Optical anisotropy in [0001]-oriented ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}/\text{AlN}$ quantum wells $\left(x>0.69\right)$

The optical polarization in [0001]-oriented ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}/\text{AlN}$ multiple quantum wells (QWs) in the deep-ultraviolet region $\left(x>0.69\right)$ was studied. Photoluminescence spectroscopy performed at 8.5 K revealed that the predominant polarization direction in QWs with a well width of $\sim 1.5\text{nm}$ switched from GaN-like $\mathit{E}\perp \left[0001\right]$ to AlN-like $\mathit{E}\parallel \left[0001\right]$ at an Al composition $x$ of $\sim 0.83$, where $\mathit{E}$ 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 $\mathit{E}\perp \left[0001\right]$ polarization. These results can be explained by the effect of strain and quantum confinement on the valence-band structures.

Figure 1

(Color online) Polarization PL spectra of $c$-oriented AlN (inset) and ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}$ $\left(\sim 1.5\text{nm}\right)/\text{AlN}$ MQWs with different Al compositions: 0.91, 0.82, and 0.69.

Figure 2

(Color online) Polarization PL spectra of $c$-oriented ${\text{Al}}_{0.82}{\text{Ga}}_{0.18}\text{N}/\text{AlN}$ MQWs with different well widths: 1.3, 8.0, and 13 nm.

Figure 3

(Color online) Plot of polarization direction of the emission from ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}/\text{AlN}$ QWs as functions of Al composition and well width.

Figure 4

(Color online) Schematic of band structures for (a) unstrained ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}$, (b) strained ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}$ on AlN, and (c) ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}/\text{AlN}$ QWs near the $\Gamma$ point. CB and LH denote the conduction band and light hole band, respectively.

Figure 5

(Color online) Contour plot of the calculated energy difference of $E\left({\Gamma }_7^{\text{QW}}\right)-E\left({\Gamma }_9^{\text{QW}}\right)$ in the unit of electron volts. Red/gray dotted thick line is the $E\left({\Gamma }_7^{\text{QW}}\right)-E\left({\Gamma }_9^{\text{QW}}\right)=0$ line for flat-band QWs without a polarization field, while (black) thick and thin lines are for QWs with polarization fields. The assumed spontaneous polarization was $-0.040\text{C}/{\text{m}}^2$.