| Summary: | 碩士 === 國立清華大學 === 電子工程研究所 === 104 === Leakage currents, which could degrade electrical properties of GaN devices, are an important issue related to epitaxial layer growth and device processing. In this thesis, according to the conduction path directions, the origins and solutions of lateral and vertical leakage currents in GaN multiple layer structures are investigated.
By exposing the GaN template surface to N2 plasma in the plasma-assisted molecular beam epitaxy (PAMBE) system, the concentration of residual oxygen and carbon at the growth interface are effectively reduced from 1.49×1019 to 8.92×1017 and from 7.22×1018 to 3.38×1016 cm-3, respectively, while preserving a good interface quality. The AlGaN/GaN high electron mobility transistor (HEMT) structure is used to study the lateral leakage current problem. The mobility of the AlGaN/GaN HEMT is increased from 469 to 730 cm2/Vs, and the lateral leakage current of AlGaN/GaN Schottky barrier diodes (SBDs) decreased from 1.54 to 0.038 A/cm2, after N2 plasma treatment at 900°C for 15 min.
Undoped GaN was grown at 750, 785, 815 and 845°C to increase the Ga evaporation rate on the surface such that the formation of Ga-decorated threading dislocations (TDs) is suppressed. Quasi-vertical GaN SBDs are utilized to investigate the vertical leakage current issue. It was found that TDs terminated as pits on the GaN surface grown at high temperatures. The average diameter of pits decreased from 507 to 379 nm as GaN growth temperature increased from 815 to 845 °C as Ga and N adatoms gain more kinetic energy to migrate on the surface. The series resistance of quasi-vertical SBDs becomes larger with increasing the growth temperature, which is attributed to the lower background doping concentration achieved in GaN. The reverse leakage current of quasi-vertical SBDs grown at high temperature is reduced from 6.5×10-2 to 1.8×10-5 A/cm2. The significant reduction of the leakage current is attributed to the suppression of Ga-decorated TDs formation in GaN films.
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