Abstract
In this study, we used simulations as a guide for experiments in order to switch freestanding nanowire growth to a laterally aligned growth mode. By means of finite element simulations, we determined that a higher volumetric flow and a reduced process pressure will result in a preferred laterally aligned nanowire growth. Furthermore, increasing the volumetric flow leads to a higher species dilution. Based on our numerical results, we were able to successfully grow laterally aligned SnO2 nanowires out of gold film edges and gold nanoparticles on a-plane sapphire substrates. In our experiments a horizontal 2-zone tube furnace was used. The generation of Sn gas was achieved by a carbothermal reduction of SnO2 powder. However, we observed no elongation of the nanowire length with an increase of the process time. Nevertheless, an alternating gas exchange between an inert gas (Ar) and an oxygen-containing process atmosphere yielded an elongation of the laterally aligned nanowires, indicating that the nanowire growth takes place in a transient period of the gas exchange.
Highlights
Since the first reports in 1964 by Wagner and Ellis about the possibility to use a vapor–liquid–solid (VLS) process to grow semiconductor nanowires (NWs), significant work has been published on the production of nanowires [1,2]
The powder for the carbothermal reduction was positioned in the middle of the furnace and the samples were put downstream with respect to the powder position [15,16]
The convection, and the diffusion are contributing transport processes of the metal particles, which were generated by carbothermal reduction of SnO2 powder
Summary
Since the first reports in 1964 by Wagner and Ellis about the possibility to use a vapor–liquid–solid (VLS) process to grow semiconductor nanowires (NWs), significant work has been published on the production of nanowires [1,2]. It was shown that for VLS-based methods laterally aligned NWs of different materials can be grown directly along the respective chosen substrate surface [10,15,16]. The growth of laterally aligned SnO2 NWs was reported within a temperature range between 800 °C and 900 °C with volumetric flow rates ranging from 3 × 102 sccm up to 2 × 104 sccm [15,16] It can be concluded from a comparison of the process parameters and the data of these studies that an increase of the process pressure requires an increase of volumetric flow rate to achieve a laterally aligned NW growth [15,16]. Significant parameters are missing, it can be concluded that a high process pressure implies a high volumetric flow [15,16] This makes it necessary to analyze these influencing parameters with respect to the laterally aligned NW growth. We found that the introduction of temperature as a variable does not affect the main conclusions
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