Al-doping and Properties of AZO Thin Films Grown at Room Temperature: Sputtering Pressure Effect

Aluminum zinc oxide (AZO) thin films were synthesized on glass substrates by radio frequency (rf) magnetron sputtering from a metallic Zn-Al (5 at. %) target at room temperature. The morphological, structural, electrical and optical properties of the films were studied as a function of the sputtering pressure, which was varied from 0.1 to 6.7 Pa. X-ray diffraction (XRD) analyses revealed that the films obtained were polycrystalline, having a hexagonal wurtzite structure with a preferential orientation in the (002) plane. In addition, the crystallite size increased as a function of sputtering pressure. Owing to the re-sputtering of the Zn atoms from the growing film, the aluminum concentration presented a maximum value of 13 at. %. At pressures close to 0.16 Pa, we obtained films with values of electrical resistivity and mobility of 2.8 10 -3 Ω cm and 17 cm 2 /Vs, respectively. Finally, our results indicate that the structure zone diagram proposed by Thornton and later modified by Kluth does not fully predict the structural/morphological behavior of the AZO films, since plasma interactions must also be taken into account. With the methodology used, transparent conductive electrodes can be deposited on substrates at low temperatures. with pressure in the range of 6 to 10 Pa. These results indicate that the effect of sputtering pressure on the film properties depends on the other deposition parameters, such as the substrate temperature and the plasma configuration. The present study reports the influence of sputtering pressure on the morphological, structural, optical and electrical properties when AZO films (thickness about 200 nm) are synthesized by RF magnetron sputtering at room temperature.


Introduction
Transparent conductive oxides (TCO) are materials based on metallic oxides capable of conducting electric current without deterioration in their optical transmission 1 , exhibiting a resistivity of roughly 10 -3 to 10 -4 Ωcm and a transmittance of about 80% in the visible region 2 . These films find diverse applications in optoelectronic devices, such as liquid crystal displays, touch-sensitive panels, flat-screen monitors, infrared optical windows and photovoltaic cells [3][4][5] . Several materials are used for the production of TCO films, indium oxide doped with tin, known as ITO (In 2 O 3 :Sn), is the most popular 2,4 . The rising demand for optoelectronic devices, however, has accelerated the depletion of reserves of indium, the main component of ITO and, consequently, increased its fabrication cost [3][4][5] . Furthermore, as pointed by Minami et al. 4 , applications in flat display panels (FDPs) technology involves the growth process of TCO thin films (thickness below 200 nm) at temperatures below approximately 200 ºC.
In this context, many investigations have sought alternatives to ITO 5 . Owing to its characteristics such as low toxicity, low production cost and stability in plasma processes 5 , AZO (Aluminium doped zinc oxide, ZnO:Al) is a strong candidate to substitute ITO. The substitution of zinc by aluminum in the ZnO matrix should increase the carrier concentration, leading to a reduction in resistivity. In addition, AZO may be synthesized by techniques such as MOCVD 6 , atomic layer deposition 7 , laser ablation 8 , electrochemical deposition 9 and sputtering [10][11][12][13][14][15] . The last of these permits the synthesis of AZO thin films at low temperatures [12][13][14] and over flexible substrates 15 .
The structural, electrical and optical properties of AZO films grown by magnetron sputtering have been investigated extensively as function of the deposition parameters, such as plasma source 3-12 , temperature 16,17 , sputtering pressure [18][19][20] and doping level 21,22 . There are reports in the literature, however, that show distinct dependencies of the film properties on a single growth parameter. For example, Kluth et al. 23 reported that the electrical resistivity increases as the pressure is increased from 0.04 to 2.70 Pa. On the other hand, Czternastek 18 reported that the electrical resistivity decreased with pressure in the range of 6 to 10 Pa. These results indicate that the effect of sputtering pressure on the film properties depends on the other deposition parameters, such as the substrate temperature and the plasma configuration. The present study reports the influence of sputtering pressure on the morphological, structural, optical and electrical properties when AZO films (thickness about 200 nm) are synthesized by RF magnetron sputtering at room temperature.

