Simulated Chemical Imaging of InGaAs/InAlAs Interfaces

Abstract

For many semiconductor devices, a detailed knowledge of interface composition on an atomic scale is important for understanding the device performance. One technique for obtaining such knowledge is “chemical imaging”. This technique has previously been applied to Si/Ge and GaAs/AlAs interfaces. Recently, there has been interest in constructing quantum wells using the (In0.532Ga0.468)As/(In0.522Al0.478)As system (hereafter InGaAs/InAlAs), which is lattice-matched to InP (lattice constant of 5:869Å).The chemical imaging technique uses high resolution electron microscopy (HREM) images taken under special conditions of microscope defocus df, and specimen thickness t. Under the correct conditions, the images contain features which are linearly dependent on composition x. For heterogeneous structures (e.g. InGaAs/InAlAs), a change in x affects <200> beams preferentially. In this case, imaging in the <100> orientation is favorable, because four <200> beams occur. It is necessary to define a function of the image intensity h(I) which is proportional to x for some values of t and df. Here we follow the approach of Thoma and Cerva and use h(I) equal to the <200> frequency component of the image intensity (I200) normalized relative to I200 for x=0 and x=1. The resolution of h(I) is limited to one <200> period, or 1/2 unit cell (2.93Å).