SnTe Film Research Articles

Growth Conditions and Interfacial Misfit Array in SnTe (111) Films Grown on InP (111)A Substrates by Molecular Beam Epitaxy.

Tin telluride (SnTe) is an IV-VI semiconductor with a topological crystalline insulator band structure, high thermoelectric performance, and in-plane ferroelectricity. Despite its many applications, there has been little work focused on understanding the growth mechanisms of SnTe thin films. In this manuscript, we investigate the molecular beam epitaxy synthesis of SnTe (111) thin films on InP (111)A substrates. We explore the effect of substrate temperature, Te/Sn flux ratio, and growth rate on the film quality. Using a substrate temperature of 340 °C, a Te/Sn flux ratio of 3, and a growth rate of 0.48 Å/s, fully coalesced and single crystalline SnTe (111) epitaxial layers with X-ray rocking curve full-width-at-half-maxima of 0.09° and root-mean-square surface roughness as low as 0.2 nm have been obtained. Despite the 7.5% lattice mismatch between the SnTe (111) film and the InP (111)A substrate, reciprocal space mapping indicates that the 15 nm SnTe layer is fully relaxed. We show that a periodic interfacial misfit (IMF) dislocation array forms at the SnTe/InP heterointerface, where each IMF dislocation is separated by 14 InP lattice sites/13 SnTe lattice sites, providing rapid strain relaxation and yielding the high quality SnTe layer. This is the first report of an IMF array forming in a rock-salt on zinc-blende material system and at an IV-VI on III-V heterointerface, and highlights the potential for SnTe as a buffer layer for epitaxial telluride film growth. This work represents an important milestone in enabling the heterointegration between IV-VI and III-V semiconductors to create multifunctional devices.

Detection of Dirac fermions in capped SnTe film via magnetotransport measurements

In this work, we present the investigation of the magnetotransport properties of a capped SnTe film, grown by molecular beam epitaxy, using Shubnikov–de Haas oscillations for the detection of Dirac fermions. The cap layer used was a 10 nm thick Sn0.98Eu0.02Te film, which can also contribute to the transport such that it is mandatory to isolate its contribution from the electrical transport measured in the sample. To separate the contribution from both layers, photoconductivity measurements were performed. A detailed analysis of the Shubnikov–de Haas oscillations is carried out using theoretical expressions and building the Landau-level indexation. We found that Dirac fermions are detected in the SnTe layer, while the cap layer contributes with trivial fermions, protecting SnTe against deterioration due to exposure to the atmosphere.

Transport property of topological crystalline insulator SnTe (100) and ferrimagnetic insulator heterostructures

• High quality SnTe/RE3Fe5O12 (RE=Eu, Y) epitaxial heterostructures were prepared. • Domain disorder is induced by Fe atom diffusion in SnTe through magnetic interface. • Two different magnetoelectrical transport in two heterostructures are observed. • Linear magnetoresistance and nonlinear Hall appear in SnTe/Eu 3 Fe 5 O 12 . Topological crystalline insulator (TCI) SnTe is a potential material for quantum electronic devices because of its attractive inherent sensitivity of band topology and highly mobile characteristic of Dirac fermions. The proximity effect at the interface of SnTe film can affect the topological surface transport and may result in novel quantum magneto-electric effects. Here, we study the magnetoelectrical transport properties of SnTe thin films grown on ferrimagnetic insulators Eu 3 Fe 5 O 12 (110) (EuIG (110)) and Y 3 Fe 5 O 12 (111) (YIG (111)) single-crystal underlayers by molecular beam epitaxy. Linear magnetic resistance (LMR) is observed in SnTe/EuIG heterostructures in the low field range, which is different from the weak antilocalization (WAL) characteristic of SnTe/YIG heterostructures. Especially, the double carrier characteristic with the coexistence of holes and electrons in SnTe/EuIG heterostructure is quite different from the holes as main carriers in SnTe/YIG, although the SnTe layer remains the same crystal plane (100) in the two heterostructures. The LMR in SnTe/EuIG is attributed to the topological surface Dirac electrons and disordered domain distribution in the SnTe layer which is in sharp contrast to the WAL of SnTe/YIG with ordered domain distribution in the SnTe layer. The present studies of transport properties not only provide a fundamental understanding of the transport mechanism of TCI and magnetite insulator heterostructure but also display the promising application probability for tunable topological electronic devices.

