An electrochemical surface treatment has been developed that decreases the reverse-bias leakage current in Schottky diodes fabricated on GaN grown by molecular-beam epitaxy (MBE). This treatment suppresses current flow through localized leakage paths present in MBE-grown GaN, while leaving other diode characteristics, such as the Schottky barrier height, largely unaffected. A reduction in leakage current of three orders of magnitude was observed for Schottky diodes fabricated on the modified surface compared to diodes fabricated on the unmodified surface for reverse-bias voltages as large as -20 V. In addition to suppressing reverse-bias leakage, the surface treatment was found to improve substantially the ideality factor of the modified surface diodes compared to that of unmodified surface diodes, suggesting that such a surface modification process could be useful for a variety of GaN-based electronic devices. (C) 2003 American Institute of Physics.
Scanning probe techniques including scanning capacitance microscopy, scanning capacitance spectroscopy, scanning Kelvin probe force microscopy, and atomic force microscopy have been used to assess structure and local electronic properties of Ga-face and N-face p-type GaN and of inversion domain boundaries in p-type GaN. Epitaxial layers of p-type GaN were grown by molecular-beam epitaxy, and by adjustment of the Ga:N flux ratio samples containing both Ga-face and N-face material were obtained. Under identical growth conditions, net incorporation of electrically active Mg acceptors was found to be more efficient for material with Ga-face polarity. Only a very small dependence of surface potential on polarity was observed, in contrast to results reported for n-type GaN, in which a substantial dependence of Schottky barrier height on polarity has been found. In addition, elevated net concentrations of ionized Mg acceptors were observed in Ga-face regions in the immediate vicinity of some, but not all, inversion domain boundaries, consistent with theoretical suggestions that incorporation of high concentrations of Mg within an inversion domain boundary can lead to increased concentrations of Mg acceptors near the inversion domain boundary. (c) 2006 American Vacuum Society.
Scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (C-AFM) have been used to image surfaces of GaN grown by molecular beam epitaxy. Detailed analysis of the same area using both techniques allowed imaging and comparison of both surface potential variations arising from the presence of negatively charged threading dislocations and localized current leakage paths associated with dislocations. Correlations between the charge state of dislocations, conductivity of current leakage paths, and dislocation type were thereby established. Analysis of correlated SKPM and C-AFM images revealed a density of negatively charged features of similar to3 x 10(8) cm(-2) and a localized current leakage path density of similar to3 x 10(7) cm(-2), With approximately 25% of the leakage paths spatially correlated with negatively charged dislocation features. Based on correlated topography and previous studies quantifying the densities of edge, screw, and mixed dislocations, our results suggested that dislocations having an edge component behave as though negatively charged while pure screw dislocations are solely responsible for the observed leakage paths and are uncharged. (C) 2003 American Institute of Physics.
Dislocation-related conduction paths in n-type GaN grown by molecular-beam epitaxy and a mechanism for local suppression of current flow along these paths are analyzed using conductive atomic force microscopy, scanning Auger spectroscopy, and macroscopic current-voltage measurements. Application of an electric field at the GaN surface in an ambient atmospheric environment is shown to lead to local formation of gallium oxide in the immediate vicinity of the conduction paths, resulting in the strong suppression of subsequent current flow. Current-voltage measurements for Schottky diodes in which local conduction paths have been suppressed in this manner exhibit reverse-bias leakage currents reduced by two to four orders of magnitude compared to those in Schottky diodes not subjected to any surface modification process. These results demonstrate that the dislocation-related current leakage paths are the dominant source of leakage current in Schottky contacts to n-type GaN grown by molecular-beam epitaxy, and elucidate the nature of a microscopic process for their suppression. (C) 2003 American Institute of Physics.
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