Zinc oxide (ZnO) is an exciting alternative wide bandgap semiconductor that has promising use in sensors and flexible electronics. Plasma treatment can be applied to prepare surfaces for ZnO deposition, improve electrical properties through organic impurity removal and surface modification, and facilitate fabrication of ZnO-based devices.
ZnO is a wide bandgap semiconductor with similar electrical and physical properties to that of GaN III-V semiconductor. Because of the relative abundance of zinc and ability to deposit ZnO at low temperatures, ZnO is appealing as a low-cost alternative to GaN for light emitting diodes (LEDs), optoelectronics, and flexible electronics [1-3].
In addition, ZnO is optically transparent in the visible spectrum and, with appropriate doping, ZnO can be tuned to be highly conductive. As a result, ZnO is also attractive as a lower cost substitute for indium tin oxide (ITO), a transparent conducting oxide, or as the semiconducting layer in thin film transistors (TFTs).
ZnO films can be deposited by a variety of methods, including sputtering or spin coating from solution. However, the as-deposited ZnO films typically require further processing to yield desirable electrical properties for optimal device performance.
Here, we explore several research studies that applied plasma treatment to induce oxygen vacancies and alter ZnO surface chemistry, and develop ZnO-based sensors and thin film transistors (TFTs) with improved electrical properties and device performance.
Zinc oxide for sensors
Sensors require high surface activity (active sites) to detect the target analyte with sufficient sensitivity. Plasma treatment alters ZnO surface morphology and produces oxygen vacancies, creating oxygen-deficient ZnO which serve as active sites for gas or chemical detection and can enhance sensor performance.
Oxygen vacancies on semiconductors can affect gas adsorption by (1) increasing carrier concentration through creation of unpaired electrons and (2) modifying the energy band structure (reducing the energy band gap in some cases) which results in more electron transfer and enhanced sensing. Increasing oxygen vacancies increases the concentration of adsorbed oxygen species on the surface, creating more unpaired electrons and active sites for gas molecule adsorption and redox reactions, which can amplify the sensor’s signal response.
In their work, Li et al. applied Ar plasma on annealed ZnO films to fabricate ZnO-based metal oxide semiconductor (MOS) gas sensor for detection of triethylamine (TEA), a commonly used organic solvent in chemical synthesis that can be harmful to human and aquatic life when released into waste streams. The researchers induced the formation of oxygen vacancies by varying the Ar plasma treatment time (1-10 min) and thus enhancing the sensor’s electrical resistance signal response.
Their XPS measurements and spectral analysis indicated that 5 min Ar plasma produced the highest oxygen vacancy content in plasma-treated ZnO films (Figure 1).
By extension, 5 min Ar plasma treatment also yielded ZnO-based gas sensors with optimal performance when operated at 250°C, with relatively high response signal (ratio of electrical resistance with TEA versus baseline resistance without TEA), fast response and recovery time, and high selectivity to 10 ppm TEA (against other organic solvents). Based on measuring the sensor’s response signal over a range of TEA concentrations (1-100 ppm), the researchers extracted a potential limit of detection of ~22 ppb, indicating the possibility to detect low levels of TEA. Their work demonstrated a promising technique to fabricate low-cost MOS gas sensors for detecting organic amines.
Figure 1. Oxygen vacancy content with plasma treatment time of ZnO films annealed at 700°C for 2 hours followed by Ar plasma treatment. Data from Li Z, Liu X, Zhou M, Zhang S, Cao S, Lei G, Lou C and Zhang J. J. Hazard. Mater. (2021) 415: 125757. DOI: 10.1016/j.jhazmat.2021.125757
In a separate study, Perera et al. developed a sensor for human biomarker detection based on oxygen-deficient ZnO films. Their sensor used human saliva and sweat as the analyte while cardiac inflammatory biomarkers interleukin 6 (IL-6) and C-reactive protein (CRP) were used as the model antigen.
Oxygen-deficient ZnO is favored because of the greater number of loosely bound oxygen, which provide more opportunities for surface functionalization and more active sites for biosensing. In addition, (3-glycidyloxypropyl)trimethoxysilane (GPS) was included as a critical functional layer in the device. GPS allows antibodies to attach and orient so that their antigen binding sites protrude from the surface, increasing exposure to incoming antigens and allowing for maximal antigen binding.
The sensor was fabricated by sputter deposition of oxygen-deficient ZnO film onto oxidized silicon substrate, followed by deposition of Au/Cr electrodes to measure electrical resistance across the exposed ZnO surface.
O2 plasma was applied to as-sputtered ZnO to remove organic impurities and introduce oxygen-containing hydroxyl groups to enhance GPS binding to the surface. As evidence, the water contact angle of as-deposited ZnO decreased from ~73° to <5° following 10 min O2 plasma exposure. The authors suggested the surface hydroxyl groups form covalent bonds with the silicon in GPS silane, such that the epoxy group of GPS can then readily bind to the amine group of an incoming antibody.
