ISSN 1063-7826, Semiconductors, 2016, Vol. 50, No. 11, pp. 1453–1457. © Pleiades Publishing, Ltd., 2016. Original Russian Text © I.V. Erofeeva, M.V. Dorokhin, V.P. Lesnikov, A.V. Zdoroveishchev, A.V. Kudrin, D.A. Pavlov, U.V. Usov, 2016, published in Fizika i Tekhnika Poluprovodnikov, 2016, Vol. 50, No. 11, pp. 1473–1478.
XX INTERNATIONAL SYMPOSIUM “NANOPHYSICS AND NANOELECTRONICS”, NIZHNY NOVGOROD, MARCH 14–18, 2016
On the Crystal Structure and Thermoelectric Properties of Thin Si1 – xMnx Films I. V. Erofeeva, M. V. Dorokhin, V. P. Lesnikov, A. V. Zdoroveishchev, A. V. Kudrin, D. A. Pavlov, and U. V. Usov Research Institute for Physics and Technology, Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod, 603950 Russia e-mail:
[email protected] Submitted April 27, 2016; accepted for publication May 10, 2016
Abstract—Thin (25 nm) Si1 – xMnx/Si(100) films are fabricated by pulsed laser deposition. According to highresolution transmission electron microscopy data, the films have a nanotextured crystalline structure and are chemically homogeneous. The temperature dependences of the resistivity and thermopower are measured in the range of 300–500 K, and the temperature dependences of the Seebeck coefficient and power factor are calculated. DOI: 10.1134/S1063782616110105
1. INTRODUCTION One of the modern trends in the technology of thermoelectric energy converters is the formation of nanolayers of thermoelectric materials [1]. Researchers have fabricated and investigated nanostructures based on different semiconductors and metals (e.g., Bi2Te3 [1], Mg2Si0.4Sn0.6 [2], InxCeyCo4Sb12 [3], and compounds of Bi and Te [4, 5]). The choice of materials is determined by requirements to the thermoelectric-converter efficiency and operating temperature range (each of the aforementioned materials exhibits the highest efficiency in a relatively narrow temperature range). A promising material for the range of 400–600 K is superstructurally ordered manganese silicide [6]. Nanostructures on the basis of bulk manganese silicide were fabricated in [7]. The use of nanostructures based on Si1 – xMnx as elements of thermoelectric converters is expected to improve the main device characteristics to the level of the best structures [1–4]. The purpose of this study is to fabricate thin Si1 – xMnx nanolayers on different substrates by pulsed laser deposition (PLD) at different growth temperatures and analyze the structural and electrical properties of these layers. 2. EXPERIMENTAL Manganese-silicide layers were formed (using a PLD system) in vacuum by the alternate sputtering of KDB-12 (p-Si:B) and Mn targets. The layer composi-
tion was set by the ratio of the sputtering times of silicon and manganese targets. We performed a series of growth experiments aimed at forming the above-described layers on Si(100) (KDB-12) and semi-insulating GaAs(100) substrates. The layer thicknesses were in the range of 20–30 nm. The deposition temperature of the manganese-silicide layers in our experiments was varied within 250–450°C. This temperature range is optimal for obtaining superstructurally ordered structures, as was found in previous experiments [2]. The same conditions were used to form a multilayer structure composed of alternating Mn and Si layers. The main variable structural parameters are listed in Table 1. The structural quality of the layers was analyzed using a high-resolution transmission electron microscope (HRTEM) JEM-2100F. The homogeneity of the layers’ chemical composition was determined by energy-dispersive X-ray spectroscopy (EDXS) using an X-max detector (Oxford Instruments), which is an element of the HRTEM. A diffraction pattern of the Si1 – xMnx/Si contact region was obtained by applying the HRTEM in the microdiffraction mode. The sheet resistance and the carrier concentration and type were determined in the temperature range of 300–600 K using the standard technique for measuring the dc resistivity and Hall effect in the van der Pauw geometry. The Hall effect was measured in a 5170-Oe dc magnetic field. The thermopower was measured in vacuum (10–3 Torr). The hot end of the sample was placed on
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Table 1. Parameters of manganese-silicide layers formed by PLD Sample
Deposition time, s/layer thickness, nm*
Deposition temperature, °C
Substrate
S 68-4
1800/20
250
Si (100)
S 59-5
1800/20
300
Si (100)
Si 0.55Mn ** 0.45 Si0.55Mn0.45
S 60-5
1800/20
400
Si (100)
Si0.55Mn0.45
S 64-8 G 59-5
9000/100 1800/20
450 300
Si (100) Si/Mn multilayer structure, 10 layers GaAs (100) Si0.55Mn0.45
G 60-5
1800/20
400
GaAs (100)
Approximate film composition
Si0.55Mn0.45
* Nominal layer thickness. ** The composition is set by the sputtering-time ratio.
