Fabrication and characteristics of a surface acoustic wave UV sensor based on ZnO thin films grown on a polycrystalline 3C-SiC buffer layer

Zinc oxide (ZnO) thin films were deposited onto a polycrystalline (poly) 3C-SiC buffer layer for surface acoustic wave (SAW) ultraviolet (UV) sensing using a magnetron sputtering system. X-ray diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO film grown on 3C-SiC/Si had a dominant c-axis orientation, a lower residual stress, and higher intensity of luminescence at 380 nm of ZnO thin film. The SAW resonator UV detector were fabricated on ZnO/Si structures with a 3C-SiC buffer layer. The SAW resonator exposed under UV illumination had a linear response with sensitivity of 85 Hz/(μW/cm²) in ZnO/3C-SiC/Si structures, as compared to 25 Hz/(μW/cm²) in ZnO/Si structures with UV intensity varied until 600 μW/cm².     

Characteristics of SAW UV sensors based on a ZnO/Si structure using third harmonic mode

This paper describes the characteristics of surface acoustic wave (SAW) ultraviolet (UV) sensors fabricated from a ZnO thin film using the third harmonic mode. A ZnO thin film was used as an active layer for UV detection, and a piezoelectric layer was sputtered using magnetron sputtering. The X-ray diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO sputtered onto Si(100) was highly (002)-oriented and had good optical properties. The two-port SAW resonator was based on an inter-digital transducer (IDT)/ZnO/Si structure and was fabricated and exposed under UV light at a wavelength of 380 nm. As a result, under a UV intensity of 3 mW/cm², the SAW UV sensor was greatly shifted by 400 kHz at the third harmonic mode compared to a frequency shift of 10 kHz in the fundamental mode.     

The effect of UV irradiation on nanocrysatlline zinc oxide thin films related to gas sensing characteristics

The effect of ultraviolet (UV) light irradiation on the nanocrystalline ZnO thin films was investigated. The degree of crystallinity, electrical conductivity, optical properties and surface properties of ZnO thin films were measured as a function of UV irradiation time. It was found that the degree of crystallinity and electrical properties of ZnO films were affected by UV irradiation, however, no noticeable change in the surface morphology was observed. The gas sensing properties of as-deposited and UV irradiated films were also measured. It was observed that the gas sensing properties were affected by the UV irradiation. The irradiation time less than 5 min has improved the sensor while the irradiation time more than 5 min degraded the sensor characteristics for a UV power density of 2.45 W cm⁻².     

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W

Nanocrystalline ZnO thin films have been deposited on rhenium and tungsten pointed and flat substrates by pulsed laser deposition method. An emission current of 1 nA with an onset voltage of 120 V was observed repeatedly and maximum current density ∼1.3 A/cm² and 9.3 mA/cm² has been drawn from ZnO/Re and ZnO/W pointed emitters at an applied voltage of 12.8 and 14 kV, respectively. In case of planar emitters (ZnO deposited on flat substrates), the onset field required to draw 1 nA emission current is observed to be 0.87 and 1.2 V/μm for ZnO/Re and ZnO/W planar emitters, respectively. The Fowler–Nordheim plots of both the emitters show nonlinear behaviour, typical for a semiconducting field emitter. The field enhancement factor β is estimated to be ∼2.15×10⁵ cm⁻¹ and 2.16×10⁵ cm⁻¹ for pointed and 3.2×10⁴ and 1.74×10⁴ for planar ZnO/Re and ZnO/W emitters, respectively. The high value of β factor suggests that the emission is from the nanometric features of the emitter surface. The emission current–time plots exhibit good stability of emission current over a period of more than three hours. The post field emission surface morphology studies show no significant deterioration of the emitter surface indicating that the ZnO thin film has a very strong adherence to both the substrates and exhibits a remarkable structural stability against high-field-induced mechanical stresses and ion bombardment. The results reveal that PLD offers unprecedented advantages in fabricating the ZnO field emitters for practical applications in field-emission-based electron sources.     

