# Presentation: 704227

## First Look at the Hamamatsu MPPC

*First Look at the Hamamatsu MPPC*

- Adam Para, Fermilab
- Research Techniques Seminar,
- November 8, 2007

## Cast of Characters

- Many people have contributed to the construction of the testing facilities, collection and interpretation of results presented here. The list includes, but is not limited to:
- Grzegorz Deptuch
- Sten Hansen
- Georgios Mavromanolakis
- Paul Rubinov
- Niki Saolidou
- Martin Trempl
- Staff of the SiDet

## Outline

- Motivation and goals
- MPPC/SiPM/PPD : the Standard Model
- Samples and Measurements
- Comparison with the Standard Model
- Determination of the parameters of the Standard Model

## Motivation 

- PPD is a novel kind of photodetectors offering new possibilities for the HEP detectors design. The most attractive/enabling  features include:
- Tiny size  ( hermeticity!)
- Immunity to magnetic fields
- Resolution ( Calorimetry)
- Fast Timing ( TOF)
- Potential contribution of Fermilab: miniature and inexpensive readout ASIC

## Goals

- Understand the response of the detectors as a function of:
- Operating conditions:
- Bias voltage
- Temperature
- Light intensity
- Temporal characteristics of the light input
- Provide measurements of electrical characteristics of the detectors as an input for the ASIC design
- Develop a procedure for the calibration of  the response of the detectors: interpretation of the detector signals in terms of the incoming light intensity

## Photodiodes, Avalanche, Geiger Mode

- _Photodiodes:_
-  p-n junction , reverse bias
-  Electron-hole pair generated by an incoming             photon  drifts to the edges of the depleted region
-  I(t) = QE * q * dN/dt(t)
-  Absolute calibration
-  No gain
-  Suitable for large signals

## Photodiodes, Avalanche, Geiger Mode

- _Avalanche Photodiodes:_
-  Photodiodes operating at higher bias voltage
-  Higher voltage –> stronger electric field -> higher energy of drifting carriers -> impact ionization -> Gain
-  (Im)Balance  between the number of carriers leaving the depletion region and the number  generated carriers per unit  time:  dNleave/dt > dNgenerated/dt
- Stochastic process: signal quenches when the ‘last’ electron/hole fails to ionize.
-  Large fluctuations of the multiplication process -> Gain fluctuations -> Excess noise factor (beyond-Poisson fluctuations)

## Photodiodes, Avalanche, Geiger Mode

- _Geiger Mode Avalanche Photodiodes:_
-  Avalanche Photodiodes operated at the elevated bias voltage.
-  Larger field -> carriers gain kinetic energy faster => shorter mean free path
-  Breakdown voltage: nothing really breaks down, but dNleave/dt  = dNgenerated/dt (on average) at this voltage
-  _Some_ electrons can generate self-sustaining avalanche (current limited eventually by the series resistance)
-  Probability of the avalanche generation increases with bias voltage (electric field)
-  Operation mode: one photon  (sometimes) continuous current

- Q=CD*(Vbias-Vbd)

## Slide 11

## Challenges of Multipixel Devices

- Edge effects: fill factor, effective quantum efficiency (PDE)
- Premature Edge Breakdown -> guard rings, other local complications
- Single pixel – standard response, Q=Cd*(Vbias-Vbd). Multiple pixels  pixel-to-pixel uniformity?
- Photon-mediated cross-talk: ~3 photons/105 electrons in the avalanche
- Impact ionization requires high fields, often incompatible with standard processes
- Dynamic range/linearity (number of pixels)

## Categories of Observed Signals in Multipixel Devices

-  Single avalanches and afterpulses exist in single and multipixel detectors
-  Afterpulses have lower amplitude if they occur befor the cell is fully recovered
-  Double signal are specific to multipixel detectors

## Examples of Practical Questions

- Which detector to use (ignoring details like price and current availability) ? Hamamatsu/CPTA/IRST/SensL? Does is matter?
- What pixel size? 25/40/50/100 microns? How many pixels?
- What bias voltage to use?
- What temperature?
- Need to stabilize temperature and voltage? Or is it sufficient to read them back? Or change voltage with the changes of temperature?
- Need external calibration? Or is the single/nth pe peak sufficient to calibrate the gain?
- How to do the large scale quality control? Is the static (DC current) measurement sufficient?
- How long integration gate? How many time samples?
- In many instances  the answer depends on the application.
- We are trying to collect data to serve as basis for such decisions.

