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Growth of ZnTe single crystals from Te solution by vertical Bridgman method with ACRT
Journal of Crystal Growth 400 (2014) 27–33
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of ZnTe single crystals from Te solution by vertical Bridgman method with ACRT Rui Yang, Wanqi Jie n, Hang Liu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2013 Received in revised form 11 March 2014 Accepted 30 March 2014 Communicated by: A. Burger Available online 5 April 2014
ZnTe ingots 30 mm in diameter and about 60 mm in length were grown from Te solution based on vertical Bridgman method with accelerated crucible rotation technique (ACRT). The composition homogeneity and the optical and electrical properties of the crystals were investigated using optical transmission microscope, energy dispersive spectrometer (EDS), scanning electron microscope (SEM), electron probe micro-analyzer (EPMA), ultraviolet–visible (UV–Vis) spectrophotometer, Fourier transform infrared (FT-IR) spectrometer and digital electrometer. The growth method was found efficient for mass transfer, and produced large size single crystal with the volume up to 10 mm 10 mm 40 mm. Single crystals formed in the earlier stage of growth process are larger in size with less Te inclusions. Secondary inclusions formed due to thermal migration and crystal cracking were found. The IR transmittance over the wavenumber range from 500 to 4000 cm 1 is about 60%, and the band gap is about 2.23 eV at room temperature (RT). Its resistivity can reach up to about 700 Ω cm, which is the highest ever reported for unintentionally doped ZnTe crystal. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A2. Growth from solutions A2. Bridgman technique A3. Single-crystal growth B1. Semiconducting II–VI materials
1. Introduction Among the wide band gap II–VI semiconductor materials, ZnTe is important for the application in the green light emitting diodes (LEDs), laser diodes (LDs), solar cells, microwave devices, etc [1–3]. Recently, ZnTe is found to be a promising crystal used for terahertz devices [4–6]. In order to commercialize LEDs, LDs and THz devices based on ZnTe, not only high quality but also large-diameter substrates are necessary [7–10]. Bulk ZnTe crystals can be grown from the melt [7,8,11–13], Te solution [9,14] and gaseous phases [15]. However, solution growth has many advantages, since the growth temperature could be significantly lower than that in melt growth, and can be easily controlled by changing the amount of additional Te. Such problems at a high temperature as contaminations from the crucible and the high zinc partial pressure would therefore be minimized. Thus, the generation of some point defects could be reduced [16]. Defects such as twins and dislocations, which are easily produced due to the low stacking fault energy in II–VI semiconductors [17], especially at a higher temperature, are more likely to be avoided. Although the vapor phase growth would also lower the temperature, the time necessary to transport sufficient material to grow large crystals is quite long. n
Corresponding author. Tel.: þ 86 29 88460445; fax: þ 86 29 88495414. E-mail address: [email protected] (W. Jie).
http://dx.doi.org/10.1016/j.jcrysgro.2014.03.047 0022-0248/& 2014 Elsevier B.V. All rights reserved.
Another advantage of Te solution growth is the purify effect owing to the solvent (Te) extraction [18]. Since excess Te will be added, crystal growth will proceed in a ever diluting system, which makes solutes transfer becomes a key issue. Thus, ACRT was applied to introduce forced convection to enhance the mass transfer, as well as to improve the interface morphology, at the same time, the sub-grains, and the inclusions etc. defects would be also restrained [19–22]. Therefore, ZnTe crystal was grown from Te solution base on the vertical Bridgman method with ACRT. Features of ZnTe crystal grown with this method were characterized.
2. Experimental Appropriate amount of starting materials of Zn (7 N) and Te (7 N) with the Molar ratio Zn:Te¼3:7 were sealed in a quartz ampoule with an inner diameter of 30 mm at the vacuum of 10 5 Pa, and synthesized in a rocking furnace at 1373 K for 48 h. Followed by a quench to RT to obtain a homogeneous Te-rich polycrystalline ingot. Then the ampoule was transferred in the vertical Bridgman furnace for the crystal growth. The temperature was lowered gradually from 1333 K to 873 K at the withdrawal rate of 0.2 mm/h and a temperature gradient of about 5.0 K/cm, followed by a cooling down to RT at the rate of about 13 K/h.
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R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
The control curve in a cycle for ACRT rotation sequence is shown in Fig. 1. Two reverse directional rotation processes were adopted, each of which includes various periods of stationarity, acceleration, steady rotation, and deceleration. After the growth, the temperature was lowered to about 873 K. The ACRT was continuously applied until the ingot is cooled down to RT, as to eliminate the influences of possible radial thermal field asymmetry on the ingot. Typical as-grown ingots, cooled from 873 K to RT with and without crucible rotation, are shown in Fig. 2(a) and (b), respectively. The wafers used for the experiments with the thickness of 2 mm were cut along the axial direction of the ingot with an inner circle cutting machine. Then, the wafers were mechanically and chemically polished to obtain a flat and smooth surface. The visible light photomicrographs were taken through an optical transmission microscope. IR transmittance images of the wafers were studied using an IR transmittance microscope system (with the optical resolution limit of 1 μm). The chemical composition of the surface layer of the ingot was characterized using EDS analysis. SEM images of the wafer surface were recorded by Hitachi S-570. Zn and Te distribution along the axial direction of the ingot is analyzed by EPMA. The transmittance spectra of the samples were recorded at RT using UV-3150 (Shimadzu UV–Vis–NIR) scanning spectrophotometer over the photon wavelength range from 200 nm to 600 nm under normal incidence. The IR transmission measurements were made at RT using Nicolet Nexus FT-IR
10
ω (r/min)
5 0 -5 -10 0
10
20
30
40
50
60
t(sec) Fig. 1. Schematic diagram of ACRT rotation sequence.
