MgxZn1-xO Films Research Articles

Improved performance of MgxZn1-xO flexible UV photodetectors by piezo-phototronic effect through alloying treatment

MgxZn1-xO films (x = 0.22, 0.39, and 0.52) with a single hexagonal wurtzite structure were grown on flexible polyethylene terephthalate (PET) substrates using radio frequency (RF) magnetron sputtering technique. The piezo-phototronic properties of flexible ultraviolet (UV) photodetectors with MgxZn1-xO films were investigated by examining the relationship between their photoelectric characteristics and strain, and the impact of Mg alloying on their piezo-phototronic effect. As the tensile strain of the detectors increased from ε = 0 to ε = 0.2, the responsivity of Mg0.22Zn0.78O, Mg0.39Zn0.61O, and Mg0.52Zn0.48O flexible UV photodetectors increased by ∼ 260 %, ∼308 %, and ∼ 441 %, respectively, showing an enhanced piezo-phototronic effect with increasing Mg content. It could be attributed to the fact that the alloying process increased the piezoelectric coefficient of the MgxZn1-xO materials, thereby increasing the piezopotential. This work provides a technical approach for the preparation and performance optimization of novel flexible optoelectronic functional devices.

Investigating mechanism of photo refractivity in Mg substituted ZnO thin films using pump and probe technique

In the current investigation magnesium (Mg) has been incorporated in the ZnO matrix to induce photo refractivity thereby adding an extra dimension to its already present multi-functionality. Undoped and MgxZn1-xO (x = 0.25, 0.30, 0.35, 0.40, 0.45) thin films have been grown using Chemical Solution Deposition (CSD) technique. Obtained X-Ray Diffraction (XRD) spectra exhibits a slight shift in the peak value along (002) plane whereas UV–visible spectra indicates corresponding widening of ZnO band gap values (3.24–3.64 eV) with increase in Mg content. Mg substituted ZnO films were then investigated for their ferroelectric properties using a P-E hysteresis loop tracer. It was noted that ZnO film with 35 % Mg content (x = 0.35) exhibited the high values for saturation (Ps = 1.8 μC/cm2) and remnant (Pr = 0.7 μC/cm2) polarization. Hence the ZnO with same Mg concentration was investigated for nonlinear optical properties (two wave mixing phenomenon) using pump and probe technique. In order to enhance the beam amplification gain and impart stability Au micro-dots were thermally evaporated onto MgxZn1-xO (x = 0.35) films. The maximum beam amplification gain was noted to shoot up to 2.7 post patterning of MgxZn1-xO (x = 0.35) film surface by Au micro-dots.

Suppressing interface recombination via element diffusion regulation towards high-efficiency Cd-free Cu(In,Ga)Se2 solar cells

Magnetron-sputtered Zn1-xMgxO (ZMO) films suffer from severe elemental diffusion to Cu(In,Ga)Se2 (CIGS), which limits the further improvement of device performance. Herein, we introduced Zn(O,S) (ZOS) as a barrier layer, and we controlled the diffusion of Zn within an appropriate range by adjusting the annealing temperature of ZOS. The results indicate that Zn can occupy copper vacancies (VCu) in CIGS and easily diffuse through VCu. A moderate amount of Zn at the interface acts as a charge compensator, reducing the band tailing effect and promoting the carrier collection efficiency of photovoltaic (PV) devices. However, excessive Zn doping induces additional defects dominated by ZnCu, ZnGa and ZnIn, leading to severe nonradiative recombination. Consequently, the Cd-free CIGS solar cell produced based on these insights exhibited a 49.4-mV reduction in the Voc deficit (Voc,def) as well as a notable increase in the power conversion efficiency (PCE) to 18.0 %. This study is the first to reveal the mechanism of defects caused by Zn diffusion, and these findings provide theoretical support for the study of Cd-free thin-film solar cells.

