UV-visible Rejection Ratio Research Articles

Mg-doped beta-Ga2O3 films deposited by plasma-enhanced atomic layer deposition system for metal-semiconductor-metal ultraviolet C photodetectors

In this study, magnesium-doped gallium oxide (Ga2O3:Mg) films with various Mg contents were deposited by a plasma-enhanced atomic layer deposition (PE-ALD) system and were used as an active layer of metal-semiconductor-metal ultraviolet C photodetectors (MSM UVC-PDs). Compared with Ga element, owing to the lower electronegativity of Mg element, Mg–O bonds were more easily formed than Ga–O bonds. Consequently, oxygen vacancies could be reduced to improve properties. By using the measurement of X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL), the results verified that Ga2O3:Mg films with Mg content of 3.28 at.% had the lowest oxygen vacancy density. The cut-off wavelength of the devices was blue-shifted from 250 nm to 220 nm by increasing Mg content from 0 at.% to 5.22 at.%. The maximum photoresponsivity of 22.06, 10.24, 6.89, and 1.67 A/W was obtained for the devices with the Mg content of 0 at.%, 1.91 at.%, 3.28 at.%, and 5.22 at.%, respectively. The corresponding UV–visible rejection ratio was 3.56 × 104, 3.86 × 104, 4.42 × 104, and 3.48 × 103, respectively. The flicker noise was the dominant noise. By appropriately doping the Mg content in Ga2O3 films, the detectivity of the MSM UVC-PDs was effectively improved.

Schottky-type GaN-based UV photodetector with atomic-layer-deposited TiN thin film as electrodes.

In this work, a GaN-based UV photodetector with an asymmetric electrode structure was fabricated by atomic layer deposition (ALD) of TiN layers. The thickness of the TiN can be monitored in situ by a quartz crystal microbalance (QCM) and precisely controlled through the modulation of deposition cycles. During the ALD process, periodic variation in the QCM frequency was observed and correlated to the physical adsorption, chemical bonding, and the excessive precursor exhaust, which included tetrakis(dimethylamino)titanium (TDMAT) and N sources. The asymmetric TiN/GaN/TiN photodetector showed excellent photosensing performance, with a UV-visible rejection ratio of 173, a responsivity of 4.25 A/W, a detectivity of 1.1×1013 Jones, and fast response speeds (a rise time of 69 μs and a decay time of 560 μs). Moreover, the device exhibits high stability, with an attenuation of only approximately 0.5% after 360 nm light irradiation for 157 min. This result indicates the potential of TiN as a transparent contact electrode for GaN-based optoelectronic devices.

Performance enhancement of a self-powered solar-blind UV photodetector based on ZnGa2O4/Si heterojunction via interface pyroelectric effect

The photodetectors based on the wide bandgap semiconductor (WBS)/Si heterojunction have attracted more and more attention in recent years due to their excellent photoelectric characteristics and easy integration capabilities. In this work, we have demonstrated a self-powered solar-blind ultraviolet (UV) photodetector based on the ZnGa2O4/Si heterojunction. A typical rectification characteristic with a rectification ratio exceeding 103 within ±5 V can be obtained. At 0 V bias, the −3 dB cutoff wavelength of ∼255 nm and the UV-visible rejection ratio of ∼3 × 102 show that the device has excellent self-powered solar-blind UV detection performance. In addition, the responsivity and the response speed of ZnGa2O4/Si heterojunction can be efficiently enhanced by a transient spike current at 0 V bias when turning on and off the 254 nm UV light. The interface pyroelectric effect of the ZnGa2O4 film should be responsible for this transient spike photocurrent phenomenon. Our findings in this work pave a feasible way to realize high-performance WBS/Si heterojunction self-powered solar-blind photodetectors.

A High Quantum Efficiency Narrow-Band UV-B AlGaN p-i-n Photodiode With Polarization Assistance

We designed and fabricated narrow-band UV-B AlGaN p-i-n photodiodes (PDs) with a full-width at half-maximum (FWHM) of 8 nm by optimizing the Al composition and thickness of the AlGaN layers. To improve the photoelectric response of the narrow-band PDs, a polarization electric field with the same direction with the applied bias field was introduced to the absorption layer by adjusting the Al composition ratio between the p-type AlGaN layer and i-type AlGaN absorption layer. The polarization enhanced narrow-band PD exhibited a higher external quantum efficiency (EQE) of 82% than the conventional one with an EQE of 67%. Meanwhile, a low dark current density of 1.7 nA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and four orders of magnitude UV-visible rejection ratio were achieved for the enhanced narrow-band PDs with a maximum photocurrent responsivity of 202 mA/W at 304 nm.

