Study of mobile ionic charges by thermally stimulated currents in 4H-SiC MOS capacitors with thick SiO_2 layers

摘要

Thick oxides deposited on 4H-SiC epilayers to form Metal-Oxide-Semiconductor capacitors have been studied by Capacitance-Voltage (C-V) and Thermally Stimulated Ionic Current (TSIC) measurements. The very negative flatband voltage V_(FB) in regards to the theoretical value indicates the presence in the oxide of a large number of positive charges. Positive mobile charges are responsible for a large part of this unusual V_(FB), as shown by the hysteresis cycle on C-V characteristics. Four traps have been detected at the gate/oxide interface and three at the oxide/SiC interface. One of them is similar to a trap already detected in MOS 3C-SiC and 6H-SiC structures. The others seem rather different. Dependences of the detrapping energy of mobile ions in these traps have been determined. Secondary Ion Mass Spectroscopy analyses have shown the presence of K_+ and Na_+ ions in the oxide in concentrations that agree well with the values obtained by TSIC measurements.