Crystal Growth and Effects of Annealing on Optical and Electrical

Sep 25, 2014 - ABSTRACT: Single crystal LiInS2 (LIS) is a promising candidate for mid-infrared (MIR) frequency conversion to obtain MIR laser output...
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Crystal Growth and Effects of Annealing on Optical and Electrical Properties of Mid-Infrared Single Crystal LiInS2 Shanpeng Wang,† Zeliang Gao,† Xiang Zhang,† Xixia Zhang,† Chunlong Li,† Chunming Dong,† Qingming Lu,‡ Minglei Zhao,§ and Xutang Tao*,† †

State Key Laboratory of Crystal Materials, ‡School of Chemistry and Engineering, and §School of Physics, Shandong University, Jinan 250100, China ABSTRACT: Single crystal LiInS2 (LIS) is a promising candidate for mid-infrared (MIR) frequency conversion to obtain MIR laser output. Large-sized LIS crystals with a diameter of 16 mm and a length of 50 mm were grown by the modified Bridgman method. The as-grown LIS crystals were annealed in different atmospheres, and the effects of annealing on the electrical and optical properties were studied in detail. The visible-near-infrared and MIR transmission spectra of the crystals were studied before and after annealing in different atmospheres. The experimental results indicate that the optimal annealing conditions for a grown crystal are to use an atmosphere of LIS vapor at 700 °C for 160 h. The electrical properties were investigated before and after annealing in the LIS vapor atmosphere. After proper postgrowth annealing, the quality of the crystal was significantly improved, which resulted in an increase in the damage threshold from 0.9 to 1.2 J/cm2.

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INTRODUCTION In recent years, there has been considerable progress in the development of coherent mid-infrared (MIR) lasers based on frequency conversion using nonlinear optical (NLO) crystals.1−4 MIR lasers have wide application in military and civilian affairs,5−7 such as LIDAR, laser counter-measures, MIR spectroscopy, and environmental monitoring. At present, the chalcopyrite MIR crystals (AgGaS2 or AGS, AgGaSe2 or AGSe, and ZnGeP2 or ZGP) are commercially produced on account of their advantages, such as high nonlinearity and/or a wide transparency range.8−10 However, the poor thermal conductivity of AGS, owing to a low-energy phonon spectrum, limits its performance, especially in continuous wave (CW) applications. ZGP has large nonlinearity (72 pm/V) and a high thermal conductivity, but strong residual absorption, and twophoton absorption of commercially available ∼1 μm laser (Nd:YAG) radiation severely limits its applications. LiInS2 (LIS)3 crystal is an excellent candidate for frequency conversion to obtain MIR laser emission. It crystallizes in a modified wurtzite structure with the mm2 point group. The lighter Li+ ions in the crystal structure enlarge the band gap of these compounds, which leads to a higher laser damage threshold. LIS has excellent properties, such as high nonlinearity, a wide transparency range (0.35−12.5 μm), nearly isotropic thermal expansion behavior, a large thermal conductivity, and phase-matching over a large MIR wavelength range. It can be pumped by commercially available 1064 nm Nd:YAG lasers, which is a very important asset for practical applications in MIR technology. However, until now, any practical MIR applications of LIS crystals have been hindered © 2014 American Chemical Society

by low optical quality. Therefore, it is highly desirable to determine the cause of the poor optical quality of the as-grown LIS crystals and to find strategies to resolve it. On one hand, much attention must be paid to the synthesis of stoichiometric polycrystalline LIS and to the crystal growth process. Because of the volatility of sulfur and the high chemical activity of lithium, it is easy for the constituents of LIS to deviate from the stoichiometric ratio. This deviation is the main reason for the formation of a large point defect density, which consists of Li vacancies (VLi), S vacancies (VS), and In−Li antisite defects (InLi). These defects result in optical absorption and scattering, both of which seriously affect the quality of the resulting LIS optical devices. On the other hand, postgrowth heat treatment is an effective strategy to reduce thermal stress and also some defects in LIS crystals. To date, a lot of research has been reported on the crystal growth and properties of LIS.3,11−19 However, there is little research reported on postgrowth annealing processes and their effect on the optical and electrical properties of LIS crystals. In this article, we report on the crystal growth, postgrowth annealing, and the effects of annealing on the optical and electrical properties of LIS crystals. Large-sized LIS crystals with a diameter of 16 mm and a length of 50 mm were grown by the modified Bridgman method using the accelerated crucible rotation technique (ACRT). A series of postgrowth annealing of the LIS crystals were carried out under different Received: August 1, 2014 Revised: August 30, 2014 Published: September 25, 2014 5957

