Sputtered ZnO Thin Films Research Articles

The Influence of Annealing Temperature on the Microstructure and Electrical Properties of Sputtered ZnO Thin Films

The Influence of Annealing Temperature on the Microstructure and Electrical Properties of Sputtered ZnO Thin Films

Polydopamine-mediated facile Silver grown on ZnO Thin Films as High Performance SERS Substrates for R6G Detection

Surface-enhanced Raman spectroscopy (SERS) is a widely known technique that uses plasmonic structures (silver, gold, etc.) to detect low-concentration molecules. However, the limited number of metallic elements with plasmonic properties leads to limitations in their application. The qualitative detection method of SERS holds considerable promise in providing novel platforms for diverse applications, owing to its utilization of hybrid structures. Mussel-inspired polydopamine offers a promising avenue for the fabrication and integration of hybrid structures suitable for the SERS platform. Here, sputtered ZnO thin films were modified with Ag Nanostructures using polydopamine to fabricate a homogeneous Ag@ZnO hybrid high-performance SERS substrate. Ag/ZnO hybrid nanostructures were analyzed by FESEM and XRD to investigate their morphological and structural characterizations. SERS measurements were performed for all silver growth times to understand the effect of the silver growth process and hybrid structure synergy on SERS performance. The Ag@ZnO hybrid structure, cultivated with 24-hour silver growth, exhibited remarkable detectability even at an ultra-low R6G concentration of 10 pM

Discriminative analysis of volatile organic compounds using machine-learning assisted Au loaded ZnO and TiO2-based thin film sensors

Noble metal-based particulate catalysts have traditionally been loaded or decorated onto the semiconductor metal oxide (SMO) surfaces to enhance the performance of gas sensors. Fabricating SMO-based sensors for selective detection of particular gases with high sensitivity remains a challenge. This study aims to enhance the efficacy of pure SMO and their heterostructure with gold (Au) decoration, along with an attempt to address the cross-sensitivity towards other gases through machine learning-assisted discriminative analysis. The sensitivity of sputtered ZnO thin films towards different VOCs such as acetone, ethanol, methanol, and toluene was improved by loading particulate Au. The highest sensitivity of 25 and low response time of ∼6 s and ∼4 s were achieved towards 10 ppm of ethanol and toluene, respectively. Further, Au-loaded ZnTiO3-TiO2 (ZTO) layered heterostructure was also adopted to achieve selectivity towards toluene. ZTO thin film sensor was highly sensitive towards 10 ppm of toluene, with a sensitivity of 49 and a response time of ∼6 s. Loading Au on ZnO thin film increases sensitivity towards multiple VOCs, whereas Au on ZTO heterostructure improves the selectivity towards toluene. To overcome the problem arising due to the cross-sensitivity, the idea of operating multiple sensors with different Au loading is proposed. The discriminative property of different sensors as an array, towards VOCs was analysed in detail using machine learning algorithms such as principal component analysis (PCA), linear discriminant analysis (LDA), logistic regression (LR), and artificial neural networks (ANN). The multilayer perceptron model implemented to discriminate multiple VOCs worked at a discrimination accuracy of 100 %.

Exploring the synergistic effects of aluminum and hydrogen impurities on high-electron-mobility sputtered-ZnO thin films

This study investigated the influence of aluminum (Al) impurity on the lattice structure, electrical, and optical properties of ZnO thin films, which were sputtered under a high vacuum environment using argon and hydrogen gases. The results revealed high electron mobility (μ) alongside a significant increase in carrier concentration (n), which deviates from the behavior typically observed in semiconductor materials. Particularly in conventional semiconductors, within the n range of 1020 cm−3, the μ exhibits a dramatic decrease with even a small increase in n. Interestingly, in this study, despite the increase in n from 2 to 3.8 × 1020 cm−3, the μ remains consistently high at around 53 cm2/Vs. Additionally, this study reveals that at an appropriate concentration, Al serves not only as a substitute for Zn to generate electrons but also as a defect passivator, similar to hydrogen. The synergistic effect of Al and hydrogen also relaxes the crystal structure, minimizing deformation and enhancing electron mobility. These effects contribute to low resistivity alongside the inherent high optical transmittance of ZnO thin films, resulting in a significantly improved figure of merit.

