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ZnO-Based Transparent Conductive Thin Films: Doping, Performance, and Processing
ZnO-based transparent conductive thin films have attracted much attention as a promising substitute material to the currently used indium-tin-oxide thin films in transparent electrode applications. However, the detailed function of the dopants, acting on the electrical and optical properties of ZnO-based transparent conductive thin films, is not clear yet, which has limited the development and practical applications of ZnO transparent conductive thin films. Growth conditions such as substrate type, growth temperature, and ambient atmosphere all play important roles in structural, electrical, and optical properties of films. This paper takes a panoramic view on properties of ZnO thin films and reviews the very recent works on new, efficient, low-temperature, and high-speed deposition technologies. In addition, we highlighted the methods of producing ZnO-based transparent conductive film on flexible substrate, one of the most promising and rapidly emerging research areas. As optimum-processing-parameter conditions are being obtained and their influencing mechanism is becoming clear, we can see that there will be a promising future for ZnO-based transparent conductive films.
Transparent conductive oxides (TCOs), capable of transporting electrical charge and transmitting visible photon, are necessary for use as transparent electrodes in flat panel displays such as liquid crystal displays (LCDs), plasma display panels, electronic paper displays, light-emitting-diodes (LEDs) , and touch panels [2, 3]. A carrier concentration on the order of 1020 cm−3 or higher and a band-gap energy above 3 eV are usually required for high conductivity and transmittance . Various TCO thin films consisting of impurity-doped SnO2 (SnO2:Sb and SnO2:F), In2O3 (In2O3:Sn, or ITO), and ZnO (ZnO:Al and ZnO:Ga) films have been researched so far in this field [5, 6]. Among them, tin-doped indium-oxide (ITO) is the one in practical use [7, 8]. However, with the market of ITO expanding, a stable supply may be difficult to ensure because of the high cost and scarcity of indium [9, 10].
Fortunately, ZnO thin films are a promising alternative to the commonly used ITO, which are low cost, nontoxic, highly durable against hydrogen plasma compared to ITO . Besides, it has a more proper work function for the transparent contact cathode electrodes of transparent OLEDs . Nevertheless, many problems associated with substituting ZnO-based TCO films for ITO still exist. Figure 1 visually describes structural, optical, electrical properties of zinc oxide. In LCD applications, stability in various environments has to be improved when films become thinner. . In OLEDs applications, preparation technology of low-temperature and plasma damage-free films on flexible substrate have to be optimized.
For the design and realization of ZnO-based devices, one of the most significant issues is doping, which necessarily involves the heavy doping with trivalent elements from the group III (Al, Ga, In). Minami summed up the resistivity of impurity-doped binary compound TCO films reported among thirty years and found that the obtained minimum resistivity of impurity-doped ZnO films is still decreasing, while SnO2 and In2O3 films have essentially remained unchanged . Such trend indicates the possible significant promotion of ZnO-based TCO films.
ZnO films have been prepared through various kinds of methods [14, 15] and each of them has its own advantages and drawbacks. In this paper, sputtering, chemical vapor deposition (CVD), and pulsed laser deposition (PLD) are described in details, since ZnO-based TCO films based on these methods can get the highest quality. The first two methods allowing large area deposition make them the most advanced and efficient deposition techniques for ZnO film. Even if the films are prepared by the same method, thickness, substrate, growth temperature, dopant, and their content will play a crucial role in structural, electrical, and optical properties of films. Therefore, whether we can optimize the fabrication of high-quality doped zinc oxide thin films at low cost determines the future of ZnO-based TCO films. The structural, optical, electrical properties of doped ZnO are summarized with particular emphasis on influence of doping on the properties of ZnO thin films, which are responsible for the properties of films. In addition, recent progress on film preparation is introduced.
2. Different Dopant Elements
Several elements are doped in ZnO films, such as B [16, 17], Al, Ga, In (from group III elements) , F (from group VII element) , and so forth. Among them the most studied are Al-doped ZnO and Ga-doped ZnO. In Section 2, we focus on recent research on Al, Ga doping, and codoping. The position of the dopant atoms and solubility limit can be related to the film preparation methods. For example, in the spray pyrolysis technique, In and Ga is more efficient than Al since the Al atoms are preferably at interstitial sites of the ZnO lattice and decrease the mobility .
Some reported electrical and optical properties of ZnO films with different dopants prepared by PLD are summarized in Table 1. Each dopant was at its appropriate content and all films were fabricated under optimum-processing-parameter conditions to achieve lower resistivity and higher transmittance.
