Light-induced Transient Grating Research Articles

Determination of carrier lifetime in thermally evaporated In2S3 thin films by light induced transient grating technique

In2S3 thin films were deposited onto soda lime glass substrates using thermal evaporation technique at a constant substrate temperature of 300 °C and the films were annealed in a sulfur ambient at 250 °C and 300 °C for 1 h. Light induced transient grating (LITG) technique was used to determine the carrier lifetime in In2S3 thin films. The determined carrier lifetime values for different excitation energy densities, I0 = 0.06–1.64 mJ/cm2 decreased from 206 to 18 ps and 150 to 14 ps for the films annealed at 250 °C and 300 °C respectively. Further, the bimolecular, Auger recombination coefficients and diffusion coefficient were determined in the films. The observed bimolecular carrier recombination origin was explained by interface and Auger recombination processes.

Crystal Structure Ideality Impact on Bimolecular, Auger, and Diffusion Coefficients in Mixed-Cation CsxMA1–xPbBr3 and CsxFA1–xPbBr3 Perovskites

Nonlinear nonradiative recombination processes are considered as a main obstacle to efficient lead halide perovskite light emitters as light emitting diodes and lasers. In this work, crystal structure ideality, described by the Goldschmidt tolerance factor, impact on Auger and bimolecular recombination rates in spin-coated mixed-cation CsxMA1–xPbBr3 and CsxFA1–xPbBr3 perovskite layers was investigated. The excitation-dependent carrier lifetime and diffusion coefficient were determined using optical pump–probe techniques: time-resolved photoluminescence and light-induced transient grating. Layers with a Goldschmidt tolerance factor closer to unity revealed Auger and bimolecular recombination rates smaller by up to an order of magnitude. The bimolecular coefficient (0.6–5 × 10–10 cm3/s) increased simultaneously with the Auger coefficient (0.05–2 × 10–27 cm6/s) upon increase of x and simultaneous decrease of tolerance factor, due to variation in the localized carrier population and Rashba splitting. A larger...

Recombination and diffusion processes in electronic grade 4H silicon carbide

Carrier dynamics in n-type 4H-SiC epilayers of varying thicknesses and low Z1/2 defect concentrations are investigated here in wide ranges of excess carrier density and temperature. Several techniques are employed to monitor carrier diffusion and recombination processes, including light induced transient grating (LITG), microwave photoconductance decay (MPCD) and free carrier absorption (FCA) using ps-laser pulses at 355 nm. The observed increase of the diffusion coefficient with the increasing excitation level is explained by the transition from the minority to the bipolar transport regime. Its subsequent decrease with even higher excitations is found to be governed by band-gap renormalization and degeneracy effects. The bulk lifetime, limited by hole traps at 0.19–0.24 eV above the valence band at lower excitations, was found to decrease from few microseconds to hundreds of nanoseconds during the transition regime from the minority to the bipolar transport. Our temperature-dependent measurements confirmed the trap activation energy and provided the approximate functional form of electron and hole lifetimes as τe = 340 × (T/300 K)3/2 ns and τh = 100 × (T/300 K)–1/2 ns, for the temperature T range 80–800 K. It was found to hold for 65 and 120 μm sample thicknesses, while the lifetimes were found to be twice shorter for the sample 35 μm thick.

Exciton diffusion in bifluorene single crystals studied by light induced transient grating technique

Exciton diffusion is considered to be of prime importance for controlling the recombination zone in organic light emitting devices. This is particularly relevant for organic lasers based on single crystals, where undesirable exciton-exciton annihilation at high concentrations is inevitable. Here, exciton diffusion is studied in bifluorene single crystals specifically designed for organic laser applications, therefore featuring exceptionally low threshold of light amplification. The non-destructive light induced transient grating (LITG) technique capable of simultaneously evaluating the exciton lifetime and diffusion coefficient along different directions of the crystal was employed for this study. Highly anisotropic singlet exciton diffusion with the diffusion coefficient varying from <0.05 up to 0.96 cm2/s (in a perpendicular direction) and the corresponding diffusion lengths changing from <60 up to 300 nm, respectively, was revealed in ethylene-bridged bifluorene crystals. The LITG results on the directionality of exciton diffusion are anticipated to be essential for devising single crystal based organic lasers.

