1. Optical Crosstalk in SiPM
Nepomuk Otte
Max-Planck-Institut für Physik, München
Humboldt Universität, Berlin
light emission during Geiger breakdown
(Sciacca, 2003)

2. Outline optical crosstalk light emission in avalanches what is SiSi?
tuning SiSi
photons responsible for optical crosstalk
simulating optical crosstalk in a back side illuminated SiPM
conclusions

3. Optical Crosstalk photons can trigger additional cells
Sketch from Cova et al. NIST 2003
Workshop on single photon detectors
 called optical crosstalk
artificial increase in signal
Excess Noise Factor of SiPM
can be quite significant
and
problem in applications

4. Light Emission in Avalanches
I will show:
This is the interesting region
several people tried their best
inconclusive results
I will show
measured spectra do not show similar behavior
emission mechanisms not clear
very few absolute measurements
W. J. Kindt
do different technologies emphasize different emission mechanisms?

5. SiPM-Simulator* full geometrical description of a SiPM:
number of cells
active volume …
simulation of avalanche photons:
black body radiation with free parameters:
temperature
intensity
isotropic emission
For this purpose I developed SiSi
full geometrical simulation of a SiPM
photoelectrons in non-depleted bulk are subject to simple diffusion model;
lifetime of electrons is free parameter
*Elisabeth ”Sis(s)i”von Wittelsbach was the empress consort of Emperor Franz Joseph of Austria. She was born 1853 in Munich, Bavaria and murdered 1898 in Geneva, Switzerland

6. Tuning of Model Parameters
SiSi has three free parameters which need to be determined
temperature of photon spectrum
intensity of photon spectrum /
probability that avalanche carrier emits photon
lifetime of electrons in non-depleted bulk

7. Model parameters tuned by reproducing measured
optical-crosstalk behavior of a SiPM with SiSi.
by MEPhI/Pulsar
optical crosstalk (dark noise) spectrum
(measured)

8. three examples of simulated distributions of optical crosstalk:
free parameters: temperature and intensity (efficiency) of the photon spectrum
lifetime of electrons in non-depleted bulk: 0 nsec
Goodness of agreement between simulation and measurement is tested with a χ²-Test

9. Simulated optical crosstalk:
Temperature 4500K
Efficiency >1.015eV: 1.45x10-4 photons/electron
electron lifetime 60 nsec
integration time 18 nsec
simulated and measured crosstalk distribution
residuals
Residuals can be explained by dark counts which are not simulated in SiSi

10.  no unique solution of model parameters but
χ² -distribution of a scan in:
temperature of the photon spectrum
intensity of photon spectrum
log scale
 no unique solution of model parameters but

11. Characteristics of Photons responsible for Optical Crosstalk
Intensity (1.15 … 1.40 eV):
~2.8*10-5 photons / avalanche electron
(uncertain by a factor of ~2)
Peak: ~1.26eV
clear correlation between temperature and photon emission efficiency
FWHM: ~0.21 eV
very narrow distribution

12. No need to precisely now spectral shape!
2000 K
No need to precisely now spectral shape!
4500 K
simulated photon spectrum

13. ~10µm – 1mm absorption length
note the
log-scale
Narrow energy distribution explained by
strong dependence of absorption lengths on energy

14. Optical Crosstalk in Back Side illuminated SiPM
photon entrance window
active volume
simulated structure:
pitch between cells 100 µm
avalanche region 10 µm diameter
100% active volume
device proposed by G. Lutz et al.

15. Crosstalk Probability in Back Side illuminated SiPM
assume breakdown probability = 0.9
For operation of back side illuminated SiPM need gain << 105
~20% crosstalk at gain 104
But:
absolute measurements always difficult!
e.g. Lacaita (1993) give five times lower intensity
 ~4% crosstalk at gain 104 instead of 20%

16. Conclusions SiSi is a nice tool to study SiPM
crosstalk behavior well described after tuning three free parameters in the simulation of which two are strongly correlated.
only photons within a narrow energy interval (1.15eV-1.40eV) give rise to optical crosstalk
measured intensity of photons: ~2.8•10-5 photons / avalanche electron (1.15eV-1.40eV)
optical crosstalk is a serious problem for back side illuminated SiPM  need low gain ~104; but:
inferred photon intensity could be too high

17. Two Examples of Crosstalk Events

18. SiPM that was used to tune SiSi
produced by MEPhI/Pulsar
576 cells
42µm
sensitive volume
21µm
2.5µm
390µm
avalanche region
non-depleted bulk
One cell of the SiPM

19. Electron Lifetimes
4500K
2000K
Intensity of the photons is reduced by ~30%
if lifetime of the electrons in the non-depleted volume is non zero