Influences of Mg concentration in ZnMgO film on energy band alignment at CIGSSe/Zn1-xMgxO interface and performances of CIGSSe solar cells

Zn1-xMgxO buffered CdS free CIGSSe solar cell has been believed to be a new simplified structure solar cell with great potential. ZnMgO film plays an important role. To obtain a high quality Zn1-xMgxO thin film becomes a key issue. In this work, Zn1-xMgxO films with different Mg concentrations were prepared by magnetron co-sputtering, and physical performances of Zn1-xMgxO films were investigated. Meanwhile, CdS free CIGSSe solar cells were prepared based on Zn1-xMgxO buffer layers. The effects of Mg concentration in Zn1-xMgxO buffer layers on device performances, energy band alignment, and interface recombination of CIGSSe solar cells were studied. The results show that although the doping of Mg reduces the crystallinity of Zn1-xMgxO films, it increases the band gap by increasing the conduction band minimum. In Zn1-xMgxO buffered CdS free CIGSSe solar cells by all-sputtering process, the photoelectric conversion efficiency increases first and then decreases with the increase of Mg concentration in the x range from 0 to 0.3, which is mainly attributed to the changes of FF and VOC. Meanwhile, the change of Mg concentration affects the energy band alignment at the CIGSSe/Zn1-xMgxO interfaces. The highest efficiency of 15.2 % of Zn0.8Mg0.2O buffered CdS free CIGSSe solar cell with the energy band difference of 0.27 eV was obtained in this work.

Fabrication of Zn1-xMgxO/AgyO heterojunction diodes by mist CVD at atmospheric pressure

We report on the preparation of heterojunction based on Zn1-xMgxO/p-AgyO semiconductor heterostructure. A series of multi-layers, containing Zn1-xMgxO (ZnMgO) films stacked on InSnO (ITO) films, were prepared using mist chemical vapor deposition system with different flow rates for the Mg carrier gas/dilution gas (c.g./d.g.). AgyO films then were deposited on the ZnMgO/ITO substrates. It was found that as the flow rate of the Mg c.g./d.g. increased, the morphology and crystallinity of the ZnMgO films were extensively impacted by the Mg content and the cross-section images of the ZnMgO/AgyO HJDs showed spontaneous order of the random alloys and partial inclinations of the multi-layers, the band gaps of ZnMgO broadened from 3.3 to 3.7 eV, and resistivity increased to 1.58 × 108 Ω·cm; the work function of AgyO films increased to 5.6 eV at the highest O2 flow rate (R[O2] = 30%) during the sputtering process. Although the energy gap variations caused abrupt interfaces, these ZnMgO/AgyO HJDs demonstrated rectifying behavior with a barrier height of around 0.98 eV, which is comparable to that achieved using similar but more expensive preparation methods.

Investigation of properties of ZnO and MgxZn1-xO films prepared by sol-gel method

ZnO is considered to be the most promising material to replace ITO. Therefore, there are many studies on ZnO at present. In this paper, the electrical and optical properties of ZnO film as transparent electrode are investigated, and the process parameters in this field are explored. ZnO films and MgxZn1-xO filmswere prepared on glass substrates by sol-gel spin coating method. X-ray diffractometer (XRD), field emission scanning electron microscope (SEM), ultraviolet-visible spectrophotometer (UV–vis) and four-probe experiment were used to characterize the films. The results show that the ZnO film gets the best comprehensive performance when the number of spin coating is 5 times and the annealing temperature is 500 °C. And the resistivity of the film decreases with the increase of Mg doping concentration. This work is the practical application research of ZnO, which is very important and meaningful.

Enhancement of H2 gas sensing properties of ZnO films by Mg alloying

To examine the effect of Mg addition on the gas sensing properties of ZnO, MgxZn1-xO epitaxial films with a constant thickness, in-plane crystal grain size, and surface morphology and varying Mg contents were prepared by pulse laser deposition, and their gas sensing properties were investigated. The results showed that the response to H2 gas improved as the number of Mg atoms substituted into Zn sites increased. In addition, as the thickness of MgxZn1-xO films with the optimal Mg content decreases, the sensor response increases remarkably. A sensor with a thickness of 9 nm showed a maximum response value of approximately 3000 to 800 ppm H2. To determine why the sensor response was improved by the addition of Mg, MgO-particle-deposited undoped ZnO films and c-axis-oriented MgxZn1-xO films with random in-plane rotation domains were prepared, and the characteristics of these sensors were compared with those of the epitaxial MgxZn1-xO films. We identified two causes for the improved sensing properties: a reduced residual electron concentration in the films due to the substitution of Mg and the chemical sensitization effect of Mg atoms. This study also suggested that a decrease in electron mobility results in a decreased sensor response value.

Effects of Thermal Annealing on the Properties of Zirconium-Doped MgxZn1-XO Films Obtained through Radio-Frequency Magnetron Sputtering.

