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Electrical transport properties of semiconducting chromium molybdenum diselenide single crystals
Materials Science in Semiconductor Processing 24 (2014) 40–43
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Electrical transport properties of semiconducting chromium molybdenum diselenide single crystals Priyanka Desai n, D.D. Patel, A.R. Jani Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India
a r t i c l e in f o
Keywords: Chromium mixed molybdenum diselenides Electrical transport properties Single crystals
abstract Mixed chromium–molybdenum diselenides (CrxMo1 xSe2 (x ¼ 0.25, 0.50, 0.75)) have been grown in single crystalline forms by the chemical vapor transport technique. Electrical transport properties like electrical resistivity (perpendicular and parallel to the c-axis), thermoelectric power measurements at high temperature and Hall effect measurements at room temperature were performed on these single crystals. Preliminary study of electrical measurements suggest a semiconducting behavior of CrxMo1 xSe2 (x ¼0.25, 0.50, 0.75) single crystals. Data of Hall coefficient and thermoelectric power have good agreement with each other and confirms the p-type nature of these crystals. Our findings should motivate an in-depth investigation of the underlying mechanisms. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recently, Transition Metal Dichalcogenides (TMDCs) have attracted to a large extent because of their possible applications as lubricants, switching devices and photoelectrochemical solar energy converters and also marked anisotropy in many of their physical properties. They form a wide range of solid solutions [1,2] with either mixed metals or mixed chalcogenide compositions. The crystals of TMDCs are layered type, each layer being a sandwich of chalcogen–metal–chalcogen sheets. The metal–chalcogen bonding is partly ionic and partly covalent whereas the interlayer bonds are of Van der Waal's type. This crystalline anisotropy leads to anisotropy in their electrical properties. The VIB–VIA group of compounds have been studied extensively for their electrical properties [3–5]. Also, studies have been made so far on mixed systems such as (Mo/W)Te2, (Mo/W)Se2 [6] and (Mo/W)(Se/Te)2 [7]. It appears from the literature that no attempt has been made to investigate the variation of properties like resistivity, Hall coefficient and thermoelectric power with the
n
Corresponding author. E-mail address: [email protected] (P. Desai).
http://dx.doi.org/10.1016/j.mssp.2014.02.052 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
composition of (Cr/Mo)Se2 solid solution in a single crystalline form. Recently we have reported the growth of single crystals of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) by the chemical vapor transport technique using dual zone horizontal furnace [8]. In this paper [8] we have studied structural parameters by mean of powder X-ray diffraction, SAED pattern, surface topography and optical response of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals. It is evident that the electrical resistivity is a physical property of enormous importance, both for the understanding of the solids and their actual applications. The temperature dependence of the electrical resistivity changes in a quite irregular manner because of various mechanisms, including the phonon scattering, mutual scattering of electrons and so forth, are involved in the electrical transport in different temperature ranges [9,10]. The thermoelectric effect proposes a distinct advantage over other methods because the measured thermoelectric voltage is directly related to the carrier concentration which makes the thermoelectric measurements simpler even for high mobility materials [4]. The study of thermoelectric power provides an independent way to determine carrier sign, density and position of Fermi level in semiconductors [11,12]. The materials with high electrical conductivity and large thermoelectric power possess outstanding thermoelectric properties. The temperature dependence of the electrical
P. Desai et al. / Materials Science in Semiconductor Processing 24 (2014) 40–43
conductivity is inclined to many influencing factors. For a semiconductor, both the carrier mobility and carrier density are temperature dependent [13]. The objective of this study was to evaluate electrical resistivity (perpendicular and parallel to the c-axis), thermoelectric power and Hall coefficient of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals. 2. Experimental Single crystals of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) were grown by the chemical vapor transport technique using iodine as a transporting agent. The growth parameters are shown in Table 1. The stoichiometric compositions of all the samples were confirmed through EDAX. The high temperature resistivity measurements parallel and perpendicular to the c-axis were carried out in the temperature range 303–423 K for all the samples using four probe setup (Model:DFP-02), SES instruments Pvt. Ltd., Roorkee. Several readings were taken over different regions of specimen and consistent results were obtained in each case. The anisotropy ratio was calculated using the values of resistivity parallel and perpendicular to the c-axis. In order to evaluate the semiconducting nature of samples, Hall effect measurements were done by using the instrument Lakshore 7504 Hall measurement system at room temperature. For the confirmation of ohmic nature of the contacts, I–V characteristics for each contact were verified. The variations of thermoelectric power (TEP) with temperature were performed in the temperature range 308–423 K with the use of thermo power setup TPSS-200, scientific solutions, Mumbai, India. This experimental setup consists of two assemblies: one is the sample chamber with two heaters and pickup probes and the other is electronic circuits to control the temperature and also the temperature gradient across the sample. The temperature is measured by the thermocouple and the gradient is measured by the deferential temperature sensor. The sample is directly mounted on the two heaters and by applying the temperature gradient (5 K) between two ends of the sample, the thermoelectric power is generated which is measured by digital voltmeter of Keithly.
