Emission mechanisms of InGaN single quantum well blue and green light emitting diodes and multiquantum well structures were investigated by means of modulation spectroscopy. Their static electroluminescence (EL) peak was assigned to the recombination of excitons localized at certain potential minima in the quantum well. The blueshift of the EL peak caused by the increase of the driving current was explained by combined effects of the quantum-confinement Stark effect and band filling of the localized states by excitons.

Group-III-nitride semiconductors have shown enormous potential as light sources for full-colour displays, optical storage and solid-state lighting. Remarkably, InGaN blue- and green-light-emitting diodes (LEDs) emit brilliant light although the threading dislocation density generated due to lattice mismatch is six orders of magnitude higher than that in conventional LEDs. Here we explain why In-containing (Al,In,Ga)N bulk films exhibit a defect-insensitive emission probability. From the extremely short positron diffusion lengths (<4 nm) and short radiative lifetimes of excitonic emissions, we conclude that localizing valence states associated with atomic condensates of In-N preferentially capture holes, which have a positive charge similar to positrons. The holes form localized excitons to emit the light, although some of the excitons recombine at non-radiative centres. The enterprising use of atomically inhomogeneous crystals is proposed for future innovation in light emitters even when using defective crystals.

Optical spectra of the bulk three-dimensional InGaN alloys were measured using the commercially available light-emitting diode devices and their wafers. The emission from undoped InxGa1−xN(x&lt;0.1) was assigned to the recombination of excitons localized at the potential minima originating from the large compositional fluctuation. The emission from heavily impurity-doped InGaN was also pointed out related to the localized states.

The emission mechanisms of strained In x Ga 1Ϫx N quantum wells ͑QWs͒ were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect ͑QCFK͒ for the wells, in which, for the first approximation, the product of the piezoelectric field, F PZ , and L exceed the valence band discontinuity, ⌬E V. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that F PZ ϫL exceeds the conduction band discontinuity ⌬E C , the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius a B. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and F PZ , produces a confined electron-hole pair whose wave functions are still overlapped ͑quantized excitons͒ provided that L Ͻa B .

We have measured polarized Raman spectra in a 2.0 fim GaN epitaxial layer of high quality, grown on a sapphire substrate. All symmetry-allowed o p t i d phonons in GaN have been assigned as follows: AI(LO), 735 cm-'; AI(TO), 533 cm-'; El(m), 743 cm-': El(m), 561 cm-'; &, 144 and 569 cm-]. Using the LyddaneSachs-teller relation, the static dielectric constants of GaN for the ordinary and extraordinary directions have been estimated as EIO = 9.28 and &IO = 10.1. We have dso observed q d -m phonons in GaN. A brief discussion on these will be given.