Annealing Effects of ZnO Thin Film on Photocatalytic Performances of Graphene Composites

The hybrid structure of Graphene and ZnO (Graphene/ZnO) is emerging as a novel material used to achieve the high performance of photocatalysis. In this study, we examined the ZnO characteristics that affect the photocatalytic activity of graphene/ZnO using a lamellar structure of graphene and ZnO thin films. Graphene samples were synthesized via chemical vapor deposition, and a typical wet process was applied to transfer them on sputter-deposited ZnO thin films with and without annealing. We confirmed that graphene-deposited ZnO demonstrated more efficient photocatalytic behavior toward the decomposition of methylene blue (as a model organic compound) with ordinary sputtered ZnO thin films. Again, ZnO thin films annealed at 1000 °C in an N2 gas atmosphere with graphene performed better than unannealed films. XRD analysis confirmed that pre-thermal treatment of a ZnO thin film promoted re-crystallization, which had less impact on the photocatalytic improvement. The attachment of graphene to the film is considered to contribute to the enhancement. Raman analysis revealed that the graphene coverage areas on the post-annealed ZnO increased by two times compared to that of an unannealed film where the unannealed film had a higher graphene layer. The results presented in this study demonstrate that an annealed ZnO thin film forms a better attachment with graphene, resulting in a larger graphene coverage area with fewer multilayers, which effectively improves the photocatalytic activity in composite structures.

Magnetron configurations dependent room temperature sputtered ZnO thin films for highly responsive, stable, and selective CO gas sensing

This letter has reported the impact of balanced and unbalanced magnetron sputtering on the CO gas sensing properties of ZnO thin films. It was noticed that the ZnO film deposited under a balanced magnetron (i.e., ZnO-BM) has superior sensing properties compared to the film deposited under an unbalanced magnetron configuration (i.e., ZnO-UBM). The optimum operating temperature (Topt) was determined as 250 °C for the ZnO-BM sample with a sensor response of ∼ 85.5 % and a faster response/recovery time (∼ 28 s/55 s). Further, the ZnO-BM sample demonstrated a notable response to even 1 ppm CO gas at Topt and exhibited a detection limit of 69 ppb. Also, it displayed remarkable selectivity and stability towards CO gas, which makes it a preferable choice for an energy-efficient CO gas sensor.

Exploring the effects of substrate and substrate temperature on the properties of radio frequency magnetron sputtered ZnO thin films

The physical and optical properties of zinc oxide thin films strongly depend on the film deposition techniques. In this work, RF magnetron sputtering was used to deposit ZnO thin films on various substrates (glass, quartz, ITO-coated glass, FTO-coated glass, Si(100), Si(100)/ SiO2) to study the role of substrate and substrate temperature on the morphological, structural and optical properties of the film grown. A detailed discussion on various aspects like surface morphology, strain, photoluminescence, bandgap, surface roughness, and material composition has been done. Compressive stress was present in the films created at room temperature and differed for various substrates based on the lattice mismatch. The nature and temperature of the substrate are found to affect the surface topography of films. Substrate and substrate temperature influenced the emission characteristics of ZnO thin films. In oxygen-rich substrates, defect emissions differed. We found that quartz is a better substrate to fabricate ZnO at room temperature and both Si and Si/SiO2 are preferable at high temperatures due to the lower lattice mismatch and thermal expansion coefficient. Comparing the electrical characteristics of ZnO thin films grown on glass and quartz substrates, ZnO on glass exhibited lower resistivity and higher carrier concentration. The Triboelectric nanogenerator (TENG) combining ZnO and PDMS, was developed to operate in vertical contact-separation mode. The obtained open circuit voltage was 30 V and 33 V for ZnO grown over ITO and FTO respectively, and the slight increase is due to the rougher film's surface on FTO than ITO. The selection of substrate is crucial in many applications and our findings provide an overview of the suitability of substrate and substrate temperature for the fabrication of ZnO thin film-based optoelectronic devices.

