Materials Science Forum Vols. 821-823

Abstract: Silicon carbide thin films were obtained on Si (100) and (111) substrates by means of vacuum laser ablation of α-SiC ceramic target. The influence of substrate temperature on composition, structure and surface morphology of experimental samples was examined using Rutherford backscattering spectrometry (RBS), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), conventional and high-resolution transmission electron microscopy (TEM/HRTEM), atomic force microscopy (AFM), selected area electron diffraction (SAED) and X-ray diffraction (XRD) methods.

Abstract: The effect of the nitrogen dilution on the optical and vibrational properties of amorphous silicon carbide (a-SiC) and silicon oxycarbide (a-SiCO) layers have been studied. The films were prepared by radio frequency (rf) reactive magnetron sputtering using an atmosphere mixture of argon (Ar) and nitrogen (N₂). The oxygen (O₂) was incorporated according to the base pressure used of each deposition process. The optical and vibrational properties of the films were characterized by means of UV/VIS transmittance measurements and Fourier transform infrared spectroscopy (FTIR), respectively. A relationship between the variation of the optical bandgap and the increase or quench of vibrational modes is established. This analysis reveals that the increase of nitrogen in both host matrices a-SiC and a-SiCO induced the formation of C=N and C N bonds thus reducing the bandgap of the material.

Abstract: The behavior of 4H-SiC power devices in severe environment with varying temperature is a key characteristic indicating their reliability. This paper shows the dependence of the ionization rates of 4H-SiC with respect to temperature. Optical Beam Induced Current (OBIC) measurements have been performed on PN junctions to determine the multiplication coefficient for temperature varying between 100 and 450K. That allows extracting the ionization rates by fitting the curves of multiplication coefficient.

Abstract: Raman spectroscopy is a well established non-destructive tool for determining crystal polytypes, strain/stress, electronic properties and material quality in SiC. Here we report on the application of ultrafast Raman imaging to a SiC wafer, allowing 870,908 spectra to be collected from a 2 inch 4H-SiC wafer, in 75 minutes. Analysis of the acquired data enabled us to locate and investigate defects and surface contamination and also allowed stress in the wafer to be characterised.

Abstract: In this work, we analyze residual stress on 4H-SiC with Raman spectroscopy that excitation wavelength is deep ultraviolet (DUV) laser 266nm. The residual stress area is created by Vickers Hardness test technique and the area is measured by 2D DUV Raman map. The result is different from visible light excited Raman, because DUV light penetration is shallower than visible light. DUV Raman signal has exactly brings only the sample surface information. We present the advantage of DUV excited Raman to analyze sample surface.

Abstract: This paper presents a comparative optical and vibrational spectroscopy study of diversely n-type 4H-SiC epilayers. It is shown that in order to determine the nitrogen doping in a wide range (10¹⁶ up to few 10¹⁹cm^-3) the two techniques are complementary. Moreover only the LTPL provides the information about the compensation and nature of the dopant species.

Abstract: New ultrafast Raman imaging methods allow high definition data to be collected from a whole wafer scale down to individual defects in 2D and 3D, on a time scale suitable for routine quality control. On the other hand, just one hour of data collection can result in datasets containing 10⁵~10⁶ spectra, and attempts to manually analyze such big data with traditional univariate methods can take days, without guarantee that all important information is revealed. Such problem can be easily overcome with fast and automated multivariate analysis methods. Here we introduce the techniques and demonstrate applications to SiC.

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: A series of aluminium doped (from 2×10¹⁶ to 8×10¹⁹ cm^-3) 4H-SiC epitaxial layers were mainly studied by Low Temperature Photoluminescence and time-resolved optical pump-probe techniques to determine the concentration of aluminium, its activation ratio, the doping related carrier lifetime, hole mobility and excess carrier diffusion length.

Abstract: The electronic quality of a Physical Vapour Transport (PVT) grown 15R-SiC crystal at different stages of growth was assessed by time-resolved optical pump-probe techniques. The measured differential transmittivity (DT) kinetics for the layers corresponding to the initial, middle and final stages of growth revealed clear differences in the decay of the DT signal, indicating a decreasing concentration of traps at the later stages of the crystal growth. The estimated trap concentration in the initial layer was N_T ≈ 10¹⁹ cm^-3, while it decreased down to less than 2×10¹⁸ cm^-3 in the top layer. The injection dependence of the diffusion coefficient at room temperature confirmed the gradual decrease of N_T in the layers corresponding to later stages of growth. Accordingly, the bipolar diffusion coefficient in the middle and the top layer was D_a ≈ 2 cm²/s, while D_a = 0.9 cm²/s was measured in the layer closest to the seed.