4H-SiC Schottky Diodes Research Articles

Dependence of the Silicon Carbide Radiation Resistance on the Irradiation Temperature

The effect of high-temperature electron and proton irradiation on SiC-based device characteristics is being investigated. Industrial integrated 4H-SiC Schottky diodes, each with an n-type base and a blocking voltage of either 600 V, 1200 V, or 1700 V, manufactured by Wolfspeed, are being studied. 0.9 MeV electron and 15 MeV proton irradiation were applied. It has been found that the irradiation resistance of silicon carbide Schottky diodes at high temperatures significantly exceeds their resistance at room temperature. This effect is attributed to the annealing of compensating defects induced by high-temperature irradiation. The parameters of radiation-induced defects are determined using the method of deep level transient spectroscopy (DLTS). Under high-temperature ("hot") irradiation, the spectrum of radiation-induced defects introduced into SiC appears to differ significantly from the spectrum of defects introduced at room temperature. It is suggested that approximately half of the compensation is due to radiation-induced defects formed in the bottom part of the bandgap.

Anomalous Electrical Behavior of 4H-SiC Schottky Diodes in Presence of Stacking Faults

In this work, we investigated the impact of crystallographic defects (specifically stacking faults, SFs) on the mechanisms of the current transport in 4H-SiC Schottky contacts. The electrical characteristics were studied under both forward and reverse bias. In particular, while the presence of SFs under the contact did not show a significant impact on the forward characteristics of the Schottky diode, a significant increase in the leakage current occurred under reverse bias in defective diodes. This anomalous behavior can be explained by a space-charge limited current model, consistent with the presence of a trapping state distribution in the 4H-SiC gap. An increase of the reverse bias above 30 V leads to a complete trap filling. The weak temperature-dependence of the leakage current observed at highest voltage suggests that a tunneling of the carriers through the barrier can be also present.

Space charge limited current in 4H-SiC Schottky diodes in the presence of stacking faults

The presence of crystallographic defects can induce notable effects on the mechanisms ruling the current transport in metal/semiconductor contacts. In this context, in this Letter, the impact of stacking faults (SFs) on the characteristics of 4H-SiC Schottky diodes was investigated under both forward and reverse bias. In particular, in the presence of SFs under the contact, while no significant effect on the ideality factor and barrier height was observed under forward bias, an anomalous increase in the leakage current occurred under reverse bias. The observed behavior of the leakage current could be explained by a space-charge limited current model, consistent with the presence of a distribution of trapping states in the gap of 4H-SiC. An increase in the reverse bias above 30 V leads to a complete trap filling. The weak temperature-dependence of the leakage current observed in this regime suggests the coexistence with a tunneling of the carriers through the barrier. The results can be useful to understand unexpected failures in 4H-SiC Schottky diodes.

Epitaxial SiC Dosimeters and Flux Monitoring Detectors for Proton Therapy Beams.

The exceptional optoelectronic properties and high radiation resistance of epitaxial silicon carbide make this material attractive for high-energy beam dosimetry and radiation monitoring, especially when strict requirements such as high signal-to-noise ratios, high time and spatial resolutions and low detectivity levels are required. A 4H-SiC Schottky diode has been characterized as a proton-flux-monitoring detector and dosimeter under proton beams for proton therapy. The diode was composed of an epitaxial film grown on 4H-SiC n+-type substrate equipped with a gold Schottky contact. The diode was embedded in a tissue-equivalent epoxy resin and then characterized in terms of capacitance vs. voltage (C-V) and current vs. voltage (I-V) characteristics in the dark in the range of 0-40 V. The dark currents at room temperature are in the order of 1 pA, while the doping and active thicknesses extracted from the C-V are 2.5 × 1015 cm-3 and 2-4 μm, respectively. Proton beam tests have been carried out at the Proton Therapy Center of the Trento Institute for Fundamental Physics and Applications (TIFPA-INFN). They have been carried out with energies and extraction currents of 83-220 MeV and 1-10 nA, respectively, as typical for proton therapy applications, corresponding to dose rates in the range of 5 mGy/s to 2.7 Gy/s. The I-V characteristics measured under proton beam irradiation at the lowest dose rate showed a typical diode photocurrent response and a signal-to-noise ratio well above 10. Investigations with null bias evidenced a very good performance in terms of the diode's sensitivity, fast rise and decay times and response stability. The diode's sensitivity was in agreement with the expected theoretical values, and its response was linear throughout the whole investigated dose rate range.

