One of the most pressing physical problems hindering further advances in nitride emitters is the presence of large piezoelectric fields in these materials. Because of the hexagonal lattice crystal and the lack of inversion symmetry, c-plane GaN has piezoelelectric properties.
This causes problems in GaN opto-electronic devices such as LED's because the use of InGaN quantum well (QW) active regions leads to lattice mis-match which creates biaxial compressive strain.
This leads to an internal piezoelectric field where electrons and holes are pulled to opposite interfaces of the QW. This spatial separation of wave functions suppresses radiative recombination with respect to non-radiative recombination thus significantly diminishing the efficiency of the device. The problem becomes worse both for thicker QWs and at higher indium content (green emission).
Semi-polar and non-polar GaN specifically address these two issues. Research has clearly shown that indium incorporation into semi-polar GaN is significantly more efficient than on c-plane GaN, i.e. it is much easier to make a green or yellow LED on semi-polar GaN than on c-plane GaN. Furthermore, research has shown that using semi-polar or non-polar GaN has the ability to reduce droop in blue LEDs.
The proposed mechanism for such a droop reduction is based on the most recent theories of the origin of droop itself i.e. non-radiative Auger recombination. Auger recombination can be minimised by reducing the carrier density in the quantum wells. However, achieving this in c-plane GaN is difficult as increasing the InGaN (QW) thickness leads to increased piezoelectric fields as described above.
Using semi-polar or non-polar GaN minimises or eliminates the piezoelectric field respectively, enabling thicker QWs and so minimising or eliminating Auger recombination.
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