2.1. Group III Elements Doping
The ionic radius of Al3+ is 0.54 Å, which is smaller than that of Zn2+ (0.74 Å). So the Al3+ can occupy the place of Zn2+ in lattice easily, leading to a reduction of the lattice parameter. In comparison with Al3+, Ga3+ has similar ionic radius to Zn2+ ions, which minimizes the ZnO lattice deformations even at higher doping concentrations [18, 21]. This may explain the reason why the best electrical properties of ZnO-based films are observed in Ga-doped ZnO thin films .
The conductive property of ZnO thin films is primarily dominated by electrons generated by oxygen vacancies and charge donation. Geng and his coworkers  investigated the effect of Al concentration on the electrical properties of ZnO:Al thin films grown on Si(100) substrate by atomic layer deposition. The crystallinity of the film degenerated while resistivity was found to decrease with increasing Al doping concentration (9.36 × 10−4 Ω·cm at maximal doping concentration 2.7 at.%).
Al is incorporated beyond the thermodynamic solubility limit, even within the limit, the relationship between Al content and resistivity is not monotonous . The resistivity of the AZO film decreased with an increase of the Al content up to 7 at.% and then increased at a high Al content of 8 at.% (Figure 2). At high Al concentration, Al exists in the form of Al2O3, resulting in deleterious effects to the electrical properties of the films. According to many other reports, the relationship between conduction characteristics and concentration of other elements doping is similar to that of Al doping.
Fluorine is another dopant to improve the conductivity of the films . Unlike aluminum and other elements in group III, effect of fluorine in ZnO is less discussed. Fluorine incorporates into the lattice by substituting oxygen atoms due to the comparability of ionic radius (F-: 1.17 Å, O2−: 1.22 Å). Actually, ZnO:F films have good combination properties. Cao et al.  prepared FZO films via pulsed laser deposition, getting a resistivity of 4.8 × 10−4 Ω·cm, carrier concentration of 5.43 × 1020 cm−3, and Hall mobility of 23.8 cm2V−1s−1, respectively. The average optical transmittance in the entire visible wavelength region was higher than 90%.
Difference of radii between doping ions and Zn2+ ions or O2− ions will result in variation of lattice constant and degeneration of crystallinity in zinc oxide. This issue could be partially resolved by codoping, which has led to an upsurge study in recent years [26–28]. High quality of films deposited by Shin et al. has confirmed the advantages of codoping . As Ga concentration increased from 0.5 to 2 at%, the peak intensity of the (0002) plane and carrier concentration for Mg and Ga codoped ZnO thin films (MGZO) increase compared with that of pure ZnO and Mg-doped ZnO thin films, as shown in Figure 3 , implying the substitution of Ga3+ ions with Zn2+ ions (ionic radius of Ga3+, Mg2+, and Zn2+ is 0.062 nm, 0.057 nm, and 0.060 nm, resp.). What is more, the MGZO structure was stabilized greatly in this method. The low electrical resistivity and wide band gap energy of MGZO thin films can be ascribed to the dopant of Ga and Mg, respectively.
3. Doping Concentration
3.1. Concentration-Dependent Conductivity
Zinc oxide, as a transparent oxide with band-gap energy of 3.37 eV , can satisfy the optical properties requirement of TCO without doping. However, the carrier concentration of intrinsic ZnO is roughly 106 cm−3 at room temperature , which is much lower than that of TCO films for practical application. In general, an introduction of dopants affords an increase in carrier concentration, that is, where is the concentration of conductive electrons, is the concentration of hole, and is s intrinsic carrier concentration in ZnO. Doping directly affects carrier concentration and influences conductivity in turn. Although introduced impurities are necessary, but not more is better. Its concentration must have a limitation; otherwise, very high doping levels will result in high free carrier absorption, high plasma resonance reflectivity, and low visible wavelength transparency (Figure 2). Even though it seems difficult to improve electrical conduction while maintaining the optical transmission, there is the potential to achieve both through improving the microstructure of the films and increasing the carrier mobility. Obviously, it is important to deconstruct the influence of intentionally introduced impurities on electrical properties of ZnO to realize any type of device applications.
3.2. Concentration-Dependent Optical Gap Shift
The optical-gap shift is contributed by two competing effects, Burstein-Moss (B-M) band-filling effect and band-gap narrowing (BGN) effect (Figure 4). The well-known optical band-gap () blue shift [32, 33] in heavily doped semiconductors occurs because the lower states in the conduction band are blocked. Conversely, band-gap narrowing is caused by exchange interactions in the free-electron gas and electrostatic interactions between free electrons and ionized impurities. The fact that BGN effect is largely compensated by B-M effect and temperature or doping are two influence factors in the behavior makes it difficult to separate these two effects by means of current experiments.
Lu and coworkers  found that , evaluated from optical absorption spectra, was mainly related to the carrier concentrations and so intrinsic to Al content. The optical gap increased with the electron concentration when cm−3, which could be fully interpreted by a modified Burstein-Moss effect; A sudden decrease in energy gap occurred at 5.4−8.4 × 1019 cm−3, consistent with the Mott criterion for a SMT. Above the critical values, the band gap increased again (see Figure 5). Further exploration in this issue is needed to promote energy band engineering in ZnO-based TCO films.
