Abstract

To date, two growth methods, hydride vapor-phase epitaxy (HVPE) and ammonothermal method, have been used to produce commercially available GaN substrates. Some manufacturers have successfully produced GaN substrates up to 4 inches in diameter, n-type and semi-insulating. However, there are significant differences in the defect density of the GaN substrates. For example, the threading dislocation density (TDD) varies in the range of 103 – 106 cm-2, depending on the supplier. The crucial aspect regarding the resulting defect density of the GaN substrates seems to be the seed approach, foreign seed or native seed, rather than the crystal growth method. Only with native GaN seeds is it possible to produce GaN substrates with the low defect density required to realize electronic and optoelectronic devices with high performance, reliability and lifetime.

In this work, the defect structure of commercial GaN substrates grown by the ammonothermal method with a foreign seed approach as well as with a native seed approach and HVPE grown GaN with a foreign seed approach is investigated. In addition, non-commercial HVPE -GaN produced by a native seed approach is being studied. For our studies, we used two different Bragg diffraction imaging techniques, namely laboratory Lang technique topography (L-XRT) and synchrotron rocking curve imaging (RCI).

It is demonstrated that ammonothermally grown GaN substrates (Am-GaN) based on native seed approach show a strong Borrmann effect, the anomalous transmission of X-rays, due to high structural perfection [1]. The same applies to HVPE -GaN grown with native seed. In contrast, Am-GaN and HVPE -GaN substrates produced with a foreign seed approach show a mosaic defect structure with high TDD . Here, the Borrmann effect is nearly fully blocked in this material. RCI is used to quantify the lattice distortion. For the GaN substrates with native seed approach, it is in the range of 2-3 arcsec, while for the GaN substrates with foreign seed approach, it reaches levels of up to 20 arcsec. This study clearly shows that low-defect GaN substrates can only be produced using low defect native seeds.