Wafer scale characterization of bulk AlN substrates and homoepitaxial growth

Abstract

Aluminum nitride (AlN) is well suited to address evolving needs of future power systems for increased density, efficiency, and voltage handling. Eventual commercialization of power devices requires maturation of substrate technology to ensure fabrication of consistent devices impacted by the spatial distribution of defects and substrate inhomogeneities. Commercial 50 mm AlN substrates grown by physical vapor transport were characterized as received and after 1 μm of homoepitaxial growth by metalorganic chemical vapor deposition. Optical profilometry determined macro-roughness to be sub-nm while atomic force microscopy reveals consistent step edges and 0.1 nm micro-roughness, despite observation of sporadic 100–400 nm wide, ∼1 nm deep depressions. Sub-bandgap photoluminescence reveals broad spectroscopic signals, which slowly vary spatially across the wafer, corresponding with faint visible coloration. Raman spectroscopy indicates uniform, high crystal quality but shows increased background scattering associated with micrometer-scale features visible in optical microscopy. High-resolution x-ray diffraction validates the crystal quality with median (0002) full-width at half maximum of 17.6 arc sec and x-ray topography identified varying regions with dislocation density from ∼102 cm−2 to ∼mid-105 cm−2. Electron microscopy revealed nano-pipes that may be the origin of the nanoscale depressions and that at least some bright optical features are aluminum platelets with coherent atomic boundaries. After epitaxial growth, surface morphology retained smooth macro- and micro-roughness with clear step edges, demonstrating viability of device-relevant epitaxial layer thicknesses for power devices. Three wafers from one boule were characterized, indicating consistent wafer-to-wafer characteristics, critical for future device commercialization.