Growth of Bi2Te3 films and other phases of Bi-Te system by MOVPE

Author’s Accepted Manuscript Growth of Bi2Te3 films and other phases of Bi-Te system by MOVPE P.I. Kuznetsov, V.O. Yapaskurt, Shchamkhalova, V.D. Shcherbakov, Yakushcheva, V.A. Luzanov, V.A. Jitov

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To appear in: Journal of Crystal Growth Received date: 17 June 2016 Revised date: 21 September 2016 Accepted date: 23 September 2016 Cite this article as: P.I. Kuznetsov, V.O. Yapaskurt, B.S. Shchamkhalova, V.D. Shcherbakov, G.G. Yakushcheva, V.A. Luzanov and V.A. Jitov, Growth of Bi2Te3 films and other phases of Bi-Te system by MOVPE, Journal of Crystal Growth, http://dx.doi.org/10.1016/j.jcrysgro.2016.09.055 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Journal of Crystal Growth Jounal of Crystall Growth 00 (2016) 1–8

Growth of Bi2 Te3 films and other phases of Bi-Te system by MOVPE P.I.Kuznetsova,, V.O. Yapaskurtb , B. S. Shchamkhalovaa , V.D. Shcherbakovb , G .G. Yakushchevaa , V. A. Luzanova , V. A. Jitova a V.

A. Kotelnikov Institute of Radioengineering and Electronics of RAS, Fryazino, Moscow district,141190, Russia of Petrology, Geological Faculty, Moscow State University, Leninskie Gory, 119991 Moscow, Russia

b Department

Abstract We have deposited films of Bi-Te system by atmospheric pressure MOVPE on (0001) Al2 O3 substrates with thin ZnTe or thick GaN buffer layers at different temperatures and Te/Bi ratio in the vapor phase. As-grown films were studied by X-ray diffractometry, SEM microscopy and Raman spectroscopy. To determine the elemental composition of the films, an energy dispersive spectrometer was used. Single-phase films of Bi2 Te3 , Bi4 Te5 , BiTe, Bi10 Te9 , Bi4 Te3 , Bi3 Te2 have been grown and growth parameter ranges for obtaining different phases were defined. It was found that under the same growth condition different phases of the Bi-Te system realize depending on the film’s thickness. Thus, when growing of Bi2 Te3 films by MOCVD method the careful control of the phase composition is required. Keywords: A1. Solid solutions, A1. X-ray diffraction, A3. Metalorganic vapor phase epitaxy, B1. Bismuth compounds, B2. Topological insulators

1. Introduction Recently thin films of bismuth telluride have been intensively investigated as a topological insulator (TI), a new material of condensed matter physics [1]. Moreover, versatile TI saturable absorbers, including those on the base of Bi2 Te3 nanoparticles, have been employed to passively mode-lock the fiber lasers at telecommunication wavelength regime [2, 3, 4]. For the latter application, thin films are needed and the high bulk resistance of TI is likely not required. Various deposition techniques, such as sputtering [5], thermal evaporation [6], electrodeposition [7], pulsed laser deposition [8], molecular beam epitaxy [9, 10] and metalorganic chemical vapor epitaxy [11, 12] have been developed to grow thin Bi2 Te3 films on different substrates. Today to best of our knowledge there is no good quality phase diagram of the Bi-Te system, which would include different phases of the homologous series mBi2 ·nBi2 Te3 , where m and n are numbers of Bi2 and Bi2 Te3 blocks per unit cell [13, 14, 15]. When depositing thin films of Bi2 Te3 other phases of the system Bi-Te may appear. The authors of the paper [16] have grown films with Bi4 Te3 phase in MBE system at the flows ratio of Te/Bi lower than 17. Caha et.al [17] observed the growth of BiTe phase on BaF2 substrates. In this paper we report the observation of many transition phases from Bi2 Te3 to Bi2 when the growth temperature and the Te/Bi ratio in vapor phase are varied upon deposition of thin films in BiMe3 -Et2 Te -H2 system. We have found that the phases Bi2 Te3 (m=0, n=3,) Bi4 Te5 (m=1, n=5), Bi10 Te9 (m=6, n=9), BiTe (m=1, n=2), Bi4 Te3 (m=3, n=3), Bi3 Te2 (m=5, n=4) and Bi2 (m=3, n=0) of infinitive adaptive series mBi2 ·nBi2 Te3 may be obtained. The formation of Bi4 Te5 , BiTe, Bi4 Te3 and Bi3 Te2 phases occurs over a wide range of Te/Bi ratio in the vapor phase at low temperatures. Thus, it is essential to identify and control the emergence of other phases of bismuth telluride when growing Bi2 Te3 . One should use with caution such a technological method as a low temperature deposition of films to suppress the free bonds of the substrate. Email address: [email protected] (P.I.Kuznetsov)

