SnSe Films Research Articles

Tin(iv) chalcogenoether complexes as single source precursors for the chemical vapour deposition of SnE2 and SnE (E = S, Se) thin films.

The molecular Sn(iv) complexes, [SnCl4{nBuS(CH2)3SnBu}] (2), [SnCl4(nBu2S)2] (3) and [SnCl4(nBu2Se)2] (4) have been prepared in good yield from reaction of SnCl4 with the appropriate chalcogenoether ligand in anhydrous hexane and, together with the known [SnCl4{nBuSe(CH2)3SenBu}] (1), employed as single source precursors for the low pressure chemical vapour deposition of the corresponding tin dichalcogenide thin films. At elevated temperatures the bidentate ligand precursors, (1) and (2), also form the tin monochalcogenides, SnSe and SnS, respectively. In contrast, (3) gave a mixture of phases, SnS2, Sn2S3 and SnS and (4) gave SnSe2 only. The morphologies, elemental compositions and crystal structures of the resulting films have been determined by scanning electron microscopy, energy dispersive X-ray spectroscopy, grazing incidence X-ray diffraction and Raman spectroscopy. Van der Pauw measurements on the SnS2, SnS and SnSe2 films confirm their resistivities to be 2.9(9), 266(3) and 4.4(3) Ω cm, respectively.

Electroreduction of carbon dioxide to formate over a thin-layered tin diselenide electrode

We first report a thin-layered SnSe2 film as a robust catalyst for CO2 electroreduction, efficiently affording formate with a faradaic efficiency of 91% and a stable activity for more than 100 h at −0.8 V versus the reversible hydrogen electrode.

Thermal Stress Effect on the Structure and Properties of Single and Double Stacked Films of GeTe and SnSe

The thermal stress effect on the structure of phase change memory materials, namely single films and double stacked films of GeTe and SnSe, is evaluated. The crystallization temperatures of GeTe and SnSe single films are 138 °C and 292 °C, respectively. The films are amorphous before annealing and crystallize in rhombohedral and orthorhombic structures afterwards. Ge is tetrahedrally bonded and Se is bivalent after deposition. Both Ge and Se have an octahedral configuration after annealing. The double stacked structure is studied in the as‐deposited state and after annealing at temperatures of 100, 210, and 350 °C. Pulsed laser deposition produces the crystallization of both as‐deposited layers when stacked, mostly of SnSe, but also some crystalline GeTe is present. GeTe fully crystallizes after annealing at 210 °C, in the face‐centred cubic structure. Annealing at 350 °C leads to the evaporation of a significant quantity of Se and to the formation of a cubic Ge0.75Sn0.25Te solid solution. Ge has an octahedral coordination, while Se is tetrahedrally bonded as a result of a combination of bivalent amorphous Se and octahedral Se from crystalline SnSe. The study shows that diffusion between layers at high annealing temperatures might suppress the memory property and determines the formation of irreversible solid solutions.

Phase-engineered SnSex toward SnSe2/SnSe heterostructure with improved thermal conductance by a low-temperature plasma-assisted chemical vapor reaction

In this work, the discovery of an abnormal p-type conductivity in SnSe2 thin films synthesized by a plasma-assisted chemical vapor reaction (PACVR) at a low temperature as low as 150°C was reported. Due to a thermodynamically driven mechanism, three distinct phases including homogeneous SnSe2 and SnSe as well as a SnSe2/SnSe heterostructure were demonstrated. The SnSe2 homogeneous film was prepared by the PACVR from a thin Sn prelayer at a high substrate temperature on a glass substrate. The SnSe homogeneous film was prepared at a low temperature on a mica substrate regardless of the Sn prelayer thickness. Furthermore, the SnSe2/SnSe heterostructure film was prepared from a thick Sn prelayer on the glass substrate. All of the phases exhibit p-type conducting behavior because of hydrogen doping at the vapor–solid reaction interface during the PACVR. Furthermore, the first experimental evidence of intrinsic thermal conductivities of SnSe2 and SnSe thin film of 2.15W/(mK) and 0.18W/(mK), respectively, together with an interfacial thermal conductance of 11.32MWm−2K−1 in between, was measured by Raman scattering behaviors. The low thermal conductivity make PACVR-synthesized SnSe2-based thin films feasible for thermoelectric applications. In particular, the SnSe2/SnSe heterostructure is the optimized phase for use in thermoelectric generators owing to not only its low thermal conductivity but also the strong accumulation of holes and phonons near the surface.

