GaN light emitting diodes (LEDs) on sapphire substrates can be improved by micro-patterning substrate to perform epitaxial over-growth which drastically reduces defects' density in the light emitting region. We patterned Al2O3 with focused ion beam and show a successful overgrowth of GaN. The exact shape of pattern milled into Al2O3 was replicated into a 0.4-mm-thick shim of Ni by electroplating. The surface roughness of Ni was ~5:5 ±2 nm and is applicable for the most demanding replication of nano-rough surfaces. This technique can be used to replicate at micro-optical elements Fresnel-axicons defined by electron beam lithography made on sub-1 mm areas without stitching errors (Raith EBL). Shimming of macro-optical elements such as car back- reflectors is also demonstrated. Ni-shimming opens possibility to make replicas of nano-textured small and large area patterns and use them for thermal embossing and molding of optically-functionalized micro-fluidic chips and macro-optical elements.
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.
We demonstrate a novel application of time-resolved transient grating technique for determination of deep trap
occupation ratio in semi-insulating crystals. Light diffraction kinetics on a transient reflection grating with very small
period (150 nm) provided conditions for studies of absorption nonlinearity and its discrimination from the coexisting free
carrier and electro-optic ones. By numerical modeling of absorption grating kinetics in subnanosecond time domain, we
determined the contributions of the recharged deep traps and two-photon absorption to diffraction kinetics and evaluated
in this way the deep trap compensation ratio in differently grown GaAs crystals. Moreover, the decay time of the
absorption grating provided the rate of carrier capture to these dominant deep traps, which have been recharged under
illumination. Using this feature, we were able to monitor the thermal annealing process in vanadium-doped CdTe
crystals: it has not effected the charge state of vanadium related deep traps, but reduced the concentration of the active
residual carrier capture centers in the crystal.
Non-equilibrium carrier generation, transport, and recombination have been investigated in Fe-doped InP crystals
experimentally in subnanosecond time domain by using time-resolved picosecond four-wave mixing technique. The
carriers were generated by below band-gap excitation at 1064 nm (hv = 1.17 eV), what allowed photoexcitation of non-equilibrium
carriers from/via Fe-related deep levels. The contributions of Fe2+/Fe3+ deep level states and of the excited
state Fe2+* have been analyzed by numerical modeling, using the relevant model which took into account carrier
generation via different defect states, as well recombination and diffusion. The modeling was helpful to get insight into
varying with excitation generation rates of holes and electrons and explained the main features of FWM kinetics and
exposure characteristics.
Time-integrated and time-resolved FWM techniques were applied for characterization of the photoelectrical properties
of undoped, S-doped, and Fe-doped InP wafers and for a mapping of their homogeneity. We performed measurements of
spatial distribution of diffraction efficiency across the wafers by nanosecond FWM and investigated a physical origin of
the observed variations by using time-resolved picosecond FWM. By analyzing the diffraction efficiency kinetics and its
dependence on excitation energy, we evaluated the impurity-assisted carrier generation, recombination, diffusion
processes, electrical activity of the defects, and their distribution across the wafers. Carrier lifetime variation from 2.5 ns
to 7.5 ns across the undoped InP wafer was found, while the diffusion coefficient value of 8 ± 0.5 cm2/s was almost
constant. In S-doped InP wafer, wafer inhomogeneity was attributed to carrier generation peculiarities governed by
spatial distribution of deep centers.
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