High-temperature-grown buffer layer boosts electron mobility in epitaxial La-doped BaSnO₃/SrZrO₃ heterostructures

Transparent conducting oxides (TCOs) have attracted attention due to their unique properties and applications in electronic devices such as transparent displays and transistors.^1,2 Recently, the transparent perovskite La:BaSnO₃ has gained interest as a novel transparent conducting oxide (TCO) due to its high mobility at room temperature (RT).^3,4 Single crystals of La:BaSnO₃ have been reported to have RT mobilities as high as 320 cm² V⁻¹s⁻¹ (mobile carrier concentration n = 8 × 10¹⁹ cm⁻³).⁴ At doping concentrations above 10¹⁹ cm⁻³, La:BaSnO₃ has the second highest RT electron mobility among TCOs and other oxide single crystals,^4–6 exceeded only by CdO.⁷ The high RT mobility in La:BaSnO₃ has been attributed to the small electron-phonon interaction and small electron effective mass (⁠$me*=0.19m0$⁠) arising from the large dispersion of the conduction band composed of Sn 5s orbitals.^8–10 However, to fully explore the potential of La:BaSnO₃, thin films with high mobilities are also required.

Recently, several efforts to achieve high carrier mobility in La:BaSnO₃ films have been reported.^11–18 Nevertheless, as compared to bulk single crystals, even the best La:BaSnO₃ epitaxial films show a reduced mobility (≤183 cm² V⁻¹s⁻¹),¹⁸ which specifically has been attributed to scattering from charge defects, such as threading dislocations (TDs),^8,18–20 and, more generally, to small carrier relaxation times.^8,9 TDs form for a large lattice mismatch between the film and the substrate, and extend perpendicularly through the films. The obvious solution to reduce TDs in La:BaSnO₃ films is to use lattice-matched substrates, but unfortunately none exists. The substrate with the closest lattice match that is commercially available is PrScO₃, mismatched by − 2.3%.²¹ With such a high mismatch, only thin commensurate layers of BaSnO₃ can be grown; at a thickness of 32 nm, the reported BaSnO₃ films are almost fully relaxed and contain high densities of TDs.¹⁹ 

To reduce the dislocation density in the La:BaSnO₃ film, we explored the insertion of an undoped BaSnO₃ buffer layer at the interface between the substrate (TbScO₃) and the La:BaSnO₃ film. As the thickness of the BaSnO₃ layer increases, the threading dislocation (TD) density decreases as the threading component of dislocations annihilate each other, leaving behind a network of misfit dislocations. This method is known and has already led to the highest mobilities in BaSnO₃ films to date, but even for thick buffer layers (330 nm), the remaining TD density is still 1.2 × 10¹¹ cm⁻².¹⁸ Conceptually, growing this undoped BaSnO₃ buffer layer at higher substrate temperatures should lower TD densities further, but due to the significant volatility of tin oxide at substrate temperatures above about 850 °C, this is not a viable option.

In this work, we report on an alternative approach to significantly reduce the dislocation density and increase the mobility of La:BaSnO₃ films. By inserting between the film and the substrate an insulating buffer layer of SrZrO₃ grown at a very high temperature, we greatly reduce the density of TDs. To avoid possible contamination, we first optimized the growth conditions of the films on (110) TbScO₃ substrates thermally prepared in situ by directly heating the substrate with a CO₂ laser. Without resorting to postgrowth treatments, we achieve a significant reduction of the density of TDs from 5 × 10¹¹ cm⁻² to 4.9 × 10⁹ cm⁻² and a maximum RT mobility of 140 cm² V⁻¹s⁻¹ for La:BaSnO₃ films prepared on a SrZrO₃ buffer (grown at 1300 °C). Extending prior reports, this RT mobility is obtained for films of small thickness (25 nm). It is the highest mobility for La:BaSnO₃ films grown by pulsed laser deposition (PLD).

