Study On The Doping Techniques And Related Mechanisms Of The Nonpolar P-type AlGaN Epi-layers

The nonpolar AlGaN-based group-?nitrides have been considered as the promising candidates for the fabrication of ultraviolet light-emitting diodes?UV-LEDs?because of the elimination of quantum confined Stark effect?QCSE?along the growth direction.In this thesis,the nonpolar?112 0?a-plane AlGaN materials grown on the semi-polar?11 02?r-plane sapphire substrates with metalorganic chemical vapor deposition?MOCVD?technology were reported and the p-type doping techniques and related mechanisms were investigated extensively.The nonpolar a-plane p-type AlGaN,AlGaN/GaN superlattice?SL?,and AlGaN/GaN distributed Bragg reflector?DBR?materials with high hole concentration were achieved successfully.These results will pave the way to fabricate nonpolar AlGaN-based UV-LED with high efficiency.The major works conducted in this research and the achievements are listed as follows:1.The reformed Mg-delta-doping technique was employed in the epitaxial growth of the nonpolar a-plane p-AlGaN films on semi-polar r-plane sapphire substrates.A hole concentration as high as 1.4×10¹⁸ cm^-3 were achieved by optimizing the pulse time of Cp₂Mg mass flow in the Mg-delta-doping process.2.The effects of the conventional one-step,high/low-temperature two-step,O₂/N₂-ambient two-step,and the newly-developed multiple-step rapid thermal annealing?RTA?techniques on the properties of nonpolar p-AlGaN materials were studied and compared.The results demonstrated that higher dissociation efficiency of the Mg-H complex for the non-polar a-plane p-AlGaN film samples could be obtained with the multiple-step rapid RTA technique.In fact,a resistivity as small as 0.7??cm and a hole concentration as high as 1.9×10¹⁸ cm^-3 were achieved by using such a technique.3.The Mg-delta-doping and the Al composition-graded AlGaN buffer layer techniques were employed in the epitaxial growth of the nonpolar a-plane p-AlGaN/GaN SLs.The characterization results demonstrate that the both surface morphology and the hole concentration for the nonpolar a-plane p-AlGaN/GaN SLs could be improved significantly with the introduction of the Mg-delta-doping process and the Al composition-graded AlGaN buffer layer.Consequently,a root mean square value as small as 0.6 nm and a hole concentration as high as 4.2×10¹⁷ cm^-33 were achieved.4.The nonpolar a-plane Mg-doped p-type AlGaN/GaN DBRs were successfully grown with MOCVD technology.The measurement results of the reflectivity demonstrate that a stopband centered at 390 nm with a bandwidth of 45 nm and a peak reflectivity as high as 55%could be achieved.Meanwhile,it was found that a relatively high reflectivity?>35%?could also be obtained within the wavelength range of 270-320 nm for the non-polar p-AlGaN/GaN DBR which was intended to be a reflector with a working wavelength at 280 nm.Additionally,a hole concentration higher than 2×10¹⁷ cm^-33 was achieved with for these two types of nonpolar p-Al_0.3Ga_0.7N/GaN DBRs grown in this study.