MgZnO Films Research Articles

The Effects of Thermal Annealing on the Structural and Optical Properties of MgZnO Films

The Effects of Thermal Annealing on the Structural and Optical Properties of MgZnO Films

Heterojunction light emitting diodes fabricated with different n-layer oxide structures on p-GaN layers by magnetron sputtering

We grew heterojunction light emitting diode (LED) structures with various n-type semiconducting layers by magnetron sputtering on p-type GaN at high temperature. Because the undoped ZnO used as an active layer was grown under oxygen rich atmosphere, all LED devices showed the EL characteristics corresponding to orange-red wavelength due to high density of oxygen interstitial, which was coincident with the deep level photoluminescence emission of undoped ZnO. The use of the Ga doped layers as a top layer provided the sufficient electron carriers to active region and resulted in the intense EL emission. The LED sample with small quantity of Mg incorporated in MgZnO as an n-type top layer showed more intense emission than the LED with ZnO, in spite of the deteriorated electrical and structural properties of the MgZnO film. This might be due to the improvement of output extraction efficiency induced by rough surface.

Annealing effect on crystallization behavior of cubic Mg xZn 1− xO films on A-plane and M-plane sapphire

Cubic Mg x Zn 1 − x O thin films with Mg composition around 70% were deposited on A-plane and M-plane sapphire substrates by rf-reactive magnetron sputtering. Measured structural and optical properties of these thin films indicated an optimal annealing temperature of 700 °C which produced high quality cubic MgZnO thin films on both substrates. Moreover, when the annealing temperature exceeded 750 °C, a much rougher surface resulted, and several large mosaic particles on the surface of the annealed films appeared. From EDX results, the Mg composition was lower than that found in other sections of the annealed films. We attributed this to thermally induced reconstruction of the crystallites. This phenomenon was more obvious for annealed MgZnO films on A-plane sapphire than that on M-plane sapphire. Thermal expansion mismatch with the substrate is the principal reason.

Epitaxial growth of high quality cubic MgZnO films on MgO substrate

Epitaxial growth of cubic Mg 0.33Zn 0.67O films on MgO (1 0 0) substrate by metalorganic chemical vapor deposition (MOCVD) was reported. X-ray diffraction (XRD) ω-scans and ϕ-scans demonstrate the films exhibit single (2 0 0) orientation and highly uniform in-plane orientation with four-fold symmetry. The epitaxial relationship between the layer and substrate is MgZnO (1 0 0)//MgO (1 0 0). Smooth surface with the rms roughness of 1.8 nm in 5×5 μm area is obtained. Atomic force microscopy (AFM) reveals formation of islands attributed to the strain in the epitaxial MgZnO films. These results demonstrate the high quality single crystal Mg 0.33Zn 0.67O films and their potential in high-performance deep ultraviolet optoelectronic devices.

Effect of magnesium incorporation in zinc oxide films for optical waveguide applications

Mg x Zn 1− x O ( x=0.05–0.33) films were successfully deposited by the sol–gel spin coating method. Zinc acetate and magnesium acetate were used as starting precursors and ethanol as solvent to prepare the sol–gel. MgZnO films were deposited on silicon (1 0 0) substrate with varying molar composition and post annealed at 450 °C. Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffractometer (XRD), Ellipsometry, Scanning Electron Microscopy (SEM) and Energy Dispersive Analysis by X-ray (EDAX) was used to characterize the deposited films. Here, effect of Mg doping concentration on the chemical, structural, mechanical and optical properties of Mg x Zn 1− x O films has been investigated. EDAX spectrum clearly shows the peak of Mg along with Zn and O indicating successful incorporation of Mg into the ZnO. From XRD spectra, it reveals that deposited film for 0.15 molar concentration was polycrystalline in nature. We succeeded in tailoring refractive index of MgZnO films from 1.44 to 1.11 corresponding to 0.05 to 0.33 Mg molar incorporation in ZnO. The refractive index was found to be decreased with an increase in Mg molar concentration and the stretching bond positions of fundamental ZnO peak was shifted from 409 to 417 cm −1. Our results explore the applicability of MgZnO as cladding layer material to form efficient heterostructure with ZnO as an active layer.

Ultraviolet Photodetector Based on a MgZnO Film Grown by Radio-Frequency Magnetron Sputtering

The metal-semiconductor-metal ultraviolet (UV) photodetector was fabricated on the Mg(0.47)Zn(0.53)O layer grown by radio-frequency magnetron cosputtering. The photodetector shows the peak response at 290 nm with a cutoff wavelength at 312 nm. It exhibits a very low dark current of about 3 pA at 5 V bias, and the UV-visible rejection ratio (R = 290 nm/R = 400 nm) is more than 4 orders of magnitude. The transient response for the detector was measured, and it was found that the rise time is 10 ns and the fall time is 30 ns. The reason for the short response time is related to the Schottky structure.