Ultrahigh Sensitivity Graphene/Nanoporous GaN Ultraviolet Photodetectors.

Integration of graphene with three-dimensional semiconductors can introduce unique optical and electrical properties that overcome the intrinsic limitation of the materials. Here, we report on the high sensitivity ultraviolet (UV) photodetectors based on monolayer graphene/nanoporous GaN heterojunctions. By investigating the reflectivity, photoluminescence, and Raman spectral characteristics of nanoporous GaN, we find that the increase in the porosity can help to improve its optical properties. The device based on the highest-porosity nanoporous GaN demonstrates rapid and linear response to UV photons, with an ultrahigh detectivity of 1.0 × 1017 Jones and a UV-visible rejection ratio of 4.8 × 107 at V = -1.5 V. We attribute such high sensitivity to the combination of the significantly enhanced light harvesting of high-porosity nanoporous GaN and the unique UV absorption, high mobility, and finite density of states of the monolayer graphene. The high performance together with a simple and low-cost fabrication process endow these graphene/nanoporous GaN heterojunctions with great potential for future selective detection of weak UV optical signals.

Ultrahigh Responsivity UV Photodetector Based on Cu Nanostructure/ZnO QD Hybrid Architectures.

Strong near-surface electromagnetic field formed by collective oscillation of electrons on Cu nanostructure a shows a strong dependence on geometry, offering a promising approach to boost the light absorption of ZnO photoactive layers with enhanced plasmon scattering. Here, a facile way to fabricate UV photodetectors with tunable configuration of the self-assembled Cu nanostructures on ZnO thin films is reported. The incident lights are effectively confined in ZnO photoactive layers with the existence of the uplayer Cu nanostructures, and the interdiffusion of Cu atoms during fabrication of the Cu nanostructures can improve the carrier transfer in ZnO thin films. The optical properties of the hybrid architectures are successfully tailored over a control of the geometric evolution of the Cu nanostructures, resulting in significantly enhanced photocurrent and responsivity of 2.26 mA and 234 A W-1 under a UV light illumination of 0.62 mW cm-2 at 10 V, respectively. The photodetectors also exhibit excellent reproducibility, stability, and UV-visible rejection ratio (R370 nm /R500 nm ) of ≈370, offering an approach of high-performance UV photodetectors for practical applications.

Photoresponses of Gallium Zinc Tin Oxide Thin-Film Transistors Fabricated by Cosputtering Method

In this article, gallium zinc tin oxide (GaZTO) thin-film transistor (TFT) was deposited by radio-frequency cosputtering. High transmittance in the visible light region and wide energy bandgap of GaZTO thin film were ascertained by UVVisNIR spectrophotometer. GaZTO TFT exhibited a threshold voltage of 2.98V, mobility of 3.86 cm2Vs, onoff ratio of 1.5 106, and subthreshold swing of 0.392Vdecade. The TFT was employed to detect UV light, and the optoelectrical properties were investigated in detail. The UV-visible rejection ratio of GaZTO TFT under illumination was 9.8 105. The results revealed that GaZTO phototransistor is versatile and able to serve as a UV sensor.

Characteristics of GaN Metal-Insulator-Semiconductor-Insulator-Metal Ultraviolet Photodiodes Using Al2O3, HfO2, and ZrO2 as Insulators

GaN-based metal–insulator–semiconductor–insulator-metal (MISIM) photodiodes using three kinds of high-k dielectrics (Al2O3, HfO2, and ZrO2 were fabricated, and their electrical, photo response, and noise properties were characterized. The GaN MISIM photodiode using ZrO2 showed the lowest dark current density of $5.43 \times 10^{-9}$ A/cm2 at 10 V bias and the highest UV-visible rejection ratio of 257 at 1 V bias. No Fowler-Nordheim tunneling phenomenon was observed in the diode under UV illumination. The noise spectral density and the trap-time-constant of the device using ZrO2 were $9.79 \times 10^{-31}~\text{A}^{2}$ /Hz for $f = 10$ Hz at 1 V and $335~\mu \text{s}$ , respectively, which were lowest compared with the other two samples. ZrO2 was identified as the better GaN, the passivation material among the three dielectrics.

High-Performance Visible-Blind Ultraviolet Photodetector Based on IGZO TFT Coupled with p-n Heterojunction.

