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Epitaxial Zinc-Blende CdTe Antidots in Rock-Salt PbTe Semiconductor Thermoelectric Matrix Michaz Szot,* Krzysztof Dybko, Piotr Dziawa, Leszek Kowalczyk, Ewa Smajek, Viktor Domukhovski, Badri Taliashvili, Piotr Dzu_zewski, Anna Reszka, Bogdan J. Kowalski, Maciej Wiater, Tomasz Wojtowicz, and Tomasz Story Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, 02-668 Warsaw, Poland ABSTRACT: The formation of zinc-blende CdTe antidots (bandgap of 1.5 eV at room temperature) embedded in a rock-salt PbTe semiconductor matrix with a narrow bandgap of 0.3 eV in properly annealed epitaxial CdTe/PbTe multilayers grown by molecular beam epitaxy on a GaAs(001) substrate is reported. Transmission microscopy and X-ray diffraction characterization revealed the monocrystalline zinc-blende crystal structure of the CdTe antidots. The CdTe antidots have a highly symmetric shape and size varying in a controlled way in the range from 5 to 30 nm, depending on the layer thicknesses in the initial multilayer CdTe/PbTe stack. The presented results indicate that the CdTe antidot growth mechanism is similar to that of PbTe dots embedded in a CdTe matrix and is driven by the nanoscale phase separation due to qualitative differences in the chemical bonding and crystal structure of PbTe and CdTe. The electrical characterization in terms of Hall effect, electrical conductivity, and Seebeck effect measurements showed that both n- and p-type conductivities can be obtained in these nanocomposite thermoelectric materials with carrier concentrations of 10171018 cm3 and mobilities of about 200 cm2/(V s) at room temperature. About a 25% increase of the thermoelectric power as compared to that of the reference bulk thermoelectric PbTe crystals was found in heterostructures with the smallest CdTe antidots.
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hermoelectric devices exploiting the Seebeck and Peltier effects for electronic conversion of heat into electricity (and vice versa) are already successfully used both as thermoelectric power generators (using SiGe-, PbTe-, or Zn4Sb3-based alloys) and Peltier coolers (using Bi2Te3-based alloys).1 The development of new thermoelectric materials is an important research direction aimed at solving contemporary energy problems, in particular by improving vehicles’ fuel economy using the waste heat of engines in complementary thermoelectric power generators. For broad-scale applications of industrial importance, thermoelectric materials with high thermoelectric efficiency are necessary, as specified by the required value of the dimensionless thermoelectric figure of merit parameter ZT g 3.2,3 The parameter ZT = S2σT/k is the key thermoelectric material characteristic, known, for example, to determine the efficiency of the thermoelectric energy generation process under given temperatures of heat source and heat sink. Here, T denotes the temperature, S is the Seebeck coefficient (thermoelectric power), σ is the electrical conductivity, and k is thermal conductivity. However, even for the best contemporary thermoelectric materials, the parameter ZT is rather low and approaches 1 at the optimal device operating temperature. The effective ZT parameter of a basic pn thermoelectric couple is usually even lower because of the thermoelectric mismatch between the p- and n-leg materials. This fact dramatically limits the application possibilities for these devices. Aside from a search for new families of thermoelectric materials, an alternative route toward materials with the improved ZT parameters r 2011 American Chemical Society
is based on the use of the already-known thermoelectric materials prepared in the form of various low-dimensional heterostructures.4 This can be achieved technologically in a variety of forms, such as epitaxial multilayer quantum well heterostructures5,6 or semiconductor nanowires7 grown by various high-vacuum deposition and chemical synthesis methods, as well as nanostructured bulk materials prepared by such industrially relevant methods as growth from the melt with thermal quenching8 or ball milling.1,911 The proper nanostructuring can be realized both in heterosystems such as semiconductor superlattices or thermoelectric matrixes with embedded nanograins of other materials and in chemically homogeneous materials with nanoscale crystalline grains.1 Two key effects are known to