Recent development in 2D materials beyond graphene

New concept ultraviolet photodetectors

Inorganic semiconductor nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating the size and dimensionality dependence of their properties for potential applications. The use of such nanostructures with tailored geometries as building blocks is also expected to play crucial roles in future nanodevices. Since the discovery of carbon nanotubes much attention has been paid to exploring the usage of inorganic semiconductor nanostructures as field-emitters due to their low work functions, high aspect ratios and mechanical stabilities, and high electrical and thermal conductivities. This article provides a comprehensive review of the state-of-the-art research activities in the field. It mainly focuses on the most widely studied inorganic nanostructures, such as ZnO, ZnS, Si, WO3, AlN, SiC, and their field-emission properties. We begin with a survey of inorganic semiconductor nanostructures and the field-emission principle, and then discuss the recent progresses on several kinds of important nanostructures and their field-emission characteristics in detail and overview some additional inorganic semiconducting nanomaterials in short. Finally, we conclude this review with some perspectives and outlook on the future developments in this area.

Inorganic semiconductor nanostructures and their field-emission applications

Material that can emit broad spectral wavelengths covering deep ultraviolet, visible, and near-infrared is highly desirable. It can lead to important applications such as broadband modulators, photodetectors, solar cells, bioimaging, and fiber communications. However, there is currently no material that meets such desirable requirement. Here, we report the layered structure of nitrogen-doped graphene quantum dots (N-GQDs) which possess broadband emission ranging from 300 to >1000 nm. The broadband emission is attributed to the layered structure of the N-GQDs that contains a large conjugated system and provides extensive delocalized π electrons. In addition, a broadband photodetector with responsivity as high as 325 V/W is demonstrated by coating N-GQDs onto interdigital gold electrodes. The unusual negative photocurrent is observed which is attributed to the trapping sites induced by the self-passivated surface states in the N-GQDs.

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Deep ultraviolet to near-infrared emission and photoresponse in layered N-doped graphene quantum dots.

Recent progress in synthesis, properties and potential applications of SiC nanomaterials

Realization of highly responsivity UV detectors is a critical challenge for accelerating the application of UV detectors. Exploiting nanoplasmonic enhancement, Ag nanoparticles have been formed on the GaN surface and the responsivity of the GaN UV detector has been enhanced about 30 times compared with that without Ag nanoparticles.

Realization of a high-performance GaN UV detector by nanoplasmonic enhancement

An asymmetric Schottky barrier metal-semiconductor-metal (MSM) ultraviolet (UV) detector with Ni/GaN/Au structure was designed and the effect of the asymmetric Schottky barrier on the detector response was investigated. This detector had response at 0 V bias and increased responsivity when a positive bias was applied to the Ni/GaN contact; however, the internal gain disappeared when a negative bias was applied to this point. This contrasts with a symmetric Ni/GaN/Ni Schottky barrier MSM UV detector which had no internal gain under positive/negative bias and almost no response at 0 V bias. The improved performance of the asymmetric Schottky barrier detector was because of the lower work function of Au causing reduction of Schottky barrier and hence enhancing a hole-accumulating and trapping process, which resulted in internal gain.

Effect of asymmetric Schottky barrier on GaN-based metal-semiconductor-metal ultraviolet detector

The influence of threading dislocations on the properties of GaN-based metal-semiconductor-metal (MSM) ultraviolet photodetectors was investigated. It was found that screw dislocations had a strong influence on the dark current of the photodetectors, while edge dislocations had the predominant effect on their responsivity. The dark current increased as the screw dislocation density increased due to their lowering of the Schottky barrier height. However, the responsivity of the photodetectors decreased with increasing edge dislocation density because of the dangling bonds along those edge dislocation lines which enhance the recombination of photogenerated electron-hole pairs. The results suggest that reducing both the screw and edge dislocation densities is an effective way to improve the photoelectric property of GaN-based MSM ultraviolet photodetectors.

Influence of threading dislocations on GaN-based metal-semiconductor-metal ultraviolet photodetectors

We studied the two-step growth of AlN using high-temperature (HT) metal–organic chemical vapour deposition (MOCVD) using a 405 nm short-wavelength in situ monitoring system. First, an AlN nucleation layer (NL) was grown on sapphire at 950 °C before deposition of the HT-AlN film at 1300 °C. The 405 nm wavelength in situ reflectance-transient curves revealed the evolution of AlN growth. The reflectance intensity of the 405 nm signal first decreased during AlN growth and then became stronger until it reached a steady state at large and equal amplitude, revealing that the AlN growth underwent a transition from nuclei to nuclei decomposed island recovery to quasi-layer-by-layer growth. The effect of different initial growth conditions on the AlN growth mode was also studied using the in situ monitoring system. The growth mechanism for the films was proposed based on the 405 nm in situ reflectance curves and atomic force microscopy observations. By optimizing the NL growth conditions, we obtained a high-quality AlN/sapphire template with full width at half maxima for the (0002) and (10−12) planes of 60 arcsec and 550 arcsec, respectively. Finally, a high performance PIN-AlGaN detector was fabricated on the AlN template, which further demonstrated its high quality.

Guoqing Miao

Papers

Realization of a high-performance GaN UV detector by nanoplasmonic enhancement

Effect of asymmetric Schottky barrier on GaN-based metal-semiconductor-metal ultraviolet detector

Influence of threading dislocations on GaN-based metal-semiconductor-metal ultraviolet photodetectors

In situ observation of two-step growth of AlN on sapphire using high-temperature metal–organic chemical vapour deposition

Influence of the growth temperature of AlN nucleation layer on AlN template grown by high-temperature MOCVD