Picosecond Transient Grating Technique Research Articles

Carrier Diffusivity in Highly Excited Bulk SiC, GaN, and Diamond Crystals by Optical Probes

Optical monitoring of diffusivity in wide bandgap semiconductors was performed by using a picosecond light-induced transient grating technique. The bandgap renormalization and carrier-carrier scattering manifested itself at room temperature as two-fold decrease of the ambipolar diffusion coefficient Da in cubic SiC and 5-fold decrease of Da in diamond at excess carrier density N > 1017cm-3, while for GaN the impact was observed only at T 19cm-3, the plasma degeneracy led to enhanced Davalues in SiC and GaN and compensated the diffusivity decrease in diamond.

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 (<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.

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.

Layer thickness dependent carrier recombination rate in HVPE GaN

We report on nonequilibrium carrier dynamics in a set of hydride vapor phase epitaxy (HVPE) GaN wafers of different thickness (11, 17, 41, 90, and 145 μm) grown on the (0001) c-plane sapphire substrates. Carrier lifetime τ R and diffusion length L D were determined by picosecond transient grating and free carrier absorption (FCA) techniques. The nonradiative recombination lifetime increased from 400 ps in the thinnest layer up to 25 ns in the thickest one, and L D varied from 0.24 to 1.9 μm, respectively. The τ R and L D values in the 145 μm-thick HVPE layer are the largest ones reported up to now in bulk GaN at room temperature. The data provided a relationship τ R ∝ d 3/2 between the carrier lifetime and wafer thickness in ∼ 10-150 μm thickness range. The latter dependence indicated the dramatic decrease of threading dislocation density in the probed subsurface area of the studied wafers from 4 × 10 9 to ∼ 10 6 cm -2 .

On the Correlation of the Structural Perfection and Nonequilibrium Carrier Parameters in 3C SiC Heterostructures

Photoelectric properties of 3C sublimation-grown epitaxial layers with different structural quality were studied by using time-resolved picosecond transient grating and free carrier absorption techniques. The layer quality was described by a parameter LTW which gives the total length of twin boundaries in a layer. Optical measurements of diffusion coefficients and carrier lifetimes in wide excess carrier density (N >1018 cm-3) and temperature range (10 K to 300 K) revealed the twin defect density dependent ambipolar mobility value at RT as well as essentially different temperature dependences of mobility of the layers. The larger value of absorption cross section in more defective layer at 1064 nm wavelength pointed out to contribution of defect-assisted absorption, which gradually vanished after the filling defect states by free carriers.

Investigation of Thermal Properties of Heavily Doped 4H-SiC Crystals by a Picosecond Transient Grating Technique

We applied a picosecond dynamic grating technique for investigation of thermal diffusivity, sound velocity, thermo-optic and photoelastic coefficients in heavily doped 4H-SiC substrates. Spatial modulation of thermal properties was achieved by intraband carrier absorption at excitation by an IR picosecond laser pulse (1064 nm) and subsequent carrier thermalisation. Decay of dynamic grating at its various periods and sample temperatures was monitored at 532 nm wavelength. The thermal diffusivity DT = 0.94 cm2/s, thermo-optic coefficient dn/dT = 3.610–5 K-1 and effective photoelastic constant = – 0.10 were determined at 300 K. DT value increased at lower temperatures and saturated below 100 K. Temperature dependences of thermo-optic coefficient and thermal expansion were found in good correlation with heat capacity peculiarities.

Characterization of Electronic Properties of Different SiC Polytypes by All-Optical Means

We applied a picosecond transient grating technique for studies of nonequilibrium carrier dynamics in differently grown or doped SiC polytypes. Optical carrier injection in 4H-SiC at two different wavelengths (266 and 355 nm) allowed us to vary the depth of the photoexcited region and determine photoelectric parameters of high density (from ~1016 to ~1019 cm-3) carrier plasma in the temperature range 10 – 300 K. A strong decrease of carrier lifetime with increasing nonequlibrium carrier density was found in 4H-SiC samples at 300 K and fitted by bimolecular recombination with coefficient B = 3 × 10-11 cm3 s-1. In 3C-SiC epilayers, the opposite tendency was observed over a wide temperature range and attributed to recharging of defect states.

