Silicon carbide opens the door to radiation-detection market

SiC detectors are made up of a layer of SiC sandwiched between two metal contacts. Incident radiation generates several electron-hole pairs that are pulled apart under an applied electric field, creating a current that is amplified and recorded.

These SiC devices will compete with established detectors for existing markets, including detectors for high-energy physics research, astronomical X-ray telescopes, and archeometry (the non-destructive identification of chemical elements in works of art).

Giuseppe Bertuccio, a researcher at Politecnico di Milano, believes that the advantages of SiC detectors include high-temperature operation, detection of a range of particles with various energies, and a potential for radiation hardness and high temporal resolution.

Bertuccio states that SiC detectors can distinguish between small differences in the energies of incident radiation even at room temperature, thanks to their negligible leakage current. In contrast, silicon and germanium detectors used for X-ray spectroscopy applications, which demand high-energy resolution, require cooling. "Typical leakage current densities of silicon detectors are of the order of 1 nA/cm2, but SiC detectors have current densities of as little as 1 pA/cm2, three orders of magnitude lower," remarked Bertuccio. This particular feature means that SiC detectors can be made with a much larger detection area than their silicon counterparts.

SiC is suitable for detecting various forms of radiation, including soft X-rays with energies of between 0.1 and 20 keV. "For X-rays with photon energies of greater than 2 keV, SiC has a similar stopping power to silicon, because the interaction is predominantly a photoelectric effect that depends on the presence of silicon atoms," said Bertuccio. However, for energies of less than 2 keV, SiC is superior to silicon, thanks to its higher attenuation coefficient. SiC detectors can also detect relativistic particles and ionizing radiation such as alpha particles, protons and ions with energies of up to a few megaelectron-volts.

Bertuccio acknowledges one downside of SiC detectors: their relatively low detection signal, which is a direct consequence of SiC s wider bandgap. However, he points out that SiC detectors have the lowest noise figures of all semiconductor-based devices. This partly explains their highly desirable signal-to-noise ratio, which is ultimately limited by the performance of front-end electronics.

Today the scientific community s efforts are directed toward building large-area detectors with a finer energy resolution. Bertuccio believes that SiC and GaAs detectors will benefit from the techniques developed for their silicon predecessors that were used to form complex detection structures such as pixel arrays and microstrip detectors. The spatial resolution of these instruments should be comparable to multielement silicon detectors.

One potential advantage that SiC detectors have over other compound semiconductor detectors is a higher temporal resolution. SiC has a high breakdown field of more than 2 MeV/cm, allowing devices to be operated under a large applied electric field. "This implies that the charge carriers can move at their saturation velocities, thus achieving very fast output current signals," said Bertuccio. The saturation velocity for electrons in SiC is roughly double that of electrons in GaAs and silicon. A further drawback of today s silicon and GaAs-based detectors, when used in high-energy physics or radiation-monitoring applications, is their limited radiation hardness.

Bertuccio believes that the unique characteristics of SiC detectors that allow them to operate at raised temperatures could spur their first commercial breakthrough. Other possible applications include detectors for X-ray spectrometers and in radiation-level monitors that are used in the nuclear industry.

According to Bertuccio, three obstacles currently prevent the commercialization of SiC detectors: growing thick epitaxial layers; producing layers with a low dopant concentration; and the fabrication of ultrapure semi-insulating bulk SiC crystals.

Today s best efforts include SiC epitaxial layers of up to 70 μm thick, and dopant levels in the region of 5 x 1014 cm-3. However, Bertuccio believes that a layer of up to 300 μm thick and a dopant level of less than 1013 cm-3 will be required to deplete the whole detector at a reasonable voltage (100-500 V) and achieve detection efficiencies similar to those in silicon devices currently used in X-ray and high-energy particle detectors.

He also notes that although semi-insulating bulk SiC wafers of up to 400 μm thick are commercially available today, they exhibit impurity and defect densities well in excess of those present in epitaxial SiC. "This limits the performance of semi-insulating SiC radiation detectors due to the trapping of signal charges at deep levels, which degrades the signal-to-noise ratio," said Bertuccio.

Compared with other semiconductor detectors, a commercial SiC device would be expensive because of the greater cost of SiC wafers: "Nevertheless," said Bertuccio, "X-ray SiC detectors do not require cooling, so running expenses for a spectrometer are lower."