Optical properties of InGaN quantum wells

The emission mechanisms of strained InGaN quantum wells (QWs) were shown to vary depending on the well thickness L and InN molar fraction x. The QW resonance energy was shifted to lower energy by the quantum confined Stark effect (QCSE) due to the internal piezoelectric field, F_PZ. The absorption spectrum was modulated by QCSE and quantum-confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of F_PZ and L (potential drop across the well) exceeds the valence band discontinuity, ΔE_V. In this case, dressed holes are confined in the triangular potential well formed at one side of the well. This produces apparent Stokes-like shift (vertical component). The QCFK further modulated the absorption energy for the wells with L greater than the three dimensional free exciton Bohr radius, a_B. For the wells having high InN content (F_PZ × L > ΔE_V, ΔE_C), electron and hole confined levels drop into the triangular potential wells formed at opposite sides of the wells, which reduces the wavefunction overlap. Doping of Si in the barriers partially screens F_PZ resulting in a smaller Stokes-like shift, shorter recombination decay time, and higher emission efficiency. Si-doping was found to improve the interface quality and surface morphology, resulting in an efficient carrier transfer from high to low bandgap energy portions of the well. Effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy potential fluctuations enhanced by the large bowing parameter and F_PZ, produces confined e-h pair whose wavefunctions are still overlapped. Their excitonic features are pronounced provided that L < a_B and F_PZ × L < E_V (quantized exciton). Several cw laser wafers exhibit stimulated emission from these energy tail states even at room temperature.