Radiation-Induced Trap Spectroscopy in Si Bipolar Transistors and GaAs Diodes

22nd RD50 WorkshopUniversity of New Mexico

June 3-5, 2013

Robert FlemingSandia National Labs

Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy’s National Nuclear

Security Administration under contract DE-AC04-94AL85000.

Outline• Introduction

– Research Objectives– DLTS & transistor fundamentals

• Silicon– BJT collector – the silicon divacancy & its relatives– BJT base – the vacancy-donor (VP)– Correlating defects and BJT gain

• GaAs– Phonon assisted tunneling – electric field enhanced

emission

Slide 2

Research Objectives• Predict performance of bipolar transistors

after exposure to neutrons– Data from ion damaged devices

• Avoid hazards of fast-burst neutron reactors– Models of defect evolution are used to connect ion data with

neutron predictions [models are not presented in this presentation, see Myers, et al., J. Appl. Phys. 104, 044507 (2008)]

• Experimental Objectives– Understand effects of defect clustering (DLTS & transistor gain)– Correlate gain degradation of BJT’s with specific defects (Si)– Understand effects of phonon-assisted tunneling (GaAs)

Slide 3

50 100 150 200 250 3000.0

0.1

0.2

0.3

C (p

F)

T (K)

DLTS: Deep Level Transient Spectroscopy

Rate = 232 s-1

Shallow(near Ec)

Deep(near midgap)

0

Vol

tage

Cap

acita

nce

Time

-V

C

)/exp( ottC

CN t

# of traps

2

)/]exp([1T

kTEEt

tc

o

Emission Rate

Filling pulse: carrier captureTransient: carrier emission

DLTS peaks occur when defect emission rate is equal to the rate window

Slide 4On the DLTS plots presented in this talk, the maximum temperature correspond to emission from ~ Eg/2

Fleming, HEART. 4/2/2009, Slide 5

0.25 0.30 0.35 0.40 0.45 0.5010-11

10-9

10-7

10-5

Silicon DLTS - Gummel Anneal.opj

Ib Undamaged

Ic

Si ion damagednpn BJT

Cur

rent

(A)

Vbe

Ib Damaged

Location of Generation-Recombination

Recombination at low currentin b/e depletion region

Recombination at high currentin b/e depletion + neutral base

IC

IB

IE

Slide 5

Gain = Ic / Ib

DLTS Locations

Recombination at low currentin b/e depletion region

Recombination at high currentin b/e depletion + neutral base

b/e junction

b/c junction

Slide 6

NEUTRON AND ION DAMAGESilicon Bipolar Transistor Collector: Doping ~ 1015 cm-3

Slide 7

Ions - End of Range

0.0

0.1

0.2

E4

V2 (-/0) + E5

V2 (=/-)

C (A

rb. U

nits

) NeutronsVO

50 100 150 200 250 300

T (K)

Ions - not End of Range

50 100 150 200 2500.0

0.1

0.2 Rate = 232 s-1

C (A

rb. U

nits

)

T (K)

25 MeV Electrons

Point defect spectra (electrons) are related to clustered defect spectra (neutrons)

Asymmetry of V2

Slide 8

50 100 150 200 250 3000.0

0.1

0.2

C (p

F)

T (K)

Three Types of “V2” defects• Normal V2 – two equal acceptor levels• “Strained V2

” – single acceptor level• “Bistable V2

”- two bistable acceptor levels

100 150 200 2500.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28Rate = 232 s-1

E5

E4V2 (-/0)

VO

V2 (=/-)

C (p

F)

T (K)

Post Rad Zero Bias, 350 K, 1 hour Injection, 300 K, 20m Difference

25 MeV Electron Damage

Slide 9Note the quotes around “V”. We do not know the exact structure or composition of the strained or bistable “V2”.

• Partial filling of the shallow V2 double acceptor level.

• Strain-induced inhibition of bond averaging of Jahn-Teller distortion of V2 Svensson, et al., Phys. Rev. B 43, 2292 (1991)

• Different structure of the defect, e.g. V2 + In

VV -/0

VV =/-

EfVO

Ec

Why is there a V2 asymmetry?

