ZnO Semiconductor Thin Films Research Articles

Study of The Electrical, Optical and Morphological Properties in Submicron and Microstructured ZnO Thin Films Obtained by Spin Coating and Chemical Bath Deposition

In the present work the synthesis of ZnO semiconductor thin films was performed successively using spin coating and chemical bath deposition techniques. The deposition was made by varying the concentration of zinc acetate and hexamethylenetetramine (HMTA: ZnAc) in the precursor solution. This process led to two preferred growth directions (002) and (101), both with very similar texture coefficients, too; a noticeable change in morphology of structured surface, variation in unit cell parameters and crystalline grain size. All the films turned out homogeneously submicro and microstructured and with a wurtzite-type hexagonal crystalline structure. Using pre-loaded Mathematica 11.3 software functions and an algorithm developed in it, the micrographies were analyzed to calculate the percentage of substrate-covered area which was always greater than 80%. Likewise, it also found that resistivity decreases at a higher percentage of covered area and that the variation in the shape of the photo luminescent emission spectrum can be considered as a qualitative indication of the concentration of charge carriers.

Structural and optical properties of pure ZnO and Al/Cu co-doped ZnO semiconductor thin films and electrical characterization of photodiodes

Abstract Nano-structured semiconductor thin films and p-type Si photodiodes were fabricated with the sol-gel spin coating technique using pure ZnO and co-doped ZnO:Alx:Cuy with x = 1 at.-%, y = 1, 2, 3, 5 at.-%. The structural and optical properties of thin films were examined using an XRD and a UV-spectrophotometer. The thin films have a hexagonal wurtzite crystal structure, and their optical band gap energies decrease with increasing Cu contribution. The electrical properties of photodiodes were assessed via I-V, C-V, (G/ω)-V and phototransient current (I-t, C-t) measurements. The Φ b(I-V), experimental zero-bias barrier height, rectification ratio, ideality factor and Ion/Ioff parameters of the diodes were calculated using thermoionic emission model. In addition, Φ b(C-V), barrier height, Vbi, built-in voltage, Vd, diffusion potential, Nd, donor concentration and Wd, depletion layer width of p-Si/ZnO:Alx:Cuy (x = 1 at.-%, y = 1 at.-%.) photodiodes were obtained using a C-2-V graph plotted at 1 MHz frequency. The photodiodes exhibit rectifying and photosensitive behaviors, and their reverse bias current increases with increasing light intensity. These results indicate that produced diodes can be employed as photodiodes or photosensors in optoelectronic circuits and electronic devices.

Enhanced Electrical Properties and Stability of P-Type Conduction in ZnO Transparent Semiconductor Thin Films by Co-Doping Ga and N

P-type ZnO transparent semiconductor thin films were prepared on glass substrates by the sol-gel spin-coating process with N doping and Ga–N co-doping. Comparative studies of the microstructural features, optical properties, and electrical characteristics of ZnO, N-doped ZnO (ZnO:N), and Ga–N co-doped ZnO (ZnO:Ga–N) thin films are reported in this paper. Each as-coated sol-gel film was preheated at 300 °C for 10 min in air and then annealed at 500 °C for 1 h in oxygen ambient. X-ray diffraction (XRD) examination confirmed that these ZnO-based thin films had a polycrystalline nature and an entirely wurtzite structure. The incorporation of N and Ga–N into ZnO thin films obviously refined the microstructures, reduced surface roughness, and enhanced the transparency in the visible range. X-ray photoelectron spectroscopy (XPS) analysis confirmed the incorporation of N and Ga–N into the ZnO:N and ZnO:Ga–N thin films, respectively. The room temperature PL spectra exhibited a prominent peak and a broad band, which corresponded to the near-band edge emission and deep-level emission. Hall measurement revealed that the ZnO semiconductor thin films were converted from n-type to p-type after incorporation of N into ZnO nanocrystals, and they had a mean hole concentration of 1.83 × 1015 cm−3 and a mean resistivity of 385.4 Ω·cm. In addition, the Ga–N co-doped ZnO thin film showed good p-type conductivity with a hole concentration approaching 4.0 × 1017 cm−3 and a low resistivity of 5.09 Ω·cm. The Ga–N co-doped thin films showed relatively stable p-type conduction (>three weeks) compared with the N-doped thin films.

Optical and Surface Properties of Nanostructured ZnO Semiconductor Thin Films Synthesized by RF Magnetron Sputtering

In this research, ZnO thin films were deposited on glass microscope slides in three separate experiments with RF input powers of 50 W, 100 W and 125 W by means of RF magnetron sputtering technique. Each deposition process was conducted for 30 minutes. Spectroscopic reflectometer , UV-VIS spectrophotometer and atomic force microscope (AFM) were used to examine the effect of sputtering power on the optical and surface properties of the produced thin films. The level of reflectivity and transparency, refractive index and band gap energy values as well as surface homogeneity and roughness were observed to rely on the RF input power. The optical band gap energy values were about 3.83-3.87 eV. The produced highly transparent ZnO thin films can be used in various optoelectronic devices and future transparent conductive electrode implementations.

