Vytautas Grivickas

Optical characterization of excess carrier lifetime and surface recombination in 4H/6H–SiC

The high-injection lifetime and surface recombination parameters have been investigated in as-grown 4H and 6H–SiC epilayers subjected to various process treatments. A depth-resolved optical transient absorption technique was utilized to evaluate the influence of film thickness and surface treatment on carrier lifetime. We demonstrate that besides polishing and ion implantation, both natural and thermal oxidation may also result in lifetime reduction due to enhanced surface losses. Moreover, a long-term stability test has revealed a substantial degradation of lifetime characteristics, consistent with a spontaneous surface oxidation and slow relaxation of SiO2/SiC interface states. We show that for common film thickness <100 μm, the effective lifetime is dominated by surface leakage, which is found, generally, to be higher in 4H compared to 6H–SiC.

Fundamental band edge absorption in nominally undoped and doped 4H‐SiC

Fundamental band edge absorption is investigated in nominally undoped (n<1014cm−3) and heavily doped (n∼8×1018cm−3) 4H‐SiC by a spectroscopy technique based on spatially and time-resolved free-carrier absorption. The spectra are extracted over a wide absorption range (0.02–500cm−1) at temperatures from 75to450K. The experimental results are supported by an indirect transition theory with a unique set of dominating momentum-conserving phonons, showing good correlation with earlier findings of differential absorption measurements at 2K. Exciton binding energy of 30±10meV is derived from fitting the data at 75K. The detected polarization anisotropy of absorption with respect to c axis is shown to be consistent with the selection rules for the corresponding phonon branches. An analytical model related to constant degree of involved phonons describes well the obtained energy gap variation with temperature. Finally, doping induced band gap narrowing is characterized above the impurity-Mott transition and compared with theoretical calculations in the random phase approximation. The shape of the fundamental absorption edge at high carrier concentrations is discussed in terms of excitonic enhancement above the Mott transition, as recently detected in Si.Fundamental band edge absorption is investigated in nominally undoped (n<1014cm−3) and heavily doped (n∼8×1018cm−3) 4H‐SiC by a spectroscopy technique based on spatially and time-resolved free-carrier absorption. The spectra are extracted over a wide absorption range (0.02–500cm−1) at temperatures from 75to450K. The experimental results are supported by an indirect transition theory with a unique set of dominating momentum-conserving phonons, showing good correlation with earlier findings of differential absorption measurements at 2K. Exciton binding energy of 30±10meV is derived from fitting the data at 75K. The detected polarization anisotropy of absorption with respect to c axis is shown to be consistent with the selection rules for the corresponding phonon branches. An analytical model related to constant degree of involved phonons describes well the obtained energy gap variation with temperature. Finally, doping induced band gap narrowing is characterized above the impurity-Mott transition and compar...

Carrier diffusion characterization in epitaxial 4H-SiC

Carrier diffusivity has been experimentally determined in low-doped n-type epitaxial 4H-SiC over a wide injection range using a Fourier transient grating technique. The data showed that, with injec ...

Temperature dependence of the absorption coefficient in 4H- and 6H-silicon carbide at 355 nm laser pumping wavelength

We report on the absorption coefficient at 355 nm laser wavelength for 4H- and 6H-SiC over a wide temperature range. The measurements were carried out using a depth- and time-resolved free-carrier- ...

Fluorescent SiC as a new material for white LEDs

Current III–V-based white light-emitting diodes (LEDs) are available. However, their yellow phosphor converter is not efficient at high currents and includes rare-earth metals, which are becoming scarce. In this paper, we present the growth of a fluorescent silicon carbide material that is obtained by nitrogen and boron doping and that acts as a converter using a semiconductor. The luminescence is obtained at room temperature, and shows a broad luminescence band characteristic of donor-to-acceptor pair recombination. Photoluminescence intensities and carrier lifetimes reflect a sensitivity to nitrogen and boron concentrations. For an LED device, the growth needs to apply low-off-axis substrates. We show by ultra-high-resolution analytical transmission electron microscopy using aberration-corrected electrons that the growth mechanism can be stable and that there is a perfect epitaxial relation from the low-off-axis substrate and the doped layer even when there is step-bunching.