improve the thermoelectric parameters of such nanostructured thermoelectrics: a decrease of the thermal conductivity due to enhanced phonon scattering on multilayer interfaces or nanograin crystal boundaries and an increase of the thermoelectric power factor (S2σ) due to the increased energy derivative of the electron density of states (DOS) at the Fermi level or the so-called electron-filtering effect by enhanced energy dependence of scattering rate. These effects have been found experimentally, for example, in PbSeTe/PbTe nanodot superlattices (NDSLs) with ZT ≈ 1.6 at 300 K,12,13 in Bi2Te3/ Sb2Te3 superlattices with ZT ≈ 2.5,5 and in ErAs:InGaAlAs Received: March 31, 2011 Revised: September 5, 2011 Published: September 08, 2011 4794
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Crystal Growth & Design nanocomposite films with ZT ≈ 1.3 at 800 K.14 Nonetheless, in the case of PbSeTe/PbTe NDSLs, the latest measurements of the thermoelectric power factor and thermal conductivity did not confirm the previous observations.15,16 Another method to engineer the thermoelectric power by shaping the DOS at the Fermi level was recently demonstrated in p-PbTe bulk crystals doped with thallium, which is supposed to form an electronic state that is resonant with the valence band in PbTe.17 Recently, very good thermoelectric properties were discovered in another PbTe-based nanostructured material, namely, bulk (PbTe)1x(CdTe)x solid solution prepared by quenching and annealing of polycrystals grown from the melt.18 Experimental studies of electric, thermoelectric, and thermal properties of this material revealed that, in n-(Pb,Cd)Te samples heavily doped with iodine to an electron concentration of about n ≈ 1019 cm3, ZT approaches 1.2 at T = 720 K. A high-resolution structural analysis of this material showed that it consists of CdTe nanograins embedded in a (Pb,Cd)Te polycrystalline matrix.18 The spontaneous formation of these CdTe nanoprecipitations is related to the very low solubility of CdTe in PbTe, as discussed below.19 The relatively high value of the parameter ZT observed in these (Pb,Cd)Te samples is due to their low lattice thermal conductivity related to the presence of the CdTe nanograins. The structural and electrical properties of the unique system of bulk, monocrystalline Pb1xCdxTe substitutional solid solutions (up to x = 0.11) grown by the self-selecting physical vapor transport method were also recently studied in intentionally undoped p-type crystals20 with a carrier concentration of p ≈ 1018 cm3. In this article, we show that, by applying a properly modified technological method originally developed by Heiss et al.21 to grow PbTe quantum dots in a CdTe matrix (new infrared optoelectronic materials system), one can obtain a novel layered thermoelectric nanomaterial composed of wide-bandgap CdTe quantum antidots (Eg = 1.5 eV at room temperature) embedded in a controlled way in a conducting narrow-bandgap PbTe crystalline matrix (E g = 0.3 eV). In contrast to the previously studied system of PbTe dots in a CdTe matrix, which is interesting optically but electrically insulating (i.e., of no relevance to thermoelectricity), the material system studied in this work exhibits the good thermoelectric properties inherent to the PbTe crystalline matrix. We applied a two-stage technological method that involves the molecular beam epitaxy (MBE) growth of a properly designed CdTe/PbTe epitaxial multilayer, followed by a nanostructureforming in situ high-vacuum annealing procedure. This method exploits the almost complete immiscibility of PbTe and CdTe below a temperature of about 300 °C, which is related to the qualitative differences in chemical bonding and crystal structure of these semiconductor materials.19,2123 Even though these compounds crystallize in different cubic lattices, they exhibit excellent matching of their lattice parameters: the zinc-blende lattice parameter of CdTe is a0 = 0.648 nm, whereas the rock-salt lattice parameter of PbTe is a0 = 0.646 nm. Thus, using this semiconductor heterosystem, it is possible to obtain stressfree thermoelectric devices. Similarly to the case of PbTe quantum dots in a CdTe matrix,23 the technological control of the size distribution of the CdTe antidots is possible by changing the thicknesses of the CdTe and PbTe layers in the initial CdTe/ PbTe multilayer. The electrical and thermal properties of a nanocomposite material can be optimized in this way. It also is very attractive for future applications to develop the methods of fabricating a nanothermoelectric cooler embedded in a single
