In Situ Study of the Temperature and Fluence Dependence of Yb²⁺ Luminescence in Yttrium Aluminum Garnet (YAG) Single Crystals

Abstract

In this study, ion-beam-induced luminescence with 2 MeV H⁺ was used to excite YAG single crystals at different temperatures. Under several constant temperatures, the luminescence intensity of Yb²⁺ monotonically decreases with increasing fluence, eventually reaching approximately 35% of the initial intensity at a fluence of 3.5 × 10¹⁴ cm⁻². The nonmonotonic evolution behavior of Yb²⁺ luminescence intensity with temperature can be effectively described by the intermediate-state model under consecutive temperature variations. The presence of an intermediate state may be the primary cause of the negative thermal quenching of Yb²⁺ luminescence. Yb²⁺ luminescence intensity decreased to 60% of the initial intensity when the temperature was continuously varied in the 100–300 K range, although the peak position remained rather stable. The luminescence of Yb²⁺ exhibits good radiation resistance and thermal stability in the experimental temperature range.

1. Introduction

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) can meet various requirements in many fields; has wide application prospects in solid-state lasers, radiation detection, medical therapy, and other fields; and is frequently used as a scintillator matrix material due to its excellent optical and mechanical properties and its amenability to doping [1,2,3,4]. Rare earth elements are an important series of activator ions in inorganic compounds with several luminescence applications. The presence of rare earth elements (e.g., Nd, Yb, and Eu) in YAG can modify the intrinsic luminescence properties of the matrix, resulting in a spectrum with strong characteristic emission of rare earth ions [5,6,7]. Due to the similar properties among rare earth elements, Y and Yb are difficult to completely separate. The effect of temperature, concentration, and the charge transfer mechanism between dopant ions on the luminescence of rare earth elements in YAG have been extensively investigated, but previous studies have mainly focused on trivalent rare earth ions, with a few studies on divalent ions, especially Yb²⁺ [8,9,10,11,12]. The presence of Yb²⁺ influences the luminescence of Yb³⁺, significantly diminishing its luminescence efficiency. Solomonov et al. observed the luminescence of Yb²⁺ with the 4f¹³6s ground state in the pulsed cathodoluminescence (PCL) spectrum of Yb:YAG ceramic samples for the first time; the spectrum included eight apparent luminescence peaks in the 590–710 nm region [13,14]. Chen et al. also observed Yb²⁺-related luminescence at 589 nm in the photoluminescence (PL) spectrum of similar Yb:YAG ceramic samples [15]. Yb²⁺ luminescence in other hosts shows evident temperature-dependent behavior as well as thermal quenching at lower temperatures [16,17,18]. However, these studies, to the best of our knowledge, did not investigate the impact of ionizing radiation and temperature on Yb²⁺ luminescence in YAG crystals.

The luminescence induced by the interaction between incident ions and materials is called ion-beam-induced luminescence (IBIL) or ionoluminescence (IL). The energy deposition of ions in materials is generally considered to be an electron energy loss (inelastic collision) and nuclear energy loss (elastic collision), with approximately 40% of the incident ion energy used to excite electrons, resulting in numerous electron–hole pairs being generated in the material [19]. IBIL not only provides a real-time analysis of in situ information when compared to conventional luminescence methods (photoluminescence (PL), cathodoluminescence (CL), and radioluminescence (RL)) but also has a more concentrated excitation area and higher detection sensitivity (10¹⁰ defects/cm²) [20,21,22]. By altering the species or energy of incident ions, IBIL can easily change the penetration depth of the incident ions, enabling it to excite different depths of the sample. Gawlik et al. used IBIL technology to investigate rare earth element-doped YAG crystals and noted that it was appropriate for the detection of rare earth elements as well as the monitoring of damage evolution during ion irradiation [5]. The evolution behavior of Yb²⁺ luminescence with fluence in the IBIL spectra was thoroughly discussed in our previous study, which clearly reflected the superior features of the IBIL technique in characterizing trace rare earth elements [23].

