| Uv detectors are much-needed key components in many fields such as missile early warning and tracking, environmental monitoring, power industry, and bioscience. Compared with the existing vacuum ultraviolet detectors, solid-state ultraviolet detector based on semiconductor materials have many advantages such as less weight, low power consumption, high quantum efficiency and facilitate integration. Among them, the AlGaN material as a wide bandgap III nitride semiconductor, which bandwidth can be adjusted from 3.4 eV to 6.2 eV and cover solar-blind wave band, is the preferred material for preparation of deep uv detectors.There exists some defects in high-Al-content AlGaN materials, such as high dislocation density, low p-type doping efficiency and carrier ionization coefficient which is far below that of GaN. So it is very difficult to obtain a high gain AlGaN solar-blind avalanche photodiode by using ordinary APD structure and preparation process. In order to improve the performance of AlGaN solar-blind APD, in this paper, we improved and optimized the traditional SAM structure and adopted the structure design of aeterojunction multiplication. By introducing three mesa process and electrochemical treatment, we successfully obtained a high performance AlGaN solar-blind avalanche photodiode. The main achievements obtained in this work are as following:1. We first proposed the heterojunction enhanced AlGaN APD structure. Due to that AlGaN with lower Al component has a higher ionization coefficient, we introduced a low-Al-content Alo.3Gao.7N layer into the conventional single high-Al-content Al0.45Ga0.55N multiplication layer to obtain a heterojunction multiplication layer and improve the average ionization coefficient of APD devices. The simulation results show that the Al composition of the low-Al-content AlGaN layer has an important impact on the performance of APDs. However, the introducing of low-Al-content AlGaN layer may lead to the loss of solar-blind characteristic. Taking the solar-blind characteristic into account, we designed a Al0.5Ga0.5N/AlN distributed Bragg reflector (DBR) structure. Considering both the low-Al-content AlGaN layer and the high reflectance region, the results show that we can get a solar-blind APD with a better performance when the Al composition of the low-Al-content AlGaN layer is 0.3. Compared with the conventional structure, the avalanche breakdown voltage shows a reduction of about 2.5 V, whereas the multiplication gain increases pronouncedly from 7.13×104 for the conventional structure to 1.14×105 for the enhanced structure correspondingly, showing an about 60% increase.2. We developed an exquisite technology to adjust and control the electric field distribution of the avalanche multiplication layer by introducing a n-type AlGaN layer into the multiplication layer. Simulation results showed that the the thickness ratio of the high/low-Al-content AlGaN layers in the multiplication region and the parameter of the n-type AlGaN alloy, such as doping density, thickness and the Al content, have an important effect on the performances of the designed APDs. When the Al content, thickness and doping density of the n-type AlGaN alloy is 0.2,20 nm, 1×1017/cm3 respectively and the thickness of the low-Al-content AlGaN layer is 40nm (the total thickness of the multiplication region is 200 nm), we obtained an optimized APD structure, which avalanche breakdown voltage reduced 2.1 V and the multiplication gain showed an about 53% increase.3. High gain AlGaN heterojunction solar-blind APD was fabricated by using the heterojunction multiplication design, which multiplication gain reached 105 order. What's more, we developed a series of new technology such as electrochemical treatment and three mesa craft and so on, which have an important impact on reducing the leakage current of APDs. |
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