Effect of tellurium on viscosity and liquid structure of GaSb melts

Journal of Alloys and Compounds 453 (2008) 458–462

Effect of tellurium on viscosity and liquid structure of GaSb melts Lei-lei Ji a , Hao-ran Geng a,∗ , Chun-jing Sun b , Xin-ying Teng a , Ya-mei Liu a a

b

School of Material Science and Engineering, Jinan University, Jinan 250022, PR China Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China Received 20 September 2006; received in revised form 19 November 2006; accepted 21 November 2006 Available online 22 December 2006

Abstract The behavior of GaSb melt with tellurium addition was investigated using viscometer and differential scanning calorimetry (DSC). Normally, the viscosity of all melts measured decreased with the increasing temperature. However, anomalous transition points were observed in the temperature dependence of viscosity for Ga–Sb–Te system. Corresponded with the abnormal points on the viscosity–temperature curves, there were thermal effect peaks on the DSC curves. Furthermore, viscous activation energy and flow units of these melts and their structural features were discussed in this paper. © 2006 Elsevier B.V. All rights reserved. Keywords: Viscosity; GaSb melt; Tellurium; Activation energy; Flow unit

1. Introduction Research on the structure of metal melts has attracted much attention recently through energy dissipation techniques [1–4], thermophysical properties measurements [5–7], XRD [8,9], and molecular dynamic simulations [10,11]. Viscosity is one of the properties sensitive to the melt structure and the friction among atoms from the microscopic viewpoint. As semiconductors are important materials in highly industrialized societies, demands for large-scale and high-scale crystal are increasing. Therefore, computational simulations on the processes of crystal growth from the melt are being attempted to produce high-quality semiconductors. Although, the simulations need reliable values of the thermophysical properties of the semiconductors for the molten and solid states, values reported for the molten state show large discrepancies. One investigator reported different viscosity values of GaSb melt when using different materials of the crucible [12]. Another investigator also measured the viscosity of GaSb melt using an oscillating viscometer to study the thermophysical properties of semiconductor melts, and concluded that most semiconductors show Arrhenius behavior and have a low viscosity [13].

∗

Corresponding author. Tel.: +86 531 82767561; fax: +86 531 82765317. E-mail address: mse [email protected] (H.-r. Geng).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.137

Single crystals of GaSb are important as substrates for long wavelength (>1.5 mm) detectors and lasers [14]. GaSb as III–V compound semiconductor reveals p-type semiconductor characteristic. Single crystal GaSb can be used as underlay materials in many parts of an apparatus if its electric property expresses n-type. Research indicated that Te was the better additional element to make electric characteristic of GaSb become n-type [15]. Some authors [16–19] also found that undoped crystals had relatively high etch pit density (EPD) and that the dislocation density of III–V compounds would be reduced by doping with various impurities [20–22]. It was approved that Te-doped GaSb crystal had lower EPD than the undoped one [23]. As we know Ga36.5 Sb63.5 is a compound component point, from which single crystal GaSb was prepared. Therefore, in this paper, tellurium was chosen as additional element and its effect on the viscosity and the microstructure of Ga36.5 Sb63.5 melt was investigated. The structural features of the melts were analyzed with respect to its viscosity–temperature curves and differential scanning calorimetry (DSC) results. 2. Experimental procedures GaSb and tellurium used in this work were produced from 99.99 wt.% Ga, 99.9 wt.% Sb, and 99.9 wt.% Te. The samples were produced in a clay-bond graphite crucible in an electric-resistant furnace. It was cast into a cylindrical shape: 27 mm in diameter and 48 mm in length. The viscosity measurements were taken with a RHEOTRONI C VIII torsional oscillation viscometer designed for high-temperature melts, made in Japan. The principal indices

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Fig. 1. Temperature dependence of dynamic viscosity and ln η–1/T curve of Ga36.5 Sb63.5 melt. of the viscometer were repetition rate ±5% and temperature precision ±3 K, with the highest measuring temperature at 1773 K. The specimen was placed in a vessel made of A12 O3 , hung by a torsional suspension. It was maintained at a measured temperature under an argon atmosphere for 1 h so that the temperature of the sample would be stable. The vessel was placed in oscillation around a vertical axis. The resulting motion gradually dissipated on account of friction. When the temperature reached the highest point required, the melt was cooled and then measured. There were four to seven experiments at every temperature. Their average was calculated. The kinetic viscosity [24] of the liquid samples is expressed as follows: v=

