrease. However, the average surface roughness increases after this annealing temperature due to increases in particle size. Fig. 4 shows the photoluminescence spectra of the ZnO thin film composed nanoparticles on porous silicon in the range of 350 nm to 900 nm. The findings show three distinct peaks at ranges of between 400 nm to 420 nm, 525 nm to 600 nm and 625 nm to 725 nm. The first peak, in the range 400 nm to 420 nm relates specifically to the ZnO; the peak at 525 nm to 600 nm is relevant to the defects of ZnO thin film; and the third peak, between 625 nm and 725 nm is due to the porous silicon [6, 24]. The peak within the range of 400 nm to 420 nm is free-exciton recombination of ZnO thin film [23]. The peak within the 525 nm to 600 nm region is considered by the native defect of ZnO thin film such as oxygen vacancies and zinc interstitial [6]. The peak within the red region ( 625 nm to 725 nm) is generated by the radiative recombination of excitons on the surface of the porous silicon [23]. The ZnO thin-film peak (400 nm to 420n m) contributed by the free-exciton recombination is less affected by post-annealing temperature in this work. However, the intensities of the peak in range 525 nm to 600 nm gradually decrease due to increases of post-annealing temperature. This is attributed to the crystalline nature of the ZnO thin film. As described earlier, by increasing the temperature the atoms are able to move to a more favorable position. Because of this the defects, such as oxygen vacancies and zinc interstitial, will also reduce. Lastly, the peaks in the range of 625 nm to 725 nm are highly affected by the post annealing temperatures. The peak shown with a post annealing temperature of 300 C is attributable to the combination of defects of ZnO and radiative recombination of excitons on the surface of the porous silicon. The increase in crystalline quality of ZnO reduces the intensities of the peaks and red-shift attributes to porous silicon surfaces. The intensities of the peaks tend to diminish after annealling at 600 C and 700 C. This suggests that the pores on the silicon surface were filled with ZnO nano-particles, causing the photoluminescence contributed by porous silicon to be restricted. ISSN /130212/06/$ Global Scientific Publishers

4 (a) (b) (c) (d) (i) (ii) (iii) (iv) a broad photoluminescence and various peaks in visible wavelength of ZnO thin film on porous silicon have been reported by other researcher [4, 6]. As ours, Fuchao et al. got the same result that the highest energy peak was centered at 410 nm (3.02 ev). They proposed that the peak centered at nm was belonging to ZnO. It might originate from the electron transition from the bottom of conduction band to the shallow acceptor level which is formed by Zinc vacancies. Besides, peak centered at nm (2.25 ev) is belonging to native defects of ZnO such as Zinc interstitial, Oxygen vacancies, and substitution Oxygen at Zinc position [25]. The emission light with peak centered at nm (1.79 ev) is originated from porous silicon [4, 26, 27]. As a comparison, photoluminescence spectrum of porous silicon is shown in fig. 6. It was found that the peaks were centered at nm (1.87 ev), the same as reported by Ramziy et. al. [28]. The peak shift from nm to nm may be related to the oxidation of porous silicon [4]. The photoluminescence property of porous silicon was very sensitive to the surface structure and was influenced easily during annealing in air especially in the presence of oxygen [29]. (e) (v) Figure 3. FESEM and three-dimensional images of ZnO thin films on porous silicon annealed at (a) and (i) 300 C, (b) and (ii) 400 C, (c) and (iii) 500 C, (d) and (iv) 600 C, and (e) and (v) 700 C. Figure 5. Fitted photoluminescence spectrum of ZnO thin film composed of nano-particles on porous silicon annealed at 500 C with three distinct peaks centered at (a) nm, (b) nm, and (c) nm. Figure 4. Photoluminescence spectra of ZnO thin film composed of nano-particles on porous silicon in the range of 350 nm to 900 nm Fig. 5 shows fitted photoluminescence spectrum of ZnO thin film annealed at 500 C. It shows three distinct peaks centered at nm, nm and nm respectively. By using different methods of synthesization, Figure 6. Photoluminescence spectrum of porous silicon. ISSN /130212/06/$ Global Scientific Publishers

5 4. Conclusions As a conclusion, the ZnO thin film was successfully deposited on porous silicon. FESEM and AFM images revealed that the thin films were composed of ZnO nanoparticles. The average ZnO particle sizes varied from 21.3 nm to 25.9 nm, depending on the post-annealing temperatures. Photoluminescence spectra showed three distinct peaks in the range of 350 nm to 900 nm. Besides, the photoluminescence position and intensities were also influenced by the annealing temperature, which were attributed to the physical changes of ZnO nano-particles. Acknowledgements The authors would like to thank the Ministry of High Education of Malaysia and Universiti Teknologi MARA (UiTM) for their financial and instrumental support. The authors would also like to thank the Faculty of Electrical Engineering (Mr. Danial and Mr. Sharil) for the use of their FESEM. Furthermore, the authors thank Mr. Salifairus Jaafar (UiTM Science Officer), Mrs. Nurul Wahida (UiTM Asst. 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