Creation and Stability of Color Centers in BaF₂ Single Crystals Irradiated with Swift ¹³²Xe Ions

Abstract

It was demonstrated that various defects can be induced in halide crystals by irradiation with swift heavy ions. Here, we irradiated barium fluoride (BaF₂) single crystals with 220 MeV xenon ions at room temperature and performed stepwise thermal annealing up to the temperature of 825 K to study the kinetics of ion-induced defects at different temperatures. Optical spectroscopy was utilized for the measurement of the wide range of absorption spectra from NIR to VUV. A sharp decrease in the F₂ absorption peak was observed for the samples annealed in the temperature range of 400–450 K. This result can be explained by their recombination with anion interstitials during thermal decay of the complex hole centers. The mobile interstitials, those did not recombine with the F₂ centers, increase the absorption peaks in the 9–10 eV region, which can be associated with interstitial aggregates.

1. Introduction

Barium fluoride is a wide-bandgap ionic material which is broadly used for radiation detection and optical applications. For applications dealing with high-energy particles, time resolution is a crucial characteristic of detectors which demand fast luminescent decay for scintillation materials [1]. BaF₂ has 195 and 220 nm cross-luminescence peaks with a pretty short decay time of 0.8 ns [2]; this makes it a promising material for scintillators [3,4]. However, the relatively slow 310 nm excitonic luminescence has a decay time of about 620 ns at room temperature [5]. Even though its intensity can be decreased by doping, the inserted impurities themselves can influence the fast luminescence [6].

Ionizing radiation, including swift heavy ions, produces point defects in the anion sublattice of many ionic crystals via decay of the electronic excitation. In the case of alkali-halides, irradiation at room temperature can easily create F centers, i.e., the anion vacancies binding electrons. By increasing the irradiation dose, the number of F centers increases until their concentration growth forces formation of complex F_n (n = 2–4) centers and larger nF aggregates. Among the studied halides, alkaline earth fluorides demonstrate lower efficiency in terms of F center formation and, consequently, larger radiation resistance [7,8]. This is mainly due to the double cubic fluorite structure, which hinders free motion of the anion sublattice defects and increases the probability of their recombination, reducing the yield of stable Frenkel pairs. On the other hand, the F centers in these crystals typically have higher mobility than in alkali halides, so the absorption spectra include bands of complex defects even at low doses. As examples, both irradiated CaF₂ and BaF₂ exhibit similar features of the prominent absorption at about 550 and 750 nm, respectively [9]. Previous bleaching experiments have shown that these two peaks cannot be ascribed to the creation of colloids where the optical bleaching is much slower than that for single-color centers or small F center clusters. Therefore, the main aim of the presented work is to reach a better understanding of the type of ion-induced color centers and their aggregation in BaF₂ using thermal annealing.

2. Materials and Methods

Monocrystals of BaF₂, produced by Epic Crystal (Kunshan, China), were irradiated with 220 MeV ¹³²Xe ions using a DC-60 accelerator (Astana, Kazakhstan) supplied by JINR (Dubna, Russia) at room temperature. The applied ion fluences ranged from 1.0 × 10¹¹ to 1.0 × 10¹⁴ ions/cm². The highest fluence took 6.0 × 10⁴ s of the beamtime, providing a moderate average flux of 1.67 × 10⁹ ions/(cm²·s) to ensure negligible heating of the samples and stabilization of the induced defects at room temperature. The calculated ion range of 17.6 µm [10] is less than the thickness of the samples (~1.0 mm). The initial stopping power of the projectiles is ~20 keV/nm and overwhelmingly comprises electronic energy loss.

Optical absorption spectra in UV, VIS, and NIR regions from 190 to 900 nm (6.5–1.4 eV) were measured via a Jasco V-660 double-beam spectrophotometer (JASCO, Hachioji, Japan). Vacuum ultra-violet (VUV) spectra up to 118 nm (10.5 eV) were obtained by means of a vacuum monochromator, BMP-2 (Institute of Physics, University of Tartu, Tartu, Estonia), equipped with a water-cooled gas discharge hydrogen lamp.

Thermal annealing was carried out in an argon atmosphere as a sequence of heating. After 5 min of retaining the given temperature T, cooling cycles followed. The annealing temperature T was increased stepwise by 20–30 K up to 825 K. All optical spectra were measured at room temperature, where they remained invariable.

