Effect of Bias on the Response of GaN Axial p-n Junction Single-Nanowire Photodetectors

ZrN nucleation layer provides backside ohmic contact to MBE-grown GaN nanowires

Self-assembled GaN nanowires are typically grown on Si substrates with convenient nucleation layers. Light-emitting devices based on arrays of GaN nanowires require that the nucleation layer is electrically conductive and optically nontransparent to prevent the absorption of generated light in the Si substrate. This study reports the molecular beam epitaxial growth of GaN nanowires on ZrN nucleation layers sputtered on sapphire and demonstrates that ZrN provides ohmic contact to dense vertical arrays of n-type GaN nanowires. The ohmic nature of the ZrN/n-type GaN nanowire contact is evidenced by the measurement of the current-voltage characteristics of individual as-grown nanowires using nanomanipulators in a scanning electron microscope. The limitations and advantages of single-nanowire measurements are discussed, and approaches to overcome these limitations are proposed. The feasibility of this concept is demonstrated by the measurement of single nanowires with a p-n junction, exhibiting highly rectifying characteristics.

Dual-functional GaN photodiode for photodetection and neuromorphic computing with high specific detectivity

The discrepancy in the photoresponse characteristics between photodetectors (PDs) and photosynapses is posing a challenge for integrating the two functions into a single device. In this work, a photodetector integrated with neuromorphic capabilities is demonstrated. Under the joint action of the photovoltaic effect and photoconductive effect, the proposed device can switch between the photodetector and photosynapse by simply altering the polarity of bias voltage. Under a reverse or zero bias voltage, the device operates as a photodetector with low dark current and high specific detectivity. On the other hand, when a positive bias voltage is applied, the device showcases synaptic functionalities such as excitatory postsynaptic current, paired-pulse facilitation, and transition from short-term memory to long-term memory. The dual-functional devices hold promise for weak light detection and neuromorphic computing in complex environments.

Highly Responsive Switchable Broadband DUV-NIR Photodetector and Tunable Emitter Enabled by Uniform and Vertically Grown III-V Nanowire on Silicon Substrate for Integrated Photonics

Low-dimensional semiconductor nanostructures, particularly in the form of nanowire configurations with large surface-to-volume-ratio, offer intriguing optoelectronic properties for the advancement of integrated photonic technologies. Here, a bias-controlled, superior dual-functional broadband light detecting/emitting diode enabled by constructing the aluminum-gallium-nitride-based nanowire on the silicon-platform is reported. Strikingly, the diode exhibits a stable and high responsivity (R) of over 200 mAW-1 covering an extremely wide operation band under reverse bias conditions, ranging from deep ultraviolet (DUV: 254 nm) to near-infrared (NIR: 1000 nm) spectrum region. While at zero bias, it still possesses superior DUV light selectivity with a high off-rejection ratio of 106. When it comes to the operation of the light-emitting mode under forward bias, it can achieve large spectral changes from UV to red simply by coating colloid quantum dots on the nanowires. Based on the multifunctional features of the diodes, this study further employs them in various optoelectronic systems, demonstrating outstanding applications in multicolor imaging, filterless color discrimination, and DUV/NIR visualization. Such highly responsive broadband photodetector with a tunable emitter enabled by III-V nanowire on silicon provides a new avenue toward the realization of integrated photonics and holds great promise for future applications in communication, sensing, imaging, and visualization.

Thermally propagated Al contacts on SiGe nanowires characterized by electron beam induced current in a scanning transmission electron microscope

Here, we use electron beam induced current (EBIC) in a scanning transmission electron microscope to characterize the structure and electronic properties of Al/SiGe and Al/Si-rich/SiGe axial nanowire heterostructures fabricated by thermal propagation of Al in a SiGe nanowire. The two heterostructures behave as Schottky contacts with different barrier heights. From the sign of the beam induced current collected at the contacts, the intrinsic semiconductor doping is determined to be n-type. Furthermore, we find that the silicon-rich double interface presents a lower barrier height than the atomically sharp SiGe/Al interface. With an applied bias, the Si-rich region delays the propagation of the depletion region and presents a reduced free carrier diffusion length with respect to the SiGe nanowire. This behaviour could be explained by a higher residual doping in the Si-rich area. These results demonstrate that scanning transmission electron microscopy EBIC is a powerful method for mapping and quantifying electric fields in micrometer- and nanometer-scale devices.

Investigation of the effect of the doping order in GaN nanowire p-n junctions grown by molecular-beam epitaxy

We analyse the electrical and optical properties of single GaN nanowire p-n junctions grown by plasma-assisted molecular-beam epitaxy using magnesium and silicon as doping sources. Different junction architectures having either a n-base or a p-base structure are compared using optical and electrical analyses. Electron-beam induced current (EBIC) microscopy of the nanowires shows that in the case of a n-base p-n junction the parasitic radial growth enhanced by the magnesium (Mg) doping leads to a mixed axial-radial behaviour with strong wire-to-wire fluctuations of the junction position and shape. By reverting the doping order p-base p-n junctions with a purely axial well-defined structure and a low wire-to-wire dispersion are achieved. The good optical quality of the top n nanowire segment grown on a p-doped stem is preserved. A hole concentration in the p-doped segment exceeding 10¹⁸ cm^-3 was extracted from EBIC mapping and photoluminescence analyses. This high concentration is reached without degrading the nanowire morphology.

