Excitonic recombination processes in III-nitride quantum wells

今天，iii族氮化物在技术上代表了宽间隙半导体的高度复杂的例子。与大带隙一起，还有其他突出的特性，特别是电子和空穴的有效质量大以及介电常数小。直接的结果是，一个强大的电子-空穴相互作用出现，导致大的激子结合能，尤其是在低维结构中。对于大多数半导体来说，激子由于其结合能小而被认为是一种低温现象。相比之下，iii -氮化物量子阱中较大的激子结合能使人们期望即使在室温下也有较大比例的激子。在本项目中，激子形成对重组过程的影响，包括辐射和非辐射重组，将被研究。我们预计结合动力学的戏剧性变化与重组的可能性大大增加。此外，激子形成对温度依赖性的强烈影响是预期的。实验上，为了阐明电子-空穴相互作用对辐射和非辐射过程的影响，需要系统地研究复合动力学作为载流子密度和温度的函数。系统的变化组成，结构，掺杂和晶体取向计划研究变化的激子结合能的影响。实验工作应辅以着重于重组过程的激子增强的模型计算。激子对复合过程的影响可能对iii -氮化物发光二极管的性能有相当大的影响。在那里，人们观察到“下垂”现象，即在大电流密度下量子效率的强烈下降。重组过程中激子增强的明确证据可能会给这些现象带来新的启示。此外，克服衰退的新策略可能会从更深入的理解中进化而来。

Today, group-III-nitrides technologically represent a highly sophisticated example of wide-gap semiconductors. Together with the large bandgap there are further outstanding properties, in particular large effective masses for electrons and holes as well as a small dielectric constant. As an immediate consequence, a strong electron-hole interaction emerges leading to large exciton binding energies, most prominently in low-dimensional structures. For most semiconductors, excitons are regarded a low-temperature phenomenon owing to their small binding energy. In contrast, the large exciton binding energy in III-nitride quantum wells makes one expect a large fraction of excitons even at room temperature. In the present project the consequences of exciton formation on recombination processes, including both radiative and nonradiative recombination, shall be studied. We expect dramatic changes of the ecombination kinetics combined with strongly increased recombination probabilities. In addition, a strong impact of exciton formation on the temperature dependence is anticipated. Experimentally, systematic studies of the recombination kinetics as a function of carrier density and temperature shall be performed, inorder to clarify the influence of the electron-hole interaction on both radiative and nonradiative processes. A systematic variation of composition, structure, doping, and crystallographic orientation is planned to study the influence of varying exciton binding energies. The experimental efforts shall be complemented by model calculations focused on the excitonic enhancement of recombination processes. The influence of excitons on recombination processes may have considerable impact on the properties of III-nitride lightemitting diodes. There, one observes the "droop" phenomenon, i.e. a strong decrease of the quantum efficiency at large current densities. An unambiguous proof of excitonic enhancement of recombination processes may shed new light onto these phenomena. Moreover, new strategies to overcome the droop may evolve from a deeper understanding.