Effect of No on the Electrical Characteristics of SiO 2 Grown on P-Type 4H SiC
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223 Mat. Res. Soc. Symp. Proc. Vol. 512 ©1998Materials Research Society
RESULTS High frequency C-V measurements were performed at temperature of 330'C to gain an overall understanding of the quality of the oxides as shown in Fig. 1. As can be seen from the figure, the C-V curve of NO nitrided sample is less stretched-out compared to N 2 annealed sample. The flatband voltage of NO nitrided sample is -5.5V while that of N 2 1.2.
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1.0 0.8
NO
0
N2 0.4 0.2 0.0" -12 -10
-8
-6
-4
-2
0
2
4
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Vg(V) Fig. 1 High frequency C-V curves measured at 330'C for both NO nitrided and N2 annealed samples annealed sample is -6.8V. Negative flatband voltage shifts show positive charges in the oxides grown on p-type SiC just like the oxides on Si. These positive fixed oxide charge is the combination of fixed charge trapped in the bulk of the oxide, charged interface states, charged oxide trap, and mobile oxide charge. It is impossible to completely separate them from each other based on C-V measurement alone. Therefore the flatband voltage refers to a net oxide charge which is the combination of all the charges existing in the oxide and in the interface of Si0 2 /SiC. However, reduction of this net oxide charges can be an indication of the improvement in the oxide quality. This implies that flatband voltage is reduced for high quality oxides grown on SiC. To get the net oxide charges in the oxide, the working function difference between SiC and Aluminium is calculated to be -2.47V at the temperature of 330TC. Thus the net oxide charges of 1.86x1012 cm- 2 and 3.50x10 12 cm 2 for NO nitrided sample and N2 annealed sample, respectively, are obtained. We have noticed that NO nitridation reduced the net oxide charge by a factor of 2 compared to N2 annealed sample. The interface trap density was determined by conductance technique[6] at high temperature. The measurement was made by biasing the MOS capacitors to their depletion region at the temperature of 330'C across a frequency range of 1KHz to 1MHz. The interface trap densities versus surface potentials for both NO nitrided and N 2 annealed samples are shown in Fig.2. We can observe from Fig.2 that NO nitrided sample shows a trend of decrease in interface trap density towards midband. The minimum interface trap density appears close to the midband for NO nitridation which is 7.2xl0 10 eV' cm-2. However, for N2 annealed sample it shows an even distribution in interface trap density across the energy
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range in the bandgap measured. The interface trap densities vary between 2.20x10" eV-' cm-2 and 2.77x 1011 eV-1 cm 2 within the energy range in the bandgap measured. The average value is about 2.5x10" eV-l cm-2. It is shown by comparing two data that NO nitridation reduced the interface trap density by a factor of about 3. If we apply the typical U-shape of interface trap density in the bandgap to NO nitrided sample, we may expect lower interface trap density in the midband. This indicates an improved interface of SiO 2/SiC for NO nitrided sample.
4xl 011 E N2
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