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2. Experimental procedure Films of Bi-Te system were grown at atmospheric pressure of hydrogen in horizontal quartz reactor of home-made MOVPE system recently used for the deposition of films Bi2 Se3 [18], Bi2 Te3−x Se x [12] and (Bi1−x Sb x )2 Se3 [19]. The films were grown on the basic plane of sapphire substrates with buffer of thin ZnTe or thick N-terminated GaN layers. ZnTe buffer layer of 20 nm thickness was grown in a single process cycle with Bi2 Te3 at a temperature of 460 ◦ C. The Te/Zn ratio in the gas phase was maintained close to unity. Rocking curve of (111) reflection of ZnTe was 0.18 degree. The GaN buffer layer was grown by MOCVD separately from Bi2 Te3 films under a reduced pressure of 50 mbar. First, at a temperature of 1150 ◦ C the sapphire was annealed during 10 min in a stream of hydrogen, then the temperature was reduced to 600 ◦ C and a thin (20 nm) layer of GaN was precipitated. The temperature was then raised to 930 ◦ C and the bulk GaN buffer of a thickness of 140 nm was grown. Such two-stage growth of the buffer layer under conditions of 800-fold excess of ammonia provided N-terminated GaN surface with hexagonal pyramids with diameter up to 10 νm truncated on the (0001) plane. Rocking curve of (002) reflection of GaN was 0.45 degree. Thrimethylbismuth (BiMe3 ), diethylzinc (ZnEt2 ) and diethyltelluride (Et2 Te), whose purity was certified as electronic grade, were used as bismuth, zinc and tellurium organometallic sources respectively. BiMe3 , ZnEt2 and Et2 Te bubblers were held at 0, +10, and +25 ◦ C, respectively. To obtain a laminar flow and an acceptable films growth rate the hydrogen with a flow rate of 0.5 cl/min was used as a carrier gas. The substrate temperature was varied from run to run in the range of 330÷463 ◦ C. The Te/Bi ratio in vapor phase was varied from 1.8 to 40 and partial pressure of BiMe3 was changed within (1.5÷6)10−5 atmosphere. We have used such a high partial pressure of BiMe3 for growing of thick films (0.2 µm or higher) required to study the films phase composition. The surfaces of the films were studied by the scanning electron microscope Jeol JSM-6480LV with a tungsten thermionic cathode. To obtain diffraction patterns, the ”DRON 3” double crystal X-ray diffractometer, using monochromatic Cu Kα (λ = 5,4051 Å) was employed. Diffraction angle measurements were carried out in the range of 2.5-45 degrees. The elemental composition of the grown films were analyzed using an energy dispersive spectrometer (EDS) X-MaxN with 50 mm2 active area, which was docked with an electron microscope, and the program INCA, ”Oxford Instruments”. To standardize and optimize the profiles of emission lines of characteristic radiation the following standards were used: crystal Bi2 Se3 (Bi-Mα Se - Lα ), ZnS (Zn - Lα ) and Al2 O3 (Al - Kα and O - Kα ). Measurement of standards and analysis of the samples were performed under identical conditions at an accelerating voltage of 10 kV, an electron probe current of 1.4 nA and a process time of 5. Accumulation time of spectra was set equal to 100 seconds. The detection thresholds for all the elements analyzed are 0.03 - 0.05 weight %. The measurements of each sample were carried out at least in 5 points and in the case of a large spread of values we made the maps of the distribution of elements concentrations. Raman scattering spectroscopy was carried out at room temperature in the backscattering configuration using a Micro-Raman spectrometer XPloRA (Horiba Scientific). A solid state green laser ((λ = 532 nm)) were used for excitation. The signal was collected through 100× objective lens and dispersed by the 1800 g/mm grating under a triple subtractive mode with a spectral resolution of ∼1 cm−1 . The laser power and an accumulation time of recording were varied from 0.3 to 3 mW and from 20 to 200 s accordingly. The lowest available frequency was 50 cm−1 . The wavenumbers of modes and their FWHM were determined by fitting spectra with Lorentzian lineshapes. 3. Results and discussion 3.1. Films growth and characterization by EDS and XRD Table 1 shows the growth conditions and the characterization data for 29 samples of Bi-Te films. The samples are grouped according to a deposition temperature, and within each group, they are arranged with increasing Te/Bi ratio in the vapor phase during their growth. The results of energy dispersive spectroscopy (EDS) analysis showed a wide variation of the Te and Bi content in the films depending on these process parameters. In the samples the Bi content varied from 40 to 100 at%, what implies the practical implementation of different phases of homologous series mBi2 ·nBi2 Te3 . The phases expected from energy dispersive spectroscopy and phases found from XRD data are slightly different because the phase composition of the epitaxial layer changes in the direction of the film growth. Most of the rhombohedral or hexagonal films are epitaxially grown with their c-axis perpendicular to the substrate surface, meaning that only the (00l) planes are visible in the symmetric ω − 2Θ X-ray diffraction scans. At temperatures of 330 and 370 ◦ C the growth runs were carried out using GaN/(0001)Al2 O3 templates with Nterminated surface as substrates. On the surfaces of the buffer GaN the hexagonal pyramids are present, the larger

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Table 1: Characterization of the grown Bi-Te films..

