Anodic Oxidation of 4H-Silicon Carbide

Reliability is an essential aspect of the development of power electronics devices. To guarantee a target lifetime spanning decades, the environmental robustness of devices must be validated through accelerated stress tests. A combination of factors that have been identified as critical include humidity, high temperature and electric field. A test that stress power devices with these factors is High Voltage-High Humidity High Temperature Reverse Bias (HV-H3TRB), with 85°C, 85% of Relative Humidity and 80% of breakdown voltage. Several mechanisms could occur in these conditions, such as ion migration, dendrites, corrosion, bubbles formation and silicon carbide (SiC) degradation due to oxidation. Because the temperatures during HV-H3TRB are substantially lower than typically necessary to thermal oxidize silicon carbide, oxidation is due to electrochemical reactions that can depend on different pH-environments. In this work, the degradation of silicon carbide that occurs during the HV-H3TRB test due to oxidation reaction is investigated. The aim of this work is to better understand the underlying mechanism of silicon carbide oxidation occurring under humid conditions. The first part of the experiment consists of verifying that anodic oxidation occurs using a 4H-SiC sample as anode in an electrochemical system in the presence of different electrolytes. In particular, rectangular pieces of 10×15 mm2 of n-type 4H-SiC, usually employed in microelectronics, were used. Each piece before any experiment was degreased by sonicating in acetone for 4 minutes. To perform anodic polarization curve, a three-electrode electrochemical system was built with 4H-SiC sample as working electrode (anode), a platinum mesh as counter electrode (cathode) and a saturated calomel electrode (SCE) as reference electrode. To study the environment effect three different electrolytic solutions have been chosen: Sodium Chloride (NaCl), Hydrochloric acid (HCl) and potassium Hydroxide (KOH). Anodic polarization curves were obtained from each of three solutions. The region where the current density of each curve growth is called active region, corresponding when the oxidation reaction occurs. The behavior of the curves is similar in all the solutions, all of them show an active region, though there is an evident shift in voltage range, especially for KOH solution. The tested samples in the three environment conditions were analyzed at SEM. The SEM images show the morphological features of the samples, and it is possible to deduce that 4H-SiC exposed to NaCl and HCl exhibits the growth of an oxide, whereas the surface of the sample exposed in KOH has a different type of coverage. From the results obtained, neutral solutions appear to favor the oxidation of 4H-SiC. To confirm the oxide formation, XPS analysis will be carried out. Furthermore, the experiment will be extended to include more structured samples, in particular samples with different passivation layers deposited on the 4H-SiC surface, with the goal to study the ability of different passivation layers to prevent the anodic oxidation phenomenon.