Quantum Efficiency

The quantum efficiency measurements have been performed in our lab using the facility developed many years ago [1] and modified to allow simultaneous measurements through an integrating sphere that guarantees uniform illumination on both the detectors: SPAD and calibrated diode. Figure 3 shows the block diagram of the QE measurement apparatus. In this way the photon flux is simply obtained by scaling for the detector area.

ST Microelectronics adopted a standard DIP 32 pin package to bond the various SPADs. Devices with different sensitive areas are available for measurements. We selected SPADs with 10, 20 and 50 ^m diameter and operated them at voltages of 10%, 20% and 30% above the breakdown voltage. The results for a 20 and 50 ^m diameter are plotted in Figure 4.

For all the SPADs we find a QE typical of silicon devices, peaking at 550-600 nm. We may note QE dependence not only from the SPAD diameter but also from the overvoltage. This last effect is under study, and one possible cause is the electric field generated by the SPAD biasing voltage.

Figure 3. Quantum efficiency measurements apparatus

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Figure 4. QE plot at three different overvoltages (10%, 20% and 30% above the breakdown voltage) for (left) a 20 ^m diameter and for (right) a 50 ^m diameter.

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Figure 4. QE plot at three different overvoltages (10%, 20% and 30% above the breakdown voltage) for (left) a 20 ^m diameter and for (right) a 50 ^m diameter.

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