1. Introduction

Middle infrared (mid-IR) spectral range of 3–5 µm has a variety of applications, such as trace gas detection, free space communication, medical diagnostics, and military applications [1–5]. Iron-doped wide band gap ZnSe/ZnS semiconductors are promising gain materials for widely tunable direct pump solid-state lasers operating in mid-IR (3–5 µm) regions [6,7]. In recent years, many significant results have been reported for Fe²⁺:ZnSe lasers. In 2015, highest output power of 35 W for Fe²⁺:ZnSe laser at 77 K was demonstrated under pumping using the radiation from Er:YAG laser [8]. In 2016, output energy of the Fe²⁺:ZnSe laser at room temperature (RT) reached 1.43 J in 150 ns pulse with 53% slope efficiency under pumping from hydrogen fluoride (HF) laser in single-pulse mode [9]. In 2017, output power of 9.2 W from continuous wave (CW) Fe²⁺:ZnSe laser was achieved in non-selective cavity at 4.15 µm [10]. Recently, 50 Hz repetition-rate Fe²⁺:ZnSe laser with maximum average power of 21.7 W was demonstrated at RT [11].

In above references, Fe²⁺:ZnSe materials were prepared via crystal growth and thermal diffusion method. Although Fe²⁺:ZnSe gain media fabricated using crystal growth methods exhibit good laser characteristics, this fabrication method is complex and cannot easily achieve homogeneous distribution of Fe²⁺ ions in ZnSe host [12,13]. For these reasons, crystal growth method has not been widely used for Fe²⁺:ZnSe materials. Being simple technology, thermal diffusion has become most commonly used method to fabricate Fe²⁺:ZnSe materials [14]. However, its main drawbacks include low doping concentration, inhomogeneous diffusion, and concentration gradient, which cause transverse parasitic oscillation [15]. In order to improve diffusion rate and length of Fe²⁺ ions in ZnSe, hot isostatic pressing (HIP) has been employed to treat Fe²⁺:ZnSe in thermal diffusion method; this is due to the fact that HIP can provide higher diffusion temperature [16–19]. In addition, Fe²⁺:ZnSe with sandwich structure was also fabricated via solid-state diffusion bonding, which can be considered as an improved thermal diffusion method to a certain extent [20,21]. For example, two-layer-doped Fe²⁺:ZnSe with a diameter of 20 mm was able to produce output energy of 480 mJ with 37% total efficiency with respect to absorbed energy [20]. Despite the presence of two doping layers, the distribution of Fe²⁺ ions was still in inhomogeneous. Thus, it is crucial to find alternative fabrication methods that could improve the distribution of Fe²⁺ ions.

Instead of thermal diffusion, ceramic technologies, such as hot pressing (HP) and spark plasma sintering (SPS), are commonly used to prepare Cr²⁺/Fe²⁺:ZnSe/ZnS transparent ceramics. In these ceramic methods, the doping of Fe²⁺ ions usually originates from powder synthesis; thus, Fe²⁺:ZnSe transparent ceramics prepared from Fe²⁺:ZnSe powders naturally exhibit uniform distribution of Fe²⁺ ions. Consequently, research groups in this field have endeavored to fabricate Cr²⁺/Fe²⁺:ZnSe/ZnS materials using ceramic method. In 2006, Gallian et al. first prepared Cr²⁺:ZnSe transparent ceramics from commercial CrSe and ZnSe powders via HP and found that ceramic laser could generate output energy of 2 mJ at slope efficiency of 5% [22]. Subsequently, Chen et al. reported hot-pressed Cr²⁺:ZnSe ceramics with Cr²⁺:ZnSe powders, which were prepared through thermal diffusion between CrSe and ZnSe powders [23]. Additionally, in 2016, Li et al. prepared Cr²⁺:ZnS transparent ceramics through the mixture of commercial Cr₂S₃ and wet-chemically synthesized ZnS powders via HP; Cr²⁺:ZnS ceramic exhibited 67% transmittance at 11.6 µm [24]. In 2017, Li et al. successfully fabricated Fe²⁺:ZnS transparent ceramics via HP and post-HIP methods using co-precipitation synthesized Fe²⁺:ZnS powders [25]. In 2018, Li et al. adopted thioacetamide (TAA) as sulfur source to synthesize Cr²⁺:ZnS nanopowders via hydrothermal method, which provides another way to prepare Cr²⁺:ZnS transparent ceramics [26]. Recently, Yu et al. prepared Fe²⁺:ZnSe ceramics from nanopowders synthesized via co-precipitation method, and the ceramic sintered at 900 °C under a pressure of 90 MPa for 2 h via SPS exhibited best optical transmission, i.e., 68% at 7.5 µm [27,28]. However, hydrogen sulfide (H₂S) and hydrogen selenide (H₂Se) gases are produced during the preparation of Cr²⁺/Fe²⁺:ZnS/ZnSe powders. Moreover, few studies have reported the fabrication of Fe²⁺:ZnSe transparent ceramics via HP method.

