Conductivity and Hall-effect in highly resistive GaN layers

Highly resistive GaN layers grown by molecular beam epitaxy are characteriz ed by temperature dependent conductivity and Hall effect measurements. Samp les with rho(300) congruent to 3 x 10(3) Ohm cm show room temperature Hall mobility of 22 and 35 cm(2) V-1 s(-1) and have a temperature dependence mu( H) similar to T-x with x = 0.9 and 0.5. This is in contradiction to a sampl e with rho(300) congruent to 32 Ohm cm which has a room temperature mobilit y of 310 cm(2) V-1 s(-1) and a mu(H) similar to T-x with x = -1.4. The same activation energy of 0.23 eV, attributed to donor-like defects, is found f or all three samples investigated. Temperature dependent conductivity data can be reasonably fitted considering band conduction. The presence of vario us hopping mechanisms is discussed. (C) 2000 American Institute of Physics. [S0003-6951(00)02825-4].