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Semiconductor detectors

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					Semiconductor detectors

      An introduction to
semiconductor detector physics
 as applied to particle physics
                     Contents
      4 lectures – can’t cover much of a huge field


•   Introduction
•   Fundamentals of operation
•   The micro-strip detector
•   Radiation hardness issues
 Lecture 4 – Radiation Damage
• Effects of radiation
   – Microscopic
   – Macroscopic
   – Annealing
• What can we do?
   –   Detector Design
   –   Material Engineering
   –   Cold Operation
   –   Thin detectors/Electrode Structure – 3-D device
      Effects of Radiation

• Long Term Ionisation Effects
  – Trapped charge (holes) in SiO2
  – interface states at SiO2 - Si interface
  – Can’t use CCD’s in high radiation environment
• Displacement Damage in the Si bulk
  – 4 stage process
  – Displacement of Silicon atoms from lattice
  – Formation of long lived point defects & clusters
    Displacement Damage

• Incoming particle undergoes collision with
  lattice
  – knocks out atom = Primary knock on atom
• PKA moves through the lattice
  – produces vacancy interstitial pairs (Frenkel Pair)
  – PKA slows, reduces mean distance between
    collisions
  – clusters formed
• Thermal motion 98% lattice defects anneal
  – defect/impurity reactions
• Stable defects influence device properties
PKA
 • Clusters formed when
   energy of PKA< 5keV
 • Strong mutual
   interactions in clusters
 • Defects outside of
   cluster diffuse + form
   impurity related defects
   (VO, VV, VP)
 • e &  don’t produce
   clusters
                     Effects of Defects


EC
       e                  e       e                           e


EV         h          h                    h

     Generation   Recombination       Trapping         Compensation




      Leakage Current             Charge Collection   Effective Doping
                                                          Density
                                             Reverse Current
                 10-1
                         n-type FZ - 7 to 25 Kcm
                         n-type FZ - 7 Kcm
                                                                               • I = Volume
                 10-2    n-type FZ - 4 Kcm
                                                                               • Material independent
I / V [A/cm3]




                         n-type FZ - 3 Kcm
                         p-type EPI - 2 and 4 Kcm
                 10-3

                   -4                                n-type FZ - 780 cm
                                                                                 – linked to defect
                 10                                  n-type FZ - 410 cm
                                                     n-type FZ - 130 cm           clusters
                 10-5                                n-type FZ - 110 cm


                   -6
                                                     n-type CZ - 140 cm
                                                     p-type EPI - 380 cm      • Annealing material
                 10 11
                               1012          1013         1014          1015
                   10
                                        eq [cm-2]                               independent
                                                                               • Scales with NIEL
    = 3.99  0.03 x 10-17Acm-1                                                • Temp dependence
after 80minutes annealing at 60C                                                                   E 
                                                                                   I T   T 2 exp  g 
                                                                                                    2kT 
                                                                                                        
                                    Effective Doping Density
                     5
                         Neutron irradiation                           300
                     4     1.8 Kcm
                           2.6 Kcm
                                      Wacker
                                      Polovodice
                                                                                                • Donor removal and
                                                                       250




                                                                             Vdep [V] (300m)
|Neff| [1012 cm-3]




                           3.1 Kcm
                     3     4.2 Kcm
                                      Wacker
                                      Topsil
                                                                       200
                                                                                                  acceptor generation
                                                                       150
                                                                                                  – type inversion: n  p
                     2
                                                                       100                        – depletion width
                     1
                                                                       50                           grows from n+
                                                                                                    contact
                     0        0.5             1              1.5   2
                                        eq [10 cm ]
                                                   14   -2
                                                                                                • Increase in full
                                                                                                  depletion voltage
  N eff    N eff 0 exp  c              – V  Neff
           = 0.025cm-1 measured after beneficial anneal
                                           Effective Doping Density
                10                                                                                               • Short-term beneficial
                    8                                                                                              annealing
 Neff [1011cm-3]




                             NA = ga eq                                     NY, = gY eq
                    6                                                                                            • Long-term reverse
                    4
                                                                              NC
                                                                                                                   annealing
                    2
                                                            gC eq                                                           – temperature dependent
                                                                              NC0
                    0
                                                                                                                1000
                                                                                                                             – stops below -10C                                      1000
                        1       10          100      1000        10000
                                                       o
                                annealing time at 60 C [min]                                                    800                                                                   800
                                                                                             Vdep (250m) [V]                                             standard silicon
                                                                                                                           operation voltage: 600 V
                                                                                                                600                                                                   600
                        RA                   BA             RA                      BA

