In-plane polarization anisotropy of the spontaneous emission of M-plane GaN/(Al,Ga)N quantum wells

We study the in-plane polarization of wurtzite GaN/(Al, Ga)N multiple quantum wells. Identical M-plane $\text{(11̄00)}$ and C-plane (0001) structures are grown by plasma-assisted molecular-beam epitaxy on $\mathrm{\gamma -}{\mathrm{LiAlO}}_{\mathrm{2}}\mathrm{(100)}$ and 6H–SiC(0001), respectively. While the emission from the conventional [0001] oriented wells is isotropic within the growth plane, we observe a strong polarization anisotropy of over 90% for the M-plane sample. The luminescence is polarized normal to [0001] and shows no spectral shift with polarization angle, i.e., it originates solely from A excitons ${\mathrm{(p}}_x$ and $p_y$ valence band states). The deviation of the polarization degree from unity is attributed to the mixing with $p_z$ valence band states due to quantum confinement.