Extending the efficiency sweet spot of 460 nm AlGaInN LEDs all across the visible spectrum proves a formidable challenge. Successive elimination of the most-likely hindrances seems the most prudent approach and reduction of macroscopic line defects certainly requires the highest priority. Next, the role of piezoelectric polarization, a most prominent uniqueness in the group-III nitrides, must be considered. All conventional high-efficiency LEDs and laser diodes are grown along the polar c-axis. Consequently, polarization control by growth along other axes, such as non-polar m-, and a-axes or various semi-polar axes are indicated.

Naturally, such approaches require substantial epitaxy development to explore the ultimate capabilities of the system. By employing homoepitaxy on thick HVPE grown bulk GaN, we achieve significant reduction of threading dislocation densities from the 10⁹ cm^-2 to the 10⁷ cm^-2. We also succeeded to grow along various crystal orientations after such surfaces have been prepared from the thick bulk GaN.

We have learned to grow blue and even green LEDs with very low line defect densities <10⁸ cm^-2. now in non-polar a- and m-plane growth as well as semi-polar directions. Unlike other approaches, stacking faults are not a concern (<10⁵ cm^-1). Pushing the emission wavelength beyond 500 nm in the non-polar orientations, yet, proves a particularly large challenge. In the absence of piezoelectric polarization, the wavelength extending aspects of the piezo-dipole cannot be employed. In turn, however, once a certain wavelength has been achieved, an LED will perform with high stability at that wavelength up to high current densities.

What remains to be seen is whether devices of such growth geometry can outperform conventional c-plane material in terms of total output power, differential, and absolute efficiency.

This work was supported by a DOE/NETL Solid-State Lighting Contract of Directed Research under DE-FC26-06NT42860.

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1. Inclined Dislocation Pair Formation as a Mechanism of Partial Strain Relaxation in GaInN/GaN Quantum Wells on Low-Dislocation Density Bulk GaN