Optimized growth conditions of epitaxial SnSe films grown by pulsed laser deposition

We have grown epitaxial tin monoselenide (SnSe) films on MgO or SrTiO3 (STO) substrates by pulsed laser deposition (PLD) at Ts = 473 or 573 K, and investigated the optimized growth condition in terms of crystal orientation, crystallinity, and electrical resistivity. For the PLD procedure, a SnSex (x = 1.0–1.6) target containing excess Se was used to compensate for the vaporization of Se. The crystal orientation and crystallinity of the SnSe films changed depending on the growth conditions, and the magnitude of the electrical resistivity ρ of the films was closely related to the crystalline nature. The SnSe film grown on the MgO substrate at Ts = 573 K using the target with x = 1.4 was the most highly a-axis-oriented and highly crystalized among all of the films investigated in this study. However, the ρ of the film in the bc-plane was about one order of magnitude larger than those of the reported single crystal and the a-axis-oriented crystalline sample fabricated by spark plasma sintering. This larger ρ was suggested to result from the lattice mismatch and/or a small amount of nonstoichiometry in the film.

(Invited) IV-VI Monochalcogenide Nanostructures: Synthesis, Doping and Thermoelectric Properties

Recently IV-VI monochalcogenide SnSe or SnS has been proposed as a promising two-dimensional (2D) material for valleytronics and thermoelectrics. Despite much theoretical interest and many experimental reports on the thermoelectric characterizations of SnSe single crystals, experimental studies on SnSe and SnS in the nanostructured form are still limited. We report the synthesis of SnSe nanoflakes and thin films with chemical vapor deposition (CVD) and their thermoelectric properties. As grown SnSe nanostructures are found to be intrinsically p-type and different types of dopants (In, Pb and Ag) were explored to control the carrier density. We will present the electrical transport property of SnSe nanoflake field effect transistor devices and the effects of doping on the electrical conductivity, Seebeck coefficient, power factor and anisotropy in SnSe films. By doping, the power factor of SnSe films can be improved by at least one order of magnitude compared to the "intrinsic" as grown materials. If time permits, we will also discuss the transport properties of SnS nanoflakes obtained by mechanical exfoliation to compare with the chemically grown SnSe nanostructures. Our work provides an initial step in the pursuit of IV-VI monochalcogenide nanostructures as novel 2D semiconductors for electronics and thermoelectrics.

Enhanced thermoelectric properties of SnSe thin films grown by pulsed laser glancing-angle deposition

SnSe single crystals have been demonstrated to possess excellent thermoelectric properties. In this work, we demonstrate a grain size control method in growing nanocrystalline SnSe thin films through a glancing angle pulsed-laser deposition approach. Structural characterization reveals that the SnSe film deposited at a normal angle has a preferred orientation along a axis, while by contrast, the SnSe film deposited at an 80° glancing angle develops a nanopillar structure with the growth direction towards the incident atomic flux. The glancing angle deposition greatly reduces the grain size of the thin film due to a shadowing effect to the adatoms, resulting in significantly increased power factor for more than 100%. The maximum Seebeck coefficient and power factor are 498.5μV/K and 18.5μWcm−1K−2, respectively. The enhancement of thermoelectric property can be attributed to the potential barrier scattering at grain boundaries owing to the reduced grain size and increased grain boundaries in the film. Given this enhanced power factor, and considering the fact that the nanopillar structure should have much lower thermal conductivity than a plain film, the zT value of such made SnSe film could be significantly larger than the corresponding single crystal film, making it a good candidate for thin film-based thermoelectric device.

Crystal Structure, Optical, and Electrical Properties of SnSe and SnS Semiconductor Thin Films Prepared by Vacuum Evaporation Techniques for Solar Cell Applications

Thin films of SnSe and SnS semiconductors had been prepared by vacuum evaporation techniques. All prepared samples were characterized on their structure, optical, and electrical properties in order to know their application in technology. The crystal structure of SnSe and SnS was determined by X-Ray Diffraction (XRD) instrument. The morphology and chemical composition were obtained by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive of X-Ray Analysis (EDAX). The optical property such as band gap was determined by DR-UV-Vis (Diffuse Reflectance-Ultra Violet-Visible) spectroscopy, while the electrical properties were determined by measuring the conductivity by four probes method. The characterization results indicated that both SnSe and SnS thin films were polycrystalline. SnSe crystallized in an orthorhombic crystal system with the lattice parameters of a = 11.47 Å, b = 4.152 Å and c = 4.439 Å, while SnS had an orthorhombic crystal system with lattice parameters of a = 4.317 Å, b = 11.647 Å and c = 3.981 Å. Band gaps (Eg) of SnSe and SnS were 1.63 eV and 1.35 eV, respectively. Chemical compositions of both thin films were non-stoichiometric. Molar ratio of Sn : S was close to ideal which was 1 : 0.96, while molar ratio of Sn : S was 1 : 0.84. The surface morphology described the arrangement of the grains on the surface of the thin film with sizes ranging from 0.2 to 0.5 microns. Color similarity on the surface of the SEM images proved a homogenous thin layer.

