Effects of Annealing on Surface Morphology and Crystallinity of Nickel doped Zinc Oxide with Nickel Seed Layer Deposited by RF Magnetron Sputtering.

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1 Effects of Annealing on Surface Morphology and Crystallinity of Nickel doped Zinc Oxide with Nickel Seed Layer Deposited by RF Magnetron Sputtering. Abdullah Haaziq Bin Ahmad Makinudin and Ahmad Shuhaimi Bin Abu Bakar Low Dimensional Research Centre Physics Department, Faculty of Science, University of Malaya, 50600, Kuala Lumpur, Malaysia. The effects of annealing on nickel doped zinc oxide with nickel seed layer (Ni/ZnO:Ni) prepared by RF magnetron sputtering deposition were investigated to understand its properties with an interest of growing transparent conducting oxide (TCO) material. Samples were prepared by varying its deposition temperature from room temperature up to 500 C. The best crystallinity shown via X-Ray diffraction (XRD) is at 300 C deposition temperature. Samples deposited in this temperature were further annealed in oxygen gas (O 2 ) and changes in their surface morphology and crystallinity are shown. The annealed samples show a higher intensity relative to as deposited samples. Surface morphology analysis via Field Emission Scanning Electron Microscopy (FESEM) shows small variation in grain size. Atomic Force Microscopy (AFM) analysis shows a rougher surface after annealing compared to non-annealed samples. Keywords: Annealing, Seed Layer, Crystallinity, Surface Roughness 1. INTRODUCTION Zinc oxide (ZnO) is one of the important wide band gap semiconductors in material science due to its specific electrical, optical and mechanical properties. One of the main properties that will be taken into account is its crystal hexagonal wurzite structure. This structure is one of the most important properties in applications in electronics, optoelectronics, photovoltaics, and sensors devices. (Ami Moezzi et al, 2007, Jr H. He et al, 2005). In optoelectronic devices ZnO have a very high potential applicability, such as light emitting diodes (LEDs), laser diodes and applications which have drawn the interests of researchers. The band gap of ZnO which is 3.37 ev, is very close to the widely known semiconductor Gallium Nitride (GaN) which is 3.39 ev. However, the exciton energy of ZnO which is 60 mev is twice larger than GaN which is only 28 mev. This makes ZnO as a more promising optoelectronic material than GaN. In the design of LEDs, large excitation binding energy is very important. Since the ZnO shows a promise in this factor, there is possibility in replacing the GaN based compounds that is mainly used in optoelectric devices. Furthermore ZnO is cheaper than GaN and is generally categorized as a nontoxic material. (S. Ghosh et al, 2008, Amir Moezzi et al, 2007) In photovoltaic technology, ZnO can be used as a transparent conducting oxide (TCO) in the front electrodes of a solar cell. This can eliminate the shadow effect related to metal-finger contacts. Other than that, it can be considered as a cheaper alternative to indium oxide electrodes which is much more expensive.(ami Moezzi et al, 2007, Jr H. He et al, 2005,) Many studies have been done by other researchers before in doping ZnO with Group 3 elements such as aluminum (Al), gallium(ga) and rare earth elements as n-type dopants. However interests have grown amongst researchers in making spintronic devices based on diluted magnetic semiconductors which led them to dope ZnO with ferum(fe), cobalt(co), nickel(ni) and others. Promising results have been shown in this area.(s. Ghosh et al, 2008) This paper investigates the effect of annealing on Ni-doped ZnO with Ni seed layer in temperature variationfrom300 o C to 500 o C in order to enhance the surface morphology and crystallinity of the film. The growth was performed using radio frequency (RF) magnetron sputtering method. The annealing process of this study was conducted using a tube furnace with oxygen gas as medium. Their material characteristics will be studied and analysed using the x-ray diffraction (XRD), atomic force spectroscopy(afm), and field-emission scanning electron microscopy(fesem). 2. EXPERIMENTAL SETUP Radio Frequency (RF) magnetron sputtering was employed to deposit a thin layer of ZnO on Si and glass substrates. The substrates were cleaned in an ultrasonic bath for 5 minutes three times with acetone and propanol before rinsed with distilled water. Nitrogen gas was sprayed to remove any excess dirt after cleaned. The substrates were then loaded into the chamber for deposition of ZnO. Deposition of the substrates were carried out at different temperatures ranging from room temperature and raised until 500 o C with the RF power and pressure kept at a constant value. The Ni seed layer was however deposited with RF power of 50 W with 15 sccm of argon (Ar) gas flow rate for 300 seconds. The growth of ZnO doped nickel were deposited with 50 and 200 W RF power with argon (Ar) to oxygen (O 2 ) ratio of 25:5 sccm for 3600 seconds, where the target gun are both released simultaneously. Pressure of the chamber was maintained at 5mTorr across the entire