In this work, the IBIL spectra of YAG single crystals were measured at several constant temperatures and consecutive temperature variations. The fluence and temperature dependence of Yb²⁺ luminescence in the IBIL spectra in the temperature range of 100 to 300 K were thoroughly discussed.

2. Experiment

The sample used in this experiment was a nominal undoped YAG single crystal produced by Hefei MTI Corporation, which was grown in an inert atmosphere with a crystal orientation of <100>, a nominal purity of 99.99%, and a crystal size of 10 × 10 × 0.5 mm³ and was polished on both sides. Qualitative analysis of the YAG crystals used in our experiments was conducted using XRF (X-ray fluorescence spectroscopy), using a sensitivity of approximately 100 ppm. The main impurities in the YAG crystal were Yb and Fe.

The IBIL spectra of the 2 MeV H⁺-excited YAG single crystals were recorded on the GIC4117 2 × 1.7 MV tandem accelerator at Beijing Normal University in the 100–300 K temperature range. A 2 MeV H⁺ could excite approximately 1.2 × 10⁵ electrons to the conduction band because the bandgap of YAG is approximately 7 eV. The beam currents of H⁺ in the IBIL experiments were less than 16 nA, and the corresponding ion fluence was in the range of 10¹⁰ to 10¹⁵ cm⁻². The ionization energy of the incident H⁺ ranged from 46 eV/(electron∙nm) at the surface to a maximum of 156 eV/(electron∙nm) at the depth of the Bragg peak. The temperature was controlled using a temperature-controlled specimen stage (Instec, Boulder, CO, USA), which could control the temperature within the temperature range of 77 to 893 K and had an accuracy of ±1 K [24]. The heating and cooling rates in all the temperature variation experiments were set at 6 K/min to ensure accurate temperature measurements.

Table 1. Temperature conditions for IBIL experiments.

Number	Temperature Conditions/K
1	100
2	150
3	200
4	100→300
5	100→300→100

lists the detailed experimental temperature conditions. An Ocean Optics QE-PRO spectrometer was used to record the IBIL spectra in the 200–1000 nm region. The integration time for each IBIL spectrum was 0.5 s. To minimize the impact of beam fluctuation on the accuracy of the experimental findings, Rutherford backscattering spectroscopy (RBS) counts were recorded using a Au-Si surface barrier detector while collecting the IBIL spectrum and were used in the spectrum calibration process.

3. Results and Discussion

3.1. IBIL Experiment: 100–300 K

The IBIL spectra of YAG single crystals measured at different temperatures (100–300 K; the temperature varies simultaneously while being irradiated with ions) are shown in Figure 1 and mainly include several luminescence components: (1) a broad emission band associated with Y_Al antisite defects (ADs) and F centers in the 300–500 nm range; (2) a series of sharp luminescence peaks in the 590–720 nm region corresponding to the transition of Yb²⁺:4f¹³5d(²F_7/2e_g) → 4f¹³6s in the form of Re-F center; and (3) a broad emission band at approximately 800 nm related to Fe³⁺ impurity. The origins of these luminescence components in the YAG single crystals have been discussed in our previous articles and will not be repeated here [23]. The IBIL spectra in Figure 1a–d undergo evident alteration. The luminescence of Y_Al ADs and Fe³⁺ quickly decays as the temperature increases, and the ion fluence accumulates and then becomes very faint as the temperature reaches 300 K. In contrast, Yb²⁺ luminescence slowly decays and is the most intense in the spectrum at 300 K.

Solomonov et al. believe that a series of sharp luminescence peaks in the 590–720 nm region is due to the transition of Yb²⁺, and our experimental results are in agreement with theirs. Solomonov et al. reported that Yb²⁺ ions in Yb:YAG are the source of certain unique luminescence and absorption features that cannot be explained by Yb³⁺ ions. The typical electron spin resonance (ESR) spectrum of Yb:YAG shows seven structured series, attributed to the optical transitions between Zeeman components of Yb²⁺ ions in a magnetic field. The ground state electronic configuration of Yb²⁺ ions in YAG is 4f¹³6s, which leads to a unique energy level structure and results in narrow absorption and luminescence bands around 1 μm. These findings support the conclusion that Yb²⁺ ions are the primary source of the observed luminescence peaks and absorption features in Yb:YAG. But within the range of 590–720 nm, the emission peaks of Eu³⁺ and Yb²⁺ show a certain degree of similarity in structure and position. However, in the XRF experiments with a sensitivity of approximately 100 ppm, the characteristic X-rays of Eu were not detected.

