Ultraviolet Photodiodes Research Articles

Demonstration of a Solar-Blind Single-Photon Imaging Lidar Based on SiC Ultraviolet Avalanche Photodiodes

In this work, a prototype solar-blind single-photon imaging lidar based on SiC ultraviolet (UV) avalanche photodiodes (APD) is demonstrated. The SiC APD with a peak responsivity at ~280 nm can greatly lessen the strict solar-blind filtering requirement for the system. When characterized by using a gated quenching circuit, the SiC APD operating in Geiger mode could exhibit a room temperature single photon detection efficiency of ~27% with a dark count probability of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.59\times 10 ^{-2}$ </tex-math></inline-formula> . Based on the SiC APD, a 266 nm nanosecond pulsed laser, a galvanometer scanner and a time-correlated single-photon counter, 3-dimensional depth image with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$128\times 128$ </tex-math></inline-formula> resolution is realized by the prototype system in daylight. The average ranging accuracy of the system is found within 28 mm based on current setup.

Fabrication High-Temperature 4H-SiC Schottky UV Photodiodes by O2 Plasma Pre-Treatment Technology

In this letter, an ultrahigh-temperature 4H-SiC Schottky ultraviolet (UV) photodiode (PD) using oxygen plasma pre-treatment (OPT) technology has been successfully fabricated and characterized. The PD has a high Schottky barrier height (SBH) of 1.94 eV. It shows excellent tolerance to extreme temperature with a dark current of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.7\times 10\,\,^{\mathbf {-8}}$ </tex-math></inline-formula> A at −25 V and 600 °C. Under UV illumination, a record operating temperature of 600 °C is realized for the first time (PDCR = 3.5 at 600 °C, −25 V). Of note, the fabricated PD exhibits high responsivity that increases with the operating temperature from 0.17 A/W at room temperature (RT) to 0.52 A/W at 600 °C, at −25 V and 275-nm. Moreover, an expeditious response time of ms level is also realized from RT to 600 °C. These results demonstrate that the proposed 4H-SiC Schottky PD is advantageous for high-temperature UV-related applications.

Ultraviolet photodiode fabricated from TiO2 nanorods/p-silicon heterojunction

In this paper, an ultraviolet (UV) photodiode in p-n heterojunction configuration is fabricated from TiO2 nanorods hydrothermally deposited on p-type silicon substrate. Under UV illumination condition, the heterojunction photodiode shows superior UV detection capability achieving a photoresponsivity of 1.41 A/W, a photosensitivity of 80 and a detectivity of 8.32 × 1011 Jones with a quick response speed of 25 ms. More significantly, the presented fabrication approach can be applicable to other metal–oxide semiconductor based optoelectronic devices.

Development of a soft x-ray (SXR) array diagnostic system on versatile experiment spherical torus (VEST).

A new soft x-ray (SXR) array diagnostic system has been developed on versatile experiment spherical torus (VEST) for measurements of 2D SXR emissivity profile and identification of poloidal mode structure. Through tomographic inversion techniques, 2D SXR emissivity profile can be acquired from the line-integrated SXR data, which enables the visualization of mode structure of plasma instability, such as the magnetohydrodynamics mode. The SXR array diagnostic system consists of two 20-channel arrays positioned at the middle and the top on the same poloidal plane for horizontal and vertical lines of sight, respectively. Each array of the diagnostic system uses absolute extreme ultraviolet photodiode array as the detector. To apply appropriate filters (up to four) for different energy regimes without breaking the vacuum, a filter wheel and its rotatable vacuum feed-through are installed behind the pinhole. SXR data are acquired with a digitizer at the sampling rate of up to 125MHz. Finally, we discuss initial measurement data obtained from Ohmic plasma in VEST.

Role of Seed Layer Crystalline Quality in Photoresponse Performance of Hydrothermally Grown ZnO Nanorods.