Investigation of Optical Nonlinearities and Carrier Dynamics in In-Rich InGaN Alloys

We present experimental studies of nonequilibrium carrier dynamics in InGaN alloys with 70–90% content of In by using picosecond transient grating technique. The observed faster recombination rate in alloys with higher Ga content and formation of a thermal grating via a lattice heating, being more pronounced for layers with larger band gap, indicated that the main reason of the heating is not the excess energy of photons, but the defect density which increases with Ga content. A gradual decrease in carrier lifetime with excitation or with increasing temperature in 50–300 K range point out the role of potential barriers in carrier recombination.

Optical evaluation of carrier lifetime and diffusion length in synthetic diamonds

The key electronic parameters of high-pressure-high-temperature and chemical-vapor-deposition grown diamonds have been determined at interband ( hν = 5.82 eV) or below bandgap ( hν = 4.68 eV) photoexcitation, using a picosecond transient grating (TG) technique. TG kinetics directly provided the values of ambipolar diffusion coefficient (6–10 cm 2/s) and carrier lifetime (in a range from 0.17 to 2.8 ns) for crystals grown under different conditions. The carrier diffusion length was found to vary from 0.5 μm in CVD layers to 1.6 μm for IIa type HPHT diamond crystal. The carrier lifetimes correlated well with the nitrogen-related defect density in both types of diamonds.

Correlation between the free carrier lifetime and total amount of deep centers in ZnO single crystals

We report on optical study of charge carrier dynamics in vapor phase grown ZnO single crystals with different content of deep defects. Deep defects were controlled by applying various heat treatment regimes to each crystal. A time-resolved picosecond transient grating technique was used to determine the carrier lifetime and diffusion coefficient, while the light absorption spectra of transparency region together with the numerical simulations provided information on deep level content, their spectral positions and concentrations. A linear dependence was obtained between the carrier recombination rate and the integral absorption coefficient in 2.0–3.1 eV spectral region. This proves the role of the deep defects as the main recombination centers of excess carriers in ZnO. Numerical simulations of absorption spectra revealed five different species of intrinsic deep defects. The most recombination-active defects were attributed to oxygen vacancy and zinc–oxygen vacancy complex.

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.

Lateral and cross-well transport of highly and moderately excited carriers in Si1−xGex/Si superlattices

Picosecond transient reflectometry and dynamic grating techniques have been applied to investigate the perpendicular and parallel transport of optically excited carriers in strained-layer Si0.83Ge0.17/Si superlattices. We present results of the carrier ambipolar diffusivity and effective lifetime measurements in the layered structure and substrate within the 1017–1020 cm−3 density range. The combined experimental data are discussed in terms of parallel and perpendicular diffusion of carriers, interface recombination, and lattice heating. The estimated lateral and cross-well diffusion constants are 10 and 0.16 cm2/s at room temperature, respectively.

Investigation of the Excited-State Dynamics of Radical Ions in the Condensed Phase Using the Picosecond Transient Grating Technique

A study of the dynamics of ground-state recovery of the perylene radical cation (Pe•+), of perylene radical anion (Pe•-), and of anthraquinone radical anion (AQ•-) is reported. In boric acid glass, the excited-state lifetime of Pe•+ is 35 ± 3 ps, while in concentrated sulfuric acid, it is smaller than 15 ps, the time resolution of the experimental setup. The excited-state lifetime of Pe•+, Pe•-, and AQ•- generated by photoinduced intermolecular electron-transfer reaction in MeCN is shorter than 15 ps. In the case of Pe•-, the uncomplete ground-state recovery is ascribed to the occurrence of electron photoejection. The free ion yield in the intermolecular electron-transfer reaction between 9,10-dicyanoanthracene (DCA) and two electron acceptors was measured in a two-pulse experiment, where the second pulse excited the ensuing DCA•-. This excitation has no influence on the magnitude of the free ion yield, indicating a short excited-state lifetime of DCA•-* relative to the time scale of back electron transfer and ionic dissociation. A red emission, ascribed to the fluorescence of protonated Pe, was detected in boric acid glass and sulfuric acid. No fluorescence that could be clearly ascribed to Pe•+* could be observed.