In this work, Fe²⁺:ZnSe transparent ceramics were successfully prepared from chemical vapor deposition (CVD) ZnSe powders via hot-pressing method. Thermal diffusion of Fe²⁺:ZnSe powders was used as doping source of Fe²⁺. The microstructure, phase compositions, and optical properties of Fe²⁺:ZnSe ceramics obtained at different sintering temperatures were systematically studied. The samples prepared in this work showed high densities and exhibited high transmittance in IR region.

2. Experimental procedure

In order to avoid the presence of harmful H₂Se gases and achieve homogeneous distribution of Fe²⁺ ions in the synthesis of Fe²⁺:ZnSe powders, high-purity CVD ZnSe and Fe²⁺:ZnSe polycrystalline were used in this work. Fe²⁺:ZnSe polycrystalline that was prepared via thermal diffusion method at 1000 °C for 240 h was used as doping source of Fe²⁺: it was grinded in an agate mortar with the CVD ZnSe crystal in a glove compartment with high-purity argon atmosphere for preparing the powders. Mixed powders were sieved using 200-mesh screen and then placed into graphite die with a diameter of 20 mm. HP method was employed to sinter the specimens at various temperatures, ranging from 850 °C to 1100 °C, for 2.0 h under vacuum level of ∼10⁻³ Pa. Heating rate was 5 °C/min, and a pressure of 90 MPa was applied during sintering. After hot-pressing, ceramic disks were mirror polished on both sides using 0.3 µm alumina powder.

The densities of Fe²⁺:ZnSe transparent ceramics were measured using Archimedes method. Phase compositions of the powders and ceramics were examined via X-ray diffraction (XRD; Bruker AXS AS D2) under CuKα radiation in 2θ range from 20° to 80°, using step size of 0.08° and scan time per step of 0.3 s. Laser granulometry measurements were performed with a Microtrac S3500 particle size analyzer. The microstructural characterization of as-obtained powders and as-sintered ceramics was performed via field-emission scanning electron microscopy (FESEM; Inspect F50, FEI). Fourier transform IR spectroscopy (Perkin Elmer; Spectrum Two) was used to measure IR transmission spectra of Fe²⁺:ZnSe ceramics in the wavelength range of 1.5–20 µm.

3. Results and discussion

Figure 1(a) shows the XRD pattern of ZnSe powder (A), grinded Fe²⁺:ZnSe powder (B), and CVD ZnSe bulk (C). All samples exhibit the cubic sphalerite structure without the presence of hexagonal structure or any impurities such as zinc oxide (ZnO). Diffraction peaks are in good agreement with standard PDF card of ZnSe (PDF#37-1463). FESEM image of mixed powders is displayed in Fig. 1(b). Inhomogeneous size distribution and micron-scale agglomerates of the powders are clearly observed. The agglomerates consist of particles of different sizes, from less than 1 µm to more than 10 µm. Generally, homogeneous, fine-sized, and high-dispersion powders are beneficial for the formation of high-density and uniform microstructure ceramics. Therefore, the microstructure of these Fe²⁺:ZnSe powders would hinder their densification. Figure 1(c) presents particle size distribution data of Fe²⁺:ZnSe powder in the form of a histogram and cumulative size distribution. Fe²⁺:ZnSe powders exhibit wide particle size distribution, ranging approximately from 0.7 to 35 µm, with 50% of particles having a size less than 2.99 µm and 90% of particles having a size within 1.06–14.2 µm.