41
and parallel to the c-axis) were studied from 303 K to 423 K. The results plotted as log ρ vs. 1000/T for parallel and perpendicular to the c-axis are shown in Figs. 1 and 2 respectively. It is seen from Figs. 1 and 2 that there is a linear decrease in resistivity with increasing temperature, which is a characteristic of semiconducting materials. Also, the values of resistivity are higher in parallel direction than the perpendicular direction which suggests the conduction of electrons is very low in parallel direction. The conductivity curves seen to be in good agreement with the models related to the thermal carrier emission across
Fig. 1. Log ρ vs. 1000/T for parallel to the c-axis.
3. Results and discussion 3.1. Resistivity measurements The resistivity variations of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals with temperature (both perpendicular
Fig. 2. Log ρ vs. 1000/T for perpendicular to the c-axis.
Table 1 Growth parameters of CrxMo1 xSe2 (x ¼ 0.25, 0.50, 0.75) single crystals. Compound
Compound preparation
Single crystal growth Temperature distribution
Cr0.25Mo0.75Se2 Cr0.5Mo0.5Se2 Cr0.75Mo0.25Se2
Temperature (1C)
Time (h)
Cold zone (1C)
Hot zone (1C)
1000 1000 1000
100 95 90
900 920 950
1050 1050 1050
Growth time (h)
Dimensions (of largest crystal) (mm3)
246 240 220
11 6 0.3 13 7 0.3 10 6 0.4
42
P. Desai et al. / Materials Science in Semiconductor Processing 24 (2014) 40–43
grain boundary barriers where a temperature dependence of conductivity is expected to yield straight line in Arrhenius plots in the whole temperature range [14]. The electrical resistivity decreases slowly with temperature, and the reason is that the carrier concentration in this temperature range is determined by the number of ionized donor liberated from the impurity level. From this region the activation energy was calculated, indicating that the donor level is positioned below the bottom of the conduction band. The thermal activation energy was calculated from the slope of the curve (Figs. 1 and 2) by using the relation Ea ρ ¼ ρ0 exp ð1Þ kB T where, Ea is activation energy, kB is Boltzmann constant, and T is absolute temperature. It is seen that there is only one activation energy value for all the samples (Table 2) over the entire temperature range investigated. The low values for activation energy suggest that the measured resistivity is the result of the extrinsic process in these crystals. This can be attributed to the high carrier concentration density which gives rise to energy levels very close to the conduction band [15]. Fig. 3 demonstrates the behavior of anisotropy variation with temperature, that clearly indicates that electronic conduction in CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals is highly anisotropic. This is quite obvious because the temperature dependence of electrical resistivity parallel and perpendicular to the c-axis is completely different. This high anisotropy in electronic behavior may be due to the Table 2 Activation energy calculated from log ρ vs. 1000/T. Samples
Activation energy (eV) (parallel to the c-axis)
Cr0.25Mo0.75Se2 0.179 Cr0.5Mo0.5Se2 0.146 Cr0.75Mo0.25Se2 0.104
Activation energy (eV) (perpendicular to the c-axis) 0.093 0.085 0.063
