Supplemental Material:
Evidence for multifilamentary valence changes in resistive switching SrTiO 3
devices detected by transmission X-ray microscopy
A. Koehl, H. Wasmund, A. Herpers, P. Guttmann, S. Werner, K. Henzler, H. Du, J. Mayer, R. Waser,
R. Dittmann
Parameters for pulsed laser deposition
The growth of the perovskite films is realized via pulsed laser deposition with a KrF eximer laser
(λ=248nm) and is monitored with reflection high energy electron diffraction. Targets of the desired film
stoichiometry are used for the deposition. In table 1 the growth parameters of the crystalline thin films
are summarized. The STO layer is grown with a laser fluence which results in slightly Sr-rich films.
Laser
Laser
Heater
frequency
fluence
temperature
La0.7Sr0.3MnO3
5Hz
2.0J/cm2
700°C
SrRuO3
10Hz
0.8J/cm2
650°C
5%Fe:SrTiO3
5Hz
1.0J/cm2
750°C
Tab. 1: Parameters for PLD growth of perovskite films.
Oxygen
pressure
0.133mbar
0.133mbar
0.100mbar
Preparation of samples for transmission geometry
In order to detach the MIM structure from the substrate two different etching procedures have been
tested. The first option is to immerse the whole sample into fluid HCl at room temperature. The LSMO
layer is dissolved typically within 12-16 minutes and the MIM structure attached to the carbon layer
floats in the acid (see Fig. S1a). This layer is collected onto a TEM grid and transferred to deionized
water to remove the acid. Afterwards it is again collected onto a TEM grid, which can be mounted to
the transmission X-ray microscope. Some samples have been successfully prepared with this method,
but while etching in fluid HCl halfly detached thin layers of carbon frequently twisted and created
samples with several layers of carbon film on top of each other, which are not suited for TXM analysis.
Alternatively we exposed the sample to HCl vapor at 80°C (see main paper and Fig S1b). In this case
the whole sample is transferred into deionized water after 15 minutes and big flat pieces of carbon film
with MIM structures are immediately detached from the substrate (probably due to surface tension
effects). In principle both techniques gave comparable results, but etching in vapor reduced the risk of
twisting of the thin layer and was therefore used for the preparation of the samples presented in the
present study.
Fig. S1: a) Sketch of etching in fluid HCl b) Etching in HCl-vapor.
Independent of the details of the etching procedure, most problematic for this sample preparation is
the adhesion of the relevant parts of the MIM structure to the carbon support layer. Although the
additional chromium layer improves the adhesion, only part of the electrodes is attached to the carbon
layer and therefore available for NEXAFS-TXM analysis. Usually the extraction replica technique is
employed for the investigation of protrusions and rough surfaces. During the electrical treatment we
typically observe an increase of the surface roughness of the Pt-pads and indeed the switched pads
are more likely to be attached to the carbon layer than the virgin pads or the film itself. Based on our
experience on several samples we are therefore confident, that all parts of the film which have been
influenced by the switching process are preserved for the NEXAFS-TXM analysis.