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heterostructure with a semiconductor laser diode, for example.24 The epitaxial system of CdTe antidots in a PbTe matrix preserves the good electrical conductivity and high thermoelectric power characteristic of PbTe and, simultaneously, is expected to exhibit a reduced thermal conductivity and, consequently, an improved figure of merit. All of the investigated CdTe/PbTe heterostructures were deposited on (001)-oriented GaAs substrate using the MBE method. First, an ultrathin ZnTe layer (just few monolayers) was deposited on the GaAs(001) semi-insulating substrate, after which a 4-μm-thick (001)-oriented monocrystalline, undoped CdTe buffer layer was grown. These initial technological steps were carried out in our IIVI semiconductor MBE facility. The role of the very thick CdTe buffer layer is to separate the electrically active CdTe/PbTe multilayer heterostructure from the GaAs/CdTe interface, which is expected to contain defects because of the large (13%) lattice-parameter mismatch between GaAs and CdTe. The presence of the ultrathin ZnTe layer (with lattice parameter a0 = 0.610 nm, i.e., between those of GaAs and CdTe) is known to stabilize the MBE growth of high-quality CdTe(001) layers on GaAs(001) substrate. After the GaAs/ CdTe hybrid substrate had been transferred (in the air) to the separate IVVI semiconductor MBE facility, it was annealed to remove a protective thin layer of tellurium and to restore a smooth epitaxy-ready (001) surface of CdTe. This was verified in situ by the observation of characteristic reflection high-energy electron diffraction (RHEED) patterns. Next, directly on the CdTe buffer, the CdTe/PbTe multilayer (with layer thicknesses varying from 10 to 25 nm for PbTe and from 1 to 6 nm for CdTe layers) was grown at a substrate temperature of 260270 °C. The growth of the heterostructures was performed using CdTe and PbTe molecular fluxes from effusion cells filled with CdTe and PbTe solid source materials. If necessary, a small additional Te2 flux from a Te effusion cell was applied. During the multilayer growth process, the two-dimensional growth mode was maintained for both PbTe and CdTe layers over the entire technological process, as observed in situ by the characteristic streaky RHEED patterns (see Figure 1a). To activate the process of CdTe antidot formation, after the MBE growth, the CdTe/ PbTe multilayers were annealed for 1 h at 300350 °C in the MBE growth chamber under high-vacuum conditions. The annealed samples were examined by X-ray diffraction (XRD) (see Figure 1c), revealing their high crystal quality with the XRD rocking curve width parameter of about 15000 for both PbTe and CdTe layers (see Figure 1b). In Figure 1c, one should note the change of the XRD peaks intensity ratio for PbTe and CdTe diffraction (006) maxima as compared to the (004) and (008) ones. This effect is expected for (001)-oriented layers with the rock-salt or zincblende crystal structure. Thereafter, the heterostructures were cleaved, and the cross-sectional (110) plane was examined by scanning electron microscopy (SEM) to reveal the nanostructure formed during the annealing process. Selected samples were also studied by transmission electron microscopy (TEM). Figure 2 shows the cross-sectional SEM images of two CdTe/ PbTe heterostructures in which the ratio of CdTe and PbTe layer thicknesses was inverted. In Figure 2a, a sample containing two PbTe layers with an initial thickness of 6 nm separated by 50-nmthick CdTe layers is shown, whereas in Figure 2b, a sample with initial 6-nm CdTe thin layers and 50-nm-thick PbTe layers is presented. The images were recorded using the SE (secondaryelectron) and BSE (backscattered-electron) modes of the SEM microscope. After the MBE growth had been carried out in 4795
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Figure 1. Crystal structure characterization of 10 [CdTe(2 nm)/PbTe(25 nm)]/CdTe//GaAs(001) multilayer: (a) RHEED patterns observed during the growth of PbTe and CdTe layers, (b) XRD (400) rocking curves, and (c) ω2θ diffraction spectra around the (400), (600), and (800) maxima of the CdTe/PbTe multilayer heterostructure after annealing (i.e., with CdTe antidots).
