Microscopic radiation damage in semiconductor detectors M. Bruzzi Univ. Firenze INFN Sezione Firenze Università di Firenze, Italy Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Motivation EnvironmentRadiation typeTypical exposure Fission reactor0.1-5MeV n5x10 17 m -2 s -1 Fusion reactor0.1-14MeV n8x10 18 m -2 s -1 Space6eV photons10 20 m -2 s -1 10keV-3MeV  5x10 12 m -2 s -1 1MeV-300MeV p4x10 9 m -2 s -1 Ion accelerator15 MeV p6x10 16 m -2 s -1 High energy physics experiments n,p, pions cm -2 radiotherapyX, e, p10kGy Semiconductor detectors have a number of potential applications where radiation induced defects plays a crucial role. Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Working principle of a semiconductor detector Insulating  >  cm Ohmic contact Schottky contact n / p semiconductor p + n or n + p junction Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

main application detect the passage of ionising radiation with high spatial resolution and good efficiency. Segmentation → position Depletion depth → efficiency ~80e/h pairs/μm produced by passage of minimum ionising particle, ‘mip’ Pitch ~ 50  m Resolution ~ 5  m Highly segmented silicon detectors have been used in Particle Physics experiments for nearly 30 years. Favourite choice for Tracker and Vertex detectors (high resolution, speed, low mass, relatively low cost, and now important, radiation hard) Segmented Detectors p + in n - Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Poisson’s equation Reminder: Reverse biased abrupt p + -n junction Electrical charge density Electrical field strength Electron potential energy effective space charge density depletion voltage Full charge collection only for V B >V dep ! Positive space charge, N eff =[P] (ionized Phosphorus atoms) Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Relevant parameters of a detector Leakage Current Capacitance Active region thickness Effective space charge in depleted region Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Semiconductor s proposed for detector applications Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Physical parameters and detector requirements Low energy for e-h generation  small gap Low leakage current  high gap Low full depletion voltage: high resisitivity High Signal / Sensitivity Radiation Hardness High speed High spatial resolution Low Noise High stability High mobility Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Radiation Damage Mechanisms in Semiconductor Devices Radiation induced defects Extended defects or clusters Carrier removal and increase of resistivity Microscopic Damage TrapsRecombination centres Macroscopic Damage Increase of the leakage current Change in N eff and space charge sign - underdepletion Decrease of minority carrier lifetime and diffusion length Midgap Fermi level pinning Decrease of the charge collection, sensitivity and energy conversion efficiency Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Microscopic Radiation Damage Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

particle Si s Vacancy + Interstitial Point Defects (V-V, V-O.. ) clusters E K > 25 eV E K > 5 keV Frenkel pair V I Simulation of Microscopic Damage Generation of hadronic interactions Transport of the produced heavy recoils Migration of V and I to form stable defects Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

10 MeV protons 24 GeV/c protons 1 MeV neutrons [Mika Huhtinen NIMA 491(2002) 194] Initial distribution of vacancies in (1  m) 3 after particles/cm 2 Vacancy amount and distribution depends on particle kind and energy Neutrons (elastic scattering) –E n > 185 eV for displacement –E n > 35 keV for cluster 60 Co-gammas –Compton Electrons with max. E   1 MeV (no cluster production) Electrons –E e > 255 keV for displacement –E e > 8 MeV for cluster Only point defects point defects & clusters Mainly clusters Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

of a radiation field (or monoenergetic particle) with respect to 1 MeV neutrons E energy of particle D(E) displacement damage cross section for a certain particle at energy E D(1MeV neutrons)=95 MeV·mb  (E) energy spectrum of radiation field The integrals are evaluated for the interval [E MIN,E MAX ], being E MIN and E MAX the minimum and maximum cut-off energy values, respectively, and covering all particle types present in the radiation field How to normalize radiation damage from different particles?  NIEL - Non Ionizing Energy Loss scaling using hardness factor  Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

NIEL – Non Ionizing Energy Loss Displacement damage functions Hypothesis: Damage parameters scale with the NIEL – Be careful, does not hold for all particles & damage parameters (see later) 1MeV 1 1 MeV neutron equivalent damage Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

