Materials Science Forum Vols. 679-680

Abstract: Dislocations in highly doped n-type 4H-SiC (n+-SiC, n>1019 cm-3) substrate have been studied by means of electron beam induced current (EBIC). Ni/n-SiC/n+-SiC/Al structure was fabricated in order to simultaneously observe the dislocations in n-SiC epilayer and n+-SiC substrate. We have found that dark dots in the EBIC image correspond to threading screw dislocations (TSDs) and threading edge dislocations (TEDs) with the former being relatively darker. Short dark lines along off-cut are attributed to basal plane dislocations (BPDs) in the epilayer; and the randomly oriented long dark lines are caused by the BPDs in the substrate. The classification of the dislocations by EBIC has been examined by wet etching in KOH+Na2O2.

Abstract: A new method for the separation of threading screw dislocations (TSD) and threading edge dislocations (TED) in a 4H-SiC epitaxial layer is proposed by measurement of the etch pit angles. The etch pit angles of the TSDs and TEDs were 28±3 and 18±3°, respectively. In the case of etch pit depths within the epitaxial layer, the values were almost constant. Almost all of the TSDs were converted from basal plane dislocations (BPDs) at the epitaxial layer/substrate interface.

Abstract: Dislocations in 4H-SiC epilayers were imaged nondestructively by means of micro photoluminescence (-PL) mapping at room temperature. The one-to-one correspondence between the individual dislocations and the -PL mapping contrast has been consistently obtained. By analyzing the reduction of the intensity in the -PL mapping image performed at 390 nm (near band-edge emission), we were able to distinguish threading screw dislocations and threading edge dislocations. Furthermore, the contrast of dislocations in PL-intensity mapping image greatly depends on the carrier lifetimes of the 4H-SiC epilayers. Lifetimes longer than 0.5 s are essential to obtain a discernible contrast for the individual dislocations.

Abstract: Interfacial dislocations are frequently observed to form during 4H-SiC epitaxy and thermal annealing. This report attempts to establish the correlation between the distribution of interfacial dislocations and the thermal stress induced by a radial temperature gradient. In addition, it is argued that they are misfit dislocations formed by the interaction between thermal strain and misfit strain.

Abstract: Frank-type defects on the basal plane in thick 4H-SiC epitaxial layers converted from threading screw dislocation (TSD) have been characterized by photoluminescence (PL) spectroscopy and PL imaging microscopy. PL emission from the stacking fault (SF) and the Frank partial of the defect was obtained at ~457 nm and >700 nm at room temperature, respectively. The PL emission peaks of two other kinds of Frank-type defects were obtained, and a correspondence between the optical properties and the microscopic structures of the three kinds of defects was clarified.

Abstract: Both 3C and 6H stacking faults have been observed in a low doped 4H-SiC epitaxial layer grown in a hot-wall CVD reactor on a heavily doped (off-axis) 4H-SiC substrate. They appear differently on the different parts of sample, with energetic dispersion ranging from 3.01 eV to 2.52 eV. Since they behave as natural type-II quantum wells in the 4H-SiC matrix, the thickness dependence of the excitonic recombination is investigated using the standard effective mass approximation. The results are discussed in terms of built-in electric field and inter-well coupling effects.

Abstract: The performance of 4H-SiC power MOSFETs is limited by the less than ideal electron inversion-layer mobility due to the poor quality of the SiC-SiO2 interface. This poor interface causes several undesirable behaviors of the electrical performance of SiC MOSFETs, including: (1) strong shifts in the threshold voltage with temperature, (2) low channel mobility and (3) strong sensitivity of the mobility to the channel doping concentration. These features are explained by a high density of interface states, the high surface electric field induced in SiC inversion layers, and the combined effectsa combination of Coulomb and surface roughness scattering.

Abstract: The effect of nitrogen (N) introduced by ion implantation at the SiO2/4H-SiC interface on the capacitance of the MOS capacitors is investigated. The Thermal Dielectric Relaxation Current (TDRC) technique and Capacitance-Voltage (C-V) measurements performed at different temperatures and probe frequencies on an N implanted sample and on a virgin sample were employed for this purpose. There are three types of defects located at or near the interface, Dit, NIToxfast and NIToxslow that can be distinguished. Only Dit and NIToxfast respond to the a.c. small, high frequency signal at temperatures above 150K. The separation of Dit from the NIToxfast states have enabled us to study the influence of the excess of interfacial Nitrogen on each of the mentioned defects. It has been found that the N-implantation process fully suppresses the formation of NIToxfast and partially NIToxslow and Dit. Theoretical C-V characteristics were computed, based on the defect distributions determined by TDRC, and compared with the experimental ones showing a close agreement.

Abstract: SiO2/4H-SiC interfaces are examined by high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and spatially resolved electron energy-loss spectroscopy (EELS). HRTEM and HAADF-STEM images of SiO2/4H-SiC interfaces reveal that abrupt interfaces are formed irrespective of the fabrication conditions. Transition regions around the interfaces reported by Zheleva et al. were not observed. Using EELS, profiles of the C/Si and O/Si ratios across an interface were measured. Our measurements did not reveal a C-rich region on the SiC side of the interface, which was reported by Zheleva et al.

Abstract: We report a strong reduction in the density of near-interface traps (NITs) at the SiO2/4H-SiC interface after dry oxidation in the presence of potassium. This is accompanied by a significant enhancement of the oxidation rate. The results are in line with recent investigations of the effect of sodium on oxidation of 4H-SiC. It is evident that both alkali metals enhance the oxidation rate of SiC and strongly influence the energy distribution of interface states.