Figure 5: Comparison of electro-reflectance (ER) spectra for the LED on silicon and sapphire.

ER spectra of (a) the LED on silicon and (b) the LED on sapphire with increasing reverse bias voltage. The flat-band voltage, where the phase angle of the ER spectra is changed by 180°, was determined to be −12 V for the LED on silicon and −20 V for the LED on sapphire.