Controlling the Epitaxial Growth of Bi2Te3, BiTe, and Bi4Te3 Pure

1 Aug 2018 - Nanoscience and Nanotechnology PhD Program, CINVESTAV-IPN , 07360 Mexico City , Mexico. ‡ Solid State Electronics Section, Electrical ...
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Controlling the Epitaxial Growth of Bi2Te3, BiTe, and Bi4Te3 Pure Phases by Physical Vapor Transport Omar Concepcioń ,*,† Miguel Galvań -Arellano,‡ Vicente Torres-Costa,§ Aurelio Climent-Font,§ Daniel Bahena,∥ Miguel Manso Silvań ,§ Arturo Escobosa,‡ and Osvaldo de Melo⊥,# †

Nanoscience and Nanotechnology PhD Program, CINVESTAV-IPN, 07360 Mexico City, Mexico Solid State Electronics Section, Electrical Engineering Department, CINVESTAV-IPN, 07360 Mexico City, Mexico § Departamento de Física Aplicada, Instituto de Ciencia de Materiales Nicolás Cabrera and Centro de Micro Análisis de Materiales (CMAM), Universidad Autónoma de Madrid, 28049 Madrid, Spain ∥ Advanced Laboratory of Electron Nanoscopy, CINVESTAV-IPN, 07360 Mexico City, Mexico ⊥ Departamento de Física Aplicada, Universidad Autónoma de Madrid, 28049 Madrid, Spain # Physics Faculty, University of Havana, 10400 Havana, Cuba

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‡

S Supporting Information *

ABSTRACT: Bi2Te3 is a well-studied material because of its thermoelectric properties and, recently, has also been studied as a topological insulator. However, it is only one of several compounds in the Bi−Te system. This work presents a study of the physical vapor transport growth of Bi−Te material focused on determining the growth conditions required to selectively obtain a desired phase of the Bi−Te system, i.e., Bi2Te3, BiTe, and Bi4Te3. Epitaxial films of these compounds were prepared on sapphire and silicon substrates. The results were verified by X-ray diffraction, Raman spectroscopy, and Rutherford backscattering spectrometry.

1. INTRODUCTION Bi2Te3 has been studied for decades as one of the most important thermoelectric materials,1,2 and recently, its properties as a topological insulator (TI) have been experimentally demonstrated.3,4 However, other Bi−Te phases that have received much less attention do exist.5−8 In fact, there are at least seven different crystal structures in the Bi−Te system. They have a layered structure (with the exception of Bi4Te3) and present a rhombohedral or trigonal symmetry that is most frequently represented using hexagonal lattices with very similar a values (∼0.44 nm) and with c varying between 1 and 15 nm.6,8 Although several Bi−Te phases have been found in the natural state, reports of their identification are recent. One of the first Bi−Te phase diagrams dates back to 1957.9 There was a discrepancy in the stability range of Bi2Te3. Brown and Lewis10 proposed a wide range of Bi2Te3 stable compositions, while Abrikosov and Bankina11 proposed the existence of other compositions. The Bi−Te system has been studied by different methods, and today, many authors6,7,12 recognize the phase diagram reported by Okamoto et al.13 Stasova and Karpinskii14 were the first to propose the presently accepted criterion that all phases in the Bi−Te system are a combination of Bi2 and Bi2Te3 blocks of the form (Bi2)m(Bi2Te3)n, where m and n are integers, as schematically © XXXX American Chemical Society

