Structural and chemical analyses of InGaN/GaN heterostructures to understand their growth mechanism by plasma-assisted molecular beam epitaxy
We have used the Nanotem instruments to investigate the growth mechanism by plasma assisted molecular beam epitaxy of axially heterostructured InGaN/GaN nanowires (NWs) as a function of the growth flux conditions. The InGaN heterostructure morphology critically depends on the In/Ga flux ratio affecting the local V/III ratio at the NW growth front. Locally N-rich conditions are associated with tapered island-like morphologies, while metal-rich conditions, leading to the formation of a stable Indium adsorbed layer at the NW growth front, promote the growth of heterostructures with a disk-like shape. Based on experimental results and theoretical predictions, we demonstrate that this indium ad-layer acts as a surfactant inducing a modification of the InGaN heterostructure growth mode. The impact of flux conditions and strain relaxation on the Indium incorporation are also addressed. The resulting insertions present abrupt interfaces and a homogeneous In distribution for In contents up to 40%.
The image shows EDX mappings and HAADF-STEM micrographs/EDX line scans along the white dashed arrow collected with the FEI Titan Themis. The micrographs illustrate the In and Ga compositional distribution within InGaN heterostructures grown with different In/Ga beam equivalent pressure (BEP): (a,b) cross-sectional NW portion embedded in a TEM lamella obtained using the dual beam FEI Scios focused ion beam (FIB) system, In/Ga BEP= 1.5; (c,d) tapered section grown at In/Ga BEP= 1, [11−20] zone axis; (d,f) In/Ga BEP= 1.5, [11−20] zone axis; (g,h) untapered section grown at In/Ga BEP = 2, [10−10] zone axis. In the axial profiles, full and dashed lines correspond to the measured (core+shell) and corrected (core) profiles, respectively.