Paper EM-ThP12
High-efficiency Semipolar GaN-based Light Emitting Diodes Fabricated by Wet Chemical Etching

Thursday, October 31, 2013, 6:00 pm, Room Hall B

Semipolar GaN semiconductors have become significantly important owing to their potential applications to high-efficiency optoelectronic and electronic devices, especially for the light-emitting diodes (LEDs) with a quantum-confined Stark effect free by eliminating the polarization-induced internal electric fields in the active regions. Another feature of semipolar GaN planes is weaker chemical inertness against chemical solutions compared to the polar (0001) c-plane, indicating that wet etching can be practically used. Indeed, several previous studies have revealed that the wet etching of semipolar GaN planes can form peculiar surface textures such as striated trigonal prisms by exposing specific crystallographic planes. These features are quite interesting in terms of the fabrication of more energy-efficient and cost-competitive LEDs. For example, first, the mesa of LEDs having semipolar orientations can be made using a wet etching process instead of conventional dry etching. Second, the crystallographic surface textures formed after wet etching can act as excellent light scatters or exits for the guided mode caused by the large mismatch of the refractive indices of GaN and the environment (air), implying that the extraction efficiency can be significantly improved. Furthermore, wet etched surfaces were found to be much more efficient in making ohmic contacts, as will be discussed in our study for the first time. These findings suggest that wet etching can be practically used in the fabrication of LEDs. In this regard, we demonstrate the first semipolar GaN-based LEDs fabricated by a wet etching process, which led to better device performance than reference LEDs.

It was shown that the wet etching of semipolar GaN was a reaction-limited process with an E_a of 11.3 kcal mol^-1, exhibiting etching rates as high as 530 Å/min under 4M-KOH solution at 90°C. In particular, the wet etched surfaces exhibited the typical trigonal prism cell structure with a (0001) c-plane and [10-10] m-planes. Notably, the ohmic contact could be better formed on the wet etched surface, which could be attributed to the enhanced carrier transport through local shallow barriers associated with the exposed crystallographic structure. The LEDs fabricated by wet etching showed excellent output performance at 1.89 times higher than that of the reference LEDs. This could be due to the role of the textured surfaces. These findings suggest that wet chemical etching can be potentially important to semipolar or nonpolar-plane LEDs, which open new ways to achieve brighter and cost-competitive lighting engines for solid state lighting.