1. New approach to simulate radiation damage to single-crystal diamonds with SILVACO TCAD
Florian Kassel, Moritz Guthoff, Anne Dabrowski, Wim de Boer

2. Outline Introduction to SILVACO TCAD Simulation of a diamond sensor
Benchmarking of the simulation
Approach to simulate the polarization effect of the diamond
Conclusion
ADAMAS – 3rd Collaboration Meeting, Trento

3. Introduction to SILVACO TCAD
SILVACO TCAD: Tool to simulate the electrical, optical and thermal behavior of semiconductor devices
Is used in CMS silicon strip community to simulate electrical properties of the sensors
Individual sensor designs of arbitrary materials possible
Simulation can be done in 2D, quasi-3D and 3D
Possibility to include a large selection of physical laws
ADAMAS – 3rd Collaboration Meeting, Trento

4. Introduction to SILVACO TCAD
Advantages of SILVACO TCAD in understanding radiation damage
Electrical field distribution is directly linked to the detector efficiency
Understanding of electrical field distribution is crucial!
Electrical field properties depend on:
Drift behavior of charge carriers
Properties of traps in diamond bulk
Creation of space charges
etc, …
Real measurement of electrical field possible with Transient Current Technique (TCT) measurement!
can be simulated with SILVACO TCAD
Exact simulation of TCT pulse necessary !
ADAMAS – 3rd Collaboration Meeting, Trento

5. Simulation of an alpha particle hit (TCT)x y z y r φ x y z
Result quality
Computing power 𝐸
Quasi -3D 2D 3D
Real charge amount
Overestimated charge density
z extension set to fixed value
Charge amount and density meet real value
Has to be rotationally symmetric
Charge amount and density meet real value
All design possible
ADAMAS – 3rd Collaboration Meeting, Trento

6. Simulation of an alpha particle hit (TCT)
Initial charge deposition simulated in SILVACO:
Based on FLUKA simulation (5.0 MeV)
Cut of charge distribution in radial length to increase simulation speed
Alpha 5.0 MeV in diamond
Cut
y – depth (cm)
ADAMAS – 3rd Collaboration Meeting, Trento

7. Benchmarking: 2D - SIMULATION
Comparing of simulation results with a TCT measurement of a real sCVD diamond.
Charge carriers density dependent on thickness of 2D slice
charge carrier influence on electric field no longer negligible
High particle density leads to electrical field free regions
Diffusion becomes dominant
ADAMAS – 3rd Collaboration Meeting, Trento

8. Benchmarking: Quasi-3D SIMULATION (200V)
Quasi-3D like model designed
Parameters:
Thickness: 530 µm
Radial length: 100µm
No traps simulated
Mobility parameters for e and h drift used from thesis of M.Pomorski
sCVD_Test1
ADAMAS – 3rd Collaboration Meeting, Trento

9. Hole drift through diamond bulk
200V
0.0 ns
0.3 ns
0.05 ns
3.0 ns
5.0 ns 𝐸
ADAMAS – 3rd Collaboration Meeting, Trento
530µm 0V
100µm 9
ADAMAS – 3rd Collaboration Meeting, Trento

10. Benchmarking: Quasi-3D SIMULATION (400V)
Deviation between simulation and measurements:
Reduced alpha particle energy simulated
Local cuts of energy distribution of alpha particle: 𝟎>𝒚>𝟏𝟓 µ𝒎 𝟎>𝒓>𝟎.𝟐 µ𝒎
Further reduction of alpha particle energy.
sCVD_Test1
Optimization of alpha particle simulation necessary !
ADAMAS – 3rd Collaboration Meeting, Trento

11. Benchmarking: 3D - SIMULATION
…still calculating
ADAMAS – 3rd Collaboration Meeting, Trento

12. Quasi-3D: Simulation of diamond polarization
Configuration:
Traps added to the diamond bulk, values assumed for first tests
Acceptor traps1: 𝐸 𝑔 =1.23 𝑒𝑉, 𝜌=1.0𝑥 𝑐𝑚³ , 𝜎𝑒ℎ=1.0𝑥 𝑐𝑚²
Donor traps: 𝐸 𝑔 =1.23 𝑒𝑉, 𝜌=1.0𝑥 𝑐𝑚³ , 𝜎𝑒ℎ=1.0𝑥 𝑐𝑚²
Simulation of a MIP particle background
Lab measurements: 3.7x106 Bq source used. Time order polarization ~1000s
SILVACO TCAD: 4x1010 Bq source simulated. Speeding up of simulation by a factor of Polarization within 0.1s
Simulation of electron or hole drift by injection of charge either on top or bottom of diamond.
1) M.Bruzzi et al., Deep levels and trapping mechanisms in chemical vapor
deposited diamond, Journal of applied Physics 91, 9 (2002),
ADAMAS – 3rd Collaboration Meeting, Trento

13. Quasi-3D: Simulation of diamond polarization𝐸
Different e/h-trap parameters lead to an asymmetrical ionization of the traps
Asymmetrical electrical field distribution
Increased recombination in low field region.
ADAMAS – 3rd Collaboration Meeting, Trento

14. Quasi-3D: Resulting TCT pulse for electron and hole drift
Example sCVD_2012:
2.5E14 24GeV peq at CMS
Thickness: 410µm
Mean after 750s (3.7MBq)
ADAMAS – 3rd Collaboration Meeting, Trento

15. Cross checking of simulation results
Irradiation study planned:
Investigation of the radiation impact on the electrical properties of a un irradiated sCVD diamond
Stepwise irradiation (~5e12 neq/cm²) till 5e13 neq/cm²
Proton irradiation performed at KIT (~23 MeV)
Systematic measurement of electrical field effects:
Transient current technique (TCT)
Charge collection efficiency (CCE)
Comparison of results with SILVACO TCAD simulation
ADAMAS – 3rd Collaboration Meeting, Trento

16. Conclusion
Quasi-3D simulation are feasible to simulate electrical properties of a diamond detector.
Simulation of an alpha particle in good agreement to real measurement
First approach of simulating the polarization effect in diamond
Increased density of donor traps assumed
MIP particle background simulated
Change of electrical field distribution and hence a change of the shape of the TCT signal observed.
Outlook:
Simulation of MIP particle in order to calculate the CCE
Modifying of traps in order to get closer to real results
Simulation of improved diamond designs:
Split pads (BCM1F), etc…
ADAMAS – 3rd Collaboration Meeting, Trento