MgZnO Films Research Articles

Magnesium-doped zinc oxide film for hydrogen production from wastewater

Recently, developing resources for H2 production has been a supreme priority to maintain the current rate of global warming in waste and drinking water. Metal-doped zinc oixde (ZnO) film-based catalyst materials are better candidates for electrochemical water splitting. Herein, it will be demonstrated that Mg-doped ZnO films can significantly enhance the electrochemical water splitting and H2 production. Therefore, Mg-doped ZnO films were synthesized using the dip coating technique. The undoped and Mg-doped ZnO films' structure, chemical, and morphological properties were characterized using detalied measurements. The electrical and optical characteristics of ZnO and Mg–ZnO samples were investigated. ZnO and Mg–ZnO films coated on the proper reactor were utilized for electrochemical water splitting. The efficiency of the electrochemical water splitting increases from 29 % to 56 % as ZnO is doped with Mg. In addition, the yielding H2 concentration increases from 9.92 × 10−7 mol/L.h to 3.65 × 10−6 mol/L.h when ZnO is doped with Mg. Hence, the work sheds new insight into new ways of fabricating significant photocatalysts for low-cost hydrogen extraction and echo-friendly agents.

Compliant substrate epitaxial MgZnO films using fluorphlogopite mica approaching homoepitaxy quality

Based on the success of high-quality epitaxial grown ZnO film on the compliant Fluorphlogopite Mica (F-Mica) substrate, the growth of MgZnO films on F-Mica substrate is studied to promote the development of flexible ZnO quantum well structures and devices. These flexible epitaxial MgZnO films show high luminous quality approaching homoepitaxial films. The phenomenon of Mg compositional fluctuations is confirmed by the research on band tail state. The effect of the compositional fluctuations on epitaxial growth dynamical and thermodynamic processes is also revealed by the comprehensively investigation of the crystal quality, optical properties, morphology, and growth rate. The study provides guidance for energy band engineering of ZnO on flexible substrates, and lays the foundation for the bendable optoelectronic devices on F-Mica substrate.

Realization of cathodoluminescence in the 180 nm spectral range by suppressing thermal stress in mist chemical vapor deposition of rocksalt-structured MgZnO films

Rocksalt-structured (RS) Mg x Zn1−x O films with x = 0.65–1.0 were grown on MgO (100) substrate using the mist CVD method. A comparative study of the RS-Mg0.92Zn0.08O films grown under slow and rapid-cooling rates apparently showed simultaneous reductions in the surface pit density, FWHM values for the X-ray diffraction peak, and defect-related cathodoluminescence (CL) for the film grown under the slow-cooling rate. CL spectra for the RS-Mg x Zn1−x O films grown under the slow-cooling rate eventually showed near-band-edge emission peaks in the 180–190 nm spectral range for MgO molar fraction x ≥ 0.92 at RT.

Investigation of phosphorus-doping of MgZnO thin films using efficient spin-on dopant process

Phosphorus doped MgZnO thin films were prepared using the RF sputtering technique on a Si wafer, followed by spin-on doping (SOD) and annealing. The SOD is a cheap and non-destructive process in which the dopant film is spun on a Si wafer and placed in the vicinity of deposited undoped MgZnO thin film at a high temperature to perform doping. After doping, the MgZnO thin films were annealed at temperatures such as 700, 800, and 900 °C, which significantly improved morphological, structural, and optical properties. The atomic force microscopy and scanning electron microscopy measurements revealed that phosphorus-doped MgZnO thin films annealed at 800–900 °C have good morphology and large grains. X-ray diffraction spectra demonstrated the (002) orientation of MgZnO thin films. The photoluminescence spectra measured at 20 K demonstrated the acceptor bound exciton peak at 3.47 eV and acceptor binding energy of around 64.34 meV, indicating the formation of shallow acceptor levels by phosphorus doping of MgZnO thin films using the SOD process. In Raman spectroscopy measurement, the peak of E2high phonons mode of MgZnO wurtzite structure was observed around 436 cm−1. The FWHM value of this peak reduces with augmentation annealing temperature, demonstrating improvement in crystallinity. X-ray photoelectron spectroscopy measurement demonstrated the presence of phosphorus atoms in the SOD processed MgZnO thin films, which is again verified by Fourier-transform infrared spectroscopy measurement showing vibration modes of P–O bonds. It was observed that the different properties of SOD-prepared phosphorus-doped MgZnO films were superior to the film prepared using the alternate costly and destructive ion-implantation technique. These findings have revealed that high-quality phosphorus-doped p-type MgZnO thin films by the SOD process are very suitable for UV optoelectronic device applications.

