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

We use infrared spectroscopic ellipsometry (IRSE) to obtain optical free-carrier parameters in p- and n-type hexagonal (α-) GaN films. Si- and Mg-doped films (40 nm to 4 μm) were grown on sapphire by MOCVD and MBE, respectively. Results from electrical (Hall) measurements are combined with those obtained from the Drude model by lineshape analysis of the IRSE data. We derive effective electron (m_e) and hole (m_h) mass parameters for GaN, where m_h depends on the free-hole concentration N_e. Undoped but n-type conductive films grown under similar conditions with different thicknesses, d, show a double-logarithmic dependence over two orders of magnitude between N_e and d: log(N_e)∝c_elog(d) with c_e<0. A similar behavior is observed for free hole concentrations in Mg-doped α-GaN. Inhomogeneously activated donors or acceptors due to a decrease in misfit dislocations 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 find that d_SL increases with decreasing N_e.