Emission Mechanism of AlN Deep-Ultraviolet Light-Emitting Diode


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. Therefore, AlN is a promising material for deep-ultraviolet (deep-UV) light-emitting devices with an ultrashort wavelength. Previously, we achieved both n-type and p-type doping in AlN and fabricated an AlN p-n junction light-emitting diode (LED) with near-band-edge electroluminescence (EL) at a wavelength of 210 nm, the shortest ever reported for any kind of LED [1]. Here we identify the origin of the near-band-edge emission and propose a high-efficiency LED structure based on the unique characteristic that the emission intensity largely changes depending on the crystal planes of its surface.
@First, a C-plane AlN p-n junction LED structure was grown on C-plane SiC substrate by metalorganic vapor phase epitaxy. By increasing the NH3 flow rate, the hole concentration in the p-type AlN was increased because of the suppressed formation of N vacancies, which act as compensating donors. As a result, the LED external quantum efficiency was increased from 8~10-6 to 1~10-4 % [2]. As shown in Fig. 1, the near-band-edge emission was dominated by an exciton emission originating from the crystal-field split-off valence band (FXCH), but another exciton emission originating from heavy/light hole valence bands (FXHH/LH) was also observed. From the emission energies, considering residual strain, the crystal-field splitting energy CR was determined to be -165 meV. Because of the negative CR, the topmost valence band is the crystal-field split off valence band with a pZ-like state and then the near-band-edge emission polarizes for the electric field parallel to the c-axis (E||c). Consequently, the near-band-edge emission is weak from the C-plane but strong from the A-plane for AlN.
@For nitride semiconductors, like AlN and GaN, the C-plane is preferable for high-quality growth and therefore a C-plane LED structure has been commonly fabricated. However, for AlN, because of the E||c polarization, an A-plane LED structure is desirable for enhancing the light extraction and improving the emission efficiency. We fabricated the A-plane LED structure by using A-plane SiC substrate and observed a near-band-edge emission at 210 nm [3]. As shown in Fig. 2, the A-plane LED showed strong emission from the LED surface (R = 0º), while the conventional C-plane LED showed weak emission. Thus, the A-plane LED is a promising structure for high-efficiency AlN deep-UV LED.

[1] Y. Taniyasu, M. Kasu, and T. Makimoto, Nature 441 (2006) 325.
[2] Y. Taniyasu and M. Kasu, Appl. Phys. Lett. 98 (2011) 131910.
[3] Y. Taniyasu and M. Kasu, Appl. Phys. Lett. 96 (2010) 221110.

Fig. 1. (a) Emission spectrum of AlN LED and
(b) optical transitions in AlN.
Fig. 2. Radiation patterns of A-plane and C-plane AlN LEDs.