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LEDs based on hexagonal InGaN/GaN quantum wells are dominant technology for many lighting applications. However, their luminous efficacy for green and amber emission and at high drive currents remains limited. Growing quantum wells instead in the cubic phase is a promising alternative because, compared to hexagonal GaN, it benefits from a reduced bandgap and is free of the strong polarization fields that can reduce the radiative recombination rate. Initial attempts to grow cubic GaN in the 1990s employed molecular beam epitaxy, but now, metal-organic chemical vapor deposition can also be used. Nonetheless, high phase purity requires careful attention to growth conditions and the quantification of any unwanted hexagonal phase. In contrast to hexagonal GaN, in which threading dislocations are key, at its current state of maturity, the most important extended structural defects in cubic GaN are stacking faults. These modify the optical properties of cubic GaN films and propagate into active layers. In quantum wells and electron blocking layers, segregation of alloying elements at stacking faults has been observed, leading to the formation of quantum wires and polarized emission. This observation forms part of a developing understanding of the optical properties of cubic InGaN quantum wells, which also offer shorter recombination lifetimes than their polar hexagonal counterparts. There is also growing expertise in p-doping, including dopant activation by annealing. Overall, cubic GaN has rapidly transitioned from an academic curiosity to a real prospect for application in devices, with the potential to offer specific performance advantages compared to polar hexagonal material.

Growing green and amber emitting InGaN/GaN quantum wells in the zincblende, rather than the wurtzite,crystal phase has the potential to improve efficiency. However, optimisation of the emission efficiency of theseheterostructures is still required to compete with more conventional alternatives. Photoluminescence timedecays were used to assess how the quantum well width and number of quantum wells affect the recombinationrates, and temperature dependent photoluminescence was used to determine the factors affecting recombinationefficiency. The radiative recombination lifetime was found to be approximately 600 ps and to increaseweakly with well width, consistent with a change in the exciton binding energy. The relative efficiency atroom temperature was found to increase by a factor of five when the number of wells was increased fromone to five. Furthermore, the efficiency increased by factor 2.2 when the width was increased from 2:5nmto 7:5nm. These results indicate that thermionic emission is the most important process reducing efficiencyat temperatures in excess of 100 K. Moreover, the weak dependence of the rate of radiative recombinationon well width means that increasing well thickness is an effective way of suppressing thermionic emissionand thereby increasing efficiency in zincblende InGaN/GaN quantum wells, in contrast to those grown in thewurtzite phase.