Effect of the nanowire diameter on the linearity of the response of GaN-based heterostructured nanowire photodetectors

High Performance GaN-Based Ultraviolet Photodetector via Te/Metal Electrodes

Photodetectors (PDs) based on two-dimensional (2D) materials have promising applications in modern electronics and optoelectronics. However, due to the intralayer recombination of the photogenerated carriers and the inevitable surface trapping stages of the constituent layers, the PDs based on 2D materials usually suffer from low responsivity and poor response speed. In this work, a distinguished GaN-based photodetector is constructed on a sapphire substrate with Te/metal electrodes. Due to the metal-like properties of tellurium, the band bending at the interface between Te and GaN generates an inherent electric field, which greatly reduces the carrier transport barrier and promotes the photoresponse of GaN. This Te-enhanced GaN-based PD show a promising responsivity of 4951 mA/W, detectivity of 1.79 × 10¹⁴ Jones, and an external quantum efficiency of 169%. In addition, owing to the collection efficiency of carriers by this Te-GaN interface, the response time is greatly decreased compared with pure GaN PDs. This high performance can be attributed to the fact that Te reduces the contact resistance of the metal electrode Au/Ti to GaN, forming an ohmic-like contact and promoting the photoresponse of GaN. This work greatly extends the application potential of GaN in the field of high-performance photodetectors and puts forward a new way of developing high performance photodetectors.

Dual-wavelength visible photodetector based on vertical (In,Ga)N nanowires grown by molecular beam epitaxy

Due to the wide applications of blue and red photodetectors, dual-wavelength (blue/red) photodetectors are promising for future markets. In this work, a dual-wavelength photodetector based on vertical (In,Ga)N nanowires and graphene has been fabricated successfully. By using the transparent graphene, both blue and red responses can be clearly detected. The rise time of response can reach 3.5 ms. Furthermore, the underlying mechanism of double responses has also been analyzed. The main reason contributing to the dual-wavelength response could be the different diameters of nanowires, leading to different In components within (In,Ga)N sections.

Correlated and in-situ electrical transmission electron microscopy studies and related membrane-chip fabrication

Understanding the interplay between the structure, composition and opto-electronic properties of semiconductor nano-objects requires combining transmission electron microscopy (TEM) based techniques with electrical and optical measurements on the very same specimen. Recent developments in TEM technologies allow not only the identification and in-situ electrical characterization of a particular object, but also the direct visualization of its modification in-situ by techniques such as Joule heating. Over the past years, we have carried out a number of studies in these fields that are reviewed in this contribution. In particular, we discuss here i) correlated studies where the same unique object is characterized electro-optically and by TEM, ii) in-situ Joule heating studies where a solid-state metal-semiconductor reaction is monitored in the TEM, and iii) in-situ biasing studies to better understand the electrical properties of contacted single nanowires. In addition, we provide detailed fabrication steps for the silicon nitride membrane-chips crucial to these correlated and in-situ measurements.

Nanoarchitectonics for Wide Bandgap Semiconductor Nanowires: Toward the Next Generation of Nanoelectromechanical Systems for Environmental Monitoring

Semiconductor nanowires are widely considered as the building blocks that revolutionized many areas of nanosciences and nanotechnologies. The unique features in nanowires, including high electron transport, excellent mechanical robustness, large surface area, and capability to engineer their intrinsic properties, enable new classes of nanoelectromechanical systems (NEMS). Wide bandgap (WBG) semiconductors in the form of nanowires are a hot spot of research owing to the tremendous possibilities in NEMS, particularly for environmental monitoring and energy harvesting. This article presents a comprehensive overview of the recent progress on the growth, properties and applications of silicon carbide (SiC), group III-nitrides, and diamond nanowires as the materials of choice for NEMS. It begins with a snapshot on material developments and fabrication technologies, covering both bottom-up and top-down approaches. A discussion on the mechanical, electrical, optical, and thermal properties is provided detailing the fundamental physics of WBG nanowires along with their potential for NEMS. A series of sensing and electronic devices particularly for environmental monitoring is reviewed, which further extend the capability in industrial applications. The article concludes with the merits and shortcomings of environmental monitoring applications based on these classes of nanowires, providing a roadmap for future development in this fast-emerging research field.

Enhancement of photoemission capability and electron collection efficiency of field-assisted GaN nanowire array photocathode

GaN has interesting prospects in applications for spectrum-tunable solid-state devices with photoelectric conversion function. Similarly, single nanowires or nanowire arrays (NWAs) proceed to exhibit good photon absorbance and photoemission characteristics as vacuum devices based on the external photoelectric effect. However, the collection of photoelectrons emitted from a nanowire surface has become the greatest impediment to the progress of GaN NWAs photocathodes. In this study, a field-assisted GaN NWA photocathode is proposed. The photoemission efficiency and electron collection efficiency of the field-assisted GaN NWA photocathode are derived. The results suggest that the external field can effectively enhance the photoemission capacity and electron collection efficiency of the photocathode. Based on the theoretical model, the structural parameters of NWAs and the field intensity are optimized. When the field intensity is 1 V μm^-1, the collected photocurrent of the GaN NWA photocathode reaches a maximum. For NWAs with an aspect ratio of 1:1, the optimal incident angle of light is 70°. This study provides a theoretical guide for the incorporation of an external field in a GaN NWA photocathode with the purpose of enhancing photoemission and electron collection capacity.

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

We present a comprehensive study of the performance of GaN single-nanowire photodetectors containing an axial p-n junction. The electrical contact to the p region of the diode is made by including a p⁺/n⁺ tunnel junction as cap structure, which allows the use of the same metal scheme to contact both ends of the nanowire. Single-nanowire devices present the rectifying current-voltage characteristic of a p-n diode but their photovoltaic response to ultraviolet radiation scales sublinearly with the incident optical power. This behavior is attributed to the dominant role of surface states. Nevertheless, when the junction is reverse biased, the role of the surface becomes negligible in comparison to the drift of photogenerated carriers in the depletion region. Therefore, the responsivity increases by about 3 orders of magnitude and the photocurrent scales linearly with the excitation. These reverse-biased nanowires display decay times in the range of ∼10 μs, limited by the resistor-capacitor time constant of the setup. Their ultraviolet/visible contrast of several orders of magnitude is suitable for applications requiring high spectral selectivity. When the junction is forward biased, the device behaves as a GaN photoconductor with an increase of the responsivity at the price of a degradation of the time response. The presence of leakage current in some of the wires can be modeled as a shunt resistance which reacts to the radiation as a photoconductor and can dominate the response of the wire even under reverse bias.