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Radiation Damage in Sentaurus TCAD David Pennicard – University of Glasgow.

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Presentation on theme: "Radiation Damage in Sentaurus TCAD David Pennicard – University of Glasgow."— Presentation transcript:

1 Radiation Damage in Sentaurus TCAD David Pennicard – University of Glasgow

2 Overview Introduction to trap models Radiation damage effects and defects P-type damage model Some example simulations Sentaurus Device command file

3 Radiation damage introduction High-energy particle displaces silicon atom from a lattice site –Results in a vacancy and an interstitial –Atom can have enough energy to displace more atoms After damage is caused, most vacancy-interstitial pairs recombine –Left with more stable defect clusters, e.g. divacancy (V 2 ) –Defect clusters affected by annealing conditions & impurities in the silicon Defect clusters give extra energy states (traps) in bandgap –Increased leakage current –Increased charge in depletion region (increase in effective p-type doping) –Trapping of free carriers Can simulate this in Sentaurus Device by modelling behaviour of trap levels directly NB – when dealing with different types and energies of particle irradiation, scale fluence (particles / cm 2 ) by non-ionizing energy loss. Standard is 1MeV neutrons. See M. Moll thesis, Hamburg 1999

4 Traps in Sentaurus Device A statement added to the Physics section can describe the traps: Parameters –Acceptor: trap has –ve charge when occupied by electron, 0 charge when occupied by hole. (Donor has +ve charge when occupied by hole) –Level: specifies how we describe energy level. Here, we give the energy below the conduction band. EnergyMid gives the energy difference –Concentration: given in cm -3 –Electron cross-section: proportional to probability of electron moving between trap and conduction band - σ e –Hole cross-section: likewise, proportional to chance of carrier moving between valence band and trap level - σ p Physics (material="Silicon") { Traps ( (Acceptor Level fromCondBand Conc=1.613e15 EnergyMid=0.42 eXsection=9.5E-15 hXsection=9.5E-14) ) }

5 Traps in Sentaurus Device For each trap level, Sentaurus simulates: Proportion of trap states occupied by electrons and holes –NB – not filled by electron=occupied by hole –This affects charge distribution, and so has to be included in Poisson equations Rate of trapping / emission between conduction band and trap, and between valence band and trap –These then have to be included in the carrier continuity equations Poisson Electron continuity Hole continuity

6 Increase in reverse leakage current Leakage current increases with fluence, independent of substrate type Leakage current reduced by annealing Also, temperature dependence. α normally given for 20C α=3.99* A/cm 3 after 80 mins anneal at 60˚C (M. Moll thesis)

7 Increase in leakage current Ec Ev E mid Trap 2 transitions involved: –Electron from valence band moves to empty trap, leaving a hole –Electron in trap moves to conduction band, giving conduction electron –Then, electron and hole are swept out of depletion region by field, avoiding recombination Rate of production limited by less frequent step (larger energy difference) –Trap above midgap limited by rate of valence band->trap –Traps below midgap likewise limited by trap->conduction band Rate drops rapidly with distance of trap from midgap –Deep level traps dominate Hole produced Free electron produced

8 Change in effective doping concentration Effective p-type doping increases (giving type inversion in n-type silicon) Dependent on material, particularly oxygen content and radiation type for p-type (n-type also has donor removal effect) My models match p-type Float Zone irradiated with protons

9 Change in effective doping concentration Additionally, have both beneficial annealing) in short term, and reverse annealing in long term Typically, test detectors after beneficial annealing, to try to find stable damage level All this implies very complicated defect behaviour!

10 Change in effective doping concentration Charge state of defect depends on whether it contains electron or hole –Acceptor: -ve when occupied by electron –Donor: +ve when occupied by hole Source of –ve charge that gives effective p-type appears to be acceptors above midgap –A small proportion of these traps are occupied by electrons –Number of traps occupied once again is highly dependent on distance from bandgap Donors below bandgap can give +ve charge, but relatively minor effect Ec Ev E mid Acceptor Trap Hole produced -

11 Number of free carriers in device decays exponentially over time Described by effective lifetime: Experimentally, effective lifetime varies inversely with fluence (this has been tested up to n eq /cm 2 ) Charge trapping G. Kramberger, Trapping in silicon detectors, Aug , 2006, Hamburg, Germany

12 Charge trapping In equilibrium, traps above E mid are mostly unoccupied Free electrons in conduction band can fall into unoccupied trap states –Likewise, traps below midgap contain electrons – can trap holes in valence band Effect is less energy-dependent –Similar equations for holes Ec Ev E mid Trap Afterwards, carrier can be released from trap –If trap levels are reasonably close to midgap, detrapping is slow –So, less effect on fast detectors for LHC

13 University of Perugia trap models * * CiOiEc+0.36Donor * * VVVEc-0.46Acceptor * * VVEc-0.42Acceptor η (cm -1 )σ h (cm 2 )σ e (cm 2 )Trap Energy (eV)Type Perugia P-type model (FZ) IEEE Trans. Nucl. Sci., vol. 53, pp. 2971–2976, 2006 Numerical Simulation of Radiation Damage Effects in p-Type and n-Type FZ Silicon Detectors, M. Petasecca, F. Moscatelli, D. Passeri, and G. U. Pignatel Ec Ev Acceptor levels: Close to midgap –Leakage current, negative charge (N eff ), trapping of free electrons Donor level: Further from midgap –Trapping of free holes

