Near-interface trap detection by CCDLTS

Isanka Jayawardhena, Ayayi C. Ahyi, Tamara Isaacs-Smith , Asanka Jayawardena, Sarit Dhar

Department of Physics, Auburn University, AL, USA

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13th Annual SiC MOS Workshop meeting at UMD

Outline

• Introduction▪ Trapping at 4H-SiC/SiO2 interfaces▪ Constant Capacitance Deep Level Transient Spectroscopy (CCDLTS)

• CCDLTS Experiments i. Effect of interface passivation using N and Pii. 4H-SiC/Al2O3 interface iii. Effect of 4H-SiC wafer orientationiv. Effect of 4H-SiC Doping concentration

• Summary

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Introduction-SiC charge trapping

• The thermal oxidation process introduces defects in the interfacial region(~2nm)o Low free carrier densityo Low channel mobilityo Poor device stability

• Interfacial nitridation using high temperature ( ~1200 ◦C) annealing in NO, N2O etc. are standard methods to passivate traps.

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Near interfacetrapping!

CCDLTS technique summary

𝒆𝒏 = 𝝈𝒏𝝂𝒕𝒉𝑵𝑪 𝒆𝒙𝒑− 𝑬𝑪 − 𝑬𝑻

𝒌𝑻4

Constant Capacitance Deep Level Transient Spectroscopy

𝑒𝑛-emission rate𝐸𝑇-trap activation energyσn- electron capture cross sectionVth- thermal velocity for electronsNc- effective density of states of CB

Electron emission

Electron capture

• Voltage pulse (Vp) used to fill the traps : Electron capture• End of the filling pulse sample return to the reversed bias

:Trapped electrons emission • Feedback loop used to hold sample at constant capacitance

: Voltage transient generated• Spectrometer produce a CCDLTS signal (ΔV) by measuring

𝑉 𝑡 at two different time t1 and t2• Thermal emission rates matches the “rate window” of the

instrument : Transient signal will maximize

=3𝑘𝑇

𝑚∗

CCDLTS AnalysisArrhenius Analysis – Trap energy and cross section

𝒍𝒏𝑻𝟐

𝒆𝒏=

𝑬𝑪 − 𝑬𝑻𝒌𝑻

+ 𝒍𝒏𝟏

σ𝒏γ𝒏

𝑵𝒊𝒕 =𝑪𝒐𝒙𝑨𝒒

𝟑∆𝑽𝒎𝒂𝒙

Nit calculation

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σn -electron capture cross section𝐸𝑇 -trap energyVth -thermal velocity for electronsNc -effective density of states of CBCox -oxide capacitanceA -areaNit -interface trap density

O1/O2 are typical signature we observe in CCDLTS for Si-face /thermal oxides

1×1016 cm3 doped 4H-SiC/ SiO2( NO annealed)

Cp=37 pFen=46.5 s

-1

O1

O2

O1

O2

Trap type

EC-ET± 0.01(eV)

σn(cm2)

Nit(cm-2)

O1 0.15 1×10-15 2.2×1011

O2 0.35 3×10-14 3.0×1011

O1 and O2 Traps

• The physical identities of these defects have been suggested to be

O1 -carbon dimers substituted for O dimers

O2 -interstitial Si atoms in the near interfacial SiO2

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Activation energies of about O1→0.15±0.05 eV O2 →0.39±0.1 eV

C- grey, Si- yellow, O- red

Knaup et al. Physical Review B 72, 115323 (2005)

Basile et al. J. Appl. Phys. 109, 064514 (2011).

