Enhanced luminescence in ZnGa<Subscript>2</Subscript>O<Subscript>4-x</Subscript>S<Subscript>x</Subscript>:Mn<Superscript>2+</Superscript> thin-film phosphors grown on MgO(100) substrate

Mn-doped ZnGa₂O_4-xS_x thin-film phosphors have been grown using a pulsed laser deposition technique at varying growth conditions. Structural characterization was carried out on a series of ZnGa₂O_4-xS_x:Mn²⁺ films grown on MgO(100) substrates using Zn-rich ceramic targets. Oxygen pressure was fixed at 100 mTorr and substrate temperatures were varied from 500 to 700 °C. The results of X-ray-diffraction patterns showed that the lattice constants of the ZnGa₂O_3.95S_0.05:Mn²⁺ thin films decrease with the substitution of sulfur for the oxygen in ZnGa₂O₄. Measurements of photoluminescence (PL) properties of ZnGa₂O_4-xS_x:Mn²⁺ thin films have indicated that MgO(100) is one of the most promising substrates for the growth of high-quality ZnGa₂O_4-xS_x:Mn²⁺ thin films. In particular, the incorporation of sulfur into the ZnGa₂O₄ lattice could induce a remarkable increase of PL. The highest green-emission intensity was observed with ZnGa₂O_3.95S_0.05:Mn²⁺ films, whose brightness was increased by a factor of 3.5 in comparison with that of ZnGa₂O₄:Mn²⁺ films. This phosphor may be promising for application to flat-panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

Photoluminescence behavior of pulsed laser deposited ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript> thin-film phosphors grown on various substrates

ZnGa₂O₄ thin-film phosphors have been grown on Si(100), Al₂O₃(0001) and MgO(100) substrates using pulsed laser deposition. The structural characterization was carried out on a series of ZnGa₂O₄ films grown on various substrates under various substrate temperatures and oxygen pressures. The films grown on these substrates not only have different crystallinity and surface morphology, but also different Zn/Ga composition ratio. The crystallinity and photoluminescence (PL) of the ZnGa₂O₄ films are highly dependent on the deposition conditions, in particular the stoichiometry ratio of Zn/Ga and the kind of substrate. The variation of Zn/Ga in the films also depends on not only the oxygen pressure but also the substrate temperature during deposition. The PL properties of pulsed laser deposited ZnGa₂O₄ thin films have indicated that Al₂O₃(0001) and MgO(100) are promising substrates for the growth of high-quality ZnGa₂O₄ thin films and that the luminescence brightness depends on the substrate. The luminescence spectra show a broad band extending from 350 to 600 nm and peaking at 460 nm. Received: 11 July 2002 / Accepted: 31 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +82-51-6206356, E-mail: jhjeong@pknu.ac.kr     

Luminescence enhancement of ZnGa2O4:Mn by Ge and Li doping

Structural and optical properties of ZnGa₂O₄:Ge⁴⁺ and ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ phosphors were investigated by using X-ray diffraction (XRD), photoluminescence (PL) and cathodoluminescence (CL) measurements. The XRD patterns show that Ge-doped ZnGa₂O₄ has a spinel phase and its lattice constant increases with respect to ZnGa₂O₄. Emission wavelength shifts from 400 to 360 nm in comparison with ZnGa₂O₄ when Ge is doped in ZnGa₂O₄ and a peak related with oxygen defect was observed in Ge-doped ZnGa₂O₄. The CL luminance of ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ phosphors is seven times brighter than that of ZnGa₂O₄:Mn²⁺. This drastic luminance improvement can be attributed to Ge doping in ZnGa₂O₄ acting as donor ion and Li doping resulting in increasing conductivity of ZnGa₂O₄. These results indicate that ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ phosphors hold promise for potential applications in field-emission display devices with high brightness operating in green spectral regions.     