- Secondary defect formation in silicon.  Primary defect generation I,I 2 higher order I  I - CLUSTER V,V 2,higher order V  V - CLUSTER  Secondary defect generation Main impurities in silicon: Carbon C s Oxygen O i I+C s  C i  C i +C s  C i C S C i +O i  C i O i C i +P s  C i P S V+V  V 2 2  V 3 V+O i  VO i  V+VO i  V 2 O i V+P s  VP s I + O 2i → IO 2i Dopants : P, B Oxygen dimer: O 2i Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Si Point-defects : The A centre oxygen-doped silicon dominant centers of vacancy capture may be isolated interstitials O i and trapping results in the formation of the V-O centre, so-called A centre EvEv EcEc V-O E A =0.18eV V-O defect ( A centre) Watkins, Corbett: Phys.Rev.,121,4, (1961),1001 Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Si Point-defects: The E centre In Phosphorous doped Si vacancy is also trapped by P to create the P-V defect, the so-called E centre. This changes the doping of the crystal, removing the doping atom P and creating an acceptor-like energy level at Et = 0.42eV → carrier removal → doped semiconductors become almost intrinsic after heavy irradiation Phosphorous-Vacancy P-V (E centre ) Corbett, Watkins et al, PRB, 60s EvEv EcEc EvEv EcEc cncn E E =0.4eV E P =0.04eV Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Si Point-defects involving more than one vacancy Point-defects can involve more than one vacancy, creating deep levels in the Si gap: V 2, V 2 O, V 3 O etc.. Divacancy V 2 V 2 O defect Lee, Corbett: Phys.Rev.B,13,6, (1976),2653 Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Annealing Defect configuration can significantly change by heating up the irradiated sample or storing at T higher than that of irradiation. I A –I E collapse of close to separated Frenkel pairs II: formation of clusters as small interstitial loops III: vacancies migrate and annihilate at interstital clusters & vacancy agglomerate in vacancy clusters IV vacancy clusters grow in size V vacancy clusters dissociate thermally and annihilate at interstitial loops: radiation damage is removed I 1, V 1 single vacancy, interstitial I 2, V 2 di-interstitials,divancancies… Example: Cu after electron irradiation annealing recovering. Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Vacancies in the fcc lattice and migration by jumping of one of the neighboring atoms to the vacancy site (a)Single vacancy in an fcc lattice (b)Vacancy migration saddle point (c)Divacancy (d)Divacancy migration saddle point (e)Trivacancy (f)Trivacancy reorientation (g)(h) tetravacancies Diffusion coefficients of isolated vacancies and interstitials are usually non negligible in the temperature ranges of interest. This leads to their migration to annihilation reactions or formation of quasimolecules. Diffusion and migration Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013 Note: In diamond, unlike in silicon, the intrinsic defects (vacancies and interstitials) created by radiation damage are immobile at room temperature

Native Impurity Defects in Diamond Natural diamonds most commonly have N present in aggregated forms, and some rare samples contain boron and are semiconducting. High-temperature–high-pressure synthetic diamonds may contain isolated substitutional nitrogen if getters have not been employed, and, where Ni is present in as part of the solvent-catalyst, a range of Ni-related defects. CVD diamond commonly contains isolated impurity defects including nitrogen, silicon and boron. Hydrogen plays a role in the formation of electrically active defects. Charge Character of defects in diamond : general trends As a general trend substitutional chalcogen impurities yield relatively deep donor levels and dangling bonds yield deep acceptor states, where all dangling bonds are saturated (VH, VNH, etc.) the defect becomes passive. For this reason diamond has a naturally slightly p-type character.

Example of conduction activated by substitutional N HPHT single crystal diamond (Sumitomo) characterized by [N ] =1.7x10 19 N-traps/cm 3 R. Mori et al. JAP, 2009 Nitrogen Defects in diamond Nitrogen related defects are of particular importance in diamond. They are most abundant since nitrogen is a prominent impurity diffusing in the material during growth. Nitrogen defect can be single substitutional impurity or be in aggregated form. The single substitutional nitrogen has an infrared local mode of vibration at 1344 cm –1. The centre is a deep electron donor, 1.7 eV below the conduction band edge. Nitrogen aggregates are, most commonly, pairs of neighboring substitutional atoms, the A aggregates, and groups of four around a vacancy, the B aggregate. Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Interaction of vacancy with Native defects 1. Nitrogen – Vacancy defect Schematic representation of the nitrogen vacancy (NV) centre structure. F. Jelezko and J. Wrachtrup: Single defect centres in diamond: A review phys. stat. sol. (a) 203, No. 13, 3207–3225 (2006) The nitrogen vacancy defect centre in diamond is traditionally observed in radiation damaged nitrogen rich diamond. It is also named GR1 centre. This defect gives rise to a strong absorption at eV (637 nm). Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

spectrum of isolated neutral substitutional nitrogen plus a weaker signal with satellites (H2) H1 EPR spectrum at 14 GHz characterized by a single line Defects H1 and H2 arise from two distinct electrically active defects produced when a single hydrogen atom enters a stretched bond at a grain boundary, or other extended misfit region in the polycrystalline CVD material. H forms a bond with one of the carbons, producing an electrically active dangling bond on the other as the two carbons relax backward. 2. Hydrogen related defects H1: A single hydrogen atom in a vacancy. Hydrogen is hypothesized to passivate most of the dangling bonds in defective regions as grain boundaries. Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Resume of calculated energy levels related to vacancy and Interstitial related centers in diamond J.P.Goss, P.R.Briddon, R.Jones, S.Sque, Donor and acceptor states in diamond, Diamond and Related Materials 13 (2004) 684–690 first-principles marker method (FPMM) - compares the electron affinity ionisation energy of a defect with that of a known reference state. Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