accordance with the technological protocol described above, both samples were annealed under the same conditions, namely, at 350 °C for 1 h. This annealing procedure resulted in the disintegration of ultrathin, continuous CdTe or PbTe layers into CdTe or PbTe nanoprecipitates. This suggests that the formation of CdTe antidots in a PbTe matrix proceeds in the same way as the formation of PbTe dots in a CdTe matrix2123 and is driven by the minimization of the CdTe/PbTe interface energy and phase separation near thermodynamic equilibrium.25,26 Therefore, one can expect a correlation between the initial thickness of the CdTe layers in the CdTe/PbTe multilayer and the size of the CdTe antidots, as was established for PbTe dots in CdTe matrix.23 To verify this idea, we prepared three samples, each containing 10 CdTe layers with thicknesses of 1, 2, and 4 nm, separated by PbTe layers with a thickness of 25 nm. Additionally, we prepared one
sample containing four CdTe layers with different initial thicknesses of 0.3, 0.6, 1, and 2 nm. After the multilayer had been grown by the MBE method, each sample was annealed in the MBE growth chamber for 1 h at 350 °C. The obtained nanostructures are depicted in Figures 3 and 4. Figure 3 shows the cross-sectional SEM images of the samples containing 10 CdTe layers with initial thicknesses of 2 nm (Figure 3a) and 4 nm (Figure 3b). As expected, for both samples, we obtained well-separated CdTe antidots with their spatial positions (along the growth direction) determined by the positions of the CdTe layers in the initial CdTe/PbTe multilayer. The sizes of the antidots in the case of the 4-nm CdTe sample are in the range 1530 nm, which is much greater then for the structure with 2-nm CdTe layers, for which the dots sizes vary from 6 to 13 nm. The correlation between the initial thickness of the CdTe layer and the final size of the CdTe antidots is 4796
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Figure 2. High-magnification cross-sectional SEM images of CdTe/ PbTe heterostructures containing (a) two layers of PbTe quantum dots in a CdTe matrix and (b) two layers of CdTe antidots in a PbTe matrix. The initial thickness of the (a) PbTe and (b) CdTe thin layers was 6 nm, and the thickness of the respective CdTe and PbTe spacer layers was 50 nm.
very clearly visible in the samples that simultaneously contain four CdTe layers of different thickness (see Figure 3c). In this case, CdTe antidots with sizes from 4 to 10 nm were obtained from the CdTe layers with thicknesses varying from 0.3 to 2 nm. Moreover, we expect that, using this method (under better-adjusted technological conditions), it should be possible to obtain CdTe dots with diameters smaller than 4 nm. Note that the lower “theoretical” limit for CdTe layers with an initial thickness of just one monolayer (0.3 nm) is about 12 nm.23 Similarly to PbTe dots,2123,25,26 the CdTe antidots very often exhibit the same highly symmetric rhombicuboctahedral shape, as shown in Figure 4. Figure 4b shows a cross-sectional TEM image of a single CdTe antidot with well visible {100}-, {110}-, and {111}-terminating facets. Such symmetric shapes of antidots are typically observed for samples with initial CdTe layer thicknesses of less than 2 nm. The dots obtained from CdTe/PbTe heterostructures with thicker CdTe layers are usually elongated in the plane of the initial CdTe layer. Statistical analysis of the cross-sectional area (using WSxM software;27 see Figure 5) and height-to-length aspect ratio of the antidots obtained from CdTe layers with different thicknesses shows that, for smaller dots (obtained from thinner CdTe layers), the aspect ratio is about 1, whereas for larger dots (originating from thicker CdTe layers), this parameter is smaller. These experimental observations support our suggestion that the driving force of the formation of CdTe antidots in a PbTe matrix and PbTe dots in a CdTe matrix is the same. For certain applications, the possibility of forming CdTe antidots in a PbTe matrix and PbTe dots in a CdTe matrix in various parts of the same heterostructure can be interesting. An example of such a heterostructure is shown in Figure 6. First, a thin PbTe layer was grown between two thick CdTe layers. Subsequently, a similar sequence of layers was deposited but with the opposite thickness
