Optical and Structural Study of ZnO Thin Films Deposited by RF Magnetron Sputtering at Different Thicknesses : a Comparison with Single Crystal

Optical characterizations of preferred orientation (002) ZnO thin films, prepared by RF magnetron sputtering under pure argon plasma, with different thicknesses have been investigated, where grain size and resistivity increase with thickness. Energy band gap and refractive indices have been calculated. A correlation between band gap values and crystalline quality, which is improved with thickness, has been discussed. The calculated refractive indices of the thicker deposited films have increased as the thickness increases. Full width Half Maxima (FWHM) of band gap Photoluminescence (PL) emission decreases with thickness increasement due to quality improvement as it has been monitored by X-Ray Diffraction (XRD). A comparison between PL spectra of 1200 nm film and ZnO single crystal at low temperature is found to be similar.


Introduction
Due to the interesting properties of ZnO composite such as its high abundance in nature, stability, nontoxicity, high transparency and wide band gap (about 3.3 eV), it is widely used as a semiconductor.It is a principle layer in a solar cells 1,2 , liquid crystal display (LCD), light emitting diode (LED) and as a thin film transistor (TFT) [3][4][5][6] .It is also used as surge protection devices [7][8][9] , UV photodetector, transparent conductor, surface acoustic wave device and gas sensor [10][11][12][13] .
AZO thin films have been prepared by different synthesis methods [14][15][16] , where high quality ZnO films can be produced with a controlled thickness by using DC or RF magnetron sputtering in the semiconductor technology manufacturing 17 .
For instance, Suvorova et al. 18 have grown an epitaxial ZnO films by RF magnetron sputtering at room temperature followed by rapid thermal annealing.But Rahmane et al. 17 managed to obtain an epitaxial growth of ZnO thin films at low temperature by choosing an appropriate substrate (good quality AlN) RF sputtering.
Park et al. 19 have studied PL emissions for ZnO single crystal nanorods at low temperature raging from 10 K to RT and found that the intensity of bound exciton peaks are increased with decreasing temperature on contrary to free exciton peak where it decreases.
In Previous work, it has been demonstrated that the increased substrate temperature accompanied with improvement of ZnO film quality where the band gap extracted by UV transmittance spectrum was about 3.2 eV at 400 o C. The electrical characteristics via C-V and I-V measurements on the basis of the heterojunction thermal emission model confirm the domination of high-density grain boundary layer existing at the interface 20 .
Therefore, in order to investigate the effect of thickness on the structural and optical properties of ZnO films, we have grown a set of films on Si and glass substrates where XRD technique was employed to explore the structural modification improvements due to increasing the thickness and EDX analysis method was utilized to reveal information about the composition of the films.PL and UV techniques were provided to monitor the optical band gap where a slight increasement is found with varied thickness.PL spectra at low temperature (LT) for ZnO thick film and single crystal have been compared.

Experimental Details
PLASSYS-MP600S deposition system was used to elaborate Zinc oxide films using RF magnetron sputtering.Si (100) and glass substrates were used to prepare the films.The purity and the diameter of the zinc oxide target were 99.99% and 15 cm respectively.The deposition rate (1.23nm/s) at RF power 600 W and morphology were obtained from a scanning electron microscope (SEM) TSCAN Vega II XMU (Czech Republic) operated at 30 kV.The Energy Dispersive X-ray Spectroscopy (EDX) is used to determine the atomic composition of the ZnO films.The thickness varied from 10 nm to 1200 nm.All the deposition conditions are summarized in a previous work 21 .The crystallographic properties of the films have been analysed by X-ray Diffraction (XRD) Stoe Stadi P Transmission X-ray diffractometer (Germany) using the Cu Kα (with λ = 0.15405 nm) radiation in a linear position sensitive detector for θ-2θ scan configuration.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 as well as LT.residual stress is a compressive stress that decreases with increasing thickness 21 .

