Isochronal annealing of n-irradiated
300 µm FZ and MCz diodes
- effect on the trapping time constant -
3rd Workshop on Defect Analysis in Radiation Damaged Silicon Detectors
Hamburg 17. – 19. April 2008
Marie Kristin Bock, Eckhart Fretwurst , Gunnar Lindström ,
Institute for Experimental Physics, Universtity of Hamburg
Marie Kristin Bock – Institute for Experimental Physics
1
Samples
FZ <111>
MCz <100>
w337-I6
w337-N1
8556-04-26
8556-04-9
3 E13
1 E14
3 E13
1 E14
Fluence [n / cm2]
286.91
288.64
299.20
299.84
Resistivity [Ohmcm]
1.98E+03
2.10E+03
1.01E+03
1.05E+03
N_eff,0 [1/ cm3]
2.17E+12
2.05E+12
4.28E+12
4.11E+12
Thickness [µm]
CV / IV measurements:
measurements:
Frequencies
10 & 100 kHz
V_dep, N_eff
Guard Ring
grounded & floating
I_leak
TCT =„ Transient Current Technique“:
Technique“:
Laser pulse:
1.2 ns width
830 nm
~ 9 µm penetration depth
Alpha particles:
V_det
244Cm
p+
n n+
V_dep, e, h
4.8 MeV deposited energy
23 µm penetration depth
„Charge Correction Method“
Tau_eff, e, h
Marie Kristin Bock – Institute for Experimental Physics
University of Hamburg
2
• material used:
•altogether 4 p+nn+ diodes: 2 standard FZ & 2 MCz
• 300 µm thickness
•irradiated with 3 E13 as well as 1 E14 neutrons/ cm2
• experimental methods:
•CV/IV with frequencies 10 & 100 kHz ;
•guard ring grounded and floating respectively
•TCT with 830 nm Laser ;
•illumination on front & rear side
•penetration depth 9 µm
with alpha particles from 244Cm source;
• illumination on front side
• 4.8 MeV deposited energy
• penetration depth 23 µm
2
Motivation
z
Which microscopic defects are responsible for the trapping of charge carriers ?
z
Annealing study with temperatures > 80 °C
z
surprising result, presented on 2nd RD50-WODEAN Workshop Vilnius 2007 by
G. Kramberger :
Abb.:
G. Kramberger,
Jozef Stefan Institut,
Ljubljana,Slowenien
Annealing of electron trapping
with increasing temperature
1.
Are these results reproducible ?
Isochronal Annealing : steps of 20°C for 30 min starting at 80°C
2.
Correlation with microscopic defects
University of Hamburg
prospective aim
Marie Kristin Bock – Institute for Experimental Physics
3
• motivation
•during the last WODEAN workshop in Vilnius 2007,
Gregor Kramberger presented first results from an isochronal
annealing study with temperatures from 80 up to 220 °C
his conclusion:
Reduction of electron trapping with rising annealing temperature !
•first aim of this work:
similar study (isochronal annealing, for 30 min 20°C steps,
starting at 80°C) to look if these results are reproducible
•prospective aim:
correlation of trapping times with annealing behaviour of
microscopic defects (as seen in TSC measurements)
3
Experimental Results:
Inversion of Space Charge Sign
ƒ base line corrected pulses: FZ 3 E13 n/cm2
before annealing:
30 min @ 80 °C:
30 min @ 100 °C:
electron
signals:
hole
signal:
Double Junction
Effect ?
nearly zero slope
of hole signal
Type Inversion:
p+ p n +
University of Hamburg
Marie Kristin Bock – Institute for Experimental Physics
4
• Pulse shapes:
FZ 3 E13 n/cm2
• each figure shows the uncorrected pulse shapes for the first 3 annealing
steps (after substraction of base line in each case)
• electron dominated current pulses in the upper row (front incidence)
hole dominated current pulses second row (rear incidence)
• because of the relatively high penetration depth of the 830 nm Laser for
each current pulse there is always a preceding peak of the respective
oppositely charge carrier
• therefore presumably there is no double junction distinguishable for the
first two annealing steps
• but after 100 °C the diode is clearly type inverted (p+pn+)
(slope of e-pulse changes from negative to positive!)
