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

Abstract

The defect structure of HVPE-GaN crystals is examined using synchrotron white-beam X-ray topography (SWXRT) and topography results are interpreted and discussed in comparison to reciprocal lattice point broadening from high resolution X-ray diffraction (HRXRD) measurements. Two as-received commercial HVPE-GaN wafers from two different vendors and one HVPE-GaN which was grown on an ammonothermal GaN-seed are investigated in this study. To our knowledge SWXRT large area back-reflection analysis of HVPE-GaN grown on an ammonothermal GaN seed has been performed for the first time. From large-area topography the formation of a cellular defect network is identified for the commercial HVPE-GaN. Large differences in the crystal lattice misorientation deformation (mosaicity) are determined for the different samples by transmission section topography. For the HVPEGaN grown on an ammonothermal GaN-seed a very low defect density was ascertained. From the contrasts of the topography threading screw-type dislocations and threading mixed-type dislocations were identified. The X-ray topography analysis shows clearly and for the first time that the nature of the defect structure and the low density of ammonothermal GaN seeds can be transferred by HVPE growth of GaN. For demanding GaN-based (opto-)electronic applications such as laser diodes or transistors for high power electronics there is a need for 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 so limiting the efficiency, performance and lifetime of devices. Freestanding GaN with low defect density is the substrate material of choice for the realization of such GaN-based device structures with superior properties. Different approaches for crystal growth of bulk GaN like ammonothermal growth, hydride vapor-phase epitaxy (HVPE) and high-and low-pressure solution growth were followed in recent years.1 The ammonothermal growth and HVPE seem to be the most promising methods to produce GaN substrates in sufficient number, size and material quality and substrates prepared with these growth methods have become available commercially in recent years. In the ammonothermal growth method, GaN crystalizes from a solution of Ga in supercritical ammonia with the addition of a mineralizer. For seed material, the ammonothermal growth method uses native GaN crystals. Benefits of the ammonothermal growth method are very low dislocation densities as low as 5 × 10 3 cm −2 and large curvature radii of the crystal planes (>100 m).2 Drawbacks of GaN ammonothermal growth method are the incorporation of impurities and the low growth rate (0.1 mm/day), even when the method can potentially produce hundreds of crystals in a single batch. In comparison, in HVPE the GaN crystallization takes place from the vapor phase by the reaction of ammonia with gallium chloride at temperatures of about 1050• C. HVPE uses GaN seed layers (templates) deposited e.g. by metalorganic v...

Evaluation of lattice curvature and crystalline homogeneity for 2-inch GaN homo-epitaxial layer

“…RSM measurements of the GaN homo-epitaxial layer were generally performed to determine the lattice parameter, homogeneity and the crystallinity along the off-specular direction. 35,36 The calculated extinction distance was 1.3 µm at the 1122 Bragg reflection based on dynamical X-ray diffraction theory. 37,38 Figures 4(c) and (d) show the HK mesh scan near the 1122 Bragg reflection.…”

Structural Analysis of Low Defect Ammonothermally Grown GaN Wafers by Borrmann Effect X-ray Topography

“…One can say that this homogeneity concerns the high dislocation density of the HVPE material grown on foreign seeds. It was shown that in commercially available HVPE-GaN the dislocation rearrangement into cell networks can take place under external or internal stress in the course of plastic relaxation [ 43 ]. The Borrmann effect can therefore hardly be observed in HVPE-GaN grown on foreign substrates due to the low material quality; it is nearly extinct.…”

Mapping of a Lattice-Plane Tilting in a GaN Wafer Using Energy-Resolved X-Ray Diffraction Topography

“…By magnifying the image through a lens and using a phosphor CCD detector, we expect to be able to achieve high image resolution; however, we may not see the overall feature on the wafer scale. The misorientation deformation (mosaicity) and dislocations in GaN are shown using synchrotron white-beam x-ray topography (SWXRT) on a GaN substrate using back reflection Laue geometry [29]. The report does not show the lattice tilting on a wafer scale even though they adopted section transmission geometry, which means that we improve the SWXRT technique for the observation of tilting on the wafer scale in the current work.…”