  Lateral inhomogeneities in the formation of 2-dimensional electron gases
(2DEG) directly influence their electronic properties. Understanding their
origin is an important factor for fundamental interpretations, as well as high
quality devices. Here, we studied the local formation of the buried 2DEG at
LaAlO3/SrTiO3 (LAO/STO) interfaces grown on STO (100) single crystals with
partial TiO2 termination, utilizing in-situ local conductivity atomic force
microscopy (LC-AFM) and scattering-type scanning near-field optical microscopy
(s-SNOM). Using substrates with different degrees of chemical surface
termination, we can link the resulting interface chemistry to an inhomogeneous
2DEG formation. In conductivity maps recorded by LC-AFM, a significant lack of
conductivity is observed at topographic features, indicative of a local
SrO/AlO2 interface stacking order, while significant local conductivity can be
probed in regions showing TiO2/LaO interface stacking order. These results
could be corroborated by s SNOM, showing a similar contrast distribution in the
optical signal which can be linked to the local electronic properties of the
material. The results are further complimented by low-temperature conductivity
measurements, which show an increasing residual resistance at 5 K with
increasing portion of insulating SrO terminated areas. Therefore, we can
correlate the macroscopic electrical behavior of our samples to its nanoscopic
structure. Using proper parameters, 2DEG mapping can be carried out without any
visible alteration of sample properties, proving LC AFM and s SNOM to be viable
and destruction-free techniques for the identification of local 2DEG formation.
Furthermore, applying LC AFM and s SNOM in this manner opens the exciting
prospect to link macroscopic low temperature transport to its nanoscopic
origin.
