Mapping the Electrostatic Potential in GaN based NW p-n junctions using Off-axis Electron Holography

Abstract

III-N semiconductor nanowires (NWs) are promising candidates for electronic and optoelectronic devices and are one of the most intensely studied materials after Si in the fields of science and technology. These compound semiconductors have the advantage of having a direct band gap unlike Si which helps in the applications including emitters across a wide spectrum, including deep ultraviolet (UV) to near-infrared (NIR) wavelengths. Molecular beam epitaxy is an efficient technique that allows the growth of these semiconductor nanowires and nanowire devices. Measuring the electric potential in such device based junctions is of great importance as it helps to understand the dopant activation under various growth parameters which can lead to the development of more efficient devices.

We have obtained potential maps from GaN nanowires containing axial n-p junctions, using off-axis electron holography. Short depletion regions associated with high carrier concentrations (1019 cm−3) were measured, but with only a fraction of the built-in voltage that was expected for uniform dopant concentrations. A strong secondary electron, surface contrast between p and n-type regions was observed, typically indicating the presence of electrical junctions, at least at the near surface region. We investigated various factors including low p-type carrier activation, nanowire surface states, and impurity passivation, but these were ruled out. The combination of an unintended, radial variation in p-type dopant concentration or a parasitic deposition of a p-type shell over the n-type region and/or the presence of Inversion Domain boundaries (IDBs) were found to be the most likely explanations.

We investigated the potential distributions in axial p-GaN/InGaN/n-GaN nanowire light emitting diodes (LED) using electron holography (EH). Two kinds of NW growth, one with long and thin morphology with changing diameter and the other with short and wide, uniform diameter were observed from the SEM image on the same substrate. The potential map from EH revealed a built-in voltage close to 3 V in case of the long, thin wires whereas there was a drastic difference found in the short ones with a built-in voltage of only 0.5 V. Potential maps also showed that the location of the junction was different in the two types of wires. We attribute the difference in built-in voltages to the short length of the n-doped part in the shorter NWs. The n segments were too short to reach a flat potential meaning an incomplete p-n junction.