UndistortedT > 20 K in c-Si or V2

= charge state

JT DistortedT < 20 K or strained Si

Slide 10

100 150 200 2500.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28Rate = 232 s-1

E5

E4V2 (-/0)

VO

V2 (=/-)

C (p

F)

T (K)

Post Irradiation Zero Bias, 350 K, 60 m Injection, 300 K, 20 m Difference

25 MeV Electron Damage

Gain and Defect Bistability

100 101 102 103 104

0.85

0.90

0.95

1.00

1.05

1.10

1.15

Gain(t) / Gain(0)

Icoll.(t) / Icoll(0)

I / I(

0)

Time (s)

Ibase(0) = 0.127 mAIcoll.(0) = 1.71 mAGain(0) = 13.48

Neutron-damaged,after 60 m 350K 0V bias

Ibase(t) / Ibase(0)

R. M. Fleming, C. H. Seager, D. V. Lang et al., Appl. Phys. Lett. 90, 172105 (2007).

Injection of minority carriers at 300 K causes E4 and E5 to become visible. Annealing at 350 K (80 C) removes E4 and E5.

Slide 11

Annealing of Bistable E4-E5

Rate = 232 s -1

350 K Injection 350 K 0V

120 160 200 240 280

T (K)

500 K Injection 500 K 0V

450 K Injection 450 K 0V

400 K Injection 400 K 0V

C

Neutron Damaged

• Bistable E4-E5 has the same asymmetry as V2

• Asymmetry of E4-E5 acceptor level amplitude is gone after 500 K

• Bistability remains until 560 K

Annealing of Bistable E4-E5

Rate = 232 s -1

350 K Injection 350 K 0V

120 160 200 240 280

T (K)

500 K Injection 500 K 0V

450 K Injection 450 K 0V

400 K Injection 400 K 0V

C

350 400 450 5000.0

0.4

0.8

1.2 V

2 acceptor levels

E4 / E5

Shal

low

/ D

eep

Rat

io

Annealing Temperature (K)

160 180 200 220 240 260 280

0.00

0.02

0.04

0.06

E5

C (p

F)

T (K)

Anneal Temp. 350 K 400 K 450 K 500 K 530 K 560 K

Difference: DLTS(Inject.) - DLTS(0V) Rate = 232 s-1

E4

Annealing above 500 K – Bistability goes away, but V2 – like defect remains

560 K 0V 560 K Injection

530 K 0V 530 K Injection

160 180 200 220 240 260

T (K)

500 K 0V 500 K Injection

Rate = 232 s-1

C

Shift of V2 is associated with transition to V2O

L Center

V2 (-/0)

100 150 200 250 3000.00

0.04

0.08

0.12

C (p

F)

T (K)

460 K -5V 488 K -5V Difference

Neutron Damaged, Reverse Bias Anneal

Rate = 232 s-1

Annealing above 500 K – Bistability goes away, but V2 – like defect remains

560 K 0V 560 K Injection

530 K 0V 530 K Injection

160 180 200 220 240 260

T (K)

500 K 0V 500 K Injection

Rate = 232 s-1

C

L Center

L Center has two levels

Above 500 K, bistable E4-E5 transforms to the stable L center with two levels.

V2 (-/0)Shift of V2 is associated with transition to V2O

Slide 15Fleming, et al., J. Appl. Phys. 104, 083702 (2008)

300 400 500 600 7000.00

0.04

0.08

0.12

0.00

0.02

0.04

0.06

C (p

F)

Annealing Temperature (K)

V2 (=/-)

V2 (-/0)

Cheng & Lori 1968

Neutron Damaged

E4 + L E5 + L

C

(pF)

Bistable E4-E5 and V2 AnnealingE4-E5 has the same annealing characteristics and level asymmetry as the strained V2 seen after neutron or other clustered damage.

Fleming, et al., J. Appl. Phys. 104, 083702 (2008)Slide 16

Kinetics of Bistable V2

Slide 17

0.00

0.02

0.04

0.06

0.08

50 100 150 200 250 3000.000

0.004

0.008

(a)

Ci

VO

E75 E5

25 MeV Electron Damage After forward bias, 300 K After 0 V bias anneal, 350 K

C /

C

E4

V2 (=/-)V2 (-/0)

(b)

T (K)

Fast Neutron Damage

E75Posit.