We investigated the effects of the crystalline state for seed layers (SLs) on the growth morphology and material characteristics for hydrothermally grown ZnO nanorods (NRs). For this, preheating (PH) at different temperatures (100–300 °C) and O2 plasma treatment (PT) for 9 min were performed during the growth of SLs on p-Si by the aqueous solution-based method to provide the characteristic change on the NR growth platform. An improvement in material properties was achieved from the ZnO NRs grown on the SL crystals of enhanced crystalline quality in terms of the increased preferred orientation (002), the higher UV emission with suppressed deep-level emissions, the recovery of O/Zn stoichiometry, and the reduction of various intrinsic defects. Ultraviolet photodiodes of a p-Si/n-ZnO-NR structure fabricated under the SL conditions of O2 PT and PH at 100 °C showed a significantly enhanced on-off current ratio of ~90 at +5 V and faster photoresponse characteristics presenting a reduction in the fall time from 16 to 9 s.

Self-Assembly Nanopillar/Superlattice Hierarchical Structure: Boosting AlGaN Crystalline Quality and Achieving High-Performance Ultraviolet Avalanche Photodetector.

As a burgeoning wide-band gap semiconductor material, AlxGa1-xN alloy has attracted great attention for versatile applications due to its superior properties. However, its poor crystalline quality has restricted the employment of AlGaN on electronic devices for a long time. Herein, we proposed a nanopillar/superlattice hierarchical structure for AlGaN epitaxy to boost the crystalline quality. The scale-controllable AlN nanopillar template is fabricated from a nickel self-assembly process. AlGaN initiates the epitaxial laterally overgrowth mode based on the nanopatterned template. In addition, the AlxGa1-xN/AlyGa1-yN superlattice structure could effectively block the propagation of threading dislocation segments. The kinetics of the dislocation and epitaxy process in the hierarchical structure is intuitively demonstrated and analyzed. Consequently, the dislocation density of AlGaN grown by this method is significantly reduced by more than 30 times compared to the AlN template. No threading dislocation segments were observed in the 4 μm TEM field of view. Moreover, based on the hierarchical structure, we also fabricated an AlGaN ultraviolet avalanche photodiode (APD). The APD exhibits superior performance, achieving a maximum gain of 1.3 × 105 and high responsivity of 1.46 A/W at 324 nm. The reliability of the nanopillar/superlattice AlGaN epitaxial procedure is anticipated to shed new light on the nitride semiconductor material, further bringing a breakthrough to wide-band gap electronic devices.

High−Performance 4H−SiC UV p−i−n Photodiode: Numerical Simulations and Experimental Results

In this work, the optical response of a high−performance 4H−SiC−based p−i−n ultraviolet (UV) photodiode was studied by means of an ad hoc numerical model. The spectral responsivity and the corresponding external photodiode quantum efficiency were calculated under different reverse biases, up to 60 V, and in the wavelength range from λ = 190 to 400 nm. The responsivity peak is R = 0.168 A/W at λ = 292 nm at 0 V and improves as bias increases, reaching R = 0.212 A/W at 60 V and λ = 298 nm. The external quantum efficiency is about 71% and 88%. The good quality of the simulation setup was confirmed by comparison with experimental measurements performed on a p−i−n device fabricated starting from a commercial 4H−SiC wafer. The developed numerical model, together with the material electrical and optical parameters used in our simulations, can be therefore explored for the design of more complex 4H−SiC−based solid−state electronic and optoelectronic devices.

Optimization of UVB photodiode based on ZnO nanorod arrays grown via the hydrothermal process

In this work, n-Type ZnO nanorods (NRs) were grown on a p-type Si (100) substrate using the hydrothermal technique. These p-n heterojunctions were made to fabricate ZnO-based ultraviolet (UV) photodiodes (PDs). To study the effect of solution volume -in the hydrothermal process-on the structural properties of the ZnO NRs arrays, three samples were prepared with different conditions. SEM characterization shows the well-aligned ZnO NRs arrays grown on all three substrates. X-ray diffraction characterization indicates the crystalline structure of all samples with a preferred orientation of c-plane (002). XRD results show the ZnO grain size enlargement concerning solution volume. To analyze the electric characterization of the samples, the current-voltage (I–V) analysis was performed under dark and illuminated conditions. The I–V measurement indicates that the increase of growth solution volume is leading to increasing in the responsivity of ZnO-based PDs. Additionally, the PL and UV–Vis characterizations exhibit that the optical properties of samples were also affected and therefore could be controlled and optimized by growth conditions.