## Samples

- We have samples of detectors from different sources.
- The results shown here are obtained with the samples of Hamamatsu MPPC. Why Hamamatsu?
- You have to start somewhere
- This is the only sample of a commercial product
- These detectors come with detailed detector-by-detector characterization. Very helpful to establish credibility of measurements
- We have samples of 25, 50 and 100 micron pixel devices. Comparison of their characteristics provides a good test of out Standard Model. Most of the detailed studies, so far, carried out for 25 micron devices. Why?
- Most attractive for the calorimetric/pulse height measurement applications (dynamic range!)
- Probably most challenging case

## Static Measurements

- Static characteristics - IV curves at fixed temperatures:
- Keithley 2400 sourcemeter
- Peltier-cooled cold plate
- Labview data acquisition program
- Forward bias  series (quenching) resistance
- Reverse bias  breakdown voltage, integral behavior of the detector s a function of the operating temperature
- Room for improvement: Peltier cold plate – source of the electrical noise.  Limiting factor at very low currents (~pA) for some detectors, preventing the measurements for others.

## Forward Bias Scan

- Exponential
- growth with V

- Limited by quenching
-  resistor

- dI/dV = 1/R

- Resistance decreases
-  with temperature
- (polysilicone)

## Quenching Resistance Summary

| Detector 
type | Quenching Resistor 
@ 25 oC, k | dR/dT
k/oC | 1/R dR/dT |
| --- | --- | --- | --- |
| 25  | 200 | 2.23 | 0.011 |
| 50  | 105 | 1.08 | 0.010 |
| 100  | 85 | 0.91 | 0.011 |

## Reverse Bias Scan  Breakdown Voltage

- Claudio Piemonte

## Reverse Bias Scan

- 100 pA

- 1 V above breakdown
- I~5x10-7A
- Gain ~ 4x106
- ‘Photodiode’  current
- level ~ 10-13 A
- How relevant  is the
- current below the
- breakdown voltage?

- Quenching
- resistance

- Temperature

- Breakdown

- Current below the breakdown
- Voltage increases with temperature
- (beware of condensation!)

## Breakdown Voltage vs T

| Detector 
type | dVbd/dT  
(mV/oC) |
| --- | --- |
| 25U | 54.7 |
| 50U | 60.9 |
| 100U | 63.9 |

## Rates Measurement

- Setup:
- Keithley 2400 source-meter
- Amplifier MITEQ, 30 dB, 1000 MHz
- HP5313A counter
- Clipping cable 5 nsec (to reduce impact of afterpulsing)
- Labview data acquisition
- Measure rates as a function of threshold (in the dark)
- Pulse height spectrum

## Dark Pulses Rates

*Dark Pulses Rates*

- Differentiate

- Define 1,2,3,4 ‘p.e.’ rate as a rate at 0.5, 1.5, 2.5, 3.5 of a single peak height

- 1 ‘p.e.’

- 2 ‘p.e.’

- 3 ‘p.e.’

- 4 ‘p.e.’

## First Peak: Single Avalanches 

- Breakdown voltage from amplitude vs V

- Width of single pe vs V

- Width of single
-  pe peak ~ 20 mV
- Electronics??
- High pixel-to-pixel
- uniformity

## Dark Pulses Rates as a Function of Bias (Over)Voltage

-  Dark pulses rate grows ~exponentially with overvoltage
-  At the same overvoltage:
- R(100)~ 3xR(50) ~ 9xR(25)
- Not quite proportional to the active area.

## Cross Talk Measurement 

- Single avalanche rate

- Single  + 1 cross talk

- Single +2 cross talk

- Single +3 cross talk

- Ratios of rates give relative probabilities of 1,2,3  extra pixels firing due to cross-talk

## Cross Talk Rates as a Function of Bias Voltage

-  Cross talk probability increases with the bias voltage
-   Cross talk probability is bigger for larger size pixels
- _But_... The cross talk is mediated by infrared photons produced in the avalanche, hence is ought to be proportional to the gain. And different size pixel detectors have different gain !

## Cross Talk Probability as a Function of Gain

-  At the same gain the cross-talk probability is much larger for smaller size pixels
-  At the operating point the Hamamatsu detectors have very small cross talk (~few %)

## Cross-talk Probability as a Function of Avalanches

- Naïve expectations:
-  with two avalanches present the number  of photons is doubled, hence the cross talk probability ought to be higher
-  Ditto for three avalanches present

- Naïve model doesn’t hold: some conspiracy between the solid angle and the photons mean free path??

