Carrier dynamics in bulk GaN

Carrier dynamics in hydride vapor phase epitaxy grown bulk GaN with very low density of dislocations, 5–8 × 105 cm−2, have been investigated by time-resolved photoluminescence (PL), free carrier absorption, and light-induced transient grating techniques in the carrier density range of 1015 to ∼1019 cm−3 under single and two photon excitation. For two-photon carrier injection to the bulk (527 nm excitation), diffusivity dependence on the excess carrier density revealed a transfer from minority to ambipolar carrier transport with the ambipolar diffusion coefficient D a saturating at 1.6 cm2/s at room temperature. An extremely long lifetime value of 40 ns, corresponding to an ambipolar diffusion length of 2.5 μm, was measured at 300 K. A nearly linear increase of carrier lifetime with temperature in the 80–800 K range and gradual decrease of D pointed out a prevailing mechanism of diffusion-governed nonradiative recombination due to carrier diffusive flow to plausibly the grain boundaries. Under single photon excitation (266 and 351 nm), subnanosecond transients of PL decay and their numerical modeling revealed fast processes of vertical carrier diffusion, surface recombination, and reabsorption of emission, which mask access to pure radiative decay

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

Luminescence properties of LiGaO2 crystal

The study was supported by the Latvia-Lithuania-Taiwan research project “Nonpolar ZnO thin films: growth-related structural and optical properties” (Latvia: LV-LT-TW/2016/5 , Lithuania: TAP LLT 02/2014 , Taiwan: MOST 103-2923-M-110-001-MY3 ).The comprehensive spectral study of lithium metagallate LiGaO2 crystal has been done including methods of pump-probe techniques, optical absorption, photoluminescence, luminescence kinetics, thermoluminescence and polarised luminescence in broad temperature region. Luminescence spectrum of the crystal contains the main emission bands at 4.43, 3.76, 2.38 and 1.77 eV. The novel data on luminescence excitation spectra including VUV area, kinetics and polarization are presented. The correlation between pump-probe experiment results and luminescence properties is found. Conclusions are done about the recombination character of all the observed emission bands, implying tunnel recombination of donor-acceptor pairs.Ministry of Science and Technology 103-2923-M-110-001-MY3; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

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

Investigation of wide-band-gap semiconductor photoelectric properties by using optical techniques with temporal and spatial resolution

The thesis is dedicated to investigation of carrier dynamics in SiC, GaN and diamond by using light-induced transient gratings, differential transmittivity, differential reflectivity and photoluminescence techniques. Also new absorption coefficient measurement and carrier lifetime microscopy techniques are presented. The experimental studies were performed in a wide range of excess carrier densities and temperatures under single- and two- photon excitation conditions. Strong diffusion coefficient temperature and injection dependences were determined. They were explained by phonon scattering, inter-carrier interaction processes and degeneracy. It was determined, that in indirect-gap SiC and diamond semiconductors at low injections lifetime is limited by point and surface defects, while in GaN recombination on grain boundaries prevails. Also nonlinear Auger recombination coefficient was observed. It was Coulombically enhanced at low injections and screened at high ones. Photoluminescence efficiency revealed radiative recombination coefficient reduction with injection in GaN. On the other hand, in SiC radiative coefficient was injection independent. Compensating defect (aluminum in SiC and boron in diamond) concentrations and activation energies were determined from their absorption saturation and recovery rate temperature dependences