S. Miasojedovas

Optical monitoring of nonequilibrium carrier lifetime in freestanding GaN by time-resolved four-wave mixing and photoluminescence techniques

Optical monitoring of nonequilibrium carrier dynamics was performed in freestanding GaN. Four-wave mixing kinetics directly provided carrier lifetime of 5.4ns in the layer, while complementary measurements by photoluminescence technique revealed the fast transients with subnanosecond decay time. Numerical modeling of photoluminescence decay taking into account the carrier spatial-temporal dynamics allowed us to attribute an origin of the fast photoluminescence transients to carrier diffusion to the bulk and to reabsorption of the backward emission. The studies demonstrated carrier diffusion limited applicability of the time-resolved photoluminescence technique for carrier lifetime measurements in a high quality thick III-nitride layers.

Luminescence decay in highly excited GaN grown by hydride vapor-phase epitaxy

Carrier recombination dynamics in GaN grown by hydride vapor-phase epitaxy has been studied by means of transient photoluminescence under high photoexcitation conditions that are close to stimulated emission regime. The luminescence transient featured an exponential decay with the time constant of 205 ps at room temperature. The transient was shown to be in good agreement with a model of saturated centers of nonradiative recombination with the trap density of ∼1017 cm−3 and carrier recombination coefficients of ∼10−8 cm3/s. In such a regime, the lifetimes of electrons and holes have a common value of 410 ps.

Carrier and defect dynamics in photoexcited semi-insulating epitaxial GaN layers

Transients of fast free-carrier recombination and of multitrapping processes, determined by different types of defects, have been traced by photoluminescence (PL) and contact photoconductivity (CPC) in semi-insulating GaN epitaxial layers. To eliminate effects caused by the electrodes, the CPC decays were supplemented with noninvasive microwave absorption transients. The lifetimes of fast recombination and initial free-carrier capture processes were evaluated using ultraviolet (UV) time-resolved photoluminescence transients. The UV PL band peaked at 3.42 eV with contributions from both stimulated and spontaneous emission was attributed to band-to-band recombination. At the highest excitations, the initial PL decay time exhibited a value of 880 ps due to nonradiative free-carrier recombination. The radiative centers were revealed in continuous-wave PL spectra, where the UV band was accompanied with the bands of blue (B) PL, peaked in the range of 2.82–3.10 eV, and yellow (Y) PL, peaked at 2.19 eV, ascribed...

Optical gain in homoepitaxial GaN

Optical gain in GaN epilayers, grown by metalorganic chemical-vapor deposition technique on bulk GaN substrates is studied by means of time-resolved luminescence spectroscopy at room temperature. Both stimulated emission and carrier recombination rate are analyzed under high photoexcitation conditions that are close to laser operation regime. Homoepitaxial GaN shows a high value of optical gain coefficient g=7200cm−1 estimated under intense pulsed excitation by a variable stripe method. For comparison, a GaN epilayer grown under identical conditions on sapphire shows a significantly lower value, g=2300cm−1. Larger values of the optical gain coefficient achieved in homoepitaxial GaN are due to the lower density of nonradiative traps. This is proved by the carrier capture time that is estimated right after exhaustion of the inverted population, and has values of τe=970 and 195ps for homo- and heterolayers, respectively.

Increase of free carrier lifetime in nonpolar a-plane GaN grown by epitaxial lateral overgrowth

Carrier recombination dynamics in epitaxial a-plane GaN and fully coalesced epitaxial laterally overgrown (ELOG) a-plane GaN films has been studied by means of time-resolved photoluminescence under high photoexcitation. The results were compared with conventional c-plane GaN films grown under similar conditions. In a-plane GaN epilayers, the total efficiency of electron-hole plasma spontaneous luminescence decreases 20 times, whereas the luminescence decay time reduces from τLU=42 to τLU⩽10ps in comparison with c-plane GaN films. Meanwhile, an essential increase in total emission efficiency (by more than two orders of magnitude) and an increase of the decay time up to τLU=430ps have been observed for an ELOG a-plane sample in comparison with a-plane GaN films. This confirms a significant reduction of the nonradiative recombination rate for nonequilibrium carriers. Assuming a saturation of the nonradiative deep-level transitions, the room-temperature free-carrier lifetime of τ=910ps for ELOG a-plane GaN sa...

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...

Carrier recombination and diffusion in GaN revealed by transient luminescence under one-photon and two-photon excitations

Carrier recombination and diffusion dynamics in a 100-μm-thick GaN grown by hydride vapor phase epitaxy on semi-insulating GaN:Mg substrate have been studied by means of transient photoluminescence under one-photon (1P) and two-photon (2P) excitations. For 2P bulk excitation the luminescence transients featured an exponential decay with the time constant of 1100ps, which was mainly due to carrier capture to nonradiative deep traps. Meanwhile for 1P surface excitation, the luminescence transients showed a nonexponential decay with the mean time constant of 440ps, which was shown to be due to both carrier in-depth diffusion and recombination.

Excitation-dependent carrier lifetime and diffusion length in bulk CdTe determined by time-resolved optical pump-probe techniques

We applied time-resolved pump-probe spectroscopy based on free carrier absorption and light diffraction on a transient grating for direct measurements of the carrier lifetime and diffusion coefficient D in high-resistivity single crystal CdTe (codoped with In and Er). The bulk carrier lifetime τ decreased from 670 ± 50 ns to 60 ± 10 ns with increase of excess carrier density N from 1016 to 5 × 1018 cm−3 due to the excitation-dependent radiative recombination rate. In this N range, the carrier diffusion length dropped from 14 μm to 6 μm due to lifetime decrease. Modeling of in-depth (axial) and in-plane (lateral) carrier diffusion provided the value of surface recombination velocity S = 6 × 105 cm/s for the untreated surface. At even higher excitations, in the 1019–3 × 1020 cm−3 density range, D increase from 5 to 20 cm2/s due to carrier degeneracy was observed.

Diffusion Enhancement in Highly Excited MAPbI3 Perovskite Layers with Additives

Carrier mobility is one of the crucial parameters determining the electronic device performance. We apply the light-induced transient grating technique to measure independently the carrier diffusion coefficient and lifetime, and to reveal the impact of additives on carrier transport properties in wet-cast CH3NH3PbI3 (MAPbI3) perovskite films. We use the high excitation regime, where diffusion length of carriers is controlled purely by carrier diffusion and not by the lifetime. We demonstrate a four-fold increase in diffusion coefficient due to the reduction of localization center density by additives; however, the density dependence analysis shows the dominance of localization-limited diffusion regime. The presented approach allows us to estimate the limits of technological improvement-carrier diffusion coefficient in wet-cast layers can be expected to be enhanced by up to one order of magnitude.

Stimulated emission and optical gain in nitride-based compounds and quantum structures

On the basis of investigation of photoluminescence (PL) dynamics of highly excited structures with different In content under strong excitation by short laser pulses and comparison with model calculations we analyze emission mechanisms in these In-containing systems. The obtained PL spectra in various In-containing samples exhibit stimulated emission line on the short-wavelength side of the spontaneous PL band. We suggest theoretical model for optical transitions in InGaN compounds with strong compositional fluctuations leading to two distinct types of the regions, one of which is In-rich islands. We assume that optical band-gap within those two regions is randomly fluctuating with Gaussian distribution. Calculated PL spectra as a function of excitation and time are in fair agreement with experimental results demonstrating all the observed peculiarities of luminescence dynamics obtained experimentally.