Excitonic recombination processes in III-nitride quantum wells

今天，第三族氮化物在技术上代表了宽禁带半导体的一个高度复杂的例子。除了大的带隙，还有更突出的性质，特别是大的电子和空穴的有效质量以及小的介电常数。直接的结果是，出现了强烈的电子-空穴相互作用，导致了大的激子结合能，最显著的是在低维结构中。对于大多数半导体来说，激子被认为是一种低温现象，因为它们的结合能很小。相比之下，III-氮化物量子阱中大的激子结合能使人们预计即使在室温下也会有很大一部分激子。在本项目中，应研究激子形成对复合过程的影响，包括辐射复合和非辐射复合。我们预计生态结合动力学会发生戏剧性的变化，同时重组几率也会大幅增加。此外，预计激子的形成对温度依赖性有很大的影响。在实验上，应对复合动力学随载流子浓度和温度的变化进行系统的研究，以澄清电子-空穴相互作用对辐射和非辐射过程的影响。为了研究不同激子结合能的影响，我们计划对激子组成、结构、掺杂和晶体取向进行系统的改变。实验工作应由侧重于复合过程的激子增强的模型计算来补充。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，人们观察到了“下垂”现象，即在大电流密度下量子效率的急剧下降。复合过程激子增强的明确证据可能会给这些现象带来新的曙光。此外，克服这种低迷的新战略可能会从更深层次的理解中演变出来。

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.