Guoqing Miao

Realization of a High-Performance GaN UV Detector by Nanoplasmonic Enhancement

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.

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

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.

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

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.

Improved performance of GaN metal-semiconductor-metal ultraviolet detectors by depositing SiO2 nanoparticles on a GaN surface

GaN metal-semiconductor-metal (MSM) ultraviolet detectors were investigated by depositing different density of SiO2 nanoparticles (SNPs) on the GaN. It was shown that the dark current of the detectors with SNPs was more than one order of magnitude lower than that without SNPs and the peak responsivity was enhanced after deposition of the SNPs. Atomic force microscopy observations indicated that the SNPs usually formed at the termination of screw and mixed dislocations, and further current-voltage measurements showed that the leakage of the Schottky contact for the GaN MSM detector decreased with deposited the SNPs. Moreover, the leakage obeyed the Frenkel–Poole emission model, which meant that the mechanism for improving the performance is the SNPs passivation of the dislocations followed by the reduction in the dark current.GaN metal-semiconductor-metal (MSM) ultraviolet detectors were investigated by depositing different density of SiO2 nanoparticles (SNPs) on the GaN. It was shown that the dark current of the detectors with SNPs was more than one order of magnitude lower than that without SNPs and the peak responsivity was enhanced after deposition of the SNPs. Atomic force microscopy observations indicated that the SNPs usually formed at the termination of screw and mixed dislocations, and further current-voltage measurements showed that the leakage of the Schottky contact for the GaN MSM detector decreased with deposited the SNPs. Moreover, the leakage obeyed the Frenkel–Poole emission model, which meant that the mechanism for improving the performance is the SNPs passivation of the dislocations followed by the reduction in the dark current.

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

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.

Enhanced spectral response of an AlGaN-based solar-blind ultraviolet photodetector with Al nanoparticles.

An enhanced spectral response was realized in an AlGaN-based solar-blind ultraviolet (SB-UV) detector using aluminum (Al) nanoparticles (NPs) of 20-60 nm. The peak responsivity of the detector (about 288 nm) with 60 nm Al NPs is more than two times greater than that of a detector without Al NPs under a 5-V bias, reaching 0.288 A/W. To confirm the enhancement mechanism of the Al NPs, extinction spectra were simulated using time-domain and frequency-domain finite-element methods. The calculation results show that the dipole surface plasmon resonance wavelength of the Al NPs is localized near the peak responsivity position of AlGaN-based SB-UV detectors. Thus, the improvement in the detectors can be ascribed to the localized surface plasmon resonance effect of the Al NPs. The localized electric field enhancement and related scattering effect result in the generation of more electron-hole pairs and thus a higher responsivity. In addition, the dark current of AlGaN-based SB-UV detectors does not increase after the deposition of Al nanoparticles. The results presented here is promising for applications of AlGaN-based SB-UV detectors.

Improved field emission performance of carbon nanotube by introducing copper metallic particles

To improve the field emission performance of carbon nanotubes (CNTs), a simple and low-cost method was adopted in this article. We introduced copper particles for decorating the CNTs so as to form copper particle-CNT composites. The composites were fabricated by electrophoretic deposition technique which produced copper metallic particles localized on the outer wall of CNTs and deposited them onto indium tin oxide (ITO) electrode. The results showed that the conductivity increased from 10-5 to 4 × 10-5 S while the turn-on field was reduced from 3.4 to 2.2 V/μm. Moreover, the field emission current tended to be undiminished after continuous emission for 24 h. The reasons were summarized that introducing copper metallic particles to decorate CNTs could increase the surface roughness of the CNTs which was beneficial to field emission, restrain field emission current from saturating when the applied electric field was above the critical field. In addition, it could also improve the electrical contact by increasing the contact area between CNT and ITO electrode that was beneficial to the electron transport and avoided instable electron emission caused by thermal injury of CNTs.

Reproducible bipolar resistive switching in entire nitride AlN/n-GaN metal-insulator-semiconductor device and its mechanism

Reproducible bipolar resistive switching characteristics are demonstrated in entire nitride AlN/n-GaN metal-insulator-semiconductor devices. The mechanism involved confirms to trap-controlled space charge limited current theory and can be attributed to the nitrogen vacancies of AlN serving as electron traps that form/rupture electron transport channel by trapping/detrapping electrons. This study will lead to the development of in-situ growth of group-III nitrides by metal-organic chemical vapor deposition as a candidate for next-generation nonvolatile memory device. Moreover, it will be benefit to structure monolithic integrated one-transistor-one-resistor memory with nitride high electron mobility transistors.

High spectral response of self-driven GaN-based detectors by controlling the contact barrier height

High spectral response of self-driven GaN-based ultraviolet detectors with interdigitated finger geometries were realized using interdigitated Schottky and near-ohmic contacts. Ni/GaN/Cr, Ni/GaN/Ag, and Ni/GaN/Ti/Al detectors were designed with zero bias responsivities proportional to the Schottky barrier difference between the interdigitated contacts of 0.037 A/W, 0.083 A/W, and 0.104 A/W, respectively. Voltage-dependent photocurrent was studied, showing high gain under forward bias. Differences between the electron and hole mobility model and the hole trapping model were considered to be the main photocurrent gain mechanism. These detectors operate in photoconductive mode with large photocurrent gain and depletion mode with high speed, and can extend GaN-based metal-semiconductor-metal detector applications.

Shift of responsive peak in GaN-based metal-insulator-semiconductor photodetectors

A gallium nitride (GaN)-based metal-insulator-semiconductor (MIS) ultraviolet photodetector (PD) was fabricated on a sapphire substrate. It was found that the responsive peak of the GaN-based MIS PD redshifted with increasing negative bias, which has not been reported before. Also, the shift of the responsive peak has been interpreted in terms of the tunneling procedure of the photo-generated holes assisted by defects in the interfaces between the GaN layers and the SiNx layers.