Determination of GaN polarity on periodically oriented surfaces.

Periodically oriented GaN surfaces have been investigated using scanning probe microscopy techniques to compare Ga- and N-polar regions on a common surface. An epitaxial layer comprising 16 μm-wide stripes of alternating surface polarity was grown by utilizing a patterned AlN layer on an N-polar GaN template. The regions of different polarity are easily distinguishable in atomic force microscopy (AFM) images, with the Ga-polar stripes, which nucleate on the patterned AlN regions being approximately 300 nm higher than the adjacent N-polar stripes. In addition, local surface potential measurements using scanning Kelvin probemicroscopy (SKPM) indicate that the N-polar regions are 0.2 to 0.5 eV higher in potential compared to Ga-polar ones, with lower surface potential contrast occurring after an HCl-based surface treatment. Using conductive AFM, electrons were injected into the surface to probe the surface charging behavior in dark. Only the N-polar regions demonstrated significant localized charging, where changes of over 1.5 eV were seen in SKPM images taken immediately after charging. This behavior was appreciably decreased by an HCl treatment, suggesting that a surface oxide in the N-polar regions plays a significant role in the charging behavior. In addition, the local surface photovoltage (SPV) was measured using above-bandgap illumination. The N-polar regions demonstrate a higher steady-state SPV value and a significantly slower restoration behavior as compared to the Ga-polar regions. The authors therefore find that several characterization techniques can readily distinguish the Ga- versus N-polar regions on this periodically oriented surface. [ABSTRACT FROM AUTHOR]