Demonstrations of AlGaN quantum well (QW) light-emitting diodes (LEDs) have opened up a new possibility of semiconductor light sources in the deep-UV region. However, their emission efficiencies are still lower than InGaN QW near-UV/visible LEDs. One intrinsic reason for the low emission efficiency is the low light extraction efficiency from the C-plane AlGaN surface due to its optical polarization anisotropy (Fig. 1) . In addition, the quantum-confined Stark effect (QCSE) has a significant impact on the internal quantum efficiency of the polar C-plane QW, as has been reported for the InGaN QWs (Fig. 2) . In this work, we study the deep-UV light emission properties of the nonpolar M-plane and polar C-plane AlGaN QWs .
The M-plane and C-plane AlGaN QW structures were epitaxially grown on M-plane and C-plane AlN bulk substrates by MOVPE, respectively. The M-plane AlGaN QWs show higher emission intensity than the C-plane ones (Fig. 3). The deep-UV light emission from the M-plane and C-plane AlGaN QWs was polarized for electric field (E) parallel to the c-axis (E||c). It turned out that the M-plane AlGaN QWs show stronger E||c polarization than the C-plane ones. Hence, the M-plane AlGaN QWs have higher light extraction efficiency than the C-plane ones. The stronger emission from the M-plane QWs can be partially attributed to the absence of the QCSE, as evidenced by emission at shorter wavelength from the M-plane AlGaN QWs than the C-plane ones (Fig. 3). Our results indicate that nonpolar AlGaN QW structures are promising approach for increasing the emission efficiency of AlGaN deep-UV LEDs due to their strong E||c polarization, along with the absence of the QCSE.
This work was partly supported by MEXT KAKENHI.