10mm Te layer
Exposed ZnTe
10mm Fig. 2. Typical as grown ZnTe ingots cooled from 873 K to RT with (a) and without (b) ACRT.
spectrometer over the wavenumber range from 500 cm 1 to 4000 cm 1 under normal incidence. The I–V curve of the ZnTe crystal was obtained using an Aglient 4339B digital electrometer after symmetrical gold contacts were formed by the reaction of gold chloride solution on both sides of the wafer.
3. Results and discussions 3.1. Mass transfer effects and single crystal size The as-grown ZnTe ingots are covered by a greyish white layer with metallic luster (Fig. 1(a) and (b)). EDS study shows that the layer is almost pure Te, the same as that on the surface of CdTe and CdZnTe ingots grown with excess Te [23]. However, only the ingot that cooled without crucible rotation was not fully covered by the Te layer, with a portion of ZnTe exposed (Fig. 2(b)). Generally, the excess Te added as solvent will be extracted to the tail of the ingot during the growth process. We think that, this Te layer found in the head of the ingot formed during the post growth cooling process. If the ingot shrinks more than do the crucible, a gap will be produced between them. Thus, the Te rich solute will flow in to form a Te layer. However, while cooling without rotation, the crystallized ZnTe ingot may lean on the inner wall of the crucible, with one portion of its surface touch against the crucible continuously. This portion will prevent the Te-rich liquid from reaching here. Typical visible light transmittance image of the as-grown ingot after its surface Te layer has been removed is shown in Fig. 3(a). It generally consists of three regions, the ZnTe single crystal region, the excess Te region and the transition region. Cracks are seen near the transition region of the ingot Fig. 3(a). It may be due to the thermal expansion coefficient difference between hexagonal Te (1.7 10 6 K 1 and 27.5 10 6 K 1, parallel and perpendicular to the c axis, respectively) and cubic ZnTe (8.2 10 6 K 1) [24]. The crystal shows transparency with reddish orange color (Fig. 3(a)). This is because the absorption edge of zinc-blende ZnTe crystal is about 550 nm at RT, corresponding to the wavelength of visible light. Owing to their opacity, the grain boundaries and cracks that present inside the ingot can be easily identified (Fig. 3(a)). The ingot was cut into two semi-cylinders along the growth direction to show the macro phase and crystal size distribution (Fig. 3(b)). The solutes were successively transferred to the head of the ingot, with only a small piece of them left in the tail. There are also some micrometre-scale polyhedron ZnTe crystals in the excess Te region. A typical SEM image of them is shown in Fig. 3(c). As they crystallized freely, they are bounded by small facets. A large size single crystal, with the cross section area of 10 mm 40 mm, in the upper side of the semi-cylinders extend from the tip all the way to the transition region of the ingot was found (Fig. 3(b)). Its size is larger that the reported result [14] with similar growth method but with relatively higher pulling rate and without the application of ACRT. The advancing growth interface (ZnTe/(ZnTe þTe)) is a little concave (Fig. 3(b)). The low thermal conductivity of ZnTe dues to its high Phillips ionicity [25] would lead to the accumulation of crystalline latent, particularly in the center of the advancing interface during growth. Thus, a concave interface would form. However, compared with the reported result [9], the present interface is with smaller curvature. This may be due to the application of ACRT, which can improve the interface morphology. Te-rich solution is more likely to accumulate in the central region of the concave interface. As the growth proceeds, the solutes are consumed gradually. Solutes transfer becomes more difficult, which will lead to the enlargement of this central Te-rich region.