Experimental Details
The AZO films were grown on 25 x 20 mm 2 glass substrates of 1 mm thickness by reactive RF magnetron sputtering at room temperature, varying the sputtering pressure from 0.1 to 6.7 Pa in a mixture of Ar+O 2 gases, where the oxygen partial pressure was maintained at about 0.015 Pa. Before deposition the substrates were ultra-sonically cleaned in distilled water with detergent, isopropyl alcohol, and subsequently dried in an air stream. The deposition system consists of a stainless-steel chamber of 270 mm diameter and 200 mm in height. A 3 in. metallic Zn-Al (5 at. %) target was used to optimize the doping effects 24 . To maximize the plasma effects 12 , a substrate to target-holder distance of 20 mm was used. The chamber was evacuated using a turbo molecular pump (Edwards T-Station 75) and the pressure was monitored using Pirani (KJL 945) and Baratron gauges. To control the gases needle valves (Edwards, model LV-10K) and a mass flow controller (MKS 1179A) were used. The plasma was activated by a radiofrequency (RF) source (13.56 MHz, 300 W) connected to the target holder. A fixed deposition time of 20 min was used at a power of 70 W.
The composition and Al content ([Al]/([Zn]+[Al]) atomic ratio) of the Zn-Al target and the AZO films were measured by Energy-dispersive X-ray spectroscopy (EDS). For this, an EDS coupled to the scanning electron microscope (Jeol JSM-6010) was used. The surface morphology was investigated using Atomic Force Microscopy (AFM) with a XE-100 Park Systems in non-contact mode over a scan area of 2µm x 2µm.
The crystalline structure of the AZO films was studied using X-ray Diffraction (XRD) in θ-2θ mode as well as at a grazing incidence angle of 2º, using a diffractometer (Panalytical X' Pert Powder) with the CuK α radiation at a wavelength of 1.5405 Å. The average crystallite size τ, was estimated using Scherrer's equation (1), where k is the shape factor (usually 0.9), λ is the X-ray wavelength, β is the FWHM and θ is the Bragg diffraction angle of the (002) peak in the θ-2θ mode. (1) Film thickness was measured using a perfilometer (Veeco Instruments, Dektak 150) and electrical resistivity measurements were made using the four-point probe linear method 25,26 . Optical reflection and transmission spectra were obtained using a Uv-Vis-NIR spectrometer (Perkin Elmer, Lambda 750) in the wavelength range of 190 to 3300 nm. The optical gaps were estimated by equation (2) 14 : where α is the absorption coefficient, hν is the incident photon energy, E g is the optical gap energy for direct allowed transitions and B is a constant dependent on the refractive index of the material, the electron effective mass and the speed of light in vacuum. Plotting (αhν) 2 as a function of the incident energy (hν) the optical gap may be estimated by extrapolating the linear portion of the line until it intercepts the x-axis, at which point αhν = 0. Figure 1 shows AFM surface images of the AZO thin films deposited at different sputtering pressures. The surface morphology is mainly characterized by small granular structures. As the pressure increases, however, surface structures of greater lateral size and height appear and the surface becomes rougher. Table 1 shows the thickness, growth rate, RMS roughness and RMS/thickness ratio for different sputtering pressures. The RMS roughness of AZO thin films was calculated from the AFM images shown in Figure 1.

