Silicon-based Electroluminescent Devices With Oxide Films Containing Trivalent Rare Earth Ions

As the fundamental material of microelectronic industry, silicon (Si) is a kind of indirect band-gap semiconductor with extremely low luminous efficiency, which severely limits the development of Si-based optoelectronic integration. Therefore, it is necessary to utilize other luminescent materials to achieve the optical sources for Si-based optoelectronic integration. Rare earth tri-valent (RE3+) ions have specific electron structure, making their luminescence with high color purity, good stability and strong immunity of environment. In this context, great efforts have been expended to investigate the luminescence of RE3+ ions. Generally, the efficient excitation of RE ions necessitates appropriate host materials. It has been proved that the oxide materials including the oxide semiconductors are desirable to host the RE3+ ions since considerably high concentration of RE3+ ions can be doped into the oxides hosts and, moreover, the oxide films can be prepared by the processes compatible with the fabrication of integrated circuits. Consequently, the realization of silicon-based emitters with the RE-doped oxide films is of great importance for extending the application of RE-related luminescence and for developing the optical sources for silicon-based optoelectronic integration. In this dissertation, the electroluminescent devices based on the TiO2 films doped with different RE3+ ions and the Tb4O7 films with inherent Tb3+ions, respectively, have been realized and their operative mechanisms have been elucidated. The primary achievements are described as follows.(1) Color-tunable electroluminescence (EL) from the Eu-doped TiO2/p+-Si (TiO2:Eu/p+-Si) heterostructured devices using different TiO2:Eu films in terms of Eu content (0.8 and 1.2 at%) and annealing temperature (550 and 650℃) is realized. It is found that the Eu-related emissions are activated by the energy transferred from TiO2 host via oxygen vacancies, at the price of weakened oxygen-vacancy-related emissions. Both the higher Eu content and the higher annealing temperature for TiO2:Eu films facilitate the aforementioned energy transfer. In this context, the dominant EL from the TiO2:Eu/p+-Si heterostructured devices can be transformed from oxygen-vacancy-related emissions into Eu-related emissions with increasing Eu-content and annealing temperature for TiO2:Eu films, exhibiting different colors of emanated light.(2) Thulium (Tm)-related NIR EL at～800 nm from the TiO2:Tm/p+-Si heterostructured device is realized. It is found that the Tm-related NIR EL is activated by the energy transferred from the TiO2 host via oxygen vacancies, at the price of weakened oxygen-vacancy-related emissions from the TiO2 host. With Tm content increasing from 0.90 to 1.60%, both the NIR EL from the Tm3+ ions and the visible emission from the TiO2 host are dramatically enhanced, which can be ascribed to the formation of more oxygen vacancies due to higher Tm-doping. Furthermore, the device based on the TiO2:Tm (1.60%) film co-doped with fluorine (F) exhibits no EL in the visible and NIR regions, which is due to that the co-doped F" ions tend to occupy the sites of the oxygen vacancies in the host. Therefore, it is again proved that the (Tm)-related NIR emission is activated by the energy transfer from the TiO2 host to Tm3+ ions via oxygen vacancies.(3) The light emitting devices (LEDs) based on the iron (Fe) and erbium (Er) co-doped TiO2/p=-Si [TiO2:(Fe, Er)/p+-Si] heterostructures, which emit only the Er-related-1540 nm light are realized. The Fe co-doping in the TiO2:Er film completely suppresses the visible EL from the TiO2 host and the Er3+ ions, while enhancing the-1540 nm emission to a certain extent. Fe-codoping introduces deep levels into the energy bandgap of TiO2. In this context, the Er-related emission is activated by the energy transfer from the TiO2 host via the Fe-related energy levels. The carrier indirect recombination via the Fe deep levels releases the energy that can only excite the electrons of Er3+ions from the ground state 4I15/2 to the lowest excited states, enabling the emission at-1540 nm in the subsequent de-excitation process.(4) Multicolor and near-infrared (NIR) EL from the devices using RE-doped TiO2 (TiO2:RE) films as the light-emitting layers are realized, with the EL onset voltages below 10 V. Such EL is ascribed to the impact excitation of RE3+ ions. The above-mentioned devices are in the structure of ITO/TiO2:RE/SiO2/Si, in which the SiO2 layer is-10 nm thick and RE includes Eu, Er, Tm, Nd, and so on. With sufficiently high positive voltage applied on the ITO electrode, the conduction electrons in Si can tunnel into the conduction band of SiO2 layer via the trap-assisted tunneling mechanism, gaining the potential energy～ 4 eV higher than the conduction band edge of TiO2. Therefore, as the electrons in the SiO2 layer drift into the TiO2:RE layer, they become hot electrons. Such hot electrons impact-excite the RE3+ ions incorporated into the TiO2 host, leading to the characteristic emissions.(5) Green EL is realized owing to the intra-4f transitions of the Tb3+ ions inherent in a Tb4O7 film sandwiched between the ITO film and heavily phosphorous-or boron-doped silicon (n+-Si or p+-Si) substrate, thus forming the so-called metal oxide-semiconductor (MOS) device. The onset voltage of such EL is below 10 V. From the current-voltage characteristic and voltage-dependent EL spectra of the aforementioned MOS device, it is derived that the Tb-related green EL results from the impact excitation of Tb3+ions by the hot electrons (holes), which stem from the electric-field acceleration of the electrons (holes) injected from the n+-Si (p+-Si) substrate via the trap-assisted tunneling mechanism.