Silicon-based Erbium Doped Zinc Oxide Thin Film Devices:Electroluminescence And Its Enhancement

It is well known that silicon suffers from extremely low luminescence efficiency due to its indirect bandgap structure,which seriously restricts the development of silicon-based optoelectronic integration.Therefore,exploring the light emitters compatible with the existing silicon-based CMOS integrated circuit technology has become an urgent issue.In the past three decades,several schemes have been put forward to realize the silicon-based light emitters.Among them,the silicon-based erbium?Er?-doped oxide semiconductor thin film light-emitting-devices?LEDs?feature the electroluminescence?EL?at¹.54?m just falling in the minimum loss window of the quartz fiber.Moreover,the fabrication processes for such LEDs are fully compatible with the CMOS processes.Consequently,the investigation on the EL from the silicon-based Er-doped oxide semiconductor thin film devices is of practical significance for the development of silicon-based light emitters.In this dissertation,the EL performances and their enhancement strategies of the silicon-based Er-doped ZnO thin film devices are systematically investigated.The primary achievements are summarized as follows:?1?The LEDs based on ZnO:Er/n-Si/n⁺-Si and ZnO:?Er,F?/n-Si/n⁺-Si heterostructures have been prepared,in which the Er-doped ZnO?ZnO:Er?and?Er,F?co-doped ZnO[ZnO:?Er,F?]films were deposited on n-Si/n⁺-Si epitaxial slices by radio-frequency?RF?magnetron sputtering,respectively.Under appropriate forward bias voltages,both LEDs exhibit only the Er-related visible and near-infrared?NIR?emissions.The EL intensities of the LED with ZnO?Er,F?film are¹0 and² times higher than those of the LED with ZnO:Er film in the visible and¹.54?m NIR regions,respectively.The carrier transportation behaviors of the two LEDs under the EL-enabling voltages are governed by the Poole-Frenkel?P-F?mechanism.Moreover,the Er-related EL from the two LEDs is ascribed to the impact-excitation of Er³⁺ions by the hot electrons.The reasons for the enhanced EL owing to the codoping of F^-ions are given as follows.Firstly,the ZnO:?Er,F?film have larger grain sizes than the ZnO:Er film,so the former possesses more optically active Er³⁺ions.Secondly,the F^-ions partially substitute for the O^2-ions around the Er³⁺ions to form ErO_6-x-x F_x octahedron,whose symmetry is lower than that of ErO₆ octahedron,thus increasing the intra-4f transition probabilities of the optically active Er³⁺ions.?2?The LEDs based on ZnO:Er/SiO_x/n⁺-Si and ZnO:?Zr,Er?/SiO_x/n⁺-Si?x?2? multilayered structures have been prepared,in which the ZnO:Er and?Zr,Er?co-doped ZnO[ZnO:?Zr,Er?]films were deposited on the pre-oxidized silicon slices with¹0nm-thick thermal oxide films by RF magnetron sputtering,respectively.At the same injection current,the EL intensities of the LED with ZnO:?Zr,Er?film are⁵0 and⁵times higher than those of the LED with ZnO:Er film in the visible and¹.54?m NIR regions,respectively.The carrier transportation behaviors of the two LEDs in the EL-enabling voltage regime follow the trap-assisted tunneling?TAT?mechanism.Moreover,the Er-related EL from the two LEDs is ascribed to the impact-excitation of Er³⁺ions by the hot electrons.The reasons why the LED with ZnO:?Zr,Er?film exhibit stronger Er-related EL are given as follows.Firstly,the Zr-codoping leads to the generation of Zn vacancies and the substitution of Zn lattice sites,which results in the transformation of‘ErO₆Zn₆'units into the‘ErO₆Zn_6-x'and‘ErO₆Zn_6-xZr_x'units.In this way,the ErO₆quasi-octahedrons containing the optically active Er³⁺ions are distorted in a manner,resulting in the decrease of crystal field symmetry and thus increasing the probabilities of the intra-4f transitions of Er³⁺ions.Secondly,the energy disperse spectrum?EDS?analysis associated with transmission electron microscopy?TEM?indicates that the incorporated Er³⁺ions are partially segregated at the ZnO/SiO_x interface within the LED with ZnO:Er film,resulting in the decrease of the optically active Er³⁺ions,while such Er segregation is not obvious in the LED with ZnO:?Zr,Er?film.?3?The LEDs based on the ZnO:Er/SiO_x/n⁺-Si and ZnO:?Ti,Er?