Native Atomic Defects Manipulation for Enhancing the Electronic Transport Properties of Epitaxial SnTe Films.

P-type SnTe-based compounds have attracted extensive attention because of their high thermoelectric performance. Previous studies have made tremendous efforts to investigate native atomic defects in SnTe-based compounds, but there has been no direct experimental evidence so far. On the basis of MBE, STM, ARPES, DFT calculations, and transport measurements, this work directly visualizes the dominant native atomic defects and clarifies an alternative optimization mechanism of electronic transport properties via defect engineering in epitaxially grown SnTe (111) films. Our findings prove that positively charged Sn vacancies (VSn) and negatively charged Sn interstitials (Sni) are the leading native atomic defects that dominate electronic transport in SnTe, in contrast to previous studies that only considered VSn. Increasing the substrate temperature (Tsub) and decreasing the Te/Sn flux ratio during film growth reduces the density of VSn while increasing the density of Sni. A high Tsub results in a low hole density and high carrier mobility in SnTe films. The SnTe film grown at Tsub = 593 K and Te/Sn = 2/1 achieves its highest power factor of 1.73 mW m-1 K-2 at 673 K, which is attributed to the optimized hole density of 2.27 × 1020 cm-3 and the increased carrier mobility of 85.6 cm2 V-1 s-1. Our experimental studies on the manipulation of native atomic defects can contribute to an increased understanding of the electronic transport properties of SnTe-based compounds.

Photothermoelectric SnTe Photodetector with Broad Spectral Response and High On/Off Ratio.

A broadband photodetector with high performance is highly desirable for the optoelectric and sensing application. Herein, we report a "photo-thermo-electric" (PTE) detector based on an ultrathin SnTe film. The (001)-oriented SnTe films with the wafer size scale are epitaxially grown on the surface of sodium chloride crystals by a scalable sputtering method. Due to the giant PTE effect under laser spot excitation on the asymmetric position between two terminals, a built-in electrical field is produced to drive bulk carriers for a self-powered photodetector, leading to a broad spectral response in the wavelength range from 404 nm to 10.6 μm far beyond the limitation of the energy band gap. Significantly, the photodetector displays a high on/off photoswitching ratio of over 105 with a suppressed dark current, which is 4-5 orders of magnitude higher than that of other reported SnTe-based detectors. Under zero external bias, the device yields the highest detectivity of ∼1.3 × 1010 cm Hz1/2 W-1 with a corresponding responsivity of ∼3.9 mA W-1 and short rising/falling times of ∼78/84 ms. Furthermore, the photodetector transferred onto the flexible template exhibits excellent mechanical flexibility over 300 bending cycles. These findings offer feasible strategies toward designing and developing low-power-consumption wearable optoelectronics with competitive performance.

Structure and topological transport in Pb-doping topological crystalline insulator SnTe (001) film

Topological crystalline insulator (TCI) as a new type of topological materials has attracted extensive research interests for its tunable topological properties. Due its symmetry topological protection essence, the structure investigation provides a solid basement for tuning its topological transport properties. On SrTiO3 (111) substrate, the SnTe film was found to be epitaxial growth only along [001] while not [111] direction. The detailed structural study was performed and a structural model was proposed to elucidate epitaxial growth of the SnTe (001) film. The transport properties of SnTe (001) film were further investigated and a typical weak anti-localization effect was observed. By Pb-doping into SnTe, the bulk carriers were inhibited and its topological surface states were strengthened to induce the enhanced surface transport contribution. With tunable multiple transport channels from the even Dirac cones, the TCI SnTe film systems will have the potential application in future spintronics devices.

