Ultraviolet Photoresponse Properties Research Articles

Ultraviolet photo-response properties of bush-like ZnO nanorods deposited by chemical bath deposition

“Bush-like zinc oxide nanorods (ZnO NRs) has been deposited on soda-lime glass substrate via two step deposition process: seed layer deposition through Atmospheric chemical vapor deposition (APCVD) followed by chemical bath deposition (CBD) for growing NRs. Depositional parameters were carefully optimized so as to obtain a porous bush-like aggregation of ZnO NRs. X-ray Diffraction confirmed the formation of ZnO and scanning electron microscope showed bush-like aggregation of ZnO NRs. Transmission Electron Microscopy revealed that mostly non-polar {101¯0} and {112¯0} surfaces were exposed to the ambience. Room temperature Photoluminescence spectroscopy indicated the presence of mostly oxygen vacancy (VO) related defect states centered at 2.32 and 2.0 eV. Ultraviolet (UV) photo-response of the specimen was measured in air and nitrogen ambience. Higher photosensitivity (3166 % in nitrogen; 2233 % in air) but slower photo-response were observed under nitrogen ambience. It was found that UV photo-response properties of ZnO NRs were mostly a surface phenomenon dominated by adsorption-desorption mechanism of ambient oxygen molecules on the ZnO surface, mediated by surface defects related to VO, such as singly ionized state of VO. Bush-like aggregation of the ZnO NRs might have aided adsorption-desorption mechanism.”

Low MOCVD growth temperature controlled phase transition of Ga2O3 films for ultraviolet sensing

The influences of low growth temperatures (420–490 °C) on phase transition of Ga2O3 films grown on Al2O3 substrates by means of metal-organic chemical vapor deposition (MOCVD) are investigated here. The changes of crystal structure, phase composition, surface and cross-sectional morphologies are carried out by X-ray diffraction, atomic force microscope and field-emission scanning electron microscope to reveal the detail evolution from ε-phase to β-phase in Ga2O3 films. The fluctuant Gibbs free energy change relying on growth temperature induces different growth mechanism of Ga2O3 films. Furthermore, the ultraviolet photoresponse properties of different phase compositions for Ga2O3 films are compared by constructing metal-semiconductor-metal photodetectors. It is found that mixture phase Ga2O3 films express better photodetection performance than pure phase Ga2O3 films due to the formation of built-in field within phase junction. This proposed regulation principle of Ga2O3 phase transition provides a new route to high-performance photoelectric material.

Morphology changes of In-doped ZnO nanosheets via ZnCl2 and InCl3 vapor formation during thermal dissipation annealing process and improved UV photoresponse properties

Ultraviolet (UV) photodetectors that are fabricated using ZnO thin films exhibit higher photocurrent stability, without a complex filter, than that exhibited by Si-based UV photodetectors. However, the ZnO-based UV photodetectors possess several drawbacks such as slow response speed, low on/off current ratio, and low sensitivity. These drawbacks can be overcome by regulating the number of free carriers and point defects that affect the photocurrent increment and the response speed of the UV photodetectors. In this study, indium (In)-doped ZnO (IZO) thin films were deposited via sol-gel spin coating. Subsequently, the amorphous IZO thin films were crystallized via thermal dissipation annealing (TDA). This facilitated the regulation of the number of point defects and free carriers, and the surface roughness of the IZO thin films decreased with the increase in the doping concentration. The UV photosensitivity of the IZO thin films was higher than that of the undoped ZnO thin films. The IZO thin films also exhibited the highest photoresponsivity and UV photoresponse speed. The technique that was devised in the study was concluded to be a feasible route for the fabrication of ZnO-based UV photodetectors with high photosensitivity, photoresponsivity, and UV photoresponse speed.

