Oxygen Effect on Structural and Optical Properties of ZnO Thin Films Deposited by RF Magnetron Sputtering

ZnO thin films with wurtzite structure have been grown on Si (100) and glass substrates using radio frequency (rf) magnetron sputtering at room temperature. The ZnO thin films have been characterised by XRD. The (002) orientation is observed at zero Oxygen flow after the (100) developed with increasing oxygen ratio. Usually, this orientation (100) is difficult to obtain. The thickness of ZnO films was confirmed by cross-section SEM, and their stoichiometry was measured by Energy Dispersive X-ray Spectroscopy (EDX) and Rutherford backscattering spectroscopy (RBS). The optical band gaps have been determined using UV spectra and found to be varied from 3.24 to 3.29 eV as a function of the oxygen ratio. Moreover, photoluminescence (PL) spectra showed more defects at higher oxygen flow. The crystalline quality of the deposited film degrades with oxygen enhancements.


Introduction
Zinc oxide (ZnO) is a promising wide band gap semiconductors due to its low electrical resistivity, high conductivity, environmental stability, nontoxicity and transparency, as well as being inexpensive.ZnO is a direct band gap semiconductor of energy E g = 3.37 eV, and exciton binding energy of ~ 60 meV at room temperature.So, it can be used as a transparent electrode in solar cells manufacturing 1 , in liquid crystal display (LCD) 2 and in light emitting diode (LED) 3 .In addition, ZnO is useful in many potential applications and can be exploited as a thin film transistor (TFT) 4,5 , and surge protection element in electronic circuitry [6][7][8] , UV photodetector 9 , transparent conductor 10 , piezoelectric transducer and surface acoustic wave device 11 .Recently, semiconducting metal oxide materials (such as ZnO) have used as gas sensors, since they develop electrical characteristics influenced by the composition of the surrounding gas ambience 12 .Different processes are used to prepare ZnO thin film synthesis such as Pulsed Laser Deposition (PLD) 13,14 , spray pyrolysis 15 , chemical vapour deposition CVD 16 , and electrochemical deposition technique (ECD) is used for the synthesis of semiconductors oxides at low temperature 17,18 .Magnetron sputtering 19,20 is widely used in the semiconductor technology manufacturing, because of producing high quality ZnO films with controlled thickness and morphology 21 .Plasma and deposition parameters such as substrate temperature, RF power and oxygen partial pressure influence on the physical (structural and optical) properties of oxide thin films 22 , where the good quality thin films were obtained with the (002) orientation at 400°C temperature in the presence of 50% of oxygen amount.
Changing the orientations is very important during the growth of nanowires 23 , it was found that the wires on the ZnO film with (002) texture grow perpendicular to the surface of the film, and the wires on the film with (100) texture grow in the plane of the substrate.
In this work, ZnO films have been deposited by vacuum RF sputtering method.The crystallographic properties of the films were characterized by XRD.The RBS analysis method was employed to reveal information about the composition of the films, and PL and UV spectra have been obtained to study the optical properties of the films.The effect of oxygen partial pressure on the optical and structural of the grown films was investigated as well.