Gaussian decomposition was performed on the Yb²⁺ luminescence spectrum to overcome the serious spectral overlap and obtain more information regarding the luminescence of Yb²⁺. The Gaussian decomposition result of Yb²⁺ luminescence at 100 K is shown in Figure 2. The seven luminescence peaks after decomposition are designated in order of increasing energy as Peak 1–Peak 7. Due to the overlap between the emission peak at 1.9 eV (649.2 nm) and the absorption peak at 640 nm, the intensity is relatively low, resulting in less-smooth fitting results compared to the evolution of the other peaks. The decomposition results at various temperatures are similar to this and are not included here.

Table 2. Peak position of Yb²⁺ luminescence after Gaussian decomposition.

Peak Number	Peak Position (eV)	R2
1	1.745
2	1.762
3	1.775	0.979
4	1.803
5	1.965
6	2.033
7	2.092

lists the peak positions of the seven luminescence peaks of Yb²⁺, which are almost consistent with the results in the literature [13]. Minor deviations from the results in the literature may be due to differences in the measured temperatures and fitting errors.

Since luminescence intensity is a critical optical parameter, the evolution of Yb²⁺ luminescence intensity with temperature (fluence) was thoroughly investigated under consecutive temperature variations. The luminescence intensity of Yb²⁺ in Figure 3 shows a trend of initially decreasing and then increasing with increasing temperature (increasing fluence), and a minimum is reached near 150 K. The experimental findings presented in Figure 4 show that increasing the ion fluence always has a negative impact on the Yb²⁺ luminescence intensity. Despite the different background temperatures, the luminescence intensities of Yb²⁺ at the three specific temperatures finally become stable at approximately 35% of the initial intensity (at a fluence of 3.5 × 10¹⁴ cm⁻²), with no apparent intensity degradation with the fluence changes. As a result, it is plausible to assume that temperature, rather than ion fluence, is the key factor responsible for the complicated evolution behavior of Yb²⁺ luminescence in the consecutive temperature variation IBIL experiment.

The integrated intensity of Yb²⁺ luminescence was calculated in the range of 590 to 720 nm to further explore the temperature dependence. The variation in the luminescence intensity with temperature is shown in Figure 5. Previous research on the negative thermal quenching of luminescence has revealed that the intermediate-state model is frequently used to describe this phenomenon, and its general form is as follows [25,26,27,28]:

$$I\left(T\right)=I_0\times {\displaystyle \frac{1+a\times \mathrm{e}\mathrm{x}\mathrm{p}\left(-\frac{E_1}{k_{B}T}\right)}{1+b \times \exp \left(-\frac{E_2}{k_{B}T}\right)+c\times \mathrm{e}\mathrm{x}\mathrm{p}\left(-\frac{E_3}{k_{B}T}\right)}},$$

(1)

where I(T) is the luminescence intensity at temperature T, I₀ is the initial luminescence intensity, E₁ represents the activation energy of the intermediate state, E₂ and E₃ represent the activation energy of the two nonradiative recombination processes, k_B is the Boltzmann constant, and a, b, and c are the fitting parameters. The numerator and denominator terms of Equation (1) represent the contributions of the negative thermal quenching and thermal quenching of the luminescence intensity, respectively.

