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Journal of Crystal Growth 311 (2008) 190–193

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

�Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Growth and characterization of ZnGeP2 single crystals by the modifiedBridgman method

Xin Zhao, Shifu Zhu �, Beijun Zhao, Baojun Chen, Zhiyu He, Ruilin Wang, Huiguang Yang,Yongqiang Sun, Jiang Cheng

Department of Materials Science, Sichuan University, Chengdu 610064, China

a r t i c l e i n f o

Article history:

Received 8 July 2008

Received in revised form

26 August 2008

Accepted 26 August 2008

Communicated by M. Schieberoscillation method (STZMOM). The as-grown single crystal was characterized by various methods,

Available online 17 October 2008

PACS:

81.10.Fq

42.70.Mp

61.05.Cp

81.70.Fy

Keywords:

A1. Characterization

A2. Single crystal growth

A2. Bridgman technique

B2. Semiconducting ternary compounds

B2. Nonlinear optic materials

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.jcrysgro.2008.08.065

esponding author. Fax: +86 28 85412745.

ail address: sfzhu@scu.edu.cn (S. Zhu).

a b s t r a c t

A good quality ZnGeP2 (ZGP) single crystal 15 mm in diameter and 40 mm in length was grown in a

vertical three-zone tubular furnace by the modified Bridgman method, i.e. real-time temperature

compensation technique (RTTCT) with descending ampoule. The starting material is high-pure and

single-phase polycrystal of ZnGeP2 synthesized by the single-temperature zone and mechanical

including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analyzer of X-

ray (EDAX), and IR and UV spectrophotometers. It is found that there is a cleavage face of (11 2) and

second-order XRD peaks of the {11 2} faces are observed, the IR transmittance of a sample of 2 mm

thickness is above 55% in the range from 6000 to 800 cm�1, the absorption edge is near 612.5 nm and

the band gap is about 2.02 eV. The absorption coefficient (a) is within 0.015–0.022 cm�1 at the spectral

region 2–8mm. The crystal has a stoichiometry ratio Zn:Ge:P=1:1.12:1.91 which is close to the ideal

stoichiometry ratio of 1:1:2. All results demonstrate that the modified growth method is a new and

promising method for the ZGP single crystal and the quality of as-grown crystal is good.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

ZnGeP2 (ZGP) has been known for a few decades as one of themost promising infrared nonlinear optical materials, which is aternary chalcopyrite that crystallizes in the tetragonal space groupI4̄2d [1,2]. It is widely used for nonlinear optical devices due to itsunique optical properties, including large nonlinear coefficient(d36=75 pm/V), wide transparency range (0.7–12mm) and highthermal conductivity (0.35 W cm�1 K�1) combined with an ex-tended phase matching range in the infrared [3–5]. However, wideapplication of ZGP crystals has been limited for years due to thelack of high-quality crack-free single crystals. There are severalmain factors to affect high-quality ZGP crystal growth. Firstly, thevapor pressure of phosphorus is very high above its melting point,so the ampoule is very prone to explode in the synthesis processof polycrystalline ZGP. Secondly, ZGP contains toxic and volatilecomponents with high vapor pressure at the melting point(1027 1C) [6], which can lead to deviations from stoichiometry.

ll rights reserved.

Thirdly, it is susceptible to stress-induced twinning and crackingduring crystal growth process [7,8]. Therefore, it is a challengingwork to obtain high-quality ZGP single crystals. In recent years,considerable effort was focused on the growth of high-qualitysingle crystals of this material. In order to overcome someproblems in ZGP crystals growth, the single crystals are grownby various methods such as the vertical Bridgman (VB) [9],vertical gradient freezing (VGF) [10], horizontal gradient freezing(HGF) [11], liquid encapsulated Czochralski (LEC) [12], high-pressure vapor transport (HPVT) [13] methods, etc. For thepresent, the most conventional techniques for the growth of ZGPsingle crystals remain the VB and HGF methods [14–16]. All thesetechniques have some shortcomings, however, such as inhomo-geneity of the temperature field, large thermal stress in thecrystals, high-density dislocations, twins, cracks and second phaseprecipitates. In addition to growth techniques, the poor quality ofthe starting polycrystalline materials is also an obstacle to high-quality single crystal growth.

In order to overcome the above shortcomings, our presentwork is focused on modifying the growth procedures for ZGPsingle crystal. The single-temperature zone and mechanicaloscillation method (STZMOM) was used to synthesize high-purity

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X. Zhao et al. / Journal of Crystal Growth 311 (2008) 190–193 191

and single-phase ZGP polycrystals that are free of voids. The real-time temperature compensation technique (RTTCT) was used togrow high-quality ZGP single crystals in a vertical three-zonetubular furnace. A crack-free ZGP single crystal 15 mm in diameterand 40 mm in length was grown by the modified verticalBridgman method (i.e. RTTCT). The quality of the as-grown crystalwas characterized using X-ray diffraction (XRD), scanning electronmicroscopy (SEM), energy dispersive analyzer of X-ray (EDAX), IRspectrophotometer, etc. All results demonstrate that the modifiedgrowth method for ZGP single crystals is promising and thequality of the as-grown crystal is good.

