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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 polarizationrelated 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.

High-speed InGaN/GaN blue light-emitting diodes (LEDs) are needed for future gigabit-per-second visible-light communication systems. Large LED modulation bandwidths are typically achieved at high current densities, with reports close to 1 GHz bandwidth at current densities ranging from 5 to 10 kA/cm2. However, the internal quantum efficiency (IQE) of InGaN/GaN LEDs is quite low at high current densities due to the well-known efficiency droop phenomenon. Here, we show experimentally that nonpolar and semipolar orientations of GaN enable higher modulation bandwidths at low current densities where the IQE is expected to be higher and power dissipation is lower. We experimentally compare the modulation bandwidth vs. current density for LEDs on nonpolar (101¯0), semipolar (202¯1¯), and polar 0001 orientations. In agreement with wavefunction overlap considerations, the experimental results indicate a higher modulation bandwidth for the nonpolar and semipolar LEDs, especially at relatively low current densities. At 500 A/cm2, the nonpolar LED has a 3 dB bandwidth of ∼1 GHz, while the semipolar and polar LEDs exhibit bandwidths of 260 MHz and 75 MHz, respectively. A lower carrier density for a given current density is extracted from the RF measurements for the nonpolar and semipolar LEDs, consistent with the higher wavefunction overlaps in these orientations. At large current densities, the bandwidth of the polar LED approaches that of the nonpolar and semipolar LEDs due to coulomb screening of the polarization field. The results support using nonpolar and semipolar orientations to achieve high-speed LEDs at low current densities.

We present the first demonstration of RF characteristics of electrically injected GaN/InGaN core−shell nanowire-based micro light-emitting diodes (μLEDs) for μLED displays and visible-light communication. A record −3 dB modulation bandwidth ∼1.2 GHz at 1 kA/cm 2 (higher than any LED grown on c-plane GaN), and a lowleakage current−voltage characteristic with excellent rectifying behavior are achieved. Analysis using a small-signal equivalent electrical circuit for the μLEDs indicates a significantly longer differential recombination lifetime (∼330 ps) compared to the measured RC time constant (∼30 ps) at 1 kA/cm 2 , confirming negligible effects from RC parasitic delay on the modulation speed. The bandwidth versus current density (J) characteristic shows a different trend compared to planar c-plane and m-plane reference μLEDs, even though the nanowires are composed of both polar cplane and nonpolar m-plane sidewalls. The anomalous behavior of the bandwidth versus J characteristic is explained by nonuniform carrier injection, coupled with nonuniform quantum well thickness and indium composition, across the nanowire. The interpretation of the RF behavior of the nanowire-based μLEDs is supported by scanning transmission electron microscopy images, a significant blue shift (∼55 nm) of the electroluminescence spectra with applied bias, and nonuniform injection revealed by COMSOL simulations.

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