# Presentation: 483607

## Characterization of Silicon Photodetectors (Avalanche Photodiodes in Geiger Mode)at Fermilab

- G. Mavromanolakis, A. Para. N.Saoulidou
- Novel PhotDetectors 07,
- Kobe, June 27, 2007

## Goals

- Develop a complete characteristics of the detector response to the external light signal
- As a function of the light source characteristics (intensity, duration, time structure)
- As a function of the operating conditions (voltage, temperature)
-  light impact point onto the detector (inter/intrapixel uniformity)
- Develop algorithm for readout strategy and calibration procedure (integration time, cross-talk, after-pulses tratement, etc..)
- Feed back the information to the manufacturers

## Step 1: Database of Static Characteristics 

- Develop an automated procedure for static characterization (breakdown  voltage, resistance) as a function of the operating temperature
- Keithley 2400 source-meter
- Dark box
- Peltier cold plate
- Labview controls/readout
- Create a database of the samples, enter the static and image data

## I-V Characteristics at Different Temperatures

- Different detectors have quite different operating point
- Dark current and the operating point depend on temperature

## Breakdown Voltage: a Knee on the I-V plot?

- Linear or logarithmic plot (derivative)?
- Two voltage scales with about 4-5 V saparation
- Operating point somewhere in the middle
-  (left half)

## Step 2: Characterization of the Detector Response to a Light Pulse

- Light source:
- Short pulse duration (<1 nsec)
- Variable light intensity (0.1 – 1000 photons)
- Absolute light calibration (still in the works)
- Readout strategy:
- Trans-impedance amplifier ( MITEQ amplifiers: AU-2A-0159, AU-4A-0150, AM-4A-000110)
- Tektronix 3054B digital scope
- LabView DAQ and analysis program
- Root-based analysis environment

## GMAPD Characterization Setup

## Disclaimer

- The following ‘results’ are primarily meant to demonstrate the scope of the data collected.
- Two analysis chain (root-based and Labview-based) have been just established
- The results and tentative conclusions are preliminary and are meant as ‘food for thought’
- Results shown will be for Hamamatsu MPPC 025U detector
- There are much more results coming, the analysis just started last week.

## Snapshot of Several Regimes at the Same Time

- -2.0 – 0 msec: ‘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

## Examples: 71.0 V

## Examples: 72.0 V

## Examples: 72.5 V

## Salient Features: Detector Instabilities 

- Often called cross-talk, afterpulsing, etc.
- These instabilities determine the nature of the response of the detector in a manner which depends on the temporal characteristic of the measured light and/or on the characteristics of the read-out electronics
- It is very important to understand their origin and to reduce their incidence
- Tens of years of R&D for the SPAD detectors should be of great help

## (Naïve Understanding) ofTwo Popular Models 

- Photon-mediated cross talk: Infrared photons created in the avalanche initiate a response in the neighboring pixels.
- Remedy: trenches for optical isolation
- Naïve expectation this cross-talk will be ‘in-time’ with the original signal.  This is probably a very small effect.
- Carriers produced in the avalanche trapped in traps.  Traps have finite lifetime and release electrons which create subsequent avalanches.
- Remedy: long recovery time of a pixel
- There are likely more effects which need to be understood. Operating voltage seems to be of critical importance.

## Time arrival of avalanches

- Response to the initial light impulse  creates a long chain of ‘afterpulsing’ spread over 0.5-1 sec
- This effect grows very rapidly with the bias voltage. Shown in 0.5 V steps.

