Enhanced Deep-UV Light Emission from Nonpolar M-planeAlGaN Quantum Wells

Ryan Banal, Yoshitaka Taniyasu, and Hideki Yamamoto
Materials Science Laboratory

 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) [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) [2]. In this work, we study the deep-UV light emission properties of the nonpolar M-plane and polar C-plane AlGaN QWs [3].
 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.

Y. Taniyasu et al., Appl. Phys. Lett. 90 (2007) 261911.
P. Waltereit et al., Nature 406 (2000) 865.
R. Banal et al., Appl. Phys. Lett. (2014) submitted.
Fig. 1.
Schematic of polarized light emission from AlGaN crystal.
Fig. 2.
Band diagram of M-plane and C-plane AlGaN QWs.
Fig. 3.
Photoluminescence (PL) from M-plane and C-plane AlGaN QWs.