Equation (1) effectively describes the evolution behavior of Yb²⁺ luminescence intensity with temperature (fluence) in the consecutive temperature variation IBIL experiment (100–300 K), and there is a satisfactory correlation between the fitting curve and the experimental data. The trap–escape mechanism of carriers in the intermediate state can explain the negative thermal quenching of luminescence intensity at temperatures ranging from 150 to 300 K. Intermediate states can be formed near the conduction band during proton irradiation. The carriers cannot escape from the intermediate state at lower temperatures. The probability of carrier release increases as the temperature increases, resulting in an increase in the luminescence intensity [25,27,28]. Thermal quenching of the luminescence intensity occurs in the 100–150 K range at the initial heating stage. The intermediate state has less of a contribution to the luminescence intensity owing to the relatively low temperature; however, the damage caused by proton irradiation and the increase in the probability of nonradiative recombination with increasing temperature leads to the thermal quenching of Yb²⁺ luminescence. According to the fitting results, there should be at least two nonradiative recombination routes that contribute to the quenching of Yb²⁺ luminescence, but the specific mechanism remains to be further studied.

The activation energies of the nonradiative recombination process and the intermediate state, which were calculated by applying the intermediate-state model to the consecutive temperature variation data between 100 and 300 K, are shown in

Table 3. Fitting parameters for the intermediate-state model.

Parameter	Activation Energy (meV)
E1	0.329
E2	0.088
E3	0.320

. The results show that proton irradiation may have created an intermediate state near the conduction band at a distance of 0.329 meV. The activation energies corresponding to this intermediate state and the nonradiative recombination are both less than 1 meV; thus, the smaller activation energy values need to be verified by further experiments. When Liu et al. used the X-ray excitation method to investigate the temperature dependence of the luminescence intensity, they discovered that the alteration in luminescence intensity was not only dependent on the intrinsic properties of the luminescence center but was also related to the speed and efficiency of the electrons and holes transferred to the luminescent center, both of which were temperature-dependent [29]. Although the intermediate-state model established based on the PL thermal quenching phenomenon provides an effective and simplified description of the complicated evolution behavior of Yb²⁺ luminescence, there may be some deviations in the activation energy obtained by fitting due to the complex interaction process between incident ions and matter and the successive accumulation of ion fluence during the spectrum acquisition process [30]. The activation energy is difficult to determine using conventional techniques since Yb²⁺ is a trace rare earth impurity. In subsequent studies, other appropriate techniques need to be used to confirm the activation energies of these two processes. For reference only, these activation energy values are provided. The simultaneous change in the ion fluence and temperature is more representative of the actual radiation environment, and the evolution behavior of Yb²⁺ luminescence under these circumstances has more realistic guiding significance for equipment design and reliability assessment.

The evolution of the peak positions of Yb²⁺ luminescence with temperature is shown in Figure 6. Notably, while there are small fluctuations in the peak position with temperature changes, the total shift can be negligible, and there is no common redshift or blueshift phenomena [29,31,32]. In general, the luminescence of the rare earth ions in YAG tends to shift to the long-wavelength direction (redshift) as the temperature increases; however, the change in Yb²⁺ luminescence with temperature is different [11,31,33,34]. The peak position associated with Yb²⁺ luminescence remains stable despite the continuous temperature changes from 100 to 300 K. The peak positions in Figure 6 almost exactly match the values in Table 2, and the individual fluctuations are potentially due to the Gaussian decomposition deviation.

Yan proposed that the thermal expansion coefficient of the YAG matrix, traces of contaminants in the sample, and the Stokes shift corresponding to the emission band are all associated with the evolution behavior of the peak position of rare earth ions in YAG with temperature [35]. The observed emission maxima will not alter with temperature if the lattice expansion-induced blueshift is offset by the effects of increased lattice distortion, increased Stokes shift, and greater self-absorption. The YAG crystal has a small thermal expansion coefficient, which results in a minor blueshift [35]. Furthermore, there is minimal thermal broadening of the emission band and a low Yb²⁺ concentration in the crystal. The effects of several variables may counterbalance each other, resulting in the peak position of Yb²⁺ not clearly shifting with temperature and displaying high thermal stability.

