Topological Dirac semimetal Na₃Bi films in the ultrathin limit via alternating layer molecular beam epitaxy

Topological Dirac semimetals (TDSs) are characterized by band touching and linear energy-momentum dispersion at specific locations inside the Brillouin zone, known as Dirac points.^1,2 Graphene is perhaps the most famous example of this class of materials, a 2D TDS with Dirac points present at high symmetry K and K′ points.³ Recently, materials such as Na₃Bi and Cd₂As₃ have been demonstrated to be 3D TDS with two Dirac points situated close to the Brillouin zone center and possessing linear dispersions in all three momentum directions [Fig. 1(a)].^4–13 Theory predicts that the quantum confinement of a 3D TDS will create a tunable bulk bandgap and alternate the system between the trivial insulator and the quantum spin Hall insulator as a function of film thickness.^14,15 For Na₃Bi, this tunability requires high-quality epitaxial growth of (0001)-oriented films on insulating substrates in the ultrathin limit (few nanometers).

To date, (0001)-oriented thin films of Na₃Bi have been grown on Si(111), Al₂O₃(0001), graphene, and SiO₂/Si(001) by molecular beam epitaxy (MBE),^16–22 and their band structures have been characterized by angle-resolved photoemission spectroscopy (ARPES). Recent scanning tunneling microscopy (STM) and ARPES studies of ultrathin (∼1 nm) Na₃Bi films on Si(111) already show evidence of bandgap opening and gap tunability through an application of electric fields.²³ In addition, Fuhrer and co-workers have performed transport measurements in situ within their MBE chamber to avoid oxidation.^19–22 They observed mobilities as high as 7000 cm²/V s, obtained as-grown carrier densities in the 10¹⁷ cm⁻³ range, and performed gate-tunable transport which exhibit a resistance maxima at the Dirac point. All of the MBE growths have been performed by codeposition in the adsorption-limited regime where excess Na flux re-evaporates and the growth rate is determined by the Bi flux. However, while transport studies have been performed on thicker Na₃Bi films on insulating substrates (20 nm or greater^19–22), transport measurements in the ultrathin regime have not been reported. In addition, even though ultrathin films of Na₃Bi on Si(111) have been used to great effect in uncovering the topological nature of the films,²³ they are not suitable for detailed magnetotransport studies due to the possibility of parallel conduction (i.e., shunting) through the substrate.

In this work, we report the growth of high quality, continuous Na₃Bi films on insulating Al₂O₃(0001) in the ultrathin regime by employing alternating layer MBE (AL-MBE) and demonstrate ohmic conductivity in films with 3.8 nm thickness (4 unit cells). We find that the sequential growth of atomic layers of Na and Bi at room temperature results in (0001)-oriented Na₃Bi films with uniform surface coverage at very low thickness. Reflection high-energy electron diffraction (RHEED) and x-ray photoelectron spectroscopy (XPS) measurements reveal that single crystal films are formed after just 2 unit cells of deposition. Scanning tunneling microscopy (STM) shows smooth films with continuous coverage, and van der Pauw measurements demonstrate charge transport on 3.8 nm films. X-ray diffraction (XRD) measurements on an 8 unit cell film exhibit a peak in the θ-2θ scan corresponding to Na₃Bi(0002). Furthermore, these AL-MBE films can also serve as seed layers for thicker Na₃Bi films grown by the conventional codeposition method with quality comparable to previously published work, as revealed by STM and the first observation of RHEED oscillations which indicate atomic layer-by-layer growth. This new growth method will enable investigation of quantum confinement and other novel physical phenomena in the ultrathin limit of Na₃Bi thin films.

Na₃Bi ultrathin films are grown on Al₂O₃(0001) substrates in an MBE chamber with a base pressure of 8 $×$ 10⁻¹¹ Torr. The Al₂O₃(0001) substrates (MTI crystal) are prepared by cleaning with isopropyl alcohol and annealing in air at 1000 °C for 3 h to saturate oxygen vacancies and smooth the surface. The substrates are then annealed in a vacuum at 600 °C for 30 min. Elemental Na (99.95%, Alfa Aesar) and Bi (99.999%, Alfa Aesar) are evaporated from thermal effusion sources whose growth rates are calibrated by a quartz deposition monitor. For codeposition in the adsorption-limited regime, typical deposition rates are ∼2.5 nm/min for Na and ∼0.17 nm/min for Bi, where the excess Na is re-evaporated due to the elevated substrate temperature (120–170 °C). The Bi deposition rate is further confirmed by measuring the period of RHEED oscillations during codeposition growth. For AL-MBE growth, the substrate is at room temperature and the Na and Bi have a typical growth rate of ∼0.2 nm/min. The chamber pressures during the alternating layer and codeposition growths are 2 $×$ 10⁻¹⁰ Torr and 3 $×$ 10⁻¹⁰ Torr, respectively. The sample temperature is monitored by a transferrable thermocouple that directly measures the temperature of the tantalum sample paddle. Since Na₃Bi will oxidize when exposed to air, most characterizations are performed in air-free environments. For in situ characterization by XPS and STM, samples are transferred from the MBE chamber into the respective systems using an ultrahigh vacuum (UHV) suitcase; XPS spectra are taken using the Al K-α cathode source; for transport characterization, samples are transferred from UHV to a glove box using a custom-made UHV-compatible sample vessel that is carried into the glove box through its antechamber; and for XRD characterization, samples are transferred from UHV to the glove box and loaded into an air-free XRD sample holder to avoid air exposure during the measurement.