Giant Linear Magnetoresistance and Carrier Density Tunable Transport in Topological Crystalline Insulator SnTe Thin Film

Carrier density control is of great importance to modulate the topological phase and topological transport for the topological crystalline insulators (TCIs). Here, the transport property modulation of TCI SnTe thin films grown on SrTiO3 (111) by tuning the carrier density is reported. The low temperature magneto‐transport in a typical SnTe film with high hole carrier density in the as grown films exhibits a giant linear magnetoresistance (GLMR) effect (up to 1849% at 2 K under 14 T) and then the magnetoresistance becomes much weaker and weak anti‐localization (WAL) appears in the same SnTe film by n‐type doping in vacuum after aging 30 days. Alternatively, the hole carrier density of the as grown SnTe films is lowered by enhancing the growth temperature to promote Sn diffusion to reduce Sn vacancies and the as grown samples exhibit a WAL with two‐dimensional (2D) characteristics. This work provides promising application for magnetoelectronic sensors and spintronics based on TCI SnTe thin film.

Non-monotonic thickness dependence of Curie temperature and ferroelectricity in two-dimensional SnTe film

Recently, the observation of atomic thin film SnTe with a Curie temperature (Tc) higher than that of the bulk [Chang et al., Science 353, 274 (2016)] has boosted the research on two-dimensional (2D) ferroic materials tremendously. However, the origin of such a phenomenon is yet to be thoroughly investigated, which hinders the understanding and design of materials with ferroic orders at the 2D limit. By using the density functional theory, we investigated the structural and ferroelectrical properties of 2D SnTe to reveal the thickness dependence. The calculated results demonstrate that 2D SnTe automatically transforms into a periodical bilayer structure, resulting from the surface effect. Moreover, based on the double-well potential and atomic distortion analysis, we found that the Tc of 2D SnTe is higher than that of the bulk counterpart, and more surprisingly, Tc exhibits a non-monotonous dependence of thickness, featuring a pronounced atomic distortion and Curie temperature maximum at 8 atomic-layers (4 unit cells). In addition, this non-monotonous dependence is sensitive to the external strain and it can be easily tuned by the external compressive strain.

Enhanced thermoelectric performance of SnTe film with optimized carrier transport induced by facile post-annealing process

Annealing treatment plays a vital role in tuning the microstructure and improving the performance of the thermoelectric film. In this work, SnTe thin films were fabricated by thermal evaporation technique and then annealed at a series of temperatures. The effects of annealing temperature on the element composite, surface morphology, crystal structure and thermoelectric properties were investigated. The electrical conductivity of the annealed films was increased while the Seebeck coefficient was declined slightly, due to the largely enhanced carrier concentrations and slightly reduced carrier mobility. In specific, the maximum Seebeck coefficient of 155 μV K−1 and power factor of 29 μW cm−1 K−2 were achieved at the annealing temperature of 400 °C. Our results may provide an effective strategy to improve the properties of thermoelectric films.

Chromium-induced ferromagnetism with perpendicular anisotropy in topological crystalline insulator SnTe (111) thin films

Topological crystalline insulator (TCI) is a recently-discovered topological phase of matter. It possesses multiple Dirac surface states, which are protected by the crystal symmetry. This is in contrast to the time reversal symmetry that is operative in the well-known topological insulators. In the presence of a Zeeman field and/or strain, the multiple Dirac surface states are gapped. The high-Chern-number quantum anomalous Hall (QAH) state is predicted to emerge if the chemical potential resides in all the Zeeman gaps. Here, we use molecular beam epitaxy to grow 12 double layer (DL) pure and Cr-doped SnTe (111) thin film on heat-treated SrTiO3 (111) substrate using a quintuple layer of insulating (Bi0.2Sb0.8)2Te3 topological insulator as a buffer film. The Hall traces of Cr-doped SnTe film at low temperatures display square hysteresis loops indicating long-range ferromagnetic order with perpendicular anisotropy. The Curie temperature of the 12DL Sn0.9Cr0.1Te film is ~ 110 K. Due to the chemical potential crossing the bulk valence bands, the anomalous Hall resistance of 12DL Sn0.9Cr0.1Te film is substantially lower than the predicted quantized value (~1/4 h/e2). It is possible that with systematic tuning the chemical potential via chemical doping and electrical gating, the high-Chern-number QAH state can be realized in the Cr-doped SnTe (111) thin film.