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for different time durations (240, 160, and 160 h, respectively) in evacuated sealed ampules, which are 30 mm in inner diameter and 150 mm in length (approximately 110 cm3 in volume). In order to make sure that the annealing procedure can be reproduced, the same amount of 1.5 g atmosphere source (S, LIS) was placed in the ampule during each experiment. The LIS source was obtained by grinding LIS crystal into the fine powder which was characterized by ICP-AES to be the composition of Li/In/S = 0.84:1:1.85.20 All quartz ampules with LIS were evacuated to 1.0 × 10−3 Pa to avoid oxidation during annealing. After annealing, the crystals were cooled down to room temperature at a rate of 50 °C/hour. For optical properties measurements, both sides of the LIS sample were carefully polished before and after annealing. Figure 2 shows annealed LIS samples that

conditions (in vacuum, in S vapor, or in LIS vapor atmosphere; at 700 or 800 °C; for 240 or 160 h). Before and after postgrowth annealing, the LIS crystals were investigated by measuring the visible-near-infrared (vis-NIR) and MIR transmission spectra and the electrical properties. The results indicate that proper postgrowth annealing is effective in reducing point defects and stress in LIS crystals and therefore significantly improve the optical quality of the crystals.

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EXPERIMENTAL SECTION

Crystal Growth. Polycrystalline LIS was synthesized from high purity elemental Li (3N), In (5N), and S (5N) using a high temperature autoclave method (HTAM). Special attention was paid to the ratio between the components. Weight losses of Li and S were caused by the volatilization and the interaction between the melt and the container walls during synthesis. Therefore, a correction to the ideal stoichiometric ratio for Li/In/S (1:1:2) was performed to give a final result of 1.02/1/2.05. The charge was loaded into a quartz ampule, which was evacuated to 1.0 × 10−3 Pa and then sealed off. The synthesis furnace was a single-temperature-zone coil furnace, which was controlled by an FP23 temperature controller (SHIMANDEN). After a step-by-step heating process to finally maintain the temperature at 1100 °C for 50 h, the furnace was slowly cooled down to room temperature. Single-phase polycrystalline LIS was obtained. LIS crystals were grown by a modified Bridgman method using the accelerated crucible rotation technique (ACRT) with the synthesized polycrystalline LIS as starting material. The charge was put in a graphite crucible with a specially polished inner wall, and the crucible was then placed inside a quartz ampule, which had a carbon-coated inner wall. The ampule was evacuated to 1.0 × 10−3 Pa and then sealed. Crystal growth started from self-nucleation at the cone-shaped tip at the bottom of the special crucible. The axial temperature gradient at the growth interface was 15−20 °C/cm. The translation speed of the growth ampule is 0.5−1 mm/h. After the charged crucible had completely translated down to the low temperature zone, the growth was completed. The furnace was then cooled to room temperature at a rate of 30 °C/h. The as-grown high integrity LIS crystals are depicted in Figure 1. Postgrowth Annealing. The LIS samples that were tested for annealing were all cut in thickness of 1.5 mm from the same as-grown LIS boule. Annealing was performed in vacuum, S vapor, and LIS vapor at various temperatures (800, 700, and 700 °C, respectively) and

Figure 2. Photos of LIS crystals annealed in (a) vacuum, (b) S vapor, and (c) LIS vapor. All LIS crystals are 1.5 mm in thickness. exhibit different colors. Sample (a) was annealed in vacuum at 800 °C for 240 h, sample (b) in S vapor at 700 °C for 160 h, and sample (c) in LIS vapor at 700 °C for 160 h. Characterizations. The optical transmittance of LIS wafers was measured both before and after annealing with a vis-NIR spectrophotometer (HITACHI U-3500) and a Fourier transform infrared (FTIR) spectrophotometer (NEXUS670). All LIS samples were cut to 2 mm in thickness and polished on two opposite sides. Electrical properties were measured using an Agilent 4294A impedance analyzer. Both sides of the LIS wafer sample were sputtered with gold so as to form electrodes. The variation of the dielectric constant and the dielectric loss with temperature were measured over the temperature range of 20 to 460 °C at frequencies of 1 kHz, 10 kHz, 100 kHz, and 1 MHz. The laser damage threshold of the LIS crystal was measured using a Q-switched Nd:YAG 1064 nm laser (LABest optronics company’s Sunlight type 200), which was operated with a 10 ns pulse width at a frequency of 1 Hz. The 1/e2 beam diameter was 0.4 mm, and M2 was less than 1.5. Surface damage was observed with an optical microscope during the measurement process.

