Crystallization of ZnO thin films via thermal dissipation annealing method for high-performance UV photodetector with ultrahigh response speed.

ZnO-based ultraviolet (UV) photodetector can be easily fabricated by using sol-gel spin-coating method, however, the crystallization of amorphous state ZnO thin films is necessary to fabricate high performance UV photodetector. Thus, we devised a thermal dissipation annealing (TDA) method in which the heat transfer to the ZnO thin films can be synchronized with the heat release from the substrate. It was found that sol-gel spin-coated ZnO thin films can be crystallized through the mobility difference of ZnO molecules placed at the surface of ZnO thin films. Also, UV photodetector based on ZnO thin films annealed with the TDA method exhibited faster rise and decay time constant (τr = 35 ms and τd = 73 ms, respectively), a higher on/off current ratio, and reproducible photocurrent characteristics compared to that of the ZnO thin films annealed by using furnace and IR lamp. Therefore, these results indicated that the TDA method is a feasible alternative route for the fabrication of ZnO based high performance optoelectronic devices.

Transparent and flexible ZnO nanorods induced by thermal dissipation annealing without polymer substrate deformation for next-generation wearable devices.

The fabrication of a transparent and flexible ultraviolet photodetector based on hydrothermally grown ZnO nanorods requires an annealing step to render the sol-gel spin-coated ZnO seed layer crystalline. As high-temperature annealing deforms low-melting-point polymer substrates, we herein devised a thermal dissipation annealing (TDA) method in which heat transfer to ZnO thin films is synchronized with heat release from the polymer substrate to crystallize the ZnO seed layer without polymer substrate deformation and melting. ZnO nanorods (NRs) were hydrothermally grown on non-annealed and annealed ZnO seed layers, and NR density and diameter were shown to be higher in the latter case, as the crystallized ZnO seed layer provided heterogeneous nucleation sites for NR growth. In addition, the larger density and diameter of ZnO NRs grown on the annealed ZnO seed layer were confirmed by analysis of O 1s signal intensities. A transparent and flexible UV photodetector based on ZnO NRs grown on the annealed ZnO seed layer exhibited a higher photocurrent/dark current ratio, photosensitivity, and photoresponsivity than that fabricated using a non-annealed seed layer. Taken together, the above results suggest that the TDA method is an effective way of fabricating transparent and flexible UV photodetectors with high photosensitivity, photoresponsivity, and photocurrent stability and it means that the next generation wearable devices can be easily realized by using the TDA method.

Crystallization of ZnO thin films without polymer substrate deformation via thermal dissipation annealing method for next generation wearable devices.

The synthesis method of transparent and flexible ZnO thin films is currently considered the most important factor for the fabrication of next generation wearable devices. To fabricate transparent and flexible devices by using sol-gel spin-coated ZnO thin films, an annealing step is necessary; however, annealing processes at high temperatures decompose polymer substrates due to their low melting temperature. It was found that sol-gel spin-coated ZnO thin films can be crystallized through the mobility difference of ZnO molecules placed at the surface of ZnO thin films. Especially, ZnO thin films can be annealed at high temperature (above 500 °C) by using a thermal dissipation annealing (TDA) method without the deformation of the polymer substrate. A transparent and flexible ultraviolet photodetector based on ZnO thin films annealed with the TDA method exhibited fast rise and decay time constants, a high on/off current ratio, and reproducible photocurrent characteristics. Thus, these results indicated that the TDA method is a feasible alternative route for the fabrication of next generation wearable devices.

Effect of the Seed Layer Type and Precursor Concentration on the Structural, Morphological, and Photoresponse Properties of Hydrothermally Grown ZnO Nanorods.

This study proposes a simple and low-cost method of improving the photoresponse properties of ZnO nanorods by selecting optimal seed layer type and precursor concentration, which are parameters significantly influencing growth rate and morphology.When 0.025 M aqueous precursor solution was used, the obtained ZnO nanorods exhibited an improved vertical orientation and growth rate, showing directional growth along the (002) plane, as confirmed by field emission scanning electron microscopy. Moreover, ZnO nanorods (NR1) obtained on the seed layer prepared by oxidation of Zn films exhibited the faster growth rate than those (NR2) synthesized on the seed layer prepared by spin-coating method due to having access to a sufficient amount of Zn2+ ions. The ZnO nanorods obtained at a precursor concentration of 0.025 M exhibited the highest photocurrent, dark current, and photoresponsivity, with the photoresponsivity and photosensitivity of ZnO NR1 exceeding those of ZnO NR2, owing to the larger surface area.

