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preface
At present, in order to reduce costs, improve performance and service life, the new generation of spacecraft will gradually use electric propulsion system to replace the traditional chemical propulsion system, the electric propulsion system has higher requirements for the performance of power devices, only the working voltage needs to reach the order of kV, conventional SiC devices are difficult to reach this level.
SiC devices are more suitable for work under complex working conditions such as high temperature, high pressure, and strong radiation field, and are more likely to meet the needs of the new generation of spacecraft for high integration, high power, strong withstand voltage, high power frequency and other performance.
SiC diodes are unidirectional two-terminal devices made of semiconductor materials, including Schottky barrier diodes, structural barrier Schottky diodes, and PIN diodes.
Based on the ground accelerator device, my team carried out the radiation effect response and mechanism of SiCJBS to medium energy protons, measured the forward and reverse I-V characteristics, reverse C-V characteristics and DLTS harmonics before and after irradiation, extracted the changes in electrical properties and defect introduction of the device, and deeply analyzed the relationship between the two.
Research methods
The two devices we use are TO-247 package, reverse DC voltage 1200V calibrated operating temperature 218~448K, before the experiment, all experimental samples are cap-opened and exposed sensitive areas (Figure 1).
#1芯片面积较小, the average forward current is 40A, #2平均正向电流20A, so the working current density of #1 is larger, and the basic electrical properties of the experimental samples are tested after opening the cap, and the devices with stable electrical performance and good uniformity are screened out to carry out irradiation experiments.
The irradiation experiment was carried out at the heavy ion single-particle effect experimental terminal of the HI13 tandem accelerator of the China Institute of Atomic Energy, and the experimental site is shown in Figure 2.
The proton irradiation experiment is carried out in the vacuum target chamber, the experimental sample is fixed on the sample holder of the target chamber, and the proton is incident vertically on the surface of the Schottky structure side of the device during the irradiation process, and the device is in a vacuum environment during the irradiation experiment.
Equivalent low-Earth orbit (LEO) irradiation damage: considering that the proportion of medium-energy protons in the space environment of low-orbit satellites is relatively large, the application scenario of low-orbit satellite operation in orbit for 10a is selected, and the proton energy of 10MeV is selected based on the displacement damage equivalent dose method.
The irradiation time under each condition is 1000s, and the irradiation conditions and corresponding numbers of 5cmX5cm with a beam area are listed in Table 1.
After irradiation, the electrical properties of the device, including forward IV, reverse I-V and C-V characteristics, were tested, and the working state of the device after full room temperature annealing for 100d was tested to evaluate the self-healing ability of room temperature annealing after proton irradiation.
Based on the equivalent dose method of displacement damage, the displacement damage generated by irradiation using the maximum injection of 10MeV proton is 1X10p/cm, which is about the displacement damage accumulated in all low, medium and high typical orbits from low earth orbit to geostationary orbit for 10 years.
Therefore, the three energies of 10, 15 and 20 MeV are carried out under unbiased conditions at room temperature, and the irradiation conditions of each sample are listed in Table 2.
The electrical characteristics of the device were analyzed before and after irradiation, including forward I-V, reverse IV and C-V characteristics, and the working state of the device after 100d full room temperature annealing was tested to evaluate the self-healing ability of room temperature annealing after proton irradiation.
To improve self-healing ability, a mathematical model needs to be established, and the American system of SiCJBS capacitor C and voltage V can be expressed by the 1/C-V relationship of Equation (1):
The Schottky barrier height of SiCJBS is described by equation (2):
Through the above mathematical model, the Schottky barrier height, effective carrier concentration and other characteristic parameters of SiCJBS before and after irradiation can be calculated, and combined with carrier characteristics and material characteristics, the degree of radiation damage generated by the device under different beam conditions and the deep physical mechanism causing macroscopic performance degradation can be theoretically analyzed.
Results and discussion
The electrical characteristics of the device before irradiation are shown in Figure 3, and it can be seen from Figure 3 that the forward and reverse electrical characteristics of the two devices within 1V are similar.
The V characteristics, the forward I-V characteristics before and after device irradiation are shown in Figure 4, and it can be seen from Figure 4 that the opening voltage of device #1 after proton irradiation increases slightly, and the forward current characteristics decrease slightly, while device #2 hardly changes.
The forward characteristics of both devices do not produce significant degradation, the forward characteristics of SiCJBS are not sensitive to proton irradiation effects, and the forward characteristics of device #2 have better stability.
