Impact of Threading Dislocations in GaN Power Switching Devices
Authors: Setera Brett , Christou Aristos 
Published: July 2021

Thick epitaxial (3-15 microns) GaN power switching devices are known to contain a high density of crystal defects, and especially threading dislocations in the epitaxial layer. The impact of these defects on device performance and reliability is a topic of ongoing research. This paper presents state-of-the-art methods for analyzing threading dislocation density, growth, and quantification of their impact on leakage current, blocking voltage, and peak E field. The effects of substrate type, epitaxial thickness, and processing techniques on dislocation density have been investigated to provide a roadmap for design and performance optimization of switching two terminal devices. Experimental and simulated data are used in conjunction to model dislocation impact on electrical performance characteristics and to identify techniques to minimize defect density.
The experimental results have been achieved for 3 and 10 micron thick epitaxy as well as initial results for 15 micron epitaxy. The substrate was bulk GaN, and the epitaxy was grown by HPVPE by Kyma Corporation. The DMOS test structure investigated showed state of the art current voltage characteristics for a 10 micron n- epitaxy. Significant deviations were observed between the measured doping profile and the simulated profile, using a critical field of 3 MV/cm. For the DMOS device with epi doping of 8 × 1015 𝑐𝑚−3 and epi thickness of 8 𝜇𝑚. The device breaks down at approximately 2020 𝑉. At 2000 𝑉, the peak field is roughly 3.11 𝑀𝑉/𝑐𝑚. For a device with epi doping of 8 × 1015 𝑐𝑚−3 and epi thickness of 15 𝜇𝑚. The device breaks down at approximately 3020 𝑉. At 3000 𝑉, the peak field is roughly 3.14 𝑀𝑉/𝑐𝑚. The observed trend is significant since it shows that by increasing the epi thickness, also results in an increase in the peak field. For thin GaN epi of 3 microns, a peak breakdown field of 2.77 MV/cm was measured. These three date points will allow one to determine a relationship between peak critical field and epi thickness, which is a major requirement for designing vertical GaN high voltage switches.
We report images from transmission x-ray topographs and TEM investigations as well as the correlation with critical breakdown field measurements. The transmission x-ray topograph shows that the GaN is characterized by a nearly uniform distribution of strain centers which are likely bundles of threading screw and edge dislocations. Strain free centers are then regions of relatively low dislocation densities. The threading dislocations are part of dislocation loops which are present in the GaN substrate and migrate through the interface. The cross-sectional TEM image shows partial dislocation loops through the thickness of the GaN sample.
Figure: X-Ray Topography of GaN/GaN showing a nearly uniform distribution of strain centers. The Grazing incidence topograph is similar to the transmission image, the grazing image also shows the nearly uniform distribution of strain centers. The X ray image shows that individual threading dislocations are not resolved showing a relatively low dislocation density in those regions.

Figure 1