larger effective mass anisotropy to the layers of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals. 3.2. Hall effect measurements The mobility of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals was determined by measuring the change in resistance R upon applying a magnetic field perpendicular to the c-axis of the samples. Hall mobility (μH), Hall coefficient (RH) and carrier concentration (n) are calculated using the following relations: μH ¼
tΔR Bρ
RH ¼ μH ρ n¼
1 RH e
ð2Þ ð3Þ ð4Þ
where, t is the thickness of crystal, B is applied magnetic field, ΔR is change in magnetic field and ρ is the resistivity. All the results obtained from Hall effect measurements are tabulated in Table 3. The positive values of Hall coefficients indicate that the majority of the charge carriers in CrxMo1 xSe2 (x ¼0.25, 0.50, 0.75) single crystals are holes. Also, carrier concentration increases with increase in chromium content which denotes extra charge carriers. 3.3. Thermoelectric power measurements The variation of thermoelectric power (S) with the inverse of temperature for CrxMo1 xSe2 (x¼ 0.25, 0.50, 0.75) single crystals is shown in Fig. 4. Absolute value of S increases steadily with an increase in temperature, confirming the typical semiconducting behavior of the samples. To study the temperature dependence of the thermoelectric power of p-type semiconductor the below expression [16] can be used. Ef kB S¼ Aþ ð5Þ e kB T where, kB is the Boltzmann constant, e is the electronic charge, A is the constant determined by the dominant scattering process and Ef is the separation of Fermi level from the top of the valence band. If thermoelectric power is plotted against the reciprocal of temperature, a straight line is expected from which Ef and A can be determined from the slope and intercept respectively. The value of scattering constant A is given by, A ¼ ð5=2Þ s
Fig. 3. Variation of anisotropy with 1000/T.
ð6Þ
where, s is scattering parameter. The observed positive values of thermoelectric power in the entire investigated temperature range follows from the fact that hole concentration is greater than electron, that is, samples are p-type. This agrees with the results obtained from Hall effect data. According to Ahmed [17] and Guin et al. [18], negative values of scattering parameters (Table 4) for CrxMo1 xSe2 (x ¼0.25, 0.50, 0.75) single crystals indicate that the scattering mechanism
P. Desai et al. / Materials Science in Semiconductor Processing 24 (2014) 40–43
43
Table 3 Hall parameters for CrxMo1 xSe2 (x¼ 0.25, 0.50, 0.75) single crystals. Parameter
Cr0.25Mo0.75Se2
Cr0.5Mo0.5Se2
Cr0.75Mo0.25Se2
Resistivity (Ω cm) (at room temperature) Hall coefficient (RH) (cm3/C) Mobility (μH) (cm2/V s) Carrier concentration (n 1016) (cm 3)
3.44 458.02 133.14 1.36
2.98 366.40 122.95 1.70
2.57 297.71 115.84 2.10
with p-type conductivity. Analysis of the electrical resistivity, Hall effect and thermoelectric power data allows us to deduce many important parameters such as activation energy, mobility, Hall coefficient, carrier concentration and scattering parameter. The values of scattering parameter indicate that carrier scattering may be dominated by acoustic phonons. This mode of investigation is an ideal way for finding out the possibility of making applications for these semiconductor crystalline compounds especially in the field of energy conversion, semiconductor devices and electronic engineering.
Acknowledgments
Fig. 4. Variation of thermoelectric power with 1000/T.