shown in Figure 1. This model, which can be extended to Bi− Se and Sb−Te systems, is supported by later works,5,6,8,15−18 although some authors recognize other different phases.6,16,19 Most authors have dealt with Bi2Te3 growth, and there have been only few reports of the preparation of other Bi−Te phases. For example, Bi2Te3 and Bi4Te3 were obtained by molecular beam epitaxy (MBE);15,20 Bi2Te3, BiTe, and Bi4Te3 were prepared using pulsed laser deposition (PLD),5,17 while Bi2Te3, Bi4Te5, Bi10Te9, Bi4Te3, and Bi3Te2 have been obtained by metal−organic vapor phase epitaxy.19 Cruz-Gandarilla et al. concluded that layers grown by close space vapor transport present an unidentified mixture of Bi−Te compounds at a substrate temperature of ≥400 °C.8 The additional Bi−Te phases mentioned above could also be TIs. Recently, Yang et al.21 demonstrated that a single Bi bilayer behaves as a two-dimensional TI because of the presence of nontrivial topological edge states, and they expect that the combination of Bi2Te3 and Bi bilayers, which forms the different Bi−Te compounds, will also exhibit TI behavior. In fact, recently, Eschbach et al.22 demonstrated that BiTe is a three-dimensional TI, and Saito et al.23 predicted, after band structure calculations, that Bi4Te3 is a bulk zero-band gap Received: May 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b01235 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Schematic representation of Bi−Te phases as a combination of Bi2 and Bi2Te3 blocks of the form (Bi2)m(Bi2Te3)n, where m and n are integers.

semimetal with a Dirac cone at the Γ point, which was confirmed using optical measurements. In previous reports about physical vapor transport (PVT)24,25 and related techniques,8,19 a single source of Bi2Te3 powder was used in which the evaporated species are mainly BiTe and Te2.26 In contrast, in the work presented here, we used elemental Bi and Te sources whereby the vapors are expected to be a mixture of Bi, Bi2, and Te2 molecules.27 That is, the elemental sources supply separated Bi and Te vapor species to the substrate, allowing us to change the vapor composition by adequate modifications of the growth parameters and so favoring the stability of a selected phase. Previously, we reported Bi2Te3 deposition on silicon and sapphire (Al2O3) by PVT in vacuum.28,29 However, in experiments at atmospheric pressure, we also detected other Bi−Te phases, depending on the experimental conditions. In this work, we present a systematic study focused on determining the adequate conditions for the preparation of pure Bi2Te3 (tellurobismuthite), BiTe (tsumoite), and Bi4Te3 (pilsenite).

copy using an HR800 Horiba Jobin Yvon system. The morphology was studied by scanning electron microscopy (SEM) using an HRSEM-AURIGA Zeiss microscope and by atomic force microscopy (AFM) in tapping mode using a JEOL JSPM-5200 environmental scanning probe microscope. Electrical measurements were taken using a Walker Scientific model HV-4H/HSV-4H1 instrument. Rutherford backscattering spectroscopy (RBS) and planar channeling experiments were performed with a 2.0 MeV α-particle beam provided by a Cockcroft-Walton tandem accelerator. The RBS spectra were simulated using the SIMNRA code31 to determine the composition of the samples. X-ray photoelectron spectra were obtained with a SPECS PHOIBOS 150 9MCD instrument. The diffraction patterns were recorded using the Bragg−Brentano θ−2θ configuration with a copper anode (Cu Kα = 0.154 nm). The X-ray beam was focused on the sample using a parabolic mirror module, and a nickel filter was used to eliminate the Cu Kβ contribution. The diffraction patterns were collected by a CCD detector with 256 pixels. The X-ray tube power was 900 W (45 kV and 20 mA). The pole diagrams were obtained with the same system. A proportional detector with a parallel beam collimator was fixed at a 2θ angle with the incident beam corresponding to the selected crystallographic plane. Maps of the diffracted beam intensity were constructed in polar coordinates as the sample was rotated 360° around the azimuthal (ϕ) axis and tilted from 0° to 80° around the tilt (ψ) axis in increments of 5°.

2. EXPERIMENTAL SECTION The PVT experimental system has been reported previously.29 The Bi and Te sources were heated to 800 and 460 °C, respectively, while the substrates were located downstream at a temperature that was adjusted between 250 and 450 °C. Nitrogen was used as a carrier gas with a flow ranging from 0.5 to 5.0 L/min. The films were deposited on Al2O3 (0 0 1) and Si (1 1 1) substrates. As reported previously,30 we noted no important differences in the properties of the films grown on either of the substrates under the same growth conditions. Samples were characterized by X-ray diffraction (XRD) using a Panalytical X’Pert Pro MRD diffractometer and by Raman spectros-

3. CHARACTERISTICS OF THE Bi−Te PHASES As explained above, different Bi−Te phases are formed by Te1−Bi1−Te2−Bi1−Te1 blocks (superscript indexes denote different chemical states) called quintuple layers (QLs) eventually inserted between Bi bilayers. The QLs are bounded via van der Waals interactions and have a thickness of approximately 1 nm (see Figure 1).18 B