Outline

• Introduction▪ Trapping at 4H-SiC/SiO2 interfaces▪ Constant Capacitance Deep Level Transient Spectroscopy ( CCDLTS)

• CCDLTS Experiments i. Effect of interface passivation using N and Pii. 4H-SiC/Al2O3 interface iii. Effect of 4H-SiC wafer orientationiv. Effect of 4H-SiC Doping concentration

• Summary

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Effect of Passivation

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Compared to as oxidized sampleO1/O2 trap densities reduced for NOand PSG samples drastically

Lower the O1/O2 densities → Higher channel mobility

For Si-face 4H-SiC/ SiO2 ( NO annealed ) MOS capacitor:

Asanka Jayawardena et al 2018 Semicond. Sci. Technol. 33 065005

O1 trap Nit(cm

-2)O2 trap Nit(cm

-2)

As-oxidized 2.6×1012 1.77×1012

NO 120 1.88×1011 2.61×1011

PSG 3.12×1010 4.14×1011

Outline

• Introduction▪ Trapping at 4H-SiC/SiO2 interfaces▪ Constant Capacitance Deep Level Transient Spectroscopy ( CCDLTS)

• CCDLTS Experiments i. Effect of interface passivation using N and Pii. 4H-SiC/Al2O3 interface iii. Effect of 4H-SiC wafer orientationiv. Effect of 4H-SiC Doping concentration

• Summary

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*From Purdue University

Different dielectrics

4H-SiC/Al2O3 interface

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Motivation: Do we see O1/O2 in other dielectric/4H-SiC interfaces?

→ O1/ O2 are inherent to 4H-SiC/ SiO2 interface! → Do we see O1/O2 in deposited SiO2? ongoing project

Cp=60 pFen=46.5s

-1

1×1017 cm-3 doped 4H-SiC/ ALD Al2O3 MOS capacitor

Changing Vp

Outline

• Introduction▪ Trapping at 4H-SiC/SiO2 interfaces▪ Constant Capacitance Deep Level Transient Spectroscopy ( CCDLTS)

• CCDLTS Experiments i. Effect of interface passivation using N and Pii. 4H-SiC/Al2O3 interface iii. Effect of 4H-SiC wafer orientationiv. Effect of 4H-SiC Doping concentration

• Summary

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Si-face C-face a-face

4H-SiC wafer orientations(0001) Si- face (000-1) C- face (11-20) a- face

Mobility*(cm2/Vs) 29 37 103

polarity polar polar Non polar

* For dry oxidation with NO (Yoshioka et al AIP Advances 5, 017109 (2015)

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4H- SiC MOS capacitors fabricated on the

a) (0001) Si- face (100% Si)

b) (11-20) a- face (50% Si, 50% C)

c) (000-1) C- face (100% C)

with

• Dry oxidation at 1150 ◦C

• NO annealed 1175 ◦C (2 hr)

a- face CCDLTS

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• Observed A1, A2 new peaks• O2 is present but masked by high A1, A2 density

A3?

a- face vs Si-face

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OrientationTrap type

EC-ET± 0.01(eV)

σn(cm2)

Nit(cm-2)

(11-20) aFace

A1 0.15 1×10-16 2.3×1011

A2 0.19 7×10-15 2.3×1011

EC-ET :emission activation energiesσn :electron capture cross section Nit :interface trap density

the error in σn about an order of magnitude and for Nit is about 10%

• Activation energy of A1, A2 traps (a-face) are in the same range as that of O1 ( Si-face) suggesting similar origin for these traps.

• O2 is present at lower density but peak could not be analyzed clearly.

Activation energiesO1→0.15±0.05 eV O2 →0.39±0.1 eV

C- face Results

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High density of energetically deeper traps masking O1/O2

• Usually O1/ O2 appears in circled areas for Si-face

• For 77K to 460 K range we did not observe any peaks for C-face

Previous results? Japanese group

Activation energiesO1→0.15±0.05 eV O2 →0.39±0.1 eV

Hatakeyama et al.: Jpn. J. Appl. Phys., 54, 111301, 2015.