Influence of a dopant source on the structural and optical properties of Mn doped ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript> thin films

Thin films of ZnGa₂O₄:Mn²⁺ were deposited on quartz substrates using an rf magnetron sputtering technique. The sputtering target, ZnGa₂O₄ doped with 2 at. % manganese, was synthesized by a high temperature solid state reaction. Two different dopant sources were used to incorporate the dopant ions into the target, namely, manganese acetate and manganese oxide. The structural and optical properties of the thin films were studied using XRD, PL and transmission spectra. Polycrystalline ZnGa₂O₄:Mn with a spinel structure could be grown at an optimized substrate–target distance even at room temperature. No luminescence was observed in the as-deposited films grown using (CH₃COO)₂Mn as the dopant source in the target. Substrate heating or post-deposition annealing in the reducing ambient didn’t impart any luminescence to the films, ruling out the possibility of Mn²⁺ incorporation in the films. However, when using MnO as the manganese source in the target, the as-deposited films exhibited green photoluminescent emission (peak maximum at 508 nm) for substrate temperatures at and above 500 °C. This suggests that, in thin films, Mn incorporation and subsequent luminescent outcome is strongly influenced by the dopant source, which is quite different from the bulk phosphor behavior. PACS 81.15.Cd; 78.55.-m; 85.60.-q     

Morphology-dependent luminescence behavior of Y<Subscript>2-x</Subscript>Gd<Subscript>x</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> thin-film phosphors grown by a laser ablation

Y_2-xGd_xO₃:Eu³⁺ luminescent thin films have been grown on Al₂O₃(0001) substrates using pulsed laser deposition. Films grown under different deposition conditions have been characterized using microstructural and luminescence measurements. The crystallinity, surface morphology and photoluminescence (PL) of the films are highly dependent on the amount of Gd present. The photoluminescence (PL) brightness data obtained from Y_2-xGd_xO₃:Eu³⁺ films grown under optimized conditions have indicated that Al₂O₃(0001) is one of the most promising substrates for the growth of high-quality Y_2-xGd_xO₃:Eu³⁺ thin-film red phosphors. In particular, the incorporation of Gd into the Y₂O₃ lattice could induce a remarkable increase of PL. The highest emission intensity was observed with Y_1.35Gd_0.60Eu_0.05O₃, whose brightness was increased by a factor of 3.1 in comparison with that of Y₂O₃:Eu³⁺ films. This phosphor may be promising for application in flat-panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

Characterization of ZnGa2O4:Mn nanophosphor synthesized by the solvothermal method in 1,4-butanediol-water system

We characterized ZnGa₂O₄:Mn²⁺ (ZnGa₂O₄—zinc gallate) nanophosphor synthesized by the solvothermal method in 1,4-butanediol-containing water to increase the amount of Mn²⁺ ions incorporated in the ZnGa₂O₄ matrix without post-heat treatment. We investigated the influence of water content in the solvent on the photoluminescence (PL) intensity and the Mn amount, the latter being measured by X-ray fluorescence analysis and electron paramagnetic resonance spectroscopy. The PL intensity per Mn amount reached the maximum at the 50 wt% water content. The addition of water promotes repeated dissolution and precipitation, resulting in homogeneous Mn²⁺ distribution in the ZnGa₂O₄ matrix. This suggests that the solvothermal method in the 1,4-butanediol-water system is useful for increasing the amount of Mn²⁺ ions incorporated in the ZnGa₂O₄ matrix without post-heat treatment. At the water content >50 wt%, the decrease in PL intensity is attributed to the optical absorption of the by-product, MnOOH.     

Structure and properties of CaNb2O6:Sm3+ thin films by pulsed laser deposition

CaNb₂O₆:Sm³⁺ films were prepared on quartz glass and α-Al₂O₃(001) substrates by pulsed laser deposition. The structural, morphological, and optical properties of the CaNb₂O₆:Sm³⁺ films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), emission-scan electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL) measurements. The results show that the structure and properties of CaNb₂O₆:Sm³⁺ films were dependent on substrates. The CaNb₂O₆:Sm³⁺ films on Al₂O₃(0001) substrate have better crystallinity. The full-width at half-maximum (FWHM) of (131) peak are 0.45 and 0.32 for the CaNb₂O₆:Sm³⁺ film on glass and Al₂O₃(001), respectively. The crystallite size of CaNb₂O₆:Sm³⁺ films grown on glass and Al₂O₃(001) was about 8.22 and 9.98 nm, respectively. The oxidation state of the Sm element on the films was Sm³⁺ state. The photoluminescence (PL) spectra were measured at room temperature, the CaNb₂O₆:Sm³⁺ films on Al₂O₃(001) substrate have a better PL intensity, the identified emission bands were by the intra 4f transitions of Sm³⁺ from the excited level to the lower levels at 567 nm for ⁴G_5/2→⁶H_5/2 transition, at 609 nm for ⁴G_5/2→⁶H_7/2 transition, and at 657 nm for ⁴G_5/2→⁶H_9/2 transition.     