Experimental Procedure
RF magnetron sputtering using a PLASSYS-MP600S deposition system has been used to produce zinc oxide films at room temperature (RT).The films were deposited on silicon Si (100) and glass substrates.The zinc oxide target's (purity 99.99%) diameter was 15 cm with a 6 cm distance between the cathode and the substrate holder.The vacuum in the deposition chamber was about 2 × 10 -7 Torr 21 .The deposition rate was monitored in situ by a quartz crystal monitor and measured ex situ by means of a scanning electron microscope (SEM) TSCAN Vega II XMU (Czech Republic) operated at 30 kV equipped with EDX, where the films thickness was 150 nm.All the deposition conditions are summarised in table 1.
The crystallographic properties of the films have been analysed by XRD Stoe transmission X-ray diffractometer Stadi P (Germany) using the Cu Kα (with λ = 0.15405 nm) a Atomic Energy Commission, Department of Physics, P. O. Box 6091, Damascus, Syria radiation in a linear position sensitive detector for θ-2θ scan configuration.
In addition, the RBS analysis has been conducted at the 3MV HVEE™ tandem accelerator at the Atomic Energy Commission of Syria (AECS) using a 4 He + beam of 2.0 MeV, and the RBS spectra were treated using the computer code SIMNRA 24 .The morphology was obtained by SEM measurements, and the atomic composition and stoichiometry of the ZnO films were determined by RBS.The optical characteristics have been examined using the UV-vis Shimadzu UV-310PC Spectrophotometer to measure the transmittance of the elaborated films, and a 325 nm wavelength of a He-Cd laser, a 1m Spex monochromator and a multialkali photomultiplier were used for the photoluminescence (PL) measurements at RT.

Deposition rate and composition measurement
The evolution of the deposition rate versus oxygen flow ranging from 0 to 20 sccm was elucidated in Figure (1-a).The deposition rate decreased from 50 to 22 nm/min as a function of the concentration of oxygen (0 to 100%) 0 sccm-20 sccm.
The increasing of the oxygen percentage induces a decrease of the deposition rate due to a decreasing of sticking coefficient 22 .On the other hand the increase of the oxygen gas flow would decrease the argon partial pressure and subsequently decrease bombarded argon ions towards targets 25,26 .Figure (1-b) shows SEM cross section for ZnO/ Si film and equals to 150 nm thickness.
The homogeneity of the films was verified by means of RBS spectra which have been fitted using the computer code SIMNRA 24 as shown in Figure (2-a).The peaks assigned to Zn indicate homogenous distribution of Zn along the films depth, and subsequently all films have the same thickness because it is linked to the RBS Zn Peak width.This result is confirmed by SEM cross-section measurements.The atomic compositions and the stoechiometry of the ZnO films were determined by EDX as shown in table 2. The ratio was found to be about 1 and the film stoichiometry was not changed during the sputtering with oxygen flux.For example figure (2-b) shows an EDX spectrum and an atomic composition where O/Zn =0.99 for 12 sccm oxygen flow film i.e. with increasing oxygen flow the deposition rate decreases which in turns affects the film crystallinity even that the stoechiometry is not changed but more interstitials and defects are produced during deposition process as it will be shown latter from PL spectra.

Structural characterization
The XRD patterns of all the sputtered ZnO films are shown in Figure (3-a).Patterns show two ZnO peaks for the (100) and (002) orientations at 31.4 o and 34.4 o respectively.The two peaks attributed to hexagonal würtzite phase.The small peak at 33 o attributed to Si substrate .The (002) orientation is obtained from 0 to 8 sccm and the intensity of the (002) peak decreases with oxygen flow so the (100) peak becomes dominant.The surface energy for the orientation (100) is higher than the orientation (002) and consequently, it is more difficult to be obtained 27 .
The FWHM of (002) ZnO peak increases significantly with increasing the oxygen flow (Figure 3-a).The grain size for ZnO films, is calculated using Scherrer's formula 28 ./ cos D 0 9m b i = Where D is the grain size, λ is the incident X-ray wavelength and β is the full-width at half maximum of the diffraction peak.
Figure (3-b) shows the grain size of (002) orientation which decreases significantly with increasing oxygen flow from 0 to 8 sccm, Moreover, the grain size of (100) orientation decreases gradually from 4 to 20 sccm.The obtained results agree with Bensmaine et al 22 .As the deposition rate decreases gradually where an oxygen gas flow increases, the crystallinity degrades 29 .The variation of the orientation could be explained by reducing of the deposition rate because the probability of atoms' rearrangement (to produce (100)) is low at high deposition rate corresponding with low oxygen flow 30 .

