- Open Access
Thermal stability of 2DEG at amorphous LaAlO3/crystalline SrTiO3 heterointerfaces
- Seon Young Moon†1, 2,
- Cheon Woo Moon†3,
- Hye Jung Chang1, 4,
- Taemin Kim3,
- Chong-Yun Kang1, 4, 5,
- Heon-Jin Choi2,
- Jin-Sang Kim1,
- Seung-Hyub Baek1, 4Email author and
- Ho Won Jang3Email author
© Moon et al. 2016
- Received: 10 December 2015
- Accepted: 10 February 2016
- Published: 1 April 2016
At present, the generation of heterostructures with two dimensional electron gas (2DEG) in amorphous LaAlO3 (a-LAO)/SrTiO3 (STO) has been achieved. Herein, we analysed thermal stability of 2DEG at a-LAO/STO interfaces in comparison with 2DEG at crystalline LaAlO3 (c-LAO)/STO interfaces. To create 2DEG at LAO/STO interface, regardless of growing temperature from 25 to 700 °C, we found that environment with oxygen deficient during the deposition of LAO overlayer is essentially required. That indicates that the oxygen-poor condition in the system is more essential than the crystalline nature of LAO layer. 2DEG at a-LAO/STO interface is depleted upon ex situ annealing at 300 °C under 300 Torr of oxygen pressure, while that in c-LAO/STO interface is still maintained. Our result suggests that the LAO overlayer crystallinity critically affects the thermal-annealing-induced depletion of 2DEG at a-LAO/STO interface rather than the generation of 2DEG. We clearly provide that amorphous TiOx can efficiently prevent the thermal degradation of 2DEG at the a-LAO/STO interface, which gives a cornerstone for achieving thermal-stable 2DEG at a-LAO/STO interface.
- 2-Dimensional electron gas
- Thermal stability
Oxide-based complex materials having strongly correlated electrons play a crucial role in a wide variety of physical phenomena and have potential for applications in exotic fields such as charge-ordered insulators, high temperature superconductivity, unconventional ferroelectricity and double exchange ferromagnets [1–3]. With improvements in atomic-scale synthesis [4, 5], heterointerfaces between complex oxides are emerging as highly interesting electronic systems owing to their unique properties [6–9]. Such interfaces can generate electron systems that are not found in nature in bulk. A recent prominent finding is that a high mobility two-dimensional electron gas (2DEG) can be generated at the interface between two insulating oxides, LaAlO3 (LAO) and SrTiO3 (STO), and the formation of 2DEG is confined within a layer of ~1 nm from the LaO/TiO2 interface [10–16]. At this polar interface both superconductivity and ferromagnetism can coexist as a result of electronic phase separation [17–20]. The interfacial conductivity shows a number of intriguing properties such as on/off switching with external electric fields [12, 21], nanoscale electronic devices [21, 22], and tunable conductivity controlled using the strong surface-interface interaction by the surface adsorbates [23, 24].
Recently, the formation of 2DEG can be found in various STO-based heterointerfaces which are different from the conventional (001) LAO/STO systems in terms of the crystal orientation and the overlayer materials [25–30]. Metallic interfaces can be realized in epitaxial LAO layers on (110) and (111) STO substrates which have no interface polar discontinuity surfaces and exhibits 2DEG transport with mobilities similar to those of (001) STO . The 2DEG can be constructed at with various amorphous overlayers of LAO, STO and yttria-stabilized zirconia (YSZ) . The formation of 2DEG was attributed to oxygen vacancies constrained near the interface, suggesting that the redox reactions on the surface of STO substrates play an important role . Remarkably, the 2DEG at spinel γ-Al2O3/perovskite STO interface showed an astonishingly high mobility of approximately 1.4 × 105 cm2 V−1 s−1 . The mobilities are much higher than those (~30,000 cm2 V−1 s−1) of the Nb-doped STO single crystal and the La-doped STO film [33–36]. These studies are expected to trigger intensive studies on high mobility complex oxide interfaces as conducted for III–V or Si–SiGe semiconductors [37–39]. Meanwhile, there is little discussion about the reliability of the 2DEG oxide interfaces. A deep understanding of the physical parameters governing the stability of 2DEG in oxide-based complex materials is crucial for real device applications. For this reason, it is important to investigate thermal stability of 2DEG in STO-based heterointerfaces for both amorphous and crystalline LAO/STO heterointerfaces, which has not been done yet.
