Materials Science Forum Vols. 717-720

Abstract: A two-way tunneling model describing simultaneous oxide trap charging and discharging in SiC MOSFETs is presented, along with a comparison with experimental results. This model can successfully account for the variation in threshold-voltage instability observed as a function of bias-stress time, bias-stress magnitude, and measurement time.

Abstract: Peak value degradation of heavy-ion induced transient currents in Metal-Oxide-Semiconductor (MOS) capacitors fabricated on n-type and p-type 6H-SiC was observed. The capacitances of MOS capacitors measured during the ion irradiation suggest that the depletion layer width decreased with increasing number of incident ions and was saturated. Since the number of incident ions obtained at the peak current saturation corresponded to that at the saturation of the capacitance, the decrease in peak current can be interpreted in terms of the decrease in the depletion layer width.

Abstract: Low frequency noise on 4H-SiC low-level signal-lateral JFETs was systematically investigated. In contrast to previous studies, which are based upon high power vertical structures, this work investigates the low-frequency noise behaviour of low-level signal-lateral devices which are more relevant to the realisation of small signal amplifiers.The JFETs studied share an identical cross section, with different gate lengths and widths. For high temperature operation between 300K and 700K at VGS = 0V, the Normalised Power Spectral Density (NPSD) of the JFETs is proportional to ƒ^-1. The NPSD increases monotonically with temperature until a critical temperature, where it starts to decline. Two unique noise origins, fluctuation from bulk and SiO₂-SiC interface traps were observed across all the devices investigated. Low frequency noise for devices with a 50μm gate width is localised at the SiO₂-SiC interface, whereas for wider devices the noise is seen to be of bulk/substrate origin, which follows Hooge’s model.

Abstract: The reliability of gate oxides is a fundamental issue for realizing SiC MOSFETs. Many reports said that crystal defects shorten the lifetime of the gate oxide. And, epi defects, the basal plane dislocations and threading screw dislocations (TSD) are considered killer defects. However, because of the high TSD density of commercial SiC wafers, the exact relationship between other kinds of dislocations with lifetime has not been revealed. On the other hand, RAF wafers that we developed have low TSD density, so it is easy to evaluate the relationship between other kinds of dislocations and lifetime. By using RAF wafers, in this study, we clarified the relationship between the lifetime of the gate oxide and crystal defects. We fabricated MOS diodes and measured their lifetimes by TDDB (Time Dependent Dielectric Breakdown) measurement. The breakdown points were defined by the photo-emission method. Finally, we classified the defects by TEM (Transmission Electron Microscopy). As the results, it was clarified that threading edge dislocation (TED) decreases the lifetime as does TSD, which earlier reports said. The lifetime of the gate oxide area, in which a TED is included, was shorter by one order of magnitude than a wear-out breakdown. And, the TSD was two orders.

Abstract: The c- and a-lattice constants of nitrogen-doped 4H-SiC were measured in the wide temperature range (RT - 1100°C). The samples used in this study were heavily doped substrates and lightly-doped free-standing epilayers. The lattice constants at room temperature are almost identical for all the samples. However, the lattice contraction by heavy nitrogen doping was clearly observed at high temperatures, which indicates that the thermal expansion coefficients are dependent on the nitrogen concentration. The lattice mismatch (Δd/d) between a lightly-doped free-standing epilayer (N_d = 6x10¹⁴ cm^-3) and a heavily-doped substrate (N_d = 2x10¹⁹ cm^-3) was calculated as 1.7x10^-4 at 1100°C. The authors also investigated lattice constants of high-dose N⁺, P⁺, and Al⁺-implanted 4H-SiC. Reciprocal space mapping (RSM) was utilized to investigate the lattice mismatch and misorientation. The RSM images show the c-lattice expansion and c-axis tilt of the ion-implanted layers, irrespective of ion species. The authors conclude that the lattice expansion is not caused by heavy doping itself, but by secondary defects formed after the ion-implantation and activation-annealing process.

Abstract: Ion implantation into 4H-SiC induces a local gradient of strain which increases with the nuclear energy losses. With the increase of temperature the strain tends to become uniform in the whole implanted area requiring the migration of particles. In case of helium implantation, defects are more stabilized and their evolutions observed post thermal annealing are concomitant with the surface swelling. The local modifications imputed to the ion process lead to the formation and the pile-up of stacking faults in the highly damaged region.

Abstract: Defect formation during the early stages of physical vapor transport (PVT) growth of 4H-SiC was investigated using high resolution x-ray diffraction (HRXRD). Characteristic lattice bending behaviors were revealed in the nearby seed crystal regions of grown crystals. The lattice bending was localized in close proximity to the seed/grown crystal interface, and the (0001) basal planes bended convexly toward the growth direction, indicative of the insertion of extra-half planes pointing toward the growth direction during the initial stages of crystal growth. This paper discusses the possible mechanisms of the observed lattice bending and sheds light on the defect formation processes during PVT-growth of 4H-SiC single crystals.

Abstract: The 3C-6H polytypic transition in 3C-SiC single crystals is studied by means of diffuse X-ray scattering (DXS) coupled with transmission electron microscopy (TEM). TEM reveals that the partially transformed SiC crystals contain regions of significantly transformed SiC (characterized by a high density of stacking faults) co-existing with regions of pure 3C-SiC. The simulation of the diffuse intensity allows to determine both the volume fraction of transformed material and the transformation level within these regions. It is further shown that the evolution with time and temperature of the transition implies the multiplication and glide of partial dislocations, the kinetics of which are quantified by means of DXS.

Abstract: This paper characterizes Al, N doped and undoped 3C-SiC samples after pulsed excimer laser anneal by using X-ray diffraction (XRD) and atomic force microscopy (AFM). In order to protect surface morphology, low energy density of 0.2444 J/cm2 per shot was applied to Al and N doped samples. The results show damage recovery is corresponding to the amount of total energy applied to the surface. The peak shift of Bragg diffraction spectra of Al doped samples are almost independent of the amount of total energy. As the energy density is reduced to 0.0667 J/cm2 per shot and applied to undoped samples, the peaks of Bragg diffraction spectra of undoped samples are shifted. However, the peaks of Bragg diffraction spectra of undoped samples annealed with combination of energy densities of 0.0667 and 0.2444 J/cm2 are not shifted.

Abstract: Technique of bulk-like 3C-SiC film (up to 300 µm) growth on undulant-Si substrate is known to be very effective to reduce stacking fault density as well as that of other planar defects. However, freestanding 3C-SiC wafer shows anisotropic warpage involving large convex curvature in the direction perpendicular to the ridge of undulation ([110] direction), and slight concave curvature in parallel direction ([-110] direction), i.e. saddle shape. In this paper the origin of the warpage of the 3C-SiC wafer is investigated. Ex-situ curvature measurements and stress calculation reveal that large compressive intrinsic stress is generated during high-temperature growth process (1623 K) in both parallel and perpendicular directions. In order to investigate the intrinsic stress distribution along the [001] direction, a reactive ion etching (RIE) is conducted for the 3C-SiC on Si substrate to observe the dependence of the SiC/Si system curvature as a function of 3C-SiC thickness. This observation shows that the intrinsic stress component perpendicular to the ridge of undulation presents nonuniform distribution in [001] direction. The remarkable change in the intrinsic stress is observed in the 50 µm-thick region from SiC/Si interface. A finite element method simulation using the obtained intrinsic stress distribution clearly explains that the anisotropic warpage of SiC wafer is induced by the intrinsic stress distribution in quantitative manner. Microstructure change induced by stacking fault reduction process (stacking fault collision) would be the cause of the intrinsic stress variation.