a graphite plate, the temperature of which was set and maintained constant (±1 K) in the range of 300– 600 K. A piece of mica was placed under the second (cold) end of the sample to separate it from the heated graphite plate and form a temperature difference of 20–40°C. The thermopower was measured between the hot and cold ends of the sample (ΔU). Based on the data obtained, the Seebeck coefficient a was calculated as (1) a = ΔU /ΔT , where ΔT is the difference in temperature between the hot and cold ends. The power factor p was calculated from the formula
p = a 2 ⋅ σ, where σ is the conductivity.
(2)
3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Composition of the SiMn Nanolayers The electron microscopy study showed the films to be nanotextured crystals. Figure 1 presents a diffraction pattern of the layers under study, obtained in the
2 1/nm Fig. 1. Electron microdiffraction pattern of Si1 – xMnx on a Si single crystal (sample S68-4 in Table 1).
HRTEM microdiffraction mode. The strong reflections in Fig. 1 correspond to electron diffraction from a silicon single crystal (substrate), while the point reflections forming a ring yield an electron-diffraction (ED) pattern characteristic of polycrystals with individual crystallites oriented at different angles (Si1 – xMnx layer). This electron diffraction can be put into correspondence with the high-resolution electron microscopy image of the transverse cut of sample S68-4 shown in Fig. 2. It can be seen that the Si1 – xMnx layer consists of nanoscale single-crystal blocks (nanocrystallites) with a characteristic size of ~10 nm. This ultrafine grained texture provides a large number of boundaries in the manganese-silicide layer. These boundaries, on the one hand, enhance carrier scattering and can reduce the electrical conductivity. On the other hand, they may significantly decrease the thermal conductivity by enhancing phonon scattering from the boundaries of blocks with different growth directions [6] and thus increasing the thermoelectric figure of merit = a2 · σ/. We note that, when designing thermoelectric materials with a high relative thermoelectric Q factor (ZT) (conditioned by their low thermal conductivity), one should either choose formation conditions and a composition which provide nanoinclusions in the material (e.g., the second phase of the chemical composition) or intentionally introduce these inclusions (for example, grow quantum dots) [6]. In our case, a structure containing scattering nanoobjects is automatically formed during growth, and the film has a single-phase chemical composition. The homogeneity of the chemical composition of Si1 – xMnx layers was confirmed by the EDXS data. In the absence of chemical-composition inhomogeneities in the layer, its entire volume contributes to the thermopower. According to the HRTEM data, the structures under study have a thickness of ~25 nm. An increase in the manganese-silicide growth temperature leads to an increase in structural quality, as follows from the data obtained in [1, 2, 7] for a number of structures with parameters close to those considSEMICONDUCTORS
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Rs, Ω/sq.