Development of human IgE biosensor using Sezawa-mode SAW devices

This paper reports Sezawa-mode surface acoustic wave (SAW) devices with via-isolated cavity to construct the allergy biosensor. To fabricate Sezawa-mode SAW devices, the RF magnetron sputtering method for the growth of piezoelectric ZnO thin films are adopted and influences of the sputtering parameters are investigated. The optimal substrate temperature of 300 °C, RF power of 120 W and sputtering pressure of 2 Pa were used to deposit piezoelectric ZnO films with a smooth surface, uniform grain size and strongly c-axis-orientated crystallization. A back-etched SAW resonator is used in this study. The wet etching of (100)-oriented silicon wafers is used to form a back-side cavity which is critical to the formation of a hopper cavity for holding bio-analytes. The remaining membrane structure silicon thickness was 25 μm. In this report, the chrome (Cr, 12 nm)/gold (Au, 66 nm) layer was initially deposited onto the sensing area of SAW devices as the binding layer for biochemical sensor. The resonance frequency of the Sezawa-mode SAW device is 1.497 GHz. The maximum sensitivity of the Sezawa-mode is calculated to be 4.44 × 10⁶ cm²/g for human immunoglobulin-E (IgE) detection. The stability for human IgE detection is calculated to be 80% and the variation of the stability ±3% was obtained after several tests.     

Effect of excimer laser annealing on the properties of ZnO thin film prepared by sol-gel method

Pristine ZnO thin films have been deposited with zinc acetate [Zn(CH₃COO)₂], mono-ethanolamine (stabilizer), and isopropanol solutions by sol-gel method. After deposition, pristine ZnO thin films have been irradiated by excimer laser (λ = 248, KrF) source with energy density of 50 mJ/cm² for 30 sec. The effect of excimer laser annealing on the optical and structural properties of ZnO thin films are investigated by photoluminescence and field emission scanning electron microscope. As-grown ZnO thin films show a huge peak of visible region and a wide full width at half maximum (FWHM) of UV region due to low quality with amorphous ZnO thin films. After KrF excimer laser annealing, ZnO thin films show intense near-band-edge (NBE) emission and weak deep-level emission. The optically improved pristine ZnO thin films have demonstrated that excimer laser annealing is novel treatment process at room temperature.     

ZnO thin film with nanorod arrays applied to fluid sensor

A ZnO guiding layer with nanorod arrays grown on a 90°-rotated ST-cut (42°45) quartz substrate was used to fabricate a Love wave fluid sensor. ZnO nanorod arrays synthesized on the guiding layer enhance the sensitivity of the flow rate. ZnO thin films were deposited by radio frequency magnetron sputtering and ZnO nanorod arrays were then synthesized on the thin films via the hydrothermal method. The crystalline structure and surface morphology of ZnO thin films and nanorod arrays were examined by X-ray diffraction and scanning electron microscopy. The effects of the thickness of ZnO thin film and the surface morphology of ZnO nanorod arrays on the sensitivity of flow rate were investigated. A linear response between flow rate and the return loss of the sensor with one-port resonator type can be obtained by adjusting the thickness of ZnO thin film and the length of nanorod arrays.     

Sensitivity enhancement of AZO-based ethanol sensor decorated by Au nano-islands

Detecting the hazardous gases for monitoring air pollution and medical diagnosis make highly sensitive gas sensors appeal to many researches. In this paper, benefiting from unique properties of noble metals, Al-doped ZnO based Ethanol sensors were fabricated and characterized in three structures including Al: ZnO thin film, Silver and Gold nano-islands on Al: ZnO thin film. The Silver and Gold thin films turn to nano-islands after a simple annealing process. The XRD analysis of the sputtered Al: ZnO layer indicates the wurtzite crystal structure of the layer with a peak at (002) plane. Moreover, the sensitivity study reveals that Nano-islands of noble metals substantially affects the sensitivity of the sensors. The decorated Gold nano-island Al: ZnO Ethanol sensor has the highest response showing an amount of 45. The response of Al: ZnO and Silver decorated Al: ZnO sensors are virtually identical to all concentrations of Ethanol, whereas the Al: ZnO gas sensor with Gold nano-islands has the substantial sensitivity for different concentrations. In addition, the response times of the sensors are 85, 70 and 90 s for Al: ZnO, Al: ZnO with Ag islands and Al: ZnO decorated by Au islands, respectively. The recovery time of Al: ZnO sensor decorated by Au islands is about 23s, while the recovery time of the Al: ZnO and Al: ZnO decorated by Silver islands are 360 and 370s, respectively. Hence, the simple annealing process on the sputtered gas sensor with a thin layer of Gold makes nano-islands on the sensor which elevates the performance of Ethanol sensing due to the high sensitivity and sensitivity of the sensor.     