##  Detector Response to Light Pulses

- Light source:
- Short pulse duration (<1 nsec)
- 1060 nm infrared laser
- ~3-4 photons detected
- Readout strategy:
- Trans-impedance amplifier
-     ( MITEQ amplifiers: AU-2A-0150)
- Tektronix 3054B digital scope
- 4 sec trace acquired, laser pulse positioned at the center
- LabView DAQ and analysis program
- Root-based analysis environment
- Most of the results shown for Hamamatsu 025U detector

## Examples of Traces

- Instabilities (afterpulsing) induced by response to laser light

- Clustering of ‘dark’ pulses

## Snapshot of Several Regimes at the Same Time

- Acquire 4 sec  long waveform with laser pulse positioned in the middle
- -2.0 – 0 sec: ‘quiet state’ of the MPPC:
- Dark rate
- Gain
- Cross talk, afterpulses
- ‘Laser gate’:
- Response to the light input
- Cross talk
- Afterpulses
- ‘Post laser gate’
- Afterpulsing, recovery

## ‘Quiet Time’ – Thermal Electrons-Induced Avalanches? 

- Have N scope traces. Count the peaks found = M
- ‘Raw’ dark rate = M/(N×t). But they should be uncorrelated  Poisson  distribution
- P(0) = Nempty/N = exp(-Nave)
- ‘True’ dark rate = Nave/t
- ‘ ‘Raw’ – ‘True’ Rates = ‘Afterpulse’ (correlated) rate
- Fraction of single pulses + Poisson statistics => another estimate of  afterpulsing probability

## Time Difference Between Dark Pulses Vbias =72.75 V

- Fraction of traces with exactly one pulse:
- Expected : 0.39
- Observed: 0.08

- Large number of pulses closely clustered in time

## ‘Dark’ Rates vs Voltage

- ‘Raw’ rate

- ‘True’ rate

- ‘True’ rate, single peak method (inferior)

-  Rate of ‘true’ dark counts increases slightly with bias voltage(reflecting the increase of the probability that a free electron will start an avalanche). This is expected as the rate of free carrier generation depends on the temperature and not the bias voltage.
-  Observed exponential growth of the dark rate is caused by afterpulsing
-  At the higher bias voltage ‘dark’ pulses come in clusters

- Probability that a single avalanche will induce at least one more avalanche (afterpulse)

## Insights About the IV plot?

- Break-down voltage of the detector

- Afterpulsing probability ~ 1, run-away

- Increase of gain x (mostly) increase of afterpulsing

## Single (Isolated) Dark Pulses: Self-Calibration of the Detector 

- With longer gate or higher voltage a long tail (afterpulses) and a double avalanche peak (cross talk) appear

## Dark Counts: Comment About the Rates 

- 71.5 V, integration gate of 50 nsec
- Dark count rate: what is the reduction when cutting at 1.5 pe?? It depends on the definition of ‘rate’:
- Factor of 30-50 (cross talk probability) if measure the amplitude, bias voltage dependent
- Factor of 5-10 if measure integral within some gate (gate dependent), dominated by afterpusling

## Laser Light Pulses

- Fractional content of the ‘zero’ bin -> average number of photons detected
- 3-5 photons detected
- Good agreement between ‘charge’ and ‘amplitude’ –based measurement
- An apparent increase of the laser intensity with the bias voltage is an indication of the increase of the PDE (avalanche initiation probability)  by a  factor of ~ 1.5 between 71 V and 72.75 V

## Light-Induced vs Dark Pulses?

- Single p.e. pulses before during and after the laser gate recorded at different bias voltages:
-  pulse shape does not depend on the avalanche origin
-  pulse shape does not depend on the voltage, apart from the overall factor (gain)

## Analysis of the ‘Laser Gate’ Data

- Two possible measures of the signal/readout strategies:
-  the peak amplitude (peak sensing readout)
- Practical for very short light pulses (lasers, Cherenkov)
- Integrate the charge within some gate (30 nsec shown thereafter)
- More typical case for HEP (like calorimetry)
- Gate length?
- Number of samples?

## Gain/linearity at Low Light Levels

*Gain/linearity at Low Light Levels*

- Integrate 1,2,3 avalanches peaks in 10 nsec gates (afterpulses vetoed)
-  Q(N) = NQ(1)
-  Q = C*(Vbias – Vbr)  C = 12fF
- Vbr = 68.5 V

## Reconstructing the Poisson Distribution (Charge and Amplitude)

## Laser Pulses  vs Bias Voltage: Amplitude

*Laser Pulses  vs Bias Voltage: Amplitude*

- Notice the decrease of the number of zero’s and the general shift to the right: increase of the mean number of detected photons as a result of the increase of detection efficiency with bias voltage. Consistent with the measurement using ‘zeros’.

