Infrared ellipsometry study of photogenerated charge carriers at the (001) and (110) surfaces of SrTiO3 crystals and at the interface of the corresponding LaAlO3

Abstract

With infrared (IR) ellipsometry and DC resistance measurements we investigated the photo-doping at the (001) and (110) surfaces of SrTiO 3 (STO) single crystals and at the corresponding interfaces of LaAlO 3 /SrTiO 3 (LAO/STO) heterostructures. In the bare STO crystals we find that the photo-generated charge carriers, which accumulate near the (001) surface, have a similar depth profile and sheet carrier concentration as the confined electrons that were previously observed in LAO/STO (001) heterostructures. A large fraction of these photo-generated charge carriers persist at low temperature at the STO (001) surface even after the UV light has been switched off again. These persistent charge carriers seem to originate from oxygen vacancies that are trapped at the structural domain boundaries which develop below the so-called antiferrodistortive transition at T* = 105 K. This is most evident from a corresponding photo-doping study of the DC transport in STO (110) crystals for which the concentration of these domain boundaries can be modified by applying a weak uniaxial stress. The oxygen vacancies and their trapping by defects are also the source of the electrons that are confined to the interface of LAO/STO (110) heterostructures which likely do not have a polar discontinuity as in LAO/STO (001). In the former, the trapping and clustering of the oxygen vacancies also has a strong influence on the anisotropy of the charge carrier mobility. We show that this anisotropy can be readily varied and even inverted by various means, such as a gentle thermal treatment, UV irradiation, or even a weak uniaxial stress. Our experiments suggest that extended defects, which develop over long time periods (of weeks to months), can strongly influence the response of the confined charge carriers at the LAO/STO (110) interface. I) INTRODUCTIONThe discovery of a nearly two-dimensional electron gas (2DEG) with a high mobility at the interface of LAO/STO heterostructures [1], and the demonstration of subsequent electric-field-effect devices [2,3] have renewed the interest in the extraordinary structural and electronic properties of STO. At room temperature, it seems to be a rather ordinary material with a simple perovskite structure with cubic symmetry [4]. It is also a band-insulator with an indirect band-gap of 3.25 eV and a direct one of 3.75 eV [5]. Thanks to the flexible valence of the Ti ions, which can be varied continuously between 4+ and 3+, one can readily dope the conduction band of STO (it has mainly Ti-3d t 2g character) with electrons to make it metallic and even superconducting at very low temperature, for example by a partial replacement of Sr 2+ with La 3+ , Ti 4+ with Nb 5+ or by creating oxygen vacancies [6][7][8][9].STO undergoes several structural phase transitions as the temperature is lowered. The first one is the so-called antiferrodistortive (AFD) transition at T* = 105 K from the cubic to a tetragonal symmetry. It involves an antiphase rotation of the neighboring TiO 6 octahedra around the c-axis and leads to ...

Optical Anisotropy of SrTiO3(110) for Different Surface Terminations

“…, and two Drude dielectric functions for and with the same plasma frequency but anisotropic scattering rates. The simulation is only indicative, as the spectral range was not wide enough for any form of quantitative fits, however, a three times higher mobility for the (here x ) compared to the [001] direction was used, consistent with the experimentally found mobility anisotropy in the SrTiO 3 /LaAlO 3 (110) interface . The observed RAS structure is, however, most likely a combination of an anisotropy in the polarizability of the near surface oxygen vacancy‐related mid gap states and the free carrier contribution.…”

“…This is encouraging for future work on SrTiO 3 /LaAlO 3 interfaces, as in contrast to electron‐based techniques it can be employed in situ during the growth of the heterostructures and can be employed to measure the properties of the interface localized electron gas while it is formed. This is particular critical as typical growth conditions for the SrTiO 3 /LaAlO 3 interface exceed 750 °C, above the onset temperature for the formation of a significant number of oxygen vacancies . This is typically mitigated by a sample cool down at higher oxygen pressures (up to 200 mbar), which was not possible in our system.…”

“…This hetero interface has been widely investigated in recent years and has shown to have even superconducting properties . While most of these studies have been performed on (001) surfaces the formation of this Q2DEG has also been seen on (110) surfaces . In all cases, the treatment of the SrTiO 3 substrate is a critical step and can influence the interface structure in any hetero‐epitaxial system.…”

“…As such RAS has the potential to measure the formation of the Q2DEG in situ during the heteroepitaxy, as both SrTiO 3 and LaAlO 3 are wide bandgap materials and the response from the Q2DEG is expected to occur in the infrared spectral region . IR‐ellipsometry has already confirmed that the Q2DEG at the LaAlO 3 /SrTiO 3 (110) interface shows inherent anisotropies in terms of the electron mobilities along the [001] and direction …”

Structural properties of ultrathin SrO film deposited on SrTiO3

“…Another important example of a special interface phenomenon is the case of the LaAlO 3 /SrTiO 3 (LAO/STO) epitaxial heterostructures, where two-dimensional electron gas (2DEG) forms unexpected at the interface of the two insulators [2,11,12]. This phenomenon has been observed only in case of TiO 2 -terminated STO substrate, and a debate about the role of polar discontinuity and strain-induced oxygen vacancies is still running [13][14][15][16].…”