Saulius Nargelas

Diffusion-driven and excitation-dependent recombination rate in blue InGaN/GaN quantum well structures

We report on diffusion-driven and excitation-dependent carrier recombination rate in multiple InGaN/GaN quantum wells by using photoluminescence, light-induced absorption, and diffraction techniques. We demonstrate gradually increasing with excitation carrier diffusivity and its correlation with the recombination rate. At low carrier densities, an increase in radiative emission and carrier lifetime was observed due to partial saturation of non-radiative recombination centers. However, at carrier densities above ∼5 × 1018 cm−3, a typical value of photoluminescence efficiency droop, a further increase of diffusivity forces the delocalized carriers to face higher number of fast non-radiative recombination centers leading to an increase of non-radiative losses.

Hole diffusivity in GaAsBi alloys measured by a picosecond transient grating technique

We applied a time-resolved transient grating technique for investigation of nonequilibrium carrier dynamics in GaAs1−xBix alloys with x=0.025–0.063. The observed decrease in carrier bipolar diffusivity with lowering temperature and its saturation below 80 K revealed a strong localization of nonequilibrium holes. Thermal activation energy ΔEa=46 meV of diffusivity and low hole mobility value μh=10–20 cm2/V s at room temperature confirmed the hybridization model of the localized Bi states with the valence band of GaAs. Nonlinear increase in carrier recombination rate with the Bi content, 1/τR∝Bi(x)3.2 indicated an increasing structural disorder in the alloy.

Spectral distribution of excitation-dependent recombination rate in an In0.13Ga0.87N epilayer

Time-resolved optical techniques of photoluminescence (PL), light-induced transient grating (LITG), and differential transmission spectroscopy were used to investigate carrier dynamics in a single 50-nm thick In0.13Ga0.97N epilayer at high photoexcitation levels. Data in wide spectral, temporal, excitation, and temperature ranges revealed novel features in spectral distribution of recombination rates as follows: at low injection levels, an inverse correlation of carrier lifetime increasing with temperature and diffusivity decreasing with temperature confirmed a mechanism of diffusion-limited nonradiative recombination at extended defects. Carrier dynamics in the spectral region below the absorption edge but ∼70 meV above the PL band revealed a recombination rate that increased with excitation, while recombination rate in PL emission band (420–430 nm) decreased after saturation of trapping centers. Monitoring of spectrally integrated carrier dynamics by LITG technique allowed us to ascribe the enhanced rec...

Dynamics of free carrier absorption in InN layers

Carrier dynamics in highly excited InN epitaxial layers was investigated in the 1550–2440 nm (0.8–0.51 eV) spectral range by using a femtosecond differential transmission technique. A transition from induced bleaching to induced absorption was observed for probing energy of 90 meV below the bandgap of the samples. The decay of the induced free carrier absorption provided the averaged lifetime of the total nonequilibrium carriers. In the carrier density range of Δn=1018–1020 cm−3, the density-dependent recombination mechanism was attributed to trap-assisted Auger recombination with decay rate 1/τ=BTAARΔn, with BTAAR in the range (4–30)×10−10 cm3 s−1 for layers with different defect densities.

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.

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).

Recombination and diffusion processes in polar and nonpolar bulk GaN investigated by time-resolved photoluminescence and nonlinear optical techniques

Optically-injected carrier dynamics were investigated in bulk polar and nonpolar GaN in 1015-to-1020 cm-3 carrier density range, exploring single- and two-photon photoexcitation conditions. The excitation decay and recombination rates were monitored by time-resolved photoluminescence and free-carrier absorption techniques, while diffusivity was investigated by light-diffraction on transient grating technique. Carrier dynamics in c- and m-plane thick freestanding HVPE GaN revealed nearly linear increase of carrier lifetime with temperature in the 80 - 800 K range whereas the bipolar carrier diffusivity decreased with temperature. This feature suggests that the measured long lifetime values of 40-50 ns at RT result from diffusion-governed carrier flow to interface defects at GaN hexagons, which act as centers of nonradiative recombination. The fast PL transients under carrier injection to submicrometer thick layer were fitted by using the determined diffusivity and lifetime values and revealed a strong impact of vertical carrier diffusion, surface recombination, and reabsorption processes. Radiative and nonradiative emission rates were analyzed by various optical techniques to discriminate contribution of excitons and free carriers at various temperatures and injected carrier densities.

Spectral dependence of carrier lifetime in high aluminum content AlGaN epitaxial layers

The spectral dependences of the nonequilibrium carrier lifetimes obtained by using time-resolved photoluminescence (PL) spectroscopy reveal new features in the carrier dynamics in AlGaN epilayers with high aluminum content. Three decay components can be traced in the PL decay. The fast, intermediate, and slow components are attributed to the decay of the free carriers, localized carriers, and the carriers trapped in deep states, respectively. The spectral dependence of the fast component is flat with a very weak dependence on temperature. At low temperatures, the intermediate decay component has the longest effective decay times in the vicinity of the PL band peak and exhibits a decrease on both high and low energy slopes. The spectral dependence of the lifetimes flattens at elevated temperatures. To interpret the decrease in the carrier lifetime at lower energies, we suggest the modification of the double-scaled potential profile in AlGaN alloys by introducing a gradual potential decrease to the low-pote...

Injection-Activated Defect-Governed Recombination Rate in InN

Excess carrier dynamics was investigated by free-carrier absorption and light-induced transient grating techniques in InN layers with residual electron density varying from n0=1.4×1018 to 4.7×1018 cm-3 in a wide excitation range (up to 1020 cm-3). Carrier lifetime τ decreased with injected carrier density ΔN≥n0 and followed the same inverse relationship as on residual electron density τ∝[B(n0+ΔN)]-1, thus confirming defect-related recombination mechanism. Its nonradiative origin was verified by τ(T) measurements and ascribed to injection-enhanced nonlinear recombination via defect-assisted Auger recombination with CTAAR= B/NT=(4.5±2)×10-28 cm6/s, assuming the defect density NT being equal to electron density. Oxygen or hydrogen impurities are proposed as possible candidates for traps assisting in Auger process.

Influence of defects and indium distribution on emission properties of thick In-rich InGaN layers grown by the DERI technique

We report on the spatial variation of optical properties in thick, In-rich InGaN layers, grown by a novel droplet elimination by radical beam irradiation (DERI) technique. The increase of layer thickness causes layer relaxation and results in double-peaked photoluminescence spectra. Spatially resolved measurements show that the defects in the strained sub-layer are distributed inhomogeneously. An increase in the layer thickness results in faster nonradiative recombination due to increasing density of nonradiative recombination centers, as evidenced by time-resolved free carrier absorption, and facilitates larger indium incorporation in the upper part of the layer.