These LED's were taken from an old light bulb, a dimmable 800 Lumen Philips EnduraLED A19 Lamp 12.5W Model 12E26A60-1, a light bulb interesting in that a "white" LED usually has phosphors integrated into its epoxy coating, whereas for this light bulb, it contains "Royal Blue" LED's with the outer plastic "shell" of the light bulb containing the phosphors. More pictures of this light bulb are further below, if you are interested. You will see two of the ceramic PCB's in the picture, and one is missing (accidentally destroyed).
This LED is on a ceramic substrate also having a black square on it, and three of these LED's are on a ceramic PCB (there are three of these PCB's -- two are shown in the picture above).
I want to know what the black square is, that is on the ceramic substrate along with the LED.
The picture below was taken with the LED barely lit.
The following picture is a little more at an angle, and with a bit more current flowing, so closer to the eventual Royal Blue color at normal power. The black square is seen at a different angle.
The black square is very shiny on top, like Marcasite.
The light bulb had 9 of these LED's in series, 23.4 volts at 100mA. So about 2.6V per LED at 100mA. The bulb ran hot to the touch, which is one reason that I phased it out. The heat sink states that the bulb was made in China.
LED-01 -- Removed from ceramic PCB, the LED has a ceramic substrate, which I broke, intending to try to measure the black chip separately:
LED-02 -- This LED has been kept whole, the epoxy lens popped off. It looks like the black chip is in parallel with the LED die:
According to comment by Tim Williams, proceeded with testing. With current limit set to 10mA, reverse voltage went to -8.1V and once the black square was removed, the voltage then reached -11.0V without conduction (or destruction). So the black square is acting like an 8.2V Zener in the reverse direction. Don't know about the forward direction.