Synthesis of Transparent and Highly c -Axis Oriented ZnO Thin Films

Zinc Oxide (ZnO) thin ﬁlms have been grown by radio frequency plasma enhanced chemical vapor deposition (PECVD) technique on silicon wafers and corning7059glass substrates kept at diﬀerent substrate temperatures. Diethylzinc (DEZ) was used as the source precursor for the preparation of ZnO ﬁlms, H 2 O and argon were used as oxidizer and carrier gases respectively. Structural and optical properties of the ﬁlms, grown with diﬀerent gas ﬂow ratio, were investigated using various characterization techniques. The as grown ZnO ﬁlms at a substrate temperature of 300 ◦ C with DEZ/H 2 O ﬂow rate ratio 1:4 at 50 W of R. F. power were found to be highly c -axis oriented with (002) preferred orientation. The elemental analysis of these ﬁlms performed using X-ray photoelectron and Auger electron spectroscopy showed the presence of zinc and oxygen only. Atomic force microscope images of the ﬁlms exhibited columnar grain growth. The ﬁlms showed a transmittance ( > 85%) in the 400-800 nm wavelength range. The optical band gap for ﬁlm deposited at 300 ◦ C estimated using Tauc’s plot was found to be ∼ 3.28 eV. [DOI: 10.1380/ejssnt.2014.334]


I. INTRODUCTION
Zinc oxide (ZnO) is a unique combination of semiconductor and piezoelectric class of material useful in a wide range of applications. As a wide band gap semiconductor material, ZnO thin films present many remarkable characteristics of high transmission in visible region and extreme excitons stability [1]. Highly c-axis oriented ZnO thin films possess good piezoelectric effect which allows them for application in surface acoustic wave devices [2]. The main concern is the optoelectronic properties of ZnO thin films. It has been observed that high quality ZnO thin films possess excellent ultraviolet emission performance, making it an ideal candidate for the fabrication of ultraviolet light-emitting devices such as ultraviolet lasers [3], ultraviolet light-emitting diodes [4].
The structural, optical and electrical properties of ZnO thin films have a strong dependence on the method of deposition and deposition parameters. ZnO thin films has been prepared by various techniques such as pulsed laser deposition [5], metal-organic chemical vapor deposition (MOCVD) [6], spray pyrolysis [7], sol-gel method [8,9], molecular beam epitaxy [10], electron beam evaporation [11] and magnetron sputtering [12] etc. There has been a major concern regarding the best technique for fabricating ZnO thin films for device applications. Plasma enhanced chemical vapor deposition (PECVD) technique has a number of key advantages such as large area de- * This paper was presented at the 12th International Conference on Atomically Controlled Surfaces, Interfaces and Nanostructures    [13][14][15][16].
In the present work, H 2 O has been explored as oxidising gas with DEZ precursor to deposit the undoped ZnO films at low temperature. For PECVD, the plasma used to aid in the decomposition of reactants and high quality thin film deposition at low temperature is possible even without the post-deposition annealing. ZnO thin films have been grown by radio frequency PECVD technique on silicon wafers and corning7059 glass substrates kept at different temperatures. The crystalline quality of the films has been examined from X-ray diffraction patterns, surface morphology, X-ray photoelectron and Auger electron spectroscopy.Optical transmittance of the films has been observed as a function of substrate temperature and the optical band gap energy has been evaluated.