Optical measurements
Figure (4-a) depicts the optical transmittance for ZnO films deposited at various sputtering oxygen flow.The optical transmittance provides useful information about the optical band gap of a semiconductor 31 .It was observed that the average transmittance of samples in the visible and near infrared range is varied from 80% to 92%.
The optical band gap of the ZnO films varied from 3.24 eV to 3.29 eV.The dependence of optical band gap on O 2 /Ar ratios in the plasma reveals that the optical band gap generally increases with increasing of O 2 in the plasma as shown in Figure (4-c).According to M. Suchea et al 32 the increase of the band gap with O 2 concentration can be associated with the decrease of the growth rates.It is well known that the optical band gap of ZnO is affected by O 2 vacancies and Zn interstitial atoms as well as the grain size [33][34][35] .There is other method, such the ectrochemical deposition 36 , where larger variation in optical band gap obtained due to the used deposition methods and/or grain size and surface morphology.That's in contrary with our previous work on other set-up where the gap and the disorder have the same behavior (better quality bigger gap) 37 .
Figure (5) shows PL spectra of ZnO films deposited at different Oxygen percent (%) O 2 /(Ar+O 2 ) as mentioned in table 1 which is varied from 0% to 100%.In general, PL spectra include a peak for direct band gap emission at about 380 nm (3.25 eV) in the UV region, two peaks at about 420 nm and 436 nm might be attributed to Zn vacancy and interstitial respectively (blue region) 38 and a broad peak at about 550 nm which might be attributed to oxygen vacancies or interstitials (green region).The width of the direct band gap emission is about 22 nm for film at 0 sccm O 2 corresponding with preferential orientation (002) where the gain size is  maxima.Therefore, we investigated the effect of oxygen flow between 0 -20 sccm on the emission and structural properties in the ZnO films on Si substrates.By increasing the oxygen flow rate, the reduction of UV emission (enhancement of deep-level emission) and additional peaks near 420, 436, 550 nm can be observed in Figure 5.The Peak at around 550 nm has experienced red shift.This result comes partly in contrast with Park et al 39 .
As the Oxygen percent increases, The FWHM increases and the related emissions of the Zn, O 2 vacancy and interstitial are clearly seen for higher oxygen flow due to decreasing of film crystallinity i.e. the intensity of UV emission can be considered as an indicator to ZnO film crystallinity, and the higher crystallinity possesses the higher intensity of UV emission 35,40 .As shown in figure 5 the red shift of oxygen interstitial peak at higher than 8 sccm is observed simultaneously with an orientation changement from (002) to (100) which is consistent with the XRD study.

Conclusions
ZnO thin films have been grown on Si (100) and glass substrates using radio frequency (rf) magnetron from ZnO target.The X-ray diffraction patterns have shown (002) orientation with the best quality for ZnO films grown at zero oxygen flow.By increasing the oxygen ratio, the deposition rate decreases and subsequently crystalline quality degrades as it related to the decrease of the grain size, Photoluminescence spectra indicated more Zn, O vacancy and interstitials at higher oxygen flow and the Peak at around 550 nm has experienced red shift.Optical band gap of the films has varied from 3.24 to 3.29 eV as a function of the oxygen percentage.

Figure 1 :
Figure 1: (a) deposition rate as a function oxygen flow and (b) SEM cross section for ZnO films deposited on Si substrate

Figure 2 :
Figure 2: (a) RBS spectra of ZnO/Si at different oxygen flow and (b) Atomic composition by EDX characterization for ZnO/Si film at 12 sccm

Figure 4 :
Figure 4: (a) Optical transmittance spectra for ZnO films on glass substrate, (b) the corresponding band gap and (c) optical gap with different oxygen flow.

Figure 5 :
Figure 5: PL spectra of ZnO thin films on Si substrate versus wavelength with different oxygen flow.

Table 2 :
Atomic percentage for ZnO film for different oxygen flow