Incorporation and effects of impurities in different growth zones within basic ammonothermal GaN

Abstract

The ammonothermal method is one of the most promising candidates for large-scale bulk GaN growth due to its scalability and high crystalline quality. However, emphasis needs to be put on understanding the incorporation and effects of impurities during growth. This article discusses how impurities are incorporated in different growth zones in basic ammonothermal GaN, and how they affect the structural, electrical and optical properties of the grown crystal. The influence of growth time on the impurity incorporation is also studied. We measure the oxygen, silicon, and carbon impurity concentrations using secondary ion mass spectrometry, and measure their effect on the lattice constant by high resolution x-ray diffraction (HR-XRD). We determine the resulting free carrier concentration by spatially resolved Fourier transform infrared spectroscopy and study the optical properties by spatially resolved low-temperature photoluminescence. We find that oxygen is incorporated preferentially in different growth regions and its incorporation efficiency depends on the growth direction. The oxygen concentration varies from 6.3 × 10 20 cm −3 for growth on the {1122} planes to 2.2 × 10 19 cm −3 for growth on the (0001) planes, while silicon and carbon concentration variation is negligible. This results in a large variation in impurity concentration over a small length scale, which causes significant differences in the strain within the boule, as determined by HR-XRD on selected areas. The impurity concentration variation induces large differences in the free carrier concentration, and directly affects the photoluminescence intensity.

Defects in Single Crystalline Ammonothermal Gallium Nitride

“…[26,[34][35][36] Ammonothermal GaN substrates have been used in homoepitaxy [35,37,38] and as substrates for InGaN LDs. [39] Although the crystalline quality of ammonothermal GaN is very high, recent studies have shown that the high concentration of impurities [22,40,41] and native point defects [42][43][44] in the as-grown material have a significant effect on the material properties. The most dominant impurities, oxygen and transition metals, cause absorption in the visible region [23,42] and their non-uniform incorporation in the growing crystal can cause build-up of strain.…”

“…The most dominant impurities, oxygen and transition metals, cause absorption in the visible region [23,42] and their non-uniform incorporation in the growing crystal can cause build-up of strain. [31,40] optical absorption [42] and compensation doping. [43] Additionally, the formation of helical dislocations during heat treatment and epitaxy process has very recently been reported and associated with interaction of dislocations and point defects.…”

“…[42] The incorporation of oxygen into the lattice and resulting generation of free electrons will also contribute to an expansion of the lattice constants. [40,[62][63][64] While this may not negatively impact thin film epitaxial layers grown on these substrates, thick, bulk growth may be challenging as minor lattice constants mismatch may lead to substantial strain buildup in the thick boules resulting in energy release via crack formation. This will be discussed in more detail in Section 3.2.…”

“…The growth is strongly anisotropic which leads to the formation of steady and non steady state growth facets with varying growth rates. [40,73,74] In basic ammonothermal growth the fastest growth rates are generally observed in the N polar -c-direction and in the a-direction, while the slow growth rate in the m-direction leads to the formation of m-plane facets bounding the grown crystal. [40,74,75] The presence of several growth facets during the process leads to the formation of distinct growth regions in the ammonothermally grown crystal.…”

“…[40,73,74] In basic ammonothermal growth the fastest growth rates are generally observed in the N polar -c-direction and in the a-direction, while the slow growth rate in the m-direction leads to the formation of m-plane facets bounding the grown crystal. [40,74,75] The presence of several growth facets during the process leads to the formation of distinct growth regions in the ammonothermally grown crystal. [40,74] It is well known that the incorporation efficiency of impurity species from the growth atmosphere to the growing crystal is strongly dependent on the growth rate and direction.…”