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Due to its large band gap and excellent electrical properties, nitride-based heterostructures are rapidly becoming a material of choice for RF and power switching applications. However, these devices require a carbon or iron doped semi-insulating buffer to deliver high breakdown voltages and suppress off-state leakage currents. We have grown semi-insulating GaN using precursor-based metal-organic chemical vapor phase epitaxy by intentionally introducing carbon and iron impurities with doping concentration ranging from 1x10^17cm-3 to 5x10^18cm-3 to compensate residual donors. Scanning probe microscopy techniques, scanning surface potential microscopy (SSPM) and bias dependent electric force microscopy (EFM) are mainly used to compare contact potential differences and local potential mapping at the vicinity of dislocation regions. For reference n-type GaN layers doped with Si and Ge, and p-type GaN layers doped with Mg are also investigated. Skew and edge type dislocation densities are estimated from tilt and twist x-ray diffraction measurements using omega-scans for the (0002) reflection and grazing incidence in-plane geometry for the (101 ̅0) reflection. The obtained values are in the range of low 108 cm-2 for screw-type and low 109 cm-2 for edge-type dislocations, independent of doping type and concentration. Locally probing dislocations by SSPM reveals a negative charge contrast with respect to the surrounding areas in C-doped samples increasing with doping concentration whereas Fe-doped samples exhibit no contrast. By investigating the contact potential by EFM, the combined effects of Fermi-level position and surface band bending due to surface states are determined. With the references of n-type and p-type GaN samples, the acceptor states introduced by carbon cause Fermi-level pinning below midgap position whereas acceptor-states by Fe impurities have to be energetically above midgap position. In vertical transport measurements, C-doped GaN layers with a dopant concentration of 4.6x10^18 cm-3 exhibit an up to 5 orders of magnitude lower dark current at room temperature and significantly higher thermal activations than Fe-doped samples with a comparable dopant concentration. In conclusion C-doped samples show superior properties in comparison to Fe-doped samples.
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