Zinc oxide family semiconductors for ultraviolet radiation emission – A cathodoluminescence study

• Study of ZnO and MgZnO grown using a variety of techniques. • UV cathodoluminescence seen in bulk ZnO and in epitaxial MgZnO. • Sputtered MgZnO shows no UV cathodoluminescence. • Atomic layer deposition appears to be suitable for growing UV-luminescent MgZnO films . Zinc oxide (ZnO) family semiconductors that include ZnO and various ternary and even quaternary semiconductors formed by the inclusion of suitable other elements to zinc and oxygen are potential candidates for making UV-emitting solid-state devices. This work is a study of the UV-emitting potential of ZnO and MgZnO through electron beam irradiation of various samples. In this work, we studied materials grown or deposited through different techniques in order to assess their suitability for use in UV-emitting devices. Our work throws light on which growth or deposition methods are more suitable for obtaining radiation emission-capable materials from this family of II-VI oxide semiconductors. We find that single crystal thin film material, produced through either metal organic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD) appear to be the best, followed by hydrothermally-grown single crystals. In contrast, sputtered material appears unsuitable for making UV-emitting devices.

A Solar-Blind Ultraviolet Photodetector With Graphene/MgZnO/GaN Vertical Structure

Graphene (Gr) has high transmittance to ultraviolet (UV) light and high mobility, which can effectively collect and transfer carriers. In this work, MgZnO (MZO) films were grown on the surface of the p-GaN by magnetron sputtering. A heterojunction solar-blind UV detector with Gr/MZO/GaN structure was constructed by introducing Gr as the window layer film. The test results show that the device has excellent detection ability for solar-blind UV light. The light response cut-off edge of the device is 263 nm, under the illumination of 255 nm and the bias voltage of −5 V, the responsivity is 14.6 mA/W, the rise time is 0.79 s, the decay time is 0.2 s, and the external quantum efficiency is 71.1%. The importance of this work lies in providing a reference for the application of Gr-based photodetectors.

Effect of magnesium content and growth temperature on structural and optical properties of USCVD-grown MgZnO films

Magnesium zinc oxide (MgxZn1-xO) films (x < 0.4) have been grown by ultrasonic spray chemical vapor deposition on soda-lime glass substrates for different growth temperature and different Mg mole fractions (x). Effects of Mg content in the liquid solution and the effect of growth temperature on optical and structural properties of MgZnO films have been investigated with X-ray diffraction, atomic force microscope, and UV–visible spectrophotometry measurements. The growth temperature has a significant effect on determining the Mg mole fractions of MgZnO films. There is a linear correlation between the measured Mg mole fraction in the resulted grown structures and Mg content in the solution. The peak of (0002) diffraction for the grown samples has been shifted to the higher diffraction angles with increase in Mg content in the liquid solution and has also become stronger. The growth temperature and Mg content in a liquid solution are crucial parameters in the controlling Mg mole fraction of samples.

Improved responsivity of MgZnO film ultraviolet photodetectors modified with vertical arrays ZnO nanowires by light trapping effect

The light trapping effect of ZnO nanowires (NWs) is attracting increasing attention as it effectively enhances the photoelectric effect. In this paper, high-density ZnO NWs are grown on a metal–semiconductor–metal structure MgZnO film UV photodetector (PD) as a light trapping unit. The photogenerated carriers diffuse along the longitudinal axis of the ZnO NWs, then diffuse onto the thin film and are collected by an applied bias electrode. When the device is connected to the NWs, the responsivity is about 12 times higher than that of the pure MgZnO film UV PD with a large light-dark current ratio (4.93 × 104). The array structure of the ZnO NWs enhances the number of photogenerated carriers at the top interface and provides a longer optical path length and a larger surface area. The resulting light trapping effect endows the device with excellent photoelectric properties. In this work, the introduction of NWs not only fundamentally improves the performance of the MgZnO thin film UV PD, but the resulting photodetector also demonstrates a sharp contrast between light trapping UV PD and the MgZnO thin film UV PD.

Synergistic effects of Ga doping and Mg alloying over the enhancement of the stress sensitivity of a Ga-doped MgZnO pressure sensor.