Fig. 1. XRD spectrum of polycrystalline ZGP (l (Cu Ka)=0.154184 nm).

2. Synthesis of polycrystalline ZGP

Polycrystalline ZGP materials are not yet available commer-cially, thus the starting materials were synthesized by us.Presently, there are two methods to synthesize ZGP polycrystals:(a) one-temperature zone method and (b) two-temperature zonemethod. In the latter it is much more difficult to fill startingmaterials into the two ends of the ampoule, but the conventionalone-temperature zone method is also not suitable for thesynthesis of polycrystalline ZGP because of the presence ofvolatile components with high vapor pressure. In our laboratory,therefore, the one-temperature zone method was modified byadding mechanical oscillation and adjusting final cooling means.Single-phase ZGP polycrystalline material for the single crystalgrowth was successfully synthesized using STZMOM, directlyfrom high-pure (6N) Zn, Ge, and red P according to thestoichiometry of ZGP with the excess of 0.2% P-rich. For thepolycrystalline synthesis process, the weighed raw materials wereloaded into a quartz ampoule which was sealed at less than1.5�10�3 Pa. Then the ampoule was placed in a horizontalfurnace which can be oscillated. The heating program wasdesigned to ensure the completion of the intermediate reactionsin order to avoid excessive vapor pressure associated with freephosphorus and zinc. The furnace’s temperature was raised asfollowing different steps: (1) raised to 530 1C at a rate of 50 1C/hand held there for 6 h; (2) raised to 750 1C at a rate of 6 1C/h andthen further raised to 950 1C at a rate of 20 1C/h and held there for5h; (3) raised to 1060 1C at a rate of 50 1C/h and then held there for30 h. Under the constant temperature of 1060 1C, mechanicaloscillation was used to make the raw materials mix uniformly andreact completely. In the cooling process of the melt, in order toseparate the residual phosphorus from the synthetic productsavailable, a modified cooling means, i.e. the cooling rate of themelt zone is less than that of the other zone, was used, which canmake the residual P vapor in the melt zone transport to the otherzone of ampoule owing to the driving force caused by the highvapor pressure difference. A polycrystalline ZGP ingot wassuccessfully obtained and was of gray–black appearance withmany shining faces.

The ingot was characterized by X-ray powder diffractiontechnique, the diffraction spectra are shown in Fig. 1. Thediffraction peaks are in agreement with the Powder DiffractionFiles (PDF no: 73-0398) of ZGP. The lattice constants werea=b=0.54628 nm, c=1.07069 nm, respectively, by calculation. Thisresult demonstrates that polycrystalline material synthesized bySTZMOM is high-purity single phase of ZGP, which can be used forthe single crystal growth.

3. Crystal growth

The crystal growth was carried out in a vertical three-zonetubular electric furnace by a modified Bridgman method, i.e.,

RTTCT with descending ampoule. FP93-type temperature-con-trollers made by Shimaden Co. Ltd. were used for controllingtemperature. For the crystal growth process, the ampoule withZGP polycrystalline materials was evacuated at room temperaturecontinuously for 3 h and sealed under 1.33�10�3 Pa and thenloaded into the upper region of the furnace. The furnacetemperature of the upper, middle and lower regions was slowlyincreased to 1080, 1050 and 900 1C, respectively, and then heldthere for 24 h in all. The temperature gradients in the growthregion were controlled at 12 1C/cm for the empty furnace and at10 1C/cm in the presence of growth ampoule, because low thermalgradients could minimize thermal stresses associated with theanisotropic thermal expansion of ZnGeP2. Then the crystal growthwas started with nucleation. The ampoule was mechanicallydescended at a rate of 0.2 mm/h during the growth process.Accompanied with the crystal growth, it was found from themonitor that the temperature of the crystallization was descend-ing gradually. It indicated that the solid–liquid interface of thecrystal growth was already removed while the temperaturedescending gradually. This phenomenon would greatly affect thequality of the grown crystal. Thus, RTTCT was used to preventremoving of the solid–liquid interface in the process of ZGP crystalgrowth. The result showed that the descending of the crystal-lization temperature was stopped effectively, the solid–liquidinterface of the crystal growth was steadied, which was good forthe quality of grown crystal.

When the solidification was completed, the crystal was slowlycooled to room temperature at the rate of 30–50 1C/h. A crack-freecrystal of ZGP 15 mm in diameter and 40 mm in length wassuccessfully obtained. The photograph of as-grown crystal isshown in Fig. 2.

4. Characterization of the crystal

4.1. XRD analysis

As-grown ZGP crystal was cleaved along the appearanceface by a blade. It was found to have a cleavage face, and XRDanalysis showed the cleavage face was the {11 2}. The XRDspectrum is shown in Fig. 3, from which it can be seen thatmultiple-diffraction peaks of the {112} face have presentedthemselves. The XRD rocking curve of the (11 2) face is shownin Fig. 4, as it can be seen the intensity of the diffraction peak is

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Fig. 2. Photograph of as-grown ZGP crystal.

