(Invited) Synchrotron White-Beam X-Ray Topography Analysis of the Defect Structure of HVPE-GaN Substrates

For demanding GaN-based applications like laser diodes or high mobility transistors there is a need for substrate material with low defect densities, since threading dislocations act as centers of non-radiative recombination, increasing the leakage current, reducing the room temperature mobility and thereby limiting the efficiency, performance and lifetime of devices. Hydride vapor-phase epitaxy (HVPE) is the most common technique for the growth of large area freestanding GaN wafers with diameters up to 50 mm and a threading dislocation density below 10⁷ cm⁻². An alternative technique with great potential for the growth of GaN bulk crystals is the ammonothermal growth method. For this method GaN threading dislocation densities as low as 5 × 10³ cm⁻²were reported [1]. We are interested examining significant differences in the defect structure of the GaN substrates which can be observed in samples obtained from different manufacturers or as a function of the growth technique.

In this work we present a detailed analysis of defect structures of freestanding GaN (0001) crystals grown by HVPE and the ammonothermal method from different vendors. Synchrotron white-beam X-ray topography (SWXRT), high resolution X-ray diffraction (HRXRD) and microphotoluminescence (µPL) were used for this combined study. SWXRT in two principal X-ray topography geometries was used in the course of this analysis, namely the large-area back-reflection mode and section transmission mode. HRXRD reciprocal space mappings and in-plane measurements were used for mosaicity analyses. The combination of these methods allows a detailed statement of the nature, density and distribution of typical defects in GaN bulk crystals with a large field of view. As an example, Fig. 1 shows a large-area back-reflection topograph (0006 reflection) of a HVPE GaN substrate. The topograph shows dislocation free areas, surrounded by rather sharp lines, indicating good material quality with low defect density. These features are interpreted as dislocation cell walls, suggesting the formation of a cellular defect network within this GaN sample, analogous to what was commonly observed in LEC GaAs [2].

With the combination of the results from SWXRT, HRXRD and µPL a consistent model for different defect structures of the freestanding GaN samples grown by HVPE and the ammonothermal method is discussed.

References

1. P.J. R. Doradzinski, R. Dwilinski, J. Garczynski, L.P. Sierzputowski, Y. Kanbara, in Technology of Gallium Nitride Crystal Growth, edited by D.Ehrentraut, E. Meissner, and M. Bockowski, (Springer-Verlag, Heidelberg, ISBN 978-3-642-04828-9), 137-158, (2010).

2. P.J. McNally, P.A.F. Herbert, T. Tuomi, M. Karilahti, and J.A. Higgins, J. Appl. Phys. (1996) 79, 8294.