Improved Ultraviolet Photoresponse Properties of ZnO Nanorods Grown by Using Hydrothermal Method Applied Rotating Precursor Solution

We grew ZnO nanorods (NRs) using a hydrothermal method while rotating the precursor solution, and report the effect of rotation on surface modification, and the optical and ultraviolet (UV) photoresponse properties of the resulting ZnO nanorods. The ZnO NRs grown in the rotating precursor solution at 100 rpm had the longest length and diameter, which decreased with increasing rotation rate above 100 rpm. Also, the intensity of the diffraction peaks from ZnO (002) and (100) was strongest and weakest for the ZnO NRs grown in a rotating solution at 150 rpm, respectively, indicating improvement in the c-axis orientation of the ZnO NRs. In the PL spectra, near-band-edge (NBE) and deep-level (DL) emissions were observed from all of the ZnO NRs. The intensity of the NBE emission gradually increased with increasing rotation rate, due to the increase in surface area. Also, the intensity of the DL emission gradually increased with increasing rotation rate because of the increasing number of interstitial oxygens. Analysis of the UV photoresponse found the photocurrent of the ZnO NRs grown in the rotating precursor solution was higher than that of ZnO NRs grown in a non-rotated precursor solution. In particular, ZnO NRs grown in a rotating precursor solution at 150 rpm exhibited the highest value of photosensitivity, with high reproducibility.

Coupling heterostructure of thickness-controlled nickel oxide nanosheets layer and titanium dioxide nanorod arrays via immersion route for self-powered solid-state ultraviolet photosensor applications

A coupling heterostructure consisting of nickel oxide nanosheets (NNS) and titanium dioxide nanorod arrays (TNAs) was fabricated for self-powered solid-state ultraviolet (UV) photosensor applications. By controlling the thickness of the NNS layer by via varying the growth time from 1 to 5 h at a deposition temperature of 90 °C, the coupling NNS/TNAs heterojunction films were formed and their structural, optical, electrical and UV photoresponse properties were investigated. The photocurrent measured from the fabricated self-powered UV photosensor was improved by increasing the thickness of NNS from 140 to 170 nm under UV irradiation (365 nm, 750 µWcm−2) at 0 V bias. A maximum photocurrent density of 0.510 µA∙cm−2 was achieved for a sample with a NNS thickness of 170 nm and prepared with a 3 h NNS growth time. Our results showed that the fabricated NNS/TNAs heterojunction has potential applications for self-powered UV photosensors.

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.

Synthesis of large-area 2D WS2 films and fabrication of a heterostructure for self-powered ultraviolet photodetection and imaging applications

High-performance self-powered ultraviolet (UV) photodetector based on a WS2/GaN heterojunction was demonstrated, which exhibits excellent UV photoresponse properties. Significantly, this photodetector has exhibited excellent UV imaging capability.

Very high photoresponse towards low-powered UV light under low-biased condition by nanocrystal assembled TiO2 film

There is an increasing interest in harvesting photoejected electrons for highly sensitive photodetectors, by interfacial engineering rather than the classic semiconductors. However, the widely employed device structures involving a p-n junction that causes photogenerated electron-hole separation to enhance the response are usually complex with a high fabrication challenge. Here, we present TiO2-based highly efficient ultraviolet (UV) photodetection by achieving its nanocrystal assembled film having high surface defects. The sol-gel derived TiO2 films have been subjected to a post-growth annealing at 500°C in air (SA) and vacuum (SB) and one subjected to UV treatment (SC) to tune the surface defects. The UV photoresponse results show that the nanocrystal assembled UV cured TiO2 film shows as high as 1.7×103 UV-to-visible rejection ratio and photo-to-dark current ratio of 1.2×104 under 10V bias and 10μW incident light power. Most interestingly, unprecedently high photo-to-dark current ratio of the order of ∼104 at as low as 1V bias condition and only 10μW incident light without device fabrication has been observed. Moreover, the films show stable response cyclibility under UV radiation. Therefore, simple UV curing improves UV photoresponse properties of TiO2 film enormously without the need to form conventional devices and opens the pathway for high-performance, low-cost, low-power consumption UV photodetector.