3.2. IBIL Experiment: 100–300–100 K

IBIL measurement on a YAG single crystal was performed under the conditions of 100–300–100 K continually varying temperature to further investigate the impact of ion fluence and temperature on Yb²⁺ luminescence and examine the reliability of previous assumptions. The evolution behavior of Yb²⁺ luminescence in the heating process of the 100–300–100 K experiment (Figure 7a) resembles that of the 100–300 K experiment (Figure 5), with a tendency of initially decreasing and then increasing and both achieving the minimum at approximately 150 K. Notably, however, while the temperature corresponding to the lowest intensity in the two experiments is the same, the associated ion fluence differs considerably. The fluence of 1.41 × 10¹⁴ ions/cm⁻² results from the lowest luminescence intensity in the 100–300–100 K experiment and corresponds to the increasing stage of the luminescence intensity in the 100–300 K experiment. This observation further demonstrates that temperature, rather than ion fluence, is the primary reason for the nonmonotonic evolution of Yb²⁺ luminescence intensity. The influence of ion fluence on Yb²⁺ luminescence can be observed with the difference in the increase in Yb²⁺ luminescence intensity when the temperature is elevated to 300 K. The luminescence intensity of Yb²⁺ only decreased to approximately 60% of its initial value in the two consecutive temperature variation experiments, which was much better than the measurement observations at specific temperatures (Figure 4). The contribution of the intermediate states to luminescence could be the primary reason for this enhancement.

In the cooling stage of 100–300–100 K, the luminescence intensity diminishes monotonically with decreasing temperature (increasing fluence), rapidly decaying at the beginning and reaching a stable stage at approximately 200 K. This behavior can potentially be attributed to a reduction in the contribution of the intermediate states resulting from decreasing temperature and the accumulation of ion irradiation damage. Experiments conducted at different fluence rates indicate that temperature is the primary factor influencing the discontinuous changes in the luminescent behavior of Yb²⁺. After our investigation, the luminescence behavior of Eu³⁺ exhibits a single variation with the occurrence of temperature changes.

The shift in the peak position of Yb²⁺ luminescence with temperature is similar to the observations at 100–300 K and exhibits good stability. The evolution behavior of the Yb²⁺ luminescence peak position at 100–300–100 K is shown in Figure 8. The stability of the peak position with temperature (fluence) indicates that Yb²⁺ luminescence can be effectively identified from the spectrum, which clearly provides an ideal scenario for the potential applications of Yb²⁺ luminescence in radiation conditions, such as space environments.

The fluence and temperature dependencies of Yb²⁺ luminescence were discussed using the IBIL outcomes at specific temperatures and consecutive temperature variations. Despite some deviations in the fitting results from the intermediate-state model, it can still qualitatively describe the complex evolution behavior of Yb²⁺ luminescence in the consecutive temperature variation IBIL experiments as well as illustrate the possible effect of ion fluence and temperature on Yb²⁺ luminescence. The IBIL measurements indicate that Yb²⁺ luminescence in YAG crystals has good radiation resistance and thermal stability in the 100–300 K temperature range.

4. Conclusions

In this study, 2 MeV H⁺ was used to excite YAG single crystals, IBIL spectra were collected under specific temperatures with consecutive temperature variations, and the fluence and temperature dependence of Yb²⁺:4f¹³5d(²F_7/2e_g) → 4f¹³6s luminescence in YAG single crystals were discussed in detail. Yb²⁺ luminescence features the slowest decay rate when compared to Y_Al ADs and Fe³⁺ and becomes the most dominant luminescence component in the spectrum at 300 K. The evolution behavior of Yb²⁺ luminescence intensity with ion fluence at several specific temperatures is highly comparable. Despite having different luminescence decay rates, all exhibit a declining trend with increasing ion fluence, decaying to approximately 35% of the initial intensity at a fluence of 3.5 × 10¹⁴ cm⁻². The intermediate-state model can effectively describe the evolution behavior of the Yb²⁺ luminescence intensity with temperature (fluence) under consecutive temperature variation circumstances, and the emergence of intermediate states is potentially the main reason for the negative thermal quenching of Yb²⁺ luminescence. The Yb²⁺ luminescence intensity decreases to 60% under the consecutive temperature variation experiments, and the peak position remains relatively stable, confirming that Yb²⁺ luminescence has good radiation resistance and thermal stability in the 100–300 K range. The outstanding features of Yb²⁺ luminescence can potentially fulfil certain requirements of the space environment and other radiation circumstances and have extremely broad application potential.