In situ temperature measurements via ruby R lines of sapphire substrate based InGaN light emitting diodes during operation

The temperature of encapsulated green and ultraviolet light emitting diodes (LEDs) in operation has been measured optically via the ruby R lines emitted by the residual Cr3+ contaminations in the sapphire substrate. These two photoluminescent R lines, which are excited by the electroluminescence of the LED itself, show a well-characterized line shift as a function of temperature and pressure. The temperature is found to rise linearly with the applied forward current of the LED at a rate of ≈1 K/mA. This optical temperature measurement based on monitoring the two ruby R lines could qualify as a sensitive method for an in situ temperature sensor for other sapphire substrate based semiconductor devices such as laser diodes and field effect transistors.