Optical Properties Of A-plane GaN And Mg-doped AigaN Films

The commercial fabrication of lightemitting diodes (LED) and laser diodes (LD) from the ultraviolet to the amber, as well as the development of highpower high electron mobility transistors (HEMT) suitable for numerous telecom applications have been driven by the tremendous improvement in the nitride materials quality. However, the device performance is hampered by the presence of strong spontaneous and piezoelectric polarization fields along the typical [0001] growth direction in the wurtzite nitride materials. Thus, nonpolar materials with no built-in electric fields, like a-plane GaN, have enormous potential and have attracted a great interest. Besides, AIGaN alloys have a wide band gap from3.4eV to6.2eV witch makes AIGaN become one of the most important materials for ultraviolet devices, p-AIGaN is the base of p-n light-emitting devices. Mg as one of the most important p-type dopant, its doping and activation mechanism are still not very clear.In this article, the in-plane optical anisotropic properties of the nonpolar a-plane GaN films grown on r-plane sapphire and the optical properties of Mg-doped AIGaN (xAi>0.5) grown on AIN interlayer were investigated by using polarised photoluminescence (PL), polarized absorption spectra, X-ray diffraction and cathodoluminescence (CL). The main conclusions we obtained are listed below:1By using polarized PL spectra the in-plane optical anisotropic properties of the nonpolar a-plane GaN films grown on r-plane sapphire were investigated. In the case of wurtzite Ⅲ-nitride structure, the top VB at the Brillouin-zone center (point) splits into three sub-bands owing to anisotropic strain main caused by lattice mismatch. There is 11meV-17meV energy difference between the splited hole-bands. And the recombination processes involving the|y>like hole band are more dominant. Meanwhile, with the decreasing film thickness, surface strain relaxation occurs, the in-plane anisotropic strains decreases, the energy difference between the splited hole-bands decreases.2The polarized absorption spectra derived from transmission show the split of the valence band from the perspective of the absorption of energy. Anisotropic anisotropic absorption coefficients, which show the same result as the former PL analysis add a view to the relationship between the absorption coefficient and the effective band-gap, were observed.3CL emission lines due to near band-edge of Mg-doped AlxGa1-xN (xAl>0.5) films have been observed, from which activation energies of Mg acceptor were deduced. The CL spectra show that the doped Mg leads to the reduction of the films quality and the main excitation process involved Mg of Mg-doped Al0.54Ga0.46N are related to bound-acceptor emission (e-A) and donor-acceptor emission (DAP). One Mg-related energy band was found to be655meV above the valence band. In addition, we explained the decreased CL intensity of the AlxGa1-xN (xAi<0.5) by using the lateral phase separation phenomenon.