Priyanka Desai and D.D. Patel are thankful to UGC, New Delhi, India for providing financial support under Meritorious Research Fellowship. References
Table 4 Values of slope, intercept, Fermi energy (EF), scattering constant (A) and scattering parameter (s) for CrxMo1 xSe2 (x¼ 0.25, 0.50, 0.75) single crystals. Samples
Slope
Intercept
EF (eV)
A
s
Cr0.25Mo0.75Se2 Cr0.5Mo0.5Se2 Cr0.75Mo0.25Se2
891.14 854.57 789.07
3451.83 2989.41 2736.46
0.891 0.854 0.789
3.451 2.989 2.736
0.951 0.489 0.236
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
corresponds to carrier scattering by acoustic phonons by assuming a single parabolic band model. 4. Conclusions
[11] [12] [13] [14]
The electrical transport properties of CrxMo1 xSe2 (x ¼0.25, 0.50, 0.75) single crystals have been studied with high temperature electrical resistivity, thermoelectric power and also room temperature Hall effect measurements. This work has shown that CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals have a semiconducting nature
[15] [16] [17] [18]
I.F. Schneemeyer, M.J. Sienko, Inorg. Chem. 19 (1980) 789. B.F. Mentzen, M.J. Sienko, Inorg. Chem. 15 (1976) 2198. J.A. Wilson, A.D. Yoffe, Adv. Phys. 18 (1969) 193. L.H. Brixner, J. Electrochem. Soc. 110 (1963) 289. R. Mansfield, A. Salam, Proc. Phys. Soc. B66 (1953) 377. E. Revolinsky, D. Beerntsen, J. Appl. Phys. 35 (1964) 2086. J.A. Champion, Br. J. Appl. Phys. 16 (1965) 1035. Priyanka Desai, D.D. Patel, A.R. Jani, J. Cryst. Growth 390 (2014) 12. C. Kittle, Introduction to Solid State Physics, 8th ed. Wiley, New York, 2004. N.W. Ashcroft, N.D. Mermin, Solid State Physics, Thomson Learning, Orlando, 1976. L.H. Brixner, J. Inorg. Nucl. Chem. 24 (1962) 257. S.N. Guin, A. Chatterjee, D.S. Negi, R. Datta, K. Biswas, Energy Environ. Sci. 6 (2013) 2603. P.Y. Yu, M. Cardona, Fundamentals of Semiconductors, 2nd ed. Springer, Berlin, 2003. A. Goswami, Thin Film Fundamentals, 1st ed. NewAge International Publishers, 1996, 556. P.A. Lee, Physics and Chemistry of Materials withLayered Structures: Optical and Electrical Properties, Reidel, Dordrecht, 1976. G. Perluzzo, A.A. Lakhani, S. Jandl, Solid State Commun. 35 (1980) 301. S.A. Ahmed, Philos. Mag. 86 (9) (2006) 1227. S.N. Guin, D.S. Negi, R. Datta, K. Biswas, J. Mater. Chem. A 2 (2014) 4324.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Electrical transport properties of semiconducting chromium molybdenum diselenide single crystals Priyanka Desai n, D.D. Patel, A.R. Jani Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India
a r t i c l e in f o
Keywords: Chromium mixed molybdenum diselenides Electrical transport properties Single crystals
abstract Mixed chromium–molybdenum diselenides (CrxMo1 xSe2 (x ¼ 0.25, 0.50, 0.75)) have been grown in single crystalline forms by the chemical vapor transport technique. Electrical transport properties like electrical resistivity (perpendicular and parallel to the c-axis), thermoelectric power measurements at high temperature and Hall effect measurements at room temperature were performed on these single crystals. Preliminary study of electrical measurements suggest a semiconducting behavior of CrxMo1 xSe2 (x ¼0.25, 0.50, 0.75) single crystals. Data of Hall coefficient and thermoelectric power have good agreement with each other and confirms the p-type nature of these crystals. Our findings should motivate an in-depth investigation of the underlying mechanisms. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Recently, Transition Metal Dichalcogenides (TMDCs) have attracted to a large extent because of their possible applications as lubricants, switching devices and photoelectrochemical solar energy converters and also marked anisotropy in many of their physical properties. They form a wide range of solid solutions [1,2] with either mixed metals or mixed chalcogenide compositions. The crystals of TMDCs are layered type, each layer being a sandwich of chalcogen–metal–chalcogen sheets. The metal–chalcogen bonding is partly ionic and partly covalent whereas the interlayer bonds are of Van der Waal's type. This crystalline anisotropy leads to anisotropy in their electrical properties. The VIB–VIA group of compounds have been studied extensively for their electrical properties [3–5]. Also, studies have been made so far on mixed systems such as (Mo/W)Te2, (Mo/W)Se2 [6] and (Mo/W)(Se/Te)2 [7]. It appears from the literature that no attempt has been made to investigate the variation of properties like resistivity, Hall coefficient and thermoelectric power with the
n
Corresponding author. E-mail address: [email protected] (P. Desai).