30
800
28
700
26
600
24
500
22
10 nm Fig. 2. HRTEM image of a cut of Si1 – xMnx layer on a Si substrate (sample S68-4 in Table 1).
ered in this study. It was shown in the aforementioned studies that the manganese-silicide layers formed on a GaAs substrate at a temperature of 300°C are epitaxial structures with a large number of two-dimensional defects (grain boundaries). Similar formation conditions were established for the layers analyzed here. 3.2. Electric Transport Properties We measured the temperature dependences (in the range from 300 to 600 K) of the electric transport properties of manganese-silicide layers deposited onto silicon (KDB-12) and semi-insulating GaAs (AGChP-100) substrates. The measured temperature dependences of the sheet resistance of the manganesesilicide layers formed at different temperatures or on different substrates are presented in Fig. 3. It was found that a change in the formation temperature leads to a significant change in the layer properties: their conductivity changes from the p- to n-type and the conductivity behavior becomes typical of metals rather than semiconductors (the resistance increases with an increase in temperature for metals and decreases for semiconductors) (Table 2). The layer formed at a temperature of 250°C exhibited p-type conductivity; its resistance somewhat decreases with increasing temperature. The range of variation of the sheet hole concentration is (1–5) × 1016 cm–2, and the mobility values are in the range of
Rs, Ω/sq.
900
32
400
20 200
300
400 T, K
500
600
Fig. 3. Temperature dependences of the sheet resistance of the Si1 – xMnx films deposited (1, 2) at a growth temperature of 300°C on (1) Si and (2) GaAs substrates and (3) at a temperature of 400°C on a Si substrate. The open squares are the Rs(T) values for Si1 – xMnx on Si at a temperature of 300°C.
2–8 cm2 V–1 s–1; such low mobilities correspond to hole conductivity in a semiconductor with a high concentration of defects (according to electron diffraction data, the Si1 – xMnx layers under study are polycrystals). With an increase in the deposition temperature to 300°C, the manganese-silicide layers retain p-type conductivity (Fig. 3, curves 1, 2), while the carrier concentration increases to (0.5–20) × 1017 cm–2 (Table 2). The mobilities are in the same range as for the layer formed at 250°C. A slight increase in the sheet resistance (~10% Rs) in the range from 250 to 500 K is followed by a sharper drop at temperatures above 500 K (open squares in Fig. 3). This nonmonotonic behavior of Rs(T) may be related to the paramagnet–ferromagnet phase transition, which manifests itself as a weak maximum in the vicinity of the Curie point. As was shown in [1], thin epitaxial Si1 – xMnx layers with a Curie point much higher than room temperature can be formed by laser synthesis.
Table 2. Results of electrical measurements of manganese-silicide layers in the temperature range of 300–500 K Sample
Deposition temperature, °C
Mobility, cm2 V–1 s–1
Sheet concentration, cm–2
Conductivity type
S 68-4
250
2−8
(1−5) × 1016
Hole
S 59-5
300
1−5
(0.5−30) × 1016
Hole
S 60-5
400
5−60
(1−4) × 1015
Electron
S 64-8
450
5−20
(1−3) × 1014
Hole
G 59-5
300
1−5
(0.5−20) × 1016
Hole
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Rs, Ω/sq. 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400
2.0 S 68-4 1.5 S 59-5
α, mV/K
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0
300
400
500 T, K
Fig. 4. Temperature dependence of the sheet resistance of a multilayer Si/Mn structure deposited onto a Si substrate.