R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
At last, a ‘V’ shape region is produced (Fig. 3(b)), consistents with the reported result [9]. This ‘V’ shape region gives rise to the opacity to visible light (Fig. 3(a)). From the phase distribution, as well as the crystal size and shape, the approximate heat extraction directions of the ingot during growth was deduced, as indicated by the arrows in Fig. 3(b). Typical axial cut wafers from the head, middle and tail of the ingot and their corresponding transmittance light images after
Grain boundaries
ZnTe
Cracks
ZnTe+Te
A large size single crystal
Te ZnTe
Te inclusions
Heat extraction directions
20mm
ZnTe Te
29
their both sides are polished are shown in Fig. 4(a)–(c), respectively. Combined with Fig. 3(b), we can see a single crystal with the size more than 10 mm 10 mm 40 mm. Twin lamella was only to be found in the head of the ingot (Fig. 4(a)), which may be due to the relatively higher growth temperature at the primary stage of growth. The last to freeze region is with poor quality, more Te inclusions, cracks, and grain boundaries are found here (Fig. 4(c)). 3.2. Composition homogeneity Typical IR transmittance images of wafer cut from the head, middle and tail of the ingot and their corresponding histogram of Te inclusion size distribution are shown in Fig. 5(a)–(c), respectively. The total concentration of Te inclusions over the scanned area was estimated to be 8 103, 1.6 104 and 1.5 104 cm 3, respectively. Inclusions with the size around or below 2 μm are the most, and are more likely to be found in the middle of the ingot. The number of Te inclusions with diameter over 40 μm increases along the axial direction of the ingot. Axial distribution of Zn and Te studied by EPMA is shown in Fig. 6. The Zn/Te composition ratio deviated from stoichiometric 1:1. With excess Te at the minimum of 1.054 at% in the head of the ingot, and then it increases gradually along the growth direction, reach its maximum of 1.554 at% in the tail of the ingot. However, in Zn–Te binary phase diagram, the maximum Te non-stoichiometry, which is due to the presence of interstitial site Te and/or Te precipitates, is only 0.0186 at% [26]. Thus, the excess Te was mainly attributed to the existence of Te inclusions. As the growth proceeds, the ever diluting solution makes the growth more difficult. Thus, more Te inclusions are trapped in the region froze later (Figs. 5 and 6). Typical IR transmittance images of where grain boundaries and large size Te inclusions clusters exist are shown in Fig. 7(a) and (b), respectively. The grain boundaries and large inclusions act as lower energy sites, which attract Te secondary phases, results in heavily Te decorated grain boundaries and the enlargement of the already large Te inclusions. However, a more ‘clear’ region near them will also be produced simultaneously. 3.3. Secondary inclusions in ZnTe
Fig. 3. Typical images of the ingot, (a) visible light transmittance image, (b) crosssectional appearance, (c) SEM image of free grown ZnTe. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Typical visible light transmittance images of Te inclusion near the edge of the ingot and those far from it is shown in Fig. 8(a) and (b), respectively. Although these inclusions both
Te inclusions Twin lamella
Grain boundaries
Te inclusions Cracks 10mm
Fig. 4. Typical axial cut wafers (first row) and their corresponding transmittance light images after polishing (second row).
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R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
1400
Counts
1200 1000 800 600 400 200
1mm
0
0
10 20 30 40 50 60 70 80 90
Diameter / μm
1400 1200
Counts
1000 800 600 400 200
1mm
0
0
10 20 30 40 50 60 70 80 90
Diameter / μm
1400 1200
Counts
1000 800 600 400
1mm
200 0
0
10 20 30 40 50 60 70 80 90
Diameter / μm Fig. 5. Typical IR transmittance images of wafer cut from the head (a), middle (b) and tail (c) of the ingot and their corresponding histogram of Te inclusion size distribution.
Fig. 6. Zn and Te distribution along the axial direction of ZnTe ingot.
present inside the grains, their shape, size and density differ greatly. The former kind has a distinctive shape, a typical one is indicated by the arrow in Fig. 8(a). It is elongated, with a faceted end towards the center of the ingot. Which is due to their thermal migration in the thermal field with large temperature gradients. And faceted surface will be produced on their hotter side [13]. The latter kind of inclusions does not show obvious evidence of thermal migration (Fig. 8(b)). Thus, a radial gradient may also exist in the inner part of the ingot. However, it should be smaller than the outside areas. Crystals grown with the vertical Bridgman method always show similar radial thermal field distribution [27]. The inclusions on the edge of the ingot are with extremely large size and more densely distributed. Because they migrated from the Te-rich layer that covering the ingot. These Te inclusions can be termed as secondary inclusions, since they are not formed by interrupted growth as the primary inclusions is [28].
R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
31
Te decorated
grain boundary
400μm
400μm
Fig. 7. Typical IR transmittance images of the area around the grain boundaries (a) and the clusters of Te inclusions (b).
Ingot edge
10mm 10mm Fig. 8. Visible light transmittance images of Te inclusions in the ingot edge (a) and those farther from the ingot edge (b).
100μm 400μm Fig. 9. A typical IR transmission image of Te inclusions filled cracks in the ingot.
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R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
100
0.00003
90
0.00002 0.00001
70
I/A
Transmittance/%
80
60 50
0.00000 -0.00001
40 -0.00002
30 20
-0.00003
10 0 500
-0.10 1000
1500
2000
2500
3000
3500
-0.05
0.00
0.05
0.10
V/ V
4000
Wavenumber/cm-1
Fig. 11. A typical I–V curve of ZnTe crystal.
4. Conclusions 70
50 40
1500
(αhv)2/cm-2eV2
Transmittance/%
60
30
1000
20 10 0
500
1000
500 0 2.230
2.231 hv/eV
1500
2.232
2000
2500
Wavelength/nm Fig. 10. Typical IR transmission spectrum (a), and ultraviolet–visible–near IR spectrum of ZnTe (b).
A crack of the ingot was found filled with Te inclusions. A typical IR transmission image and an enlarge segment of it are shown in Fig. 9(a) and (b), respectively. At the temperature above Te melt point, no matter where does the cracks initiate, when they propagate to meet the Te-rich liquid in front of the solid/liquid interface, the liquid will enter in the open fracture. Thus, high density of Te inclusions is formed after cooling (Fig. 9). It is another kind of secondary inclusions. With the presence of temperature gradients, they may migrate to other parts of the ingot, thus further degrade the crystal homogeneity.
