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Carrier recombination and emission lifetimes in heavily irradiated pad–detectors and their impact on operational characteristics of pin
diodes
E.Gaubas1, T.Čeponis1, R.Grigonis2, A.Uleckas1, and J.Vaitkus1
1Vilnius University, Institute of Applied Research, Vilnius, Lithuania (VU)2Vilnius University, Laser Research Center, Vilnius, Lithuania (VU)
Outline
Carrier capture (MW-PCD/E) and emission (I-V) lifetime variations
Barrier capacitance variations with fluence (BELIV)
Time and spectral resolved priming of BELIV transients
Summary
Recombination lifetime during and after irradiation
During protons irradiation/ protons pre-irradiated (MW-PCD) - wafer samples
After irradiation - wafer and diode samplesirradiated by neutrons and protons
100
101
102
10-2
10-1
100
experiment
pre-irradiated >1012
cm-2
6 MeV protons fixed 1.5 nA
simulated
Ncl,0
=1012
cm-2
R (s
)
Exposure time (s)
1013
1014
1015
1016
100
101
102
103non-irradiated
Fluence (cm-2)
Neutron irradiated FZ diodes n-epi diodes p-epi diodes
(ns
)
10-2 10-1 100 101 10210-2
10-1
100
101
102
=1
=1/3
=2
Exposure time under irradiation by protons (s)
R
(s
)
MCZ Si wafer8 MeV protons at 285K :
fixed 4nA varying current by steps
10-1 100 101 10210-2
10-1
100
101
102
103
experiment varying current
0.2 -4 by steps fixed 4 nA
simulated
R,cluster,
c=0.33, n-fixed, 0=10-18 cm2
R (s
)
Exposure time (eqv. 4 nA) (s)
- diode samples with applied field
n<n0
n<ni<<n0
10 1000
2
4
6
ICDC MW-PCT-E R
ICD
C, M
W-P
CT
-E (s
)
texposure
(s)
recombination prevails
recombination competes with multi-trapping effect
1012 1013 1014 1015 101610-1
100
101
102
103
104
Okmetic MCZ<100> 1kOhm·cm 300 m MWR, CIS 8556- 14 wafers 1 - 63 passivated CIS 8556- 14 wafers measured by DG technique CIS 8556-01 diodes neutron irradiated
R (
ns)
Fluence (n/cm2)
M=0/en0
Impact of recombination within ENRwith R M might be crucial (if n0~1/) on functionality of junction
Carrier capture-recombination-generation lifetimes (simple S-R-H approach)
ni/ND
20 10 0 10 201
10
10015
1
taug x( )
taur x 0.01( )
taur x 0.001( )
taur x 0.0001( )
2020 x(EDL-Ei)/kT
Gen
erat
ion
lif
etim
e
Rec
om
bin
atio
n li
feti
me
re
c, g
en
]cosh(2
1[1
kT
EE
n
n
NvR
n iDLei
DLrec
DL
iDL
igen NvkTEE
Gn
)cosh(2
/
at nvn pvp
Single-type prevailing traps
of M=NDL<<n0
n-Si
Photo-injection URT
ne pe > ni2
ne · pe < ni2ne · pe - ni
2 =0
n-Si
Photo-injection URT
ne pe > ni2
ne · pe < ni2ne · pe - ni
2 =0
Without applied E field
With applied dc E field
Eqv.R>0
G<0
S-R-H is limited by conditions:i) M<<n0. then traps are filled by n0 without change in n0;
ii) single type centers dominateiii) traps do not interactiv) charge on traps can be ignored relatively to dopants one
Thus, validity of S-R-H conditions should be estimated in applications
J.S. Blakemore, in: Semiconductor Statistics, Ch. 8, Pergamon Press, (1962)
A.Rose. Concepts in photoconductivity and allied problems. Interscience Publishers, John Wiley & Sons, New York-London, 1963.