It shows that the displacement damage accumulated during 10 years of operation in LEO orbit without power and room temperature will basically not cause serious degradation of the forward characteristics of these two devices.
The reverse I-V curve after device irradiation is shown in Figure 5, and it can be seen from Figure 5 that the leakage current of device #2 after proton irradiation increases significantly, and the 10MeV proton causes severe degradation of the device breakdown voltage.
The decrease of reverse current at small test voltage may be caused by the decrease of tunneling current and thermal diffusion current, which may be related to the increase of Schottky barrier, the decrease in the concentration of carriers captured by irradiation defects, and the accumulation of negative charges at the interface.
The annealing characteristics are shown in Figure 6 after annealing at room temperature.
After proton irradiation, the reverse rated breakdown voltage of devices #1 and #1-1 drops by 570V, the reverse rated breakdown voltage of other devices drops below 800V, the reverse rated breakdown voltage of devices #2, #2-2 and devices drop below 780 and 880V, respectively, and the reverse rated breakdown voltage of device #2-4 drops below 810V.
It can be seen that the reverse characteristics of SiCJBS are sensitive to the proton displacement damage effect, and the characteristics of the device become unstable after irradiation.
It may be that the accumulation of interfacial charge leads to the increase of the peak electric field at the semiconductor interface, while the irradiation defect causes the Schottky junction and PN junction interface to be damaged and enhances the tunneling effect, and a combination of factors leads to the damage of the breakdown characteristics of the device.
Compared with Figure 5, it can be seen that the proton irradiation damage ability of proton irradiation under #1器件抗低注量 and high-energy conditions is stronger, but the ability to eliminate room temperature annealing defects after irradiation is poor, which may be related to the excessive doping concentration of its materials, and also shows that room temperature annealing will not completely eliminate the damage caused by irradiation, and the device performance is difficult to recover.
The V-characteristics, the C-V characteristic curve before and after device irradiation, are shown in Figure 7.
Device #1 achieves a capacitance degradation rate of 32.6% after proton irradiation, with little change in other devices.
The capacitance degradation rates of #2-2 and #2-4 devices of device #2 reached 25.2% and 24.3%, respectively, and their capacitance characteristics were significantly correlated with proton energy and charge.
Taking the CV data of Figure 7 with Equation (1), the 1/C-V relationship curve is obtained and the characteristic parameters of the device are calculated in Table 3.
For #1 device #1-1, the cause of abnormal capacitance change may be related to its factory reliability, and the carrier concentration and Schottky barrier height of other devices change little, which may not show the law of #2 device due to the limitation of test and calculation accuracy.
Microscopic defects, based on the results of the deep energy level transient spectrum test results, the type and density of defects caused by proton irradiation of the device can be obtained, and the DLTS test results before and after device irradiation are shown in Figure 8.
It can be seen that the intensity of the defect peaks of these two SiCJBS is not exactly the same, but there are basically three main defect peaks.
The analysis believes that the three peaks are all convex upwards, the peak strength is positive, and they belong to the main defect energy level, which will capture most carrier electrons and reduce their mobility, which will lead to changes in electrical properties such as the decrease of current under the same bias voltage.
Analysis of DLTS results showed that 10MeV and 20MeV proton irradiation enhanced the original defect energy level peaks, indicating that proton irradiation led to an increase in the density of C defects and S defects, but the Si atom dislocation energy was greater than the C atom dislocation que energy, so proton irradiation could introduce more C defects, which led to multiple C defect-related peaks being enhanced after proton irradiation.
The appearance of new defect peaks on the Z/side of #2-2 and the broadening of the original defect peaks of EH may be related to the new defects induced by proton irradiation in SiC devices, but they coincide with the original defect peaks, so no separate defect peaks are shown.
At the same time, DLTS were significantly deformed after irradiation #1-4 and #2-4, which may be related to the introduction of more serious C-defect group damage in the device by 20MeV higher energy proton irradiation, due to the increase in defect energy level density, the increase in trapped carrier capacity and the decrease in carrier concentration, which is consistent with the C-V test results.
It is proved that proton irradiation leads to more defects in the device, increases the defect density, increases carrier recombination, and triggers carrier removal effect, which leads to significant degradation of the reverse electrical performance of the device.
The forward IV characteristics of the device after the V feature is used to increase the amount of proton irradiation column are shown in Figure 9.
The ideal factor calculation formula shows that the ideal factor of the device increases compared with before irradiation, and reaches a maximum at 10 MeV, because compared with higher energy protons, 10 MeV proton irradiation causes the most serious non-ionizing energy loss in SiCIBS.
