Radiation tolerance analysis of 4H-SiC PIN diode detectors for neutron irradiation

Silicon carbide (SiC) detector shows sufficient merits for the pplication of radiation measurement in harsh neutron radiation fields due to the strong radiation tolerance, good environment adaptability, and excellent electrical properties. In order to meet the demand for higher precision measurements, the eliability of SiC detectors under harsh neutron irradiation nvironments must be characterized. In this work, electrical characteristics, α-particle response, and pulse response speed of 4H-SiC PIN diode detectors were carefully investigated under MeV equivalent neutron irradiation fluence from \(5 × 10^{12} cm^{−2}\) to \(1 × 10^{1} cm^{−2}\) . Before neutron irradiation, SiC detector showed dark current as low as 10 pA at 200 V bias, with superior charge collection efficiency of more than 98%, perfect energy resolution within 0.9%, and ltrafast response speed with a rise time and decay time of 2.1 s and 5.9 ns, respectively. After a neutron irradiation at luence of 1 × 1014 cm−2, noticeable changes were found in the forward IV characteristics of SiC detector, but the dark current was stable below 1 nA, with only 5.7% and 0.26% degradation in charge collection efficiency and energy resolution respectively, and the pulse response speed remained unchanged. These results demonstrate 4H-SiC PIN diode detector has superior neutron radiation tolerance, and the obtained evolution characteristics are very valuable for improving the measuring accuracy of SiC detectors in the intense neutron field.

Further reading: R.L. Gao et al., “Effect of Very High-Fluence Proton Radiation on 6H-SiC Photoconductive Proton Detectors,” in Sensors and Actuators A: Physical,Volume 333,2022, doi: 10.1109/LED.2019.2949174.

Effect of Very High-Fluence Proton Radiation on 6H-SiC Photoconductive Proton Detectors

In this work, the effect of very high-fluence 100 MeV proton radiation on the performance of 6H-SiC photoconductive proton detectors is studied. The irradia- tion fluence is up to \(1.6\times10^{17}cm^{−2}\) and the degradation process of the SiC detector is continuously monitored for the first time. Before proton irradiation, the detector shows a low dark current of ∼ \(0.8 nA/cm^{2}\) at 1000 V bias. As the irradiation fluence increases, the dark current continuously drops, which should be caused by irradiation damage induced carrier compensation defects. Meanwhile, the output current of the SiC detector shows an exponential decay behavior and tends to saturate at irradiation fluence up to \(5\times10^{16}cm^{−2}\). At the end of the very high fluence irradiation test, the detector still exhibits ∼20% of its original output current value, suggesting the excellent radiation hardness of SiC for proton detection.

Further reading: Q. Liu et al., “Effect of Very High-Fluence Proton Radiation on 6H-SiC Photoconductive Proton Detectors,” in IEEE Electron Device Letters, vol. 40, no. 12, pp. 1929-1932, Dec. 2019, doi: 10.1109/LED.2019.2949174.

Correlation between structural defects and carrier lifetime

In 4H SiC substrate and epitaxial films, due to the influence of epitaxial process and growth conditions, there are many structural defects in the SiC epitaxial layer, such as thread dislocation TSD, thread edge dislocation TEDS, base plane dislocation BPDS, microtubule super TSD, 10 °low angle grain boundary, endogenous stacking faults, carrot shaped morphological defects, etc. These defects will affect the performance of the device more or less.

Of course, the characterization means of many defects are gradually improved. In order to eliminate the influence of defects, it is necessary to understand these means. First and foremost, time resolved photoluminescence (TRPL) is a very effective means to measure the radiative recombination between excitons and donors, and cathodoluminescence is also a good means to characterize the lifetime.

By providing electron beams to samples, the lifetime distribution can be well obtained. Other methods can provide a good relationship between the needle sample and the life. In addition, the comparison between the defects and the life diagram can well show that these structural defects do have a certain impact on the effective life of local carriers.

