Zifeng Zhang

Separated-absorption-multiplication 4H-SiC avalanche photodiodes with adjustable responsivity and response time

4H-SiC based avalanche photodiodes (APD) with a small multiplication region onto a large absorption region, which can be regarded as a particular separated-absorption-multiplication structure, are proposed and their optoelectronic performances are modeled. The avalanche breakdown voltage, energy band diagram, spectral responsivity and response time of the devices are found to be dependent on the area of multiplication region. The performance of the device could be similar to a conventional separated-absorption-multiplication APD or p–i–n diode by designing various multiplication areas. Characteristics of the APD are fully studied and explained by the analysis of energy band diagrams of the devices.

GaN light-emitting diodes with an Al-coated graphene layer as a transparent electrode

We fabricated GaN light-emitting diodes with a layer of graphene as a transparent electrode. A 3-nm-thick Al layer was deposited on the graphene layer by electron-beam evaporation. This Al layer plays an important role in protecting the graphene layer during the device fabrication process. Moreover, this Al layer can also enhance the light emission of GaN light-emitting diodes through the investigation of electroluminescence spectra. The significantly improved light emission is attributed to the current expansion, the enhanced plasmonic density of states, and the decreased non-radiative recombination rate.

Nanoscale avalanche photodiode with self-quenching and ultrahigh ultraviolet/visible rejection ratio

A 4H-SiC based separate-absorption-multiplication (SAM) avalanche photodiode with a nanoscale multiplication region and a bulk absorption region is proposed and its optoelectronic performance is modeled. The results show that the avalanche breakdown voltage of the device is found to be dependent on the illumination condition. This is attributed to the existence of an illumination-dependent hole potential well in the upper center of the absorption region. Based on the illumination-dependence of avalanche breakdown voltage, a self-quenching and an ultrahigh UV/visible rejection ratio have been realized in this structure.

TiNbO 2 -Based Photodetectors With Low Dark Current and High UV-to-Visible Rejection Ratio

We reported on metal-semiconductor-metal ultraviolet (UV) photodetector (PD) based on pulse-laser-deposited TiNbO₂ alloy film. The TiNbO₂ PD exhibited a low dark current density of ~0.46 nA/cm², a high UV-to-visible rejection ratio of ~10⁶, and a high responsivity of ~0.22 A/W under the illumination of 310-nm light (~1.86 mW/cm²). Interestingly, the TiNbO₂ alloy PD presents a blue-shift cutoff wavelength of 350 nm due to quantum size effect and alloy characteristics, which would be promising for the further research in the UV detection.