3. Interaction of Vacancy with Doping Levels P doped n-type single-crystalline diamond with thermal activation energy of 0.58 eV and highest mobility of 0.35 cm 2 /Vs [Kato et al.]. Vacancy and H defects may bind to the neighboring P atom forming PVH related complex to influence the electronic properties of doped diamond [Yan et al.]. P–V, P–V–H,P–2V–H complexes introduce acceptor levels providing hole carriers in diamond. P–V defect P–V–H complex defect H. Kato, S. Yamasaki, H. Okushi, Appl. Phys. Lett. 86 (2005) C.X. Yan et al. Theoretical characterization of carrier compensation in P-doped diamond Applied Surface Science 255 (2009) 3994–4000 Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Divacancy complex P–2V–2H and trivacancy complex P–3V, introduce energy levels near the middle of the band gap, which may serve as recombination centers. P–3V P–2V-2H Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Electrical activity of energy levels related to defects Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Defects are usually characterised by energy levels within the forbidden gap interacting with both conduction and valence bands, through capture and emission of electron or hole. N t = total concentration of energy levels; n t = concentration of occupied levels, n = concentration of free electrons, p = concentration of free holes; = rms electron thermal velocity; = rms hole thermal velocity  n = electron capture cross section;  p = hole capture cross section; If a trap is exposed to a flux of free electrons per unit area: n, then number of electrons captured by the unoccupied states in interval  t is:  n t =  n n (N t -n t )  t then capture rate is: Similarly for holes: EvEv EcEc cpcp cncn EtEt Capture coefficients Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Occupancy of the level is determined by the competition of emission and capture processes. Electrons are emitted and holes captured by energy levels occupied with electrons (n t ), while electrons are captured and holes emitted by unoccupied energy levels (N t -n t ). If e n,p = rate of emission for electrons and holes, rate of change of occupancy is: In thermal equilibrium emission and capture process must balance i.e. the rates of capture and emission must be equal both for electrons and holes: Therefore the occupancy of traps is determined by: Emission constants EvEv EcEc epep enen EtEt Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

In thermal equilibrium the occupancy of traps is defined by the Fermi-Dirac distribution. For a deep state with degeneracy g 0 when empty with electrons and g 1 when occupied, in a system with Fermi energy E f, the occupancy is defined as: Assuming g 0 /g 1 = 1 we obtain: Finally, as: we get: Thermal Equilibrium Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Out of equilibrium excess carriers are generated, and we set n=(n 0 +  n), p=(p 0 +  p), with  n=  p, of course. In this conditions np > n i 2 and there is a net flow of current through the bands. In particular, at steady state, it is subject to the condition R e =R h =U, where U is the recombination-generation rate. The result is: The following short notation has been used: U determines the rate at which excess carriers recombines through the defect level: Non-equilibrium : Shockley-Read-Hall statistics Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Some particular cases are identified depending on excess carrier concentration. We consider, as an example, high resistivity p-type : p 0 >>p 1,n 1 : Low injection level. In this situation p 0 >>  n. Recombination-generation rate simplifies into:  e is the lifetime of electrons. Because electrons are in this material the minority carriers, this characteristic time is customarily called "minority carriers lifetime". Minority Carrier Lifetime Diffusion length: D e  e K·T/e Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Generation - Recombination phenomenon via midgap levels Indirect recombination via midgap levels is a two-step process where both electron and hole are captured by the centre As N t grows with irradiation  decrease with the accumulated dose. recombination leads to a decrease in sensitivity during device lifetime. Generation via midgap levels can lead to an increase of leakage current (reverse voltage applied) : e n,c n e p,c p Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Deterioration of the minority carrier lifetime with irradiation in Si 10MeV p* *10 MeV protons produces a 3000 times larger equivalent damage compared to 1 MeV electrons as I leak ≈ qAUW=qWn i /  Volumetric leakage current linearly dependent on fluence Experimentally observed: Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

 Damage parameter  Leakage current per unit volume and particle fluence 80 min 60  C Generation from midgap levels → linear increase with fluence of the Leakage Current  is constant over several orders of fluence and independent of impurity concentration in Si  can be used for fluence measurement Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Undoped high band gap materials are intrinsically radiation hard because even at high fluences of irradiation <1pA/cm 2 Silicon: Partial depletion T = -30°C; V = 600V, W = 300  m Diamond SiC Si M. Bruzzi ;H. Sadrozinski ; A.Seiden NIM A (2007 ) Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Note: Same model to explain ccd degradation in irradiated Diamond ccd = average distance h and e drift apart before recombination Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Microscopic Radiation Damage Analysis: non spectroscopic methods Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

a. Electrical Resistivity Measurements II II z =  n /  p

b. Hall Coefficient Analysis I z =  n /  p B V Hall =R H J x B z h Example of Si doped with In B,P. Slope in free hole concentration changes due to defect activation Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Irradiated Si (1MeV n-equivalent ) showing pinning of the Fermi level at E v eV Combining resistivity and Hall Coefficient results one can determine the Fermi level position in the forbidden gap Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

