Effective carrier mass and mobility versus carrier concentration in p- andn-type alpha-GaN determined by infrared ellipsometry and Hall resistivity measurements

We use infrared spectroscopic ellipsometry (IRSE) to obtain optical free-ca rrier parameters in p- and it-type hexagonal (alpha-) GaN films. Si- and Mg -doped films (40 nm to 4 mum) were grown on sapphire by MOCVD and MBE, resp ectively. Results from electrical (HaII) measurements are combined with tho se obtained from the Drude model by lineshape analysis of the IRSE data. We derive effective electron (m,) and hole (m,) mass parameters for GaN, wher e m, depends on the free-hole concentration N-e. Undoped but n-type conduct ive films grown under similar conditions with different thicknesses, d, sho w a double-logarithmic dependence over two orders of magnitude between N-e and d: log(N-e) proportional to c(e)log(d) with c(e)< 0 with. A similar beh avior is observed for free hole concentrations in Mg-doped alpha -GaN. Inho mogeneously activated donors or accepters due to a decrease in misfit dislo cations along the growth direction could explain the thickness dependence. A low electron mobility and large lattice mode broadening were observed at the layer-substrate interface in n-type GaN films. For high N-e values, the films reveal surface carrier depletion layers with thickness d(SL). We fin d that d(SL) increases with decreasing N-e. (C) 2001 Elsevier Science B.V. All rights reserved.