1. Introduction

White light-emitting diodes (wLEDs) are now replacing traditional incandescent bulbs and fluorescent tubes for general illumination and backlight, which promise energy saving, high efficiency, long life time, and environmental friendly [1–3]. However, wLEDs also have limitations due to (i) efficiency droops of LED chips that make it hard to achieve high efficiency/high power by using one single chip; (ii) thermal dissipation problem that causes degradation of devices, and (iii) point-like light source that leads to uncomfortable glaring. Therefore, new lighting technologies are essential to complement existing ones.

Field emission lamps (FELs) are considered as an emerging flat-panel light source for display backlights and general lighting, which combines field emission electron sources and cathodoluminescence (CL) phosphors [4–11]. Unlike LED chips, the field emission electron sources are low-power, temperature-insensitive, and irradiation-unaffected, which makes FELs more reliable and energy-saving. Furthermore, the versatile CL phosphors also make FELs easier to control or tune the emission spectra. Nakamoto et al. reported a white FEL with the luminous efficacy of 28 lm/W driven at 6kV and 300 µA/cm² (cw), by using a phosphor mixture of ZnS:Ag, Cl (blue), SrGa₂S₄:Eu²⁺ (green) and Y₂O₃: Eu³⁺ (red) [5]. Further, he developed white color flat FELs using high efficiency SrGa₂S₄:Eu²⁺ and SrGa₂S₄:Ce³⁺ (blue) [6]. Engelsen et al. used ZnO: Zn (cyan) and Y₂O₃:Eu³⁺ (red) as luminescent layers, and reported 18-21 lm/W in white FELs driven at 5 kV and 2 µA/cm² [7]. Cao et al. reported 12,000 cd/m² in a FEL driven at 6.4 kV by using a white CL phosphor [8]. Recently, Arakawa et al. developed white FEL headlamps for environment-friendly vehicle by using a phosphor blend of ZnS: Ag, Al (blue), ZnS:Cu, Al (green) and Y₂O₂S: Eu (red), which has the luminance of 42,700 cd/m² at 7 kV and 233 μA (1.56 W) [12]. Till now, sulfide phosphors have been usually used in white FELs due to their very high efficiency [6, 13]. On the other hand, they suffer from luminance saturation and degradation under high energy electron bombardment, generating corrosive gases that contaminate emission tips and shorten the device lifetime [5, 14]. Therefore, there is a great demand to develop robust CL phosphors for highly reliable FELs.

Rare earth-doped nitride phosphors, such as AlN:Eu²⁺, Ca-α-sialon:Eu²⁺, β-sialon:Eu²⁺ and CaAlSiN₃:Eu²⁺ have attracted great attentions for use in wLEDs due to their promising photoluminescence properties such as abundant emission colors, high conversion efficiency, and small thermal quenching [2, 15–19]. Moreover, they were also reported to show interesting cathodoluminescence [20–23]. For example, we demonstrated that AlN:Eu²⁺ was a blue CL phosphor with high color purity and reliability, enabling to develop field emission displays (FEDs) exhibiting the lifetime 10 times longer than the device using Y₂SiO₅:Ce [20]. Kargin et al. investigated the cathodoluminescence of Ca-α-sialon:Eu²⁺, and evidenced it as a yellow-emitting CL phosphor [21]. It is therefore possible to prepare robust white FELs by using the phosphor blend of AlN:Eu²⁺ and Ca-α-sialon:Eu²⁺, or of AlN:Eu²⁺, β-sialon:Eu²⁺ (green) and CaAlSiN₃:Eu²⁺ (red). In this work, we attempt to investigate and discuss the optical properties of these white FELs.

