Abstract
The optical polarization in [0001]-oriented ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}/\text{AlN}$ multiple quantum wells (QWs) in the deep-ultraviolet region $(xg0.69)$ was studied. Photoluminescence spectroscopy performed at 8.5 K revealed that the predominant polarization direction in QWs with a well width of $\ensuremath{\sim}1.5\text{ }\text{nm}$ switched from GaN-like $\mathbit{E}\ensuremath{\perp}[0001]$ to AlN-like $\mathbit{E}\ensuremath{\parallel}[0001]$ at an Al composition $x$ of $\ensuremath{\sim}0.83$, where $\mathbit{E}$ is the electric field vector of emitted light. This Al composition is much higher than the previously reported critical compositions for polarization switching phenomena. Furthermore, decreasing the well width from more than 10 to 1.5 nm promoted $\mathbit{E}\ensuremath{\perp}[0001]$ polarization. These results can be explained by the effect of strain and quantum confinement on the valence-band structures.
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