Ambipolar diffusion coefficient in molecular‐beam‐epitaxy‐grown silicon layers

The noncontact transient grating technique under constant electron‐hole pumping to the density 4×1019 cm−3 was used to characterize the ambipolar diffusion coefficient Da in Si:Al, Si:In, and Si:Sb molecular‐beam‐epitaxy‐grown layers. Da was found to be almost constant at a value of ≊8 cm2/s up to an equilibrium carrier density of 2×1019 cm−3 in the layer and was independent of the doping type. At higher doping density, evidence for a sharp increase in Da was observed. For example, Da increased to a value of 20 cm2/s at a doping density of about 1020 cm−3. The Da behavior is in reasonable agreement with results of the high‐density ambipolar diffusion theory of Young and van Driel and is incompatible with majority‐carriers diffusion coefficients according to the formula Da = 2DnDp/(Dn + Dp). An explanation for this behavior is given.

Excess carrier recombination lifetime of bulk n-type 3C-SiC

Transient absorption technique was used to determine carrier lifetimes in 3C-SiC grown on Si and 6H-SiC substrates. A slow lifetime component originated from minority carrier traps and pointed out to the trap saturation with increasing injection. Recombination lifetime in different samples varied between 0.5–120 ns. Its value decreased with excess carrier density in the transition range between minority-carrier-lifetime and high-injection lifetime but abnormally increased above the carrier density of 2×1017 cm−3. Negligible contribution of surface and Auger recombination to recombination lifetime peculiarities was observed. Possible mechanisms of the observed lifetime variation are discussed.

Band edge absorption, carrier recombination and transport measurements in 4H-SiC epilayers

Abstract A depth-resolved technique based on probe-pump free carrier absorption (FCA) is especially useful for measurements in thin layers. This technique is used here to characterize optical and electrical properties under a wide range of injection levels, 10 14 –10 18 cm −3 , in low-doped n -type 4H-SiC epilayers. Our results reveal valuable anisotropy of the band-band absorption at the photon energy about 0.2 eV above the indirect band gap. While the absorption coefficient is found nearly independent of the injection level. The ambipolar carrier diffusion coefficient is measured by an FCA-detected transient grating. Carrier diffusion length, bulk lifetime and surface recombination velocity is also estimated.

Properties of CdS nanocrystallites embedded in to thin ZrO2 waveguides

Abstract In this paper we present results on CdS nanocrystal (NC) formation in ZrO2 thin film grown by sol–gel process. Raman and waveguiding Raman scattering spectroscopies were applied to determine CdS nanocrystal precipitation in our films. RBS (Rutherford back scattering) and optical absorption spectroscopy were applied to determine CdS distribution homogeneity through the layer depth, NC size and dispersion. Thus we obtained low size dispersed CdS nanocrystals with preparation temperature controlled size. Moreover, Raman and high-resolution transmission electron microscopy (HRTEM) evidenced simultaneous correlated crystallisation of hexagonal CdS and tetragonal/monoclinic ZrO2 phases. We applied also light induced dynamic grating technique under high excitation conditions in order to analyse dynamic properties of nonequilibrium charge carriers in thin films.

New contactless method for carrier diffusion measurements in silicon with a high precision

A novel, contactless transient grating method is demonstrated for excess carrier diffusion coefficient determination at an arbitrarily chosen injection density. The method is based on free‐carrier absorption decay measurements along a grid of carriers excited in the bulk of the sample by a YAG laser pulse. The focused infrared probe beam, with an excess‐carrier detection limit as low as 1012 cm−3, monitors the lateral interdiffusion of carriers by scanning across the carrier grating. Measurements on a 1015 cm−3, p‐doped Si sample at injection levels in the range 1013–1017 cm−3 show a considerable reduction of the carrier diffusion coefficient at injection levels Δn ≥ 1016 cm−3.