An electro-optical system for improving graphene nanopore DNA Sequencing

I propose a new electro-optical system for sequencing single-stranded DNA molecules. The localized plasmons in graphene combined with the nanopore-based DNA translocation have been suggested for sequencing DNA molecules and decreasing the translocation speed. These localized surface plasmon resonances in the graphene nanopore have three dominant modes. Every-mode peak wavelength is shifted while the DNA nucleobases are presented to the nanopore. The ultraviolet photodiodes with narrow-band filters can separate each mode individually. Then, the electrical-current to voltage convertor, differential pairs based on the bipolar transistors, combine all modes in the effective output voltage. The output voltage level of the circuit is sensitive to the DNA presence and is unique for each DNA nucleobase. Analyzes have been performed for different central wavelengths and spectral width of the light source. Results show that the best sensitivity to the DNA molecule is 6.04, and the best selectivity is 1.1. These factors are directly related to the method's applicability for DNA translocation sensing and DNA sequencing; they are enormously improved. The proposed method and results shed light on a higher selectivity for DNA nucleobases which is the main bottleneck for nanopore DNA sequencing.

Monolithically Integrated UV Photoelectric Switch Based on GaN-on-Silicon Platform

We report a monolithically integrated ultraviolet (UV) photoelectric switch based on a GaN-on-silicon platform for the first time. The novel integrated switch consists of a U-shaped trench metal-oxide-semiconductor field effect transistor (UMOSFET), an InGaN/GaN multiple quantum wells (MQW) UV photodiode (PD), and a thin-film resistor. A common blue light-emitting diode epi wafer is adopted to design and fabricate the switch without an extra epitaxy growth or ion implantation process, which greatly simplifies the fabrication. The backside of the chip is designed to receive the incident light. The shielding of the silicon substrate indicates the UMOSFET and thin-film resistor are immune to interference from the incident light, except for the PD. Under UV illumination, the induced photocurrent in the MQW PD is converted into a voltage by the thin-film resistor and is used to drive the UMOSFET for output. The measured results indicate that the response speed of the photoelectric switch with 21 Hz cut-off frequency is mainly limited by the fall time of PD photocurrent.

Boosting the External Quantum Efficiency of AlGaN-Based Metal–Semiconductor–Metal Ultraviolet Photodiodes by Electrode Geometry Variation

Fast aluminum-gallium-nitride–based metal–semiconductor–metal ultraviolet photodiodes were successfully designed, fabricated, and characterized using conventional photolithography techniques. Various electrode geometries were fabricated to investigate the influence of metal contact shapes on the device performance indices with emphasis on the response speed and bias-voltage–independent efficiency. Based on the independently measured Hall mobility and electric field, we evaluated the carrier transit time of the devices as 1.31 ps with bias-voltage–independent external quantum efficiency of 1198% in photoconductive mode at 19.5 V and 70% at 60 V for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> -doped and intrinsic devices, respectively.

The Study of Optical and Electrical Properties of Nanostructured Silicon Carbide Thin Films Grown by Pulsed-Laser Deposition

In this paper, nanostructured silicon carbide (SiC) thin films are deposited onto glass substrate using pulsed laser deposition technique. Electrical and optical characterizations such as conductivity, resistivity, transmission, Seeback effect, absorption, absorption coefficient, energy band gap, and extinction coefficient as a function of photon energy, and the effect of thin films thickness on transmission are carried out to characterize the prepared samples. Results showed that the prepared SiC thin film is an n-type semiconductor with an indirect bandgap of ~3 eV, 448 nm cutoff wavelength, 3.4395 × 104 cm−1 absorption coefficient and 0.154 extinction coefficient. The surface morphology of the SiC thin films is studied using scanning electron microscope at a substrate temperature of 400 °C and it is found that the grain size of the prepared SiC thin film is about 30 nm. As such, the nano thin films optical and structural characteristics enable the films to be used as gases sensors in many optoelectronic devices such as the environment and ultraviolet photodiode.