Excitation and temperature dependent exciton-carrier transport in CVD diamond: Diffusion coefficient, recombination lifetime and diffusion length

Time-resolved induced absorption (IA) and light induced transient grating (LITG) techniques were applied for the investigation of nonequilibrium exciton-carrier diffusion and recombination processes in a high-purity CVD diamond. Injection range from 1015 to 1020cm−3 carrier density was achieved by combining one photon and two photon excitations. The measurements were performed in the 10–750K temperature range. The LITG diffusion coefficient peaked at 44cm2/s value at room temperature under low injection conditions. At lower temperatures it transferred to much lower exciton diffusion coefficient. A strong decrease of diffusion coefficient under higher injection conditions was explained by exciton formation with a low diffusion coefficient and many body effects, as polyexciton and electron-hole droplet formation. High temperature phonon-limited diffusion coefficient was weakly injection dependent. Low excitation carrier lifetime was about 700ns above 200K. At lower temperatures, the decay time reduced by two orders of magnitude, which was explained by the formation of biexcitons. At lowest temperatures, an increase of the carrier recombination rate at high injection was attributed to Auger recombinations of polyexcitons and electron-hole droplets. While at high temperatures, the increase of the recombination rate with 490meV activation energy was observed. The combination of IA and LITG measurements provided effective diffusion lengths in a 0.3–50µm range, being strongly dependent on the excess carrier density and temperature.

Photo-electrical and transport properties of hydrothermal ZnO

We performed the studies of optical, photoelectric, and transport properties of a hydrothermal bulk n-type ZnO crystal by using the contactless optical techniques: photoluminescence, light-induced transient grating, and differential reflectivity. Optical studies revealed bound exciton and defect-related transitions between the donor states (at ∼60 meV and ∼240 meV below the conduction band) and the deep acceptor states (at 0.52 eV above the valence band). The acceptor state was ascribed to VZn, and its thermal activation energy of 0.43 eV was determined. A low value of carrier diffusion coefficient (∼0.1 cm2/s) at low excitations and temperatures up to 800 K was attributed to impact the recharged deep acceptors. Electron and hole mobilities of 140 and ∼80 cm2/Vs, correspondently, were determined at room temperature. The decrease of carrier lifetime with excitation was ascribed to increasing rate of radiative recombination at low temperatures and nonradiative recombination above the room temperature.

Photomodification of carrier lifetime and diffusivity in AlGaN epitaxial layers

Nonradiative recombination rate and diffusivity of nonequilibrium carriers were modified by intense laser pulses in AlGaN epilayers with Al content ranging from 16 to 71%. The epilayers were examined before and after the photomodification using light-induced transient grating and photoluminescence spectroscopy techniques. The photomodification resulted in (i) enhancement of the nonradiative recombination rate and (ii) large changes of the diffusion coefficient of the nonequilibrium carriers, without imposing any macroscopic structural damage to the epilayers. The photomodification effect on the recombination rate was stronger in the layers with higher Al content indicating the involvement of the Al atoms in this process. The carrier diffusivity exhibited a rapid initial increase as a consequence of the photomodification followed by a slow decline, as the photomodification duration was increased. The enhancement of the diffusion coefficient of up to 2.4 times was accompanied by 13% decrease in the carrier lifetime.

Nonradiative Recombination, Carrier Localization, and Emission Efficiency of AlGaN Epilayers with Different Al Content

The interplay between nonradiative recombination and carrier localization in AlxGa1−xN epilayers (0.11 < x < 0.78) was studied using the photoluminescence (PL) and light-induced transient grating (LITG) techniques. The carrier localization conditions were estimated from the temperature dependence of the PL band peak position. The room-temperature carrier lifetimes estimated by LITG were limited by the competition between the carrier localization and nonradiative recombination. Strong carrier localization was shown to be insufficient to achieve high internal quantum efficiency in AlGaN epilayers.

Nondestructive Evaluation of Photoelectrical Properties of a PVT Grown Bulk 15R-SiC Crystal

Investigation of excess carrier dynamics in a 15R-SiC bulk layer grown by physical vapour transport (PVT) on 15R-SiC substrate has been carried out using pump-probe techniques: an interband carrier injection by a picosecond laser pulse and measuring the induced absorption and diffraction of a probe beam. For this task, differential transmittivity (DT) and light induced transient grating (LITG) techniques were used. Room temperature carrier lifetime varied in the 3 ns 8 ns range at excess carrier densities above ΔN0 = 7×1017 cm-3 and was ascribed to the recovery time of optically recharged carrier traps, and their activation energy of Ea = 75 meV was determined. The presence of recharged traps caused the injection-dependence of the diffusion coefficient D, whereby its value dropped below 0.1 cm2/s at ΔN0 &lt; 1×1018 cm-3 and gradually increased up to 0.7 cm2/s at higher injections. At elevated temperatures (300 K &lt; T &lt; 700 K), when the traps are thermally activated, the diffusivity increased up to ~ 1.5 cm2/s and was independent on ΔN0. The overgrown layer parameters were comparable to those of the used 15R PVT seed.

