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Abstract
In the recent two decades, Cr2+:ZnSe crystals have been widely used as a gain media for 2∼3 μm mid-infrared lasers. However, it still remains a huge challenge for researchers to meet more and more requirements on the crystals with high qualities and large sizes. In this work, one Cr2+:ZnSe single crystal with a diameter of about 15 mm was successfully grown by chemical vapor transporting (CVT) with NH4Cl in a closed quartz ampoule without any seed. The transmission of as-grown crystal is up to 70% in the UV-VIS-NIR region and is with an intense characteristic absorption of Cr2+ near 1770 nm. The X-ray photoelectron spectroscopy (XPS) shows that the incorporation of chlorine anions does not significantly affect the valence distribution of components in the crystal. The mid-infrared photoluminescence spectra show a strong and broad emission band centered at 2400 nm with a width of 600 nm under the 1770 nm laser excitation at room temperature. The calculated cross sections of absorption and emission were 1.31×10−18 and 1.4×10−18 cm2, respectively. The measured photoluminescence decay time was about 6.9 μs at room temperature. It is confirmed that the method of CVT with NH4Cl is suitable for the growth of Cr2+:ZnSe single crystals, which are expected to have a promising prospect in mid-infrared laser applications in further.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Middle infrared (Mid-IR) lasers in the range of 2∼5 μm have been widely used in molecular spectroscopy, trace gas analysis, remote sensing, medical communication as well as industrial control [1]. In the recent two decades, transition metal (TM) doped II-VI compounds, especially Cr2+:ZnSe materials, have proven to be the most promising candidates for room-temperature tunable Mid-IR lasers [2]. For the Cr2+:ZnSe laser materials, the divalent chromium ion incorporated into the ZnSe lattice occupies the zinc site, which is coordinated by four selenium anions. Because of the presence of the heavier anions Se, which decreases the phonon cut-off frequency, there is almost no temperature quenching in the Mid-IR band and the quantum efficiency is close to 100% at room temperature. The absorption and emission cross-sections of this materials are about in the order of 10−18 cm2. All these excellent luminescence properties make it be the excellent choice for Mid-IR lasers [3–5]. The use of single-crystal Cr2+:ZnSe may offer improved performance, but a major breakthrough in crystal growth would be required for such an advance.
At present, Cr2+:ZnSe laser crystals are commonly obtained by doping during the growth of single crystal [6–8], and thermal diffusion after sputtering metal film on single crystals [9,10]. For diffusion single crystals [11], there exists a large concentration gradient that is detrimental to laser efficiency [12]. The post-treatment of thermal diffusion has been studied in order to reduce the concentration gradient and to increase the uniformity of the dopant [13–15]. However, the role of post-processing is still limited and its complexity undoubtedly brings the high cost of TM-doped crystal fabrication. Thus, doping during the crystal growth is considered as a convenient route. Due to high melting point and the solid phase transition at 1426 °C [16], ZnSe single crystal is preferred to be commonly grown by vapor transport methods. Chemical vapor transporting (CVT) is regarded as an excellent technique for single crystal growth at low temperature. Among applications of CVT techniques, as being more stable at room temperature, NH4Cl is an excellent transporting agent for growing intrinsic ZnSe single crystal [17] and CdTe crystal [18] etc. To the best of our knowledge, there is a few literatures which thoroughly reported the CVT growth and properties of Cr2+:ZnSe single crystals.
In this work, one Cr2+:ZnSe single crystal was successfully grown by CVT method with a transporting agent of NH4Cl. The doping of chromium ions is illustrated by the possible chemical vapor transport reactions in the CrSe-HCl system. Optical properties of as-grown crystals, such as transmission, XPS, Raman and Mid-IR luminescence, are characterized. The effects of unintended incorporated chlorine ions, the cross sections of absorption and emission as well as the luminescence lifetime are presented in detail.
2. Experimental
2.1 Synthesis
In this experiment, high pure elements Cr (4N), Zn (7N) and Se (7N) were used as raw materials for synthesizing CrSe and ZnSe polycrystalline. All syntheses were carried out in quartz ampoules sealed under a vacuum of 5×10−5 Pa. The solid state reaction of Cr and Se elements was performed at 1000 °C for 1 week. Then the obtained materials were ground in an agate and transferred into one quartz ampoule. The procedure of solid state reaction was carried out again. After that, CrSe polycrystalline were prepared successfully. The Zn and Se elements pre-reacted under the flame of H2-O2 and then the quartz ampoule was kept at 1000 °C for 1 week to obtain ZnSe polycrystalline.