An Efficient 4H-SiC Photodiode for UV Sensing Applications

In this paper, we report experimental findings on a 4H-SiC-based p-i-n photodiode. The fabricated device has a p-type region formed by ion-implantation of aluminum (Al) in a nitrogen doped n-type layer. The dark reverse current density reaches 38.6 nA/cm2 at −10 V, while the photocurrent density rises to 6.36 µA/cm2 at the same bias under λ = 315 nm ultraviolet (UV) radiation with an incident optical power density of 29.83 μW/cm2. At the wavelength of λ = 285 nm, the responsivity is maximum, 0.168 A/W at 0 V, and 0.204 A/W at −30 V, leading to an external quantum efficiency of 72.7 and 88.3%, respectively. Moreover, the long-term stability of the photodiode performances has been examined after exposing the device under test to several cycles of thermal stress, from 150 up to 350 °C and vice versa. The achieved results prove that the examined high-efficiency UV photodiode also has a stable responsivity if subjected to high temperature variations. The proposed device is fully compatible with the conventional production process of 4H-SiC components.

Multiple-quantum-well-induced unipolar carrier transport multiplication in AlGaN solar-blind ultraviolet photodiode

AlGaN solar-blind ultraviolet (SBUV) detectors have potential application in fire monitoring, corona discharge monitoring, or biological imaging. With the promotion of application requirements, there is an urgent demand for developing a high-performance vertical detector that can work at low bias or even zero bias. In this work, we have introduced a photoconductive gain mechanism into a vertical AlGaN SBUV detector and successfully realized it in a p - i - n photodiode via inserting a multiple-quantum-well (MQW) into the depletion region. The MQW plays the role of trapping holes and increasing carrier lifetime due to its strong hole confinement effect and quantum confinement Stark effect. Hence, the electrons can go through the detector multiple times, inducing unipolar carrier transport multiplication. Experimentally, an AlGaN SBUV detector with a zero-bias peak responsivity of about 0.425 A/W at 233 nm is achieved, corresponding to an external quantum efficiency of 226%, indicating the existence of internal current gain. When compared with the device without MQW structure, the gain is estimated to be about 10 3 in magnitude. The investigation provides an alternative and effective approach to obtain high current gain in vertical AlGaN SBUV detectors at zero bias.

Self-powered ultraviolet photodiode based on lateral polarity structure GaN films

In this work, we report on a self-powered ultraviolet photodiode realized using lateral polarity structure (LPS) GaN films. The opposite nature of the polarization charge yields different barrier heights at the standard Ni/Au Schottky contact interface of N-polar and III-polar GaN films. As a result, a natural nonzero built-in potential is obtained in the LPS GaN photodiode, which showed photoresponsivity even at 0 V applied bias. The self-powered mechanism inside such an LPS GaN photodiode is discussed in detail by a combination of simulation prediction and experimental validation. Furthermore, a variation in the doping concentration of the adjacent III- and N-polar GaN domain is shown to improve the photoresponsivity compared to the conventional III-polar photodiode. Thus, this work validates that the LPS GaN photodiode is a promising candidate to realize self-powered operation and a general design rule for the photodiode with in-plane built-in potential.

Self-Powered MXene/GaN van der Waals Heterojunction Ultraviolet Photodiodes with Superhigh Efficiency and Stable Current Outputs.

A self-powered, high-performance Ti3 C2 Tx MXene/GaN van der Waals heterojunction (vdWH)-based ultraviolet (UV) photodiode is reported. Such integration creates a Schottky junction depth that is larger than the UV absorption depth to sufficiently separate the photoinduced electron/hole pairs, boosting the peak internal quantum efficiency over the unity and the external quantum efficiency over 99% under weak UV light without bias. The proposed Ti3 C2 Tx /GaN vdWH UV photodiode demonstrates pronounced photoelectric performances working in self-powered mode, including a large responsivity (284 mA W-1 ), a high specific detectivity (7.06 × 1013 Jones), and fast response speed (rise/decay time of 7.55 µs/1.67 ms). Furthermore, the remarkable photovoltaic behavior leads to an impressive power conversion efficiency of 7.33% under 355 nm UV light illumination. Additionally, this work presents an easy-processing spray-deposition route for the fabrication of large-area UV photodiode arrays that exhibit highly uniform cell-to-cell performance. The MXene/GaN photodiode arrays with high-efficiency and self-powered ability show high potential for many applications, such as energy-saving communication, imaging, and sensing networks.

Modular bolometric/soft x-ray diagnostic in Sino-UNIted Spherical Tokamak.

