Anisotropy of free-carrier absorption and diffusivity in m-plane GaN

Polarization-dependent free-carrier absorption (FCA) in bulk m-plane GaN at 1053 nm revealed approximately 6 times stronger hole-related absorption for E⊥c than for E||c probe polarization both at low and high carrier injection levels. In contrast, FCA at 527 nm was found isotropic at low injection levels due to electron resonant transitions between the upper and lower conduction bands, whereas the anisotropic impact of holes was present only at high injection levels by temporarily blocking electron transitions. Carrier transport was also found to be anisotropic under two-photon excitation, with a ratio of 1.17 for diffusivity perpendicular and parallel to the c-axis

Light-induced reflectivity transients in black-Si nanoneedles

© 2015 Elsevier B.V. All rights reserved. The change in reflectivity of black-Si (b-Si) upon optical excitation was measured by the pump-probe technique using picosecond laser pulses at 532 (pump) and 1064 nm (probe) wavelengths. The specular reflection from the random pattern of plasma-etched b-Si nano-needles was dominated by the photo-excited free-carrier contribution to the reflectivity. The kinetics of the reflectivity were found to be consistent with surface structural and chemical analysis, performed by scanning and transmission electron microscopy, and spectroscopic ellipsometry. The surface recombination velocity on the b-Si needles was estimated to be ~102cm/s. Metalization of b-Si led to much faster recombination and alteration of reflectivity. The reflectivity spectra of random b-Si surfaces with different needle lengths was modeled by a multi-step refractive index profile in the Drude formalism. The dip in the reflectivity spectra and the sign reversal in the differential reflectivity signal at certain b-Si needle sizes is explained by the model

Carrier dynamics in highly-excited TlInS2: Evidence of 2D electron-hole charge separation at parallel layers

We report a comprehensive study of the time-resolved photoluminescence (PL), carrier recombination, and carrier diffusion under diverse laser pulse excitations in TlInS2. The 2Dlayered crystals were grown by Bridgman method without or by a small Erbium presence in the melt. The investigation expose large differences in two crystal types, although, a linear nonradiative lifetime and carrier diffusivity attain close values at high excitation with no contribution of the Auger recombination and absence of the band gap narrowing effect. Moreover, at high pulse power, we detect imprinted transient grating fringes which are attributed to new crystal phase formed by 2D electron-hole charge separation on local layers. The versatile model of the spontaneously polarized 2D-crystal has been developed to explain observed features and ergodicity of charge dynamic processes. The model embraces the planar stacking faults (PSFs) as a distortion which edge is acting as sink of strong recombination. The reduced occurrence of the PSFs in the Erbium doped TlInS2 is the main attribute which determines enhancement of PL by a factor of 50, and improves carrier diffusion along 2D-layers. The simulation permits to evaluate the PSFs sizes of about 0.7 m. Presented results allow improving 2D-crystal growth technology for novel sensor devices with separated excess charges