                                                                                                                400                                                                   400
                                  A                                      B
                                                  Ne ff = Zero                                                  200                                                                   200
                                                                                                                                                                oxygenated silicon
                                                                                                                       0       1      2      3        4     5    6    7      8   9   10
                                           Increasing Radiation
                                                                                                                                                      time [years]
    After type inversion                                           Before type inversion
    Signal speed from a detector
•   Duration of signal = carrier collection time
•   Speed  mobility & field
•   Speed  1/device thickness
•   PROBLEMS
    – Post irradiation mobility & lifetime reduced
       •  lower  longer signals and lower Qs
    – Thick devices have longer signals
            Signal with low lifetime
                   material
• Lifetime, , packet of charge Q0 decays

                                             t 
• In E field charge drifts     Q (t )  Qo e
• Time required to drift distance x:
                  x x
               t 
                  v E
• Remaining charge:                     x E            x L
   – Drift length, L       Q( x)  Q0 e       Q 0 e
    is a figure of merit.
                     Induced charge
• Parallel plate detector:

        1
    Qs   Q( x)dx   Q0e  x L dx
        d0
           d
                    1
                    d0
                        d                    L
                                      Qs  Q0 1  e d L
                                             d
                                                           
                                                              Qs L
                                                     L  d :   
• In high quality silicon detectors:                          Q0 d
•   10ms, e = 1350cm2V-1s-1, E = 104Vcm-1
  L  104cm (d ~ 10-2cm)

   – Amorphous silicon, L  10m (short lifetime, low mobility)
   – Diamond, L  100-200m (despite high mobility)
   – CdZnTe, at 1kVcm-1, L  3cm for electrons, 0.1cm for holes
            What can we do?
•   Detector Design
•   Material Engineering
•   Cold Operation
•   Electrode Structure – 3-D device
               Detector Design
• n-type readout strips on n-type substrate
    – post type inversion  substrate p type  depletion
      now from strip side
    – high spatial resolution even if not fully depleted
•   Single Sided
•   Polysilicon resistors
•   W<300m thick  limit max depletion V
•   Max strip length 12cm  lower cap. noise
         Multiguard rings

                     • Enhance high
                       voltage operation
                     • Smoothly decrease
                       electric field at
                       detectors edge    back plane
  Poly                                      bias
               strip bias
Guard
rings          V
           Substrate Choice
• Minimise interface states
• Substrate orientation <100> not <111>
  – Lower capacitive load
  – Independent of ionising radiation
• <100> has less dangling surface bonds
              Metal Overhang

     • Used to avoid breakdown performance
       deterioration after irradiation
                                                                           2
p+                       SiO2




                                     Breakdown Voltage (V)
        (1)   (2)        n
n+                                                                                     1
               4m

                             0.6m
                    p+
                                                                   Strip Width/Pitch
                                                             <111> after 4 x 1014 p/cm2
      Material Engineering

• Do impurities influence characteristics?
• Leakage current independent of impurities
• Neff depends upon [O2] and [C]
                         1E+13
                                                                          [C] = 0.0437
                         9E+12
                                                                                                   500




                                                                                                         VFD for 300 m thick detector [V]
                         8E+12
                                     Standard (P51)
                         7E+12       O-diffusion 24 hours (P52)                                    400
                                     O-diffusion 48 hours (P54)            St = 0.0154
          |Neff| [cm ]
          -3




                         6E+12
                                     O-diffusion 72 hours (P56)
                         5E+12       Carbon-enriched (P503)                                        300

                         4E+12
                                                                                                   200
                         3E+12                                             [O] = 0.0044 0.0053

                         2E+12
                                                                                                   100
                         1E+12

                             0                                                                     0
                                 0    1E+14        2E+14          3E+14          4E+14         5E+14
                                                                           -2
                                        Proton fluence (24 GeV/c ) [cm ]
O2 works for charged hadrons

• Neff unaffected by O2 content for
  neutrons
• Believed that charge particle irradiation
  produces more isolated V and I
                                                                                                              V + O  VO
                      7
                                standard FZ
                      6
                                                                                                              V + VO  V2O
                                   neutrons                                          400
                                   pions



