1. Introduction

Light emitting diodes (LEDs) based on GaN and alloys of GaN with InN and AlN materials systems have enabled a revolution in solid state lighting,[1, 2] and have already gradually started to replace incandescent bulbs and fluorescent lamps in general lighting applications. Phosphors for solid state white lighting are the crucial component to achieve high luminous efficiency and good color rendering index (R_a) in LED + phosphor assemblies.[3, 4, 5, 6] In this regard, the efficiency of a phosphor-converted LED (pcLED) can be expressed[7]

where η_pcLED is the final pcLED efficiency, η_LED is the LED efficiency, η_phosphor is the phosphor quantum efficiency, η_package is the package efficiency, and η_S is the Stokes conversion efficiency. Thus, the eventual performance of white LED based devices strongly depends on the luminescence properties of the phosphors used.[8] Approaches to increase η_package in white LEDs have included the insertion of low-refractive index layers (TiO₂/SiO₂) as a reflecting layers,[9] and the use of photonic crystal layers in the LED package.[10]

As phosphor research progresses towards the goal of obtaining highly efficient white LEDs, various phosphors having different absorption maxima are required depending on specific LED emission characteristics, including the emission wavelength, (=λ _max)[6, 13, 14] which in theory can range from 0.6 eV to 6.0 eV through alloying of InN, GaN, and AlN compound semiconductors.[1, 2]

In previous studies, we reported successful solid solutions between two distinct phosphor hosts, with concurrent modification of both anionic groups as well as cations, allowing greater tunability and the simultaneous optimization of multiple parameters.[11] We have used the same idea here in attempting to prepare the solid solution series Gd_1-xSr_2+xAlO_5-xF_x:Ce³⁺ (GSAF:Ce³⁺) which combines two distinct phosphor host systems. The first is GdSr₂AlO₅ (GSA)[12] and the second is the previously unreported (for phosphors) system Sr₃AlO₄F (SAF). SAF was first reported by Kennedy et al. and is structurally similar to the reported compound GSA (I4/mcm, S.G.# 140).[15, 16] Despite distinct chemical compositions, we can expect that a complete solid solution series based on the structural similarity, and results of our previous work.[11, 17]

Herein, the new green-emitting phosphor SAF:Ce³⁺, which crystallizes in a tetragonal host structure in space group I4/mcm (S.G. #140),[15, 16] can provide several attractive properties for Ce³⁺ doped solid solution phosphors: (1) the introduction of F ions into the host lattice can decrease the extent of thermal quenching of the phosphor during LED operation as a consquence of the softer phonon modes, (2) both crystal field splitting and covalency of the Ce-O bonds, which result in a shift of emission band positions and widths, can be controlled by changing solid solution amount x, and (3) finally we can expect to tune the efficiency of the phosphor, since as we determine, the SAF:Ce³⁺ phosphor has somewat high quantum efficiency.

In this study, a series of phosphors with formulae Gd_1-xSr_2+xAlO_5-xF_x:Ce³⁺ (GSAF:Ce³⁺) (x=0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) were prepared and their structural and optical properties investigated. The new solid solution phosphors GSAF:Ce³⁺ are highly tunable under 370 nm to 450 nm excitation sources. Furthermore, white LEDs based InGaN LED chips (λ_max=405 nm and 450 nm) with the GSAF:Ce³⁺ phosphor samples were fabricated and studied.

2. Experimental

Samples of SASF:Ce³⁺ were prepared by solid state reactions using high purity (≥99.9%) Gd₂O₃, SrCO₃, SrF₂, Al₂O₃, and CeO₂ as starting materials, by firing between 1200°C and 1500°C in a reducing atmosphere of 5%H₂/95%N₂ for 4 h. The concentration of Ce³⁺ was fixed to 0.025, which was the optimum concentration of Ce³⁺ in GSAF:Ce³⁺. The X-ray diffraction (XRD) data were obtained over the scattering angle of 20°≤2θ≤100° with steps of 0.016° using Cu-Kα radiation (Philips X’Pert). In order to obtain lattice parameters, the LeBail pattern-matching method was employed, as implemented in the General Structure Analysis System (GSAS) program.[18] PL spectra were obtained on a Perkin Elmer LS55 spectrophotometer. Prototype white LEDs were fabricated by applying an intimate mixture (40 %/60 %) by weight of the phosphor powder and transparent silicone resin on InGaN LEDs (grown and packaged at UCSB, λ_max=405 nm and 450 nm) with an active area of 300×300 µm². Luminescence spectra from the device were acquired with diodes under forward bias with various constant currents, with the light output collected using an integrating sphere. To assess the efficiency of luminescence, we measured the quantum efficiency (η_phosphor) using excitation source of 405 nm from an argon laser, using an experimental configuration described by Greenham et al.[19]

