(Invited) Development of 3-inch AlN Single Crystal Substrates
Authors: Bondokov Robert T , Hogan Kasey , Norbury Griffin Q. , Mark Justin , Branagan Sean P. , Ishigami Naoki , Grandusky James , Chen Jianfeng (Jeff) 
Published: November 2022

Aluminum nitride (AlN) holds great promise as a substrate for electronic devices such as high-power switches, high power density, high frequency and high operating temperature devices for RF. This promise is largely due to its superior properties, for instance its ultra-wide bandgap (UWBG) of 6.2 eV, high thermal conductivity (> 290 W/m*K), high melting point (> 2800°C), relatively good chemical resistance, similarity of its lattice structure and parameters to that of Gallium nitride (GaN), high hardness value of about 12 GPa (at 1 N using Vickers indenter), etc. Presently AlN substrates are successfully used for fabrication of ultraviolet (UV) light emitting diodes (LEDs) operating at wavelengths shorter than 280 nm, i.e., in the UVC region of the spectrum. The AlN-based UVC LEDs are employed in water disinfection, surface cleaning, air purification, and environmental sensing. Several factors including substrate availability, size, production capacity, quality, and cost influence the commercialization of the aluminum nitride both as an UWBG and as an opto-electronic substrate. In this work we present the present and future outlook for the AlN substrates made using Physical Vapor Transport (PVT) growth technique.
The PVT crystal growth method of aluminum nitride is carried out in a Tungsten crucible that accommodate the AlN source material as well as the (0001) oriented AlN seed. The thermal gradient is provided by an rf-heating plus appropriate shielding and insulation. In this setup, the sublimed source material is transported to the Al-polar face of the seed where it incorporates into a single crystal. The AlN crystals are then oriented, sliced, and polished. The resulting 2-inch substrates are tested to meet certain specification needed for further epitaxial growth and LED processing. The two-inch AlN wafer specification as well as number of physical properties and purity were previously reported [1].
In addition to the 2-inch AlN crystal growth there is an ongoing effort to increase the substrate diameter. Large-diameter (e.g. 100 mm) AlN substrates are a practical requirement benefiting high power electronic devices. Again, PVT growth method is used to expand the crystal size and diameter. However, several challenges have yet to be resolved en route to 100 mm diameter crystals. These challenges are associated with increased thermally induced stress, the need for better control of the axial/radial thermal gradients, vendor ability to provide materials, maintaining crystal quality during expansion, etc.
Our PVT growth technique demonstrates high crystal growth yield which allows to contain relatively low production cost. Furthermore, we have established a high-capacity process capable to produce many thousands of 2-inch AlN substrates per year. Our current crystal growth process also demonstrated greater than 3-inch diameter single-crystal AlN substrates; properties of such wafers will be reported.

Robert T Bondokov et al 2021 ECS Trans.
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