Through the reverse IV characteristic shown in Figure 10, it can be seen that device #1 and device #2 have a decrease in leakage current at a small test voltage after being exposed to proton radiation.
With the increase of the test voltage, the reverse leakage current of the #1-1 device irradiated by 10MeV proton in group #1 began to increase sharply at 560V, and the reverse leakage current of the #2-1 device irradiated by 10MeV proton irradiation in group #2 also increased slowly after 200V.
The reverse I-V characteristics of annealing characteristics after room temperature annealing are shown in Figure 11, and the reverse leakage current of device #1 is further increased, the electrical performance is significantly degraded, and the reverse characteristics of device #2 are relatively stable.
Among them, the reverse leakage current of the #1-3 device irradiated by 20MeV proton begins to fluctuate at 600V, the reverse leakage current of the #1-2 device irradiated by 15MeV proton begins to fluctuate at 370V, and the reverse leakage current of the #1-1 device irradiated by 10MeV proton begins to fluctuate at 370V and completely breaks down at 580V, and the reverse leakage current of the #1-1 device irradiated by 10MeV proton completely breaks down at 420V.
It is explained that the higher injection volume proton irradiation calculated by the displacement damage equivalent dose method introduces a large number of irradiation defects into the SiCJBS device, and the irradiation defect in the #1 device cannot be significantly repaired with room temperature annealing, and the rated breakdown voltage of the device has dropped to less than 50%, so it cannot meet the needs of aerospace applications.
The V-characteristic device, C-V test results are shown in Figure 12.
As can be seen from Figure 12, the capacitance of both devices #1 and #2 has degraded. Device #1 has a small degradation, wherein the capacitance degradation rate of #1-1 at 0V is greater than 10%, but the degradation of the capacitance after the irradiation of the three energy protons is small, and the degradation of device #2 is more obvious, among which #2-1 after 10MeV proton irradiation has the highest capacitance degradation rate at 0V and is greater than 30%.
Because the capacitance of SiCJBS is related to the carrier concentration, interface state, device material and structural characteristics of the device, #2芯片面积大, the carrier concentration is low, and the same amount of proton irradiation introduces more defects and interface states in the #2 device, resulting in a further decrease in the carrier concentration, a smaller space charge region, and a decrease in barrier capacitance, so the C-V characteristics change drastically.
Table 4 shows the built-in potential Vi effective carrier concentration N and Schottky barrier height De calculated from the CV data of Figure 12 and Equation (1).
This is because proton irradiation induces more irradiation defects, which increase the temporary capture and recombination ability of carriers, resulting in a decrease in carrier concentration, that is, carrier removal effect, resulting in degradation of electrical properties.
Similarly, the C-V performance degradation of the device is the most serious after 10MeV proton irradiation, which is consistent with the results of I-V characteristics, mainly related to the fact that the low-energy proton NIEL is larger, causing more lattice atoms to leave the equilibrium position to form irradiation defects.
conclusion
Aiming at the 10 Mev single-energy proton irradiation corresponding to the displacement damage dose accumulated in the orbit of a typical commercial spacecraft for 10 years, the ground simulation irradiation experiment of the 10~20MeV medium-energy proton accelerator was carried out to obtain the forward electrical characteristics, reverse electrical characteristics and microscopic defect characteristics of SiCJBS before and after irradiation, and the relationship between the irradiation conditions and the degradation of device characteristics was analyzed.
At present, commercial high-performance SiCJBS, especially its reverse electrical performance, is still sensitive to the effect of displacement damage. Based on the 10MeV equivalent displacement damage dose method, SiCJBS will have different degrees of degradation due to the displacement damage defects caused by medium energy proton irradiation in medium injection amounts, laying hidden dangers for its reliable operation.
However, 10MeV low energy and large injection volume of medium energy proton irradiation will cause SiCJBS to produce permanent defects that are difficult to recover, which can cause its breakdown voltage to drop sharply or even directly damage. If it is applied to space missions, it is difficult to meet the needs of space missions.
In the future, the radiation effect of the new SiCJBS in the aerospace environment needs to be further studied, and the relationship between the device structure, process and its radiation resistance is deeply analyzed by reverse technology, and a reliable evaluation technical standard and evaluation platform are established to ensure the smooth and rapid development of the aerospace industry.
Citations
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Wang Chenglong, Wang Qingyu, Zhang Yue, etc. Molecular dynamics study of irradiation performance at SiC/C interface[J].Acta Physica Sinica,2014(15)