Zirconium-doped MgxZn1−xO (Zr-doped MZO) mixed-oxide films were investigated, and the temperature sensitivity of their electric and optical properties was characterized. Zr-doped MZO films were deposited through radio-frequency magnetron sputtering using a 4-inch ZnO/MgO/ZrO2 (75/20/5 wt%) target. Hall measurement, X-ray diffraction (XRD), transmittance, and X-ray photoelectron spectroscopy (XPS) data were obtained. The lowest sheet resistance, highest mobility, and highest concentration were 1.30 × 103 Ω/sq, 4.46 cm2/Vs, and 7.28 × 1019 cm−3, respectively. The XRD spectra of the as-grown and annealed Zr-doped MZO films contained MgxZn1−xO(002) and ZrO2(200) coupled with Mg(OH)2(101) at 34.49°, 34.88°, and 38.017°, respectively. The intensity of the XRD peak near 34.88° decreased with temperature because the films that segregated Zr4+ from ZrO2(200) increased. The absorption edges of the films were at approximately 348 nm under 80% transmittance because of the Mg content. XPS revealed that the amount of Zr4+ increased with the annealing temperature. Zr is a potentially promising double donor, providing up to two extra free electrons per ion when used in place of Zn2+.

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.

Solar-Blind Photodetector with Lower Dark Current and Higher Ilight/Idark Ratio Based on Mg0.38Zn0.62O Film Deposited by Pulsed Laser Deposition Method.

In this study, pulsed laser deposition method (PLD) was employed to grow MgxZn1-xO films on quartz substrates. The optimal deposition temperature of 300 °C for MgxZn1-xO film was decided and Mg0.38Zn0.62O, Mg0.56Zn0.44O and Mg0.69Zn0.31O films were grown respectively using MgxZn1-xO targets with different Mg contents (x = 0.3, 0.5 and 0.7). As-deposited Mg0.38Zn0.62O film possessed the mixed-phase (hexagonal and cubic phase) structure, appropriate band gap of 4.68 eV and smaller surface roughness of 1.72 nm, and the solar-blind photodetector (PD) based on it was fabricated. The key features of our PD are the cutoff wavelength of 265 nm lying in solar-blind band, lower dark current (Idark) of 88 pA, higher peak responsivity of 0.10 A/W and bigger Ilight/Idark ratio of 1688, which provide the new idea for the application of solar-blind PDs based on MgxZn1-xO films.

Microstructural and optical properties of high-quality Mg–Zn oxide thin films

The microstructural and optical properties of Mg–Zn oxide thin films deposited by aerosol assisted chemical vapor deposition onto fused silica substrates were studied as a function of the Mg concentration in the precursor solution, which was varied from 0 to 100 mol %. The thickness of all the samples was around 150 ± 20 nm. Scanning electron microscopy and grazing incidence x-ray diffraction were used for the analysis of the morphology, composition, and crystalline structure. Outcomes showed a gradual evolution of the microstructural and optical properties of the films as the Mg content increased. The systematic increase of the lattice parameter a, and the decrease of parameter c were observed with the increase of Mg concentration, nevertheless, the unit cell volume remained nearly constant, around 0.0471 ± 0.0007 nm3. Energy dispersive x-ray microanalysis results indicated that Mg /(Zn + Mg) atomic ratio (x) up to 0.59 was found for a single-phase hexagonal MgxZn1-xO film. This value is higher than most of the reported Mg content in wurtzite ZnO. At higher Mg loadings (>70 mol % in solution), the segregation of periclase MgO phases occurred. X-ray photoelectron spectroscopy confirmed the composition of selected Mg doped ZnO films and the substitution of Zn+2 by Mg+2 atoms. The Williamson-Hall analysis of diffracted peaks was employed to determine the micro-strain in the films, it was found ∈ = 0.2 ± 0.1 % with a slight tendency to decrease ∈ as x increases. For single-phase hexagonal MgxZn1-xO films, the computation of the dispersion relations of the optical constants was achieved using direct transmittance and near-normal absolute reflectance. For this objective, TFcalc software was used to fix the Sellmeier dispersion relation parameters by fitting the modeled transmittance and reflectance spectra of the stack film-substrate with corresponding experimental spectra. A detailed analysis of the optical band gap energy and its correlation with the microstructural characteristics of the films was performed, in particular with lattice parameters. The modification of optical band gap energy and lattice parameters with the inclusion of Mg in the wurtzite structure can be explained because the MgO4 unit has a compressed tetrahedral geometry with a much shorter height than those of ZnO4 tetrahedra, and due to the change in electronegativities and ionic radius between Mg and Zn, the bonding character becomes more ionic, and the increased localization of Zn 3d and O 2p orbitals. The optical band gap energy was tuned by changing the Mg content, from about 3.3 eV of undoped ZnO up to 4.3 eV of hexagonal MgxZn1−xO (x ~ 0.59). Moss relation was used to analyze the relationship of optical band gap energy and refractive index.

