Ultraviolet Optoelectronic Devices Research Articles

Fabrication and optical property of ZnO nanorod array by hydrothermal method

We synthesize a large area of ZnO nanorod array by using the RF magnetron sputtering and low-temperature hydrothermal method. The sample morphology, crystal structure and optical properties are characterized and analyzed by means of the SEM, XRD and fluorescence spectrophotometer. The results show that ZnO nanorod array has the advantages of large aspect ratio, ordered orientation, uniform and compact at a lower OH−/Zn2+ concentration. The XRD results show that the samples are all hexagonal wurtzite structures and the photoluminescence (PL) pattern shows that an obvious ultraviolet peak of the samples appears around 375 nm, and an emission band of strong yellow green light appears in the wavelength between 450–650 nm. With the increase of the concentration ratio of OH−/Zn2+, the ultraviolet peak and the emission band of strong yellow green light both show a blue shift. The above results indicate that the ZnO nanorod array is a type of potentially preferred materials for ultraviolet optoelectronic devices.

Structural, Surface Topography and Optical Characterization of Nanocrystalline MgxZn1-xO Thin Films Grown by Modified Chemical Bath Deposition (SILAR) Method

A series of MgxZn1-xO thin films were grown on glass substrates using modified chemical bath deposition (m-CBD) called successive ionic layer adsorption and reaction (SILAR) technique. The crystal structure, surface topography and the optical characterization of the prepared films were studied as a function of Mg/Zn (x) content. It is observed that the deposited films have polycrystalline structure in nature and grown in two phases Hexagonal and cubic. The preferential orientation of the films was absorbed along (002) plane. Structural parameter such as crystallite size, number of dislocation density and micro-strain were also investigated. The crystallite size and surface roughness are increased with the increase of Mg2+ ions content. Thus the results showed that the surface topography and the surface quality of the deposited films can be controlled by Mg2+ ions content. The optical transmittance spectra analysis showed that transmittance increase with the increase Mg2+ content to about 85% for x = 0.75, and the energy band gap increases (2.82 - 3.17) eV as the Mg2+ content increases x = (0.25 - 0.75). These results indicate that the MgxZn1-xO thin films can be potentially used in high-performance ultraviolet optoelectronic devices.

Single-layer graphene electrode enhanced sensitivity and response speed of β-Ga2O3 solar-blind photodetector

High-sensitivity, rapid response β-Ga2O3 film-based metal-semiconductor-metal (MSM) solar-blind photodetectors were obtained by replacing conventional Au/Ti electrodes with single-layer graphene (SLG) nanosheets. Compared with the photodetector with Au/Ti electrodes, the photodetector using SLG electrodes demonstrates enhanced deep ultraviolet photoresponse properties, including a more rapid response speed (0.078 s), a higher light-to-dark current ratio (~1.41 × 104%), a higher R254/R365 rejection ratio (~1020), and the larger detectivity (~6.29 × 1011 Jones). The integration of SLG with β-Ga2O3 offers a large transparent area to the incident photons, and narrows the barrier at the contact interface upon ultraviolet illumination, strongly suggesting an unlimited potential for deep ultraviolet optoelectronic devices.

GaN based UV‐LEDs with Ni/Au Nanomeshes as Transparent p‐type Electrodes

Ni/Au nanomesh (NM) is utilized as the p‐type contact in the ultraviolet (UV)‐LEDs for improving the light extraction efficiency. The substitution of conventional planar Ni/Au p‐type electrode with highly ordered, large scalable Ni/Au NMs results in enhanced light transmission in the p‐type contact region of the UV‐LEDs. Transmission over 75% is demonstrated for Ni/Au‐NM thin film coated sapphire substrate, whereas light is totally blocked by conventional planar Ni/Au thin film. Linearized I–V characteristic between p‐GaN and Ni/Au NMs is achieved by appropriate thermal annealing of the contact, and specific contact resistivity of 0.90 Ω cm2 is reported. UV‐LED incorporated with Ni/Au NMs exhibits higher output power than that with planar contact due to enhanced light transmission through the metal network structure. Finally, simulation results suggest good agreement with experimental data, illustrating that Ni/Au NMs can serve as a potential candidate in the development of high efficiency UV optoelectronic devices.