We demonstrate the synergistic effects of Ga doping and Mg alloying into ZnO on the large enhancement of the piezopotential and stress sensing performance of piezotronic pressure sensors made of Ga-doped MgZnO films. Piezopotential-induced pressure sensitivity was enhanced through the modulation of the Schottky barrier height. Doping with Ga (0.62 Å) of larger ionic radius and alloying with Mg (0.57 Å) of smaller ionic radius than Zn ions can synergistically affect the overall structural, optical and piezoelectric properties of the resulting thin films. The crystal quality of Ga-doped MgZnO films either improved (XGa ≦ 0.041) or deteriorated (XGa ≧ 0.041) depending on the Ga doping concentration. The band gap increased from 3.90 eV for pristine MgZnO to 3.93 eV at XGa = 0.076, and the piezoelectric coefficient (d33) improved from ∼23.25 pm V−1 to ∼33.17 pm V−1 at an optimum Ga concentration (XGa = 0.027) by ∼2.65 times. The change in the Schottky barrier height ΔΦb increased from −4.41 meV (MgZnO) to −4.81 meV (XGa = 0.027) and decreased to −3.99 meV at a high Ga doping concentration (XGa = 0.041). The stress sensitivity (0.2 kgf) enhanced from 28.50 MPa−1 for the pristine MgZnO to 31.36 MPa−1 (XGa = 0.027) and decreased to 25.56 MPa−1 at higher Ga doping concentrations, indicating the synergistic effects of Ga doping and Mg alloying over the pressure sensing performance of Ga-doped MgZnO films.

A New Approach to the Fabrication of Memristive Neuromorphic Devices: Compositionally Graded Films.

Energy-efficient computing paradigms beyond conventional von-Neumann architecture, such as neuromorphic computing, require novel devices that enable information storage at nanoscale in an analogue way and in-memory computing. Memristive devices with long-/short-term synaptic plasticity are expected to provide a more capable neuromorphic system compared to traditional Si-based complementary metal-oxide-semiconductor circuits. Here, compositionally graded oxide films of Al-doped MgxZn1−xO (g-Al:MgZnO) are studied to fabricate a memristive device, in which the composition of the film changes continuously through the film thickness. Compositional grading in the films should give rise to asymmetry of Schottky barrier heights at the film-electrode interfaces. The g-Al:MgZnO films are grown by using aerosol-assisted chemical vapor deposition. The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the films show self-rectifying memristive behaviors which are dependent on maximum applied voltage and repeated application of electrical pulses. Endurance and retention performance tests of the device show stable bipolar resistance switching (BRS) with a short-term memory effect. The short-term memory effects are ascribed to the thermally activated release of the trapped electrons near/at the g-Al:MgZnO film-electrode interface of the device. The volatile resistive switching can be used as a potential selector device in a crossbar memory array and a short-term synapse in neuromorphic computing.

Tailoring microstructure and optical properties of MgZnO film on glass by substrate temperature

Magnesium-doped zinc oxide (MGZO) films were deposited on glass substrates by magnetron sputtering system. The effects of substrate temperature on microstructure and optical properties of MGZO films were firstly investigated. Based on XRD results, the full width at half maximum (FWHM) of MGZO film reaches the value of 0.27° at 300 ℃, illustrating the film quality is enhanced via substrate temperature increasing. The band-gap of MGZO film increases with substrate temperature increasing. Alternatively, the photoluminescence quality of MGZO film can be enhanced by the substrate temperature, resulting from the increasing ratio of I380nm to I540nm. The ratio achieves the value of 3.2 at 500 ℃. This work can offer a feasible approach to tailor the microstructure and optical properties of MGZO film.

Fabrication of flexible UV-B photodetectors made of MgxZn1-xO films on PI substrate for enhanced sensitivity by piezophototronic effect