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ratio, namely, a thin CdTe layer between two thick PbTe layers. Next, the sample was annealed in situ at 350 °C for 1 h. As a result we obtained, in a single technological process, one layer of CdTe antidots (upper row of dots in Figure 6) and one layer of PbTe dots (lower row), separated by a PbTe/CdTe spacer layer. Such a sequence of layers can be repeated to produce multilayered nanodot composite materials. We have also found that, if the thickness of PbTe spacer is too small, CdTe material from a few neighboring layers can accumulate during the annealing process to form one layer of large CdTe antidots. Figure 7 shows a sample that initially contained four CdTe layers of 2-nm thickness with only 10-nm-thick PbTe spacers. As a result of annealing, we obtained one layer of CdTe antidots with dimensions of about 30 nm. It is also possible that, in this case, the time of annealing (1 h) was too long or the temperature of 350 °C was too high. We verified that 20-nmthick PbTe spacers are sufficient for a controlled layer-to-dots transformation in CdTe/PbTe multilayers under such annealing conditions. To experimentally confirm the anticipated influence of CdTe nanoprecipitates on the electric and thermoelectric properties of the CdTe/PbTe heterostructures, an additional set of multilayer samples designed for electron transport experiments was prepared. These heterostructures contained CdTe antidot layers separated by 20-nm-thick PbTe spacers. The thickness of the initial CdTe layers was changed in the range from 0.6 to 6 nm. All of these multilayers were annealed in the MBE growth chamber for 1 h at the same temperature of 360 °C. Then, dc measurements of the electrical conductivity, Hall effect, and thermoelectric power were carried out in the temperature range of T = 4300 K in a continuous-flow liquid-He cryostat. Hall effect measurements were performed in the standard Hall bar geometry in a magnetic field of up to 7 kGs on samples with a typical size of 3 15 mm2 and small indium contacts fabricated at the edge of the specimen. The Seebeck coefficient was measured on the same samples as the electrical measurements. For determination of the thermopower, the sample was located between a copper block (cold rod) and a resistance heater to control the temperature gradient along the sample. To measure the change in voltage with temperature gradient, two contacts were placed on the top surface of the sample close to the subminiature LakeShore GaAlAs calibrated thermometers used for temperature gradient determination. To improve the heat-transfer efficiency in the system, silver paste was applied to fix the thermometers and heater to the sample and the sample to the copper block. The Seebeck coefficient was determined as the slope of the linear dependence of the voltage as a function of the applied temperature gradient. Most of our samples exhibited n-type conductivity with roomtemperature carrier concentrations varying in the range of 10171018 cm3, although we also obtained a p-type sample with a low concentration on the order 1017 cm3. The heterostructures were not intentionally doped, and the observed conducting carriers mostly originated from native defects, as is usually observed in bulk crystals and homogeneous thin layers of PbTe. The room-temperature mobility of carriers in the best CdTe/PbTe samples with the highest carrier concentration was about 200 cm2/(V s). For samples with low carrier concentrations, we found carrier mobilities that were a factor of 2 lower and a rapid decrease of carrier concentration with decreasing temperature down to about 1014 cm3 at T = 80 K. These samples were not suitable for further thermoelectric measurements. 4797
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Figure 3. High-magnification cross-sectional (top) SEM and (bottom) TEM dark-field images of the CdTe/PbTe heterostructures initially containing 10 CdTe layers of (a) 2- and (b) 4-nm thickness with 25-nm-thick PbTe spacers. (c) TEM bright-field image of a sample originally containing four CdTe layers with different thicknesses, as indicated.