Cr2+:ZnSe polycrystalline were synthesized by the solid phase diffusion method, as the following procedure. The mixture of CrSe and ZnSe powders, in which the nominal concentration of Cr was about 1.5×1019 atoms/cm3, ground in an agate mortar to distribute CrSe as evenly as possible. The ground powders were sealed in quartz ampoule and then the sealed ampoule was placed in tubular resistance furnace. The tube was heated to 1000 °C at the rate of 50 °C/h to keep at that temperature for 72 h, and then cooled to room temperature at the rate of 50 °C/h. The obtained sample was reground, and the heat treatment descripted above was performed twice.
2.2 Crystal growth
As-synthetized Cr2+:ZnSe polycrystalline and an appropriated amount of NH4Cl were sealed into a quartz ampoule. The dosage of transporting agent NH4Cl was about 1.0 mg/mL. The tip of the ampoule was designed as a cone with a little curvature, which was used to select one single crystal at the beginning of the crystal growth. The two-zone tube furnace with double-layer heating resistance wires was used, as shown in Fig. 1(a). A reverse transporting operation was performed: the source zone and growth zone were kept at 900 °C and 950 °C respectively for 12 h to remove the raw material from the tip. After that, the growth of Cr2+:ZnSe single crystal was carried out for one month with the temperature around 1000 °C. Temperature profile along the axial direction of the tube furnace is shown in Fig. 1(b). After growth, the furnace was cooled to room temperature at the rate of 50 °C/h. One Cr2+:ZnSe single crystal was grown by physical vapor transport (PVT) method as a part of comparison. The as-grown crystals were cut into wafers with a thickness of 1.5∼2 mm. Sandpaper, magnesium oxide suspension, silicone gel and hydrogen peroxide were used to polish and clean the surface of wafers.
Fig. 1. (a) Schematic diagram of CVT growth system of Cr2+:ZnSe single crystal in two-zone tube furnace, (b) the temperature profile along the axial direction of the tube furnace.
2.3 Characterization
X-ray photoelectron spectroscopy (XPS) test was taken on Axis Supra to determine the location of chromium binding energy. The sample surface before the XPS test was etched by Ar+ beam for one minute to remove some impurities present on the surface. UV-VIS-NIR spectrophotometer (SHIMADZU UV-3150) was taken to feature transmission and Cr2+ concentration of the sample with a thickness of 1.2 mm at room temperature. The Raman spectra of the samples were recorded at the laser excitation wavelength of 785 nm. Steady state and transient luminescence spectra of the crystal were obtained on FSP920C spectral detection system at room temperature, the excitation source was taken on 1770 nm OPO laser with a power of 27 mW and a frequency of 20 Hz, and InSb detector was used to receive data.
3. Results and discussion
3.1 As-grown crystal
The result of our CVT growth experiment is shown in Fig. 2. The crystal is uniform dark red in color from the tip to the tail. Some spiral growth steps on the planes can be observed under the optical microscope or by naked eyes. Several low index planes with the spiral steps intersect the surface, which indicates that the crystal growth is established on the mechanism of two-dimensional nucleation. One polished wafer cut from the middle part of as-grown crystal is shown in the inset. It is dark red in color and transparent uniformly. It is believed that Cr ions are distributed evenly in the as-grown crystal.
The UV-VIS-NIR spectrum of as-grown crystal is shown in Fig. 3. The thickness of wafer tested is 1.2 mm. The transmission is up to 70%, the absorption edge is at about 567 nm and its corresponding energy is lower than the known band gap of 2.67 eV for ZnSe at room temperature. The shift of the absorption edge towards longer wavelength is due to the optical transitions between different charge states of the chromium ion, i.e., Cr2+ → Cr+ [19,20]. There is a wide-band and strong absorption at a wavelength of 1770 nm, which corresponds to spin-allowed 5T2 and 5E transition within the 3d4 shell of Cr2+ ions. The calculated concentration of Cr2+ based on absorption is 1.31×1019 atoms/cm3, the calculation method of concentration refers to other study [6]. This characteristic absorption of Cr2+ at 1770 nm is sufficient to produce Mid-IR luminescence of Cr2+:ZnSe crystals, which will be discussed in the next content.
