Synthesis of Conductive Sol-Gel ZnO Films and Development of ZnO Printed Electronics

1. Introduction

Printing electrically functional inks has emerged as an important research topic to drive device technologies into the future. It has some advantages compared to conventional fabrication techniques in terms of low cost and applicability for flexible devices. This is promising for wearable, implantable, patch-like, and textile-integrated electronics, advancing the device field. With the right ink and substrate, it would be possible to achieve lightweight, flexible, transparent devices with a good electrical performance, which will revolutionize our daily lives.

Until recently, organic inks have monopolized printing technology because of their printability, flexibility, and electronic functionality. However, developing printable inorganic inks would allow for higher performance—like conventional devices—at a much lower cost than conventional fabrications, such as atomic layer deposition (ALD), pulsed laser deposition (PLD), chemical vapor deposition (CVD), physical vapor deposition (PVD), molecular beam epitaxy (MBE), and sputtering. In addition, printing techniques allow for deposition at low temperatures and at specified locations, a controllability of both parameters that no other deposition techniques can claim. These advantages make printing much easier and more compatible with flexible substrates, as some polymers cannot withstand the high processing temperatures of some deposition techniques or the harsh chemicals and UV radiation in the photolithography necessary to construct a functional device.

Some facile printing technologies are inkjet (IJP) and aerosol jet printing(AJP). Inkjet printing (IJP) is very quick and a simple drop-on-demand or continuous stream technique with micrometer precision [1, 2]. Aerosol jet printing (AJP) is favored as a clean and precise technique, using a continuous stream to print features down to 10 μm [3]. Nonetheless, both techniques are new and exciting ways to fabricate electronic devices. Both AJP and IJP techniques are currently used to print a wide variety of organic and inorganic inks for use as flexible photodetectors, transistors, and other circuit board components [4, 5, 6, 7, 8, 9, 10]. ZnO is an exciting material for electronics due to its direct wide bandgap (3.2 eV at 298 K), strong UV absorption, and electrical tunability. Many researchers have successfully fabricated ZnO devices at low cost and relatively low temperature by way of printing sol-gel precursor and other nanoparticle-based inks [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. In addition, the authors of this chapter have successfully fabricated ZnO transparent conductive oxides (TCOs) using a simple sol-gel, spin coating technique [21]. Research efforts utilizing sol-gel-derived ZnO thin films for device applications have greatly increased, recently [22, 23, 24]. Through these methods, the resultant ZnO material properties can be tuned by introducing group III metal ions during the precursor sol synthesis. In this chapter, we present the use of a sol-gel technique to develop ZnO printed electronics.

To understand the applications of ZnO sol-gel precursors, we employ spin coating, IJP, and AJP deposition with thermal and photonic sintering to synthesize and tune ZnO thin films. Extrinsic shallow donors, such as Al³⁺, In³⁺, and Ga³⁺, have similar ionic radius to that of Zn²⁺ and thus can easily replace it with little effect on the lattice structure [20, 25, 26, 27, 28, 29]. These donors are introduced during the precursor solutions’ synthesis and various atmospheric post-processing heat treatments are applied to introduce, eliminate, or passivate intrinsic defects that greatly alter the electrical conductivity of the films. The ZnO thin film properties are studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet-visible range (UV-VIS) absorbance, Van der Pauw and Hall effect measurements, and positron annihilation spectroscopy (PAS).

2. Sol-gel synthesis

To make a ZnO sol-gel precursor, we dissolve zinc acetate (99.99%) in 2-methoxyethanol (99.8%)—using ethanolamine (99%) as a stabilizer—to obtain a 0.75 M solution, with zinc acetate and ethanolamine at a 1:1 molar ratio. To dope ZnO thin films, aluminum(III) nitrate nonahydrate (99.997%), gallium(III) nitrate hydrate (99.9998%), and indium(III) acetate hydrate (99.99%) metal salts were implemented to replace some zinc acetate in the mixture to obtain a doping level of 1% in solution while keeping the 1:1 molar ratio with ethanolamine and the molarity at 0.75 M. Undoped ZnO, aluminum-doped ZnO (AZO), gallium-doped ZnO (GZO), and indium-gallium-codoped ZnO (IGZO) precursor solutions were prepared in an open-air environment, then covered with plastic paraffin film, heated to 60°C, and magnetically stirred for 2 h to obtain a transparent homogenous solution and then left to cool before deposition (Figure 1).

