Effect of structural defects and chemical impurities on Hall mobilities in low pressure MOCVD grown GaN

We have studied the effect that structural defects and chemical impurities have on the electron mobility in GaN films grown in a production scale metalorganic chemical vapor depositon system. Structural defects such as dislocations, stacking faults, twins, and amorphous regions in the buffer layer have been examined. In general, we have found that the structural defects are not the primary contributor to low mobility. However, there is one type of defect (''nanopipe'') that may be an important indirect contributor to mobility degradation by acting as a conduit into the film interior for impurities such as carbon and oxygen. We have also investigated the role that the principal impurities play in determining the electrical performance; Of particular concern was the presence of carbon resulting from an incomplete dissociation of trimethylgallium precursor gas. Also present in the films were traces of oxygen, hydrogen, and aluminum, while heavy metals such as iron, chromium, and molybdenum were detected at or near the resolution limit of secondary ion mass spectrometry analysis. We present evidence for compensation by carbon at low carrier concentrations, which would help to explain the anomalous mobility behavior in GaN.