Investigation of inversion domains in GaN by electric-force microscopy

Inversion domains in III-nitride semiconductors degrade the performance of devices fabricated in them. Consequently, it is imperative that we understa nd their electrostatic manifestation, the growth conditions under which suc h domains form, and an effective means of their identification. In what is nominally referred to as Ga-polarity samples, N-polarity domains have a pol arization that is reversed with respect to the remainder of the surface, an d therefore, have a different potential under strain. We have used surface- potential electric-force microscopy (SP-EFM) to image the electrostatic sur face potential of GaN grown on sapphire, which is strained due to the therm al mismatch between the substrate and GaN. Employing a control sample with side-by-side Ga- and N-polarity regions, we have established the EFM mode n ecessary to identify inversion domains on GaN samples grown by molecular-be am epitaxy. This method is not sensitive to topology and has a spatial reso lution of under 100 nm. The measured surface potentials for Ga-face and N-f ace regions are +25 +/- 10 and -30 +/- 10 mV, respectively, with respect to the sapphire substrate, where the sign is consistent with Ga- and N-polari ty GaN under compressive strain due to thermal mismatch with the sapphire s ubstrate. (C) 2001 American Institute of Physics.