I 2 (δ − Tδ0 /T0 )2 π(MR)2 TW 2

(1)

in which W =1−

3 3 2nR Δ − Δ2 − a + (b − cΔ) 2 8 H

(2)

in which Δ = δ/2π; M represents the mass of the liquid samples, and a, b, and c are constants. I is the moment of inertia of the suspended system, r the radius of the vessel, H the height of the liquid specimen in the vessel, S the logarithmic damping decrement, T the time period of the oscillations, in which the subscript 0 refers to an empty vessel, and n is the number of solid planes horizontally in contact with the liquid specimen (e.g. in the case of a vessel having its lower end closed and its upper surface free, n = 1; if the vessel encloses both the fluid top and bottom, n = 2). The dynamic viscosity is calculated by the equation: η = v/ρ

800–850 ◦ C. For clarification, regarding the anomalous changes, the ln η–1/T curve of GaSb is given here. And it is obvious that the curves are divided into two parts: a low-temperature area and a high-temperature area. It is known that Ga36.5 Sb63.5 is a compound component point from the binary phase diagram. As covalence semiconductor compound, GaSb crystal has tetrahedron coordination structure. However, its electrical resistivity and its temperature coefficient show typical metal property. One investigator had studied the structure of GaSb [25], and found that most of the coordination number in GaSb melt was 5 and 6, about 15% tetrahedron coordination still remained. From the experimental result, it was considered that the tetrahedron network structure was destroyed after GaSb melts, and the short-range order structure was reformed. Therefore the nearest atom space and the coordination number increased, which induced the anomalous variation of viscosity. 3.2. Effect of Te on viscosity of Ga–Sb–Te melts Fig. 2 shows the temperature dependence of the viscosity of Ga–Sb–Te melts during the cooling process. The content

(3)

in which ρ is the density of the specimen. The DSC equipment used in our experiments is a STA 409 EP-type, high-temperature differential scanning calorimetry made by NETZSCH Co., Germany. It measures from room temperature to 1500 ◦ C. The rate of heating and cooling was 10 ◦ C per minute under argon atmosphere, with a sample weight of ∼15 mg. In this experiment, the highest temperature used was 900 ◦ C.

3. Results and discussion 3.1. Viscosity of Ga36.5 Sb63.5 melt Fig. 1 shows the temperature dependence of the viscosity of Ga36.5 Sb63.5 melt during the cooling process. The viscosity value decreases with the increase of the temperature. GaSb melt has a low viscosity, from 1.464 to 1.172 mPa s at the temperature area of measurement. The viscosity–temperature curve is not continuous, and there is anomalous change at the temperature of

Fig. 2. Viscosity of Ga–Sb–Te melts as a function of temperature.

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Fig. 3. Logarithm of viscosity for Ga–Sb–Te melts as a function of reciprocal temperature. (a) ln η–1/T curve of Ga36 Sb63.5 Te0.5 melt, (b) ln η–1/T curve of Ga35 Sb63.5 Te1.5 melt, and (c) ln η–1/T curve of Ga34 Sb63.5 Te2.5 melt.

of Te is 0.5, 1.5, and 2.5%, respectively. The viscosity values of all melts decrease with the increase of the melt temperature. From Fig. 2, it is obvious that the effect of tellurium on the viscosity of GaSb melt is different with its different doped content. The more tellurium was mixed, the smaller the corresponding viscosity values were. However, compared with GaSb melt, 0.5% Te and 1.5% Te additions increased the viscosity value, but 2.5% Te addition decreased the viscosity value. It is considered that there exists a content range of Te, in which the addition of tellurium may increase the viscosity value of GaSb melt, and this will be studied further in the future. From Fig. 3, the anomalous change occurs with 0.5% Te addition at the temperature of 820–850 ◦ C and with 2.5%Te addition at 800–830 ◦ C. These anomalous changes, respectively, divide the viscosity curves into two segments: a low-temperature area and a high-temperature. At the same time the viscosity–temperature curve shows good Arrhenius behavior with 1.5% Te addition.

The viscosity of the melt is estimated well by the following expression [26]:   h E , A= , (4) η = A exp RT vm in which, η is the viscosity, R the Boltzmann constant, h the Planck’s constant, T the absolute temperature, vm the flow unit volume, and E is the activation energy (which is the required energy to move a flow unit from one balanced position to another in melt). With respect to the expression (4), the logarithm for both equal sign was made as follow: L n η=L n A+

Ev RT

(5)

From Eq. (5), we can calculate the values of the viscous activation energy E and the size of flow unit, as shown in Table 1. For Ga36 Sb63.5 Te0.5 melt both its values of the viscous activa-

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461

Table 1 E and vm data of Ga–Sb–Te melts at different temperature zones High temperature area

Ga36 Sb63.5 Te0.5 Ga35 Sb63.5 Te1.5 Ga34 Sb63.5 Te2.5

(10−25

E (eV)

vm

0.0438 0.0551 0.0321

9.38 9.302 10.22

cm−3 )

