Phonon signatures for polaron formation in an anharmonic semiconductor
Abstract
Mechanistic studies on lead halide perovskites (LHPs) in recent years have suggested charge carrier screening as partially responsible for long carrier diffusion lengths and lifetimes that are key to superior optoelectronic properties. These findings have led to the ferroelectric large polaron proposal, which attributes efficient charge carrier screening to the extended ordering of dipoles from symmetry-breaking unit cells that undergo local structural distortion and break inversion symmetry. It remains an open question whether this proposal applies in general to semiconductors with LHP-like anharmonic and dynamically disordered phonons. Here, we study electron-phonon coupling in Bi 2 O 2 Se, a semiconductor which bears resemblance to LHPs in ionic bonding, spin-orbit coupling, band transport with long carrier diffusion lengths and lifetimes, and phonon disorder as revealed by temperature-dependent Raman spectroscopy. Using coherent phonon spectroscopy, we show the strong coupling of an anharmonic phonon mode at 1.50 THz to photo-excited charge carriers, while the Raman excitation of this mode is symmetry-forbidden in the ground-state. Density functional theory calculations show that this mode, originating from the A 1g phonon of out-of-plane Bi/Se motion, gains oscillator strength from symmetry-lowering in polaron formation. Specifically, lattice distortion upon ultrafast charge localization results in extended ordering of symmetry-breaking unit cells and a planar polaron wavefunction, namely a two-dimensional polaron in a three-dimensional lattice. This study provides experimental and theoretical insights into charge interaction with anharmonic phonons in Bi 2 O 2 Se and suggests ferroelectric polaron formation may be a general principle for efficient charge carrier screening and for defect-tolerant semiconductors.
Document Details
- Document Type
- Pub Defense Publication
- Publication Date
- Jul 21, 2022
- Source ID
- 10.1073/pnas.2122436119
Entities
People
- Feifan Wang
- Hailin Peng
- Jin Zhao
- Jue Wang
- Lucas Huber
- Teng Tu
- Weibin Chu
- Xiaoyang Zhu
- Yanan Dai
Organizations
- Columbia University
- National Science Foundation
- Office of Naval Research
- Peking University
- University of Science and Technology of China
- University of Southern California