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Reconstructions and stabilities of PbTe(1 1 1) crystal surface from experiments and density-functional theory
Applied Surface Science 356 (2015) 742–746
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Reconstructions and stabilities of PbTe(1 1 1) crystal surface from experiments and density-functional theory Haifei Wu a,∗ , Jianxiao Si b , Yonghong Yan a , Qing Liao d , Yunhao Lu c a
Department of Physics, Shaoxing University, Shaoxing 312000, People’s Republic of China College of Mathematics, Physics and Information Engineering, Zhejiang Normal University, Jinhua 321004, People’s Republic of China Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China d Department of Physics, Hezhou University, Hezhou 542899, People’s Republic of China b c
a r t i c l e
i n f o
Article history: Received 27 June 2015 Received in revised form 13 August 2015 Accepted 15 August 2015 Available online 19 August 2015 Keywords: Surface reconstruction Reflection high energy electron diffraction First principal calculations
a b s t r a c t Surface properties of epitaxially grown PbTe(1 1 1) thin films on BaF2 (1 1 1) substrates were systematically investigated by the characterizations of in situ reflection high energy electron diffraction (RHEED) and atomic force microscope (AFM). First principal calculations were performed to facilitate the interpretation of experimental observations. Our results indicate that substrate temperature (Tsub ) and chemical environment are crucial in determining the type of surface structure during the growth of PbTe(1 1 1) thin films. When PbTe is grown at Tsub = 250 ◦ C, the metastable (1 × 1) structure is formed on the PbTe(1 1 1) surface. However, whether Tsub is elevated to 300 ◦ C or the PbTe(1 1 1)-(1 × 1) sample grown at Tsub = 250 ◦ C is post-annealed at 300 ◦ C, the stable (2 × 1) reconstructions appear on the PbTe(1 1 1) surface. In Pb-rich environment, the most stable (2 × 1) reconstruction results from the substitution of the half of the 2nd Te atoms by Pb atoms, while in Te-rich environment, the most stable (2 × 1) reconstruction originates from the missing of half of the 1st Pb atoms. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Narrow gap lead telluride (PbTe) has attracted tremendous attention for its applications in mid infrared optoelectronic devices and thermoelectric devices within the temperature ranges of 300–900 K [1–3]. Since the optoelectronic and thermoelectric performances of devices fabricated on dot-patterned PbTe compound underlying layer can be improved effectively by quantum confinement, massive interest has been devoted to the PbTe based quantum dots, superlattices, nanowires, and embedded nanocrystals in recent years [4–7]. The qualities of these low dimensional structures are generally dominated by the surface properties of PbTe film, such as the surface roughness and surface structures. Furthermore it has been reported that PbTe transforms from normal insulator to nontrivial topological insulator (TI) under surface strain [8]. Hence, understanding the nature of the PbTe film surfaces becomes closely relevant to the design and fabrication of excellent PbTe based wires, dots, and superlattices, and moreover to further promotion of PbTe based devices. Up to now, a few of studies have
∗ Corresponding author. E-mail address: [email protected] (H. Wu). http://dx.doi.org/10.1016/j.apsusc.2015.08.126 0169-4332/© 2015 Elsevier B.V. All rights reserved.
been focused on the surfaces of the rocksalt-type PbTe. For example, Springholz et al. and Ma et al. reported that the low indexed surfaces of the lead-salt compounds do not exhibit any surface reconstructions [9,10], whereas a 2 × 2 periodicity was observed on the (1 1 1) surface of PbTe in a Rutherford backscattering and RHEED experiments in 1998 [11]. Dissimilarly, Fuchs et al. reported a 2-fold reconstruction pattern in the [1 1 0] direction of the PbTe(1 1 1) surface by RHEED [12], inferring the surface reconstruction different from 2 × 2. Recently, Volker et al. reported that the 2 × 2 and 2 × 1 reconstructions are lower in surface energy compared to both Pband Te-terminated pristine PbTe(1 1 1) by first-principle simulations, and the 2 × 2 reconstruction could be reconciled with the previous Rutherford backscattering experiment [13]. Thus, it can be concluded that although various surface structures on PbTe(1 1 1) have been reported, the detailed surface formation mechanisms are still unclear. In this letter, we carry out systematic study on the surface properties of PbTe(1 1 1) thin films grown under different conditions by molecular beam epitaxy (MBE). The surface lattice period and morphology of PbTe were characterized by RHEED and AFM, respectively. In conjunction with ab initio calculation results, the influence of growth conditions on the evolution of the PbTe(1 1 1) surface was analyzed.
