Abstract-Terahertz nano probing of semiconductor surface dynamics

Geunchang Choi, Young-Mi Bahk, Taehee Kang, Yoojin Lee, Byung Hee Son, Yeong Hwan Ahn, Minah Seo, and Dai-Sik Kim

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b03289

Most semiconductors have surface dynamics radically different from its bulk counterpart due to surface defect, doping level, and symmetry breaking. Due to the technical challenge of direct observation of the surface carrier dynamics, however, experimental studies have been allowed in severely shrunk structures including nanowires, thin films, or quantum wells where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nano probing system to investigate the surface dynamics of bulk semiconductors, using metallic nano gap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically decrease close to the limit of THz time resolution (~1 ps) as the gap size decreases down to nano scale, and that they return to their original values once the nano gap patterns are removed. Our THz nano probing system will open up pathways towards direct, and nondestructive measurements of surface dynamics of bulk semiconductors

Abstract-Crystallization Kinetics of Lead Halide Perovskite Film Monitored by In-situ Terahertz Spectroscopy

Sae-June Park, Ah-Ram Kim, Jung Taek Hong, Ji-Yong Park, Soonil Lee, and Yeong Hwan Ahn

J. Phys. Chem. Lett., Just Accepted Manuscript

DOI: 10.1021/acs.jpclett.6b02691

Publication Date (Web): January 4, 2017

Copyright © 2017 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpclett.6b02691?journalCode=jpclcd

Vibrational modes in the terahertz (THz) frequency range are good indicators of lead halide perovskite's crystallization phase. We performed real-time THz spectroscopy to monitor the crystallization kinetics in lead halide perovskite films. First, THz absorptance was measured while the perovskite film was annealed at different temperatures. During this process, we observed the dynamical phase transition of the perovskite structure from an intermediate phase to the tetragonal phase. By analyzing the Avrami exponent, we observed an abrupt dimensionality switch (of interfacial growth from 1D to 2D) with increasing temperature starting at approximately 90°C. More interestingly, we monitored the laser-induced enhancement of the crystallinity of the pre-annealed perovskite film. The THz absorptance increased initially, then subsequently decayed over a couple of hours, although the enhancement factor varies depending on the film crystallinity. In particular, the Avrami analysis implied that the enhanced crystallization upon light illumination was assisted by the 1D diffusion processes. The perovskite film conductivity increased by the light irradiation more dramatically than the thermal annealing due to percolation threshold behaviors. The activation energy for light-induced crystallization was measured of 2.3 eV, which indicated that enhanced crystallization originated from the photo-induced structural change of residual lead iodide at the grain boundary.