Abstract
We present a combined resonant soft X-ray reflectivity and electric transport study of hbox {LaAlO}_3/hbox {SrTiO}_3 field effect devices. The depth profiles with atomic layer resolution that are obtained from the resonant reflectivity reveal a pronounced temperature dependence of the two-dimensional electron liquid at the hbox {LaAlO}_3/hbox {SrTiO}_3 interface. At room temperature the corresponding electrons are located close to the interface, extending down to 4 unit cells into the hbox {SrTiO}_3 substrate. Upon cooling, however, these interface electrons assume a bimodal depth distribution: They spread out deeper into the hbox {SrTiO}_3 and split into two distinct parts, namely one close to the interface with a thickness of about 4 unit cells and another centered around 9 unit cells from the interface. The results are consistent with theoretical predictions based on oxygen vacancies at the surface of the hbox {LaAlO}_3 film and support the notion of a complex interplay between structural and electronic degrees of freedom.
Highlights
We present a combined resonant soft X-ray reflectivity and electric transport study of LaAlO3 /SrTiO3 field effect devices
The field effect is due to a transfer of charge between the gate electrode and the conducting channel, i.e. a charge redistribution within a field effect device (FED)
The analysis reveals that the interfaces of our FEDs are sharp, with an interface roughness σi = (1 ± 0.1) unit cell (UC) and a surface roughness σs = (0.7 ± 0.1) UC
Summary
We present a combined resonant soft X-ray reflectivity and electric transport study of LaAlO3 /SrTiO3 field effect devices. To our knowledge, such a high-resolution depth profile has not yet been reported This is exactly the motivation of the present study, where we combine electric transport and resonant X-ray reflectivity (RXR) measurements into one experiment and use this to explore the electronic depth profile in a LAO/STO heterostructure as a function of the applied back-gate voltage and temperature. In this way we aim at clarifying the relation between the macroscopic charge transport of our device and the microscopic depth distribution of the 2DEL. In order to characterize the hysteretic behavior of the LAO/STO FED6,11,18 and to initialize it according to the commonly adopted approach, we applied the cycle shown in Fig. 1(a), while keeping the beam off and the temperature at 11 K
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