H. Wu et al. / Applied Surface Science 356 (2015) 742–746
2. Experimental and calculation details PbTe films were epitaxially grown on freshly cleaved BaF2 (1 1 1) substrates in a solid source molecular beam epitaxy (SSMBE) system with a base pressure better than 1.5 × 10−10 Torr. An extra Te beam flux was supplied during the epitaxial growth process of PbTe to adjust the stoichiometry of Pb and Te. The freshly cleaved BaF2 (1 1 1) substrate was preheated at about 200 ◦ C for 40 min in the load and fast-entry lock chamber (residual pressure P = 10−7 Torr) before it was transferred into the growth chamber. Before growth initiation, the substrate was further cleaned in the growth chamber by heating for 10 min at 550 ◦ C, the cleanliness and the orientation of the substrate were checked by RHEED. The variation of beam flux was controlled through adjustment of the effusion cell temperature. The beam flux rates and layer thickness was monitored using quartz-crystal thickness monitor calibrated by optical ellipsometry. To study the role of growth temperature played on the evolution of surface properties, samples were grown at different Tsub varying from 250 to 450 ◦ C, the growth temperature Tsub is thermocouple measured one at the back of sample holder. During the growth of these samples, a fixed vapor pressure ratio of Te to PbTe (Rf ) was used. Likewise, to study the evolution of surface properties with Pb-to-Te atomic ratio, samples were prepared with different Rf while keeping a same Tsub . The growth rate was about 1 m/h and the thicknesses of as-grown PbTe films were about 1 m for all samples. Morphological characterization was performed ex situ by AFM using NT-MDT AFM in semi-contact mode. The experimental results are qualitatively discussed in terms of DFT calculations. Structural optimization and surface energy calculations were performed within the spin-polarized DFT formalism using the generalized-gradient approximation (GGA) and the projector-augmented wave method as implemented in VASP [14–16]. We treated the outermost s and p electrons of Pb and Te as valence electrons and the rest as cores. The energy cutoff was set up to 400 eV and Monkhorst–Pack k-point sampling for the Brillouin zone with a Gaussian smearing of 0.1 eV was used to make sure the surface energy difference between consecutive cycles was within 10−4 eV [17]. Symmetrical seventeen atomic layers slab was introduced to model the Pb-terminated PbTe(1 1 1) surface, which is indicated by the experimental results [18,19]. A vacuum region of at least 20 A˚ was used to avoid interaction between slabs. All the atomic coordinates except for those in the central three layers of the slabs were relaxed using the conjugate-gradient algorithm with a tolerance factor of 0.01 eV/A˚ for the force minimization
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[20]. Spin–orbital coupling (SOC) was not included since it is found that the inclusion of SOC did not have significant influence on the structural properties. Calculated surface energy under various growth conditions: the Te-rich condition corresponds to the chemical potential of Te (Te ) set to that of a standalone Te atom since Te can be vaporized as a single atom in experiment, whereas the Pb-rich condition corresponds to Pb set to that of bulk Pb, with the constraint Te + Pb equal to the total energy per formula bulk PbTe. The surface energy is defined as: Esurf = (Etotal − nTe − mPb )/2A, where n and m are the number of Te and Pb atom included in the slab, and A is the surface area [21,22]. 3. Results and discussions Fig. 1 shows RHEED patterns along [0 −1 1]-direction of growing PbTe films (1.0 m) on BaF2 (1 1 1) at different substrate temperatures with Rf to be 0.3. From Fig. 1(b)–(e), PbTe films were grown at ascending substrate temperature series, viz. 250, 300, 400 and 450 ◦ C, respectively. A typical RHEED pattern along [0 1 −1]direction for a clean BaF2 (1 1 1)-(1 × 1) substrate is shown in Fig. 1(a) for comparison. When rotating the BaF2 (1 1 1)-(1 × 1) and PbTe sample grown at Tsub = 250 ◦ C about their normal axises respectively, a 6-fold rotational symmetry is observed along the <1 0 −1> and <1 1 −2> azimuths (not shown here) for both of them, and the spacing between the streaky lines of their RHEED patterns are almost the same (see Fig. 1(a) and (b)). Considering the small lattice mismatch between BaF2 and PbTe (+4.2%), it can be speculated that a smooth single crystal PbTe film without surface reconstruction is obtained at Tsub = 250 ◦ C and Rf = 0.3. Compared to Fig. 1(b), Fig. 1(c)–(e) are characterized by the appearance of additional streaky lines in the middle of original diffraction streaks in the RHEED patterns along the [0 −1 1] direction of PbTe films as marked by arrows in Fig. 1. Moreover, it is worthy to note that for any epitaxial PbTe films grown at Tsub ≥ 300 ◦ C, the additional diffraction streaks can only be observed along [0 −1 1] and [0 1 −1] directions with an angle difference of 180◦ when samples are rotated about their normal axis, implying a 2-fold rotational symmetry along the <0 −1 1> azimuths. Meanwhile, none additional streaks are observed along the <1 1 −2> azimuths. These features of RHEED patterns from Fig. 1(c)–(e) directly indicate that a (2 × 1) surface reconstruction is formed once PbTe films are grown at Tsub ≥ 300 ◦ C and Rf = 0.3. Actually, in our previous investigation, the (2 × 1) surface reconstruction was also detected in PbSe film grown on BaF2 (1 1 1) at Tsub = 450 ◦ C by scanning tunneling microscopy (STM) [23].
Fig. 1. RHEED patterns along [0 −1 1]-direction of growing PbTe films (1.0 m) on BaF2 (1 1 1) with Rf = 0.3 at Tsub of: (b) 250 ◦ C; (c) 300 ◦ C; (d) 400 ◦ C and (e) 450 ◦ C. A typical RHEED pattern along [0 1 −1]-direction for a clean BaF2 (1 1 1)-(1 × 1) substrate is shown in (a).
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Fig. 2. The AFM images of growing PbTe films (1.0 m) on BaF2 (1 1 1) with Rf = 0.3 at Tsub of: (a) 250 ◦ C; (b) 300 ◦ C; (c) 400 ◦ C and (d) 450 ◦ C.