3. Results and Discussion

The energy deposition of the utilized Xe ions is enough for considerable structural modifications in the vicinities of the ion trajectories (latent tracks), producing hillock-like structures, as investigated by AFM and TEM [11,12,13]. However, the present study is concentrated on the created point defects and their aggregates using optical spectroscopy. The absorption spectra of BaF₂ crystals, irradiated with various fluences of the 220 MeV xenon ions, are presented in Figure 1. The results show a clear increase in the optical density with ion fluence in the whole measured spectral region. The absorption peaks of the stable color centers at room temperature are shown in Table 1. The F centers are absorbing at 2.3 and 2.046 eV [8,14]. However, these color centers are mobile at room temperature and cannot be found in the spectra. Therefore, the simplest observed defect is the F₂ center, which consists of two anion vacancies and two captured electrons and exhibits absorption maximum at 1.62 eV [7,11]. More complex F_n centers of various configurations seemingly absorb in the higher energy range [15].

Few peaks in the 9–10 eV region can be associated with the exciton absorption. However, it seems more plausible to ascribe them to aggregates of interstitial defects. This is similar to the 121 nm peak, observed in lithium fluoride after irradiation with swift heavy ions [16]. Being close to the fundamental absorption of the crystal [17,18,19], they can also originate via the highly disordered regions in the track [20,21] as the part of the Urbach tail. Other known absorption peaks, stable at room temperature, are listed in Table 1.

Table 1. Color centers in barium fluoride at room temperature [8,9,11,22].

Center	Absorption Maximum (eV)
F2	1.62
F3	1.83, 3.04
F4	2.24
V2 (F3−)	6.5

Table 1. Color centers in barium fluoride at room temperature [8,9,11,22].

Center	Absorption Maximum (eV)
F2	1.62
F3	1.83, 3.04
F4	2.24
V2 (F3−)	6.5

Absorption spectra of the stepwise annealed crystal, irradiated with 220 MeV Xe to the fluence of 1 × 10¹⁴ ions/cm², are shown in Figure 2.

The observed sharp decrease in the number of F₂ centers in the narrow temperature interval of 400–450 K (Figure 3) corresponds to the known thermoluminescence (TSL) intensity peak at 420 K [19,23]. The TSL spectrum is considered as the superposition of 330 nm and more intense 370 nm bands [19], which might be provided by recombination of the electrons with the hole centers, but the mechanism of such recombination remains unclear. This process can be associated with decay of the F₂ centers and further migration of the F centers and/or release of the electrons [24]. However, we believe that it is the hole centers (fluorine interstitials) that become mobile and recombine with the F₂ centers. This assumption is based on the observation of simultaneous decrease in the broad band at 6.5 eV and the increase in the 9–10 eV absorption bands, as shown in Figure 2, supposing the origin of the latter as the interstitial fluorine aggregates. We fit the measured optical density OD(T) versus the annealing temperature to the Arrhenius law:

$$OD\left(T\right)=A\exp \left(-{\displaystyle \frac{E_a}{kT}}\right),$$

(1)

where A is the pre-exponential factor, and k is the Boltzmann constant, shown as the blue dashed curve in Figure 3. This gives the activation energy E_a of 0.32 eV for the annealing process in this temperature range. The second decrease in the F₂ peak above 670 K (red dashed curve) has an activation energy of 0.53 eV, which is very close to the calculated migration energy of the single F centers [25], and can be explained by the direct thermal decay of the divacancies. Interestingly, these values are larger than the one of the ion-induced surface hillocks [11,12]. This result supports the assumption that hillocks are correlated to the creation of more defect aggregates rather than single defects [26]. This is also confirmed by the creation of pyramidal etch pits using selective chemical etching after irradiation with GeV uranium ions [13]. The etchability of ion tracks is a good indication of the aggregates [27].

4. Conclusions

Energetic xenon ions create reach variety of the optical absorption bands of particular defects. The F₂ centers, absorbing at 1.62 eV, are the simplest stable color centers at room temperature, since the single F centers are mobile. Interestingly, a significant decrease in F₂ centers is observed during stepwise annealing of the irradiated crystals in the narrow temperature region of 400–450 K. We believe this decrease originates from the decay of hole (interstitial) centers of the broad absorption band at ~6.5 eV. This explanation is supported by the simultaneous increase in the peaks in the 9–10 eV region, which presumably corresponds to anion interstitial aggregates. The activation energies of the F₂ peak decreases were estimated in two temperature intervals and compared with annealing of the surface hillocks.