II. EXPERIMENTAL
ZnO films were grown in a parallel-plate capacitive coupled (13.56 MHz) PECVD system. The upper stainless steel electrode equipped with showerhead was coupled to the R. F. power supply. Substrates were kept on the heater placed at grounded lower electrode and the substrate temperature was controlled using a thermocouple attached to the heater by temperature controller. The gap between the substrate and the showerhead was main- The Surface Science Society of Japan (http://www.sssj.org/ejssnt) tained at 15 mm. DEZ [(C 2 H 5 ) 2 Zn] was used as the Zn source (kept in stainless steel bubbler at a temperature of < 4 • C), while H 2 O and argon were used as oxidizer and carrier gas respectively. Corning (7059) glass and silicon wafers were used as the substrates. The degree of cleanliness of the glass substrate affects the appearance and the morphology of the film. The glass substrates were cleaned ultrasonically in methanol and acetone for 10 minutes each and then cleaned with DI water and dried with dry nitrogen. Dipping the Si substrates ultrasonically in trichloroethylene, acetone and isopropyl acetate consecutively for 10 minutes and then draining them with DI water cleaned the silicon substrates. Prior to film deposition, etching in HF acid and then rinsing with DI water removed the native oxide layer on Si substrate. The R. F. power was fixed at 50 W using a matching network. The reactor pressure was kept constant between 2-6 Torr. Substrate temperature was varied from 100 to 400 • C and the DEZ flow rate was varied from 30-80 ml/min. H 2 O was introduced through argon gas at the top of the reactor with a fixed flow rate of 200 ml/min. High ratio of H 2 O gas as oxident to DEZ precursor is useful for the reduction of carbon levels in the deposited films. It has been reported that water vapors act as protonating agent in the removal of the carbonaceous ligands giving very low carbon contamination [17]. The flow rates of DEZ and H 2 O using argon gas were controlled by mass flow controllers. Thickness of the films was measured with the ellipsometer and were in the range of 0.3-0.9 µm depending upon the flow rate ratio of DEZ to H 2 O. Typical results for ZnO films of 0.48 µm thickness are presented in this paper. X-ray diffractometer (Rint-2500 Regaku) using Cu Kα radiation with a wavelength of 1.54Å was used for recording the diffraction spectra of the films. Elemental analysis was carried out using X-ray photoelectron (Gamma Data Scienta, ESCA-200) and Auger Electron Spectroscopy(JAMP-7800). SPI-3700 Atomic Force Microscope (AFM) was used to observe the surface morphology of the films. A Shimadzu model 2500PC UV-VIS Spectrophotometer was employed to obtain optical transmission in the wavelength range of 200-900 nm.

