Cathodoluminescence degradation and surface characterization of SrGa₂S₄:Ce³⁺ power and thin films
Moleme, Pulane Adelaide
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The structure, morphology and luminescent properties of commercial SrGa2S4:Ce3+ phosphor powder and thin films were investigated. The phosphor shows bright blue under ultraviolet (UV) excitation. Measurements were carried out using various characterization techniques such as Xray diffraction (XRD), scanning electron microcopy (SEM) and X-ray energy dispersive spectroscopy (EDS). The XRD data were collected using a D8 advance powder X-ray diffractometer with CuKα radiation. Morphology and elemental composition were done using Shimadzu Super Scan SSX-550 coupled with EDS. Photoluminescence (PL) data were collected using Varian Cary Eclipse Fluorescence Spectrophotometer with a monochromatized Xenon lamp (60-75 W) as excitation source and measurements were carried out in air at room temperature, and cathodoluminescence (CL) data were collected with S2000 Ocean Optics Spectrometer. The absorption spectra were recorded using Perkin Elmer Lambda 950 UV-VIS spectrometer. The same characterization tools were used to characterize the thin films. XRD data confirmed the orthorhombic structure of SrGa2S4 that was consistent with the standard JCPDS file no. (77-1189). The SEM images of the SrGa2S4:Ce3+ powder showed particles with irregular shapes and EDS detected presence of the major elements. Both PL and CL showed the broad emission peaks around 444 nm and 485 nm which are due to Ce3+ radiative transitions (5d (T2g) → 4f (2F5/2) and 5d (T2g) → 4f (2F7/2)). Cathodoluminescent ageing characteristics of the SrGa2S4:Ce3+ powder and thin films under prolonged electron beam bombardment were studied and presented. The cathodoluminescent intensity with increasing Coulomb loading was observed to degrade under different primary electron beam voltages for the powder. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to monitor the surface chemical changes both during electron beam bombardment and after the degradation process. Auger peak to peak heights monitored during the ageing process suggest a decrease in S and C Auger peak intensity and an initial increase in oxygen concentration on the surface. XPS results indicate the formation of an SrO overlayer due to electron stimulated surface chemical reactions (ESSCRs). For preparation of films, silicon (Si) (100) substrates were used. A pellet was prepared from the standard SrGa2S4:Ce3+ powder. The Lambda Physik EMG 203 MSC 309 nm XeCl excimer laser was used to grow the films. The films growth was carried out in a chamber which was first evacuated to a base pressure of 8 x 10-5 mbar before backfilling to pressures of 1.0 x 10-2 mbar Ar and 1.0 x 10-2 mbar O2, where Ar and O2 were used as cross pulse gases. The films were deposited at different substrate temperatures ranging from 400°C to 600°C with 28 800 and 57 600 pulses respectively. The laser beam was operated at 8 Hz repetitive rate. The substrate temperature, number of pulses and the working pressure are the parameters that were varied during the preparation of the thin films. A highly crystalline SrGa2S4 layer was obtained at the growth temperature of 400°C. XRD patterns also showed that the properties of the films were sensitive to substrate temperature. PL and CL spectra were characterized by a broad band that can be fitted by two Gaussian peaks according to the two Ce3+ radiative transitions. At high substrate temperature a shift to Ce3+ emission in SrS occurred as well as in Ar atmosphere for both UV and high energy electrons excitation. The atomic force microscopy (AFM) images before annealing exhibited smooth surface at low substrate temperature, which became rough at high substrate temperature and after annealing in vacuum at 700°C temperature. Non-uniformity in particles (big and small) of the films and smooth films were observed from the SEM images.