Title : The use of thin AlAs capping layers in the InAs/GaAs quantum dot system for intermediate band solar cells
Recently, the covering of InAs quantum dots (QDs) with AlAs capping layers (CLs) has attracted special attention as it has been shown to be an excellent way to enhance the photovoltaic efficiency in QD solar cells (QDSCs), with the removal of the wetting layer (WL) being the putative cause of this improvement. Certainly, analysis by DCTEM imaging using chemistry-sensitive g200 DF conditions evidences a gradual fading of the contrast of the InAs WL as the thickness of the AlAs CL increases being finally almost replaced by that of AlAs itself. However, the changes in the structure and recombination mechanisms after deposition of AlAs CL on the InAs QDs layer remain unclear. In this work, a comprehensive compositional analysis of several layers of AlAs/InAs/GaAs QDs with different CL thicknesses (0, 1, 2, 2, 3 and 5 MLs) is carried out by STEM-related techniques on a Titan Cubed3 FEI operated at 200 kV. In particular, energy dispersive X-ray spectroscopy (EDX) using ChemiSTEM technology and low-loss electron energy loss spectroscopy (EELS) were performed to obtain compositional and sample thickness mappings at the nanoscale level to analyse different regions of both CLs and QDs. First, nanoscale EDX analysis demonstrates that Al atoms do not replace In atoms, but rather there is an overlapping In/Al distribution from the second CL monolayer that hides the presence of the WL when using the DCTEM technique. Secondly, the size, composition and density of QDs are evaluated as they are key properties in the efficiency of QDSCs. A significant increase of QDs heights with the AlAs thickness is found, in agreement with previous works. The average volume and density of the QDs together with the measurements of the In content in the WL allowed us to calculate a linear increase of the average In content with the CL widening. Finally, the disposition of Al around the QD is analysed. Elemental analysis of the AlAs layer showed that the real Al content above the QD apex increases when the CL thickness is raised, which is consistent with higher and In-richer QDs. Initially there is an Al accumulation at the QD edges at the expense of Al deposited at the apex, that results in a low base diameter expansion. Only from the layer with 5 MLs onwards the Al levelling in the QD apex and in the regions between QDs is achieved.
Audience Take Away:
- The audience will be able to use this work to learn about the enormous possibilities offered by new aberration-corrected STEM-related techniques in the analysis of nanostructured semiconductor materials at the atomic scale.
- Our work represents a great information that will help in the design of new quantum dot based nanostructures for the implementation of high efficiency intermediate bandgap solar cells.
- In this work it has been shown that performance improvements in these devices are not produced by an elimination of the wetting layer but by the large mixing from the first monolayers between the wetting layer and the capping layer. Quantification procedures of 3 elements by different STEM techniques are presented which have allowed the best structural and compositional description of the system.