Silicon-based solar panels, first developed in the 1950s, are still evolving by virtue of pioneering new research into the high temperature stability of Silicon
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.
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 heated, Raman spectroscopy was used 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.
(Top) Silicon can be in various metastable phases depending on the annealing temperature
(Bottom) Chemical structures of two of these silicon phases: hd-Si and r8-Si
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.”.
(Left to Right) Dr Sherman Wong, Prof. Jim S. Williams and Prof. Jodie E. Bradby in their lab at the Australia National University
One of the key studies in this work was the comparison of three different annealing methods: furnace, laser, and hotstage ramped annealing. The researchers attribute their new, more accurate temperature readings for the silicon phase transitions 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 hotstage 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.
Electron diffraction reflections from samples that had undergone (a) laser and (b) hot stage annealing. (c) Measured Raman peaks identifying each silicon phase at various and (d). The intensity ratio in Raman spectra for the bc8 and r8 phase as a function of temperature change
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 to have an absorption spectrum which overlaps more with the solar spectrum than standard diamond cubic silicon.
References
Wong, Sherman, et al. "Thermal evolution of the indentation-induced phases of silicon." Journal of Applied Physics 126.10 (2019): 105901.
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