Synthesis and investigation of BiSbTeSe single crystal doped with Zr produced using Bridgman method

Emina Požegaa,*, Pantelija Nikolićb, Slavko Bernikc, Lidija Gomidželovića, Nebojša Labusb, Milan Radovanovićd, Saša Marjanoviće

a Mining and Metallurgy Institute Bor, Zeleni Bulevar 35, 19210 Bor, Serbia

b Institute of Technical Sciences of SASA, Knez Mihailova 35/IV, 11000 Belgrade, Serbia

c Jožef Stefan Institute, Jamovacesta 39, 1000 Ljubljana, Slovenia

d Faculty of Technical Sciences, University of Novi Sad, Trg Dositeja Obradovića 6, 21000 Novi Sad, Serbia

e University of Belgrade, Technical Faculty Bor, VJ 12, 19210 Bor, Serbia

(*Corresponding author: emina.pozega@irmbor.co.rs)

 

ABSTRACT

Single crystal ingot of BiSbTeSe doped with Zr was synthesized using Bridgman method. Energy dispersive spectrometry (EDS) analysis was used to determine chemical composition of studied samples as well as to check and confirm samples homogeneity. X-ray diffraction (XRD) measurements proved that obtained crystal ingot is a single cristal and confirms Bi2Te3-type compound with orientation (00l) of single crystal. Melting point was determined by dilatometrically measured shrinkage during heating. Mobility, concentration, resistivity/conductivity and Hall coefficient of BiSbTeSe doped with Zr samples were determined using a Hall Effect measurement system based on the Van der Pauw method. The Hall Effect was measured at room temperature with an applied magnetic field strength of 0.37 T at different current intensities. The measured ingot samples were cut and cleaved from different regions. Calculated results obtained using a Hall Effect measurement system (Ecopia, HMS-3000) were mutually compared for cleaved and cut samples. Changing of transport and electrical parameters with the increase of the current intensity was also monitored. The results confirmed that electrical and transport properties of single crystal depend on crystal growth direction and mobility was also significantly improved in comparison with theoretical value of Bi2Te3 and available literature data.

 

RESUMEN

Síntesis e investigación del mono cristal BiSBTeSe dopado con Zr obtenido mediante el método de Bridgman. El lingote del mono cristal de BiSbTeSe dopado con Zr se sintetizó utilizando el método de Bridgman. La composición química se determinó mediante análisis con espectroscopía de dispersión de energía (EDS). Mediante difracción de rayos X (DRX) se demostró que el lingote de cristal obtenido es un mono cristal y confirmó que se trata de un compuesto del tipo Bi2Te3, con orientación del mono cristal (001). El punto de fusión se determinó por medidas dilatométricas. La movilidad, concentración, resistividad/conductividad, y el coeficiente de Hall del BiSbTeSe dopado con Zr, se determinaron utilizando un sistema de medición de efecto Hall basado en el método de Van der Pauw. El efecto Hall se midió a temperatura ambiente con una intensidad de campo magnético aplicada de 0,37 T a diferentes intensidades de corriente. Las muestras de lingotes medidos se cortaron y se rompieron de diferentes regiones.

Las muestras de lingotes utilizadas en las medidas fueron obtenbidas mediante corte y escisión en distintas zonas. Los resultados obtenidos utilizando un sistema de medición de efecto Hall (Ecopia, HMS-3000) en las distintas muestras, se compararon entre sí. Los resultados confirmaron que las propiedades eléctricas y de transporte del mono cristal dependen de la dirección del crecimiento del cristal. La movilidad se mejoró significativamente en comparación no solo con el valor teórico de Bi2Te3 sino también con los datos existentes en la literatura.

 

Submitted: 22 June 2016; Accepted: 25 April 2017; Available On-line:

Citation/Citar como: Požega, E.; Nikolić, P.; Bernik, S.; Gomidželović, L.; Labus, N.; Radovanović, M.; Marjanović, S. (2017) “Synthesis and investigation of BiSbTeSe single crystal doped with Zr produced using Bridgman method”. Rev. Metal. 53(3):e100. http://dx.doi.org/10.3989/revmetalm.100

KEYWORDS: Bridgman method; Dilatometric analysis; EDS; Hall Effect measurement system; Single crystal; XRD

PALABRAS CLAVE: Análisis dilatométrico; EDS; Método Bridgman; Monocristal; Sistema de medición de efecto Hall; XRD

ORCID ID: Emina Požega (http://orcid.org/0000-0001-6797-2435); Pantelija Nikolić (http://orcid.org/0000-0002-2196-4736); Slavko Bernik (http://orcid.org/0000-0001-7330-3094); Lidija Gomidželović (http://orcid.org/0000-0002-0783-0328); Nebojša Labus (http://orcid.org/0000-0003-1557-0711); Milan Radovanović (http://orcid.org/0000-0002-9702-3879); Saša Marjanović (http://orcid.org/0000-0001-6930-2232)

Copyright: © 2017 CSIC. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) Spain 3.0.


