Materials Science Forum Vols. 556-557

Abstract: Micro-photoluminescence can be used to image electrically active structural defects in SiC. Under suitable excitation conditions it is possible to observe both band-edge PL and near bandedge PL from recombination via a shallow boron acceptor. The intensity of the band-edge emission is related to the carrier lifetime – and is reduced by the presence of structural or interfacial defects. The intensity of the deep level PL is a complex function of the number of radiative centers and the number of centers limiting carrier lifetime. Micro-PL mapping can provide information on the spatial distribution of electrically active defects in SiC.

Abstract: Three types of defects, namely defect I, defect II, defect III, in the 4H-SiC homoepilayer were investigated by micro-raman scattering measurement. These defects all originate from a certain core and are composed of (I) a wavy tail region, (II) two long tails, the so called comet and (III) three plaits. It was found that there are 3C-SiC inclusions in the cores of defect II and defect III and the shape of inclusion determines the type of defect II or defect III. If the core contains a triangle-shaped inclusion, the defect III would be formed; otherwise, the defect II was formed. No inclusion was observed in the core of the defect I. The mechanisms of these defects are discussed.

Abstract: The microscopic origin of the Nx EPR-lines observed in heavily nitrogen doped 4H-SiC and 6H-SiC is discussed with the help of EPR parameters calculated from first principles. Based on the symmetry of the g-tensors we propose a model with distant NC donor pairs on inequivalent lattice sites which are coupled to S=1 centers but with nearly vanishing zero-field splittings, giving rise to an essentially S=1/2 like spectrum. The proposed aggregation in neutral donor pairs can contribute to the saturation of the free concentration observed in heavily nitrogen doped SiC.

Abstract: We applied picosecond dynamic grating technique for studies of carrier dynamics in ntype DPB(double positioning boundary)-free 3C-SiC (111) epilayer grown by VLS (vapour-liquidsolid) mechanism on 6H-SiC (0001). The measurements of bipolar diffusion coefficient D and carrier lifetime τR in the samples at various pump energies (0.5 – 3.0 mJ/cm2) and temperatures (9 – 300 K) provided the values of bipolar mobility of ~ 80 cm2/Vs and τR = 1.5 - 2.0 ns at 300 K. The ionized impurity scattering, dominant at T < 100 K, and carrier-density dependent lifetimes in 10- 300 K range were attributed to contribution of trapping centers which electrical activity saturates at high carrier density.

Abstract: The objective of the present work is to examine the preparation and characterization of the SiC sponge obtained by direct synthesis of high purity graphite and silicon powder in inert atmosphere at 1900 - 2000 0C for 2-3 hours. To burn out the carbon excess the samples were subsequently heated in oxygen atmosphere at 800 -900 0C resulting in a weight loss of SiC. The relative density of SiC sponge is 1.5-1.7 g/cm 3. The advantages of the proposed method are the high rate of ceramic yield and capability to create the ceramic samples of preliminary selected shape by using a graphite cell of different configurations. Raman scattering spectroscopy and electron paramagnetic resonance (EPR) data have shown that SiC sponge produced by direct synthesis of graphite and silicon powder consists of 6H and 3C SiC polytypes.

Abstract: We report an optical investigation of cubic Silicon Carbide (3C-SiC) layers grown on 6H-SiC substrates by Chemical Vapour Deposition and Vapour-Liquid-Solid mechanism. Micro- Infrared reflectance ('-IR), micro-Raman ('-Raman) and low temperature photoluminescence spectroscopies were used for the characterisation of such layers. '-IR measurements showed unusual optical behaviour of 3C-SiC layers. The difference of refraction index between the 3C-SiC film and the 6H-SiC substrate cannot explain this result. The experimental '-IR reflectance spectrum was modelled by introducing a thin (thickness ≤ 0.5 'm) metallic-like (doping ≥ 1020 at.cm-3) interfacial film between the layer and the substrate. The photoluminescence spectra revealed the presence of a peak which may be attributed to recombination at the 3C/6H interface. All these results suggest the presence of a two dimensional electron gas at the interface.

Abstract: Photoluminescence properties of a freestanding nanoporous SiC layer obtained from bulk 6H-SiC substrate as well as SiC nanopowder consisting of numerous separated nanoparticles has been investigated. The nanoporous SiC layer is obtained by UV radiation assisted electrochemical etching of the 6H-SiC wafer and the SiC nanopowder is formed by mechanical grinding of the nanoporous SiC free layer. A comparison of low temperature PL spectra of the SiC nanostructures and initial SiC bulk substrate has been performed. The evolution of PL spectra of the SiC nanostructures with respect to their surface states and excitation laser power has been studied. In particular, the well pronounced high energy tail above the excitonic bandgap in the PL spectra of the nanostructured SiC is attributed to quantum confinement effects. The strong PL signal obtained below the bandgap is explained by radiative transitions involving surface states, N-Al donoracceptor recombination levels and deep levels corresponding to volume defects in the SiC nanocrystallites.

Abstract: The room-temperature photoluminescence (RTPL) was investigated in commercial nitrogen-doped 4H-SiC substrates. In a typical RTPL spectrum of n-type 4H-SiC substrate, the ‘band-edge’ emission was similar to PL signatures that are typically attributed to free-exciton recombination in high-quality thick epitaxial layers. The origin of the deep-defect ‘red’ emission and its influence on recombination properties of SiC remain unclear. In most of the substrates in which the ‘red’ RTPL band was strong, clear reverse correlation between the ‘red’ and ‘band-edge’ RTPL intensities was observed. In contrast, direct correlation was observed between the ‘bandedge’ PL map and distribution of the net free electron concentration. There is a possibility that incorporation of nitrogen donors is influenced by (or influences) incorporation of lifetime-limiting deep defects.

Abstract: A lightly doped n-type homo-epitaxial layer was grown by CVD onto a heavily doped n-type 4H-SiC substrate for which half of the surface had been made porous by photoelectrochemical etching. Raman spectra are obtained in the optic phonon region using three scattering geometries. An effective medium model for the porous layer is used to assist in the interpretation of the spectra. This work demonstrates that the contributions to the Raman spectra of the various layers in a sample with multiple 4H-SiC layers can be extracted.

Abstract: With development of very-low-micropipe-density substrate, reduction of other device killer defects becomes important for large size power devices. We employed reverse biased electrochemical etching (RECE) method in order to elucidate where the current leaks out in Schottky barrier diode. Low concentration 4H-SiC epi-layer with Al implanted guard rings was electrochemically etched under reverse bias voltage up to 400V in HF-based electrolyte. The surface of the substrate was observed with Nomarski microscopy before and after RECE. In guard ring area, holes appeared which are aligned toward off-direction of the substrate. The length of the aligned holes is about 25μm. The guard ring surrounding the holes were etched uniformly but limited as though there exists a boundary parallel to the steps. In Schottky contact area, not all but some of carrot defects showed an etched feature after RECE. Such etching features are also observed at different position without carrot. We consider that threading screw dislocation is one cause of leakage current in SBD. An etching feature like tangled strings also appeared. Its outer dimension is more than 1mm with thickness of about 50μm. The origin of tangled-string-like feature is not clear yet.