Microcathodoluminescence and electrical properties of GaN epitaxial layers grown on thick freestanding GaN substrates

Abstract

Microcathodoluminescence (MCL) spectra, MCL imaging, electron beam induced current imaging, capacitance-voltage (C-V) profiling, and deep level transient spectrum measurements with optical injection (ODLTS) were performed on 3-μm-thick GaN epilayers prepared by metal organic chemical vapor deposition on top of thick freestanding GaN crystals grown by hydride vapor phase epitaxy. The GaN substrates showed a dislocation density of around 5×106cm−2 near the back surface and ∼106cm−2 near the Ga surface on which the epitaxy was performed. Dislocations are revealed as broad dark spots in MCL spectra taken on (0001) surface and as dark broad lines on the cleaved surface. The structure of the GaN film is more complicated, showing a pattern of arrowheadlike features of alternating dark-bright contrast. MCL spectra taken in the dark and bright regions indicate that the shallow donors in the dark region are more heavily compensated. The carrier lifetime near defects is greatly reduced. ODLTS spectra of the films show the presence of well known hole traps with activation energy of 0.85eV often associated with the yellow luminescence band in GaN.