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RESULTS AND DISCUSSION IR Spectrum Analysis. Figures 3 and 4 present the transmittance spectra of the samples annealed in (a) vacuum, (b) LIS vapor, (c) sulfur vapor, and (d) as-grown LIS. The Fresnel losses shown in Figure 3e were calculated from the Sellmeier equations by supposing zero absorption and multiple reflections. One can see from the transmittance spectra that the transparency range covers from 0.35 to 13.1 μm, which corresponds to the zero transmission at short and long wavelength cutoff edges, respectively. The window from 800 nm to 10 μm is a high transmission region without obvious absorption peaks. Absorption at λ < 0.35 μm is due to fundamental band edge absorption, whereas the region at λ > 13.1 μm is associated with multiphonon absorption. The short and long cutoff edges do not change after annealing, which suggests that the band gap and phonon frequencies of LIS do not change after annealing. From the vis−NIR transmission spectra of the annealed and as-grown LIS samples that are shown in Figure 3, one can see

Figure 1. Photo of large as-grown LIS crystals with diameter of 16 mm and length of 50 mm. 5958

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the LIS crystal grown in our lab has a lithium deficit.20 While for the crystal annealed in LIS vapor, the sulfur vacancies (Vs), lithium vacancies (VLi), and/or antisite InLi, can supposedly be reduced as resulting in the transmission increase. Figure 4 indicates that the transmittance of LIS crystals annealed in LIS or S vapor is higher than that of the as-grown sample. Furthermore, the transmittance increase of the sample annealed in LIS vapor is greater than that of the sample annealed in S vapor. It can clearly be seen that the MIR transmittance is improved by nearly 10% for the sample annealed in LIS vapor. The transmittance increase of the crystals after annealing may be due to the following two reasons. First is the reduction of thermal stress in the crystals and second is the decrease in number of the scattering particles or point defects to some extent. Annealing in LIS vapor supplies both Li and S to compensate for the deficits in lithium and sulfur in order to approach stoichiometry in the sample. While annealing in S vapor alone can only compensate for the sulfur deficit in the sample. For the crystal annealed in vacuum, some S or Li from the compound LIS will be released under counter-pressure and result in a Li and/or S deficit. Therefore, transmission will be significantly decreased after annealing in vacuum. In conclusion, annealing in LIS vapor is the most effective postgrowth heat treatment that can be used to improve LIS transmittance. Annealing in S and LIS vapor produces a weak absorption band at 8−9 μm, while there are no such absorption bands in the asgrown and vacuum annealed sample. This observation may be associated with S−S vibration absorption because some S atoms may occupy interstitial positions or cation (Li or In) sites after annealing in S and LIS vapor. Fortunately, the weak absorptions do not affect the application of LIS in the range of 3−8 μm. Electrical Properties Analysis. The dielectric constants can be obtained from values of sample capacitances by the equation: C = εrA/ε0t. A is the area of the plates, t is the thickness of crystal sample, and εr and ε0 (8.85 × 10−12 F·m−1) are the absolute and relative dielectric constants, respectively. We measured the dielectric constants and dielectric loss of asgrown and annealed LIS at different electric field frequencies. To determine the dependence of the dielectric permittivity and dielectric loss on temperature, their variations are plotted as a function of temperature at different electric field frequencies (i.e., 1 kHz, 10 kHz, 100 kHz, and 1 MHz). From Figure 5a,b, at any frequency, it is clear that the relative dielectric permittivity and dielectric loss do not increase with temperature over the range of 20−200 °C. Then the dielectric permittivity and dielectric loss both rapidly increase up to 450 and 400 °C, respectively. The relative dielectric permittivity and dielectric loss were measured to be 15.6 and 0.021, respectively, at room temperature (20 °C) at a frequency of 1 MHz. The dielectric permittivity of a crystal is due to contributions from the electronic, ionic, dipole, and space charge polarizations. At low frequencies and high temperatures, all these polarizations are active, and space charge polarization is generally active. Therefore, the increase in the dielectric permittivity is greater with a lower frequency electric field at high temperatures. Dielectric loss is the energy loss in an electrically insulating material. Materials with low dielectric loss dissipate a relatively small amount of heat when subjected to an oscillating electric field. As shown in Figure 5b, the dielectric loss is not appreciable for frequencies above 100 kHz. However, we do not observe a ferroelectric−paraelectric transition with increasing temperature. The peaks in the relative dielectric

