Impact of Digital Alloy Capping Layers on Bilayer InAs Quantum Dot Heterostructures

The influence of digital alloy capping technique on the strain-coupled bilayer InAs quantum dots (QDs) has been presented. Multiple capping layers of different composition have been utilized instead of a single thick capping layer to reduce the overall strain inside the heterostructure. In bilayer QDs, the propagation of residual strain from the lower QD layer to the top QD layer introduces defects and dislocations. By using the digital alloy technique over the conventional or analog alloy capping technique, the surface defects could be minimized and the crystallinity could be improved. In this study, four different capping materials have been considered, viz. two ternary materials (In
_x
Ga
_1-x
As and GaAs
_1-x
Sb
_x
) and two quaternary materials (In
_x
Al
_y
Ga
_1-x-y
As and In
_x
Ga
_1-x
As
_1-y
Sb
_y
) for analysis of both analog alloy and digital alloy techniques. The strain and energy band profile has been computed for all the heterostructures using
Nextnano
software. The biaxial and hydrostatic strain have been computed and compared to understand the strain distribution within the heterostructures. The ground state emission wavelength was found red shifted for the InAs bilayer QD heterostructure with digital alloy capping layers as compared to that of the analog alloy capping. Moreover, the choice of capping materials exhibits both type-I and type-II energy band alignment which is suitable for different optoelectronic applications. This comparative study of analog and digital alloy capping layer for the bilayer QDs would definitely help in optimization of the device heterostructures with minimal strain and defects.