STEM strain analysis at sub-nanometre scale using millisecond frames from a direct electron read-out CCD camera

We report on strain analysis by nano-beam electron diffraction with a spatial resolution of 0.5nm and a strain precision in the 4–7·10⁻⁴ range. Series of up to 160000 CBED patterns have been acquired in STEM mode with a semi-convergence angle of the incident probe of 2.6mrad, which enhances the spatial resolution by a factor of 5 compared to nearly parallel illumination. Firstly, we summarise 3 different algorithms to detect CBED disc positions accurately: selective edge detection and circle fitting, radial gradient maximisation and cross-correlation with masks. They yield equivalent strain profiles in growth direction for a stack of 5 In_xGa_1−xN_yAs_1−y/GaAs layers with tensile and compressive strain. Secondly, we use a direct electron read-out pnCCD detector with ultrafast readout hardware and a quantum efficiency close to 1 both to show that the same strain profiles are obtained at 200 times higher readout rates of 1kHz and to enhance strain precision to 3.5·10⁻⁴ by recording the weak 008 disc.