 

CONTENT

1. INTRODUCTIONTOP

Bismuth telluride was widely studied as a thermoelectric material, particularly in temperature range around 27 ºC (Fleurial et al., 1988) Jariwala et al., 2015). Bi2Te3 has useful application in the field of thermoelectric devices as solid state coolers for cooling of electronic components and in space technology for military purposes (Požega et al., 2015), like generators Bi2Te3 have found application in space craft. Therefore, studies on various properties of bismuth telluride are interesting for both basic and applied research. Thermoelectric performance of Bi2Te3 is characterized by dimensionless figure of merit, ZT. Study of bismuth telluride showed a sharp maximum in figure of merit as a function of stoichiometric deviation (Fleurial et al., 1988). Doping of semiconducting compounds is largely used as a method of controlled synthesis. Transport property measurements in doped Bi2Te3 single crystals with Se and Fe have been studied by measuring the thermoelectric power and electrical conductivity in the temperature range 30 ºC to 200 ºC (Jariwala et al., 2015). They focused on improving the transport properties by better control of the stoichiometry through a uniform impurity distribution during the growth process. Se doped Bi2Te3 single crystals have been reported to be thermoelectric materials with maximum figure of merit, ZT~1.04 at 125 ºC (Yan et al., 2010; Soni et al., 2012; Guo et al., 2014; Jariwala et al., 2015). (BiSb)2(TeSe)3 belongs to the family of narrow-band semiconductors A2V B3VI (A = Bi, Sb and B = Te, Se) with the tetradymite structure (Kašparová et al., 2005; Rowe, 2005). All semiconductors of the general formula (BiSb)2(TeSe)3 are solid solutions, which are isostructural with the Bi2Te3. They have layered structure of Bi2Te3 type. Layered structure of Bi2Te3 compound is a result of much stronger interatomic bonds between Te atoms in a plane layer of the same bond between layers. The reduction in the lattice component of thermal conductivity, λL can be achieved through the use of solid solutions (Rosi et al., 1959) of bismuth telluride with the isomorphous compounds antimony telluride and bismuth selenide (Goldsmid, 2014). Also improvement of power factor (α2σ), where α is Seebeck coefficient, σ is the electrical conductivity, can be obtained by the adoption of nanostructures (Goldsmid, 2014). Transport properties of the quaternary systems of BiSbTeSe have been measured as function of temperature in the temperatura range of 27 ºC to 227 ºC (Farag et al., 2010). They obtained a concentration of majority charge carriers of 1019 cm-3. Problem of optimizing impurity content of bismuth telluride alloys for specific temperatures were studied (Rowe, 2005). Generator materials up to 800 ºC have been reviewed, specifying Bi2Se0.6Te2.4 and Bi0.5 Sb1.5Te3, as the n-type and p-type materials up to 300 ºC, but without discussing of carrier concentration optimization (Dashevsky et al., 1997). The investigated properties of Bi0.4Sb1.6Te3 with 0.3 wt% of PbTe showed that ZT rises from about 1.0 at 50 ºC to 1.1 at 107 ºC and then fell to 0.8 at 350 ºC (Kusano and Hori, 2002). The properties of n-type BixSb2−xTe3, with x between 0.5 and 0.7, were studied (Gerovac et al., 2010). Mechanochemical synthesis of a nanocrystalline thermo electric compound were investigated and found that if the particle size is being reduced, structural defects will be increased and hence solid state chemical reactions will be enhanced (Rojas-Chávez et al., 2010). Various investigations were performed improvement of thermoelectric performance of Bi2Te3 alloy, too (Wu et al., 2013).

In this paper BiSbTeSe doped with Zr was investigated. Attention was paid to the improvement of the electrical performance of BiSbTeSe. After review of available literature data, it has been concluded that this type of multi-component material has not been synthesized and studied until now.

