Polarized Emission of AlN Deep-UV LED by a Negative Crystal-field Splitting Energy
Yoshitaka Taniyasu and Makoto Kasu
Materials Science Laboratory
@ Aluminum nitride (AlN) is a direct-bandgap semiconductor with a bandgap energy of 6 eV, the largest among semiconductors, and is therefore promising for light-emitting devices with the shortest wavelength. We have succeeded in p-type and n-type doping of AlN and have fabricated an AlN light-emitting diode (LED). In this study, to gain insight into the light emission mechanism, we characterized the radiation properties.
@ An AlN LED was grown on SiC (0001) substrate by metalorganic vapor phase epitaxy. Figure 1 shows the radiation properties of the AlN LED. The radiation angle Ę is defined as the angle from the surface normal (c-axis direction) to the detection direction. As the radiation angle increases, the intensity of the near-band-edge emission at 210 nm increases. To clarify the reason, we analyzed the relationship between the emission intensity and the radiation angle as shown in Fig. 2. Solid lines are calculated results for different polarization ratios P. P = 1 corresponds to polarized light parallel to the c-axis (E||c), P = |1 corresponds to polarized light perpendicular to the c-axis (EŪc), and P = 0 corresponds to unpolarized light. The experimental values follow the calculated one for P = 0.995. This indicates that the emission strongly polarizes for E||c. As a result, the emission intensity has the maximum perpendicular to the c-axis and the minimum parallel to it. Therefore, as the radiation angle increases from the c-axis, the emission intensity increases as shown in Fig. 1 .
@ The origin of the strong polarization can be explained by the band structure. Because AlN has strong ionicity, the lattice is strongly distorted. As a result, the crystal-field splitting energy has a negative value and the top valence band therefore becomes the C7V band. A transition between the top valence band C7V and conduction band C7C is allowed for E||c but is almost completely prohibited for EŪc. Therefore, the near-band-edge emission strongly polarizes for E||c. Using a crystal-field splitting energy ¢CRY = 230 meV and a spin orbital splitting energy ¢SO = 20 meV, P was calculated to be 0.997, which is in good agreement with the experimentally obtained value. Thus, we found that the strong polarization results from the negative crystal-field splitting energy.
@ This work was partly supported by Grant-in-Aid for Young Scientists (A), 19686003, from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
 Y. Taniyasu, M. Kasu, and T. Makimoto, Appl. Phys. Lett. 90 (2007) 261911.
Fig. 1. Radiation properties of AlN LED.
Fig. 2. Light emission intensity as a
function of radiation angle.