Deep-UV Light Emission from AlN/GaN Short-Period Superlattices
Yoshitaka Taniyasu and Makoto Kasu
Materials Science Laboratory
@AlGaN alloys are promising materials for ultraviolet (UV) light-emitting diodes (LEDs) with emission wavelengths ranging from 210 nm (AlN) in deep-UV to 365 nm (GaN) in near-UV regions. At present, the emission efficiency of deep-UV LEDs is much lower than that of near-UV LEDs. One intrinsic reason for the lower emission efficiency is attributed to the valence band structure . For GaN the topmost valence band is the heavy hole (HH) band. The transition between the conduction band (CB) and HH band is allowed for electric field perpendicular to the c-axis (EŪc), and the emission intensity is therefore strong along the c-axis (from C-plane). On the contrary, for AlN the topmost valence band is the crystal-field split-off (CH) band. The transition between the CB and CH band is allowed for electric field parallel to the c-axis (E||c), and the emission intensity is therefore weak along the c-axis (from C-plane). Because the LEDs are fabricated with the C-plane orientation, which is suitable for high-quality growth, the emission intensity is intrinsically low for deep-UV LEDs using high-Al-content AlGaN. Recently, we demonstrated that AlN/GaN short-period superlattices (SLs) show stronger deep-UV light emission from the C-plane than the AlGaN alloys .
@Transmission electron microscopy (TEM) images of AlN/GaN short-period SLs are shown in Fig. 1. The GaN well thickness was 0.48 nm (1.8 monolayesr) and the AlN barrier thickness was 1.82 nm (7.3 monolayers). The growth of the SL structure with abrupt interfaces between AlN and GaN was confirmed. The emission properties of the AlN/GaN short-period SLs were characterized by photoluminescence (PL). As the GaN well thickness decreased from 2.5 to 0.9 monolayers, the emission wavelength decreased from 275.8 to 236.9 nm due to the quantum size effect. To investigate the polarization property, angle-dependent PL was carried out (Fig. 2). For an AlGaN alloy, the emission along the c-axis (Ę = 0º) was weak because of the E||c polarization. On the contrary, for the AlN/GaN short-period SLs, the emission along the c-axis was strong. This is because the quantized energy level for holes originates from the HH band of GaN and therefore the transition between the electron and hole quantized energy levels is allowed for EŪc. Due to the different polarization properties, the AlN/GaN shortperiod SL shows four times higher emission intensity than the AlGaN alloy. We also fabricated a deep-UV LED with AlN/GaN short-period SLs as an emission layer and confirmed strong deep-UV light emission from C-plane.
 Y. Taniyasu and M. Kasu, Appl. Phys. Lett. 90 (2007) 261911.
 Y. Taniyasu and M. Kasu, Appl. Phys. Lett. 99 (2011) 251112.
Fig. 1. (a) Cross-sectional TEM image of the AlN/GaN
short-period SLs and (b) its higher magnification.
Fig. 2. PL of (a) the AlN/GaN short-period
SLs and (b) the AlGaN alloy.