A well known semiconductor Cd3As2 has reentered the spotlight due to its unique electronic structure and quantum transport phenomena as a topological Dirac semimetal. For elucidating and controlling its topological quantum state, high-quality Cd3As2 thin films have been highly desired. Here we report the development of an elaborate growth technique of high-crystallinity and high-mobility Cd3As2 films with controlled thicknesses and the observation of quantum Hall effect dependent on the film thickness. With decreasing the film thickness to 10 nm, the quantum Hall states exhibit variations such as a change in the spin degeneracy reflecting the Dirac dispersion with a large Fermi velocity. Details of the electronic structure including subband splitting and gap opening are identified from the quantum transport depending on the confinement thickness, suggesting the presence of a two-dimensional topological insulating phase. The demonstration of quantum Hall states in our high-quality Cd3As2 films paves a road to study quantum transport and device application in topological Dirac semimetal and its derivative phases. Despite many achievements in the topological semimetal Cd3As2, the high-quality Cd3As2 films are still rare. Here, Uchida et al. grow high-crystallinity and high-mobility Cd3As2 thin films and observe quantum Hall states dependent on the confinement thickness.

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Quantum Hall states observed in thin films of Dirac semimetal Cd3As2

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.

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Gate-tunable quantum oscillations in ambipolar Cd 3 As 2 thin films

Opto(electrical) properties of new aromatic polyazomethines with fluorene moieties in the main chain for polymeric photovoltaic devices

The temperature-dependent optical absorption of 3D arrays of close-packed strongly quantized ZnSe QDs, deposited in thin film form, is studied in the interval from 11 to 340 K. Because of the particle size distribution and interdot coupling between proximal QDs within the QD arrays, the excitonic peaks are not visible at all, even at temperatures as low as 11 K. The temperature coefficient of the band-gap energy in the strongly quantized QD arrays was found to be twice larger than the value characteristic of a bulk ZnSe specimen. The Debye temperature, on the other hand, is shown to decrease by about 15% in comparison with the bulk value, which is attributed to the phonon confinement effects. It is shown that the sub-band-gap exponential absorption tails in the strongly quantized 3D QD arrays obey the Urbach−Martienssen rule. The temperature dependence of the Urbach energy and the relation between this quantity and the band-gap energy of the films could be excellently fitted to the predictions of the Cody...

G. Pocztowski

Papers

Optical properties of amorphous thin films of the ZnP system

Quantum size effect in thin Cd3As2 films

Some properties of thin amorphous Cd3As2 films related to deposition conditions

2.20 Size effects in thin films of cadmium arsenide