In order to reveal the surface structure formation mechanism of PbTe epitaxy, we further performed AFM measurements on these four as-grown PbTe samples, and the surface morphology evolution with ascending substrate temperature is presented in Fig. 2. At Tsub = 250 ◦ C, the surface structure is dominated by typical growth spirals formed around threading dislocations that originate from the growth on the 4.2% lattice-mismatched substrate [23]. At Tsub ≥ 300 ◦ C, besides spirals structures, triangular pits are observed and randomly distributed on the surface. With increasing Tsub , the density of triangular pits increase. According to the previous investigations by Wu et al. and Zhang et al. [23,24], the formation of pure spirals without any other large-sized defects in the growth of PbTe needs appropriate Tsub and an atomic ratio of Pb/Te ≈ 1 atmosphere. However, when an atomic ratio of Pb/Te is increased to make the films slightly Pb-rich, triangular pits will appear. On the other hand, when an atomic ratio of Pb/Te is decreased to make the films slightly Te-rich, regular shape defects will be formed on the film surface. Consequently, one can expect that at Tsub = 250 ◦ C, the reevaporated Te atoms from the PbTe epitaxial layers were just compensated for by the supply of additional Te flux from a separate effusion cell with Rf = 0.3, resulting in the growth of PbTe films with a balanced stoichiometry and pure spirals on the surface (see Fig. 2(a)). However, when Tsub is elevated up to 300 ◦ C while keeping Rf = 0.3, much more Te atoms will reevaporate from the PbTe epitaxial layers, which eventually lead to a Pb enrichment during the growth process. The higher the growth temperature is, the more serious the Te reevaporation will become. That is why one can see the density of triangular pits, originating from excessive Pb atoms congregating in the dislocation core area and relatively slow growth rate of (1 0 0) facets [23], increases with growth temperature from 300 ◦ C to 450 ◦ C in Fig. 2(b)–(d). As is shown in the RHEED results above, PbTe film grown at Tsub = 250 ◦ C with Rf = 0.3 does not exhibit any surface
reconstructions, while PbTe films grown at Tsub ≥ 300 ◦ C with identical Rf exhibit (2 × 1) surface reconstruction. It seems that the formation of PbTe(1 1 1) surface reconstruction is associated with higher Tsub (Tsub ≥ 300 ◦ C) and Pb-rich growth atmosphere. To determine which factor plays a key role on the appearance of the (2 × 1) surface reconstruction, another two samples were prepared. The first sample was grown at Tsub = 250 ◦ C without additional Te flux to create a Pb-rich ambient, while the other was grown at Tsub = 300 ◦ C with Rf = 0.5 to create an atomic ratio of Pb/Te ≈ 1 ambient. The AFM and RHEED measurement results of these two samples are shown in Fig. 3. It can be seen from Fig. 3 that at Tsub = 250 ◦ C, although PbTe film was grown with Pb-rich atmosphere, confirmed by the appearance of triangular pits in the AFM image in Fig. 3(a), the additional streaky lines still does not appear in the diffraction pattern. While at Tsub = 300 ◦ C, although PbTe film was grown in stoichiometric atmosphere, confirmed by the appearance of pure spirals without triangular pits or any other defects in the AFM image in Fig. 3(b), the (2 × 1) surface reconstruction can be easily detected by RHEED. Therefore, it can be deduced that the appearance of the (2 × 1) surface reconstruction is not relevant to whether PbTe film was grown with Pb-rich chemical atmosphere or not. Subsequently, we annealed the sample grown at Ts = 250 ◦ C with Rf = 0.0 at a temperature of 300 ◦ C for 10 min. Interestingly, the 2-fold reconstruction streaks in the [0 −1 1] direction appeared in the diffraction pattern. Further, the 2-fold reconstruction patterns still exist on the surface of the sample (not shown here) annealed for 10 min at Tsub = 400 ◦ C and even 450 ◦ C, respectively. Consequently, we can draw the conclusion that the formation of the (2 × 1) surface reconstruction on PbTe(1 1 1) film surface depends directly on Tsub . As long as PbTe(1 1 1) films are grown at Tsub ≥ 300 ◦ C or just postannealed up to 300 ◦ C, the (2 × 1) reconstruction will form on the PbTe(1 1 1) surface and become very stable. No matter the film is
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Fig. 3. The AFM and RHEED images of PbTe films (1.0 m) under growth conditions of: (a) Tsub = 250 ◦ C and Rf = 0.0; (b) Tsub = 300 ◦ C and Rf = 0.5.
further annealed up to 450 ◦ C or cooled down to room temperature, the reconstruction is still present on the surface. To verify the surface structure of PbTe(1 1 1) from the experimental RHEED patterns and inspect the details of the surface mechanism, we further carried out a series of theoretical
calculations using DFT. Since it is indicated in the experiment that the stable (1 1 1) surface of PbTe grown at elevated Tsub is Pb terminated [18,19], we only focused on the Pb-terminated PbTe(1 1 1) surface. Based on the experimental results above, possible surface structures of PbTe(1 1 1) have been sketched in Fig. 4(a). The
Fig. 4. (a) Four possible surface structures of PbTe(1 1 1): (1 × 1) represents the pristine Pb-terminated PbTe(1 1 1)-(1 × 1) structure, (2 × 1)−1 represents the (2 × 1) reconstruction where half of the 2nd Te atoms are substituted by Pb atoms, (2 × 1)−2 represents the (2 × 1) reconstruction where half of the 1st Pb atoms are missing, in addition, (2 × 1)−3 represents the (2 × 1) reconstruction where half of the 1st Pb atoms are missing and half of the 2nd Te atoms are substituted by Pb atoms. (b) Calculated surface energy under various growth conditions: the Te-rich condition corresponds to the chemical potential of Te (Te ) set to that of a standalone Te atom since Te can be vaporized as a single atom in experiment, whereas the Pb-rich condition corresponds to Pb set to that of bulk Pb, with the constraint Te + Pb equal to the total energy per formula bulk PbTe. The surface energy is defined as: Esurf = (Etotal − nTe − mPb )/2A, where n and m are the number of Te and Pb atom included in the slab, and A is the surface area.