Na₃Bi has a hexagonal crystal structure (P63/mmc space group) with lattice constants a = 0.5448 nm and c = 0.9655 nm.²⁴ As shown in Fig. 1(b), it consists of honeycomb NaBi layers which are sandwiched between hexagonal Na layers above and below along the c-axis, making up half of a unit cell. Since the underlying Al₂O₃(0001) substrate has an in-plane lattice constant of a = 0.4785 nm, Na₃Bi films with (0001) orientation and 30° in-plane rotation have a lattice mismatch of only 1.4% [Fig. 1(c)].

The AL-MBE growth of Na₃Bi on Al₂O₃(0001) is performed at room temperature and consists of alternating the growth of a flux-matched NaBi monolayer followed by a pure Na bilayer and repeating this sequence. Figure 2 summarizes the evolution of RHEED patterns throughout the AL-MBE growth of Na₃Bi. Figure 2(a) shows the Al₂O₃(0001) substrate prior to growth along the $⟨112¯0⟩$ and $⟨101¯0⟩$ azimuths, indicating a clean and smooth starting surface. After depositing a 1 unit cell of Na₃Bi, a faint RHEED pattern of Na₃Bi(0001) appears along both directions, while the Al₂O₃ Kikuchi lines are suppressed [Fig. 2(b)]. After depositing an additional unit cell, the diffraction pattern is very clear along both directions with no hint of the underlying substrate [Fig. 2(c)]. Terminating the AL-MBE growth after 4 unit cells leads to the patterns shown in Fig. 2(d), which remain largely unchanged even at higher thickness. To compare the AL-MBE growth with the conventional codeposition growth,^19–22 we perform codeposition growth directly onto an Al₂O₃(0001) substrate at 120 °C (the Na:Bi flux ratio is ∼20:1). These conditions are chosen to match the recipe for the 2 nm seeding layer used in previous studies. Figure 2(e) shows the RHEED patterns obtained after codepositing 4 unit cells of Na₃Bi using this method. The pattern along Al₂O₃ $⟨101¯0⟩$ looks similar to the pattern obtained by AL-MBE, while the $⟨112¯0⟩$ azimuth shows hints of the Al₂O₃ substrate peaks (white arrows) superimposed on the Na₃Bi diffraction pattern.

To characterize the film morphology, we perform a combination of in situ STM and ex situ AFM measurements. Figure 3(a) shows an AFM image of an Al₂O₃(0001) substrate after a 1000 °C air anneal which produces flat surfaces with ordered atomic steps. Following the growth of 5 unit cells of Na₃Bi by AL-MBE, we image the topography by in situ STM [Fig. 3(b)] under constant current conditions (200 pA tip current, 200 mV sample bias). The images exhibit a smooth surface morphology, with terraces having a typical width of ∼5 nm and a step height of ∼0.5 nm corresponding to a single monolayer (half unit cell). Moreover, the films are macroscopically flat, with a height variation of ∼1 nm [Fig. 3(b) inset]. Our attempts to perform in situ STM on 4 unit cell films deposited at 120 °C through codeposition failed due to tip crash, which suggests that the deposited films are not continuous and thus not electrically conducting. Therefore, we utilize ex situ AFM measurements to perform the comparison. Figures 3(c) and 3(d) show AFM images of 5 unit cell Na₃Bi films deposited by AL-MBE and codeposition growth, respectively. It is worthwhile to note that the air exposure does cause the AL-MBE film to be rougher in the AFM image [Fig. 3(c)] compared to the in situ STM image [Fig. 3(b)], as one would expect from sample oxidation. Nevertheless, the AFM comparison provides a clear indication of the improved morphology and continuous film structure for AL-MBE growth [Fig. 3(c)], as compared to the codeposition growth which is highly non-uniform with bare substrate patches separating disconnected islands [Fig. 3(d)].

The continuity of the AL-MBE films is further verified by electrical transport measurements which are performed inside a glove box. The room temperature resistivity measurement of a 4 unit cell Na₃Bi film shows a linear I-V curve with a resistivity of ∼7 × 10⁻⁴ Ω m [Fig. 3(g)]. This is higher than ∼5 $×$ 10⁻⁵ Ω m previously reported for thicker 20 nm films, which may suggest bandgap opening or the importance of surface scattering in the ultrathin films. Further studies are needed to understand the sources of electron momentum scattering in ultrathin films.

Additional structural characterization is provided by XRD performed on an 8 unit cell AL-MBE film [Fig. 3(e)]. The θ-2θ scan shows the expected Al₂O₃(0006) substrate peak at 2θ = 41.68° as well as a peak at 2θ = 17.60° [detailed scan in Fig. 3(f)] which corresponds to the (0002) diffraction peak of Na₃Bi and thus verifies the (0001) crystallographic orientation of the film. No additional peaks are detected within the scan range, indicating an absence of other phases.