In the case of such samples, two activation energies, Ea, can be derived from analysis of the n(T) and p(T) dependences. For an n-type sample with an initial CdTe layer thickness of 1 nm, we found Ea1 = 102 meV at high temperatures and Ea2 = 75 meV at low temperatures. The larger activation energy is about one-half of the PbTe energy bandgap at low temperatures. The smaller activation energy is possibly related to a deep donor or acceptor center in PbTe. A very similar temperature behavior was observed for 0.5-μm-thick n-PbTe layers grown in the same technological regime and also for high-resistivity thin layers of p-PbTe.28 It should be noted that the electrical properties of thick n-PbTe layers are comparable to those of bulk n-type crystals only after intentional doping with Bi. Figure 8a shows the temperature dependence of the Seebeck coefficient for three representative samples containing CdTe antidots formed from CdTe layers with thicknesses of 0.6, 2, and 4 nm. For all of these samples, we observed a linear increase in the thermoelectric power with increasing temperature, as expected for a system with a degenerate electron gas. The Seebeck coefficient for the sample containing the largest antidots (open circles)
is about 1.5 times lower than those for samples with smaller antidots (red squares and blue triangles), which is related to the approximately 2 times higher carrier concentration in this sample (n = 5.9 1018 cm3 at 300 K) compared to samples with smaller dots (n = 3.8 1018 cm3 and n = 3.2 1018 cm3 at 300 K, respectively). Comparing the thermoelectric power observed in our CdTe/PbTe antidot multilayers with the reference values for bulk n-PbTe crystals (see Figure 8b), we found that, for the samples with large CdTe antidots (open circles and open diamonds), the Seebeck coefficient is slightly higher than that expected for bulk PbTe crystals. However, in CdTe/PbTe multilayers with smaller CdTe antidots (red squares, black triangles, and blue triangles), we observed about 25% increases of the Seebeck coefficient S as compared to the reference Pisarenko plot (solid line).15 This increase in S is comparable to that observed experimentally in nanostructured samples of PbTe containing EuTe inclusions29 and in thin films of InGaAlAs nanocomposite containing ErAs nanoparticles.14 The enhancement of S in the case of both cited materials was interpreted as the effect of electron energy filtering induced by modification of the 4798
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Figure 4. (a) Cross-sectional TEM image of CdTe antidots obtained from a sample containing 10 CdTe layers with an initial layer thicknesses of 1 nm. (b) Single CdTe antidot with highly symmetric rhombicuboctahedral shape with three types of {100}-, {110}-, and {111}-terminating facets.
scattering mechanism in nanostructured samples with respect to the bulk ones. The two-phase heterostructures studied in this work can be viewed as nanometer-sized regions of PbTe (matrix material) with embedded CdTe antidots. Thus, we attribute the enhancement of the Seebeck coefficient in our samples to the increased DOS at the Fermi level for partially confined electrons in a matrix material, that is, to the mechanism considered in the case of PbSeTe/PbTe NDSLs. We suggest that the increase of the DOS is more appreciable for the heterostructures with smaller CdTe antidots. This can be qualitatively understood taking into account the experimentally observed linear relation between the CdTe dot diameter D and the thickness of the initial CdTe layer dCdTe (D ≈ 3dCdTe). For a decreasing CdTe dot diameter, one expects an increase of the area density of dots, ND, and a decrease of the mean interdot distance, RD. For a PbTe/CdTe multilayer with d CdTe = 2.5 nm, the interdot distance of RD ≈ 10 nm is small enough to reveal dimensional effects in PbTe. However, as the dot density (ND ≈ 1012 cm2) in this case corresponds to a volume concentration 10 18 cm 3 (i.e., the native defect concentration in PbTe), we cannot neglect a priori the possibility that the S enhancement in our small-dot structures is caused by an increase of the scattering parameter λ due to the electron scattering on CdTe grain boundaries, as takes place in the electron-filtering mechanism mentioned above. Our interpretation has recently received strong support in theoretical modeling of CdTe nanoclusters embedded in PbTe by tightbinding and ab initio DFT methods.30 We note that, in contrast to PbSeTe/PbTe system, no interface mixing is expected in PbTe/CdTe dot heterostructures because of the extremely low mutual solubility of CdTe and PbTe. Therefore, this materials system is particularly suitable for experimental studies of this effect. Furthermore, we expect rather good thermal stability of
Figure 5. Statistical distribution of the cross-sectional areas of CdTe antidots observed in CdTe/PbTe multilayers with varying initial thicknesses of CdTe layers. The analysis was carried out for the (110) crosssectional plane region of an area of about 500 300 nm2 using WSxM software.27.