Pulsed laser deposition of high Mg-content MgZnO films: Effects of substrate temperature and oxygen pressure

High Mg-content, wurtzite MgZnO films were grown on sapphire (0001) substrates by pulsed laser deposition. It was found that the compositions, defect states, optical, and electrical properties of MgZnO films depend critically on the substrate temperature and oxygen pressure. As the substrate temperature increases, Mg content in single-phase wurtzite MgZnO films increases significantly from 27 close to 46 at. %, and corresponding band gap is widened from 3.49 to 3.88 eV. X-ray photoelectron spectroscopy studies show that plenty of Zn interstitials are present in MgZnO films grown at lower oxygen pressures, which lead to the lattice expansion and the high electron concentration as native donor defects; while the films grown at higher oxygen pressure exhibit high resistivity with indeterminate conductivity type and the relatively wide band gap.

Single-crystalline cubic MgZnO films and their application in deep-ultraviolet optoelectronic devices

By employing a relatively low growth temperature and oxygen-rich conditions, single-crystalline cubic MgZnO films were prepared. A solar-blind deep ultraviolet (DUV) photodetector was finished on the MgZnO film. The maximum responsivity of the photodetector is 396 mA/W at 10 V bias, which is almost three orders of magnitude larger than the highest value ever reported in MgZnO-based solar-blind photodetectors. The dark current density is 1.5×10−11 A/cm2, comparable with the smallest value ever reported in solar-blind photodetectors. The improved performance reveals that the single-crystalline cubic MgZnO films have great potential applications in DUV optoelectronic devices.

Solar-blind 4.55 eV band gap Mg 0.55Zn 0.45O components fabricated using quasi-homo buffers

A route for synthesizing high Mg content single-phase wurtzite MgZnO films having band gaps in the solar-blind region is demonstrated by employing molecular beam epitaxy on Al 2O 3 substrates. Importantly, a low Mg content “quasi-homo” buffer, Mg 0.17Zn 0.83O, was applied to accommodate a host of structural discrepancies and therefore, avoiding phase separation in a high Mg content film, Mg 0.55Zn 0.45O, as proved by X-ray diffraction. The Mg fraction in the overgrown single-phase epilayer, Mg 0.55Zn 0.45O, was confirmed by Rutherford backscattering spectrometry.

Dependence of the MgO sputtering power on the characteristics of MgZnO thin films grown by radio-frequency magnetron sputtering

The dependence of the MgO sputtering power on the structural and optical properties of epitaxially grown MgZnO thin films on GaN/sapphire substrates by radio-frequency magnetron sputtering was investigated. The photoluminescence investigation showed blue shift of 170 meV in MgZnO film grown at the MgO power of 300 W, compared with the ZnO films grown at the MgO power of 0 W, which was attributed to the enhancement of the Mg incorporation at higher power. In addition, increase in Mg mole fraction with increase in sputtering power of MgO was observed from the PL results, and a maximum of 6.6 at.% Mg was obtained at the MgO power of 300 W. The high-resolution X-ray diffraction and transmission electron microscopy (TEM) investigations revealed that the threading dislocation density in the MgZnO thin films increased with increase in sputtering power. Furthermore, microstructural analysis performed by TEM revealed formation of a thin cubic-like phase in the interface between GaN template and MgZnO thin film, together with increased thickness of the interfacial layer with sputtering power.

Characteristics of cubic MgZnO thin films grown by radio frequency reaction magnetron co-sputtering

Cubic MgZnO thin films were prepared on fused quartz substrate by radio frequency (rf) reaction magnetron co-sputtering technique, and composition modulation of cubic MgZnO films could be accomplished by changing rf power on Zn metal target. During growth process crystal quality of our MgZnO films can be improved by increasing the ratio of O 2/Ar. Unlike reported growth of (1 1 1) plane cubic MgZnO films, our cubic MgZnO films show (2 0 0) plane growth properties. This phenomenon was assigned to a preferred selection growth characteristic of MgZnO on non-crystalline quartz substrate. Band gaps of cubic MgZnO films calculated by empirical formula are different from that obtained in the absorption spectrum. It is suggested that parts of Mg do not contribute to the band gaps of Mg x Zn 1− x O thin films.

Enhanced photoluminescence caused by localized excitons observed in MgZnO alloy

Temperature-dependent photoluminescence of MgZnO alloy film has been studied, and it is found that the emission intensity increases significantly at a certain temperature range and then decreases when increasing temperature further. The anomalous increase is resulted from the localized excitons in MgZnO alloy, as revealed by the enhanced second-order longitudinal optical phonon in the Raman spectrum of the MgZnO film. A schematic model was suggested to depict the carrier transportation process in the MgZnO film considering the existence of localized exciton states. The results reported in this paper indicate that localized excitons in MgZnO alloy can result in greatly enhanced emission efficiency, which is eagerly wanted for the application of ZnO-based materials in high-efficiency light-emitting devices.

