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 . 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 % . 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 ΔCR was determined to be -165 meV. Because of the negative ΔCR, 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 . As shown in Fig. 2, the A-plane LED showed strong emission from the LED surface (θR = 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.
 Y. Taniyasu, M. Kasu, and T. Makimoto, Nature 441 (2006) 325.
 Y. Taniyasu and M. Kasu, Appl. Phys. Lett. 98 (2011) 131910.
 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.
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