A. Kadys

Black silicon: substrate for laser 3D micro/nano-polymerization

We demonstrate that black silicon (b-Si) made by dry plasma etching is a promising substrate for laser three-dimensional (3D) micro/nano-polymerization. High aspect ratio Si-needles, working as sacrificial support structures, have flexibility required to relax interface stresses between substrate and the polymerized micro-/nano- objects. Surface of b-Si can be made electrically conductive by metal deposition and, at the same time, can preserve low optical reflectivity beneficial for polymerization by direct laser writing. 3D laser polymerization usually performed at the irradiation conditions close to the dielectric breakdown is possible on non-reflective and not metallic surfaces. Here we show that low reflectivity and high metallic conductivity are not counter- exclusive properties for laser polymerization. Electrical conductivity of substrate and its permeability in liquids are promising for bio- and electroplating applications.

Impact of carrier localization, recombination, and diffusivity on excited state dynamics in InGaN/GaN quantum wells

We apply a number of all-optical time-resolved techniques to study the dynamics of free carriers in InGaN quantum structures under high excitation regime. We demonstrate that carrier lifetime and diffusion coefficient both exhibit a substantial dependence on excitation energy fluence: with increasing carrier density, carrier lifetime drops and diffusivity increases; these effects become more apparent in the samples with higher indium content. We discuss these experimental facts within a model of diffusion-enhanced recombination, which is the result of strong carrier localization in InGaN. The latter model suggests that the rate of non-radiative recombination increases with excitation, which can explain the droop effect in InGaN. We use the ABC rate equation model to fit light induced transient grating (LITG) kinetics and show that that linear carrier lifetime drops with excitation (i.e. excess carrier density). We do not observe any influence of Auger recombination term, CN3, up to the maximum carrier density that is limited due to the onset of very fast stimulated recombination process. To support these conclusions, we present spectrally resolved differential transmission data revealing different recombination rates of carriers in localized and extended states.

Optical and structural properties of BGaN layers grown on different substrates

Growth of BGaN epitaxial layers by metalorganic chemical vapor deposition (MOCVD) using triethylboron (TEB) as a boron source was studied on 6H-SiC substrate and on GaN and AlN templates on sapphire. X-ray diffraction, atomic force microscopy and photoluminescence spectroscopy were exploited to characterize the structural quality, surface morphology, luminescence efficiency, and boron content. Silicon carbide was shown to be slightly superior to AlN/sapphire and considerably better than GaN/sapphire as the most favorable substrate to incorporate a possibly higher boron content. Increasing TEB flow rate at correspondingly optimized growth temperature and V/III ratio enabled us to achieve the boron content of up to 5.5%, though at the expense of structural quality. We showed that the band gap bowing parameter is similar for the epilayers deposited on all the three templates/substrates under study and is approximately equal to 4 eV, substantially lower than reported before.

Growth of InN and In-Rich InGaN Layers on GaN Templates by Pulsed Metalorganic Chemical Vapor Deposition

InN and In-rich InGaN layers have been grown on GaN templates using the pulsed metalorganic chemical vapor deposition (MOCVD) technique and compared with analogous layers grown by conventional MOCVD. Structural investigations were performed using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Surface morphology was studied using atomic force microscopy. Electrical and optical properties were studied using Hall measurements and photoluminescence (PL) spectroscopy. All layers were free of metal droplets. InN grown using pulsed MOCVD showed fairly low background electron concentration (ne = 8 × 1018 cm−3 to 9 × 1018 cm−3) and high electron mobility (μe = 644 cm2 V−1 s−1). Structural studies revealed increase of size (from 100 nm to 500 nm) and decrease of density of InN islands at higher growth temperatures. For In-rich InGaN layers (In content 68% and 80%) the density of islands was similar to that in InN, while the diameter varied from 50 nm to 150 nm. Inhomogeneities of In and Ga distribution in the layers resulting in broadened XRD lines and PL bands are discussed.

