radiation-induced trap spectroscopy in si bipolar transistors and gaas diodes
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Radiation-Induced Trap Spectroscopy in Si Bipolar Transistors and GaAs Diodes. 22 nd RD50 Workshop University of New Mexico June 3-5, 2013 Robert Fleming Sandia National Labs. Outline. Introduction Research Objectives DLTS & transistor fundamentals Silicon - PowerPoint PPT PresentationTRANSCRIPT
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