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

MURI meeting June’06

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

MURI meeting June’06

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

MURI meeting June’06

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

MURI meeting June’06

Acknowledgements

Work supported by the Air Force Office of Scientific Research

through the MURI program and DARPA

Thank you

MURI meeting June’06

Back up slides