Microstructural and compositional analyses of GaN‐based nanostructures

Abstract

Composition and microstructure of GaN-based island structures and distributed Bragg reflectors (DBRs) were investigated with transmission electron microscopy (TEM). We analysed free-standing InGaN islands and islands capped with GaN. Growth of the islands performed by molecular beam epitaxy (MBE) and metal organic vapour phase epitaxy (MOVPE) resulted in different microstructures. The islands grown by MBE were plastically relaxed. Cap layer deposition resulted in a rapid dissolution of the islands already at early stages of cap layer growth. These findings are confirmed by grazingincidence X-ray diffraction (GIXRD). In contrast, the islands grown by MOVPE relax only elastically.Strain state analysis (SSA) revealed that the indium concentration increases towards the tips of the islands. For an application as quantum dots, the islands must be embedded into DBRs. Structure and composition of Al y Ga 1-y N/GaN Bragg reflectors on top of an AlGaN buffer layer and In x Al 1-x N/ GaN Bragg reflectors on top of a GaN buffer layer were investigated. Specifically, structural defects such as threading dislocations (TDs) and inversion domains (IDs) were studied, and we investigated thicknesses, interfaces and interface roughnesses of the layers. As the peak reflectivities of the investigated DBRs do not reach the theoretical predictions, possible reasons are discussed.

InGaN quantum dot growth in the limits of Stranski–Krastanov and spinodal decomposition

“…The average diameter of the nano islands is 8 nm with a height of 3.8 nm. The high resolution TEM pictures obtained from this sample showed coherent growth of quantum dots without misfit dislocations [38]. This result confirm that an island growth based on a StranskiKrastanov like surface transformation is taking place.…”

“…Only steps of mono layer height are visible on the surface. Additional transmission electron microscopy (TEM) investigations revealed a layer thickness of 4-5 unit cells and an In concentration of 22 AE 3% [38]. After 35 s first occurrence of nano islands is visible in Fig.…”

Characterization of InGaN/GaN quantum well growth using monochromated valence electron energy loss spectroscopy

“…24 The QDs are then grown strain-free by relaxing strain in the ambient structure, e.g., plastically, by the introduction of misfit dislocations at the interface between wetting layer and dot. 25,26 Here, a pseudomorphic growth was observed, hence no misfit dislocations are present to reduce the strain under the QDs. Typically, this is only possible by increasing the strain energy of the underlying substrate, 27 resulting in a redshift of E p of the underlying substrate.…”

Growth and characterization of nitride‐based distributed Bragg reflectors

“…A residual absorption is present in this kind of structures, which reduce the maximum reflectivity that can be achieved. Investigation by transmission electron microscopy reveal the presence of inversion domains developing at the interface from sapphire to the AlGaN template layer 17. These inversion domains are probably optically active in the visible spectral region, giving a possible explanation for the reduced peak reflectivity of the DBRs.…”