Aluminum and vanadium co-doping effects on the optical and electrical properties of oriented ZnO films

The fabrication of bifunctional zinc-oxide thin films remains a challenge. Here, we investigate the effects of aluminum-vanadium co-doping on the electrical conductivity and the optical transparency of zinc oxide films. We find that by co-doping, aluminum enhances film transparency via zinc-vacancy-defect substitution, while vanadium enhances electrical conductivity. The roles of two dopants and defects are interesting information that is useful to applications of transparent conductive oxides.

Doping is known as useful method to modify electrical properties of ZnO. Vanadium doping is known to enhance ZnO electrical conductivity by promotion of high-valent cation doping. The carrier density of vanadium doped ZnO (VZO) are incorporated into substitutional zinc site. Since vanadium has high affinity to oxygen compared with zinc, the doping of vanadium induces the formation of oxygen vacancy defects. These oxygen vacancy defects cause n-type conductivity and reduction of optical transmittance [16][17][18][19]. Aluminumdoped ZnO (AZO) also has conductivity and optical transparency and considered as a candidate because of its low cost and wide availability [20][21][22][23][24][25]. However, the electrical conductivity of the ZnO film begins to decrease at higher concentration of aluminum due to increase in the solubility limit of dopant quantity [26]. For further improving the properties, co-doping of metal elements is considered as a solutions [27]. Aluminum and vanadium co-doped ZnO has optical transparency and electrical conductivity [28,29], however, investigations of crystal orientation and dopant concentrations on the properties are remained issue. Here, we investigate the effects of aluminum and vanadium co-doping on the properties of ZnO. Because we expect aluminum atoms to migrate to and substitute light-absorbing defects, we aim to determine whether aluminum doping can produce an aluminum-vanadium co-doped ZnO (AVZO) that is both transparent and conductive.

Experiment
A radio frequency (RF) sputtering system was used for film deposition; the crystallographic, electrical, and optical properties have been investigated from viewpoint of dopant concentration and deposition temperature. All film depositions were performed by a RF magnetron sputtering system (EIKO, ES-350SU). Quartz was used Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. as a substrate. The base pressure was kept below 10 −6 Pa. Argon gas was used for plasma generation and its pressure was fixed to 1.0 Pa during film deposition. The applied RF power was 150 W. The film deposition temperature was varied from room temperature to 200°C. The distance between the target (ceramic ZnO, 99.99%) and the substrate was set to 110 mm.
Both the vanadium doping and the aluminum doping were performed by co-sputtering of the metal chips onto an erosion area of the target. The doping concentration was controlled by changing the number of metal chips on the target. The dopant concentrations were measured by x-ray fluorescence spectroscopy (RIGAKU RIX2100) using rhodium radiation. Variation of the atomic concentration measurement is within 0.1%. The film thicknesses were measured by stylus profiler (Kosaka Laboratory Ltd ST 4000-M). The film crystallinity measurements were done by x-ray diffraction (XRD, RIGAKU SmartLab) using Cu K α radiation (operation power of XRD was 40 kV×30 mA). The film thickness was set to 500 nm on quartz substrate. The Hall-effect measurements were done using self-made device under 0.4 T of magnetic field using indium electrodes. The electrical and optical properties were measured by four-probe method (ITSUBISHI CHEMICAL ANALYTECH Loresta AX MCP-T370) and ellipsometry (J.A. Woollam Co. M-2000), respectively.

