Mapping of the Electrostatic Potentials in a Fully Processed Led Device with nm-Scale Resolution by In Situ off-Axis Electron Holography

The optoelectronic properties of a fully processed red emitting AlGaInP micro-diode device is measured using standard I-V and luminescence measurements. A thin specimen is then prepared for in situ transmission electron microscopy analysis by focused ion beam milling, then the changes of electrostatic potential as a function of applied forward bias voltage are mapped by off-axis electron holography. We demonstrate that the quantum wells in the diode sit on a potential gradient until the threshold forward bias voltage for light emission is reached; at which point the quantum wells are aligned at the same potential. From simulations, a similar effect for the band structure can be demonstrated, where the quantum wells are aligned at the same energy level, and contain electrons and holes that are available for radiative recombination at this threshold voltage. We demonstrate that off-axis electron holography can be used to directly measure the potential distribution in optoelectronic devices, and is a powerful tool to help better understand their performance and to improve simulations.