Sample code

Te/Bi ratio in vapor

Buffer layer

EDS results Te,at% Bi,at.%

Phases expected from EDS data

Phases found from XRD data

FWHM of rocking curve of main peak, degree

Growth temperature 330◦ C TI375 TI344 TI359 TI373 TI374 TI370 TI358 TI372 TI376 TI371 TI369

3.2 5.7 8.3 10.8 11.1 12.1 16.2 17.8 20.9 24.4 32.8

GaN ZnTe GaN GaN GaN GaN GaN GaN GaN GaN GaN

1.1÷24.2 38.0 43.7 49.4 47.6÷52.2 52.4 52.0 55.5 55.6÷57.2 58.9 59.5

75.8÷98.9 Bi8 Te3 , Bi4 Te, Bi9 Te, Bi 62.0 Bi3 Te2 56.3 Bi4 Te3 50.6 BiTe 47.8÷52.4 Bi8 Te9 , BiTe, Bi10 Te9 47.6 Bi8 Te9 48.0 Bi8 Te9 44.5 Bi4 Te5 42.8÷44.4 Bi3 Te4 , Bi4 Te5 41.1 Bi2 Te3 , Bi10 Te9 40.5 Bi2 Te3 Growth temperature 370◦ C

multiphase Bi3 Te2 + trace Bi2 Bi4 Te3 BiTe BiTe BiTe BiTe Bi4 Te5 Bi4 Te5 +trace Bi2 Te3 Bi2 Te3 + trace Bi4 Te5 Bi2 Te3

TI364

1.8

GaN

0÷42

58÷100

TI363

3.7

GaN

0÷40

TI368

4.6

GaN

0÷44

Two phase Bi2 and Bi2 Te3 Two phase Bi2 and Bi4 Te3 , multiphase

TI366

5.1

GaN

0÷44

TI343

5.7

ZnTe

0÷40

TI362 TI365 TI361 TI360

6.8 8.1 10.9 15.6

GaN GaN GaN GaN

52.1 56.2 59.8 59.7

Bi4 Te3 ,Bi3 Te2 , Bi2 Te, Bi7 Te3 , Bi4 Te, Bi9 Te, Bi2 60÷100 Bi3 Te2 , Bi2 Te, Bi7 Te3 , Bi4 Te, Bi9 Te, Bi2 56÷100 Bi4 Te3 , Bi3 Te2 , Bi2 Te, Bi7 Te3 , Bi4 Te, Bi9 Te, Bi2 56÷100 Bi4 Te3 , Bi3 Te2 , Bi2 Te, Bi7 Te3 , Bi4 Te, Bi9 Te, Bi2 60÷100 Bi3 Te2 , Bi2 Te, Bi7 Te3 , Bi4 Te, Bi9 Te, Bi2 47.9 Bi8 Te9 43.8 Bi4 Te5 40.2 Bi2 Te3 40.3 Bi2 Te3 Growth temperature 420◦ C

BiTe Bi4 Te5 Bi2 Te3 Bi2 Te3

1.04 1.26 1.5 + 0.42 1.84+ 0.45

TI357 TI355 TI353 TI354 TI377

2.0 3.0 4.8 10.3 40.4

ZnTe ZnTe ZnTe ZnTe GaN

39.5 47.9 53.8÷59.5 60.1 59.5

60.5 Bi3 Te2 52.1 Bi10 Te9 40.5÷46.2 Bi2 Te3 , Bi4 Te5 , Bi6 Te7 39.9 Bi2 Te3 40.5 Bi2 Te3 Growth temperature 463◦ C

Bi3 Te2 Bi10 Te9 multiphase Bi2 Te3 Bi2 Te3

0.25 0.26

TI346

0.8

ZnTe

23.3-42.1

57.9-76.7

Bi4 Te3 ,Bi3 Te2 , Bi2

TI347 TI348 TI349 TI350

1.9 4.5 14.7 39.5

ZnTe ZnTe ZnTe ZnTe

49.6 59.6 59.5 60.1

50.4 40.4 40.5 39.9

Bi4 Te3 , Bi3 Te2 , Bi2 Te, Bi7 Te3 BiTe Bi2 Te3 Bi2 Te3 Bi2 Te3

multiphase

1.01 0.87 2.72 +0.56 2.42+0.93 2.9 + 0.55 3.2+0.54 0.81 1.25

-

-

multiphase

BiTe Bi2 Te3 Bi2 Te3 Bi2 Te3

0.33 1.1

0.15 0.24 0.2 0.33

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Figure 1: Rocking curves around (0015) reflection of BiTe films grown at temperatures of 330 ◦ C (TI370) and 463 ◦ C (TI347).