Crystalline-phase-dependent red emission behaviors of Gd2O3:Eu3+ thin-film phosphors

Gd₂O₃:Eu³⁺ luminescent thin films have been grown on Al₂O₃(0001) substrates using pulsed-laser deposition. The films grown at different deposition conditions showed different crystalline phases, surface morphologies and luminescent characteristics. Both cubic and monoclinic crystalline phases were observed for the Gd₂O₃:Eu³⁺ films, and the crystalline structure and the surface morphology of the films were highly dependent on the oxygen pressure and substrate temperature. The cubic system showed a higher luminescence than the monoclinic system. The luminescence characteristics were strongly influenced by not only the crystalline structure but also the surface morphology of the films. The photoluminescencebrightness data obtained from Gd₂O₃:Eu³⁺ films indicate that Al₂O₃(0001) is a promising substrate for growth of high-quality Gd₂O₃:Eu³⁺ thin-film red phosphor. In particular, the Gd₂O₃:Eu³⁺ films showed a much better photoluminescence behavior than a Y₂O₃:Eu³⁺ films with the same thickness. PACS 78.20.-e; 78.55.-m; 78.66.-w     

Energy transfer among three luminescent centers in full-color emitting ZnGa2O4:Mn, Cr phosphors

The ZnGa₂O₄:Mn²⁺, Cr³⁺ phosphors show three colors; the blue band of 380 nm from the charge transfer between Ga-O, the green band of 505 nm from Mn²⁺ and the red band of 705 nm from Cr³⁺. As a variation of Mn²⁺ or Cr³⁺ concentrations in ZnGa₂O₄:Mn²⁺, Cr³⁺, the relative emission intensity can be tuned. This phenomenon is explained in terms of the energy transfer based on four factors: the spectral overlap between the energy donors (Ga-O) and the energy accepters of Mn²⁺ or Cr³⁺, the absorption cross section of the energy accepters, the distance between them, and the decay time of the energy donors. ZnGa₂O₄:0.0025Mn²⁺, 0.010Cr³⁺ shows the CIE coordinates of x=0.4014, y=0.3368, which is a pure white light. The single-phased full-color emitting ZnGa₂O₄:Mn²⁺, Cr³⁺ phosphors can be applied to illumination devices.     

Improved photoluminescence of pulsed-laser-ablated Y<Subscript>1-x</Subscript>Gd<Subscript>x</Subscript>VO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> thin film phosphors by Gd substitution

Gd-substituted Y_1-xGd_xVO₄:Eu³⁺ luminescent thin films have been grown on Al₂O₃(0001) substrates using pulsed-laser deposition. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. The crystallinity, surface morphology, and photoluminescence (PL) of the films are highly dependent on the amount of Gd. The photoluminescence (PL) brightness data obtained from Y_1-xGd_xVO₄:Eu³⁺ films grown under optimized conditions have indicated that the PL brightness is more dependent on the surface roughness than the crystallinity of the films. In particular, the incorporation of Gd into the YVO₄ lattice could induce a remarkable increase of PL. The highest emission intensity was observed with Y_0.57Gd_0.40Eu_0.03VO₄ thin film whose brightness was increased by a factor of 2.5 and 1.9 in comparison with that of YVO₄:Eu³⁺ and GdVO₄:Eu³⁺ films, respectively. This phosphor have application to flat panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

First applicability of ZnGa2O4 : Ge, Li, Mn phosphor for a plasma display panel

The green emission intensity of ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ excited by the vacuum ultraviolet line of 147 nm reaches 70% of commercial green Zn₂SiO₄:Mn²⁺. The vacuum ultraviolet excitation spectra consist of four peaks. In a plasma display test bed filled with Ar and Ne plasma discharged by a radio-frequency generator of 13.6 MHz, ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ and commercial Zn₂SiO₄:Mn²⁺ phosphor screens show a linear increase in luminance with increasing self bias voltages. Increasing gas pressures cause the luminance to increase. Also, on increasing the self bias voltages and the gas pressures, the current densities of ZnGa₂O₄:Ge⁴⁺, Li⁺, Mn²⁺ phosphor screens are increased; this is the same behavior as that of the commercial phosphor.     

