Strain Driven Shape Evolution of Stacked (In,Ga)N Quantum Disks Embedded in GaN Nanowires

Nanowire axial heterostructures of lattice mismatched systems offer a way to elastically relax the epitaxial strain, preventing the formation of misfit dislocations and other associated defects at larger mismatches and thicknesses compared to their planar counterparts. However, the fabrication of axial multi-quantum wells or disks with a homogeneous size and shape distribution along the whole stack is still an unresolved challenge. Very often the disks are embedded inside the nanowires and exhibit a systematic change in their morphology. Despite this phenomenon being associated to the radial growth of the nanowires, its ubiquity points to a more fundamental origin.

(a) High resolution transmission electron micrograph and corresponding strain map of an (In,Ga)N multi-quantum disk stack embedded in a GaN nanowire. (b) Strain distribution of three selected growth steps of the nanowire as calculated by finite element method. The strain state of the last barrier varies from one growth step to the next due to the change in shape of the QDs. (c) Experimental aspect ratio of the QDs of the nanowire shown in (a) (black squares) and calculated ones considering variable or constant QD volumes (red dots or green triangles, respectively).

In this work we investigate the problem in detail using as a case study the highly mismatched (In,Ga)N/GaN system. A stack of six nominally identical (In,Ga)N quantum disks (QDs) were grown by MBE on GaN nanowires. The samples showed a systematic decrease in the aspect ratio of the QDs along the stacking sequence, High-resolution transmission electron microscopy and geometrical phase analysis reveal a correlation between the strain state of each GaN spacer and the aspect ratio of the corresponding disk above, evidencing the role of the strain in the observed evolution of the shape of the QDs along the stack. 


The experimental observation is explained by assuming that the final shape of each QD is defined by the dynamic equilibrium between its surface tension and misfit stress. If the misfit stress varies in a systematic way from one QD to the next, their equilibrium shape will change correspondingly. Calculations performed by finite element method show that while the stress induced by each QD does not pile up over distances larger than one GaN spacer, the change in morphology of one QD is sufficient to change the stress state of the next QD, leading to the systematic shape variation. 

1 Author J. Bartolomé , M. Hanke , D. van Treeck , A. Trampert

Strain Driven Shape Evolution of Stacked (In,Ga)N Quantum Disks Embedded in GaN Nanowires

Source Nano Lett. , 17 , 4654 ( 2017 )
DOI : 10.1021/acs.nanolett.7b01136 | 2969 Cite : Bibtex RIS
J. Bartolomé, M. Hanke, D. van Treeck, and A. Trampert