Local Strain and Crystalline Defects in GaN/AlGaN/GaN(0001) Heterostructures Induced by Compositionally Graded AlGaN Buried Layers

Abstract

Plastic strain relaxation in epitaxial layers is one of the crucial factors that limits the performance of III-nitride-based heterostructures. In this work, we report on strain relaxation and crystalline defects in heterostructures consisting of compositionally graded AlGaN epitaxial layers tensile-strained between a GaN-buffer and a GaN-cap. We demonstrate the effects of Al concentration and the shape of the concentration-depth profile in the buried graded layers on the accumulated elastic strain energy and how this influences the critical thickness for crack generation or fracture. It is shown that this fracture leads to the formation of partially relaxed regions with their degree of strain relaxation directly related to the density of cracks. Nevertheless, even though the in-plane coherency between the AlGaN layer and the GaN-buffer is broken, the in-plane coherency within the AlGaN layer is preserved for all regions. Furthermore, the tensile strain released in the buried graded AlGaN layers is consistent with compressive strain induced in the GaN-cap layers. Finally, the localized stress and the densities of threading dislocations are correlated with the features of the resulting fractured heterostructures. These results are important toward the control of complex plastic strain relaxation and further facilitate the growth of high quality compositionally graded AlGaN-based devices.

Low-Temperature and Ammonia-Free Epitaxy of the GaN/AlGaN/GaN Heterostructure

“…The strained in-plane and out-of-plane unit cell parameters from AlGaN together with those belonging to GaN were also extracted from the RSMs. From these, the values of the in-plane and out-of-plane strain (ε ∥ and ε ⊥ ) of AlGaN were determined according to the following equations 22 (1)…”

“…parameters from AlGaN, together with those belonging to GaN, were also extracted from the RSMs. From these, values of in-and out-of-plane strain (ε‖ and ε⟂) of AlGaN were determined according to following equations: 22 𝜀 ∥ = 𝑎 𝐴𝑙𝐺𝑎𝑁 −𝑎 0 (𝑥) 𝑎 0 (𝑥)…”

Defect-rich GaN interlayer facilitating the annihilation of threading dislocations in polar GaN crystals grown on (0001)-oriented sapphire substrates

“…[20][21][22] In a recent publication, Stanchu et al described the positive effect of compositionally graded AlGaN buried layers in GaN/AlGaN/GaN stacks on the lattice strain relaxation and on the annihilation of TDs. 23 Among the nowadays actively developed techniques which support bending and bunching of the TDs in GaN, the selective area growth (SAG), [24][25][26][27][28] the epitaxial lateral overgrowth (ELO), [29][30][31][32][33] and the facet-controlled epitaxial lateral overgrowth (FACELO) [34][35][36] are to be mentioned.…”

Low-temperature and ammonia-free epitaxy of GaN/AlGaN/GaN heterostructure

“…parameters from AlGaN, together with those belonging to GaN, were also extracted from the RSMs. From these, values of in-and out-of-plane strain (ε‖ and ε⟂) of AlGaN were determined according to following equations: 22 ∥ = − 0 ( )…”