Outline

• Introduction▪ Trapping at 4H-SiC/SiO2 interfaces▪ Constant Capacitance Deep Level Transient Spectroscopy ( CCDLTS)

• CCDLTS Experiments i. Effect of interface passivation using N and Pii. 4H-SiC/Al2O3 interface iii. Effect of 4H-SiC wafer orientationiv. Effect of 4H-SiC Doping concentration

• Summary

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2x1016 cm-3 & 1x1017 cm-3 epitaxial layer doping concentration

Results Summary (high doped 4H-SiC)

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Trap type

EC-ET(eV)

σn(cm2)

Nit(cm-2)

O1 0.17± 0.02 2×10-15 1.3×1011

O2 0.33± 0.03 2×10-14 2.1×1011

O3(new) 0.26± 0.02 2×10-15 2.5×1011

Nit calculations for Vp=Vfb+5V

Si- face 4H-SiC/SiO2 MOS capacitors with high (1x1017 cm-3) epitaxial layer

doping concentration (NO annealed)

Activation energiesO1→0.15±0.05 eV O2 →0.39±0.1 eV

Trap type

EC-ET± 0.01(eV)

σn(cm2)

Nit(cm-2)

O1 0.15 1×10-15 2.2×1011

O2 0.35 3×10-14 3.0×1011

For 2x1016 cm-3 sample:

Summary

For 4H-Silicon carbide MOS system

✓O1/O2 inherent to 4H-SiC/SiO2 interface: not observed in ALD Al2O3✓Depending on wafer orientation dominating trap types are changing

• Si-face → O1 and O2 are present

• a-face → A1,A2 traps have similar activation energies as O1

→ O2 is present with lower concentration; but hard to quantify

• C-face → Both O1, O2 could be present with lower concentrations but higher density of deeper traps dominate the CCDLTS signal

✓Depending on 4H-SiC epitaxial doping concentration NIT types are changing

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Acknowledgements

➢Dr. Sarit Dhar, Dr. Ayayi C. Ahyi, Tamara Isaacs-Smith, Dr. AsankaJayawardena

➢Dr. C. Jiao , Department of Electrical and Computer Engineering, Purdue University, IN

➢Dr. P.M. Mooney from Simon Fraser University, BC, Canada

➢This work is supported by II–VI Foundation

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Thank You!

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References

1] A. F. Basile, J. Rozen, J. R. Williams, L. C. Feldman, and P. M. Mooney, J. Appl. Phys., 109, 064514, 2011.

[2] P. Deák, J. M. Knaup, T. Hornos, C. Thill, A. Gali, and T. Frauenheim,” J. Phys. Appl. Phys., 40, 6242, 2007.

[3] P. M. Mooney, Z. Jiang, A. F. Basile, Y. Zheng, and S. Dhar, J. Appl. Phys., 120, 034503, 2016.

[4] T. Hatakeyama, M. Sometani, Y. Yonezawa, K. Fukuda, H. Okumura, and T. Kimoto, Jpn. J. Appl. Phys., 54, 111301, 2015.

http://www.evincetechnology.com/whydiamond.html

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No saturation of signal with higher trap filling voltage(Vp) indicates traps are distributed spatially near interfacial region

Trap type

EC-ET± 0.01(eV)

σn(cm2)

Nit(cm-2)

O1 0.15 1×10-15 2.2×1011

O2 0.35 3×10-14 3.0×1011

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• N passivation 1. Leaves N only very near the SiO2/SiC interface

• P passivation 1. SiO2 transforms to PSG (different gate dielectric)

2. P uptake ( P coverage in monolayer at SiC interface and in Bulk PSG ) effect on passivation.

• Theoretical calculation

• N can passivate for both C (or Si) dangling bonds, three fold coordinated C atoms, Si-Si dangling bonds(SiO2/SiC interface) S.Wang et al Physics Review Letters 98,026101(2007)

• P can passivate three fold coordinated Carbon atoms (SiO2/SiCinterface) Y.K Sharma et al.: Solid State Electronics 68, 103(2012)

