Momentarily trapped exciton polaron in two-dimensional lead halide perovskites

Two-dimensional (2D) lead halide perovskites with distinct excitonic feature have shown exciting potential for optoelectronic applications. Compared to their three-dimensional counterparts with large polaron character, how the interplay between long- and short- range exciton-phonon interaction due to polar and soft lattice define the excitons in 2D perovskites is yet to be revealed. Here, we seek to understand the nature of excitons in 2D CsPbBr3 perovskites by static and time-resolved spectroscopy which is further rationalized with Urbach-Martienssen rule. We show quantitatively an intermediate exciton-phonon coupling in 2D CsPbBr3 where exciton polarons are momentarily self-trapped by lattice vibrations. The 0.25 ps ultrafast interconversion between free and self-trapped exciton polaron with a barrier of ~ 34 meV gives rise to intrinsic asymmetric photoluminescence with a low energy tail at room temperature. This study reveals a complex and dynamic picture of exciton polarons in 2D perovskites and emphasizes the importance to regulate exciton-phonon coupling.

Near band-edge and excitonic behavior of GaAsN epilayers grown by Chemical Beam Epitaxy

ABSTRACTPhotoluminescence and absorption spectroscopy experiments were performed on as grown and thermally annealed GaAs1-xNx with nitrogen content in the range of 0.75–7.1%. At low temperature, the photoluminescence spectra exhibits two set of features: (i) a relatively broad peak at low energy and near to the vicinity of the predicted band gaps and (ii) a sharp excitonic feature at higher energy (about 100 meV for x>4%). Post growth thermal annealing processes systematically favor stronger excitonic emissions, and a notable intensity reduction of the deeper (defect related) luminescence. The low temperature binding energy of the higher energy excitonic peak is found to be consistent with the increase of the electronic effective masses. A careful examination of the data obtained in this work suggests that for higher nitrogen content (x>4%), the fundamental band gap of GaAsN is located at significantly higher energies than those commonly accepted for these alloys.

Photoluminescence Detection of Native Defects in The Surface Region of Bulk CdTe

ABSTRACTThe native defects introduced by Br2/CH3 OH etching and aging under atmospheric conditions have been investigated in In doped, bulk CdTe using photoluminescence (PL) spectroscopy. The results indicate a large enhancement of the Cd vacancy related 1.5896 eV excitonic feature with chemical treatment and aging. Hence, the primary perturbation is interpreted to be a small loss of Cd within the sampling region. This result is compared and contrasted with previous studies of etching induced modifications. The implications of Cd depletion on interpreting PL spectra, device processing and long term stability are considered.