Materials Science Forum Vol. 1062

Abstract: Silicon carbide (SiC) is a wide band-gap semiconductor of great technological importance, showing promise for application areas ranging from quantum computing and communication to power devices. Vital in both the contexts of power devices and quantum technology is the understanding of intrinsic defects that are introduced during various device processing steps, both immediately after their formation and over the course of defect evolution with temperature. Here we monitor the formation and evolution of intrinsic point defects in n-type 4H-SiC after proton irradiation at room temperature and subsequent annealing in the temperature range 300-1000 °C, and discuss the nature and origin of the EH₄ and EH₅ deep level defects observed by deep level transient spectroscopy around 400-500 K. In particular, the controversy on the nature of the EH5 trap in particular is addressed, where we propose the presence of two overlapping defect peaks: one metastable level that appears after low energy electron irradiation below the silicon displacement limit, and one more stable level that gradually decreases in concentration until an annealing temperature of 1000°C. We argue that the former is likely related to carbon interstitials, while the latter was recently tentatively attributed to the carbon antisite-vacancy pair.

Abstract: Diamond and Silicon Carbide (SiC) are promising wide band-gap semiconductors for power electronics, SiC being more mature especially in term of large wafer size (200 mm). Nitrogen impurities are often used in both materials for different purpose: increase the diamond growth rate or induce n-type conductivity in SiC. The determination of the nitrogen content by secondary ion mass spectrometry (SIMS) is a difficult task mainly because nitrogen is an atmospheric element for which direct monitoring of N^± ions give no or a weak signal. With our standard diamond SIMS conditions, we investigate ¹²C¹⁴N^- secondary ions under cesium primary ions by applying high mass resolution settings. Nitrogen depth-profiling of diamond and SiC (multi-) layers is then possible over several micrometer thick over reasonable time analysis duration. In a simple way and without notably modifying our usual analysis process, we found a nitrogen detection limit of 2x10¹⁷ at/cm³ in diamond and 5x10¹⁵ at/cm³ in SiC.

Abstract: Recently high-k gate dielectrics for SiC power MOSFETs attracted increasing research interest thanks to promising results related to improved specific channel resistances and threshold voltage stability. We investigated high-k gate stacks for 1.2kV and 3.3kV SiC power MOSFETs regarding on-state performance and stability during high temperature gate bias tests. Furthermore, we studied the high-k/SiC interface quality and the effect of burn-in pulses using SiC MOSCAPs. High-k SiC power MOSFETs show significant improvement in on-state performance and threshold voltage stability. We found that the burn-in pulses can be shorter for high-k gate dielectrics compared to SiO₂-based devices.

Abstract: Electrically active defects at the SiO₂/SiC interface can have detrimental effects on the device performance of SiC MOSFETs. Capacitance-or conductance-based analysis techniques are commonly used to extract the density of interface defects, despite having the disadvantage of requiring dedicated test structures for the analysis. Here, we discuss confocal sub-bandgap photoluminescence (PL) microscopy as a fast and reliable alternative to conventional electrical characterization techniques. For this purpose, the quality of the SiO₂/SiC interface after post-oxidation annealing in N₂O is studied both by confocal imaging as well as by the high-low and C-Ψ capacitance technique. We find excellent agreement between the optical and electrical analysis and observe a significant increase of the interface defect density for annealing temperatures below 1050 °C. Keywords: interface defect density, photoluminescence, capacitance-voltage

Abstract: In this study, we investigated the origin of line-shape defect in 4H-SiC epitaxial wafers. The inspection results revealed that such defects resulted from the substrate entirely and accompanied with dislocation lines during the epitaxial process. Although the defect surface condition with nanometer level of roughness seemed to do little harm to the initial electrical characteristics of power devices, dislocation lines possibly resulted in high leakage current when reverse voltage was applied. To reduce line-shape defects, it is essential to reduce defects and threading dislocations in substrates and to develop a nondestructive method for wafer screening.