Figure 3. Vis-NIR transmittance spectra of LIS crystals annealed in (a) vacuum, (b) LIS vapor, (c) S vapor, and (d) as-grown LIS crystal; (e) theoretically calculated maximum transmittance obtained by supposing zero absorption and multiple reflections.

Figure 4. MIR transmittance spectra of LIS crystals annealed in (a) vacuum, (b) LIS vapor, (c) S vapor, and (d) as-grown LIS crystal; (e) theoretically calculated maximum transmittance obtained by supposing zero absorption and multiple reflections.

that the transmission of all the crystals at λ > 0.35 μm slowly increases as the wavelength increases, due to strong Rayleigh scattering by submicron inclusions of side phases or defects. The Rayleigh scattering cross section can be written as σ ≈ λ−4r6(m2 − 1)2/(m2 + 2)2, where λ is the wavelength, r is the particle radius with r ≪ λ, and m = npar/nmed is the ratio of the refractive index of the scattering particles to that of the surrounding medium.21 From the Rayleigh equation, the Rayleigh cross-section is clearly proportional to λ−4, and the scattering is maximal at short wavelengths and decreases toward the MIR region. One can see from Figure 3 that the transmission of the LIS sample annealed in LIS vapor increases, whereas the LIS sample annealed in vacuum or sulfur vapor decreases, especially the sample annealed in vacuum. It is understandable that some amount of sulfur atoms in the LIS structure will be released when annealed in vacuum due to the pumping effect under counter-pressure. The sulfur deficit in the LIS crystal results in sulfur vacancies, which in turn give rise to the transmittance decrease. As reported in our previous work, 5959

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crystal, although the two diffuse dielectric anomalies still exist, the dielectric peaks are strongly suppressed. The weakened dielectric anomalies may be related to an obvious decrease of VS and VLi defects, which indicates that annealing LIS crystals in LIS vapor should be an effective postgrowth treatment to improve the crystal quality. Damage Threshold Measurement. The high laser damage threshold of NLO crystals is a very important parameter for their practical applications. In this work, the laser damage threshold of LIS before and after annealing was studied. The schematic setup for damage threshold measurement is shown in Figure 6. LIS samples used for damage threshold measurements were polished on two sides before and after annealing. The experimental results are listed in Table 1.

Figure 6. Schematic setup for laser damage threshold measurements.

Table 1. Optical and Thermal Properties of Several Important MIR Crystals

crystal as-grown LIS3 annealed LIS AGS27,28

Figure 5. (a) Relative dielectric permittivity and (b) dielectric loss as a function of temperature at different frequencies for LIS before and after annealing.

transparency range (μm) 0.4−13.1 0.4−13.1 0.47−13

NLO coefficient (pm/V) 16 (±4) (d33) 16 (±4) (d33) 13 (d14)

thermal conductivity at 300 K (W/m K)

laser damage threshold at 1064 nm (J/cm2)

7.6⊥(001)

0.9 (±0.05)

7.8⊥(001)

1.2 (±0.05)

1.5⊥(001)

0.4

The lattice vibrational frequency and Debye temperature of LIS crystal are much larger than that of commercially available AgGaS2 crystals, which can be attributed to the smaller ionic radius and lower mass of the lithium ions. Therefore, the LIS crystal has a high damage threshold, which makes it promising for pumping with high power lasers. From Table 1, one can see that the damage threshold of annealed LIS is higher than that of as-grown LIS, which suggests that the quality of LIS is improved after proper annealing.