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Inorganic Chemistry Bi2Te3, the most commonly encountered phase, can be represented as (Bi2)0(Bi2Te3)3, and its elemental cell is formed by 15 atomic layers (QL−QL−QL). It belongs to the R3̅m rhombohedral space group but can be represented as a hexagonal lattice with parameters of a = 0.438 nm and c = 3.044 nm.32 BiTe, i.e., (Bi2)1(Bi2Te3)2, was first obtained experimentally in 1978.33 It is formed by 12 atomic layers (QL−Bi−Bi−QL) belonging to the trigonal P3̅m space group and can be also represented as a hexagonal structure with lattice parameters of a = 0.443 nm and c = 2.405 nm. Meanwhile, Bi4Te3, without van der Waals bonds, can be expressed as (Bi2)3(Bi2Te3)3, consists of 21 atomic layers (QL−Bi−Bi−QL−Bi−Bi−QL−Bi−Bi), and belongs to the R3̅m space group with hexagonal lattice parameters of a = 0.445 nm and c = 4.189 nm.20 The atomic coordinates, lengths, and angles among the different bonds for Bi2Te3, BiTe, and Bi4Te3 have been reported previously.34−36

4. RESULTS AND DISCUSSION To identify the conditions for obtaining different pure Bi−Te phases in our growth system, we explored the variation of the growth parameters over a wide range. Preliminary experiments demonstrated that the obtained phase was not influenced by changes in the source temperature (in the range between 760 and 840 °C for Bi and 420 and 500 °C for Te), the growth time (in the range of 7−240 min), or the type of substrate [Si (1 1 1) and Al2O3 (0 0 1)]. In contrast, the control of the phase was determined by an adequate combination of carrier gas flow and substrate temperature. 4.1. High Substrate Temperature. Mixture of Bi−Te Phases. Depositions made at a substrate temperature of 450 °C always presented a mixture of different phases under all the tested growth conditions. As an example, Figure 2a shows a diffractogram of a film grown at 450 °C on Al2O3 for 60 min with a carrier gas flow of 3 L/min. If we discard the reflections corresponding to Al2O3, the observed peaks can be attributed to different phases, i.e., Bi2Te3, BiTe, Bi4Te3, and Bi.37−40 Because of the structural similarities, all of these phases present some overlapping reflections that make their identification difficult. Because most of the peaks correspond to {0 0 l} planes, one could expect a fiber texture or epitaxial nature of the films.41,42 However, some contribution of randomly oriented crystallites is indicated by the peak at ∼27° (not indexed in the figure). It coincides with the most intense reflection reported for the different phases [(0 1 2) Bi, (0 1 4) BiTe, (0 1 5) Bi2Te3, or (0 1 7) Bi4Te3] and by the reflection at ∼56° that corresponds to the (0 2 4), (0 2 8), (0 2 10), and (0 2 14) planes of the same phases. Diffractograms of films grown on Si (1 1 1) under the same growth conditions displayed similar features. Raman spectroscopy has proven to be a useful complementary tool for identifying the different compounds. Numerous reports of Raman measurements of Bi2Te3 can be found in the literature, while only a few studies of the rest of the phases exist. For example, Russo et al.5 calculated that Bi2Te3, BiTe and Bi4Te3 have 4, 12, and 6 Raman modes, respectively. Other experimental or theoretical Raman shift positions found in the literature for the different phases of the Bi−Te system are summarized in Table 1. There is a discrepancy around the Raman shift of the Bi4Te3 peaks. Russo et al.5 and Kuznetsov et al.19 report that one peak of Bi4Te3 is located around 83 cm−1, but Xu et al.20 localized it around 87 cm−1. Our results, included in Table 1, are in

Figure 2. (a) X-ray diffractogram and (b) Raman spectrum of a sample grown at 450 °C on Al2O3. Miller indices of the different crystallographic planes (h k l) and Raman shift are indicated. The different Bi−Te phases are (●) Bi2Te3, (■) BiTe, and (▲) Bi4Te3.

