Recently the GaN/AlN multi-quantum-well structure avalanche photodiode (MAPD) has been demonstrated with PMT-like multiplication gain larger than 1E4. In this work, the photocurrent of GaN/AlN MAPD has been investigated and negative differential conductance (NDC) is found in the photocurrent characteristic of MAPD. Through self-consistent calculation, conduction band structure and discrete energy states in each quantum well layer have been obtained for MAPD. The discrete states drop down and align with the conduction band edge of absorption layer around the NDC peak voltage, so the NDC feature is proposed as resonant tunneling of photoelectrons into MQW structure. The proposed resonant tunneling process is confirmed by the observation of resonant tunneling peaks in a specially designed resonant tunneling diode simulating the band profile of MAPD. The finding of NDC feature is beneficial for understanding and increasing the quantum efficiency of MAPD, since the photoelectron blocking at AlN barrier is greatly reduced by the resonant tunneling process.
Nanoantenna enhanced fluorescence is a promising method in many emergent applications such as single molecule detection. However, the excitation wavelengths and the emission wavelengths of emitters could be well-separated depending on their Stokes-shifts, preventing optimal fluorescence enhancement by a rudimental nanoantenna. Here we propose an Ag-Si hybrid stack nanoantenna, which comprises an Ag bottom cylinder and a Si top cylinder, to match the Stokes-shift of the fluorescence emitter. The Ag cylinder is designed to resonate at the excitation wavelength of the emitter, yielding a large field enhancement to boost the excitation rate of the emitter. Meanwhile, the resonance of the low loss Si cylinder is designed to match the emission wavelength of the emitter, boosting the radiative decay rate by more than one order of magnitude and maintaining a high quantum yield. As a result, all-round enhancements in the fluorescence emission are achieved. Preliminary studies show that the hybrid stack nanoantenna can produce two times more fluorescence enhancement, and 20 times larger far field intensity comparing to those of a pure metallic nanoantenna. On top of that, around 70% of the overall radiation has been directed towards the dielectric cap side, which would be beneficial to the collection efficiency. This design fully leverages the advantages of both metal and dielectric, which could be useful in the fluorescence enhancement applications.
The high-gain photomultiplier tube (PMT) is the most popular method to detect weak ultra-violet signals which attenuate quickly in atmosphere, although the vacuum tube makes it fragile and difficult to integrate. To overcome the disadvantage of PMT, an AlN/GaN periodically–stacked-structure (PSS) avalanche photodiode (APD) has been proposed, finally achieving good quality of high gain and low excessive noise. As there is a deep г valley only in the conduction band of both GaN and AlN, the electron transfers suffering less scattering and thus becomes easier to obtain the threshold of ionization impact. Because of unipolar ionization in the PSS APD, it works in linear mode. Four prototype devices of 5-period, 10-period, 15-period, and 20-period were fabricated to verify that the gain of APD increases exponentially with period number. And in 20-period device, a recorded high and stable gain of 104 was achieved under constant bias. In addition, it is proved both experimentally and theoretically, that temperature stability on gain is significantly improved in PSS APD. And it is found that the resonant enhancement in interfacial ionization may bring significant enhancement of electron ionization performance. To make further progress in PSS APD, the device structure is investigated by simulation. Both the gain and temperature stability are optimized alternatively by a proper design of periodical thickness and AlN layer occupancy.
In this paper, we demonstrate the laser simulation adequacy both by theoretical analysis and experiments. We first explain the basic theory and physical mechanisms of laser simulation of transient radiation effect of semiconductor. Based on a simplified semiconductor structure, we describe the reflection, optical absorption and transmission of laser beam. Considering two cases of single-photon absorption when laser intensity is relatively low and two-photon absorption with higher laser intensity, we derive the laser simulation equivalent dose rate model. Then with 2 types of BJT transistors, laser simulation experiments and gamma ray radiation experiments are conducted. We found good linear relationship between laser simulation and gammy ray which depict the reliability of laser simulation.