Potential imaging of operating light-emitting devices using Kelvin force microscopy

We report on the measurements of two-dimensional potential distribution with nanometer spatial resolution of operating light-emitting diodes. By measuring the contact potential difference between an atomic force microscope tip and the cleaved surface of the light emitting diode, we were able to measure the device potential distribution under different applied external bias. It is shown that the junction built-in voltage at the surface decreases with increasing applied forward bias up to flatband conditions, and then inverted. It is found that the potential distribution is governed by self-absorption of the sub-band-gap diode emission.