concordance with refs 5 and 19 and were confirmed by XRD. A Raman measurement of the same sample is shown in Figure 2b. The peaks at 83.7 and 113.1 cm−1 correspond to the vibration modes of Bi4Te3; those at 89.7 and 119.8 cm−1 correspond to BiTe. The peak at 136.1 cm−1 corresponds to Bi2Te3, and the peak at 100.9 cm−1 is the convolution of vibrational modes of Bi2Te3, BiTe, and Bi4Te3. Bi modes can be identified as an overlap of the peaks at 83.7, 89.7, and 113.1 cm−1 between Bi and the corresponding phases (see Table 1). Bi cannot be easily identified by Raman, because of the overlap with the signals from other compounds, but its presence has been confirmed by XRD. Because the diffractograms do not provide information about the in-plane orientation of the films, pole diagrams were C

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Inorganic Chemistry Table 1. Raman Shifts for Different Phases in the Bi−Te System, Including Pure Bi and Te Elements Raman shift (cm−1)

phase Bi2Te3

36.5

62.0 58.4 59.7

38 BiTe

56 56.5 60

82

Bi4Te3 37.2

88 89.6 92 89.1

83 82.5

51.1 57.1

87.7 83.9

Bi4Te5 Bi3Te2 Bi10Te9 Bi Te

56.5

88.3 77.6

56.7 65

80

87.7 91 92 92 91.3

102.3 99.3 101.1 101.2 100 100.1 102 101.8 98 99.8 100 100.1 98 96.9 100.4

ref 134 131 132.7 132.9 117 119.6 120 117.1

127

109.8 114.9 109.5

132.2 125.9

115.8 115.3

105 102.2

121 119.7

142 139.5

5 19 20 a 5 19 43 a 5 19 20 a 19 19 19 5 20 44 45

This work at 350 °C.

a

made to determine if the films present either a fiber texture or an epitaxial structure. The presence of a given phase can be also verified if the tilt angle of a selected reflection equals the calculated angle between the surface orientation and the corresponding crystalline plane. The selection of the adequate reflection to be used should avoid overlap between reflections of the different phases. Considering the reflection intensities and tilt angles of the different phases, the crystalline planes (0 1 11) Bi2Te3 at 40.3° (2θ), (1 1 12) BiTe at 62.5°, (0 1 35) Bi4Te3 at 84.7°, and (0 1 5) Bi at 44.6° were chosen.37−40 Figure 3 shows pole diagrams taken at those 2θ angles of the same sample used for the measurements shown in Figure 2. The presence of Bi2Te3, BiTe, Bi4Te3, and Bi can be confirmed by the spots shown in the pole diagrams. In all cases, the tilt angle corresponds to the expected value (see the stereograms shown in the Supporting Information). Because only spots with 6-fold symmetry were detected in all cases, we can conclude that the crystallites have a unique a-axis orientation. The spots corresponding to (1 1 12) BiTe reflections in Figure 2b are rotated 30° with respect to the (0 1 l) spots of the other phases in the other diagrams, as expected. Because the diagrams were measured with a low-resolution experimental setting, the pole diagram taken at 84.7° shows not only the peaks corresponding to (0 1 35) Bi4Te3 planes but also those of (1 1 9) Bi and (2 2 3) Al2O3 at 85.0° and 84.4°, respectively. It can also be seen that the (0 1 35) Bi4Te3 spots are rotated by 30° with respect to the (2 2 3) Al2O3 spots, confirming that Bi−Te crystallite a-axes of the films are aligned with the a-axes of the Al2O3 substrate (see stereographic projections in the Supporting Information). This behavior is also shown in Figure 3d in which the reflections of (0 1 5) Bi and (1 1 3) Al2O3 are displayed. Thus, an epitaxial structure can be assumed. EDS measurements revealed different Bi/Te ratios for different electron beam energies in the same region of the samples. However, Bi/Te ratios were found to be the same in different regions of the surface when a similar electron beam

Figure 3. Pole diagrams of the sample grown at 450 °C on Al2O3 for different (h k l) Bi−Te planes: (a) Bi2Te3, (b) BiTe, (c) Bi4Te3, and (d) Bi.