2. Experimental procedure

Si-codoped AlN:Eu²⁺ (0.1 mol% Eu and 2.9 mol% Si) was prepared by using raw materials of α-Si₃N₄ (SN-E10, Ube Industry Co., Ltd., Tokyo, Japan), AlN (Type F, Tokuyama, Shunan-shi, Japan), and Eu₂O₃ (Shin-Etsu Chemical, Tokyo, Japan). The powders were mixed in a mortar by hand and packed in boron nitride crucibles. The powder mixture was fired in a gas-pressure sintering furnace with a graphite heater at 2050 °C for 4 h under 1.0 MPa N₂ atmosphere. 1 g (Sr,Ca)AlSiN₃:Eu²⁺ (Sr_0.8Ca_0.192Eu_0.008AlSiN₃) was prepared by firing the powder mixture of 0.2651g α-Si₃N₄ (SN-E10, Ube Industry Co., Ltd., Tokyo, Japan), 0.2329g AlN (Type F, Tokuyama, Shunan-shi, Japan), 0.0538g Ca₃N₂ (Cerac, 99%, 200 mesh), 0.4406g Sr₃N₂ (Cerac, 99%, 200 mesh) and 0.0075g EuN (lab-made) at 1800^oC for 2 h under 1.0 MPa N₂ atmosphere. After firing, the phosphor powders were pulverized by hand using silicon nitride mortar and pestle. The average particle size of the phosphors, determined by the scanning electron microscope (SEM) observation, is about 10 μm for AlN:Eu²⁺ and 12 μm for CaAlSiN₃:Eu²⁺. There is no impurity phase identified in the fired AlN:Eu²⁺ and (Sr,Ca)AlSiN₃:Eu²⁺ samples. Ca-α-sialon:Eu²⁺ (λ_em = 592 nm) and β-sialon:Eu²⁺ (λ_em = 531 nm) are commercially available from DENKA Co. Ltd, both having an average particle size of 15 μm. The external quantum efficiency of AlN (under 365 nm excitation), β-sialon:Eu²⁺, Ca-α-sialon:Eu²⁺ and CaAlSiN₃:Eu²⁺ (under 450 nm excitation) is 51, 55, 64 and 68%, respectively.

CL spectra and time-dependent luminescence intensity of individual phosphors were measured using an ultrahigh vacuum SEM with Gemini electron gun (Omicron, Bavaria, Germany) equipped with a CL system [24]. The probe diameter of the electron gun is in the order of 10 nm. The phosphor powders were put on a conductive carbon tape and then covered with a copper grid. The specimens were irradiated for 1 h with the electron beam of 5 kV and 1000 pA, with the aim to stabilize the luminescence intensity. The observed area of the phosphors was scanned by an electron beam for every 90 ms.

The energy efficiency (η, %) upon electron excitation is calculated by [25]

Figure 1 schematically shows the configurations for measuring CL properties of phosphor blends. As seen in Fig. 1(a), the emission spectra and stability of white FELs were also recorded by the high sensitive spectroradiometer (HS-100, Photal Otsuka Electronics, Japan). The phosphor blends of 1 g with different B:Y (0.8:0.2, 0.75:0.25, 0.7:0.3, 0.65:0.35) or B:G:R (0.36:0.12:0.44, 0.28:0.20:0.52, 0.20:0.20:0.60, 0.16:0.16:0.68, 0.12:0.16:0.72) ratios (in weight) for achieving white balance were homogeneously dispersed in ethanol, and then precipitated on an Al plate naturally. After drying, the phosphor blends were then pressed tightly on the Al plate, and finally loaded into the vacuum chamber (CITECHNO INC., Yokohama, Japan) for measurements (Fig. 1(b)). The sample size was fixed at 10 × 10 mm². The phosphors were excited by electron beams under the DC mode. The irradiation area of the electron beam and the emitted area of the phosphor were 2.14 cm² and 16.0 mm², respectively. For comparison, a reference warm wLED with a color temperature of 2,780 K was fabricated by combining Ca-α-sialon:Eu²⁺ and a blue LED (λ_em = 450 nm).

 

Fig. 1 (a) Schematic configuration for measuring field emission properties of the phosphor blends and (b) photographs of the vacuum system for CL phosphors.

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A prototype of white FEL using the B:Y ratio of 0.75:0.25 was prepared by co-precipitating the phosphor blend on a silica glass, then coating the phosphor by the Al film with the thickness of 200 nm, and finally sealed with glass, as schematically shown in Fig. 2. The luminous efficacy of the white FEL was measured by MCPD-7000 (Otsuka Electronics, Tokyo, Japan). A high power voltage supply (Mastsusada Precision Inc., Tokyo, Japan) was used to generate high voltages (e.g., 7 kV) in FELs.

 

Fig. 2 Schematics of the white FEL structure in this work.