Traps with barriers, multitrappingmulti-charge/multi-valency defects
Traps with exp distributed levels Redistribution of carriers via interaction of traps Photo-, thermo- quenching effects
EC
EVx
n0
M
Carrier recombination lifetimes (for M>>n0) Single-species (type) traps
1
0
1
000
1
00000
11
1)(
n
N
N
nMnp
N
nMNnPP
cM
cM
cMcMpvMn
p
1
0
1
000
1
00000
11
1)(
p
P
P
pMnp
P
pMPpNn
vM
vM
vMvMncMp
n
M0, n 0, S-R-H
11100
0
p
PM
M
n
NM
e
Mm
vMcMkT
FM
00
00
00
00 pn
Pp
pn
Nn vMn
cMpnp
M, M>>n0, S-R-H invalid
000
01
1 pp
cMpp mn
N
MmMp
P
nn
n
vMnn
1
)(
11 0
000
MMv nrec
11
C
MC
C
MC
gen NkT
EE
NvkT
EE
)exp()exp(
Although relaxation to equilibrium/steady-stateis kept by M=pM+nM
n p
J.S. Blakemore, in: Semiconductor Statistics, Ch. 8, Pergamon Press, (1962)
cE
cefhf dEEgEPEEPNr
c
c
E
ce
E
ce
n
dEEgEP
dEEgEPE
Capture coefficient <n> should be used
instead of v within rigorous analysis
fhfn EPNnr EC
EVx
Only shallow dopants of proper densityare able to support fast
operation (w(U)) of a diode ~1/M <1/em
to ensure maj. carriers on level are in equilibrium with band
Carrier recombination=capture lifetimes (for M>>n0)
Single-species (type) traps
100 150 200 250 3000
100
200
300
400
DLS
(m
V)
T (K)
Electron irradiation
Cz Si (doped 1014 cm-3)
1015 e-/cm2
1016 e-/cm2
Only deep slow levelscan be filled
and observed as carrier emitters
0.0 0.1 0.2 0.3 0.4 0.5 0.610-13
10-11
10-9
10-7
10-5
10-3
10-1
R
SC
cp
M
em, cp
, S
C, R
, M
(s)
E (eV)
ND=1012 (cm-3)
em
cp
, NT=8x1011 cm-3
cp
, NT=1014 cm-3
SC
R, N
T=8x1011 cm-3
M
em
NT=8x1011 cm-3
NT=1014 cm-3
dX Xinvalid
Space chargegeneration current
Interaction of several type centers appearsdue to carrier redistribution through bands
Carrier recombination lifetimes for Mi,s>n0 multi-valency(i)/multi-species(s) centers
Interaction of the whole system of centers appears due to carrier redistribution through bands, by inter-center recombination (capture-emission) and via charging /configurational transforms of defects
System neutrality is supported by free and localized charges/fields.Relaxation is long and complicated. It is similar to the random-walk processes in disordered materials.
0 2000 4000 6000 8000
10-1
100
2
1
1 CPC excited with 355 nm light pulse of 30 ps2 MWA excited with 355 nm light pulse of 30 ps
t (s)
Nor
mal
ized
pho
tore
spon
se U
CP
C, M
WA
/Um
ax
0 2 4 6100
101
experimental decayin stretched-exponent time scale
UC
PC
(ar
b.un
its)
t=0.7 (ms)
Relaxation is similar to multi-exponential in any narrow display segmentDifferent techniques may give different lifetime values
A single lifetime parameter se can be extracted only when stretched-exponent time scale is employed
The stretched-exponent model is widely used UCPC =U0exp[–(t/se)]
S.Havlin and D.Ben-Avraham, Advances in physics 51, 187 (2002).L.Pavesi, J. Appl. Phys. 80, 216 (1996).