14 Experimental trapping times for p-type silicon (V. Cindro et al., IEEE NSS, Nov 2006) up to n eq /cm 2 –β e = 4.0*10 -7 cm 2 s -1 β h = 4.4*10 -7 cm 2 s -1 Calculated values from p-type trap model –β e = 1.6*10 -7 cm 2 s -1 β h = 3.5*10 -8 cm 2 s -1 University of Perugia trap models Aspects of model: –Leakage current – reasonably close to α=4.0* A/cm –Depletion voltage – matched to experimental results with proton irradiation with Float Zone silicon (M. Lozano et al., IEEE Trans. Nucl. Sci., vol. 52, pp. 1468–1473, 2005) –Carrier trapping – Model reproduces CCE tests of 300 m pad detectors But trapping times dont match experimental results

15 Altering the trap models Priorities: Trapping time and depletion behaviour –Leakage current should just be sensible: α = 2-10 * A/cm Chose to alter cross-sections, while keeping σ h /σ e constant Carrier trapping: Space charge: Modified P-type model * * CiOiEc+0.36Donor * * VVVEc-0.46Acceptor * * VVEc-0.42Acceptor η (cm -1 )σ h (cm 2 )σ e (cm 2 )Trap Energy (eV)Type

16 Comparison with experiment Comparison of Radiation Hardness of P-in-N, N-in-N, and N-in-P Silicon Pad Detectors, M. Lozano et al., IEEE Trans. Nucl. Sci., vol. 52, pp. 1468– 1473, 2005 α=3.75* A/cm Compared with experimental results with proton irradiation Depletion voltage matches experiment Leakage current is 30% higher than experiment, but not excessive Experimentally, α=3.99* A/cm 3 after 80 mins anneal at 60˚C (M. Moll thesis) α=5.13* A/cm

17 N+ on p strip detector: CCE At high fluence, simulated CCE is lower than experimental value –Looked at trapping rates using 1D sim – as expected –Trapping rates were extrapolated from measurements below n eq /cm 2 –In reality, trapping rate at high fluence probably lower than predicted PP Allport et al., IEEE Trans. Nucl. Sci., vol 52, Oct V bias, 280 m thick From β values used, expect 25μm drift distance, 2ke- signal

18 Example - Double-sided 3D detector Electrode columns etched from opposite sides of silicon substrate –Short distance between electrodes –Expect reduced depletion voltage and faster collection (less trapping)

19 Example - Double-sided 3D at n eq /cm 2 Plotted electric field in cross-section at 100V bias Where the columns overlap, (from 50 m to 250 m depth) the field matches that in the full-3D detector At front and back surfaces, fields are lower as shown below Region at back is difficult to deplete at high fluence A. B. Undepleted 100V n eq /cm 2, front surface10 16 n eq /cm 2, back surface

20 Example - Collection with double-sided 3D Slightly higher collection at low damage But at high fluence, results match standard 3D due to poorer collection from front and back surfaces. 20% greater substrate thickness

21 Sentaurus Device command file See Sentaurus/Seminar/RadDamage: –StripDetectorRadDamage_des.cmd –StripDetectorRadDamage_Param_des.cmd Traps added to silicon –Insert appropriate concentrations, or use a Fluence variable in Workbench Physics (material="Silicon") { # Putting traps in silicon region only Traps ( (Acceptor Level EnergyMid=0.42 eXsection=9.5E-15 hXsection=9.5E-14) (Acceptor Level EnergyMid=0.46 eXsection=5E-15 hXsection=5E-14 ) (Donor Level EnergyMid=0.36 eXsection=3.23E-13 hXsection=3.23E-14 ) ) }

22 Sentaurus Device command file Extra variables can be added to Plot Warning – trap models are sensitive to changes in the bandgap and temperature –Dont change the effective intrinsic density model – alters bandgap –Likewise, keep using default 300K temp. (Strictly speaking this is slightly wrong, since the standard test temp should be 20C.) Plot { ……… eTrappedCharge hTrappedCharge eGapStatesRecombination hGapStatesRecombination } Physics{ # Standard physics models - no radiation damage or avalanche etc. Temperature=300 Mobility( DopingDep HighFieldSaturation Enormal ) Recombination(SRH(DopingDep)) EffectiveIntrinsicDensity(Slotboom)

23 Sentaurus Device command file Oxide charge increases after irradiation –Electron-hole pairs produced in oxide – holes become trapped in defects in oxide, giving positive charge –Saturates fairly rapidly – cm -2 is a normal value after irradiation, though some papers claim up to 3*10 12 cm -2 –X-ray irradiation causes oxide charging, but little bulk damage Other points –More complicated physics tends to give slower solving, and poorer convergence: may need to alter solve conditions (smaller steps etc) –For charge collection simulations, need to correct the integrated current to remove the leakage current –CV simulations give strange results! Physics(MaterialInterface="Oxide/Silicon") { Charge(Conc=1e12) }

24 Example files See Sentaurus/Seminar/RadDamage StripDetectorRadDamage_des.cmd –Basic MIP simulation at n eq /cm 2 –This has already been run –You can look at the output files in the same folder.dat files taken during IV ramp.dat files taken during the MIP transient.plt files StripDetectorRadDamage_Param_des.cmd –_des.cmd file for a Workbench project –Use parameter Fluence to control the radiation damage –Uses #if statements to omit Traps statement and use lower oxide charge if Fluence is zero –Works with simple StripDetector.bnd/cmd files in Workbench folder


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