A visible-blind ultraviolet (UV) photodetector was designed based on a three-terminal electronic device of thin-film transistor (TFT) coupled with two-terminal p-n junction optoelectronic device, in hope of combining the beauties of both of the devices together. Upon the uncovered back-channel surface of amorphous indium-gallium-zinc-oxide (IGZO) TFT, we fabricated PEDOT:PSS/SnO x/IGZO heterojunction structure, through which the formation of a p-n junction and directional carrier transfer of photogenerated carriers were experimentally validated. As expected, the photoresponse characteristics of the newly designed photodetector, with a photoresponsivity of 984 A/W at a wavelength of 320 nm, a UV-visible rejection ratio up to 3.5 × 107, and a specific detectivity up to 3.3 × 1014 Jones, are not only competitive compared to the previous reports but also better than those of the pristine IGZO phototransistor. The hybrid photodetector could be operated in the off-current region with low supply voltages (<0.1 V) and ultralow power dissipation (<10 nW under illumination and ∼0.2 pW in the dark). Moreover, by applying a short positive gate pulse onto the gate, the annoying persistent photoconductivity presented in the wide band gap oxide-based devices could be suppressed conveniently, in hope of improving the response rate. With the terrific photoresponsivity along with the advantages of photodetecting pixel integration, the proposed phototransistor could be potentially used in high-performance visible-blind UV photodetector pixel arrays.

Solution-processed visible-blind UV-A photodetectors based on CH3NH3PbCl3 perovskite thin films

Solution-processed visible-blind UV-A photodetectors based on smooth, pinhole-free CH3NH3PbCl3 perovskite thin films with a high UV-visible rejection ratio.

Performance enhancement of MgZnO ultraviolet photodetectors using ultrathin Al2O3 inserted layer

In this study, the magnesium zinc oxide (MgZnO) films and ultrathin alumina (Al2O3) inserted layers were subsequently deposited on sapphire substrates using a plasma-enhanced atomic layer deposition system, and applied in metal-semiconductor-metal ultraviolet (UV) photodetectors (MSM-UPDs). The dark current of the MgZnO MSM-UPDs was decreased from 1 to 0.34 nA with an increase in Al2O3 layer thickness from 0 to 5 nm. The ultrathin Al2O3 inserted layer effectively passivated the dangling bonds on the MgZnO surface and blocked leakage current. At a bias voltage of 5 V, the maximum UV-visible rejection ratio of the MgZnO MSM-UPDs was 1.78 × 103 with 5-nm-thick Al2O3 inserted layer. Furthermore, the noise equivalent power and detectivity of MgZnO MSM-UPDs with 5-nm-thick Al2O3 inserted layer were improved from 1.26 × 10−14 W and 2.50 × 1013 cm Hz1/2 W−1 to 0.93 × 10−14 W and 3.40 × 1013 cm Hz1/2 W−1 in comparison with MgZnO MSM-UPDs without Al2O3 inserted layer. The high performances of MgZnO MSM-UPDs were achieved by using ultrathin Al2O3 inserted layer.

A surface oxide thin layer of copper nanowires enhanced the UV selective response of a ZnO film photodetector

A UV detector with a high UV-visible rejection ratio achieved by a warped Schottky junction between Cu NWs and ZnO.

Ultraviolet Photodetectors With 2-D Indium-Doped ZnO Nanostructures

In this paper, we developed a simple method to fabricate an indium-doped ZnO nanostructure ultraviolet (UV) photodetector (PD) on a glass substrate. Through the aqueous chemical solution method, the indium-doped ZnO nanostructure uniformly grew on a glass substrate. The average length and diameter were 3.06 $\mu \text{m}$ and 38 nm, respectively. At 1 V bias, the photo-to-dark current ratio of the indium-doped ZnO PD was 740 under UV light irradiation (365 nm). The sample exhibited fast response and recovery time. The increase and decrease times were 3.02 and 1.53 s, correspondingly. When irradiated at 360 nm, the UV-visible rejection ratio of the indium-doped ZnO nanostructure PD was approximately 312. The indium-doped ZnO nanostructure PD exhibited high sensitivity, fast response and recovery times, and good orientation properties.

Annealing effects on MgZnO films and applications in Schottky-barrier photodetectors