Fabrication and characterization of ultra-thin ZnO based bottom gate thin film transistor for UV detection

An RF sputtered ZnO thin film was utilized in a thin film transistor (TFT) fabrication. The film was fabricated on SiO2 gate oxide that was thermally grown on Si Substrate. Few lithography steps were used to fabricate the device and device structure was proved to be a bottom gate TFT. The grown film was ultra thin of 25 nm thickness and the Si wafer was n-type. The structural and optical properties of ZnO films were investigated by Field Emission Scanning Electron Microscopy (FESEM), X-Ray Diffraction (XRD) and Photo luminance (PL) spectroscopy. The photo luminance study confirmed a high-quality absorbance in UV spectra and relatively good transmittance in the visible region. The electrical performance of the transistor was studied using Proxima B1500 and the on/off current ratio was measured to be ∼107. The ZnO bottom gate TFT is simulated using the commercial TCAD tool Silvaco ATLAS™. Later the simulated I–V characteristic of the device is compared with the experimental one and both are in fairly good agreement. Further the UV detection capability of the fabricated device was studied by using UV LED light of 365 nm wavelength. The measured value of threshold voltage was found to be 9V under dark light and 10V under UV light.

Effect of substrate temperature and RF power on the structural and optical properties of sputtered ZnO thin films

ZnO thin films have been deposited on glass substrates by radio frequency (RF) magnetron sputtering from a zinc oxide target in order to investigate the effect of RF power and substrate temperature on the properties of the deposited films. The structural and optical properties of the films were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV-Vis transmission spectra and photoluminescence (PL). All ZnO thin films exhibited diffraction peak of (002) corresponding to c-axis orientation and the film deposited at 450°C exhibited the larger grain size as a result of stress relaxation. It is observed that the increase in substrate temperature or sputtering power can facilitate the growth of ZnO in (100) and (101) direction. PL emission was obtained at UV and visible region for the excitation wavelengths of 280 nm and 390 nm respectively. PL study indicated that both the crystal quality and stoichiometry can influence the UV PL emission in ZnO thin films. Dependence of temperature and power on transmission spectra was studied and the optical band gap was calculated.

Structural, Optical, and Electrical Properties of Sputtered ZnO Thin Films and Their Application in Transparent Memory Devices

Structural, Optical, and Electrical Properties of Sputtered ZnO Thin Films and Their Application in Transparent Memory Devices

Substrate dependent structural variations of biomimetic carbonated hydroxyapatite deposited on glass, Ti and sputtered ZnO thin films

The formation of hydroxyapatite films on glass, titanium (Ti), and sputtered ZnO is attempted to understand the crystal growth and its manipulation by substrates of a different kind. The substrates used are commercially available glass and polished Ti (99.9% purity), as-deposited, and annealed nanostructured ZnO thin films developed by pulsed DC magnetron sputtering, which are immersed in the calcifying solution for the nucleation and growth of the hydroxyapatite. The calcifying solution is obtained by mixing equal volumes of aqueous calcium (5 mM) and carbonate-containing phosphate (3 mM) solutions prepared at 37 °C and with a pH of ~7.4. X-ray diffraction, Raman spectroscopy, and Field Emission Scanning & Transmission Electron Microscopy (FESEM & FETEM) analyses reveal that the structure and morphology of the hydroxyapatite films are dependent on the growth duration, substrate type, and even on the post-deposition annealing of the sputtered ZnO substrates. The Ti substrate induces greater crystallinity in hydroxyapatite, whereas sputtered ZnO thin films cause a significantly reduced crystallinity. Moreover, the thicknesses of HAp are exacerbated by increasing deposition time irrespective of the substrate. FETEM analyses reveal the formation of needle-shaped hydroxyapatite nanocrystals grown on Ti and ZnO thin films. Thicker needle-shaped hydroxyapatite nanocrystals are observed when grown on Ti (~6–12 nm) than ZnO thin films (~4–6 nm). Further, the amorphous calcium phosphate phase coexists with nanocrystalline hydroxyapatite when ZnO thin films deposited at 50 and 70 W sputtering powers are used as the substrates. Both charged surfaces and the supersaturation of the calcifying solution impact the nucleation of hydroxyapatite onto substrates.

