Fabrication And Properties Of GaN-based P-i-n Ultraviolet Photodetectors

III-nitride semiconductors, for example, gallium nitride(Ga N), are promising materials for fabricating ultraviolet(UV) detectors due to their excellent physical properties such as wide direct bandgap, excellent thermal and chemical stability. With the advantages of low energy consumption, small size and strong radiation resistance, Ga N-based UV detectors have been widely used in military and civil fields, becoming a hot research direction of Ga N-based optoelectronic devices. However, the lack of high quality homogeneous single crystal substrates and the existence of high density dislocations in hetero-epitaxial Ga N films seriously prohibit the development and applications of high performance UV detectors. Therefore, it is critical to explore effective methods to reduce the defect density for improving the device performance.Firstly, the current research status and existing problems of Ga N-based UV detectors are described in this thesis. Then, basic properties and preparation methods of Ga N materials are briefly introduced. Based on the discussions of common structures, working principles and main characteristic parameters of Ga N-based UV detector, the following three parts of work are completed.1. In order to reduce the defect density in the Ga N materials and improve the quality of epitaxial layers, Ga N-based p-i-n UV detectors with a thin p-type layer of 60 nm are fabricated on the patterned sapphire substrates(PSS). X-ray diffraction results indicate that the epitaxial layers grown on the PSS have lower defect density than those grown on the standard sapphire substrates(SSS). Moreover, Ga N-based Schottky UV detectors are fabricated on the PSS for comparing performances of different detectors.2. To evaluate the detector performances, a spectral response testing system is designed and built in the lab. In order to achieve the required testing accuracy, the large noise problem is analyzed and solved by using the following methods: moving the annular lamp switch out of the shielding box, changing the connecting cables between the sourcemeter and probe, and grounding the probe stage, sourcemeter and shielding box together. The optimized system exhibits a noise level decreased to the orders of 10-12, meeting the testing requirements. Furthermore, the optical power attenuation problem of the spectral system is investigated. To solve the problem that the intensity of the light transmitted to the device under test is too low to induce an obvious photoresponse, a UV optical fiber and a universal magnetic holder are adopted, and the holder can guarantee the optical fiber close enough to the device under test. Finally, the completed spectral response testing system provides a satisfactory platform for evaluating detector performances.3. The performances of fabricated Ga N-based UV detectors are tested and analyzed. The results show that, the Ga N-based p-i-n UV detectors with a thin p-type layer on the PSS have lower dark current and maintain good enough quantum efficiency even in the deep UV range. The comparison results between p-i-n and Schottky UV detectors show that they exhibit similar performances, but the Schottky UV detectors have higher dark current due to the larger device areas. In practical applications, the Schottky UV detectors are more restrained by the depletion layer width and the barrier height, while the p-i-n UV detectors have more advantages in suppressing the dark current.