In situ XPS characterization provides insight into the chemical composition of the Na₃Bi films (Fig. 4). Survey scans from 4 unit cell films grown by AL-MBE [Fig. 4(a), green dashed line] and codeposition [Fig. 4(a), red solid line] show peaks from Na 1s, Bi 4f, O 1s, and Al 2p core levels, as well as Auger peaks (Na KLL). The Al 2p and O 1s peaks are expected due to the finite escape depth of photoelectrons from the Al₂O₃ substrate.²⁵ Analysis of the Na 1s peaks [Fig. 4(b)] and the Bi 4f peaks [Fig. 4(c)] from the two samples reveals the stoichiometry of Na_3.76Bi for the AL-MBE growth and Na_4.54Bi for the codeposition, indicating that both films are Na rich. By comparing the two samples, the AL-MBE growth has sharper and taller Na and Bi peaks, while the codeposition growth has relatively smaller Na and Bi peaks. The spectra on codeposited films also have shoulder peaks at 1074.20 eV, 164.24 eV, and 158.72 eV, which are indicative of Na₂O and Bi₂O₃ formation.^26,27 Furthermore, comparing the relative heights of the O 1s peaks [Fig. 4(d)] and the Al 2p peaks [Fig. 4(e)] provides additional evidence for a smoother film morphology by AL-MBE growth. The much stronger O and Al peaks for codeposition compared to AL-MBE growth indicate an increased contribution from the Al₂O₃ substrate due to patchy growth for codeposition, which is consistent with the discontinuous island morphology observed in the AFM image [Fig. 3(d)]. These results indicate that AL-MBE growth leads to material of higher quality than codeposition in the ultrathin film regime.

In addition, ultrathin films synthesized by the AL-MBE method can be used as a seed layer for subsequent codeposition growth. The advantage of codeposition in the adsorption-limited regime is the better control over stoichiometry by re-evaporation of excess Na. We have investigated this growth mode at two different temperatures, 120 °C and 170 °C. In both cases, we initiate growth on Al₂O₃(0001) with a 4 unit cell Na₃Bi film grown by AL-MBE at room temperature. The substrate temperature is then ramped to 120 °C, and codeposition growth is initiated by opening the Na and Bi shutters (the Na:Bi flux ratio is ∼20:1). Notably, we observe RHEED intensity oscillations for growth at 120 °C [Fig. 5(a)] which indicates atomic layer-by-layer growth. Interestingly the oscillation period is a full unit cell, or two monolayers, which indicates that the lattice structure of the unit cell remains intact. For the 170 °C growth, the substrate temperature is ramped from 120 °C to 170 °C during the initial stages of growth, as shown in Fig. 5(b). We observe RHEED oscillations during the initial stages of growth, which disappear when the substrate temperature exceeds 150 °C. STM measurements compare the morphology of 35 nm thick films grown under these two conditions. Figure 5(c) shows a typical STM image for a 120 °C sample, showing ∼0.5 nm atomic steps and typical terrace widths of 40 nm. Figure 5(d) shows a typical STM image for a 170 °C sample, showing ∼0.5 nm atomic steps and typical terrace widths of 150 nm. The inset of Fig. 5(d) is a high resolution image of the atomic lattice whose measured in-plane lattice constant is 0.55 nm, which is consistent with Na₃Bi (0.5448 nm). Finally, XPS spectra taken on films grown at 170 °C indicate the stoichiometry of Na_3.50Bi, which is slightly better than the AL-MBE grown films in Fig. 4. These results show that films codeposited at 170 °C on the top of an AL-MBE seed layer have similar morphology to the ones synthesized by direct codeposition in previously published studies.^19–22 Interestingly, codeposition at 120 °C proceeds in a layer-by-layer fashion with a trade-off for smaller atomic terrace size, likely due to the reduced atom mobility on the surface during growth. More detailed characterization would be necessary to determine if layer-by-layer growth results in higher electronic quality of the films.

In summary, we demonstrate that alternating layer MBE growth at room temperature can be used to synthesize uniform and continuous Na₃Bi films on Al₂O₃(0001) substrates in the ultrathin regime (few nm). The morphology is greatly improved compared to codeposition of Na₃Bi films, which grow as disconnected islands when deposited directly on Al₂O₃(0001). We demonstrate in-plane electrical conductance on films as thin as 3.8 nm (4 unit cells), which enables transport studies of topological and quantum phenomena in these systems. In addition, the ultrathin AL-MBE films can be used as seed layers for atomic layer-by-layer growth by codeposition in the adsorption-limited regime. These results facilitate the investigation of Na₃Bi films in the ultrathin limit, where the effects of quantum confinement can lead to novel topological phenomena such as the thickness-tunable quantum spin Hall effect.

We acknowledge the technical assistance of Guanzhong Wu. Funding for this research was provided by the Center for Emergent Materials: an NSF MRSEC under Award No. DMR-1420451.