3.2 Crystal growth process analysis
As shown in Fig. 3, the incorporated Cr ions into ZnSe matrix are confirmed by the wide-band and strong absorption. In order to further improve the quality of Cr2+:ZnSe crystals, the doping process during their CVT growth should be illustrated systematically. However, due to the shortage of thermodynamics data of CrSe and CrCl2, here we just simply discuss the possible reactions involving the stable state of CVT growth in Zn(Cr)Se-NH4Cl system. The nucleation and the establishment of the stable growth can be found in our previous works [21].
It is well known that NH4Cl totally dissociates into HCl, H2 as well as N2 at the temperature above 337.8 °C [18,22]. Starting from the decomposition of NH4Cl, four typical chemical reactions which occurred in Zn(Cr)Se-NH4Cl system are listed as:
The entire crystal growth process of Cr2+:ZnSe single is constituted by four stages as marked in Fig. 1(a). The First stage occurs in the source zone with a higher temperature T1. The molecules of HCl gas react with the source material of Cr2+:ZnSe polycrystalline and the mixed gases of ZnCl2, CrCl2 and H2Se are generated. This stage is marked by the process ① in Fig. 1(a). The next stage is the transporting of the mixed gases from the high temperature zone T1 to the low temperature zone T2, which is labeled by the process ②. The driven force of the second stage results from the concentration difference due to the consuming of the mixed gases at the growth zone with a lower temperature T2. In the third stage——the process ③, the reverse procedures of reactions (2) and (3) take place, i.e., the reaction (4) does, which is considered as the crystal growth and doping process. In this stage, the gas of HCl is produced and its concentration increases at the growth zone. As a similar driven force of concentration difference, the gas of HCl is transported from the low temperature zone T2 to the high temperature zone T1, which is labeled by the process ④. It can be determined that the generated nitrogen does not participate in the transport. However, chlorine atoms participate in the transport process and promote the transport of ZnSe, so it may dope into the crystal during crystal transporting growth.
3.3 XPS and Raman analysis
In order to investigate the effect of doped Cl element on the valence state of chromium ions, a comparison XPS analysis between as-grown crystals obtained by CVT and PVT was performed, as shown in Fig. 4. The detailed peak for Zn 2p3/2 with binding energies at 1021.5 eV is shown in Fig. 4(a), which corresponds to divalent Zn [23–25]. The Se 3d peak with the binding energy of about 54.5 eV is shown in Fig. 4(b), which corresponds to divalent Se [26]. The Cr 2p3/2 peak and the Cr 2p1/2 peak generated due to the spin-orbit splitting were marked. Cr 2p3/2 peak with the binding energy of about 575.1 eV attributed to Cr2+ (575.3 eV [27]) is shown in Fig. 4(c). The binding energy of the XPS peak is used to determine whether there is a deviation in the state of chromium between as-grown crystal and PVT crystal. These results show that the peaks of the two crystals are in the same position, so the microscale introducing of NH4Cl transport agent during crystal growth does not affect the valence states of elements.
Fig. 4. X-ray photoelectron spectroscopy (XPS) comparison of as-grown crystal with PVT Cr2+:ZnSe single crystal.(a) The detail scan of X-ray spectroscopy (XPS) spectra of Zn 2p3/2 and Zn 2p1/2. (b) The detail scan of X-ray spectroscopy (XPS) spectra of Se 3d. (c) The detail scan of X-ray spectroscopy (XPS) spectra of Cr 2p3/2 and Cr 2p1/2.
The Raman spectrum of sample is shown in Fig. 5, the transverse–optical (TO) and longitudinal–optical (LO) [28], double transverse–acoustical phonon (2TA) and 2TA + TO [29] modes from 100 cm-1 to 550 cm-1 were marked. In the region between TO and LO phonons, an obvious additional Raman mode between 220 cm-1 to 230 cm-1 cans be observed. This peak is LO (CrZn-Se) mode introduced by the dope of Cr2+ in ZnSe crystal field, referring to the literatures [30,31]. Consequently, for the obtained Cr2+:ZnSe single crystal, Cr ions successfully occupy Zn sites, which introduces the characteristic mode of crystal lattice vibration. For an ideal perfect crystal, the phonon propagates in the mode of a non-attenuating plane wave. Usually, the Raman scattering follows the momentum conservation, but if there are other defects such as dislocations or some inclusions in the crystal, the propagation of the phonon wave is attenuated. In the Raman spectrum, the asymmetry of the peak shape, the broadening of impurity peak, and the weakening of the intensity in some phonon peak mean that the crystal quality is poor. The LO peak corresponding to the first-order Raman scattering peaks is the characteristic peak for ZnSe crystal, which is dominated by the crystallinity of ZnSe crystal. The CVT crystal has a symmetrical LO peak, and the relative intensity (${I_{LO}}/{I_{TO}}$) is about 3.7. A high relative intensity indicates Cr2+:ZnSe single crystal grown by this method has a high crystal quality. Although the chloride ions may be doped into the crystal, they do not cause serious lattice distortion.
