A Formation of SiO 2 /4H-SiC Interface by Oxidizing Deposited Poly-Si and High Temperature Hydrogen Annealing
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contrast, we found that hydrogen annealing at low pressure ( 8.5x102 Pa ) did not shift C-V characteristics of 4H-SiC MOS structures toward negative voltage from the ideal C-V characteristics and decreased Di, [6,7]. The positive charge which occurs at the SiO 2/4H-SiC interface in hydrogen annealing at the pressure of 1.1 X 10' Pa is considered to be due to the excess hydrogen. Therefore, if the combination of oxidizing deposited poly-Si and high temperature hydrogen annealing at low pressure ( 8.5x102 Pa ) is applied to the fabrication of SiC MOSFETs, two problems as mentioned above will be solved. In this study, we have investigated a formation of the SiO/4H-SiC interface by oxidizing deposited poly-Si and high temperature hydrogen annealing at low pressure ( 8.5x10 2 Pa ) and effect of the oxidation temperature on high-frequency C-V characteristics. EXPERIMENTAL The
8° off-angled n-type
4H-SiC (0001) substrates from Cree Research with a 4.9! m -thick n-type epitaxial layer were used for this study. The n-type dopant was nitrogen. The effective carrier density ( Nd-N, ) was 1.7 X 1016 cm-3. After the standard RCA cleaning, 10 nm thick sacrifice oxide films were grown at 1100'C in dry 02, and then they were removed by 5% HF solution before loading in ultra-high-vacuum ( UHV ). The 22 nm thick poly-Si layers were deposited on 4H-SiC substrates in a Si molecular-beam-epitaxy ( MBE ) apparatus. The poly-Si layers were thermally oxidized at 1050'C, 1150'C and 1250'C for 30 min in dry 02. The samples were moved from the hot zone to the cool zone and cooled down to 20'C rapidly. The post-oxidation annealing ( POA ) of samples oxidized at 1050'C and 1150'C were performed in hydrogen ( 8.5 X 102 Pa ) at 1000'C for 30 min and POA of samples oxidized at 1250'C were performed in hydrogen ( 8.5 X 102 Pa ) , argon ( 8.5 X 10' Pa ) and vacuum (1 X 10-4 Pa ) at 1000'C for 30 min. Aluminum on top of oxide films and on the backs of the samples was evaporated to make gate electrodes and ohmic contacts of MOS structures, respectively. Highfrequency C-V measurements were performed using an HP 4274 LCR meter in a shielded dark box at room temperature. The thickness of SiO 2 films grown on 4H-SiC substrates by oxidizing deposited poly-Si as shown in Fig. 1 were measured using the surface profilometer ( Dektak IIA ). The thickness of SiO 2 films of MOS structures, estimated from capacitance-voltage ( C-V ) characteristics, was 60 ± 5nm. RESULTS AND DISCUSSION
Oxidation rate Figure 1 shows the oxidation time dependence of the Si0 2 thickness formed by oxidizing deposited poly-Si on SiC substrates, single crystal Si [8] and single crystal SiC [9] at 1050'C. The oxidation rate of a poly-Si is approximately 100 times faster than that of single crystal SiC
106
[9] and approximately 1.2 times faster than that of single crystal Si as expected. available for practical use.
12 0 . . . , . .
100
.. . . .
.. . .
This value is
.
T=1050C 0
= 80 o
60 -
".
40 -
U0
20 0
10
20
30
40
50
70
Oxidation Time(min) Figure 1 Oxidation
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