Doped ZnO Films Research Articles

The Effect of Doping Zinc Oxide Thin Films With (0.5 wt. %) Carbon Nanotubes by Vacuum Evaporation

The study included the production of nano-thin films made of zinc oxide doped with carbon nanotubes (ZnO:CNTs) at a doping ratio of 0.5 wt%. The materials are applied onto glass substrates using the vacuum evaporation process. The structural characteristics of the thin films of undiluted and doped ZnO were assessed using X-ray diffraction (XRD). Additionally, the surface properties of the films were examined using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The optical characteristics of the thin films were analyzed and described using UV-Vis spectroscopy. The ZnO thin films exhibited crystalline formations. The thin films exhibited significant orientations along the (100), (002), and (101) crystallographic planes, indicating their hexagonal phase structures. The crystal size of ZnO thin films exhibited a range of 30.87 nm to 19.96 nm after the process of doping. The study findings demonstrate that the addition of carbon nanotubes leads to an increase in the absorption ratio. Zinc-doped carbon nanotube thin films has features that make them suitable for many applications, such as gas detectors and UV detectors.

Influence of Wrinkle Structure on Properties of Na-doped ZnO Films

Un-doped ZnO and Na-doped ZnO (NZO) thin films with various Na doping concentrations were successfully fabricated on glass substrates by a sol-gel process. The effect of Na ions at concentrations of 0, 1, 2, 3, 4, and 5% on the crystalline structure, surface morphology, and optoelectronic properties of ZnO films was studied using appropriate measurement techniques. XRD analysis revealed a polycrystalline hexagonal wurtzite structure in the NZO films. The NZO films' average crystal size ranged from 17.7 nm to 19.6 nm. SEM micrographs showed that wrinkle network appeared in the pure and doped ZnO films due to solvent evaporation during the thermal annealing process. UV-Vis spectroscopy pointed out that the average transmittance of NZO films is as large as ca. 78%. The optical bandgap of the films varied slightly between 3.235 eV for the pure ZnO and 3.246 eV for the Na 5% doped NZO film. When Na ions were doped into the NZO films, the electrical conductivity improved. Moreover, the absorption figure of merit exhibited the largest value of 0.981 Ω-1cm-1 for the fabricated NZO films, which enables the use of these films for solar cells and for other optoelectronic applications.

Thin Ga-doped ZnO Film with Enhanced Dual Visible Lines Emission

Introduction: In this study, a simple and facile route was employed to prepare a highly transparent and luminescent ultra-thin gallium doped ZnO film (GZO). Methods: The thin GZO film has been deposited using the simultaneously ultrasonic vibration and sol-gel spin-spray coating technique. The structural and optical properties of pure and doped thin films were investigated by various methods, such as X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), scanning electron microscopy (SEM), UV-Vis, and PL spectroscopy. Results: XRD results indicated that both pure and doped ZnO films had a hexagonal wurtzite structure with (101) preferred orientation. XPS and EDX studies confirmed the incorporation and presence of Ga ions into the ZnO lattice structure. The doped sample showed nearly 90% of transparency, and a strong blue-green emission in the visible region. Conclusion: The obtained results proved that the prepared thin film could be a novel candidate for optoelectronic applications.

The enhancement of physical properties of the (Ni-Co) doped ZnO films assisted by UV irradiation

The enhancement of physical properties of the (Ni-Co) doped ZnO films assisted by UV irradiation

A combined theoretical and experimental study on vertically aligned ZnO and ZnO: Sn