http://dx.doi.org/10.1016/j.mssp.2014.02.052 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
composition of (Cr/Mo)Se2 solid solution in a single crystalline form. Recently we have reported the growth of single crystals of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) by the chemical vapor transport technique using dual zone horizontal furnace [8]. In this paper [8] we have studied structural parameters by mean of powder X-ray diffraction, SAED pattern, surface topography and optical response of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals. It is evident that the electrical resistivity is a physical property of enormous importance, both for the understanding of the solids and their actual applications. The temperature dependence of the electrical resistivity changes in a quite irregular manner because of various mechanisms, including the phonon scattering, mutual scattering of electrons and so forth, are involved in the electrical transport in different temperature ranges [9,10]. The thermoelectric effect proposes a distinct advantage over other methods because the measured thermoelectric voltage is directly related to the carrier concentration which makes the thermoelectric measurements simpler even for high mobility materials [4]. The study of thermoelectric power provides an independent way to determine carrier sign, density and position of Fermi level in semiconductors [11,12]. The materials with high electrical conductivity and large thermoelectric power possess outstanding thermoelectric properties. The temperature dependence of the electrical
P. Desai et al. / Materials Science in Semiconductor Processing 24 (2014) 40–43
conductivity is inclined to many influencing factors. For a semiconductor, both the carrier mobility and carrier density are temperature dependent [13]. The objective of this study was to evaluate electrical resistivity (perpendicular and parallel to the c-axis), thermoelectric power and Hall coefficient of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals. 2. Experimental Single crystals of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) were grown by the chemical vapor transport technique using iodine as a transporting agent. The growth parameters are shown in Table 1. The stoichiometric compositions of all the samples were confirmed through EDAX. The high temperature resistivity measurements parallel and perpendicular to the c-axis were carried out in the temperature range 303–423 K for all the samples using four probe setup (Model:DFP-02), SES instruments Pvt. Ltd., Roorkee. Several readings were taken over different regions of specimen and consistent results were obtained in each case. The anisotropy ratio was calculated using the values of resistivity parallel and perpendicular to the c-axis. In order to evaluate the semiconducting nature of samples, Hall effect measurements were done by using the instrument Lakshore 7504 Hall measurement system at room temperature. For the confirmation of ohmic nature of the contacts, I–V characteristics for each contact were verified. The variations of thermoelectric power (TEP) with temperature were performed in the temperature range 308–423 K with the use of thermo power setup TPSS-200, scientific solutions, Mumbai, India. This experimental setup consists of two assemblies: one is the sample chamber with two heaters and pickup probes and the other is electronic circuits to control the temperature and also the temperature gradient across the sample. The temperature is measured by the thermocouple and the gradient is measured by the deferential temperature sensor. The sample is directly mounted on the two heaters and by applying the temperature gradient (5 K) between two ends of the sample, the thermoelectric power is generated which is measured by digital voltmeter of Keithly.
41
and parallel to the c-axis) were studied from 303 K to 423 K. The results plotted as log ρ vs. 1000/T for parallel and perpendicular to the c-axis are shown in Figs. 1 and 2 respectively. It is seen from Figs. 1 and 2 that there is a linear decrease in resistivity with increasing temperature, which is a characteristic of semiconducting materials. Also, the values of resistivity are higher in parallel direction than the perpendicular direction which suggests the conduction of electrons is very low in parallel direction. The conductivity curves seen to be in good agreement with the models related to the thermal carrier emission across
Fig. 1. Log ρ vs. 1000/T for parallel to the c-axis.
3. Results and discussion 3.1. Resistivity measurements The resistivity variations of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals with temperature (both perpendicular
Fig. 2. Log ρ vs. 1000/T for perpendicular to the c-axis.
