Detailed interpretation of electron transport in $n-GaN$

We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on two silicon doped GaN epitaxial layers grown by two different metalorganic chemical vapor deposition sources on sapphire substrates. Reactive ion etching has been used to enable Hall measurements to be taken as a function of film thickness, for 1.62 and 3.92 μm thick films. Temperature-dependent Hall experiments indicate classical donor freeze-out in the doped region, while the depth profile measurements show that in the undoped layer, the Hall electron density passes through a minimum before increasing again at lower temperatures. Such behavior is indicative of impurity conduction in this region. Using a model based upon a doped layer with one type of shallow donor, plus compensating acceptors, in parallel with an interface layer which shows impurity-band conduction, a simultaneous fitting of mobility and carrier concentration has been undertaken to quantify the contribution of different scattering mechanisms, and the densities of the donors and acceptors in the doped region were also found. An important result is that for the carrier concentration fitting, when the temperature dependence of the activation energy is taken into account, the fitted energies are closer to those predicted by free and bound carrier screening. Finally the Fermi level was found to be in the range ∼56–84 meV below the conduction band minimum at room temperature and moved closer to the donor levels at low temperatures.