A HAADF investigation of AlAs/GaAs interfaces using SuperSTEM

Abstract

Z-contrast imaging using a high-angle annular dark field detector can be used to characterise III-V heterostructures. GaAs/AlAs heterostructures were grown using MBE and prepared for TEM using a cross-sectional method. SuperSTEM 1 was used to investigate both the GaAs-on-AlAs and the AlAs-on-GaAs interfaces as a function of specimen thickness. The analysis of the images showed that the apparent interface widths varied with thickness in an unexpected manner. The GaAs-on-AlAs widths remained constant with thickness while the AlAs-on-GaAs interface widths increased. The actual interfacial width can be a result of either surface stepping during MBE growth or inter-diffusion of the column III atoms. To assist the interpretation of this result a series of interfacial models were created and explored using a frozen phonon multislice simulation. The models consisted of terraced, vicinal and diffused interfaces. The results show that while the width of the diffused interface reduces with specimen thickness, the widths of the stepped interfaces vary in a more complex manner. This variability is due in part to the different channeling depths of the Ga and Al columns, but the measured width is also affected by the depth of the material before the first step and whether GaAs or AlAs is on top. These effects can cause the apparent interfacial width to vary markedly from the actual width. Even measuring the width itself is non-trivial.