Vertically aligned ZnO and Sn (5, 10 v/v%) doped ZnO microrods were produced using chemical bath deposition technique on ZnO seed layers fabricated on glass substrates by spin coating method. The thickness of the seed layer is 365 nm and the length of the ZnO and ZnO:Sn rods are in the range of 1–5 μm. The length of the rods decreases with increasing Sn concentration. The structural, morphological, elemental compound, electrical and optical properties of ZnO and ZnO:Sn films were investigated using x-ray powder diffraction, field emission scanning electron microscopy, energy-dispersive x-ray spectroscopy, two probe method and UV–Vis absorption spectroscopy, respectively. The electronic structure of ZnO and Sn doped ZnO is calculated depending Density Functional Theory thanks to the WIEN2k program. Based on experimental and theoretical calculations, the band gap energy decreases with increasing Sn concentration. The band gap energy of seed layer, pure ZnO, Sn (5%) and Sn (10%) doped ZnO films was estimated as 3.27 eV, 3.18 eV, 3.14 eV and 3.12 eV, respectively. The band gap energy of Sn (1 atom) and Sn (2 atom) doped ZnO calculated theoretically was found as 0.392 eV and 0.245 eV, respectively. The Fermi Energy shifting towards the conduction band was observed upon Sn doping. Sn (5%) doped ZnO film shows the highest electrical conductivity although its band gap energy is higher than that of Sn (10%). The highest conductivity is attributed to the lowest dislocation density and less oxygen vacancies in the structure.

Fabrication of alcohol sensor using undoped and Al doped ZnO nanostructure film with polymer electrolyte gating

We report the fabrication of two terminal and three terminal gas sensor using Al-doped ZnO nanostructured-films and polymer electrolyte gate dielectric on glass substrate using vacuum free chemical method. The Al doped ZnO films are characterized by UV–vis Spectrometer, SEM, EDX and XRD. The characterization results have revealed the polycrystalline structure of both undoped and doped ZnO; with loosely packed, porous, and spherical granny nanostructure with mean grain size 20–10 nm and bandgap of the films is within the range of 3.12–3.16 eV. The conductivity of the ZnO film is tuned by Al concentration and the maximum value of conductivity was observed in 3 % Al doped ZnO films. Similarly, the best performance index of TFT such as current ON/OFF ratio, high transconductance and low threshold voltage was observed in 3 % Al doping concentration. The ordinary (two-terminal) sensor and three-terminal (FET) sensors' responses towards three different concentrations 50, 250, 500 ppm of ethanol and methanol vapors have been studied. The sensitivity of the film is modulated by Al concentration and higher value of sensitivity was achieved at 3 % Al doped ZnO films. The use of polymer electrolyte enhanced the sensitivity of the device which is more effective in methanol vapor. The Response-Recovery time of the sensor is significantly improved in three terminal devices than the two terminal devices.

Photocatalytic activity of rare earth elements (Gd and ce) co-doped ZnO nanostructured films

Organic dyes, which are one of the main sources of surface and groundwater pollution, cause health problems as well as environmental hazards. For this reason, ZnO, which stands out among the promising semiconductor photocatalysts in wastewater treatment due to its advantages, was preferred. In particular, Gd–Ce co-doped ZnO nanostructured films, which have been studied for the first time in the literature, were produced by ultrasonic spray pyrolysis (USP) technique and photocatalytic degradation of Methylene blue (MB) dye was carried out. Ultraviolet–Visible Spectrophotometry (UV–Vis) and Field Emission Scanning Electron Microscopy (FESEM) were used to obtain the transmittance and morphology of thin films. According to the FESEM analysis results, morphological changes in the type of nanostructure were observed after the doping. While the photocatalytic degradation percentage of the ZnO film with Gd doping was 65.84 %, when both Gd and Ce were doped, the degradation percentage increased significantly, reaching 91.58 %. Photocatalytic studies revealed that Gd–Ce co-doped ZnO films exhibited better photocatalytic efficiency compared to only Gd doped ZnO films. Therefore, when all the results are evaluated, it is concluded that ZnO:Gd:Ce photocatalysts may have a significant potential application in environmental remediation.

Fabrication and characterization of V-doped ZnO films implemented to lead-free piezoelectric micromachined devices

