Multiple parallel conduction paths observed in depth-profiled n-GaN epilayers

We have combined plasma etching with the Hall effect and resistivity measurements between 10 and 300 K to study the depth distribution of conduction in silicon (Si)-doped GaN epitaxial layers grown on sapphire substrates by two different metalorganic chemical vapor deposition processes. Reduction of the epitaxial layer thickness produces a linear decrease of the sheet carrier density with depth in the doped region, whilst in one sample, in the region less than ∼0.3 μm from the interface, the sheet carrier density tends to flatten out to a value of ∼3×1013 cm−2. The former is indicative of a uniform dopant distribution in the epitaxial material, and the latter reveals the existence of mobile charge near the interface. These experiments allow the properties of the doped material to be deconvoluted from those of the interface region, and the temperature dependence of these properties indicates the presence of two parallel conduction paths in the doped material: the conduction band and an impurity band. Thus a full analysis of GaN epitaxial layers is shown to require consideration of multiple parallel conduction processes, at the interface and in the bulk.