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 less than or equal to 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 x 10(18) cm(-3) for x = 0.1, 0.2, 0.3, 0.4, and 0.5, respectively, compared with the nominal doping value of 10(18) 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. (C) 2002 Elsevier Science B.V. All rights reserved.