Paper EM1-MoA3
Local Surface Electrical Characterization of Polar GaN Surfaces

Monday, October 31, 2011, 2:40 pm, Room 209

While GaN is a widely-used material in optoelectronic devices, localized surface-related electrical properties are not well-understood. These properties affect the operational performance and lifetimes of GaN-based devices. Here, several atomic force microscopy (AFM) techniques were used to characterize the Ga-polar, +c [0001], and N-polar, -c [0001bar], surfaces of free-standing bulk GaN. Samples were prepared by either a chemical-mechanical polish (CMP) or mechanical polish (MP) of HVPE-grown GaN. AFM data showed that the Ga-polar surfaces (MP and CMP) were uniformly flat with rms roughness of less than 1 nm over a 5x5 micron image. In contrast, the N-polar surfaces were significantly rougher (~5 nm rms) with scratch-like features (100 nm wide, microns long), where the CMP treatment resulted in the presence of surface protrusions (~100 nm dia.) in proximity of the scratches. We then examined the local electrical properties using conducting AFM (C-AFM) to map surface conductivity and to obtain I-V spectra. C-AFM images at forward-bias (<6V) showed small contrast variations for all samples except the N-polar CMP surface. In that case, we observed less conducting behavior on the protrusions as compared to the surrounding surface. Local I-V data also revealed a higher forward-bias, turn-on voltage for the N-polar vs. Ga-polar samples. To investigate the local surface charging behavior, we used a two-step technique. First, a metallized AFM tip was used to locally charge the surface by applying a DC voltage, and then the resulting change in surface potential was monitored as a function of time with scanning Kelvin probe microscopy (SKPM). These surface charging data showed a smaller change in surface potential for the N- vs. Ga-polar samples, which appears to be consistent with the lower onset of conduction for the N-polar orientation. Finally, we measureed the photo-induced changes in surface potential under UV light exposure (100W Hg lamp), otherwise known as the surface photovoltage effect (SPV). The N-polar samples had a smaller SPV compared to Ga-polar, which indicates a smaller amount of band bending at the surface. Additionally, N-polar GaN restored to dark-state conditions at a much faster rate, regardless of CMP or MP treatment. In summary, we observed differences in morphology and electrical behavior for the two polar, c-plane GaN surfaces, as well as differences in behavior due to CMP and MP treatments. These data suggest a less pronounced surface charging behavior on N-polar vs. Ga-polar GaN.