Electroluminescence, Charge Trapping and Clustering in Rare-Earth Implanted SiO2-Si Light-Emitting Diodes

In this work a comparative study of charge trapping, electroluminescence intensity (ELI) and clustering in SiO2 implanted by different rare-earth (RE) impurities (Eu, Tb, Gd, Er, Tm) with following high-temperature annealing is performed to clarify the connection between the electrical properties, the structure of the luminescent centers, the ELI and the EL spectra. RE impurities were implanted into the bulk of thermally grown SiO2 on n-type Si. The implanted doses were chosen in such a way that the maximum concentration corresponded to 0.1, 0.5, 1.5 and 3.0 at %. To activate the RE implanted impurities a post implantation furnace anneal in the temperature range of 800-1100 °C for 30 min and flash lamp annealing (FLA) for 20 ms at 1000 °C in a nitrogen ambient have been carried out. The ITO layer was used as a transparent electrode. Charge trapping was studied by the shifting of the high-frequency CV characteristics and the changing of the applied voltage during constant current electron injection from Si into SiO2. The EL signal was recorded at the same injection regime at room temperature in the wavelength range of 300 to 750 nm. Some control structures were studies by transmission electron microscopy with high resolution (XTEM).
It was shown that RE impurities such as Tb, Gd, Er and Tm implanted into SiO2 cause mainly net positive charge trapping in the range of the injected charge from 1x1015 to 2x1017 e/cm² and stable ELI of their main luminescence lines in green, UV, IR and blue spectral region of the EL spectra, respectively. Above 1x1018 e/cm² of the injected charge an electron trapping in the bulk of the oxide and a hole trapping at the SiO2-Si interface is observed for all types of the RE impurities. The electron trapping correlates with the EL quenching of the main EL lines for all studied RE implanted structures with the exception of the Eu implanted one. The Eu implanted oxide demonstrates effective electron trapping up to 1x1020 e/cm² without EL quenching of the main studied EL lines: in red spectral region with a maximum at 618 nm (5D0-7F2 transition for Eu3+ ions); in the blue-green spectral region around 460-470 nm and in blue-violet one at 410 nm (corresponding to a 4f6d-4f7 transition of the Eu2+ ion). The XTEM measurements discovered that the clustering in the Eu-implanted SiO2 is enhanced considerably in comparison with the Tb-implanted one. It is suggested that the enhanced electron trapping in the Eu implanted structures is associated with enhanced clustering, which is partly caused by low valency (2+) oxides existing for the Eu impurity such as EuO and Eu3O4. The use of FLA for the Eu implanted SiO2 results in a decrease of the nanocluster size and an increase of the ELI in the red region of the spectrum.