Papers by Keyword: Radiation Hardness

Abstract: Radiation-hardened SiC power devices are essential to prevent leakage degradation and catastrophic failures such as SEE and SEB. Lateral device structures lower the risk of contact shorting by providing greater physical separation between conductive regions. Wider device geometries also improve radiation tolerance, as larger dimensions can accommodate charge buildup with less effect on device performance. In addition, RESURF structures enhance robustness by shifting the high electric field into the bulk, which reduces the impact of radiation-sensitive interface states on breakdown.

Abstract: Due to their low leakage current, low noise levels, high thermal conductivity, and potential radiation hardness, SiC devices offer various advantages over Si devices in certain applications. As a result, they are being considered for operation in harsh environments, such as plasma diagnostic systems in future nuclear fusion reactors or in high energy physics applications. We report on relevant results of the GRACE project, which seeks to deliver a new generation of SiC sensors with graphene-enhanced contacts. Such devices are aimed to be radiation-hard and functional at high temperatures. The work presented in this paper focuses on the optimisation of the electrical contacts, along with the electrical characterisation and radiation-tolerance assessment of the first sensor prototypes produced.

Abstract: Forward and reverse current-voltage (I-V) characteristics of commercial rectifying Schottky diodes (SDs) based on silicon carbide (4H-SiC, base layer doping level 3·10¹⁵ cm^-3) have been studied under irradiation with 0.9 MeV electrons and 15 MeV protons. The starting diodes were characterized by a barrier height of ~1.5 eV and nearly ideal forward and reverse I-V characteristics. It was found that, at doses exceeding the threshold dose D_th, the series differential resistance R_s of the diodes grows as R_s ~ D^m (m = 10-15) and shows no tendency toward saturation. D_th ≈7·10¹⁵ cm^-2 under electron irradiation, and D_th ≈ 4·10¹³ cm^-2 in the case of irradiation with protons. Heating to 200oC results in that R_s decreases with activation energy of ~1.1 eV and R_s is partly annealed-out with activation energy of ~0.7 eV. The starting Schottky diodes changes only slightly under irradiation, but, possibly, the irradiation leads to an over-compensation of the n-type layer and formation of an additional barrier in the form of a pn junction.

Abstract: Neutron irradiation (~1 MeV, dose 10¹⁴-5.6∙10¹⁵ neutron/cm²) of packaged diodes based on 6H-SiC pn structures (with the base n-layer doped to ~5∙10¹⁶ cm^-3) has been studied. In addition to the well-known rise in the series resistance of the diodes, the effect of a partial suppression of the excess current in both forward-and reverse-biased diodes and that of an increase in the recombination current, probably associated with the decrease in the nonequilibrium carrier lifetime, were discovered and discussed. These effects are common to 6H-and 4H-SiC pn structures.

Abstract: The effect of 4.5 MeV electron irradiation on static characteristics of commercially available 5 A/1700 V SiC power MOSFETs is investigated. Results show that in the low dose range (up to 20 kGy) the threshold voltage decreases rapidly with irradiation dose but devices keep full functionality. This effect is caused by embedding of the positive charge into the gate oxide. When electron dose reaches 200 kGy, the threshold voltage moves back close to its original value, however, the ON‑state resistivity increases and transconductance is lowered. This is caused by introduction of deep acceptor centers into the low doped drift region of MOSFET. This effect can be considered as a cause of the final failure of the device (the lost of the ON-state capability).

Abstract: The overall radiation response to X-ray exposure of metal-oxide-semiconductor (MOS) capacitors, subjected to two different post-deposition-annealing (PDA) processes in N₂O or POCl₃ atmospheres, was investigated by capacitance-voltage (C-V) analyses. The production rate and saturation density of electrically active defects, different for the two oxides, demonstrated an additional contribution to the defects formation coming from the annealing treatements. The higher susceptibility of the POCl₃-annealed oxide respect to the N₂O annealed is discussed.

Abstract: This paper aims to establish a new method to characterize the interface between 4H-SiC and passivating dielectric layers. The investigations are made on MOS test structures utilizing Al₂O₃ and SiO₂ dielectrics on 4H-SiC. These devices are then exposed to various fluences of Ar⁺ implantation and then measured by the new method utilizing optical free carrier absorption (FCA) technique to assess the interface traps. A program has been developed using MATLAB to extract surface recombination velocity (SRV) at the oxide/epi-layer interface from the optical data. Capacitance-voltage (CV) is done to extract the density of interface traps (D_it) and a comparison was made. It is observed that SiO₂ samples show a large rise of SRVs, from 0.5×10⁴ cm/s for a reference sample to 8×10⁴ cm/s for a fluence of 1×10^₁₂ cm^-2, whereas Al₂O₃ samples show more stable SRV, changing from 3×10⁴ cm/s for the un-irradiated reference sample to 6×10⁴ cm/s for a fluence of 1×10¹² cm^-2. A very similar trend is observed for D_it values extracted from CV measurements and it can therefore be concluded that the FCA method is a suitable technique for the interface characterization.

Abstract: Silicon sensors, widely used in high energy and nuclear physics experiments, suffer severe radiation damage that leads to degradations in sensor performance. These degradations include significant increases in leakage current, bulk resistivity, space charge concentration, and free carrier trapping. For LHC (Large Hadron Collider) applications, where the total fluence is in the order of 1x10¹⁵ neq/cm² for 10 years, the increase in space charge concentration has been the main problem since it can significantly increase the sensor full depletion voltage, causing either breakdown if operated at high biases or charge collection loss if operated at lower biases than full depletion. For LHC Upgrade, or the sLHC, however, with an increased total fluence up to 1x10¹⁶ neq/cm², the main limiting factor for Si detector operation is the severe trapping of free carriers by radiation-induced defect levels.

Abstract: Polarization effect characteristically occurs in detectors based on wide-bandgap materials at considerable concentrations of radiation defects. The appearance of an electromotive force in the bulk of a detector is due to the long-term capture of carriers at deep levels related to radiation centers. The kinetics and strength of the polarization field have been determined. The capture can be controlled by varying the detector temperature, with a compromise reached at the "optimal" temperature between the generation current and the position of the deepest of the levels whose contribution to the loss of charge via capture is negligible. It has been found that the depth of a level (related to the energy gap width) is close to 1/3, irrespective of a material. The optimal temperatures are strictly individual for materials.

Abstract: The radiation hardness of Al2O3 as a dielectric for SiC surface passivation is studied and compared to SiO2 for potential application in radiation hard SiC devices. SiO2 is deposited on 4H-SiC by PECVD and post annealed in N2O, whereas Al2O3 is deposited by atomic layer deposition (ALD). The oxides are bombarded with Ar ions in an energy range to produce maximum damage near the oxide/SiC interface. Metal-insulator-semiconductor structures are prepared and their dielectric characteristics are analyzed using capacitance-voltage measurements. Additionally, the effect of the interface damage on surface recombination is studied using the optical free carrier absorption method for the same samples. The results indicate that the SiO2/SiC interface is significantly affected at 1×1011 cm-2 fluence of Ar ions, however, the dielectric properties of Al2O3/SiC interface remain unaffected even for ten times higher fluences. Similar observations are made for the surface recombination measurements.