In this study, we report both the formation and thermal-annealing-induced annihilation of 2DEG at amorphous LAO/STO (a-LAO/STO) heterointerface. The experimental results for a-LAO/STO system are compared with those for the most standardized crystalline LAO/STO (c-LAO/STO) system. Our electrical measurements reveal that the metallic conductivity at a-LAO/STO heterointerfaces is obtained in oxygen-deficient growth conditions and that there is a critical thickness of the a-LAO overlayer for the 2DEG formation, which are very similarly observed in c-LAO/STO heterointerfaces. However, a-LAO/STO shows much pronounced degradation in 2DEG conductivity compared with c-LAO/STO with increasing ex situ annealing temperature, demonstrating that the crystallinity of the overlayer has little impact on the 2DEG formation but significantly affect the thermal stability of 2DEG in the STO-based heterointerfaces.
All LAO overlayers were grown on TiO2-terminated STO substrates by pulsed laser deposition (PLD) in an oxygen atmosphere. Two types of LAO overlayers were grown. In the first, we deposited a LAO overlayer at room temperature, resulting in an amorphous LAO (a-LAO). Structural characterizations of the heterostructures were performed using a scanning transmission electron microscope (STEM, Titan, FEI) operated at 300 kV. In the second, we deposited a LAO overlayer at 700 °C, resulting in a crystalline LAO (c-LAO). Both types of heterostructures were grown with various LAO overlayer thicknesses varying from 0 to 12 nm. Both types were grown at the oxygen pressure of 1 mTorr. After the deposition of c-LAO overlayers at 700 °C, the samples were cooled down to room temperature maintaining the oxygen pressure at 1 mTorr. The laser energy density of 1.5 J cm−2 and the repetition rate of 2 Hz were used. The distance from the target surface to the sample was 50 mm. In addition, we deposited various amorphous capping layer such as TiO2, SnO2, SiO2, and Al2O3 on the a-LAO/STO heterostructure at room temperature using RF magnetron sputtering and PLD. The thickness of all amorphous capping layers is 100 nm. The interfacial conductivity at room temperature was evaluated using current–voltage (I–V) measurements which had been carried out using indium ohmic contacts on the diagonal corners of 5 mm × 5 mm samples. The sheet resistance and carrier concentration were measured by a Hall measurement system using the van der Pauw configuration in the temperature range from 300 K down to 10 K. And the electron mobility was calculated from the relationship between sheet resistance and carrier concentration. To test the stability of the a-LAO/STO and c-LAO/STO heterostructures, they were ex situ annealed at temperatures ranging from 100 to 700 °C for 1 h under the oxygen pressure of 300 Torr.
In this work, we investigated the thermal stability of 2DEG at a-LAO/STO interface. 2DEG is formed under oxygen deficient growth condition and the crystallinity of LAO overlayer is rarely related to the 2DEG formation. The electrical properties of 2DEG at a-LAO/STO interface are analogous to those at c-LAO/STO interface. We found that the thermal stability of 2DEG upon annealing in high oxygen pressure is strongly dependent on the crystallinity of LAO overlayer. We suggested that preserving the surface oxygen vacancies is critical in achieving thermal stable 2DEG interface at elevated temperatures. It is demonstrated that an additional overlayer like a-TiOx on a-LAO/STO heterostructure can enhance the thermal stability of 2DEG at the heterointerface.
SYM conducted the formation of 2DEG and measurement. CWM writed and formatted of the full manuscript according to the journal’s instruction. TK prepared materials and substrates for the procedures. HJC carefully performed the transmission electron microscopy analysis. CYK, HJC and JSK supported apparatus for the measurement and characterization. SHB and HWJ carefully revised the manuscript. All authors read and approved the final manuscript.
This work was supported by a research program of Korea Institute of Science and Technology (2E25440), a KIST-UNIST partnership program (2V04450), an Outstanding Young Researcher Program the National Research Foundation of Korea, and an Aspiring Researcher Program through Seoul National University in 2013.
The authors declare that they have no competing interests.
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