of which are truncated on the (0001) plane. The crystalline perfection of the films grown on such substrates is poor, what follows from the full width at half-maximum (FWHM) of the rocking curves around of main XRD peaks (near 45 degrees) given in Table 1. The rocking curve of sample TI370 with BiTe phase shown in Fig. 1 illustrates this also. For multiple phases of homologous series mBi2 ·nBi2 Te3 the main reflexes of XRD spectra are almost superimposed on each other and only broadening of one side of the reflexes indicates a presence of another phase. The XRD spectra in Fig. 2a, b, c show this. For many samples in Table 1 we see a significant deviation of the content of bismuth and tellurium from stoichiometry of the main phase found from XRD data. In Fig. 2b for sample TI 369 one sees that even at slight deviation from stoichiometry Bi2 Te3 , a marked shoulder of (0011) reflection of the Bi4 Te5 phase is superimposed on the peak (006) of the main phase. Raman spectroscopy indicated that the phase Bi2 Te3 is atop one. An asymmetry of the (0011) reflex is visible even in the XRD of the sample TI 372, the composition of which coincides perfectly with the stoichiometry of Bi4 Te3 phase, Fig. 2c. The phase of the deposited film strongly depends on Te excess in the reactor chamber. At a temperature of 330 ◦ C under the large excess of Et2 Te in the vapor phase, the films with pure Bi2 Te3 phase can be obtained. SEM image of the surface of the TI 369 film with the main phase Bi2 Te3 grown at 33- fold excess of Et2 Te is presented in Fig. 3a. It is seen that nor film is deposited on vicinal surface of GaN pyramids and the epitaxy is only going on (0001) surfaces of the truncated pyramids. Such an island epitaxy led to numerous low-angle boundaries and poor crystalline perfection of the grown films. Under the conditions of 18-fold Et2 Te excess a quite pure Bi4 Te5 phase is obtained, the sample TI 372. According to XRD spectra four films with the main BiTe phase were implemented at Te/Bi ratios from 11 to 16, although the results of EDS analysis indicate the stoichiometric composition close to Bi8 Te9 for two samples of them (TI 370 and TI 358). Further reduction of the tellurium excess led first to the deposition of Bi4 Te3 phase (TI 359), then Bi3 Te2 phase (TI 344), and finally, to multiphase TI 375 sample. EDS shows that in various points of the multiphase TI 375 film the content of Bi ranges from 76 to 99 at%. This is more than in Bi7 Te3 phase (70%) and is the largest of the known Bi content in Bi-Te system what indicates the presence the Bi2 phase. It should be noted that the film deposition temperature of 330 ◦ C is significantly higher than the melting temperature of metallic bismuth, therefore the latter goes in the drops during the deposition. Crystallized drops are clearly visible on SEM image of the sample TI 375, Fig. 3b. As EDS outside the crystallites indicated a stoichiometric composition of GaN we concluded that the telluride phases are located under the crystallites. The XRD spectrum allows us to identify the Bi2 phase, reflections from telluride phase is not visible, perhaps due to its poor crystallinity. The sample TI 364 with the bismuth content close to that of the sample TI 375 was grown at a higher temperature of 370 ◦ C. In the XRD spectrum of the sample TI 364 except of high intensity reflections (003) and (006) of the epitaxial bismuth there are wide reflections of the telluride phase, the main of which can be attributed to reflections (0013) and (0030) of the Bi3 Te2 phase (m=5, n=4, c predicted = 9.95 Å ). At a temperature of 370 ◦ C a quite pure phase

