MgZnO Alloy Research Articles

Magnesium zinc oxide detectors for fast ultraviolet detection

We explored a Mg-alloyed ZnO material (or MgZnO alloy) on a lattice-matched scandium aluminum magnesium oxide substrate for obtaining most effective photodetectors with highest (∼10 A/W) and fastest (up to 3 GHz) responses and two orders of magnitude UV/VIS rejection ratio. Device operation was related to its material electronic properties (carrier lifetime, diffusivity, and diffusion length) via photoluminescence and light-induced pump–probe techniques. Defect-related response is found suitable for sensitive logarithmic detectors, while exciton-related response is favorable to ultrafast linear UV solar blind detectors. Two-contact planar detector design allows their application in low-cost environmental detection systems.

DFT Study of Structural and Electronic Properties of MgZnO Alloy

Structural and electronic properties of magnesium zinc oxide alloy with different Mg content and different configurations are studied by density functional theory. The energy track of rocksalt and wurtzite phases of MgZnO with different Mg content indicates that below 63% the wurtzite phase is energetically stable and above 63% the rocksalt phase is stable. Distribution of Mg in the MgZnO alloy is important to understand the phase segregation. By analyzing the distribution of Mg in the MgZnO alloy, we find that the energy required for two Mg to stay together is slightly larger than that for two Mg to stay apart, meaning that Mg has the potential to be uniformly distributed in the MgZnO alloy. By comparing with other dopants, we find that Mg introduces smaller lattice distortion. The corresponding electronic properties are studied by analyses of the density of state and the band structure. From the decomposed-projected band structure, we find that the energy level of O 2p orbitals is modulated to lower energy by introducing Mg, indicating that the enlargement of the band gap is partly caused by the subsidence of the valence band. For comparison, Cd modulates O 2p orbitals to a higher energy level, consistent with the presence of a narrow band gap.

Structural and electronic properties of wurtzite MgZnO and BeMgZnO alloys and their thermodynamic stability

Structural and electronic properties of MgZnO and BeMgZnO alloys are studied by the ab-initio Density Functional Theory method. Large band gap bowings are found for both kinds of alloys. The total energies as functions of the lattice constants are calculated and used to determine the ranges of composition in which the alloys are stable in the wurtzite structure. It is shown that the addition of 6% of Be can already help in stabilization of the MgZnO alloy in the wurtzite structure. The band gap can reach 7 eV for the wurtzite BexMg0.5Zn0.5-xO alloys with x approaching 0.5 and about 5.0 eV for Be0.125MgxZn0.875-xO type alloys for x approaching 0.6. Varying the alloy composition according to the presented stabilization diagram showing ranges of the x, y, for which BexMgyZn1-x-yO is stable in the wurtzite phase, one may tune band gaps over a wide spectral range, which provides flexibility in band gap engineering.

High responsivity solar blind photodetector based on high Mg content MgZnO film grown via pulsed metal organic chemical vapor deposition

Metal-semiconductor-metal (MSM) structured solar blind photodetectors were fabricated based on various Mg content wurtzite MgxZn1−xO epitaxial films grown via pulsed metal organic chemical vapor deposition. The response spectra of the devices showed a peak position that shifts from ∼383nm to 276nm for Mg content, x, between 0.0 and 0.51, covering a wide portion of the ultra-violet spectral region, extending well into the solar blind window. At 10V bias, a large responsivity of ∼1.8×104A/W was obtained at 276nm for a device based on a high Mg content (x=0.51) MgZnO film. To the best of our knowledge, this responsivity is the highest ever reported for a MgZnO based device and its origin is attributed to large internal gain resulting from carrier trapping at the MgZnO/Ni/Au interface. This is confirmed by the presence of an asymmetric Schottky barrier height on the two MSM contacts. Conversely, the response speed of the devices was slow with the 10%–90% rise and fall times measured to be in the millisecond range. The results reported in this work show the realization of high responsivity MgZnO based solar blind photodetectors, providing a significant step in the development of MgZnO alloy based of detector.