Enhanced thermoelectric performance of SnTe thin film through designing oriented nanopillar structure

SnTe, an ideal lead-free substitute of PbTe, recently has attracted much attention for thermoelectric heat to electricity generation. Previous reports mainly focused on band engineering and all-scale phonon scattering for enhancing the thermoelectric performance of bulk SnTe. Herein, we adopt strategy of interface engineering to design oriented nanopillar structure in SnTe film via one-step thermal evaporation method. The analysis of the electrical and thermal properties illustrates that the SnTe film with highly oriented growth shows enhanced thermoelectric performance. The excellent properties are mainly attributed to the special nanopillar structure, which may facilitate the electrons' transport while significantly scatter the phonons. The maximum power factor of 19.8 μW cm−1 K−2 and the thermal conductivity of 3.54 W m−1 K−1 are obtained. This work provides a promising approach to enhance the thermoelectric performance of SnTe film, which can be easily extended to other thermoelectric films.

Topological Crystalline Insulator SnTe/Si Vertical Heterostructure Photodetectors for High-Performance Near-Infrared Detection.

Due to the gapless surface state and narrow bulk band gap, the light absorption of topological crystalline insulators covers a broad spectrum ranging from terahertz to infrared, revealing promising applications in new generation optoelectronic devices. To date, the photodetectors based on topological insulators generally suffer from a large dark current and a weaker photocurrent especially under the near-infrared lights, which severely limits the practical application of devices. Owing to the lower excitation energy of infrared lights, the photodetection application of topological crystalline insulators in the near-infrared region relies critically on understanding the preparation and properties of their heterostructures. Herein, we fabricate the high-quality topological crystalline insulator SnTe film/Si vertical heterostructure by a simple physical vapor deposition process. The resultant heterostructure exhibits an excellent diode characteristic, enabling the construction of high-performance near-infrared photodetectors. The built-in electric field at SnTe/Si interface enhances the absorption efficiency of near-infrared lights and greatly facilitates the separation of photogenerated carriers, making the device capable of operating as a self-driven photodetector. The as-grown SnTe film acts as the hole transport layer in heterostructure photodetectors, promoting the transport of holes to electrode and reducing electron-hole recombination effectively. These merits enable the SnTe/Si heterostructure photodetector to have a high responsivity of 2.36 AW-1, a high detectivity of 1.54 × 1014 Jones, and a large bandwidth of 104 Hz in the near-infrared wavelength, which makes the detector have a promising market in novel device applications.

ARPES study of the epitaxially grown topological crystalline insulator SnTe(111)

SnTe is a prototypical topological crystalline insulator, in which the gapless surface state is protected by a crystal symmetry. The hallmark of the topological properties in SnTe is the Dirac cones projected to the surfaces with mirror symmetry, stemming from the band inversion near the L points of its bulk Brillouin zone, which can be measured by angle-resolved photoemission. We have obtained the (111) surface of SnTe film by molecular beam epitaxy on BaF2(111) substrate. Photon-energy-dependence of in situ angle-resolved photoemission, covering multiple Brillouin zones in the direction perpendicular to the (111) surface, demonstrate the projected Dirac cones at the Γ¯ and M¯ points of the surface Brillouin zone. In addition, we observe a Dirac-cone-like band structure at the Γ point of the bulk Brillouin zone, whose Dirac energy is largely different from those at the Γ¯ and M¯ points.

Topologically nontrivial narrow bands in ultrathin SnTe films with defect superstructures

It is shown that a two-dimensional topological insulator can be realized and the band topology (equivalently, the edge states) may be further controlled by charge doping in an ultrathin SnTe film with a defect superstructure. Based on first-principles density functional theory (DFT), we predict that a Sn-Te bilayer, if exfoliated from three-dimensional bulk SnTe in the (1 1 1) direction, has a trivial band topology in its pristine form, but is made topologically nontrivial by introducing an appropriate array of defects. The emergence of the topological state is ascribed to the formation of topologically nontrivial narrow bands near the Fermi level by spin–orbit splitting of defect-induced bands. In addition, we demonstrate that a transition between a topological insulator and a normal insulator is possible under the electron or hole doping which can be useful for controlling the topological edge states.

Topological surface transport in epitaxial SnTe thin films grown on Bi2Te3

The topological crystalline insulator SnTe has been grown epitaxially on a Bi2Te3 buffer layer by molecular beam epitaxy. In a 30-nm-thick SnTe film, p- and n-type carriers are found to coexist, and Shubnikov--de Haas oscillation data suggest that the n-type carriers are Dirac fermions residing on the SnTe (111) surface. This transport observation of the topological surface state in a p-type topological crystalline insulator became possible due to a downward band bending on the free SnTe surface, which appears to be of intrinsic origin.