energy was used. This indicates a composition dependence in depth, so these results seem to indicate that the different phases are not distributed along the substrate surface but are stacked, one on top of the other, as a consequence of the insertion of Bi bilayers between QLs. 4.2. Carrier Gas Flow Influence. Bi−Te Pure Phases. A very different behavior was observed at lower temperatures when the carrier gas flow was varied between 0.5 and 5.0 L/ min. Figure 4 shows diffractograms and Raman spectra of layers grown on Al2O3 at 250 and 350 °C as a function of the carrier gas flow. It can be seen that both techniques show that D

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Figure 4. XRD and Raman spectra of samples grown on Al2O3 at (a and b) 250 °C and (c and d) 350 °C for different carrier gas flows. For the sake of simplicity, Miller indices of the different crystallographic planes and Raman shift values for the peaks are shown only for pure phases. Different Bi−Te phases can be identified by the same symbol code used in Figure 2.

The identified phases at 250 and 350 °C are summarized in Table 2. It can be observed that well-defined regions were found in which pure phases Te, Bi2Te3, BiTe, Bi4Te3, and Bi

the obtained phases depend strongly on the flow: in the range of 0.5−4.5 L/min, with an increase in the flow, two-phase coexisting regions alternate with pure one-phase ones. E

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°C, the same dependence of the composition of the films on the carrier gas flow is observed but Bi condensation is reduced and Te2 decomposition enhanced; thus, only the intermediate phases are observed. At 450 °C, Bi condensation can be disregarded and the level of Te2 dissociation increases; thus, no dependence of the composition on the flow rate was seen. The mixture of observed phases can be understood considering the insertion of Bi bilayers between the QLs, favored by the higher temperature during the deposition process. 4.3. Characterization of Bi−Te Pure Phases. RBS measurements allow us to determine the composition of a mixture from the relation among the scattering yields of the relevant elements without the need of using standards; consequently, it is a very valuable technique for determining the composition of alloys or compounds. To avoid charge, buildup conducting substrates are preferred; thus, layers deposited on Si were selected for this study. Figure 5 shows

Table 2. Identified Phases for Samples Grown at Different Substrate Temperatures and Carrier Gas Flows flow (L/min)

250 °C

350 °C

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Te Te Te Bi2Te3 Bi2Te3 and BiTe BiTe BiTe and Bi4Te3 Bi4Te3 Bi Bi

Bi2Te3 Bi2Te3 Bi2Te3 and BiTe Bi2Te3 and BiTe BiTe BiTe and Bi4Te3 BiTe and Bi4Te3 Bi4Te3 Bi4Te3 and Bi Bi4Te3 and Bi

were obtained contrasting with layers grown at 450 °C, where a mixture of different phases was identified for all flow values, as stated above. Published Bi−Te phase diagrams6,7,12,13 predict other phases besides those we found in this work. We did not detect Bi4Te5, Bi6Te7, Bi2Te, or Bi7Te3 in our experiments for any growth conditions. Russo et al.5 obtained the same phases found in this work by PLD, while Fulop et al.15 obtained Bi2Te3, Bi4Te3, and a mixture of both under different growth conditions using MBE. Russo et al.5 concluded that, except for the most stable Bi2Te3, Te rich phases do not form because Te atoms, being lighter and more volatile than Bi ones, can be dispersed after deposition by incoming energetic particles and easily reevaporate. Bassi et al.17 concluded that phases with a Bi concentration of >57% (Bi4Te3) are not energetically favorable or have low stability, at least when deposited by PLD. The deposited phase is determined by the actual Bi/Te ratio on the substrate surface. To understand the influence of the carrier gas flow and the substrate temperature in obtaining one Bi−Te phase or another, the vapor properties of Bi and Te were considered. Bi vapors are composed by both Bi atoms and Bi2 molecules in nearly equal proportions,27 while 95% of the Te vapor is composed by Te2 molecules.46 Te2 is a relatively stable molecule and has to dissociate on the substrate surface during the deposition process or can re-evaporate. Decomposition of Te2 can be enhanced by higher temperatures or by larger permanence times caused by a lower carrier gas flow. It is also important to note that Bi has a vapor pressure that is ∼3 orders of magnitude lower than that of tellurium and can condense before reaching the substrate, leading to a lower Bi concentration on the substrate.47 At a 250 °C substrate temperature and a lower carrier gas flow (top left corner of Table 2), it is reasonable to accept that Bi is consumed by condensation in the reactor walls before reaching the substrate along the path in which the reactor temperature decreases from 800 °C (Bi source temperature) to 250 °C (substrate temperature). Te2 stagnates on the surface and has enough time to decompose. This results in the deposition of pure Te at the substrate surface. As the carrier gas flow increases, Bi vapor species will be more quickly dragged out from the source region and will be able to reach the substrate before condensation. As the Bi/Te ratio is increased, different compounds will be formed: Bi2Te3 at a rate of 2.0 L/min, BiTe at a rate of 3.0 L/min, and Bi4Te3 at a rate of 4.0 L/min. Two adjacent phases can coexist for intermediate flow values. A further increase in the carrier gas flow results in a shorter Te2 permanence time, and eventually, it is wiped away, before it can react; thus, only Bi deposition is observed. At 350

