Doping characteristics of Al_0.15Ga_0.85N epilayers with various Mg incorporations

We have investigated the p-type doping characteristics such as crystallinity, electrical resistivity and optical properties of Al_0.15Ga_0.85N:Mg epilayers grown by MOCVD with various Mg incorporations. While the mirror-like surface without any defects such as cracks and hillocks can be seen in the undoped Al_0.15Ga_0.85N epilayer which was grown without any intentional doping of Mg, many hexagonal pyramids and small islands are observed in the p-Al_0.15Ga_0.85N:Mg which was grown with Mg incorporation rate of 50 nmol/min. Mg-doped Al_0.15Ga_0.85N layers exhibit little increase in the electrical resistivity with Mg incorporation until 40 nmol/min and then shows the abrupt increase above 50 nmol/min. The cathodeluminescene spectra of every Mg-doped layer show that the donor-to-acceptor (D-A) pair transition is dominated at 362 nm, which is red shifted by 19 nm against band edge luminescence emission of the undoped Al_0.15Ga_0.85N layer. By calculating the relative intensity between D-A pair transition of Mg-doped Al_0.15Ga_0.85N and band edge emission of the undoped, the change of optical characteristics with different Mg doping level was evaluated. This result suggested that the residual Mg-H complexes in highly Mg-doped Al_0.15Ga_0.85N are not cracked at a certain activation process and are responsible for the abrupt decrease of optical luminescence characteristics.