## Charge Induced by the Laser Pulse in 30 nsec Gate

*Charge Induced by the Laser Pulse **in 30 nsec Gate*

-  With the increasing bias voltage  afterpulses increase the response, but degrade the ability to detect individual avalanches.
-  Poisson shape destroyed
- This is caused by additional pulses, often occurring before a pixel is fully recovered,  or parts of thereof sneaking into the integration gate.

## Calibration Challenge

- All the response plots shown are:
- For the same detector
- Operating at different bias voltages
- Operating at the ‘same’ (i.e. room) temperature
- For the same light input
- Challenge:
-  find a prescription to convert all these measurements to the same ‘number’ (like 4.5 for these plots)
- While giving the correct average response at other light levels
- And minimizing the resolution

## Charge of the Laser pulse in 10 nsec gate with afterpulse veto

*Charge of the Laser pulse in 10 nsec gate with afterpulse veto*

-  Require that   [Q(30) Q(10)]<0.15xQ(10), i.e. no afterpulse immediately following the laser pulse.
-  Ability to count individual avalanches restored.
-  This is not a very practical solution in real life applications, though. It may be, perhaps, of some use in situations where:
-  Arrival time of the light pulse is known (timing of the gate)
-  Input light pulse has small duration (~ 1-2 nsec)

## Output Pulse Shape as a Function of Bias Voltage

-  Average pulse shape of the response to the laser light as a function of the bias voltage (red – Vbias =71 V, blue – Vbias = 72.75 V)
-  Clear evidence for afterpulsing component growing with the voltage making pulses bigger and longer.

## Variation of ‘observables’ with Bias Voltage 

-  Different measures of the signal show different variation with the bias voltege (at fixed temperature and the same light signal).
-  For 1.5 V variation of the bias voltage the peak amplitude grows by a factor of about 2.5, whereas the integral of charge in 100 nsec gate changes by a factor of 7
-  Need to keep the voltage (and temperature) very stable or need to devise a precise calibration procedure.

## Choosing the Gate for the Readout?

- Observed signal grows with the bias voltage. This growth has several components:
-  increase of the gain
-  increase of afterpulsing.
- The latter is a much bigger effect. So what?? Afterpulses provide a kind of additional gain. True, but this contribution fluctuates  degrades the charge measurement resolution (excess noise factor).
- Relative width of the observed pulse height spectrum slightly decreases with bias voltage for 10 nsec gate (presumably a reflection of the increased number of detected photons), but it increases for longer gates.
- Bottom plot shows a contribution to resolution from fluctuations of the afterpulses contribution in different gates.

## Detector Recovery / Afterpulsing

- Pulse arrival distribution: clear afterpulsing for about~ 1 sec
- At least two components:
- 1=39 nsec
- 2=202 nsec
- These components probably correspond to traps with different lifetimes

## 25/50/100 micron pixel devices: Capacitance

- From the gain vs Vbias dependence

## Pulse Shapes of Different Devices

- Average responses to the laser pulse
- RC =
- 4.9E-9 for 025
- 1.44E-9 for 050
- 5.42e-8 for 100
- Detector pulse shape related to the RC constant, but ~40% faster

## Next Steps

- More analysis
- Better data (controlled temperature)
- Wider dynamic range of the laser pulses
- Independent measurement of the relative intensities of laser light
- Studies of temperature dependence

## What Happens with Increased Temperature:Expectations

- Dark rate increases ~ exp(-Eg/kT)
- Breakdown voltage increases, gain decreases
- Increased collisions with lattice, lower kinetic energy of electrons
- Quenching resistance decreases
- Pulses get faster
- Recovery time shortens
- Traps lifetime decreases

## Summary

- We are completing a general test facility
- Detailed studies of various aspects of the response of the PPD’s allow good understanding of the detectors behaviour
- Hamamatsu detectors (25 microns) have relatively low dark noise rates
- They have very short recovery time (5 nsec)
- (Owing to a short recovery time) Response of the detectors is dominated at higher bias voltages by the afterpulsing
- Cross talk is at the few percent level and it is always much smaller than afterpulsing
- Stay tuned, more to come
- Better yet: come and have fun