Tuning the polarization-induced free hole density in nanowires graded from GaN to AlN

TL;DR

This study systematically investigates how grading the composition in AlGaN nanowires influences free hole density induced by polarization, revealing the conditions for optimal p-type doping in nanowire LEDs.

Contribution

It provides a detailed analysis of polarization-induced doping in graded AlGaN nanowires, establishing the relationship between composition gradient and free hole density, and demonstrating the robustness of p-type polarization doping.

Findings

Free hole density reaches up to 1.6x10^19 cm^-3 at high gradients.

Gradients ≥ 1.30 %Al/nm effectively suppress deep donor compensation.

Capacitance and band modeling confirm the correlation between polarization charge and free holes.

Abstract

We report a systematic study of p-type polarization induced doping in graded AlGaN nanowire light emitting diodes grown on silicon wafers by plasma-assisted molecular beam epitaxy. The composition gradient in the p-type base is varied in a set of samples from 0.7 %Al/nm to 4.95 %Al/nm corresponding to negative bound polarization charge densities of 2.2x10^18 cm^-3 to 1.6x10^19 cm^-3. Capacitance measurements and energy band modeling reveal that for gradients greater than or equal to 1.30 %Al/nm, the deep donor concentration is negligible and free hole concentrations roughly equal to the bound polarization charge density are achieved up to 1.6x10^19 cm^-3 at a gradient of 4.95 %Al/nm. Accurate grading lengths in the p- and n-side of the pn-junction are extracted from scanning transmission electron microscopy images and are used to support energy band calculation and capacitance modeling. These results demonstrate the robust nature of p-type polarization doping in nanowires and put an upper bound on the magnitude of deep donor compensation.