The impact of Mg content on the structural, electrical and optical properties of MgZnO alloys: A first principles study

The structural, electronic and optical properties of wurtzite MgZnO with Mg concentration ranging from 0 to 0.5 are studied using the first principle calculations. It's found that the lattice constants c and specific volume V of the MgZnO alloys decrease and their band gap widens as Mg concentration increases, which are in agreement with our experimental results. A particular Mg concentration is found to exist at around 0.375, equals to which the corresponding MgZnO alloy has the minimum width of the top valence band. This indicates that Mg concentration may be used to tune the electronic properties of MgZnO alloy. Meanwhile, it's also found that the energy response range of the optical spectrums decreases with the increase of Mg concentration. There are different energy shifts toward high energy (blue shift) of the peaks in the optical spectrums with the increase of Mg concentration, which are explained by the variations of the density of states in details. So the electronic and optical properties of MgZnO may be tuned through Mg concentration, and our research results may provide meaningful references to the development and design of photoelectric devices.

Improved photoluminescence performance of MgZnO films by alloying beryllium

Abstract We investigate the photoluminescence properties of MgZnO and BeMgZnO thin films that have been grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. Alloying MgZnO with Be results in the suppression of defect-related visible emission confirmed by the photoluminescence measurements and the decrease of background electron concentration of MgZnO confirmed by Hall measurements. Calculated formation energy of V O defects based on density functional theory (DFT) reveals that Be incorporation can suppress the formation of V O defects and results in the improvement of optical and electrical properties in MgZnO alloy. The Be source temperature plays an important role in determining the surface morphology, electrical and optical properties of BeMgZnO film. An optimized alloy is achieved at the Be source temperature 880 °C. The present work is of interest for developing a method to improve PL performance of MgZnO films.

Formation of MgZnO alloy under thermodynamic conditions

MgxZn1−xO ceramics with x=0.10, 0.15, 0.20, 0.25 and 0.30 were sintered at Ts=700–1250°C. Photoluminescence (PL) and PL excitation (PLE) spectra as well as X-ray diffraction patterns were measured at 293K. Bandgap width was evaluated from the position of PLE peak. Non-monotonous dependence of PLE peak position on Ts was observed for all used x. This effect was concluded to be caused by the formation of cubic phase side by side with hexagonal one and the enhancement of this process with increasing Ts, which resulted in the decrease of Mg content in the hexagonal phase at Ts>1000°C. Temperature range of 1000–1050°C was found to be the optimum one for the formation of hexagonal MgZnO alloy at used x. It was shown that x=0.20 was the solubility limit of MgO in hexagonal MgxZn1−xO ceramics.

Ultraviolet Photodetectors Based on MgZnO Thin Films

In recent years, MgZnO alloy becomes one of the most suitable materials for the fabrication of ultraviolet detectors. In this paper, we have fabricated three metal semiconductor metal (MSM) photodetectors on Mg0.42Zn0.58O film grown by radio frequency magnetron sputtering. The interdigital electrodes are 500 um long and 5 um wide with an interelectrode spacing 2, 5 and 10 um, respectively. The structural, electrical and optical properties of epilayers were characterized by various techniques. At 5 V bias, a peak responsivity of 1.09 mA/W was achieved at 283 nm for the device with 2 um interelectrode spacing. The peak responsivity at 283 nm increased with the reduction of the finger pitch for three devices and the ultraviolet-visible rejection ratio (R283 nm/R400 nm) was more than one order of magnitude at 5 V bias.

Solar-blind wurtzite MgZnO alloy films stabilized by Be doping

MgxZn1−xO alloy films were deposited on c-plane sapphire substrates by radio frequency plasma-assisted molecular beam epitaxy (rf-PMBE). The phase segregation occurred when x was larger than 33%. Be doping was found experimentally able to stabilize the high-Mg-content MgZnO alloy. By alloying 1–2% Be into MgZnO, the band gap of as-prepared quaternary alloys can be raised to the solar-blind range (4.5 eV). Calculated formation energy of the alloys based on first principle reveals that a small amount of Be incorporation can reduce the formation energy of high-Mg-content MgZnO alloys and results in a more stable system, which justifies our experimental observations.