3. Substrate preparation: piranha etching and atmospheric plasma treatment

Quartz, cyclic olefin copolymer (TOPAS), polyethylene terephthalate (PET), and polyimide (Kapton) substrates were selected based on their transparency and/or flexibility. Before ZnO deposition, substrates were cleaned and etched to improve substrate/solution compatibility. Quartz substrates were treated in piranha baths to clean residual contaminants from the substrate and induce a surface charge. The substrate was placed in a 3:1 H₂SO₄:H₂O₂ bath at 80°C for 15 min, rinsed with deionized water, placed in a 3:1 NH₄OH:H₂O₂ bath at 80°C for 15 min, rinsed with deionized water again, and placed in an oven at about 100°C to dry. Before IJP and AJP, quartz, TOPAS, PET, and Kapton substrates were prepared by swabbing with acetone and isopropanol, drying with a nitrogen gun, and applying atmospheric plasma treatment from a corona discharge wand with the transformer set at 200 W at a standoff distance of 5 mm from the ground electrode.

4. Thin film synthesis

Once the sol-gel precursors and substrates have been prepared, ZnO thin films were deposited using spin coating, IJP, and AJP. Table 1 summarizes ZnO sol-gel precursors, substrates, deposition methods, sintering conditions, and post-processing environments and thicknesses.

5. Scanning electron microscopy

Spin-coated AZO thin films that were post-processed in H₂ and Zn were imaged by SEM. Low-magnification surface images show worm-like structures (Figure 2), while a higher magnification shows round particles with an average particle size of 20 nm (Figure 3). Platinum was then deposited on the film surface by a focused ion beam and a trench was milled through the sample to obtain a high-resolution cross-sectional image (Figure 4). It can be seen that the film is deposited as distinct individual layers (each layer is ~40 nm thick). The images also reveal non-uniform thickness, with ~25% variation across the film and indicate that it is difficult to obtain uniform thickness using sol-gel methods. These images represent the first high-resolution cross-sectional images for sol-gel films. They illustrate that the distinct individual layers and the non-uniformity in thickness are inherent of the spin coating method, but they may be reduced by further annealing. This non-uniform layering leads to interference in UV-VIS transmission spectra, a well-known feature in sol-gel films.

6. X-ray diffraction

Film crystallinity was studied using a Rigaku X-ray diffractometer to determine the ZnO crystal phase (hkl values) and average grain size. XRD patterns for spin-coated AZO films indicate polycrystalline thin films, with peaks corresponding to the (100), (002), and (101) planes (Figure 5). These diffraction patterns match the ZnO hexagonal wurtzite structure, without any secondary phase impurities in the films. Furthermore, we can see a change in polycrystallinity based on post-processing in H₂ and Zn environments. When processed in H₂, films become less polycrystalline, while processing in Zn leads to the opposite effect.

XRD patterns for IJP ZnO films (Figure 6) reveal that ZnO phases form at temperatures as low as 150°C, with more peaks appearing at increased sintering temperature. Although there is a lack of ZnO phase formation for ZnO⁴ on Kapton and an impurity phase for ZnO³ on TOPAS, the results demonstrate that ZnO thin films can be successfully fabricated by inkjet printing and thermal and photonic sintering processes.

XRD patterns for AJP ZnO reveal amorphous nature at 200°C and an increasing polycrystallinity with the sintering temperature. In addition, increasing the sintering temperature increases the average grain size, which is consistent with previous reports [30, 31]. XRD patterns for AJP IGZO XRD show a ~50% increase in grain size due to the low doping concentration of In³⁺ and Ga³⁺, with a minimal effect on the polycrystalline structure. Table 2 presents the average grain sizes of the aforementioned thin films. The grain size D was calculated for each 2θ peak using the Scherrer equation:

where λ = 1.54 Å is the X-ray wavelength, β is the full width at half maximum (FWHM) of the corresponding peak, and θ is the Bragg angle.

