Energy Band Engineering And Luminescence Properties Of Green-Yellow InGaN/GaN Quantum Wells

Due to the large lattice mismatch between InN and GaN, the InGaN alloy phenomena and pit formation would become severe with an increase of indium content, leading to poor crystalline quality and increase of nonradiative recombination rate for green-yellow LEDs. In addition, strong piezoelectric field within high indium content InGaN on GaN leads to a local separation of electrons and holes in InGaN quantum wells, leading to a significant decrease of radiative recombination rate.In this thesis, a modified InGaN active layer structure was proposed and achieved by indium pretreatment of the bottom GaN barriers and control of the growth temperature profile of the InGaN active layers. Various characterization techniques such as AFM, HR-TEM, HAADF-STEM, HR-XRD, XPS, and PL combined with APSYS simulation were employed to investigate the modified triangular QWs and to explore the origin of the luminescence properties experimentally and theoretically. The high-quality modified QWs shows efficiency and droop improvements, which are attributed to the improvement in overlapping of electron and hole wave functions, the promotion of hole injection, the increased radiative recombination and the enhanced interband transitions involving the excited electron state. In addition, the broad-band dual-wavelength green-yellow emission, which was observed in PL spectrum, may be applicable for monolithic or RGB white light sources. Brief summaries are as follows:1) The modified InGaN/GaN QWs showed improved crystalline quality and interface abruptness. High-quality InGaN/GaN QW structure composed of0.52nm Ino.35Gao.65N "wetting layers",1.56nm Ino.35-o.22Gao.65-0.78N graded layers, and1.56nm Ino.22Ga0.78N layers along the growth direction was achieved.2) Energy band structures showed enhanced overlap of electron and hole wave functions, and enhanced interband transitions involving the excited electron state "e2" for the modified triangular QWs compared with that of the conventional triangular QWs.3) Broad-band dual-wavelength green-yellow emission was observed, which may be applicable for monolithic or RGB white light sources.4) With increase of injection current, major contribution of the interband transition between the excited electron state "e2" and the first quantized hole state "hl" to the emission spectra led to the spectral broadening and broad-band green-yellow emission. Further, the filling of excited electron state with electrons at high carrier injection density and the reduction in PF-induced band bending suppressed the carrier leakage or electron overflow loss.5) APSYS simulation showed efficiency and droop improvements, which were attributed to the improvement in overlapping of electron and hole wave functions, the promotion of hole injection, the increased radiative recombination and the enhanced interband transitions involving the excited electron state for the modified InGaN active layers.