The discovery of three-dimensional (3D) topological insulators (TIs) has attracted much attention in recent years due to their rich physics and promising applications in electronic and spintronic devices.^[1–9] The exotic gapless surface states of TIs are protected by time-reversal symmetry (TRS), with locked spin-momentum which is immune to backscattering. On the other hand, the breaking of TRS will open up a gap in the Dirac surface states of TIs, leading to a massive Dirac fermion state which can give rise to many novel phenomena.^[10–15] Introducing ferromagnetism into TIs through proximity effect with a ferromagnetic insulator (FMI) is an effective way to break the TRS, and it avoids bringing extra impurities compared with doping method.^[16–27]

The prototype topological insulator, Bi₂Se₃, is a layered material with relatively large bulk energy gap among 3D TIs.^[3,4] Experiments on the growth of Bi₂Se₃ thin films on different kinds of substrates have been reported, including Si(111), GaAs(111), graphene, SrTiO₃(111), Al₂O₃(0001), and so on.^[28–33] However, experiments on insulating ferromagnetic substrates are limited to a few kinds of materials, since the existing FMIs are very scarce in nature.^[34] So far, the commonly used FMIs mostly possess low-symmetry structures associated with a poor growth quality, and the few known high-symmetry materials either have very low Curie temperatures (≤ 17 K), or require chemical doping of an otherwise antiferromagnetic matrix.^[35]

LaCoO₃ (LCO) single-crystalline thin film under tensile strain is a newly reported high-temperature perovskite ferromagnetic insulator.^[35–38] The tensile-strained LaCoO₃ film is shown to be a rare undoped high-symmetry FMI with a remarkably high T_C of up to 85 K. Both experiments and first-principles calculations demonstrate tensile-strain-induced ferromagnetism due to the spin-state transition of Co³⁺ ion, which does not exist in bulk LaCoO₃. More importantly, the strained LaCoO₃ thin film can be prepared epitaxially on substrate such as the SrTiO₃ (STO) with an atomically flat surface,^[35] which is crucial for growing high-quality Bi₂Se₃ thin films onto it. Recently, we have found ferromagnetism in the Bi₂Se₃ film grown on LaCoO₃/SrTiO₃ by the proximity effect, implying that this system would be a promising platform for studying novel transport phenomena in TI/ferromagnetic structures.^[39]

In this work, we systematically studied the growth behaviors of topological insulator Bi₂Se₃ thin films on LaCoO₃/SrTiO₃ by molecular beam epitaxy (MBE). We find that the optimal condition for Bi₂Se₃ to grow on LCO/STO is at a much lower substrate temperature, around 150 °C, as compared with the optimal temperature on bare STO. The thus obtained Bi₂Se₃ thin films display a well-organized morphology and high carrier mobility, similar to those grown on bare STO substrates at higher temperature. While at higher growth temperatures, Bi₂Se₃ films on LCO exhibit disordered film morphology and decreased carrier mobility. The magnetoresistance and characteristic weak antilocalization effect are also reduced at higher growth temperatures. These unusual behaviors may be attributed to the subtle strain stress and large thermal-expansion coefficient of LCO. Our exploration and realization of the Bi₂Se₃/LCO heterostructure will be promising for developing versatile spintronic device applications.

Figure 1 shows the evolution of RHEED patterns of Bi₂Se₃ film on LCO/STO substrate during growth. Before the growth of Bi₂Se₃ film, straight stripes of RHEED pattern shown in Fig. 1(a) were observed, reflecting a clean and flat surface of the LaCoO₃ film. When the Bi₂Se₃ film started to grow, the original stripes disappeared, and a new characteristic pattern corresponding to Bi₂Se₃ occurred, as shown in Fig. 1(b). After the growth of 2–3 quintuple-layers (QLs) of Bi₂Se₃, the streaky RHEED pattern becomes clear, indicating a crystalline film forming. As the film grew thicker, the stripes in RHEED pattern became bright and sharp accordingly, as illustrated in Figs. 1(c) and 1(d). The clear and sharp stripes indicate highly crystalline and smooth surface of Bi₂Se₃ film. Besides, QL-by-QL growth mode is determined by a periodic oscillation of diffraction intensity in RHEED pattern during film growth.

Fig. 1.

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	Fig. 1. RHEED patterns recorded during the deposition of Bi2Se3 film on LaCoO3 substrate. (a) The surface of LaCoO3 thin film after degassing, and before the Bi2Se3 film growth. (b), (c), and (d) were taken when the Bi2Se3 film has grown for 10 min (3 QL), 20 min (6 QL), and 60 min (18 QL), respectively.