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H. Wu et al. / Applied Surface Science 356 (2015) 742–746
pristine PbTe(1 1 1)-(1 × 1) surface is an alternative Pb- and Telayer structure, and here the 1st layer means the outmost Pb layer of PbTe(1 1 1) surface while the 2nd layer means the sublayer Te. In Fig. 4(a), (1 × 1) represents the pristine Pb-terminated PbTe(1 1 1)-(1 × 1) structure, (2 × 1)−1 represents the (2 × 1) reconstruction where half of the 2nd Te layer atoms are substituted by Pb atoms, (2 × 1)−2 represents the (2 × 1) reconstruction where half of the 1st Pb atoms are missing, and (2 × 1)−3 represents the (2 × 1) reconstruction where half of the 1st Pb atoms are missing and half of the 2nd Te atoms are substituted by Pb atoms. As the chemical environment is crucial in determining the type of surface structure in a crystal film growth, to explore the feasibility of surface structure engineering, we calculated the surface energies of the relevant surface structures in Fig. 4(a) and showed their dependence on the various growth conditions in Fig. 4(b). Apparently, the surface energies of the entire structures increase with the chemical environment from Pb enrichment to Te enrichment except for (2 × 1)−2 , and PbTe(1 1 1)-(1 × 1) structure is thermodynamically unfavorable as it always has a higher surface energy than the (2 × 1) motifs. It is the metastable structure under all growth conditions, which is in agreement with the above experimental results. Further inspection of Fig. 4(b) quickly reveals that in the Pb-rich environment, (2 × 1)−1 is the most stable structure, while in the Te-rich environment, (2 × 1)−2 becomes the most stable structure. It is noteworthy that in the above experiments, the metastable PbTe(1 1 1)-(1 × 1) structure has indeed been produced at Tsub = 250 ◦ C. This is because molecular beam epitaxy growth is a dynamic process, rather than a thermal equilibrium process and metastable structure could exist due to thermal fluctuation. To obtain stable structure, thermal energy is needed to overcome the energy barrier between metastable and stable structures. That is why the (2 × 1) reconstruction only occurred at higher growth temperature (Tsub ≥ 300 ◦ C) or PbTe(1 1 1)-(1 × 1) films post-annealed up to 300 ◦ C or higher temperatures. 4. Conclusions In summary, a series of PbTe film samples were grown on BaF2 (1 1 1) by molecular beam epitaxy. The surface properties of PbTe(1 1 1) were investigated by RHEED and AFM. DFT calculations were carried out to facilitate the interpretation of experimental observations. The experimental results generally suggest that the formation of the (2 × 1) surface reconstruction in PbTe(1 1 1) films depends directly on the substrate temperature. As long as PbTe(1 1 1) films are grown at Tsub ≥ 300 ◦ C or just post-annealed up to 300 ◦ C, the (2 × 1) reconstruction will form on the PbTe(1 1 1) surface and become very stable. No matter the film is further annealed up to 450 ◦ C or cooled down to room temperature, the reconstruction is still present on the surface. The DFT calculations show that
PbTe(1 1 1)-(1 × 1) structure is metastable. Once sufficient energy is provided in the Pb-rich environment, the substitution of the half of the 2nd Te atoms by Pb atoms will result in the most stable (2 × 1) reconstruction on the surface, while in the Te-rich environment, the missing of half of the 1st Pb atoms leads to another stable (2 × 1) surface reconstruction. Therefore, in principle, the surface structures of PbTe(1 1 1) can be manipulated by careful control of the experimental conditions. In this regard, it is helpful for the fabrication of quantum dots, superlattices, nanowires, and embedded nanocrystals based on PbTe. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant nos. 51202149, 11374009, 51302248, 11204180, and 11464051), the Zhejiang Provincial Natural Science Foundation of China (Grant no. LQ12F04001), and the Ministry of Science and Technology of China. References [1] T.A. Costi, V. Zlatic, Phys. Rev. Lett. 108 (2012) 036402. [2] A. Hochreiner, T. Schwarzl, M. Eibelhuber, W. Heiss, G. Springholz, V. Kolkovsky, G. Karczewski, T. Wojtowicz, Appl. Phys. Lett. 98 (2011) 021106. [3] Y. Zhang, X.Z. Ke, C.F. Chen, J. Yang, P.R.C. Kent, Phys. Rev. B 80 (2009) 024304. [4] E.I. Rogacheva, O.N. Nashchekina, S.N. Grigorov, M.A. Us, M.S. Dresselhaus, S.B. Cronin, Nanotechnology 14 (2003) 53. [5] E.I. Rogacheva, O.N. Nashchekina, A.V. Meriuts, S.G. Lyubchenko, M.S. Dresselhaus, G. Dresselhaus, Appl. Phys. Lett. 86 (2005) 063103. [6] X.F. Qiu, Y.B. Lou, A.C.S. Samia, A. Devadoss, J.D. Burgess, S. Dayal, C. Burda, Angew. Chem. Int. Ed. 44 (2005) 5855. [7] B. Zhang, J. He, T.M. Tritt, Appl. Phys. Lett. 88 (2006) 043119. [8] R. Buczko, Ł. Cywinski, Phys. Rev. B 85 (2012) 205319. [9] D. Khokhlov, Lead Chalcogenides: Physics and Applications, CRC Press, 2002, pp. 160. [10] J.X. Ma, Y. Jia, E.J. Liang, X.C. Wang, F. Wang, X. Hu, Acta Phys. Sin. 52 (2003) 3155. [11] J. Fuchs, Z. Feit, H. Preier, Appl. Phys. Lett. 53 (1988) 894. [12] K. Nakajima, K. Kimura, M. Mannami, Nucl. Instrum. Methods Phys. Res. B 135 (1998) 350. [13] V.L. Deringer, R. Dronskowski, J. Phys. Chem. C 117 (2013) 24455. [14] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [15] P.E. Blöchl, Phys. Rev. B 50 (1994) 17953. [16] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 1169. [17] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. [18] H.F. Wu, H.J. Zhang, Q. Liao, J.X. Si, H.Y. Li, S.N. Bao, H.Z. Wu, P. He, Surf. Sci. 604 (2010) 882. [19] G. Springholz, G. Bauer, Appl. Phys. Lett. 60 (1992) 1600. [20] W.H. Press, B.P. Flannery, S.A. Teukolsky, W.T. Vetterling, Numerical Recipes, Cambridge University Press, New York, 1992. [21] Y.P. Jiang, Y.Y. Sun, M. Chen, Y.L. Wang, Z. Li, C.L. Song, K. He, L.L. Wang, X. Chen, Q.K. Xue, X.C. Ma, S.B. Zhang, Phys. Rev. Lett. 108 (2012) 066809. [22] G. Wang, X.G. Zhu, Y.Y. Sun, Y.Y. Li, T. Zhang, J. Wen, X. Chen, K. He, L.L. Wang, X.C. Ma, J.F. Jia, S.B. Zhang, Q.K. Xue, Adv. Mater. 