Kenneth E. Morgan

Defect Content Evaluation in Single-Crystal AlN Wafers

ABSTRACT Aluminum nitride (AlN) offers exceptional properties necessary to explore the development of large area substrates for nitride based electronics and photonics. Recent studies on AlN bulk growth using the sublimation-recondensation method developed at Crystal IS demonstrated high-quality crystals with low dislocation density and crystallographic uniformity. The diameter enlargement of these AlN boules is often associated with extensive defect generation. The goal of this study is to evaluate the origin and distribution of growth defects in AlN bulk crystals. AlN crystals were grown using the sublimation-recondensation technique and then they were sliced into wafers. The defect evaluation in this study was performed using x-ray topography, differential image contrast and polarized-light optical microscopy, atomic force microscopy (AFM) and etch pit pattern delineation. Special attention was paid to crack development and propagation, grain boundary distribution, micro-scale inhomogenities as well as to the origin and density of dislocations. The major cause of growth defect appears to be non-linearity of both axial and radial temperature gradients. Growth optimization results in lower defect density and improved crystallinity of the AlN crystals.

Fabrication and Characterization of 2-inch diameter AlN Single-Crystal Wafers cut From Bulk Crystals

Aluminum nitride (AlN) boules larger than 2 inches in diameter were grown by the sublimation-recondensation technique. X-ray Laue diffraction was used to characterize the crystallinity and orientation of the boules, and 2” dia. substrates were sliced with typical thickness of ∼500 μm. The wafers were then polished in order to meet the common standards for wafer thickness and flatness. The Al-terminated surface was finished with a proprietary chemical-mechanical process and showed RMS roughness of 0.5 nm or less as measured by atomic force microscopy (5×5 μm area). Currently, the substrates have some polycrystalline regions that are highly textured but about 85% of the total area is monocrystalline. The dislocation density in the crystalline regions of the substrate was measured by preferential chemical etching and then determining the resulting etch pit density (EPD). The etching technique involves potassium hydroxide and has been qualified through correlation with x-ray topography measurements of the dislocations. Measured EPD varied from 250 cm −2 to 3×10 4 cm −2 . Other structural defects such as low angle grain boundaries, prismatic slip bands, inversion domains, have also been observed. The rare appearance of these defects will be discussed even though their role in the epitaxial growth of GaN and AlGaN is yet to be clarified.

Growth of Self-Seeded Aluminum Nitride by Sublimation-Recondensation and Substrate Preparation

Bulk aluminum nitride boules have been grown at driving rates of 0.9mm/h by the self-seeded sublimation-recondensation technique. Up to 15mm diameter substrates cut from those boules present large single crystal grains that have been analyzed using different techniques. X-ray double crystal diffraction shows a full-width-at-half-maximum of around 100 arcsec and X-ray topography reveals extensive areas with a density of dislocations less than 10 4 cm −2 . These substrates have been prepared by chemical mechanical polishing techniques to obtain a surface roughness of 1.4-1.6nm.