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Blue GaN based light emitting diode structure was deposited by metal organic chemical vapour deposition technique. The conventional photolithography technique followed by dry etching and contact evaporation was used to fabricate the mesas of the optoelectronic device. The produced structures were characterized by photoluminescence, electroluminescence, X-ray diffraction, scanning electron and atomic force microcopy. Electron beam induced current was used to study defects produced during the growth process. The electron beam induced current measurements showed that defect structures formed at different growth stages can be examined by changing incident electron beam energy and thus beam penetration depth. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitation-intensity dependent photoluminescence (PL) as well as effective recombination time of a series of c-plane GaN layers grown on sapphire substrates were measured at room temperature. It was shown that at low excitation level a high background free-electron concentration causes an increase of radiative recombination rate in the near band edge region of GaN that results in an increase of the PL efficiency. In the investigated series, the GaN layers with lower crystal quality but having higher free-electron concentration showed higher PL efficiency at excitation levels under ∼50 kW/cm2. At elevated excitation levels the impact of free-carrier concentration decreases and the PL efficiency is mainly determined by nonradiative recombination time. From simulations of excitation-intensity dependent PL efficiency, the Auger recombination coefficient in the investigated GaN samples was estimated to be much less then 10–30 cm6s–1. A drop of PL efficiency was observed at excitation levels higher than 2500 kW/cm2 when PL was registered from sample's surface. The reason of the drop was stimulated emission propagating along the in-plane direction and causing the saturation of photogenerated carrier density. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependent Hall effect, magnetoresistivity and time resolved photoconductivity studies have been performed on GaN epilayers and GaN/InGaN multiple quantum wells (MQWs) grown on sapphire films by Metal Organic Chemical Vapor Deposition (MOCVD) method. The MQW structures consisted of different width or different amount of quantum wells. We related the electrical parameters with the quantum structure parameters and found that the geometrical variation of the quantum wells may be substituted by the variation of the amount of the quantum wells. The Hall and the magnetoresistivity measurements showed the complex behavior, which was attributed to the inhomogeneities of the samples, similarly to the case of the crystals with great size defects (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Two optical techniques for the determination of a bipolar diffusion length LD of optically excited carriers in GaN epitaxial layers, namely a time-resolved picosecond four-wave mixing (FWM) on free carrier grating and time-integrated photoluminescence (PL) are presented and examined. The PL technique is based on time-integrated photoluminescence (PL) spectra measurements from the front and back sides of the sample under cw and nanosecond pulsed laser excitation. The another method utilizes time-resolved picosecond four-wave mixing (FWM) at various light-induced grating periods to extract diffusion coefficient and carrier recombination lifetime. The value of the diffusion length derived by means of FWM decreases with GaN layer thickness from LD = 260 nm (for 1.7 μm-thick layer) to LD = 100 nm (for 0.3 μm-thick layer). The integral PL measurements give the value of LD = 120-130 nm for the 620 nm layer under pulsed excitation intensities up to 200 kW/cm2. It increases to 150-170 nm at the excitation intensity enhancement to 1 MW/cm2. These values are close to the value of the diffusion length equal to 160 nm obtained using FWM for this layer thickness evidencing the compatibility of both methods. The changes in the value of LD are discussed in terms of the defect distribution in the epitaxial GaN layer. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the emission properties of nonpolar a -plane GaN layers grown by metalorganic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE). Together with the typical band edge exciton emission, the low-temperature photoluminescence spectra reveal several emission bands in the spectral range 3.29-3.42 eV. These emissions appear with different intensities in layers grown by different techniques and show different thermal quenching and recombination dynamics. The 3.42 eV and 3.35 eV emissions are found to be of an intrinsic origin and are associated with carriers localized at stacking faults. The emission at 3.29 eV, which is present only in MOCVD layers, shows a donor-acceptor pair behavior suggesting that impurities attached to structural defects are involved. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct measurements of carrier recombination dynamics in In0.1Ga0.9N/In0.03Ga0.97N multiple quantum well (QW) structures with different well thickness (d = 5.5, 17, 34 nm) are presented. The measurements by light-induced picosecond transient grating and time-resolved photoluminescence techniques revealed three contributing carrier recombination mechanisms at high carrier injections (“efficiency droop” conditions): stimulated emission (SE), bimolecular, and Shockley-Read-Hall. The study indicates on absence of Auger recombination rate with quadratic dependence, 1/τ ∝ N2, which was not observed up to ∼5x1019 cm-3 excess carrier density (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on photoelectrical and optical properties of the GaN layers overgrown on the columnar structure with diameter of columns varying from 250 to 600 nm. Excess carriers were injected to 2-μm thick overgrown layer by an interference pattern of two picosecond laser beams at 355 nm. Carrier recombination and diffusion was monitored by time-resolved transient grating technique and provided 3-4 fold increase of carrier lifetime (up to ∼500 ps) with respect to that in the GaN template. Even stronger increase of carrier lifetime at excess carrier density of ∼1019 cm-3 was observed both in the overgrown layers and substrate and attributed to saturation of electron trapping centers by free carriers. A factor of 7-8 stronger time-integrated photoluminescence intensity (at 363 nm) also confirmed the increased structural quality of the overgrown layers, what correlated well with the enhanced electrical parameters (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We analyze conditions when the deep-impurity governed carrier generation and transport in semi-insulating crystals create the coexisting free carrier, photorefractive, and absorptive nonlinearities. By monitoring the nonequilibrium carrier and space charge field dynamics via the probe beam diffraction on transient gratings, number of electrical parameters in bulk semi-insulating crystal have been determined, as carrier lifetime, diffusion coefficient, diffusion length, and sign of carriers, photogenerated from deep traps. We show a novel possibility to record a transient reflection grating via spatial modulation of the deep trap occupation and applicability of this technique to evaluate the compensation ratio of these traps as well as their recombination properties. Experimentally measured diffraction kinetics on free carrier, photorefractive and absorptive gratings in few nanosecond time interval provided the values of diffusion coefficient and carrier lifetime in bulk CdTe and ZnTe crystals, doped by V, Ge, Al, Sc as well in InP:Fe. Carrier transport peculiarities in these crystals revealed the type (n or p) of dominant carriers, generated from the deep impurity levels. In n-type HVPE GaN crystals, we observed impact of light-induced space-charge field to carrier diffusion. The transient reflection gratings in semi-insulating CdTe:V allowed discrimination of deep trap governed carrier recombination rate from the other recombination channels. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The carrier diffusion and recombination were investigated by light induced transient grating technique (LITG) in a HVPE grown GaN at intense band-to-band excitation. The given measurements in high structural quality layers revealed the features of an intrinsic crystal – carrier-density dependent recombination and diffusion. It was found that ambipolar carrier diffusion coefficient D and carrier lifetime τ were dependent on excitation intensity and therefore varied during the grating decay. The values of D = 1.5 cm2/s and τ = 3.8 ns were found for carrier densities ∼1018 cm–3, while at the highest excitation used (n∼1020 cm–3) the D value reached 4.1 cm2/s, and τ decreased to 1.3 ns. The numerical modelling of carrier dynamics confirmed that the observed increase of diffusion coefficient with carrier density was due to degeneracy of carrier plasma, while short carrier lifetime at high carrier density was ruled by a bimolecular recombination. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)