Optical polarization and light extraction from UV LEDs

Abstract

In solid-state devices, light is generated by radiative transitions between the lowest conduction band and the highest valence subband. In wurtzite group III-nitride compound materials, the valence band is split into three subbands (heavy hole band, light hole band, and crystal field split-off band) each having a unique polarization state for electronic transitions. The ordering of these bands and hence the optical polarization is determined by the strain state, the aluminum mole fraction x of the AlxGa1−xN alloy, and the quantum confinement. This has consequences for the optical polarization properties and for the extraction efficiency of the spontaneously emitted light generated in light-emitting diodes (LEDs). Furthermore, the high refractive index of the semiconductor material and the presence of absorbing metal contacts and p-semiconductor layers result in UV LEDs having a low extraction efficiency. In this chapter, the basic mechanisms of light extraction and the generation of polarized light in UV LEDs are discussed. Different concepts for improving the light extraction efficiency are presented. This includes reflective contacts, contact design, surface patterning and surface roughening, photonic crystals, plasmonics, and encapsulation techniques.