Surface morphology
The growth rate remained between 8.8 and 13 nm/min. For easier visualization, we plot the growth rate as a function of the sputtering pressure in Figure 2. The growth rate reaches a maximum at around 2 Pa. For lower pressures, the sputtering rate of Zn atoms from the target is limited by the low density of Ar + ions in the plasma. When the pressure is increased, the sputtering of Zn becomes more effective. At higher pressures, however, the scattering coefficient increases due to collisions in the plasma and, consequently, the number of sputtered Zn atoms reaching the substrate decreases again. In fact, according to equation (3) the mean free path, λ, is given by 27 : where λ is expressed in mm e P is the pressure in Pa. Then for a pressure of 2 Pa the mean free path is λ ~3.4 mm. Since the target-substrate distance is 20 mm, sputtered atoms and ions suffer multiple collisions with the process gas particles before reaching the film surface. Furthermore, the plasma density over the growing surface increases at higher pressures. This effect is considerable when the unbalanced configuration is used or when, as in our case, the target is very close to the substrate. Consequently, desorption of Zn adatoms is enhanced and the growth rate is further reduced.
The RMS roughness presents a maximum value around 10.2 nm at 5.3 Pa, as shown in Table 1. We should remember, however, that the surface roughness also evolves with film thickness 28 . Thus we have analyzed the behavior of the roughness/thickness ratio instead of the RMS roughness, as shown in Figure 2. One can observe that the RMS/thickness increases continuously up to 4 Pa and thereafter it stabilizes around the value of 0.05. The rise in the RMS roughness/  For all samples, the aluminum concentration is greater than that found in the target used as the precursor (5 at.%). This result can be attributed to the bombardment of the growing film surface by plasma species, mainly by oxygen ions (O -). As discussed before, the plasma plume promotes the bombardment and intensifies the re-sputtering rate of Zn adatoms, producing an increase in the concentration of Al in the film since the binding energy of Al-O is higher than that of Zn-O. This excess of Al concentration in the film was also reported by Cornelius et. al 21 . In this case, the Zndesorption and, consequently, the higher Al-concentration,   is stressed mainly by the substrate temperature instead of plasma interactions. Figure 3 shows the results of the grazing incidence XRD measurements. All the spectra exhibit the peaks characteristic of wurtzite 17 . Specifically, it is possible to observe peaks associated with the (002), (100), (101), (102), (110) and (103) planes, the (002) peak being the most intense (JCPDS 36-1451). This indicates that the crystal grows preferentially along the c-axis. The ratio of the intensities of the peaks associated with the (101) and (002) planes is also given in Table 3, and it increases from 0.05 to 0.22 as the pressure increases from 1.3 to 5.3 Pa, falling slightly thereafter. A small (101) peak was observed at 0.16 Pa. However, if we compare the values with the ZnO polycrystalline powder, we can conclude that the (002) growth alignment is changed slightly with sputtering pressure.

Composition and structural properties
According to the Structure Zone Diagram (SZD) model proposed by Kluth et al. 23 , alignment of the ZnO crystallites should be increased with sputtering pressure. However, this SZD does not explicitly include plasma and ion effects on the growing film. At low pressures, sputtered species are more energetic, and may bombard the growing film surface, causing defects in the lattice and limiting the columnar growth. In particular, a fine-grained, nanocrystalline with preferred orientation is expected according to the SZD proposed by A. Anders 29 . On the other hand, operating at higher pressures the distribution function is shifted to lower energies. Moreover, the shadowing mechanism 30 , that favors rougher surfaces but not larger structures, is less effective at higher pressures, since sputtered atoms and ions are likely to experience more collisions before arriving at the growing surface. Consequently, the growth of slightly misaligned tapered crystallites takes place, as indicated by the I 101 /I 002 values. Table 3 also displays the lattice constant estimated from the θ-2θ XRD measurements. The constant-c data of the AZO films exhibit a tendency to decrease as the sputtering pressure increases. This reduction could be due to a reduction in the energy of the particles (ions and atoms) that reach the film surface. Indeed, studies by Kappertz et al 31 revealed that Oions may have kinetic energies, great enough to produce implantation in the growing film. This mechanism can generate dislocations and increases the lattice constant-c of the AZO films. On the other hand, an increase in pressure can produce a greater number of collisions in the plasma, reducing the kinetic energy of these incoming particles. Thus, the reduction in the constant-c observed at higher pressure may be caused by a reduction in the efficiency of Oion implantation. Figure 4 shows the crystallite size as a function of the sputtering pressure. For comparison, we include several data published in the literature 10,14,20,32 20 grown at temperatures greater than 523K, the crystallite size is usually larger, between 30 to 70 nm.
Two aspects are worth emphasizing: (i) crystallite size increases with temperature. This is mainly due to activation of the surface diffusion mechanism expected at higher temperatures. (ii) The crystallites increase slightly in size as a function of pressure when films are grown at room temperature. However, the inclination is reversed at higher temperatures. These points also are in agreement with the SZD proposed by A. Anders 29 , where columnar grains take  place at higher temperatures but tapered crystallites should be predominant at lower temperatures when the sputtering pressure is raised.