2D boron dichalcogenides from the substitution of Mo with ionic B2 pair in MoX2 (X = S, Se and Te): high stability, large excitonic effect and high charge carrier mobility

B2 pairs-substituted MoX2 monolayers provide options for future 2D electronic devices.

Enhancement of photoluminescence from zinc oxide by aluminum nanoparticle surface plasmon

During the past few decades, surface plasmons (SPs) have become a research hotspot. The SPs are the collective oscillations of free electrons at the interface between metal and dielectric surrounding. Localized surface plasmon resonance (LSPR) for metal nanoparticles (NPs) has a wide application in the light emission enhancement by the selective photon absorption and by increasing local electromagnetic field. Nowadays, many achievements of SP-enhanced-emissions are applied to light emitting diodes. With the advantages of the direct wide band gap (3.37 eV) and large exciton binding energy (60 meV), zinc oxide (ZnO), which is considered as a potential material, has a wide range of applications, especially in ultraviolet (UV) optoelectronic devices. However, the low photoluminescence efficiency of ZnO limits the commercial applications of ZnO-devices. The relevant research shows that the selection of different metal NPs, such as platinum (Pt), aluminum (Al), argentum (Ag), aurum (Au), is one of the approaches to improving the UV emission from ZnO. In this study, two-dimensional arrays of Al NPs are used to improve the LSPR photoluminescence efficiency from ZnO grown by the atomic layer deposition (ALD). The two-dimensional arrays of Al NPs are fabricated on the surfaces of p-type Gallium nitride (GaN) substrates by colloid lithography. With the air-liquid interface self-assembly, the monolayer masks for colloid lithography are obtained on the substrates of p-type GaN. Then, after a 50-nm Al layer is deposited by thermal evaporation, the Al NPs’ arrays are gained by being dipped into toluene and extra sonication to remove the masks. Finally, 15 nm Al<sub>2</sub>O<sub>3</sub> and 200 nm ZnO films are deposited in sequence by ALD at a temperature of 125 ℃. The extinction spectra of Al NPs’ arrays are acquired by an ultraviolet-visible spectrophotometer. The results of the extinction spectra suggest that the radiative recombination rate is increased by the resonance coupling between the localized surface plasmons (LSP) of the Al NPs arrays and the excitons of the ZnO. A 1.91-fold enhancement of photoluminescence integral intensity in band-edge emission is measured because of the Al NP arrays coupled with ZnO. The result means that the LSP of the Al NPs’ arrays can increase the UV-emission of the ZnO. Therefore, this cost-effective and facile approach can be used in high-performance optoelectronic devices.

Prediction of new ZnS–CaS alloys with anomalous electronic properties

ZnS–CaS alloys have great potential for applications in ultraviolet optoelectronic devices due to their large and tunable band gaps.

Low Temperature Fabrication of Transparent Conductive Electrode With High Ultraviolet Transmittance Down to Wavelength of 250 nm

Indium tin oxide (ITO) is a broadly deployed transparent conductive electrode (TCE) with various applications in solar energy harvesting and optoelectronics. However, the low transmittance of ITO in the ultraviolet range has motivated researchers to opt for alternative materials as TCE. In this paper, the fabrication of a hybrid TCE made up of silver nanowire (AgNW) and peroxo‐constructed amorphous strontium stannate (pcaSS) is reported, employing facile spin coating solution‐processed method at 150 °C. The fabricated TCE demonstrated low sheet resistance of 32.8 Ω sq−1 and high transmittance of 88.2% at 550 nm. Furthermore, benefitting from the ultra‐high band gap of as‐synthesized pcaSS (4.5 eV), this TCE maintained its high transmittance (above 80%) inside ultraviolet range down to wavelength of 250 nm. This research contributes to the fabrication of more efficient solar energy and ultraviolet optoelectronic devices.

