Papers by Author: Patrik Ščajev

Abstract: We present investigation of carrier recombination and optical trap recharge in sublimation grown n- and p-type 3C layers by using time-resolved nonlinear optical techniques. Carrier lifetime and recharged trap recovery were measured by differential transmittivity technique. By monitoring nonequilibrium carrier dynamics, we analyzed impact of carrier density and temperature on carrier lifetime and recharged trap recovery rate. Large carrier lifetime and small diffusivity at low injections in highly compensated samples and their dependences on injection were explained by optical recharge of compensating aluminum impurities. The complete recharge of the compensating impurities by optical means allowed us to calculate the compensating aluminum density in n-type samples and compensating nitrogen in p-type ones.

Abstract: 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 > 10¹⁷ cm^-3, while for GaN the impact was observed only at T 19 cm^-3, the plasma degeneracy led to enhanced D_a values in SiC and GaN and compensated the diffusivity decrease in diamond.

Abstract: 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.

Abstract: Free carrier dynamics has been studied in 4H- and 3C-SiC in a wide temperature range using time-resolved photoluminescence, free carrier absorption, and light induced transient grating techniques. Considerably high carrier lifetime was observed in 3C-SiC epitaxial layers grown on 4H-SiC substrates using hot-wall CVD with respect to previously reported values for 3C-SiC grown either on Si or on 6H-SiC substrates. The temperature dependences of carrier lifetime and diffusion coefficient for 4H- and 3C-SiC were compared. Shorter photoluminescence decay time with respect to free carrier absorption decay time was observed in the same 4H-SiC sample, while these techniques revealed similar trends in the carrier lifetime temperature dependencies. However, the latter dependences for hot-wall CVD-grown 3C layers were found different if measured by time resolved photoluminescence and free carrier absorption techniques.

Abstract: The electrical and optical techniques have been applied for investigation of carrier transport and recombination features in thick free-standing 3C-SiC layers. Temperature dependencies of Hall mobility, magneto-resistivity, and conductivity indicated presence of high potential barriers, up to 0.4 eV. The carrier mobilities and equilibrium densities were calculated in the barrier and inter-barrier regions. Contactless measurements of the excess carrier ambipolar mobility and lifetime at 1016-18 cm-3 injection levels revealed carrier scattering solely by phonons in 80 – 800 K range. A correlation between the temperature dependencies of carrier lifetime and ambipolar mobility pointed out that diffusion-limited surface recombination at extended defects contributes significantly to the carrier lifetime.

Abstract: 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.

Abstract: We applied time-resolved free carrier absorption (FCA) to monitor non-equilibrium carrier dynamics in 4H epilayers and 3C SiC bulk crystals at excess carrier densities in the N = 1017 - 1019 cm-3 range. The numerical fitting of FCA decay kinetics provided the linear and nonlinear carrier recombination rates in the 40-390 K range and the absorption cross-sections eh at 1064 nm. In 4H, the decrease of the bulk lifetime (800 ns) with excitation provided the bimolecular and Auger coefficients B=(1.2±0.4)×10-12 cm3/s and C=(7±4)×10-31cm6/s, respectively, at room temperature. These values for 3C were 55-150 ns, (2.0±0.4)×10-12 cm3/s, and (2±1)×10-32 cm6/s, respectively. The rate of linear and nonlinear recombination increased at lower temperatures. A value of eh =4.4×10-18 cm2 for 3C SiC at 1.064 m was found 2.3 times smaller than that for 4H SiC.

Abstract: 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.