Abstract: This paper presents a macro-and nanoscale electrical investigation of Schottky and metal-oxide junctions with hetero-epitaxial 3C-SiC layers grown on Si. Statistical current-density-voltage (J-V) characterization of Pt/3C-SiC Schottky diodes showed an increase of the reverse leakage current with increasing the devices diameters. Furthermore, C-V and J-V analyses of SiO₂/3C-SiC capacitors revealed non-idealities of the thermal oxide, such as a high trapped positive charge (3×10¹² cm⁻²) and a reduced breakdown field (E_BD=6.5 MV/cm) compared to ideal SiO₂. Nanoscale electrical characterizations by conductive atomic force microscopy (CAFM) and scanning capacitance microscopy (SCM) allowed to shed light on the origin of non-ideal behavior of Schottky and thermal oxide junctions, by correlating the morphological features associated to 3C-SiC crystalline defects with local current transport and carrier density.

Abstract: The detection and classification of SiC Epitaxial extended defects was refined to separate out defective areas that influence device characteristics. Die level defect localization along with defect area calculations were performed on millions of die across product groups. A clear impact of non-killer defects was observed, especially with increasing density and defective area in the die. Specifically, all types of stacking faults caused higher leakage, lower blocking voltage, and increases in ON resistance and threshold leakage. Furthermore, MOSFET devices were affected to a much larger extent than diode devices. Testing die with higher numbers of defects provides insight on device reliability. Analyzing devices with specific counts of BPDs let us quantify the amount of bipolar degradation caused drift by product/voltage classes.

Abstract: In this work, we focus on the electrical characterization of Ni Schottky contact on n-type heavily doped (N_D>10¹⁹ cm⁻³) 4H-SiC layer, achieved by P-ion implantation. In particular, the forward current–voltage characterization of Schottky diodes showed a reduced turn-on voltage for the Ni/heavily-doped 4H-SiC if compared to a reference Ni/4H-SiC Schottky contact fabricated under similar conditions but without implant. Moreover, it was observed the predominance of a thermionic-field-emission (TFE) mechanism for the current transport through the interface. From a current-voltage-temperature (I-V-T) study, the temperature-dependence of the Schottky barrier and doping concentration were evaluated, obtaining a reduction of the barrier (from 1.77 to 1.66 eV), while the doping concentration maintains constant around 1.96×10¹⁹ cm^-3. This study provides useful insights for a deeper comprehension of the electrical behavior of Ni contacts and can have possible applications in 4H-SiC Schottky diode technology.

Abstract: Laser annealing process for ohmic contact formation on 4H-SiC has attracted increasing attention in the last years, because it enables the fabrication of SiC power devices on very thin substrates. We have investigated the formation of Nickel-based ohmic contact on 4H-SiC by using a Yb:YAG laser in scanning mode, with a wavelength of 515 nm and a pulse duration of 1200 ns. A 100 nm thick Ni layer has been deposited on SiC and irradiated at different process conditions. The reaction process has been studied, as a function of fluence and scan number of laser annealing, by means of X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) analyses. The electrical properties of the annealed layers have been evaluated on Schottky Barrier Diodes (SBDs) devices, confirming the ohmic behavior of the reacted contact and showing improved performances respect to RTA approach. The compatibility of thermal budget of the process in the front side has been verified by means process simulation. A strong relationship between structural properties of reacted layers and electrical behavior of SBDs devices has been revealed. Solid-state laser annealing process, with wavelength in green light region, can indeed represent a suitable solution for ohmic contact formation of 4H-SiC power devices, fabricated on thin substrates.

Abstract: MeV level aluminum implants into 4H-SiC were performed as part of superjunction diode fabrication. Measurement of resistance test structures produced resistivities well above expected values with large decreases at elevated temperatures. Capacitance-voltage measurements indicate a high activation rate of the implanted aluminum. Temperature dependent Hall measurements produce reasonable hole mobilities with acceptor ionization energies of approximately 330meV, well above the 200meV expected for low concentration aluminum doping in 4H-SiC.