Yttrium aluminium garnet (Y₃Al₅O₁₂, YAG) is important host material for phosphors, scintillators, and solid state lasers^[1⇓-3]. With the valence 4f electrons shield by 5s and 5p electron orbits, the rare earth ions in the solid state often emit in narrow and strong bands^[4]. The rare earths of most interest for selective emitters are ytterbium (Yb), thulium (Tm), erbium (Er), holmium (Ho) and dysprosium (Dy). Rare earth doped YAG crystal have been used in photonic applications ranging from near infrared (IR) emitters to cathode ray phosphors and scintillators since Y³⁺ being vicariate with trivalent lanthanide ions^[5]. Yb³⁺ ions can absorb the light between 900 and 1000 nm with energy transfer (ET) from ²F_7/2 to ²F_5/2 level. Then, the energy can be transferred to the ⁴I_11/2 level of Er³⁺, leading to laser emission at 2.94 μm wavelength. Therefore, Er and Yb doped glass, ceramics, fiber and single crystal materials have been synthesized for studying up-conversion luminescence and active lasing performances^{[6⇓⇓⇓-10]}. Er³⁺,Yb³⁺: YAG crystals of a diameter of about 25 mm and a length of 60 mm were obtained using Cz method^[7]. It was reported that 8 inch (20.32 cm) Yb:YAG single crystal was grown by Cz method^[8]. The Er³⁺/Yb³⁺ co-doped YAG fiber (about 3 mm in length and 0.8 mm in cross section diameter) have been grown by laser heated pedestal growth^[9]. Recently, Er,Yb co-doped YAG single crystals (70 mm×150 mm×15 mm) was grown by a modified horizontal directional crystallization method in Ar+(CO, H₂) atmosphere using Mo crucible^[11]. The growth of larger size of Er,Yb co-doped YAG single-crystals was still in demand.

However, it is a challenge to grow bulk YAG single crystals with low defects. We have used the chemical bonding theory of single crystal growth quantitatively to describe the anisotropic bonding behaviors of constituent atoms during YAG crystallizing, and ϕ3 inch(1 inch=2.54 mm) YAG crystal were obtained^[12-13]. In this work, we grew Er,Yb:YAG single crystal with a diameter of 80 mm and length of 230 mm with fast growth method. The structure, defect, optical and luminescence performances were studied.

The Er,Yb:YAG polycrystals were synthesized with the powders of Er₂O₃ (99.999%), Yb₂O₃ (99.999%), Y₂O₃ (99.999%) and Al₂O₃ (99.999%) as raw materials. First, all ingredients were dried to remove residual moisture from the powder to ensure accurate weighing. The doping concentrations of Yb³⁺ and Er³⁺ were 0.05% and 0.15% (in mass), respectively. The four powders were mixed and blended in a mixer for 24 h to ensure that the ingredients were well mixed. The mixed powders were then packed into a mould and compacted. Finally, the pressed cylinders were transferred to a muffle furnace and sintered at 1100 ℃ for 12 h to form polycrystalline Er,Yb:YAG.

The growth of Er,Yb:YAG single crystal was carried out by the Cz method in a nitrogen atmosphere. The sintered polycrystalline Er,Yb:YAG was put into an iridium crucible equipped with an intermediate frequency induction heating system Cz furnace, heated until the polycrystalline material is completely melted and held for a period. The YAG in the <111> direction was used as the seed crystal and seeding was performed manually. A series of subsequent crystal growth processes were controlled by an automatic control system with the following specific growth parameters: a pulling rate of 2-4 mm·h^-1 and rotation rate of 5-10 r/min^[12].

The crystal phase of single crystal Er,Yb:YAG were examined with X-ray diffraction (XRD, Rigaku, Smartlab 3kW). The elemental analysis of the Er,Yb:YAG crystal was characterized by Energy-dispersive X-ray spectroscopy (EDS, Oxford, Ultim Max 100) and glow discharge mass spectrometry (GDMS, AstruM, Nu Instrument). Raman spectra were excited using Raman spectrometer (Horiba, LabRAM HR Evolution) with a 325 nm laser. Etched Er,Yb:YAG wafers were performed by optical microscope (Nikon, LV150). The refractive index of Er,Yb:YAG wafer was measured by Prism Coupling device (Metricon, Model 2010). The optical absorption and PL spectra were determined by UV-Vis spectrophotometer (Metash, UV-9000) and fluorescence spectrometer (Edinburgh, FLS 1000), respectively.

Fig. 1(a) shows the photograph of a large-size Er,Yb: YAG single crystal grown by the Cz method and annealed in air. Fig. 1(b) shows the picture of a wafer with a thickness of 1.5 mm cut perpendicular to the orientation of <111> and polished on both sides. The length and diameter of the crystal are 230 and 80 mm, respectively. The shape is characteristic of typical <111> orientation, and the crystal is colourless transparent and has no defects such as cracks and inclusions^[14-15].

Grind the annealed crystal into powder for XRD analysis, and the results are shown in Fig. 1(c). A comparison of the XRD results with the standard PDF cards (PDF# 00-033-0040) in the database shows that all peaks are well indexed to the cubic structure Y₃Al₅O₁₂ (space group: $Ia\bar{3}d$), and no impurity phases are found, which means that the dopant ions are well substituted for the Y³⁺ lattice positions^[16-17]. XRD tests on the YAG-doped wafer in Fig. 1(b) show only a sharp diffraction peak of the (444) crystal plane at 2θ=52.76^o and no diffraction peaks of the other crystal planes, confirming its good quality.