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3. Results and discussion

The CL spectra of individual nitride phosphors and their mixtures are presented in Fig. 3, measured under 3 kV and 100 μA at room temperature. As seen, all phosphors show a broad emission band, owing to electron transitions of 4f⁶5d → 4f⁷ of Eu²⁺. The CL emission maximum and the full width at half maximum (FWHM) are 471 and 63 nm for AlN:Eu²⁺, 531 and 55 nm for β-sialon:Eu²⁺, 592 and 99 nm for Ca-α-sialon:Eu²⁺, and 625 nm and 81 nm for CaAlSiN₃:Eu²⁺, respectively. The CL spectra of the nitride phosphor investigated are in line with the photoluminescence one reported in the literature [2, 15-18, 22]. Moreover, these nitride phosphors show a little decrease in luminescence intensity under electron excitation (Fig. 3(a)). Under the electron irradiation for 1 h, the CL intensity declines only by 10, 13, 15 and 18% for AlN:Eu²⁺, Ca-α-sialon:Eu²⁺, β-sialon:Eu²⁺, and CaAlSiN₃:Eu²⁺, respectively. The luminescence decline under electron beam irradiation is ascribed to (i) the surface charging of phosphors which reduces the arrival energy of the exciting electron, because the nitride phosphors investigated in this work are insulating materials, and (ii) the accumulation of carbon at the surface during the electron bombardment, which prevents low energy electrons from reaching the phosphor grains [26, 27]. The luminescence quenching of nitride phosphors is comparable to that of some oxide phosphors such as Y₂SiO₅:Ce³⁺ and ZnO:Zn [27], but it is much less than that of sulfide phosphors [28, 29]. The CL quenching of phosphors can be reduced by surface engineering, such as surface coating of conductive or protective layers [27].

 

Fig. 3 Emission spectra of (a) individual AlN:Eu²⁺ (B), β-sialon:Eu²⁺ (G), Ca-α-sialon:Eu²⁺ (Y) and CaAlSiN₃:Eu²⁺ (R) phosphor, and (b) white FELs with different ratios of B: Y, and (c) white FELs with different ratios of B: G: R, measured at 3 kV and 100 μA. The black spectrum of white LED by using a blue chip and Ca-α-sialon:Eu²⁺ is also included for comparison. The inset in Fig. 3(b) shows the photograph of white light from Sample B.

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At 2.0 kV, the energy efficiency was measured as 2.32, 2.30, 3.00 and 1.93% for AlN:Eu²⁺, β-sialon:Eu²⁺, Ca-α-sialon:Eu²⁺ and CaAlSiN₃:Eu²⁺, respectively. The efficiency of nitride phosphors is much lower than that of traditional sulfides (ZnS:Ag, ~25%), but is at the same level of oxide CL phosphors (for example, Y₂SiO₃:Ce³⁺, 7.7%) [30, 31]. Broxtermann et al. reported that the energy efficiency of Pr³⁺ doped Y₂SiO₅, YPO₄ and YBO₃ was 2.7, 9.2 and 5.1%, respectively.[25] Usually, the CL phosphors have lower energy conversion efficiency compared to LED phosphors, as most part (> 80%) of the incident electron energy losses via non-radiative pathways such as heat losses and lattice vibrations. The low energy efficiency of nitride phosphors is attributable to the large electronegativity of N^3- anion (3.04) that leads to strong binding of a charge to anion and a high effective mass of a free hole [32].

As shown in Table 1 and Figs. 3(b)-3(c), the difference in phosphor blend ratio (B:Y and B:G:R) leads to variations in color temperature of white light. A warm white with the color temperature of 3,430 K is achieved in Sample D with the blue to yellow ratio of 0.65: 0.35. The color rendering index (Ra) varies in the range of 73 – 77, which is much higher than that of the reference wLED (Ra = 57). The larger Ra of white FELs is ascribed to the fact that AlN:Eu has a wider FWHM than InGaN, both of which are blue component in the emission spectra of lighting devices. Much higher color rendering index of 93-96 can be achieved by replacing Ca-α-sialon:Eu²⁺ with β-sialon:Eu²⁺ and CaAlSiN₃:Eu²⁺, but sacrificing the luminous efficacy of white FELs. This trade-off between color rendering and luminous efficacy is also usually observed in white LEDs.

