# Presentation: 765602

## Evaluation of HgCdTe APDs for Single Photon Counting

- NIST Workshop on Single-Photon Counting
- 1 April 2003

**Work supported  in part by NIST contract #SB1341-02-C-0015**

-                        Jim Gates
- 	760-730-9768
- 	jimgates00@aol.com

## Outline of the Presentation

- NIST objectives
- Approaches to photon counting
- APD selection criteria and test results
- Performance model for single photon counting at 100K (linear mode)
- Resistive transimpedance amplifier (RTIA) simulation
- Future plans
- Summary and conclusions

## NIST Objectives for Voxtel SBIR

- 1.55-micron photon counting module
- Small size (beer can or smaller)
- 200-micron diameter APD
- Fast counting rate (1-10 Mcps goal)
- Prefer Geiger mode but linear mode is an option
-  Low dark counts (cooling required)
- Two modules for correlated photon counting
- Open to other approaches (SBIR Research)

## Geiger vs. Linear Mode

**Geiger Mode (gated): achieves 300Kcps – 10 Mcps (at RT)**
**Features: mature and APD noise not important**
**Challenges : high effective QE (detection efficiency), dark counts, afterpulsing, APD failure (need better APDs and quenching circuits).**
**Linear mode just being developed at 1.55 microns**
**Features: no afterpulsing, no gating, high QE**
**Challenges: low excess noise, TIA circuits and high linear mode APD gain, better APDs (dark current)**
**This presentation will discuss linear mode with HgCdTe APDs**

## Photon Counting Detective Assembly

- Large Diameter APD 200-micron (or smaller with micro lens)
- RTIA amplifier (cooled)
- Packaged in vacuum sealed dewar with TE cooler (or Stirling minicooler)

**J1**

**J2**

**Detector**

**RTIA**

**AD590 Temp**

**Sensor**

**High Voltage**

**Bypass**

**TEC Power**

**Filter Board**

**  ****Temperature Sensor**

**High Voltage Bias**

**           ****RTIA **

**        ****Power**

**Output Signal**

**Cooler Power**

**TE Cooler with**

**Thermistor**

**Cooler Controller**

**TEC **

**Coldstage**

**Vacuum**

**Dewar**

**TIA **

**Circuit**

**Board**

## Miniature Stirling Cooler (required if APD cooled below 200K)

- Cooler Weight: 270 gr.
- Input Voltage: 12 VDC
- Input Power (170mW @80K @23°C): 4.5W
- Cool down Time (190J @23°C): 5 Minutes
- MTTF > 5,000 Hours (Expected)
- Cost (3-10K$)

## HgCdTe Advantages for APDs

- High Q.E Over Wide Spectral Range 1-2 m or Larger
- Can be Optimized for Specific Wavelengths
- QE >90% (with AR coat)
- Lattice matched growth
- QE maintained when cooled
- Back illuminated Vertical Design Enables
- GHz Bandwidths and double pass
- Low Excess Noise
- k can be engineered to low values < 0.1
- Low k tracks with detector temperature
- Good Uniformity due to  material control
- Low Idark due to material purity, GRings  and passivation
- Synergy with High Volume Manufacturing of IR Detectors

## InGaAs λc vs. T (InGaAs needs to be above 100K  for 1.55 micron)

## Need low k (less than 0.1) with high gain in a linear mode

## HgCdTe APD Design

## K-value engineering

## Eg and Delta tuning to achieve low excess noise in the gain region

## Resonance at 0.928 eV (1.375 micron cutoff used for gain region)

## 1.60 and 1.38 micron λc vs. T  (Does not change much with T)

## APD Test Procedure

- 

-   APD attached to a “Test’ TIA and signal

- current resulting from the optical pulse determine

- 

-   Flux measured using a calibrated diode (W)

- 

-  Dark current is subtracted from light current

- and the

- responsivity

-  (A/W Determined)

- 

-  QE measured voltage at which the APD is fully

- depleted ~ 40V but not showing gain.

- 

-   Gain is determined as the ratio of

-  responsivity

- at high voltage to

- responsivity

- at ~ 40V

- 

- Noise is measured ~ 10MHz

- and integrated over 42MHz Bandwidth

**APD Test Procedure**

## Excess Noise of the APD versus Gain fit to the McIntyre Model for APDs 

## Quantitative Measurements Use Dedicated APD Test Facility

**Quantitative Measurements Use Dedicated APD Test Facility**

- Laser Illuminated Wafer Probe

- Calibrated Lasers

- Acquisition Software and Data Base

- Pulse response/detector rise time

- Noise Spectrum

- Optics/Display

## HCT APD Gain vs. Vdet (APD operates at low temperatures)

## Id/G vs. G @100K (100 μm diameter APD Id very low at 100K)

## Linear mode 1.55 micron single photon counting is possible

## RTIA Simulation (low noise and high speed are possible)

## Future Plans

- Test more HgCdTe APDs
- Test APDs in linear mode with RTIA (HgCdTe and InGaAs/Si)
- Test APDs in Geiger mode (InGaAs/InP, InGaAs/Si and HgCdTe)
- Make a selection of APD and mode
- Build two single photon counting modules

## Summary and Conclusions

- HgCdTe appears promising in linear mode operation to achieve single photon counting. The APD achieves high gain at 100K. Id and excess noise is low.
- InGaAs/Si also has low noise and may work very well in a linear mode and Geiger mode.
- New APD detectors need to be evaluated for Geiger and linear mode to meet NIST requirements.
- Innovations in circuitry and APD detectors will improve single photon counting at 1.55 microns.
- Linear mode may distinguish between one and two photons.