Comparison of optical properties in GaN and InGaN quantum well structures

Static, field-modulated, and time-resolved spectroscopic measurements were carried out to compare the electronic structures between AlGaN/GaN binary and GaN/InGaN ternary single quantum wells (SQWs). The internal field exists across the quantum well (QW) naturally induces quantum-confined Stark effects, namely the redshift of the QW resonance energy and separation of electron-hole wavefunction overlap. Thus AlGaN/GaN SQWs exhibited a weak luminescence peak due to the presence of nonradiative channels. However, optical absorption and degenerate pump-probe measurements revealed that excitonic character still remains for the thin QWs having the well width nearly the same as the bulk free exciton Bohr radius even under high electric field as high as 0.73 MV/cm. A slightly In-alloyed InGaN SQW exhibited bright luminescence peak in spite of the pronounced effective bandgap inhomogeneity in the QW, which was confirmed by the point excitation and monochromatic cathodoluminescence mapping methods to have the lateral potential interval smaller than 40 nm. Therefore the light emitting area of the potential minima has the size defined as 'quantum-disk' [M. Sugawara, Phys. Rev. B, 51, 10743 (1995)]. Carriers generated in the InGaN QWs are effectively localized in these region to form localized QW excitrons exhibiting highly efficient spontaneous emissions.