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of Bi2 Te3 may be grown at an excess of Te in the vapor phase above 11, see samples TI 361 and TI 360 in Table 1. At lower values of Te/Bi ratio the phase composition changes dramatically. So at 8-fold excess of Et2 Te the phase Bi4 Te5 (TI 365) is obtained and at 7-fold excess the phase BiTe (TI 362) is obtained, while at values of Te/Bi ratio less than 6 the multiphase samples are obtained. In general, the crystalline perfection of the single-phase films grown at 370 ◦ C are higher than that of the films deposited at 330 ◦ C, but the FWHM of the main peak rocking curves still exceeds one degree. In multiphase TI 363 film, with the surface morphology substantially different from the surface of the TI 364 film (compare Fig. 3c and Fig. 3b), the bismuth content ranged from 60 to 100 at.% also. In the XRD spectrum there are intensive reflections (009) and (0021) of the Bi4 Te3 phase (m=3, n=3, c=41.91Å) except of a weak and broad bismuth reflection (003). The concentration map of the distribution of elements on the area of 100×125µm2 was taken for the sample TI 363. In most regions of the concentration map the film composition almost perfectly agreed with stoichiometry of the phase Bi4 Te3 with bismuth content of 57.1 at%. In some regions of the map bismuth content was 100 at%, and in other regions, tellurium and bismuth were absent altogether. The result of the conversion of concentration map of Bi, Te, GaN distribution in the phase map is shown in Fig. 3d. It is seen that the phases Bi2 and Bi4 Te3 are spatially separated, and on a substantial part of the surface there is nor deposition. The diagram in Fig. 3f shows the spatial distribution of the phases Bi2 , Bi4 Te3 and GaN. With increasing growth temperature the samples crystalline perfection significantly improved and the number of realized phases decreased. When at 420 ◦ C three phases (Bi2 Te3 , Bi10 Te9 , Bi3 Te2 ) were obtained, at 463 ◦ C only two phases (Bi2 Te3 and BiTe) were obtained. The phase Bi4 Te5 is the low temperature one; no traces of this phase have been seen in the samples grown at temperatures of 420 ◦ C and 463 ◦ C. It should be noted that of all the samples shown in Table the best crystalline perfection has sample TI 347 with the BiTe phase. In the XRD spectrum from 6 to 90 degrees are seen reflections from surfaces (00l) with l from 2 to 21, and the rocking curve in the vicinity of the (0012) reflection equals 0.15 degree. The authors of [17] observed reflections with l from 2 to 17. Fig. 4 shows XRD of the films with phases Bi2 Te3 , Bi4 Te5 , Bi4 Te3 , Bi3 Te2 , Bi10 Te9 and BiTe obtained in a relatively pure form during this study. Results of identification of the XRD reflection peaks are given in the figure near the peaks. Only the reflections of (00l) series are seen, what evidences the epitaxial growth of the films with c axis perpendicular sapphire surface. The calculated parameters for these five rhombohedral and one hexagonal (BiTe) crystal lattices are 30.42, 54.32, 41.91, 59.90, 114.02 and 24.00 Å respectively. For the first three phases and BiTe, there are numerous data in ICDD PDF Database. Our XRD spectra and calculated parameters are in good agreement with them. There are no data for phases Bi10 Te9 (m=6, n=9) and Bi3 Te2 (m=5, n=4) in this Database. But the values of the parameter c defined from XRD spectra, which are equal to 154.14 and 59.95 Å respectively, well coincide with those calculated according to the formula from [13] c=(mc’+nc”)/3, with c’=11.859 Å, c”=30.474 Å. 3.2. Raman spectra Raman spectra of four films grown at a temperature of 463 ◦ C and various Te/Bi ratio in vapor phase are shown in Fig. 5a. According to the EDS and XRD data the upper spectra belongs to BiTe phase and the rest three correspond to Bi2 Te3 phase. Raman spectra of Bi2 Te3 films show A1g , E1g and A2g peaks of high intensity at wavenumbers listed in Table 2, which are slightly lower than those of active lattice vibrations in bulk Bi2 Te3 [20], namely 62 cm−1 (A1g ), 102,3 cm−1 (E1g ) and 134 cm−1 (A2g ). The red shift of these vibrations in Bi2 Te3 films were observed by many authors, for example authors of the paper [21] found the vibrations with wavenumbers of 56, 98 and 128 cm−1 . We did not observed the differences in wavenumbers of modes of a set of samples defined by excitation laser powers of 0.3 and 3 mW (see samples TI 348, TI 377, TI 373, TI 372 and TI 359 in Table 2), what evidences that the red shift is not the thermal effect. In the spectrum of the TI 350 sample grown at 40-fold excess of tellurium there is a peak at 85 cm−1 , the origin of which is not known. We can’t associate this peak with tellurium precipitates because we have not observed the peak with wavenumber 115 cm−1 , which was related to impurities of Te by authors of the papers [24, 25]. In the spectrum of BiTe phase in Fig. 5a four modes with wavenumbers of 55, 88, 98.2 and 117.6 cm−1 are well defined, but their intensities are more than an order of magnitude smaller than the intensities of the modes of Bi2 Te3 films. Lower intensities of the modes can’t be associated with a poor crystalline perfection, because it was high for the film TI 347 (see Table 1). According to the calculations of authors of the paper [21] in the Raman spectrum of BiTe may occur 12 active modes. However, in the spectrum of polycrystalline BiTe film they found four broad bands, two of them labeled by wavenumbers 88 and 117 cm−1 . Raman spectrum of BiTe epitaxial film on BaF2 (111) substrate