Epitaxial La:BaSnO₃ films with a La-doping content of 6% and a thickness of 25 nm were grown on several (110) oriented TbScO₃ single crystalline substrates (5 × 5 × 1 mm³). All samples were grown by PLD (λ = 248 nm) at a target-substrate distance of 56 mm, using a CO₂ laser substrate heating system. The La:BaSnO₃ films were grown at 850 °C and 1.5 J cm⁻² at 1 Hz in 1 × 10⁻¹ mbar of O₂. Films were grown on either SrZrO₃ or BaSnO₃ buffer layers. The buffer layers were deposited at 4 Hz to a thickness of 100 nm. We chose SrZrO₃ as a candidate because its lattice parameter value (4.101 Å)^22,23 is between that of La:BaSnO₃ (4.116 Å) and TbScO₃ (3.955 Å).²⁴ Also, like TbScO₃, SrZrO₃ has a low vapor pressure and can therefore be grown at high temperatures. SrZrO₃ layers were deposited at temperatures ranging from 850 °C to 1600 °C with a laser fluence of 2 J cm⁻² at 1.4 × 10⁻² mbar of O₂. The BaSnO₃ buffer layers were grown at 850 °C with a fluence of 1.5 J cm⁻² at 1 × 10⁻¹ mbar of O₂. Following Ref. 18, BaSnO₃ was chosen as it has roughly the same lattice parameter as La:BaSnO₃.

Prior to deposition, the TbScO₃ substrates were in situ terminated at a high temperature with the CO₂ laser. Figure 1(a) shows a typical atomic force microscopy (AFM) image of thermally prepared substrates heated at 1300 °C for 200 s. The substrate surface is smooth and well-ordered, with clear and uniform terraces. More details on the in situ thermal preparation of the substrates and related oxide surfaces are provided in Ref. 25. Figure 1(b) shows a photograph of the growth chamber during the deposition of the SrZrO₃ buffer layer at 1600 °C.

The deposition of both the active layer (La:BaSnO₃) and the buffer layer (SrZrO₃) was in situ monitored by reflection high-energy electron diffraction (RHEED) [Fig. 1(c)]. The SrZrO₃ layer was deposited at 1300 °C. Immediately after the deposition, the sample was cooled to 850 °C at 2 K/s for the subsequent growth of the La:BaSnO₃ film. We observe that the intensity of the RHEED oscillations remains the same throughout the deposition of the SrZrO₃ layer. For the La:BaSnO₃ film, the intensity drops after several monolayers and then stabilizes. The intensity drop suggests a relaxation in the film after a critical thickness has been reached (see Fig. S1 of the supplementary material). The RHEED data indicate that the SrZrO₃ buffer layer and the La:BaSnO₃ films are grown in a layer-by-layer mode with a smooth surface as also demonstrated by streaky RHEED patterns for both layers [see Fig. 1(d)].

The crystalline quality and phase purity of the films were characterized by X-ray diffraction (XRD) using Cu K_α radiation. Figure 2(a) shows the 2θ − ω scans around the 002 diffraction planes for the La:BaSnO₃/TbScO₃ (sample A1), La:BaSnO₃/BaSnO₃/TbScO₃ (sample A2), and La:BaSnO₃/SrZrO₃/TbScO₃ (sample A3) films. Laue thickness fringes and phase-pure La:BaSnO₃ 00l peaks are observed, indicating smooth growth and high crystallinity [Fig. 2(a), see also Fig. S2 of the supplementary material]. The film thicknesses were extracted from the Laue thickness fringes and the Kiessig fringes observed by XRD. The extracted out-of-plane lattice parameters are c = 4.134 Å, 4.120 Å, and 4.113 Å for samples A1, A2, and A3. These values are consistent with the out-of-plane lattice constants reported previously for La:BaSnO₃ films^20,26 and are close to the bulk lattice parameter (∼4.126 Å) of polycrystalline 6% La:BaSnO₃.^27,28 Figures 2(b) and 2(c) show reciprocal space maps (RSM) around the asymmetric (⁠$1¯03$⁠)_p reflection peaks of the films (samples A2 and A3) and the substrate. The in-plane and out-of-plane lattice constants of the film in sample A2 are a = 4.117 Å and c = 4.120 Å, respectively, indicating that the film is almost completely relaxed. The relatively small deviation of the lattice constants is attributed to the large lattice mismatch (3.9%) between BaSnO₃ and TbScO₃ [Fig. 2(b)]. The broadening of the film peak indicates the presence of a large density of dislocations. For sample A3, the extracted in-plane and out-of-plane lattice constants are a = 4.103 Å and c = 4.113 Å, indicating that the film is fully strained in the a direction (−0.56%) and partially relaxed in the c direction. From the XRD data (see Fig. S3 of the supplementary material) of a control sample of SrZrO₃ grown at 1300 °C (in the same growth conditions as the buffer layer in sample A3), in-plane and out-of-plane lattice constants of a = 4.085 Å and c = 4.128 Å are obtained, indicating that the buffer layer is almost fully strained to the substrate (3.966 Å), suggesting a small number of dislocations in film A3 as discussed below.