Properties of Al-Ga co-doped ZnO semiconductor thin films deposited on polyethylene terephthalate substrates by radio frequency magnetron sputtering

Al and Ga co-doped ZnO (AGZO) semiconductor thin films were deposited on polyethylene terephthalate (PET) substrates by radio frequency magnetron sputtering. Prior to oxide thin film deposition, the hydrophilicity of the surface of the PET substrate was increased by corona discharge treatment. In this study, the influence of corona discharge power on the surface properties of the PET substrates and the effect of substrate temperature on the physical properties of PET/AGZO thin film samples were investigated. After corona discharge treatment to modify the surfaces of the PET substrates, drastic decreases in the contact angle of water on the PET substrates demonstrated that the corona discharge treatment changed the surface wettability of the PET substrates. Experimental results showed that increasing the substrate temperature from room temperature to 135 °C significantly enhanced the crystallinity, reduced the surface roughness, and improved the electrical properties of the AGZO thin films. The AGZO thin films deposited on PET substrates at 135 °C had the highest Hall mobility of 5.5 cm2/Vs.

Low temperature deposition of ZnO semiconductor thin films on a PEN substrate by a solution process

Low-temperature processed ZnO semiconductor films were deposited onto polyethylene naphthalate (PEN) plastic substrates by a spin coating technique using ZnO nanoparticle (NP) dispersion. The ZnO nanoparticles (NPs) were synthesized by the hydrothermal method, and solution processable dispersion was used to disperse the ZnO NPs in a mixed aqueous solution with a polyvinylpyrrolidone (PVP) dispersant agent. The effects of annealing temperature (from 150°C to 250°C) on the electrical properties of glass/ZnO film samples are reported. The optimized annealing condition (200°C) was applied for ZnO film deposited on a PEN substrate. Comparative electrical properties of the PEN/ZnO film samples before and after bending tests are also presented. Experimental results show that the electrical resistivity of the PEN/ZnO film sample was 1.91 × 104 Ω cm with a Hall mobility of 45.9 cm2/Vs. After bending tests, the electrical resistivity was raised to 1.26 × 105 Ω cm and the Hall mobility was reduced to 31.0 cm2/Vs.

Local structure and local conduction paths in amorphous (In,Ga,Hf)–ZnO semiconductor thin films

The local structure and local conduction paths of Ga–In–Zn–O (GIZO) and Hf–In–Zn–O (HIZO) amorphous thin films were investigated by the extended X-ray absorption fine structure (EXAFS). We found that the local hindrance paths of In–Ga and In–Hf exist in the conduction paths of amorphous GIZO and HIZO semiconductor thin films, respectively.

Structural and optoelectrical properties of Ga-doped ZnO semiconductor thin films grown by magnetron sputtering

Transparent conductive gallium-doped zinc oxide (Ga-doped ZnO) films were prepared on glass substrate by magnetron sputtering. The influence of substrate temperature on structural, optoelectrical and surface properties of the films were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), spectrophotometer, four-point probe and goniometry, respectively. Experimental results show that all the films are found to be oriented along the c-axis. The grain size and optical transmittance of the films increase with increasing substrate temperature. The average transmittance in the visible wavelength range is above 83% for all the samples. It is observed that the optoelectrical property is correlated with the film structure. The Ga-doped ZnO film grown at the substrate temperature of 400 °C has the highest figure of merit of 1.25 × 10−2 Ω−1, the lowest resistivity of 1.56 × 10−3 Ω·cm and the highest surface energy of 32.3 mJ/m2.

Effect of dopants on the structural, optical and electrical properties of sol–gel derived ZnO semiconductor thin films

Undoped, Ga-, In-, Zr-, and Sn-doped ZnO transparent semiconductor thin films were deposited on alkali-free glasses by sol–gel method. 2-methoxyethanol (2-ME) and diethanolamine (DEA) were chosen as a solvent and a stabilizer, respectively. The doping concentration was maintained at 2 at.% in the impurity doping precursor solutions. The effects of different dopants on the structural, optical, and electrical properties of ZnO thin films were investigated. XRD results show that all annealed ZnO-based thin films had a hexagonal (wurtzite) structure. ZnO thin films doped with impurity elements obviously improved the surface flatness and enhanced the optical transmittance. All impurity doped ZnO thin films showed high transparency in the visible range (>91%). The Ga- and In- doped ZnO thin films exhibited higher Hall mobility and lower resistivity than did the undoped ZnO thin film.