Table 1 Growth parameters of CrxMo1 xSe2 (x ¼ 0.25, 0.50, 0.75) single crystals. Compound
Compound preparation
Single crystal growth Temperature distribution
Cr0.25Mo0.75Se2 Cr0.5Mo0.5Se2 Cr0.75Mo0.25Se2
Temperature (1C)
Time (h)
Cold zone (1C)
Hot zone (1C)
1000 1000 1000
100 95 90
900 920 950
1050 1050 1050
Growth time (h)
Dimensions (of largest crystal) (mm3)
246 240 220
11 6 0.3 13 7 0.3 10 6 0.4
42
P. Desai et al. / Materials Science in Semiconductor Processing 24 (2014) 40–43
grain boundary barriers where a temperature dependence of conductivity is expected to yield straight line in Arrhenius plots in the whole temperature range [14]. The electrical resistivity decreases slowly with temperature, and the reason is that the carrier concentration in this temperature range is determined by the number of ionized donor liberated from the impurity level. From this region the activation energy was calculated, indicating that the donor level is positioned below the bottom of the conduction band. The thermal activation energy was calculated from the slope of the curve (Figs. 1 and 2) by using the relation Ea ρ ¼ ρ0 exp ð1Þ kB T where, Ea is activation energy, kB is Boltzmann constant, and T is absolute temperature. It is seen that there is only one activation energy value for all the samples (Table 2) over the entire temperature range investigated. The low values for activation energy suggest that the measured resistivity is the result of the extrinsic process in these crystals. This can be attributed to the high carrier concentration density which gives rise to energy levels very close to the conduction band [15]. Fig. 3 demonstrates the behavior of anisotropy variation with temperature, that clearly indicates that electronic conduction in CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals is highly anisotropic. This is quite obvious because the temperature dependence of electrical resistivity parallel and perpendicular to the c-axis is completely different. This high anisotropy in electronic behavior may be due to the Table 2 Activation energy calculated from log ρ vs. 1000/T. Samples
Activation energy (eV) (parallel to the c-axis)
Cr0.25Mo0.75Se2 0.179 Cr0.5Mo0.5Se2 0.146 Cr0.75Mo0.25Se2 0.104
Activation energy (eV) (perpendicular to the c-axis) 0.093 0.085 0.063
larger effective mass anisotropy to the layers of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals. 3.2. Hall effect measurements The mobility of CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals was determined by measuring the change in resistance R upon applying a magnetic field perpendicular to the c-axis of the samples. Hall mobility (μH), Hall coefficient (RH) and carrier concentration (n) are calculated using the following relations: μH ¼
tΔR Bρ
RH ¼ μH ρ n¼
1 RH e
ð2Þ ð3Þ ð4Þ
where, t is the thickness of crystal, B is applied magnetic field, ΔR is change in magnetic field and ρ is the resistivity. All the results obtained from Hall effect measurements are tabulated in Table 3. The positive values of Hall coefficients indicate that the majority of the charge carriers in CrxMo1 xSe2 (x ¼0.25, 0.50, 0.75) single crystals are holes. Also, carrier concentration increases with increase in chromium content which denotes extra charge carriers. 3.3. Thermoelectric power measurements The variation of thermoelectric power (S) with the inverse of temperature for CrxMo1 xSe2 (x¼ 0.25, 0.50, 0.75) single crystals is shown in Fig. 4. Absolute value of S increases steadily with an increase in temperature, confirming the typical semiconducting behavior of the samples. To study the temperature dependence of the thermoelectric power of p-type semiconductor the below expression [16] can be used. Ef kB S¼ Aþ ð5Þ e kB T where, kB is the Boltzmann constant, e is the electronic charge, A is the constant determined by the dominant scattering process and Ef is the separation of Fermi level from the top of the valence band. If thermoelectric power is plotted against the reciprocal of temperature, a straight line is expected from which Ef and A can be determined from the slope and intercept respectively. The value of scattering constant A is given by, A ¼ ð5=2Þ s
Fig. 3. Variation of anisotropy with 1000/T.
