| In the development of rare earth(RE)functional materials,rare earth luminescent materials are particularly compelling.Luminescence is the most prominent feature of the RE compounds,receiving people's a lot of attention.Due to the special electronic shell structures,rare earth elements have incomparable optical properties over general elements.RE luminescence almost covers the whole solid light-emitting category,as long as referring to light,it almost can't exclude RE elements.Rare earth elements have abundant electronic energy levels and long life excited states,they can emit various wavelength from ultraviolet,visible to infrared areas of electromagnetic radiation,making they become the great luminescence treasury.Doping RE ions into the semiconductor materials as luminescent center is the important way of making light-emitting devices and amplification devices,efforts to enhance the visible and near-infrared luminescence of RE ions are on-going.Er3+luminescence at 1.54 ?m,due to the intra-4f transition from the first excited to the ground state(4I13/2-4I15/2),nas attracted great interest since this emission waveiengtui is coincident with the absorption minimum of silica-based optical fibers.Fluorescence spectra of RE doped solid materials have obvious quenching effect,namely,with the increase of annealing temperature and RE inos density,the luminescence intensity will significantly decrease.Since the research found that the quenching effect decreased with the increase of the matrix materials' forbidden band width,the wide bandgap semiconductor materials become the ideal materials to host RE elements.And it has been reported that oxygen codoping and the use of wide-bandgap materials as the host material are effective to enhance the Er-related 1.54 ?m emission.ZnO is considered to be a promising candidate as a host material for Er doping because of being an oxide with a wide bandgap of about 3.37 eV.In addition,ZnO has a strong ability to resist radiation damage,so it has particular preponderance in the ion implantation processing technology of semiconductor devices,such as doping and insulation.And ZnO has great potential in the production of high temperature resistant,high frequency and high power optoelectronic devices.However,the efficiency of 1.54?m emission in Er:ZnO thin films has not yet been satisfied.One of the important reasons is the charge nature differences between the trivalent Er ion and bivalent Zn ion in matrix material,giving rise to the low optical activity of Er ions.Moreover,since trivalent Er ion can't directly displace the cation of ZnO material,making its solubility low in ZnO.Several studies believe that the addition of Si nanocrystals(nc-Si)can increase the effective absorption cross section of Er3+ ions.Through external light excitation,nc-Si absorbs photons and then inspires the excitons in Si,the exciton radiative recombination process emits photons.If Er ions near the nc-Si,the exciton radiative recombination process can transfer energy to Er3+,and inspire Er3+ luminescence.It has been confirmed that through the addition of Si nanocrystals into SiO2,strong Er3+ photoluminescence(PL)at room temperature could be obtained.Ge nanocrystals(nc-Ge)have many properties,such as wide size-dependent emission tunability,larger Bohr radius of about 24.3 nm,which is superior to nc-Si.It means that compared with nc-Si,the quantum confinement effect of nc-Ge is stronger,this effect will generate when the size of the nanoparticle is less than the Bohr radius.The luminescence efficiency of Er3+ may be enhanced by the introduction of nc-Ge through the change of local environment and recombination of photogenerated excitons in nanocryscals and subsequent energy transfer to Er3+.One of the research contents in this paper is to enhance the pholuminescence(PL)intensity of Er ions through codoping Ge and Er ions into ZnO material.Ge nanoparticles are formed by annealing process,the additional levels produced by the quantization character of nc-Ge and larger light capture cross section of the nanostructures,makes it become an effective medium transferring energy to Er3+,achieving the goal of improving the efficiency of the infrared photoluminescence.At the same time,the addition of nanoparticles destroy the symmetry of Er ions surrounding environment,enhancing the transition rates,and resulting in the enhancement of 1.54 ?m PL intensity.In this paper the experimental results show that the presence of nc-Ge greatly improves the efficiency of the Er3+ fluorescence.Another research content is to fabricate photonic crystal and waveguide structure in ZnO thin film,their light limit effect can improve the absorption transition probability of rare earth ions,reducing the scattering loss of light emitting area,which is also beneficial to the improvement of luminescence.The main research contents in this article are shown as follows:The first section investigates the growth conditions of Er doped ZnO thin film(Er:ZnO)by magnetron sputtering method.The influence on membrane