Piezoresponse force microscopy (PFM) characterization of GaN nanowires grown by Plasma assisted Molecular beam epitaxy (PA-MBE)

In this work, we have grown GaN nanowires using Plasma assisted Molecular beam epitaxy (PA-MBE) on Si (111) substrate. High resolution X-ray diffraction (HRXRD) characterization and scanning electron microscopy (SEM) studies were carried out to investigate the crystal structure, morphology and uniformity of the grown nanowires. These studies confirm wurtzite crystal structure and uniform growth. The diameter of nanowires was observed to be in the range of 100 – 200 nm with length between 1 – 2 µm. Piezoresponse force microscopy (PFM) was used in Dual AC Resonance Tracking (DART) mode for imaging the height and phase response from the grown nanowires. A switching spectroscopy PFM (SS-PFM) was employed to measure the piezoresponse from individual nanowires. It shows repetitive Displacement-voltage (D-V) loops at multiple points which demonstrates the defect free and high quality growth. An effective piezoelectric coefficient, d
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in the range of 14 – 20 pm/V was calculated for GaN nanowires. Further, we have also simulated piezoresponse of GaN thin films and nanowires for comparison and confirmed that size dependence (polarization/volume) is a dominant factor for increase in the piezoresponse. This study demonstrates the usability of GaN nanowires for sensing, actuation and energy harvesting for high temperature applications.