Microwave losses of undoped n-type silicon and undoped 4H-SiC single crystals at cryogenic temperatures

Abstract

We investigated microwave losses of single-crystalline Si and 4H-SiC at cryogenic temperatures at 8.6–24 GHz using a method involving a dielectric resonator with high-T _c superconductor YBa₂Cu₃O_7-δ films used to improve the measurement sensitivity. The loss tangent of our undoped n-type Si appeared to be extremely low at temperatures below 20 K with a value of 1 × 10⁻⁶ at 24 GHz at 10 K, which is more than 100 times lower than the value of 2 × 10⁻⁴ at 6.8 GHz at 10 K reported by Krupka et al. [IEEE Trans. Microw. Theory Tech. 54, 3995 (2006)] for undoped p-type Si. Meanwhile, the loss tangent of pristine 4H-SiC appeared to be very high with a value of 0.01 at 10 K at 8.6 GHz, which is 4000 times higher than that of our undoped Si. When the pristine 4H-SiC was irradiated with thermal neutrons, the loss tangent was enhanced by seven times due to the significantly reduced electrical resistivity. Our results show that, at temperatures below 20 K, the loss tangent of undoped n-type Si is low enough for various cryogenic applications and that thermal neutron irradiation could provide a useful means of reducing the electrical resistivity of SiC possibly by means of neutron transmutation doping.