Modulation of the Bi 3+ 6s 2 Lone Pair State in Perovskites for High-Mobility p-Type Oxide Semiconductors.

Oxide semiconductors are key materials in many technologies from flat-panel displays，solar cells to transparent electronics. However, many potential applications are hindered by the lack of high mobility p-type oxide semiconductors due to the localized O-2p derived valence band (VB) structure. In this work, the VB structure modulation is reported for perovskite Ba 2 BiMO 6 (M = Bi, Nb, Ta) via the Bi 6s 2 lone pair state to achieve p-type oxide semiconductors with high hole mobility up to 21 cm 2 V -1 s -1 , and optical bandgaps widely varying from 1.5 to 3.2 eV. Pulsed laser deposition is used to grow high quality epitaxial thin films. Synergistic combination of hard x-ray photoemission, x-ray absorption spectroscopies, and density functional theory calculations are used to gain insight into the electronic structure of Ba 2 BiMO 6 . The high mobility is attributed to the highly dispersive VB edges contributed from the strong coupling of Bi 6s with O 2p at the top of VB that lead to low hole effective masses (0.4-0.7 m e ). Large variation in bandgaps results from the change in the energy positions of unoccupied Bi 6s orbital or Nb/Ta d orbitals that form the bottom of conduction band. P-N junction diode constructed with p-type Ba 2 BiTaO 6 and n-type Nb doped SrTiO 3 exhibits high rectifying ratio of 1.3 × 10 4 at ±3 V, showing great potential in fabricating high-quality devices. This work provides deep insight into the electronic structure of Bi 3+ based perovskites and guides the development of new p-type oxide semiconductors.