Tin diselenide as a new saturable absorber for generation of laser pulses at 1μm.

The nonlinear optical responses of two-dimensional (2D) few-layer tin diselenide (SnSe2) have been investigated in this work. By applying the Z-scan technique, the saturable absorption of SnSe2 thin film has been observed at a wavelength of ~1μm, which enables SnSe2, a new saturable absorber for pulsed laser generation. A detailed exploration of the nonlinear absorption of SnSe2 film and their dependence on excitation pulse energy is carried out. Our results show that saturation intensity of the nonlinear absorption is ~23.78 GW/cm2 and corresponding nonlinear absorption coefficients are at a range from -13596 to -2967.3 cm/GW. By employing the SnSe2-coated mirror as a saturable absorber, the passively Q-switched lasing has been achieved on a crystalline waveguide platform at ~1 μm.

Depth-resolved and temperature dependent analysis of phase formation processes in Cu–Zn–Sn–Se films on ZnO substrates

The constitution of secondary phases in kesterite Cu2ZnSnSe4 (CZTSe) thin films is still a limiting factor for their application in solar cells. Therefore an enhanced understanding of phase formation processes during the fabrication of CZTSe films is required. In this study we present a temperature and film-depth dependent phase analysis of Zn/Sn/Cu precursors on ZnO substrates selenized at different temperatures. A special sample preparation step using a focused ion beam was applied to prepare shallow angle cross sections for depth-resolved Raman profiling of the thin films. At low selenization temperatures multiphase structures are demonstrated and a first formation of CZTSe besides secondary phases at only 250 °C is detected. At high selenization temperatures an accumulation of ZnSe at the interface of CZTSe and ZnO substrates is observed. Furthermore indications for the formation of a thin SnO2 interface layer were found by X-ray diffraction, secondary electron microscopy and energy dispersive X-ray spectrometry.

Molecular beam epitaxy of large-area SnSe2 with monolayer thickness fluctuation

The interest in layered materials is largely based on the expectation that they will be beneficial for a variety of applications, from low-power-consuming, wearable electronics to energy harvesting. However, the properties of layered materials are highly dependent on thickness, and the difficulty of controlling thickness over a large area has been a bottleneck for commercial applications. Here, we report layer-by-layer growth of SnSe2, a layered semiconducting material, via van der Waals epitaxy. The films were fabricated on insulating mica substrates with substrate temperatures in the range of 210 °C–370 °C. The surface consists of a mixture of N and (N ± 1) layers, showing that the thickness of the film can be defined with monolayer accuracy (±0.6 nm). High-resolution transmission electron microscopy reveals a polycrystalline film with a grain size of ∼100 nm and clear Moiré patterns from overlapped grains with similar thickness. We also report field effect mobility values of 3.7 cm2 V−1 s−1 and 6.7 cm2 V−1 s−1 for 11 and 22 nm thick SnSe2, respectively. SnSe2 films with customizable thickness can provide valuable platforms for industry and academic researchers to fully exploit the potential of layered materials.

Effect of deposition temperature on structural, optical and electrical properties of nanocrystalline SnSe thin films

SnSe thin films have been prepared by chemical bath deposition method at different substrate temperature. The influence of deposition temperature on structural, optical and electrical properties of polycrystalline SnSe films have been investigated using X-ray diffraction, optical absorbance and conductivity measurements. The X-ray diffraction study reveals the orthorhombic structure of the SnSe films oriented along the (111) direction. The structural parameters such as lattice spacing, crystallite size, strain, number of crystallites per unit area and dislocation density have been evaluated. Optical absorbance measurement indicates the existence of direct allowed band gap in the range 1.50–1.91 eV. The dark conductivity (σ d ) and photoconductivity (σ ph ) measurement in the temperature range (278–385 K), indicates that the conduction in these materials is through an activation process having one activation energy. σd and σ ph values decrease with the decrease of crystallite size. The value of photosensitivity, carrier life time and trap depth have also been calculated.