Ultraviolet Photoresponse Properties of Li-Doped ZnO Thin Films Prepared by Sol–Gel Spin-Coating Method

Ultraviolet Photoresponse Properties of Li-Doped ZnO Thin Films Prepared by Sol–Gel Spin-Coating Method

High-speed photoresponse properties of ultraviolet (UV) photodiodes using vertically aligned Al:ZnO nanowires

Ultraviolet (UV) photodiodes with fast photoresponse properties were fabricated using vertically aligned aluminum doped ZnO nanowires. Stable p-type ZnO have been achieved by doping equimolar concentration of P-N (0.75 at.%) simultaneously in ZnO. The vertically aligned and electrically conducting n-type Al (3 at.%) doped ZnO nanowires were grown by a simple aqueous chemical growth process. The structural, morphological, optical, and electrical properties were investigated. For the fabrication of UV photodiodes, the optimum p-type ZnO layers and n-type ZnO nanowires were stacked upon ITO substrate. A 250 nm thin NiO was deposited as an electron blocking layer (EBL) in between the ZnO p–n junctions. The current density–voltage (J–V) characteristic of the fabricated UV photodiode was measured under dark and UV illumination conditions. Under a reverse bias of 3 V, the device exhibits a high photoresponsivity (R) value of 15.07 (A/W) upon illumination of UV light (λ = 365 nm). The fabricated photodiode exhibits a fast photoresponse switching characteristics with a response and recovery time calculated as 61 ± 11 and 455 ± 41 ms, respectively. The role of vertically aligned nanowires in the formation of oxygen interstitial (Oi) defects and its impact on improving the UV photoresponse properties were investigated.

Fabrication of a Graphene/ZnO based p-n junction device and its ultraviolet photoresponse properties

Graphene with a zero-bandgap energy is easily doped using a chemical dopant, and a shift upwards or downwards in the Fermi level is generated. Moreover, the integration of inorganic material into the doped graphene changes the physical and chemical properties of the material. For this purpose, we successfully fabricated a p-n junction device by depositing an n-typed ZnO layer on p-doped graphene and studied the ultraviolet (UV) photoresponse properties under a photocurrent (UV light on) and a dark current (UV light off). Two devices, lateral and vertical, were developed by alternating the thickness of the ZnO layer, and the photoresponse mechanisms were described on the basis of the contact potential difference.

Deep ultraviolet photoconductive and near-infrared luminescence properties of Er3+-doped β-Ga2O3 thin films

Highly oriented (2¯01) Er3+-doped β-Ga2O3 (Er:Ga2O3) thin films with different doping concentrations were grown on (0001) sapphire substrates using radio frequency magnetron sputtering. The crystal structure, optical absorption, near-infrared luminescence, and ultraviolet photoresponse properties of Er:Ga2O3 films were systematically studied. The evolution of lattice and energy band gap with increasing doping level confirms the chemical substitution of Er3+ ions into the Ga2O3 crystal lattice. The down-shifting near-infrared luminescence (∼1538 nm: ascribed to Er3+: 4I13/2−4I15/2) was observed under ultraviolet excitation. Moreover, an obvious deep ultraviolet photoresponse was also obtained in the formed thin films.

Optical, Electrical, and UV Photoresponse Properties of Fluorine-Doped ZnO Thin Films Grown on Flexible Mica Substrates

Fluorine-doped ZnO (FZO) thin films have several potential applications, for instance, in low-cost optoelectronic devices; understanding how their optical, electrical, and photoresponse properties depend on and can be controlled via the synthesis conditions is essential for application of these systems. In this study, FZO thin films with different annealing temperatures were grown on muscovite mica substrates via sol–gel spin-coating. In photoluminescence measurements, a strong peak in the ultraviolet (UV) region and a broad peak in the visible region were observed for all films, being strongly dependent on the annealing temperature. The transmittance of the annealed films was slightly higher than that of as-grown film, and the absorption edges in the transmittance spectra red-shifted with increasing annealing temperature. The optical bandgap and Urbach energy of the films were calculated from the absorption coefficient values, using the Tauc and Urbach relations, respectively. Finally, the electrical (i.e., resistivity and carrier concentration) and photoresponse properties of the films were investigated to assess their applicability for use in FZO-based UV detectors.

Ultraviolet photoelectrical properties of a n-ZnO nanorods/p-diamond heterojunction

The n-ZnO nanorods/p-diamond heterojunction was fabricated and it shows high performance ultraviolet photoresponse properties.