23 (2011) 2929. [23] H.F. Wu, H.J. Zhang, Y.H. Lu, T.N. Xu, J.X. Si, H.Y. Li, S.N. Bao, H.Z. Wu, P. He, J. Cryst. Growth 294 (2006) 179. [24] B.P. Zhang, C.F. Cai, L. Hu, X.D. Wei, H.Z. Wu, Appl. Surf. Sci. 257 (2011) 1986.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Reconstructions and stabilities of PbTe(1 1 1) crystal surface from experiments and density-functional theory Haifei Wu a,∗ , Jianxiao Si b , Yonghong Yan a , Qing Liao d , Yunhao Lu c a
Department of Physics, Shaoxing University, Shaoxing 312000, People’s Republic of China College of Mathematics, Physics and Information Engineering, Zhejiang Normal University, Jinhua 321004, People’s Republic of China Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China d Department of Physics, Hezhou University, Hezhou 542899, People’s Republic of China b c
a r t i c l e
i n f o
Article history: Received 27 June 2015 Received in revised form 13 August 2015 Accepted 15 August 2015 Available online 19 August 2015 Keywords: Surface reconstruction Reflection high energy electron diffraction First principal calculations
a b s t r a c t Surface properties of epitaxially grown PbTe(1 1 1) thin films on BaF2 (1 1 1) substrates were systematically investigated by the characterizations of in situ reflection high energy electron diffraction (RHEED) and atomic force microscope (AFM). First principal calculations were performed to facilitate the interpretation of experimental observations. Our results indicate that substrate temperature (Tsub ) and chemical environment are crucial in determining the type of surface structure during the growth of PbTe(1 1 1) thin films. When PbTe is grown at Tsub = 250 ◦ C, the metastable (1 × 1) structure is formed on the PbTe(1 1 1) surface. However, whether Tsub is elevated to 300 ◦ C or the PbTe(1 1 1)-(1 × 1) sample grown at Tsub = 250 ◦ C is post-annealed at 300 ◦ C, the stable (2 × 1) reconstructions appear on the PbTe(1 1 1) surface. In Pb-rich environment, the most stable (2 × 1) reconstruction results from the substitution of the half of the 2nd Te atoms by Pb atoms, while in Te-rich environment, the most stable (2 × 1) reconstruction originates from the missing of half of the 1st Pb atoms. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Narrow gap lead telluride (PbTe) has attracted tremendous attention for its applications in mid infrared optoelectronic devices and thermoelectric devices within the temperature ranges of 300–900 K [1–3]. Since the optoelectronic and thermoelectric performances of devices fabricated on dot-patterned PbTe compound underlying layer can be improved effectively by quantum confinement, massive interest has been devoted to the PbTe based quantum dots, superlattices, nanowires, and embedded nanocrystals in recent years [4–7]. The qualities of these low dimensional structures are generally dominated by the surface properties of PbTe film, such as the surface roughness and surface structures. Furthermore it has been reported that PbTe transforms from normal insulator to nontrivial topological insulator (TI) under surface strain [8]. Hence, understanding the nature of the PbTe film surfaces becomes closely relevant to the design and fabrication of excellent PbTe based wires, dots, and superlattices, and moreover to further promotion of PbTe based devices. Up to now, a few of studies have
∗ Corresponding author. E-mail address: [email protected] (H. Wu). http://dx.doi.org/10.1016/j.apsusc.2015.08.126 0169-4332/© 2015 Elsevier B.V. All rights reserved.
been focused on the surfaces of the rocksalt-type PbTe. For example, Springholz et al. and Ma et al. reported that the low indexed surfaces of the lead-salt compounds do not exhibit any surface reconstructions [9,10], whereas a 2 × 2 periodicity was observed on the (1 1 1) surface of PbTe in a Rutherford backscattering and RHEED experiments in 1998 [11]. Dissimilarly, Fuchs et al. reported a 2-fold reconstruction pattern in the [1 1 0] direction of the PbTe(1 1 1) surface by RHEED [12], inferring the surface reconstruction different from 2 × 2. Recently, Volker et al. reported that the 2 × 2 and 2 × 1 reconstructions are lower in surface energy compared to both Pband Te-terminated pristine PbTe(1 1 1) by first-principle simulations, and the 2 × 2 reconstruction could be reconciled with the previous Rutherford backscattering experiment [13]. Thus, it can be concluded that although various surface structures on PbTe(1 1 1) have been reported, the detailed surface formation mechanisms are still unclear. In this letter, we carry out systematic study on the surface properties of PbTe(1 1 1) thin films grown under different conditions by molecular beam epitaxy (MBE). The surface lattice period and morphology of PbTe were characterized by RHEED and AFM, respectively. In conjunction with ab initio calculation results, the influence of growth conditions on the evolution of the PbTe(1 1 1) surface was analyzed.
