Al0.1Ga0.9N p-i-n ultraviolet avalanche photodiodes with suppressed surface leakage current and uniform avalanche breakdown.

We report high-performance Al0.1Ga0.9N p-i-n ultraviolet (UV) avalanche photodiodes (APDs) based on sapphire substrates with stable breakdown voltages (VBR) around 113.4 V, low dark current densities (JBR) below 9 × 10-4 A/cm2 and a high avalanche gain over 2 × 106. A two-step deposition method was employed to reduce passivation-induced plasma damage while maintaining high dielectric film quality. Consistent JBR for various mesa sizes at the VBR are demonstrated, which reveals the suppression of the surface leakage current. Uniform electroluminescence (EL) distributions during the avalanche multiplication processes are displayed, which confirms the elimination of edge breakdown. Pure bulk leakage current distributions and uniform body avalanche breakdown behaviors are observed for the first time in AlGaN APDs. The emission spectra of the EL at various current levels are also presented.

High-responsivity dual-band ultraviolet photodetector based on Ga2O3/GaN heterostructure.

Ultraviolet photodetectors have aroused wide concern based on wide-band-gap semiconductors, such as GaN and Ga2O3. Exploiting multi-spectral detection provides unparalleled driving force and direction for high-precision ultraviolet detection. Here we demonstrate an optimized design strategy of Ga2O3/GaN heterostructure bi-color ultraviolet photodetector, which presents extremely high responsivity and UV-to-visible rejection ratio. The electric field distribution of optical absorption region was profitably modified by optimizing heterostructure doping concentration and thickness ratio, thus further facilitating the separation and transport of photogenerated carriers. Meanwhile, the modulation of Ga2O3/GaN heterostructure band offset leads to the fluent transport of electrons and the blocking of holes, thereby enhancing the photoconductive gain of the device. Eventually, the Ga2O3/GaN heterostructure photodetector successfully realizes dual-band ultraviolet detection and achieves high responsivity of 892/950 A/W at the wavelength of 254/365 nm, respectively. Moreover, UV-to-visible rejection ratio of the optimized device also keeps at a high level (∼103) while exhibiting dual-band characteristic. The proposed optimization scheme is anticipated to provide significant guidance for the reasonable device fabrication and design on multi-spectral detection.

Enhanced Thermoelectric Properties of Coated Vanadium Oxynitride Nanoparticles/PEDOT:PSS Hybrid Films.

Thermoelectric (TE) materials transform thermal energy into electricity, which can play an important role for global sustainability. Conducting polymers are suitable for the preparation of flexible TE materials because of their low-cost, lightweight, flexible, and easily synthesized properties. Here, we fabricate organic-inorganic hybrids by combining vanadium oxynitride nanoparticles coated with nitrogen-doped carbon (NC@VNO) and poly(3,4-ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS). We find that the electrical conductivity, Seebeck coefficient, and power factor of the NC@VNO/PEDOT:PSS film can be enhanced up to 4158 S/cm, 45.8 µV/K, and 873 µW/mK2 at 380 K, respectively. The large enhancement of the power factor may be due to the facilitation of the interfacial charge transport tunnel between the NC@VNO nanoparticles and PEDOT:PSS. The improvement of the Seebeck coefficient may be due to the energy filter effect as induced by interfacial contact and internal electric field between the NC@VNO nanoparticles and PEDOT:PSS. Our measurement suggests that the high binding energy of pyrrolic-N enhances the Seebeck coefficient, and the high binding energy of oxide-N increases electrical conductivity.

Simulation Study on the Structure Design of p-GaN/AlGaN/GaN HEMT-Based Ultraviolet Phototransistors.

This work investigates the impacts of structural parameters on the performances of p-GaN/AlGaN/GaN HEMT-based ultraviolet (UV) phototransistors (PTs) using Silvaco Atlas. The simulation results show that a larger Al content or greater thickness for the AlGaN barrier layer can induce a higher two-dimensional electron gas (2DEG) density and produce a larger photocurrent. However, they may also lead to a larger dark current due to the incomplete depletion of the GaN channel layer. The depletion conditions with various Al contents and thicknesses of the AlGaN layer are investigated in detail, and a borderline between full depletion and incomplete depletion was drawn. An optimized structure with an Al content of 0.23 and a thickness of 14 nm is achieved for UV-PT, which exhibits a high photocurrent density of 92.11 mA/mm, a low dark current density of 7.68 × 10-10 mA/mm, and a large photo-to-dark-current ratio of over 1011 at a drain voltage of 5 V. In addition, the effects of other structural parameters, such as the thickness and hole concentration of the p-GaN layer as well as the thickness of the GaN channel layer, on the performances of the UV-PTs are also studied in this work.

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.

Progress on AlGaN-based solar-blind ultraviolet photodetectors and focal plane arrays.

Solar-blind ultraviolet (UV) photodetectors (PDs) have attracted tremendous attention in the environmental, industrial, military, and biological fields. As a representative III-nitride material, AlGaN alloys have broad development prospects in the field of solar-blind detection due to their superior properties, such as tunable wide bandgaps for intrinsic UV detection. In recent decades, a variety of AlGaN-based PDs have been developed to achieve high-precision solar-blind UV detection. As integrated optoelectronic technology advances, AlGaN-based focal plane arrays (FPAs) are manufactured and exhibit outstanding solar-blind imaging capability. Considering the rapid development of AlGaN detection techniques, this paper comprehensively reviews the progress on AlGaN-based solar-blind UV PDs and FPAs. First, the basic physical properties of AlGaN are presented. The epitaxy and p-type doping problems of AlGaN alloys are then discussed. Diverse PDs, including photoconductors and Schottky, metal-semiconductor-metal (MSM), p-i-n, and avalanche photodiodes (APDs), are demonstrated, and the physical mechanisms are analyzed to improve device performance. Additionally, this paper summarizes imaging technologies used with AlGaN FPAs in recent years. Benefiting from the development of AlGaN materials and optoelectronic devices, solar-blind UV detection technology is greeted with significant revolutions. Summarizing recent advances in the processing and properties of AlGaN-based solar-blind UV PDs and FPAs as well as AlGaN growth and doping techniques.

Back-illuminated AlGaN heterostructure solar-blind avalanche photodiodes with one-dimensional photonic crystal filter.

AlGaN heterostructure solar-blind avalanche photodiodes (APDs) were fabricated on a double-polished AlN/sapphire template based on a separate absorption and multiplication (SAM) back-illuminated configuration. By employing AlGaN heterostructures with different Al compositions across the entire device, the SAM APD achieved an avalanche gain of over 1×105 at an operated reverse bias of 92 V and a low dark current of 0.5 nA at the onset point of breakdown. These excellent performances were attributed to the acceleration of holes by the polarization electric field with the same direction as the reverse bias and higher impact ionization coefficient of the low-Al-content Al0.2Ga0.8N in the multiplication region. However, the Al0.2Ga0.8N layer produced a photocurrent response in the out of the solar-blind band. To retain the solar-blind detecting characteristic, a periodic Si3N4/SiO2 photonic crystal was deposited on the back of the AlN/sapphire template as an optical filter. This significantly improved the solar-blind characteristic of the device.