Electrical and optical investigation of MBE grown Si-doped AlxGa1−xN as a function of Al mole fraction up to 0.5

Si-doped AlxGa1−xN grown by gas source MBE was investigated as a function of Al mole fraction up to 0.5 using temperature dependent Hall effect (TDH) and cathodoluminescence measurements. The band gap energies for the AlxGa1−xN layers estimated from the bound exciton peaks agree well with the linearly extrapolated band gaps only for x≤0.3. TDH measurements reveal the presence of a highly degenerate n-type region at the AlxGa1−xN/sapphire interface. The donor concentrations of AlxGa1−xN layers are estimated to be 4.5, 5.0, 15, 19, and 8×1018 cm−3 for x=0.1, 0.2, 0.3, 0.4, and 0.5, respectively, compared with the nominal doping value of 1018 Si cm−3. The activation energies of the Si donors are 6, 11, 40, 60, and 68 meV for x=0.1 0.2, 0.3, 0.4, and 0.5, respectively. Both the electrical and optical measurements indicate that good quality AlxGa1−xN films can be grown for x up to 0.3 by gas source MBE using a sapphire substrate and an AlN buffer layer, but lesser quality films are obtained for x>0.3.