Passivation by N Implantation of the SiO 2 /SiC Acceptor Interface States: Impact on the Oxide Hole Traps and the Gate O
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Passivation by N Implantation of the SiO2/SiC Acceptor Interface States: Impact on the Oxide Hole Traps and the Gate Oxide Reliability A. Poggi, F. Moscatelli, S. Solmi and R. Nipoti CNR-IMM Department of Bologna, via Gobetti 101, 40129 Bologna, Italy ABSTRACT This study compares p-MOS capacitors fabricated on N+ implanted and on virgin 4H-SiC. The former sample have N at the SiO2/SiC interface, the latter have not. To investigate the presence of deep and shallow hole traps at the SiO2/SiC interface, high frequency and quasistatic capacitance voltage measurements under dark have been compared for bias sweeping from accumulation to depletion and from depletion to accumulation, the latter after white light illumination. The presence of N has an effect on the density of the shallow donor like traps but no effect on the deep ones. The positive charge trapped in the oxide and/or at the oxide interface after equivalent tunneling hole injection have been compared and are equivalent. Time dependent dielectric breakdown tests have been compared too. The oxide grown on N+ implanted SiC broken at lower electric field. INTRODUCTION SiC based MOSFETs on 4H-SiC appear promising for switches in high power electronics due to the high values of the electron bulk mobility. A significant stepping stone towards the feasibility of SiC MOSFETs has been the incorporation of nitrogen (N) at the SiO2/SiC interface which yields a reduction of the acceptor traps in the upper half of the SiC band-gap and a significant increase of the channel conductivity. The oxidation or re-oxidation in NO or N2O [13] is one of the effective process to improve the interface properties of SiC MOS devices. Furthermore, a N+ shallow implantation into SiC before surface oxidation leads to an accumulation of N atoms at the SiO2/SiC interface and causes a reduction of the acceptor-like interface states density [4,5] and an increase of the electron channel mobility [6]. Even though there is still substantial room for reducing the series resistance in the ON state of MOSFETs, scrutiny now turns toward the impact of the nitridation process on the reliability of gate oxide and SiO2/SiC interface. In this work the effects of a SiC N+ implantation before gate oxidation is investigated on p-MOS capacitors fabricated at the same time and on the same chips of planar n-channel MOSFETs. As above mentioned, N+ implantation improves the performances of the n-channel 4H-SiC MOSFETs. In particular, threshold voltage decreases, maximum field effect mobility increases and density of acceptor-like interface state decreases for increasing N concentration at the SiO2/SiC interface. The electrical characterization of p-MOS devices allow us to gain informations on hole traps at the SiO2/SiC interface and on oxide reliability. EXPERIMENT A p-type 4H-SiC homo-epitaxial wafer (8o off-axis, 10 µm epi-layer thickness and
7.5 × 1015 cm-3 net acceptor doping, purchased from CREE Research Inc.) was employed for the fabrication of n-channel MOSFETs and test devices among which p-M
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