ZnO, AZO, GZO, and IGZO sol-gel precursors are viable options to achieve a ZnO wurtzite structure at low sintering temperatures. Films are generally inhomogeneous in thickness and amorphous or polycrystalline in nature, with grain size and polycrystallinity increasing with the sintering temperature. A low doping concentration does not inhibit ZnO wurtzite formation, but the incorporation of In³⁺ and Ga³⁺ dopants effectively increases the average grain size. Furthermore, post-processing in H₂ and Zn environments can change the polycrystallinity of the films.

7. Ultraviolet-visible range spectroscopy

A dual-beam Perkin Elmer UV-VIS spectrometer was used to record the transmission and absorbance spectra of spin-coated and printed ZnO films. A blank substrate was placed in line with that reference beam, while the sample spectra were recorded.

Transmission measurements for spin-coated ZnO⁰ and AZO show the band edge near 380 nm with a high visible range transparency (Figure 8). The individual layering, as observed from SEMs, leads to interference effects in the spectra, which can be reduced by a greater H₂ concentration during annealing and lead to improved transparency (Figure 8a). This can be explained by a decrease in polycrystallinity observed in XRD analysis. However, the opposite effect occurs after annealing in a Zn environment (Figure 8b), which is due to the increase in polycrystallinity.

Printed ZnO and IGZO films also show a band edge near 380 nm from optical absorbance measurements (Figure 9a). The bandgap is near 3.2 eV for all printed films, as calculated by the Tauc method (Figure 9b). AJP IGZO films are more transparent than IJP films due to better overall print quality. Figure 10 compares the IJP and AJP techniques, as seen by the naked eye. It is clear that AJP films are more transparent because of less light scattering from surface roughness and striations in the IJP films.

These results established that the sol-gel precursor method can produce films with good visible range transparency in spin coating and printing techniques. Interference in the visible range absorbance can be reduced by the post-processing conditions. Here, AJP yields a better print quality than IJP and offers similar visible range transparency to spin coating.

8. Van der Pauw and Hall effect measurements

At 300 K, the resistivity was obtained via van der Pauw measurements using an MMR Hall effect system. All spin-coated and printed ZnO, AZO, GZO, and IGZO films show high electrical resistivity after the initial sintering. All spin-coated films were too resistive to initially measure, and the printed films measured resistivity >10⁴ Ω cm. However, post-processing of spin-coated GZO, AZO, and ZnO⁰ in H₂ and Zn environments induced a large conductivity. Van der Pauw and Hall effect measurements for ZnO, AZO, GZO, and IGZO films are summarized in Table 3.

AZO thin films annealed in both H₂ and Zn offer the lowest electrical resistivity (1.71 × 10⁻² Ω cm) and the highest carrier concentration (3.01 × 10²¹). We emphasize that the electrical conductivity results only after the post-processing steps. The large decrease in resistivity is attributed to the passivation of defect states, which will be discussed further in the PAS section of this chapter. The low resistivity coupled with the high visible range transparency offers solution-processed ZnO as a viable option of TCO in printed electronics.

In³⁺ and Ga³⁺ dopants were also investigated through electrical measurements by comparing printed IGZO² and ZnO⁵ thin films. Both use IJP deposition on Kapton substrates and have a thickness of 600 nm. Unsurprisingly, IGZO² has a lower resistivity (3.06 × 10⁴ Ω cm) than ZnO⁵ (4.59 × 10⁵ Ω cm). It is well understood that In³⁺ and Ga³⁺ increase conductivity in ZnO, an effect studied in other In- and Ga-doped sol-gel ZnO [32, 33].