The resulting sensor measured the difference in electrical resistance of an antigen-bound surface to that of an as-prepared surface, where the percentage change in electrical resistance (ΔR, %) indicates the antigen concentration bound to the surface. The measured ΔR was proportional to IL-6 and CRP antigen concentration. The device exhibited relatively fast incubation period, where ΔR stabilized within 10 min of analyte exposure, suggesting optimal immobilization times of 10 min for both antigens.
The successful demonstration of the ZnO biosensor in detecting IL-6 and CRP showed promise that other antigen specific devices may also be fabricated with the appropriate selection of functionalized layers and antibody to the target antigen to illicit appropriate antibody/antigen interactions.
zinc oxide thin film transistors
Meena et al. developed a solution-cast ZnO as the charge transport and semiconducting (channel) layer in TFTs fabricated on flexible polyimide (PI) substrates. Their work focused on applying low power O2 plasma to control oxygen vacancies in ZnO and improve charge carrier mobility and carrier concentration.
ZnO was spin-coated from a zinc acetate precursor solution followed by O2 plasma treatment at varying times (1-5 min) and subsequent thermal annealing at 250°C for 1 hour to passivate the surface. The plasma treatment removed organic impurities and created oxygen vacancies to further provide free charge carriers.
AFM imaging showed that plasma-treated ZnO films were relatively smooth (RMS surface roughness of 1-2 nm) with a continuous (crack-free) surface. Optical transmittance spectra showed that plasma-treated ZnO on PI substrates maintained 80-85% transmittance in the visible spectrum.
XPS spectra indicated a shift in native carbon C-C peak associated with the underlying PI substrate while FTIR spectra showed continual decrease and eventual absence of absorbance bands associated with volatile organic groups. Both spectra suggested that O2 plasma removed loosely bound carbon layers and organic impurities and, with that, the authors proposed the removal of acetate groups from the zinc acetate precursor and formation of ZnO.
Electrical measurements were performed on the TFTs with plasma-treated ZnO as the channel layer. From the drain current versus gate voltage ID-IG curves, the field effect carrier mobility and carrier concentration were extracted for varying plasma treatment times (Figure 2). Both mobility and carrier concentration peaked at 2 min plasma exposure and monotonically decreased with longer plasma treatment (up to 5 min).
The researchers proposed that short plasma exposure was associated with removing organic impurities (acetate precursor) and creation of excess oxygen vacancies to provide free electron for conduction in the ZnO channel layer, resulting in increased charge carrier mobility. They suggested the subsequent degradation in mobility and carrier concentration was due to a decrease in oxygen vacancies with additional plasma exposure. Reactive oxygen species from O2 plasma can fill oxygen vacancies, capturing free electrons and thereby decreasing the supply of charge carriers.
Their work concluded that short plasma treatments of up to 3 min were optimal to enhance ZnO electrical properties, demonstrating the feasibility of applying low-temperature fabrication techniques to develop TFTs on flexible substrates.
Figure 2. Charge carrier mobility and carrier concentration as a function of O2 plasma treatment time in plasma-treated ZnO-based thin film transistors on flexible polyimide substrate. Data from Meena JS, Chu M-C, Chang Y-C, You H-C, Singh R, Liu P-T, Shieh H-PD, Chang F-C and Ko F-H. J. Mater. Chem. C (2013) 1(40): 6613-6622. DOI: 10.1039/C3TC31320D
Relevant Articles from Harrick Plasma Users
- Li Z, Liu X, Zhou M, Zhang S, Cao S, Lei G, Lou C and Zhang J. “Plasma-induced oxygen vacancies enabled ultrathin ZnO films for highly sensitive detection of triethylamine.” J. Hazard. Mater. (2021) 415: 125757. DOI: 1016/j.jhazmat.2021.125757
- Perera GS, Ahmed T, Heiss L, Walia S, Bhaskaran M and Sriram S. “Rapid and Selective Biomarker Detection with Conductometric Sensors.” Small (2021) 17: 2005582:1-12. DOI: 1002/smll.202005582
- Meena JS, Chu M-C, Chang Y-C, You H-C, Singh R, Liu P-T, Shieh H-PD, Chang F-C and Ko F-H. “Effect of oxygen plasma on the surface states of ZnO films used to produce thin-film transistors on soft plastic sheets”. J. Mater. Chem. C (2013) 1(40): 6613-6622. DOI: 1039/C3TC31320D
Supplemental References (Do not report using Harrick Plasma Instruments)
 Kołodziejczak-Radzimska A and Jesionowski T. “Zinc Oxide – From Synthesis to Application: A Review.” Materials (2014) 7: 2833-2881.
 Janotti A and Van de Walle CG. “Fundamentals of zinc oxide as a semiconductor.” Rep. Prog. Phys. (2009) 72: 126501.
 Borysiewicz MA. “ZnO as a Functional Material, a Review” Crystals (2019) 9: 505.