III. RESULTS AND DISCUSSION
The X-ray diffraction (XRD) pattern were recorded for ZnO films deposited on silicon and glass substrates at different substrate temperature ranging from 100 to 400 • C. The films grown at room temperature were found to be amorphous as no peak was observed on both silicon and glass substrates. The films grown at and above 100 • C show wurtzite hexagonal structure with a c-axis preferential growth along the (002) orientation. No other phase related to other compound was detected. In case of Si substrate the intensity of (002) peak was very high as compared to that on glass substrate. The relative intensity of (100) and (101) planes as compared to (002) was also found to be very low. Figure 1(a) shows the XRD pattern of the ZnO films deposited at 300 • C with flow rate ratio of DEZ : H 2 O= 1:4 on Si and glass substrates exhibiting a strong (002) reflection at 2θ = 34.42 • which is very close to unstressed ZnO powder value. XRD pattern of the films deposited at different substrate tem- peratures (100-400 • C) and with different DEZ flow rate (30, 50, 60 and 80 ml/min) at 300 • C for Si substrate are reported elsewhere [18]. It has been shown that the increase of DEZ flow rate results in the reduction of the intensity of the (002) peak significantly and the additional peaks corresponding to (100) and (101) planes also appear. The full width at half maximum (FWHM) of the films decreased with substrate temperature increment until 300 • C,and then with further increase of temperature FWHM increases slightly, showing the possibility to get good crystalline films at 300 • C. These results are consistent with the results of reflection high energy electron diffraction pattern [18]. The degradation of quality of the film at higher growth temperature can be attributed to the migration of reactants or due to possible gas phase reaction, as the growth rate was also found to decrease at this temperature. The change in the 2θ value of the (002) peak position at different substrate temperature signifies that the Bragg's plane spacing along the growth direction is not uniform. This can be due to the lattice mismatch between ZnO and Si substrate. The crystalline nature of the films is strongly dependent on growth temperature and the DEZ flow rate ratios. The grain size (t) of the films corresponding to (002) peak using FWHM values has been calculated using the Scherrer's formula [19].
where λ is the x-ray wavelength (1.54Å) and B is the FWHM of the XRD pattern. Figure 1(b) shows the variation of grain size with substrate temperature of ZnO films deposited on silicon and glass substrates. The average grain size of the film deposited on silicon at 300 • C was estimated to be 30 nm. The chemical composition and state of the films were subsequently investigated by X-ray photoelectron spectroscopy (XPS). Prior to the analysis, Ar + sputter cleaning has been carried out for 30 seconds. The XPS analysis of the ZnO films grown on Si wafer substrate at 300 • C have been surveyed in the binding energy range 0-1100 eV as shown in Fig. 2(a). No contamination species were detected within the sensitivity of the instrument apart from the adsorbed atmospheric oxygen and carbon due to exposure of the film to air; possibly due to radicals retained on the surface of the polymerized film after growth showing a very weak peak O a and C 1s respectively. The sharp Zn 2p and O 1s peaks have been observed in the XPS spectrum. The Zn 2p consists of two peaks due to splitting. O 1s peak appear at energy of 530.82 eV, and the energy of appearing Zn 2p 3/2 and Zn 2p 1/2 are 1021.91 eV and 1044.95 eV respectively. The energy difference between the two peaks Zn 2p 1/2 and Zn 2p 3/2 is 23.04 eV as shown in Fig. 2(b) expanded spectrum, which is close to the reported theoretical value of 23.1 eV for ZnO [20]. The observed Zn 2p 3/2 peak at 1021.91 eV is a typical characteristic of the Zn +2 in zinc oxide. Figure 3(a) shows the typical AES spectrum of the ZnO film grown at 300 • C on Si wafer. O (KLL) and Zn (LMM) signals clearly indicate the formation of ZnO films. Since DEZ, a metal organic compound has been used as a source material in the r. f. plasma, there is always possibility of inclusion of carbon and related compounds in the films due to the decomposition of DEZ. However no carbon signals have been found in the AES spectrum of present ZnO films. AES depth profile of ZnO film on Si substrate is also shown in Fig. 3(b). The depth profile results suggest that no region of silicon oxide is detected at the interface between Si and ZnO. This indicates that the amount of SiO 2 grown in the initial growth stage of ZnO is negligible. Figures 4(a) and (b) show the 2D and 3D surface morphology images of the ZnO film deposited on silicon substrate as examined by Atomic Force Microscope. The RMS roughness is 21nm as obtained by sectional analysis of the film surface.Form the 3D surface morphology image, it can be seen that ZnO thin films comprise columnar grains which grow along the c-axis direction perpendicular to the substrate surface. This is in agreement with the results of X-ray diffraction. From the two dimensional surface morphology images, it can be seen the grain boundaries are clear and the grains are round shape in plane. Figure 5 shows the transmittance spectra of ZnO films grown at different substrate temperature. It is found that the films are transparent, with more than 85% transparency in the visible region. The average transmittance of film was found to decrease slightly with the increase of substrate temperature. Moreover, distinct interference fringes are observed below the band gap, which indicates the smooth nature of the film surface and the interface between film and substrate. The optical absorption coefficient 'α' is defined as where I is the intensity of transmitted light, I 0 is the intensity of incident light, 'd' is the thickness of the film and the transmittance is defined as I/I 0 . One can obtain α from transmission spectrum by using the following equation: where 'T ' is the transmittance. In a direct transition semiconductor, α and optical energy band gap E g are given by [21] (αhν) = A(hν − E g ) 1/2 , where A is a constant for direct transition, h being the Planck's constant and ν the frequency of incident photon. By extrapolating (αhv) 2 to zero, the optical band gap 'E g ' is determined. The optical band gap for the film deposited at 300 • C estimated using Tauc's plot as shown in inset of Fig. 5 was found ∼3.28 eV.

IV. CONCLUSION
ZnO thin films with a dominant c-axis texture were deposited by PECVD at 300 • C temperature on both glass and silicon substrates using DEZ and H 2 O sources with 1:4 flow rate ratio. It has been observed that the film orientation can be controlled by appropriate ratio of DEZ to H 2 O and substrate temperature. Elemental analysis results by XPS and AES have also confirmed the presence of zinc and oxygen. No other substantial concentration of other impurity has been detected in these films. Films are transparent in the visible region and have sharp absorption band edge.