ð6Þ
where, s is scattering parameter. The observed positive values of thermoelectric power in the entire investigated temperature range follows from the fact that hole concentration is greater than electron, that is, samples are p-type. This agrees with the results obtained from Hall effect data. According to Ahmed [17] and Guin et al. [18], negative values of scattering parameters (Table 4) for CrxMo1 xSe2 (x ¼0.25, 0.50, 0.75) single crystals indicate that the scattering mechanism
P. Desai et al. / Materials Science in Semiconductor Processing 24 (2014) 40–43
43
Table 3 Hall parameters for CrxMo1 xSe2 (x¼ 0.25, 0.50, 0.75) single crystals. Parameter
Cr0.25Mo0.75Se2
Cr0.5Mo0.5Se2
Cr0.75Mo0.25Se2
Resistivity (Ω cm) (at room temperature) Hall coefficient (RH) (cm3/C) Mobility (μH) (cm2/V s) Carrier concentration (n 1016) (cm 3)
3.44 458.02 133.14 1.36
2.98 366.40 122.95 1.70
2.57 297.71 115.84 2.10
with p-type conductivity. Analysis of the electrical resistivity, Hall effect and thermoelectric power data allows us to deduce many important parameters such as activation energy, mobility, Hall coefficient, carrier concentration and scattering parameter. The values of scattering parameter indicate that carrier scattering may be dominated by acoustic phonons. This mode of investigation is an ideal way for finding out the possibility of making applications for these semiconductor crystalline compounds especially in the field of energy conversion, semiconductor devices and electronic engineering.
Acknowledgments
Fig. 4. Variation of thermoelectric power with 1000/T.
Priyanka Desai and D.D. Patel are thankful to UGC, New Delhi, India for providing financial support under Meritorious Research Fellowship. References
Table 4 Values of slope, intercept, Fermi energy (EF), scattering constant (A) and scattering parameter (s) for CrxMo1 xSe2 (x¼ 0.25, 0.50, 0.75) single crystals. Samples
Slope
Intercept
EF (eV)
A
s
Cr0.25Mo0.75Se2 Cr0.5Mo0.5Se2 Cr0.75Mo0.25Se2
891.14 854.57 789.07
3451.83 2989.41 2736.46
0.891 0.854 0.789
3.451 2.989 2.736
0.951 0.489 0.236
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
corresponds to carrier scattering by acoustic phonons by assuming a single parabolic band model. 4. Conclusions
[11] [12] [13] [14]
The electrical transport properties of CrxMo1 xSe2 (x ¼0.25, 0.50, 0.75) single crystals have been studied with high temperature electrical resistivity, thermoelectric power and also room temperature Hall effect measurements. This work has shown that CrxMo1 xSe2 (x¼0.25, 0.50, 0.75) single crystals have a semiconducting nature
[15] [16] [17] [18]
I.F. Schneemeyer, M.J. Sienko, Inorg. Chem. 19 (1980) 789. B.F. Mentzen, M.J. Sienko, Inorg. Chem. 15 (1976) 2198. J.A. Wilson, A.D. Yoffe, Adv. Phys. 18 (1969) 193. L.H. Brixner, J. Electrochem. Soc. 110 (1963) 289. R. Mansfield, A. Salam, Proc. Phys. Soc. B66 (1953) 377. E. Revolinsky, D. Beerntsen, J. Appl. Phys. 35 (1964) 2086. J.A. Champion, Br. J. Appl. Phys. 16 (1965) 1035. Priyanka Desai, D.D. Patel, A.R. Jani, J. Cryst. Growth 390 (2014) 12. C. Kittle, Introduction to Solid State Physics, 8th ed. Wiley, New York, 2004. N.W. Ashcroft, N.D. Mermin, Solid State Physics, Thomson Learning, Orlando, 1976. L.H. Brixner, J. Inorg. Nucl. Chem. 24 (1962) 257. S.N. Guin, A. Chatterjee, D.S. Negi, R. Datta, K. Biswas, Energy Environ. Sci. 6 (2013) 2603. P.Y. Yu, M. Cardona, Fundamentals of Semiconductors, 2nd ed. Springer, Berlin, 2003. A. Goswami, Thin Film Fundamentals, 1st ed. NewAge International Publishers, 1996, 556. P.A. Lee, Physics and Chemistry of Materials withLayered Structures: Optical and Electrical Properties, Reidel, Dordrecht, 1976. G. Perluzzo, A.A. Lakhani, S. Jandl, Solid State Commun. 35 (1980) 301. S.A. Ahmed, Philos. Mag. 86 (9) (2006) 1227. S.N. Guin, D.S. Negi, R. Datta, K. Biswas, J. Mater. Chem. A 2 (2014) 4324.