Introduction- Wide Bandgap semiconductors

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Property 4H-SiC

Bandgap Eg (eV) 3.26

Dielectric constant , 𝜀𝑟 10.1

Electric breakdown field, Ec (MV/cm) 2.5

Thermal conductivity, λ (W/cm K) 4.9

Structure Hexagonal

Most technologically advanced wide bandgap semiconductor

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Stacking sequences of the Crystal structures of (a) 3C SiC, (b) 4H SiC, and (c) 6H SiC.

number of layers in the

sequence, followed by H, R, or

C to indicate whether the type

belongs to the hexagonal,

rhombohedral, or cubic class.

Introduction- Wide Bandgap semiconductors

Property Si 4H-SiC GaAs GaN Diamond

Bandgap Eg ( eV) 1.12 3.26 1.43 3.50 5.45

Electron mobility, 𝜇𝑛 (cm2/Vs) 1500 1000 8500 1250 2200

Dielectric constant , 𝜀𝑟 11.9 10.1 13.1 9 5.5

Electric breakdown field, Ec (MV/cm) 0.3 3.0 0.4 3.3 10

Thermal conductivity, λ (W/cmK) 1.5 4.9 0.4 2.3 20

Baliga’s Figure of Merit (𝜀𝑠𝜇𝑛𝐸𝑐3) 1 340 15 1250 24644

Crystal Structure Diamond Hexagonal Zincblende Hexagonal Diamond

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Effect of nitridation

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Sample

O2 Trap

EC-ET(eV)

σn(cm2)

Nit(cm-2)

As-oxidized 0.34±0.1 5×10-16 1.77×1012

Std NO 0.35±0.1 1×10-14 2.61×1011

Discussion• O1/O2 inherent to 4H-SiC/SiO2 interface

• NO annealing results in higher N concentration in both C-face and a-face compared to Si-face → Higher N concentration could be related to reduction of O2 traps

• Depending on wafer orientation dominating NIT types are changing

• Depending on

28Dhar et al. J. Appl. Phys. 97, 074902 (2005)

Characterization(hi-lo CV)

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Draw again

𝒍𝒏 𝑻𝟐/𝒆𝒏 =𝑬𝑪 − 𝑬𝑻𝒌𝑻

+𝒍𝒏(𝟏/(𝝈𝒏𝜸𝒏))

𝒍𝒏𝑻𝟐

𝒆𝒏=

𝑬𝑪−𝑬𝑻

𝒌𝑻+ 𝒍𝒏

𝟏

σ𝒏γ𝒏

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Sample

O1 Trap O2 Trap

EC-ET(eV)

σn(cm2)

Nit(cm-2)

EC-ET(eV)

σn(cm2)

Nit(cm-2)

As-oxidized0.15±0.05[1

]1×10-15[1]

2.6×1012[1]

0.34±0.1 5×10-16 1.77×1012

NO 120 0.19±0.05 1×10-13 1.88×1011 0.35±0.1 1×10-14 2.61×1011

PSG [7] 0.15±0.01 1×10-15

3.12×1010 0.38±0.01 7×10-13

4.14×1011

BSG 0.14±0.05 1×10-16 4.06×1011 0.33±0.1 2×10-15 8.65×1011

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O1 trap Nit(cm

-2)O2 trap Nit(cm

-2)

As-oxidized 2.6×1012 1.77×1012

NO 120 1.88×1011 2.61×1011

PSG 3.12×1010 4.14×1011

Sample

O2 Trap

EC-ET(eV)

σn(cm2)

Nit(cm-2)

As-oxidized 0.34±0.1 5×10-16 1.77×1012

NO 120 0.35±0.1 1×10-14 2.61×1011

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34http://www.rohm.com/web/global/electronics-basics/sic/types-of-sic-power/