structure and Er3+ fluorescence of temperature,pressure,oxygen argon ratio and substrate type are studied respectively.It is found that when the temperature of substrate is low,the crystallization of the film has poor quality,the grain size of the thin film is small.With the increase of temperature,the ZnO diffraction peak intensity will dramatically increase,it means that the crystallization of the film gradually gets better.With the increase of argon gas content,the size of grain grows,and film quality improves.Similarly,the film C axis preferred orientation gradually enhances when the sputtering pressure increases.By measuring the infrared PL spectrum,it is found that as the crystallization of ZnO involves in annealing process,the local symmetry and structure around Er3+ are improved,which is not favor for Er3+ PL.Thus properly decreasing the substrate temperature,reducing the working pressure,and increasing the sputtering argon oxygen ratio,are advantageous to the Er3+ 1.54 ?m emission.The second section investigates the enhancement effect of nc-Ge in Er3+ PL efficiency of the Ge:Er:ZnO film.The different co-doping methods of Ge are investigated respectively,including direct sputtering growth and subsequent ion implantation.Experimental and theoretical studies show that nc-Ge occurs quantum confinement effect,with the decrease of the size,the carrier(electrons,holes)movement are limited which leads to the increase of kinetic energy and the discrete of energy levels.The blue shift of the PL peak and broadening of the PL spectrum come up as the average size of nc-Ge decreases,coinciding with the quantum confinement effect.Sample containing nc-Ge shows strong visible PL with a peak at 582-593 nm,which is consistent with the calculated energy of the exciton of the?5 nm sized nc-Ge.The intensity of 1.54 ?m changes significantly due to the existence of nc-Ge and shows an obvious dependence on nanocrystal size.The size of?5 nm size nc-Ge makes the most contribution to the energy transfer between nc-Ge and Er3+ PL centers,resulting in the significant enhancement of Er-related 1.54 ?m PL.The size of?5 nm nc-Ge dominates in the 600 ? sample,the quantum confinement effect encourages excitation of nc-Ge at?2.2 eV,and this energy is resonant with 4S3/2-4I15/2 of Er3+.When external excitation is applied,excited nc-Ge can either give rise to the visible emission,or resonantly transfer energy to the nearby Er3+ and cause an energy level transition of 4I15/2-4S3/2.The excited intra-4f electrons of Er3+ ions then jump down to the first excited level4113/2 through a non-radiative relaxation and give 1.54?m emission by following the de-excitation transition to the ground level 4l15/2.A large absorption cross-section of nc-Ge and resonant energy transfer from nc-Ge to Er3+could make the 1.54?m emission more efficient.The existence of nc-Ge also strongly affects the local environment of Er,which enhances the transition rates,resulting in the enhancement of the 1.54 ?m PL intensity.And it is found that the Er3+ PL in Ge:Er:ZnO film is more governed by nc-Ge and less sensitive to the influence of surrounding potential field of ZnO.Er-doped ZnO thin film is fabricated on sapphire substrate by radio frequency magnetron sputtering technology.The as-deposited Er:ZnO film has good film quality and exhibits excellent single-mode waveguide characteristic.A photonic crystal structure in the Er:ZnO film is fabricated by focused-ion-beam etching.By selecting appropriate parameters,a photonic band gap(PBG)at wavelength from 1.51 ?m to 1.57 ?m can be achieved.The Er3+ PL frequency just falls within PBG of this PC,Er-related 1.54 ?m emission can propagate along the only path confined by the introduced line defects in this PC.When the film is stimulated by a 532 nm laser,PL at 1.54 ?m can be excited.Based on this PC,a minimized Er:ZnO light emitter or transmission device can be expected.Simulation results show that the propagation of 1.54 ?m is well restricted along a certain direction in the photonic crystal structure.It provides a novel way to control and confine the transmission of light in ZnO waveguide and will be applicable for the application of Er:ZnO photonic devices.A single-mode waveguide at 1.54?m in Er-doped ZnO thin film is fabricated by RF magnetron sputtering technology and FIB etching.It is found that the film has VIII good film quality with excellent single-mode characteristic.Ridge waveguide has rapid attenuation of the evanescent waves on both side walls,which endows it a stronger ability of confining guided wave.It is suitable for highly sensitive devices,especially in the miniaturization and integration of optical devices.When the film is stimulated by 532 nm laser,Er-related 1.54 ?m luminescence and directional transmission can be achieved in this Er:ZnO channel waveguide structure.Due to simple geometric structure and clear operating principle,this single-mode channel waveguide could be a candidate for future optical communications. |
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