High quality of the Bi₂Se₃ film is also verified by XRD measurement on Bi₂Se₃/LCO samples. In Fig. 2, the Bi₂Se₃ film exhibits (00l) (l = 3n, where n is an integer) type of reflections over the 10°–80° scan range without other set of peaks of bulk Bi₂Se₃, indicating that the film is of single crystalline quality along the growth direction. The Bragg peak positions of Bi₂Se₃ film grown on LCO/STO(001) agree very well with the reference data of ICSD standard card for Bi₂Se₃ single crystal, suggesting that the film is in good relaxation and the lattice parameter is consistent with its bulk value. In addition, all films grown on LCO within a range of different conditions show similar features in XRD patterns, indicating that single crystalline phase of Bi₂Se₃ can be obtained among these samples.

Fig. 2.

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	Fig. 2. X-ray diffraction spectrum of a 20 QL Bi2Se3 film on LaCoO3/SrTiO3(001) (red line) and the data of an as-prepared LaCoO3 film on SrTiO3(001) substrate (black line) for comparison. The standard data (ICSD card) of Bi2Se3 single crystal is also shown (marked as orange star symbols) as a reference.

To investigate the influence of different substrate temperatures during growth on the film morphology, we scanned the surface of the Bi₂Se₃ films through AFM technique. Figure 3 shows the AFM images of Bi₂Se₃ films grown on LCO/STO substrates with different substrate temperatures ranged from 130 °C to over 300 °C during growth. Comparing the images shown in Figs. 3(a)–3(h), we find that the optimal morphology is obtained at a substrate temperature of 150 °C (Fig. 3(b)), which displays the characteristic pyramid-shaped layered structure in good order. At a substrate temperature as low as 130 °C, the film is not well crystallized (shown in Fig. 3(a)) due to insufficient diffusion of Bi and Se atoms. While at a substrate temperature higher than 150 °C, the pyramid-shaped islands on the surface of the film become distorted gradually, and the disorder in morphology becomes more visible at higher temperatures above 200 °C. The high temperature may influence the underlying stain stress from LCO substrate, and the underneath Bi₂Se₃ layers seem to become inflated and coarse, indicating it is unfavorable for the Bi₂Se₃ film to grow at such high substrate temperature. When the substrate is heated to over 300 °C, the film will decompose because the Se atoms are desorbed from the substrate. In addition, at a larger scale, the integral morphology of Bi₂Se₃ film grown at 150 °C is also well-organized and uniformly distributed, with a small root-mean-square roughness of 1.36 nm.

Fig. 3.

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	Fig. 3. AFM images (size: 1 μm×1 μm) of a series of Bi2Se3 films grown on LaCoO3/SrTiO3(001) substrate at different substrate temperatures: (a) 130 °C, (b) 150 °C, (c) 175 °C, (d) 190 °C, (e) 200 °C, (f) 225 °C, (g) 250 °C, and (h) 270 °C. All the films have been grown for 60 min.

For comparison, the morphologies of Bi₂Se₃ films on pure STO(001) substrates were investigated under the same growth conditions. As shown in Fig. 4(a), at a substrate temperature of 150 °C, Bi₂Se₃ film on STO exhibits almost the same morphology as that in Fig. 3(b). While at higher temperatures (see Figs. 4(b)–4(d)), Bi₂Se₃ films on STO display a better morphology with increased triangle island size and larger terrace area, and each QL on the surface is clearly resolved, indicating the overall surface is much flatter. The significant difference between samples grown on LCO and pure STO implies that higher substrate temperature is not suitable for Bi₂Se₃ film growing on LCO, and the most favorable temperature is about 150 °C, much lower than that on STO substrate alone.

Fig. 4.

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	Fig. 4. AFM images (size: 1 μm × 1 μm) of Bi2Se3 film grown on SrTiO3(001) substrate at different substrate temperatures: (a) 150 °C, (b) 190 °C, (c) 210 °C, and (d) 230 °C. All the films have been grown for 60 min, under the same conditions as that in Fig. 3.

