Study On Rare Earth Doped Lu₂O₃/CdS Composite Thin Films Based X-Ray Detectors

X-ray detectors can detect invisible high-energy X-rays,so it has a wide range of applications in industrial inspection,nuclear energy,nuclear medicine,and safety inspection.X-ray detectors based on scintillation crystals have the advantages of portability,high efficiency,and economy,so it is one of the important development directions.Rare earth oxide has the advantages of high density,stable physical and chemical structure,high thermal conductivity and low phonon energy.It can control the fluorescence emission and efficiency by doping rare earth ions.However,the melting points of rare earth oxides are all higher than1400℃.Common single-crystal pulling and pressed ceramics techniques are difficult to achieve the planar thin film preparation,which hinders the application of X-ray detectors.In this paper,Lu₂O₃:Re³⁺(Eu³⁺,Tb³⁺,Tm³⁺)thin film prepared by near-room temperature solution electrochemical deposition method is used to control the optical properties by controlling the type,concentration and deposition time of doped ions to obtain efficient red-green-blue（RGB）three primary colors Uniform dense and transparent Lu₂O₃:Re³⁺rare earth oxide scintillator film with down-converted fluorescence.A layer of metal interdigital electrode and CdS photosensitive film was further electrodeposited on the surface of Lu₂O₃:Re³⁺film to realize the numerical detection of X-ray.The main research contents are as follows:（1）Preparation of Lu₂O₃:Re³⁺thin film scintillator and study of its growth mechanism.The cyclic voltammetry method was used to select the best deposited reference potential to be-1.2 V,and the solution was electrochemically deposited at near room temperature to deposit a rare earth oxide precursor film.The TG-DTA curve and XRD pattern studies before and after thin film annealing revealed that during the deposition process,H₂O molecules and NO₃^-in the solution produced a large amount of OH^-near the FTO working electrode,which promoted the rare earth ion to grow a layer of hydrated rare earth compounds on its surface Re₂（OH）_6-x（NO₃）_x·y H₂O,transformed into pure cubic Lu₂O₃:Re³⁺scintillation film after annealing at 400-600℃.The thickness of the Lu₂O₃ film was changed from 333.8±9.3 nm to16.13±0.35μm by changing the deposition time from 3 to 15 min.With the increase of the deposition time,the transmittance of the Lu₂O₃:Re³⁺scintillation film in the visible light band decreased from 95.75%to 81.08%.SEM and AFM analysis found that the Lu₂O₃:Re³⁺scintillation film had a dense cross section and a clear interface,a dense and uniform surface,and a surface roughness（Ra）of the order of 100 nanometers.The red,green and blue（RGB）three-primary fluorescence emission of Lu₂O₃:Re³⁺film was obtained by doping Eu³⁺,Tb³⁺and Tm³⁺.Under the excitation of 245 nm UV light,the Lu₂O₃:Re³⁺film emits red fluorescence in the 600-625 nm band,and the red light emission comes from the radiation transition of the ⁵D₀→⁷F₀ energy level.Under the excitation of 254 nm ultraviolet light,the Lu₂O₃:Tb³⁺thin film emits green fluorescence in the 530-575 nm band,which originates from the radiation transition of the ⁵D₄→⁷F₅ energy level.Under the excitation of 355 nm UV light,the Lu₂O₃:Tm³⁺film emits blue fluorescence in the 475-500 nm band,which originates from the radiation transition of the ¹G₄→³H₆ energy level.Spectroscopy analysis found that the fluorescence intensity of the Lu₂O₃:Re³⁺thin film increased with the deposition time first,and then changed very little after 11 minutes.Lu₂O₃:Re³⁺scintillator film with transparent and efficient three primary colors.（2）Preparation and optical properties of CdS photosensitive films.The cyclic voltammetry curve was used to select a reference potential of-1.7 V.By adjusting the molar concentration ratio of Cd²⁺to S₂O₃^2-in the electrolyte,a CdS photoelectrically-responsive semiconductor film was deposited at room temperature（50℃）in a water bath.By analyzing the XRD pattern,it was found that when the molar ratio of Cd²⁺to S₂O₃^2-was 5:1,the prepared film was a cubic and six-direction mixed phase CdS film,and no impurities such as S,Cd,and Cd O existed.Analysis of the CdS thin film by SEM and AFM spectroscopy revealed that the cross section was dense,the interface was clear,the surface was dense and uniform,the surface roughness（Ra）was 75～90 nm,and the thickness increased with deposition time 445.02±5.14 nm to 1.6±0.04μm.XRD pattern analysis showed that the hexagonal phase ratio increased and the tetragonal phase ratio decreased in the CdS photosensitive film after annealing.Through UV-Visible-NIR absorption spectrum analysis,it was found that the CdS photosensitive film has strong absorption for light with a wavelength less than 650 nm,and the band gap is～2.17 e V.CdS light-sensitive films under pulsed light irradiation at 615 nm,545 nm,and 490 nm,with a bias voltage from 0.5-2.5 V,the photocurrent increased rapidly from 40 n A to 90 n A.The rise timeτ_rise of photoelectric detection reaches 0.714～1.334 s and the decay time,τ_decay,is 1.232～3.717 s.（3）Lu₂O₃:Re³⁺scintillator/CdS composite thin film X-ray detector assembly and application research.The planar composite of CdS photosensitive film and Lu₂O₃:Re³⁺scintillation fluorescent film was realized by electrochemical deposition.It was found that the fluorescence intensity decreased sharply,indicating that CdS photosensitive film has strong absorption and photoelectric effect in the visible light region.An X-ray source with an operating voltage of 60 k V and adjustable power from 0 to 12 W was used as the excitation source.The X-ray excitation fluorescence spectrum was analyzed and found that the Lu₂O₃:Re³⁺scintillator film emits strong RGB three-primary fluorescence.The spectrum is roughly the same as that of ultraviolet excitation,and only some new emission peaks appear in the short wavelength region,mainly from deep-level defect emission.The X-ray linear responsivity of the RGB three-primary fluorescence is 0.997,0.999,and 0.998,respectively.Under X-ray excitation,the detector has a responsivity of up to 81 n A/W,a current switching ratio of about 30 and a response time of about 0.15 s.The data show that the X-ray linear response of the rare earth oxide scintillator and the photoelectrically-responsive semiconductor composite film is good,which can directly realize X-ray human eye visualization and numerical current detection.