Structural study
Van der Pauw technique 22 has been used to estimate the room temperature resistivity ρ of the films where it increases with thickness as shown in figure 1(b).The conductivity of the films gets lower as the thickness increases due to reduction in the carrier concentration, which is inversely proportional to the resistivity, where it decreases from 2.975 to 0.2184 Ω cm for 1200 down to 30 nm thickness respectively 21 .This manner is also noticed by Myoung et al. 23 .This means that the thicker films have less defects concentrations (bigger grain size and fine grain boundary).The behavior of increasing the resistivity with average grain size, as shown in figure 1(b), comes in consistent with other work 24 .

Optical properties 3.2.1 UV study
Figure 2(a) shows the UV transmissions spectra for thinner films (10 -200 nm) and figure 2(b) for higher thickness (500 -1200 nm).It has been observed that the average transmittance of samples in the visible range rises from 83 % to 95 % as shown in the spectra of ZnO films.The optical transmittance provides useful information about the optical band gap of the semiconductor 20 where it extend the linear part of α 2 as a straight asymptotic line that intercepts the horizontal axis α 2 = 0 yielding Eg from hν value as shown in figure 2(c) and (d).
The obtained values of band gap for different thicknesses are summarized in table 1 where, in general behavior, there is a value increasement with the thickness increase accompanied by an improvement of the crystalline quality, where it is attributed to the strain in the film introduced during the deposition 21 , i.e. the thicker films suffer less strain than thinner ones and consequently higher band gab values.This comes in a good agreement with our results where thicker films have lower stress value and better quality.
The obtained values of energy gap show that the sample thickness influences to some extent the gap.However, this influence is not sufficiently important.
Figure 3 shows the evolution of the refractive index (n) as a function of wavelength for different thicknesses of the films raging from (500 -1200 nm).The optical constant (Refractive index n) of ZnO films on glass substrate were calculated from transmittance measurement by using Manifacier's envelop method 25 .The variation of optical constant (n) with wavelength (λ) shows a decrease with increasing wavelength (λ) due to decreased absorption of traps and intraband transitions 26 .These results are in conformity with the results 27 .
According to an investigation by Moustaghfir et al. 28 , the refractive index (n at λ = 633 nm) of the radio frequency (RF.) magnetron reactive sputtered ZnO film (a thickness of ~800 nm) fabricated under the sputtering was 1.89 and Grain size has been deduced from XRD pattern using Scherrer's formula for (002) peaks of ZnO films and it was found to increase from 10 nm to 27 nm with increasing the thickness.Rocking curve results showed an improvement of the films crystallinity, whereas the FWHM decreases from 16° to 2.9° for films at 50 and 1200 nm, respectively.The   becomes 1.91 by further annealing at 400 °C/1 h.In our result the refractive index at 633 nm was about 2.0425 at 1000 nm film.Additionally, an earlier study of the RF sputtered ZnO film (a thickness of 1000 nm) by Mehan et al. 29 revealed that the refractive index at λ = 550 nm was 1.980 but it was 2.006 for the ZnO bulk.The quality of sputtered ZnO films can influence refractive index.Our value at the same λ and thickness was 2.0425 which is close to the bulk ZnO value (table (1)).
The increase of n with thickness can be ascribed to the increase in the grain size and the enhancement of the crystalline quality.We have seen during the crystallographic study that the grain size and the crystallinity were enhanced as the film thickness increased.The samples of lower thickness must contain much more defects like oxygen vacancies which constitute an important source of free carriers.As the grain size increases the amount of oxygen vacancies decreases to some extent leading, thus, to a decrease in the free carrier density 30 .
Our results show that the refractive index increases for 500, 1000 and 1200 nm films as shown in figure 3 (except for 900 nm).This behavior comes in agreement with ref 31 .This could be explained by the improvement of the crystalline quality (grain size figure 1(b) and lower stress with thickness 21 .

