Appl. Phys. Express 6, 062102 (2013)

At present, c-plane GaN crystals are widely used to fabricate violet and blue light-emitting diodes (LEDs). However, achieving efficient operation of green devices at longer wavelengths (>500 nm) is challenging as the large polarization fields in their active regions reduce their radiative recombination rates. This long-standing problem has led to a lack of suitable green semiconductor sources, and is often referred to as the 'green gap'. Now, Yuji Zhao and co-workers at the University of California in the USA and Mitsubishi Chemical Corporation in Japan report miniature (0.005 mm2) green InGaN/GaN LEDs on semipolar (2021) bulk GaN substrates that show an extremely low wavelength shift and a narrow spectral linewidth up to very high current densities (10,000 A cm−2), indicating reduced polarization effects. This contrasts with LEDs having the same indium concentration fabricated on (2021) substrates. Theoretical simulations indicate that the reduced polarization effects in the (2021) LEDs are caused by electric-field cancelling and Coulomb screening in (2021) InGaN quantum wells. The team also fabricated small-area (0.144 mm2) (2021) InGaN LEDs and found that they had a smaller wavelength shift and a narrower linewidth than green LEDs fabricated on other planes. These results imply that certain semipolar orientations may be advantageous for realizing stable-emission LEDs and laser diodes used in display and lighting applications.