Laser–MBE growth of high-quality ZnO thin films on Al2O3(0001) and SiO2/Si(100) using the third harmonics of a Nd:YAG laser

High-quality ZnO thin films were grown on single-crystalline Al₂O₃(0001) and amorphous SiO₂/Si(100) substrates at 400–640 °C using laser molecular beam epitaxy. For film growth, the third harmonics of a pulsed Nd:YAG laser were illuminated on a ZnO target. The ZnO films were epitaxially grown on Al₂O₃(0001) with the narrow X-ray diffraction full width at half maximum (FWHM) of 0.04° and the films on SiO₂/Si(100) exhibited a preferred c-axis orientation. Furthermore, the films exhibited excellent optical properties in photoluminescence (PL) measurements with very sharp excitonic and weak deep-level emission peaks. At 15 K, PL FWHM values of the films grown on Al₂O₃(0001) and SiO₂/Si(100) were 3 and 18 meV, respectively. Received: 8 May 2001 / Accepted: 18 September 2001 / Published online: 20 December 2001     

EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.     

Optical and structural properties of nanosized ZnGa2O4:Cr phosphor

Nanosized ZnGa₂O₄:Cr³⁺ powder is synthesized through hydrothermal method. The average particle size is 20 nm and they are spherical in shape. The excitation band from the charge transfer between Cr³⁺-O²⁻ shows a blueshift behavior due to quantum confinement effect. X-ray diffraction pattern, Fourier transform-infrared spectrum, and electron paramagnetic resonance signal indicate that nanosized ZnGa₂O₄:Cr³⁺ phosphor shows many defect-related energy states and heavy lattice distortion in comparison with bulk ZnGa₂O₄:Cr³⁺ phosphor. Many defect states result in more nonradiative loss and shorter decay time.     

Synthesis and color-tunable luminescence properties of novel calcium aluminate silicate chloride phosphors

Eu²⁺ and Mn²⁺ co-doped calcium aluminate silicate chloride phosphors with the chemical composition of Ca₃Al₂Si₂O₈Cl₄:Eu²⁺, Mn²⁺ have been prepared by a solid-state method, and their luminescence properties have been investigated by tuning the En²⁺/Mn²⁺ ions concentration. The phase formation and microstructure of Ca₃Al₂Si₂O₈Cl₄:Eu²⁺, Mn²⁺ phosphors have been illuminated by XRD and SEM analysis. Photoluminescence (PL) spectrum reveals that Ca₃Al₂Si₂O₈Cl₄:Eu²⁺ exhibits a strong blue emission band centered at 431 nm, while Ca₃Al₂Si₂O₈Cl₄:Eu²⁺, Mn²⁺ can emit bluish-white light by adjusting the Mn²⁺ content appropriately. The energy transfer mechanism involving Eu²⁺–Mn²⁺ have also been investigated.     

Cathode luminescence characteristics of ZnGa2O4 phosphor thin films with the doped activator

The zincgallate (ZnGa₂O₄) phosphor thin film was grown using RF magnetron sputtering system at various process parameters. A ZnGa₂O₄ phosphor thin film was deposited on Si(1 0 0) substrate and annealed by a rapid thermal processor (RTP). The X-ray diffractometer (XRD) patterns indicate that the Mn-doped ZnGa₂O₄ phosphor thin film shows a (3 1 1) main peak and a spinel phase. A ZnGa₂O₄ phosphor thin film has better crystallization due to increased substrate, annealing temperature and deposition time. Also the ZnGa₂O₄:Mn phosphor thin film shows green emission (510 nm, ⁴T₁→⁶A₁), and the ZnGa₂O₄:Cr phosphor thin film shows red emission (705 nm, ⁴A₂→⁴T₂).     

Morphology and crystalline-phase-dependent luminescence of Li-doped Gd2O3:Eu3+ thin films grown by pulsed laser deposition