Effect of Electric Current on Oxidation of Zn Films to ZnO Films Using Electrochemical System.

In this study, we fabricated ZnO films from Zn films using an electrochemical system under different oxidation currents of 0.2, 0.5, 1, and 2 mA. Most of the samples showed fluctuating potential values near 0 V, and the pH value also showed the same tendency. The morphology of the ZnO films showed a polycrystalline film form and short hexagonal pillars at the underside and upside, respectively, due to the preferred growth of (101) orientation. In the photoluminescence spectra, the highest value of the near band-edge and deep-level emission ratio indicated that a high-quality ZnO film was obtained. To investigate the photoresponse properties, we measured the dark current and photocurrent using a current-voltage measurement system. The ZnO films with the highest quality showed a large number of photocurrent generations and high responsivity value. We expect that this method can offer a novel method of studying the growth of high-quality ZnO films for application to optoelectronic devices.

Yellow-red light-emitting diodes using periodic Ga-flow interruption during deposition of InGaN well.

We report a possible way to extend the emission wavelength of InyGa1-yN/InxGa1-xN quantum-well (QW) light-emitting diodes (LEDs) to the yellow-red spectral range with little degradation of the radiative efficiency. The InyGa1-yN well with high indium (In) content (HI-InyGa1-yN) was realized by periodic Ga-flow interruption (Ga-FI). The In contents of the HI-InyGa1-yN well and the InxGa1-xN barrier were changed to manipulate the emission wavelength of the LEDs. An In0.34Ga0.66N/In0.1Ga0.9N-QW LED, grown by continuous growth mode (C-LED), was prepared as a reference. The photoluminescence (PL) wavelengths of the HI-InyGa1-yN/InxGa1-xN QW LEDs were changed from 556 to 597 nm. The PL intensity of the HI-InyGa1-yN/InxGa1-xN LED with a peak wavelength of 563 nm was 2.7 times stronger than that of the C-LED (λ = 565 nm). The luminescence intensity for the HI-InyGa1-yN/InxGa1-xN QW LED emitting at 597 nm was stronger than those of the other LED samples with shorter wavelengths. Considering the previous works on degradation in crystal quality and increase in the quantum-confined Stark effect with increasing In content in InGaN, the approach in this work is very promising for yellow-red InGaN LEDs.

Enhanced Light Emission from Monolayer Semiconductors by Forming Heterostructures with ZnO Thin Films.

Monolayer transition-metal dichalcogenides (1L-TMDs) are atomically thin direct band gap semiconductors, from which the emission of light is determined by optical transitions of exciton complexes such as neutral excitons and trions. While the quantum yields of 1L-TMDs are quite low, the ability to control the populations of exciton complexes in 1L-TMDs through various doping processes is an interesting advantage, and provides ample possibilities for engineering the optical properties of these semiconductor monolayers. Here we demonstrate a simple method of controlling the populations of excitons and trions to enhance the light emission of 1L-TMDs by having them form heterostructures with ZnO thin films (TFs). 1Ls of MoS2 or MoSe2 showed up to 17-fold increases in photoluminescence (PL) when they were placed on ∼50 nm thick ZnO TFs. This enhancement of the PL was due to charge exchanges occurring through the 1L-TMD/ZnO interface. The PL enhancements and changes in the PL spectra of the 1L-TMDs were greater when the 1L-TMD/ZnO heterostructures were subjected to 355 nm wavelength laser excitation than when they were excited with a 514 nm wavelength laser, which we attributed to the onset of energy transfer by photoexcited excitons and/or the additional p-doping by photoexcited holes in ZnO. The p-doping phenomenon and the enhanced light emission of 1L-TMD/ZnO heterostructures were unambiguously visualized in spatially resolved PL and Raman spectral maps. Our approach using the 1L-TMD/ZnO TF heterostructure suggests that a rich variety of options for engineering the optical properties of 1L-TMDs may be made available by carrying out simple and intuitive manipulations of exciton complexes, and these endeavors may yield practical applications for 1L-TMDs in nanophotonic devices.

