Chemical vapour transport in open systems: a technique for the "in situ" control of crystal morphologies in iron- and cobald silicides

This dissertation addresses chemical vapor transport (CVT) for producing nanostructured materials. To date, the lack of a comprehensive understanding of nanowire growth has hindered the potential integration of advanced metal silicide structures into semiconductor technology.
This study aimed to define the thermodynamic reaction conditions necessary for forming one-dimensional structures within the Fe-Si-I and Co-Si-I transport systems. To elucidate aspects of the nanowire formation mechanism, CVT reactions of all chemical species in these systems were investigated. The transport systems were adapted into an open CVT system, derived from the original “Mitführmethode”, to compare, for the first time, experimental results of binary phase materials with thermodynamic predictions, calculated using the Gmin-method (TRAGMIN software).
The experiments focused on three key parameters: source supply type, growth substrate, and flow rate, revealed limitations of the thermodynamic model, particularly due to the unaccounted saturation effect. Furthermore, crystal analysis showed that flow rate played a crucial role in controlling composition and morphology, enabling targeted growth regulation.
Through this parameter study, one-dimensional crystal growth was linked to the presence of liquid metal iodides. Liquid-assisted nucleation and growth pointed toward a vapor-liquid-solid (VLS) mechanism. Additionally, transport conditions of supersaturation were compared with universal growth behaviors observed in snowflake studies. The findings indicate that nanostructured growth processes are fully accessible through conventional CVTs, enabling in situ control of growth morphology throughout the process. In conclusion, this investigation presents promising opportunities to fine-tune metal silicides for applications in electronics and photovoltaics.