Anisotropy time dependence of photoexcited C60 and C70 in transient grating experiments. Solvent effect

Anisotropy time dependence of photoexcited C50 and C70 has been measured by picosecond transient grating techniques at room temperature in various solvents. The monoexponential anisotropy decay was observed for the C60 molecule and biexponential anisotropy decay was observed for the C70 molecule. Both for the C60 and for the C70 molecules anisotropy time decay is very fast. The data obtained were analyzed in terms of the rotational reorientation of fullerene molecules in solvents. Hynes-Kapral-Weinberg theory reasonably explains the observed reorientation times of fullerenes molecules. The dielectric friction effect on the C70 rotational reorientation on the short axis is reported.

Orientational dynamics of C70 molecules in chlorobenzene

Dynamics of anisotropy relaxation of C70 singlet excited molecules in chlorobenzene was measured at room temperature by the picosecond transient grating technique. The time-ependent diffraction efficiency exhibits a two-stage decay: a fast component (τ1 = 12±5 ps), which is comparable with the corresponding signal of C60 in chlorobenzene (τ = 8±2 ps), and a slow one (τ2 = 30±5 ps). It is supposed that relaxation of anisotropy is related to the orientational mobility of excited C70 molecules relative to two axes of the molecular framework. The results obtained cannot be described by the Einstein-Stokes-Debye theory. The Hynes-Kapral-Weinberg theory, which takes into account microscopic interactions between molecules upon collisions, agrees satisfactorily with the experiment. The influence of dielectric friction on the orientational mobility of C70 in chlorobenzene was estimated.

Orientational behavior of C 70 molecules in chlorobenzene

The orientational behavior of C 70 singlet excited molecules in a chlorobenzene solution has been measured at room temperature by the picosecond transient grating technique. A two-stage decay signal was observed: a fast part ( τ = 12 ± 5 ps) which is comparable with the corresponding signal of C 60 in chlorobenzene ( τ = 8 ± 2 ps), and a slow part ( τ = 30 ± 5 ps). The data obtained are analyzed in terms of the rotation of C 70 molecules. The influence of the dielectric friction on the C 70 rotation is reported.

Rotational reorientation dynamics of C 60 in various solvents. Picosecond transient grating experiments

The picosecond transient grating technique has been used to study the rotational reorientation of C 60 in various solvents: in toluene 7± 1.5 ps, o-dichlorobenzene 10.3± 1.5 ps, o-xylene 13±2 ps and in decalin 3.5± 1.5 ps. The data obtained cannot be described by hydrodynamic Debye theory. Rough-sphere fluid theory predicts the correct values for C 60 rotation in toluene, o-dichlorobenzene and in decalin. The deviations for o-xylene are probably connected with the specifics of the local solvent structure or with the stronger interaction of C 60 with solvent molecules. The rotation of C 60 in decalin is rapid and approaches the rotation in the gas phase determined by inertia.

Singlet exciton dynamics of guest impurities in molecular crystals

The relaxation dynamics of excited guest singlet states in high-concentration p-terphenyl: pentacene crystals was investigated using picosecond transient grating techniques. The temperature and concentration characteristics of the grating decays were rationalized to arise from the competition between resonant transport and trapping of the excitation energy by lower-lying sites.

Enhanced ambipolar in-plane transport in an InAs/GaAs hetero-n-i-p-i

In-plane transport in an InAs/GaAs semiconductor hetero-n-i-p-i has been investigated using picosecond transient grating techniques and an order-of-magnitude enhancement of the ambipolar transport relative to that measured in a similar undoped sample has been demonstrated. Both the magnitude and the density dependence of this enhanced transport are consistent with an additional driving force that is associated with an in-plane modulation of the screened n-i-p-i field. This modulation is the result of the spatial separation by perpendicular transport of electrons and holes that also have an in-plane density modulation.

Picosecond photoionization and geminate recombination in an organic donor—acceptor complex

The initial steps involved in the charge separation process in thin films of the organic charge-transfer complex formed from poly-N-vinylcarbazole (PVK) and 2,4,7-trinitro-9-fluorenone (TNF) are observed directly using polarization selective pico-second transient grating techniques. When forming a grating with parallel polarized writing beams, a nonexponential decay of the transient grating signal is observed. When forming a grating with perpendicularly polarized writing beams, the decay of the transient grating signal is observed to increase and to be described adequately as a single exponential. These transient grating signals have been investigated as a function both of an applied electric field and as a function of the TNF:PVK ratio. All of these results are consistent with a four-level kinetic model that includes charge hopping, geminate recombination, and field-assisted charge separation.