Fig. 3. XRD spectrum of the {11 2} faces.

Fig. 4. XRD rocking curve of the (112) face.

Fig. 5. SEM image of growth steps on the cleavage face of (11 2).

Fig. 6. Photograph of a ZGP wafer.

X. Zhao et al. / Journal of Crystal Growth 311 (2008) 190–193192

high and the shape of the peak has good symmetry. These resultsdemonstrate that the crystallinity of the as-grown ZGP crystal isgood.

4.2. Observation and analysis on the cleavage face of (11 2)

The shape of cleavage face on the (112) of as-grown ZGPcrystal was observed by SEM and the image is shown in Fig. 5. Asit is seen from the image that there are growth steps on the (112),which are parallel to each other and the intervals are about thesame. The stoichiometry of the crystal was determined by EDAX.The result shows that the crystal has a stoichiometry ratioZn:Ge:P=1:1.12:1.91, which is close to the ideal stoichiometry ratioof 1:1:2. The above-mentioned characterizations demonstratethat the grown crystal is more homogeneous.

4.3. IR and vis–NIR transmission tests

A wafer 6�7�2 mm3 cut from the middle of as-grown ZGPcrystal was annealed for 100 h at 550 1C in ZGP powder. Aftercoarse grinding and polishing, it was used for IR and vis–NIRtransmission test. The photograph of a ZGP wafer is shown inFig. 6. IR transmission spectrum recorded by IRPrestige-21spectrophotometer is shown in Fig. 7. It is found from thespectra that the infrared transmission is above 55% in the regionfrom 6000 to 800 cm�1. Vis–NIR spectrum recorded by UV-1700spectrophotometer is shown in Fig. 8. The result shows that theabsorption edge of the crystal is near by 612.5 nm and calculated

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Fig. 7. IR transmission spectrum of as-grown ZGP crystal.

Fig. 8. Vis–NIR transmission spectrum of as-grown ZGP crystal.

Table 1Absorption coefficients of as-grown ZGP crystal at different wavelengths

Wavelength l (mm) Absorption coefficient a (cm�1)

Experimental Reported [18]

1 1.63

1.06 1.55 1.06–1.52

2 0.015 0.15

2.05 0.017 0.09–0.26

4 0.007 0.05–0.10

6 0.013 0.1

8 0.022 0.15

10 0.133 0.45

10.6 0.298 0.65–0.9

X. Zhao et al. / Journal of Crystal Growth 311 (2008) 190–193 193

band gap is about 2.02 eV. The following formula was used toobtain the value of the absorption coefficient:

a ¼ �1

Lln

ð1� RÞ2

2TR2

" #2

þ1

R2

8<:

9=;

1=2

�ð1� RÞ2

2TR2

where L is the thickness of the sample, T the transmission, andR=(n�1)2/(n+1)2 the Fresnel power reflection coefficient of ZGP.

The refractive indices for the various pump wavelengths werederived from G. Ghosh’s Sellmeier coefficients [17]. Someexperimental values and reported values of absorptioncoefficient (a) of ZGP crystal are shown in Table 1, from which itcan be seen that absorption coefficients (a) of as-grown ZGPcrystal is within 0.015–0.022 cm�1 at the spectral region 2–8mmand less than the values of earlier reports in Ref. [18]. All resultsfrom IR and vis–NIR transmission tests show that transparency ofthe as-grown ZGP crystal is high at the mid-IR and NIRwavelengths and the transparency range is about 0.65–12.5mm.

5. Conclusion

STZMOM is an effective and convenient technique to synthe-size high-pure and single-phase polycrystalline ZGP materials.Modified Bridgman method, i.e., real-time temperature compen-sation technique (RTTCT) with descending ampoule, is a new andpromising growth method for high-quality ZGP single crystals.Using the polycrystalline materials synthesized by STZMOM, agood quality ZGP single crystal with size of F15 mm�40 mm hasbeen grown by the modified new technique in a three tempera-ture-zone tubular furnace. As-grown crystal was characterized bydifferent methods, such as XRD, IR and UV spectrophotometer,SEM, etc. The results show that there is a cleavage face of (11 2)and the second-order XRD peaks have presented themselves onthe {112} face, IR transmittance of the crystal is above 55% in theregion from 6000 to 800 cm�1, the absorption coefficient (a) iswithin 0.015–0.022 cm�1 at 2–8mm. Absorption edge of thecrystal is near by 612.5 nm and calculated band gap is about2.02 eV. The characterization results demonstrate that quality ofas-grown ZGP crystal is good. It is acceptable for the fabrication ofthe infrared nonlinear optical devices.

Acknowledgements

This work is supported by the National Natural ScienceFoundation General and Key Programs of China (Nos. 50672061and 50732005) and the 863 High-Tech program of China (No.2007AA03Z443).

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