4H-SiC Schottky diode radiation hardness assessment by IBIC microscopy

We report findings on the Ion Beam Induced Charge (IBIC) characterization of a 4H-SiC Schottky barrier diode (SBD), in terms of the modification of the Charge Collection Efficiency (CCE) distribution induced by 20 MeV C ions irradiations with fluences ranging from 20 to 200 ions/μm2.The lateral IBIC microscopy with 4 MeV protons over the SBD cross section, carried out on the pristine diode evidenced the widening of the depletion layer extension as function of the applied bias and allowed the measurement of the minority carrier diffusion lengths.After the irradiation with C ions, lateral IBIC showed a significant modification of the CCE distribution, with a progressive shrinkage of the depletion layer as the fluence of the damaging C ions increases.A simple electrostatic model ruled out that the shrinkage is due to the implanted charge and ascribed the perturbation of the electrostatic landscape to radiation-induced defects with positive charge state.

Effect of Proton Irradiation on the Properties of High-Voltage Integrated 4H-SiC Schottky Diodes at Operating Temperatures

Effect of Proton Irradiation on the Properties of High-Voltage Integrated 4H-SiC Schottky Diodes at Operating Temperatures

Effect of proton irradiation on the properties of high-voltage integrated 4H-SiC Schottky diodes at operating temperatures

The effect of proton irradiation (proton energy 15 MeV) on the parameters of high-voltage 4H-SiC integrated Schottky diodes (JBS) was studied for the first time in the operating temperature range Ti (23 and 175oC). The blocking voltage of the diodes under study, Ub, was 600 and 1700 V. For devices with U_b = 600 V, the fluence range was 5·1013-1·1014 cm-2; for devices with U_b=1700 V, the fluence range was 3·1013-6·1013 cm-2. An increase in the irradiation temperature leads to a noticeable decrease in the effect of irradiation on the current-voltage characteristics of the diodes. The effect of annealing on the current-voltage characteristics of irradiated devices is studied. Keywords: Silicon carbide, Schottky diodes, proton irradiation, current-voltage characteristics, annealing.

Large-area vertical Schottky barrier diodes based on 4H-SiC epilayers: Temperature-dependent electrical characteristics

As an alternative to conventional silicon-based devices, 4H-phase silicon carbide (4H-SiC) is emerging as a promising semiconductor for particle detectors worked at high temperatures. In this work, 4H-SiC Schottky diodes with a vertical geometry and a sensitive area of 7 × 7 mm2 were fabricated. The electrical characterization parameters of devices were calculated from 300 to 473 K using I-V and Cheung’s function H(J)–J, in order to reveal the temperature-dependent performances. The results indicate that the ideality factor (n), barrier height (Φb), and leakage current are positively related to temperature. On the contrary, the on-resistance (Ron), series resistance (Rs), and built-in potential voltage (Vbi) are negatively dependent on temperature. In addition, these diodes with large areas exhibit a quite low leakage current and quite large rectification ratio simultaneously, demonstrating the high quality of these devices. Finally, the response characteristics of a Ni/4H-SiC detector were investigated under the irradiation of a 241Am alpha source. This work may provide a further understanding of the SiC-based detectors with large sizes for radiation detection applications at high temperature.

Features of the Carrier Concentration Determination during Irradiation of Wide-Gap Semiconductors: The Case Study of Silicon Carbide

In this paper, the features of radiation compensation of wide-gap semiconductors are discussed, considering the case study of silicon carbide. Two classical methods of concentration determination are compared and analyzed: capacitance-voltage (C-V) and current-voltage (I-V) characteristics. The dependence of the base resistance in high-voltage 4H-SiC Schottky diodes on the dose of irradiation by electrons and protons is experimentally traced in the range of eight orders of magnitude. It is demonstrated that the dependence of the carrier concentration on the irradiation dose can be determined unambiguously and reliably in a very wide range of compensation levels, based on the results of measuring the I-V characteristics. It is shown that the determination of the carrier removal rate using the I-V characteristics is more correct than using the C-V characteristics, especially in the case of high radiation doses.

