APD Transimpedance amplifier stability
The parasitic electric fields across the feedback resistor need to be increased, to compensate for the PhotoDiode hanging on Pin- of the OpAmp.
Just like a scope-probe has 2 resistors (one in the probe and one in the scope) and 2 capacitors in parallel with each resistor, with the capacitors selected or adjusted to implement 2 identical RC timeconstants, for best flatness of pulse response.

Take a couple pieces of solid insulated wire, twist them together to increase the capacitance, and solder across the feedback resistor.

You have just tuned the circuit; now untwist part of the gimmick-capacitor, and watch the pulse response change.