Takeshi Mitani

Raman investigation of strain in Si∕SiGe heterostructures: Precise determination of the strain-shift coefficient of Si bands

Raman scattering experiments were carried out on Si∕SiGe heterostructures. The strain in both the top Si layer, and the Si1−xGex buffer layers with various Ge compositions was evaluated using several excitation sources, together with x-ray diffraction and secondary ion mass spectrometry. The strain-shift coefficient, which is a necessary quantity to evaluate the strain by Raman spectroscopy, was precisely determined. The dependence of the Si–Si band frequency on the Ge composition in the SiGe alloy was also examined. We found that the strained top-Si layers with a thickness below 25nm experience coherent growth on Si1−xGex buffer layers with composition x<0.35.

Deep ultraviolet Raman scattering characterization of ion-implanted SiC crystals

Multiple energy phosphorous ions were implanted into 4H-SiC at room temperature and at an elevated temperature (500 °C) followed by annealing at various temperatures. Deep ultraviolet Raman microscopy was used to analyze the effect of the implantation dose and postannealing temperature on the recovery of surface layers damaged by the implantation. The Raman analysis showed that the recovery rate of the crystallinity increased with an increase in the annealing temperature. However, for highly dosed samples, recovery was not complete even with annealing temperatures up to 1700 °C. With room-temperature implantation, part of the implanted layer was converted into a 3C structure with heavy stacking faults. New Raman bands were observed at below 500cm−1 in samples heavily dosed with 4.0×1016cm−2 after annealing, which revealed that excess phosphorus precipitates. A downshift of the phonon Raman bands and a reduction in the LO–TO-phonon frequency splitting were observed in as-implanted samples and ones that are...

Depth profiling of strain and defects in Si∕Si1−xGex∕Si heterostructures by micro-Raman imaging

We have reported depth and in-plane profiling of strain, Ge composition, and defects in strained-Si∕Si1−xGex∕Si heterostructures using micro-Raman imaging. Raman profiling in the depth direction was carried out with a depth resolution of ∼15nm using a small-angle beveled sample and ultraviolet (UV) excitation. Depth profiles of the Ge composition and Raman bandwidth clearly show that the defect density depends strongly on the Ge-grading rate in a compositionally graded Si1−xGex layer. The in-plane strain variation at a given depth in each layer has been evaluated. The in-plane strain variations in the Si1−xGex are closely related to clustering of misfit dislocations in the graded Si1−xGex layer. For the top strained-Si layer, two-dimensional UV-Raman images of the frequency and bandwidth of the Si band reveal that film crystallinity is correlated with the magnitude of in-plane strain. The close correlation between the frequency and bandwidth is attributed to inhomogeneous strain fields associated with mis...

Growth Rate and Surface Morphology of 4H-SiC Single Crystal Grown under Various Supersaturations Using Si-C Solution

We have investigated growth rate and surface morphology of 4H-SiC single crystal grown from Si-C solution with various supersaturation levels at growth temperature in the range from 1840 to 2140 °C. The growth rate depends linearly on the amount of supersaturated carbon, irrespective of the growth temperature. This indicates that the growth is limited by the transfer of solute element onto the crystallization front. The adequate condition for stable solution growth are discussed with respect to high growth rate and surface morphology.

Depth Profiling of Al Ion-Implantation Damage in SiC Crystals by Cathodoluminescence Spectroscopy

Cross-sectional CL measurements have been performed on the cleaved surface of the Al-ion implanted 4H-SiC. The strong L1 luminescence that originates from the DI defect has been observed even in the deep region (~10 μm) where implanted ions do not penetrate. In the implanted layer, CL results show that high-density non-radiative defects remain even after activation annealing. Generation of the DI defect in the deep region is presumably attributed to the diffusion of point defects from the implanted layer.

Raman characterization of damaged layers of 4H-SiC induced by scratching

Recent development of device fabrication of SiC is awaiting detailed study of the machining of the surfaces. We scratched 4H-SiC surfaces with a sliding microindenter made of a SiC chip, and characterized machining affected layers by micro-Raman spectroscopy. The results of the Raman measurement of the scratching grooves revealed that there were residual stress, defects, and stacking faults. Furthermore, with heavy scratching load, we found clusters of amorphous SiC, Si, amorphous carbon, and graphite in the scratching grooves. Analysis of the Raman spectra showed that SiC amorphization occurs first and surface graphitization (carbonization) is subsequently generated through the phase transformation of SiC. We expect that the Raman characterization of machined surfaces provides information on the machining mechanism for compound semiconductors.

Determination of carrier concentration by Fano interference of Raman scattering in heavily doped n-type 4H-SiC

For n-type 4H-SiC crystals with carrier concentrations between 2 × 1017 and 2.5 × 1020 cm−3, Fano interference of the folded transverse acoustic (FTA) doublet modes was observed. The Fano line-shape parameters were shown to vary with carrier concentration. It is proposed that the peak shifts in the FTA modes resulting from interference with an electronic continuum state can be used to measure carrier concentration for n-type 4H-SiC up to 1020 cm−3. In addition, the relative intensity of the FTA doublet modes varies markedly with carrier concentrations above 5 × 1018 cm−3. This suggests that mode coupling occurs between the FTA doublet components. The variation in the intensity ratio is attributed to the intensity transfer between the FTA doublet components. This mode coupling arises from a phonon-phonon interaction via electronic continuum state-phonon interactions.

Structures of Comets in a Homoepitaxially Grown 4H-SiC Film Studied by DUV Micro-Raman Spectroscopy

Deep-ultraviolet (DUV) micro-Raman spectroscopy was applied to study the micro structures of surface defects in a 4H-SiC homoepitaxially grown film. From DUV Raman spectrum, inclusions of 3C-SiC was found in comet defects. The shape of 3C-structure in comets was investigated and it was found that 3C inclusions in comets can be classified into two types. In addition, spectrum broadening due to the coupling of nonfolded longitudinal optical phonon mode and the photo-excited carriers was also found. The formation mechanisms of 3C inclusion in comets were discussed.

Cathodoluminescence Microcharacterization of Radiative Recombination Centers in Lifetime-Controlled Insulated Gate Bipolar Transistors

Cross-sectional cathodoluminescence (CL) measurements were applied to the study of electron-irradiated punch-through insulated gate bipolar transistors (IGBTs) to investigate the relationship between radiative recombination centers and electrical characteristics. IGBTs were additionally annealed at temperatures of 200–400 °C for 1 h. As annealing temperature rose, collector–emitter saturation voltage (VCES) decreased and current fall time (tf) increased. The cross-sectional CL measurements showed sharp luminescent peaks at 1018 meV (W or I1), 1040 meV (X or I3), and 790 meV (C) and a broad band at approximately 0.90–1.05 eV. As annealing temperature rose, the intensity of the W line decreased and that of the X line increased, suggesting that small self-interstitial clusters agglomerate and form stable, large self-interstitial clusters reducing the total number of self-interstitial clusters. The C line, which originated from an interstitial oxygen and carbon complex, showed no significant change. We consider that self-interstitial clusters play important roles in the electrical characteristics of lifetime-controlled IGBTs.

4H-SiC Growth from Si-Cr-C Solution under Al and N Co-Doping Conditions

We have investigated the solution growth under various Al-N co-doping conditions. Both p-type and n-type 4H-SiC were successfully grown under Al-N co-doping conditions, while using the effect of Al-addition to stabilize both growth surface and polytype. The doping and electrical properties were investigated systematically. Interaction between Al and N in the incorporation process and electrical property under heavily co-doped conditions were discussed.