An increase in the deposition temperature to 400°C leads to a change in the manganese-silicide conductivity type to electronic, as follows from the change in the Hall-constant sign. The resistance, as for the sample obtained at 250°C, decreases with an increase in temperature (Fig. 3, curve 3), which is typical of semiconductor-like conductivity. The nonmetal character of conductivity is also confirmed by the smaller value ((1–4) × 1015 cm–2) of the sheet carrier concentration (see Table 2). The temperature dependences of the sheet resistance of the Si1 – xMnx layers formed at a deposition temperature of 300°C on Si and GaAs substrates are presented in Fig. 3 (curves 1 and 2, respectively). These dependences are similar, with a small difference in the sheet resistance value (within 10%), which can be explained by the influence of the substrate on the structural quality of the deposited layer, which, in turn, determines the sheet resistance. The multilayer Si/Mn/Si structure (S64-8 in Table 1) exhibits an unsteady temperature dependence of the resistance, which apparently can also be related to the paramagnet–ferromagnet phase transition at a point above room temperature (Fig. 4). The concentration value (1–3) × 1014 cm–2 corresponds to the semiconductor-like conductivity of the structure. The observed changes in the electrical properties of the manganese-silicide layers (Fig. 3) can be explained by the effect of the atomic and crystalline structures of manganese silicide on its properties. For example, the conductivity depends on the degree of structural quality of the layers (the latter affects the carrier mobility) [8]. The higher the degree of structural quality, the lower the semiconductor resistance is. The carrier concentration in the Si1 – xMnx layer is determined by the degree of compensation of holes by defect centers of a different nature. The possible defect centers in manganese silicide are grown boundaries or interstitial Mn atoms (which are double donors) [9, 10]. The presence of such defects can be expected
1.0
p, mW/(K2 m)
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0.5 0 350
400
450 500 Taver., K
550
600
Fig. 5. Dependences of the Seebeck coefficient (filled symbols) and power factor (open symbols) on the average temperature between the hot and cold ends for the samples formed at different growth temperatures: Tgr = 250°C (sample S68-4) and 300°C (S59-5).
in the structures formed at 400°C (which exhibited n-type conductivity). Thus, the results of our study indicate that one can control the conductivity type (n or p) of Si1 – xMnx layers by varying the layer deposition temperature. The physical nature of this effect is related to the specific features of the atomic (related to the impurity incorporation) and crystalline (related to ordering) layer structures. The obtained regularities make the proposed method of manganese-silicide formation promising for thermoelectric energy converters, because a change in the deposition temperature makes it possible to vary the electrical parameters of the layers in wide ranges. 3.3. Thermoelectric Properties The temperature dependences of the Seebeck coefficient in the temperature range of 300–500 K were measured for the structures under study at different values of the main variable parameters. The results are presented in Figs. 5 and 6. The temperature dependences of the Seebeck coefficient and power factor are qualitatively similar for all structures containing a Si1 – xMnx layer (Figs. 5, 6) and are in agreement with the data of publications on the characteristics of higher manganese silicides [11]. The power factor steadily increases with increasing temperature in the range of 300–500 K. We note the inverse sign of the Seebeck coefficient observed for structure S60-5 (with a Si1 – xMnx layer formed at a temperature of 400°C) (Fig. 6). The change in sign is related to the n-type conductivity of the Si1 – xMnx layer (all other samples had p-type conductivity, see Table 2). The dependences of the thermopower sign and Seebeck coefficient on the conductivity type is a fundamental property of semiconductors [12]. The SEMICONDUCTORS
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multilayer heterostructures (specifically, Si1 – xMnx/Si) to provide higher power factors due to the conservation of high electrical conductivity along the intermetallic layers and an increase in the Seebeck coefficient with increasing number of layers.
0.04 0.2 0.03 0.02
0
0
−0.01
p, mW/(K2 m)
0.1
0.01 α, mV/K
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−0.1
−0.02 −0.03
−0.2
−0.04 350
400
450 Taver., K
500
550
Fig. 6. Dependences of the Seebeck coefficient (solid line) and power factor (symbols) on the average temperature between the hot and cold ends for the sample formed at T = 400°C (S60-5).