Effects of Precursor Concentration on Structural and Optical Properties of ZnO Thin Films Grown on Muscovite Mica Substrates by Sol-Gel Spin-Coating.

The structural and optical properties of the ZnO thin films grown on mica substrates for different precursor concentrations were investigated. The surface morphologies of all the samples indicated that they consisted of granular structures with spherical nano-sized crystallites. The thickness of the ZnO thin films increased significantly and the optical band gap exhibited a blue shift with an increase in the precursor concentration. It is remarkable that the highest I(NBE)/I(DLE) ratio was observed for the ZnO thin film with 0.8 M precursor concentration, even though cracks formed on the surface of this film.

Influences of Si-doped graded short-period superlattice on green InGaN/GaN light-emitting diodes.

We report significant improvement in optical and electrical properties of green InGaN/GaN light-emitting diodes (LEDs) by using Si-doped graded short-period InGaN/GaN superlattice (SiGSL) formed by so called indium-conversion technique. For comparison, a conventional LED without the superlattice (C-LED) and a LED with undoped graded superlattice (unGSL-LED) were prepared, respectively. The photoluminescence (PL) intensity of the SiGSL-LED was increased more than 3 times at room temperature (RT) as compared to C-LED. The PL intensity ratios of RT to 10K for the C-LED, unGSL-LED, and SiGSL-LED were measured to be 25, 40.9, and 47.5%, respectively. The difference in carrier lifetimes between 10K and RT for the SiGSL-LED is relatively small compared to that of the C-LED, which is consistent with the variation in PL intensity. The output power of a transistor-outline type SiGSL-LED was increased more than 2 times higher than that of the C-LED.

Structural and electrical properties of catalyst-free Si-doped InAs nanowires formed on Si(111).

We report structural and electrical properties of catalyst-free Si-doped InAs nanowires (NWs) formed on Si(111) substrates. The average diameter of Si-doped InAs NWs was almost similar to that of undoped NWs with a slight increase in height. In the previous works, the shape and size of InAs NWs formed on metallic catalysts or patterned structures were significantly changed by introducing dopants. Even though the external shape and size of the Si-doped NWs in this work were not changed, crystal structures inside the NWs were significantly changed. For the undoped InAs NWs, both zincblende (ZB) and wurzite (WZ) structures were observed in transmission-electron microscope images, where the portion of WZ structure was estimated to be more than 30%. However, only ZB was observed with an increase in stacking fault (SF) for the Si-doped NWs. The undoped and Si-doped InAs NWs were used as channels of four-point electrical measurements with Al/Ni electrodes to investigate electrical properties. The resistivity calculated from the current-voltage curve of a Si-doped InAs NW showed 1.32 × 10(-3) Ωcm, which was dramatically decreased from 10.14 × 10(-3) Ωcm for the undoped InAs NW. A relatively low resistivity of catalyst-free Si-doped InAs NWs was achieved without significant change in structural dimensions.

Electrostatic Discharge Characteristics of InGaN/GaN Light-Emitting Diodes with Si-Doped Graded Superlattice.

We report the influences of a Si-doped graded superlattice (SiGSL) on the electrostatic discharge (ESD) characteristics of an InGaN/GaN light-emitting diode (LED). For comparison, a conventional InGaN/GaN LED (C-LED) was also investigated. The luminous efficacy for the SiGSL-LED was 2.68 times stronger than that for the C-LED at the injection current of 20 mA. The resistances estimated from current-voltage (I-V) characteristic curves were 16.5 and 8.8 Ω for the C-LED and SiGSL-LED, respectively. After the ESD treatment at the voltages of 4000 and 6000 V, there was no significant change in the I-V curves for the SiGSL-LED. Also, there was small variation in the I-V characteristics for the SiGSL-LED at the ESD voltage of 8000 V. However, the I-V curves for the C-LED were drastically degraded with increasing ESD voltage. While the light emission was not observed at the injection current of 20 mA from the C-LED sample after the ESD treatment, the emission spectra for the SiGSL-LED sample were clearly measured with the output powers of 10.47, 9.66, and 7.27 mW for the ESD voltages of 4000, 6000, and 8000 V respectively.

