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InP-SPAD Afterpulsing SPW2011

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InP-SPAD Afterpulsing SPW2011 Powered By Docstoc
					                      What Does SPAD Afterpulsing Actually
                          Tell Us About Defects in InP?


                        Mark Itzler, Mark Entwistle, and Xudong Jiang




SPW2011 – June 2011
   Presentation Outline

           50 MHz photon counting with RF matched delay line scheme
           Afterpulse probability (APP) dependence on hold-off time
           Fitting of APP data
                     inadequacy of legacy approach assuming one or few traps
                     new fitting based on broad trap distribution
           Implications of APP modeling for trap distributions
           Summary




SPW2011 – June 2011                          Princeton Lightwave Inc.           2
   Afterpulsing: increased DCR at high rate
        Single photon detection by avalanche multiplication in SPADs
        Avalanche carriers trapped at defects in InP multiplication region
        Carrier de-trapping at later times initiates “afterpulse” avalanches
        Serious drawback of afterpulsing → limitation on counting rate
                                                                                        p-contact metallization       SiNx passivation
                                                                                        +
                                                                                       p -InP diffused region                                   E

    # of trapped                                                           i-InP cap
                                                                                       multiplication region
         carriers                                                            n-InP charge
                                                                             n-InGaAsP grading
                                                                             i-InGaAs absorption
         primary                                                              +
                                                                             n -InP buffer
       avalanche                                                             n+-InP substrate
                                                                                            anti-reflection coating   n-contact metallization
                 short hold-off                                                                               optical input
                          time    afterpulses
                                                                                       trap sites located in
                                                                                       multiplication region
    # of trapped
         carriers
                                                                                                                                    Ec
                                                                                                                                    Ev


                           Long hold-off time
SPW2011 – June 2011                             Princeton Lightwave Inc.                                                                   3
    New results for RF delay line circuit

        Enhance matched delay line circuit to operate at higher repetition rate
           Inverted and non-inverted RF reflections cancel transients
           Based on existing PLI product platform




                                                          Cancel transient response synchronous
                                                            with photon arrival
                                                          Temporally gate out leading and trailing
                                                            transients
                                                          Set threshold for remaining avalanche
                                                            signal




                                                        Bethune and Risk,
                                                        JQE 36, 340 (2000)

SPW2011 – June 2011                        Princeton Lightwave Inc.                                  4
                Matched delay line solution to 50 MHz
                            Extension of cancellation scheme to higher frequencies
                                  More precise cancellation for reduced detection threshold → detect smaller avalanches
                                  Higher speed components to enable 50 MHz board-level operation
                            Measure cumulative afterpulsing using odd gates “lit”, even gates “dark”
                                  Take all counts in even gates above dark count background to be afterpulses

                                 OLD Performance (1 ns gate duration)                                                   NEW Performance (1 ns gate duration)
                          1E+0
                                                                                                                         PER DETECTED PHOTON
                          1E-1
                                                                                                                 1E-1




                                                                                        Afterpulse Probability
Afterpulse Probability
  (per 1 ns gate pulse)




                          1E-2
                                                                                                                 1E-2
                          1E-3
                                                                            10 MHz
                          1E-4                                              5 MHz                                1E-3                                                 50 MHz
                                                                            2 MHz
                                                                                                                                                                      33 MHz
                          1E-5                                              1 MHz
                                                                                                                                                                      10 MHz
                                                                            0.5 MHz                                                                                   1 MHz
                          1E-6                                                                                   1E-4
                                 5%     10%        15%        20%         25%     30%                                   0%           10%         20%            30%        40%
                                              Photon Detection Efficiency                                                              Photon Detection Efficiency

                     •       Absence of afterpulsing “runaway” indicates higher frequencies can be achieved
SPW2011 – June 2011                                                         Princeton Lightwave Inc.                                                                           5
    “Double-pulse” afterpulse measurement
         Use “time-correlated carrier counting” technique to measure afterpulses
                                                                                      Cova, Lacaita, Ripamonti,
         Trigger single-photon avalanches in 1st gate                                      EDL 12, 685 (1991)

         Measure probability of afterpulse in 2nd gate at Tn
         Use range of Tn to determine dependence of afterpulse probability on
          time following primary avalanche



