The most successful example of large lattice-mismatched epitaxial growth of semiconductors is
the growth of III-nitrides on sapphire, leading to the award of the Nobel Prize in 2014 and great
success in developing InGaN-based blue emitters. However, the majority of achievements in the
field of III-nitride optoelectronics are mainly limited to polar GaN grown on c-plane (0001)
sapphire. This polar orientation poses a number of fundamental issues, such as reduced quantum
efficiency, efficiency droop, green and yellow gap in wavelength coverage, etc. To date, it is still
a great challenge to develop longer wavelength devices such as green and yellow emitters. One
clear way forward would be to grow III-nitride device structures along a semi-/non-polar
direction, in particular, a semi-polar orientation, which potentially leads to both enhanced indium
incorporation into GaN and reduced quantum confined Stark effects. This review presents recent
progress on developing semi-polar GaN overgrowth technologies on sapphire or Si substrates,
the two kinds of major substrates which are cost-effective and thus industry-compatible, and also
demonstrates the latest achievements on electrically injected InGaN emitters with long emission
wavelengths up to and including amber on overgrown semi-polar GaN. Finally, this review
presents a summary and outlook on further developments for semi-polar GaN based
optoelectronics
