  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
magnetoresistance 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 (SdH) 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-principles calculations.
The present results offer new insights towards 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.