In spite of intense research in MgZnO films for photodetector application, growth of MgZnO over a flexible substrate especially utilizing piezophototronic effect could hold a more promising approach. We demonstrate high performance UV-B photodetectors fabricated by using high quality, single phase, wurtzite MgxZn1-xO thin films deposited on a polyimide substrate by magnetron sputtering. The Mg content in the MgxZn1-xO films varies from 38.5 at.% to 44 at.% by changing the substrate temperature from room temperature to 250 °C. The MgZnO films have columnar nanorod like morphology and are highly oriented along the c-axis. The metal-semiconductor-metal photodetectors of the MgZnO films have achieved significantly high light to dark current ratio (~1630). The peak responsivity is 0.3 mA/W at 292 nm with a cutoff wavelength of 305 nm at 9 V bias voltage, selectively detecting the UV-B region (280 nm-320 nm) of electromagnetic spectrum and the UV-B-visible rejection ratio (~2.34×105) is much higher than MgZnO PDs on rigid substrates. The performance of the MgxZn1-xO photodetector is further enhanced under applied strain leading to significantly higher photocurrent (~4 µA) and light to dark contrast ratio (~11000), resulting in large boost in the sensitivity to light by 20 % depending on Mg concentration. The sensitivity to light (~20%) at a fixed strain of +0.29 % or -0.29% is also four times higher as that with 0 strain (~5%) demonstrating the synergistic effect of the piezophototronic effect along with the flexible substrate over the performance of the PDs.

Novel ultraviolet photodetector with ultrahigh photosensitivity employing SILAR-deposited ZnS film on MgZnO

Abstract A novel MgZnO/ZnS heterojunction-based ultraviolet (UV) photodetector (PD) with high performance is fabricated by a facile sol-gel process and a successive ionic layer adsorption and reaction (SILAR) method. ZnS is coated onto the MgZnO film as an interface modification layer, which overcomes the drawbacks of the pristine MgZnO photosensitive layer, such as lower carrier mobility and more traps in the material, and greatly enhances UV-light absorption. The type-II heterostructure constructed by work function differences near the interface facilitate the separation of photogenerated carriers. Compared with the MgZnO PD, the optimized heterojunction PD (MgZnO/ZnS-10) yields a dramatically decreased dark current (≈1 nA), a remarkable responsivity (900 A/W) and an ultrahigh photo-to-dark current ratio (up to 2.3 × 105) under 325 nm light illumination at 5 V bias. These results provide a cost-efficient means for improving the properties of MgZnO PDs, and show the advantages of MgZnO/ZnS heterojunction PDs in UV detection. This study demonstrates that rational construction of novel heterojunctions holds great potential for fabricating high-performance photodetectors.

A comparative analysis for effects of solvents on optical properties of Mg doped ZnO thin films for optoelectronic applications

Nanocrystalline Magnesium doped ZnO thin films (MgZnO) were synthesized on glass substrate using five different solvents i.e. 1-Propanol (1Pd), 2-Propanol (2Pd), Ethanol (Ed), Methanol (Md) and 2-Methoxyethanol (MEd) by sol-gel spin coating method. This study targeted the best solvent with MgZnO film for optoelectronic applications. The magnesium doping percentage was fixed at 5% for all samples to discover comparatively better doped results among different solvents. All the films synthesized by different solvents exhibited polycrystalline structure, particularly films with 1-Propanol and 2-Methoxyethanol showed preferred c-axis orientation at 180 °C pre-heating condition revealed by XRD pattern. Highest average crystallite size calculated by XRD study was for 2-Methoxyethanol solvent which possessed good crystalline quality. Crystalline quality was also confirmed by Raman analysis. FESEM images elucidated that all the films were crack free, had uniformly smooth grains and highest grain size for 2-Methoxyethanol solvent. EDS and FTIR spectra confirmed the magnesium presence in ZnO thin films for all solvents. The optical absorbance, transmittance and reflectance spectrum were analyzed by UV–vis spectroscopy. The highest transmittance (94 %), band gap value (3.36 eV) and better quality by PL analysis in comparison were found for 2-Methoxyethanol solvent. The data for optical constants were measured by spectroscopic ellipsometry. Furthermore real and imaginary parts of complex dielectric constant and energy loss functions were examined.

Effect of Annealing on the Electrical and Optical Properties of MgZnO Films Deposited by Radio Frequency Magnetron Sputtering

The properties of phase‐separated MgZnO induced by heat treatment are investigated, because the electrical conductivity of MgZnO deposited by sputtering can be improved by heat treatment. The absorption edge shows a red shift, but the transmittance in the ultraviolet (UV)‐C region increases after heat treatment. This is due to the formation of wurtzite (WZ) MgZnO with a high Zn content and a rock salt (RS) structure with a low Zn content induced by phase separation. This suggests that the current path is formed by the WZ crystals with low resistivity, and the transmittance in the UV region is increased by the RS crystals with high transmittance. It is confirmed that the RS structure is partially mixed with the WZ crystal in the in‐plane direction. In contrast, different distributions of crystals are observed in the stacking direction. A WZ structure is formed after RS‐MgZnO is formed on the substrate interface. It is found that Mg is incorporated into the RS structure by heat treatment, and Mg‐rich MgZnO is formed.

