Strain effects on $n$-type doping in AlN

Abstract

Controllable doping in AlN and its alloys is essential for deep-ultraviolet light sources. Ionization energies for donors in AlN ($\mathrm{Si_{Al}}$, $\mathrm{S_N}$, $\mathrm{Se_N}$) are high. We report first-principles calculations demonstrating that strain engineering can result in a reduction in ionization energies. The donor levels for $\mathrm{S_N}$ and $\mathrm{Se_N}$ shift closer to the conduction-band minimum (CBM) under in-plane tensile strains, driven by a downward shift of the CBM. The most widely used donor, $\mathrm{Si_{Al}}$, forms a $DX$ center in AlN. We find that a 2.5% in-plane tensile strain (which would be induced by pseudomorphic growth on GaN in experiment) shifts the ($+/-$) transition level from 271 meV to 98 meV below the CBM, which would enhance the electron concentration by three orders of magnitude. These results demonstrate that strain engineering offers an effective route to enhance doping levels in AlN.

Figures (3)

• Figure 1: Atomic structures of (a) $\mathrm{Si_{Al}^{+}}$ at the substitutional site and (b) $\mathrm{Si_{Al}^{-}}$ in the most stable $DX$ configuration in unstrained AlN. The Si atom is shown as a dark blue sphere, Al atoms as light blue spheres, and N atoms as small white spheres. The yellow lobes represent the charge density associated with the occupied $DX$ state within the bandgap, with isosurfaces set to 5% of the maximum value. (c) Ionization energy of $\mathrm{Si_{Al}}$ in AlN and electron concentration $n$ as a function of in-plane tensile strain. The shaded area indicates the strain regime where $\mathrm{Si_{Al}^-}$ can assume a substitutional configuration.
• Figure 2: Atomic structures of (a) $\mathrm{S_{N}^{+}}$, (b) $\mathrm{S_{N}^{0}}$ and (c) $\mathrm{S_{N}^{-}}$ in unstrained AlN. The sulfur atom is shown as red sphere. The yellow lobes in (b) and (c) represent the charge density associated with the occupied impurity states in the bandgap, with isosurfaces set to 5% of the maximum value. (d)-(e) Ionization energy and electron concentration $n$ as a function of in-plane tensile strain for (d) $\mathrm{S_{N}}$ and (e) $\mathrm{Se_{N}}$ in AlN. The shaded areas indicate the appearance of a delocalized substitutional configuration of neutral and negative charged states of $\mathrm{S_{N}}$ and $\mathrm{Se_{N}}$.
• Figure 3: ($+/-$) of $\mathrm{Si_{Al}}$, ($+/0$) of $\mathrm{S_N}$ and $\mathrm{Se_N}$, and the CBM energy $E_c$ as a function of strain shown on an absolute energy scale. The top axis shows the fractional volume change under corresponding strain.