The chemical compositions of Er,Yb:YAG crystal were determined by EDS and GDMS. It can be seen in Fig. 1(d) that Y, Al and O peaks are well detected. Because the element contents of Yb and Er are relatively low, and the characteristic energies of these elements are located at 1.521 and 1.405 keV, which are close to the peaks of Al (1.486 keV) and overlap, they cannot be distinguished well. GDMS is one of the effective means of analysing trace elements in conductors and semiconductors^[18-19]. The actual doping concentrations of Er³⁺ and Yb³⁺ ions in YAG crystals were analysed using GDMS. The results showed that the actual doping content of Er³⁺ and Yb³⁺ was 0.13% and 0.049% (in mass), demonstrating the successful doping of Er³⁺ and Yb³⁺.

In order to study the defects present in the grown Er,Yb:YAG crystal, the polished wafers were chemically etched. The etching was carried out using concentrated phosphoric acid (H₃PO₄) at 180 ℃ for 60 min. A comparison of the wafer surface before and after corrosion is shown in Fig. 2. The corrosion pits are evenly distributed over the entire corrosion surface, and no dislocation corrosion pit features are observed, which means that the crystals are near-perfect^[20-21].

For cubic Y₃Al₅O₁₂ (YAG) with space group $O_{h}^{10}-Ia\bar{3}d$, the irreducible representations as follows: Γ = 3A_1g + 5A_2g + 5A_1u + 5A_2u + 8E_g + 10E_u + 14T_1g + 14T_2g +18T_1u + 16T_2u. 25 modes are Raman active (3A_1g, 8E_g and 14T_2g) while 18 modes are IR active (18T_1u)^[22]. The Raman spectra of rare earth doped YAG have been recorded in the 100-1200 cm⁻¹ spectral range at room temperature (Fig. 3 and Fig. 4(a)), which shows strong Raman bands at 259, 371, 402, 784 cm⁻¹, medium bands at 161, 218, 548, 558, 692, 718, 859 cm⁻¹, and weak bands at 294, 338, 754 cm⁻¹. The low-frequency region (100-500 cm⁻¹) can be attributed to the lattice vibrational mode and the translational motion of the RE³⁺(Y,Er,Yb), and also the mixing of translational, rotational and ν₃ molecular mode of the [AlO₄] unit. The intermediate- frequency region (500-600 cm⁻¹) accounts to the ν₂ molecular mode splitting of the [AlO₄] unit while the high-frequency region (600-900 cm⁻¹) can be assigned to the ν₁ and ν₄ internal modes of the tetrahedral [AlO₄] unit^[23-24]. According to the results of factor group analysis, the three A_1g modes are related to the [AlO₄] internal vibrations^[25]. There is a weak Raman peak at 1100 cm⁻¹, which may be the Yb-O vibration^[26-27]. To investigate the doping uniformity of Er,Yb:YAG crystal grown by the Cz method, two sets of points were taken outside the center of the wafer (the inset of Fig. 4(b)). Raman spectra of these points showed that the peak intensity of the C4 point is the highest at the 783 cm⁻¹ band. Raman spectra of other points are similar to those of C1, C2 and O3 (Fig. 4(a)). Lorentz fitting was performed on all selected points at the 783 cm⁻¹ band (Fig. 4(b)).

The peak position obtained by B5 point fitting is 784 cm⁻¹, which means that B5 point is subjected to higher compressive stress than other points. The peak position varies from 781.5 to 784.1 cm⁻¹, and FWHM varies from 20 to 22.2 cm⁻¹. The peak position and FWHM of each point were not greatly changed. Therefore, it can be considered that the doping uniformity of the wafer is relatively acceptable.

The room-temperature UV-Vis absorption spectra in 200-1050 nm of Er,Yb:YAG in Fig. 1(b) are shown in Fig. 5. The absorption bands centered at 914 and 941 nm are attributed to the transitions from the ground state to the excited state (²F_7/2→²F_5/2) of Yb³⁺ ion^{[11,28 -29]}. It can be seen from the illustration in Fig. 5 that Er,Yb:YAG crystal has strong UV absorption band when the wavelength less than 230 nm, which is caused by charge transfer (CT) luminescence from O²⁻ to Yb^3+[30-31]. Except for the optical absorption of Yb³⁺ and CT luminescence, all remaining peaks in the absorption spectra are attributed to the transitions of Er³⁺ ion^[11,32]. The assignment of the absorption lines of the Er³⁺ ion is presented in Table 1. The refraction index of Er,Yb:YAG is 1.83.

Fig. 6 illustrates the emission spectra of crystals excited at different wavelengths at room temperature, 382 nm for Fig. 6(a) and 260 nm for Fig. 6(b). It can be seen from Fig. 6(a) that the green emission at 554 nm has the strongest emission peak, which corresponds to the ⁴S_3/2→⁴I_15/2 energy levels transition in Er³⁺ ion^[37]. The violet emission at 405 nm shown in Fig. 6(b) is attributed to the ²H_9/2→⁴I_15/2 in Er³⁺ ion^[38]. The decay curves of the fluorescence lifetimes of the emission peaks at 554 and 405 nm are presented in Fig. 6(c, d). The fitting results show that the fluorescence lifetimes are 17.92 and 52.09 μs, respectively.

In this work, cubic-phase Er,Yb:Y₃Al₅O₁₂ crystal with a diameter of 80 mm and length of 230 mm was obtained by the fast Cz growth method. Raman results of 17 points in cutting crystal wafer showed uniform peak number and position, which can be considered that the structure uniformity of the wafer is relatively acceptable. The refraction index of Er,Yb:YAG is 1.83. The absorption bands are attributed to the transitions from the ground state to the excited state of Yb³⁺ and Er³⁺ ions. Expect for the optical absorption of Yb³⁺ and CT luminescence, all remaining narrow peaks in the absorption spectra are attributed to the transitions of Er³⁺ ion. It is concluded that the fast growth method was effective for dual rare earth ions doping in YAG crystal.