Resistivity (ρ), electron mobility (μ_e), and carrier concentration (n) were characterized in a physical property measurement system (PPMS) in a van der Pauw geometry by wire bonding aluminum wires to the samples’ corners. An excitation current of 1 μA was used. Using the procedure discussed in Refs. 29 and 30, the carrier concentration was calculated as n = 1/eR_H, where R_H is the measured Hall coefficient; and the electron mobility was extracted from μ_e = R_H/ρ, where ρ was determined by the van der Pauw method. Figure 3(a) presents the temperature dependence of the resistivity at zero magnetic field for samples A1, A2, and A3. All films exhibit metallic behavior over the entire temperature range, consistent with previous reports on La:BaSnO₃.^3,4 The temperature dependence of the carrier density shows the behavior of a degenerate semiconductor.³ The concentration of the negatively charged carriers is temperature independent at high temperatures but starts to decrease below 50 K following the freeze out at the La⁺³ ions [Fig. 3(b)].³¹ The temperature dependence of the electron mobility is presented in Fig. 3(c). For all three samples, μ_e increases down to the lowest temperature (2 K), contrary to previous studies which reported a saturation below 50 K.^18,19 This behavior suggests a significant reduction of phonon scattering for these films at low temperatures. For the sample directly grown on the substrate (A1), a mobility of 76 cm² V⁻¹s⁻¹ (RT) was measured. In the sample with the BaSnO₃ buffer layer (A2), the RT μ_e is improved to 117 cm² V⁻¹s⁻¹ but apparently still limited by the high density of TDs, as discussed below. To reduce the density of TDs, we switched to SrZrO₃ buffer layers. Several La:BaSnO₃/SrZrO₃ heterostructures were prepared by varying the growth temperatures of the SrZrO₃ layer from 850 °C to 1600 °C, while keeping the other growth parameters constant. We find 1300 °C as the optimal growth temperature of the SrZrO₃ buffer layer to achieve high mobility in the La:BaSnO₃ layers [Fig. 3(d)]. Above 1300 °C, the buffer layers start to become more nonstoichiometric, i.e., the ratio of Sr:Zr deviating from 1 in SrZrO₃ due to the volatility of constituents³² [see Fig. S3(d) of the supplementary material]. This results in an enhanced defect density (e.g., shear defects and/or point defects arising from nonstoichiometry¹⁸), limiting the mobility of the La:BaSnO₃ layers deposited on buffer layers grown at temperatures above 1300 °C. Sample A3 prepared on a buffer layer grown at 1300 °C has the highest mobility μ_e = 140 cm² V⁻¹s⁻¹ (RT) and 375 cm² V⁻¹s⁻¹ (2 K) [Fig. 3(c)]. For PLD grown La:BaSnO₃ thin films, this RT μ_e is ∼30% higher than the previous record (100 cm² V⁻¹s⁻¹) achieved without postgrowth treatment on BaSnO₃ substrates²⁶ and 15% higher than the reported enhanced mobility (122 cm² V⁻¹s⁻¹) after postannealing processes in H₂ forming gas at 950 °C on SrTiO₃.³³ 