“negative” sign of the Seebeck coefficient observed for the multilayer Si/Mn/Si structure in the low-temperature range is also related to the change in the conductivity type during heating. The largest values of the power factor (Fig. 5) were obtained for structures S68-4 and S59-5: 1.8 mW/(K2 m) and 1.2 mW/(K2 m), respectively. According to the data of [13], they are record values for thermocouples. An increase in the structure formation temperature significantly reduces the power factor for the structure with semiconductor conductivity type. This is in agreement with the data of [14], where it was shown that the maximum values of the power factor should be expected for heavily doped semiconductor structures, whereas the transition to the metal conductivity type reduces the aforementioned parameter. 4. CONCLUSIONS It was shown that pulsed laser deposition can be used to form layers of thermoelectric materials with ultrafine grained polycrystalline structure. The structure and homogeneity of the layers depend on the formation temperature. Apparently, the optimal temperature range for obtaining homogeneous polycrystalline layers with a large number of grain boundaries is 250–300°C. The above-reported values of the thermoelectric parameters are below the known maximum values; however, they were obtained for manganese-silicide layers only 25 nm thick. One would expect the growth of SEMICONDUCTORS
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ACKNOWLEDGMENTS This study was performed within a state contract (project nos. 8.1054.2014/K and 3423) with the Ministry of Education and Science of the Russian Federation and supported by the Russian Foundation for Basic Research (project no. 15-38-20642mol_a_ved) and grant MK-8221.2016.2 of the President of the Russian Federation. REFERENCES 1. E. S. Demidov, Yu. A. Danilov, V. V. Podol’skii, V. P. Lesnikov, M. V. Sapozhnikov, and A. I. Suchkov, JETP Lett. 83, 568 (2006). 2. E. S. Demidov, V. V. Podol’skii, V. P. Lesnikov, E. D. Pavlova, A. I. Bobrov, V. V. Karzanov, N. V. Malekhonova, and A. A. Tronova, JETP Lett. 100, 719 (2014). 3. S. V. Novikov, Extended Abstract of Cand. Sci. Dissertation (St. Petersburg, 2014). 4. A. V. Shevel’kov, Course of Lectures (Khim. Fakul’t., Mosk. Gos. Univ. im. M. V. Lomonosova, Moscow, 2010) [in Russian]. 5. M. V. Dorokhin, D. A. Pavlov, A. I. Bobrov, Yu. A. Danilov, P. B. Demina, B. N. Zvonkov, A. V. Zdoroveishchev, A. V. Kudrin, N. V. Malekhonova, and E. I. Malysheva, Phys. Solid State 56, 2131 (2014). 6. A. F. Ioffe, Semiconductor Thermoelements and Thermoelectric Cooling (Infosearch Ltd., London, 1957). 7. E. S. Demidov, E. D. Pavlova, and A. I. Bobrov, JETP Lett. 96, 706 (2012). 8. S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981; Mir, Moscow, 1984), Vol. 1, rus. p. 53. 9. A. F. Orlov, L. A. Balagurov, I. V. Kulemanov, Yu. N. Parkhomenko, A. V. Kartavykh, V. V. Saraikin, Yu. A. Agafonov, and V. I. Zinenko, Semiconductors 44, 28 (2010). 10. Y. H. Kwon, T. W. Kang, and C. J. Park, Solid State Commun. 140, 14 (2006). 11. V. K. Zaitsev, S. V. Ordin, K. A. Rakhimov, and A. E. Engalychev, Sov. Phys. Solid State 23, 353 (1981). 12. A. A. Detlaf, B. M. Yavorskii, and L. B. Milkovskaya, Course of Physics, Vol. 2: Electricity and Magnetism (Vysshaya Shkola, Moscow, 1977), p. 376 [in Russian]. 13. F. Yu. Solomkin, V. K. Zaitsev, S. V. Novikov, A. Yu. Samunin, D. A. Pshenai-Severin, and G. N. Isachenko, Tech. Phys. 59, 1209 (2014). 14. Z.-G. Chen, G. Han, and L. Yang, Progr. Nat. Sci.: Mater. Int. 22, 535 (2012).
Translated by Yu. Sin’kov
