Comparison of optical properties of GaN/AlGaN and InGaN/AlGaN single quantum wells

Static, field-modulated and time-resolved spectroscopies were carried out t o compare the electronic states between GaN/AlGaN binary and InGaN/AlGaN te rnary single quantum wells (SQWs). The internal field that exists across th e quantum well (QW) naturally induces a quantum-confined Stark effect (QCSE ), namely the redshift of the QW resonance energy and decrease of the elect ron-hole wavefunction overlap. The GaN/A1GaN SQW exhibited a weak emission due to QCSE. However, optical absorption and degenerate pump-probe measurem ents revealed that excitonic character was maintained for thin QWs with the well width nearly the same as the bulk free exciton Bohr radius even under an electric field as high as 0.73 MV/cm. A slightly In-alloyed InGaN SQW e xhibited a bright luminescence peak in spite of an effective bandgap inhomo geneity in the QW, which was confirmed by the point excitation and monochro matic cathodoluminescence (CL) mapping method. The lateral interval of each light-emitting area was estimated from the spatial resolution of the CL ma pping to be smaller than 60 nm. Such local potential minima is considered t o be formed due to the presence of a structure similar to quantum-disks [M. Sugawara: Phys. Rev. B 51 (1995) 10743]. Carriers generated in the InGaN Q Ws are effectively localized in these regions to form localized QW excitons exhibiting highly efficient spontaneous emissions.