E. Gaubas, S. Juršenas, S. Miasojedovas, J. Vaitkus, and A. ŽukauskasJOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 8 15 OCTOBER 2004
Carrier generation/emission lifetime (for M>>n0)
1013
1014
1015
1016
10-6
10-5
10-4
10-3
IL|UR=100V
IL|UR=200V
1/IL|UR=100V
I L (A
)
(n/cm2)
103
104
105
106
g~1/
I L (A
-1)
-200 -150 -100 -50 0 5010-13
10-11
10-9
10-7
10-5
10-3
I
(A)
U (V)
=1013 cm-2
=1014 cm-2
=1016 cm-2
b
Qualitative emission lifetime dependence on fluencecan be estimated from I-V
1012 1013 1014 1015 101610-1
100
101
102
103
104
Okmetic MCZ<100> 1kOhm·cm 300 m MWR, CIS 8556- 14 wafers 1 - 63 passivated CIS 8556- 14 wafers measured by DG technique CIS 8556-01 diodes neutron irradiated
R (
ns)
Fluence (n/cm2)
Nearly linear reduction of generation lifetimewith enhancement of fluence is similar to that of
of recombination lifetime characteristic
Examined MW-PCT characteristics imply prevailing of intricate system of defects and reduction of majority carriers. The recombination capture lifetimes become shorter than dielectric relaxation time.
Carrier emission lifetime decrease (increase of leakage current – at UR in I-V), follows capture lifetime reduction (increase of serial resistance R~1/n0 - at UF in I-V), and both manifest a close to a linear decrease with enhancement of fluence
Items to clarify: Whether diode/detector is functional under heavy irradiations? What is a system of defects and levels, which governs extraction of carriers (UR) and state of material? Which models are acceptable for prediction of charactreristics?
The Barrier Evaluation by Linearly Increasing Voltage (BELIV) transient technique has been employed to clarify, how a reduction of carrier capture lifetime and emission affects junction and material
GLIVpin
diode
RL=50 oscilloscope
GLIVGLIVpin
diode
RL=50 oscilloscope
2/30
)1(
21
)()(
bi
bib
bbC
U
At
U
At
ACU
CUC
t
U
dt
dqti
BELIV technique
Abrupt junction
Reverse bias
3/4,0,
],
1[
],3
21[
LgUbi
AtLgUbi
At
ACi LgbLgC
Linearly grade junction
2/10
2/30 )1()1(
)1(
21
)()()()(big
iTBk
eAt
diff
bi
bibgdiffCR U
AtSwenei
U
At
U
At
ACtitititi
])0()0(2
3)
4
)0(()0(
4
)0([
)0(2
gcc
gC
g
bie ii
ii
i
Ai
Ut
te
dxxt
xitit
RCC
RCCM
0]
)(exp[)(
1)(
dt
tdUCtUn
d
Se
dt
dUC
ndeS
dt
dU
d
S
dt
dp
dt
dndeSti C
geomCnC
geomtr
CFD
)()(
22)(
2|)(| 0
00
Variations of BELIV transients with temperature and priming by steady-state IRBI
as well as dc UF
Short carrier capture lifetime reduces n0ND and increases a serial resistance of ENR. Supply of majority carriers from rear electrode by dc UF priming (due to shrinkage of depletionw width, forward current) restores a junction.
