Depth-resolved correlation between Er distribution and photoluminescence in Si quantum dots in SiO₂

Light-emitting devices based on Si quantum dots (QDs) have been demonstrated, yet fabrication of light-emitting devices and lasers with wavelengths in the 1400-2500nm range is significantly more challenging due to multiple factors, including tensile stress and imperfections at the quantum dots/ silica matrix interfaces.  This research focuses on introducing low density of erbium (Er) to the array of Si QDs with the goal of having better control of emission in the near-infrared region. More specifically, it aims to understand better the effects of physical proximity between erbium ions (Er3+) and Si QDs on the optical properties of implanted SiO2 thin film samples, particularly the photoluminescence of Er centers at 1540 nm. It is known that Si quantum dots (QDs) are an effective strategy to enhance Er luminescence in SiO2 matrices, and the resulting emission strongly depends on Er-to-Si QD distance [1]. Quantitative models suggest that only ions located within a 1 to 3 nm of Si QDs contribute effectively to the 1540 nm emission, where the ones located within ~1.5 nm of the QDs surface undergo fast energy transfer, while those up to 2.5 nm and further away utilize slower mechanisms [1,2]. Our goal is to study spatial proximity as well as Er redistribution by diffusion, clustering or segregation during the thermal annealing required for Si QD nucleation and how it influences optical emission. 

We fabricated a set of samples by implanting Er ions at energies of 30, 80, 150 and 220 keV, to a dose of 5×10¹⁴ cm⁻², producing systematically shifted Er depth profiles (Fig. 1). A constant Si implantation of 2.8×10¹⁶ cm⁻² at 37 keV was done in all samples to generate a common Si-rich region for subsequent Si-QD formation. This implantation design (Fig.1) provides a system in which the overlap of Er and Si QDs distributions and the optical effects of physical proximity between them can be compared after thermal diffusion. 

Fig.1 Sample preparation process. 

For characterization, Rutherford backscattering spectrometry (RBS) is used to verify implanted doses. Medium energy ion scattering (MEIS) will be employed to quantitatively determine the depth distributions of Er and excess Si before and after thermal annealing, consequently showing annealing induced Er redistribution. Secondary ion mass spectrometry (SIMS) will be used as a complementary technique to validate the depth distributions obtained by MEIS. Such depth-resolved monitoring is essential as Er tends to segregate through precipitation or clustering when it exceeds solid solubility limits, leading to luminescence quenching [3]. Earlier studies have suggested that Er segregates away from Si-QD-rich regions, which reduces the fraction of optically active Er centers and limits energy transfer efficiency [2,4]. Finally, photoluminescence spectroscopy (PL) measurements will be performed to evaluate both Si NC and Er optical activity after annealing.  

In our preliminary results with a similar set of samples, we have observed an ~20 times increase in emission intensity (Fig.2). The knowledge gained from this study will guide the optimization of parameters and may lead to significant further improvements in Er –based LED devices. 

Fig.2 PL emission increase of ~x20 observed in previous samples. 

By combining quantitative depth profiling with optical characterization, we can establish correlations between Er luminescence intensity and the Er to Si depth overlap measured by MEIS. Previous results indicate that achieving strong coupling through such overlap can increase the effective Er excitation cross-section by up to four orders of magnitude compared to direct pumping [1]. The results will demonstrate how depth-resolved analysis can provide insight into dopant redistribution processes that directly affect optical performance. This work also highlights the capability of MEIS to connect depth-dependent compositional changes with functional properties in ion-implanted photonic materials and provides guidance for the optimization of Si QD enhanced Er emitters. 

References. 

[1] The mechanism of energy transfer from Si nanocrystals to Er ions in SiO2. K. Imakita, M. Fujii and S. Hayashi. Eur. Phys. J. D 34 (2005) pp 161-163. 

[2] Energy transfer in Er-doped SiO2 sensitized with Si nanocrystals. I. Izeddin, D. Timmerman, T. Gregorkiewicz, A. S. Moskalenko, A. A. Prokofiev and I. N. Yassievich. Phys. Rev. B 78 (2008) 035327. 

[3] Formation of Si–Er–O structures in Si by Ar-irradiation for harvesting light at the 4f intraband transition. S. Abedrabbo, M. M. Zeidan, A. Abdullah, I. A. Qattan, J. Hassan and A. T. Fiory. Radiat. Phys. Chem. 231 (2025) 112607. 

[4] Erbium in silicon. A. J. Kenyon. Semicond. Sci. Technol. 20 (2005) pp R65-R84.