Results and discussion
First, the roles of dopants are discussed in terms of defect geometry. Native point defects in ZnO films consist of vacancy, interstitial, and antisite defects [19]. Zinc vacancy defects introduce partially occupied state in the band gap, and therefore act as acceptors. Oxygen vacancies in ZnO films have low formation energy and are always present in low concentrations under equilibrium condition. This situation is changes when vanadium is doped [30]. In our case, the roles of the oxygen vacancy should be considered.
We do not consider the interstitial defects in this paper for the following reason. There are two types of interstitial defects: zinc and oxygen interstitials. Zinc interstitials are further categorized into two: tetrahedral and octahedral. The tetrahedral interstitial is unstable and has one zinc and one oxygen as nearest-neighbor atoms along the c-axis, affecting geometrical orientation. The octahedral is in the interstitial channel along the caxis. These two zinc interstitials reduce the electrical conductivity in ZnO and increase carrier concentration, however, excess zinc is required for defect formation. Co-sputtering of ZnO and metallic zinc is a method to obtain the zinc interstitial contained film [31]. Oxygen interstitials forms under excess oxygen condition, and the formation energy of oxygen interstitials are high in equilibrium.
Antisite defects, where an oxygen atom wrongly occupies a site on the zinc sublattice or vice versa, has a high formation energy under equilibrium condition, which make them unlikely to form under our experimental conditions. Therefore, the defects to discuss here are focusing upon the vacancies: oxygen and zinc vacancy.
Next, the effects of aluminum and vanadium co-doping on ZnO crystallization are discussed. To investigate a substitution effect by vanadium and aluminum doping, the AVZO crystallinity is probed by XRD for different aluminum-dopant concentrations, at the fixed vanadium-dopant concentration of 1.5 at%. Figure 1(a) shows the resulting XRD peaks at around 34°originating from the ZnO (002) diffraction. As indicated by the peak-intensity increase between the black and red curves in figure 1(a), aluminum doping at concentrations as low as 0.4 at% enhances AVZO crystallinity. The AVZO crystallinity is further enhanced by an increase in aluminum concentration from 0.4 to 0.6 at% (red and blue curves in figure 1(a)). No further crystallinity enhancement is observed when the aluminum concentration is increased to 1.3 at% (blue and green curves in figure 1(a)).
Moreover, figure 1(b) shows the ZnO (002) diffraction-peak angle shifts with increasing aluminum concentrations from ∼34.3°up. This angle shift indicates an elongation along the c-axis, suggesting the substitution of zinc-vacancy defects by aluminum atoms [32]. Figure 1(b) also indicates that the diffraction peak angle shift is saturated when 0.6 at % of aluminum is co-doped with 1.5 at% vanadium. The combined results from figures 1(a) and (b) are evidence supporting the presence of a co-doping effect.
Next, the effects of vanadium and aluminum co-doping on the properties are discussed. Figure 2(a) shows vanadium concentration dependence on the electrical resistivity and optical transmittance at the fixed aluminum doping concentration. In this paper, the average transmittance is the average of wavelength range from 450 to 800 nm. Low resistivity of approximately 0.4 mΩcm is observed at the vanadium concentration of 1.4 at%, while optical transmittance indicates monotonically decrease.
A different trend is observed in figure 2(b). This figure shows the transparency is recovered by aluminum doping and is maintained from 0.5 to 1.4 at%. The local minimum of the resistivity is observed at 0.6 at% of aluminum. Vanadium has high oxygen affinity compared with zinc, thus the doping of vanadium induces oxygen vacancy defect, resulting in the reduction of the optical transparency [33]. From here on, the investigation focuses on the effects of dopant addition by comparing AZO, VZO, and AVZO. The concentration of aluminum in AZO and vanadium in VZO are fixed at 0.6 and 1.5 at%, respectively. The dopants of AVZO is 0.6 at% of aluminum and 1.5 at% of vanadium. When a cation (aluminum or vanadium) is doped in the ZnO, the resistivity begins to increase beyond a certain cationic dopant concentration, which is attributed to the solubility limit. Cation-cation co-doping, AVZO in this paper, is effective in increase the solubility of the total dopants beyond the individual solubility limit of the cation elements [28]. This physical view agrees with the resistivity changes in figure 2. Figures 3(a) to (c) compare the effects of deposition temperature on the electrical properties of these films. The carrier density measurements against deposition temperature in figure 3(a), although with significant variations, shows