Multi-quantum-well polaritons in semiconductor nanocolumns

本项目的目的是观察GaN和InGaN/GaN纳米柱中的多量子阱极化子耦合。然而，由于纳米柱中的量子阱具有不同的共振能量，我们没有观察到极化子耦合。由于纳米柱的形貌，我们观察到了特殊的光学性质。我们研究了GaN纳米柱的特定光学性质，重点研究了柱状结构的影响。在大直径（~ 100nm） GaN纳米柱的光致发光（PL）光谱中，有两个峰属于自由A激子(A)和束缚激子(I)。随着直径的减小，出现了能量位置对应于B激子(B)和C激子(C)的高能峰。为了阐明这一现象的原因，我们计算了考虑激子极化效应的GaN纳米柱的PL光谱；即考虑纳米柱顶部和侧面的传播和辐射。结果表明，B和C激子的峰值随着直径的减小而变得显著，这是因为来自侧面的PL变得重要。我们还观察到大直径GaN纳米柱中的双激子发射，发现双激子结合能高于体样品。虽然结合能增加的机制目前尚不清楚，但我们认为这是由于纳米柱形态的空间约束效应引起的。

The purpose of this project was to observe multi-quantum-well polariton coupling in GaN and InGaN/GaN nanocolumns. However we have not observed the polariton coupling, because quantum wells in the nanocolumns have different resonance energies. We have observed peculiar optical properties due to the nanocolumn morphology. We have studied specific optical properties of the GaN nanocolumn, focusing on the influence of the columnar shape.In photoluminescence (PL) spectrum of GaN nanocolumns with large diameters (〜100nm) there are two peaks attributed to free A-excitons (A) and bound excitons (I). With decreasing the diameter, higher-energy peaks whose energy positions correspond to B-excitons (B) and C-excitons (C) appear. In order to elucidate the origin of this, we calculated PL spectra of GaN nanocolumns considering the polaritonic effect of the excitons; i.e. considering propagation and radiation from both the top and the side surfaces of the nanocolumns. The results show that the peaks of B and C excitons become remarkable with decreasing the diameter because PL from the side surface becomes important.We also observed biexciton emissions in GaN nanocolumns with large diameters, and found that the biexciton binding energies are higher than that in bulk samples. Although the mechanism for the increase in the binding energy is not clear at the present, we suppose that it arises from spatial confinement effect due to the nanocolumn morphology.