The physical and chemical properties of the evaporated metal films depend on the crystal orientation and morphology of the films, which is influenced by the deposition conditions [13]-[20]. Therefore, it is important to capture the changes in crystal orientation and morphology of the film due to changes in deposition conditions to improve the corrosion resistance of Zn thin films. To investigate the effect of crystal orientation due to changes in the vacuum level, the Zn thin film prepared by changing only the Ar gas pressure at each bias voltage was analyzed by XRD. The relative intensities of the X-ray diffraction peaks are shown in Figure 3.

Figure 3: 
X-ray diffraction patterns of Zn films deposited at various bias voltages

At a vacuum of 5×10^-4 Torr, a small amount of residual Ar gas is present. Whereas, at 5×10^-2 Torr gases like oxygen, nitrogen, and water are also present along with Ar in the vacuum chamber. These gases adsorb strongly on the (002) face which has a high surface energy, therefore the crystal growth caused by the deposition of particles can be hindered, and thus the growth rate would be slower than that of the (101) face having relatively low surface energy.

As a result, the area occupancy of a surface having high surface energy would become high. Figure 3 shows that when the Ar gas pressure was 5×10^-1 Torr, the strength of the (002) face increased significantly compared to the (101) face, as compared to that at 5×10^-4 Torr and 5×10^-2 Torr. This could be the result of preferential adsorption of the residual gas on (002) face, which has high surface energy. In other words, the adsorption of residual Ar gas on the crystal nucleus would lead to the formation of multiple nuclei rather than continuous growth. As a result, the morphology of the film is expected to be particulate in nature.

Next, the effect of bias voltage on crystal structure is analyzed. As shown in Figure 3, the ratio of the intensity of (002) face to that of (101) face increases when the bias voltage is decreased from 0 V to -200 V. However, when the pressure increases to 5×10^-1 Torr, the effect of adsorption of Ar gas on the film deposition dominates in comparison to the bias voltage. This could be due to the increased availability of Ar gas for adsorption from all sides. In particular, the high affinity of Ar gas towards (002) face leads to a surface dominated by (002) orientation. Figure 4 shows SEM images of the morphology of the top surface and cross-section of Zn thin films prepared by increasing the Ar gas pressure while keeping the bias voltage constant. When the bias voltage is 0 V, as the Ar gas pressure increases some defects are observed in the crystal grains and the size of the grains on the top surface and the cross-section become small. Also, a decrease in the Zn film thickness is observed as the gas pressure increases. When the bias voltage is reduced to -200 V, the size of the crystal grains on the top surface becomes fine while that on the cross-section becomes small and dense. A similar trend in the morphology is observed when the bias voltage is further reduced to -400 V.

Figure 4: 
SEMphotographs of Zn films deposited at different Ar gas pressures

Now we analyze the results of morphology in relation to the X-ray diffraction results shown in Figure 3 to examine the process of thin-film formation. When the Ar gas pressure is high (in the case of low vacuum – 5×10^-1 Torr), Volmer-Weber growth occurs in a direction perpendicular to the substrate, such as the (101) face, where the surface energy is relatively lower than the (002) face. Also, the growth is suppressed on (002) face due to the residual gas in the vacuum container. When the bias voltage is increased in the negative direction, the atom transfer effect, heating effect, and the sputtering effect increases causing the adsorbent atom to desorb from the growth surface on the substrate. Here, an increase in the atomic mobility facilitates surface diffusion. Therefore, as the bias voltage is increased in the negative direction, the Zn atoms diffuse to the film formation surface, so the crystal grains on the surface become large and the width of the columnar structure becomes thick. As the bias voltage is increased, the sputtering effect, the moving effect, and the heating effect are also increased by the ionized and activated Ar gas, which leads to a thick columnar cross-section. In the case of the Ar gas pressure of 5×10^-1 Torr, the surface free energy of Zn particles is reduced due to the presence of Ar gas and columnar crystal growth is suppressed. Thus, as the nuclear growth is suppressed the crystal grains on the surface are small and the cross-section has a granular crystal structure.

Figure 5 shows the results of the electrochemical anodic polarization measurement of Zn thin films in a deaerated 3% NaCl solution. In the case of the bias voltage of 0 V, the Zn thin film specimen prepared under Ar gas pressure of 5×10^-1 Torr showed better corrosion resistance than the 99.99% Zn-ingot and AZ91D.

Figure 5: 
Anodic polarization curves of Zn films deposited at various bias voltages measured in deaerated 3% NaCl solution

Additionally, as the gas pressure increased the corrosion resistance of the Zn thin film improved on account of low passivation current density. When the bias voltage was -200 V, the Zn thin film prepared under all gas pressure conditions showed better corrosion resistance than both the 99.99% Zn-ingot and AZ91D sample.

In addition, the corrosion resistance improved as the gas pressure increased, similar to the case where the bias voltage is 0 V. The best corrosion resistance was observed when the bias voltage is -200 V and the Ar gas pressure was 5×10^-1 Torr. Even under the bias voltage of -400 V, the Zn thin film showed good corrosion resistance as compared to the 99.99% Zn-ingot and AZ91D sample.

The corrosion behavior of the Zn thin film is analyzed in relation to the morphology and crystal orientation of the top surface and cross-section. As observed in the SEM images, the size of the grains of the top surface of the Zn film as well as the cross-section decreased as the Ar gas pressure increased. The fact that the grain size of the top surface is small means that the area occupied by the grain boundary on the top surface is large. It is well-known that the grain boundary acts as an anode because it is regarded as a kind of defect. In the case of a Zn film having a relatively large area occupied by the grain boundary, the grain boundary acts as an active anode in the environment. This is similar to the formation of an oxide film on the surface of the aluminum. Moreover, when the grain boundary is small and the area occupied by the grain boundary is large, a dense passivation oxide film forms relatively quickly. When the passivation film is formed on the grain boundary as described above, the film on the orientation face having a higher surface energy than that of the (101) face and the (002) face acts as a grain boundary to promote the formation of the passivation film. This is an active form because the face having lower surface energy is chemically unstable. In other words, the Zn thin film with a small crystal grain on the surface and oriented toward the (002) face is excellent for corrosion resistance because it makes a dense passivation film in a deaerated 3% NaCl solution. In addition, when pitting corrosion occurs, the granular structure with fewer defects than the columnar structure is expected to have a small dissolution rate. This is because the area of the granular structure exposed to the solution is smaller than the columnar structure morphology.