              Double-pulse (“pump-probe”) method
                         T1




                                                                   probability
                                                                   Afterpulse
                                         ≈

                              T2
                                         ≈



                                                                                 T1       T2
                                                                                  Time



SPW2011 – June 2011                     Princeton Lightwave Inc.                                            6
    FPGA-based data acquisition
         Use FPGA circuitry to control gating and data collection
         Generalize double-pulse method to many gates
             Capture afterpulse counts in up to 128 gates following primary avalanche
             Temporal spacing of gates determined by gate repetition rate
         Allows capture of afterpulse count in any gate after avalance
             No need to step gate position as in double-pulse method


                                                                                 1 ns gates
                                 20 ns

          50 MHz:
                         1   2    3    4      5   6
                                                      ≈    126 127 128   1   2


                                      40 ns
          25 MHz:
                                                      ≈



                         1        2           3                  128     1


SPW2011 – June 2011                           Princeton Lightwave Inc.                        7
                                FPGA-based afterpulse measurements
                                  Obtain afterpulsing probability data at 5 frequencies for 128 gates
                              1E-1                                                                         1E-1                                                                                              1E-1
                                                                      33 MHz                                                                                            25 MHz                                                                       10 MHz




                                                                                  Afterpusle probability




                                                                                                                                                                                    Afterpusle probability
   Afterpusle probability




                              1E-2                                                                         1E-2                                                                                              1E-2

                              1E-3                                                                         1E-3                                                                                              1E-3

                              1E-4                                                                         1E-4                                                                                              1E-4
                                          33 MHz                                                                                            25 MHz                                                                       10 MHz

                              1E-5                                                                         1E-5                                                                                              1E-5
                                     10              100               1000                                        10                                  100               1000                                       10              100               1000
                                          Time following primary avalanche (ns)                                                             Time following primary avalanche (ns)                                        Time following primary avalanche (ns)

                              1E-1
                                                                      40 MHz                                                               1E-1
Afterpusle probability




                              1E-2                                                                                                                     All frequencies                                                     PDE = 20%
                                                                                                                                                                                                                           1 ns gates
                                                                                                              detected photon per gate
                                                                                                              Afterpusle probability per

                              1E-3
                                                                                                                                           1E-2
                              1E-4
                                          40 MHz                                                                                                                                                                                                  APP ~ 1%
                              1E-5
                                                                                                                                                                                                                                                   at 30 ns
                                     10              100               1000
                                          Time following primary avalanche (ns)                                                            1E-3
                              1E-1
                                                                      50 MHz                                                                                 50 MHz
     Afterpusle probability




                              1E-2                                                                                                                           40 MHz
                                                                                                                                           1E-4              33 MHz
                              1E-3
                                                                                                                                                             25 MHz
                              1E-4
                                          50 MHz                                                                                                             10 MHz
                              1E-5                                                                                                         1E-5
                                     10              100               1000
                                          Time following primary avalanche (ns)                                                                   10                100              1000
      SPW2011 – June 2011                                                                                                                               Time following primary avalanche (ns)
                                                                                                                                           Princeton Lightwave Inc.                                                                                              8
    Legacy approach to afterpulse fitting
       Try to fit afterpulse probability (APP) data with exponential fit
                                 Physically motivated by assumption of single dominant trap




                                              APP1  exp(-t/τ1)
                                1E-1
                                                                              PDE = 20%
                                                                              1 ns gates
                                                                                             Single exponential curve generally
                                                                                             fits range of ~5X in time
   detected photon per gate
   Afterpusle probability per




                                1E-2



                                1E-3

                                            50 MHz
                                1E-4        40 MHz
                                            33 MHz
                                            25 MHz
                                            10 MHz
                                1E-5
                                       10                100               1000
                                              Time following primary avalanche (ns)
SPW2011 – June 2011                                                   Princeton Lightwave Inc.                               9
    Legacy approach to afterpulse fitting
       Try to fit afterpulse probability (APP) data with exponentials
                                 Physically motivated by assumption of single dominant trap
       Single exponential not sufficient; assume second trap