H. Wu et al. / Applied Surface Science 356 (2015) 742–746
2. Experimental and calculation details PbTe films were epitaxially grown on freshly cleaved BaF2 (1 1 1) substrates in a solid source molecular beam epitaxy (SSMBE) system with a base pressure better than 1.5 × 10−10 Torr. An extra Te beam flux was supplied during the epitaxial growth process of PbTe to adjust the stoichiometry of Pb and Te. The freshly cleaved BaF2 (1 1 1) substrate was preheated at about 200 ◦ C for 40 min in the load and fast-entry lock chamber (residual pressure P = 10−7 Torr) before it was transferred into the growth chamber. Before growth initiation, the substrate was further cleaned in the growth chamber by heating for 10 min at 550 ◦ C, the cleanliness and the orientation of the substrate were checked by RHEED. The variation of beam flux was controlled through adjustment of the effusion cell temperature. The beam flux rates and layer thickness was monitored using quartz-crystal thickness monitor calibrated by optical ellipsometry. To study the role of growth temperature played on the evolution of surface properties, samples were grown at different Tsub varying from 250 to 450 ◦ C, the growth temperature Tsub is thermocouple measured one at the back of sample holder. During the growth of these samples, a fixed vapor pressure ratio of Te to PbTe (Rf ) was used. Likewise, to study the evolution of surface properties with Pb-to-Te atomic ratio, samples were prepared with different Rf while keeping a same Tsub . The growth rate was about 1 m/h and the thicknesses of as-grown PbTe films were about 1 m for all samples. Morphological characterization was performed ex situ by AFM using NT-MDT AFM in semi-contact mode. The experimental results are qualitatively discussed in terms of DFT calculations. Structural optimization and surface energy calculations were performed within the spin-polarized DFT formalism using the generalized-gradient approximation (GGA) and the projector-augmented wave method as implemented in VASP [14–16]. We treated the outermost s and p electrons of Pb and Te as valence electrons and the rest as cores. The energy cutoff was set up to 400 eV and Monkhorst–Pack k-point sampling for the Brillouin zone with a Gaussian smearing of 0.1 eV was used to make sure the surface energy difference between consecutive cycles was within 10−4 eV [17]. Symmetrical seventeen atomic layers slab was introduced to model the Pb-terminated PbTe(1 1 1) surface, which is indicated by the experimental results [18,19]. A vacuum region of at least 20 A˚ was used to avoid interaction between slabs. All the atomic coordinates except for those in the central three layers of the slabs were relaxed using the conjugate-gradient algorithm with a tolerance factor of 0.01 eV/A˚ for the force minimization
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[20]. Spin–orbital coupling (SOC) was not included since it is found that the inclusion of SOC did not have significant influence on the structural properties. Calculated surface energy under various growth conditions: the Te-rich condition corresponds to the chemical potential of Te (Te ) set to that of a standalone Te atom since Te can be vaporized as a single atom in experiment, whereas the Pb-rich condition corresponds to Pb set to that of bulk Pb, with the constraint Te + Pb equal to the total energy per formula bulk PbTe. The surface energy is defined as: Esurf = (Etotal − nTe − mPb )/2A, where n and m are the number of Te and Pb atom included in the slab, and A is the surface area [21,22]. 3. Results and discussions Fig. 1 shows RHEED patterns along [0 −1 1]-direction of growing PbTe films (1.0 m) on BaF2 (1 1 1) at different substrate temperatures with Rf to be 0.3. From Fig. 1(b)–(e), PbTe films were grown at ascending substrate temperature series, viz. 250, 300, 400 and 450 ◦ C, respectively. A typical RHEED pattern along [0 1 −1]direction for a clean BaF2 (1 1 1)-(1 × 1) substrate is shown in Fig. 1(a) for comparison. When rotating the BaF2 (1 1 1)-(1 × 1) and PbTe sample grown at Tsub = 250 ◦ C about their normal axises respectively, a 6-fold rotational symmetry is observed along the <1 0 −1> and <1 1 −2> azimuths (not shown here) for both of them, and the spacing between the streaky lines of their RHEED patterns are almost the same (see Fig. 1(a) and (b)). Considering the small lattice mismatch between BaF2 and PbTe (+4.2%), it can be speculated that a smooth single crystal PbTe film without surface reconstruction is obtained at Tsub = 250 ◦ C and Rf = 0.3. Compared to Fig. 1(b), Fig. 1(c)–(e) are characterized by the appearance of additional streaky lines in the middle of original diffraction streaks in the RHEED patterns along the [0 −1 1] direction of PbTe films as marked by arrows in Fig. 1. Moreover, it is worthy to note that for any epitaxial PbTe films grown at Tsub ≥ 300 ◦ C, the additional diffraction streaks can only be observed along [0 −1 1] and [0 1 −1] directions with an angle difference of 180◦ when samples are rotated about their normal axis, implying a 2-fold rotational symmetry along the <0 −1 1> azimuths. Meanwhile, none additional streaks are observed along the <1 1 −2> azimuths. These features of RHEED patterns from Fig. 1(c)–(e) directly indicate that a (2 × 1) surface reconstruction is formed once PbTe films are grown at Tsub ≥ 300 ◦ C and Rf = 0.3. Actually, in our previous investigation, the (2 × 1) surface reconstruction was also detected in PbSe film grown on BaF2 (1 1 1) at Tsub = 450 ◦ C by scanning tunneling microscopy (STM) [23].
Fig. 1. RHEED patterns along [0 −1 1]-direction of growing PbTe films (1.0 m) on BaF2 (1 1 1) with Rf = 0.3 at Tsub of: (b) 250 ◦ C; (c) 300 ◦ C; (d) 400 ◦ C and (e) 450 ◦ C. A typical RHEED pattern along [0 1 −1]-direction for a clean BaF2 (1 1 1)-(1 × 1) substrate is shown in (a).
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Fig. 2. The AFM images of growing PbTe films (1.0 m) on BaF2 (1 1 1) with Rf = 0.3 at Tsub of: (a) 250 ◦ C; (b) 300 ◦ C; (c) 400 ◦ C and (d) 450 ◦ C.
