Polarization Engineering of III-Nitride Nanostructures for High-Efficiency Light Emitting Diodes

The concept of polarization engineering of InGaN quantum wells are discussed as an approach for improving the radiative recombination rate of III-Nitride based active region. Two quantum wells were discussed as follow: 1) staggered InGaN quantum well, and 2) type-II InGaN-GaNAs quantum well. Staggered InGaN quantum wells (QW) grown by metalorganic chemical vapor deposition was demonstrated as; improved active region for visible light emitters. Fermi's Golden Rule indicates that InGaN QW with step-function like In distribution leads to significantly improved radiative recombination rate and optical gain due to increased electron-hole wavefunction overlap, in comparison to that of conventional InGaN QW. Spontaneous emission spectra of both conventional and staggered InGaN QW were calculated based on energy dispersion and transition matrix element obtained by 6-band k.p formalism for wurtzite semiconductor, taking into account valencc-band-states mixing, strain effects, and polarization-induced electric fields. The calculated spectra for the staggered InGaN QW showed enhancement of radiative recombination rate, which is in good agreement with photoluminescence and cathodoluminescence measurements at emission wavelength regime of 425-nm and 500-nm. Experimental results of light emitting diode (LED) structures at 450-nm utilizing staggered InGaN QW show improvement in output power much higher than what is predicted theoretically. In addition to the staggered InGaN QW, type-II InGaN-GaNAs QW was also investigated theoretically with potential of implementation for highefficiency LEDs.