Photoluminescence features and carrier dynamics in InGaN heterostructures with wide staircase interlayers and differently shaped quantum wells

We present a comprehensive study of photoexcited carrier dynamics in differently grown InGaN/InGaN multiple quantum well (MQW) structures, modified by insertion of a wide interlayer structure and subsequent growth of differently shaped quantum wells (rectangular, triangular, trapezoidal). This approach of strain management allowed the reduction of dislocation density due to gradually increasing In content in the interlayer and shaping the smooth quantum well/barrier interfaces. A set of c-oriented MQW structures emitting at 470 nm were grown at Vilnius University, Institute of Applied Research, using a closed coupled showerhead type MOCVD reactor. Photoluminescence (PL) spectra of MQW structures were analysed combining continuous wave and pulsed PL measurements. Reactive ion etching of the structures enabled discrimination of PL signals originated in the InGaN interlayer structure, underlying quantum wells, and quantum barriers, thus providing growth-related conditions for enhanced carrier localization in the wells. Time-resolved PL and differential transmission kinetics provided carrier lifetimes and their spectral distribution, being the longest in triangular-shape QWs which exhibited the highest PL intensity. The light-induced transient grating (LITG) technique was used to determine the spatially averaged carrier lifetime in the entire heterostructure, in this way unravelling the electronic quality of the LED internal structure at conditions similar to device performance. LITG decay rates at low and high excitation energy densities revealed increasing with photoexcitation nonradiative recombination rate in the triangular and trapezoidal wells.

Application of excite-probe techniques for determination of surface, bulk and nonlinear recombination rates in cubic SiC

Non-equilibrium carrier dynamics was studied in a free-standing 160 μm thick 3C-SiC wafer using time-resolved differential transmittivity (DT), differential reflectivity (DR), and light induced transient grating (LITG) techniques. To enable investigation of carrier recombination and diffusion features in the middle part of the wafer, a wedge-shaped strip was prepared by polishing out the most-defective part of the wafer on the substrate side. Measurements of DT and DR decay kinetics on both sides of the wedged sample and their numerical fitting provided bulk lifetimes of 180 ns in the middle part and 60 ns in the edge part (∼50 μm thick subsurface region). On the as-grown side, the surface recombination velocity was equal to 3 × 103 cm/s. Carrier diffusivity, determined by LITG technique, and recombination rate were found injection-dependent. The low-injection value of diffusion length, LD = 8 μm, was found the longest in the middle part of the wafer, while at high injections it decreased to 1 μm due to an impact of Auger recombination with a coefficient, C = (4 ± 1) × 10−32 cm6/s. The 2.7 times lower diffusion length value in the edge part was explained by an impact of point defects to the carrier lifetime and diffusivity.

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 recombination rate to bimolecular recombination and determine its coefficient B = 7 × 10−11 cm3/s. Complementary measurements unveiled the cause of PL efficiency saturation at injection levels above 5 × 1018 cm−3, attributable to bandgap renormalization in the extended states above the PL emission band, which encumbers carrier transfer from high-to-low energy states. As the degree of localization, and therefore, the total number of band tail states is expected to increase with In content, their impact to dependence of PL efficiency on excitation density could even be stronger for higher In compositions. These results provided insight that spectrally resolved carrier generation-recombination rates are excitation-dependent and would play a critical role in saturation of internal quantum efficiency in InGaN alloys used in light emitters, such as light emitting diodes.

The Influence of Coherent Transient Energy Transfer on Femtosecond Time-Resolved Z-Scan Measurements

We investigate the influence of the coherent effect on femtosecond time-resolved Z-scan measurements using a degenerate pump-probe Z-scan technique. The time response of the light-induced transient grating (LITG) of Bi2O3-B2O3-SiO2 (BI) glass shows a valley-peak variation, which has an obvious influence on the time-resolved Z-scan measurements. The valley-peak variation of the LITG signals with the delay time in BI glass is due to coherent transient energy transfer between positively chirped pump and probe pulses. The influence of the LITG effect on the closed-aperture time-resolved Z-scan measurements could be reduced by adjusting the position of the aperture transversely.

On Applicability of Time-Resolved Optical Techniques for Characterization of Differently Grown 3C-SiC Crystals and Heterostructures

We applied a number of time-resolved optical techniques for investigation of optical and photoelectrical properties of cubic SiC grown by different technologies on different substrates. The excess carriers were injected by a short laser pulse and their dynamics was monitored by free-carrier absorption, light-induced transient grating, and photoluminescence techniques in a wide excitation range. Combining an optical and electrical probe beam delay, we found that free carrier lifetimes in differently grown layers vary from few ns up to 20 μs. Temperature dependences of carrier diffusivity and lifetime revealed a pronounced carrier trapping in thin sublimation grown layers. In free-standing layers and thick sublimation layers, the ambipolar mobility was found the highest (120 cm2/Vs at room temperature). A linear correlation between the room-temperature band edge emission and carrier lifetime in differently grown layers was attributed to defect density, strongly dependent on the used growth conditions.