Further reading: J. Bergman,O. Kordina,E. Janzén,“Time Resolved Spectroscopy of Defects in SiC”,in,phys.stat.sol.(a)162,65(1997) Serguei I.Maximenko,Jaime A. Freitas Jr.,Paul B. Klein,Amitesh Shrivastava,and TangaliS. Sudarshan, “Cathodoluminescence study of the properties of stacking faults in 4H-SiC homoepitaxial layers”,in,Appl.Phys.Lett.94,092101(2009) J.Hassana and J.P.Bergman,“Influence of structural defects on carrier lifetime in 4H-SiC epitaxial layers:Optical lifetime mapping”,in,Journal of Applied Physics 105,123518(2009) Nadeemullah A. Mahadik, Robert E.Stahlbush, Joshua D. Caldwell,Michael O’Loughlin, and Albert Burk, “Correlation of Extended Defects on Carrier Lifetime in Thick SiC Epilayers”,in,Materials Science Forum Vols.717-720(2012)pp297-300

Growth habits of SiC on two different types of substrates

First,growth on nonplanar substrates。The experimental conditions are as follows： 1.Growth method: gas phase crystalline method

2.Countertop groove: a method of etching by reactive ions. 1-10um wide, 2um deep

3.Original materials: SiH4 and C3H8 provide silicon and carbon sources, triethylaluminum for p-shaped doping ，Heterogeneous, N2 for n-shaped doping,

4.Growth conditions: 1620 degrees Celsius, reaction internal pressure 800 mbar This paper studies the difference between the two crystal orientations and the growth layer under the two C/Si。The conclusion is as follows：

1.Parallel to the [1 ̄100] direction, for different C/Si, (C/Si=1.2 and C/Si=4), the top of the step can be fitted and grow slowly, when the ratio is high, hanging objects appear, and the hanging objects of 6H structure are more obvious than 4H.

2.Parallel to the [11 ̄20] direction, for different C/Si, (C/Si=1.2 and C/Si=4), they grow symmetrically and are not affected by deviation angles

Second,growth on planar substrates,The experimental conditions are as follows： （N-shaped 4H-SiC substrate inclined toward [1120] and [1100].） 1.Table groove: using the method of reactive ion etching, etching 3um deep and 3-60um wide groove, after the groove is formed, Al+ is injected into the SiC lining at 1000 degrees Celsius, 0.3um thick, concentration \(5.5\times10^{14}cm^{−2}\), no postimplantation annealing process.

2.Growth conditions: 1500 degrees Celsius, atmospheric pressure, chemical vapor deposition method

3.Raw material: SiH4C3H8 mixed gas, the carrier of the carrier gas is H2, using horizontal water-cooled reactor.

4.Gas ingress rate: SiH4: 0.15-0.30sccm, C3H8:0.15-0.30sccm,H2:3.0slm

5.Growth time: 20-120min

The growth habit of n-shaped Sic homogeneous epitaxy without doping was studied under the condition of C/Si=2-4。The experimental conditions are as follows： The SiC substrate is obliquely oriented to [1120], and the groove is parallel to [1120], which can be grown There is a smooth surface and an epitaxial layer with high symmetry. The symmetry of the epitaxial layer perpendicular to the inclination direction is not high; High symmetry is shown parallel to this direction. And there is a triangle defect in the [1100] direction.

Further reading: “Structures, Electronic Properties, and Gas Permeability of 3D Pillared Silicon Carbide Nanostructures”,in,Journal Article “SiC Migration Enhanced Embedded Epitaxial (ME3) Growth Technology”,in,Journal Article

Reduction of traps and improvement of carrier lifetime in 4H-SiC epilayers by ion implantation

The carrier traps Z1/2 and EH6/7 can be created by displacement of carbon atoms caused by electron radiation. After implanting carbon atoms(concentration:1.5e17cm-3) into the shallow surface of 4H-SiC epilayer and annealing, DLTS and TRPL was used for characterization. The carrier lifetime increased from 122 to 218ns and the trap concentration decreased from 3e13 to below 5e11cm-3. The reverse correlation between carrier lifetime and trap concentration agrees well in annealing temperature of 800°C-1800°C. Samples implanted with silicon atoms showed similar results after 1600°C annealing.

It can be inferred that carbon interstitials in the implanted layer can indiffuse during annealing and recombine with carbon vacancies in the epilayer, so that point defects disappear. Thus the Z1/2 and EH6/7 traps relate to the carbon vacancies, which was supported by later research.

Further reading:L.Storasta and H.Tsuchida, “Reduction of traps and improvement of carrier lifetime in 4H-SiC epilayers by ion implantation” ，Appl.Phys.Lett.90,062116(2007)