Photo and cathodoluminescence of commercial YAG:Ce based phosphors in UV region

Abstract

The luminescence spectral kinetic characteristics in UV and visible spectral regions of a group of commercial YAG:Ce based phosphors are investigated under excitation by UV radiation in the spectral range of 300–210 nm (photoluminescence, PL) and high-energy electrons (0.25 MeV, pulse cathodoluminescence, PCL). It was established that UV luminescence appears in all phosphors, whose spectrum consists of bands in the region of 312, 320, 380 nm and 530–565 nm. The kinetics of the PCL UV luminescence decay in all phosphors shows at least one, two components in different phosphors but is differ from kinetics of luminescence in the visible region. The possible structure of luminescence centers responsible for UV luminescence is discussed.

Introduction

Yttrium aluminum garnet Y₃Al₅O₁₂ (YAG) together with Gd₃Ga₅O₁₂ and Lu₃Al₅O₁₂ belongs to the family of complex oxide materials with the garnet structure of the composition A₃B₂C₃O₁₂, which have been successfully grown as single crystals or synthesized as nano and micro-materials through different methods [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. When these materials are doped with one or two rare-earth ions (Ce³⁺, Eu³⁺, Tb³⁺ and etc) [1], [2], [3], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], which are considered to be excellent luminescence dopants, they exhibit unique and outstanding optical and luminescent properties, thereby receiving tremendous attention due to their potential application as laser crystals, fast scintillators, ionizing radiation dosimeters, light emitting devices and converters of optical radiation [1], [2], [3], [8], [9], [10], [12], [20], [21], [22], [23]. In particular, YAG doped by cerium Ce³⁺ is excellent phosphor used in white LED as an efficient light converter, providing a high efficiency of UV (450–460 nm) light conversion into a visible light (500–780 nm) [1], [2], [3]. The same spectral range is the light emission of the most efficient LED chip based on InGaN blue chip [21], [22]. The combination of light emission from such a chip with YAG phosphor luminescence allow one to obtain an efficient white light LED. It is also important to note that YAG:Ce based single crystals and composites are also promising materials for radiation detection and dosimetry [23]. When YAG phosphors were excited by the electron beam

When YAG phosphors were excited by the electron beam of nanosecond pulse duration with an electron energy of 250 keV, the luminescence for YAG phosphor doped with cerium ions is located at 2.17 eV (570 nm), while for undoped YAG phosphor at 2.12 eV (585 nm) [12]. Furthermore, the luminescence band of YAG:Ce phosphors has an asymmetric shape and could be decomposed into the two Gaussian components with maxima at 2.4 eV and 2.19 eV [12]. The excitation of YAG:Ce in the exciton absorption region [24], [25] leads to the luminescence appearance in the region of 250–450 nm with bands at 260, 301, 372, 394 nm, which are different for crystals, ceramics, and films. The UV luminescence was observed [26] at 460 nm when excited by radiation at 258 and 292 nm. Furthermore, in the cathodoluminescence spectrum of (LuYGd)₃(AlGa)₅O₁₂:Ce, a broad luminescence band was observed [27] in the region of 200–400 nm and narrow at 312 nm. It was also reported [28], that X-ray excitation induces the luminescence in Lu₃Al₅O₁₂ (LuAG) and Y₃Al₅O₁₂ (YAG) crystals at 315 and 380 nm, the band at 315 nm being dominant. Two well-resolved emission bands at 300 and 360 nm were found in [29] in the x-ray luminescence spectrum. Lastly, UV luminescence in the region of 300–500 nm was observed in ref. [30] with bands at 380, 415, 435 nm.

Thus, the available information about YAG:Ce luminescence, especially on crystal structure/defects responsible for luminescence centers in the UV region is still fragmentary, and so far has not been studied enough. Thus, the detailed study of UV luminescence is of great interest both for understanding the processes of transfer of excitation energy to luminescence centers and for the development of scintillators and specialized LEDs

This article presents the results of the study of the UV excitation spectra and luminescence in a series of commercial YAG phosphors of different firms. In the article, the UV excitation area is defined as the spectrum range from 200 to 310 nm, the UV luminescence −300–450 nm. The visible spectrum luminescence used in an LED to produce white light, in the text is called VL, UV luminescence- abbreviated as UVL.