Research On The Epitaxial Growth And Characterization Of Non-Polar And Semi-Polar Gan-Based Nitrides

In recent years, light-emitting diode (LED)-based solid lighting sources have been widely used. Extensive researches were performed on nitride semiconductors because of their advantages for the fabricating excellent devices. Morever, the LED devices fabracited with non-polar GaN-based semiconductors have high luminous efficiency since the quantum-confined stark effect (QCSE) was eliminated due to the absence of polarization electrical field in the non-polar semiconductors. The growth of non-polar GaN-based semiconductors was conducted using metal origanic vapor phase epitaxy (MOVPE) in this study. And the structural, electrical, and optical properties as well as the effect of Si-doping on the properties were studied. The main research contents of this dissertation are as follows:(1) singal crystal GaN films were achieved on a-, m- and r-plane sapphire substrates respectively. And it was determined unambiguously that the orientations of the GaN epi-layers grown on c-, a-, m-, and r-plane sapphire substrates were along the (0001), (11 22), and (11 20) directions, respectively according to the peak positions in the XRD spectra. It was found that the crystal quality was enhanced by the increasement of ?/? ratio for the GaN film grown on a-plane sapphire substrate. The growth condition for GaN on a-plane sapphire was similar to that for GaN on m-plane sapphire substrate. Furthermore, a HT-grown AIN was found to be more useful than a LT-grown AIN to improve the crystal quality of GaN grown on r-plane sapphire substrate.(2) The growth of non-polar (11 20) AlGaN was performed on r-plane sapphire substrate. It is notable that smooth surface and good crystal quality are hardly achieved simultaneously. A ?/? ratio of 10000 for the AIN nucleation layer and a 200nm-thick GaN buffer layer on AIN result in a smooth surface morphology but a poor crystal quality. And a ?/? ratio of 2567 for the AIN nucleation layer and a 400nm-thick GaN buffer layer on AIN lead to a good crystal quality but an unsmooth surface. It was also found that a high tempeture-grown AIN nucleation layer and a high ?/? ratio are benefical to improve the crystal quality of non-polar AlGaN epitaxial layer. The growth of semi-polar (11 22) AlGaN was conducted on m-plane sapphire substrate. And it was found that using the AlGaN multilayers with varied Al compositions as buffer is effective to improve the crystal quality of semi-polar AlGaN epi-layer.(3) The effects of Si-doping on the structural and electrical properties of the wurtzite AlGaN epi-layers grown on polar, semi-polar, and non-polar sapphire substrates were studied. The characterization results showed that both the surface morphology and the crystal quality of the polar AlGaN samples grown on ?-plane sapphire substrates were improved with increasing Si concentration due to the Si-induced increase in dislocation movement. It was also found that the folds on the surfaces of the semi-polar and non-polar AlGaN samples grown on m- and r-plane sapphire substrates, respectively were significantly reduced in consequence of the growth suppression along c direction by Si-doping. Moreover, owing to the enhanced crystal quality, an increase in both the mobility and the carrier density for the polar AlGaN samples grown on a-plane sapphire substrates was achieved as the Si-doping level was increased. In addition, a relatively high electron concentration was obtained from the undoped semi-polar AlGaN samples grown on m-plane sapphire substrate, which is helpful to fabricate high quality semi-polar AlGaN-based ultra-violet light emitting diodes (UV-LEDs).