Papers by Author: Lucia Calcagno

Abstract: Silicon carbide (SiC) has emerged as a leading material for high-power applications. However, the high density of interface states (D_it) at the SiO₂/SiC interface still constrains the performance and reliability of MOSFET devices. In this work, lateral 4H-SiC MOSFETs subjected to post-deposition annealing (PDA) in nitric oxide (NO) of different durations were investigated through capacitance-voltage measurements, supported by an analytical model and an iterative MATLAB-based D_it extraction algorithm. The results demonstrate that NO PDA effectively reduces D_it not only near the conduction band edge but also towards the valence band, yielding improved channel mobility (µ_FE) and enhanced threshold voltage stability.

Abstract: Solid State Detectors (SSD) are crucial for fast neutron detection and spectroscopy in tokamaks due to their solid structure, neutron-gamma discrimination, and magnetic field resistance. They provide high energy resolutions without external conversion stages, enabling compact array installations in the harsh environment of a tokamak. Research comparing diamond and 4H-SiC detectors highlights thickness as a key efficiency factor. A 250 μm SiC epilayer with low doping, grown using a high-growth-rate process, exhibits sharp interfaces and minimal defects, essential for neutron detectors. The study evaluates detector designs, and performance using a 4H-SiC substrate. Various detector designs, such as Schottky diodes and p/n diodes, are assessed via I-V and C-V measurements, addressing high depletion voltage challenges. Preliminary neutron irradiation tests validate detector functionality, energy resolution and confirming detector reliability.

Abstract: In this work we have studied hydrogen etching of Silicon Carbide (SiC) chips at high temperatures and in confined limited regions, to elucidate and control the formation and propagation of terraces on the surface of SiC (0001) 4° off-axis samples. This process is very important for the development of high-power transistors. The effects of process parameters on the etching of 4H-SiC (0001) have been extensively investigated using several types of surface analysis (Atomic Force Microscopies (AFM), Scanning Electron Microscope (SEM) and High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). We correlated the growth of terraces with etching temperature and time. Moreover, we found the average width of terraces increases decreasing the dimension of the structure from 20 µm to 1 µm using the same process parameters. The nanofacet formation of typical hill-and-valley structure has been observed in localized region on SiC (0001) basal plane.

Abstract: The defects produced by 7.0 MeV C+ irradiation in 4H-SiC epitaxial layer were followed by Deep Level Transient Spectroscopy, current-voltage measurements and Transmission Electron Microscopy in a large fluence range (109-51013 ions/cm2). At low fluence (109 -1010 ions/cm2), the formation of three main level defects located at 0.68 eV, 0.98 eV and 1.4 eV below the conduction band edge is detected. The trap concentration increases with ion fluence suggesting that these levels are associated to the point defects generated by ion irradiation. In this fluence range the leakage current of the diodes does not change. At higher fluence an evolution of defects occurs, as the concentration of traps at 0.68 eV and 1.4 eV decreases, while the intensity of the level at 0.98 eV remains constant. In this fluence range complex defects are formed and an increase of a factor five in the leakage current is measured.

Abstract: The defects produced by irradiation with 7 MeV C+ induce a change in the electrical properties of 4H-SiC Schottky diodes. Capacitance-voltage and Current-voltage characteristics of the diodes fabricated in epilayers doped with different nitrogen concentrations were monitored before and after irradiation with different fluences. The Capacitance-voltage curves show free carrier compensation after low fluence irradiation and it was found that the reduction of carriers per ion induced vacancy increases with nitrogen content. The forward current-voltage characteristics of the diodes show an increase in the series resistance after irradiation. This change is mainly related to the high compensation occurring around the end of the ion range.

Abstract: The growth rate of 4H-SiC epi layers has been increased up to 100 µm/h with the use of trichlorosilane instead of silane as silicon precursor. The epitaxial layers grown with this process have been characterized by electrical, optical and structural characterization methods. Schottky diodes, manufactured on the epitaxial layer grown with trichlorosilane at 1600 °C, have higher yield and lower defect density in comparison to diodes realized on epilayers grown with the standard epitaxial process.

Abstract: The growth rate of 4H-SiC epi layers has been increased by a factor 19 (up to 112 μm/h) with respect to the standard process with the introduction of HCl in the deposition chamber. The epitaxial layers grown with the addition of HCl have been characterized by electrical, optical and structural characterization methods. An optimized process without the addition of HCl is reported for comparison. The Schottky diodes, manufactured on the epitaxial layer grown with the addition of HCl at 1600 °C, have electrical characteristics comparable with the standard epitaxial process with the advantage of an epitaxial growth rate three times higher.

Abstract: High growth rate of 4H-SiC epitaxial layers can be reached with the introduction of HCl in the deposition chamber. The effect of the Cl/Si ratio on this epitaxial growth process has been studied by optical and electrical measurements. Optical microscopy shows an improvement of the surface morphology and luminescence measurements reveal a decrease of epitaxial layer defects with increasing the Cl/Si ratio in the range 0.05–2.0. The leakage current measured on the diodes realized on these wafers is reduced of an order of magnitude and DLTS measurements show a decrease of the EH6,7 level concentration in the same range of Cl/Si ratio. The value Cl/Si=2.0 allows to grow epitaxial layers with the lowest defect concentration.

Abstract: The defects formation in ion-irradiated 4H-SiC was investigated and correlated with the electrical properties of Schottky diodes. The diodes were irradiated with 1 MeV Si+-ions, at fluences ranging between 1×109cm-2 and 1.8×1013cm-2. After irradiation, the current-voltage characteristics of the diodes showed an increase of the leakage current with increasing ion fluence. The reverse I-V characteristics of the irradiated diodes monitored as a function of the temperature showed an Arrhenius dependence of the leakage, with an activation energy of 0.64 eV. Deep level transient spectroscopy (DLTS) allowed to demonstrate that the Z1/Z2 center of 4H-SiC is the dominant defect in the increase of the leakage current in the irradiated material.

Abstract: The influence of the epitaxial layer growth parameters on the electrical characteristics of Schottky diodes has been studied in detail. Several diodes were manufactured on different epitaxial layers grown with different Si/H2 ratio and hence with different growth rates. From the electrical characterization a maximum silicon dilution ratio can be fixed at 0.04 %. This limit fixes also a maximum growth rate that can be obtained in the epitaxial growth, with this process, at about 8 μm/h. Several epitaxial layers have been grown, using this dilution ratio, with different temperatures (1550÷1650 °C). At 1600 °C the best compromise between the direct and the reverse characteristics has been found. With this process the yield decreases from 90% for a Schottky diode area of 0.25 mm2 to 61% for the 2 mm2 diodes. Optimizing the deposition process to reduce the defects introduced by the epitaxial process, yield of the order of 80% can be reached on 1 mm2 diodes.