In order to reveal the surface structure formation mechanism of PbTe epitaxy, we further performed AFM measurements on these four as-grown PbTe samples, and the surface morphology evolution with ascending substrate temperature is presented in Fig. 2. At Tsub = 250 ◦ C, the surface structure is dominated by typical growth spirals formed around threading dislocations that originate from the growth on the 4.2% lattice-mismatched substrate [23]. At Tsub ≥ 300 ◦ C, besides spirals structures, triangular pits are observed and randomly distributed on the surface. With increasing Tsub , the density of triangular pits increase. According to the previous investigations by Wu et al. and Zhang et al. [23,24], the formation of pure spirals without any other large-sized defects in the growth of PbTe needs appropriate Tsub and an atomic ratio of Pb/Te ≈ 1 atmosphere. However, when an atomic ratio of Pb/Te is increased to make the films slightly Pb-rich, triangular pits will appear. On the other hand, when an atomic ratio of Pb/Te is decreased to make the films slightly Te-rich, regular shape defects will be formed on the film surface. Consequently, one can expect that at Tsub = 250 ◦ C, the reevaporated Te atoms from the PbTe epitaxial layers were just compensated for by the supply of additional Te flux from a separate effusion cell with Rf = 0.3, resulting in the growth of PbTe films with a balanced stoichiometry and pure spirals on the surface (see Fig. 2(a)). However, when Tsub is elevated up to 300 ◦ C while keeping Rf = 0.3, much more Te atoms will reevaporate from the PbTe epitaxial layers, which eventually lead to a Pb enrichment during the growth process. The higher the growth temperature is, the more serious the Te reevaporation will become. That is why one can see the density of triangular pits, originating from excessive Pb atoms congregating in the dislocation core area and relatively slow growth rate of (1 0 0) facets [23], increases with growth temperature from 300 ◦ C to 450 ◦ C in Fig. 2(b)–(d). As is shown in the RHEED results above, PbTe film grown at Tsub = 250 ◦ C with Rf = 0.3 does not exhibit any surface
reconstructions, while PbTe films grown at Tsub ≥ 300 ◦ C with identical Rf exhibit (2 × 1) surface reconstruction. It seems that the formation of PbTe(1 1 1) surface reconstruction is associated with higher Tsub (Tsub ≥ 300 ◦ C) and Pb-rich growth atmosphere. To determine which factor plays a key role on the appearance of the (2 × 1) surface reconstruction, another two samples were prepared. The first sample was grown at Tsub = 250 ◦ C without additional Te flux to create a Pb-rich ambient, while the other was grown at Tsub = 300 ◦ C with Rf = 0.5 to create an atomic ratio of Pb/Te ≈ 1 ambient. The AFM and RHEED measurement results of these two samples are shown in Fig. 3. It can be seen from Fig. 3 that at Tsub = 250 ◦ C, although PbTe film was grown with Pb-rich atmosphere, confirmed by the appearance of triangular pits in the AFM image in Fig. 3(a), the additional streaky lines still does not appear in the diffraction pattern. While at Tsub = 300 ◦ C, although PbTe film was grown in stoichiometric atmosphere, confirmed by the appearance of pure spirals without triangular pits or any other defects in the AFM image in Fig. 3(b), the (2 × 1) surface reconstruction can be easily detected by RHEED. Therefore, it can be deduced that the appearance of the (2 × 1) surface reconstruction is not relevant to whether PbTe film was grown with Pb-rich chemical atmosphere or not. Subsequently, we annealed the sample grown at Ts = 250 ◦ C with Rf = 0.0 at a temperature of 300 ◦ C for 10 min. Interestingly, the 2-fold reconstruction streaks in the [0 −1 1] direction appeared in the diffraction pattern. Further, the 2-fold reconstruction patterns still exist on the surface of the sample (not shown here) annealed for 10 min at Tsub = 400 ◦ C and even 450 ◦ C, respectively. Consequently, we can draw the conclusion that the formation of the (2 × 1) surface reconstruction on PbTe(1 1 1) film surface depends directly on Tsub . As long as PbTe(1 1 1) films are grown at Tsub ≥ 300 ◦ C or just postannealed up to 300 ◦ C, the (2 × 1) reconstruction will form on the PbTe(1 1 1) surface and become very stable. No matter the film is
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Fig. 3. The AFM and RHEED images of PbTe films (1.0 m) under growth conditions of: (a) Tsub = 250 ◦ C and Rf = 0.0; (b) Tsub = 300 ◦ C and Rf = 0.5.
further annealed up to 450 ◦ C or cooled down to room temperature, the reconstruction is still present on the surface. To verify the surface structure of PbTe(1 1 1) from the experimental RHEED patterns and inspect the details of the surface mechanism, we further carried out a series of theoretical
calculations using DFT. Since it is indicated in the experiment that the stable (1 1 1) surface of PbTe grown at elevated Tsub is Pb terminated [18,19], we only focused on the Pb-terminated PbTe(1 1 1) surface. Based on the experimental results above, possible surface structures of PbTe(1 1 1) have been sketched in Fig. 4(a). The
Fig. 4. (a) Four possible surface structures of PbTe(1 1 1): (1 × 1) represents the pristine Pb-terminated PbTe(1 1 1)-(1 × 1) structure, (2 × 1)−1 represents the (2 × 1) reconstruction where half of the 2nd Te atoms are substituted by Pb atoms, (2 × 1)−2 represents the (2 × 1) reconstruction where half of the 1st Pb atoms are missing, in addition, (2 × 1)−3 represents the (2 × 1) reconstruction where half of the 1st Pb atoms are missing and half of the 2nd Te atoms are substituted by Pb atoms. (b) Calculated surface energy under various growth conditions: the Te-rich condition corresponds to the chemical potential of Te (Te ) set to that of a standalone Te atom since Te can be vaporized as a single atom in experiment, whereas the Pb-rich condition corresponds to Pb set to that of bulk Pb, with the constraint Te + Pb equal to the total energy per formula bulk PbTe. The surface energy is defined as: Esurf = (Etotal − nTe − mPb )/2A, where n and m are the number of Te and Pb atom included in the slab, and A is the surface area.