E5

E4

n-type, Rate = 232/s

50 100 150 200 250 300

0.000

0.004

0.008

0.012 After zero bias 350 K After injection 300 K

Unrelated bistablepeaks

CO

C /

C

T (K)

Neutron Damage

H1

V2 (0/+)

p-type, Rate = 232/s

100 101 102 103 104

0

25

50

75

100

Neutron DamagedAnneal-In 320 K

Inverse gain H1 DLTS E5 DLTS

Am

plitu

de (%

)

Time (s)

Fleming, et al., J. Appl. Phys. 111, 023715 (2012)

Stable phase: E75Metastable phase: E4, E5, H1

0.48 0.52 0.5610-4

10-3

Neutron Damaged, Anneal-InRate exp(qV / 1.2kT)

Rat

e (s

-1)

Forward Bias Voltage (V)

NEUTRON AND ION DAMAGESilicon Bipolar Transistor Base: Doping ~ 1017 cm-3

Slide 18

Neutron – Ion Equivalence using EOR Silicon Ions

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 SPR 4.5 MeV Si 36 MeV Si

I B/I C

(AS

TM)

DLTS Integrated Area

SPR Slope 0.61IBL 4.5 MeV Si Slope 0.71IBL 36 MeV Si Slope 0.55

50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8VP + V2 -/0 +V*2 -/0

Neutron Ion DLTS Equiv.opj

Nt /

(c

m-1)

SPR Neutrons 4.5 MeV Si Ions x 1.2e5

T (K)

pnp base, Neutron/ion equiv. = 1.2e5DLTS Rate = 23.2/s, -5V bias

V2 =/-

Electric field broadening

Slide 19

Exact match of neutron & ion DLTS spectra in pnp base

Linear relationship between inverse gain and DLTS amplitude

Si ion energy chosen to place the ion end-of-range (EOR) at the emitter-base junction

Correlating DLTS & Gain

Slide 20

0.25 0.30 0.35 0.40 0.45 0.5010-11

10-9

10-7

10-5

Silicon DLTS - Gummel Anneal.opj

Ib undamaged

Ic

Si ion damagednpn BJT

Cur

rent

(A)

Vbe

Ib 300 K

Ib 550 KIb 600 K

In the npn collector after 550 K, normal V2 remains while the strained V2 has annealed away. Little additional gain recovery occurs after 600 K where the normal V2 anneals.

Similar results are obtained for the pnp transistor. Correlate DLTS and gain using the pnp base.

50 100 150 200 250 300

0.0

0.1

0.2

Silicon DLTS - Gummel Anneal.opj

600 K

550 K

C /

C (a

rb. u

nits

)

T (K)

Si ion damagednpn collectorDLTS Rate = 232/s

300 K

Remove defects by annealing to assign relative importance of defects to gain reduction.

Slide 21Fleming, et al., J. Appl. Phys. 107, 053712 (2010).Fleming, et al., J. Appl. Phys. 108, 063716 (2010).

350 400 450 500 550 600 6500.00

0.02

0.04

0.06

0.08

C (p

F)

Annealing Temperature (K)

Zero Bias Anneal Reverse Bias Anneal Deep Peak Deep Peak VO / VP2 VO / VP2

V2(=/-) V2 (=/-)

Neutron Damaged: pnp base

300 350 400 450 500 550 6000.0

0.4

0.8

1.2

SPR pnp SPR npn ACRR pnp ACRR npn

NT E

-B D

eple

tion

(Nor

mal

ized

)

Annealing Temperature (K)

50 100 150 200 250 3000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

After 530 K

After 350 K

Gain ~ 0

Gain ~ 2/3

Gain ~ 1/3

VP ~= 1.5 X V2 (42%)

V*2 ~= 1 X V2 (29%)

V2 (29%)

V2 (-/0)

VO + VP2

V2 (=/-)

Fig 2: SPR

C (p

F)

T (K)

Si pnp Neutron Damage: Base

VONeutron damaged: NT from transistor currents

NT from DLTS

C(TA) ~ [VP] fvp(TA) + [V2*] fv(TA) + [V2]

Polynomial empiricallydetermined from pnp base

Polynomial empiricallydetermined from npn collector = constant

DEFECTS IN GaAs – ELECTRON & NEUTRON DAMAGE

Slide 22

DLTS in GaAs : Analysis is more complex

Slide 23

Silicon GaAsDefect library from prior work Extensive (from EPR/DLTS) Minimal (EPR not effective)

Additional defect species in clusters

A few, e.g. bistable V2, strained V2 Unknown at present, work in progress

Electric-field enhanced emission from defects (phonon assisted tunneling)

Minimal Extensive – Results in broad DLTS features after clustered damage (U-band, L-band)

• Neutron spectra is radically different from electron damage.

