† Corresponding author. E-mail:
Project supported by the National Basic Research Program of China (Grant No. 2013CB632506) and the National Natural Science Foundation of China (Grant Nos. 11374186, 51231007, and 51202132).
Density functional theory within the local density approximation is used to investigate the effect of the oxygen vacancy on the LaGaO3/SrTiO3 (001) heterojunction. It is found that the energy favorable configuration is the oxygen vacancy located at the 3rd layer of the STO substrate, and the antiferrodistortive distortion is induced by the oxygen vacancy introduced on the SrTiO3 side. Compared with the heterojunction without introducing oxygen vacancy, the heterojunction with introducing the oxygen vacancy does not change the origin of the two-dimensional electron gas (2DEG), that is, the 2DEG still originates from the dxy electrons, which are split from the t2g states of Ti atom at interface; however the oxygen vacancy is not beneficial to the confinement of the 2DEG. The extra electrons caused by the oxygen vacancy dominantly occupy the 3dx2−y2 orbitals of the Ti atom nearest to the oxygen vacancy, thus the density of carrier is enhanced by one order of magnitude due to the introduction of oxygen vacancy compared with the density of the ideal structure heterojunction.
In the last decades, heterostructures based on perovskite transition metal oxides have attracted much attention, particularly due to the discoveries of extraordinary electronic and magnetic properties of the internal interface. For example, the two-dimensional electron gas (2DEG) with high mobility was observed at the interface between LaAlO3 (LAO) and SrTiO3 (STO), which exhibit insulating electrical properties in bulk materials.[1,2] Since then, some novel physical properties have been observed at the interface of oxide heterojunction, such as magnetism,[3] superconductivity[4,5] and the coexistence of magnetism and superconductivity.[6–8] The mechanism of the 2DEG formed at the interface between insulated oxides has been widely investigated both in theory[9–15] and experiments.[16–20] Especially, the heterostructures of LaGaO3 (LGO) and STO have received more and more attention due to their two-dimensional electrical properties at interface.[21–26]
In 2010, Perna et al.[21] first observed the 2DEG formed at the interface of LGO/STO, which exhibits some properties similar to those of LAO/STO, such as superconducting at low temperature (Tc = 150 mK for both heterostructures). The onset of conductivity takes place at 4 unit cells (u.c.) for both LAO/STO[2] and LGO/STO[21] interface. With using the electronic reconstruction model, our previous calculations show that the origin of the 2DEG is partly filled dxy orbital split from t2g states of Ti atom of the interface caused by symmetry broken. Though the electronic reconstruction has performed to explain the mechanism of the 2DEG formed at ideal LAO/STO interface,[27,28] there are also abundant evidences that the creations of oxygen vacancies during the growth of the oxide heterostructures play an important role in determining the transport properties for the LAO/STO system.[2,16,17,29] Experimentally, Amoruso et al. found that the interface produced at an oxygen pressure of 10−1 mbar, at which the interface should be free from oxygen defects, did not show a 2DEG, unlike the conductive interface obtained in low oxygen pressure.[23] However, the experiments reported by Aruta et al. demonstrated a direct link between the target-to-substrate distance and the interface sheet resistance, that is to say, the insulating interface was caused by the increase of the target-to-substrate distance.[24] Anyway, the mechanism of the electrical property effect of the oxygen vacancy has not been fully understood. A better understanding of the effect of oxygen vacancies on interface electronic states of LGO/STO and the distribution character of the induced charge carriers is lacking.
In this article, the first-principles calculations are employed to investigate the LGO/STO n-type interface with and without oxygen vacancy (VO). The values of formation energy of the VO located at different layers of the STO substrate are calculated to find the most suitable configuration for stabilizing the system. Then, the role of the VO in generating the 2DEG is explored and the electrical property effect of the VO on the LGO/STO heterojunction is discussed.
We investigate an n-type LGO/STO supercell with a 2 × 1 surface unit cell consisting of 5 u.c. layers of STO substrate and 7 u.c. layers of LGO film along the (001) direction as shown in Fig.
In order to evaluate the stability of the defective system, the values of formation energy of the VO at various positions are calculated. In this calculation, only the VO located on STO side named by n1–n5 is investigated for the LGO/STO heterojunction, which is shown in Fig.
The values of formation energy of the VO are calculated by DFT method, and shown in Fig.
In order to compare the effect of the relaxation, the atomic distortions which are exclusively along the c-axis for the bulk, i.e., the supercell without VO and for the supercell with one VO in the 3rd TiO2 layer, are displayed in Fig.
For the supercell with one VO in the 3rd layer, an unexpected pattern is discovered. Oxygen octahedron rotates oppositely in adjacent cells. The value of rotation angle is shown in Fig.
In order to explore the effect of such an AFD distortion on the type of electronic states around the Fermi level and their spatial distribution, the density of states projected on the ten layers of LGO and STO for (LGO)7/(STO)5 with one VO in the 3rd layer are obtained in our calculations, and shown in Fig.
In order to investigate the 2DEG effect of introducing the oxygen vacancy in the heterojunction, the orbital decomposed DOSs for the Ti atoms at the interface and in the 3rd layer are calculated, and the results are shown in Fig.
The local density approximation methods are employed to investigate the effect of the VO on the LGO/STO heterojunction. Based on the values of formation energy of the VO at the positions located at different layers, it is found that the energy favorable configuration is the VO at the position located at the 3rd layer of the STO substrate, and the AFD distortion can be caused by the VO. The introduction of the VO does not change the origin of the 2DEG, namely the Ti dxy electrons. However, the VO is not beneficial to the confinement of the 2DEG. The density of the carriers is enhanced by the VO by one order of magnitude compared with that of the ideal heterojunction. At the Ti nearest to the VO, a strong eg splitting happens and the dx2−y2 state becomes occupied dominantly by the extra electrons from the VO.
[1] | |
[2] | |
[3] | |
[4] | |
[5] | |
[6] | |
[7] | |
[8] | |
[9] | |
[10] | |
[11] | |
[12] | |
[13] | |
[14] | |
[15] | |
[16] | |
[17] | |
[18] | |
[19] | |
[20] | |
[21] | |
[22] | |
[23] | |
[24] | |
[25] | |
[26] | |
[27] | |
[28] | |
[29] | |
[30] | |
[31] | |
[32] | |
[33] | |
[34] | |
[35] |