A Decade of Nonpolar and Semipolar III-Nitrides: A Review of Successes and Challenges

The performance of III-nitride devices is degraded by polarization in a wurtzite crystal structure. Nonpolar and semipolar III-nitrides have been extensively studied since the early 2000s as a solution to the polarization-related issues. Removal of polarization is expected to improve the radiative efficiency, optical gain, charge transport, and potentially offer solutions to the challenging problems in III-nitride light-emitting diodes (LEDs) known as efficiency droop and the green gap. In addition, use of nonpolar and semipolar orientations is also predicted to offer polarization pinning in some devices due to anisotropic in-plane strain. Despite the many potential advantages over c-plane, nonpolar and semipolar optoelectronic devices have not successfully replaced conventional c-plane devices in the commercial sector. Here, nonpolar and semipolar III-nitrides are reviewed after more than a decade of development. The successes and challenges of nonpolar and semipolar orientations for applications such as LEDs, laser diodes, superluminescent diodes, and vertical-cavity surface emitting lasers are discussed. New potential avenues for nonpolar and semipolar III-nitrides are also highlighted, including visible-light communication and power electronics. The availability of low-cost, high-crystal-quality substrates with nonpolar or semipolar orientation is discussed and alternative approaches for realizing these orientations are presented, including selective-area bottom-up nanostructures.