Distinguishing negatively-charged and highly conductive dislocations in gallium nitride using scanning Kelvin probe and
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Distinguishing negatively-charged and highly conductive dislocations in gallium nitride using scanning Kelvin probe and conductive atomic force microscopy Blake S. Simpkins and Edward T. Yu Department of Electrical and Computer Engineering and Program in Materials Science University of California at San Diego La Jolla, CA 92093-0407 Patrick Waltereit and James S. Speck Materials Department, University of California, Santa Barbara, Santa Barbara, California ABSTRACT Scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (CAFM) are used to image surfaces of GaN grown by molecular beam epitaxy (MBE). Numerical simulations are used to assist in the interpretation of SKPM images. Detailed analysis of the same area using both techniques allows imaging of surface potential variations arising from the presence of negatively charged dislocations and dislocation-related current leakage paths. Correlations between the charge state of dislocations, conductivity of leakage current paths, and possibly dislocation type can thereby be established. Approximately 25% of the leakage paths appear to be spatially correlated with negatively charged dislocation features. This is approximately the level of correlation expected due to spatial overlap of randomly distributed, distinct features of the size observed, suggesting that the negatively charged dislocations are distinct from those responsible for localized leakage paths found in GaN. The effects of charged dislocation networks on the local potential profile is modeled and discussed. INTRODUCTION Group III-nitride semiconductors have been the subject of intensive research in recent years for optoelectronic and high power, high-speed electronic devices [1,2]. Although progress has been made in improving material quality and device performance, substantial challenges remain. In particular, the lack of readily available homoepitaxial substrates necessitates growth on either sapphire or SiC, both of which lead to degradation in material quality predominantly through the presence of high dislocation densities. These dislocations in GaN are known to degrade device performance through carrier scattering [3], non-radiative recombination [4], and increased reverse-bias leakage current [5,6]; however, the correlation among dislocation type, electronic properties such as charge and conductivity, remain subjects of active investigation. Negatively charged dislocation features have previously been imaged using scanning Kelvin probe microscopy (SKPM) on both AlGaN/GaN heterostructures grown by MBE [7] and GaN grown by hydride vapor phase epitaxy (HVPE) [8]. These charged features have also been imaged using scanning capacitance microscopy (SCM) on MOCVD GaN [9] and MOCVD AlGaN/GaN HFETs on SiC [10]; however, the precise crystallographic nature of these charged dislocations was not determined. Mobility degradation in GaN films has been analyzed by modeling coulombic carrier scattering events at negatively charged dislocations [3] while pure screw dislocations have be
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