Publisher’s Note: “Band-edge emission enhancement in sputtered ZnO thin films with ultraviolet surface lattice resonances” [J. Appl. Phys. 130, 223103 (2021)

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A Study on the ZnO Thin Film Deposited by RF Sputtering Method as an Electron Transport Layer in Quantum Dot Light-Emitting Diodes

We report a highly efficient quantum dot light emitting diode (QLEDs) with a radio frequency (RF) sputtered ZnO thin film as an electron transport layer (ETL) instead of the conventional ZnO nanoparticles (NPs) by solution process. ZnO NPs have been used as a key material to improve the performance of QLEDs, but the charge imbalance in ZnO NPs resulting from fast electron injection, and their limited uniformity are significant disadvantages. In this study, ZnO layers were deposited by RF sputtering with various O2 partial pressures. All of the ZnO films showed preferential growth along the (002) direction, smooth morphology, and good optical transmittance. To test their feasibility for QLEDs, we fabricated devices with RF sputtered ZnO layers as an ETL, which has the inverted structure of ITO/RF sputtered ZnO/QDs/CBP/MoO3/Al. The optical/electrical characteristics of two devices, comprised of RF sputtered ZnO and ZnO NPs, were compared with each other. QLEDs with the sputtered ZnO ETL achieved a current efficiency of 11.32 cd/A, which was higher than the 8.23 cd/A of the QLEDs with ZnO NPs ETL. Next, to find the optimum ZnO thin film for highly efficient QLEDs, deposition conditions with various O2 partial pressures were tested, and device performance was investigated. The maximum current efficiency was 13.33 cd/A when the ratio of Ar/O2 was 4:3. Additional oxygen gas reduced the O vacancies in the ZnO thin film, which resulted in a decrease in electrical conductivity, thereby improving charge balance in the emission layer of the QLEDs. As a result, we provide a way to control the ZnO ETL properties and to improve device performance by controlling O2 partial pressure.

A Study on the ZnO Thin Film Deposited by RF Sputtering Method as an Electron Transport Layer in Quantum Dot Light-Emitting Diodes

We report a highly efficient quantum dot light emitting diode (QLEDs) with a radio frequency (RF) sputtered ZnO thin film as an electron transport layer (ETL) instead of the conventional ZnO nanoparticles (NPs) by solution process. ZnO NPs have been used as a key material to improve the performance of QLEDs, but the charge imbalance in ZnO NPs resulting from fast electron injection, and their limited uniformity are significant disadvantages. In this study, ZnO layers were deposited by RF sputtering with various O2 partial pressures. All of the ZnO films showed preferential growth along the (002) direction, smooth morphology, and good optical transmittance. To test their feasibility for QLEDs, we fabricated devices with RF sputtered ZnO layers as an ETL, which has the inverted structure of ITO/RF sputtered ZnO/QDs/CBP/MoO3/Al. The optical/electrical characteristics of two devices, comprised of RF sputtered ZnO and ZnO NPs, were compared with each other. QLEDs with the sputtered ZnO ETL achieved a current efficiency of 11.32 cd/A, which was higher than the 8.23 cd/A of the QLEDs with ZnO NPs ETL. Next, to find the optimum ZnO thin film for highly efficient QLEDs, deposition conditions with various O2 partial pressures were tested, and device performance was investigated. The maximum current efficiency was 13.33 cd/A when the ratio of Ar/O2 was 4:3. Additional oxygen gas reduced the O vacancies in the ZnO thin film, which resulted in a decrease in electrical conductivity, thereby improving charge balance in the emission layer of the QLEDs. As a result, we provide a way to control the ZnO ETL properties and to improve device performance by controlling O2 partial pressure.