Growth of topological crystalline insulator SnTe thin films on Si(111) substrate by molecular beam epitaxy

We present the growth of atomically flat topological crystalline insulator (TCI) SnTe films on Si(111) substrate by molecular beam epitaxy (MBE). The growth condition for achieving high quality SnTe film was established by a combination of reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) studies. In-situ angle resolved photoemission spectroscopy (ARPES) measurements elucidate the topological nature of the SnTe films. The electronic structure of SnTe films can be tuned by film thickness and Pb doping. The success in growing high quality SnTe thin films with tunable electronic structure is crucial for potential device applications.

Facile synthesis and characterization of SnTe films

In this paper, we reported a novel, simple, and cost-effective route to SnTe films. The films were prepared by a chemical bath method, at room temperature and ambient pressure, using conventional chemicals as starting materials with or without surfactant. The films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and field-emission scanning electron microscopy, respectively. The SnTe film deposited without surfactant consists of nanoparticles (∼100nm). The film deposited using polyethyleneglycol (PEG) as the surfactant consists of nanoparticles with size of ∼25nm, whereas the film deposited using polyvinylpyrrolidone (PVP) as the surfactant consists of rough rod-like nanostructures (∼50 in diameter and ∼500nm in length), besides nanoparticles (∼40–180nm). The SnTe film deposited with PEG is smoother and denser. The formation mechanism of the SnTe films was proposed.

Oscillations in the thickness dependences of the room-temperature Seebeck coefficient in SnTe thin films

The dependences of the Seebeck coefficient S on the thickness ( d = 5–110 nm) of SnTe thin films grown by thermal evaporation in vacuum on (001)KCl substrates were obtained at room temperature. Distinct oscillations in the S( d)-dependences were observed and attributed to the size quantization effect in SnTe thin films. The monotonic component of the d-dependences of S decreases with increasing thickness. In this connection it is suggested that the equilibrium concentration of non-stoichiometric cation vacancies depends on the SnTe film thickness.

Current-voltage spectra of metal/oxide/SnTe diodes II: Analysis

The second-derivative current-voltage spectra of metal/oxide/SnTe diodes, recorded by Stoeckel and Finkenrath, are analysed. The spectra show quasi-periodic structures. The period depends on the SnTe film thickness in a way which indicates size quantization. The spectra are classified as regular, irregular and ( U c− U) -1; the last term refers to the line shape and may indicate a saddle point in a two-dimensional subband. The irregular spectra may be superpositions (beats) of regular spectra, coming from separate patches of film with different thickness. However, not all the features are consistent with a one-particle size quantization model, nor indeed with any other of several models which we discuss.

Temperature Dependence of the Optical Absorption Edge ofp-Type SnTe

The optical-absorption coefficient and index of refraction of SnTe were measured at 300, 77, and 12\ifmmode^\circ\else\textdegree\fi{}K over the photon energy range between 0.14 and 0.64 eV. The specimen was a single-crystal SnTe film with a hole concentration of 3.6 \ifmmode\times\else\texttimes\fi{} ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The threshold of absorption by bound carriers at 12\ifmmode^\circ\else\textdegree\fi{}K occurs at a photon energy less than 0.38 eV. This suggests that the smallest direct gap is considerably less than the gap of 0.30 eV which Esaki and Stiles have deduced from tunneling measurements. In contrast to the temperature dependence of the tunneling gap, the temperature dependence of the absorption edge suggests that the temperature coefficients of the direct gaps that lie in the photon energy range investigated are positive. However, these coefficients decrease rapidly with decreasing photon energy, suggesting that direct gaps between states sufficiently close to the band edge will have negative temperature coefficients. The electric-susceptibility effective mass was also determined. Its variation from $0.066{m}_{0}$ at 300\ifmmode^\circ\else\textdegree\fi{}K to $0.057{m}_{0}$ at 12\ifmmode^\circ\else\textdegree\fi{}K is consistent with the observation of direct gaps having positive temperature coefficients. We propose new features for the band structure of SnTe which account for these results, as well as previously reported results for the PbTe-SnTe alloy system.