Abstract
Electrostatic doping in materials can lead to various exciting electronic properties, such as metal–insulator transition and superconductivity, by altering the Fermi level position or introducing exotic phases. Cd3As2, a three-dimensional (3D) analog of graphene with extraordinary carrier mobility, was predicted to be a 3D Dirac semimetal, a feature confirmed by recent experiments. However, most research so far has been focused on metallic bulk materials that are known to possess ultra-high mobility and giant magneto-resistance but limited carrier transport tunability. Here we report on the first observation of a gate-induced transition from band conduction to hopping conduction in single-crystalline Cd3As2 thin films via electrostatic doping by solid electrolyte gating. The extreme charge doping enables the unexpected observation of p-type conductivity in a ∼50-nm-thick Cd3As2 thin film grown by molecular beam epitaxy. More importantly, the gate-tunable Shubnikov–de Haas oscillations and the temperature-dependent resistance reveal a unique band structure and bandgap opening when the dimensionality of Cd3As2 is reduced. This is also confirmed by our first-principle calculations. The present results offer new insights toward nanoelectronic and optoelectronic applications of Dirac semimetals in general and provide new routes in the search for the intriguing quantum spin Hall effect in low-dimension Dirac semimetals, an effect that is theoretically predicted but not yet experimentally realized. The tunable quantum transport capabilities of cadmium arsenide thin films may unlock new applications for graphene-like semiconductors. Cadmium arsenide has similar electronic properties to graphene, but is easier to work with thanks to its three-dimensional crystal structure. Faxian Xiu of Fudan University in Shanghai and co-workers have now mapped out this material's band structure in confined 50-nanometre-thin film structures. By using a source-drain layout with an unconventional gate electrode — a droplet of ionic electrolyte that electrostatically dopes cadmium arsenide and changes its Fermi level — they saw remarkable conductivity switching behaviour, which is useful for ambipolar field effect transistors. Applying magnetic fields during device operation also revealed the possibility of generating quantum spin Hall effects — the team observed intriguing quantum oscillation conductivity when the Fermi level was pushed into the high-mobility conduction band. Cd3As2, which is known as a topological Dirac semimetal, has been grown on mica substrates by molecular beam epitaxy with high mobility. The temperature-dependent resistance of as-grown Cd3As2 thin films showed semiconducting behavior, indicating the band gap opening as opposed to the bulk counterpart. By solid electrolyte gating, the ambipolar effect and gate-tunable quantum oscillations were clearly demonstrated. These features make the Cd3As2 thin film system a promising platform to observe various exotic phenomena and realize new electronic applications.
Highlights
Dirac materials, such as graphene and topological insulators, have attracted substantial attention owing to their unique band structures and appealing physical properties originated from two-dimensional (2D) Dirac fermions with linear energy dispersion.[1,2,3,4] Recently, the existence of three-dimensional (3D) Dirac fermions has been theoretically predicted while several potential candidates, including β-BiO2,5 Na3Bi6 and Cd3As27 were explored as topological Dirac semimetals (TDSs), in which the Dirac nodes are developed via the point contact of conduction-valence bands
We demonstrate the tunable transport properties, including ambipolar effect and quantum oscillations of wafer-scale Cd3As2 thin films, deposited on mica substrates by molecular beam epitaxy
The surface morphology of the as-grown thin films was probed by atomic force microscopy with a root mean square of ~ 0.3 nm (Figure 1c)
Summary
Dirac materials, such as graphene and topological insulators, have attracted substantial attention owing to their unique band structures and appealing physical properties originated from two-dimensional (2D) Dirac fermions with linear energy dispersion.[1,2,3,4] Recently, the existence of three-dimensional (3D) Dirac fermions has been theoretically predicted while several potential candidates, including β-BiO2,5 Na3Bi6 and Cd3As27 were explored as topological Dirac semimetals (TDSs), in which the Dirac nodes are developed via the point contact of conduction-valence bands. Among 3D TDSs, Cd3As2 is considered to be an excellent material owing to its chemical stability against oxidation and extremely high mobility.[11,12,13,14] the electrical, thermal and optical properties of Cd3As2 have been widely investigated, hampered by the complicated crystal structure its band structure remains a matter of controversy.[14,15] Recently, first-principle calculations have revealed the nature of 3D topological Dirac semimetal state in Cd3As2.2,7,8 Soon after the prediction, its inverted band structure with the presence of Dirac fermions was experimentally confirmed.[11,13,16,17,18,19] More importantly, beyond the relativistic transport of electrons in bulk Cd3As2, a theoretically predicted topological insulator phase may eventually emerge upon the breaking of crystal symmetry.[7]
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