Post on 18-Dec-2015
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 Program2Vanderbilt Institute of Nanoscale Science and Engineering
3Department of Physics & Astronomy4Department of Electrical Engineering & Computer Science
Vanderbilt University, Nashville, TN - 37235
MURI meeting June’06
Radiation damage in SiO2/SiC interfaces
Objective Motivation Introduction Experimental Results Conclusion
MURI meeting June’06
Outline
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
Objective
High power device
MURI meeting June’06
Only compound semiconductor whose native oxide is SiO2
MotivationDeep-space missions - Electronic switches and
circuits
High power and high temperature systems
Concerns: Weight, efficiency and reliability
High breakdown field strength
Wide Band Gap
High thermalconductivity
High currentdensities
Inert
4H SiliconCarbide
Eg ~ 3.23 eV at RT
~ 4.5 Wcm-1s-1~ 2.0 MV cm-1
~ 2.0 x 107 cm s-1
MURI meeting June’06
Introduction
Thermal oxidation of SiC
SiC
SiO2Transition layer
sub-oxide bondsoxycarbidesfree carbon
SiO2/SiC interface is different from SiO2/SiTypical oxidation temperatures ~ 1100 - 1300 °C
At Ec-E ~ 0.1 eV,Dit ~ 1013 cm-2 eV-1 ---> as-oxidized
Dit ~ 1012 cm-2 eV-1 ---> nitrided
~ 1nmN N N N N N N N N N N N N N
Nitrided
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SiO2
n-type 4H-SiC
Au back contact
40 nmSiO2 thermally grown at 1150°C, 4 hrs
Sputter deposited Mo/Au dots
Sputter deposited back contact
NO Passivation at 1175°C, 2 hrs
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]
Sample preparation:
10 keV X-rays
Experimental
n-type 4H-SiC
MURI meeting June’06
As-oxidized samples
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)
Results
Biased irradiationGrounded irradiation
MURI meeting June’06
Nitrided samples
Turn around also observed
Low dose regime (<1 Mrad) nitrided samples show more positive charge buildup
Indications of Fermi level Pinning was observed
Results
Biased irradiationGrounded irradiation
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Bandgap argument
Band diagram SiO2 and p-Si
Eg ~ 9.0 eV
SiO2
EC
EV
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
MURI meeting June’06
Bandgap argument
Band diagram SiO2 and p-type 4H-SiC
Eg ~ 9.0 eV
SiO2
EC
EV
Eg ~ 3.23 eV
EF
EC
EV
Emg
4H-SiC
2.7 eV
3.05 eV
Time constant p(E) = (1/pTNv)exp(E-Ev)/kT
0.2 - 0.6 eV
EC
EV
Emg
Si
4.7 eV
3.2 eV
EF
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Vmg vs dose (Grounded samples)
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)
Nitrided/As-oxidized comparison
Results
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 2231-2235, 2002
MURI meeting June’06
Vmg vs dose (Biased samples)
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
Nitrided/As-oxidized comparison
Results
MURI meeting June’06
Photo-CV results
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
Photo-CV (Biased and grounded capacitors)
MURI meeting June’06
1. Significantly higher negative charge trapping for SiO2/SiC MOS devices compared to typical SiO2/Si
2. 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
Conclusions
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Acknowledgements
Work supported by the Air Force Office of Scientific Research
through the MURI program and DARPA
Thank you
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Back up slides