R. Aleksiejūnas

Determination of free carrier bipolar diffusion coefficient and surface recombination velocity of undoped GaN epilayers

Time-resolved nondegenerate four-wave mixing experiments were performed on 2.6-μm-thick GaN epilayers grown by metalorganic chemical-vapor deposition using picosecond pulses at 355 and 1064 nm for grating recording and probing, respectively. Measurements of free carrier grating kinetics at various grating periods Λ in range from 3 to 10 μm allowed determination of nonequilibrium carrier bipolar diffusion coefficient D=1.7 cm2/s at 300 K. Substitution of the D value into two-dimensional carrier transport model allowed fitting of the whole set of grating kinetics at various Λ varying surface recombination velocity S and linear recombination time τR. This procedure provided us a value of S=5×104 cm/s as well as carrier lifetime of ∼1 ns.

The determination of high-density carrier plasma parameters in epitaxial layers, semi-insulating and heavily doped crystals of 4H-SiC by a picosecond four-wave mixing technique

We applied a picosecond four-wave mixing technique for measurements of carrier lifetimes and diffusion coefficients in highly excited epitaxial layers, semi-insulating and heavily doped 4H-SiC substrates. Optical carrier injection at two different wavelengths (266 and 355 nm) allowed us to vary the depth of the excited region from 1–2 µm to 50 µm, and thus determine photoelectric parameters of carrier plasma in the density range from 2 × 1016 to 1019 cm−3. Strong dependence of carrier lifetime and mobility on carrier density was found in the epitaxial layers. The origin of fast decay components, not resolved previously by photoluminescence and free-carrier absorption techniques in SiC, was attributed to nonlinear carrier recombination. Numerical modelling provided a value of bimolecular recombination coefficient equal to B = (2–4) × 10−11 cm3 s−1 and verified a surface recombination velocity S = 4 × 104 cm s−1. In heavily doped crystals, nonlinear carrier recombination reduced the carrier lifetime down to 1.1 ns, while in semi-insulating ones a lifetime of 1.5–2.5 ns was measured. Temperature dependences of four-wave mixing provided monopolar carrier mobility in heavily doped and bipolar one in semi-insulating crystals, and revealed the contribution of ionized impurity and phonon scattering mechanisms.

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.

Diffusion-limited nonradiative recombination at extended defects in hydride vapor phase epitaxy GaN layers

Time-resolved free-carrier absorption and transient grating techniques were applied to determine carrier lifetimes and diffusion coefficients in a set of hydride vapor phase epitaxy GaN layers of various thickness (from 10 to 145 μm). A linear increase in nonradiative carrier lifetime in 80–800 K range found to be in a correlation with decrease of the bipolar carrier diffusion coefficient. This correlation confirmed that recombination rate is governed by carrier diffusive flow to the grain boundaries of columnar defects. A model of diffusion-governed nonradiative lifetime was proposed for fitting the measured lifetime values in the layers of different thickness as well as lifetime dependence on temperature or threading dislocation density.

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.

Fast optical nonlinearity induced by space-charge waves in dc-biased GaAs

Time-resolved four-wave mixing in dc-biased GaAs crystals exhibit GHz frequency oscillations when the applied dc field and nonequilibrium carrier density are high enough to form a photorefractive high-field domain grating. Its propagation through the spatially modulated carrier density structure creates a photorefractive space-charge wave and leads to an oscillatory behavior of the probe-beam diffraction efficiency with period T=Λ/ν≈200 ps, determined by the grating spacing Λ and domain drift velocity ν. Formation of the space-charge wave in dc-biased GaAs was verified experimentally by using a picosecond transient grating technique. The experimental data were found to be in good agreement with numerical calculations based on the hot electron hydrodynamic model.

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.