Piezoelectric micromachined ultrasonic transducers (p-MUTs) have been extensively utilized in medical imaging, range-finding, gesture recognition, and so on. However, the piezoelectric layer is dominated by the toxic Pb(Zr, Ti)O3, other materials possess inferior piezoelectric coefficients, and the traditional clamped diaphragm restricts the p-MUT response. In this work, lead-free ZnO films are doped by the vanadium nanostructures and are implemented to beam-island structure membranes, which are aimed to achieve non-toxic and high-performance p-MUTs. Firstly, the doping mechanism of ZnO is analyzed and the p-MUT structure is designed. Secondly, simulation based on the finite element method is conducted to evaluate the dynamic displacement of p-MUTs, after which prototypes are fabricated by the standard micromachined process. The effects of key fabrication parameters including O2 flow rates, sputtering targets, and annealing temperatures on V-doped ZnO films are investigated in detail. By using atomic force microscopy (AFM) and X-ray diffraction (XRD), the surface morphology and crystal structure of the films are analyzed respectively. Moreover, the piezoelectric properties are measured by piezo response force microscopy (PFM). The results indicate a piezoelectric coefficient as high as 194.5 pm/V, which is superior to most doped ZnO films. Finally, an experimental testing system is established to examine the p-MUT performance. Compared with the clamped diaphragm, the beam-island structure can acquire better electromechanical coupling and achieve range-finding successfully. This work provides a fine application prospect for enhancing the performance of lead-free p-MUTs.

Structural insight into nanoscale inhomogeneity of electrical properties in highly conductive polycrystalline ZnO thin films doped using methane

Recently, methane has been demonstrated as an effective n-type dopant for ZnO thin films deposited using the RF-magnetron sputtering method. It was shown that the major electrical doping effect of methane is caused by hydrogen released during methane decomposition. This work investigates the origin of the observed increase in conductivity of methane-doped ZnO films with the increase in thickness. The study is aimed at describing the nature of this thickness-dependent effect through a detailed analysis of the thickness-dependent morphology and crystalline structure. A combination of structural, electrical, and optical characterization revealed a transition from fine-grained films with a random orientation at early stages to partially (002)-textured films with columnar grains at later stages of growth. It is demonstrated that grain/sub-grain boundaries increase the electrical conductivity and that the contribution of such buried inner boundaries increases with increasing thickness. It is proposed that hydrogen diffuses along the grain and sub-grain boundaries during growth, leading to continuous doping of the buried interfaces. This hydrogen diffusion mechanism results in an apparent ‘additional doping’ of thicker films. The results provide new insights into the thickness-dependent conductivity of doped polycrystalline ZnO films mediated by hydrogen diffusion along internal interfaces.

Novel cathode buffer layer enabling over 21.6%/20.9% efficiency in wide bandgap/inorganic perovskite solar cells

Different types of hybrid organic/inorganic halide perovskite solar cells (PSCs) have attracted extensive research attention and showed extremely considerable practical application potential in current energy field, for instance, wide bandgap (WBG) PSCs are widely used in fabricating high-efficiency tandem devices, while inorganic perovskite solar cells (IPSCs) had been sought after by researchers due to its excellent thermal stability. However, either WBG PSCs or IPSCs were severely influenced by the carrier extraction and transport between the electron transfer layer and anode/cathode. On this basis, an in-situ oxygen plasma treated Ti doped ZnO film named TZO (O) was used as cathode buffer layer to regulate the energy-level alignment between the SnO2 layer and ITO electrode, optimize the crystal quality of perovskite and passivate perovskite defects through providing superior SnO2 film. It is found that the more suitable energy levels for the interlamination availably polished up carrier transport effect to a great extent, which could be seen in steady-state and time-resolved photoluminescence spectra, in addition to this, the perovskite film covered on TZO (O)/ SnO2 demonstrated larger perovskite crystals and less surface defects. As a consequence, the power conversion efficiency (PCE) of the optimized device with a bandgap of ≈ 1.66 eV up to 21.62%, and the efficiency of CsPbI2.85Br0.15 IPSCs is enhanced from 19.21% to 20.92%. Moreover, the unencapsulated TZO (O)-treated wide bandgap PSCs shows enhanced stability, retaining 89.47% of its initial efficiency after 1000 h in a N2 filled environment, while the unencapsulated IPSCs-based TZO (O) still remains 83.77% of its initial efficiency under continuous illumination in air for 325 h. This work exhibited that the buffer layer TZO (O) is a promising interfacial modification material for both n-i-p type wide bandgap PSCs and IPSCs.

