The growth and luminescence properties of Y3Al5O12:Ce3+ single crystal by doping Gd3+ for W-LEDs

doi:10.1016/j.matlet.2016.02.001

Materials Letters

Volume 170, 1 May 2016, Pages 58-61

https://doi.org/10.1016/j.matlet.2016.02.001 Get rights and content

Highlights

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The highly transparent Ce and Gd co-doped YAG single crystal was grown by Czochralski method.
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The luminescence properties of Ce and Gd co-doped YAG single crystal are investigated in detail.
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Such Ce,Gd:YAG single crystal-based W-LED exhibits excellent optical performance.
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A very low light loss of Ce,Gd:YAG single crystal under a high temperature (in the temperature 378 K and 438 K).

Abstract

Transparent Ce³⁺ and Gd³⁺ co-doped Y₃Al₅O₁₂ (Ce,Gd:YAG) single crystal (SC) was successfully grown by the Czochralski method. Meanwhile, the structures and optical properties of single crystal wafers (SCWs) were investigated. When Y³⁺ ions were partially substituted by Gd³⁺ ions, the emission peak shifted towards the long wavelength direction. Compared with commercial phosphors, SCWs not only showed excellent thermal stability, but also exhibited superior optical performance with a luminous efficacy of 144.11 lm/W, a correlated color temperature of 5639 K and a color rendering index of 69.9.

Introduction

White-light emitting diodes (W-LEDs), which are considered as an ideal candidate for next-generation lighting sources, are rapidly developed for applications in general illumination, backlight, traffic light and landscape lighting owing to their excellent merits such as high brightness, energy saving, eco-friendliness and long lifetime [1], [2], [3], [4]. Currently, commercial W-LEDs are commonly generated by the combination of a blue InGaN chip and the yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (Ce:YAG) phosphors [5], [6], [7], [8]. However, the W-LEDs obtained by this approach would suffer from a poor color rendering index(CRI) [9] and a high correlated color temperature (CCT) [10] due to the shortage of red-emitting component in the visible spectral region [11]. Furthermore, such a design for high-power W-LEDs may age easily and turn yellow due to high local heat emitted from the chip, which will inevitably induce luminance loss and chromaticity shift [12], [13]. Therefore, inorganic materials such as transparent ceramics [14] and glass ceramics [15], have been suggested recently as practical alternatives to the organic polymer binders as they had several advantages such as high chemical/physical stability and excellent weather resistance. Unfortunately, low reproducibility is an unavoidable challenge to the mass productive of the transparent ceramics, and so far glass ceramics still suffer from the low luminous efficacy (LE).

Single crystal is an important phosphor material with excellent thermal and chemical stabilities. Generally, substituting Y³⁺ with larger ions (e.g., Tb³⁺, La³⁺, Gd³⁺) can increase the crystal field splitting and modify Ce³⁺ emission to the long wavelength regions, therefore the shortage of red-emitting component can be easily avoided. As far as we know, Latynina et al. once reported the luminescence properties of Ce,Gd:YAG SC, but the energy transfer process and the performance index of the W-LED devices were not investigated in detail [16].

Herein, by appropriately designing molar ratio of Ce/Gd, Ce³⁺/Gd³⁺-doped YAG SC was successfully grown by the Czochralski method. Optical, thermal and electrical properties of the as-grown SC and a prototype of W-LEDs based on SCWs were discussed with commercial LED references, the corresponding results demonstrate that Ce,Gd:YAG SC is expected to be an innovative candidate for W-LEDs.

Section snippets

Experimental

Ce and Gd co-doped YAG SC is grown by the Czochralski method. High purity oxide powders, Y₂O₃ (99.999%), Al₂O₃ (99.999%), CeO₂ (99.999%) and Gd₂O₃ (99.99%), are used as starting materials. The molar ratio of Ce:Gd is 0.02:0.02. The whole growth process is in the nitrogen atmosphere. [111] oriented YAG SC is used as a seed, due to its fast growth rate according to the chemical bonding theory of single crystal growth [17], [18]. The pulling rate of 1.2 mm/h and the rotation rate of 15 rpm are

Results and discussion

The luminescence SCWs and W-LEDs fabricated by the combination of InGaN chips and SCWs were provide in Fig. 1(a). Apparently, the as-grown Ce,Gd:YAG SC with ~27 mm in diameter and ~70 mm in length exhibits excellent transparency and bright yellow color. The powders XRD patterns of Ce:YAG, Gd:YAG and Ce,Gd:YAG SC are compared and shown in Fig. 1(b). All the diffraction peaks are in agreement with JCPDS card no. 033-0040 and no extra peak is detected, indicating that the doped Ce and Gd ions do not

Conclusion

In summary, Ce,Gd:YAG SC, which shows many advantages, such as high transparency and thermal conductivity, has been successfully grown by the Czochralski method. More importantly, the results of temperature-dependent emission spectra indicate that the as-grown SC exhibits excellent thermal stability. The optimal SCW-based W-LED yields a LE of 144.11 lm/W, a CCT of 5639 K and a CRI of 69.9, under an operating current of 20 mA. It is demonstrated that the combination of the SCW and the free-standing

Acknowledgements

This work was financially supported from National Natural Science Foundation of China (Nos. 51172165 and 51372172).

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Materials Letters

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgements

References (23)

Mater. Lett.

Ceram. Int.

Light: Sci. Appl.

Radiat. Meas.

J. Eur. Ceram. Soc.

J. Alloy. Compd.

Appl. Phys. Lett.

J. Mater. Chem.

ACS Appl. Mater. Interfaces

Opt. Lett.

Mater. Sci. Eng. R

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Spectroscopic investigation of K<inf>5</inf>La(MoO<inf>4</inf>)<inf>4</inf>:Sm<sup>3+</sup> red phosphor with excellent thermal stability and color purity for white LEDs

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