One-step electrochemical synthesis of Cu—Zn—Sn—Se precursor thin films from solutions containing lactic acid

The influence of the conditions of electrochemical deposition of Cu—Zn—Sn—Se precursor thin films from solutions containing lactic acid on their optical and electrophysical properties was studied. It was found that an additional annealing step of precursor films in a selenium atmosphere at 550 °C is necessary for the preparation of single-phase polycrystalline photoconducting Cu—Zn—Sn—Se films with a band gap energy equal to ~1 eV. The obtained semiconductor CZTSe materials satisfy the composition and property requirements imposed on absorbing layers in solar panels.

Large lateral photovoltaic effect with ultrafast relaxation time in SnSe/Si junction

In this paper, we report a large lateral photovoltaic effect (LPE) with ultrafast relaxation time in SnSe/p-Si junctions. The LPE shows a linear dependence on the position of the laser spot, and the position sensitivity is as high as 250 mV mm−1. The optical response time and the relaxation time of the LPE are about 100 ns and 2 μs, respectively. The current-voltage curve on the surface of the SnSe film indicates the formation of an inversion layer at the SnSe/p-Si interface. Our results clearly suggest that most of the excited-electrons diffuse laterally in the inversion layer at the SnSe/p-Si interface, which results in a large LPE with ultrafast relaxation time. The high positional sensitivity and ultrafast relaxation time of the LPE make the SnSe/p-Si junction a promising candidate for a wide range of optoelectronic applications.

Optimized substrate temperature range for improved physical properties in spray pyrolysis deposited Tin Selenide thin films

Tin diselenide (SnSe2) thin films have been deposited on glass substrate using the spray pyrolysis technique by varying substrate temperatures (Tsub) from 250 °C to 400 °C. These films were characterized for structural, morphological, electrical and thermoelectric properties by using X-ray Diffraction, Field Emission Scanning Electron Microscopy (FESEM), four probe resistivity/conductivity and thermoelectric measurement respectively. X-Ray Diffraction patterns revealed that substrate temperature plays a critical role in phase formation and a single phase SnSe2 film were formed at a critical temperature range 275–375 °C. FESEM study of single phase film at 300 °C shows uneven and irregular shape grains throughout the film and the EDX shows the stoichiometric ratio of deposit. The variations of the micro-structural parameters, such as crystallite size (D), dislocation density (δ), and micro-strain (ε), were investigated for all films. The results showed that the crystallite sizes increase with an increase in substrate temperature, whereas the dislocation density and micro-strain decreases with increasing substrate temperature. The resistivity (ρ) of SnSe2 film was found to be decreasing with increasing temperature and also with substrate temperature. The predefined substrate temperature range (275–375 °C) was further narrowed down using 4-probe resistivity measurement. It shows that, although 275–375 °C is enough formation of single phase SnSe2, but there is a small temperature range 325–375 °C where one will get low resistive sample, which influence the flow of charge carrier. Low electrical resistance or inversely high electrical conductivity is one of the prime requirements of thermoelectric samples. The 4-probe resistivity measurement of single phase thin films shows that the lowest electrical resistance obtained for substrate temperature of 350 °C. Thermoelectric measurement confirms that the coated films were n-type semiconductor materials and the thermo-emf increases with an increase in substrate temperature. Among all the films the film with substrate temperature of 350 °C shows enhanced behaviour to be used in industrial applications.

Cubic‐structured tin selenide thin film as a novel solar cell absorber

Tin selenide thin film with a simple cubic crystalline structure (SnSe‐CUB) of unit cell dimension a = 11.9632 Å is obtained via chemical deposition on a tin sulfide (SnS‐CUB) thin film base layer of simple cubic structure of a = 11.5873 Å. The SnSe‐CUB films obtained this way are thermally stable while heating to 300 °C. Its optical band gap is 1.4 eV. A thin film of 200 nm in thickness of this material in a solar cell may lead to a light generated current density of 23 mA cm−2 and a maximum of 29 mA cm−2. Thin film of SnSe‐CUB possesses p‐type electrical conductivity of 5 × 10−5 Ω−1 cm−1, which is three orders of magnitude lower than that of SnSe films of orthorhombic crystalline structure. Overall, these characteristics make SnSe‐CUB thin film a novel solar cell absorber material.