Switching the selectivity of ZnO thin films for ultra-sensitive acetaldehyde gas sensors through Co doping

Pure and Co doped (1–5 wt%.) ZnO thin films were synthesised using the chemical spray pyrolysis method for the detection of acetaldehyde at 300 K. The gas sensing properties of these films were studied. The preferential orientation of the films along the c-axis stated its high crystalline nature. The responsiveness of the Co doped films switched to acetaldehyde from ammonia of the pure ZnO film. The 5 wt%. Co doped ZnO films could effectively detect 5 ppm of acetaldehyde at 300 K with a better response of around 238, compared to the pure and other doping concentrations. This high response is due to the catalytic property of Co, whose presence changed the electrical properties of the film. Co2+ created Zn2+ vacancies over the surface and generated defects, promoting the gas sensing mechanism. The constructed sensing layer is stable and repeatable implying its potential application as an acetaldehyde gas sensor.

Enhancing the hydrogen photo-production using zinc oxide films doped with iron, tin, and aluminum

Hydrogen is recognized as the most promising renewable energy source and can be efficiently produced through water photo-electrolysis green process. This study investigates the performance of ZnO nanorods doped with Fe, Al, and Sn at a 2 % molar ratio for hydrogen photo-production. The films were prepared using a combination of successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD) methods. X-ray diffraction (XRD) analysis confirmed the hexagonal structure of the doped ZnO films with preferential growth along the (002) orientation, while energy-dispersive X-ray spectroscopy (EDX) confirmed their high purity. The doping with Al maintains the ZnO nanorods morphology while Fe, and Sn doping induced changes to overlapped nanoflakes/nanoflowers morphology. Optical analysis revealed a clear trend in the energy band gap, with Fe-ZnO film exhibiting the lowest value (2.95 eV), followed by Al-ZnO (3.03 eV), Sn-ZnO (3.07 eV), and ZnO (3.11 eV). The Fe-ZnO film also demonstrated the best performance as a photoelectrode for hydrogen generation, achieving optimized incident photon-to-current efficiency (IPCE) and applied bias photon-to-current efficiency (ABPE) values of 1.45 % and 0.20 % respectively, under 500 nm monochromatic illumination, with high reproducibility, and durability. These findings represent one step forward to developing ZnO-doped photoanodes for efficient and sustainable hydrogen green production.

Improved sputtering method to deposit high conducting doped ZnO films without substrate heating

Magnetron sputtering deposition with Zn supply was utilized to deposit Ga-doped ZnO (GZO) films to minimize acceptor-like crystalline defects. This deposition technique significantly increased the carrier concentration of GZO films. In addition, the impact of partial oxygen pressure on the deposition atmosphere on carrier concentration was remarkably reduced. As a result, the resistivity of the films decreased to as low as 4 × 10−4 Ωcm without the need for intentional substrate heating. Consequently, the deposition with Zn supply shows great potential for producing ZnO-based transparent conducting films with practically low resistivity on polymer substrates that have lower heat tolerance.

Unravelling the crossover amongst band and various hopping conduction mechanisms in Mo doped ZnO thin films owing to carrier localization at defects

Understanding the carrier transport mechanism of transparent conducting oxides is crucial for its application in optoelectronic devices. Although, electrical transport mechanisms of various trivalent cation doped ZnO films have been reported adequately, a substantial study is needed to understand the transport mechanism in ZnO doped with higher valence (>+3) dopants. Unlike Al, In and Ga doped ZnO where metal-insulator transition is commonly observed, in this study, the crossover amongst band and various hopping mechanisms has been observed in the temperature range of 300–10 K in Mo doped ZnO films deposited at various substrate temperatures. All the films show electrons as the majority charge carriers and notably, the film deposited at 573 K shows the highest conductivity value of 167.22 (Ω.cm)−1. In-depth analyses of the conduction mechanism reveal grain boundary scattering dominated band conduction at and not far below the room temperature, which is shifted towards nearest neighbor hopping (NNH), and then to Mott variable range hopping (VRH) type of conductions as the temperature is decreased further. Unfamiliarly, a modified Efros-Shklovskii (ES) VRH type of conduction mechanism is shown to prevail in the lowest measured temperature range (∼30–10 K). Unprecedentedly, it is demonstrated here that the activation energies for various conduction mechanisms are higher at lower vacuum condition (∼10−1 mbar) as compared to those measured at higher vacuum condition (∼10−3 mbar) implying an added role of the surface traps in the charge transport process. The temperature dependent conductivity and X-ray photoelectron spectroscopy (XPS) studies proclaim that the film deposited at 573 K possess fewer defects related disorder for carrier localization as compared to the films deposited at 523 K and 698 K.