 

Fig. 1. (a) XRD patterns of (A) ZnSe powders, (B) Fe²⁺:ZnSe powders, and (C) CVD ZnSe bulk; (b) FESEM image of Fe²⁺:ZnSe powders; (c) particle size and cumulative size distribution of Fe²⁺:ZnSe powders.

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In order to observe their densification, Fe²⁺:ZnSe ceramics were sintered at various temperatures, from 850 °C to 1100 °C, for 2 h under 90 MPa in a vacuum of ∼10⁻³ Pa using HP method. Figure 2 shows the XRD pattern of Fe²⁺:ZnSe ceramics sintered at different temperatures. These ceramics display pure cubic phase of ZnSe, which is the same as that of the powders. Since phase transition temperature of ZnSe crystal from cubic sphalerite to hexagonal wurtzite structure is around 1425 °C [29], phase transition was not observed during sintering process. It can be observed that the intensity of the (111) diffraction peak of Fe²⁺:ZnSe ceramics is in general higher than that of the (220) peak. However, the intensity of the (220) diffraction peak is higher than that of the (111) peak for Fe²⁺:ZnSe ceramics sintered at 1050 °C and 1100 °C, despite the (111) diffraction peak of Fe²⁺:ZnSe powders being significantly higher than the (220) peak (Fig. 1(a)). This can be attributed to high pressure changing preferential orientation of the grains. Consequently, it is possible that the grains of these ceramics grow preferentially along the [220] direction.

 

Fig. 2. XRD patterns of Fe²⁺:ZnSe transparent ceramics sintered at different temperatures.

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Figure 3 shows the FESEM images of the microstructures of hot-pressed Fe²⁺:ZnSe ceramics. Both transgranular fracture and layered structure are observed in the images. Large particle size and non-uniform distribution of grinded Fe²⁺:ZnSe powders cause very large and extremely irregular distribution of grains in these ceramics. From Fig. 3(a), it can be seen that some residual pores are localized both inside the grains (indicated by red arrows) and outside the grains (indicated by white circles). Hence, it can be inferred that insufficient sintering temperature of 850 °C leads to pores within the grains. However, the specimen displays dense microstructure with relative density higher than 99%. Fe²⁺:ZnSe ceramic sintered at 900 °C has few pores at grain boundaries and slightly increased grain size, as shown in Fig. 3(b). Similar microstructure, with some micropores outside the grains and large grains, is also observed for Fe²⁺:ZnSe ceramics sintered at 950 °C and 1000 °C (Fig. 3(c) and (d)), respectively. When sintering temperature exceeds 1000 °C, the specimens present very large grains (>50 µm), as indicated by yellow arrows in Fig. 3(e) and (f). These ceramics are considered to have large (220) grains because of high-intensity (220) diffraction peak observed in corresponding XRD pattern, particularly at sintering temperature of 1050 °C.

 

Fig. 3. FESEM images of Fe²⁺:ZnSe ceramics sintered at different temperatures: (a) 850 °C, (b) 900 °C, (c) 950°C, (d) 1000 °C, (e) 1050 °C, and (f) 1100 °C.

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Photographs of 1.0-mm-thick Fe²⁺:ZnSe ceramics obtained at different sintering temperatures for 2 h are displayed in Fig. 4. They present dark yellow color and non-uniform black foggy clusters, which indicates possible presence of carburization from graphite die, as well as other defects, such as pores and impurities. The carbons can easily diffuse from the graphite die to Fe²⁺:ZnSe ceramics during sintering due to long dwelling time and high temperature and can thus form defects in Fe²⁺:ZnSe ceramics. In previous studies, Fe²⁺:ZnSe ceramics were sublimated at a sintering temperature above 1000 °C, even with high pressure, which led to vacancies in Fe²⁺:ZnSe ceramics [28]. Figure 5 reveals the energy-dispersive X-ray spectroscopy (EDS) analysis of ZnSe ceramic sintered at 1100 °C for 2 h. According to EDS results, hot-pressed ZnSe ceramic does not show any impurities (carbon element). Thus, the carbons in the graphite die do not diffuse into the ZnSe ceramic. It can be considered that black clusters in ZnSe ceramic are caused by Se vacancies, owing to the Zn:Se ratio being about 54:46. These Se vacancies may affect the mid-IR laser generation of these ceramics, and they might be eliminated by annealing at high temperature with Se element.

