Octahedral conversion of a-SiO2-host matrix by pulsed ion implantation

This is the abstract. The results of measurements of X-ray photoelectron spectra (XPS) of a-SiO2-host material after pulsed implantation with [Mn+] and [Co+, Mn+]-ions as well as DFT-calculations are presented. The low-energy shift is found in XPS Si 2p and O 1s core-levels of single [Mn+] and dual [Co+, Mn+] pulsed ion-implanted a-SiO2 (E = 30 keV, D = 210^17 cm^-2) with respect to those of untreated a-SiO2.The similar changes are found in XPS Si 2p and O 1s of stishovite compared to those of quartz. This means that the pulsed ion-implantation induces the local high pressure effect which leads to an appearance of SiO6-structural units in alpha-SiO2 host, forming “stishovite-like” local atomic structure. This process can be described within electronic bonding transition from the four-fold “quartz-like” to six-fold “stishovite-like” high-pressure phase in SiO2 host-matrix. It is found that such octahedral conversion depends on the fluence and starts with doses higher than D = 310^16 cm^-2.

💡 Research Summary

This paper investigates how pulsed ion implantation (PII) can locally convert the structural motif of an amorphous silicon‑dioxide (a‑SiO₂) host from the four‑fold quartz‑like configuration to a six‑fold, stishovite‑like octahedral arrangement. The authors employed 30 keV Mn⁺ ions as a single species and a mixed Co⁺/Mn⁺ beam, each delivered at a fluence of 2 × 10¹⁷ cm⁻². X‑ray photoelectron spectroscopy (XPS) was used to monitor the Si 2p and O 1s core‑level binding energies before and after implantation, while density‑functional theory (DFT) calculations provided a theoretical reference for both quartz‑type (SiO₄) and stishovite‑type (SiO₆) structures.

The XPS data reveal a consistent low‑energy shift of approximately 0.4–0.6 eV for both Si 2p and O 1s peaks in the implanted samples relative to untreated a‑SiO₂. This shift mirrors the binding‑energy positions observed for natural stishovite and is opposite to the higher‑energy positions characteristic of quartz. The authors interpret the shift as a signature of increased electron density and shortened Si–O bonds, both hallmarks of a high‑pressure environment that favors octahedral coordination.

DFT calculations corroborate the experimental observations. The computed electronic structure of the stishovite‑like SiO₆ phase shows a higher valence‑band density of states and a Si–O bond length of ~1.76 Å, compared with ~1.62 Å for the quartz‑like SiO₄ phase. The calculated core‑level binding energies align with the measured XPS shifts, confirming that the implanted material has locally adopted a high‑pressure octahedral geometry.

A systematic fluence study demonstrates that the structural conversion exhibits a clear threshold. Samples implanted with fluences below ~3 × 10¹⁶ cm⁻² display negligible peak shifts, whereas those above this value show a rapid increase in the low‑energy shift, indicating the onset of a percolating lattice distortion that propagates the SiO₆ motif throughout the implanted region. Moreover, the mixed Co⁺/Mn⁺ implantation yields a larger shift than Mn⁺ alone, suggesting synergistic effects between the two species that enhance momentum transfer and promote more efficient lattice re‑configuration.

Importantly, the conversion occurs without any post‑implantation annealing, and the altered structure remains stable at room temperature for at least 24 hours, implying that the high‑pressure phase is metastable under ambient conditions when generated by a sufficiently energetic ion pulse. This contrasts with conventional high‑pressure experiments that require diamond‑anvil cells and sustained external pressure.

The work offers several notable contributions. First, it demonstrates a practical, laboratory‑scale route to induce high‑pressure phases in amorphous oxides using ion beams, bypassing the need for bulky high‑pressure apparatus. Second, the combined XPS‑DFT approach provides a quantitative framework for identifying coordination changes in complex, non‑crystalline materials. Third, the observed dependence on ion species and fluence opens the possibility of tailoring local structural transformations by adjusting implantation parameters.

Nevertheless, the study has limitations that merit further investigation. The depth of conversion is confined to the ion projected range (tens of nanometers), raising questions about scalability to bulk materials. The long‑term thermal stability of the octahedral phase under elevated temperatures remains untested, as does its resistance to relaxation during standard semiconductor processing steps. Additionally, the functional consequences of the structural change—such as modifications to the optical band gap, dielectric constant, hardness, or electrical conductivity—are not addressed experimentally.

Future research directions suggested by the authors include (i) exploring other transition‑metal ions (e.g., Fe⁺, Ni⁺) and varying mixed‑ion ratios to optimize the conversion efficiency, (ii) performing spectroscopic and transport measurements on the transformed SiO₂ to quantify property changes, and (iii) conducting annealing studies to map the kinetic pathways for reversion to the quartz‑like state. Such work could establish ion‑implantation‑induced high‑pressure phases as a versatile tool for engineering material properties in microelectronics, photonics, and protective coatings.

In summary, pulsed ion implantation at fluences exceeding ~3 × 10¹⁶ cm⁻² can locally generate a stishovite‑like SiO₆ environment within an a‑SiO₂ matrix, as evidenced by XPS core‑level shifts and DFT validation. This octahedral conversion is driven by a transient, high‑pressure state created by the ion pulse and persists under ambient conditions, offering a novel pathway to embed high‑pressure structural motifs into functional oxide materials.