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

Outline Introduction to SILVACO TCAD Simulation of a diamond sensor Benchmarking of the simulation Approach to simulate the polarization effect of the diamond Conclusion 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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 ! 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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) 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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 19.11.2014 530µm 0V 100µm 9 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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 ! 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

Benchmarking: 3D - SIMULATION …still calculating 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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 10 13 1 𝑐𝑚³ , 𝜎𝑒ℎ=1.0𝑥 10 14 𝑐𝑚² Donor traps: 𝐸 𝑔 =1.23 𝑒𝑉, 𝜌=1.0𝑥 10 14 1 𝑐𝑚³ , 𝜎𝑒ℎ=1.0𝑥 10 14 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 10.000 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), 5765 - 5774 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

Quasi-3D: Simulation of diamond polarization 𝐸 Different e/h-trap parameters lead to an asymmetrical ionization of the traps Asymmetrical electrical field distribution Increased recombination in low field region. 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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) 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento

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… 19.11.2014 ADAMAS – 3rd Collaboration Meeting, Trento