Combining Te solution growth method and vertical Bridgman method with ACRT, ZnTe ingots 30 mm in diameter and about 60 mm in length were grown. The phase/composition distributions of the whole ingot were discussed in detail. The solutes were transferred efficiently during growth, and large size single crystal with the volume up to 10 mm 10 mm 40 mm was produced. Single crystals formed in the early stage of growth possess less Te inclusions. Compared with the reported results, the relatively lower pulling rate and the application of ACRT may help to obtain a growth interface with smaller curvature, as well as larger size ZnTe single crystals. The as-grown ZnTe crystal has a band gap of 2.23 eV, and the IR transmittance of about 60% over the wavenumber range from 500 to 4000 cm 1. Its resistivity can reach up to about 700 Ω cm, which is the highest ever reported for unintentionally doped ZnTe crystals.
Acknowledgements This work has been financially supported by the National Basic Research Program of China (No. 2011CB610406) and National Natural Science Foundation of China under Grants no 51202197. It is also supported by 111 Project of China (No. B08040), Specialized Research Fund for the Doctoral Program of Higher Education of China (20116102120014), NWPU Foundation for Fundamental Research and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU). References
3.4. Optical and electrical properties of ZnTe The IR transmittance over the wavenumber range from 500 cm 1 to 4000 cm 1 is about 60% (Fig. 10(a)), consists well with the reported result [29]. The transmittance spectra of ZnTe over the wavelength range from 200 nm to 2600 nm is shown in Fig. 10(b), from which the band gap is determined to be about 2.23 eV at RT. It also did not show large deviation from the reported results of ZnTe grown from Te solution (2.24 eV [30]), melts (2.28 eV [31]) and vapor phase (2.20 eV [32] and 2.26 eV [33]). A typical I–V curve of the as-grown ZnTe crystal is shown in Fig. 11. From which the resistivity is determined to be about 700 Ω cm. To our best knowledge, it is the highest ever reported for unintentionally doped ZnTe crystal. Scarcely can this value reaches 100 Ω cm among the reported results, and most of them only falling into the range of 0–10 Ω cm [34–37]. Which may indicate that we have grown ZnTe crystals with fewer defects.
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[10] S.A. Ku, C.M. Tu, W.C. Chu, et al., Saturation of the free carrier absorption in ZnTe crystals, Opt. Express 21 (12) (2013) 13930–13937. [11] T. Asahi, T. Yabe, K. Sato, ZnTe single crystal growth by the liquid encapsulated pulling method, Phys. Status Solidi B 241 (3) (2004) 648–651. [12] T. Asahi, T. Yabe, K. Sato, Growth of large-diameter ZnTe single crystals by liquid-encapsulated melt growth methods, J. Electron. Mater. 33 (6) (2004) 651–653. [13] A. Arakawa, T. Asahi, K. Sato, Growth and characterization of large diameter ZnTe single crystals, Phys. Status Solidi B 229 (1) (2002) 11–14. [14] W.Y. Uen, S.Y. Chou, H.Y. Shin, et al., Characterizations of ZnTe bulks grown by temperature gradient solution growth, Mater. Sci. Eng., B 106 (1) (2004) 27–32. [15] V.E. Kisel, N.A. Toistik, V.G. Shcherbitsky, et al., Growth and spectroscopic characterization of Cr:ZnTe laser crystals, in: Conference on Lasers and ElectroOptics Europe, Conference on IEEE (2005) 81. [16] P. Rudolph, Non-stoichiometry related defects at the melt growth of semiconductor compound crystals-a review, Cryst. Res. Technol. 38 (7-8) (2003) 542–554. [17] S. Takeuchi, K. Suzuki, K. Maeda, Stacking-fault energy of II–VI compounds, Philos. Mag. A 50 (1984) 17–178. [18] K. Zanio, Purification of CdTe from tellurium-rich solutions, J. Electron. Mater. 3 (2) (1974) 327–351. [19] H.J. Scheel, E.O. Schulz-Dubois, Flux growth of large crystals by accelerated crucible-rotation technique, J. Cryst. Growth 8 (3) (1971) 304–306. [20] V.E. Distanov, A.G. Kirdyashkin, The influence of accelerated crucible rotation mode on the melt temperature field in the Stockbarger technique, J. Cryst. Growth 222 (3) (2001) 607–614. [21] P. Capper, The role of accelerated crucible rotation in the growth of Hg1 xCdxTe and CdTe/CdZnTe, Prog. Cryst. Growth Charact. Mater. 28 (1) (1994) 1–55. [22] Y.C. Liu, B. Roux, C.W. Lan, Effects of accelerated crucible rotation on segregation and interface morphology for vertical Bridgman crystal growth: visualization and simulation, J. Cryst. Growth 304 (1) (2007) 236–243. [23] P. Rudolph, A. Engel, I. Schentke, et al., Distribution and genesis of inclusions in CdTe and (Cd,Zn)Te single crystals grown by the Bridgman method and by the travelling heater method, J. Cryst. Growth 147 (1995) 297–304.