How to detect electrically active defects ? Defect Spectroscopy in semiconductors 1. Thermally Stimulated Currents TSC 2. Deep Level Transient Spectroscopy DLTS 3. Photo Induced Current Transient Spectroscopy PICTS Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Main trap parameters: E t,  N t Activation energy Cross section Concentration Emission coefficient: Capture coefficient : EvEv EcEc enen cpcp cncn epep EtEt EtEt Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Space charge due to deep levels in depleted region Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

TSC peak time cooling primingheating T time V cool V fill V TiTi V bias time I fill -Cooling with applied reverse V cool or null bias -Forward voltage applied V fill at T i or illumination with optical source - V bias applied -Thermally stimulated current read-out Thermally Stimulated Current: TSC Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Example: TSC at Low Temperature to evidence Shallow Donor Removal in n-type Si irradiated with neutrons P Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Deep levels in chemical vapor deposited diamond M. Bruzzi et al. JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 9 1 MAY 2002 TSC of native deep levels in polycrystalline CVD diamond A contacts area,  carrier mobility,  electric field, e electronic charge,.  = ccd average distance an hole and electron drift apart before recombination, can be measured directly. Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

t1t1 t2t2 CC T3T3 T2T2 T1T1 T 1 > T 2 > T 3 V rev time C t1t1 t2t2 T 2 max /e n 1/T Slope E t Intercept  Deep Level Transient Spectroscopy DLTS Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Example: DLTS in Silicon f =10 11 cm MeV neutrons ROSE Coll. NIM A 466 (2001) Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Clusters observed by DLTS in Silicon A potential barrier is usually screening the extended defect Main effect of clustering is a widening of the DLTS signal Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Photo Induced Current Transient Spectroscopy PICTS Similarly as DLTS, trap priming is performed by exposing to an optical excitation with h  g and current transient is measured M. Bruzzi et al. Photo Induced deep level analysis in undoped CVD diamond films, DRM 9 (2000) Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

M. Bruzzi et al. Photo Induced deep level analysis in undoped CVD diamond films, DRM 9 (2000) Photo Induced Current Transient Spectroscopy PICTS in unirradiated polycrystalline CVD diamond Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Impact of Defects on Detector properties Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Shockley-Read-Hall statistics (standard theory) Impact on detector properties can be calculated if all defect parameters are known:  n,p : cross sections  E : ionization energy N t : concentration Trapping (e and h)  CCE shallow defects do not contribute at room temperature due to fast detrapping charged defects  N eff, V dep e.g. donors in upper and acceptors in lower half of band gap generation  leakage current Levels close to midgap most effective enhanced generation  leakage current  space charge Inter-center charge transfer model (inside clusters only) Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Change of Depletion Voltage V dep (N eff ) “Type inversion”: N eff changes from positive to negative (Space Charge Sign Inversion) after inversion before inversion n+n+ p+p+ n+n+ p+p+ Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Deterioration of Charge Collection Efficiency (CCE) by trapping Increase of inverse trapping time (1/  ) with fluence Trapping is characterized by an effective trapping time  eff for electrons and holes: where Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Effect of trapping on the Charge Collection Distance: Q tc  Q 0 exp(-t c /  tr ), 1/  tr = . v sat,e x  tr = av G. Kramberger et al., NIMA 476(2002),  e =  cm -2 /ns  h =  cm -2 /ns Expected collection distance at saturation velocity av : after 1x10 15 n eq cm -2 : 240µm expected charge ~19ke. av after 1x10 16 n eq cm -2 : 25µm expected charge <1.3ke : quite inefficient detector! Expected signal from charge trapping Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Signal to Noise ratio Example with diodes Figure of Merit: Signal-to-Noise Ratio S/N. Radiation damage severely degrades the S/N. Landau distribution has a low energy tail - becomes even lower by noise broadening Noise sources: (ENC = Equivalent Noise Charge) - Capacitance - Leakage Current - Thermal Noise (bias resistor) less signal more noise What is signal and what is noise? Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Leakage Current and N eff (after hadron irradiation)  Leakage current decreasing in time (depending on temperature)  Strong temperature dependence Consequence: Cool detectors during operation! Example: I(-10°C) ~1/16 I(20°C) 80 min 60  C …. with time (annealing): Changes with time and temperature after irradiation (annealing) Short term: “Beneficial annealing” Long term: “Reverse annealing” - time constant depends on temperature: ~ 500 years(-10°C) ~ 500 days( 20°C) ~ 21 hours( 60°C) - Consequence: Detectors must be cooled even when the experiment is not running! …. with time (annealing): E≈0.6eV Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Parameterization of Annealing Results Annealing components:  Short term annealing  N A ( ,t(T))  Stable damage  N C (  ) N C = N C0 (1-exp(-cΦ eq ) + g C Φ eq note: g C negative for EPI (effective positive space charge generation!)  Long term (reverse) annealing: Two components:  N Y,1 ( ,t(T)), first order process  N Y,2 ( ,t(T)), second order process Change of effective “doping“ concentration:  N eff = N eff,0 – N eff ( ,t(T)) Standard parameterization:  N eff = N A ( ,t(T)) + N C (  ) + N Y ( ,t(T))