In general, Bi₂Se₃ thin films can be layer-by-layer grown on different kinds of substrates with even slight lattice mismatch, due to the van der Waals epitaxy. In these cases, including on the pure STO(001) substrate, Bi₂Se₃ thin film tends to have a smoother surface at a relatively higher temperature above 200 °C, which allows sufficient diffusion of the Bi and Se atoms.^[30–33] While in the case of LCO/STO, higher substrate temperature seems to have a negative influence on the film quality of Bi₂Se₃. Compared with other substrates, LCO has a larger thermal-expansion coefficient.^[36] The expanded lattice of underlying LCO layer and the accompanied influence on strain stress due to lattice mismatch becomes more obvious at high substrate temperature, which results in the disordered morphology of films. Therefore, by exploring a wide range of possible growth temperatures, we find that only at the relatively lower temperature can Bi₂Se₃ film be grown onto LCO/STO in free-standing QL-by-QL mode, as it does on other substrates.

For transport measurements, we chose a thickness of 20 QL for the Bi₂Se₃ films, which is well above the thickness threshold of 6 nm. Below this threshold, the top and bottom surface of Bi₂Se₃ may be coupled by quantum tunneling and lead to a hybridization gap near Dirac point, similar to the result brought by strong magnetic interaction.^[40] In Fig. 5(a), we plot the Hall resistance (R_xy) of Bi₂Se₃/LCO samples grown at different substrate temperatures measured in the perpendicular applied magnetic field at 1.7 K. For all the samples, the Hall resistance has a nearly linear dependence on the magnetic field, suggesting that one type of carriers dominate the transport. Using the one-band model, we can obtain that the carriers in all the Bi₂Se₃ films are n type, and the initial carrier densities are in the range of 2.5 × 10¹³–3.5 × 10¹³ cm⁻², which are consistent with the results for Bi₂Se₃/STO samples.^[41,42]

Fig. 5.

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Fig. 5. Magnetotransport properties of the Bi2Se3/LCO heretostructure devices. All the data are measured at 1.7 K. (a) Hall resistance (Rxy) of several Bi2Se3/LCO samples as a function of the perpendicular magnetic field. (b) Initial carrier density (black squares) and carrier mobility (red circles) derived from several Bi2Se3/LCO samples grown at different substrate temperatures (denoted by Tsub) ranging from 150 °C to 225 °C. (c) Magnetoresistance (MR) measured in perpendicular magnetic field of four Bi2Se3/LCO samples (in solid symbols) grown at Tsub = 150 °C, 175 °C, 200 °C, 225 °C, respectively, and a control sample (in pink open symbol) of Bi2Se3/STO grown at Tsub = 150 °C. (d) Magnetoconductivity in low-field range of the corresponding samples in (c) as well as two additional curves of the Bi2Se3/STO control sample with Tsub = 150 °C at gate voltage 30 V and −100 V, respectively. The HLN fitting results of each curve are plotted in solid (orange) lines. (e) The fitting parameters of HLN equation for each Bi2Se3/LCO sample shown in (d). (f) MR of a typical Bi2Se3/LCO sample grown at optimal Tsub = 150 °C measured in tilted field. The tilted angle is defined as the angle between the applied magnetic field and the film plane.

The initial carrier density and carrier mobility of four Bi₂Se₃/LCO samples grown at different substrate temperatures (T_sub) are plotted in Fig. 5(b). The sample with best morphology that grown at optimal T_sub = 150 °C has a carrier density 2.7 × 10¹³ cm⁻² and mobility 659 cm² · V⁻¹·s⁻¹. Such carrier mobility is as high as that of Bi₂Se₃/STO control samples,^[43] indicating high film quality with pristine good transport properties is achieved at the optimal growth condition. On the other hand, the carrier density of the film increases gradually when T_sub goes up, probably due to the increasing density of Se vacancies at higher substrate temperature. This behavior for Bi₂Se₃ films grown on ferromagnetic LCO/STO is the same as that on non-magnetic STO substrates. However, unlike the usual case on other substrates, the carrier mobility of the film decreases when T_sub goes up, implying more scattering and disorder inside the film. This may be ascribed to the lattice distortion due to the expanded lattice of LCO and defects such as the dislocations arisen at higher substrate temperatures, just as inferred from the film morphology.

