Very low dislocation density AlN substrates for device applications

Native Aluminum Nitride (AlN) single-crystal substrates with ultra-low dislocation density are very promising for use in III-nitride epitaxial growth required for ultraviolet (UV) electro-optical applications and high power radio frequency (RF) devices. They offer a better lattice and thermal expansion match to AlGaN alloys, especially those with high Al content, than foreign substrates such as SiC or sapphire. An additional advantage of bulk substrates is the possibility of slicing and preparing surfaces with the desired orientation, such as non-polar and pre-determined, specific misorientations, which will permit the fabrication of devices with specific, special properties. In this paper we present chemical and electrical characterization of the AlN material. Secondary Ion Mass Spectroscopy (SIMS) measurements show that oxygen is the main impurity, with concentrations in the order of mid 1018 cm-3. The electrical resistivity of the AlN was measured, giving a lower limit of 1012Ω-cm at room temperature. The prepared surface of substrates with different orientations, as well as of homo-epitaxial and hetero-epitaxial layers of AlGaN with different Al:Ga ratios were measured by Atomic Force Microscopy. The observation of atomic steps in the bare substrates and step flow in the epilayers are an indication of the good surface preparation. The crystalline quality of the epilayers was assessed by measuring the full width at half maximum (FWHM) of both symmetric and asymmetric X-ray rocking curves.