Papers by Author: Marek Skowronski

Abstract: Improvements in the quality and consistency of 4H-SiC epitaxy wafers are now starting to enable growth of commercial SiC power device applications in areas such as inverters for photo-voltaic systems and power supplies. Recent work has achieved very low epitaxy surface roughness and very low BPD (Basal plane dislocation) in the on 4 degree off-axis substrates. In this paper, we report characterization of the very low BPD epitaxy wafers and a newly observed triangular defect.

Abstract: The effectiveness of an in-situ growth interrupt in nitrogen doped 8° off-cut epilayers was investigated using ultraviolet photoluminescence imaging. Low-doped n-type epilayers (<1016 cm-3) exhibited an abrupt increase in BPD to TED conversion at the growth interrupt and achieved 96-99% conversion overall (< 10 BPDs/cm-2), while high-doped epilayers had minimal conversion at the interrupt (< 1%) and overall (< 30%). This large discrepancy suggests nitrogen prohibits or alters the conversion mechanism at the growth interrupt. Therefore, a novel SEM technique was developed to "freeze-in" the interface morphology and help elucidate the conversion mechanism. Preliminary results suggest that preferential etching at the point of BPD intersection with the surface is greatly reduced in highly doped layers, which inhibits the conversion mechanism.

Abstract: The understanding of the structure and associated defect level of point defects in SiC is important because the material is to be used both as a semiconductor and semi-insulator. Production of the latter is achieved by compensation of unavoidable impurities using defects that require more energy for ionization than the unintentional donors or acceptors. The purpose of the present work is to measure the defect energy level of one center in high resistivity 4H SiC using photo-induced electron paramagnetic resonance (photo-EPR). The center is identified as SI-5, an EPR signal that others have attributed to the negative charge state of the carbon vacancy-carbon antisite pair, − C Si V C . Samples containing this defect exhibit two different photo thresholds, which depend on the resistivity activation energy, Ea. For samples with Ea less than 0.8 eV, a photothreshold at 0.75+/- 0.05 eV is observed, but for those with Ea greater than 0.8 eV, the threshold is between 2 and 2.5 eV. Previous work focused on the former case. Here, the SiC substrates with the larger Ea are emphasized, showing that the photo-threshold likely measures the neutral to negative defect level, − / 0 C Si V C .

Abstract: Material defects such as Si-core and C-core partial dislocations (PDs) and threading screw dislocations (TSDs) and threading edge dislocations (TEDs) are being investigated for their contributions to device performances in 4H-SiC. Non-destructive electroluminescence and photoluminescence techniques can be powerful tools for examining these dislocations. In this report, these techniques were used to reveal the different spectral characteristics for the mentioned dislocations. At higher injection levels, both the Si-core and C-core PDs possessed a spectral peak at 700 nm. However, at lower injection levels, the spectral peak for the Si-core PD remained at 700 nm while the peak for the C-core moved to longer wavelengths. For the threading dislocations, TSDs possessed a peak between 800 and 850 nm while the TEDs possessed a peak at 600 nm independent of the injection levels.

Abstract: The evolution of basal plane dislocations (BPDs) in 4H-SiC epitaxy during its growth is investigated by using two types of interrupted growth in conjunction with ultraviolet photoluminescence (UVPL) imaging of the dislocations. For the first, each epitaxial growth was stopped after 10-20 μm and a UVPL map was collected. For the second, changing the gas flow interrupted the growth and the BPDs were imaged at the end. The first sequence made it possible to track the formation of half-loop arrays and show that they arise from BPDs that glide perpendicular to the offcut direction. For both types, each interruption causes between 30 – 50% of the BPDs to be converted to threading edge dislocations (TEDs). This result suggests that using interrupted growth may be an alternate method to producing epitaxial layers with low BPD concentration.

Abstract: Halide chemical vapor deposition (HCVD) allows for rapid growth while maintaining the purity afforded by a CVD process. While several shallow and deep defect levels have been identified in 6H HCVD substrates using electrical techniques, here we examine several different point defects found in 4H n-type HCVD SiC using electron paramagnetic resonance (EPR) spectroscopy. One spectrum, which exhibits axial symmetry and broadens upon heating, may represent a collection of shallow defects. The other prominent defect has the g tensor of the negatively charged carbon vacancy, but additional hyperfine lines suggest a more complex center. The role of these defects is not yet determined, but we note that the concentrations are similar to those found for the electrically detected defect levels, making them a reasonable source of electrically active centers.

Abstract: We detail a comprehensive approach to preparing epiwafers for bipolar SiC power devices which entails etching the substrate, growing a semi-sacrificial basal plane dislocation (BPD) conversion epilayer, polishing away a portion of that conversion epilayer to recover a smooth surface and then growing the device epilayers following specific methods to prevent the reintroduction of BPDs. With our best processing, we achieve a BPD density of < 10 cm-2 and an extended defect density of < 1.5 cm-2. Specifics of low BPD processing and particular concerns and metrics will be discussed in regard to process optimization and simplification.

Abstract: The selectivity, material removal rate, and the residual subsurface damage of colloidal silica (CS) chemi-mechanical polishing (CMP) of silicon carbide substrates was investigated using atomic force microscopy (AFM) and plan view transmission electron microscopy (TEM). Silica CMP, in most process conditions, was selective. In the damage region surrounding remnant scratches, the vertical material removal rate exceeded the planar material removal rate, which resulted in an enhancement of the scratches over the duration of the polishing process. The material removal rate was low, about 20 nm / hr. In addition, the selectivity leads to a slow removal of residual subsurface damage from mechanical polishing. The silica CMP polished surface exhibits significant subsurface damage observed by plan view TEM even after prolonged polishing of 16 hours.

Abstract: A new chemical mechanical polishing process (ACMP) has been developed by the Penn State University Electro-Optics Center for producing damage free surfaces on silicon carbide substrates. This process is applicable to the silicon face of semi-insulating, conductive, 4H, 6H, onaxis and off-axis substrates. The process has been optimized to eliminate polishing induced selectivity and to obtain material removal rates in excess of 150nm/hour. The wafer surfaces and resultant subsurface damage generated by the process were evaluated by white light interferometery, Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and epitaxial layer growth. Residual surface damage induced by the polishing process that propagates into the epitaxial layer has been significantly reduced. Total dislocation densities measured on the ACMP processed wafers are on the order of the densities reported for the best as grown silicon carbide crystals [1]. Characterization of high electron mobility transistors (HEMTs) grown on these substrates indicates that the electrical performance of the substrates met or exceeded current requirements [2].

Abstract: Undoped 6H- and 4H-SiC crystals were grown by Halide Chemical Vapor Deposition (HCVD). Concentrations of impurities were measured by various methods including secondary-ion-mass spectrometry (SIMS). With increasing C/Si ratio, nitrogen concentration decreased and boron concentration increased as expected for the site-competition effect. Hall-effect measurements on 6H-SiC crystals showed that with the increase of C/Si ratio from 0.06 to 0.7, the Fermi level was shifted from Ec-0.14 eV (nitrogen donors) to Ev+0.6 eV (B-related deep centers). Crystals grown with C/Si > 0.36 showed high resistivities between 1053 and 1010 4cm at room temperature. The high resistivities are attributed to close values of the nitrogen and boron concentrations and compensation by deep defects present in low densities.