4.5 Summary

The impact of various high-temperature processing steps on the charge state of the SiC/SiO₂ interface was investigated using 4H-SiC n-MOSCAPs and CV measurements. By comparing the measured CV curves to an ideal, simulated CV curve, it was shown that during the processing of SiC/SiO₂ based semiconductor structures a significant accumulation of positive charges occurs at the semiconductor-insulator interface. The build-up of positive charges starts after the deposition of the polycrystalline gate contact and continues in all additional processing steps, in which the sample is exposed to a high thermal budget with temperatures above 500 °C. The positive charge accumulation leads to a significant shift of the flatband voltage . The observed flatband voltage shift ranges from after the deposition of the poly-Si gate contact at , to approximately after the formation of the metal contact at a temperature of approximately .

The origin of these charges is still unknown. However, two possible candidates have been discussed:

• • First, positive impurities (ions) with low diffusion coefficients at room temperature, which are randomly distributed throughout the SiO₂. After subjecting the sample to a high temperature, these ions might be able to diffuse with a preferred direction along the oxide field, which points in the direction of the SiC/SiO₂ interface.

• • Second, high amounts of hydrogen are incorporated during the deposition of the polycrystalline gate contact, which can be trapped in various configurations in the amorphous SiO₂. Assuming the diffusivity of hydrogen in the the amorphous SiO₂ is not rate limiting [70], an energy barrier of approximately was extracted from the experimental data, which is in good agreement with proposed hydrogen-related trapping mechanisms like the hydrogen release model [48, 58, 132, 133].

To summarize, a significant amount of positive charge accumulates at the SiC/SiO₂ interface during high- processing steps. The atomic origin of the charge build-up is still unknown but likely linked to hydrogen, which is incorporated during the poly-Si deposition. However, to enable a better understanding, further studies on the observed behavior are mandatory. Here, especially a combination with measurement techniques like secondary ion mass spectrometry (SIMS), which enable the analysis of the exact composition of the specimen, should be advantageous.