Figure 5(c) shows the magnetoresistance (MR) of Bi₂Se₃/LCO samples grown at different T_sub, together with a Bi₂Se₃/STO control sample measured at 1.7 K under a perpendicular magnetic field. The MR, defined as MR = [ρ_xx (H) − ρ_xx (0)]/ρ_xx(0), where ρ_xx(H) is the longitudinal resistivity as a function of the magnetic field H, is positive for all curves measured here. In the range near zero field, MR is characterized by the cusp-like shape due to the weak antilocalization (WAL) effect, which is a characteristic sign for strong spin-orbit coupled TI materials.^[24,44,45] On the surface of TI materials, backscattering is prohibited due to time-reversal symmetry when the magnetic field is absent. With an increasing magnetic field, which breaks the time-reversal symmetry, backscattering starts to occur and leads to a rise in the MR.^[45] The WAL effect is pronounced in the Bi₂Se₃/STO control sample. However, for the Bi₂Se₃/LCO samples, MR are reduced significantly, meaning that the WAL effect is suppressed due to the presence of underlying magnetic layer.^[19,21] Moreover, with the increasing T_sub, MR decreases monotonously for the Bi₂Se₃/LCO samples, implying further suppression of the WAL effect. The further reduced MR is caused by extra scattering from lattice disorder in the film with higher T_sub, which becomes dominated near zero field and thus result in a relatively decreased MR in higher field. Besides, it should be noted that, for the non-magnetic Bi₂Se₃/STO samples, MR increases with raised T_sub, which is contrary to the behavior of Bi₂Se₃/LCO samples.

Further analysis of the WAL effect is carried out by a quantitative fitting of low-field magnetoconductance (MC) data. The magnetoconductivity is defined as Δσ(H) = σ_xx(H) − σ_xx(0), where the sheet conductance (σ) is calculated by the relation σ_xx = ρ_xx/(ρ_yx² + ρ_xx²). As proposed in the literature,^[41–46] the WAL quantum corrections to the low-field magnetoconductivity in the strong spin–orbit coupling (SOC) limit can be described by the Hikami–Larkin–Nagaoka (HLN) equation^[47]

As plotted in Fig. 5(d), the magnetoconductivity of samples has an inverted shape as that in Fig. 5(c), and the fitting curves in orange lines agree very well with the experimental data. The reduced negative magnetoconductivity is an indicator of suppressed WAL effect in the magnetic samples. From the fitting data of HLN equation, we can obtain the two parameters α and l_φ, which are plotted in Fig. 5(e). For the Bi₂Se₃/LCO samples, both α and l_φ have an inverse dependence on the T_sub. The smaller values of α and l_φ at higher T_sub suggests more scattering inside the films. While for the Bi₂Se₃/STO control sample, α takes a value very close to 0.5, consistent with the previous studies.^[42,44] The coherence length l_φ has a larger value at higher T_sub, which is in contrast to the decreasing trend in Bi₂Se₃/LCO samples. The reduced coherence length and associated more scattering with increased T_sub further indicate a negative influence on the transport properties for Bi₂Se₃/LCO samples.

Transport properties in the tilted magnetic field are also measured for the Bi₂Se₃/LCO samples, as shown in Fig. 5(f). The MR has the largest value when it is measured in a perpendicular magnetic field, and is featured with a parabolic shape in higher field range, which is originated from the Lorentzian deflection of carriers.^[44] When the applied field is tilted from the perpendicular direction to a smaller angle, the MR decreases, but maintains the similar shape. While when the field approaches the parallel direction, the MR decreases significantly, and it is no longer in the quadratic relationships with the magnetic field. In a parallel field, the WAL effect is manifested due to the absence of large parabolic background, which is similar to the results on STO substrates.^[48]

The above data of magnetotransport measurements further indicate a crucial role played by the substrate temperature during growth, where the disorder arising from higher substrate temperature is proven to be disadvantageous for the transport properties of samples. These results are in consistence with the analysis of film morphology, and corroborate the conclusion that low substrate temperature is favorable for the growth of Bi₂Se₃/LCO.

In conclusion, we have performed a systematic investigation on the growth behaviors of epitaxial Bi₂Se₃ thin films on a ferromagnetic insulating LCO/STO substrate, which forms a new TI/FMI heterostructure. The Bi₂Se₃ films with highly crystalline structure and well-ordered surface are obtained at a relatively low substrate temperature of 150 °C during growth. Transport measurement confirms high carrier mobility in the sample grown at optimal conditions. An unusual dependence of film morphology and transport properties on the growth temperatures are observed, and can be attributed to the large thermal-expansion coefficient of LCO substrate and the increased strain stress at the interface at high substrate temperature. Our results and methodology of fabricating such high-quality TI/FMI heterostructure provide not only a platform for investigating the proximity-induced magnetism in TIs, but a versatile approach to explore the TI-based spintronic devices in applications.