3. Results and discussion

The XRD evolution of GSAF:Ce³⁺ (x=0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) solid solutions all show the GSA (or SAF) phase at room temperature, are shown in Figure 1(a). The XRD patterns of the x=0 and 1.0 agree well with the expected structures of GSA and SAF, respectively. Increasing solid solution content x produced diffraction patterns that were similar to that of the initial models of GSA or SAF. The panel on the right of Figure 1 (a) shows the reflections near 2θ=37° which clearly confirm the phase evolution as a function of x. The isostructural end-members, despite their differing compositions, are clearly structurally very compatible.

Figure 1(b–c) displays the variation of the lattice parameters with x. The lattice parameters were obtained from LeBail pattern matching analysis and clearly follow the Végard law, proving complete solid solution between the two distinct end-member compounds. With the increase in x, the structures remained the space group I4/mcm with reflection peaks monotonically shifting toward larger (or smaller) angles. The lattice parameters of pure GSA:Ce³⁺ (x=0) are a=b=6.733(1) Å and c=10.952(3) Å, and those of SAF:Ce³⁺ (x=1.0) are a=b=6.7720(1) Å, and c=11.1485(2) Å. An increase of parameters could arise from the replacement of the smaller Gd³⁺ (r ₈=1.053 Å) by the larger Sr²⁺(r ₈=1.26 Å) taking place concurrently with the replacement of the larger O²- (r ₄=1.38 Å) by the smaller F^- (r ₄=1.31 Å).[20] The larger Sr atoms should dominate the increase in lattice parameters since the difference between O²- and F^- in ionic radius is relatively small.

 

Fig. 1. (a) XRD patterns of the GSAF:Ce³⁺ (x=0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0). The panel on the right details the evolution of reflections near 2θ=37°. Expected reflection positions for the end-members are displayed at the top. Evolution of (b) a and (c) c lattice parameters as a function of x as obtained from LeBail pattern-matching of XRD data.

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Table 1. Spectral parameters of Gd_1-xSr_2+xAlO_5-xF_x:Ce³⁺ (x=0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0).

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Figure 2(a–b) shows PL emission of the solid solution series when they are excited at their maximum excitation. All the emission spectra are broad due to the transition of Ce³⁺ from the 5d (² D _3/2,² D _5/2) excited state to the 4f (² F _5/2,² F _7/2) ground state.[23] With increasing solid solution amounts x, the emission positions gradually shift to a short wavelength in the range from 574 nm to 474 nm, corresponding to GSA:Ce3+ (x=0) and SAF:Ce³⁺ (x=1.0), respectively. (see Table 1) Since the crystal field splitting is proportional to 1/R ⁶[21], where R is the distance between the central ion and its ligands, the emission wavelength of the phosphor can be tuned by changing the solid solution amount x. As shown in Figure 1(b–c), the lattice parameters of a-, c-axis increase linearly, which results in longer interatomic distance in the phosphors. Consequently the emission wavelength of these move to higher energy region up to 474 nm at x=1.0. The lattice ionicity of the host is another important variable that influences the emission energies. Through the solid solution series, Gd atom is substituted by Sr atom (χ _Gd=1.2, χ _Sr=0.95, Pauling scale) and F atom substitutes O atom (χ _F=3.98, χ _O=3.44) at the same time, where χ is electronegativity.[22] As net values of |∑χ_anion-∑χ_cation| for the solid solution series x increases, there is decreased covalency, the 4 f and 5d levels of Ce³⁺ become well separated, and the 5d→4f emission occurs at shorter wavelengths. Depending on the solid solution amount x, there is a large change in emission wavelength, as much as 100 nm, which gives this solid solution series high tunability.

 

Fig. 2. (a-b) Excitation and emission spectra of Gd_1-xSr_2+xAlO_5-xF_x:Ce³⁺ (x=0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0). (c) Relative emission intensities under various excitation wavelengths across the solid solution series.