• The broad features after

neutron/ion damage are known as the “U-band” (n-GaAs) and the “L-band” (p-GaAs)

50 100 150 200 250 300 350 4000.00

0.02

0.04

0.06

0.08

0.10

Electron damage

Neutron damage U-band

No EL2with MBEgrowth

E5E4

E3

MNB Diodes DLTS.opj

C (p

F)

T (K)

n-GaAs Nd = 2 x 1016 cm-3E2

0 50 100 150 200 250 300 350 4000.0

0.5

1.0

1.5

2.0

E5E4

E3

E3a

E2

LTMF S-III Anneal.opj

3 MeV 8 Mrad LMTF1e16 n-GaAs, Rate = 11.6/s

NT (

1014

cm

-3)

T (K)

E1

Clustered & Non-clustered Damage

0 50 100 150 200 250 300 350 4000.0

0.2

0.4

0.6

0.8

ACRR S-III Anneal.opj

Nt /

(c

m-1)

T (K)

ACRR Neutrons1e16 p-GaAsRate = 11.6/s

0 50 100 150 200 250 300 350 4000.0

0.2

0.4

0.6

0.8

ACRR S-III Anneal.opj

Nt /

(c

m-1)

T (K)

ACRR Neutrons1e16 n-GaAs, Rate = 11.6/s

0 50 100 150 200 250 300 350 4000.0

0.4

0.8

1.2

1.6

2.0

2.4

H3?

Be RelatedH1

LTMF S-III Anneal.opj

3 MeV 8 Mrad LMTF1e16 p-GaAs, Rate = 11.6/s

NT (1

014 c

m-3)

T (K)

H0

Slide 24

Neutron Damaged GaAs

0.0 0.2 0.4 0.6 0.8 1.0 1.20.0

0.5

1.0

1.5 Neutron Damaged GaAs 10 ms Filling Pulse

Density of States.opj

Ave

rage

Nt (1

014 c

m-3)

E (eV)

p-GaAs n-GaAs

• Neutron damage causes expansion of the GaAs lattice• Decrease of GaAs bandgap

• Bandgap decrease of 0.16 eV results in overlap of U- and L-bands & apparent continuous distribution of defect states across midgap

• Cause?– Inhomogeneous broadening , e.g. strain broadened defect levels– Homogeneous broadening, e.g. electric-field dependent emission rate

Slide 25Fleming, et al., J. Appl. Phys. 107, 123710 (2010).

Broad U- & L-bands at high doping (neutron damage)

0 50 100 150 200 250 300 350 4000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.63e15

1e16

3e161e17

Nt per neutron, n-GaAs

Nt /

(c

m-1)

T (K)

3e17

50 100 150 200 250 300 350 4000.0

0.4

0.8

1.2

1.6

2.0

2.4 Nt per neutron, p-GaAs

Nt /

(c

m-1)

T (K)

3e151e16

3e16

1e17

3e17

• U- and L-bands broaden as doping is increased

• Electric fields are higher in high-doped diodes (depletion widths are narrower)

• Broadening is suggestive of electric-field enhanced emission due to tunneling

Slide 26

Double DLTS (DDLTS)

Slide 27

Elastic tunneling

Phonon assisted tunneling

Thermal emission

• If emission from a trap is controlled by tunneling, the emission rate will depend on the E-field (and on doping).

• Standard DLTS involves emission from regions with varying E-field, hence DLTS peak shapes will be distorted if tunneling is important.

-5 0 5 10 15 20 25 30 35 40 450

2

4

6

8

10

Ele

ctric

Fie

ld (a

rb u

nits

)

x (arb. units)

Standard DLTSE-field varies from max. value to zeroThe DLTS signal is weighted by x, hence emission at regions near the depletion edge with smaller values of E is favored

-5 0 5 10 15 20 25 30 35 40 450

2

4

6

8

10

Ele

ctric

Fie

ld (a

rb u

nits

)

x (arb. units)

Double DLTS (DDLTS)A differential technique that usestwo DLTS spectra to resolve defects located within a defined (high field) regionof the depletion zone. Region sampled iscontrolled by varying the filling pulse voltage