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

- Count pulses – ‘dark rate’
- Look at the time-difference between pulses: evidence for correlation at the scale of ~100 nsec
- Fraction of single pulses + Poisson statistics => calculate afterpulsing probability

## Dark Count Rate vs Voltage 

- Raw dark count rate rises dramatically with voltage
- Afterpulsing probability rises almost linearly (in the studied range) with voltage
- All of the increase of the dark rate is consistent with being the result of increased afterpulsing, whereas the rate of the ‘true’ dark pulses is~75K, voltage independent

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

- Detect pulses in the ‘quiet time’
- Plot the peak value of the detected pulses:
- V/V ~ 8-10%
- Integrate the charge within some gate (8ns)
- To reduce impact of the afterpulsing require no other pulse within 50 nsec
- Q/Q ~ 10-15%
- Width of the ‘calibration pulses’ represents uniformity of the response over the front face of the detector

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

- With longer gate or higher voltage a long tail and a double avalanche peak appear

## Detector Gain Vs Voltage

- Gain = Q/e
- Q = C (V-Vbr)

## Insights about the IV plot?

- Break-down voltage of the detector

- Afterpulsing probability ~ 1, run-away

- Increase of gain x (mostly) increase of afterpulsing

## 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 if measure the amplitude, bias voltage dependent
- Factor of 5-10 if measure integral within some gate (gate dependent)

## Dark Counts: Afterpulsing Mechanisms?

- 71.5 V, integration gate of 50 nsec
- An unique laboratory for studies of afterpulses: starting with a single electron avalanche, single pixel
- Evidence for ‘instantenous’ (like photon mediated) cross talk at few percent level, (depending on the bias voltage)
- Integrated charge: ‘fractional’  + ‘whole’ avalanche. Combination of two effects:
- Smaller charges produced. Avalanche in not-fully-revered pixel?
- Cutting the pulse at the edge of the gate
- Merits further studies.

## Amplitude/Peak Value in the Laser Gate vs Voltage

## ‘Laser Gate’:Integrated Charge vs Bias Voltage

## Analysis of the ‘Laser Gate’ Data

- Two independent methods:
- Take the peak value
- Integrate the charge within a given gate (30 nsec) shown thereafter
- Use Fourier analysis to determine the fundamental frequency
- Automatically partition the spectrum into 0-1st-2nd-3th-etc... peak
- Compare with the expected Poisson distribution. Any additional contributions (like afterpulses) will shift the distributions towards the higher values

## Detection Efficiency vs Bias Voltage

- Fractional content of the ‘zero’ bin -> average number of photons detected
- Good agreement between ‘charge’ and ‘amplitude’ –based measurement
- PDE increases by a  factor of ~ 1.5 between 71 V and 72.75 V

## Reconstructing the Poisson Distribution (Charge and Amplitude)

## Charge/amplitude Spectrum at 71 V

- Both charge (integral in 30 nsec gate) and the amplitude spectrum follows Poisson distribution

## Charge/Amplitude Spectrum at 71.5 V

- Amplitude spectrum follows Poisson distribution, wheras integrated (30 nsec) charge distribution shows deviations due to afterpulsing distribution

## Charge/amplitude distribution at 72.5 V

- Charge distribution completely dominated by afterpulsing (to the point that automated Fourier analysis fails miserably), whereas amplitude spectrum still shows no deviations from Poisson

## Lessons From the Charge/Amplitude Comparison

- Amplitude spectrum is not sensitive to afterpulsing but it is sensitive to the instantaneous cross-talk (like photon-mediated).
- Amplitude spectrum shows no deviation from Poisson, hence the ‘instantaneous’ cross-talk is a very small effe
- Charge distribution is very sensitive to the integration gate. Even with 30 nsec gate it shows huge effects (and deviations from Poisson distribution).
- This effect is a very strong function of the bias voltage. The time scale (30 nsec) seems to indicate that these afterpulses are generated in different pixels (no trapping model)
- What is the mechanism of cross talk at the timescales of the order of tens of nsecs???

## Improvements ‘in the Works’

- Environmetal chamber for the temperature control and study the temperature dependence
- Independent measurement of the input light intensity

## Conclusions

- Waveforms produced by GMAPD’s under different conditions provide wealth of information  about properties and behavior of the detectors
- Detector instabilities (cross-talk, afterpulsing) are major contribution to the detector response. Their practical consequences depend on the readout details and the experimental conditions (temporal structure of the measured light pulses)
- Physics of afterpulsing is likely to be rather complicated
- A lot more understanding can be achieved from the existing setups/data, but (perhaps) special detector samples (different recovery time, different number of pixels of a given size) could be very helpful in disentangling various contributions.