Change of inverse trapping time Decrease of inverse trapping time (1/  ) with annealing for electrons Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Appearance of a Double Junction at electrodes Double level model: Acceptor in second half – Donor in first half of bandgap originate the double junction. Levels are neutral in bulk, ionised close to contacts. Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Radiation Damage of the Neutral bulk Despite the high space charge at electrodes the neutral bulk in between the two junctions has almost intrinsic resistivity and slightly p-type conductivity due to removal of shallow dopants and deep defect formation M. Bruzzi Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

65 P-sideN-side In reality, after irradiation electric fields show a double junction structure with a non- depleted bulk in the middle of the sensor below the full depletion voltage See G. Casse, et. al., NIMA 426 (1999) and G. Kramberger, et. al., NIMA 579 (2007) for details ISE-TCAD simulation after p cm -2 Double Junction Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Material Engineering Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

8 North-American institutes Canada (Montreal), USA (BNL, Fermilab, New Mexico, Purdue, Rochester, Santa Cruz, Syracuse) 1 Middle East institute Israel (Tel Aviv) 38 European institutes Belarus (Minsk), Belgium (Louvain), Czech Republic (Prague (3x)), Finland (Helsinki, Lappeenranta), Germany (Dortmund, Erfurt, Freiburg, Hamburg, Karlsruhe, Munich), Italy (Bari, Florence, Padova, Perugia, Pisa, Trento), Lithuania (Vilnius), Netherlands (NIKHEF), Norway (Oslo (2x)), Poland (Warsaw(2x)), Romania (Bucharest (2x)), Russia (Moscow, St.Petersburg), Slovenia (Ljubljana), Spain (Barcelona, Valencia), Switzerland (CERN, PSI), Ukraine (Kiev), United Kingdom (Glasgow, Lancaster, Liverpool) 257 Members from 47 Institutes Detailed member list: RD50 - Development of Radiation Hard Semiconductor Devices for High Luminosity Colliders Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Approaches to develop radiation harder solid state tracking detectors Defect Engineering of Silicon Deliberate incorporation of impurities or defects into the silicon bulk to improve radiation tolerance of detectors – Needs: Profound understanding of radiation damage microscopic defects, macroscopic parameters dependence on particle type and energy defect formation kinetics and annealing – Examples: Oxygen rich Silicon (DOFZ, Cz, MCZ, EPI) Oxygen dimer & hydrogen enriched Si Pre-irradiated Si Influence of processing technology New Materials – Silicon Carbide (SiC), Gallium Nitride (GaN) – Diamond (CERN RD42 Collaboration) – Amorphous silicon Device Engineering (New Detector Designs) – p-type silicon detectors (n-in-p) – thin detectors, epitaxial detectors – 3D detectors and Semi 3D detectors, Stripixels – Cost effective detectors – Monolithic devices Scientific strategies: I.Material engineering II.Device engineering III.Change of detector operational conditions CERN-RD39 “Cryogenic Tracking Detectors” operation at K to reduce charge loss Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Oxygen Enrichment for Radiation Hardening Main Hypothesis: Oxygen beneficial as sink of vacancies V-O i complex concentration increase reduction of deeper levels mainly divacancy related RD48 (ROSE) and RD50 CERN Collaborations V2OV2O EcEc EVEV VO V 2 in clusters Typical oxygen concentration in Si: -FZ [Oi] cm -3 -Diffusion oxygenated FZ : DOFZ [O i ] cm -3 -Czochralski Si: [O i ] up to cm -3 Note: as VO is a point defect the beneficial effect of oxygen is expected especially when cluster formation by irradiation is less important than point defect formation. Microscopic radiation damage in semiconductor detectors M. Bruzzi, Legnaro, 15 Aprile 2013

Material: Float Zone Silicon (FZ) Using a single Si crystal seed, melt the vertically oriented rod onto the seed using RF power and “pull” the monocrystalline ingot Wafer production  Slicing, lapping, etching, polishing Mono-crystalline Ingot Single crystal silicon Poly silicon rod RF Heating coil Float Zone process Highly pure crystal  Low concentration of [O] and [C] cm -3 Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