The lowest resistivity for AJP ZnO is in the thin film that was annealed at 300°C. The overall resistivity of ZnO depends on its structural properties, which are affected by the oxygen concentration in the film. At higher sintering temperatures, resistivity increases with annealing temperature [34, 35]. However, at temperatures below 300°C, we see the opposite effect [36]. While increasing the sintering temperature, we are removing more solvent, forming a ZnO structure, and increasing the grain size, which creates more pathways for conduction. As we further increase temperature, the grain size continues to increase—as seen in XRD—but more oxygen is being introduced to the ZnO. Increasing the grain size is expected to decrease the resistivity [37], while introducing more oxygen may increase the resistivity [38], resulting in a local minimum in the resistivity as a function of sintering temperature.

In general, doping, sintering, and post-processing all play a vital role in the conductivity of sol-gel ZnO films because of their effects on the ZnO lattice structure and defect formation. First, shallow donors can increase the free carriers in the conduction band. Second, the solvent must be completely evaporated, and the grain boundary concentration and adsorbed oxygen must be minimized to increase the mobility and carrier concentration, respectively. And lastly, post-processing techniques can be utilized to further improve the polycrystallinity and passivate defect charge states.

9. Photoconductivity

The MMR Hall effect system was equipped with a 365-nm light-emitting diode (LED), positioned 1.8 cm from the sample stage, to measure the resistivity as a function of light intensity. After dark measurements were taken, the LED light intensity was increased in steps up to 24 mW (~4.4 × 10¹⁶ photons·cm⁻² s⁻¹), allowing the light and temperature to stabilize for at least 1 min prior to each measurement. Although light from the LED produces localized heating, the temperature was maintained at 300 K using a Joule-Thompson refrigerator located directly beneath the sample stage, operating in combination with a heating element. Photoconductivity was observed in IJP ZnO⁵; IJP IGZO²; and AJP ZnO⁶, ZnO⁷, and ZnO⁸ (Figure 11).

Upon initial UV LED illumination at 0.98 mW (~4.4 × 10¹⁶ photons·cm⁻² s⁻¹), there is a sharp decrease in resistivity. We credit this to ZnO absorbing light and promoting an electron from the valence band to the conduction band because the incident UV photons are of greater energy (~3.4 eV) than the ZnO bandgap (~3.2 eV). The photoresponse is due to oxygen chemisorption, where light illumination causes oxygen desorption and the release of trapped electrons to the conduction band [39]. Here, the greatest conductive response is seen at the greatest sintering temperature in AJP ZnO. This may be because larger grains desorb more oxygen when illuminated. In addition, the larger grain size would allow for better electron mobility.

With increased light intensity, IJP ZnO thin films quickly saturate, as there is no more oxygen to desorb. But, AJP ZnO—which has a greater photoconductive response—does not saturate at higher light intensity. As the intensity increases, photogenerated holes can be produced and then trapped at charged boundary states, while excess electrons can be promoted to the conduction band, increasing the free carrier concentration. In addition to the effect of the grain boundary, the charge state of defects may also undergo a change upon illumination and lead to an increase or a decrease in electron scattering affecting electron mobility. For instance, a change in the charge state of defects could increase electron scattering, decreasing the electron mobility and compromising the conductivity. Both the carrier concentration and electron mobility strongly affect the transport properties of ZnO films, and different photo-induced processes could lead to the observed non-linear behavior with increasing light intensity.