Table 1. Optical properties of samples with different mixing ratios of phosphors (B:Y) and (B:G:R).

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Sample D was selected to investigate the luminance saturation. As shown in Fig. 4(a), the luminance of white light increases monotonically with increasing the accelerating voltage, indicating that no luminance saturation occurs in white FELs using AlN:Eu²⁺ and Ca-α-sialon:Eu²⁺ (The maximum excitation density here is 70,00 W/m²) under the current conditions. Cathodoluminescence saturation is usually observed for many kinds of CL phosphors, caused by several mechanisms such as luminescence thermal quenching due to specimen heating, ground-state depletion, build-up of space charge and interaction between activators [33, 34]. Compared to sulfides, nitride phosphors doped with Eu²⁺ showing the simple 4f⁶5d¹-4f⁷5d⁰ electronic transitions have better thermal stability (smaller thermal quenching), which therefore leads to quite small saturation. In a previous report, the authors showed that the saturation of AlN:Eu²⁺ was much less than that of Y₂SiO₅:Ce³⁺[20]. A linear relation between the luminance and voltage is deviated in Sample D’ using three phosphors. This deviation is due possible to the re-absorption of blue and green light from AlN:Eu²⁺ and β-sialon:Eu²⁺ by the CaAlSiN₃:Eu²⁺ red phosphor because the latter has strong absorptions in the blue-to-green spectral region, which reduces the luminescence intensity of both the blue and green phosphors. This is evidenced by a slight increase in the x-chromatic coordinate (red shift of the color point).

 

Fig. 4 Brightness of (a) Sample D with the B:Y ratio of 0.65:0.35 and (b) Sample D’ with the B:G:R ratio of 0.16: 0.16: 0.68 as a function of the accelerating voltage. The anode current was fixed at 100 μA.

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The luminance is 3,412 cd/m² at 7 kV and 100 μA. The low brightness of white FELs in the current work is owing to the lower efficiency of both AlN:Eu²⁺ and Ca-α-sialon:Eu²⁺ phosphors under electron bombardment in comparison to that of commercial sulfide phosphors. In addition, there is no variation in chromatic coordinates as the accelerating voltage increases (see the inset of Fig. 4(a)), indicating that robust nitride phosphors would yield highly reliable and long lifetime white FELs.

A white FEL device was prepared by using the composition of Sample B, as shown in Fig. 5. The flat white light from the FEL is uniform and without glaring effects. The luminance degradation of the white FEL can be neglected when it is switched on for 3,000 hours. Figure 6 illustrates emission spectra and irradiance of white FELs, measured at varying anode currents and voltages. As seen, the irradiance intensity of white FELs increases linearly as the applied voltage and current increase, again validating the absence of luminance saturation in FELs using nitride phosphors. Driven at 7 kV and 167 μA, the white FEL has the luminous efficacy of ~5 lm/W, color temperature of 4,883 K and the color rendering index of Ra = 76. Compared to those white FELs using commercial oxide (Y₂O₃, Y₂O₂S, ZnO) and sulfide (ZnS, and SrGa₂S₄) phosphors, the white FEL using nitride phosphors shows a rather lower luminous efficacy, which is attributable to the low CL efficiency of laboratory-made nitride phosphors. Anyhow, it is the first time to apply robust nitride phosphors in FELs, which makes an extension of the applications of these materials, although it is necessary to minimize the surface-charging by coating conductive layers to further enhance CL intensity.

 

Fig. 5 Prototypes of white FELs (ϕ60 x 95 mm) with (a) front view, (b) rear view, (c) power off, and (d) power on.

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Fig. 6 Emission spectra (a) and luminance (b) of the white FEL (Sample B) measured at varying anode currents.

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4. Conclusions

In summary, we have developed white flat field emission lamps by using CL nitride phosphors at the first time. The prepared white FELs show almost no luminance saturation under the current measurement conditions. Although using nitride phosphors as emissive materials in white FELs leads to somewhat lower luminous efficacy than using commercial CL phosphors, it opens up a new application field for nitride phosphors that are usually used for white light-emitting diodes. In addition, white FELs with high reliability and long lifetime can be anticipated by using robust nitride phosphors.