Czochralski silicon (Cz) & Epitaxial silicon (EPI)  Pull Si-crystal from a Si-melt contained in a silica crucible while rotating.  Silica crucible is dissolving oxygen into the melt  high concentration of O in CZ  Material used by IC industry (cheap)  Recent developments (~5 years) made CZ available in sufficiently high purity (resistivity) to allow for use as particle detector. Czochralski silicon Epitaxial silicon  Chemical-Vapor Deposition (CVD) of Silicon  CZ silicon substrate used  in-diffusion of oxygen  growth rate about 1  m/min  excellent homogeneity of resistivity  up to 150  m thick layers produced (thicker is possible)  price depending on thickness of epi-layer but not extending ~ 3 x price of FZ wafer RD50: Defect Engineering of Si Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

Standard FZ, DOFZ, MCz and Cz silicon 24 GeV/c proton irradiation  Standard FZ silicon type inversion at ~ 2  p/cm 2 strong N eff increase at high fluence Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

Standard FZ, DOFZ, MCz and Cz silicon 24 GeV/c proton irradiation  Standard FZ silicon type inversion at ~ 2  p/cm 2 strong N eff increase at high fluence  Oxygenated FZ (DOFZ) type inversion at ~ 2  p/cm 2 reduced N eff increase at high fluence Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

Standard FZ, DOFZ, MCz and Cz silicon 24 GeV/c proton irradiation  Standard FZ silicon type inversion at ~ 2  p/cm 2 strong N eff increase at high fluence  Oxygenated FZ (DOFZ) type inversion at ~ 2  p/cm 2 reduced N eff increase at high fluence  CZ silicon and MCZ silicon  “no type inversion“ in the overall fluence range (for experts: there is no “real” type inversion, a more clear understanding of the observed effects is obtained by investigating directly the internal electric field; look for: TCT, MCZ, double junction)  Common to all materials (after hadron irradiation, not after  irradiation):  reverse current increase  increase of trapping (electrons and holes) within ~ 20%

Mara Bruzzi and Michael Moll on behalf of the RD50 CERN Collaboration – LHCC, November 16, Characterization of microscopic defects -  and proton irradiated silicon detectors : Major breakthrough on  -irradiated samples – macroscopic changes of the depletion voltage and leakage current can be explained by electrical properties of measured defects 2005: Shallow donors generated by irradiation in MCz Si and epitaxial silicon after proton irradiation observed [APL, 82, 2169, March 2003] Almost independent of oxygen content:  Donor removal  “Cluster damage”  negative charge Influenced by initial oxygen content:  I–defect: deep acceptor level at E C -0.54eV (good candidate for the V 2 O defect)  negative charge Influenced by initial oxygen dimer content (?):  BD-defect: bistable shallow thermal donor (formed via oxygen dimers O 2i )  positive charge Levels responsible for depletion voltage changes after proton irradiation:  D-defect [G. Lindstroem, RD50 Workshop, Nov..2005] MCz n-type  MeV p irradiated,  =4×10 14 cm -2 [D. Menichelli, RD50 Workshop, Nov..2005] Epi 50  m  GeV p irradiated,  =4×10 14 cm -2

2003:  -irradiated samples For the first time macroscopic changes of the depletion voltage and leakage current can be explained by electrical properties of measured defects ! [APL, 82, 2169, March 2003]

2004- proton irradiated silicon detectors Almost independent of oxygen content: Donor removal “Cluster damage”  negative charge Influenced by initial oxygen content: deep acceptor level at E C -0.54eV (good candidate for the V 2 O defect)  negative charge I nfluenced by initial oxygen dimer content (?): BD-defect: bistable shallow thermal donor (formed via oxygen dimers O 2i )  positive charge Levels responsible for depletion voltage after 23 GeV proton irradiation: [I.Pintilie, RESMDD, Oct.2004] TSC after irradiation with 23 GeV protons with an equivalent fluence of 1.84x10 14 cm -2 recorded on Cz and Epi material after an annealing treatment at 600C for 120 min.

Divacancy has two charge states at 0.24 and 0.43 eV corresponding to 135 K and 233 K DLTS transitions. Bistability of the V 2 = peak observed after forward bias ( appearance of a peak at 195K corresponding to a decrease in the shallower the shallow peak at 135 K) explained as partial filling of the level due to band bending within a cluster (R. M. Fleming et al APL, 90, ) neutron irradiated silicon detectors : reverse current correlates with defects in clusters. The issue of Defect Clusters in neutron irradiated silicon

Hole traps H116 K, H140 K, and H152K (cluster related defects not present after  - irradiation and observed in neutron irradiated n-type Si diodes during 80 °C annealing) are responsible of long term annealing I. Pintilie, E. Fretwurst, and G. Lindström, APL 92, The issue of long term annealing: which traps ? Hole traps concentration is in fact in agreement with Neff changes during 80 °C annealing.

