Growth and silicon doping of AlGaN films in the entire alloy composition by molecular beam epitaxy

In this paper we report the growth and Silicon doping of AlGaN films in the entire alloy composition by RF-plasma-assisted MBE. The films (0.7 to 0.8 mu m thick) were grown with the (0001) polarity on C-plane sapphire substrates using a thin AlN buffer and were doped n-type with the silicon cell at a constant temperature (1200 degrees C). ne composition and the structure of the films were determined by XRD, the silicon and oxygen impurities were determined by SIMS measurements, and the transport coefficients were obtained from Hall effect measurements. The FWHM of the on- and off-axis XRD rocking curves indicate that the microstructure of the films is better at low AlN mole fraction (< 20%) and high AlN mole fraction (> 70%). The resistivity of the films varies from 10(-3) to 25 Omega.cm as the AlN mole fraction increases from 0 to 96%. Reliable Hall effect measurements were carried out in films with up to 77% Al and were found to be degenerately n-type doped with room temperature carrier concentration varying from 1x10(20) to 7.7x10(18) cm(-3) as the AlN mole fraction increased from 0 to 77%. Correspondingly, the electron mobility is reduced by almost an order of magnitude, a result which is consistent with alloy scattering. The effective mass of some of the samples was determined by combining infrared reflectivity and Hall Effect measurements and these data were used to determine the Si ionization energies. We concluded that the Si is the main donor in the entire alloy composition and that the gradual reduction of conductivity with Al content is due to the increase of the Si ionization energy.