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[24] A. Bonvalet, M. Joffre, J.L. Martin, et al., Generation of ultrabroadband femtosecond pulses in the mid-infrared by optical rectification of 15 fs light pulses at 100 MHz repetition rate, Appl. Phys. Lett. 67 (20) (1995) 2907–2909. [25] J.S. Phillips, Ionicity of the chemical bond in crystals, Rev. Mod. Phys. 42 (3) (1970) 317. [26] T. Feltgena, J.H. Greenbergb, A.N. Guskovc, et al., P–T–X phase equilibrium studies in Zn–Te for crystal growth by the Markov method, Int. J. Inorg. Mater. 3 (2001) 1241–1244. [27] T. Jasinski, A.F. Wint, On control of the crystal–melt interface shape during growth in a vertical Bridgman configuration, J. Cryst. Growth 71 (2) (1985) 295–304. [28] J.W. Mullin, Crystallization, fourth ed., Butterworth-Heinemann (2001) 285–286. [29] R. Wang, W. Fang, P. Zhao, et al., Growth and characteristics of ZnTe single crystal for THz technology, J. Semicond. 29 (5) (2008) 940–943. [30] R. Wang, W. Fang, P. Zhao, et al., Growth and Characterization of 〈1 1 0〉 oriented ZnTe single crystal, Photonics Asia (2007) 683519-1–683519-8. [31] K. Sato, S. Adachi, Optical properties of ZnTe, J. Appl. Phys. 73 (2) (1993) 926–931. [32] K.D. Patel, G.K. Solanki, J.R. Gandhi, et al., Growth and characterization of ZnTe crystals grown by physical vapor transport technique, Chalcogenide Lett. 6 (9) (2009). [33] G. Krishnaiah, B.K. Reddy, N. Madhusudhana Rao, et al., Optical and electrical properties of vapour phase grown Zn1 xCrxTe crystals, Thin Solid Films 518 (10) (2010) 2599–2602. [34] M. Nishio, K. Tsuru, H. Ogawa, Chemical vapor growth of ZnTe on GaAs by the closed-tube method, Jpn. J. Appl. Phys. 18 (10) (1979) 1909–1914. [35] S. Bhunia, D.N. Bose, Microwave synthesis, single crystal growth and characterization of ZnTe, J. Cryst. Growth 186 (4) (1998) 535–542. [36] M. Dudley, G.D. Yao, W. Zhou, Growth of ZnTe by physical vapor transport and traveling heater method, J. Cryst. Growth 128 (1) (1993) 627–632. [37] Z. Dong, Y. Yan, W. Shi, et al., Growth of ZnTe single crystals from vapor under controlled partial pressure of zinc component, J. Synth. Cryst. 17 (1) (1998) 16–20.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of ZnTe single crystals from Te solution by vertical Bridgman method with ACRT Rui Yang, Wanqi Jie n, Hang Liu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 December 2013 Received in revised form 11 March 2014 Accepted 30 March 2014 Communicated by: A. Burger Available online 5 April 2014
ZnTe ingots 30 mm in diameter and about 60 mm in length were grown from Te solution based on vertical Bridgman method with accelerated crucible rotation technique (ACRT). The composition homogeneity and the optical and electrical properties of the crystals were investigated using optical transmission microscope, energy dispersive spectrometer (EDS), scanning electron microscope (SEM), electron probe micro-analyzer (EPMA), ultraviolet–visible (UV–Vis) spectrophotometer, Fourier transform infrared (FT-IR) spectrometer and digital electrometer. The growth method was found efficient for mass transfer, and produced large size single crystal with the volume up to 10 mm 10 mm 40 mm. Single crystals formed in the earlier stage of growth process are larger in size with less Te inclusions. Secondary inclusions formed due to thermal migration and crystal cracking were found. The IR transmittance over the wavenumber range from 500 to 4000 cm 1 is about 60%, and the band gap is about 2.23 eV at room temperature (RT). Its resistivity can reach up to about 700 Ω cm, which is the highest ever reported for unintentionally doped ZnTe crystal. & 2014 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A2. Growth from solutions A2. Bridgman technique A3. Single-crystal growth B1. Semiconducting II–VI materials
1. Introduction Among the wide band gap II–VI semiconductor materials, ZnTe is important for the application in the green light emitting diodes (LEDs), laser diodes (LDs), solar cells, microwave devices, etc [1–3]. Recently, ZnTe is found to be a promising crystal used for terahertz devices [4–6]. In order to commercialize LEDs, LDs and THz devices based on ZnTe, not only high quality but also large-diameter substrates are necessary [7–10]. Bulk ZnTe crystals can be grown from the melt [7,8,11–13], Te solution [9,14] and gaseous phases [15]. However, solution growth has many advantages, since the growth temperature could be significantly lower than that in melt growth, and can be easily controlled by changing the amount of additional Te. Such problems at a high temperature as contaminations from the crucible and the high zinc partial pressure would therefore be minimized. Thus, the generation of some point defects could be reduced [16]. Defects such as twins and dislocations, which are easily produced due to the low stacking fault energy in II–VI semiconductors [17], especially at a higher temperature, are more likely to be avoided. Although the vapor phase growth would also lower the temperature, the time necessary to transport sufficient material to grow large crystals is quite long. n
Corresponding author. Tel.: þ 86 29 88460445; fax: þ 86 29 88495414. E-mail address: [email protected] (W. Jie).
http://dx.doi.org/10.1016/j.jcrysgro.2014.03.047 0022-0248/& 2014 Elsevier B.V. All rights reserved.
