The ε-GaSe single crystal is an attractive nonlinear optical crystal. Compared to other infrared nonlinear optical crystals, it is characterized by large nonlinear coefficient ( d₂₂=70-80 pm/V), lower absorption coefficient and large birefringence (0.35), wide transparency range (0.6- 18 μm), which determines its potential applications in broader tunable output laser^{[ 1]}. GaSe crystals have been used for generating tunable coherent laser in the range of 3-24 μm by Optical Parametric Oscillator (OPO)^{[ 2]} and have been used for Different Frequency Generation (DFG) in the ranges of 2.7-28.7 μm (type-I PM) and 4.14-28 μm (type-II PM)^{[ 3]}.

However, pure GaSe crystal is not suitable for practical frequency conversion systems because of very low hardness. To avoid this shortcoming, many GaSe-based solid solutions were grown and tested for optical properties^{[ 4, 5, 6]}. Fortunately, the mechanical property of pure GaSe crystal can be improved by doping other elements without destroying its optical properties. To our knowledge, the vertical Bridgman method is an effective approach to produce ε-GaSe single crystal. However, it is difficult to obtain single crystal with good quality, because of the strong temperature oscillation near the growth interface^{[ 7]} and the cracking of crystals as growth of large laminar pure and doped GaSe crystals^{[ 8]}. What’s more, the high vapor pressure and volatile component may lead to deviation from stoichiometry. Apparently, improving growth technology of pure GaSe crystal could provide a guide for growth of GaSe-based solid solutions. The single GaSe crystals had been produced by the high-pressure vertical zone melting technique^{[ 9]} and the temperature difference method under controlled vapor pressure (TDM-CVP) technique^{[ 10]}. But these methods are complex and costly due to their demanding on furnace or design of crucible. In addition, a conventional Bridgman method using seed aided technique has been used for producing GaSe single crystal in our lab^{[ 11]}. Unfortunately, the repeatability is very poor because of the temperature oscillation and the cracking of crystal. In the present work we report a modified Bridgman method for growth of large size GaSe crystal together with its structural and optical performance characteristics.

The high purity (99.9999%) Ga and Se were prepared in the stoichiometric amounts and the GaSe compound was synthesized by two zone horizontal furnace. After evacuation to 10^-4Pa and sealing, the carbon-coated ampoule was placed in the furnace with Ga in hot zone and Se in cold zone. The hot zone was raised to 680℃ at a rate of 15 ℃/h, and then the cold zone reached 500℃. After maintained for about 15 h, both of zones were further raised to 970℃ at a rate of 25 ℃/h and maintained for 10 h. Finally, the furnace was cooled to room temperature at a rate of 30 ℃/h, and the synthesized alloy ingot was taken out carefully. Then, single crystal was grown by the modified Bridgman method with spontaneous nucleation.

The furnace device is shown in Fig. 1. There are two aspects of the modified Bridgman method. Firstly, a sealed and unvacuum quartz tube is set in carbon-coated ampoule which is evacuated and sealed. When the temperature is gradually raised to melting point, the quartz tube will expand and limit the space occupied by melt as small as possible. By limiting the free space of melt, the partial pressures of Se vapor will increase, thus it restrains volatilization of Se from GaSe melt effectively. Secondly, a controllable temperature field is designed by using different materials of furnace-tubes and multiple-hearth furnace. At the hot and cold zone, the high thermal conductivity carborundum furnace-tubes are used for getting uniform temperature distribution. The adiabatic zone is composed by a mullite furnace-tube of lower thermal conductivity. As shown in Fig. 1, the long carborundum furnace-tube located in hot zone is winded by two pieces of resistive heater which could be used for lengthening hot zone and getting uniform temperature field. The carborundum furnace-tube located in cold zone could build a small temperature gradient which could reduce the cracking of crystals. The mullite furnace-tube located in adiabatic zone could be used for producing a large temperature gradient. Although the large Prandtl number (2.78) of GaSe melt lead to strong convection in melt^{[ 12]}, we can only weaken the convection of melt by decrease the Grash of number which can be achieved by adjusting the temperature distribution in the hot and adiabatic zone. The temperature gradient in the mullite furnace-tubes was about 20 ℃/cm and the growth speed was 1 cm/d during the growth of GaSe. When the growth was completed, as-grown crystal was cooled at a rate of 30 ℃/h. Through this method, a crack-free crystal of GaSe 19 mm in diameter and 65 mm in length was successfully obtained, and then cleaved and cut into an oblong shape. Fig. 2 shows the boule of GaSe grown in our lab and some samples cleaved along (001) plane.

Fig. 1

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	Fig. 1 The modified Bridgman furnace device

Fig. 2

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	Fig. 2 The photograph of as-grown GaSe boule and samples

To identify the elemental component of as-grown crystal, a sample cleaved from the crystal was test with 9 independent points of 3×3 matrix-arranged distribution in an area of 1 cm² by an energy dispersive spectrometer (EDS). Figure 3 displays the energy dispersive X-ray spectrum (EDXS) from which we cannot see any other spectral peaks except Ga and Se, indicating fine purity of the product. Besides, the average atomic ratio of Ga to Se is 50.65:49.35, close to the stoichiometric proportion. It should be noted that the tiny deviation of constituents may be from the volatilization of Se in the synthesis and growth processes. A small piece of as-grown crystal was grinded into powder and its X-ray powder diffraction data were recorded by an χ’Pert Pro MPD Diffractometer (Cu Kα, 0.15406 nm). The 2 θ ranged from 10° to 70°, and the scan step was 0.02°/s at room temperature. The diffraction patterns (Fig. 4) and lattice parameters ( a= b= (0.3755±0.0001) nm, c=(1.5928±0.0077) nm) are very close to the standard PDF card data No. JCPDS 37-0931. The (001) plane was cleaved for single crystal XRD measurement. Figure 5 shows the XRD rocking curve of (002) face which displays the high and sharp diffraction peak. The basically symmetrical and sharp peak illustrates good quality of as-grown crystals.

Finally, the IR transmittance spectra (0.65-16 μm) of GaSe sample were measured by FT-IR. Figure 6 shows the high transmittance and wide transparent spectral range of GaSe sample. The optical absorption coefficient was calculated by the equation in Ref. [13]. The absorption coefficient is as low as 0.2 cm^-1 in the range of 0.9-14 μm, indicating the perfect optical quality of GaSe sample.

Fig. 3

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	Fig. 3 EDX spectrum of a GaSe sample

Fig. 4

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	Fig. 4 X-ray diffraction patterns of GaSe powder

Fig. 5

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	Fig. 5 XRD rocking curve of the (002) face

Fig. 6

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	Fig. 6 The measurement of infrared transmittance spectrum

The GaSe single crystal in φ19 mm×65 mm size was grown by modified vertical Bridgman method. The phase purity and high crystallinity were confirmed by powder XRD and single crystal XRD, respectively. The FT-IR measurements indicated wide transparency over the spectral range (0.65-16 μm) and low absorption coefficient (<0.3 cm^-1) in the grown GaSe crystal. The results show that the GaSe crystal with volatile component and large Prandtl number can be successfully grown by the modified vertical Bridgman method. The GaSe single crystal with good structural and optical properties can be used for nonlinear optical frequency conversion. It is noted that this modified vertical Bridgman method may be suited for growth of other semiconductors with the same properties as GaSe single crystal.