In this paper, MgZnO films were annealed at various temperatures (700–900 °C) and their effects on the electrical characteristics of Schottky-barrier photodetectors (SB-PDs) were studied. We show that the content of Mg atom increases with annealing temperature. On 900 °C annealing, the Mg content is determined to be as high as 49.2 at. %. However, no cubic-phase MgZnO segregation occurs in such high-temperature annealed MgZnO films, revealing the MgZnO film grown by radio-frequency sputtering is rather stable. Moreover, high-temperature (900 °C) annealing results in denser MgZnO film and smaller gap between the grains, leading to higher mobility and lower sheet resistance compared with low-temperature annealed MgZnO films. The enhanced performance results in the 900 °C annealed SB-PD exhibiting higher rectification ratio of 326 compared to the 800 °C annealed SB-PD that has a rectification ratio only 40. Further, lower sheet resistance reduces the turn-on voltage from >200 V (for SB-PD with as-deposited MgZnO film) to 0.4 V (for SB-PD with 900°C annealed MgZnO film). Even at low (−10 V) bias-voltage, the 900 °C annealed SB-PD presents the highest ultraviolet (UV) responsivity (0.088 A/W) and UV-visible rejection ratio (2264) compared with the SB-PDs with low-temperature annealed MgZnO films at very high (−100 V) bias-voltage.

Nanomesh electrode on MgZnO-based metal-semiconductor-metal ultraviolet photodetectors.

In this work, the nano-scaled mesh electrodes are fabricated by obliquely depositing metals through the highly ordered polystyrene nanosphere mask. Furthermore, the intrinsic MgZnO film is deposited as the absorption layer for the metal-semiconductor-metal ultraviolet photodetectors (MSM-UV-PDs) using the vapor cooling condensation system. The 100-nm-linewidth nanomesh electrodes with metal occupying a roughly 10% of the device surface region consequently render PDs with a high transmittance in the ultraviolet (UV) wavelength range. The photoresponsivity of MgZnO-based MSM-UV-PDs evaluated at the wavelength of 330 nm with the operating bias voltage of 5 V is elevated from 0.135 to 0.248 A/W when the thin metal electrode is replaced by the nanomesh electrode, and the corresponding quantum efficiency is improved from 50.75 to 93.23%. Finally, adopting the nanomesh electrode also helps to enhance the UV-visible rejection ratio (R330nm/R450nm) and the detectivity from 1663 and 1.78 × 1010 cmHz0.5W−1 to 2480 and 2.43 × 1010 cmHz0.5W−1, respectively.

Reduction of persistent photoconductivity in ZnO thin film transistor-based UV photodetector

We report a ZnO-based thin film transistor UV photodetector with a back gate configuration. The thin-film transistor (TFT) aspect ratio W/L is 150 μm/5 μm and has a current on-off ratio of 1010. The detector shows UV-visible rejection ratio of 104 and cut-off wavelength of 376 nm. The device has low dark current of 5 × 10−14 A. The persistent photoconductivity is suppressed through oxygen plasma treatment of the channel surface which significantly reduces the density of oxygen vacancy confirmed by XPS measurements. The proper gate bias-control further reduces recovery time. The UV-TFT configuration is particularly suitable for making large-area imaging arrays.

Homo- and hetero-epitaxial growth of hexagonal and cubic MgxZn1−x O alloy thin films by pulsed laser deposition technique

In this work, we describe the homo- and hetero-epitaxial growth of hexagonal and cubic MgxZn1−xO thin films on lattice matched substrates of c-Al2O3, ZnO, MgO and SrTiO3. The crystalline quality, composition and epitaxial nature of the alloy films are obtained by x-ray diffraction and Rutherford backscattering spectroscopy (RBS) techniques. The RBS channeling yields are in the range 3–8% for homoepitaxial and hetero-epitaxial thin films. The metal–semiconductor–metal and ultraviolet detectors were fabricated on hexagonal and cubic MgxZn1−xO thin films and the leakage current and UV-visible rejection ratio are correlated with the epitaxial relationship between the film and substrates.

Superior Characteristics of RuO2/GaN Schottky-Type UV Photodetector

A RuO2/GaN Schottky diode was designed and fabricated for application as a UV photodetector. The fabricated RuO2/GaN Schottky diode showed a breakdown at over —50 V, plus a leakage current of 9 nA/cm2 at —5 V with a good uniformity that was one order of magnitude lower than that of a Pt/GaN Schottky diode under the same epitaxial conditions. The proposed RuO2/GaN Schottky photodiode also exhibited a UV-visible rejection ratio of 3 × 105 and responsivity of 0.23 A/W at 330 nm.

Superior Characteristics of RuO2/GaN Schottky-Type UV Photodetector

A RuO2/GaN Schottky diode was designed and fabricated for application as a UV photodetector. The fabricated RuO2/GaN Schottky diode showed a breakdown at over —50 V, plus a leakage current of 9 nA/cm2 at —5 V with a good uniformity that was one order of magnitude lower than that of a Pt/GaN Schottky diode under the same epitaxial conditions. The proposed RuO2/GaN Schottky photodiode also exhibited a UV-visible rejection ratio of 3 × 105 and responsivity of 0.23 A/W at 330 nm.