It has been shown that using a Scanning Electron Microscope (SEM), equipped with a Field Emission Gun (FEG) and in-lens specimen position (ultrahigh resolution SEM), operating in the backscattered electron (BSE) mode, it is possible to obtain correct characterization of a superlattice with an image contrast related to the atomic number variation (1).In order to check the performance of a JEOL JSM 890 SEM in the BSE imaging mode, a GaAs/ Ga1-xAlxAs superlattice structure, whose cross section is reported in Fig. 1, has been characterized. On the top there are layers with a fixed value of the mole fraction of Al (x =0.3) and thickness variable between 1 and 20 nm. Below, all the layers are 5 nm thick and the Al mole fraction varies in the range 0.05<x<0.40. Observations at different accelerating voltages show that the image contrast decreases by increasing the electron energy, whereas the resolution is improved. According to our experiments, in these specimen, the best compromise between resolution and contrast is in the energy range 10 - 15 kV. Fig. 2 shows the BSE image, taken at 13 kV, of the top superlattice structure; the GaAs layers appear bright and those of Ga0.7Al0.3As are dark. The resolution obtained on this structure, where the mean atomic number varies by ΔZ=2.7 from layer to layer (corresponding to a contrast C= 4.4% ), is 2 nm. A better evidence of this resolution is given by Fig. 3, which shows a superstructure of 2 nm AlAs / 2nm GaAs, ( ΔZ = 9, C=16%). The image of fig. 4 refers to the superlattice on the bottom of Fig. 1 and allows to specify the minimum detectable ΔZ for a fixed resolution of 5nm. The fringe contrast drops linearly, as well known, with the mean atomic number variation between the layers. As the number of visible fringes is 7, we deduce that the minimum detectable mean atomic number variation is 0.8, (C = 1.3 %).
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