Assessment of Polarity in GaN Self-Assembled Nanowires by

Letter pubs.acs.org/NanoLett

Assessment of Polarity in GaN Self-Assembled Nanowires by Electrical Force Microscopy A. Minj,† A. Cros,*,† N. Garro,† J. Colchero,‡ T. Auzelle,§,⊥ and B. Daudin§,⊥ †

Materials Science Institute, University of Valencia, P.O. Box 22085, E46071 Valencia, Spain Facultad de Química, Departamento de Física, Universidad de Murcia, 30100 Murcia, Spain § Université Grenoble Alpes, INAC-SP2M, F-38000 Grenoble, France ⊥ CEA, INAC-SP2M, F-38054 Grenoble, France ‡

S Supporting Information *

ABSTRACT: In this work, we demonstrate the capabilities of atomic force microscopies (AFMs) for the nondestructive determination of the polarity of GaN nanowires (NWs). Three complementary AFMs are analyzed here: Kelvin probe force microscopy (KPFM), light-assisted KPFM, and piezo-force microscopy (PFM). These techniques allow us to assess the polarity of individual NWs over an area of tens of μm2 and provide statistics on the polarity of the ensemble with an accuracy hardly reachable by other methods. The precise quantitative analysis of the tip−sample interaction by multidimensional spectroscopic measurements, combined with advanced data analysis, has allowed the separate characterization of electrostatic and van der Waals forces as a function of tip−sample distance. Besides their polarity, the net surface charge density of individual NWs was estimated. KEYWORDS: Nanowire, GaN, polarity, Kelvin probe force microscopy, surface photovoltage, piezo force microscopy

S

Assessment of polarity can be achieved by different methods, most of them based on electron microscopy such as aberration corrected scanning transmission electron microscopy,9−11 convergent beam electron diffraction,6,12−14 electron energyloss spectroscopy,5,14 low energy electron diffraction, or X-ray photoelectron diffraction.15 Many of these techniques require demanding sample preparation, some of them are not adequate to assess a statistically significant number of NWs, and others only give information on the dominant growth orientation but do not deal with the characteristics of single NWs. Other techniques, such as KOH etching,12,13 result in sample damage. By contrast, scanning probe microscopy methods, such as Kelvin probe force microscopy (KPFM),16 piezoresponse force microscopy (PFM),6 or photoassisted KPFM,17 allow the determination of the polarity of single NWs over micrometer large surface areas with nanometer resolution and without the need of any special sample preparation; moreover, these techniques are already available in many commercial atomic force microscopes. However, the morphological characteristics of self-organized polar NWs are quite challenging for AFM characterization: they usually grow aligned along the polar axis and present small diameters (