Impact of doping on carrier recombination and stimulated emission in highly excited GaN:Mg

Light-induced transient grating and photoluminescence measurements were employed for carrier recombination studies in variously Mg doped GaN layers. Carrier lifetime and ambipolar diffusion coefficient were found to decrease with doping from 210 to 20 ps and from 2.0 to 0.9 cm 2/s,respectively, which proved the degradation of electrical quality of the layers. A threshold of stimulated emission was found to depend non-monotonously on doping and had the lowest value of 0.19 mJ/cm 2 in the most doped layer. This dependence was explained in terms of degeneracy of the hole system.

Nondestructive Evaluation of Photo-Electrical Properties of 3C-SiC (111) Homoepitaxial Layers Grown by CVD

Free carrier absorption (FCA) and picosecond light-induced transient grating (LITG) techniques were applied to study the photoelectrical properties of 3C-SiC(111) homoepitaxial layers grown by CVD method on VLS (vapour-liquid-solid) grown seeds. The thickness of the CVD layers was ~10.5 µm with non-intentional type doping of n (~ 1017 cm-3) or p (&lt;1015 cm-3). The carrier lifetime and the diffusion coefficient were measured as the function of the sample temperature, the injected excess carrier density at different growth parameters. At room temperature the ambipolar diffusion coefficient was Da=2.5-3 cm2/s, while the lifetime was in the range of 12-18 ns. The best structural and electrical properties were obtained for a CVD layer grown at high, 1600 °C temperature.

Carrier lifetime, diffusion length and mobility in (100) CVD diamond samples pre-treated in an O2/H2-plasma

ABSTRACTIn this article, (100) Ib diamond substrates are pre-treated for different durations in an O2/H2-plasma, influencing the etching of defects like unepitaxial crystals, flat top hillocks, and pyramidal hillocks [1-3]. Such procedure is used to prevent those to incorporate into the ~ 100μm thick CVD diamond film, which is subsequently grown on top. While the surface morphology and structure of substrates and films are studied by SEM, the time-resolved Light-Induced Transient Grating (LITG) technique provides information on the excess carrier parameters close to the front surface of the grown layers. This technique is particularly useful as it does not requires a separation of the CVD film from its substrate. It will be shown that O2/H2-plasma treatments of more than 150 minutes but less than 240 minutes largely reduce the incorporation of defects in the bulk of the grown film. This, in turn, influences the carrier dynamics as measured by LITG, but also the surface roughness and growth rate as shown by SEM.

Determination of carrier diffusion coefficient and lifetime in single crystalline CVD diamonds by light‐induced transient grating technique

AbstractWe report on a contactless, all‐optical study of carrier diffusion and recombination kinetics in single‐crystalline diamond layers using the light‐induced transient grating (LITG) technique. Decay times of transient diffraction grating yielded carrier lifetime of τR ≈ 3 ns and bipolar diffusion coefficient Da = 12 cm2/s at 300 K. The latter value of Da was found to be 4–5 times lower than the ambipolar diffusivity based on electron and hole mobilities, measured by photo‐electrical time‐of‐flight (ToF) technique. This discrepancy was attributed to the bandgap renormalization at high excess carrier densities and its impact on carrier diffusion. The significant decrease of low temperature diffusivity pointed out to a contribution of many‐body effects which are tentatively attributed to the formation of electron–hole liquid (EHL) at T &lt; 150 K.

Polarization dependence of optical Kerr effect in metallophthalocyanine-doped inorganic–organic hybrid materials

Ultrafast optical Kerr effect of metallophthalocyanine-doped inorganic–organic materials was investigated using a femtosecond optical Kerr shutter at wavelength of 800 nm. Experimental results showed that the dependence of the Kerr signals on the polarization angle between the pump beam and the probe beam could be controlled by changing the pump-probe intensity ratio. The pump-intensity dependence of the polarization characters of the Kerr signals probably arose from the contribution of light induced transient grating (LITG) to the Kerr signals.

Excited state dynamics in silver nanoparticles embedded in phosphate glass

Excited state dynamics in silver nanoparticles embedded in aluminophosphate glass was studied by ultrafast optical pump–probe technique. The absorption process of pump radiation and the electron–phonon relaxation on the 10 −13–10 −11 s scale were analyzed in the framework of two-temperature model. The time evolution of the light-induced transient diffraction grating shows an uncommon relaxation on the nanosecond time scale. This relaxation is assigned to phonon–phonon scattering process as well as to the energy transfer from photoexcited electronic states in glass matrix to silver nanoparticles.