SCSI “Type Inversion” after neutrons but not after protons due to donor generation enhanced after proton irradiation [Pintilie, Lindstroem, Junkes, Fretwurst, NIM A 611 (2009) 52–68] Epi-Si irradiated with 23 GeV protons and reactor neutrons

I.Pintilie, NSS, 21 October 2008, Dresden Summary – defects with strong impact on the device properties at operating temperature Point defects E i BD = E c – eV  n BD =2.3  cm 2 E i I = E c – eV –  n I =2.3  cm 2 –  p I =2.3  cm 2 Clu ster related centers E i 116K = E v eV  p 116K =4  cm 2 E i 140K = E v eV  p 140K =2.5  cm 2 E i 152K = E v eV  p 152K =2.3  cm 2 E i 30K = E c - 0.1eV  n 30K =2.3  cm 2 V 2 -/0 VO -/0 P 0/+ H152K 0/- H140K 0/- H116K 0/- C i O i + /0 BD 0/++ Ip 0/-Ip 0/- E30K 0/+ B 0/- 0 charged at RT +/- charged at RT Point defects extended defects Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

I.Pintilie, NSS, 21 October 2008, Dresden Summary – defects with strong impact on the device properties at operating temperature Point defects E i BD = E c – eV  n BD =2.3  cm 2 E i I = E c – eV –  n I =2.3  cm 2 –  p I =2.3  cm 2 Cluster related centers E i 116K = E v eV  p 116K =4  cm 2 E i 140K = E v eV  p 140K =2.5  cm 2 E i 152K = E v eV  p 152K =2.3  cm 2 E i 30K = E c - 0.1eV  n 30K =2.3  cm 2 V 2 -/0 VO -/0 P 0/+ H152K 0/- H140K 0/- H116K 0/- C i O i + /0 BD 0/++ Ip 0/-Ip 0/- E30K 0/+ B 0/- 0 charged at RT +/- charged at RT Point defects extended defects Reverse annealing (neg. charge) leakage current + neg. charge (current after  irradiation) positive charge (higher introduction after proton irradiation than after neutron irradiation) positive charge (high concentration in oxygen rich material) Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

© A. Junkes Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with reactor neutrons N eff : Neutron irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

© A. Junkes N eff : Neutron irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with reactor neutrons Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

© A. Junkes N eff : Neutron irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with reactor neutrons Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

© A. Junkes N eff : Neutron irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with reactor neutrons Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

© A. Junkes N eff : Neutron irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with reactor neutrons

© A. Junkes N eff : Proton irradiation Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with 23 GeV Protons By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence

© A. Junkes N eff : Proton irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with 23 GeV Protons

© A. Junkes N eff : Proton irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with 23 GeV Protons

© A. Junkes N eff : Proton irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with 23 GeV Protons

© A. Junkes N eff : Proton irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with 23 GeV Protons

© A. Junkes N eff : Proton irradiation By A. Junkes, presented by U. Parzefal on behalf of the RD50 Collaboration, RESMDD10, Oct. 2010, Florence Epitaxial silicon ( EPI-DO, 72  m, 170  cm, diodes) irradiated with 23 GeV Protons

p-on-n silicon, under-depleted: Charge spread – degraded resolution Charge loss – reduced CCE p + on-n RD50: Device engineering p-in-n versus n-in-p (or n-in-n) detectors n-on-p silicon, under-depleted: Limited loss in CCE Less degradation with under-depletion Collect electrons (3 x faster than holes) n + on-p n-type silicon after high fluences: (type inverted) p-type silicon after high fluences: (still p-type) Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

Charge Multiplication by impact ionization Charge collected at electrodes in a semiconductor can not only be generated by ionising radiation but also by the acceleration of charge carriers by high electric fields, a phenomenon called impact ionization. This way electrons and holes promoted in conduction/valence bands by ionising radiation (primary charge) attain enough energy to create new electron-hole pairs (secondary charge). This mechanism is also origin of current breakdown in diodes when very high reverse voltage are applied. In a segmented device electric field is increased in the nearby of the collecting electrode due to accumulation of field lines. In irradiated devices the electric field is also enhanced by radiation induced defects. Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

Charge Multiplication Increase of the electric field close to the strips causing impact ionization/carrier injection when high concentrations of effective acceptors are introduced at very high fluences. CCE measured with p-type Si microstrip detectors at very high fluences shows evidence of a charge multiplication effect: 100% CCE seen after 3x10 15 n/cm 2, electrons after n/cm 2

Clinical dosimetry in radiotherapy is well known matter but high conformal radiotherapy modalities (IMRT, Stereotactic treatments with photons and protons, IMPT) pose problems due to the small radiation fields with high dose gradients, to the variation in space and time of the dose rate and to the variation in space and time of the beam energy spectrum. Semiconductor Devices in Clinical Radiotherapy

a) null bias; b) DC coupling; c) Sampling time and reset fixed by digital electronics (usually T ≈ 10ms); d) Only integrated charge measured. 1.5Gy/min Sensitivity of the device scales with diffusion length: Si dosimeter working principle: Photovoltaic Mode Mara Bruzzi, Danno da radiazione in semiconduttori Scuola Nazionale rivelatori ed elettronica per fisica delle alte energie, astrofisica 12 Aprile 2011, Legnaro, Italy

G.Rikner et al.Phys. Med. Biol. 28, 1983, Commercial single-pad Si dosimeters suffer of a strong dependence of the sensitivity on the accumulated dose. This issue is of concern in clinical radiotherapy applications. A significant improvement of long term stability of the dosimetric response has been achieved by us. Radiation Damage of Si dosimeters Standard Si dosimeters  Pre-irradiation up to 10kGy  Frequent Calibration needed S  L =  (D  ), D diffusion coefficient,  minority carrier lifetime: 1/  = 1/  0 + K   accumulated dose, with   minority carrier lifetime at zero dose.