Optical and electrical properties
All films exhibit transmittances above 80% in the visible region of the light spectrum. This implies that the variation in the sputtering pressure did not change the film transparency. Figure 5 presents square of the absorption coefficient for the AZO films deposited at different pressures. The optical gap presented values of 3.44 to 3.58 eV. For comparison, the optical gap energy of the intrinsic ZnO thin film grown at room temperature was 3.28 eV 30 . This shift in the energy of the optical gap to greater values is attributed to the Burstein-Moss effect expressed by Equation (4) 33 : where m* is the effective mass of the electron, which for ZnO is equal to 0.38 m 0

33
, h is Planck's constant and N represents the carrier density. We have applied formula (4) to estimate the free electron density in the conduction band.
With the measurements of electrical resistivity obtained using the four-point probe method and the carrier density obtained using the Burstein-Moss equation, electrical mobilities were obtained by employing the relation ρ= (neµ) −1 .
The electrical resistivity, carrier density, electrical mobility and Al-doping efficiency are shown in Table 4. Our best electrical resistivity values were obtained for the film synthesized at 0.16 Pa, namely 2.8 10 -3 Ωcm. In this case, carrier density, electrical mobility and Al-doping efficiency were 1.3 10 20 cm -3 , 17 cm 2 /Vs and 5.2%, respectively. This low value of carrier density can be related to the high oxidation rate of Al present in the alloy during the process, producing neutral Al defects, for example, the Al 2 O 3 (ZnO) m homologous phase 21,34,35 . Furthermore, Al-doping efficiency is further reduced at higher sputtering pressure, indicating that homologous phase formation is favored. Increasing the pressure in the deposition chamber, the lifetime of O species is increased 36 , favoring reactions with the Zn and Al. This can decrease the ionization efficiency of Al doping, favoring the formation of homologous phases. Figure 6 compares our electrical resistivity data with those reported by Czternastek 18 and Kluth et al. 19 . Kluth et al. reported the electrical resistivity for a wide range of experimental conditions. Particularly, we compare with the data for samples grown using a metallic ZnAl target at substrate temperature of 543K and sputtering pressure ranging from 0.05 to 2 Pa. In a Czternastek's 18 work, the total pressure was varied from 2 to 14 Pa for distinct Al doping levels and the substrate temperature was kept at 470K. Specifically, we compare only with the data for ZnO:Al (3 at.%) films.
The electrical resistivity observed for Kluth 19 is around one order of magnitude less than that observed here. This    According to the SZM model proposed by Anders, at higher temperature the grains are enlarged because the adatom mobility becomes the dominant growth mechanism. As a consequence, electrical mobility increases. Moreover, both data show that the electrical resistivity increases with sputtering pressure, at least up to 5 Pa. The data of Czternastek 18 show that the electrical resistivity decreases when pressure is increased from 5 to 10 Pa. This result can be explained in the following way. In our previous work 30 , we observed a structural transition of ZnO thin films at a substrate temperature of around 473K and a sputtering pressure of 1.3 Pa. In these conditions, ZnO grains tend to grow more misaligned and the film density decreases According to the SZM proposed by Anders 29 , however, this structural transition should be shifted to higher temperatures when the sputtering pressure is increased, and columnar growth could recur. As a consequence, the electrical mobility should be enhanced. For Czternastek's 18 study, when the sputtering pressure goes from 5 to 10 Pa at 470K, AZO films became more columnar and, consequently, more conductive electrically.

Conclusions
AZO films were synthesized onto glass substrates using RF magnetron sputtering at room temperature. The effects of the argon sputtering pressure in the range 0.1 to 6.7 Pa on the physical properties of the films were examined. All AZO films presented a high optical transmittance (above 80%) in the visible region. The best electrical properties were obtained at a pressure of 0.16 Pa, yielding an electrical resistivity of 2.8x10 -3 Ω cm. X-ray diffraction analyses of the films as a function of the sputtering pressure revealed a preferential orientation in the (002) plane. Additionally, the crystallite size tends to increase with sputtering pressure in the range of 0.1 to 6.7 Pa. This behavior differs from the model proposed by Kluth et al. 19 . It can be better explained when plasma effects are taken into account in the SZD model, as proposed by A. Anders 29 . Finally, an increase in sputtering pressure produced a decrease in both carrier concentration and electrical mobility. As a consequence, the electrical resistivity of the films increases as a function of the sputtering pressure for films grown at room temperature. With the results obtained, transparent conductive electrodes can be produced on substrates that require low temperature deposition.  19 are also plotted.