Enhanced Spatial Light Confinement of All Inorganic Perovskite Photodetectors Based on Hybrid Plasmonic Nanostructures

3D incident light confinement by radical electromagnetic fields offers a facile and novel way to break through the performance limit of inorganic perovskite CsPbBr₃ quantum dots (QDs). Herein, metallic nanoparticles decorated anodic aluminum oxide (AAO) hybrid plasmonic nanostructures with geometric control are first proposed for cyclic light utilization of perovskite photodetectors, enabled by spatially extended light confinement. The drastic multiple interference induced by plasmonic coupling within AAO matrixes are generated as a function of pore sizes, which can effectively collect the transmitted photons back to the surface. In addition, the self-assembled metallic nanoparticles simultaneously concentrate the incident and reflected light beams into the CsPbBr₃ QD layers. The light confinement inherently stems from the metallic nanoparticles due to the variation of the near surface electromagnetic fields. As a result, perovskite photodetectors based on Al nanoparticles/AAO hybrid plasmonic nanostructures with a pore size of 220 nm exhibit enhanced photoresponse behavior with remarkably increased photocurrent by ≈43× and maintain low dark current under 490 nm light illumination at 1 V.

Photodetecting properties of single CuO-ZnO core-shell nanowires with p-n radial heterojunction

CuO-ZnO core-shell radial heterojunction nanowire arrays were obtained by a simple route which implies two cost-effective methods: thermal oxidation in air for preparing CuO nanowire arrays, acting as a p-type core and RF magnetron sputtering for coating the surface of the CuO nanowires with a ZnO thin film, acting as a n-type shell. The morphological, structural, optical and compositional properties of the CuO-ZnO core-shell nanowire arrays were investigated. In order to analyse the electrical and photoelectrical properties of the metal oxide nanowires, single CuO and CuO-ZnO core-shell nanowires were contacted by employing electron beam lithography (EBL) and focused ion beam induced deposition (FIBID). The photoelectrical properties emphasize that the p-n radial heterojunction diodes based on single CuO-ZnO core-shell nanowires behave as photodetectors, evidencing a time-depending photoresponse under illumination at 520 nm and 405 nm wavelengths. The performance of the photodetector device was evaluated by assessing its key parameters: responsivity, external quantum efficiency and detectivity. The results highlighted that the obtained CuO-ZnO core-shell nanowires are emerging as potential building blocks for a next generation of photodetector devices.

Constructing Caesium-Based Lead-Free Perovskite Photodetector Enabling Self-Powered Operation with Extended Spectral Response

Since the discovery of the state-of-the-art hybrid halide perovskites, their application in optoelectronic systems has drawn considerable attention. However, the toxicity from lead (Pb) and the volatility induced by organic constituents hinder their future large-scale market development. Herein, a fully inorganic Pb-free halide perovskite based on robust Cs₃Bi₂I₉ is synthesized and realized its potential in photodetector application. The material property investigation suggests the good crystalline quality with strong absorption coefficient suitable for photodetection. An interesting feature based on the extended absorption is obtained, which is the characteristic of a weak phonon-assisted transition. Additionally, the morphological features display the beautifully grown micrometer-sized crystals of Cs₃Bi₂I₉. The fabricated photodetector demonstrated the self-powered operation (zero-bias state) with a very low dark current of 0.46 pA. Profiting from this, an improved photosensitivity of 1.4 × 10⁴ is achieved. Moreover, along with self-powered photodetection, the photodetector exhibits a broad spectral response (450-950 nm), high detectivity (1.2 × 10¹⁰/1.6 × 10¹² Jones), high responsivity (0.59 μA W^-1/3.8 mA W^-1), and fast response speed (ms) under a weak optical signal of 0.1 mW cm^-2 with a larger active area of 0.25 cm². The photodetector shows high photostability which was well retained for almost 2000 repetitive cycles without degradation. More strikingly, based on the core stability of the perovskite film, an excellent long-term stability of 3 months (90 days) is achieved for the photodetector even after exposure to moist air (75% relative humidity). This study thus highlights one of the few Pb-free all-inorganic perovskite photodetectors employing a simple device architecture with a larger active area that outshines by showing efficient and comparable performance under the self-powered mode under low light conditions.

Ferroelectric Enhanced Performance of a GeSn/Ge Dual-Nanowire Photodetector

GeSn offers a reduced bandgap than Ge and has been utilized in Si-based infrared photodetectors with an extended cutoff wavelength. However, the traditional GeSn/Ge heterostructure usually consists of defects like misfit dislocations due to the lattice mismatch issue. The defects with the large feature size of a photodetector fabricated on bulk GeSn/Ge heterostructures induce a considerable dark current. Here, we demonstrate a flexible GeSn/Ge dual-nanowire (NW) structure, in which the strain relaxation is achieved by the elastic deformation without introducing defects, and the feature dimension is naturally at the nanoscale. A photodetector with a low dark current can be built on a GeSn/Ge dual-NW, which exhibits an extended detection wavelength beyond 2 μm and enhanced responsivity compared to the Ge NW. Moreover, the dark current can be further suppressed by the depletion effect from the ferroelectric polymer side gate. Our work suggests the flexible GeSn/Ge dual-NW may open an avenue for Si-compatible optoelectronic circuits operating in the short-wavelength infrared range.