a general decrease in density with the increasing deposition temperature. Similarly, the electrical resistivity measurements also show a variation, with a slight decreasing trend with the increasing deposition temperature as shown in figure 3(c) (for AVZO, the electrical resistivity is measured to be less than 0.5 mΩcm at 150°C). In contrast, the mobility measurements show an increasing trend with the increasing temperature ( figure 3(b)). This observation is explained by the fact that, since a higher temperature suppresses defect formation, a higher deposition temperature also reduces carrier scattering at these defect sites, thus enhancing the mobility. From figure 3, we deduce that the optimum deposition temperature window for obtaining high transmittance and low resistivity AVZO ranges between 150°C and 175°C.
Next, the effects of dopants on film visible-light transmittance are discussed. Figure 4(a) compares the optical transmission of AZO, VZO, and AVZO. Without aluminum, the transmittance of VZO is measured to be ∼40% in visible light region. This lower transmittance is caused by the light absorbing defects at zinc sites. These defects are compensated by aluminum doping, resulting in increases in visible-light transparency by comparable extents of AVO and AVZO in figure 4(a).
The compensation of the vacancy defects by aluminum doping is effective in the absence of vanadium, resulting in AZO has highest transmittance which is significant below 150°C. Both AZO and AVZO have comparative transmittance in the temperature range from 150 to 175°C ( figure 4(b)). The difference between AZO and AVZO below 150°C is explained by oxygen vacancy defects. The oxygen vacancy defects in ZnO-based film induces large optical absorption. This result indicates that the doping of vanadium promotes the oxygen vacancy formation [30,34,35]. Oxygen vacancy formation by vanadium doping occurs preferentially over the compensation by aluminum at low temperature condition, and both reactions are comparative at higher condition.  While AZO and VZO in figure 4(a) show similar absorption edge, the edge of AVZO is shifted to shorter wavelengths, indicating that AVZO has high density of the carrier. This observation is further confirmed by figure 3(a), which shows an high carrier density similar to that of the optical band gap [36][37][38].
Next, the effects of deposition temperature on crystallinity are discussed. Figures 5(a) to (d) show the XRD patterns of AZO, VZO, AVZO, and ZnO, respectively. The deposition temperatures are room temperature, 150°C , and 200°C. The ZnO (002) diffraction peak for the doped films show increase in intensity at higher temperature, indicating the migration is enhanced at higher temperatures [39]. In addition, a second diffraction peak at ∼36°is observed in AZO and VZO at higher temperature, originating from ZnO (101). This tilted orientation indicates that high temperature deposition of VZO results in bi-product formation, and the tilted orientation forms easily in the absence of vanadium [40]. Figure 5(e) shows correlation between XRD diffraction intensity and optical transmittance of AZO, VZO, and AVZO. This higher crystallinity of VZO compared with AZO is explained by the enhanced adatom surface migration caused by vanadium doping [41]. As is observed, the diffraction intensity of AVZO is much higher than other films. Both AZO and AVZO have similar optical transmittance at high deposition temperature (150°C .), however, the tilted orientation in AZO is unavoidable as shown in figure 5. Therefore, the co-doping mainly affects crystal orientation.
Due to the high oxygen affinity, the vanadium dopant induces formation of oxygen vacancy, resulting in increasing of the conductivity and reduction of transmittance. Then, the oxygen vacancy defects are compensated by aluminum doping. The zinc vacancy is complementarily compensated by both aluminum and vanadium. Thus, this co-doping-enhanced crystallinity is explained by aluminum and vanadium acting as complementary substitution species at vacancy sites, improving crystallinity and properties.

Conclusion
This work investigated the crystallographic, electrical, and optical properties of aluminum and vanadium codoped ZnO. We found that co-doping enhances crystallization of ZnO due to substitution of aluminum at zinc site. This effect is most pronounced when the co-doped ZnO films are grown at 150-175°C. These results suggested a method for producing co-doped ZnO that has both visible light transparency and high conductivity. Our investigation yielded interesting information on the formation process of conductive oxide films, which can be applied to a wide range of fields.

Acknowledgments
This research is supported by The Murata Science Foundation.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.

Contributions of each author
Takeru Okada: Writing-original draft and Investigation