10. Positron annihilation spectroscopy

It is impossible to understand the effect of annealing on the transport properties without investigating the presence of point defects in the films. PAS is a well-established technique for measurements of cation vacancies, which strongly influence the transport properties [40, 41, 42, 43, 44]. In fact, many works have applied PAS and identified Zn vacancies in ZnO films and bulk single crystals [39, 40, 41]. The sensitivity of PAS to open volume defects such as vacancies can be understood as follows. The lack of positive ion cores at vacancies forms an attractive potential that traps positrons leading to characteristic changes in the measured positron annihilation parameters. Therefore, PAS is a very useful tool to further improve the development of sol-gel ZnO film. Here, depth-resolved Doppler broadening of PAS measurements was applied to elucidate the aforementioned effect of annealing on the electrical properties of ZnO films. The measurements were carried out on AZO films before and after annealing in forming gas (5% H₂, 95% N₂). Figure 12a and b shows S and W parameters, respectively, for the films as a function of incident positron energy and mean implantation depth. The S and W parameters represent the annihilation fraction of positrons with valence and core electrons, respectively, and they provide an indication about defect density [45, 46, 47]. The S parameter was obtained from the annihilation peak by dividing the counts in the central peak by the total counts in the peak, while the W parameter was obtained by dividing the counts in the wings of the peak by the total counts in the peak. Trapped positrons at defects are more likely to annihilate with low-momentum valence electrons causing an increase in S parameter and a decrease in W parameter [45, 46, 47]. In Figure 12a and b, an increase in S parameter and a decrease in W parameter at low positron energy (0–5 keV) are due to positron annihilation at the surface of the films. The figures show a significant decrease in S parameter and an increase in W parameter after H₂ processing, which can be interpreted as follows. A Zn vacancy has a negative charge state and is therefore an effective trapping center for positrons, while an O vacancy or interstitial defects cannot trap positrons. Therefore, a decrease in S parameter and an increase in W parameter after H₂-annealing are a clear indication for the reduction or passivation of Zn vacancy-related defects. Annealing in forming gas cannot eliminate Zn vacancies; however, hydrogen can partially or completely fill Zn vacancies modifying their negative charge state, which prevents positron trapping. Similarly, Zn interstitials can fill Zn vacancies decreasing positron trapping.Figure 13 shows the S-parameter versus W-parameter plot for ZnO films after air annealing, ZnO films after forming gas annealing, and ZnO bulk single crystals. The points on the graph represent the data points corresponding to energy values at which positrons annihilate only in the middle of the film without any influence from the surface or the substrate. The line in the S-parameter versus W-parameter plot runs through the bulk value, which is an indication that there is only one dominant defect type in the samples [40, 46]. This illustrates that H₂ annealing did not create new defects but only reduced Zn vacancies, the dominant trapping defect for positrons in ZnO. PAS studies here illustrated that Zn vacancy-related defects are dominant in sol-gel ZnO films and provided strong evidence that hydrogen passivates Zn vacancies, eliminating their deep acceptor state, which leads to a large increase in carrier concentration and high-induced conductivity in the films.

11. Conclusion

In conclusion, ZnO films were deposited on various substrates using a simple sol-gel precursor method. This precursor has proven compatible spin coating, IJP, and AJP techniques to fabricate TCOs and photodetectors. SEM measurements reveal surface roughness and non-uniformity that are inherent to the sol-gel process. However, these drawbacks can be overcome to optimize the UV-VIS and electrical properties. XRD analysis shows polycrystallinity that can be tuned by sintering temperature and processing atmosphere. The post-processing step and the addition of In³⁺ and Ga³⁺ have both shown to enhance the electrical conductivity of ZnO either through the suppression of acceptor vacancies or the addition of shallow donors. Resistive ZnO thin films also exhibited an overall photoconductive response of 10⁶. PAS was executed to study the role of hydrogen passivation of cation vacancies in the electrical properties of sol-gel ZnO thin films and to illustrate its need for the development of conductive sol-gel ZnO films. Overall, this work demonstrates the compatibility of sol-gel ZnO with printed electronics and other devices and presents fundamental research to understand the structural, optical, and electrical properties of the material system.

Acknowledgments

The authors would like to thank the following collaborators for their contributive efforts: Wolfgang Anwand and Andreas Wagner at the Institute of Radiation Physics; Pooneh Saadatkia, Erik Flesburg, and Micah Haseman at Bowling Green State University; and Emily M. Heckman, Eric Kreit, Roberto S. Aga, Brett Wenner, Kevin Leedy, Steve Tetlak, David C. Look, Jeff Allen, and Monica Allen at the Air Force Research Laboratories at Wright-Patterson Air Force Base and Eglin Air Force Base.

Funding for this work was provided by multiple AFRL and DAGSI projects.

Conflict of interest

The authors declare that they have no conflicts of interest.