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was shown also in the paper [16]. The wavenumbers of four modes coincide with our data for the film TI 347, but there were additional bands at 82 and 127 cm−1 . In Fig. 5b we present the Raman spectra of four films grown at 370 ◦ C. According to X-ray data the TI 360 and TI 361 samples have the Bi2 Te3 phase, the TI 365 sample has the pure Bi4 Te5 phase and the TI 362 sample consists of BiTe phase although its composition (47.8 at % Bi) significantly differs from stoichiometric one. The Raman spectrum of the Bi4 Te5 sample is close to spectra of Bi2 Te3 samples. We have not found any data on the Raman spectra of the Bi4 Te5 films or bulk crystals, therefore we do not know do they differ from the spectra of Bi2 Te3 or not. The weak peaks around 75 cm−1 seen in the spectra of the TI362 and TI347 films with BiTe phase are related to an oxidation [22]. In Fig. 5c the Raman spectra of six samples identified as pure phases of homologous series mBi2 ·nBi2 Te3 are shown. Under the spectra in the left the samples codes are given. These are the Bi2 Te3 , Bi4 Te5 , BiTe, Bi10 Te9 , Bi4 Te3 and Bi3 Te2 phases with the bismuth content of 40, 44.4, 47.1, 50, 52.6, 57.1 and 60 at.% respectively. The experimental results on bismuth content in these films are very close to the stoichiometry (see Table 1).The spectra are arranged in order of decreasing bismuth content and one can see the strong transformation of the spectra. Note the strong broadening of the modes of all the samples compared to the modes of the Bi2 Te3 sample TI 369 and the weak intensity of modes in the spectrum of the BiTe sample TI 358. Parameters of peaks of the Raman spectra of all selected phases are given in Table 2. Raman spectra of Bi-Te films with different compositions were reported in the paper [21], however, only compounds with the bismuth content of 40, 50 and 57 at.% were attributed to the single phase. The wavenumbers of some modes for Bi2 Te3 (56, 97, 128 cm−1 ) and BiTe (88 and 117 cm−1 ), Bi4 Te3 (83 cm−1 ) are in good agreement with our data, but it should be noted that there are some additional well-defined modes in our spectra. In the paper [23] the frequencies 57, 88 and 115 cm−1 are given for the Bi4 Te3 film grown on Si by MBE. In our opinion, the authors of the paper [23] have grown BiTe films and not Bi4 Te3 film. 4. Conclusion In conclusion, thin films of several phases of Bi-Te system were grown by atmospheric pressure MOVPE. The bulk and local structure as well as the surface quality of the grown films were systematically investigated using XRD, SEM, EDS and Raman techniques. All realized phases, namely Bi2 Te3 , Bi4 Te5 , BiTe, Bi10 Te9 , Bi4 Te3 , Bi3 Te2 and Bi2 , belong to the homologous series mBi2 ·nBi2 Te3 of space groups R-3m or P-3ml, they may be described by the hexagonal crystal lattice and tend to crystallize along the axis c perpendicular to the (0001) plane of sapphire substrate. To best of our knowledge the epitaxial films of Bi4 Te5 , Bi3 Te2 and Bi10 Te9 phases are obtained for the first time. Upon growing the very thin films of Bi2 Te3 , when it is difficult to accurately determine the elemental and phase composition, one should take into account the probability of other phases appearing. Acknowledgements The authors gratefully acknowledge the support Russian Foundation of Basic Research under grants 14-07-00242 and 16-02-00677. [1] Y. Ando, J.Phys.Soc.Jap. 82 (2013) 102001, http://dx.doi.org/10.7566/JPSJ.82.102001 [2] Y. -H. Lin, C. -Y. Yang, S. -F. Lin, W. -H. Tseng, Q. Bao, C. -I. Wu, and G. -R. Lin, Laser Phys. Lett. 11, (2014) 055107, doi:10.1088/16122011/11/5/055107 . [3] J. Lee, J. Koo, Y. M. Jhon, and J. H. Lee, Optic express, 22, (2014) 6167,doi:10.1364/OE.22.006165 . [4] P. Yan, R. Lin, S. Ruan, A. Liu, H. Chen, Y. Zheng, S. Chen, C. Guo, and J. Hu, Sci. Rep. 5 , (2015) 8690, doi: 10.1038/srep08690 [5] D. -Ho Kim, E. Byon, G. -H. Leea, S. Cho, Thin Solid Films 510, (2006) 148, doi:10.1016/j.tsf.2005.12.306 [6] J. Dheepa, R. Sathyamoorthy, S. Velumani, A. Subbarayan, K. Natarajan, P. J. Sebastian, Solar Energy Materials and Solar Cells 81, (2004) 305, doi:10.1016/j.solmat.2003.11.008 [7] M. Takahashi, M. Kojima, S. Sato, N. Ohnisi, A. Nishiwaki, K. Wakita, T. Miyuki, S. Ikeda, and Y. Muramatsu, J. Appl. Phys. 96, (2004) 5582, doi: 10.1063/1.1785834 [8] A. Bailini, F. Donati, M. Zamboni, V. Russo, M. Passoni, C. S. Casari, A. Li Bassi, and C. E. Bottani, Appl. Surf. Sci. 254, (2007) 1249, doi:10.1016/j.apsusc.2007.09.039 [9] S. E. Harrison, S. Li, Y. Huo, B. Zhou, Y. L. Chen, and J. S. Harris, Appl.Phys. Lett. 102, (2013) 171906, http://dx.doi.org/10.1063/1.4803717 [10] L. He, X. Kou, and K. L. Wang Phys.Stat.Sol. RRL 7, (2013) 50, doi 10.1002/pssr.201307003 [11] H. Cao, R. Venkatasubramanian, C. Liu, J. Pierce, H. Yang, M. Z. Hasan, Y. Wu, and Y. P. Chen, Appl. Phys. Lett.101, (2012) 162104, doi: 10.1063/1.4760226

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Table 2: Details of Raman spectra of different phases of Bi-Te system.

