Two-dimensional surface band structure of operating light emitting devices

We report on 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 surface potential distribution. These measurements enable us to accurately locate the metallurgical junction of the light emitting device, and to measure the dependence of the built-in voltage on applied external bias. As the device is forward biased, the junction built-in voltage decreases up to flat band conditions, and then inverted. It is shown that the potential distribution across the pn junction is governed by self-absorption of the sub-bandgap diode emission.