Analysis of electron traps at the 4H–SiC/SiO2 interface; influence by nitrogen implantation prior to wet oxidation

Abstract

Electron states at the SiO2/4H–SiC interface have been investigated using capacitor structures and especially, the influence of excess nitrogen, introduced by ion implantation, at the interface is studied in detail. Implanted and nonimplanted n-type samples with an interfacial concentration of nitrogen of ∼1019 cm−3 and 1016 cm−3, respectively, were analyzed by capacitance-voltage (C-V) measurements, performed at different temperatures and probe frequencies, and thermal dielectric relaxation current (TDRC) measurements performed in the temperature range of 35–295 K. Three main categories of electron states are disclosed, true interface states (Dit), fast near interface states (NIToxfast) and slow near interface states (NIToxslow). The density versus energy distributions of Dit and NIToxfast have been deduced from the TDRC data and they are shown to give a close quantitative agreement with the shape and frequency dependence of the C-V curves. Further, the amount of NIToxslow extracted from TDRC is demonstrated to be responsible for the parallel shifts and hysterezis effects occurring in the C-V characteristics. All three categories of electron states are reduced in concentration in the implanted samples. This holds particularly for NIToxfast with a peak at ∼0.1 eV below the conduction band edge of 4H–SiC that is suppressed by at least two orders of magnitude relative to the nonimplanted samples. The decrease for Dit is also substantial (a factor of ∼10) while the loss for NIToxslow is considerably smaller (only ∼30%). The results provide firm evidence that NIToxfast and NIToxslow do not originate from the same kind of defect center.