permittivity and dielectric loss at higher temperatures can be attributed to dielectric relaxation.22,23 The effect of annealing on the relative dielectric permittivity and dielectric loss was investigated under different frequency electric fields over the temperature range of 20−460 °C. In Figure5a, at a frequency of 1 kHz at the same temperature, the relative dielectric permittivity decreases significantly, and the dielectric relaxation peak almost disappears after annealing. It is suggested that the defects in the LIS crystal are gradually annealed out, producing a large number of saturated bonds. The reduction of defects gives rise to a decrease of the localized density of states in the band structure and consequently increases the activation energy. In Figure 5b, it can be seen that the as-grown LIS crystal exhibits two diffuse dielectric anomalies in the temperature range of 200−470 °C. One dielectric anomaly appeared at around 300 °C and the other one at around 400 °C. Since LIS is not ferroelectric and possesses no structural phase transitions in the measured temperature range, its diffuse dielectric anomalies cannot be related to the diffuse phase transitions.24 Considering the possible existence of sulfur vacancy (VS) and lithium vacancy (VLi) in the as-grown LIS crystal, we propose that the two dielectric anomalies may be ascribed to the thermally activated VS and VLi, respectively.25,26 Thus, the amplitude of dielectric peak, which must have a certain relationship with the density of vacancies, should be noticed.25 In the case of annealed LIS

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CONCLUSIONS Large LIS crystals with a diameter of 16 mm and a length of 50 mm were grown by the modified Bridgman method. A series of postgrowth annealing processes were carried out in different annealing atmospheres, at different temperatures, and for different time durations. The optimal annealing parameters for LIS crystals grown in our lab employed a LIS vapor atmosphere at 700 °C for 160 h. After annealing, the NIR and MIR transmission of the LIS crystals was improved. When the applied electric field frequency is set at 1 kHz, the dielectric relaxation peak almost completely disappears after annealing. For an applied electric field frequency of 1 and 10 kHz, the dielectric loss obviously decreases, and the peak in the dielectric loss shifts to higher temperatures. The damage threshold of the annealed LIS crystal measured with a 10 ns, 1 Hz, Q-switched 1064 nm Nd:YAG laser is 1.2 J/cm2, which is 0.3 J/cm2 higher than that of the as-grown LIS crystal. These results indicate that 5960

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(19) Wang, S.; Liang, Q.; Tao, X.; Dekorsy, T. Opt. Mater. Express 2014, 4, 575−586. (20) Wang, S.; Tao, X.; Liu, G.; Dong, C.; Jiang, M. J. Synth. Cryst. 2009, 38, 851−855. (21) Cox, A. J.; DeWeerd, A. J.; Linden, J. Am. J. Phys. 2002, 70, 620−625. (22) Szwagierczak, D.; Kulawik, J. J. Eur. Ceram. Soc. 2005, 25, 1657− 1662. (23) Zhou, L.; Vilarinho, P. M.; Baptista, J. L. J. Appl. Phys. 1999, 85, 2312−2317. (24) Kuwabara, M.; Goda, K.; Oshima, K. Phys. Rev. B 1990, 42, 10012−10015. (25) Bidault, O.; Goux, P.; Kchikech, M.; Belkaoumi, M.; Maglione, M. Phys. Rev. B 1994, 49, 7868−7873. (26) Kang, B. S.; Choi, S. K.; Park, C. H. J. Appl. Phys. 2003, 94, 1904−1911. (27) Lin, X.; Zhang, G.; Ye, N. Cryst. Growth Des. 2008, 9, 1186− 1189. (28) Dmitriev, V. G.; Gurzadyan, G. G.; Nikogosyan, D. N. Handbook of Nonlinear Optical Crystals; Springer: New York, 1999; Vol. 64.

proper postgrowth annealing is effective in reducing point defects and stress in the crystals and thus significantly improves the optical quality of the LIS crystals.

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AUTHOR INFORMATION

Corresponding Author

*(X.-T.T.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We gratefully acknowledge the financial support from the State National Natural Science Foundation of China (Grant No. 51321091, 51272129, 50802054, and 51227002) and the 973 Program of the People’s Republic of China (Grant No. 2010CB630702). Also thanks to Shandong Provincial Natural Science Foundation, China (ZR2010EQ010). Thanks are extended to Prof. Boughton for his help in revising the manuscript.

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