Perspectives on SnSe-based thin film solar cells: a comprehensive review

Currently, selenium (Se)-based compound semiconductors (CISe, CIGSe and CZTSe) are considered as the active materials in the photovoltaic world. However, these materials exhibit couple of issues related to stoichiometry maintenance and scarcity of their constituent elements (In, Ga), which limit their massive production for future energy demands. These issues could be rectified by introducing a non-toxic, inexpensive and earth-abundant binary material. One such material is a tin monoselenide (SnSe), which exhibits a high chemical stability along with attractive physical properties namely, suitable band gap (1.3 eV), high absorption coefficient (105 cm−1) and p-type conductivity. These properties indicate SnSe as a competitive substitute in place of conventional absorbers in thin film solar cells. Despite of its remarkable properties, only a few reports were published on the fabrication of SnSe-based solar cells with poor efficiency (≤1 %). This indicates a need to review on the physical properties of SnSe and its device structures in a deeper sense. In this context, the present review describes the different methods of preparation of SnSe films and their physical properties along with the details of photovoltaic device fabrication. We highlighted the different factors that are limiting the efficiency of SnSe solar cells, and a few suggestions were included to overcome these problems for further improvement of these cells. This review will enrich and stimulate the readers to further investigate the growth of SnSe thin films and their devices, for the development of >20 % efficient SnSe solar cells.

Thin films of n-type SnSe2 produced from chemically deposited p-type SnSe

Tin(IV) selenide (SnSe2) thin films 120–280nm in thickness are obtained through heating SnSe films at 350°C, in the presence of Se-vapor. Thin film of SnSe deposited from a chemical bath is of orthorhombic crystal structure, with a band gap, Eg, of 0.94eV, and p-type conductivity of 0.3Ω−1cm−1. Thin film of SnSe2 280nm in thickness of hexagonal crystalline structure, with Eg of 1.2eV and n-type electrical conductivity of 2Ω−1cm−1 is produced from SnSe film 260nm in thickness. The SnSe2 film shows a Hall mobility of 10cm2/(Vs), carrier (electron) concentration 1017–1018cm−3 and thermoelectric power of −390μV/K. The thin film of SnSe2 formed this way is stable during further heating at temperatures up to 430°C, but it reverts to p-type SnSe thin film when heated at 530°C. Applications of these SnSe and SnSe2 thin films are considered.

Heteroepitaxial growth of SnSe films by pulsed laser deposition using Se-rich targets

Epitaxial growth of SnSe was studied using pulsed laser deposition on three kinds of single-crystalline substrates, MgO (100), NaCl (100), and SrF2 (100), along with silica glass. For polycrystalline SnSe grown on glass, we found that a 20% Se-rich polycrystalline target is effective for obtaining stoichiometric SnSe films. The crystal structure of all the obtained films was the same as that of bulk SnSe, and the optimum temperatures for epitaxial growth were 400–500 °C, where the highest temperature was limited by re-evaporation. All the epitaxial films exhibited an a-axis orientation. For in-plane epitaxial structures, a quasi cube-on-cube heteroepitaxy was observed for MgO, whereas a 45°-rotated heteroepitaxy was observed for NaCl and SrF2, which can be explained by in-plane lattice matching. The anisotropic lattice mismatching and thermal expansion explain the compressive strain induced along the in-plane c-axis and the tensile strains along the in-plane b-axis and the out-of-plane a-axis. The epitaxial films on NaCl exhibited the best crystallinity, but the highest hole mobility of 60 cm2/(V s) was obtained on MgO. The indirect optical gap of the best films was determined to be 0.8 eV. Plausible interface models were proposed based on density functional calculations.

Quantum Spin Hall Effect in Strained (111)-Oriented SnSe Layers

Recently, the quantum spin Hall effect has been predicted in (111)-oriented thin films of SnSe and SnTe topological crystalline insulators. It was shown that in these films the energy gaps in the two-dimensional band spectrum depend in an oscillatory fashion on the layer thickness -- the calculated topological invariant indexes and edge state spin polarizations show that for films 20-40 monolayers thick a two-dimensional topological insulator phase appears. Edge states with Dirac cones with opposite spin polarization in their two branches are obtained for both materials. However, for all but the (111)-oriented SnTe films with an even number of monolayers an overlapping of bands in $\overline{\Gamma}$ and $\overline{\mathrm{M}}$ diminishes the final band gap and the edge states appear either against the background of the bands or within a very small energy gap. Here we show that this problem in SnSe films can be removed by applying an appropriate strain. This should enable observation of the Quantum Spin Hall effect also in SnSe layers.