Figure 5. RBS spectra for three samples grown on Si (1 1 1) with different pure phases. From top to bottom: Bi2Te3, BiTe, and Bi4Te3, respectively. Dotted and solid lines represent experimental and simulated spectra, respectively. The Bi compositions resulting from the simulations are also presented. The regions corresponding to the different elements are highlighted.

measured and simulated spectra for samples grown on the Si (1 1 1) substrate that were previously identified as Bi2Te3, BiTe, and Bi4Te3 by the analysis of their diffractograms. The contribution of the different elements to the scattering is also shown. It can be observed that Bi composition values determined from these spectra are in excellent agreement with the stoichiometry of the three different compounds: 0.39 instead of 0.40 for Bi2Te3, 0.49 instead of 0.50 for BiTe, and 0.57 for Bi4Te3. RBS results were also corroborated by particleinduced X-ray emission (see the Supporting Information). Channeling behavior was studied to determine the crystallographic relation between the substrate and layer as this technique can be an alternative to texture measurements. To study the planar channeling behavior, the sample was located in the goniometer of the substrate holder with the normal to the surface pointing in the opposite sense of the α particle beam. A total of 200 RBS spectra were measured, while angles δ and β (see Figure 6a,b) varied in a coordinated way with F

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Figure 6. (a) Geometrical arrangement for the planar channeling study. (b) Angles δ and β varied in a coordinated way with steps of 0.025° between 2.5° and −2.5°. The arrow indicates the starting point of the movement. (c) Integrated intensity of the spectra in the regions of Si (bottom) and Bi4Te3 (top) as a function of spectrum number.

Figure 7. Scanning electron microscopy and atomic force microscopy images for three samples with different pure phases: (a) Bi2Te3, (b) BiTe, and (c) Bi4Te3.

steps of 0.025° between 2.5° and −2.5°. With this movement, the normal to the surface of the sample will describe a cone with an aperture angle of 5°. Planar channeling of monocrystalline samples is expected to occur for certain δ−β values in which a crystal plane is parallel to the beam direction and the α particles propagate through the crystal channels without being dispersed. If the deposited films were polycrystalline, the beam would be dispersed and the effect would not be observed. Figure 6c shows the integrated intensity of the spectra in the regions of Si (bottom) and Bi4Te3 (up) as a function of spectrum number. The spectra were counted from the position indicated by the arrow in Figure 6b and following, in a counterclockwise, the trajectory in the δ−β space. The Si signal shows six deep minima corresponding to the (1̅ 1 0), (1 1̅ 0), and (0 1̅ 1) crystalline planes. In the case of the Bi4Te3 spectra, the observed minima are at the same angles, and because the c-axis or ⟨0 0 1⟩ direction is normal to the surface, the minimum will correspond with the planar channeling for (1 0 0), (1 1̅ 0), and (0 1̅ 0) and the hexagonal base. This observation indicates an epitaxial relation between the film and substrate with the edges of the hexagonal lattice of Bi4Te3 oriented along {1 1 0} planes of Si. Similar results were observed for the sample with the BiTe phase by RBS measurements and pole diagrams (see the Supporting Information). Because the epitaxial deposition of Bi2Te3 has already been established,29 channeling studies for this material were not performed. Panels a−c of Figure 7 show SEM and AFM results for Bi2Te3, BiTe, and Bi4Te3 samples, respectively. The three compounds present different morphologies. Figure 7a shows the typical spiral growth of Bi2Te3 with triangular pyramids, with ∼1 nm steps and ∼100 nm terraces. In contrast, BiTe and Bi4Te3 are composed of rounded grains (Figure 7b,c) in the