4
CV/ IV : FZ 3 E13 n/cm2
Experimental Results:
FZ 3*1013 n/cm2 : Depletion Voltage
2
13
C_end [pF]
120
U de p [V ]
FZ 3E13 n/cm : Capacitance
Guard Ring floating
TCT Front
deconv 10 kHz
serial
160
TCT Back
deconv 10 kHz
serial
80
40
12.5
12
11.5
CV gr grd 10
kHz serial
0
0
20
40
60
80
11
before
80
100
120
140
160
180
T_anneal [°C]
100 120 140 160 180
Anneal.Temp [°C]
2
FZ 3E13 n/cm : Effective Doping Concentration
Guard Ring grounded
N_eff [1/cm 3]
1.E+12
0.E+00
-1.E+12
-2.E+12
-3.E+12
0
20
40
60
80
100
120
140
160
180
T_anneal [°C]
University of Hamburg
•
Marie Kristin Bock – Institute for Experimental Physics
5
Results from CVIV measurements:
FZ 3 E13 n/cm2
•
bottom diagram: as shown on the slide before, the effective doping
concentration gets negative somewhere between 80 and 100°C annealing
temperature because of type inversion
•
in the upper left picture the annealing behaviour of the depletion voltage
U_dep is plotted
•
comparison of the U_dep values determined by
1. CV measurement, 10 kHz serial mode
2. TCT (collected charge vs. square root of applied detector voltage):
a) front illumination (electron dominated)
b) rear illumination (hole dominated)
except for the last annealing step all values from TCT are in
very good agreement with those from CV
•
upper right diagram: contrary to expectation the capacitance at U_dep shows
a slight decrease; this fact was accounted for in the analysis of TCT pulses
concerning the deconvolution of the measured pulse shapes before applying
the „Charge Correction Method“
5
Experimental Results:
Inversion of Space Charge Sign
ƒ base line corrected pulses: FZ 1 E14 n/cm2
electron signals:
30 min @ 80 °C:
before annealing:
30 min @ 100 °C:
already
type inverted
T_anneal [°C]
University of Hamburg
•
Marie Kristin Bock – Institute for Experimental Physics
6
Pulse shapes:
FZ 1 E14 n/cm2
•
again, in the upper row the electron dominated pulses (front illumination) after
the first three annealing steps are shown
•
in this case, due to the 3 times higher fluence the diode is obviously type
inverted from the beginning
(positive slope of the pulse shape, except for the short hole contribution)
•
in the bottom graph the green line shows the depletion voltage during
annealing study measured with CV technique, compared to U_dep from TCT
measurements.
6
CV/ IV : FZ 1 E14 n/cm2
Experimental Results:
Relaxation of Depletion Voltage, bistable effect:
T_anneal [°C]
2
N_eff [1/cm 3]
FZ 1E14 n/cm : N_eff
Guard Ring grounded; after 24 h RT
0.E+00
-5.E+12
-1.E+13
-2.E+13
0
20
40
University of Hamburg
60 80 100 120 140 160 180
T_anneal [°C]
Marie Kristin Bock – Institute for Experimental Physics
7
•Results from CVIV measurements:
FZ 1 E14 n/cm2
• in this slide the relaxation of the depletion voltage due to the „bistable
defect“ is demonstrated, an especially important feature for the FZ material
• the graph in the upper half shows a comparison of the depletion voltage
(from C/V at 10 kHz) measured directly after annealing and after keeping
the diode for 24 h at room temperature (RT);
additionally the U_dep values evaluated from C/V at 100 kHz are
included (green triangles before & magenta squares after 24 h RT)
• the lower left picture shows the same relaxation effect as pltted for the
effective doping concentration
• conclusion: the decrease of depletion voltage within 24 h at RT after each
annealing step is a pronounced effect in FZ material and increases with
increasing temperature
• note: except for one value, the capacitance at U_dep stays nearly constant
7
Experimental Results:
TCT pulses & CV/IV MCz 3 E13 n/cm2
ƒ base line corrected pulses: MCz 3 E13 n/cm2
electron signals:
before annealing:
30 min @ 60 °C:
no type inversion up to now
T_anneal [°C]
University of Hamburg
Marie Kristin Bock – Institute for Experimental Physics
8
•Pulse shapes & CVIV measurements:
MCz 3 E13 n/cm2
• this diode is so far only annealed at 60 and 80 °C
• no type inversion yet
• after a short increase for the first annealing step at 60 °C, the depletion
voltage starts to decrease
• type inversion expected at around 160 °C
8
Experimental Results:
Inversion of Space Charge Sign
ƒ baselinecorrected pulses: MCz 1 E14 n/cm2
before annealing:
30 min @ 80 °C:
Double Junction
Effect
Type Inversion:
electron signal
p+ p n +
hole signal
30 min @ 100 °:
University of Hamburg
Marie Kristin Bock – Institute for Experimental Physics
9
•Pulse shapes:
MCz 1 E14 n/cm2
• obviously the electron dominated current pulses measured before and after
30 min annealing at 80 °C show a typical double peak structure indicating a
double junction of the electric field inside the silicon bulk
• the two figures below show the electron and hole dominated pulses
respectively after the annealing step at 100 °C. The pulse slopes for front
and rear illumination are the results of type inversion.