Intersection of 4H-SiC Schottky diodes I–V curves due to temperature dependent series resistance

We theoretically and experimentally analyzed the non-obvious intersections of Schottky diode I–V curves measured at different temperatures caused by increasing the series resistance of the diode with increasing temperature. We considered a homogeneous diode and an inhomogeneous diode with two ways of influencing the I–V curve by the series resistance. In each case we developed a numerical method that enabled anticipation of the I–V intersection point. We studied the Ni/Au/4H-SiC diode for which such an intersection was measured. For homogeneous diodes and temperature interval 300–400 K we found a voltage dispersion of intersection points of only ∼0.002 V, which is in accordance with experimental observations and suppositions in the literature that the curves intersect at almost the same I–V point. Even for an inhomogeneous diode with a common series resistance we obtained a dispersion of the intersection voltage of only ∼0.02 V which is hardly discernible by the common visualization of the I–V curves. The largest dispersion of intersection points was obtained for an inhomogeneous diode composed of non-interacting diode patches.

Current-voltage characteristics and DLTS spectra of high voltage SiC Schottky diodes irradiated with electrons at high temperatures

The effect of the electron irradiation at high temperatures Ti (300 and 500 °C) and of the subsequent annealing on the current–voltage characteristics and DLTS spectra of high-voltage integrated 4H-SiC Schottky diodes is compared for the first time. The optimal annealing modes of the structures irradiated with 0.9 MeV electrons are determined. It is shown that the irradiation with a relatively low fluence Φ = 1016 cm−2 at 300 °C leads to an increase in the base resistance Rb by approximately an order of magnitude, and the initial value of Rb can be completely restored by a single annealing in a nitrogen atmosphere at a temperature of 250 °C for 60 min. At the same time, however, annealing affects only slightly the DLTS spectra measured after the irradiation. With increasingΦ, the longer annealing is required to fully restore the Rb. However, with the fluence increasing to Φ ≥ 5 × 1016 cm−2, the effect of a “reverse annealing” was observed in SiC for the first time: the resistance of the base grows as a result of the annealing. Irradiation at temperature of 500 °C leads at Φ ≥ 5 × 1016 cm−2 to an even more pronounced effect of reverse annealing. It is demonstrated that the presence of DLTS-detected acceptor centers arising as a result of the “hot” irradiation in the upper half of the band gap can explain neither the change in the current–voltage characteristics nor the results of annealing.

Neural network estimations of annealed and non-annealed Schottky diode characteristics at wide temperatures range

In this study, Artificial Neural Network (ANN) model has been proposed to characterize the annealed and the non-annealed Schottky diode from experimental data. The experimental current values of Ni/n-type 6H–SiC Schottky diode for the voltages applied to the diode terminal starting from 80 K with 20 K steps up to 500 K temperature were measured for both non-annealed and annealed Schottky diodes. The applied voltage has been varied starting from -2 V with 10 mV steps up to +2 V for each temperature value. The modeling performance has been assessed according to the varying number of neurons in the hidden layer, starting from 5 to 50 neurons, thereafter the optimum number of neurons has been obtained for both annealed and non-annealed ANN models. The minimum Root Mean Square Error (RMSE) and Mean Absolute Error (MAE) indices values for both annealed and non-annealed diodes have been obtained with 40 neurons for both the training and test phase.

Ni/Heavily-Doped 4H-SiC Schottky Contacts

In this work, we focus on the electrical characterization of Ni Schottky contact on n-type heavily doped (ND>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.

Annealing High-Voltage 4H-SiC Schottky Diodes Irradiated with Electrons at a High Temperature

Annealing High-Voltage 4H-SiC Schottky Diodes Irradiated with Electrons at a High Temperature

Effects of deposition temperature on Mo/SiC Schottky contacts

We report on the results of our investigation of the effect of deposition temperature on molybdenum (Mo) used as Schottky contacts to fabricate silicon carbide (SiC) Schottky barrier diodes. The contacts were deposited using magnetron sputtering on n-type 4H-SiC at temperatures ranging from 25 to 900 °C. The fabricated diodes were characterized by current–voltage, current–voltage–temperature, and capacitance–voltage measurements. The diodes showed average Schottky barrier heights that decreased from 1.54 to 1.00 eV with increasing deposition temperature of the contacts. The average values of the ideality factors similarly decreased from 1.21 to 1.06 with increasing deposition temperature. However, the diodes with contacts deposited at 400–900 °C nearly maintained a constant ideality factor value of 1.06. The x-ray diffraction analysis indicated that silicides were formed at the interface for contacts deposited at or above 400 °C. The improved properties of the diodes with contacts deposited at higher temperatures are correlated with the formation of the interfacial alloys possessing improved electrical properties. This work demonstrates that high temperature deposition is another method for optimizing the properties of Mo contacts for the fabrication of 4H-SiC Schottky diodes for electronic applications in high power and high temperature environments.