The Optical Blueshift Saturation Behavior of Mg-=SUB=-x-=/SUB=-Zn-=SUB=-1-x-=/SUB=-O Films

A phenomenon of blueshift saturation was observed from cathodoluminescence (CL) spectra of MgxZn1-xO films (0.069≤x≤0.8) which were grown by pulsed laser deposition technology. By analyzing the results of composition-dependent X-ray diffraction spectra, scanning electron microscopy images, absorption spectra and CL spectra, the crystalline structural disorder was determined via Urbach energy value. Furthermore, a competition mechanism between band-filling effect and band tail states was proposed in the composition-dependent CL spectra. This competition is believed to be responsible for the composition-induced blueshift saturation often observed in the disordered alloy semiconductors. The results of this research can provide important reference in energy-band engineering.

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.

Zn1−xMgxO second buffer layer of Cu2Sn1−xGexS3 thin-film solar cell for minimizing carrier recombination and open-circuit voltage deficit

Zn1−xMgxO (ZMO) second buffer was applied to replace conventional ZnO second buffer layer of Cu2(Sn,Ge)S3 (CTGS) solar cells to reduce interface carrier recombination and open-circuit voltage deficit (VOC,def). Structure of the solar cell is glass/Mo/CTGS/CdS first buffer/ZMO second buffer/ZnO:Al (AZO)/Ni-Al. Bandgap energy (Eg) of the ZMO films is changed from 3.20 to 3.65 eV by varying Mg/(Mg + Zn) ratio from 0 (for pure ZnO) to 0.26. The increase in the Eg of ZMO films is attributable to the move of conduction band minimum (EC). It is determined that ZMO second buffer with increase in Eg from 3.20 to 3.55 eV yields the smoothing conduction band offset (CBO) at ZMO/CdS interface varying from −0.26 to +0.09 eV. Saturation current density (J0) is therefore reduced to the lowest value, and activation energy of recombination (EA) is the closet to the Eg of the CTGS absorber layer, implying the lowest interface carrier recombination (reduced VOC,def), and thereby increasing conversion efficiency (η) to 4.5% (Eg of ZMO: 3.55 eV). On the other hand, the ZMO second buffer with Eg larger than 3.55 eV results in the CBO at ZMO/CdS interface of +0.19 eV (Eg of ZMO: 3.65 eV), making the well of EC structure in ZMO/CdS/CTGS stacking layers, thus increasing interface carrier recombination, and severely decreasing the η to about 0.2%.

Solar-Blind Photodetector with Lower Dark Current and Higher Ilight/Idark Ratio Based on Mg0.38Zn0.62O Film Deposited by Pulsed Laser Deposition Method.

In this study, pulsed laser deposition method (PLD) was employed to grow MgxZn1-xO films on quartz substrates. The optimal deposition temperature of 300 °C for MgxZn1-xO film was decided and Mg0.38Zn0.62O, Mg0.56Zn0.44O and Mg0.69Zn0.31O films were grown respectively using MgxZn1-xO targets with different Mg contents (x = 0.3, 0.5 and 0.7). As-deposited Mg0.38Zn0.62O film possessed the mixed-phase (hexagonal and cubic phase) structure, appropriate band gap of 4.68 eV and smaller surface roughness of 1.72 nm, and the solar-blind photodetector (PD) based on it was fabricated. The key features of our PD are the cutoff wavelength of 265 nm lying in solar-blind band, lower dark current (Idark) of 88 pA, higher peak responsivity of 0.10 A/W and bigger Ilight/Idark ratio of 1688, which provide the new idea for the application of solar-blind PDs based on MgxZn1-xO films.

