Three-Dimensional $\mathrm{GaN}\mathrm{\text{−}}{\mathrm{Ga}}_2{\mathrm{O}}_3$ Core Shell Structure Revealed by X-Ray Diffraction Microscopy

In combination of direct phase retrieval of coherent x-ray diffraction patterns with a novel tomographic reconstruction algorithm, we, for the first time, carried out quantitative 3D imaging of a heat-treated GaN particle with each voxel corresponding to $17\times 17\times 17{\mathrm{nm}}^3$. We observed the platelet structure of GaN and the formation of small islands on the surface of the platelets, and successfully captured the internal $\mathrm{GaN}\mathrm{\text{−}}{\mathrm{Ga}}_2{\mathrm{O}}_3$ core shell structure in three dimensions. This work opens the door for nondestructive and quantitative imaging of 3D morphology and 3D internal structure of a wide range of materials at the nanometer scale resolution that are amorphous or possess only short-range atomic organization.

Figure 1

Schematic layout of the 3D x-ray diffraction microscope, combining ab initio phase recovery with tomographic image reconstruction.

Figure 2

Isosurface rendering of a reconstructed 3D GaN quantum dot particle, showing (a) the front view, (b) the back, and (c) the side view. (d) SEM image of a different GaN nanoparticle. The GaN quantum dot particles were heat-treated under ${\mathrm{N}}_2$ at $900°\mathrm{C}$ for 24 h. The size of the platelet indicated by a blue arrow in (a) is $\sim 260\times 120\times 50{\mathrm{nm}}^3$. The formation of small islands on the surface of the particles is clearly visible (some of them are labeled with green arrows).

Figure 3

3D internal structure of the GaN quantum dot particle, where the $z$ axis is perpendicular to the front view [Fig. 2a] and points outward. The 3D $\mathrm{GaN}\mathrm{\text{−}}{\mathrm{Ga}}_2{\mathrm{O}}_3$ core shell structure is clearly visible where the low electron density corresponds to $\beta \mathrm{\text{−}}{\mathrm{Ga}}_2{\mathrm{O}}_3$ and the high density corresponds to the GaN cores.