UV photoresponse properties of ZnO nanorods arrays deposited with CuSCN by SILAR method

Ultraviolet (UV) photoresponse properties of ZnO nanorods arrays (NRs) deposited with copper thiocyanate (CuSCN) by different successive ionic layer adsorption and reaction (SILAR) cycles were investigated. Compared to the pristine ZnO NRs, the UV photosensitivity increases from 1.65 to 131.53 through lowering the dark current and the photoresponse speed has been obviously improved. As the number of SILAR cycles reaches 120, self-powered UV photoresponse property with fast response speed was observed, which was attributed to the photovoltaic behavior of ZnO/CuSCN.

Synthesis of WO3 nanostructures and their ultraviolet photoresponse properties

Tungsten oxide (WO3) nanostructures such as nanowires, nanorod bundles and nanotube bundles are synthesized by a facile hydrothermal method. The ultraviolet (UV) photoresponse characteristics of devices containing these WO3 nanostructures are investigated for the first time and new photosensitive mechanisms involving both photo-generated electron–hole pairs and reversible electrochemical reactions are proposed. We find that h-WO3 nanowires with large specific surface areas and fewer defects exhibit excellent UV photoresponse properties with switch ratios (defined as Iphoto/Idark) as high as 60, which is due to the existing large tunnels serving as channels and intercalation sites for mobile ions and active electrochemical reactions, and our findings provide a new family and more selectivity for UV photosensitive nanomaterials in the future.

Improvement of ultraviolet photoresponse of bent ZnO microwires by coupling piezoelectric and surface oxygen adsorption/desorption effects

Localized ultraviolet photoresponse properties of bent ZnO microwires bridging two perfect Ohmic contacts in both atmospheric and high vacuum (8 × 10(-6) torr) environments have been investigated for the first time to explore the bending strain effect on the photoelectrical properties of ZnO. It is found that the ZnO microwire has higher photoconductivity and faster rising speed when photo-excitation is localized at the bending region in an atmospheric environment, while the rising speeds are almost the same when photo-excitations are localized at the bending and straight regions under vacuum. The bending strain induced improvement of the UV photoresponse in air was well explained by considering the coupling of piezoelectric effects and the surface oxygen adsorption/desorption procedure on the bent ZnO microwire. Our results are valuable for designing and fabricating strain modulated photoelectrical micro/nano-devices.

Ultraviolet Photoresponse Properties of Single-Walled Carbon Nanotubes Decorated with Thickness-Controlled ZnO Layer by Pulsed Laser Deposition

We fabricated single-walled carbon nanotubes (SWNTs) covered with a thickness-controlled ZnO layer (ZnO–SWNTs) by pulsed laser deposition (PLD) and investigated their UV photoresponse, induced by the photodesorption of oxygen molecules from the ZnO surface. The magnitude of the negative photocurrent and the recovery time were strongly dependent on the thickness and morphology of the ZnO layer, and were highest when the ZnO layer was 3–4 nm thick. The observed recovery curves of the negative photocurrent were fitted with double-exponential-function curves, which indicate the coexistence of two types of adsorption sites for oxygen molecules on the ZnO surface. The light intensity dependence of the negative photocurrent was also measured.

Ultraviolet Photoresponse Properties of Single-Walled Carbon Nanotubes Decorated with Thickness-Controlled ZnO Layer by Pulsed Laser Deposition

Ultraviolet Photoresponse Properties of Single-Walled Carbon Nanotubes Decorated with Thickness-Controlled ZnO Layer by Pulsed Laser Deposition

The growth and the ultraviolet photoresponse properties of the horizontal growth ZnO nanorods

The horizontal ZnO nanorods (NRs) were grown by using a low temperature hydrothermal method between the lithographic ZnO interdigital electrodes. In order to horizontally grow the ZnO nanorods, the vertical growth was restrained by coating with the photoresist on the surface nucleation sites. By controlling the distance between the electrodes, only the electrodes for an interval of 7 μm can be connected by the horizontal nanorods to form device. The electrical property of the device was measured. The detector showed a narrow ultraviolet photoresponse with a response peak at 379 nm, which was according with the peak of the photoluminescence. The mechanism of photoresponse was discussed.