Because the surface plays an important role in the electrical and optical properties of GaN devices, an improved understanding of surface effects should help optimize device performance. In this work, atomic force microscopy (AFM) and related techniques have been used to characterize three unique sets of n-type GaN samples. The sample sets comprised freestanding bulk GaN with Ga polar and N polar surfaces, epitaxial GaN films with laterally patterned Ga- and N-polar regions on a common surface, and truncated, hexagonal GaN microstructures containing Ga-polar mesas and semipolar facets. Morphology studies revealed that bulk Ga-polar surfaces treated with a chemical-mechanical polish (CMP) were the flattest of the entire set, with rms values of only 0.4 nm. Conducting AFM (CAFM) indicated unexpected insulating behavior for N-polar GaN bulk samples, but showed expected forward and reverse-bias conduction for periodically patterned GaN samples. Using scanning Kelvin probe microscopy, these same patterned samples demonstrated surface potential differences between the two polarities of up to 0.5 eV, where N-polar showed the expected higher surface potential. An HCl cleaning procedure used to remove the surface oxide decreased this difference between the two regions by 0.2 eV. It is possible to locally inject surface charge and measure the resulting change in surface potential using CAFM in conjunction with SKPM. After injecting electrons using a 10 V applied voltage between sample and tip, the patterned polarity samples reveal that the N-polar regions become significantly more negatively charged as compared to Ga-polar regions, with up to a 2 eV difference between charged and uncharged N polar regions. This result suggests that the N-polar regions have a thicker surface oxide that effectively stores charge. Removal of this oxide layer using HCl results in significantly decreased surface charging behavior. A phenomenological model was then developed to fit the discharging behavior of N-polar GaN with good agreement to experimental data. Surface photovoltage (SPV) measurements obtained using SKPM further support the presence of a thicker surface oxide for N polar GaN based on steady state and restoration SPV behaviors. Scanning probe microscopy techniques have therefore been used to effectively discriminate between the surface morphological and electrical behaviors of Ga- vs. N-polar GaN.
| Identifer | oai:union.ndltd.org:vcu.edu/oai:scholarscompass.vcu.edu:etd-4088 |
| Date | 26 April 2013 |
| Creators | Ferguson, Josephus Daniel, III |
| Publisher | VCU Scholars Compass |
| Source Sets | Virginia Commonwealth University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | © The Author |
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