50 100 150 200 250 3000

2

4

6

8

10

12

SPR pnp DDLTS.opj

E (107 V/m) 2.3 2.0 1.7 1.1 0.7 0.3

C1 -

C2 (f

F)

T (K)

Neutron Damaged n-SiNd = 1e17 cm-3

50 100 150 200 250 300 350 400

0.0

0.5

1.0

1.5

2.0

2.5

eBN56016 DDLTS 4.opj

E (107 V/m) 0.8 0.6 0.4 0.2 Low Field

C1 -

C2 (f

F)

T (K)

DDLTS n-GaAs Nd=1e17 cm-3, Electron Damage

E1 E2

E3

E4E5

E3 emission rateincreases with field

E4/E5 emissionrates increasewith field

DDLTS: Measure Emission Rate at ~ Constant E-field

• Electron damage, no clusters• Emission rates increases with

applied E-field • Attributed to phonon-assisted

tunneling– Schenk, Solid State Electronics 35, 1585 (1992).

• Much smaller rate increase in silicon (Frenkel-Poole emission)

E-field dependent emission rate in GaAs:

Goodman, et al., Jpn. J. Appl. Phys. 33, 1949 (1994).Auret, et al., Semicond. Sci. Technol. 10, 1376 (1995).

Slide 28

Higher Fields:E5 Resembles U-band

50 100 150 200 250 300 350 400

0.0

0.5

1.0

1.5

2.0

2.5

E (107 V/m) 1.8 1.5 1.3 1.0 Low Field

C1 -

C2 (f

F)

T (K)

DDLTS n-GaAs Nd=1e17 cm-3, Electron Damage

E1 E2

E3

E4E5

Slide 29

Higher Fields:E5 Resembles U-band

50 100 150 200 250 300 350 400

0.0

0.5

1.0

1.5

2.0

2.5

E (107 V/m) 1.8 1.5 1.3 1.0 Low Field

C1 -

C2 (f

F)

T (K)

DDLTS n-GaAs Nd=1e17 cm-3, Electron Damage

E1 E2

E3

E4E5

Neutron Damage(low applied field)

Slide 30

Consequences of Tunneling

Slide 31

0 10 20 30 40 50 60 70100

102

104

106

108

1010

GaAs ACRR Currents.opj

200 K240 K300 K

J /

W (A

/cm

3 )

qV / kT

400 K

Neutron DamagedSolid: 3 x 1017 cm-3

Open: 3 x 1016 cm-3

Lines: SRH

2 4 6 8 1010-29

10-24

10-19

Neutrons, C 10 MeV,Ge 28 MeV, Si 28 MeV

SRH3 MeV e_

IV 3e17.opj

Cur

rent

/ V ca

lc (A

cm

3 )

1000 / T (K-1)

n-GaAs Nd = 3 x 1017 cm-3

+0.8V bias

20 MeV e_

• Larger GR currents at high doping due to higher E-fields• Larger GR currents at lower T SRH is small, but tunneling ~ constant

with temperature• Decrease in excess GR currents as forward bias increases due to

lower E-field

Conclusions: Silicon• Clustered damage

– DLTS spectra of clustered damage similar to point defect spectrum

• EOR Si ions produce DLTS spectrum close to that of neutron damaged– Additional V2-like defects – exact structures are not known

• “Strained V2” – No double acceptor, very broad annealing profile• “Bistable V2” – Annealing kinetics match gain annealing kinetics

– Metastable levels: E4, E4, H1 (like V2) Single stable level: E75 (very shallow)

• Use annealing to de-convolve defects in BJT base that affect gain: VP, V2* and V2 – Requires knowledge of annealing in low-doped collector

• Establish ion-neutron equivalence– Simple scale factor relates neutrons & EOR Si ions

Slide 32

Conclusions: GaAs• Emission from GaAs defects is highly electric-

field dependent– Consequences on both DLTS and device currents

• DLTS– Electric-field broadened DLTS of electron-damaged diodes

resembles DLTS of neutron damage diodes– U- and L-bands (neutron & ion damage) are likely a

combination of field-broadened DLTS & the effects of additional electric fields from damage clusters

• Device currents– Excess currents in forward biased diodes beyond standard

SRH

Slide 33

Acknowledgements

D. King K. McDonaldE. Bielejec G. PatriziJ. Campbell D. SerklandS. Myers G. VizkeletheyN. Kolb W. Wampler

Special thanks to:D. V. Lang C. H. Seager

Slide 34