Structural and electrical evaluation of the influence of carbon delta layers for defect reduction on epitaxial growth of thin, relaxed germanium layers on silicon substrates

人们对将锗集成到硅基半导体技术的兴趣是由这样一种想法推动的，即通过使用具有增强的沟道迁移率的锗MOSFET来继续硅MOS技术的小型化，或者通过集成基于锗的光电电路来扩展所述技术的应用组合。此外，由于晶格常数的差异导致了三维孤立化和在几个单层之后产生大量的晶体缺陷，因此在硅上直接生长锗用于器件应用是不可能的，因为在硅上松弛的锗层适合作为III-V材料的虚拟衬底，从而为光伏提供了额外的好处。使用分级缓冲层仍然可以获得具有器件能力的薄膜质量。强烈降低生长温度也可以得到光滑的Ge薄膜，随后必须在更高的温度下进行热处理。第一种方法存在缓冲层厚度过大的问题，而第二种方法则受生长后退火过程中硅和锗之间开始互扩散的影响。或者，表面活性剂的使用可以抑制生长过程中的孤岛现象。不幸的是，最好的表面活性剂也是硅和锗的掺杂剂，因此在生长过程中的掺入最终会导致背景掺杂。本项目的目标是在硅衬底上外延薄的弛豫锗薄膜时使用碳三角层作为缺陷过滤器。它起源于我们最近开发的碳介导外延方法，该方法将低温生长与防止岛屿生长的碳的亚单层覆盖相结合。这种方法可以使硅上极薄、光滑、松弛的锗薄膜避免潜在的本底掺杂。此外，我们的最新结果表明，碳三角层可以阻止位错在锗薄膜中的垂直传播。本项目致力于研究这一效应的基本机制。在考虑粗糙度和缺陷密度的情况下，优化碳三角层的数目和距离以及每个三角层中的最佳碳量将导致具有最佳结构性能的Ge薄膜。第二个关键方面是碳介导外延生长的锗薄膜的电学特性。特别是，我们集中于碳对载流子迁移率和复合过程的影响，这些过程可能会在pn结中引起额外的泄漏。作为最终的结果，对碳介质外延生长的弛豫锗薄膜的可实现的优值因子和应用潜力进行了评估。

The interest in the integration of germanium into silicon-based semiconductor technology is driven by the idea to continue the miniaturization in silicon MOS technology by use germanium MOSFETs with enhanced channel mobility or to extend the application portfolio of said technology by the integration of optoelectronic circuits based on germanium. Moreover, relaxed germanium layers on silicon are suited as virtual substrates for III-V materials yielding additional benefits for photovoltaics.Direct growth of germanium on silicon for device applications is ruled out as the difference in lattice constant leads to three-dimensional islanding and generation of numerous crystal defects after only a few monolayers. Device capable film quality can still be obtained using graded buffer layers. A strong reduction of growth temperature also leads to smooth germanium films, which have to be annealed subsequently at higher temperatures. While the first approach suffers from the large buffer thickness, the second is afflicted with the onset of interdiffusion between silicon and germanium during post-growth annealing. Alternatively, the use of surfactants can suppress islanding during growth. Unfortunately, the best surfactants are also dopants for silicon and germanium, thus incorporation during growth ends up with background doping. This project aims at the use of carbon delta-layers as defect filters in the epitaxy of thin relaxed germanium films on silicon substrates. It originates in our recently developed method of carbon-mediated epitaxy, which combines low-temperature growth with an island-preventing submonolayer coverage of carbon. This method enables extremely thin smooth relaxed germanium films on silicon avoiding potential background doping. In addition, our recent results indicate that carbon delta-layers can block the vertical propagation of dislocations in the germanium film. This project focuses on the investigation of the fundamental mechanisms of this effect. An optimized number and distance of carbon delta-layers together with an optimum amount of carbon in each delta-layer will lead to germanium films with the best possible structural properties considering roughness and defect density. A second key aspect is the electrical characterization of germanium films grown by carbon-mediated epitaxy. In particular, we center on the impact of carbon on carrier mobility and recombination processes that could evoke additional leakage in pn-junctions. As final result, an evaluation of the achievable figures of merit and the application potential of relaxed germanium films grown by carbon-mediated epitaxy is aspired.