Figure 6. Cross-sectional TEM image of a CdTe/PbTe multilayer containing one layer of CdTe antidots (top layer) and one layer of PbTe dots (bottom layer) in a properly designed single CdTe/PbTe multilayer subjected to thermal treatment. 4799
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Figure 7. SEM image of CdTe antidots obtained from a CdTe/PbTe multilayer initially containing four 2-nm-thick CdTe layers with 10-nm PbTe spacers.
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temperatures, the reorganization of the CdTe antidot distribution in the sample due to total energy minimization of the system is possible and requires further investigation. In conclusion, we have developed technological procedures for preparing a new layered thermoelectric material composed of the zinc-blende CdTe antidots embedded in the rock-salt PbTe semiconductor thermoelectric matrix. We applied a special annealing regime for multilayer CdTe/PbTe heterostructures grown by MBE on a GaAs(001) substrate with a very thick CdTe buffer layer. Depending on the ratio of the initial thicknesses of the CdTe and PbTe layers in the heterostructure, both the growth of CdTe antidots in a conducting PbTe matrix and the growth of PbTe dots in an insulating CdTe matrix was achieved. Electron microscopy investigations revealed the highly symmetric shape of CdTe antidots, with diameters varying in the range from 5 to 30 nm depending on the initial CdTe layer thickness. Our experimental observations indicate that the growth mechanism of CdTe antidots in a PbTe matrix is analogous to the growth of PbTe dots in a CdTe matrix and is driven by the phase separation originating from the qualitative differences in chemical bonding and crystal structure of PbTe and CdTe. Both types of electrical conductivity were achieved in nanocomposite CdTe/ PbTe multilayer materials through only crystal stoichiometry control with carrier concentrations in the range 10171018 cm3 and carrier mobilities of about 200 cm2/(V s) at room temperature. In CdTe/PbTe multilayer nanocomposites with relatively small CdTe antidots, an increase of the room-temperature Seebeck coefficient of about 25% as compared to that of reference thermoelectric n-PbTe crystals was observed. This experimental finding together with the reduction of the thermal conductivity expected in nanocomposites makes this material attractive for further thermoelectric investigations, in particular, in heavily n- and p-type doped CdTe/PbTe antidot multilayers with optimized thermoelectric parameters.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (+48 22) 843-56-26. Fax: (+48 22) 843 09 26. E-mail:
[email protected].
Figure 8. Seebeck coefficient S as a function of (a) temperature and (b) electron concentration for CdTe/PbTe antidot heterostructures with initial CdTe layer thicknesses as indicated in the figure. Our experimental data for CdTe/PbTe antidot multilayers (symbols) are compared to the S(n) dependence calculated for n-PbTe bulk crystals (solid line, the so-called Pisarenko plot15).
this nanocomposite, making it applicable to thermoelectric generators, which usually work at higher temperatures. Our expectations are based on recent synchrotron XRD studies of bulk PbTeCdTe nanocomposites carried out to very high temperatures of 1100 K, which showed the full reproducibility of the rock-salt lattice parameter of this two-phase materials system upon thermal cycling to 500 K.31 The nanocomposite structural transformation was found to begin at about 700 K. However, at still higher
’ ACKNOWLEDGMENT This material is based on work supported by the U.S. Army Research Laboratory and the U.S. Army Research Office under Contract W911NF-08-1-0231, as well as partially supported by the European Union within the European Regional Development Fund, through an Innovative Economy grant (POIG.01.01.02-00108/09). ’ REFERENCES (1) Snyder, G. J.; Toberer, E. S. Complex thermoelectric materials. Nat. Mater. 2008, 7, 105–114. (2) Majumdar, A. Thermoelectricity in semiconductor nanostructures. Science 2004, 303, 777–778. (3) DiSalvo, F. J. Thermoelectric cooling and power generation. Science 1999, 285, 703–706. (4) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R.; Lee, H.; Wang, D.; Ren, Z.; Fleurial, J. P.; Gogna, P. New Directions for LowDimensional Thermoelectric Materials. Adv. Mater. 2007, 19, 1043–1053. (5) Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B. Thin-film thermoelectric devices with high room-temperature figures of merit. Nature 2001, 413, 597–602. 4800
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