Photoluminesce study 3.2.2.1 PL at Room Temperature (RT)
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, 436 nm might be attributed to Zn vacancy and interstitial respectively (blue region) 32 , and a broad peak at about 550 nm which might be attributed to oxygen vacancies or interstitials (green region).
The intensity of band gap emission increases with thickness (figure 4(a)) correspondingly FWHM decreases (figure 4(b)) which indicate better quality i.e.The crystalline quality was found to improve, while the stress values decreased with increasing thickness 21,23,33 .

PL at low temperature
PL spectra reveal a blue emission band attributed to localized donor states (for single crystal and 1200 nm film).At low temperatures (<80 K) PL spectra are dominated by the emission attributed to the recombination of excitons bound to neutral donors (for SC) figure 5(a).At higher temperatures (>100 K), defect related emissions in the visible range become comparable with the excitonic emission (for SC).But for the 1200 nm film the PL spectra at LT the intensity of the free exciton emission (370 nm) and green emission (~ 550 nm) are comparable but at RT the green emission become dominate due to defects in the prepared films figure 5(b).
In the region between 380 nm and 450 nm (blue region), two weak emission bands at about 413 (3.0 eV) and 436 nm (2.84 eV) appeared in the studied film but is not exist in the SC 34 .
Two broad band appear clearly in the region (~ 550 nm) for ZnO studied film (figure 5(b)) are attributed to the deep level emissions.The band located at about 607 nm (2.04 eV), is known as yellow emission in ZnO.Wu et al. 35 proposed that the single negatively charged interstitial oxygen (O-i) is responsible for the yellow luminescence.
Also PL intensity and its FWHM at low temperature for ZnO single crystal and 1200 nm film showed similar behavior figure 5(c) and figure 5(d).The increasement of the PL intensity at low temperatures is due to increasing of exciton and defects bindings.This was accompanied with a decreasing of FWHM 34 .This supports that ZnO film is a good quality due to its sharp excitonic emission 19 .

Conclusions
High transparent ZnO films, using RF magnetron sputtering method, have been prepared with different thicknesses on silicon and glass substrates.The influence of thickness on the optical and structural properties has been studied where quality improvement (grain size from XRD) has compared with PL and UV measurements.FWHM of band gap PL emission decrease from 27 nm to 14 nm for films deposited on silicon substrate.Optical band gap and refractive indices values have been deduced from UV spectra for films deposited on glass.Also FWHM of PL at low temperature for 1000 nm film and ZnO single crystal showed similar behavior where it increases with temperature.The resistivity as a function of grain size was found to be changed from 0.2184 Ω cm to 2.975 Ω cm for studied films.

Acknowledgement
Authors would like to thank Prof. I. Othman, the Director General of AECS for encouragement and financial support and M. Alsabagh for PL measurements.

a
Department of Physics, Atomic Energy Commission of Syria, P. O. Box 6091, Damascus, Syria

Figure 1 (
Figure 1(a) shows the XRD patterns for ZnO films with different thicknesses raging from 1200 nm down to 50 nm.A (002) preferential orientation peak at 34.42 o is found, which could be assigned to the hexagonal ZnO würtzite phase with the c-axis perpendicular to the substrate surface, and small (100) peak at 31.4 o .The stoichiometry (O/Zn about 1) was verified by EDX analysis.

Figure 1 .
Figure 1.(a) XRD patterns, (b) Resistivity as a function of grain size of (002) peaks for ZnO films with different thickness.

Figure 3 .
Figure 3.The evolution of the refractive index (n) as a function of wavelength for different film thicknesses raging from (500 -1200 nm).

Figure 4 .
Figure 4. (a) PL spectra at RT, (b) FWHM for band gap emission as a function of ZnO/Si film thickness.

Figure 5 .
Figure 5. PL spectra at low temperature (a) for single crystal and (b) for ZnO film, (c) and (d) intensity and FWHM for band gap emission for SC and prepared film respectively.

Table 1 .
Optical band gap and refractive index at 550 nm for ZnO films on glass at different thickness.