To further investigate the role of TDs that act as scattering centers and trap electrons,^8,13,20,34 we studied the defect structure of samples A2 and A3 using cross-sectional transmission electron microscopy (TEM). Figures 4(a) and 4(b) present the weak-beam dark-field TEM (WB-DFTEM) images of the entire film thickness for samples A2 and A3, respectively. For both samples, misfit dislocations are visible along the interface as bright dots, indicated in Figs. 4(a) and 4(b) by red arrows. To accommodate the large lattice mismatch between the substrate and the film, these misfit dislocations generate edge-type defects, which extend vertically through the film [bright contrast indicated by white arrows in Fig. 4(a)]. For a TEM specimen of 16 nm thickness, the extracted density of TDs in sample A2 was 5 × 10¹¹ cm⁻². This density of TDs is in agreement with previous reports on La:BaSnO₃ films prepared on DyScO₃¹⁸ and SrTiO₃²⁰ substrates. Interestingly, TDs were barely observed in the highest mobility sample A3, as shown in the WB-DFTEM image in Fig. 4(b). Even for a TEM specimen of 205.5 nm thickness, the TD density was 4.9 × 10⁹ cm⁻² which is two orders of magnitude lower than the one in sample A2 and previously reported TD densities for La:BaSnO₃ films.^18,20 Figures 4(c) and 4(d) depict the high resolution TEM images of samples A2 and A3, respectively. A fully relaxed interface between the TbScO₃ substrate and the BaSnO₃ layer is seen in sample A2 as indicated by white circles. Additional structural defects such as stacking faults are also visible in the film [see yellow arrows in Fig. 4(c)]. On the other hand, a strained interface is seen in sample A3 with no apparent structural defects [Fig. 4(d)].

The low density of TDs in sample A3 is attributed to the high temperature used for the SrZrO₃ buffer layer growth. One of the effective ways of minimizing the TD density is to enhance the motion and reaction of TDs by high thermal stress. This method was demonstrated to be efficient in epitaxial semiconductor GaAs films on Si substrates.^35,36 At high temperatures, the velocity of the dislocation glide motion and the concentration of vacancies that support the climb motion of the dislocations are exponentially enhanced.³⁷ Thus, the observed significant reduction of the TD density in sample A3 is attributed to the enhancement of the glide and climb motion at high temperatures. The stress that increases the glide motion originates from the difference in the thermal expansion coefficients of ∼3.1 between SrZrO₃ and TbScO₃^38,39 and also possible grown-in strain in the SrZrO₃ layer. The latter is supported by the fact that SrZrO₃ undergoes a phase transition from tetragonal to cubic at high temperatures,³⁹ indicating epitaxial strain at the interface during cool down, yielding a reduction of the lattice mismatch. Our results suggest that the high temperature grown SrZrO₃ epilayer deposited not only on TbScO₃ but also on other oxide substrates (e.g., SrTiO₃, DyScO₃, MgO) can be utilized as a template for subsequent growth of high mobility La:BaSnO₃ films with fewer TDs.

Although we have demonstrated that by inserting a high-temperature grown SrZrO₃ buffer layer between the film and the substrate the density of TDs significantly reduces with an associated improvement of the electron mobility in La:BaSnO₃ films, we point to several observations. Normally, scattering by TDs not only diminishes the electron mobility but also reduces the number of free charge carriers.²⁰ However, we observe that the carrier density of sample A2 (with the high density of TDs) is higher than that of sample A3 (with a low TD density) [Fig. 3(b)]. Given that the active layer in our samples is thin (25 nm), the lower number of free charge carriers in sample A3 suggests that not only TDs are trapping electrons, but also effects such as surface scattering or interface traps are lowering the density of mobile carriers. For sample A2, these contributions are expected to be less pronounced as the buffer layer (BaSnO₃) and the active layer consist of the same materials.

In summary, we explore the possible use of buffer layers grown at very high temperatures for the reduction of TDs and improvement of electron mobility in epitaxial La-doped BaSnO₃/SrZrO₃ heterostructures grown on (110) TbScO₃ substrates. For the La:BaSnO₃ films prepared on a SrZrO₃ buffer layer grown at 1300 °C, a RT mobility of 140 cm² V⁻¹s⁻¹ has been achieved, together with a reduction of the density of TDs to 4.9 × 10⁹ cm⁻², all without postgrowth sample treatment. With the insertion of a high temperature grown buffer layer between the La:BaSnO₃ film and the TbScO₃ substrate, the mobility has been doubled, which opens a new road toward high mobilities in La:BaSnO₃ based electronic devices.

See supplementary material for analysis of RHEED oscillations, additional XRD measurements of La:BaSnO₃/SrZrO₃ films, characterization of SrZrO₃ buffer layers, and an overview of transport characteristics and lattice parameters of different La:BaSnO₃/SrZrO₃/TbScO₃ heterostructures for several growth temperatures of the SrZrO₃ buffer layer.

We acknowledge valuable discussions with Darrell G. Schlom and Hans Boschker and thank Helga Hoier and Marion Hagel for technical assistance. W. Sigle and P. van Aken acknowledge funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 823717 -ESTEEM3.