Plastic deformation of ZnO thin films through edge and screw dislocation movements

AbstractColumnar wurtzite grains were formed in sputtered ZnO thin films deposited on a plastic polyethylene terephthalate substrate. Selected‐area diffraction patterns reveal that the columnar grains in the sputtered films present two preferred growth planes, namely, the basal (0002) and prismatic (100) growth planes. The diffraction patterns obtained also confirm that the microstructure of sputtered indium tin oxide thin films is amorphous in nature. Tensile tests indicate that the fracture strain of the ZnO thin film occurs between 1.73% and 2.14%, while the fracture strain of the indium tin oxide thin film occurs between 0.24% and 0.67%. Thus, the fracture toughness of the sputtered ZnO thin film is greater than that of the sputtered indium tin oxide thin film. High‐resolution transmission electron microscopic images demonstrate that edge and screw dislocations could be identified in the sputtered ZnO thin films. Moreover, edge and screw dislocation movements may, respectively, be observed in the basal‐ and prismatic‐oriented ZnO columnar grains of the sputtered ZnO thin films. Our results indicate that movements of the edge and screw dislocations in the basal‐ and prismatic‐oriented ZnO columnar grains account for the plastic deformation of the investigated ZnO thin films under tensile stress.

High optoelectronic performance of magnetron sputtered ZnO thin films co-doped with F and Ga: Experiment and first-principles calculations

In this study, F-and-Ga co-doped ZnO (FGZO) films were deposited on glass substrates by radio frequency (RF) magnetron sputtering and the effect of film thickness on the structural, morphological, electrical, optical, and chemical properties was systematically investigated. A minimum resistivity of 5.95 × 10−4 Ωcm, carrier concentration of 2.39 × 1020 cm-3, and mobility of 44.0 cm2/Vs were obtained at the film thickness of 1450.4 nm. The transmittance of FGZO films is close to that of FTO and ITO films in the optical range of 400−1000 nm. After 70 s’ etching with diluted HCl, the haze values at 550 nm, 800 nm, and 1100 nm are 82.3 %, 48.2 %, and 22.2 %, respectively. After co-doping with F and Ga, the electron density of states at the Fermi level increased significantly, and the free carriers were mainly attributed to the hybridized F 2p, Ga 4p, Ga 4 s, O 2p, and Zn 4 s electronic orbitals.

Phosphorus doping of ZnO using spin‐on dopant process: A better choice than costly and destructive ion-implantation technique

Radio frequency sputtered ZnO thin films doped with phosphorus (ZnO:P) have been prepared employing spin-on dopant process. In the SOD process, the dopant film has been spin-coated on a silicon substrate and positioned close to the as-deposited undoped ZnO film at high temperature to perform the phosphorus doping. The high-resolution X-ray diffraction measurement reveals that the prepared ZnO:P films are good in crystalline quality which improves further by annealing. It is found that the full-width half-maximum corresponding to (002) peak of SOD processed thin films is much narrower than previously reported ion-implanted thin films, indicating the better crystalline quality of SOD processed phosphorus-doped ZnO thin films. The X-ray photoelectron spectroscopy measurement signifies that the P2O5 decomposes into two phosphorus atoms behaving like an acceptor dopant and five oxygen atoms which may fill in oxygen vacancies at high-temperature annealing. The photoluminescence spectra discover the acceptor bound exciton peak at 3.35 eV and free electron to acceptor level transitions at 3.31 eV. The calculated acceptor binding energy is 127 meV for the phosphorus dopant which works as a shallow acceptor level. It is found that the phosphorus-doped ZnO thin films prepared using the SOD process have much superior structural and optical properties in comparison to previously reported ion-implanted film. This study demonstrates that the SOD process is much superior than the ion-implantation process to produce high-quality ZnO:P thin films for very stable p-type conduction.