Low temperature area E (eV)

vm (10−25 cm−3 )

0.0457 0.0551 0.0330

10.02 9.302 9.77

tion energy E and its size of the flow unit increase with the decrease of temperature. The variation of the E values indicates that the energy required to move from one melt state to another is different. That is to say, when the restricted particles change into free ones, different amount of energy is needed. At higher temperature, the bondage between flow units is smaller, so correspondingly the energy of destroying this bondage is smaller. vm implies the size of flow unit in the melt, the bigger vm is the more the particles (atom, hydronium, atomic groups, etc.) gathering inside of the flow units are. The variation of vm implies that the liquid structure changes from short-range order to longrang order during the cooling process, so vm increases with the decrease of temperature. The value of the viscous activation energy E of Ga34 Sb63.5 Te2.5 melt also increases with the decrease of temperature, but its size of the flow unit decreases with the decrease of temperature. For Ga34 Sb63.5 Te2.5 , it is considered that the shrinkage degree of flow units suddenly decreases and cavity thickness between flow units increases at 800–830 ◦ C, which makes atom move difficultly and decreases the fluidness of the melt, then sudden increase of the melt viscosity was taken place. The activation energy and the size of the flow unit for Ga35 Sb63.5 Te1.5 keep the same values in whole temperature area of measurement, which apparently shows good Arrhenius linearity. And it is indicated that no changes occur in the structure of Ga35 Sb63.5 Te1.5 melt. Fig. 4 is the DSC curve of Ga–Sb–Te system during the heating process. From the DSC curve of Ga36 Sb63.5 Te0.5 , it shows that there is one endothermic peak on the curve at 780–820 ◦ C, and also there is endothermic peak on the DSC curve of Ga34 Sb63.5 Te2.5 at 780–820 ◦ C. From the results, it is evident that the anomalous range of viscosity–temperature curve corresponds preferably with the range of endothermic peak. They therefore suggest that there may be liquid–liquid structure change. Regarding the temperature discrepancy between the range of anomalous points and endothermic peak, it was thought that it might possibly be attributed to the variation of experimental conditions. In addition, there is no endothermic peak or exothermic peak on the DSC curve of Ga35 Sb63.5 Te1.5 , corresponding with the characteristic of its viscosity curve. The change of liquid structure is the rearranging of atoms or atomic clusters. The whole process deals with the break of old bonds and the formation of new bonds. The break of old bonds requires energy in order to overcome the interatomic force, which corresponds to the endothermic peak on the DSC curve. It releases energy and corresponds to the exothermic peak. This thermal effect is caused by the energy change induced by the breaking of old bonds and the formation of new bonds.

Fig. 4. DSC curves of Ga–Sb–Te melts (10 K/min). (a) DSC curve of Ga36 Sb63.5 Te0.5 , (b) DSC curve of Ga35 Sb63.5 Te1.5 , and (c) DSC curve of Ga34 Sb63.5 Te2.5 .

4. Conclusions (i) The viscosity of Ga36.5 Sb63.5 increases with the decrease of temperature, and there is an overall exponential relationship. There is anomalous point on the curve at the temperature of 800–850 ◦ C.

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(ii) The effect of Te addition on GaSb melt is different. Compared with GaSb melt, 0.5% Te and 1.5% Te addition increase the viscosity value, but 2.5% Te addition decreases the viscosity value. (iii) The anomalous change occurs with 0.5% Te addition at the temperature of 820–850 ◦ C, and with 2.5% Te addition at 800–830 ◦ C. The ranges of anomalous change correspond preferably with the ranges of thermal effect peak on DSC curve. The viscous activation energy and the size of the flow units of the Ga–Sb–Te melt are obtained and analyzed at different temperature zones. Acknowledgements The authors gratefully acknowledge the support of the National Natural Science Foundation of China (No. 50371047) and the province National Science Foundation of Shan Dong (No. Y2006F55). References [1] F.Q. Zu, Z.G. Zhu, L.J. Guo, B. Zhang, J.P. Shui, C.S. Liu, Phys. Rev. B 64 (2001) 180203(R). [2] F.-Q. Zu, L.-J. Guo, Z.-G. Zhu, Y. Feng, Chin. Phys. Lett. 19 (1) (2002) 94. [3] F.Q. Zu, Z.G. Zhu, B. Zhang, Y. Feng, J.P. Shui, J. Phys., Condens. Matter. 50 (13) (2001) 11435. [4] F.-Q. Zu, Z.-G. Zhu, L.-J. Guo, X.-B. Qin, H. Yang, W.-J. Shan, Phys. Rev. Lett. 89 (12) (2002) 125505. [5] H. Geng, H. Geng, X. Xue, Mater. Charact. 51 (2003) 29–33.

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