Facile synthesis and enhanced ultraviolet emission of ZnO nanorods prepared by vapor-confined face-to-face annealing.

In this study, we report a novel regrowth method of sol-gel-prepared ZnO films using a vapor-confined face-to-face annealing (VC-FTFA) technique in which mica was inserted between two films, followed by annealing with the FTFA method. The ZnO nanorods are regrown when zinc acetate dihydrate and zinc chloride (ZnCl2) are used as the solvent, because these generate ZnCl2 vapor. The near-band-edge emission intensity of the ZnO nanorods was enhanced through the VC-FTFA method, increasing significantly by a factor of 56 compared to that of ZnO films annealed in open air at 700 °C. Our method may provide a route toward the facile fabrication of ZnO nanorods.

Oxidation Temperature Effects on ZnO Thin Films Prepared from Zn Thin Films on Quartz Substrates.

We investigated the structural and optical properties of the ZnO thin films formed by oxidation of Zn thin films. Zn thin films were deposited by thermal evaporation and were then annealed from 300 to 800 degrees C to prepare ZnO thin films. We found that ZnO thin films were formed by thermal oxidation of Zn thin films at oxidation temperatures over 400 degrees C. The grain size of ZnO thin films increased with the oxidation temperature and the highest ZnO (002) intensity was obtained at 600 degrees C. In the PL spectra, the intensity of the near-band-edge peak increased with the oxidation temperatures until 400 degrees C. However, these values gradually decreased with a further increase in the oxidation temperatures over 400 degrees C. The transmittance of the ZnO thin films was more than 90% for the visible wavelength region, and the optical band gap was red-shifted with increase in the oxidation temperature.

Effect of Oxidation Temperature on Characteristics of Thermally Oxidized ZnO Thin Films on Mica Substrates.

Muscovite mica is one of the promising alternatives to polymer substrates because of its good thermal resistivity, flexibility, and transparency. In this study, metallic Zn films with a thickness of 300 nm were deposited on mica substrates through thermal evaporation; the thin films were then oxidized by annealing at temperatures ranging from 350 to 550 degrees C. The structural and optical properties of thermally oxidized ZnO thin films were investigated. Diffraction peaks for ZnO (100) and (002) planes were observed only for the ZnO thin films oxidized at temperatures above 450 degrees C. These films consisted of relatively rough film-like structures, and the average transmittance of the films was greater than 70% in the visible region. The highest near-band-edge emission was observed for the ZnO thin films oxidized at 500 degrees C. Upon increasing the oxidation temperatures to 500 degrees C, the optical band gap was blue-shifted.

Effects of Fluorine Doping on Structural and Optical Properties of Hydrothermally Grown ZnO Nanorods.

Pure and fluorine-doped ZnO nanorods were hydrothermally grown on quartz substrates. The nanorod thickness and density were changed by varying the fluorine concentration. The full width at half-maximum and position of the diffraction peak corresponding to the ZnO (002) plane were used to calculate the decreased residual stress and increased c-axis lattice constant of the FZO nanorods. The near-band-edge PL emission peaks intensified and the deep-level emission peaks gradually weakened because more fluorine atoms occupied the ZnO lattice vacancies with increasing nanorod F concentration, thereby decreasing the nanorod surface-defect concentration.

Effect of Metallic Au Seed Layer Annealing on the Properties of Electrodeposited ZnO Nanorods.

This study focuses on the effect of annealing the Au seed layer (ASL) on the structural and optical properties of electrodeposited ZnO nanorods. ZnO nanorods were fabricated in a three-step approach. In the first step, ASLs were deposited using an ion sputter technique. In the second step, layers were annealed in air at various temperatures ranging from 400 degrees C to 600 degrees C. Finally, ZnO nanorods were grown using an electrodeposition method. The field-emission scanning electron microscopy analysis showed that better aligned ZnO nanorods are fabricated on the annealed ASL compared with non-annealed ASL The X-ray diffraction analysis showed a notable improvement in directional growth along the (002) crystallographic plane when ZnO nanorods were grown on the annealed ASL. The photoluminescence analysis showed that the UV emission peak of ZnO nanorods on the annealed ASL at 400 degrees C was blue-shifted and increased.