Surface hole gas enabled transparent deep ultraviolet light-emitting diode

The inherent deep-level nature of acceptors in wide-band-gap semiconductors makes p-ohmic contact formation and hole supply difficult, impeding progress for short-wavelength optoelectronics and high-power high-temperature bipolar electronics. We provide a general solution by demonstrating an ultrathin rather than a bulk wide-band-gap semiconductor to be a successful hole supplier and ohmic contact layer. Free holes in this ultrathin semiconductor are assisted to activate from deep acceptors and swept to surface to form hole gases by a large electric field, which can be provided by engineered spontaneous and piezoelectric polarizations. Experimentally, a 6 nm thick AlN layer with surface hole gas had formed p-ohmic contact to metals and provided sufficient hole injection to a 280 nm light-emitting diode, demonstrating a record electrical-optical conversion efficiency exceeding 8.5% at 20 mA (55 A cm−2). Our approach of forming p-type wide-band-gap semiconductor ohmic contact is critical to realizing high-efficiency ultraviolet optoelectronic devices.

Defects induced broad spectral photoresponse of PVT-grown bulk AlN crystals

Aluminum nitride (AlN) is a wide bandgap semiconductor with promising application prospect in deep ultraviolet optoelectronic devices. Here, bulk AlN crystals in millimeter size were grown using an improved physical vapor transport technique. Schottky diodes based on these crystals demonstrated good photoresponse to a broad spectral range from 261 nm to 1064 nm. The photoresponse is attributed to the defects induced midgap energy states. The device demonstrated a photo responsivity up to 3.7 AW−1 at 360 nm with bias voltage of −10 V. This research paves a new way for broad spectral photodetectors.

First-principles investigation of size-dependent piezoelectric properties of bare ZnO and ZnO/MgO core-shell nanowires

Using first-principles density functional theory (DFT) calculations, we evaluate and compare the piezoelectric properties in bare ZnO and ZnO/MgO core-shell nanowires. The study confirms that effective piezoelectric coefficient in bare ZnO nanowires is a highly diameter sensitive parameter. In the thinnest ZnO nanowire, with only one layer of atoms, the effective piezoelectric coefficient attains a maximum value of 12.21 C/m2. On the other hand, for the first time, we report that piezoelectricity in MgO-passivated ZnO nanowires exhibits a lesser size dependence than in the bare ones, a finding that may prove useful in the design of emerging ultraviolet optoelectronic devices.

Localized-State-Dependent Electroluminescence from ZnO/ZnS Core–Shell Nanowires–GaN Heterojunction

ZnO is a very important material for excitonic ultraviolet optoelectronic devices operating above room temperature due to its wide band gap and high exciton binding energy. In this paper, the influences of different degrees of the localized state on the photoluminescence and electroluminescence properties of the ZnO/ZnS core–shell nanowires–GaN heterojunction are systematically discussed. The physical model for radiative recombination of localized carriers was proposed to explain these phenomena. Our results indicate that surface-coating of ZnS nanoparticles on ZnO nanowires (NWs) is one of the effective ways to manipulate the localized states, and only the appropriate localized state will result in the optimal optoelectronic properties.

α-Ga2O3 grown by low temperature atomic layer deposition on sapphire

α-Ga2O3 is a metastable phase of Ga2O3 of interest for wide bandgap engineering since it is isostructural with α-In2O3 and α-Al2O3. α-Ga2O3 is generally synthesised under high pressure (several GPa) or relatively high temperature (∼500 °C). In this study, we report the growth of α-Ga2O3 by low temperature atomic layer deposition (ALD) on sapphire substrate. The film was grown at a rate of 0.48 Å/cycle, and predominantly consists of α-Ga2O3 in the form of (0001)-oriented columns originating from the interface with the substrate. Some inclusions were also present, typically at the tips of the α phase columns and most likely comprising ε-Ga2O3. The remainder of the Ga2O3 film – i.e. nearer the surface and between the α-Ga2O3 columns, was amorphous. The film was found to be highly resistive, as is expected for undoped material. This study demonstrates that α-Ga2O3 films can be grown by low temperature ALD and suggests the possibility of a new range of ultraviolet optoelectronic and power devices grown by ALD. The study also shows that scanning electron diffraction is a powerful technique to identify the different polymorphs of Ga2O3 present in multiphase samples.