Another advantage of Te solution growth is the purify effect owing to the solvent (Te) extraction [18]. Since excess Te will be added, crystal growth will proceed in a ever diluting system, which makes solutes transfer becomes a key issue. Thus, ACRT was applied to introduce forced convection to enhance the mass transfer, as well as to improve the interface morphology, at the same time, the sub-grains, and the inclusions etc. defects would be also restrained [19–22]. Therefore, ZnTe crystal was grown from Te solution base on the vertical Bridgman method with ACRT. Features of ZnTe crystal grown with this method were characterized.
2. Experimental Appropriate amount of starting materials of Zn (7 N) and Te (7 N) with the Molar ratio Zn:Te¼3:7 were sealed in a quartz ampoule with an inner diameter of 30 mm at the vacuum of 10 5 Pa, and synthesized in a rocking furnace at 1373 K for 48 h. Followed by a quench to RT to obtain a homogeneous Te-rich polycrystalline ingot. Then the ampoule was transferred in the vertical Bridgman furnace for the crystal growth. The temperature was lowered gradually from 1333 K to 873 K at the withdrawal rate of 0.2 mm/h and a temperature gradient of about 5.0 K/cm, followed by a cooling down to RT at the rate of about 13 K/h.
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R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
The control curve in a cycle for ACRT rotation sequence is shown in Fig. 1. Two reverse directional rotation processes were adopted, each of which includes various periods of stationarity, acceleration, steady rotation, and deceleration. After the growth, the temperature was lowered to about 873 K. The ACRT was continuously applied until the ingot is cooled down to RT, as to eliminate the influences of possible radial thermal field asymmetry on the ingot. Typical as-grown ingots, cooled from 873 K to RT with and without crucible rotation, are shown in Fig. 2(a) and (b), respectively. The wafers used for the experiments with the thickness of 2 mm were cut along the axial direction of the ingot with an inner circle cutting machine. Then, the wafers were mechanically and chemically polished to obtain a flat and smooth surface. The visible light photomicrographs were taken through an optical transmission microscope. IR transmittance images of the wafers were studied using an IR transmittance microscope system (with the optical resolution limit of 1 μm). The chemical composition of the surface layer of the ingot was characterized using EDS analysis. SEM images of the wafer surface were recorded by Hitachi S-570. Zn and Te distribution along the axial direction of the ingot is analyzed by EPMA. The transmittance spectra of the samples were recorded at RT using UV-3150 (Shimadzu UV–Vis–NIR) scanning spectrophotometer over the photon wavelength range from 200 nm to 600 nm under normal incidence. The IR transmission measurements were made at RT using Nicolet Nexus FT-IR
10
ω (r/min)
5 0 -5 -10 0
10
20
30
40
50
60
t(sec) Fig. 1. Schematic diagram of ACRT rotation sequence.
10mm Te layer
Exposed ZnTe
10mm Fig. 2. Typical as grown ZnTe ingots cooled from 873 K to RT with (a) and without (b) ACRT.
spectrometer over the wavenumber range from 500 cm 1 to 4000 cm 1 under normal incidence. The I–V curve of the ZnTe crystal was obtained using an Aglient 4339B digital electrometer after symmetrical gold contacts were formed by the reaction of gold chloride solution on both sides of the wafer.
