Nitrogen Passivation of the Interface States Near the Conduction Band Edge in 4H-Silicon Carbide

摘要

This paper describes the development of a nitrogen-based passivation technique for interface states near the conduction band edge [D_(it)(E_c)] in 4H-SiC/SiO_2. These states have been observed and characterized in several laboratories for n- and p-SiC since their existence was first proposed by Schorner, et a. [1]. The origin of these states remains a point of discussion, but there is now general agreement that these states are largely responsible for the lower channel mobilities that are reported for n-channel, inversion mode 4H-SiC MOSFETs. Over the past year, much attention has been focused on finding methods by which these states can be passivated. The nitrogen passivation process that is described herein is based on post-oxidation, high temperature anneals in nitric oxide. An NO anneal at atmospheric pressure, 1175 deg C and 200-400sccm for 2hr reduces the interface state density at E_c-E approx= 0.1eV in n-4H-SiC by more than one order of magnitude - from > 3X 10~(13) to approximately 2 X 10(12) cm~(-2) eV~(-1). Measurements for passivated MOSFETs yield effective channel mobilities of approximately 30-35cm~2/V-s and low field mobilities of around 100cm~2/V-s. These mobilities are the highest yet reported for MOSFETs fabricated with thermal oxides on standard 4H-SiC and represent a significant improvement compared to the single digit mobilities commonly reported for 4H inversion mode devices. The reduction in the interface state density is associated with the passivation of carbon cluster states that have energies near the conduction band edge. However, attempts to optimize the the passivation process for both dry and wet thermal oxides do not appear to reduce D_(it)(E_c) below about 2 X 10~(12)cm~(-2)eV~(-1) (compared to approximately 10~(10)cm~(-2)eV~(-1) for passivated Si/SiO_2). This may be an indication that two types of interface states exist in the upper half of the SiC band gap - one type that is amenable to passivation by nitrogen and one that is not. Following NO passivation, the average breakdown field for dry oxides on p-4H-SiC is higher than the average field for wet oxides (7.6MV/cm compared to 7.1MV/cm at room temperature). However, both breakdown fields are lower than the average value of 8.2MV/cm measured for wet oxide layers that were not passivated. The lower breakdown fields can be attributed to donor-like states that appear ear the valence band edge during passivation.