Real-space imaging of atomic arrangements and vacancy layers ordering in laser crystallized Ge2Sb2Te5 phase change thin films

Andriy Lotnyk*, Sabine Bernütz, Xinxing Sun, Ulrich Ross, Martin Ehrhardt, Bernd Rauschenbach

Leibniz Institute of Surface Modification (IOM), Permoserstr. 15, D-04318 Leipzig, Germany

*

Supporting Information (SI)

Details on the compensation of specimen drift and evaluation procedure

The signal to noise ratio of STEM micrographs was improved by the averaging over many STEM image acquisitions (228 frames for the image size of 256x256 pixels and 78 frames for the image size of 512x512 pixels), recorded with a short dwell time (2µs per pixel) by the Esprit software (Bruker). While the TEM uses parallel recording, the serial acquisition of STEM images can encounter problems with specimen drift, which results in image distortions. The specimen drift can be minimized by optimized room design [37] and by developing stable columns or by using multiple rapid exposures and software based drift correction techniques [38,39]. This results in accuracies that are comparable for TEM and STEM [41]. In order to compensate the specimen drift in our study, an image drift correction implemented in the Esprit software was applied during the acquisitions. It should be noted that typical specimen drift of the Titan microscope is less than 0.45 nm/min and the spot drift is 0.03 nm/min. This demonstrates that the instrument itself and the ambient room are highly stabilized. Since the specimen drifts cannot be entirely eliminated, the correction of residual image distortions (angles in non-scanning directions) was performed by using the Jitterbug software (HREM Research Inc.) [40].

The real-space structure of the GST lattice was directly evaluated from atomic-resolution STEM images. The interatomic distances along the major crystallographic axes and intensity ratios for individual atomic column positions were calculated by detailed fitting and evaluation of intensity profiles using the Origin software. At least 200 measurement points for a particular crystallographic axis were used for the analysis. In order to calculate the interatomic distances, the STEM images were processed by a radial difference filter for noise reduction, whereas the intensity ratios were calculated using raw data from non-filtered images. The filter is available from HREM Research Inc. as a plug-in for the Gatan Digital Micrograph software suite.

Fig. S1. Atomic-resolution MAADF-STEM image of Si in [110] viewing direction (left) and measured in-plane interatomic distances with standard deviation (σ) over multiple measurements. The experimentally measured values are 1.5% higher than the literature values. This was used for the correction of experimental data.

Fig. S2. EDX line scan taken from metastable GST thin film crystallized by ns-laser pulse. The composition of GST is very close to the expected 2:2:5 composition.

Fig. S3 (a) Bright-field cross-sectional TEM image of metastable GST thin film crystallized by a single fs-laser pulse at a laser fluence of 19 mJ/cm2. (b) Selected area electron diffraction pattern of GST thin film shown in (a) and real lattice spacing from selected diffraction rings 1, 2 and 3 in the reciprocal space. (c) Bright-field cross-sectional TEM image of GST thin film crystallized by a single ns-laser pulse at a laser fluence of 130 mJ/cm2. (d) Dependence of grain width on laser fluence.

Fig. S4. Simulated atomic-resolution LAADF-STEM images of metastable GST in [100] viewing direction at 0 nm defocus and various specimen thicknesses (t). The Te/GeSb/V atomic columns appear dark on a bright background. The projected structure model of GST with exclusively o-Ge (marked by “o”) is inserted in all images. Note: additional contrast appears in the LAADF-STEM image at the (1/4, 1/4, 1/4) lattice site for specimens with thickness larger than 40 nm.

Fig. S5 (a) Nanobeam electron diffraction pattern of amorphous GST (a-GST). (b) Reduced density function calculated from (a). The nearest neighbor peak at r = 2.8 Å and bond angle 93.4° are an indication for defective octahedral sites of Ge and Sb in a-GST.