                                                                                           Vdep [V] (300m)
 |Neff| [1012 cm-3]




                      5            protons                        oxygen rich FZ
                      4
                                                                        neutrons
                                                                        pions
                                                                        protons
                                                                                     300
                                                                                                              V2O  reverse annealing
                      3                                                              200
                      2

                      1
                                                                                     100                      High [O] suppresses V2O
                      0   0.5       1         1.5        2        2.5         3    3.5
                                                                                                              formation
                                          eq [10 cm ]
                                                    14       -2
Charge collection efficiency

• Oxygenated Si enhanced due to lower
  depletion voltage

  CCI ~ 5% at 300V
  after 3x1014 p/cm2


                        CCE of MICRON ATLAS prototype
                       strip detectors irradiated with 3 1014 p/cm2
                                                             ATLAS operation
                   Damage for ATLAS barrel layer 1

                   1000                                                                  1000

                   800                                                                   800
                                                             standard silicon
Vdep (250m) [V]




                              operation voltage: 600 V                                          Use lower resistivity Si to
                   600                                                                   600
                                                                                                increase lifetime in neutron field
                   400                                                                   400
                                                                                                Use oxygenated Si to increase
                   200                                                                   200
                                                                                                lifetime in charge hadron field
                                                                   oxygenated silicon
                          0       1      2      3        4     5    6    7      8   9   10
                                                         time [years]
                                   Charge collection loss at SLHC
                                              fluences
                                                                                                  Collected charge at 1000V as a
                                                                                          30000
                                                                                                  function of radiation fluence




                                                                       Charge collected
                                                                                          25000

                       Collected charge as a function of                                  20000
                       bias voltage for different irradiation                             15000
                       fluences                                                           10000

                                                                                          5000

                                                                                              0
                       30000
Collected charge (e)




                                                                                                  0   2        4       6      8         10   12
                       25000                                                                                                       -3
                                                                                                          Fluence (1E15 1MeV neq cm )
                       20000

                       15000                                                                  Charge collected is more than
                       10000                                                                  expected from previous equations
                        5000                                                                  and fits to lifetime with fluence
                          0                                                                   The reason is explained due to
                               0    200   400    600     800    1000                          avalanche multiplication under the
                                                                                              strip implant at the very high
                                           Bias Voltage (V)                                   electric fields in the detector
              Cold Operation
• Know as the “Lazarus
  effect”
• Recovery of heavily
  irradiated silicon
  detectors operated at
  cryogenic temps
  – observed for both
    diodes and microstrip
    detectors
                             The Lazarus Effect
• For an undepleted heavily irradiated detector:                                                            undepleted region             d
                     2
                        t drift                                                  1                        active region                     D
                d                                                        d2 
          CCE    exp        
                 D                                            where          N eff
                        trap 
                           T = 300 K                                                                        T = 77 K
      e                                  conduction band                                  e
                                                                                                                           conduction band
      electron trapping               electron de-trapping


                                                                                                  e
                                                                                              trap filled


                                                                                                                 trap filled

                                                                                                                     h
                      hole trapping             hole de-trapping
                                                                                                                               valence band

      h                                        valence band
                                                                                          h
• Traps are filled  traps are neutralized
                    Neff compensation (confirmed by experiment)
          B. Dezillie et al., IEEE Transactions on Nuclear Science, 46 (1999) 221
   Reverse Bias




Measured at 130K - maximum CCE
CCE falls with time to a stable value
                 Cryogenic Results
• CCE recovery at cryogenic temperatures
   – CCE is max at T ~ 130 K for all samples
   – CCE decreases with time till it reaches a stable value

• Reverse Bias operation
   – MPV ~5’000 electrons for 300 m thick
       standard silicon detectors irradiated with
       21014 n/cm2 at 250 V reverse bias and T~77 K
   – very low noise

• Forward bias is possible at cryogenic temperatures

• No time degradation of CCE in operation with forward bias or in
  presence of short wavelength light
   – same conditions: MPV ~13’000 electrons
          Electrode Structure
• Increasing fluence
  – Reducing carrier lifetime
  – Increasing Neff
     • Higher bias voltage
     • Operation with detector under-depleted
• Reduce electrode separation
  – Thinner detector  Reduced signal/noise
    ratio
  – Close packed electrodes through wafer
The 3-D device
       • Co-axial detector
          – Arrayed together
       • Micron scale
          – USE Latest MEM
            techniques
       • Pixel device
          – Readout each p+ column
       • Strip device
          – Connect columns together
                                   Operation
                                                    -ve              -ve               -ve
      SiO2   +ve             -ve   +ve
                                               p+