Decrease in sensitivity with the accumulated dose due to the generation of a dominant trap acting as lifetime killer. 1/  - 1/  0 =  v th N t, N t = a  ; a = trap generation rate  capture cross section ; v th carrier thermal velocity. N t trap concentration. a DOFZ < a S FZ  increased radiation hardness of the device to radiotherapic beams. Improved radiation hardness of DOFZ Si a DOFZ = 5.0x10 7 cm -3 Gy -1, a STFZ = 8.1x10 7 cm -3 Gy -1 Material engineering concepts have been applied also to Silicon dosimeters for radiotherapy M. Casati et al. NIM A 2005

Another radiation hardness solution (≈ 1980) was: working with p-type materials. In fact, dominant center produced by electron irradiation has cross sections: This means that for this center is easier to capture holes. As diffusion is ruled by minority carriers, to get a transport less influenced by irradiation minority carriers must be electrons, thus material has to be p-type.

Our recipe: Low resistivity epitaxial p-type Si on MCz substrates Concept: active region is limited in any direction to a value shorter than L e at the highest dose of interest. Epitaxial Layer is used to limit active depth, guard-ring to limit active area. D. Menichelli et al., Nucl. Instr. Meth, 2007, vol. 583, C.Talamonti et al. Nucl. Instr. Meth A, vol. 658, p (2011).

first large area 2D map of dose with epi rad hard Si 103 Map of dose (head) in 19x12 cm 2 IMRT field shows very good agreement with TPS TPS Our 2D Si device M. Bruzzi et al., IEEE NSS MIC Symposium Conference Record, C. Talamonti et al. Presented at RESMDD12, October BUT … Silicon no water equivalent 

104 La tessuto-equivalenza Numero atomico effettivo Z a i = numeri frazionali di elettroni per grammo appartenenti ai materiali di numero atomico Z i 3 < m < 4 Il materiale del rivelatore deve interagire con la radiazione in modo simile al tessuto umano

105 Chemical Vapour Deposited polycrystalline Diamond 50  m 200  m Courtesy of Element Six After polishing and Material removal - Columnar growth – increased quality at growth side DEF Florence Mara Bruzzi, Semiconductor Detectors for Clinical Radiotherapy Scuola Fisica Medica, Univ. Firenze, 5 Novembre 2012

106 diamond is almost water equivalent it doesn’t perturb the radiation field → small fields the energy is absorbed as in the water → no correction factors high radiation hardness → long term stability high density → high sensitivity → small dimensions non toxic on-line application of pCVD diamond ☺  High defect density give rise to priming effects, polarization and in general instability of the signal. Diamond Dosimeters for clinical radiotherapy and beam monitoring

E. Borchi et al., TSC response of irradiated CVD diamond films, NIM A 426 (1999) As an off- line dosimeter ThermoLuminescent response and Thermally Stimulated Current can be read-out to evaluate the exposure dose Main TSC-TL component at 520K

108 Current response of the unirradiated sample (Au contact) showing the priming effect during the first 6 successive irradiations. Priming due to defect passivation High quality Low quality Morphologic quality influences sensitivity, dynamics and reproducibility pCVD diamond CVD under a Co 60  - (0.2Gy/min) Diamond: Native defects affecting dosimetric properties

R. Mori, M. Bruzzi ADAMAS, December 2012, GSI 1 MeV neutrons on pCVD Results: 1 MeV neutrons deactivate high temperature electrically active defects. This corresponds to have faster dynamics in dosimetry. [7] Bruzzi et al., DRM, 2001, Electrical properties and defect analysis of neutron irradiated undoped CVD diamond films.

110 M. Bruzzi et al., Appl. Phys. Lett, (2002) Decrease of TSC peaks due to deactivation of high temperature defects Defect Removal brings to better dynamics after neutron irradiation, good for conventional radiotherapy

Conclusions Microscopic view of radiation Damage is a useful tool to quantitatively explain macroscopic radiation damage in semiconductor devices Material/Device Enginering successful Best to work on the subject interdisciplinarly RD50 forum for development of Ultra Radiation Hard Semiconductor Detectors Performance can improve after irradiation (see diamond dosimeters) due to passivation of native defects.