Combined priming of BELIV transientsby temperature reducing (increased e) and by IR illumination
(n0) leads to restore of a junction
Reduction of temperature increases (e) and decreases space charge generation current, however, Cb0Cg at UC<0.3 V
Priming by IR illumination increases n0 and barrier capacitance observed in BELIV transientsrestores a junction but enhances leakage current
when fast carrier capture/emission is present
Barrier capacitance as a function of fluence extracted at 300 K
1012 1013 1014 1015 1016
101
102
103
Cgeom
C
b0 (
pF)
Fluence (n/cm2)
Cb0
: BELIV, PL
=1.4 sNeutron irradiated diodes
reverse bias forward bias
50 100 150 200 250101
102
Neutron irradiated diodesf=100 kHz, U
ac=1V
Cp, =1012 cm-2
Cs, 1012 cm-2
Cp, 1014 cm-2
Cs, 1014 cm-2
Cp, 1016 cm-2
Cs, 1016 cm-2
C (
pF)
UR (V)
0 50 100 150 U (V)10
100
C (pF)
MCz Si diodai, T=300KU
ac=20 mV, f=100 kHz
=1012 cm
-2:
Cs Cp
=1013 cm
-2:
Cs Cp
=1014 cm
-2:
Cs Cp
=1016 cm
-2:
Cs Cp
C-V’s as a function of fluence at 100 kHz and 300 Kdisplacement in barrier capacitance is controlled by (LRC) measurements of phase shift for the ac test signal at fixed frequency in routine C-V
Applicability of LRC measured C-V is doubtful
for diodes irradiated with > 1E13 n/cm2
Barrier partially recovers by n0 priming with IR, dc UF and combined priming with temperature (emission lifetime) decreasing only in diodes irradiated with fluence of <1014 n/cm2. Short carrier capture and emission times determine low barrier capacitance (capability to to collect charge (transient) at fixed voltage) and large space charge generation (leakage) current in heavily irradiated diodes. The space charge generation current prevails in heavily irradiated diodes over barrier charging (displacement, which is controlled by measurements of phase shift for the ac test signal in routine C-V), therefore applicability of C-V technique is doubtful for control of heavily irradiated detectors. Carrier capture and emission lifetimes are short, and barrier capacitance decreases to geometrical its value at low (U< Ubi) applied voltage of the diodes with enhancement of fluence. Operation of a diode is similar to that of capacitor.
Items to clarify: Whether diode/detector of the present design is functional after heavy irradiations?- doubtful What is the system of defects and levels, which governs extraction of carriers (UR) and state of material?- material becomes similar to insulator. Additonal issues: - if there are filled levels those compensate material; - how rapidly these levels are able to response to external voltage changes Which models are acceptable for prediction of characteristics?
The BELIV technique with spectrally resolved fs pulsed IR (1.1 – 10 µm) biasinghas been employed to clarify what is a system of levels and if these levels are filled
EC
EV
EC
EV
Variations of BELIV transients by pulsed IR of varied spectrum
mono-polar photoconductivity
change of charge state- capacitance
OPO DFG fs pulse
To approve principlesstructure with known technological defects
For qualitative understanding(within depletion approximation):
Cb~(ND=n0)1/2~1/w(n0)
Cb(t) ~1/M=(en0|tr)/0
(using extended depletion approximation)
OPO DFG wavelength/quantum for which appears suppression of traps, i.e. recover of BELIV transient of Cb,indicates a system of filled single type deep levels
0 20 40 60
0.0
0.1
0.2
UB
ELI
V (
a.u.
)
t (s)
Si junctionpulsed bias illumination OPO-DFG
pulse= 100 fs, rep rate 1kHz
ex
= 4 m, Epulse = 30 J
IR limit h= 0.31eV
Carrier capture rate
)]()([22
22,
0
0
ENREEe
e
Une CC
drM
for transient processes
Variations of BELIV transients by pulsed IR of varied spectrumin neutron irradiated detectors
The shallowestE1 h 0.3 eV
E3 h 0.41eV
E4 h 0.51eV
E2 h 0.36 eV
Variations of BELIV transients by pulsed IR of varied spectrumin neutron irradiated detectors
0 5 10 15 20
0.00
0.03
0.06
0.09
0.12
MCz Si diode irradiated 1014 n/cm2
ULIV
= 12V
ex
= 1.15m 1.3 1.5 2.4 2.5 2.6
BE
LIV
sig
nal (
a.u.
)
t (s)
The most shallow filled Ered threshold =h 0.5 eV
Edeepest -h 1.08eV
0 3 6 9
0.00
0.03
0.06
0.09
0.12
MCz Si diode irradiated 1016 n/cm2
ULIV
= 12V
ex
= 1.2m 1.5 2.4 2.6 3.0
BE
LIV
sig
nal (
a.u.