Determination of electrical types in the P-doped ZnO thin films by the control of ambient gas flow

Phosphorus (P)-doped ZnO thin films with amphoteric doping behavior were grown on c-sapphire substrates by radio frequency magnetron sputtering with various argon/oxygen gas ratios. Control of the electrical types in the P-doped ZnO films was achieved by varying the gas ratio without post-annealing. The P-doped ZnO films grown at a argon/oxygen ratio of 3/1 showed p-type conductivity with a hole concentration and hole mobility of 1.5 × 10¹⁷ cm⁻³ and 2.5 cm²/V s, respectively. X-ray diffraction showed that the ZnO (0 0 0 2) peak shifted to lower angle due to the positioning of P³⁻ ions with a larger ionic radius in the O²⁻ sites. This indicates that a p-type mechanism was due to the substitutional P_O. The low-temperature photoluminescence of the p-type ZnO films showed p-type related neutral acceptor-bound exciton emission. The p-ZnO/n-Si heterojunction light emitting diode showed typical rectification behavior, which confirmed the p-type characteristics of the ZnO films in the as-deposited status, despite the deep-level related electroluminescence emission.     

Gas sensing properties of single crystalline ZnO nanowires grown by thermal evaporation technique

The ZnO NWs were applied as effective material for the fabrication of ethanol (C₂H₅OH) and carbon monoxide (CO) gas sensor. The ZnO NWs were grown by thermal evaporation techniques on non-catalytic Si (100) substrates. The average width and length of ZnO NWs was 60 nm and 20 μm, respectively and they were single crystalline in nature. The maximum response was 51.64 at 300 °C for 1000 ppm of CO gas, while 104.23 at 400 °C for 250 ppm of ethanol gas. The response of ZnO NWs was very high for ethanol compared to the CO, whereas the recovery time for ethanol was very poor compare to CO gas. The response of ZnO NWs was about 25 times higher for ethanol compare to CO, at 400 °C for 100 ppm of each gas. The high response for ethanol is related to electron donating effect of ethanol (10e^?) which was higher than the CO gas (2e^?). The high response of ZnO NWs was attributed to large contacting surface area for electrons, oxygen, target gas molecule, and abundant channels for gas diffusion.     

Magnetoelectrical properties of W doped ZnO thin films

ZnO thin films were deposited on the Si(100) substrate by rf sputtering using a 99.999% pure commercially bought and a home made target under 100 W power. The home made ZnO target, including 1–2% tungsten, was synthesized via solid state reaction. Thin films were deposited under a flow of 70% argon and 30% O₂ gas mixture followed by post-deposition annealing under 1 Torr oxygen atmosphere. Both deposition and post-deposition annealing were done at 420±1 °C. The structural analyses show that the films were in the [0002] preferred direction and that W atoms are bound to the oxygen atoms by replacing the Zn host atoms. Although no specific change was observed in the magnetic properties as a result of W doping, significant changes in the electrical properties were observed, as determined by the longitudinal and transversal magneto-electrical measurements. It was found that the W impurities induce better insulating properties due to lower carrier concentration and higher resistivity values. On the other hand, the enhanced positive magnetoresistivity and the existence of polarized spin currents, which were not specific for pure ZnO thin films, were observed in W doped ZnO films below 10 K.     