                        h+
                                                                         h+

                                          Bulk n
                                                                                                          E
                   e-
                                                                    e-                          W2D

                                               n+
                   W3D               Equal detectors
                                       thickness                    +ve
                E                      W2D>>W3D
Carriers swept horizontally                                               Carriers drift total
Travers short distance between electrodes                                 thickness of material

                                           Proposed by S.Parker, Nucl. Instr. And Meth. A 395 pp. 328-343(1997).
               Advantages
• If electrodes are close
  – Low full depletion bias
  – Low collection distances
  – Thickness NOT related to collection distance
  – No charge spreading
  – Fast charge sweep out


                                                
                  2
          qN DW                    L
   V fd                    Qs  Q0 1  e d L
            2                     d
                    A 3-D device
• Form an array of holes
• Fill them with doped poly-
  silicon
• Add contacts
   – Can make pixel or strip
     devices
• Bias up and collect charge
         Real spectra

                                   80




              Signal (ADC value)
                                   70
                                   60
                                   50
                                   40
                                   30
                                   20
                                   10
                                    0
                                        0   10   20         30           40   50   60
                                                      Bias Volatge (V)




Very good energy                            At 20V
   resolution                               Plateau in Q collection
                                            Fully active
                                    3-D Vfd in ATLAS

                         2000                                                       s ta n d a rd s ilic o n                       2000
   ( 2 0 0 m ) [ V ]




                         1500                                                                                                      1500
                                                                 6 0 0 0 e f o r B - la y er

                         1000                                                                                                      1000
                   dep




                                    o p e r a ti o n v o l t a g e : 6 0 0 V
        V




                          500                                                                                                      500

                                                                                               o x y g e n a te d s ilic o n
                                                                                                                               •    3D detector!
                                0          1           2           3            4     5       6          7       8       9     10
                                                                               tim e [y e a rs ]


Damage projection for the ATLAS B-layer
(3rd RD48 STATUS REPORT CERN LHCC 2000-009, LEB Status Report/RD48, 31 December 1999).
                                                                3D charge collection
                                              • Small electrode spacing
                                                      – Increases charge collection due to lower
                                                        drift distance
                                                      – Reduces bias voltage
                                                      – Increases fields and therefore enhances
                                                        charge multiplication effects
                                                                                                                measured collected (solid line
                                                                                                           TheThe collected charge charge
                                              30000
                         35000                                                                             from 285 um thick p-type 3D signal
                                                                                                               and open circles) and the
Collected Charge (e)




                                                                                                           Signal to noise ratio
                                                                                                      50
                       Collected charge (e)




                           25000
                         30000
                                                                                                               to noise ratio at a bias of
                                                                                                           detectors operate(dashed line and
                         25000
                           20000                                                                      40
                                                                                                               solid than 150 V (solid line
                                                                                                           no more diamonds) as a function of
                         20000                                                                        30
                           15000                                                                                open circles) and the double
                                                                                                           andirradiation dose for320 um
                         15000
                                              10000                                                   20
                                                                                                               side 3D planar detectors
                                                                                                           thick p-type detectors bias to their
                         10000
                                      5000
                                               5000                                                   10   operated at a bias up to 1000 V
                                                                                                               maximum sensible bias voltage
                                                0 0                                                   0
                                                                                                           (dashed line and closed 250V and
                                                                                                               (which was between
                                                  0   0   5         510        15     10    20   25
                                                                                                           diamonds) as a function of
                                                                                                               350V).
                                                                          15               -2
                                                           Fluence / 15 (1MeV cm )
                                                          Fluence / 1010(1 MeV neq neq-2cm )               irradiation dose.
                 Summary
• Tackle reverse current
   – Cold operation, -20C
   – Substrate orientation
   – Multiguard rings
• Overcome limited carrier lifetime and
  increasing effective doping density
   – Change material
   – Increase carrier lifetime
   – Reduce electrode spacing
                Final Slide
•   Why?
•   Where?
•   How?
•   A major type
•   A major worry

				
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posted:11/15/2012
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