Gd₂O₃:Eu³⁺ and Li-doped Gd₂O₃:Eu³⁺ luminescent thin films have been grown on Si(100) substrates using pulsed laser deposition. The films grown at different deposition conditions show different crystalline and morphology structures and luminescent characteristics. Although both cubic and monoclinic crystalline structures were observed in both Gd₂O₃:Eu³⁺ and Li-doped Gd₂O₃:Eu³⁺ films, the cubic structure becomes more dominant for Li-doped Gd₂O₃:Eu³⁺ films. The photoluminescence brightness data obtained from Li-doped Gd₂O₃:Eu³⁺ films indicate that Si(100) is a promising substrate for growth of high-quality Li-doped Gd₂O₃:Eu³⁺ thin-film red phosphor. In particular, the incorporation of Li⁺ ions into the Gd₂O₃ lattice induced a change of crystallinity and enhanced surface roughness. Two major factors to determine photoluminescence brightness for Li-doped Gd₂O₃:Eu³⁺ films were crystalline phase and surface roughness. The highest emission intensity was observed with Gd_1.84Li_0.08Eu_0.08O₃, whose brightness was a factor of 2.1 larger than that of Gd₂O₃:Eu³⁺ films. This phosphor is promising for applications in flat-panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

Surface characterization and luminescent properties of SrAl2O4:Eu,Dy nano thin films

SrAl₂O₄:Eu²⁺, Dy³⁺ thin films were grown on Si (1 0 0) substrates in different atmospheres using the pulsed laser deposition (PLD) technique. The effects of vacuum, oxygen (O₂) and argon (Ar) deposition atmospheres on the structural, morphological and photoluminescence (PL) properties of the films were investigated. The films were ablated using a 248 nm KrF excimer laser. Improved PL intensities were obtained from the unannealed films prepared in Ar and O₂ atmospheres compared to those prepared in vacuum. A stable green emission peak at 520 nm, attributed to 4f⁶5d¹→4f⁷ Eu²⁺ transitions was obtained. After annealing the films prepared in vacuum at 800 °C for 2 h, the intensity of the green emission (520 nm) of the thin film increased considerably. The amorphous thin film was crystalline after the annealing process. The diffusion of adventitious C into the nanostructured layers deposited in the Ar and O₂ atmospheres was most probably responsible for the quenching of the PL intensity after annealing.     

Role of Ti in the luminescence process of Al2O3:Si,Ti

Al₂O₃:Si,Ti, prepared under oxidizing condition at high temperature, gives PL emission around 430 nm when excited with 240 nm. The Al₂O₃:C, TL/OSL phosphor, also shows emission around 430 nm, which corresponds to characteristic emission of F-center. Thus, to identify the exact nature of luminescent center in Al₂O₃:Si,Ti, fluorescence lifetime measurement studies were carried out along with the PL,TL and OSL studies. The PL and TL in Al₂O₃:Si,Ti show emission around 430 nm and the time-resolved fluorescence studies show lifetime of about 43 μs for the 430 nm emission, which is much smaller than the reported lifetime of ∼35 ms for the 430 nm emission (F-center emission) in Al₂O₃:C phosphor. Therefore, the emission observed in Al₂O₃:Si,Ti phosphor was assigned to Ti⁴⁺ charge transfer transition. Fluorescence studies of Al₂O₃:Si,Ti do not show any traces of F and F⁺ centers. Also, Ti⁴⁺ does not show any change in the charge state after gamma-irradiation. On the basis of the above studies, a mechanism for TSL/OSL process in Al₂O₃:Si,Ti is proposed.     

Luminescence characterization and electron beam induced chemical changes on the surface of ZnAl2O4:Mn nanocrystalline phosphor

Luminescence characteristics and surface chemical changes of nanocrystalline Mn²⁺ doped ZnAl₂O₄ powder phosphors are presented. Stable green cathodoluminescence (CL) or photoluminescence (PL) with a maximum at ∼512 nm was observed when the powders were irradiated with a beam of high energy electrons or a monochromatic xenon lamp at room temperature. This green emission can be attributed to the ⁴T₁ → ⁶A₁ transitions of the Mn²⁺ ion. Deconvoluted CL spectra resulted in two additional emission peaks at 539 and 573 nm that may be attributed to vibronic sideband and Mn⁴⁺ emission, respectively. The luminescence decay of the Mn²⁺ 512 nm emission under 457 nm excitation is single exponential with a lifetime of 5.20 ± 0.11 ms. Chemical changes on the surface of the ZnAl₂O₄:Mn²⁺ phosphor during prolonged electron beam exposure were monitored using Auger electron spectroscopy. The X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition of the possible compounds formed on the surface as a result of the prolonged electron beam exposure. The XPS data suggest that the thermodynamically stable Al₂O₃ layer was formed on the surface and is possibly contributing to the CL stability of ZnAl₂O₄:Mn phosphor.