Effect of plasmon–phonon interaction on the infrared reflection spectra of MgxZn1-xO/Al2O3 structures

The effect of plasmon–phonon interaction on the vibrational properties of the MgxZn1-xO films on anisotropic substrate is studied versus Mg content (x), film thickness (0.5–20 μm), free carrier concentration (1×1016–5×1018 cm−3) and damping coefficients using infrared reflection spectroscopy. The mathematical model with additive and phenomenological contribution of several oscillators to dielectric permittivity of MgxZn1-xO material was developed. The infrared reflection spectra were simulated in the range of “residual rays” of the film and the substrate for MgxZn1-xO/Al2O3 structures using self-consisted parameters of bulk ZnO, MgO and Al2O3 materials. Based on the Kramers–Kronig relationship, the frequency range where film reflectivity is sensitive to the variation of film doping and thickness was determined. The frequencies and damping coefficients of TO and LO modes of the oscillators, static and high-frequency dielectric permittivity for orientation E⊥c were obtained with high accuracy. Main attention was paid to the compositions which were in hexagonal structure. Experimental infrared reflection spectra were recorded for the films with x = 0.25 additionally doped with manganese and their simulation was performed based on the model developed. The free carrier concentration and mobility as well as film conductivity were determined. The results obtained showed the utility of infrared reflection spectroscopy for the investigation of textured alloy films. This non-destructive and contactless method can be implemented for the determination of optical properties of other semiconductor films.

Enhanced efficiency of quantum dot light-emitting diode by sol-gel derived Zn1-xMgxO electron transport layer

Abstract In this study, sol-gel derived Zn1-xMgxO (ZMO) is proposed as an electron transport layer (ETL) for solution-processed quantum-dot light-emitting diodes (QLEDs). It is demonstrated that the increase of Mg content in Zn1-xMgxO films from 0% to 20% causes a dramatic suppression of electron current, which is attributed to the lifting of conduction band minimum and reduction of electron mobility. As a result of Mg-doping, the charge carrier balance might be achieved in the QLED with the Zn0.85Mg0.15O layer resulting in maximum external quantum efficiency of 5.74% and current efficiency of 18 cd A−1, which are over 3-fold higher than in the case of the device with ZnO layer. Improved device performance is further explained by reduced exciton quenching at QDs/ZMO interface, which is confirmed by time-resolved PL experiments. Obtained results indicate that sol-gel derived ZMO is a promising candidate for ETL in quantum-dot based optoelectronic devices.

Effect of thermal annealing on microstructure and optical properties of Zn1−xMgxO thin films grown by hydrothermal method

Zn1−xMgxO films were prepared by hydrothermal method to investigate the effect of thermal annealing on the microstructure and optical properties. The results show that the films exhibit mixed cubic-hexagonal phases. With increasing of calcination temperature from 500 to 1000 °C, the peak intensities of MgO (200) and ZnO in the XRD patterns increases, the absorption edge shows a blue shift. And the band gap broadens from 3.43 to 4.03 eV while the average transmittance from 450 to 750 nm decreases from 88 to 79 at.%. The first-principles calculations reveal that the Zn1−xMgxO is direct band gap semiconductor. When the Mg content in the Zn1–xMgxO films increases with rising the calcination temperature, the CBM determined by the Zn-4s and Mg-2p states shifts significantly to the high energy range and the VBM determined by the O-2p state holds constant at Gamma point. This is the main reason for the band gap broadening of the films with increasing the calcination temperature.

Excitation-current-density and temperature dependences of deep UV cathodoluminescence in rocksalt-structured MgxZn1-xO films

Excitation-current-density and temperature-dependent cathodoluminescence (CL) spectra were measured for rocksalt-structured MgxZn1-xO films with x = 0.92–0.61. A deep UV luminescence (DUVL) band was predominantly observed in an energy range of 4.91–5.86 eV for all the films at 6–300 K. The analysis based on a rate equation model confirms that the DUVL band is attributed to the near-band-edge emission. Temperature-dependent CL spectra were quantified in terms of peak energy shifts and full width at half maximum values. Relatively high equivalent internal quantum efficiencies of 2.5%–11% were obtained for the RS-MgxZn1-xO films.

Characterization of co-sputtered MgxZn1-xO thin films and their application in CdTe solar cells

In this paper, MgxZn1-xO (MZO) films with x from 0 to 0.25 were prepared by radio frequency (RF) magnetron co-sputtering, and the structure, surface morphology, optical and electronic properties of MZO films were investigated. With the increase of Mg content, the crystallinity of MZO films became worse with the appearance of grain agglomeration, and the optical energy band gap (Eg) linearly changed from 3.228 eV to 3.824 eV. The MZO films with different Mg content were used as the window layer for CdTe solar cells with or without CdS layer. The efficiency of MZO/CdTe devices is much lower than MZO/CdS/CdTe devices. Incorporating a buffer layer CdS into CdTe solar cells can significantly improve the devices efficiency, and the devices still maintain a high spectral response in the short wavelength region.