Paper SS+EM-WeA1
Local Characterization of Laterally Patterned GaN Polar Surfaces

Wednesday, October 31, 2012, 2:00 pm, Room 22

Using scanning probe microscopy techniques, we have investigated the topographic and electrical properties of lithographically defined Ga- and N-polar regions grown on the same surface of a GaN epilayer. These unique structures were created on N-polar substrates grown by either hydride vapor phase epitaxy (HVPE) or metalorganic chemical vapor deposition (MOCVD). A patented selective epitaxy process [1,2] was used to prepare the alternating polarity surfaces on the two separate substrate types. To produce adjacent stripes of alternating polarity on a single substrate, a thin inversion layer comprising AlN was selectively grown inside a nitride mask. After removing the mask, Ga- and N-polar GaN were simultaneously grown over the inversion layer and bare N polar substrate, respectively, using MOCVD. Atomic force microscope (AFM) topography images reveal that Ga-polar regions are smoother than N-polar ones by about two orders of magnitude (rms of ~0.5 nm and ~50 nm, respectively). Local current/voltage (I/V) spectra obtained by conductive AFM indicate a lower turn-on voltage and higher conductivity for N polar regions, whereas Ga polar regions are insulating. Scanning Kelvin probe microscopy (SKPM) data show a surface potential drop of ~0.5 V across the interface domain boundary (< 200 nm lateral resolution) from the N- to the Ga polar regions on the HVPE substrate. A lower potential drop (~0.2 V) is seen across the N- to Ga- interface on the template, as well as inconsistent surface potential values (+/-0.1 V) for Ga-polar stripes. Using SKPM, the surface photovoltage (SPV), or the change in surface potential upon exposure to above-bandgap light, may also be measured for both surfaces simultaneously. Both samples show similar characteristics to previously studied polar GaN surfaces, where Ga-polar regions have higher initial SPV values (~0.3 V for HVPE, ~0.5 V for MOCVD) than N-polar regions (~0.2 V for HVPE, ~0.3 V for MOCVD). Restoration of the SPV signal after illumination is faster for Ga-polar regions, which is not consistent with previously studied bulk Ga-polar films. In summary, scanning probe methods can be used to distinguish Ga- versus N-polar GaN surface regions grown on the same GaN epilayer.

1. J.K. Hite, M.E. Twigg, M.A. Mastro, C.R. Eddy, Jr. and F.J. Kub, “Initiating Polarity Inversion in GaN Growth Using an AlN Interlayer”, Physica Status Solidi A 208, 1504-1506 (2011).

2. J.K. Hite, N.D. Bassim, M.E. Twigg, M.A. Mastro, F.J. Kub and C.R. Eddy, Jr., “GaN Vertical and Lateral Polarity Heterostructures on GaN Substrates”, Journal of Crystal Growth 332, 43-47 (2011).