Evidence for voltage drops at misaligned wafer-bonded interfaces of AlGaInP light-emitting diodes by electrostatic force microscopy

Electrostatic force microscopy (EFM) with phase detection has been applied to cleaved cross sections of wafer-bonded transparent substrate (TS) AlGaInP light-emitting diode (LED) structures. EFM was performed with the LED under active bias to image the voltage drops across the device layers. Measurements on a nonwafer-bonded, absorbing substrate (AS) AlGaInP LED wafer, showed a voltage drop only at the p–n junction. A TS wafer with high forward voltage (Vf ) showed a much larger voltage drop at the wafer-bonded interface, compared with a normal TS LED wafer. Secondary ion mass spectrometry profiles of these wafers revealed ∼1×1013 cm−2 of carbon at the bonded interface in the high Vf sample, compared to ∼3×1012 cm−2 in the normal wafer. The unwanted voltage drop at the bonded interface was likely caused by a combination of carbon acting as a p-type dopant and the presence of interface states due to a ∼3° in-plane rotational misalignment at wafer bonding.