Sample code TI371 TI369 TI360 TI354 TI377 TI377 TI348 TI348 TI349 TI350 TI361 TI358 TI347 TI362 TI374 TI370 TI373 TI373 TI376 TI372 TI372 TI365 TI355 TI355 TI359 TI359 TI364

Phase from Xray data Bi2 Te3 Bi2 Te3 Bi2 Te3 Bi2 Te3 Bi2 Te3 Bi2 Te3 Bi2 Te3 Bi2 Te3 Bi2 Te3 Bi2 Te3 Bi2 Te3 BiTe BiTe BiTe BiTe BiTe BiTe BiTe Bi4 Te5 Bi4 Te5 Bi4 Te5 Bi4 Te5 Bi10 Te9 Bi10 Te9 Bi4 Te3 Bi4 Te3 Bi3 Te2

Exposure parameters power,mW time,sec 3 200 3 200 3 200 3 200 3 20 0.3 200 3 20 0.3 200 3 200 3 200 3 200 3 200 3 10 3 200 3 200 3 200 3 200 0.3 200 3 200 3 20 0.3 200 3 3 0.3 3 3

10 200 500 200 20

Peaks position/FWHM,cm−1

51.7/8.5 50.2/6.4 56.5/6.1 55/10 54.4/11.2 55/5.5 55.2/5 53/6.5 52.6/8.7 56.5/4.8 55.1/5.9 55.6/6.5 58.8/4.7 54.5/8.9 56/7.8 49.7/6.6 51.1/5.4

57.3/5.1 58.4/4.1 55.5/6 58.5/4.8 57.7/5.3 57.6/5.4 60.6/4.6 59.2/10.5 60.2/4.2 61.1/4.6 58/5.4 74.4/3.8 76/3.2

57.6/5.4

74.7/3.2 75.3/3.1

77.6/6.4

90.4/8.2 68.5/5.5 71.4/9.5

77.4/30.8 85.1/10.5 70.3/7 89.6/7.3 88/9.4 88.4/6.5 87.5 / 7.2 87.8 / 6.8 84.9 / 8.1 86 / 8.8 sh 88.3 / 5.8 sh 88.3 / 9.8 sh 9.8/10.6 sh 90 / 6.4 87.2 / 6.3 87.7 / 5.7 82.9 / 8 82.5 / 5.5

99/7.1 99/3/4.8 96.2/5.6 99/5.1 98.6/5.7 98.6/5.5 100.9/6.1 100.5/9.8 101/4.8 102.2/5.7 98.4/5.1 100.1/10 98.2/10 99.5/8.1 98.1 / 9.9 97.6 / 10.3 99.1 / 12.2 100 / 9.1 98 / 6.9 97.1 / 6.5 97.7 / 5.7 99.8 / 4.8 98.6 / 33 100.4 /8.4 99.4 / 6.7 99.8 / 8.4 96.9/15.2

∗ The

positions of the peak in the samples TI371 and TI360 are noticeably shifted than in other samples of Bi2 Te3 phase. This assumes the existence of an unidentified phase in the surface layer. ∗∗) The essential shift of the peaks points that near the surface there is another phase, not BiTe. ∗∗∗) the position of peaks points that the phase Bi Te is the upper one, not the phase Bi Te . 2 3 4 5 x)

At the laser power of 3 mW additional peaks at 171 and 204 cm−1 appears (not shown) as a result of strong oxidation.

126,4/12.8∗ ) 131/9.3 128/10.7∗) 130.6/10.3 130.1/9.8 130.3/16.4 132/11.1 133.1/14.9 132.5/11.1 131.1/14.6 130.2/10.4 119.6/15.9 117.6 / 14.6 119.1 / 11.2 117.6 / 12.4 118.6 / 13.9 111.9 / 13∗∗) 113.1 / 14∗∗) 125.9 / 13.3 123.3 / 16.3 124.2 / 16.4 130 / 13.4∗∗∗) 116.2 / 8.2 115.8 / 9.4 106.1 / 22.4 109.8 / 8 x) )