Ultrasound-assisted synthesis of 3D flower-like zinc oxide decorated fMWCNTs for sensitive detection of toxic environmental pollutant 4-nitrophenol

Sonochemical synthesis of functionalized multi-walled carbon nanotubes (fMWCNTs) embellished 3D flower-like zinc oxide (ZnO) nanocomposite based novel electrochemical sensor for the detection of toxic environmental pollutant 4-nitrophenol (4-NP) is detailed in this paper. We have used laser-assisted synthesis technique in the development of 3D flower-like ZnO nanoparticles (NPs) and ultrasonication method was employed in preparation of ZnO NPs@fMWCNTs nanocomposite using a high-intensity ultrasonic bath DC200H with power of 200 W/cm² and 40 KHz frequency. The nanocomposite was meticulously fabricated on screen printed carbon electrode (SPCE) to carry out various electrochemical analysis. Different characterizations such as Raman spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, UV visible spectroscopy (UV–Vis), X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) of the materials used in this work were taken. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques are used in electrochemical investigations. We have observed well-defined oxidation and reduction peak currents representing electrochemical mechanism of 4-NP at very low potentials for ZnO NPs@fMWCNTs/SPCE. Furthermore, we were able to achieve efficient electrochemical determination of 4-NP using the developed sensor with a high sensitivity of 11.44 μA μM⁻¹ cm⁻² and very low detection limit (LOD) of 0.013 μM in a broad linear range of 0.06–100 μM. All the significant features of a good sensor including anti-interference, good stability, excellent repeatability, and reproducibility were exhibited by the sensor. Moreover, we have tested practical feasibility of sensor by carrying out real sample analysis on different water samples.     

Effects of oxygen pressures on pulsed laser deposition of ZnO films

Zinc oxide (ZnO) thin films on Si (1 1 1) substrates were deposited by pulsed laser ablation of ZnO target at different oxygen pressures. A pulsed Nd:YAG laser with wavelength of 1064 nm was used as laser source. The deposited thin films have been characterized by X-ray diffraction (XRD), Atomic force microscopy (AFM), and Raman spectroscopy. XRD measurements indicate that the ZnO thin films deposited at the oxygen pressure of 1.3 Pa have the best crystalline quality. AFM results show that the surface roughness of ZnO film increases with the increase of oxygen pressure. The Raman results indicate that oxygen ambient plays an important role in removing defects due to excess zinc.     

Influence of the active layer thickness on the electrical properties of ZnO thin film transistors fabricated by radio frequency magnetron sputtering

We report on the electrical properties of bottom-gate ZnO thin film transistors (TFTs) with different active layer thicknesses. The ZnO active layer films with thickness varied from 20 to 100 nm were deposited by radio frequency (rf) magnetron sputtering on SiO₂/p-Si substrate and annealed at a high temperature of 950 °C. The transistor with 40 nm thick ZnO exhibited the best performance, with a field effect mobility of 27.5 cm²/V s, a threshold voltage of −2.4 V and an on/off ratio of 7×10³.     

Characterization of magnesium oxide gate insulators grown using RF sputtering for ZnO thin-film transistors

A ZnO thin-film transistor (TFT) with an MgO insulator was fabricated on a silicon (100) substrate using a radiofrequency magnetron sputtering system. The MgO insulator was deposited using the same deposition system; the total pressure during the deposition process was maintained at 5 mTorr, and the oxygen percentage of O₂/(Ar + O₂) was set at 30%, 50%, or 70%. The process temperature was maintained at below 300 °C. The dielectric constant of the MgO thin layer was approximately 11.35 with an oxygen percentage of 70%. This ZnO TFT displayed enhanced transistor properties, with a field-effect mobility of 0.0235 cm² V⁻¹ s⁻¹, an I_ON/I_OFF ratio of ∼10⁵, and an SS value of 1.18 V decade⁻¹; these properties were superior to those measured for the MgO insulators synthesized using oxygen percentages of 30% and 50%.     

Influence of YSZ electrolyte thickness on the characteristics of plasma-sprayed cermet supported tubular SOFC

Novel Ni–Al₂O₃ cermet-supported tubular SOFC cell was fabricated by thermal spraying. Flame-sprayed Al₂O₃–Ni cermet coating played dual roles of a support tube and an anode current collector. Y₂O₃-stabilized ZrO₂ (YSZ) electrolyte was deposited by atmospheric plasma spraying (APS) to aim at reducing manufacturing cost. The gas tightness of APS YSZ coating was achieved by post-densification process. The influence of YSZ coating thickness on the performance of SOFC test cell was investigated in order to optimize YSZ thickness in terms of open circuit voltage of the cell and YSZ ohmic loss. It was found that the reduction of YSZ thickness from 100 μm to 40 μm led to the increase of the maximum output power density from 0.47 W/cm² to 0.76 W/cm² at 1000 °C. Using an APS 4.5YSZ coating of about 40 μm as the electrolyte, the test cell presented a maximum power output density of over 0.88 W/cm² at 1030 °C. The results indicate that SOFCs with thin YSZ electrolyte require more effective cathode and anode to improve performance.     

