Christian Zollner has a featured article on Applied Physics Letters

Reduced dislocation density and residual tension in AlN grown on SiC by metalorganic chemical vapor deposition 

Appl. Phys. Lett. 115, 161101 (2019); https://doi.org/10.1063/1.5123623
Crack-free AlN films with threading dislocation density (TDD) below 109 cm−2 are needed for deep-UV optoelectronics. This is typically achieved using pulsed lateral overgrowth or very thick buffer layers (>10 μm), a costly and time-consuming approach. A method for conventional metalorganic chemical vapor deposition growth of AlN/SiC films below 3 μm with greatly improved quality is presented. Focusing on substrate pretreatment before growth, we reduce average film stress from 0.9 GPa (tension) to −1.1 GPa (compression) and eliminate cracking. Next, with optimized growth conditions during initial deposition, AlN films with x-ray rocking curve widths of 123 arc-sec (00020002) and 304 arc-sec (2021202¯1) are developed, and TDD is confirmed via plan view transmission electron microscopy (TEM) to be 2 ×× 108 cm−2. Film stress measurements including x-ray 2θ-ω, reciprocal space mapping, and curvature depict compressively stressed growth of AlN on 4H-SiC due to lattice mismatch. The thermal expansion coefficient mismatch between AlN and SiC is measured to be Δα=αAlNαSiC=1.13×106°C1Δα=αAlN−αSiC=1.13×10−6 °C−1 and is found to be constant between room temperature and 1400 °C. TEM confirms the existence of dense misfit dislocation (MD) networks consistent with MD formation near SiC step edges and low MD density regions attributed to nearly coherent AlN growth on SiC terraces. These low-TDD, crack-free AlN/SiC buffers provide a platform for deep-UV optoelectronics and ultrawide bandgap electronics.
This work was supported by the National Science Foundation Graduate Research Fellowship Program (Grant No. 1650114). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Extensive use was made of shared facilities of the UCSB Materials Research Laboratory, an NSF MRSEC (No. DMR 1121053), and member of the Materials Research Facilities Network, as well as the UCSB Nanofab. Special thanks to S. Keller and Y. Li for insightful conversations and to C. A. Taylor at KSA for technical support.

 

FIG. 1. Atomic force microscopy (AFM) images of commercially sourced SiC substrates: (a) Supplier A (mechanically polished), (b) Supplier B (chemomechanically polished), and (c) Supplier B after NH3 pretreatment. AFM after 75 nm (d) and 2.95 μm (e) of AlN growth by MOCVD shows the initiation layer and smoothing layer morphologies. Scale bars equal 1 μm, and data color scales are 5 nm (a), 300 pm [(b) and (c)], 500 pm (d), and 1 nm (e).
FIG. 1. Atomic force microscopy (AFM) images of commercially sourced SiC substrates: (a) Supplier A (mechanically polished), (b) Supplier B (chemomechanically polished), and (c) Supplier B after NH3 pretreatment. AFM after 75 nm (d) and 2.95 μm (e) of AlN growth by MOCVD shows the initiation layer and smoothing layer morphologies. Scale bars equal 1 μm, and data color scales are 5 nm (a), 300 pm [(b) and (c)], 500 pm (d), and 1 nm (e).

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