Photoluminescence of Er³⁺-implanted amorphous hydrogenated silicon suboxides

Erbium ions incorporated into amorphous hydrogenated silicon suboxides (a-SiO_x:H) allow to overcome the disadvantages of Er³⁺ in c-Si such as the limited solubility, the strong quenching of the luminescence at room temperature, and the need for co-doping with electronegative atoms. a-SiO_x:H alloys have an enhanced Er solubility and easily variable oxygen content, thereby providing favorable atomic environments for an efficient Er luminescence and reduced excitation backtransfer due to deeper localized band-tail states. In the present study, Er³⁺ doses up to 7×10¹⁴ cm^-2 were implanted into a-SiO_x:H with oxygen content between 0 and 44 at. %. Optical properties such as the absorption coefficients and the photoluminescence (PL) spectra of the Er³⁺ ions and of the SiO_x host were investigated as a function of erbium implantation dose, oxygen content, defect density, temperature, and annealing treatment. It was found that annealing is a requirement for activating the characteristic Er PL at 1.54 μm mainly due to a reduction of implantation induced defects. The intensity of both the intrinsic SiO_x and the Er PL was found to be inversely proportional to the defect density as measured by electron spin resonance or subgap absorption. The Er PL is additionally enhanced upon annealing, probably as a result of better structural arrangements of the Er ions. The Er PL intensity increases approximately linearly with the implantation dose. An increase of the oxygen content (and correspondingly of the optical band gap) of a-SiO_x:H causes no drastic changes in the erbium luminescence energy and intensity, whereas the intrinsic PL shifts to higher photon energies according to the larger band gap. Already low O concentrations of a few percent provide favorable Er environments. The main advantage of a-SiO_x:H as a host matrix is revealed by temperature-dependent PL measurements. For high oxygen contents, the thermal quenching of both the Er³⁺ and the intrinsic PL is strongly reduced. In an a-SiO_x sample with 44 at. % oxygen, the Er PL is only quenched by 20% between 77 and 300 K. In contrast, the quenching of the intrinsic PL for all [O] is roughly one order of magnitude stronger than that of the Er PL. These PL measurements were complemented by PL excitation experiments over a wide spectral range. We have observed that the Er³⁺ PL is excited about one order of magnitude more efficiently when pumped with sub-band-gap light compared to band-to-band excitation. The experimental results are discussed with regard to the two currently proposed Er3+ excitation models, the defect-related Auger effect and the Förster transfer mechanism.