3. Results and discussions 3.1. Mass transfer effects and single crystal size The as-grown ZnTe ingots are covered by a greyish white layer with metallic luster (Fig. 1(a) and (b)). EDS study shows that the layer is almost pure Te, the same as that on the surface of CdTe and CdZnTe ingots grown with excess Te [23]. However, only the ingot that cooled without crucible rotation was not fully covered by the Te layer, with a portion of ZnTe exposed (Fig. 2(b)). Generally, the excess Te added as solvent will be extracted to the tail of the ingot during the growth process. We think that, this Te layer found in the head of the ingot formed during the post growth cooling process. If the ingot shrinks more than do the crucible, a gap will be produced between them. Thus, the Te rich solute will flow in to form a Te layer. However, while cooling without rotation, the crystallized ZnTe ingot may lean on the inner wall of the crucible, with one portion of its surface touch against the crucible continuously. This portion will prevent the Te-rich liquid from reaching here. Typical visible light transmittance image of the as-grown ingot after its surface Te layer has been removed is shown in Fig. 3(a). It generally consists of three regions, the ZnTe single crystal region, the excess Te region and the transition region. Cracks are seen near the transition region of the ingot Fig. 3(a). It may be due to the thermal expansion coefficient difference between hexagonal Te (1.7 10 6 K 1 and 27.5 10 6 K 1, parallel and perpendicular to the c axis, respectively) and cubic ZnTe (8.2 10 6 K 1) [24]. The crystal shows transparency with reddish orange color (Fig. 3(a)). This is because the absorption edge of zinc-blende ZnTe crystal is about 550 nm at RT, corresponding to the wavelength of visible light. Owing to their opacity, the grain boundaries and cracks that present inside the ingot can be easily identified (Fig. 3(a)). The ingot was cut into two semi-cylinders along the growth direction to show the macro phase and crystal size distribution (Fig. 3(b)). The solutes were successively transferred to the head of the ingot, with only a small piece of them left in the tail. There are also some micrometre-scale polyhedron ZnTe crystals in the excess Te region. A typical SEM image of them is shown in Fig. 3(c). As they crystallized freely, they are bounded by small facets. A large size single crystal, with the cross section area of 10 mm 40 mm, in the upper side of the semi-cylinders extend from the tip all the way to the transition region of the ingot was found (Fig. 3(b)). Its size is larger that the reported result [14] with similar growth method but with relatively higher pulling rate and without the application of ACRT. The advancing growth interface (ZnTe/(ZnTe þTe)) is a little concave (Fig. 3(b)). The low thermal conductivity of ZnTe dues to its high Phillips ionicity [25] would lead to the accumulation of crystalline latent, particularly in the center of the advancing interface during growth. Thus, a concave interface would form. However, compared with the reported result [9], the present interface is with smaller curvature. This may be due to the application of ACRT, which can improve the interface morphology. Te-rich solution is more likely to accumulate in the central region of the concave interface. As the growth proceeds, the solutes are consumed gradually. Solutes transfer becomes more difficult, which will lead to the enlargement of this central Te-rich region.
R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
At last, a ‘V’ shape region is produced (Fig. 3(b)), consistents with the reported result [9]. This ‘V’ shape region gives rise to the opacity to visible light (Fig. 3(a)). From the phase distribution, as well as the crystal size and shape, the approximate heat extraction directions of the ingot during growth was deduced, as indicated by the arrows in Fig. 3(b). Typical axial cut wafers from the head, middle and tail of the ingot and their corresponding transmittance light images after
Grain boundaries
ZnTe
Cracks
ZnTe+Te
A large size single crystal
Te ZnTe
Te inclusions
Heat extraction directions
20mm
ZnTe Te
29
their both sides are polished are shown in Fig. 4(a)–(c), respectively. Combined with Fig. 3(b), we can see a single crystal with the size more than 10 mm 10 mm 40 mm. Twin lamella was only to be found in the head of the ingot (Fig. 4(a)), which may be due to the relatively higher growth temperature at the primary stage of growth. The last to freeze region is with poor quality, more Te inclusions, cracks, and grain boundaries are found here (Fig. 4(c)). 3.2. Composition homogeneity Typical IR transmittance images of wafer cut from the head, middle and tail of the ingot and their corresponding histogram of Te inclusion size distribution are shown in Fig. 5(a)–(c), respectively. The total concentration of Te inclusions over the scanned area was estimated to be 8 103, 1.6 104 and 1.5 104 cm 3, respectively. Inclusions with the size around or below 2 μm are the most, and are more likely to be found in the middle of the ingot. The number of Te inclusions with diameter over 40 μm increases along the axial direction of the ingot. Axial distribution of Zn and Te studied by EPMA is shown in Fig. 6. The Zn/Te composition ratio deviated from stoichiometric 1:1. With excess Te at the minimum of 1.054 at% in the head of the ingot, and then it increases gradually along the growth direction, reach its maximum of 1.554 at% in the tail of the ingot. However, in Zn–Te binary phase diagram, the maximum Te non-stoichiometry, which is due to the presence of interstitial site Te and/or Te precipitates, is only 0.0186 at% [26]. Thus, the excess Te was mainly attributed to the existence of Te inclusions. As the growth proceeds, the ever diluting solution makes the growth more difficult. Thus, more Te inclusions are trapped in the region froze later (Figs. 5 and 6). Typical IR transmittance images of where grain boundaries and large size Te inclusions clusters exist are shown in Fig. 7(a) and (b), respectively. The grain boundaries and large inclusions act as lower energy sites, which attract Te secondary phases, results in heavily Te decorated grain boundaries and the enlargement of the already large Te inclusions. However, a more ‘clear’ region near them will also be produced simultaneously. 3.3. Secondary inclusions in ZnTe
Fig. 3. Typical images of the ingot, (a) visible light transmittance image, (b) crosssectional appearance, (c) SEM image of free grown ZnTe. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Typical visible light transmittance images of Te inclusion near the edge of the ingot and those far from it is shown in Fig. 8(a) and (b), respectively. Although these inclusions both
Te inclusions Twin lamella
Grain boundaries
Te inclusions Cracks 10mm
Fig. 4. Typical axial cut wafers (first row) and their corresponding transmittance light images after polishing (second row).
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R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
1400
Counts
1200 1000 800 600 400 200
1mm
0
0
10 20 30 40 50 60 70 80 90
Diameter / μm
1400 1200
Counts
1000 800 600 400 200
1mm
0
0
10 20 30 40 50 60 70 80 90
Diameter / μm
1400 1200
Counts
1000 800 600 400
1mm
200 0
0
10 20 30 40 50 60 70 80 90
Diameter / μm Fig. 5. Typical IR transmittance images of wafer cut from the head (a), middle (b) and tail (c) of the ingot and their corresponding histogram of Te inclusion size distribution.
