Properties of quantum well excitons in GaN/AlGaN and InGaN/GaN/AlGaN UV, blue, green, and amber light emitting diode structures

Optical spectra of GaN, In-doped GaN, and InGaN single quantum well (SQW) structures were compared to explore the role of In for the emission mechanisms in the SQW light emitting diodes (LEDs). The internal electric field, F, due to spontaneous and piezoelectric polarization in strained quantum wells (QWs) naturally induces the quantum confined Stark effect (QCSE) that reduces the electron-hole (e-h) wavefunction overlap due to rapid ionization of excitons causing a serious decrease of the emission efficiency especially for the GaN SQW, although a clear QW excitonic absorption peak was observed. The low energy tail of the emission spectrum of InGaN amber SQW LEDs extends below the bulk InN bandgap, indicating that F is not completely screened by injected carriers. However, doping and alloying of In in GaN improve the emission efficiency despite the fact that the effective bandgap inhomogeneity is increased with increasing InN mole fraction. In is considered to support effective localization of QW excitons, and the Coulomb interaction between the e-h pair is maintained provided that the QW thickness is smaller than the free exciton Bohr radius.