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>
Fluence [n / cm 2 ]
Thickness [µm]
Resistivity [Ohmcm]
N_eff,0 [1/ cm3] w337-I6
3 E13
286.91
1.98E+03
2.17E+12
CV / IV measurements : :
Frequencies 10 & 100 kHz
Guard Ring grounded & floating w337-N1
1 E14
288.64
2.10E+03
2.05E+12
TCT =„ Current Technique “:
Laser pulse:
Alpha particles:
1.2 ns width
830 nm
~ 9 µm penetration depth
244 Cm
4.8 MeV deposited energy
23 µm penetration depth
MCz <100>
8556-04-26
3 E13
299.20
1.01E+03
4.28E+12
V_det p+ n n+
8556-04-9
1 E14
299.84
1.05E+03
4.11E+12
„Charge Correction Method“
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
V_dep, N_eff
I_leak
V_dep, e, h
Tau_eff, e, h
• 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/ cm 2
• 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 244 Cm source;
• illumination on front side
• 4.8 MeV deposited energy
• penetration depth 23 µm
2

Motivation
Which microscopic defects are responsible for the trapping of charge carriers ?
Annealing study with temperatures > 80 °C z surprising result, presented on 2nd RD50-WODEAN Workshop Vilnius 2007 by
G. Kramberger :
Abb.:
Annealing of electron trapping with increasing temperature
G. Kramberger,
Jozef Stefan Institut,
Ljubljana,Slowenien
1.
Are these results reproducible ?
Isochronal Annealing : steps of 20°C for 30 min starting at 80°C
2.
Correlation with microscopic defects prospective aim
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
• 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/cm 2 before annealing: 30 min @ 80 °C: electron signals:
30 min @ 100 °C: 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
•
Pulse shapes:
FZ 3 E13 n/cm 2
• 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

Experimental Results: CV/ IV : FZ 3 E13 n/cm 2
FZ 3*1013 n/cm2 : Depletion Voltage
160
120
80
40
0
0 20 40 60 80 100 120 140 160 180
Anneal.Temp [°C]
TCT Front deconv 10 kHz serial
TCT Back deconv 10 kHz serial
CV gr grd 10 kHz serial
FZ 3E13 n/cm
2
: Capacitance
Guard Ring floating
13
12.5
12
11.5
11 before 80 100 120
T_anneal [°C]
140 160 180
FZ 3E13 n/cm
2
: Effective Doping Concentration
Guard Ring grounded
1.E+12
0.E+00
-1.E+12
-2.E+12
-3.E+12
0 20 40 60 80 100
T_anneal [°C]
120 140 160 180
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
•
Results from CVIV measurements:
FZ 3 E13 n/cm 2
• 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/cm 2 electron signals: before annealing: 30 min @ 80 °C: 30 min @ 100 °C: already type inverted
University of Hamburg
T_anneal [°C]
Marie Kristin Bock – Institute for Experimental Physics
•
Pulse shapes:
FZ 1 E14 n/cm 2
• 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

Experimental Results: CV/ IV : FZ 1 E14 n/cm 2
Relaxation of Depletion Voltage, bistable effect:
T_anneal [°C]
FZ 1E14 n/cm
2
: N_eff
Guard Ring grounded; after 24 h RT
0.E+00
-5.E+12
-1.E+13
-2.E+13
0 20 40 60 80 100 120 140 160 180
T_anneal [°C]
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
• Results from CVIV measurements:
FZ 1 E14 n/cm 2
• 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/cm 2 electron signals: before annealing: 30 min @ 60 °C: no type inversion up to now
University of Hamburg
T_anneal [°C]
Marie Kristin Bock – Institute for Experimental Physics
•
Pulse shapes & CVIV measurements:
MCz 3 E13 n/cm 2
• 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/cm 2 before annealing: 30 min @ 80 °C:
Double Junction
Effect
30 min @ 100 °: electron signal
Type Inversion: p + p n + hole signal
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
•
Pulse shapes:
MCz 1 E14 n/cm 2
• 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
FZ 3E13 n/cm
2
: Damage Rate;
Guard Ring grounded
6.00
5.00
4.00
3.00
2.00
1.00
0.00
20 40 60 80 100 120
T [°C]
140 160 180 200
FZ 3 E13 n/cm2
FZ 1E14 n/cm2
MCz 3 E13 n/cm2
MCz 1 E14 n/cm2
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
•
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;
0.04
0.03
0.02
0.01
0
20 40 60 80 100 120 140 160 180 200 220 240
Anneal.tem p. [°C]
1*10^14 cm^-2
7.5*10^13 cm^-2
3*10^13 cm^-2
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
•
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/cm 2 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
0.1
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]
1*10^14 cm^-2
7.5*10^13 cm^-2
3*10^13 cm^-2
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
•
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
0.05
0.04
0.03
0.02
0.01
0
20 40 60 80 100
Anneal.tem p. [°C]
120 140 160
1*10^14 cm^-2
3*10^13 cm^-2
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
•
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
MCz: Back 830 nm
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
20 40 60 80 100
Anneal.tem p. [°C]
120 140 160
1*10^14 cm^-2
3*10^13 cm^-2
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
•
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
MCz 1E14 n/cm
2
TCT Alpha
0.22
0.2
0.18
0.16
0.14
0.12
0.1
0 100 200 300 400
U_det [V]
500 600 700 800
Non Irradiated
Before Annealing
30@80
30@100
30@120
30@140
University of Hamburg Marie Kristin Bock – Institute for Experimental Physics
•
Collected Charge vs. Bias-Voltage for alpha injection on front side
MCz 1 E14 n/cm 2 :
• 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
Comparison of Trapping Probabilities
1E14 n/cm
2
& Front 830 nm
0.05
0.04
0.03
0.02
0.01
0
20 40 60 80 100 120 140
Anneal.temp. [°C]
160 180 200
MCz
FZ
G.K.: 7.5*10^13 cm^-2
Comparison of Trapping Probabilities
1E14 n/cm
2
& Back 830 nm
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
20 40 60 80 100 120 140 160 180 200
Anneal.temp. [°C]
MCz
FZ
G.K.: 7.5*10^13 cm^-2
University of Hamburg
ƒ annealing of electron trapping verified !
ƒ but...
™ unexpected behaviour of hole trapping in MCz
™ no explanation on microscopic level so far
™ CCM at temperatures > 180 °C not applicable (for investigated diodes)
Marie Kristin Bock – Institute for Experimental Physics
• 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