Fine luminescent patterning on ZnO nanowires and films using focused electron-beam irradiation

ZnO thin films and nanowires (NWs) were precisely treated by focused electron-beam (E-beam) irradiation with a line width between 200 nm and 3 μm. For both ZnO films and NWs, an increased green emission was clearly observed for the E-beam-treated parts. Using a high-resolution laser confocal microscope, the photoluminescence intensities for E-beam-treated ZnO structures increased with increasing dose 1.0 × 10¹⁷–1.0 × 10¹⁸ electrons/cm². The resistivity of a single ZnO NW increased from 56 to 1800 Ω cm after the E-beam treatment. From the results for the annealed ZnO thin films, we analyzed that the variations in PL and resistivity were due to the formation of vacancies upon focused E-beam irradiation.     

Substitution mechanism of Ga for Zn site depending on deposition temperature for transparent conducting oxides

High quality transparent conductive gallium-doped zinc oxide (GZO) thin films were deposited on glass substrates using rf-magnetron sputtering system at the temperature ranging from room temperature (RT) to 500 °C. The temperature-dependence of Ga doping effect on the structural, optical and electrical properties in ZnO has been investigated. For the GZO thin films deposited at over 200 °C, (103) orientation was strongly observed by X-ray diffraction analysis, which is attributed to the substitution of Ga elements into Zn site. X-ray photoelectron spectroscopy measurements have confirmed that oxygen vacancies were generated at the temperature higher than 300 °C. This might be due to the effective substitution of Ga³⁺ for Zn site at higher temperature. It was also found that the optical band gap increases with deposition temperature. The optical transmittance of GZO thin films was above 87% in the visible region. The GZO thin films grown at 500 °C showed a low electrical resistivity of 4.50 × 10^?4 Ω cm, a carrier concentration of 6.38 × 10²⁰ cm^?3 and a carrier mobility of 21.69 cm²/V.     

Electrochemical evaluation of Ta2O5 thin film for all-solid-state switchable mirror glass

We investigated the electrochemical property of Ta₂O₅ thin film for all-solid-state switchable mirror glass. The film was deposited by reactive dc magnetron sputtering in a mixture gas of argon and oxygen. The current density of the film covered WO₃/ITO/glass was decreased with decreasing argon/oxygen ratio and working pressure measured by cyclic voltammetry. The film deposited at argon/oxygen ratio of 4.7 and working pressure of 1.0 Pa had better electrochemical property than that of other deposition condition. Its estimated proton conductivity was 2.1 × 10^? 9 S/cm by conventional ac impedance method. However, the device using the film showed poor optical switching property. The transmittance change of the device at a wavelength of 670 nm was only 16% by applying voltage. On the other hand, the device using the film deposited at working pressure of 0.7 Pa was able to switch its optical switching property from reflective of 0.1% to transparent states of 44% within 15 s. These results indicate that the suitable deposition condition of the Ta₂O₅ thin film existed to be used for all-solid-state switchable mirror glass.     

Optimization of parameters for deposition of Ga-doped ZnO films by DC reactive magnetron sputtering using Taguchi method

Ga-doped ZnO (ZnO:Ga) transparent conductive films were deposited on glass substrates by DC reactive magnetron sputtering. Taguchi method was used to find the optimal deposition parameters including oxygen partial pressure, argon partial pressure, substrate temperature, and sputtering power. By employing the analysis of variance, we found that the oxygen and argon partial pressures were the most influencing parameters on the electrical properties of ZnO:Ga films. Under the optimized deposition conditions, the ZnO:Ga films showed acceptable crystal quality, lowest electrical resistivity of 2.61 × 10⁻⁴ Ω cm, and high transmittance of 90% in the visible region.