Transparent-flexible thermoelectric module from In/Ga co-doped ZnO thin films

Transparent-flexible thermoelectric thin films have immense potential as power supplies for future small-sized consumer electronics, the internet of things, and wearable devices. Here, we report the thermoelectric properties of dual Ga and In doped ZnO films (IGZO) deposited on a polyimide substrate with post-thermal treatment in vacuum along with fabricating 4-unileg flexible IGZO thermoelectric devices. All the as-deposited and annealed IGZO films are the preferred (002) orientation and under tensile stress. The post-thermal treatment controls the dopant substitution/diffusion in the host ZnO lattice affecting the film crystallinity, residual stress, and thermoelectric properties. Among the films, the IGZO film annealed at 250 °C has the best power factor of 16.9 μWm−1K−2 with the largest crystal size, lowest tensile stress, highest carrier concentration, and lowest density-of-state effective mass. The practical application of flexible IGZO films was also reported via a 4-unileg-IGZO films thermoelectric module, which achieved an output power about 3.2 nW at ΔT = 120 K.

The influence of post-annealing temperatures on XPS, XZ height of nanoparticles, and optical properties of Cu–Al doped ZnO films

In this article, using a radio frequency magnetron sputtering system at room temperature, Cu–Al doped ZnO films were deposited on quartz substrates. XPS studies have shown that the broad bands at around 932.5, 943, and 952 eV correspond to the Cu 2 P3/2 peak of Cu+1, Cu 2p3/2 of Cu+2, and Cu 2p1/2, respectively. In the case of Al2p3/2, the peak with binding energy 74.2 eV may be attributed to the presence of Al3+ oxide. The optical density, D opt, of films annealed at 600 °C has a maximum value about of 0.25. Films annealed at 600 °C have a minimum value of the fractal dimensions, D f, about 2.65. It is observed that films annealed at 500 °C, have a maximum value of the single layer. The minimum value of dissipation factor occurred in films annealed at 500 °C. It can be seen that with doping Cu and Al atoms into ZnO films, the oxygen vacancies were nearly removed and the quality of films was increased; hence the porosity which is introduced in ZnO films can be removed by Cu–Al doping of ZnO films. Due to high evaporation in films annealed at 600 °C, they were rougher and they have a maximum value of RMS roughness at about 4.68 nm. Films annealed at 400 °C have a minimum value of dissipation factor.

The impact of oxygen on Ga doped ZnO film.

The Ga doped ZnO (GZO) film is one of the promising alternative films to replace ITO film, but its properties suffer from degradation when it is deposited under oxygen-rich conditions. This degradation has been investigated by depositing the films under different oxygen partial pressures. XRD results showed that all GZO films had wurtzite structure and the lattice parameter-c contracted when oxygen was introduced into the argon deposition atmosphere, but the parameter-c nearly remained constant when oxygen partial pressures were further increased. The contraction of parameter-c was caused by the increasing concentrations of VZn (Zn vacancy). It was the first time to observe that the impurity phase of Ga2Zn6O9 appeared and disappeared in GZO films during the increase of oxygen partial pressures. Analogously, conductivity decayed and optical bandgap decreased abruptly as oxygen was introduced, which enhanced self-compensation of donors and acceptors. The energy band structures of GZO and ZnO films were determined by using UPS, and the results showed that oxygen had little effect on the electron affinity of the GZO film, but a significant difference in electron affinity between the ZnO and GZO films was observed. This result indicated that although the electron affinity of ZnO could be effectively tuned by doping with Ga, it remained quite stable for GZO under oxygen-rich conditions.