Stabilization and enhanced energy gap by Mg doping in ε-phase Ga2O3 thin films

Mg-doped Ga2O3 thin films with different doping concentrations were deposited on sapphire substrates using laser molecular beam epitaxy (L-MBE) technique. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and ultraviolet-visible (UV-vis) absorption spectrum were used to characterize the crystal structure and optical properties of the as-grown films. Compared to pure Ga2O3 thin film, the Mg-doped thin films have transformed from the most stable β-phase into ε-phase. The absorption edge shifted to about 205 nm and the optical bandgap increased to ∼ 6 eV. These properties reveal that Mg-doped Ga2O3 films may have potential applications in the field of deep ultraviolet optoelectronic devices, such as deep ultraviolet photodetectors, short wavelength light emitting devices and so on.

Effect of Material Structure on Photoluminescence of ZnO/MgO Core‐Shell Nanowires

AbstractZinc oxide (ZnO) nanowires are widely studied for use in ultraviolet optoelectronic devices, such as nanolasers and sensors. Nanowires (NWs) with an MgO shell exhibit enhanced band‐edge photoluminescence (PL), a result previously attributed to passivation of ZnO defects. However, we find that processing the ZnO NWs under low oxygen partial pressure leads to an MgO‐thickness‐dependent PL enhancement owing to the formation of optical cavity modes. Conversely, processing under higher oxygen partial pressure leads to NWs that support neither mode formation nor band‐edge PL enhancement. High‐resolution electron microscopy and density‐functional calculations implicate the ZnO m‐plane surface morphology as the key determinant of core‐shell structure and cavity‐mode optics. A ZnO surface with atomic steps along the m‐plane in the c‐axis direction stimulates the growth of a smooth MgO shell that supports guided‐wave optical modes and enhanced UV PL. On the other hand, a smoother ZnO surface leads to nucleation of a rough cladding layer which supports neither enhanced UV PL nor optical cavity modes. Finite‐element analysis shows a clear correlation between allowed Fabry‐Perot and whispering gallery modes and enhanced UV‐PL. These results point the way to fabricating ZnO/MgO core‐shell nanowires for more efficient UV nanolasers, scintillators, and sensors.

Growth and characterization of grid-like β-Ga2O3 nanowires by electric field assisted chemical vapor deposition method

Gallium oxide (Ga2O3) has five crystalline polymorphs, i.e. corundum (α-phase), monoclinic (β-phase), spinel (γ-phase), bixbite (δ-phase) and orthorhombic (ε-phase). Among these phases, the monoclinic structured β-Ga2O3 is the most stable form, and is a ultraviolet (UV) transparent semiconductor with a wide band gap of 4.9 eV. It is a promising candidate for applications in UV transparent electrodes, solar-blind photodetectors, gas sensors and optoelectronic devices. In recent years, one-dimensional (1D) nanoscale semiconductor structures, such as nanowires, nanobelts, and nanorods, have attracted considerable attention due to their interesting fundamental properties and potential applications in nanoscale opto-electronic devices.Numerous efforts have been made to fabricate such devices in 1D nanostructures such as nanowires and nanorods. Comparing with the thin film form, the device performance in the 1D form is significantly enhanced as the surface-to-volume ratio increases. In order to realize β-Ga2O3 based nano-optoelectronic devices, it is necessary to obtain controlled-synthesis and the high-quality β-Ga2O3 nanomaterials. According to the present difficulties in synthesizing β-Ga2O3 nanomaterials, in this paper, the grid-like β-Ga2O3 nanowires are prepared on sapphire substrates via electric field assisted chemical vapor deposition method.High-purity metallic Ga (99.99%) is used as Ga vapor source. High-purity Ar gas is used as carrier gas. The flow rate of high-purity Ar carrier gas is controlled at 200 sccm. Then, oxygen reactant gas with a flow rate of 2 sccm enters into the system. The temperature is kept at 900℃ for 20 min. The effect of the external electric voltage on the surface morphology, crystal structure and optical properties of β-Ga2O3 nanowires are investigated. It is found that the external electric voltage has a great influence on the surface morphology of the sample. The orientation of the β-Ga2O3 nanowires grown under the action of an applied electric field begins to improve. Only a grid composed of three different growth directions appears. And with the increase of applied voltage, the distribution of nanowires becomes denser and the length increases significantly. In addition, it is found that the chemical vapor deposition method assisted by this external electric field can significantly improve the crystallization and optical quality of the samples.