Characteristics improvement of metalorganic chemical vapor deposition grown MgZnO films by MgO buffer layers

MgZnO films with a small quantity of Mg were grown on c-sapphire substrates coated with a thin MgO buffer layers by metalorganic chemical vapor deposition. The MgO buffer layer causes improvement in the structural, optical, and electrical properties of subsequently deposited MgZnO thin films, when compared to MgZnO films deposited without a buffer layer. The MgZnO films with a MgO buffer layer grown at 330 °C showed the best performance. Transmission electron microscopy revealed that the cubic phase MgO buffer layer promoted the epitaxial behavior of MgZnO, where the planar relationships of the wurtzite-MgZnO/cubic-MgO/sapphire heterostructures mainly were MgZnO(0001)//MgO(001)//sapphire(0001) and MgZnO(11̄00)//MgO(110)//sapphire(112̄0). It resulted in lower lattice mismatch between MgO and MgZnO by domain epitaxy of 2/1 and enhancement in preferred growth of the MgZnO films along the c-axis.

Annealing temperature dependent electrical and optical properties of ZnO and MgZnO films in hydrogen ambient

Un-doped ZnO and MgZnO thin films were deposited on c-plane sapphire substrates by molecular-beam epitaxy (MBE) and subsequently annealed in hydrogen ambient at 200–500 °C with a step of 100 °C. Hall-effect measurements show that annealing temperature has great effect on the electrical property of both ZnO and MgZnO films. The electron concentration of both ZnO and MgZnO films increases with annealing temperature ranging from 200 °C to 400 °C, and then decreases, which is attributed to incorporation of H into ZnO as a shallower donor during the annealing process and change of solid solubility of hydrogen in ZnO and MgZnO films with annealing temperature. The D 0X emission is related to the hydrogen in MgZnO film and the donor level of the H is estimated to be 33.5 meV. It is also found that the controversial luminescence band at 3.310 eV can be formed in un-doped ZnO film upon annealing and its intensity increases with increasing annealing temperature, implying that this band may be not related to p-type doping.

Fabrication of MgZnO films by molecular precursor method and their application to UV‐transparent electrodes

AbstractMgZnO thin films were fabricated by the molecular precursor method (MPM) for the realization of cost‐effective near‐ultraviolet (UV) transparent electrodes for GaN‐based UV light‐emitting diodes (LEDs). The fabrication by MPM requires a common solution. It was clarified that ammonia aqueous is a common solvent for Mg, Zn and Ga precursors. MgZnO thin film was successfully fabricated on a quartz glass substrate using MgZnO precursor solution. The solid phase composition of Mg in the film is 40‐50% of its molar fraction in the liquid phase. The X‐ray diffraction patterns indicate the films have a hexagonal single phase, the same as in the case of ZnO. Ga doping of the MgZnO films enables their resistivity control. The possibility of applying MgZnO films for UV transparent electrodes on GaN‐based UV LEDs is discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural, optical and electrical properties of post annealed Mg doped ZnO films for optoelectronics applications

Mg x Zn1-x O films with 0.15 mole composition of Magnesium were successfully deposited by the spin coating sol–gel method. Zinc acetate dihydrate and Magnesium acetate were used as starting precursors to prepare the solution in ethanol solvent. The MgZnO films were deposited on microscopic glass substrates and post annealed at three different temperatures. X-ray Diffractometer (XRD), Scanning Electron Microscopy (SEM) and UV–VIS Spectrophotometer were used to characterize the deposited films for studying structural and optical properties. Energy dispersive analysis by X-ray (EDAX) was used to determine incorporation of Mg content in ZnO films. XRD spectrum reveals that, the deposited Mg doped ZnO films were polycrystalline in nature. The intensity of c-axis in the XRD spectrum goes on decreasing as Mg composition slightly increasing corresponding to increase in annealing temperature. EDAX spectra clearly showed the incorporation of Mg into the ZnO films. Semiconductor characterization system was used for the I–V characterization of MgZnO films. I–V characteristics show decrease in current as increase in the biased voltage. Optical band gap of MgZnO films was found to be increased from 3.2 to 3.38 eV as estimated from the absorption coefficients.

