PC-Mon-P7 - Defect analysis comparing polarized stress imaging and X-ray topography methods on gallium nitride epitaxial layers on silicon and sapphire
2. Physics and characterizationTeodóra Nagyné Kovács2, Zsolt O. Kovács2, Zsolt Kovács2, Roland Weingärtner1, Sven Besendörfer1
1 Fraunhofer Institute for Integrated Systems and Device Technology IISB, Erlangen, Germany
2 Semilab Semiconductor Physics Laboratory Co. Ltd., Budapest, Hungary
Abstract text
Gallium nitride (GaN) is a wide bandgap semiconductor for power electronics and high frequency applications. Today, the most industrially relevant types of GaN devices are high electron mobility transistors (HEMTs) fabricated on foreign substrates such as (111)-oriented silicon (Si), silicon carbide or sapphire. Lattice and thermal mismatch between the substrate and the epitaxial layers induce stress in both the substrate and the GaN layer, resulting in wafer bowing and formation of macroscopic defects and dislocations. The visualization of stress fields and macroscopic defects on full wafer scale is important for the optimization of epitaxial growth conditions and the design of potential strain relief layers as well as for the understanding of low device yield.
In this work, we present a comparison of polarized stress imaging (PSI) and monochromatic X-ray topography (XRT) for visualizing stress fields and macroscopic defects on full wafer scale of epitaxially grown GaN on Si(111) and sapphire substrates. While XRT allows for a relatively slow measurement of both the substrate and the epitaxial layer individually, PSI is a fast method performed in transmission and always contains integral information. Since both methods are sensitive to the same features but use different imaging methods this comparison helps gaining. a more complete view of wafer defects like stress fields around dislocations, slip lines at the interface between substrate and epitaxial layers or scratches. A detailed analysis of the results from both measurement techniques highlights their respective strengths and limitations in understanding the stress-related properties of the material.
We provide an overview about different types of crystal defects that can be seen in XRT and show how the corresponding stress images measured with PSI appear. Our analysis focuses on the conclusions that can be drawn from the similarities and differences between the images. Particular attention is given to the comparative analysis of the individual XRT measurements of substrate and epi-layer and the PSI image. The results will contribute to the development of industrially relevant high-throughput wafer testing, either as a quality control procedure or to sort out wafers unsuitable for further device processing.