Evaluation of carrier concentration reduction in GaN-on-GaN wafers by Raman spectroscopy and Kelvin force microscopy

The carrier concentration in a gallium nitride (GaN) substrate of a GaN-on-GaN wafer grown by hydride vapor phase epitaxy (HVPE) was evaluated by Raman spectroscopy and Kelvin probe force microscopy (KFM). On the basis of the longitudinal optical phonon-plasmon coupled (LOPC) mode of Raman spectra and surface potential measurements by KFM, the carrier concentration at the periphery of the HVPE-GaN substrate was found to be about one order of magnitude lower than that at the center. The decrease in carrier concentration is considered to be due to the out-diffusion of dopants during the metal organic chemical vapor deposition (MOCVD) of the epitaxial layer. In silicon (Si) epitaxial wafers, the autodoping of out-diffused dopants introduces nonuniform device characteristics. This undesirable effect needs to be suppressed to successfully move from prototype GaN-on-GaN power devices to commercial products.