3.4 Mid-IR luminescence characterization
The Mid-IR steady-state photoluminescence spectrum of as-grown Cr2+:ZnSe single crystal tested under the excitation wavelength of 1770 nm is shown in Fig. 6(a). The strong emission band is in the range from 1800 nm to 3200 nm with a FWHM width of 600 nm, which is attributed to 5E→5T2 emission transition of chromium ions. The high oscillator strength of Cr2+ ions in ZnSe crystal field leads the strong absorption (Fig. 3) and the wide emission width (Fig. 6). It is one of reasons why Cr2+:ZnSe crystals are attracting so much attention. The 3D transient photoluminescence spectrum plotted in variable times and wavelengths is shown in Fig. 6(b), and its top view is shown in Fig. 6(c). The decay time of the crystal in 2100∼2500 nm emission wavelength band is longer, while the decay times in 1800∼2100 nm and 2500∼3200 nm are slightly reduced. It is revealed that Cr2+:ZnSe single crystal exhibits a single exponential decay process, and the fitted emission lifetime (1/e) of the crystal at the wavelength of 2400 nm is about 6.9 μs. This lifetime is slightly higher than those in Ref. [32] (5 μs) and [33](5.5 μs) of the diffusion polycrystals at room temperature. The emission lifetime is longer due to no obvious concentration quenching, which indicates that the distribution of doped Cr ions is relatively uniform. From the point of view of crystal growth, NH4Cl can well transport various components of Cr2+:ZnSe.
Fig. 6. Mid-IR photoluminescence properties of as-grown Cr2+:ZnSe single crystal: (a) Steady state photoluminescence spectrum, (b) and (c) The color picture present the changes of PL intensities at variable times and wavelengths, (d) Photoluminescence kinetics at the wavelength of 2400 nm and the lifetime fitting (inset).
The cross sections of absorption and emission are the basic parameters guiding the design of the laser device. From the absorption spectrum (Calculated from the transmission spectrum, see Fig. 3), the absorption cross section can be obtained:
Where N is the concentration of Cr2+ in crystal, l is sample thickness,$\; {I_0}$ and I are the intensity of the light before and after it passes through the sample respectively. The calculated absorption cross section is 1.31×10−18 cm2 at 1770 nm. The emission cross section of the sample was estimated using the F-L (Fuchtbauer-Ladenburg) formula [1,34].The strong and symmetrical luminescence peaks as well as large cross sections of emission and absorption of as-grown crystals indicate that less other impurity ions are incorporated and the crystal growth technique is suitable. In view of the versatility of growing ZnSe single crystals by this method, it is also possible via this technique to grow high quality ZnSe single crystal doped with other transition elements such as Fe, Co, Ni etc.
4. Conclusions
In this work, one Cr2+:ZnSe single crystal was successfully produced by CVT technique with using NH4Cl as the transporting agent. The possible reactions involving doping process of chromium ions in the Cr2+:ZnSe CVT growth are suggested. Optical properties of as-grown crystals are characterized by spectra of UV-VIS-NIR transmission, XPS, Raman, and Mid-IR photoluminescence. In the wavelength of 200∼2600 nm at room temperature, the crystal shows a high transmittance and a high doping concentration. XPS contrast results show a similar elements state between as-grown CVT and PVT crystals. The Cl unintended incorporated does not affect the valence states of elements of as-grown crystals. Photoluminescence spectra were obtained under the excitation of 1770 nm OPO laser, absorption and emission cross sections of the crystal were calculated. Compared with other reports on Cr2+:ZnSe crystal, high doping concentration, large absorption and emission cross section as well as long luminescence lifetime prove that this CVT method with NH4Cl is valid for Cr2+:ZnSe single crystal growth.
Funding
National Natural Science Foundation of China (Nos. 52072300); Graduate School, Northwestern Polytechnical University (ZZ2019083).
Disclosures
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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