 

Fig. 4. Photographs of Fe²⁺:ZnSe ceramics sintered under 90 MPa for 2 h at different temperatures: (a) 850 °C, (b) 900 °C, (c) 950°C, (d) 1000 °C, (e) 1050 °C, and (f) 1100 °C.

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Fig. 5. EDS spectra of ZnSe ceramic sintered at 1100 °C.

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RT IR transmission spectra of Fe²⁺:ZnSe ceramics are illustrated in Fig. 6; they were measured in the wavelength range of 1.5–20 µm. The sample sintered at 850 °C shows a relatively low transmittance, which is ascribed to residual pores at grain boundaries. With the increase of sintering temperature, the ceramics become dense, and their transmittance increases. As can be observed, Fe²⁺:ZnSe ceramics sintered at 900 °C exhibit best optical quality in the range from 1.5 to 20 µm, with a transmittance of ∼63% at 5 µm and ∼69% at 14 µm. Additionally, this sample had an actual density value of 5.261 g/cm³, which was equal to a relative density of 99.83%. The transmittance of specimens sintered at 950 °C and 1000 °C is basically the same (∼67% at 14 µm). While sintering temperature continues to increase, the transmittance of the ceramics decreases, especially for the sample sintered at 1100 °C, which showed a low transmittance in the mid-IR range. This is due to the fact that these Fe²⁺:ZnSe ceramics were over-sintered at a high temperature. Owing to ⁵E→⁵T₂ transition of Fe²⁺ ions in ZnSe material, these ceramics exhibit an absorption peak at approximately 3 µm, which is red-shifted compared with 2.86 µm value reported in the literature [25]. Due to the inaccurate concentration of Fe²⁺ ions in thermally diffused Fe²⁺:ZnSe, the absorption intensity of Fe²⁺:ZnSe ceramics is greatly different, as shown in Fig. 7.

 

Fig. 6. IR transmittance spectra of 1-mm-thick Fe²⁺:ZnSe transparent ceramics sintered at different temperatures.

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Fig. 7. Absorption spectra of hot-pressed Fe²⁺:ZnSe transparent ceramics sintered at different temperatures.

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These hot-pressed Fe²⁺:ZnSe ceramics can be considered as promising mid-IR laser gain materials. The luminescence lifetime of Fe²⁺:ZnSe ceramics has been reported to be in a good agreement with the lifetime of conventional thermally diffused Fe²⁺:ZnSe polycrystalline sample, and the uncalibrated photoluminescence spectrum of Fe²⁺:ZnSe ceramic sample covers the spectral range of 3.5–5.5 µm [14].

4. Conclusions

Fe²⁺:ZnSe powders were prepared by grinding a mixture of CVD ZnSe and thermally diffused Fe²⁺:ZnSe crystal in an agate mortar. As-obtained Fe²⁺:ZnSe powders exhibited pure cubic phase, and they were composed of small particles (0.7 µm) and large particles (35 µm). Fe²⁺:ZnSe transparent ceramics were sintered at a temperature ranging from 850 °C to 1100 °C for 2 h under a pressure of 90 MPa via hot-pressing method. As-prepared Fe²⁺:ZnSe ceramics showed cubic ZnSe phase and dense microstructure. Insufficient sintering temperature of 850 °C led to some residual pores being localized both inside and outside the grains. However, over-sintering occurred when sintering temperature exceeded 1000 °C. These ceramics presented dark yellow color with non-uniform black foggy clusters. Fe²⁺:ZnSe ceramic sintered at 900 °C for 2 h under pressure of 90 MPa displayed best optical transmittance of ∼63% at 5 µm and ∼69% at 14 µm. All Fe²⁺:ZnSe ceramics showed an absorption peak at approximately 3 µm.