Co Cluster Formation Induced by Cu Codoping in Co:ZnO Semiconductor Thin Films

Here we report that the occupation sites of Co atoms in ZnO matrix could be effectively tuned by the concentration of Cu codopant. The Co–Cu codoping effect has been revealed by a combination of X-ray diffraction and X-ray absorption fine structure spectroscopy techniques for Zn0.95–xCuxCo0.05O (0.005 ≤ x ≤ 0.08) thin films grown by pulse laser deposition at 923 K. Specifically, at the low Cu doping levels (x ≤ 0.02), the Co(+2) ions are substantially incorporated into the ZnO lattice; upon increasing the Cu concentration to 0.03 or higher, partial formation of Co(0) species occurs, and its proportion rises rapidly with the Cu concentration. Further analysis shows that the Cu codopants are precipitated to form Cu(0) metallic phase in all the samples. We propose a competition mechanism between the Co(0)–Cu(0) metallic interactions and the dissolution of Co ions in ZnO to interpret these findings.

Effect of preheating process on crystallization and optical properties of sol-gel derived ZnO semiconductor thin films

Highly transparent ZnO semiconductor thin films were deposited onto alkali-free glass substrates by a sol-gel spin coating process. This research investigated the effects of different preheating rates (4 or 10°C/min) on the various surface morphologies, crystallization characteristics, and optical properties of these thin films. The ZnO sol was synthesized by dissolving the zinc acetate dihydrate in isopropanol (IPA), and then adding monoethanolamine (MEA). These as-coated films were preheated at 300°C for 10 min and annealed in an air ambiance at 500°C for 1 h. Experimental results revealed that the as-prepared films had a hexagonal wurtzite structure, and that the heating rates of the preheating process obviously affected the surface morphologies, crystallization qualities, and transparency levels of the thin films. A ZnO thin film preheated at 10°C/min exhibited preferential orientation along the (002) plane, a flat surface, and also achieved a high transmittance value, 92.5%, for light with a wavelength of 550 nm.

Microstructural and optical properties of Ga-doped ZnO semiconductor thin films prepared by sol–gel process

Transparent thin films of Ga-doped ZnO (GZO), with Ga dopant levels that varied from 0 to 7 at.%, were deposited onto alkali-free glass substrates by a sol–gel process. Each spin-coated film was preheated at 300 °C for 10 min, and then annealed at 500 °C for 1 h under air ambiance. The effects of Ga dopant concentrations on crystallinity levels, microstructures, optical properties, and electrical resistivities of these ZnO thin films were systematically investigated. Photoluminescence spectra of GZO thin films were examined at room temperature. XRD results revealed that the undoped ZnO thin films exhibited a preferred orientation along the (002) plane and that the ZnO thin films doped with Ga showed degraded crystallinity. Experimental results also showed that Ga doping of ZnO thin films could markedly decrease surface roughness, improve transparency in the visible range, and produce finer microstructures than those of undoped ZnO thin films. The most promising films for transparent thin film transistor (TTFT) application produced in this study, were the 3 and 5 at.% Ga-doped ZnO thin films, both of which exhibited an average transmittance of 90.6% and an RMS roughness value of about 2.0 nm.

Sol–gel derived ZnO/PVP nanocomposite thin film for superoxide radical sensor

Pure ZnO films and ZnO nanoparticle-dispersed polyvinylpyrrolidone (PVP) composite films are prepared on Pyrex glass substrates by the sol–gel dip-coating technique utilizing zinc acetate precursor. The thin films are extensively characterized for surface morphology, chemistry, and nanocrystallite size using various advanced analytical techniques including Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). For the processing conditions considered, ZnO semiconductor thin films with nanocrystallite size 20–30 nm are obtained. The ZnO nanoparticle size in the PVP composite film increases with increase in ZnO content. The resistance of both the synthesized ZnO and ZnO/PVP thin films decrease significantly after exposure to solution containing superoxide anion radicals (SOR). The results thus indicate that ZnO and ZnO/PVP composite thin films can be used as biosensors for SOR and potentially for characterizing the antioxidant properties of fluids.

Synthesis and Characterisation Of Sol–gel Derived Nanostructured Composite of ZnO/PVP Thin Films

Pure ZnO films and ZnO nanoparticle dispersed polyvinylpyrrolidone (PVP) films were prepared on a Pyrex glass substrate by sol–gel dip coating using a zinc acetate precursor. The thin film is extensively characterised for its surface morphology, chemistry, thickness and nanocrystallite size, using various advanced analytical techniques such as SEM, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Under the given processing conditions, ZnO semiconductor thin films with nanocrystallite size 20–30 nm were obtained, and the ZnO nanoparticle size in the PVP matrix increased with increase in ZnO content.