9
Experimental Results:
CV/ IV : normalised current
2
FZ 3E13 n/cm : Damage Rate;
Guard Ring grounded
6.00
FZ 3 E13 n/cm2
alpha [10^17 As/cm]
5.00
FZ 1E14 n/cm2
4.00
MCz 3 E13 n/cm2
3.00
MCz 1 E14 n/cm2
2.00
1.00
0.00
20
40
60
80
100
120
140
160
180
200
T [°C]
University of Hamburg
Marie Kristin Bock – Institute for Experimental Physics
10
• Damage Rate:
• for all four diodes the current related damage rate α (reverse current,
normalised to diode volume and fluence), follows the same trend, starting
around 5.00 E-17 A/cm and decreasing steadily
• Behaviour as expected
10
Experimental Results:
Electron Trapping
FZ: Front 830 nm;
1/Tau [1/ns]
0.04
1*10^14 cm^-2
7.5*10^13 cm^-2
0.03
3*10^13 cm^-2
0.02
0.01
0
20
40
60
80 100 120 140 160 180 200 220 240
Anneal.tem p. [°C]
Marie Kristin Bock – Institute for Experimental Physics
University of Hamburg
•
11
Trapping Probability:
electrons in FZ:
1. The trapping probabilities (inverse trapping time constant) are increasing with
higher fluence, including the values for 7.5 E13 n/cm2 obtained by G.
Kramberger (green dots)
2. For all FZ diodes the trapping probablity decreases with increasing annealing
temperature
11
Experimental Results:
Hole Trapping
FZ: Back 830 nm;
0.12
1*10^14 cm^-2
7.5*10^13 cm^-2
1/Tau [1/ns]
0.1
3*10^13 cm^-2
0.08
0.06
0.04
0.02
0
20
40
60
80 100 120 140 160 180 200 220 240
Anneal.tem p. [°C]
Marie Kristin Bock – Institute for Experimental Physics
University of Hamburg
12
•Trapping Probability:
holes in FZ:
• especially for the lowest fluence it is clear, that the trapping probability
stays nearly constant during the annealing study, a very small negative
trend to be observed for the larger fluences, needing further checks
12
Experimental Results:
Electron Trapping
MCz: Front 830 nm
0.06
1*10^14 cm^-2
3*10^13 cm^-2
1/Tau [1/ns]
0.05
0.04
0.03
0.02
0.01
0
20
40
60
80
100
120
140
160
Anneal.tem p. [°C]
Marie Kristin Bock – Institute for Experimental Physics
University of Hamburg
•
13
Trapping Probability:
electrons in MCz:
1. Also in the case of the MCz diodes the trapping probability is decreasing with
increasing annealing temperture in a similar way as shown for FZ.
2. Moreover also here the trapping is in general increasing with larger fluence.
Exception: The first point for the diode irradiated with the lower fluence is
three times larger than expected.
3. there is no explanation for this unexpected value up to now.
This measurement was repeated with a similar diode (same wafer, same
fluence) resulting in the same value!
13
Experimental Results:
Hole Trapping
1/Tau [1/ns]
MCz: Back 830 nm
0.14
1*10^14 cm^-2
0.12
3*10^13 cm^-2
0.1
0.08
0.06
0.04
0.02
0
20
40
60
80
100
120
140
160
Anneal.tem p. [°C]
Marie Kristin Bock – Institute for Experimental Physics
University of Hamburg
•
14
Trapping Probability:
holes in MCz:
1. for the holes the situation is completely different and unexpected
as compared to FZ
2. the trapping probability stays nearly constant only up to 80 °C,
increasing during further annealing and seems to saturate for higher
temperatures
14
Experimental Results:
Charge Collection
2
MCz 1E14 n/cm TCT Alpha
0.22
Non Irradiated
Q_coll [10^-16 As]
0.2
Before Annealing
30@80
0.18
30@100
30@120
0.16
30@140
0.14
0.12
0.1
0
100
200
300
400
500
600
700
800
U_det [V]
University of Hamburg
Marie Kristin Bock – Institute for Experimental Physics
15
•Collected Charge vs. Bias-Voltage for alpha injection on front side
MCz 1 E14 n/cm2:
• even after starting the annealing procedure the collected charge drops
to relatively low values and then doesn‘t seem to vary much with increasing
temperature
• totally unexpected behaviour; no explanation so far
15
Summary
ƒ annealing of electron trapping
Comparison of Trapping Probabilities
2
1E14 n/cm & Front 830 nm
0.05
MCz
0.04
verified !
1/Tau [1/ns]
FZ
0.03
G.K.: 7.5*10^13
cm^-2
0.02
ƒbut...
0.01
™ unexpected behaviour of hole
0
20
40
60
80
100
120
140
160
180
trapping in MCz
200
Anneal.temp. [°C]
™ no explanation on microscopic
level so far
Comparison of Trapping Probabilities
2
1E14 n/cm & Back 830 nm
0.14
MCz
0.12
FZ
1/Tau [1/ns]
0.1
0.08
G.K.: 7.5*10^13
cm^-2
0.06
™ CCM at temperatures > 180 °C
not applicable (for investigated
diodes)
0.04
0.02
0
20
40
60
80
100 120 140
Anneal.temp. [°C]
University of Hamburg
160
180
200
Marie Kristin Bock – Institute for Experimental Physics
16
• to have it in a nutshell:
• the decrease of electron trapping probability was verfied
• totally unexpected behaviour of hole trapping in MCz
• so far no promising candidates found by means of TSC
measurements which would explain the observed trapping times
16