Mobility Collapse in Undoped and Si-Doped GaN Grown by LP-MOVPE

The room temperature carrier mobility in bulk GaN layers is found to improve drastically by up to a factor of 20 once the incorporated silicon is higher than a critical value. A theoretical model taking into account several scattering mechanisms has been developed to account for the temperature dependence of conduction band mobility mu(T) and carrier density n(T) deduced from Van der Pauw Hall measurements. For significantly Si-doped layers, both mu(T) and n(T) can be quite accurately reproduced. In lightly doped and undoped samples, mu(T) cannot be explained just using this model. TEM observation shows the presence of a more diffuse distribution of threading dislocations in the Si-doped material compared to the undoped one. The grain boundaries in this latter case are likely to correspond to high energetic barriers that carriers can overcome only by some tunneling process resulting in very low mobility.