Investigation of thickness effect on NO2 gas sensing properties of ZnO/Na thin films

This present communication studies the dependences of film thickness, electrical properties, and film morphology on the gas detection capability of Na/ZnO thin films. The phase formation, microstructure, surface composition, and electrical parameters of these films were analyzed by XRD, FESEM-EDAX, and Hall measurement. XRD measurements reveal that the crystal quality of Na doped ZnO films is enhanced with the increase in the sensing layer thickness. The morphology of samples shows a transition of grain shape from spherical to irregular as the films become thicker. The film thickness shows variations in the range from 132 nm to 356 nm. Hall measurement concludes that Na doping concentration induces p-type conductivity in ZnO. The Na/ZnO film of a thickness of 132 nm emerged as the suitable NO2 sensor with better sensitivity and the fastest res/rec property. The obtained discrepancy in responsivity is attributed to the difference in grain size and carrier concentration with a change in film thickness.

Deposition of Sol-Gel ZnO:Mg Films and Investigation of Their Structural and Optical Properties.

This work presents a facile sol-gel method for the deposition of ZnO and ZnO:Mg films. The films are spin coated on silicon and quartz substrates. The impact of magnesium concentrations (0, 0.5, 1, 2 and 3 wt%) and post-annealing treatments (300-600 °C) on the film's structural, vibrational and optical properties is investigated. Undoped ZnO films crystallize in the wurtzite phase, with crystallite sizes ranging from 9.1 nm (300 °C) to 29.7 nm (600 °C). Mg doping deteriorates the film crystallization and shifting of 002 peak towards higher diffraction angles is observed, indicating the successful incorporation of Mg into the ZnO matrix. ZnO:Mg films (2 wt%) possess the smallest crystallite size, ranging from 6.2 nm (300 °C) to 25.2 nm (600 °C). The highest Mg concentration (3 wt%) results into a segregation of the MgO phase. Lattice constants, texture coefficients and Zn-O bond lengths are discussed. The diminution of the c lattice parameter is related to the replacement of Zn2+ by Mg2+ in the ZnO host lattice. The vibrational properties are studied by Fourier transform infrared (FTIR) spectroscopy. IR lines related to Mg-O bonds are found for ZnO:Mg films with dopant concentrations of 2 and 3 wt%. The optical characterization showed that the transmittance of ZnO:Mg thin films increased from 74.5% (undoped ZnO) to about 89.1% and the optical band gap energy from 3.24 to 3.56 eV. Mg doping leads to a higher refractive index compared to undoped ZnO films. The FESEM (field emission scanning electron microscopy) technique is used for observation of the surface morphology modification of ZnO:Mg films. The doped ZnO films possess a smoother grained surface structure, opposite to the wrinkle-type morphology of undoped sol-gel ZnO films. The smoother surface leads to improved transparency of ZnO:Mg films.

Influence of oxygen partial pressure on the strain behaviour of reactively co-sputtered Ga doped ZnO thin films

Gallium doped ZnO films (GZO) were deposited by reactive co-sputtering of Zn and GaAs at different partial pressures of oxygen in Ar-O2 mixture. The GZO films were deposited at 375 °C, with GaAs area coverage of 0 - 3 % and 4 - 10 % O2 in sputtering atmosphere. The composition of GZO films was studied by X-ray photoelectron spectroscopy, which showed that the films deposited at 4 % and 5 % O2 display Ga/Zn ratio of ∼ 0.1 and contain oxygen vacancies, while those deposited at 6 % and 7 % O2 display Ga/Zn ratio of ∼ 0.01 and contain chemisorbed/interstitial oxygen. X-ray diffraction measurements supported by Raman studies revealed that undoped ZnO films are strongly c-axis oriented and display decrease of crystallite size along c-axis, along with substantial increase of hydrostatic tensile strain with increase of O2 percentage, which are attributed to the presence of interstitial oxygen. In sharp contrast, GZO films deposited at 4 % and 5 % O2 under limited oxygen availability, display smaller and mis-oriented crystallites along c-axis and large compressive strain, which is also hydrostatic, owing to the incorporation of Ga. The GZO films deposited at 6 % and 7 % O2 display improved crystallinity and decrease of compressive strain, which are attributed to reduced Ga content of the films and sufficient availability of oxygen during deposition. This study reveals the critical influence of the partial pressure of oxygen on the stoichiometry, doping concentration, crystallinity and the nature of strain in reactively sputtered GZO films.