4. Effect of Growth Conditions
4.1. Preparation Methods
Among various preparation methods, pulsed laser deposition (PLD) is more appropriate for precise controlling of morphology and composition of the grown materials. Park et al. and Agura et al. [22, 39] prepared AZO and GZO thin films on glass substrates with a low resistivity on the order of 10−5 Ω·cm by PLD, respectively. Resistivity on the order of 10−5 Ω·cm is the best result for ZnO films ever reported and there is a general agreement on the better performances in terms of conductivity for ZnO films grown by PLD than those obtained via other techniques. However, the problems of stability and cost most probably constrained the future development of this method in manufacture on a large scale.
Magnetron sputtering is one of the deposition methods in practical use. It was found that the lowest resistivity prepared by this method is still on the order of 10−4 Ω·cm, higher than the resistivities obtain by PLD several years ago. Chemical-vapor-deposition is another industrially most advanced deposition techniques. How to increase growth rate and applied to continuous, in-line manufacturing is a vital issue. Illiberi et al. deposited ZnO:Al films on glass by an industrially scalable metal organic chemical-vapor-deposition (MOCVD) technique at atmospheric pressure [40, 41]. In view of an industrial up-scale, the vaporized precursors, that is, diethylzinc (DEZ), Tertiary-butanol (t-BUT), and trimethylaluminum (TMA), can be injected in different deposition zones, where each layer is deposited. Then, each zone installed along the same production line continuous, realizing in-line manufacturing. Schematic of an industrial atmospheric-pressure MOCVD system is shown in Figure 6. A maximum growth rate was found at a deposition temperature of 480°C, for which :Al films show good crystalline quality, high conductivity ( Ω/□ for film thicknesses above 1050 nm), and good transmittance (>85% in the visible range).
Sol-gel method is an attractive liquid-phase routes for obtaining thin films due to its advantages of easy control of the film composition and fabrication of a large area thin film with low cost. Luo et al.  developed a versatile film-deposition process combining microwave-assisted nonaqueous sol-gel process with dip-coating. The resistivity of obtained films was a little high (minimum resistivity is 2.35 × 10−2 Ω·cm) but their results are still promising on the way to the development of liquid-phase routes. To produce TCO films of good quality, every step of sol-gel method, including synthesis of nanoparticles, preparation of dispersions, thin-film processing, and postannealing, has to be optimized.
4.2. Substrate Type
Up to now, transparent conductive films have been prepared on plenty of different substrates, such as glass, sapphire, and polymer. Polymeric substrates, polyethylene-terephthalate (PET) , polycarbonate (PC) , aspolyphthalamide (PPA) are drawing increasing attention due to their merits of light weight, low cost, ability to flex, curve, roll, fold small volume and promising application in organic light-emitting diodes (OLEDs) [48, 49].
Substrates nature alone may still affect microstructures (crystallization, morphology) and electrical properties of films. Esmaili-Sardari et al. have a research on conductivity type dependent on different substrates . All films grown on boron doped p-Si have n-type conductivity at low Hall Effect temperatures and p-type conductivity at high Hall Effect temperatures but films grown n-type substrate show n-type behavior for the entire Hall temperatures range from 80 to 350 K, suggesting that the differences in nearly substrates surfaces could affect intrinsic defects and concentrations as well as the ZnO (0001) surface termination.
Shin et al. prepared thin films of Ga-doped ZnO (GZO) on glass and Al2O3 (0001) substrates by using RF magnetron sputtering under the same processing conditions and found that individual grains with columnar shape are clearly observed in the GZO film on the glass substrate, as shown in Figure 7(a). However, it is difficult to distinguish individual grains in the GZO film on the Al2O3 substrate (Figure 7(b)). The superior crystal quality of the epitaxially grown GZO films on Al2O3 substrates compared to the polycrystalline GZO films on glass substrates afforded better electrical characteristics.
Clatot et al. measured the film crystallinity and stoichiometry, resistivity, and transmittance curves of Si-doped ZnO thin films deposited both on glass and PET substrates. Their results confirmed the strong dependence of the electrical and optical properties on substrate nature . As the theoretical and experimental studies of the impact of hydroxylation, vacancies, interstitials, and extrinsic doping on the electronic properties of the polar ZnO (0002) surface done by Li et al. , we can have a study on the influence of substrate type on growth, defects, and the electronic properties of the ZnO thin films.
