Mobility collapse in undoped and Si-doped GaN grown by LP-MOVPE

The room temperature carrier mobility in bulk GaN layers is found to improv e drastically by up to a factor of 20 once the incorporated silicon is high er than a critical value. A theoretical model taking into account several s cattering mechanisms has been developed to account for the temperature depe ndence of conduction band mobility mu(T) and carrier density n(T) deduced f rom 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 un doped samples, mu(T) cannot be explained just using this model. TEM observa tion shows the presence of a more diffuse distribution of threading disloca tions in the Si-doped material compared to the undoped one. The grain bound aries in this latter case are likely to correspond to high energetic barrie rs that carriers can overcome only by some tunneling process resulting in v ery low mobility.