Synthesis and studying of structural, surface topography and optical properties of Mgx Zn1-xO thin Films by spray pyrolysis method

In the present paper, MgxZn1-xO thin films were synthesized with a wide composition range x=(0.25, 0.5, 0.75) using a spray polyposis technique at 450 °C. The X-ray data analysis revealed that the films were polycrystalline with hexagonal and cubic structures. The preferential orientation of all films was absorbed along (002). Structural parameter such as average crystallite size, dislocation density and micro-striation were also investigated. AFM results show that the surface topography and the surface quality of the deposited thin film can be controlled by the Mg2+ content of MgxZn1-xO thin films. The UV-Visible analysis show that the optical absorption values decrease with the increase of Mg2+ concentration. It is observed that the band gap increases as the Mg2+ concentration increases. The reflectance spectra of MgxZn1-xO thin films shifted towards the lower wavelength with the increase of Mg2+ concentration. The extinction coefficient was slightly decreased and shifted towards higher energies with the increase of Mg2+ contents. These results demonstrate the high quality of single crystal MgxZn1-xO films and their potential in high-performance deep ultraviolet optoelectronic devices.

Epitaxial growth and characterization of CuGa2O4 films by laser molecular beam epitaxy

Ga2O3 with a wide bandgap of ∼ 4.9 eV can crystalize in five crystalline phases. Among those phases, the most stable monoclinic β-Ga2O3 has been studied most, however, it is hard to find materials lattice matching with β-Ga2O3 to grown epitaxial thin films for optoelectronic applications. In this work, CuGa2O4 bulk were prepared by solid state reaction as target, and the films were deposited on sapphire substrates by laser molecular beam epitaxy (L-MBE) at different substrate temperatures. The influences of substrate temperature on structural and optical properties have been systematically investigated by means of X-ray diffraction, Transmission electron microscope and UV-vis absorption spectra. High quality cubic structure and [111] oriented CuGa2O4 film can be obtained at substrate temperature of 750 °C. It’s also demonstrated that the CuGa2O4 film has a bandgap of ∼ 4.4 eV and a best crystal quality at 750 °C, suggesting that CuGa2O4 film is a promising candidate for applications in ultraviolet optoelectronic devices.

Recent Advances on p-Type III-Nitride Nanowires by Molecular Beam Epitaxy

p-Type doping represents a key step towards III-nitride (InN, GaN, AlN) optoelectronic devices. In the past, tremendous efforts have been devoted to obtaining high quality p-type III-nitrides, and extraordinary progress has been made in both materials and device aspects. In this article, we intend to discuss a small portion of these processes, focusing on the molecular beam epitaxy (MBE)-grown p-type InN and AlN—two bottleneck material systems that limit the development of III-nitride near-infrared and deep ultraviolet (UV) optoelectronic devices. We will show that by using MBE-grown nanowire structures, the long-lasting p-type doping challenges of InN and AlN can be largely addressed. New aspects of MBE growth of III-nitride nanostructures are also discussed.

Crystal and band structures of ZnS, MgS, and ZnS-MgS alloys

ZnS-MgS alloys have great potential applications in ultraviolet optoelectronic devices due to their large bandgaps. The structures of ZnS, MgS, and ZnS-MgS alloys are systematically investigated for different compositions and pressures, using first-principles evolutionary simulations. We successfully found the experimentally known structures (B1, B3, and Cmcm for ZnS and B1 for MgS) and the novel stable structures (P4/nmm for ZnS and P213 and R3¯ for MgS). The phase-transition pressures are in good agreement with the experimental results. We also predicted the existence of thermodynamically stable ZnS-MgS alloys at high pressures (P4/nmm-MgZnS2, P21/m-MgZn3S4, Pmm2-MgZn7S8, and R3-Mg8ZnS9) with unusual bonding and electronic properties. Furthermore, we revealed that the structures of Zn-rich alloys are constructed by the supercells of metal P4/nmm-ZnS, leading to the abrupt band-gap increase with the increase in Mg concentration, whereas the structures of Mg-rich alloys are based on those of R3¯-MgS and the bandgap decreases as the Mg concentration further increases.