2. MATERIALS AND METHODSTOP

Single crystal ingot of BiSbTeSe doped with Zr was synthesized using the Bridgman method at the maximum temperature of 600 ºC. High purity elements (5 N and 6 N) were used as the source material. Bismuth (Sigma-Aldrich, 99.999%), antimony shot (Koch Light Labaratories LTD, 99.9999%), selenium shot (Alfa Aesar, 99.999%) and tellurium (Sigma-Aldrich, 99.999%) were taken in determined proportion and sealed in quartz ampoule under pressure of 10–5 Pa. Zirconium (KEFO) was of 99.98% purity. Quartz ampule was lowered at a speed of 2.2 mmh–1 in the field of vertical furnace (Nikolić et al., 2010; Dordević et al., 2012). A slow rate of crystallization was desirable to avoid the internal stresses and microcracks. P type single crystal sample was synthesized in a vertical configured furnace with very precise temperature regulation. Single crystal of Zr doped BiSbTeSe with dimensions of 80 mm in length and 9.2 mm in diameter were obtained. Obtained ingot was cut into plates with dimensions of 2.2 mm and cylinders with 9 mm in length. Cylinders were cleaved longitudinally in order to obtain square shape plates with thickness between 1.5 to 2 mm and with very flat and glossy surface. Quartz ampoule had the conical shape in the bottom for easier formation of seed. The inside of the ampoule was protected in order to prevent contact between the growing sample and quartz ampoule wall. Analyses of samples composition were performed by an energy dispersive spectroscopy (EDS), type Oxford Inca 3.2, coupled with the scanning electron microscope Jeol JSM 5800, operated at 20 keV. X-ray intensity measurement was carried out on a PHILIPS PW 1710 automated diffractometer using monochromatized Cu Kα radiation. Melting point of the sample was determined on the dilatometer Bähr GmbH Thermoanalyse 802 s. Dilatometric analyses were carried out under following conditions: air atmosphere, heating rate 10 oCmin-1 and Tmax = 600 °C. Rectangle cross-section sample with dimensions 7.74x4.82 mm with thicknes of 1.71 mm was used for the investigations. Al2O3, with dimensions 2x1.26 mm and l0 of 0.45 mm, was used as a reference material. Hall measurements were performed on a Hall Effect measurement system (Ecopia, HMS-3000) at room temperature and different electric currents intensities for the applied magnetic field of 0.37 T. Software program of Hall Effect measurement system (Ecopia, HMS-3000) automatically calculated bulk and sheet carrier concentration, specific resistance, conductivity and Hall coefficient. Calculations were done on the basis of voltage obtained by Van der Pauwlaws (Pauw Van der, 1958) and entered input data into the software (sample thickness D, current intensity I, the magnetic induction of permanent magnet B). Data from Hall measurements at room temperature were collected using Schottky diode contacts. The measured samples were cut and cleaved from different regions of the ingot, with cutting route perpendicular (⊥) and cleaved parallel (׀׀) to the plane of crystallisation. In the following text these samples will be marked as 8/3 (⊥), 8/4 (⊥), 8/5 (׀׀), 8/6 (⊥) and 8/8 (׀׀), respectively. Samples cut perpendicular (⊥) to the growth of the ingot were in the form of discs and samples cleaved parallel (׀׀) to the plane of crystallisation were square cross-section. Cut samples were then highly mechanically polished. The measured samples were cleaned in acetone before the measurements.

3. RESULTS AND DISCUSSIONTOP

3.1. Scanning electron microscopy with energy dispersive spectrometry (SEM-EDS)TOP

Samples 8/5 (׀׀) and 8/8 (׀׀) were attached on the cut samples 8/3 (⊥), 8/4 (⊥) and 8/6 (⊥) since all the samples were from the same ingot. Therefore, energy dispersive spectrometry measurements were performed only on 8/5 (׀׀) and 8/8 (׀׀) samples to estimate composition of ingot. The experimental results of the samples 8/5 (׀׀) and 8/8 (׀׀) composition, done by energy dispersive spectrometry analysis, are presented in Table 1, where also the presence of Zr can be observed. The ingot top contains more zirconium, bismuth and tellurium, as shown in Table 1. At the end of the crystal there is more antimony and selenium. Based on energy dispersive spectrometry analysis it is observed a good homogeneity of single crystal composition with variation inside experimental error of energy dispersive spectrometry analysis.

