To assess the impact of random alloying on the optical properties of the InGaN alloy, high-quality InxGa1−xN (0 < x < 0.18) epilayers grown on c-plane free-standing GaN substrates are characterized both structurally and optically. The thickness (25–100 nm) was adjusted to keep these layers pseudomorphically strained over the whole range of explored indium content as checked by x-ray diffraction measurements. The evolution of the low temperature optical absorption (OA) edge linewidth as a function of absorption energy, and hence the indium content, is analyzed in the framework of the random alloy model. The latter shows that the OA edge linewidth should not markedly increase above an indium content of 4%, varying from 17 meV to 30 meV for 20% indium. The experimental data initially follow the same trend with, however, a deviation from this model for indium contents exceeding only ∼2%. Complementary room temperature near-field photoluminescence measurements carried out using a scanning near-field optical microscope combined with simultaneous surface morphology mappings reveal spatial disorder due to growth meandering. We conclude that for thick high-quality pseudomorphic InGaN layers, a deviation from pure random alloying occurs due to the interplay between indium incorporation and longer range fluctuations induced by the InGaN step-meandering growth mode.
Fe doped GaN was studied by time-resolved photoluminescence (PL) spectroscopy. The shape of PL transients at different temperatures and excitation powers allowed discrimination between electron and hole capture to Fe3+ and Fe2+ centers, respectively. Analysis of the internal structure of Fe ions and intra-ion relaxation rates suggests that for high repetition rates of photoexciting laser pulses the electron and hole trapping takes place in the excited state rather than the ground state of Fe ions. Hence, the estimated electron and hole capture coefficients of 5.5 × 10−8 cm3/s and 1.8 × 10−8 cm3/s should be attributed to excited Fe3+ and Fe2+ states. The difference in electron capture rates determined for high (MHz) and low (Hz) (Fang et al., Appl. Phys. Lett. 107, 051901 (2015)) pulse repetition rates may be assigned to the different Fe states participating in the carrier capture. A weak temperature dependence of the electron trapping rate shows that the potential barrier for the multiphonon electron capture is small. A spectral feature observed at ∼420 nm is assigned to the radiative recombination of an electron in the ground Fe2+ state and a bound hole.
A multimode scanning near-field optical microscopy technique that allows the mapping of surface morphology, photoluminescence (PL) spectra in illumination and illumination-collection modes, and PL dynamics, all in one scan, has been developed along with a method to use it for evaluation of carrier diffusion. The method allows measuring diffusion lengths as small as ∼100 nm and their anisotropy and spatial distribution, parameters remaining inaccessible to conventional far-field techniques. The procedure has been applied to study ambipolar carrier diffusion in a nonpolar m-plane InGaN/GaN quantum well. The diffusion was found to be highly anisotropic with diffusion coefficients along and perpendicular to the wurtzite c axis equal to 0.4 and 1.9 cm 2 /s, respectively. The large diffusion anisotropy confirms band structure calculations that suggest that the topmost valence band in an m-plane InGaN quantum well is highly anisotropic.
Time-resolved scanning near-field photoluminescence (PL) spectroscopy was applied to map carrier lifetimes in wide m-plane InGaN/GaN quantum wells grown on on-axis and miscut substrates. Both radiative and nonradiative lifetimes were found to be spatially nonuniform. Lifetime variations were smaller for quantum wells grown on miscut, as compared to on-axis substrates. Correlation with surface topography showed that largest deviations of recombination times occur at +c planes of pyramidal hillocks of the on-axis sample. Observed correlation between radiative lifetimes and PL peak wavelength was assigned to a partial electron localization.
Time-resolved transmission and reflection measurements were performed for bulk GaN at room temperature to evaluate the energy of the first conduction band satellite valley. The measurements showed clear threshold-like spectra for transmission decay and reflection rise times. The thresholds were associated with the onset of the intervalley electron scattering. Transmission measurements with pump and probe pulses in the near infrared produced the intervalley energy of 0.97 ± 0.02 eV. Ultraviolet pump and infrared probe reflection provided a similar value. Comparison of the threshold energies obtained in these experiments allowed estimating the hole effective mass in the upper valence band of 1.4m0. Modelling of the reflection transients with rate equations suggests that intra-and intervalley electron -LO phonon scattering times are about 30 fs and 8 fs, respectively.
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