Phonon–plasmon coupled modes in GaN vertical power structures: Carrier concentration and mobility analysis

Abstract

Confocal Raman microscopy is proposed as a non-invasive method for determining carrier density and mobility in GaN structures used in vertical power devices. The electronic properties of two GaN epilayers grown on 100 mm c-plane sapphire substrates via metal–organic chemical vapor deposition are investigated by analyzing the longitudinal phonon–plasmon (LPP) modes of the highly conductive GaN bottom layer, the drain layer of the device. Due to the wide range of device architectures, two scenarios are considered: (1) direct optical access to the drain, where LPPs are measured from the side, and (2) no direct access, where LPPs are measured from the top, in a buried layer. In the latter case, the effect of aberrations introduced by the refractive index mismatch at the air–GaN interface is discussed. The LPPs are fitted using a theoretical model incorporating strain and three electron–phonon interaction mechanisms: electro-optical, deformation potential, and charge density fluctuations. The relative impact of these contributions is discussed, revealing that charge density fluctuations dominate and must be considered for the accurate correlation of the experiment and model. Finally, the carrier density and mobility values are extracted and compared with available data, showing the method’s potential for characterizing vertical GaN-based power-device structures.