Table 1. Representation of elements in samples in atm.% obtained by SEM-EDS analysis
Element  
Sample Bi Sb Te Se Zr
8/5 (׀׀) 11.83 24.32 63.10 0.23 0.52
8/8 (׀׀) 11.48 24.75 63.01 0.26 0.50

3.2. X-ray diffraction analysis (XRD)TOP

X-ray diffractogram for the single crystal of Zr doped BiSbTeSe sample is depicted in Fig. 1. Only Bragg reflections are given in this diffraction pattern. Using the values obtained for the lattice parameters of the Bi2Te3 compound and ASTM tables, it was calculated that these are Bragg reflections of the planes with Miller’s indices (003), (006), (009), (0012), (0015) and (0018). Single crystal plate was cleaved, from the ingot, and Bragg’s diffractions confirmed it orientation (00l).

Figure 1.  X-ray diffractogram of single crystal BiSbTeSe doped with Zr.

 

3.3. Dilatometric analysisTOP

Obtained temperature (572 ºC) on Fig. 2 is close to melting temperature of Bi2Te3 compound, but slightly decreased, which can be contributed to the presence of Se, Sb and Zr. Declared melting point helped us to determine the temperature which has to be passed during synthetitc heating. Also, high melting point of Zr was intriguing since it can be found in the form of oclusions in telluride phase. Energy dispersive spectrometry analysis proved that Zr aded as metal in zero valent state is incorporated in formed lattice of telluride.

Figure 2.  Dilatometric curve of Zr doped BiSbTeSe single crystal.

 

3.4. Hall Effect measurements at room temperatureTOP

Hall Effect measurements at room temperature were carried out by passing a 0.1, 0.5, 1 and 5 mA current through the samples under magnetic field of 0.370 T. All calculated data from Hall measurements at room temperature for samples: 8/3 (⊥), 8/4 (⊥), 8/5 (׀׀), 8/6 (⊥) and 8/8 (׀׀) are presented in Figs. 3-8.

Figure 3.  Dependence of: (a) bulk and sheet carrier concentration and (b) specific resistance and conductivity from current intensity for sample 8/3 (⊥) at room temperature.

 

Figure 4.  Dependence of: (a) bulk and sheet carrier concentration and (b) specific resistance and conductivity from current intensity for sample 8/4 (⊥) at room temperature.

 

Figure 5.  Dependence of: (a) bulk and sheet carrier concentration and (b) specific resistance and conductivity from current intensity for sample 8/5 (׀׀) at room temperature.

 

Figure 6.  Dependence of; (a) bulk and sheet carrier concentration and (b) specific resistance and conductivity from current intensity for sample 8/6 (⊥) at room temperature.

 

Figure 7.  Dependence of: (a) bulk and sheet carrier concentration and (b) specific resistance and conductivity from current intensity for sample 8/8 (׀׀) at room temperature.

 

Figure 8.  The variation of: (a) Hall coefficient RH and (b) mobility with current for samples: 8/3 (⊥), 8/4 (⊥), 8/5 (׀׀), 8/6 (⊥) and 8/8 (׀׀) at room temperature.

 

The results of specific resistance and conductivity are displayed in Figs. 3b, 4b, 5b, 6b and 7b. Figs. 3b, 4b, 5b, 6b and 7b show how resistivity and conductivity varies in the current range 0.1 – 5 mA. From these figures, it can be observed that specific resistivity is decreasing with the current intensity, i.e. electrical conductivity increases with the current intensity. Values of specific conductivity vary from 104 to 2615 Ω–1cm–1and are higher for samples 8/5 (׀׀) and 8/8 (׀׀). The Hall Effect measurements show that the carrier concentration increases. The results are displayed in Figs. 3a, 4a, 5a, 6a and 7a, giving the bulk and sheet carrier concentration, as a function of the current intensity, A.