Effects of Fermi pinning and quantum mechanisms on leakage current of 4H-SiC Schottky diodes

Reverse bias leakage current through 4H-SiC Schottky diodes was modeled using quantum transmission theory and the theoretically calculated values were compared with the measured values from fabricated Schottky diodes. To account for interface defects, energy barriers due to Fermi pinning from previously observed defects were used in place of the ideal barrier structure predicted by the Schottky–Mott rule. Incorporating barriers with energy values set due to Fermi pinning at known defect energies resulted in better experimental agreement by many orders of magnitude.

Electron irradiation hardness of high-voltage 4H-SiC Schottky diodes in the operating temperature range

Effect of irradiation with 0.9 MeV electrons on the parameters of 4H-SiC Schottky diodes with a limiting blocking voltage Ub=600 and 1700 V was studied for the first time in the range of operating temperatures Ti (23 and 175oC). The range of fluences Phi was 1·1016-2·1016 cm-2 for devices with Ub=600 V and 5·1015-1.5·1016 cm-2 for devices with Ub=1700 V. Irradiation at room temperature increases significantly the differential resistance of the base of the diodes. Irradiation with the same doses at Ti=175oC --- i.e. at limiting operating temperature of devices, does not affect practically the parameters of current-voltage characteristics. Nevertheless, the DLTS spectra demonstrate a significant increase in the concentration of deep levels in the upper half of the band gap not only after irradiation at room temperature, but also after irradiation at Ti=175oC. Keywords: silicon carbide, Schottky diodes, electron irradiation, current-voltage characteristics, DLTS spectra.

Отжиг высоковольтных 4H-SiC диодов Шоттки, облученных электронами при высокой температуре

The effect of annealing on the parameters of 4H-SiC Schottky diodes irradiated with electrons at high temperatures has been studied for the first time. The electron energy was 0.9 MeV. The irradiation was carried out at temperatures of 23, 300, and 500oС at fluences  in the range 1×1016 – 1.3×1017 cm-2. The results of annealing after the irradiation at high temperatures are qualitatively different from the results of annealing after conventional irradiation at room temperature with the same fluencies. The results obtained indicate that the spectrum of radiation defects introduced into SiC under a high-temperature (“hot”) irradiation differs significantly from that of defects introduced by irradiation at room temperature. At irradiation temperatures of 300 and 500°C and large, the effect of "reverse annealing" was revealed, when the base resistance grows rather than falling as a result of annealing.

Materials and Processes for Schottky Contacts on Silicon Carbide.

Silicon carbide (4H-SiC) Schottky diodes have reached a mature level of technology and are today essential elements in many applications of power electronics. In this context, the study of Schottky barriers on 4H-SiC is of primary importance, since a deeper understanding of the metal/4H-SiC interface is the prerequisite to improving the electrical properties of these devices. To this aim, over the last three decades, many efforts have been devoted to developing the technology for 4H-SiC-based Schottky diodes. In this review paper, after a brief introduction to the fundamental properties and electrical characterization of metal/4H-SiC Schottky barriers, an overview of the best-established materials and processing for the fabrication of Schottky contacts to 4H-SiC is given. Afterwards, besides the consolidated approaches, a variety of nonconventional methods proposed in literature to control the Schottky barrier properties for specific applications is presented. Besides the possibility of gaining insight into the physical characteristics of the Schottky contact, this subject is of particular interest for the device makers, in order to develop a new class of Schottky diodes with superior characteristics.

Radiation tolerance comparison of silicon and 4H–SiC Schottky diodes

In this paper, silicon and 4H-silicon carbide (4H–SiC) based Schottky barrier diodes (SBD) have been compared for their radiation hardness capabilities through technology computer aided design (TCAD) simulations, using the non-ionizing energy loss (NIEL) mechanism. Both the devices are modeled for radiation effects after choosing same design points through interplay of drift layer doping and thickness for a fair comparison. Device simulation results show the superior radiation tolerance of 4H–SiC SBDs when compared to silicon devices under proton irradiation. On-state simulations reveal that the performance of Si and 4H–SiC diodes begin to degrade at a 1 MeV proton fluence of ~5 × 1011 and ~7 × 1012 cm−2, respectively. The higher radiation tolerance of 4H–SiC SBDs is also observed in off-state simulations. It is found that the reverse breakdown voltage decreases in silicon, whereas it increases in 4H–SiC devices under the effect of irradiation, mainly due to doping compensation mechanism. These observations validate experimental results available in literature and provide a way forward to use TCAD simulations for studying radiation effects in other devices.