Dual-band photodetectors operating within infrared (IR) and ultraviolet (UV) wavelength ranges are highly applicable for tracking and surveillance of targets applications. Because of the large band-gap and conduction band offset, nitride heterostructures are good candidates for monolithically integrated IR and UV dual-band detection. However, suffering from the considerable lattice mismatch and strong polarization effect of nitride heterostructures, photodetectors with complicated structures increase the difficulties of material epitaxy and device fabrication. Here, we present a relatively simple structure design for the detection of IR and UV signals on the same sensing area simultaneously. The responses of IR and UV signals originate from the intersubband and interband transitions in the GaN/AlN superlattice sandwiched by n-doped GaN contact layers, respectively. Experimental results show that the grown sample exhibits an absorption response peaked at 1.5 μm for TM-polarized input lights. Meanwhile, the prototype sample also has a strong photocurrent response at wavelengths shorter than 350 nm, which is mainly decided by the band-gap of the GaN/AlN superlattice. These results prove the feasibility of the proposed structure of detecting IR and UV dual-band signals.
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.
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.
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