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The emission bandwidth, associated with the color rendering R_a, is important to obtain pleasant white light when incorporated in LED lamps. For GSAF:Ce³⁺ (x=1.0), the full width at half maximum (FWHM) of the PL spectra is around 88 nm. In the solid solution, the values of FWHM were larger than that for x=0, ranging from 108 nm to 122 nm due to the compositional disordering of Gd³⁺/Sr²⁺ and O²-/F- in the host compound.(see Table 1)

The dependence of PL intensities in GSAF:Ce³⁺ (x=0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) under various excitation wavelengths are shown in Figure 2(c). The solid solution series phosphor displayed optical properties under excitation in the range of 400 nm, 430 nm, and 450 nm. The initial results shown here suggest PL intensities that are about 36% (x=0, λ _ex=453 nm), 61% (x=0.3, λ _ex=445 nm), and 103% (x=1.0, λ _ex=400 nm) that of commercial YAG:Ce³⁺ (λ_ex=450 nm). The values of η_phosphor in GSAF:Ce³⁺ (x=0, 0.3, 1.0) were measured to be around 39 %, 50 %, 83% at room temperature, in contrast to that for commercial YAG:Ce³⁺, which is around 75 %. In particular, Gd_0.7Sr_2.3AlO_4.7F_0.3:Ce³⁺ (x=0.3), is assigned to an GSA-like environment, had the highest PL intensity (λ_ex=445 nm) in this solid solution series. The origin of the remarkable enhancement at x=0.3 is attributed to addition of Sr²⁺ and F^- elements into the host lattice and clearly arises from balancing environmental effects around Ce³⁺. The basis structure of GdSr₂AlO₅ (x=0) consists of frameworks of AlO₄, SrO₈, and GdO₁₀ polyhedra, where the polyhedra are being filled with Sr²⁺ and F^- atoms via solid solution. Consequently SrO₈ and GdO₁₀ polyhedra change to SrO₆F₂ and Sr/GdO₈F₂ polyhedron as x increases.

Table 2. Optical properties of commercial YAG:Ce³⁺ and Gd_1-xSr_2+xAlO_5-xF_x:Ce³⁺ (x=0, 1.0) phosphors in white LEDs with a current of 20 mA.

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CCT=Correlated color temperature

Figure 3 displays electroluminescence (EL) spectra and the Commission International del’Eclairage (CIE) chromaticity from the device fabricated with (a) GSAF:Ce³⁺ (x=0), (b) GSAF:Ce³⁺ (x=1.0), (c) GASF:Ce³⁺ (x=1.0) + commercial red phosphor respectively on InGaN LEDs (λ _max=450 nm and 405 nm) under different forward-bias currents in the range of 2mA to 40 mA. In Figure 3(b–c), we attempted to reduce the emission peak around 405 nm from LED chip since near-UV light is not a likely candidate for lamp applications. The measured luminous efficacy and R_a are in the range of 20mA listed in Table 2. White LEDs based with GSAF:Ce³⁺ phosphors (x=1.0), were fabricated the same conditions of that of YAG:Ce³⁺, displayed higher values of efficacy than that of YAG:Ce³⁺ since the GSAF:Ce³⁺ phosphors (x=1.0) have excellent values of η_phosphor in Eq. 1. As the peak position of the GSAF:Ce³⁺ phosphors (x=1.0) was located at around 474 nm, the commercial red phosphor added to the (b) phosphor (x=1.0) + LED assemble, which obtained 87 value of R_a. However, the low value of luminous efficacy was obatained due to a reabsorption between the GSAF:Ce³⁺ (x=1.0) and red phosphor.

4. Conclusion

We have prepared the series of GSAF:Ce³⁺ solid solution phosphors from the end members GdSr₂AlO₅ and Sr₃AlO₄F. Due to subtle changes of interatomic distance and ionicity of host lattice, the emission wavelength of the phosphors can be tuned in the range from 574 nm to 474 nm. Applying GSAF:Ce³⁺ (x=1.0) on InGaN LEDs (λ_max=405 nm), white LEDs displaying 30 lm/W at 20mA were obtained. We believe this highly tunable solid solution phosphor family has the potential to strongly impact phosphor-converted white solid state lighting.

 

Fig. 3. Luminescence of the InGaN LED + phosphor, under different forward bias currents (indicated): (a) InGaN (λ_max=450 nm) + GSAF:Ce³⁺ (x=0), InGaN (λ_max=405 nm) + GSAF:Ce³⁺ (x=1.0), and InGaN (λ_max=405 nm) + GSAF:Ce³⁺ (x=1.0) + commercial red phosphor. (d) CIE chromatic coordinates of the device under different forward bias currents [as in panel (a), (b), and (c)]. The Planckian locus line and the points corresponding to color temperatures of 3500K and 6500K are indicated.

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