absence of facets. As demonstrated by pole diagrams and channeling experiments, these layers are epitaxially oriented with the substrate; then, the rounded morphology should be explained by lower roughening temperatures of these phases with respect to those of Bi2Te3.48 These phases are known to have lower melting temperatures, which are directly related to the roughening temperature. Hall measurements of pure phase samples showed that room-temperature mobility decreases with an increase in Bi concentration from 6.1 cm2 V−1 s−1 (Bi2Te3) to 2.3 cm2 V−1 s−1 (BiTe) and 0.7 cm2 V−1 s−1 (Bi4Te3). All samples were ntype with resistivities on the order of 0.01 Ω cm and carrier concentrations between 1019 and 1020 cm−3. Because of roughness values of ∼30% and poor adherence of the layers, it was difficult to measure their thickness; thus, precise concentration and resistivity values could not be determined. A cross section and a surface profile can be observed in the Supporting Information. 4.4. Surface Oxidation. When Bi−Te films are exposed to ambient conditions, they eventually become oxidized. The analysis of the different phases by X-ray photoelectron spectroscopy (XPS) showed that the samples presented a considerable amount of adsorbed carbon and oxygen species. To discriminate integrated oxygen species from adventitious ones, a soft Ar+ ion beam sputtering step was applied (0.5 keV and 1 mA) for 1 min. Spectra were recorded after irradiation with no monochromatic Al Kα source acquired at normal takeoff angles with a pass energy of 75 eV for survey spectra and 25 eV for core level spectra. The data were processed with Casa XPS software using Gaussian/Lorentzian fractions of 30, a Shirley baseline, and core level energies referenced to the C 1s line at 285.0 eV. Constrictions were imposed to certify the accomplishment of the spin−orbit splitting intensity ratios in the Te 3d and Bi 4f peaks in view of the partial overlap of the G

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Figure 8. (a) Survey XPS spectra for the different phases with an indication of the most relevant element peaks used for quantification of the surface composition. (b) Te 3d core level spectrum of the Bi2Te3 sample with experimental data (■) and assigned deconvolution (lines). (c) Bi 4f core level spectrum of the Bi2Te3 sample with experimental data (■) and assigned deconvolution (lines). (d) Estimation of the Te/Bi atomic ratio at the surface of the different phases taking into account the total Te and Bi composition (■), the fraction of reduced Te and Bi (red circles), and the fraction of oxidized Te and Bi (blue triangles).

elements were observed to closely fit the expected values for the samples containing more Bi (BiTe and Bi4Te3) but gave rise to a slight Te substoichiometry for Bi2Te3 (see the red circles in Figure 8d). The estimation of the surface composition for the different phases by XPS supports the ability of the process to control the stoichiometry in spite of the apparent deviation of the value obtained for Bi2Te3. It is convenient to recall that the sensitivity factors are prone to variation due to matrix effects, and it is consequently normal to detect overestimations or underestimations of the stoichiometry expected from XRD when using fixed sensitivity factors. In our case, the measured matrix changes intrinsically, due to the design of the different Bi/Te ratios, and also extrinsically as a result of the preferential oxidation of Bi, which justifies the reported underestimation of the Bi2Te3 surface composition. The complementary analysis of the Te/Bi atomic ratio for the oxidized fraction of these elements shows a more drastic depletion of Te (triangles in Figure 8d) and confirms the greater susceptibility of Bi to preferential oxidation compared to that of Te. This, in fact, can be expected from their respective electron affinity. We can conclude that the phases show a logic trend also upon being analyzed at the surface level, but the reactivity with oxygen, mainly due to the high affinity of Bi for O, tends to deplete the Te surface composition.

