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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
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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
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
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 msec • 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: • DV/V ~ 8-10% • Integrate the charge within some gate (8ns) • To reduce impact of the afterpulsing require no other pulse within 50 nsec • DQ/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? Increase of gain x (mostly) increase of afterpulsing Afterpulsing probability ~ 1, run-away Break-down voltage of the detector
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.
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.