A systematic study of light extraction efficiency enhancement depended on sapphire flipside surface patterning by femtosecond laser

We report on the investigation of light extraction efficiency enhancement at the flip-side surface of sapphire patterned by using femtosecond laser pulse irradiation. The enhancement was measured by means of fluorescence and confocal microscopies. Theoretical simulation (FDTD) gave the results. The variation of patterning geometry provided by different gap and pit size, by triangle and square array configuration revealed compactness of the pit network as an optimum condition for the enhancement. Maximum enhancement over 40% was obtained for the triangle array configuration. The appearance of ripples at the bottom of the pit corresponded to the enhancement. A deeper insight about consequences to radiative effects influenced by the ripples is presented.

Impact of Diffusivity to Carrier Recombination Rate in Nitride Semiconductors: From Bulk GaN to (In,Ga)N Quantum Wells

We combined light induced transient grating and free carrier absorption techniques to investigate temporal and spatial carrier dynamics in nitrides. Inverse correlation of diffusivity and nonradiative recombination rate in GaN epilayers was ascribed to carrier diffusive flow to the internal boundaries of hexagonal grains and recombination on dislocations there, while the similar dependence in InGaN quantum wells (QWs) was a consequence of carrier delocalization caused either by carrier injection or thermal escape. Numerical modeling of recombination rates in light emitting diode structures with In content up to 13% revealed increasing with excitation nonradiative recombination rate which is a consequence of higher overall carrier mobility at higher densities. We propose the injection-enhanced in-plane diffusivity as the most probable mechanism explaining the increase of non-radiative losses in high power light emitting diodes (LEDs).

Effects of photo-quenching-buildup and saturation of the induced impurity grating in ZnSe:Cr crystals

A model is proposed and calculations are performed to elucidate the effects of anomalous kinetics and dynamics of transient light self-diffraction signals (“a dip and a buildup” upon the termination of the grating generation pulse and saturation effects) observed in experiments on four-wave mixing in crystals doped with deep impurities. These observations are explained by the effects of optical depletion of local sites by the same grating generation pulse with only the subsequent buildup of the impurity grating through the capture of band carriers by these sites to form the previously created light-dynamic grating.

Low-temperature redistribution of non-thermalized carriers and its effect on efficiency droop in AlGaN epilayers

The carrier dynamics in AlGaN epilayers with different degrees of carrier localization were studied using low-temperature photoluminescence spectroscopy at different excitations. We observed a nonmonotonous band peak energy shift with increasing excitation, which is attributed to carrier-density-dependent carrier redistribution within localized states. The carrier redistribution enhances the carrier mobility and increases the nonradiative recombination rate resulting in efficiency droop. These results indicate the significant role of nonradiative recombination even at low temperatures and low carrier densities, despite strong carrier localization. The obtained results are consistent with the excitonic-type nonradiative recombination.

Evaluation of photoelectric processes in photorefractive crystals via the exposure characteristics of light diffraction

We demonstrate a novel way to analyse carrier recombination and transport processes in photorefractive semiconductors via the exposure characteristics of light induced diffraction. The results of a picosecond four-wave mixing on free carrier gratings in semi-insulating GaAs crystals at various grating periods and modulation depths of a light interference pattern are discussed. The role of a deep-trap recharging in carrier diffusion and recombination is sensitively revealed through a feedback effect of a space-charge field to non-equilibrium carrier transport.

Dependence of radiative and nonradiative recombination on carrier density and Al content in thick AlGaN epilayers

Dynamics of radiative and nonradiative recombination of non-equilibrium carriers is investigated in thick AlGaN epitaxial layers with Al content ranging from 0.11 to 0.71. The internal quantum efficiency (IQE) in the epilayers was obtained using two approaches: either estimated from PL measurements or calculated using the recombination coefficients of a simple ABC model, retrieved by fitting the kinetics of light induced transient gratings (LITG). At photoexcited carrier densities below ~1019 cm−3, both approaches provided similar IQE values indicating that the simple ABC model is applicable to analyze carrier recombination at such carrier densities. The increase in IQE at higher carrier densities slowed down for the values extracted from PL considerably faster than for those obtained from LITG transients. This discrepancy is explained in terms of the mixed nature of the rate coefficient B caused by the onset of the density-activated nonradiative recombination at high carrier densities.