Ultraviolet Photodetector Based on Mg<formula formulatype="inline"> <tex Notation="TeX">$_{\rm x}$</tex></formula>Zn<formula formulatype="inline"> <tex Notation="TeX">$_{1 - {\rm x}}$</tex></formula>O Thin Films Deposited by Radio Frequency Magnetron Sputtering

The Mg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O (0 les x les 0.36) films were deposited on an Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate by radio frequency magnetron sputtering. The X-ray diffraction analysis showed the deposited MgZnO films have a single-phase hexagonal wurtzite structure. The optical bandgap of Mg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O films exhibited a linear increase from 3.25 to 4.04 eV with the Mg content x increasing from 0 to 0.36. Correspondingly, the cutoff wavelength of the resultant detectors varies from 380 to 310 nm, indicating that the detecting wavelengths of the Mg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O metal-semiconductor-metal ultraviolet photodetectors (MSM-UPDs) can be controlled by simply adjusting the Mg contents in the film. The resultant MgZnO MSM-UPDs with various Mg contents exhibit promising performances. At a bias voltage of 20 V, the dark currents of all MSM-UPDs are smaller than 0.6 nA. The rejection ratio of the MSM-UPDs varies from 701 to 769, slightly dependent on the Mg contents.

Effect of growth ambient on the structure and properties of MgxZn1−xO thin films prepared by radio-frequency magnetron sputtering

MgxZn1−xO thin films were grown by the radio-frequency magnetron sputtering technique in Ar + N2 and O2 + N2 ambient with different nitrogen partial pressure ratios on quartz substrates at 773 K. The correlations between the growth ambient and the resultant films properties were discussed. It is found that Mg concentration, structure and band gap of the MgZnO film can be tuned by increasing the nitrogen partial pressure ratio of the Ar + N2 ambient, but cannot be tailored by changing the nitrogen partial pressure ratio of the O2 + N2 ambient. X-ray diffraction shows that the MgxZn1−xO films grown in the Ar + N2 ambient consist of wurtzite phase at the nitrogen partial pressure ratios from 0% to 50%, a mixture of wurtzite and cubic phases at the ratios of 78–83%, and a cubic phase at 100%, whereas such structure transformation did not exist in the MgxZn1−xO film deposited in the O2 + N2 ambient by changing the nitrogen partial pressure ratio. The Mg concentration increases linearly with the increasing nitrogen partial pressure ratio of Ar + N2, but does not vary with the increasing nitrogen partial pressure ratio of O2 + N2. The band gap increases linearly from 3.64 to 4.02 eV for the wurtzite MgxZn1−xO deposited in the Ar + N2 ambient as nitrogen partial pressure ratio increases from 0% to 50%, but the band gap of the MgxZn1−xO film deposited in the O2 + N2 ambient keeps a constant of 3.63 eV with oxygen gas and reaches 6.30 eV abruptly in pure nitrogen ambient (without oxygen gas). The MgxZn1−xO films, which are grown in the Ar + N2 and O2 + N2 ambient at nitrogen partial pressure ratio from 50% to 100% and 83% to 100%, respectively, show p-type conduction after annealing.

Annealing effects on electrical properties of MgZnO films grown by pulsed laser deposition

Electrical properties and deep trap spectra are reported for MgZnO(P) films grown by pulsed laser deposition on undoped n-ZnO substrates. The as-grown films are n type with a “bulk” donor concentration of ∼1018cm−3 and have a compensated high resistivity layer near the surface. Deep trap spectra in these films are dominated by electron traps with an activation energy of 0.3eV, hole traps with an activation energy of 0.14eV, and some unidentified electron traps with a barrier for capture of electrons. After annealing in oxygen at 800°C the MgZnO(P) becomes p type, with the dominant hole traps having an activation energy of 0.2eV. The space charge region of the formed p-n junction is mainly located in the n-ZnO substrate. The main hole traps in this part of the heterojunction have activation energies of 0.14 and 0.84eV, while the main electron traps have activation energies of 0.15 and 0.3eV.

Photoluminescence and acceptor level state of p-type nitrogen-doped MgZnO films

A wurtzite nitrogen-doped MgZnO (MgZnO:N) film was grown by plasma-assisted molecular-beam epitaxy (PAMBE) on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown film shows n-type conduction at room temperature, but transforms into p-type conduction after annealed. Photoluminescence (PL) spectrum measured at 80 K is dominated by neutral donor-bound exciton emission (D0X) located at 3.522 eV for the n-type MgZnO:N film, but by neutral acceptor-bound exciton emission (A0X) located at 3.515 eV for the p-type MgZnO:N film. By fitting exciton emission intensity of temperature-dependent PL spectra, the binding energies of the D0X and A0X were estimated to be 32 and 43 meV, respectively. Based on the energy shift of exciton emission, the band gap of the MgZnO:N film is estimated to be 3.613 eV, which is 179 meV larger than that of ZnO. Using the Haynes rule, the acceptor energy level of the MgZnO:N film was evaluated to be about 176 meV above the valence band.