4.3. Growth Temperature
At high temperature, films exhibited higher doping efficiency, higher carrier concentration, lower resistivity, and higher Hall mobility. Lee et al.  investigated the temperature dependence of electrical resistivity and optical band gap of GZO thin films. The resistivity of films decreases from 9.61 × 10−3 to 4.50 × 10−4 Ω·cm with the increases of deposition temperature from 100 to 500°C (Figure 8), and this can be attributed to substitutional doping increase and the grain boundary density decrease as the growth temperature rose.
Low growth temperature is required for the flexible substrates in OLEDs. However, the films always show poor properties at low growth temperature. This relationship does not mean all hope for low-temperature growth lost. In recent years, the growth of high quality films at low temperature on flexible substrates has attracted extensive attention owing to the rather low cost with many unique applications [51, 52]. Clatot et al. successfully deposited Si-doped zinc oxide (SZO, Si 3%) thin films at a relatively low temperature (≤150°C) by pulsed laser deposition (PLD). The SZO films deposited on PET substrates at 100°C exhibit a very promising resistivity of 9.0 × 10−4 Ω·cm . Reference [44, 53] introduced a process combining room-temperature PLD and direct pulsed laser recrystallization (DPLR). During DPLR, the AZO thin film undergoes recrystallization and growth while the underlying flexible substrate is retained at low temperature. Schematic of DPLR technique and temperature evolution of AZO film on different substrates carried out by multiphysics EM-HT simulation is shown in Figures 9(a) and 9(b), respectively. Recrystallization and growth of films reduce internal imperfections, resulting in higher conductivity, Hall mobility (increases from 6.56 to 10.12 cm2/Vs), and higher transmittance. It is also worthy to note that the merit of no damaging on the underlying substrates makes this technology promising in various low temperature applications.
4.4. Ambient Atmosphere
In deposit technique, ablated species will loss kinetic energy due to collisions with the background gas molecules, resulting in different film density and morphology. Gas pressure, especially oxygen partial pressure, has received a lot of attention [24, 25]. Gondoni et al.  found that when the background gas (O2) pressure changed from 0.01 Pa to 10 Pa, resistivity remains the same and increases to a magnitude of 10−1 Ω·cm upon reaching 10 Pa abruptly (black squares in Figure 10). The increasing resistivity at higher pressures derived from a lower concentration of oxygen vacancy, which reduces the number of free electrons in turn. In addition, the annealing treatment in air was observed to cause resistivity to increase (red dots in Figure 10), diminishing residual dependence on deposition conditions because oxygen vacancies are saturated. This variation relation has been also observed in other preparation methods [5, 54].
In CVD processing, the precursor pressure in CVD processing also showed strong influence on resistivity. Kim et al.  prepared AZO layers on glass substrates by LP-CVD and found Al precursor, (Trimethylaluminum) showed strong influence on Al contents and resistivity of AZO.
4.5. Film Thickness
Film thickness is another factor for the electrical and optical properties in ZnO-based TCO films, which change from ~50 nm to ~2000 nm. In magnetron sputtering and other deposition technique, ZnO films with various thicknesses are obtained by varying the deposition time, temperature, and power [42, 55]. In sol-gel spin coating, films with various thicknesses are obtained with different repeating times of coating procedure [26, 45, 56].
As we all know that electrical conductivity becomes better while transmittance may become poorer as the film thickness increases . Shin et al.  investigated the effect of film thickness on the structural and electrical properties of Ga-doped ZnO thin films. It was also found that the electrical resistivity difference between the films on the two substrates decreased from 9.48 × 10−4 Ω·cm to 1.45 × 10−4 Ω·cm with increasing the film thickness from 100 nm to 1000 nm. In Figure 7, the conductivity increases because of both the increase of grain size and the decrease of grain boundary scattering. On the other hand, more photons are adsorbed when the thickness increases according to the following:
Here is the intensity of transmitted light, is the intensity of incident light, is absorption coefficient, and can be treated as the film thickness. Due to these two opposite trends, it is difficult to balance the need for high conductivity with the need for high transparency. How to get thinner films with both excellent electrical and optical property is one of the main research tasks.
In this paper, how dopants and growth conditions, such as substrate type, growth temperature, influence electrical and optical properties of ZnO-based TCO films were discussed in a panoramic view. we reviewed recently published work on new, cheap, low-temperature, and high-speed deposition technology, highlighting methods of producing film on flexible substrate, the most promising and rapidly emerging research areas. However, more experimental works are needed to clarify the relationship between doping, processing, and property of ZnO thin films.
This work was supported by the Natural Science Foundation of China (Nos. 61222403), the Jiangsu Introduction Program of High-Level Innovative and Pioneering Talents, the Fundamental Research Funds for the Central Universities (Nos. 4015-56XIA12004 and 4015-56YAH11047), and by the Opened Fund of the State Key Laboratory on Integrated Optoelectronics No. 20********.
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