Papers by Keyword: Free Carrier Absorption

Abstract: For high voltage SiC bipolar devices, carrier lifetime is an important parameter, and for optimization of device performance, we need to control distribution of the carrier lifetime in a wafer. So far, there have been limited systems for depth-resolved carrier lifetime measurements without cross sectional cut. In this study, we adopted a free carrier absorption technique and made local overlapping of the probe laser light with excitation laser light to develop depth-resolved carrier lifetime measurements. We named the developed system a microscopic FCA system and demonstrated measurement results for samples with and without intentional carrier lifetime distribution.

Abstract: Excess carrier dynamics in 6H-SiC substrates with n- and p-type moderate doping were detected using femtosecond pump-probe measurements with supercontinuum probing. Band-to-band recombination and carrier trapping were determined as the main recombination processes in both materials. Spectral fingerprints corresponding to each of these recombination components were obtained using the global and target analysis. It was shown that, in spite of background doping, the band-to-band recombination in 6H-SiC is dominated by the excess electron absorption component and the carrier trapping is dominated by the excess hole absorption.

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: 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: 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.

Abstract: An alternative approach based on non-equilibrium free-carrier density measurements was used to characterize the fundamental absorption edge of 3C-SiC at room and 77 K temperatures. At 77 K temperature the extracted absorption edge compared well to the previous literature data revealing characteristic thresholds due to the phonon emission assisted transitions. At room temperature the absorption tail due to the phonon absorption assisted transition was revealed up to the value of 0.01 cm-1 exceeding the previous 5 cm-1 limit induced by unintentional sample doping.

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: The effects of measurement technique and measurement conditions (injection level, temperature) on the measured carrier lifetimes in n- 4H-SiC epilayers are investigated. For three optical measurement techniques, it is shown that the high and low injection lifetimes can vary dramatically. Differences in the lifetime for varying injection level and temperature are approached both experimentally and via carrier dynamics simulations, assuming Z1/Z2 as the dominant defect. Reasonable agreement between measured and calculated behavior is obtained, as is insight into the recombination kinetics associated with the lifetime limiting defect.