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

Improved quantum efficiency in InGaN light emitting diodes with multi-double-heterostructure active regions

InGaN light emitting diodes(LEDs) with multiple thin double-heterostrucutre (DH) active regions separated by thin and low energy barriers were investigated to shed light on processes affecting the quantum efficiency and means to improve it. With increasing number of 3 nm-thick DH active layers up to four, the electroluminescence efficiency scaled nearly linearly with the active region thickness owing to reduced carrier overflow with increasing total thickness, showing almost no discernible efficiency degradation at high injection levels up to the measured current density of 500 A/cm2. Comparison of the resonant excitation dependent photoluminescence measurements at 10 K and room temperature also confirmed that further increasing the number of DH layers beyond six results in degradation of the material quality, and therefore, increasing nonradiative recombination. Using multiple DH active regions is shown to be a superior approach for quantum efficiency enhancement compared with simply increasing the single DH thickness or the number of quantum wells in LED structures due to better material quality and larger number of states available

On Optical Characterization of Carrier Lifetimes in GaN Layers by Time-Resolved Four-Wave Mixing and Photoluminescence Techniques

We provide numerical analysis of nonequilibrium carrier dynamics in GaN layers at interband photoexcitation by a picosecond light pulse. By solving the continuity equation for bipolar carrier plasma, we analyze spatial and temporal evolution of carrier density. We show that fast carrier diffusion to the bulk determines the carrier in-depth profile in GaN epilayers with a thickness larger than the carrier diffusion length. By integrating the carrier spatial profiles at experimental conditions, corresponding to time-resolved four-wave mixing and time-resolved photoluminescense we simulate the four-wave mixing and time-resolved photoluminescense kinetics in subnanosecond time domain. The modeling data using parameters of the studied GaN epilayers (their thickness, diffusion coefficient, carrier lifetime, and absorption coefficients at emission wavelengths) were compared with the experimental results. The analysis provided conditions at which the discrepancy between the measured carrier lifetime by time-resolved photoluminescense and time-resolved four-wave mixing may occur. For hydride-vapor phase epitaxy GaN layers with a large diffusion length, the fast photoluminescense kinetics are confirmed by modeling and experiments that they are due to diffusion governed carrier in-depth redistribution, while four-wave mixing kinetics remain insensitive for carrier in-depth redistribution

High-Speed Quadratic Electrooptic Nonlinearity in dc-Biased InP

We present experimental data on degenerate four-wave mixing as well as simulation results of fast optical nonlinearities in highly-excited semiinsulating InP under applied dc-field. Hot-electron transport governed enhancement of optical nonlinearity is obtained by applying a dc-field of 10-14 kV/cm at full-modulation depth of a light-interference pattern. The hydrodynamic model, which incorporates both free-carrier and photorefractive nonlinearities is used to explain the experimentally observed features. We show that the enhancement of optical nonlinearity is due to the quadratic electrooptic effect

On Oscillating Carrier Dynamics in Highly Excited InP:Fe Crystals

The numerical analysis and experimental data on time-resolved four-wave mixing confirmed a novel origin of oscillations in subnanosecond carrier dynamics in highly excited InP:Fe crystals. The effect was attributed to simultaneous presence of electron and hole gratings, which drift in the space charge field and contribute constructively or destructively to refractive index modulation in time domain

Time-Resolved Transient Grating Spectroscopy for Studies of Nonequilibrium Carrier Dynamics in Wide Band-Gap Semiconductors

Using interdisciplinary fields relevant to a highly excited semiconductor - nonequilibrium phenomena in high density plasma, light-induced changes of optical properties, and dynamic holography, we developed time-resolved four-wave mixing technique for monitoring the spatial and temporal carrier dynamics in wide band-gap semiconductors. This opened a new possibility to analyse fast electronic processes in a non-destructive "all-optical" way, i.e. without any electrical contacts. This technique allowed evaluation of recombination and transport processes and the determination of important carrier parameters which directly reveal the material quality: carrier lifetime, bipolar diffusion coefficients, surface recombination rate, nonlinear recombination rate, diffusion length, threshold of stimulated recombination. The recent experimental studies of differently grown group III-nitrides (heterostructures and free standing films) as well silicon carbide epilayers by nondegenerate picosecond four-wave mixing are presented