)t (s)
The most shallow filled, or a half of two step generation
Ered threhold =h 0.52 eV
Edeepest -h 1.08eV
For 1E14 n/cm2 and h> 0.83 eV possiblepartial recovering of a barrier, while for h≤ 0.5 eV space charge generation current prevails (rapidcapture/emission processes
For 1E16 n/cm2 only space charge generation current increases (extremely rapidcapture/emission processes) while barrier capacitanceis close to Cgeom
A clear structure of deep levels is absent in heavily irradiated diodes >1014 n/cm2 while carriers are generated by inter-band excitation.
Items to clarify: Whether diode/detector of the present design is functional after heavy irradiations?- doubtful What is the system of defects and levels, which governs extraction of carriers (UR) and state of material?- material becomes similar to insulatorAdditonal issues: - if there are filled levels those compensate material; - no, high density of various species levels is more probable those are only weakly filled by small n0
- how rapidly these levels are able to response to external voltage changes - fast capture of excess carrier and fast space charge generation current response
But system relaxes to equilibrium state very slowly – as estimated from I-V point-by-point measurements at T<150 K
Which models are acceptable for prediction of characteristics?
The disordered material models-?
Summary
Examined MW-PCT characteristics imply prevailing of intricate system of defects and reduction of majority carriers. The recombination capture lifetimes become shorter than dielectric relaxation time. Carrier emission lifetime (increase of leakage current – at UR in I-V), follows capture lifetime (increase of serial resistance R~n0 - at UF in I-V)
Barrier capacitance decreases to geometrical value of the diodes with enhancement of fluence. Carrier capture and emission lifetimes are short. The pointed system of deep levels can be revealed only in diodes irradiated with fluence of <1014 n/cm2.
A clear structure of deep levels is absent in heavily irradiated diodes >1014 n/cm2 while carriers can be generated by inter-band excitation.
Thanks to G.Kramberger for neutron irradiations.E.Tuominen, J.Harkonen and J.Raisanen are appreciated for samples (substrates and pin diodes) as well as for proton irradiations.
Thank You for attention!
)(1
02
2
xdx
d
xy
yy
dzzzdzzyx
)()(
1)()(
0
)()( xnNex d
kT
xenN
e
dx
xdd
)(exp
)(0
02
2
kT
xeN
e
kT
e
kTxN
exE dd
)(exp)(
2)(
0
2
)(
0
1 )(xx
x
dEdxd
ekTx /)( assumption
)(2
)(0
2 xeN
xE d
2
0
)(2
)( xxeN
x dd
2
02 dd x
eNV
)()(
0
xxeN
dx
dxE d
d
Depletion aproximation for material
containing only dopants:
only numerical integration
n
p
d
m
x
x
x
x
dzzdzz 0)()(
n
p
x
x
dxxxV )(1
)(0
2
2
1exp1)(
D
dd L
xxeNx
2
2
1exp
D
dd L
xxN
Limitations:for steady-state
2
0
2
)(2
exp)( xxkT
NeNxn d
dd
e
kT
L
xV
D
d
2
2
12
1
20
d
D Ne
kTL
Utrtr
M 22
2
tr
for transients
gives depletion approximation
P.Blood and J.W.Orton. The electrical characterization of semiconductors: majority carriers and electron states, (Academic Press, London –San Diego-New York, 1992).
Transient currents in depletion region
in deep traps containing material
x
x
c dt
dnxxedx
dt
dnexJ
2
))(()()( 2
dt
dpxWe
dt
dnxexJ c )()()(
)()()(),(00
tNe
tnNNxWe
txE ctt
dt
dNe
dt
dnxWe
t
D ct
x
)(
WtNW
tnNNe
V ctt )(2
))((2
0
dt
dnW
dt
dN tc
2
dt
dnWxe
t
D t
2
dt
dp
dt
dneW
dt
dn
dt
dpWx
dt
dpxW
dt
dnxetJ
22)()(
trTest harmonic signalUac<kT/e, to evaluate Cb,, by control of phase shift avoiding xd>tr , i.e.desirable regime EC(x)-EC(xd)<kT.
P.Blood and J.W.Orton. The electrical characterization of semiconductors: majority carriers and electron states, (Academic Press, London –San Diego-New York, 1992).