Te 4s peak (secondary peak not used for quantification) with the main Bi 4f peak. The resulting survey spectra for the different phases are shown in Figure 8a. The samples contain different amounts of Bi and Te, as well as C and O. Additional core level spectra were acquired for each element, as exemplified in panels b and c of Figure 8 for the Te 3d and Bi 4f core levels, respectively, of the Bi2Te3 sample. Peak deconvolution allowed the estimation of the chemical state of the Bi and Te phases at the surface level. For each spin−orbit splitting, Te denotes three different chemical states, while Bi appears in only two states. According to refs 49 and 50, the Te 3d5/2 contributions can be attributed to pure Bi−Te (572 eV), Bi−Te−O (573.5 eV), and O−Te− O (576 eV) while the Bi 4f7/2 contributions are assigned to Bi−Te (157.5 eV) and O−Bi−O (159 eV). By using the survey spectra, we were able to estimate the Te/Bi atomic ratio by using the standard procedures of estimation of area and selecting the appropriate relative atomic sensitivity factors.51 The results of the obtained Te/Bi atomic ratio are plotted as black squares in Figure 8d and denote a generalized tendency of all the synthesized phases toward a Te depletion on the surface, which is especially acute for Bi2Te3. By using the chemical state analysis of Bi and Te elements on the different phases, we estimated the Te/Bi atomic ratios by separately taking into account the presence of reduced and oxidized phases. The Te/Bi atomic ratios of the reduced H

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Inorganic Chemistry

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5. CONCLUSIONS The ability of the PVT technique to control the stability of different phases in the Bi−Te system by optimizing the carrier gas flow and the substrate temperature was demonstrated. Samples were characterized by XRD, Raman spectroscopy, RBS, channeling, and XPS. The results of these techniques agree well in identifying, apart from pure Bi and Te, the epitaxial growth of three different phases (Bi2Te3, BiTe, and Bi4Te3), which can be found in the pure state in well-defined regions of the temperature flow diagram. Any specific phase can be obtained by using an adequate combination of carrier gas flow and substrate temperature. For substrate temperatures of >400 °C, a mixture of Bi2Te3, BiTe, Bi4Te3, and Bi was obtained irrespective of the carrier gas flow. All these different phases, equally oriented with respect to the substrate, are probably stacked, alternating one on top of the other.

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ACKNOWLEDGMENTS The authors thank A. Tavira for XRD measurements and J. Roque for SEM images and acknowledge financial support from CONACYT and the program “Cátedras de Excelencia de la Comunidad de Madrid”.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01235.

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Article

Pole diagram at a 2θ of 84.7° of a sample grown at 450 °C and stereographic projections of Bi4Te3, Al2O3, and Bi (Figure S1), Bi percent concentrations obtained by EDS measurements for different e-beam energies (Figure S2), PIXE spectra of the pure samples (Figure S3), integrated intensity of the spectra in the regions of Si (bottom) and BiTe (top) as a function of spectrum number and pole diagram of a pure BiTe sample at a 2θ of 62.5° (Figure S4), and cross section determined by SEM of a Bi2Te3 sample grown on a Si substrate and profilometry measurement (Figure S5) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Nanoscience and Nanotechnology PhD Program, Cinvestav, Av. Instituto Politécnico Nacional 2508, Gustavo A. Madero, San Pedro Zacatenco, 07360 Ciudad de México, Mexico. Email: [email protected]. Phone: +52 55 6941 3845. ORCID

Omar Concepción: 0000-0001-8197-7523 Vicente Torres-Costa: 0000-0001-5066-2799 Miguel Manso Silván: 0000-0002-5063-1607 Osvaldo de Melo: 0000-0002-8502-5711 Author Contributions

O.C., A.E., and O.d.M. designed the experiments, wrote the paper, and contributed to the interpretation of the results. O.C. performed the growth experiments. M.G.-A. and D.B. contributed with microscopy and Raman measurements. V.T.-C., A.C.-F., and O.d.M. performed and designed the RBS and channeling experiments and contributed to their interpretation. M.M.S. performed the XPS measurements and processed and interpreted the spectra. All authors contributed to the revision of the paper and have approved the final version of the manuscript. Notes

The authors declare no competing financial interest. I

DOI: 10.1021/acs.inorgchem.8b01235 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01235 Inorg. Chem. XXXX, XXX, XXX−XXX