Vulnerability of epitaxial layers and substrates of 4H-SiC to ionizing radiation and thermal treatments

Abstract

Silicon Carbide (SiC) is a wide-bandgap crystalline semiconductor with an indirect bandgap of 2.3-3.3 eV depending on polytype. 4H-SiC is a very promising semiconductor for high-power devices and high-temperature devices thanks to its superior physical and electrical properties: it exhibits 10 times higher breakdown electric field strength and 3 times higher thermal conductivity than silicon, high chemical inertness, thermal conductivity, mechanical strength, saturation drift velocity [1]. Furthermore, SiC has high radiation hardness thanks to the large values of the threshold energy for defects formation. Consequently, SiC is the most promising semiconductor for fabrication of devices which can operate in extreme conditions, as high levels of irradiation, elevated temperatures, and high chemical activity. Nonetheless, the characteristics of SiC-based devices are influenced by the presence of impurities as well as extended and point defects. In general, the effects of irradiation causes formation of vacancies, interstitials and related defects which may give rise to states in the bandgap influencing the electrical and optical properties of the material [2-3]. Moreover, it has been observed that treatments at temperature of about 2000°C induce the sublimation of Si and the growth of graphene layer on top of SiC in different atmospheres or in vacuum [4-5]. These studies prove the vulnerability of the material. In this work some complementary non-invasive techniques (μ-Raman, steady-state and time resolved photoluminescence spectroscopy) have been used to study the vulnerability of epitaxial layer of 4H-SiC to ionizing radiation (β-rays and X-rays) and to the effects of thermal treatments in a low temperature range (below 2000°C in Ar atmosphere) on substrates of 4H-SiC. The dose range spanned from 1 kGy up to 100 kGy. In the samples irradiated with β-rays the lifetime of the excitonic band decreases when the deposited dose increases. In particular, in the samples with higher native defectiveness the effect starts from lower deposited doses. Conversely, in the samples irradiated with X-rays there aren’t effects at the same deposited dose as β-rays. These findings suggest that irradiation with electrons induces defects related to atomic displacement. The effects of thermal treatments in air, from 100°C up to 900°C, have been explored to study the recovery properties of epitaxial layer of 4H-SiC. The effect of thermal treatments conducted on the substrates were monitored by μ-Raman spectroscopy. Traces of damaged graphene and graphitization have been observed. [1] T. Kimoto P. and J.A. Cooper, Fundamentals of Silicon Carbide Technology. John Wiley & Sons Singapore Pte. Ltd, 2014. [2] A. A. Lebedev et al., Materials Science Forum, 433-436, 2003 [3] A. Le Donne et al., Diamond & Related Materials, 14, 2005 [4] K. V. Emtsev et al., Nature Materials, 8, 203–207, 2009 [5] J. L. Tedesco et al, Appl. Phys. Lett., 96, 222103, 2010