/SiO_x/n⁺-Si?x?2?multilayered structures have been prepared,in which the ZnO:Er and?Ti,Er?co-doped ZnO[ZnO:?Ti,Er?]films were deposited on pre-oxidized silicon slices with¹0 nm-thick thermal oxide films by RF magnetron sputtering,respectively.At the same injection current,the EL intensities of the LED with ZnO:?Ti Er?film are⁵0 and 20times higher than those of the LED with ZnO:Er film in the visible and¹.54?m NIR regions,respectively.The carrier transportation behaviors of the two LEDs in the EL-enabling voltage regimes are dominated by the TAT mechanism and the resulting Er-related EL is originated from the direct impact-excitation of Er³⁺ions incorporated in the ZnO matrix by the hot electrons.It should be pointed out that the reasons for the enhanced EL due to the Ti codoping are similar to those due to the Zr codoping as mentioned above.?4?The LED based on Au/PMMA/ZnO:Er/n⁺-Si MIS structure has been prepared,in which⁰.1 at.%Er-doped ZnO:Er film was deposited on the silicon substrate by RF magnetron sputtering and the PMMA film was spin-coated onto ZnO:Er film.Such MIS-structured LED exhibits the ultraviolet?UV?random lasing?RL?under relatively low bias voltages.While it exhibits the near-band-edge?NBE?spontaneous UV emission of ZnO host itself at³80 nm and the Er-related visible emission under relatively high bias voltages.When the MIS device is applied with sufficient but relatively low forward bias voltages,the difference between the quasi-fermi level of the electrons(E_Fn)and the quasi-fermi level of the holes(E_Fp)in the region near the PMMA/ZnO:Er interface is larger than the bandgap of ZnO,namely,E_Fn-E_Fp>E_g,which satisfies the condition of stimulated emission and therefore optical gain.In polycrystalline ZnO:Er films,the NBE UV emission from the ZnO host is subject to multiple scattering.In certain multiple scattering processes,the optical gain can be larger than the optical loss,thus resulting in RL.When the MIS device is applied with relatively high forward bias voltages,a considerable number of holes are swept out of the region near the PMMA/ZnO:Er interface and drift into the inner region of the ZnO:Er film,where they recombine with the electrons drifting from n⁺-Si substrate.The direct recombination results in the NBE UV emission from the ZnO host at³80nm,while the defect-assisted indirect recombination transfers the energy to the Er³⁺ions nearby the defects,leading to the Er-related EL.?5?The LED based on ZnGa₂O₄:Er/SiO_x/n⁺-Si?x?2?structure has been prepared,in which the Er-doped ZnGa₂O₄?ZnGa₂O₄:Er?film was deposited on the pre-oxidized silicon slice with¹0 nm-thick thermal oxide film by RF magnetron sputtering.Under sufficiently high forward bias voltages,the LED exhibits the visible and¹.54?m NIR emissions of Er³⁺ions.Such emissions are ascribed to the direct impact-excitation of Er³⁺ions incorporated into the ZnGa₂O₄ lattice by the hot electrons.Under sufficiently high forward bias voltages,the electrons in n⁺-Si enter the conduction band of SiO_x via the TAT mechanism and then jump down to the conduction band of ZnGa₂O₄driven by the electric field to become hot electrons.These hot electrons directly impact-excite the Er³⁺ions incorporated into the ZnGa₂O₄ host,resulting in the characteristic emissions of Er³⁺ions.