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Hi doped ( HI NO

device Vp HT/CL E σ Nit E σ Nit E σ Nit

G9 Vfb+5 HT 0.180326 2.61549E-15 1.41146E+11 0.368658 4.24977E-14 2.55975E+11 0.269723 1.12688E-15 2.53543E+11

Vfb+5 CL 0.163807 1.13479E-15 1.40011E+11 0.31704 3.945E-15 2.50982E+11 0.239682 2.4134E-16 2.48923E+11

Vfb+2 HT 0.130087 2.05776E-17 0.30358 3.22211E-16 0.2700 9.90496E-16

Vfb+2 CL 0.115558 1.45037E-17 0.271153 4.07975E-16 0.185479 1.75001E-17

B9 Vfb+5 CL 0.192279 1.94423E-14 0.324443 1.07559E-14 0.230083 1.14343E-16

Vfb+5 HT 0.209875 4.50355E-14 1.22197E+11 no no 1.19477E+11 0.257184 3.14255E-16 2.54354E+11

Vfb+2 CL 0.200904 4.83976E-14 0.311267 2.32205E-15 0.273954 3.58031E-15

Vfb+2 HT 0.172209 5.27003E-16 0.374225 6.14025E-14 0.284751 3.31166E-15

F5 Vfb+2 HT 0.14316 2.5899E-17 0.29816 6.14129E-16 0.282485 2.64442E-15

Vfb+2 CL 0.1794 3.07E-15 0.330845 8.78E-17 0.2658 1.78E-15

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BackupsO1 O2 O3

0.163807 0.368658 0.269723

0.130087 0.31704 0.239682

0.180326 0.30358 0.2700

0.192279 0.330845 0.2658

0.209875 0.324443 0.230083

0.200904 0.29816 0.257184

0.172209 0.311267 0.273954

0.14316 0.374225 0.284751

0.1794 0.282485

average 0.174672 0.328527 0.26374

std dev 0.024487 0.026682 0.016525

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SiO2Al

4H-SiC

Experiment: Wafer Orientation

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4H- SiC MOS capacitors fabricated on the

a) (0001) Si- face (100% Si)

b) (11-20) a- face (50% Si, 50% C)

c) (000-1) C- face (100% C)

with

• Dry oxidation at 1150 ◦C

• NO annealed 1175 ◦C (2 hr) Si-facetox=62 nm

C-facetox=76 nm

a-facetox=54 nm

414H-SiC

SiO2

Dangling bonds

C-C bonds

Si-Si bonds

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ID

t1=2.8 ID t2=7.0 ID

t

t=0

ID: Initial delay

Initial Delay(ID)ms Emission rate(en)s-1 τ(s)

50 4.651 0.215

20 11.628 0.086

10 23.256 0.043

5 46.512 0.0215

2 116.279 0.0086

1 232.558 0.0043

0.5 465.116 0.00215

0.2 1162.790 0.00086

0.1 2325.581 0.00043

0.05 4651.163 0.000215

𝒆𝒏 = Τ𝟏 (𝟒. 𝟑 × 𝑰𝑫)

• 𝟏/𝒆𝒏 =𝐞𝐱𝐩( 𝑬𝑪−𝑬𝑻 /𝒌𝑻)

𝝈𝒏𝜸𝒏𝑻𝟐

• The amplitude of the CCDLTS signal is directly proportional to the trap density, and the areal density of the near-interface traps was calculated by [18]

• 𝑁𝑖𝑡= 3 ∙ (CoxA⁄) ∙ ΔV(To) ∙ΔW𝑞 (3)

• where the factor of 3 arises from the sampling times t1 and t2 (t2=2.5t1) of the voltage transient in the DLTS spectrometer,[18] ΔV(To) is the maximum CCDLTS signal at the peak temperature To and ΔW is a broadening factor, which is the ratio of the integral of the experimental CCDLTS signal ΔV over the measured temperature range to the integral of the intensity of a theoretical CCDLTS peak for a trap having a single energy level.

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