Growth of III-nitrides by molecular beam epitaxy for heterojunction field effect transistors and optoelectronic applications.

摘要

Group III-nitride semiconductors, aluminum nitride (AlN), gallium nitride (GaN), and Indium nitride (InN) have emerged as the next generation materials especially for the development of blue/UV range light emitter or detector and high power high frequency electronic devices, thanks to their large band gap (except InN) and unique materials properties such as large polarizations.; In this research, Molecular Beam Epitaxy (MBE) growth techniques for group III-nitride semiconductors (especially GaN and AlGaN) have been developed with RF nitrogen plasma source. Research concentration was given to growth of AlGaN/GaN heterojunction field effect transistors (HFETs) and development of n- and p-type doping techniques for GaN and AlGaN.; High Quality GaN films were obtained by employing low temperature nitridation of sapphire, AIN nucleation layer, and Ga rich growth condition. Over 1000 cm2Ns electron mobility were routinely obtained in 2 dimensional electron gas of AlGaN/GaN structure with sheet electron density, ∼1.0 x 10-3 cm-2. Heterojunction field effect transistors having 0.3mum x 100mum T-shaped gates were fabricated on one of the MBE grown AIGaN/GaN structures, which exhibited 800 mA/mm DC full channel current and 0.8 W/mm saturated output power density of at 8 GHz in class A operation. Breakdown voltage and cutoff frequency (ftau ) of this HFET were measured to be 60 V and 44.6 GHz respectively.; The maximum channel current at 3 GHz was limited to only half of DC current due to RF dispersion, but a thin AlN passivation layer deposited by RF plasma induced molecular beam epitaxy on the surface of post-processed AlGaN/GaN (HFET)s effectively reduced the RF dispersion. RF full channel current of the same 0.3mum x 100mum HFETs at 8 GHz increased from 400 mA/mm to 800 mA/mm and the saturated output power density was also doubled from 0.8 W/mm to 1.6 W/mm.; Very high levels of type doping of GaN and AlxGa1-x N alloys were successfully achieved by degenerate doping technique with RF plasma induced Molecular Beam Epitaxy growth and Si dopant. Electron concentrations were obtained up to 1.25 x 1020 cm-3 when the Al more fraction was 50%, and 8.5 x 1019 cm -3 electrons were measured even when the Al mole fraction was 80%. Despite the very high S doping density, there was no significant degradation of materials properties, and, in fact, the layers showed superior optical and structural qualities suitable for device applications.