                                                     APP2  exp(-t/τ2)
                                1E-1
                                                                              PDE = 20%
                                                                              1 ns gates
                                                                                             Single exponential curve generally
                                                                                             fits range of ~5X in time
   detected photon per gate
   Afterpusle probability per




                                1E-2



                                1E-3

                                            50 MHz
                                1E-4        40 MHz
                                            33 MHz
                                            25 MHz
                                            10 MHz
                                1E-5
                                       10                100               1000
                                              Time following primary avalanche (ns)
SPW2011 – June 2011                                                   Princeton Lightwave Inc.                               10
    Legacy approach to afterpulse fitting
       Try to fit afterpulse probability (APP) data with exponentials
                                 Physically motivated by assumption of single dominant trap
       Single exponential not sufficient; assume second trap
       Still need third exponential to fit full data set

                                                                 APP3  exp(-t/τ3)
                                1E-1
                                                                              PDE = 20%
                                                                              1 ns gates
                                                                                             Single exponential curve generally
                                                                                             fits range of ~5X in time
   detected photon per gate
   Afterpusle probability per




                                1E-2



                                1E-3

                                            50 MHz
                                1E-4        40 MHz
                                            33 MHz
                                            25 MHz
                                            10 MHz
                                1E-5
                                       10                100               1000
                                              Time following primary avalanche (ns)
SPW2011 – June 2011                                                   Princeton Lightwave Inc.                               11
    Legacy approach to afterpulse fitting
       Can achieve reasonable fit with several exponentials
       …but choice of time constants is completely arbitrary!
         → depends on range of times used in data set
       Our assertion: No physical significance to time constants in fitting
         → simply minimum set of values to fit the data set in question

                                1E-1
                                                                                PDE = 20%
                                                                                1 ns gates
   detected photon per gate
   Afterpusle probability per




                                1E-2
                                                                                                   APP = C1exp(-t/τ1)
                                            τ1 = 30 ns
                                                                                                         + C2exp(-t/τ2)
                                1E-3                                                                       + C3exp(-t/τ3)
                                                         τ2 = 120 ns
                                            50 MHz
                                1E-4        40 MHz
                                            33 MHz                     τ3 = 600 ns
                                            25 MHz
                                            10 MHz
                                1E-5
                                       10                100               1000
                                              Time following primary avalanche (ns)
SPW2011 – June 2011                                                     Princeton Lightwave Inc.                            12
    What other functions describe APP?
   Good fit for simple power law T-α with α ≈ -1
               → Is power law behavior found for other afterpulsing measurements?
               → Is the power law functional form physically significant?

                                1E-1
                                                                              PDE ~ 20%
                                               y = 0.52x-1.07                 1 ns gates
   detected photon per gate
   Afterpusle probability per




                                1E-2
                                                                                                APP = C T-α

                                1E-3

                                            50 MHz
                                            40 MHz
                                1E-4
                                            33 MHz
                                            25 MHz
                                            10 MHz
                                1E-5
                                       10               100               1000
                                             Time following primary avalanche (ns)
SPW2011 – June 2011                                                  Princeton Lightwave Inc.                 13
    Afterpulsing data from Univ. Virginia

     Good fit for power law T-α with α ≈ -1.0 to -1.1


                                                                                               data from Joe Campbell, UVA
                                         1E+0

                                                                  y = 3.44x-1.03                 UVA data
                                                                                                  ~30% PDE
                                                                       y=   2.20x-1.05
                                         1E-1                                                                Double-pulse
                Afterpulse probability




                                                                              y = 0.74x-1.09                   method
                                                                                                             PLI SPADs
                                         1E-2



                                         1E-3        3 ns gate
                                                     2 ns gate
                                                     1 ns gate
                                         1E-4
                                                10                         100                           1000
                                                         Time following primary avalanche (ns)
SPW2011 – June 2011                                               Princeton Lightwave Inc.                               14
    Afterpulsing data from NIST

     Good fit for power law T-α with α ≈ -1.15 to -1.25

                                                                                                         data from Alessandro Restelli
                                                                                                             and Josh Bienfang, NIST
                                           1E+0
                                                                              y = 2.92x-1.16                NIST data
                                                                                                                ~15% PDE
                                               1E-1                                  y = 0.49x-1.21
                                                                                                                           Double-pulse
                      Afterpulse Probability