2 Intensity (a.u) Intensity (a.u) JURNAL FIZIK MALAYSIA VOLUME 35, number deposition process. In order to ensure a uniform deposition, the sample stage is rotated at 6 rotation per minute (RPM). The distance between the substrates and the target was fixed at 140mm. For further accuracy, the substrates went through a presputtering process for 300 seconds before the actual deposition in order to remove any unwanted contaminations on target surface. All the samples were analysed and a specific sample was chosen to undergo the annealing process due to its most desired surface morphology and crystallinity. The surface structure and morphology of the samples were studied via a field emission scanning electron microscopy (FESEM). The crystal structures of the films were determined by an x-ray diffraction (XRD). An atomic force microscopy (AFM) was used to study the surface roughness of the samples. where λ is the wavelength, β is the full width at half maximum(fwhm) and with the help of Bragg's equation in order to find the λ, λ = 2dsinθ (2) where θ is the Bragg angle. When a certain deposition temperature (500 C) is reached, the Ni:ZnO/Ni intensity is reduced and almost not present via the XRD plot. This is due to the deposition temperature of 500 C which causes the Ni:ZnO/Ni sample to turns into amorphous.from the plot of the peak intensities, samples deposited in the temperature of 300 C shows the highest peak intensity compared to other deposition temperature. Further analysis of the spectra also shows this deposition temperature has the best crystallinity structure. Hence this specific sample is further annealed to see if there is any effect on its structure. 3. RESULTS AND DISCUSSION (002) (103) (002) (103) 500 o C 400 o C Anneal at 500 C 300 o C Anneal at 400 C 200 o C 100 o C FIG. 1. XRD patterns of Ni:ZnO/Ni as deposited at different temperatures. X-ray diffraction patterns of the samples were plotted viaoriginpro and analysis of the data were calculated via XpertPro. From figure 1, it is shown that higher deposition temperature gives higher peak intensity. The diffraction peak around 34.4 C corresponds with the index of (002) is clearly seen. This (002) peak in the pattern shows that the Ni:ZnO/Ni nano films structure is the wurtzite crystal structure.peak (103) however shows no presence after deposition at any deposition temperatures. The crystallinity size can be calculated using Sherrer's equation, 2 RT (1) Anneal at 300 C No Anneal FIG. 2. XRD patterns of Ni:ZnO/Ni as deposited and annealed at different temperatures. From the plot it is seen that there is a shift in the (002) peak as-deposited, annealed at 300 C, annealed at 400 C, and annealed at 500 C. This shows that the angle shifts towards higher angles due to increasing annealing temperature. Table 1: Summary of the XRD analysis of annealed samples. T ( C) 2θ ( ) d ( ) FWHM D (nm) As-deposited C 400 C 500 C

3 The results of the annealed sample are as shown in figure 2. This XRD pattern also indicates that the as deposited nano films have a weak c-axis orientation. However, after annealed with increasing annealing temperature results in a lower full width half maximum (FWHM) which gives a higher crystalline size. The peak (103) was not present in the XRD plot of the as deposited samples. However due to annealing, the XRD plot shows a small presence in the (103) peak. (a) (b) (c) (d) FIG. 3. FESEM images of (a) as-deposited (b) annealed at 300 C (c) annealed at 400 C and (d) annealedat 500 C. The surface morphology of the samples (asdeposited and annealed) was analysed via FESEM and AFM. Figure 3 shows the FESEM images of the surface of the Ni:ZnO/Ni as-deposited and annealed at 300 C, 400 C and 500 C. From the figure, only slight difference can be detected amongst the samples. As reported in ZnO studies, annealing of ZnO below 600 C shows not much difference in surface morphology. Annealing of the samples above 600 C should show a better result on the surface morphology.(jing Li et al, 2006). The presence of Ni seed layer and Ni dopant in the sample have enhanced the visibility of surface morphology in FESEM and AFM characterizations. Change in the surface morphology of the Ni:ZnO/Ni films after the oxygen annealing treatment was subsequently investigated using an AFM, and the resulting surface images are shown in Fig.4 for various annealing temperatures.from the AFM results, there exist some differences between as-deposited and annealed at the three different temperatures.this proves the existence of Ni due to Ni doping during deposition. The surface roughness of the samples shows some variation. The root mean square (rms) roughness for as deposited is2.834 nm, while the sample annealed at 300 C,400 C, and 500 C, is 3.417, 3.687and nm, respectively. As seen from the measurements, when annealed at 300 C and 400 C, the roughness show similar values, however when annealed at 500 C the roughness increased drastically. Fromthe figure, higher annealing temperatures shows rougher surface in which reported in many studies before indicates that ZnO samples annealed at 800 C give better surface roughness. Samples were annealed at a maximum of 500 C due to substrates that are not suitable for high annealing temperatures.

4 (a) (b) (c) FIG. 4. Three dimensional AFM images of Ni:ZnO/Ni films over 10nm 10nm area: (a) annealed at 300 C (b) annealed at 400 C (c) annealed at 500 C 4. CONCLUSION References The Ni:ZnO/Ni nano films prepared via RF magnetron sputtering technique on glass and Si substrates on different deposition temperatures were analyzed. It is shown by XRD that the samples deposited at 300 C have the best crystalline structure. This specific sample is then further annealed at different temperatures. The characterization of the surface structure and crystallinity were conducted via FESEM, XRD and AFM. Increase in annealing temperatures from 300 C to 500 C showed a slight increment in the grain size and higher peak intensities which proves that higher annealing temperatures gives better surface crystallinity. The results showed that the preferred crystallinity structure and surface morphology can be altered by annealing. (d) [1] S. Ghosh, P. Srivastava1, B. Pandey, M. Saurav, P. Bharadwaj, D.K.. Avasthi, d. Kabiraj and S.M. Shivaprasad, (2008) Study of ZnO and Ni-doped ZnO synthesized by atom beam sputtering technique. Appl. Phys. A, 765 [2] Amir Moezzi, Andrew M. McDonagh, Michael B. Cortie, (2007) Zinc oxide particles: Synthesis, properties and applications, Chemical Engineering Journal, [3] Jr H. He, Chang S. Lao, Lih J. Chen, DragomirDavidovic, and Zhong L. Wang, (2005) Large-Scale Ni-Doped ZnO Nanowire Arrays and Electrical and Optical Properties,

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