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

点击翻译按钮获取中文摘要

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.