(Color online) Dependences of light-induced diffraction efficiency, η, and differential transmission DT on excitation energy fluence, I 0, in GaN under two-photon carrier injection. The indices γ are the slopes of the curves in log-log scale: expected ones (γ = 2 and 4, dashed lines) and experimental (γexp < γ, solid lines). Notice the deviation from expected behavior with increasing excitation fluence.
(Color online) Diffusive decay of LITG kinetics for two-photon carrier injection in bulk GaN with a grating period of 1.74 μm. Various decay times reveal dependence of diffusivity on temperature.
(Color online) (a) The measured dependences of diffusivity D on injected carrier density at different temperatures (symbols) and the modeled data (solid curves); at 80 K, the modeled curves with/without the impact of band gap renormalization (BGR) are compared. (b) Experimental data compared with theoretically calculated temperature dependence for hole (D h, theor) and ambipolar (D a = 2D h, theor) diffusivity (solid and dash dot curves, respectively), which account for acoustic (ac) and polar and nonpolar optical phonon (pop and npo, respectively) scattering (dotted curves). γ = 0.79 is the average slope (dashed line) of the dependence D h(T) in the log-log plot.
(Color online) Kinetics of FCA decay at various temperatures (a) and carrier lifetime vs. temperature for different injection levels (b). Fitting of τR(T) is composed of diffusion and interface defect governed recombination terms.
(Color online) Evolution of carrier depth profiles in bulk GaN after carrier injection to a 50 nm thick layer by a Ti-sapphire laser pulse at 267 nm wavelength. Surface recombination velocity S = 1.1 × 104 cm/s and D value varying from the ambipolar to the monopolar one (depending on the instant carrier density as determined before) were used for calculations.
(Color online) The calculated TRPL kinetics revealing impact of cases involving only diffusion (αR = 0, S = 0), diffusion and surface recombination (αR = 0, S ≠ 0), diffusion and reabsorption (αR ≠ 0, S = 0), and all three factors, diffusion, surface recombination, and reabsorption (αR ≠ 0, S ≠ 0). The expected slow PL decay component, governed by bulk recombination only (IPL∝ exp (−2t/τR) with τR = 40 ns) is shown by a dashed line.
(Color online) Comparison of carrier decay transients revealed by various optical techniques at single and two-phonon excitation conditions. PL transients recorded upon carrier injection to a thin surface layer by a strongly absorbed laser pulse (single photon carrier injection conditions). The inset shows the time-integrated PL spectrum, where the four spectral regions used to obtain the PL transients are marked. TRPL decays for four spectral regions (1 to 4) are compared with the calculated best fits (solid lines) using reabsorption coefficients of 7000 and 400 cm−1 in spectral regions 1 and 4, respectively, and S = 1.1 × 104 cm/s. The recombination-governed LITG decay (grating period Λ = 7.8 μm), at 1P injection conditions, 2 × 1019 cm−3, leads to carrier density decay instantaneous time of 3.2 ns (dashed line). For comparison, FCA decay at 2 P injection conditions (1017 cm−3) indicates a 40 ns lifetime (dashed line).
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