Synthesis and optical characteristics of yttrium-doped zinc oxide nanorod arrays grown by hydrothermal method.

Yttrium-doped ZnO (YZO) nanorods were synthesized by hydrothermal growth on a quartz substrate with various post-annealing temperatures. To investigate the effects of post-annealing on the optical properties and parameters of the nanorods, X-ray diffractometry (XRD), photoluminescence (PL) measurement, and ultraviolet (UV)-visible spectroscopy were used. From the XRD investigation, the full width at half maximum (FWHM) and the dislocation density of the nanorods was found to increase with an increase in the post-annealing temperature. In the PL spectra, the intensity of the near band edge (NBE) emission peak in the UV region also increases with an increase in the temperature of post-annealing. The deep level emission (DLE) peak in the visible region changes with various post-annealing temperatures, and its intensity increases remarkably with post-annealing at 800 degrees C. In this paper, changes in the optical parameters of the nanorods caused by variation in the behavior of Y during post-annealing was investigated, with properties such as absorption coefficients, refractive indices, and dispersion parameters being obtained from transmittance and reflectance analysis.

Influence of Sn doping on structural and optical properties of zinc oxide nanorods prepared via hydrothermal process.

Hydrothermally grown ZnO nanorods were doped with various concentrations of Sn, ranging from 0 to 2.5 at%. Scanning electron microscopy (SEM), X-ray diffractometer (XRD), ultraviolet (UV)-visible spectroscopy, and Photoluminescence (PL) measurements were used to determine the effect of Sn doping on the structural and optical properties. In the SEM images, the nanorods have hexagonal wurtzite structure and the diameter of the nanorods increases with an increase in the Sn content. The optical parameters of the Sn-doped ZnO (SZO) nanorods such as the absorption coefficients, optical bandgaps, Urbach energies, refractive indices, dispersion parameters, dielectric constants, and optical conductivities were determined from the transmittance and reflectance results. In the PL spectra, the intensity of the NBE peak in the UV region decreases and is blue-shifted with an increase in the Sn content, while the DLE peaks of the nanorods in the visible region shift toward the low-energy region with the introduction of Sn.

Optical parameters of boron-doped ZnO nanorods grown by low-temperature hydrothermal reaction.

Sol-gel spin-coating was used to deposit ZnO seed layers onto quartz substrates, and ZnO nanorods doped with various concentrations of B (i.e., BZO nanorods) ranging from 0 to 2.0 at% were hydrothermally grown on the ZnO seed layers. The effects of B doping on the absorption coefficient, optical band gap, Urbach energy, refractive index, extinction coefficient, single-oscillator energy, dispersion energy, average oscillator strength, average oscillator wavelength, dielectric constant, and optical conductivity of the hydrothermally grown BZO nanorods were investigated. The optical band gaps were 3.255, 3.243, 3.254, 3.258, and 3.228 eV for the nanorods grwon at 0, 0.5, 1.0, 1.5 and 2.0 at% B, respectively. B doping increased the Urbach energy from 40.7 to 65.1 meV for the nanorods grown at 0 and 2.0 at% B, respectively, and significantly affected the dispersion energy, the single-oscillator energy, the average oscillator wavelength, the average oscillator strength, the refractive index, and the optical conductivity of the hydrothermally grown BZO nanorods.

Effect of Co doping concentration on structural properties and optical parameters of Co-doped ZnO thin films by sol-gel dip-coating method.

The structural and optical properties of Co-doped ZnO thin films prepared by a sol-gel dip-coating method were investigated. X-ray diffraction analysis showed that the thin films were grown with a c-axis preferred orientation. The position of the (002) peak was almost the same in all samples, irrespective of the Co concentration. It is thus clear that Co doping had little effect on the position of the (002) peak. To confirm that Co2+ was substituted for Zn2+ in the wurtzite structure, optical measurements were conducted at room temperature by a UV-visible spectrometer. Three absorption peaks are apparent in the Co-doped ZnO thin films that do not appear for the undoped ZnO thin film. As the Co concentration was increased, absorption related to characteristic Co2+ transitions increased because three absorption band intensities and the area underneath the absorption wells between 500 and 700 nm increased with increasing Co concentration. The optical band gap and static dielectric constant decreased and the Urbach energy and extinction coefficient increased with increasing Co concentration.