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[12] P. I. Kuznetsov, G. G. Yakushcheva, V. A. Luzanov, A. G. Temiryazev, B. S. Shchamkhalova, V. A. Jitov, and V. E. Sizov, J. Cryst. Growth 409, (2015) 56, http://dx.doi.org/10.1016/j.jcrysgro.2014.09.035 ; [13] J. W. G. Bos, H. W. Zandbergen, M. -H. Lee, N. P. Ong, and R. J. Cava, Phys. Rev. B 75, (2007) 195203, doi: 10.1103/PhysRevB.75.195203 [14] J. -W. G. Bos, F. Faucheux, R. A. Downie, and A. Marcinkova, Solid State Chemistry 193, (2012) 13, doi:10.1016/j.jssc.2012.03.034 [15] Y. Feutelais, B. Legendre, N. Rodier, and V. Agafonov, Materials Res. Bull. 28, (1993) 591, doi:10.1016/0025-5408(93)90055-I [16] A. F¨ul¨op, Y. Song, S. Charpentier, P. Shi, M. Ekstr¨om, L. Galletti, R. Arpaia, T. Bauch, F. Lombardi, and S. Wang, Appl. Phys. Express 7, (2014) 045503, http://dx.doi.org/10.7567/APEX.7.045503 [17] O. Caha, A. Dubroka, J. Huml´ıcˇ ek, V. Hol´y, H. Steiner, M. Ul-Hassan, J. S´anchez-Barriga, O. Rader, T. N. Stanislavchuk, A. A. Sirenko, G. Bauer, and G. Springholz, Cryst. Growth Des. 13, (2013) , 3365, DOI: 10.1021/cg400048g [18] P. I. Kuznetsov, V. A. Luzanov, G. G. Yakusheva, A. G. Temiryazev, B. S. Shchamkhalova, V. A. Zhitov, and L. Yu. Zakharov, Communications Technology and Electronics 61, (2016), 183, doi: 10.1134/S1064226916010083 [19] P. I. Kuznetsov, G. G. Yakushcheva, B. S. Shchamkhalova, V. A. Luzanov, A. G. Temiryazev, and V. A. Jitov, J. Cryst. Growth 433, (2016) 114, http://dx.doi.org/10.1016/j.jcrysgro.2015.10.006 [20] W. Kullmann, J. Geurts, W. Richter, N. Lehner, H. Rauch, U. Steigenberger, G. Eichhorn, and R. Geick, Phys. Stat. Sol. B 126, (1984) 131, doi:10.1002/pssb.2221250114 [21] V. Russo, A. Bailini, M. Zamboni, M. Passoni, C. Conti, C. S. Casari, A. Li Bassi, and C. E. Bottani, Raman Spectroscopy 39, (2008) 205, doi: 10.1002/jrs.1874 [22] H. Xu, Y. Song, Q. Gong, W. Pan, X. Wu, and S. Wang, Modern Phys. Lett. B 29, (2015) 1550075, doi:10.1142/S021798491550075X [23] H. Xu, Y. Song, W. Pan, Q. Chen, X. Wu, P. Lu, Q. Gong, AIP Advances 5, (2015) 087103, http://dx.doi.org/10.1063/1.4928217 [24] J. Yuan, M. Zhao, W. Yu, Y. Lu, C. Chen, M. Xu, S. Li, K. P. Loh, and Q. Bao, Materials, 8 (2015), 5007, doi:10.3390/ma8085007 [25] S. Sen, K. P. Muthe, N. Joshi, S. C. Gadkari, S. K. Gupta, Jagannath, M. Roy, S. K. Deshpande, and J. V. Yakhmi, Sensors and Actuators B 98 (2004) 154, doi:10.1016/j.snb.2003.10.004

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Figure 2: XRD spectra of: multiphase sample (a); sample with main Bi2 Te3 phase and traces of Bi4 Te3 phase (b); sample with main Bi4 Te5 and small traces of Bi4 Te3 phases (c); and sample with pure Bi2 Te3 phase. Growth temperatures (Tgr ) and sample codes are shown in the insets.The dashed line represent the fitting of peaks by Gaussian function to find the peak positions.

P.I. Kuznetsov et.al. / Jounal of Crystall Growth 00 (2016) 1–8

a)

c)

10

b)

d)

f) e) Figure 3: SEM images of samples grown on GaN surface at different growth parameters: (a) TI 369 (Tgr =330 ◦ C, Te/Bi=32.8), (b) TI375 (Tgr =330 ◦ C, Te/Bi=3.2), (c) TI 363 (Tgr = 370 ◦ C, Te/Bi=3.7), (e) TI 377 (Tgr =420 ◦ C, Te/Bi=40.4); phase map (d) and areas under different phases (f) of the TI 363 sample.

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Figure 4: XRD spectra of samples with pure phases of Bi3 Te2 ,Bi4 Te3 ,Bi10 Te9 ,BiTe,Bi4 Te5 ,Bi2 Te3 (from up to down). Peaks corresponding the buffer layer and substrate are indicated by dashed lines.

P.I. Kuznetsov et.al. / Jounal of Crystall Growth 00 (2016) 1–8

8 8 .4 9 9 .5

1 1 9 .1

5 4 .4

c ) TI364

Bi3Te2

TI359

Bi4Te3

1 3 0

4 3 .8

TI355

Bi10Te9

×10

TI358

BiTe

4 0 .2

TI376

Bi4Te5

4 0 .3

TI369

Bi2Te3

1 2 8 .2

9 6 .2

T I3 6 1

8 5 .1

5 5 .5

A

E

A

1

2

2

g

1 g

1 g

1 3 0 .2

R a m a n s ig n a l ( a .u .)

9 9 .8

4 7 .9

T I3 6 5 5 8

40.5

TI349

R a m a n s ig n a l ( a .u .)

40.4

5 8 .8

50.4

TI348

x 2 0

T I3 6 2

9 8 .4

×10

TI347

R a m a n s ig n a l ( a .u .)

B i a t %

b )

7 6

1 1 7 .6

7 5

9 8 .2

5 5

8 8

Bi at %

a )

12

TI350

4 0

8 0

39.9

1 2 0

S to k e s s h ift ( c m

1 6 0 -1

)

T I3 6 0

4 0

8 0

1 2 0

S to k e s s h ift ( c m

1 6 0 -1

)

4 0

8 0

1 2 0

S to k e s s h ift ( c m

1 6 0 -1

)

Figure 5: Raman spectra of films: Bi2 Te3 (TI350, TI349, TI348) and BiTe (TI347) grown at temperature of 463 ◦ C (a); of Bi2 Te3 (TI360, TI3361), Bi4 Te5 (TI365) and BiTe (TI362) grown at temperature of 370 ◦ C(b); (c) different phases of B-Te system which are realized in this work. Details of the Raman spectra see in Table 2.