Karolis Gulbinas

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.

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.

4H-silicon carbide-dielectric interface recombination analysis using free carrier absorption

In this paper, an alternative method to characterize the interface between 4H polytype of Silicon Carbide (4H-SiC) and passivating dielectric layers is established. The studies are made on dielectr ...

Internal Stress in Freestanding 3C‐SiC Grown on Si and Relation to Carrier Lifetime

Residual stress and carrier lifetime variation have been measured in free‐standing n‐type 3C‐SiC wafer grown on undulated Si substrate. We identify extended regions of residual stress that lie parallel to epilayer surfaces. The opposite polarity of stress is identified toward the interface and toward the top surface. Integrated carrier lifetime has been determined by random defect density distribution which is enhanced in the areas of double‐positioning boundary defects. It is shown that carrier lifetimes are severely reduced by the presence of residual stress towards epilayer surfaces. In this way lifetime depth‐distribution can be mistakenly attributed to enhanced surface recombination.

Raman Scattering and Carrier Diffusion Study in Heavily Co- doped 6H-SiC Layers

Thick 6H-SiC epilayers were grown using the fast sublimation method on low-off-axis substrates. They were co-doped with N and B impurities of ≈1019 cm−3 and (41016–51018) cm−3 concentration, respectively. The epilayers exhibited donor-acceptor pair (DAP) photoluminescence. The micro-Raman spectroscopic study exposed a compensated n-6H-SiC epilayer of common quality with some 3C-SiC inclusions. The compensation ratio of B through 200 μm thick epilayer varied in 20-30% range. The free carrier diffusivity was studied by transient grating technique at high injection level. The determined ambipolar diffusion coefficient at RT was found to decrease from 1.15 cm2/s to virtually 0 cm2/s with boron concentration increasing by two orders.

Carrier Lifetimes and Influence of In-Grown Defects in N-B Co-Doped 6H-SiC

The thick N-B co-doped epilayers were grown by the fast sublimation growth method and the depth-resolved carrier lifetimes have been investigated by means of the free-carrier absorption (FCA) decay ...

Temperature Dependencies of Free-Carrier-Absorption Lifetime in Fluorescent 6H-SiC Layers

The nonradiative decay of majority electrons has been studied over a wide temperature range from 80 K to 600 K using the time-resolved free-carrier-absorption (FCA) technique. At high injection level of the highly-luminescent N-B codoped 6H-SiC epilayer, we revealed three main relaxation components of injected free electrons over ps-to-ms time ranges. By means of temperature dependency, two components can be ascribed to thermal activation of holes from a shallow (200 meV) and a deep (500 meV) acceptor. The third one, which has a hundred us-time scale, we attribute to minority hole recombination from the valance band into the electron trap (53 meV). This recombination channel seems to compete with the deep-acceptor (Boron) to-donor (Nitrogen) pair visible emission at and below 300 K.

Conversion of Laser Pulse Optical Energy to Photo-acoustic Wave in nm-Scale Layered TlGaSe2 Crystals

Experiments are presented that reveal an efficient optical energy conversion from the visible to the infrared wavelengths range as a result of photo-acoustic response (PAR) after light pulse incites onto the free surface of TlGaSe2 crystal. Excitation was carried out with a tunable wavelength of ns-pulse laser and the PAR was detected laterally with a focused cw- probe. The observed properties can be related to variety of successive factors: high electron- hole-phonon deformation potential, a high factor of refraction coefficient dependency on pressure, the absence of surface recombination and the band filling effect, in relation with low absorption coefficient due to the forbidden direct-band optical transition in TlGaSe2. All these ensure that the acoustic energy remain well confined under a wide pulse power and energy range suggesting that TlGaSe2 is a promising material for dynamic optical energy conversion.

Examination of Photoluminescence Temperature Dependencies in N-B Co-doped 6H-SiC

Two overlapping photoluminescent (PL) bands with a peaks (half-width) at 1.95 eV (0.45 eV) and 2.15 eV (0.25 eV), correspondingly at 300 K, are observed in heavily B-N co-doped 6H-SiC epilayers under high-level excitation condition. The low energy band dominates at low temperatures and decreases towards 300 K which is assigned to DAP emission from the nitrogen trap to the deep boron (dB) with phonon-assistance. The 2.15 eV band slightly increases with temperature and becomes comparable with the former one at 300 K. We present a modelling comprising electron de-trapping from the N-trap, i.e. calculating trapping and de-trapping probabilities. The T-dependence of the 2.15 eV band can be explained by free electron emission from the conduction band into the dB center provided by similar phonon-assistance.

Anisotropy of band gap absorption in TlGaSe2 semiconductor by ferroelectric phase transformation

The depth-resolved free-carrier absorption and the photo-acoustic response are used to examine the band-gap absorption in 2D-TlGaSe2 layered semiconductor after its transformation into the ferroelectric F-phase below 107 K. The absorption exhibits unusual behavior with a biaxial character in respect to the light polarization on the layer plane. A spectral analysis shows that the anisotropy is associated to the lowest Γ-direct optical transition. The Γ-absorption and the localized exciton at 2.11 eV are dipole-prohibited or partially allowed in two nearly perpendicular polarization directions. The shift of anisotropy axis in respect to crystallographic a- and b-directions demonstrates the non-equivalent zigzag rearrangement of the interlayer connecting Tl+ ions, which is responsible for occurrence of the F-phase.