Silicon is a key component for optoelectronic devices such as solar panels and transistors. New observations on the stability of exotic silicon phases have revealed that some types of Si are stable to a much higher temperature than previously thought. Since high temperature processing is common in the development of optoelectronic devices, this has positive implications for how these new types of silicon can be used in future solar energy materials.
In a paper published in the Journal of Applied Physics, researchers at The Australian National University in collaboration with the University of Melbourne, Oak Ridge National Laboratory and the Universidad de La Laguna found that metastable silicon states achieved by indentation remained stable up to 450 °C. The research has clarified how these indentation-formed phases of silicon evolve through metastable structures such as r8-Si, to nanocrystalline phases such as hd-Si and Si-XIII.
In this work, researchers used a combination of high-pressure indentation and high temperature annealing to ensure the silicon would undergo phase transitions into the desired phases. After the initial indentation, pressure is gradually released, and the phases are subsequently formed during the annealing step.
As the sample is heated, they used Raman spectroscopy to map characteristic peaks in the silicon and thus identify the phase. As shown in the figure below, the silicon phases follow the pathway bc8/r8→Si-XIII/hd-Si→hd-Si→dc-Si, with the key finding here being that Si-XIII is seen at 100 °C and remains until 240 °C, and the crystalline hd-Si appears at around 200 °C and impressively remains beyond 450 °C.
Sherman Wong, who performed the research The Australian National University (now working as a researcher at RMIT University), said: “This work allowed us to show differences in the Raman spectra of the silicon samples as a function of temperature, which lets us see when different phases are starting to appear or disappear. We were particularly interested in when Si-XIII started appearing, as it is a completely new phase, and how high a temperature we needed to go before hd-Si disappeared. It was exciting to find that hd-Si is stable at 450°C, as modern Si devices are processed at this temperature.”.
One of the key studies in this work was the comparison of three different annealing methods: furnace, laser, and hot-stage ramped annealing. The researchers attribute their new, more accurate temperature readings for the silicon phase transitions in part due to the improved accuracy of their temperature measurement as different phases absorbed the laser’s heat at different rates, as well as a better understanding of thinning behaviour in the samples. For the hot stage annealing, a Linkam THMS600 temperature control stage was used to precisely control the annealing temperature within a nitrogen environment. The temperature stage was also used to perform in-situ Raman microscopy at a range of temperatures, as pictured in the figure below. The Raman peaks were used to identify the silicon phases, and the change in Raman intensity was then observed as a function of temperature to show when phases started changing from one to another.
Silicon has been at the heart of the semiconductor industry since the mid-20th century due to its ability to be doped to achieve better electrical properties. Dopants such as phosphorus or boron are introduced into the silicon lattice to create an electron-rich (n-type) or electron-depleted (hole rich, p-type) semiconducting material. Current and voltage can then be generated from incident light via the photovoltaic effect. Silicon itself has various phases which can be induced by high pressure and temperature. By controlling the atomic structure, it is possible to enhance the absorption of incident light, which raises the photovoltaic efficiency. For example, r8-Si is predicted have an absorption spectrum which overlaps more with the solar spectrum than standard diamond cubic silicon.
For more information on this research, please visit the profile of Dr Sherman Wong, now at RMIT: https://www.rmit.edu.au/contact/staff-contacts/academic-staff/w/wong-dr-sherman.
For more information on the Linkam instrument, please contact Linkam’s Application Specialist, Dr Robert Gurney: email@example.com.