                                                                                                                             method
                                               1E-2
                                                                                                                           PLI SPADs
                                               1E-3


                                               1E-4
                                                          1.50 ns gate
                                               1E-5       1.00 ns gate                  y = 0.13x-1.24
                                                          0.63 ns gate
                                                          0.50 ns gate                         y = 0.06x-1.25
                                               1E-6
                                                      1                  10              100               1000
                                                                Time following primary avalanche (ns)
SPW2011 – June 2011                                                      Princeton Lightwave Inc.                                      15
    Afterpulsing data from Nihon Univ.

     Good fit for power law T-α with α = -1.38


                                                                                                    data from Naota Namekata, Nihon U.
                                                            1
                                                                                                        Nihon U. data
                      Normalized Afterpulse Probability




                                                                                                                213 K

                                                                                                                        Autocorrelation
                                                           0.1                                                            test of time-
                                                                                                                          tagged data
                                                                                                                        PLI SPADs


                                                          0.01                   y = 237.66x-1.38




                                                0.001
                                                                 10             100                1000
                                                                      Time following primary avalanche (ns)
SPW2011 – June 2011                                                          Princeton Lightwave Inc.                                16
    Literature on InP trap defects

       Literature on defects in InP describes dense spectrum of levels

       Instead of assuming one or a few dominant trap levels:
        → consider implications of a broad distribution for τ


       Deep-level traps in
       multiplication region
                                                                                                                                           Ec – 0.24 eV
                                                                                                                                           Ec – 0.30 eV
                                                 SiNx passivation
                      p-contact metallization
                                                                                                                                           Ec – 0.37 eV
                     p+-InP diffused region                                Electric field                                                  Ec – 0.40 eV
         i-InP cap
                     multiplication region                                                                                                 Ec – 0.55 eV
           n-InP charge
           n-InGaAsP grading
           i-InGaAs absorption
            +
           n -InP buffer
           n+-InP substrate                                                                           Early      Later      Radiation
                       anti-reflection coating   n-contact metallization                              work       work       effects
                                         optical input

                                                                                                                  W. A. Anderson and K. L. Jiao, in “Indium Phosphide
                                                                                                                  and Related Materials: Processing, Technology, and
                                                                                                                  Devices”, A. Katz (ed.) (Artech House, Boston, 1992)




SPW2011 – June 2011                                                                         Princeton Lightwave Inc.                                                     17
    Implications of trap distribution on APP
       Develop model for APP with distribution of detrap rates R ≡ 1/τ
             – APP related to change in trap occupation: dN/dt ~ R exp(-t R)
             – Integrate over detrapping rate distribution D(R)
                             → APP ~ ∫ dR D(R) R exp(-t R)

         D(R)                     δ(R – R0)                   D(R)            “Uniform”
                                  single trap



narrowest                                                                                      widest
distribution                 R0          R                                           R    distribution

                      D(R)               Normal       D(R)              “Inverse”
                                                                        D(R) α 1/R




                                    R0           R                             R
SPW2011 – June 2011                          Princeton Lightwave Inc.                            18
    Implications of trap distribution on APP

       “Single trap” leads to exponential behavior
             – Fitting requires multiple exponentials and is arbitrary

       Normal distribution is similar to single trap
             – Gaussian broadening of δ(R – R0) doesn’t change exponential behavior

       “Uniform” and “inverse” distributions can be solved analytically
             – Require assumptions for a few model parameters
                      Minimum detrapping time:               τmin = 10 ns
                      Maximum detrapping time:               τmax = 10 µs   just sample values;
                      Number of trapped carriers:            n=5            can be generalized
                      Detection efficiency:                  20%




SPW2011 – June 2011                          Princeton Lightwave Inc.                         19
    Modeling results for APP
           APP results for Uniform and Inverse detrap rate distributions D(R)
           APP behavior fit well by T-α for 10 ns to 10 µs
                                           – Value of α depends on model parameter values, but α is well-bounded