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pristine PbTe(1 1 1)-(1 × 1) surface is an alternative Pb- and Telayer structure, and here the 1st layer means the outmost Pb layer of PbTe(1 1 1) surface while the 2nd layer means the sublayer Te. In Fig. 4(a), (1 × 1) represents the pristine Pb-terminated PbTe(1 1 1)-(1 × 1) structure, (2 × 1)−1 represents the (2 × 1) reconstruction where half of the 2nd Te layer atoms are substituted by Pb atoms, (2 × 1)−2 represents the (2 × 1) reconstruction where half of the 1st Pb atoms are missing, and (2 × 1)−3 represents the (2 × 1) reconstruction where half of the 1st Pb atoms are missing and half of the 2nd Te atoms are substituted by Pb atoms. As the chemical environment is crucial in determining the type of surface structure in a crystal film growth, to explore the feasibility of surface structure engineering, we calculated the surface energies of the relevant surface structures in Fig. 4(a) and showed their dependence on the various growth conditions in Fig. 4(b). Apparently, the surface energies of the entire structures increase with the chemical environment from Pb enrichment to Te enrichment except for (2 × 1)−2 , and PbTe(1 1 1)-(1 × 1) structure is thermodynamically unfavorable as it always has a higher surface energy than the (2 × 1) motifs. It is the metastable structure under all growth conditions, which is in agreement with the above experimental results. Further inspection of Fig. 4(b) quickly reveals that in the Pb-rich environment, (2 × 1)−1 is the most stable structure, while in the Te-rich environment, (2 × 1)−2 becomes the most stable structure. It is noteworthy that in the above experiments, the metastable PbTe(1 1 1)-(1 × 1) structure has indeed been produced at Tsub = 250 ◦ C. This is because molecular beam epitaxy growth is a dynamic process, rather than a thermal equilibrium process and metastable structure could exist due to thermal fluctuation. To obtain stable structure, thermal energy is needed to overcome the energy barrier between metastable and stable structures. That is why the (2 × 1) reconstruction only occurred at higher growth temperature (Tsub ≥ 300 ◦ C) or PbTe(1 1 1)-(1 × 1) films post-annealed up to 300 ◦ C or higher temperatures. 4. Conclusions In summary, a series of PbTe film samples were grown on BaF2 (1 1 1) by molecular beam epitaxy. The surface properties of PbTe(1 1 1) were investigated by RHEED and AFM. DFT calculations were carried out to facilitate the interpretation of experimental observations. The experimental results generally suggest that the formation of the (2 × 1) surface reconstruction in PbTe(1 1 1) films depends directly on the substrate temperature. As long as PbTe(1 1 1) films are grown at Tsub ≥ 300 ◦ C or just post-annealed up to 300 ◦ C, the (2 × 1) reconstruction will form on the PbTe(1 1 1) surface and become very stable. No matter the film is further annealed up to 450 ◦ C or cooled down to room temperature, the reconstruction is still present on the surface. The DFT calculations show that
PbTe(1 1 1)-(1 × 1) structure is metastable. Once sufficient energy is provided in the Pb-rich environment, the substitution of the half of the 2nd Te atoms by Pb atoms will result in the most stable (2 × 1) reconstruction on the surface, while in the Te-rich environment, the missing of half of the 1st Pb atoms leads to another stable (2 × 1) surface reconstruction. Therefore, in principle, the surface structures of PbTe(1 1 1) can be manipulated by careful control of the experimental conditions. In this regard, it is helpful for the fabrication of quantum dots, superlattices, nanowires, and embedded nanocrystals based on PbTe. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant nos. 51202149, 11374009, 51302248, 11204180, and 11464051), the Zhejiang Provincial Natural Science Foundation of China (Grant no. LQ12F04001), and the Ministry of Science and Technology of China. References [1] T.A. Costi, V. Zlatic, Phys. Rev. Lett. 108 (2012) 036402. [2] A. Hochreiner, T. Schwarzl, M. Eibelhuber, W. Heiss, G. Springholz, V. Kolkovsky, G. Karczewski, T. Wojtowicz, Appl. Phys. Lett. 98 (2011) 021106. [3] Y. Zhang, X.Z. Ke, C.F. Chen, J. Yang, P.R.C. Kent, Phys. Rev. B 80 (2009) 024304. [4] E.I. Rogacheva, O.N. Nashchekina, S.N. Grigorov, M.A. Us, M.S. Dresselhaus, S.B. Cronin, Nanotechnology 14 (2003) 53. [5] E.I. Rogacheva, O.N. Nashchekina, A.V. Meriuts, S.G. Lyubchenko, M.S. Dresselhaus, G. Dresselhaus, Appl. Phys. Lett. 86 (2005) 063103. [6] X.F. Qiu, Y.B. Lou, A.C.S. Samia, A. Devadoss, J.D. Burgess, S. Dayal, C. Burda, Angew. Chem. Int. Ed. 44 (2005) 5855. [7] B. Zhang, J. He, T.M. Tritt, Appl. Phys. Lett. 88 (2006) 043119. [8] R. Buczko, Ł. Cywinski, Phys. Rev. B 85 (2012) 205319. [9] D. Khokhlov, Lead Chalcogenides: Physics and Applications, CRC Press, 2002, pp. 160. [10] J.X. Ma, Y. Jia, E.J. Liang, X.C. Wang, F. Wang, X. Hu, Acta Phys. Sin. 52 (2003) 3155. [11] J. Fuchs, Z. Feit, H. Preier, Appl. Phys. Lett. 53 (1988) 894. [12] K. Nakajima, K. Kimura, M. Mannami, Nucl. Instrum. Methods Phys. Res. B 135 (1998) 350. [13] V.L. Deringer, R. Dronskowski, J. Phys. Chem. C 117 (2013) 24455. [14] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [15] P.E. Blöchl, Phys. Rev. B 50 (1994) 17953. [16] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 1169. [17] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. [18] H.F. Wu, H.J. Zhang, Q. Liao, J.X. Si, H.Y. Li, S.N. Bao, H.Z. Wu, P. He, Surf. Sci. 604 (2010) 882. [19] G. Springholz, G. Bauer, Appl. Phys. Lett. 60 (1992) 1600. [20] W.H. Press, B.P. Flannery, S.A. Teukolsky, W.T. Vetterling, Numerical Recipes, Cambridge University Press, New York, 1992. [21] Y.P. Jiang, Y.Y. Sun, M. Chen, Y.L. Wang, Z. Li, C.L. Song, K. He, L.L. Wang, X. Chen, Q.K. Xue, X.C. Ma, S.B. Zhang, Phys. Rev. Lett. 108 (2012) 066809. [22] G. Wang, X.G. Zhu, Y.Y. Sun, Y.Y. Li, T. Zhang, J. Wen, X. Chen, K. He, L.L. Wang, X.C. Ma, J.F. Jia, S.B. Zhang, Q.K. Xue, Adv. Mater. 23 (2011) 2929. [23] H.F. Wu, H.J. Zhang, Y.H. Lu, T.N. Xu, J.X. Si, H.Y. Li, S.N. Bao, H.Z. Wu, P. He, J. Cryst. Growth 294 (2006) 179. [24] B.P. Zhang, C.F. Cai, L. Hu, X.D. Wei, H.Z. Wu, Appl. Surf. Sci. 257 (2011) 1986.