Preparation and Electrical Characterization of B-N Codoped p-type MgZnO Film

Preparation and Electrical Characterization of B-N Codoped p-type MgZnO Film

Performances of surface plasmon enhanced MgZnO ultraviolet photodetector based on parallel effect

The responsivity of surface plasmon (SP) modified metal–semiconductor-metal ultraviolet photodetectors (PDs) was increased from 1.15 × 10−3 to 2.5 × 10−3 A/W by varying the number of interdigitated electrode pairs. MgZnO films with Pt nanoparticles (NPs) having a grain size of 6.26 nm in a 25-pair interdigitated electrode arrangement showed improved responsivity and signal to noise ratio in comparison with the bare MgZnO PD. This improvement was attributed to both the enhanced conductivity and increased absorption resulting from the near-field enhancement of the SP. Parallel effect physical simulation results were in reasonable agreement with our experimental results.

MgZnO/SiO2/ZnO metal–semiconductor–metal dual-band UVA and UVB photodetector with different MgZnO thicknesses by RF magnetron sputter

The MgZnO/SiO2/ZnO metal–semiconductor–metal (MSM) dual-band UVA and UVB photodetectors (PDs) with different MgZnO thicknesses were fabricated by RF sputter. From the dark current, it was found that the PD with 200 nm thick MgZnO had a lower leakage current, which implies less defect density and better crystal quality. Therefore, the FWHM of the X-ray diffraction and grain size of the scanning electron microscope image for PDs with a thicker MgZnO thickness were narrower and larger than those of the others. From the photoluminescence (PL) at room temperature, the main defect types of the MgZnO/SiO2/ZnO thin film included Ov, Oi, and Zni. Then, a variable and voltage-controlled tunable wavelength of UV PD from UVB to UVA can be well accomplished by using a SiO2 blocking layer inserted between the MgZnO and ZnO thin film. Therefore, at a lower and higher bias voltage, the PD with a 200 nm thick MgZnO can detect the UVB and UVA range, respectively.

Back-to-back asymmetric Schottky-type self-powered UV photodetector based on ternary alloy MgZnO

A back-to-back asymmetric Schottky self-powered UV photodetector (PD) with fast speed was realized by a facile combination of ternary alloy MgZnO film and asymmetric Au electrodes. The high-crystalline-quality MgZnO films were synthesized by metal organic chemical vapor deposition. Due to the incorporation of Mg, the band gap of MgZnO film was confirmed to be 3.76 eV. In addition, owing to the large Schottky barrier difference between the active layer and the two asymmetric Au electrodes, the responsivity of the device could reach up to 2.22 mA W−1 at the optical wavelength of 330 nm without any power supply, and with a sharp cutoff at the wavelength of 343 nm. The device exhibits a very low dark current of 247 pA at −5 V. More interestingly, the decay time of our device is only around 92 µs, which is much quicker than any other previously reported UV PDs even with bias voltage applied. Our findings provide a promising approach to realize a high-performance self-powered UV PD.

Synthesis and characterization of F-doped MgZnO films prepared by RF magnetron co-sputtering

Fluorine-doped Mg0.21Zn0.79O (MZO:F) thin films were successfully deposited on glass substrates at room temperature by radio frequency (RF) magnetron co-sputtering method with Mg0.21Zn0.79O, MgF2, and ZnF2 targets. Low resistivity (1.26 × 10-1 Ω⋅cm), high Hall mobility (3.55 cm2/Vs) and high carrier concentration (1.39 × 1019 cm−3) were simultaneously realized in the annealed MZO:F film. The effects of doping concentration and annealing temperature on the structural, morphological, electronic, electrical and optical properties of MZO:F films were systematically investigated in this study. XRD and XPS analysis confirmed that F atoms have entered into crystal lattices of MZO and acted as donors when they occupied O sites in the hexagonal lattice. Moreover, it was observed that both doping concentration and annealing temperature greatly affected the crystallinity, grain size, growth orientation and surface morphology of the MZO:F thin films. In addition, the optical gap changed dramatically with the F content and annealing temperature, which could be attribute to the Burstein-Moss effect accompanied by the effects of fluctuating potential arising from the randomly situated impurities.