                                          1E+0
    Normalized afterpulsing probability




                                                                         Inverse
                                          1E-1

                                          1E-2                                               y = 25.23x-1.18
                                                      y = 200.93x-2.00                                         Inverse D(R):
                                          1E-3                                                                    T-α with 1.05 < α < 1.30

                                          1E-4                               Uniform
                                                                                                               Uniform D(R):
                                          1E-5                                                                    T-α with 1.9 < α < 2.1

                                          1E-6
                                                 10             100              1000                     10000
                                                         Time following primary avalanche (ns)
SPW2011 – June 2011                                                            Princeton Lightwave Inc.                                    20
    Insights from modeling of APP
     Inverse distribution provides correct power law behavior
           – More traps with slower release rates D(R) α 1/R          D(R)
           – Other distributions considered do not agree with data



                                                                                                   R
     Inverse distribution not necessarily a unique solution
           – But it provides more accurate description than single trap or uniform
     Slower falloff of APP with hold-off time for Inverse distribution
           – Need longer hold-off times to achieve same relative decrease in total AP
     Other possible explanations for power law behavior to explore
           – Role of field-assisted detrapping, especially in non-uniform E-field
           – Model in literature cites power law behavior for “correlated” detrapping
                                                                                    A. K. Jonscher,
                                                                       Sol. St. Elec. 33, 139 (1990)




SPW2011 – June 2011                      Princeton Lightwave Inc.                                      21
    Afterpulsing data on Silicon SPADs

     Neither power law nor exponential provide particularly good fit!
            Nature of defects in Si SPADs may be categorically different than for InP

                                                                                                                      data from Massimo Ghioni,
                                        -2
                                                                     Power law                                               Politecnico di Milano
                                  1.E-02
                                     10
                  Afterpulsing Probability Density (ns-1)




                                                                                                              Silicon SPADs
                                                                          y = 0.21x-1.54
                                                            -3                                                T = -25 C, Pap = 6%
                                  1.E-03
                                     10
                                                                                                                                    Double-pulse
                                                                                                                                      method
                                                            -4
                                  1.E-04
                                     10
                                                                 y = 0.0002e-0.005x

                                        -5                               Exponential
                                  1.E-05
                                     10


                                        -6
                                  1.E-06
                                     10


                                  1.E-07-7
                                     10
                                          10                                               100                     1000
                                                                                                  Time (ns)
SPW2011 – June 2011                                                                    Princeton Lightwave Inc.                                 22
   Summary
    Reached 50 MHz photon counting with RF matched delay line scheme
          Significant further repetition rate increases should be feasible

    Fitting of APP data with multiple exponentials not physically meaningful
          Extracted detrapping times are arbitrary, depend on hold-off times used
          Literature on defects in InP suggests possibility of broad distribution of defects

    Consistent power law behavior of APP data found by various groups
          APP vs. time T described by T-α with α ~ 1.2 ± 0.2

    Assumption of “inverse” distribution D(R) α 1/R for detrapping rate R
     provides best description of data among distributions considered so far
          Not unique, but establishes general behavior
          May be other models that predict power law APP behavior for dominant trap
          Further modeling can predict behavior for different operating conditions




SPW2011 – June 2011                          Princeton Lightwave Inc.                           23
                      BACK-UP SLIDES




SPW2011 – June 2011       Princeton Lightwave Inc.   24
   Electric field engineering in APDs
       Vertical structure to realize SAGCM structure for well-designed APD
             Multiplication gain: high field for impact ionization
             Carrier drift in absorber: low but finite absorber field
             Avoid of tunneling in all layers
             Eliminate interface carrier pile-up
       Control of 3-D electric field distribution to avoid edge breakdown


                                  p-contact metallization SiNx passivation
                                                                                                E
                                p+-InP diffused region

                 i-InP cap
                                   multiplication region
                      n - InP charge
                      n -InGaAsP grading

                      i - InGaAs absorption
                       +
                      n - InP buffer
                       +
                      n - InP substrate                                                Schematic design for
                                  anti-reflection coating    n-contact metallization   InGaAs/InP SPAD for
                                                                                       1.5 μm photon counting
                                                    optical input

SPW2011 – June 2011                                         Princeton Lightwave Inc.                        25

				
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