A bolometer/soft x-ray diagnostic based on modular absolute extreme ultraviolet photodiodes has been built to obtain the local emission profile of a poloidal cross section in Sino-UNIted Spherical Tokamak (SUNIST). The system consists of seven photodiode arrays of 112 channels located around the low field side of a poloidal cross section. All arrays are mounted into 81 × 36 × 33mm3 modules, which could be installed flexibly on the wall of the vacuum vessel. Tungsten shields with adjustable pinholes restrict the radiation flux. Compact flexible printed circuits with multilayer copper polygon poured on both sides are designed to transmit signals with electronic noise, which is reduced to less than 1/10 before. A two-stage transimpedance amplifier array of 700 kHz bandwidth is fixed on the atmosphere side of a vacuum electrical feedthrough and is powered by battery packs. A synchronized switch shuts down the circuit to reduce the heat production and increase the battery life. A line-integrated radiation emission profile with 2cm spatial resolution and temporal resolution up to 2 µs is obtained, and the local emission profile of SUNIST is reconstructed with the help of a newly developed tomography scheme for movable detector modules.

Absolute calibration of the Lyman-α measurement apparatus at DIII-D.

The LLAMA (Lyman-Alpha Measurement Apparatus) diagnostic was recently installed on the DIII-D tokamak [Rosenthal et al., Rev. Sci. Instrum. (submitted) (2020)]. LLAMA is a pinhole camera system with a narrow band Bragg mirror, a bandpass interference filter, and an absolute extreme ultraviolet photodiode detector array, which measures the Ly-α brightness in the toroidal direction on the inboard, high field side (HFS) and outboard, low field side (LFS). This contribution presents a setup and a procedure for an absolute calibration near the Ly-α line at 121.6nm. The LLAMA in-vacuum components are designed as a compact, transferable setup that can be mounted in an ex situ vacuum enclosure that is equipped with an absolutely calibrated Ly-α source. The spectral purity and stability of the Ly-α source are characterized using a vacuum ultraviolet spectrometer, while the Ly-α source brightness is measured by a NIST-calibrated photodiode. The non-uniform nature of the Ly-α source emission was overcome by performing a calibration procedure that scans the Ly-α source position and employs a numerical optimization to determine the emission pattern. Nominal and measured calibration factors are determined and compared, showing agreement within their uncertainties. A first conversion of the measured signal obtained from DIII-D indicates that the Ly-α brightness on the HFS and LFS is on the order of 1020 Ph sr-1m-2s-1. The established calibration setup and procedure will be regularly used to re-calibrate the LLAMA during DIII-D vents to monitor possible degradation of optical components and detectors.

Uniform and High Gain GaN p-i-n Ultraviolet APDs Enabled by Beveled-Mesa Edge Termination

In this letter, a beveled-mesa edge termination technology was developed to improve the performance of GaN pi-n ultraviolet avalanche photodiodes (APDs). Simulation results showed that the beveled-mesa is effective in reduction of the electric field at the mesa sidewall. With a photoresist thermal-reflow process, 12°-angle beveled-mesa APDs were fabricated from homoepitaxial p-i-n structures on GaN substrate. The most beneficial property of the beveled-mesa termination is the uniform breakdown-characteristic of the beveled-mesa APDs compared to the vertical-mesa APDs. Record-high VBR uniformity of 95.4 V ± 0.2 V and average avalanche gain up to 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> were observed for the beveled-mesa APDs within a linear array across 4 mm side-length. These results indicate the potential of the beveledmesa GaN APD array for ultraviolet light imaging applications.

128-pixel arrays of 4H-SiC UV APD with dual-frequency PECVD SiNx passivation.

In this paper, high-performance 1×128 linear arrays of 4H-SiC ultraviolet (UV) avalanche photodiode (APD) with dual-frequency plasma enhanced chemical vapor deposition (PECVD) passivation are demonstrated for the first time. The results show that SiNx dielectric deposited by dual-frequency PECVD can effectively reduce the leakage current at high bias voltages. Due to the improved 4H-SiC epi-layer material and SiNx passivation, the fabricated 22 mm-long 1×128 4H-SiC APD linear arrays exhibit an excellent performance with a high pixel yield of 100% and a small breakdown voltage variation of 0.2 V, which is the best result ever reported. At room temperature, the pixels have a gain of over 105 and a maximum quantum efficiency of 53.5% @ 285 nm. Besides the high uniformity of breakdown voltage for 128 pixels, the dark currents at 95% of breakdown voltage are all below 1 nA.