As shown in Fig. 3a, bulk and sheet carrier concentration for sample 8/3 (⊥) increases with the current intensity up to 1 mA. There after, bulk and sheet carrier concentration decreases. From Fig. 3b it can be observed that specific resistivity is decreasing with the current intensity increasing, i.e. electrical conductivity increases with the current intensity. Dependence of bulk and sheet carrier concentration from the current intensity for simple 8/4 (⊥) are displayed in Fig. 4a. Sheet carrier concentration is fairly constant and was found that is 1018 cm−2. Bulk carrier concentrations at the current intensity of 0.1 mA and 0.5 mA are nb = 1.889x1018 cm−3, nb = 1.034x1019 cm−3 respectively. At the current intensity of 1 mA bulk carrier concentration decreases to value of 8.583x1018 cm−3. Thereafter bulk carrier concentration increases as the current intensity increases. The specific resistance of sample 8/4 (⊥) shown in Fig. 4b decreases as the current intensity increases from 0.1 to 1 mA and after that specific resistance increases. The plot of the specific conductivity dependence from the current intensity for sample 8/4 (⊥) is shown in the Fig. 4b. As it can been seen the specific conductivity increases as the current intensity increases from 0.1 to 1 mA. At the current intensity of 1 mA specific conductivity is σ = 5.559x102 Ω−1cm−1. After that specific conductivity decreases. Dependence of bulk and sheet carrier concentration from the current intensity for sample 8/5 (׀׀) is displayed in Fig. 5a. Bulk and sheet carrier concentration are fairly constant in range of 0.1 – 1 mA. The diagram show that bulk and sheet carrier concentrationare in the range of 1017 – 1020 cm−3. Power factor is related to carrier concentration and is maximized at n ~ 1020 cm−3 in semiconductors (Snyder and Toberer, 2008). On Fig. 5b is displayed dependence of specific resistance and conductivity from the current intensity for simple 8/5 (׀׀). It may be noted that the specific resistance decreases as the current intensity increases and is fairly constant in range of 1–5 mA. Specific conductivity increases as the current intensity increases. Dependence of bulk and sheet carrier concentration from the current intensity for sample 8/6 (⊥) are displayed in Fig. 6a. Bulk and sheet carrier concentration are fairly constant in range of 0.1–1 mA. As it can be seen from Fig. 6b the specific conductivity increases and the specific resistanceof simple 8/6 (⊥) decreases as the current intensity increases. Bulk and sheet carrier concentration for simple 8/8 (׀׀) increases with the current intensity (Fig. 7a). The results of specific resistance and conductivity dependence from the current intensity for sample 8/8 (׀׀) are show in Fig. 7b. We see that the resistance decreases (except for current intensity of 1 mA) with the current intensity, showing metallic character of the material. A little deviation in derived measurement results of specific resistance, conductivity, bulk and sheet carrier concentration can be contributed to interval time between measurements.

The variation of Hall coefficient RH with the current intensity for all samples is shown in Fig. 8a. Values of Hall coefficient are positive for all samples and decreases with current. It means that our single crystal is p-type in nature and majority charge carriers are holes. This can be support by Seebeck coefficient measurements (Deshpande et al., 2009).

A change in carrier mobility with the current increase is also researched. The cleaved samples 8/5 (׀׀) and 8/8 (׀׀) had highest mobility of free carriers-holes.

As shown on Fig. 8b mobility calculated from Hall measurements was found to decrease with the increase of the current intensity for all samples. In this case, the mobility for the samples 8/5 (׀׀) and 8/8 (׀׀) is higher than theoretical value for Bi2Te3 which is 510 cm2V−1s−1(Gol’cman et al., 1972). It is about a factor of 4 better for samples 8/5 (׀׀) and 8/8 (׀׀) than theoretical value for Bi2Te3.

4. CONCLUSIONSTOP

–  BiSbTeSe single crystal doped with Zr was successfully synthesized. XRD analysis confirmed Bi2Te3 structure and (00l) orientation. Scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) indicates that single crystal has a homogeneous composition.
–  The content of Bi, Sb, Te, Se and Zr in samples was obtained using Energy Dispersive X-ray Spectroscopy (EDS) analysis. Along the ingot, the concentration of Zr decreased from the top to the end of the ingot. Bragg’s diffractions confirmed orientation (00l) for cleaved sample. Melting point is determined using dimensions from graphics change in function of time (Fig. 2). The results of Hall measurements in two different directions on high quality single crystal obtained by standard Bridgman method were presented. Measurements were taken in the direction of crystal growth and normal to the direction of crystal growth.
–  Using the Hall Effect measurement system, it was proved that all samples were of the “p” type with high carrier mobility of 103 cm2Vs−1 at 25 ºC and high carrier concentration of 1019 cm−3. Mobility varies from 2.173x103, 1.723x103, 1.752x103 and 4.648x101 for sample 8/5 (׀׀). For sample 8/8 (׀׀) mobility is in range 1.926x103 - 2.146x102 cm2Vs−1. Measurements of single crystal electrical properties confirmed their high quality and potential for practical application.

 

ACKNOWLEDGMENTSTOP

Author wishes to thank Stevan Vujatović for manufacturing high-quality monocrystalline ingots and Dr. Dorđe Veljović for his assistance in the characterization of samples. The authors are grateful to the Ministry of Education, Science and Technological Development of the Republic of Serbia for financial support of project: “Development of ecological knowledge-based advanced materials and technologies for multifunctional application” (Grant Nº TR34005).

 

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