Probing embedded structural inhomogeneities in MgZnO alloys via selective resonant Raman scattering

The issue of phase segregation, inherent to the MgZnO alloy system, was investigated via selective resonant Raman scattering. We demonstrate that it is a highly sensitive technique for the detection of embedded structural inhomogeneities. MgZnO thin-films with bandgaps that span the UV-range of 3.2–5.7 eV were realized. Under resonant conditions facilitated via different laser excitation energies, the LO-phonon behavior indicated that the phase segregation is in the range of 35%–65% Mg, in which domains of hexagonal-wurtzite and cubic-NaCl structures coexist. The scattering of the forbidden LO-mode of the cubic phase is discussed in terms of inversion-symmetry relaxation due to alloying.

Sol–Gel Route to MgZnO Alloy Particles via Diethanolamine and Triethanolamine as Stabilizers

Sol–Gel Route to MgZnO Alloy Particles via Diethanolamine and Triethanolamine as Stabilizers

MgZnO/MgO strained multiple-quantum-well nanocolumnar films: Stress-induced structural transition and deep ultraviolet emission

MgZnO/MgO strained multiple-quantum-wells (S-MQWs) with different structures are grown by pulsed laser deposition. The S-MQW films are composed of closely arranged, vertically oriented nanocolumns. Z-contrast scanning transmission electron microscopy observations and line-scan compositional analysis reveal that the nanocolumns are compositionally modulated along their length and have a MQW structure. A coherent epitaxial relationship with a sharp interface is established between MgZnO and MgO layers with large lattice mismatch. It is worth noting that the MgZnO layer undergoes a structural transition from hexagonal to cubic phase with its thickness decreasing. The calculations reveal that a large in-plane compressive stress dominates such an interesting phase transition process, stabilizes the low Mg-content MgZnO alloy in the anomalous cubic phase, and also leads to a broadening of the band gap. As a result, the wavelength-tunable deep-ultraviolet emission in the range of 261–314 nm is obtained from these S-MQW nanocolumnar films. The 261 nm is known to be the shortest emission wavelength ever reported for MgZnO material.

N-doped MgZnO alloy thin film prepared by sol–gel method

In this work, nitrogen-doped MgZnO thin films were deposited on glass substrate by spin-coating technique and thermal annealing treatment. The X-ray diffraction (XRD), scanning electron microscope (SEM) and optical transmission measurement were carried out to investigate the effect of growth temperature and nitrogen doping on the physical and optical properties of the films. XRD and SEM results revealed that the crystallinity and preferred c-axis orientation was enhanced with increasing annealing temperature and certain concentration of N-doping. In addition, the films incorporated with nitrogen doping exhibit significant blue shift in optical band gap and improvement in its transparency.

Effects of Mg concentration on solubility and chemical state of N in N-doped MgZnO alloy

Solubility and chemical state of N in an N-doped Mg(x)Zn(1-x)O film were studied by using Raman and x-ray photoelectron spectroscopy. Three anomalous Raman peaks are observed at 272, 580, and 642 cm(-1), respectively, and are demonstrated to be only related to substitution of N for O site (N(O)) but not to substitution of N(2) for O site (N(2))(O). The solubility of the N(O) is dominated by Mg concentration and chemical potentials of N and O in growth condition. The chemical state of the N can change from coexistence of (N(2))(O) and N(O) to single (N(2))(O) with increasing Mg concentration.

Effects of Mg Incorporation on Microstructure and Optical Properties of ZnO Thin Films Prepared by Sol-gel Method

Well-crystallized MgZnO alloy thin films with hexagonal wurtzite structure were fabricated by sol-gel method. With the band gap increases, the surface roughness and the grain size reduces. It is worth noting that the intensity of the band-edge luminescence of Mg doped films enhances with the increase of the Mg content. The microstructure and photoluminescence mechanism have been discussed based on X-ray diffraction patterns, atomic force microscopy images, ultraviolet-visible absorption spectra, photoluminescence spectra and Fourier transform infrared spectra.