Zinc Oxide Thin-Film Transistor with Catalytic Electrodes for Hydrogen Sensing at Room Temperature

Palladium-Titanium (Pd-Ti)/n-ZnO Schottky contact based thin-film transistor (TFT) was fabricated on a thermally oxidized n-Si substrate with a compatible fabrication process and the hydrogen-sensing response was evaluated under room temperature (RT) without the presence of any heating element which reduces design complexity as well as power consumption significantly. Structural, mechanical, chemical and optical properties of the RF (Radio Frequency) sputtered ZnO thin film with 150 nm thickness as a gas sensing element have been investigated. Electrical characteristics and concentration dependent (500 ppm to 4500 ppm) RT sensor response (in the context of increasing drain current as the Schottky barrier height decreases) of the fabricated device with the maximum sensitivity of 70.8% (at 4500 ppm H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) including a minimum response time of 20 s (at 4500 ppm H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) and recovery time of 40 s (at 500 ppm H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) have been demonstrated. Sufficiently impressive performance of the sensor has been observed with respect to repeatability, selectivity, time-depending stability and humidity analysis at RT.

Development of porous ZnO thin films for enhancing piezoelectric nanogenerators and force sensors

The applications of piezoelectric nanogenerators (PENGs) as a sustainable power source for portable electronic devices are still limited due to low voltage output ranging from mini- to several volts. In this study, we first develop a porous ZnO structure by annealing (650–950 °C) sputtered ZnO thin film. The porous ZnO thin film enables a significant enhancement in the output voltage of primitive PENG (up to 3 mV), i.e., by 7.5 times than that of pristine ZnO (0.4 mV). Concurrently, the introduction of pores in ZnO thin films leads to a 6.9-fold enhancement in the sensitivity of piezotronic force sensor compared to the pristine one. These excellent performances are further supported by up to two-fold enhancement in the piezoelectric coefficient (2.62 pm/V) than that of pristine ZnO (1.12 pm/V). Herein, we conducted transmission electron microscopy characterizations and found that this phenomenon can be ascribed to the lattice contraction in the c-direction induced by the pores. The diffusion-controlled pore formation mechanism is elucidated. • Investigation of porous ZnO thin film based piezotronic pressure sensorand piezoelectric nanogenerator. • Large boost in sensitivity of pressure sensor and output voltage in piezoelectric nanogenerator. • Piezoelectric coefficient is increased from 1.03 pm/V in pristine ZnO to 2.57 pm/V for porous ZnO thin film. • C-axis lattice contraction caused from pores are scrutinized through extensive TEM works. • Detail pore formation mechanism is illustrated.

Influence of magnetron configurations on the structure and properties of room temperature sputtered ZnO thin films

Under the unbalanced magnetron (UBM) sputtering process, not only the plasma is confined near the target like in the conventional balanced magnetron (BM) sputtering process, but also extends towards the substrate and support the ion-assisted deposition (surface of thin films is bombarded by energetic Ar+ ions during the sputtering process). Here, we report the influence of magnetron configurations on the structure and properties of room temperature sputtered ZnO thin films while keeping other process parameters fixed. The UBM configuration has significantly improved various properties of ZnO thin films in comparison to the BM configuration. The crystalline quality with dominant orientation (002) and uniform distribution of grains is observed while an increase in the band gap from 3.25 eV (BM) to 3.33 eV (UBM) is obtained. The lower defects as investigated from Zn2p and O1s core level XPS spectra, which is well supported by Photoluminescence measurements. In addition to that, surface hydrophobicity has been increased from 121.2° (BM) to 125.5° (UBM). Thus, the unbalanced magnetron configuration in the sputtering process significantly enhanced the structural, optical and surface properties of ZnO thin films even at room temperature and low plasma power without any post annealing treatments, which is highly desired for the device fabrication.