Fig. 6. Zn and Te distribution along the axial direction of ZnTe ingot.
present inside the grains, their shape, size and density differ greatly. The former kind has a distinctive shape, a typical one is indicated by the arrow in Fig. 8(a). It is elongated, with a faceted end towards the center of the ingot. Which is due to their thermal migration in the thermal field with large temperature gradients. And faceted surface will be produced on their hotter side [13]. The latter kind of inclusions does not show obvious evidence of thermal migration (Fig. 8(b)). Thus, a radial gradient may also exist in the inner part of the ingot. However, it should be smaller than the outside areas. Crystals grown with the vertical Bridgman method always show similar radial thermal field distribution [27]. The inclusions on the edge of the ingot are with extremely large size and more densely distributed. Because they migrated from the Te-rich layer that covering the ingot. These Te inclusions can be termed as secondary inclusions, since they are not formed by interrupted growth as the primary inclusions is [28].
R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
31
Te decorated
grain boundary
400μm
400μm
Fig. 7. Typical IR transmittance images of the area around the grain boundaries (a) and the clusters of Te inclusions (b).
Ingot edge
10mm 10mm Fig. 8. Visible light transmittance images of Te inclusions in the ingot edge (a) and those farther from the ingot edge (b).
100μm 400μm Fig. 9. A typical IR transmission image of Te inclusions filled cracks in the ingot.
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R. Yang et al. / Journal of Crystal Growth 400 (2014) 27–33
100
0.00003
90
0.00002 0.00001
70
I/A
Transmittance/%
80
60 50
0.00000 -0.00001
40 -0.00002
30 20
-0.00003
10 0 500
-0.10 1000
1500
2000
2500
3000
3500
-0.05
0.00
0.05
0.10
V/ V
4000
Wavenumber/cm-1
Fig. 11. A typical I–V curve of ZnTe crystal.
4. Conclusions 70
50 40
1500
(αhv)2/cm-2eV2
Transmittance/%
60
30
1000
20 10 0
500
1000
500 0 2.230
2.231 hv/eV
1500
2.232
2000
2500
Wavelength/nm Fig. 10. Typical IR transmission spectrum (a), and ultraviolet–visible–near IR spectrum of ZnTe (b).
A crack of the ingot was found filled with Te inclusions. A typical IR transmission image and an enlarge segment of it are shown in Fig. 9(a) and (b), respectively. At the temperature above Te melt point, no matter where does the cracks initiate, when they propagate to meet the Te-rich liquid in front of the solid/liquid interface, the liquid will enter in the open fracture. Thus, high density of Te inclusions is formed after cooling (Fig. 9). It is another kind of secondary inclusions. With the presence of temperature gradients, they may migrate to other parts of the ingot, thus further degrade the crystal homogeneity.
Combining Te solution growth method and vertical Bridgman method with ACRT, ZnTe ingots 30 mm in diameter and about 60 mm in length were grown. The phase/composition distributions of the whole ingot were discussed in detail. The solutes were transferred efficiently during growth, and large size single crystal with the volume up to 10 mm 10 mm 40 mm was produced. Single crystals formed in the early stage of growth possess less Te inclusions. Compared with the reported results, the relatively lower pulling rate and the application of ACRT may help to obtain a growth interface with smaller curvature, as well as larger size ZnTe single crystals. The as-grown ZnTe crystal has a band gap of 2.23 eV, and the IR transmittance of about 60% over the wavenumber range from 500 to 4000 cm 1. Its resistivity can reach up to about 700 Ω cm, which is the highest ever reported for unintentionally doped ZnTe crystals.
Acknowledgements This work has been financially supported by the National Basic Research Program of China (No. 2011CB610406) and National Natural Science Foundation of China under Grants no 51202197. It is also supported by 111 Project of China (No. B08040), Specialized Research Fund for the Doctoral Program of Higher Education of China (20116102120014), NWPU Foundation for Fundamental Research and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU). References
3.4. Optical and electrical properties of ZnTe The IR transmittance over the wavenumber range from 500 cm 1 to 4000 cm 1 is about 60% (Fig. 10(a)), consists well with the reported result [29]. The transmittance spectra of ZnTe over the wavelength range from 200 nm to 2600 nm is shown in Fig. 10(b), from which the band gap is determined to be about 2.23 eV at RT. It also did not show large deviation from the reported results of ZnTe grown from Te solution (2.24 eV [30]), melts (2.28 eV [31]) and vapor phase (2.20 eV [32] and 2.26 eV [33]). A typical I–V curve of the as-grown ZnTe crystal is shown in Fig. 11. From which the resistivity is determined to be about 700 Ω cm. To our best knowledge, it is the highest ever reported for unintentionally doped ZnTe crystal. Scarcely can this value reaches 100 Ω cm among the reported results, and most of them only falling into the range of 0–10 Ω cm [34–37]. Which may indicate that we have grown ZnTe crystals with fewer defects.
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