Growth and Characterization of GaBi Thin Films

Among the well-known III-V materials such as the phosphides, arsenides, and antimonides, this material class also contains a group of materials known as bismides. Bismides include the ternary compounds GaSb_1-xBi_x, GaAs_1-xBi_x, InSb_1-xBi_x, and the binary compounds AlBi, GaBi, and InBi. Density functional theory (DFT) calculations indicate that GaSb_0.5Bi_0.5, GaAs_0.5Bi_0.5, and InSb_0.5Bi_0.5 are metastable Dirac semimetals (DSMs) and with complete Bi substitution, GaBi and InBi are topological semimetals and AlBi is a topological insulator. Unlike other topological materials, bismides are part of the III-V family, and as such, their band structures and Fermi energies will be tunable through alloying, doping, and gating as in other III-V materials synthesized via molecular beam epitaxy (MBE). Another benefit of exploring topological materials in the well-developed III-V materials system is that binary bismides have been theoretically demonstrated to form in the simple zincblende crystal structure, making these materials easier to grow as thin films than current topological materials such as TaAs, Cd₃As₂, and NbP.<br/> <br/>Like the ternary compounds, the binary compounds have also been extensively studied via DFT. DFT calculations indicate that GaBi and InBi are topological semimetals that are distinct from Weyl or Dirac semimetals. GaBi and InBi show non-trivial Z₂ topology like that of topological insulators. DFT calculations have also shown that under strain, GaBi and InBi have no global gap between the valence band and conduction band, indicating that utilizing alloying to induce strain in these materials could modify their topological band structure.<br/> <br/>Despite the extensive DFT studies on the binary compounds, there has been little published on experimental synthesis of these compounds. There have been two published attempts to synthesize InBi as a thin film on GaAs(100) and Si(111) substrates. In the case of InBi grown on GaAs(100), this attempt was unsuccessful likely due to the large lattice mismatch between InBi and the GaAs substrate. The report of InBi on Si(111) was more successful, showing coalesced films formed in the tetragonal structure, although from x-ray diffraction there is still evidence of crystalline Bi present in these samples also likely due to the large lattice mismatch between the InBi film and the Si(111) substrate.<br/> <br/>Here, we present our recent work on synthesis of GaBi thin films via MBE. We demonstrate using an InSb substrate, we can stabilize GaBi in the zincblende phase and grow this material as a thin film. Through x-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), we demonstrate the effect of the Ga:Bi flux ratio on the coalescence and crystallinity of films. We also report uncoalesced films with droplets with substrate temperatures ranging from 150C to 300C, indicating a narrow window for coalesced growth. The best results based on XRD and EDS were at a substrate temperature of 200C, which is where we study the effect of the Ga:Bi flux ratio. Additionally, we report surface morphology characteristics such as root mean square roughness from atomic force microscopy. Overall, the demonstrated growth of GaBi as thin films offers new insights into the bismides material system and creates a jumping off point to study the topological nature of GaBi among the other binary bismides AlBi and InBi.