Low Temperature Growth and P-type Doping Studies of Gallium Nitride Epilayers

• Main Authors: Kuan-Ting Liu, 劉冠廷
• Other Authors: Shoou-Jinn Chang
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/36685132034276544856

博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 93 === In this dissertation, the low temperature growth and characterization of GaN epitaxy layers have been investigated by radio frequency plasma assisted molecular beam epiaxy (RF-MBE). A novel modulated beam growth method was proposed to alternately grow Ga-enriched and N-enriched surfaces during the growth for improving the crystal quality of the low temperature grown GaN layers. The experimental evidence was presented that the vertical striations in the cleaved sections, which are often observed in low temperature grown GaN layers and bring a significant degradation in the optical and electrical properties of nitride-based devices, have disappeared. We could achieve a low residual carrier concentration of 5.7 × 1016 cm-3 at the growth temperature as low as 730℃ and a better surface morphology, X-ray diffraction (XRD) and photoluminescence (PL) characteristics by using the modulated beam growth method compared with conventional MBE growth. These results could all be attributed to the enhanced lateral growth of the modulated beam growth method. Furthermore, the migration enhanced epitaxy (MEE) method involving an alternative supply of Ga atoms and N2 plasma was also used to grow GaN layers under low growth temperature. It was found that we could achieve the better crystalline characteristics of the GaN layers grown by MEE than by MBE at the growth temperature of 600℃. The observed results suggest the enhanced migration of surface adatoms from terrace to kink in the step during growth and the occurrence of a step flow growth. In addition, we also found that the crystalline characteristics of MEE growth were dependent on the thickness of GaN layers, which was probably due to the increase of grains size as growth time and coalescence each other. Another subject of this dissertation is the p-type doping study in GaN by the technics of the doping during epitaxial growth and post-growth ion implantation. It was found that the deterioration of p-type conductive characteristics in Be-doped GaN layers were related to the surface polarity change compared to Mg-doped GaN layers grown under the identical growth conditions by RF-MBE. When the undoped or Mg-doped GaN layers grown on a sapphire (0001) substrates, the surface polarity was N-terminated. However, Be-doped GaN layers always show Ga-terminated surfaces regardless of the polarity of the underlying layers. By growing Be-doped GaN on the Ga-terminated GaN surface, we have successfully grown Be-doped p-type GaN layer with a hole concentration of 1.8 × 1017 cm-3 at room temperature. Moreover, we also introduce co-doping method through ion implantation for improving p-type conductivity of GaN. Four kinds of co-implanted GaN samples including Mg/P, Mg/N, Be/N and Be/C have been systematically investigated compared with single acceptor doping. It was found that co-implanting acceptors with the selected dopants were not only to increase the activation efficiency of acceptors but also to decrease the surface barrier height under a proper dopant concentration ratio and post-annealing condition. These phenomena could be attributed to the reduction of self-compensation and the termination of N-vacancy related surface defects because the additional impurities may partially occupy on the N vacancies and enhancing the acceptors activation. The difference in the observed co-doping effects for various co-implanted samples was essentially dependent on the depths of additional dopants relative to those of acceptors. For the Mg/P co-implanted GaN, we found a new PL line as results of P-related transition, evidently due to the recombination of electrons from the shallow native donors with holes previously captured by isoelectronic P traps (D-I pair). Besides, a high activation efficiency of Mg acceptor and hole concentration were obtained to be ~9.6% and 4.8 × 1018 cm-3, respectively, for the Mg/N co-implanted sample. Based on the PL results of Be/N and Be/C co-implanted samples, we have found that the Be is an effective p-type dopant in GaN and have confirmed that the Be acceptor level (145~155 meV) is shallower than Mg one. These experimental results demonstrated that co-doping P or N with Mg atoms, and N or C with Be atoms in GaN is an effective method to improve p-type conductivity and to reduce the surface barrier height, which can help to decrease the metal contact resistivity to p-type GaN.