Fabrication and Characterization of Mg-Doped ZnO Nanorod Arrays

Photoelectronic characteristics are investigated in well-aligned MgO-coated ZnO nanorods (MgO/ZnO nanocables) grown on Si substrates buffered with ZnO film at a low temperature by solution techniques. Transmission electron microscopy shows that a rough surface was observed for the MgO-coated ZnO nanorods due to deposition of MgO nanoparticles on the surface of the ZnO nanorods. However, after annealed at high temperatures, the surface of the MgO-coated ZnO nanorods was flattened to form Mg-doped ZnO nanorods. Photoluminescence spectra of Mg-doped ZnO nanorods displayed a blue shift of the near-band-edge emission with increasing annealing temperature indicative of an increase in the band gap of the MgZnO alloy due to diffusion of the Mg atoms into the ZnO nanorods. In contrast, no blue shift was detected for the samples annealed in H2/N2 (5%/95%) reduction atmosphere but a blue emission was detected at 800 degrees C, indicating that MgO diffusion process may produce a new luminescent center to emit the blue emission in H2/N2 reduction atmosphere.

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.

Optical and structural properties of sol–gel prepared MgZnO alloy thin films

Mg x Zn 1− x O ( x = 0–0.5) alloy thin films were prepared by a sol–gel dip-coating method. Mg 0.1Zn 0.9O and Mg 0.5Zn 0.5O films prepared were annealed in the range of 400–900 °C to investigate their thermal stability and temperature-dependent optical properties. The Mg 0.1Zn 0.9O films were thermally stable in the investigated annealing temperature range and exhibited the maximum ultraviolet emission at 800 °C. The segregation of MgO occurred in the Mg 0.5Zn 0.5O films, and the near-band-edge ultraviolet emission of this alloy was enhanced with increasing annealing temperature. The Mg saturation content in the sol–gel prepared MgZnO alloys was found to be about 0.23 where the band gap extended to 3.48 eV.

Observation of negative thermo-optical coefficient in cubic MgZnO thin films

We report on the investigation of thermo-optical coefficient (dn∕dT) in cubic MgZnO thin films through temperature-dependent (10–295K) optical transmission measurements. As a result of potential fluctuation, we observe an anomalous thermal behavior of the band gap in the ternary MgZnO alloy, which limits the observation of negative dn∕dT up to 200K. The negative dn∕dT is evidenced by the phase shift from the prominent Fabry-Pérot interference fringes and modeled by taking into account the thermal and electronic background contribution, as well as the Wannier exciton effect. It is found that the large thermal expansion in cubic MgZnO plays a key role in the observed negative dn∕dT.

ZnO and related materials: Plasma-Assisted molecular beam epitaxial growth, characterization and application

This paper will address features of plasma-assisted molecular beam epitaxial growth of ZnO and related materials and their characteristics. Two-dimensional, layer-by-layer growth is achieved both on c-plane sampphire by employing MgO buffer layer growth and on (0001) GaN/Al2O3 template by predepositing a low-temperature buffer layer followed by high-temperature annealing. Such two-dimensional growth results in the growth of high-quality heteroepitaxial ZnO epilayers. Biexciton emission is obtained from such high quality epilayers The polarity of heteroepitaxial ZnO epilayers is controlled by engineering the heterointerfaces. We achieved selective growth of Zn-polar and O-polar ZnO heteroepitaxial layers. The origin of different polarities can be successfully explained by an interface bonding sequence model. N-type conductivity in Gadoped ZnO epilayers is successfully controlled. High conductivity, enough to be applicable to devices, is achieved. MgxZn1-xO/ZnO heterostructures are grown and emission from a ZnO quantum well is observed. Mg incorporation in a MgZnO alloy is determined by in-situ reflection high-energy electron diffraction intensity oscillations, which enables precise control of the composition. Homoepitaxy on commericial ZnO substrates has been examined. Reflection high-energy electron diffraction intensity oscillations during homoepitaxy growth are observed.