1. Radiation damage in SiO2/SiC interfaces
S. Dixit1,2, S. Dhar2,3, J. Rozen1,2, S. Wang3, S. T. Pantelides3,
D. M. Fleetwood4,3, R. Schrimpf4 and L. C. Feldman1,2,3
1Interdisciplinary Materials Science Program
2Vanderbilt Institute of Nanoscale Science and Engineering
3Department of Physics & Astronomy
4Department of Electrical Engineering & Computer Science
Vanderbilt University, Nashville, TN
MURI meeting June’06

2. Objective Motivation Introduction Experimental Results Conclusion
Outline
Objective
Motivation
Introduction
Experimental
Results
Conclusion
MURI meeting June’06

3. Objective Effect of ionizing radiation on SiO2/4H-SiC MOS devices.
Comparison between as-oxidized and nitrided oxides.
Comparison between positively-biased (electrons swept) and grounded irradiations.
MURI meeting June’06

4. Motivation Only compound semiconductor whose native oxide is SiO2
Deep-space missions - Electronic switches and circuits
High power and high temperature systems
Concerns: Weight, efficiency and reliability
Eg ~ 3.23 eV at RT
Wide Band Gap
~ 2.0 MV cm-1
~ 4.5 Wcm-1s-1
High thermal
conductivity
High breakdown
field strength
High power
device
4H Silicon
Carbide
~ 2.0 x 107 cm s-1
High current
densities
Inert
Only compound semiconductor whose native oxide is SiO2
MURI meeting June’06

5. Thermal oxidation of SiC
Introduction
Thermal oxidation of SiC
SiO2/SiC interface is different from SiO2/Si
Typical oxidation temperatures ~ °C
SiO2
Transition layer
N N N N N N N N N N N N N N
~ 1nm
SiC
sub-oxide bonds
oxycarbides
free carbon
Nitrided
At Ec-E ~ 0.1 eV,
Dit ~ 1013 cm-2 eV-1 ---> as-oxidized
Dit ~ 1012 cm-2 eV-1 ---> nitrided
MURI meeting June’06

6. Experimental Sample preparation: Irradiation: 10 keV X-rays SiO2
40 nm
n-type 4H-SiC
SiO2
SiO2 thermally grown at 1150°C, 4 hrs
NO Passivation at 1175°C, 2 hrs
n-type 4H-SiC
Sputter deposited Mo/Au dots
Sputter deposited back contact
Au back contact
Irradiation:
10 keV X-rays, RT radiation
31.5 krad(SiO2)/min dose rate
Function of dose
Function of positive bias [~1.5 MV/cm, ~2.3 MV/cm]
MURI meeting June’06

7. Results As-oxidized samples
Grounded irradiation
Biased irradiation
Positive charge buildup (like SiO2/Si) followed by a turnaround to net negative charge trapping
For high doses (>1Mrad(SiO2)), grounded samples show net negative charge while the biased samples show net positive charge (like SiO2/Si)
MURI meeting June’06

8. Results Nitrided samples Turn around also observed
Grounded irradiation
Biased irradiation
Turn around also observed
Low dose regime (<1 Mrad) nitrided samples show more positive charge buildup
Indications of Fermi level Pinning was observed
MURI meeting June’06

9. Band diagram SiO2 and p-Si
Bandgap argument EC EF EC EV
Emg Si
4.7 eV
3.2 eV
At midgap, interface traps are charge neutral
Vmg = Vot
(Shifts in the CV curve)
Vit = Vinv- Vmg
(stretch-outs in the CV curve) mg
inv
Eg ~ 9.0 eV
SiO2 EV
Band diagram SiO2 and p-Si
MURI meeting June’06

10. Bandgap argument EC EF EC EV Emg 4H-SiC SiO2 EV
Time constant p(E) = (1/pTNv)exp(E-Ev)/kT EC
Eg ~ 3.23 eV EF EC EV
Emg
4H-SiC
2.7 eV
3.05 eV EC EV
Emg Si
4.7 eV
3.2 eV EF
Eg ~ 9.0 eV eV
SiO2 EV
Band diagram SiO2 and p-type 4H-SiC
MURI meeting June’06

11. Nitrided/As-oxidized comparison
Results
Vmg vs dose (Grounded samples)
Nitrided/As-oxidized comparison
Positive charge trapping reported for the first time in grounded 4H-SiC capacitors for low dose irradiations
At higher doses, negative charge trapping (consistent with previous work 1) observed due to creation of oxide trapped charge and deep interface states (Ec-E~0.6 eV to mid-gap)
1 T. Chen, Z. Luo, J. D. Cressler, T. F. Isaacs-Smith, J. R. Williams, G. Chung, and S. D. Clark, “The effects of NO passivation on the
radiation response of SiO2/4H-SiC MOS capacitors,” Solid State Electron., vol. 46, pp , 2002
MURI meeting June’06

12. Nitrided/As-oxidized comparison
Results
Vmg vs dose (Biased samples)
Nitrided/As-oxidized comparison
Increased positive charge trapping observed in the case of nitrided samples under bias
This might suggest that the nitrided samples have less electron traps due to NO passivation as compared to the as-oxidized ones at low doses
MURI meeting June’06

13. Photo-CV (Biased and grounded capacitors)
Photo-CV results
Photo-CV (Biased and grounded capacitors)
The hysteresis and prominent ledges are indicative of slow interface traps and near-interface (border) traps
Density of these traps are a factor of 2 higher for grounded as compared to positively-biased case, consistent with higher negative charge trapping
MURI meeting June’06

14. Conclusions
Significantly higher negative charge trapping for SiO2/SiC MOS devices compared to typical SiO2/Si
This can be attributed to
rapid buildup of interface states analogous to the hole-H interaction in SiO2/Si
wider bandgap of SiC which exposes the oxide related defects
3. Higher net positive charge buildup in the nitrided samples, consistent with a lower density of pre-existing interface and near-interface electron traps
MURI meeting June’06

15. Thank you Acknowledgements
Work supported by the Air Force Office of Scientific Research
through the MURI program and DARPA
Thank you
MURI meeting June’06

16. Back up slides
MURI meeting June’06