Tellurium Films Research Articles

Direct Observations of Spontaneous In-Plane Electronic Polarization in 2D Te Films.

Single-element polarization in low dimensions is fascinating for constructing next-generation nanoelectronics with multiple functionalities, yet remains difficult to access with satisfactory performance. Here, spectroscopic evidences are presented for the spontaneous electronic polarization in tellurium (Te) films thinned down to bilayer, characterized by low-temperature scanning tunneling microscopy/spectroscopy. The unique chiral structure and centrosymmetry-breaking character in 2DTe gives rise to sizable in-plane polarization with accumulated charges, which is demonstrated by the reversed band-bending trends at opposite polarization edges in spatially resolved spectra and conductance mappings. The polarity of charges exhibits intriguing influence on imaging the moiré superlattice at the Te-graphene interface. Moreover, the plain spontaneous polarization robustly exists for various film thicknesses, and can universally preserve against different epitaxial substrates. The experimental validations of considerable electronic polarization in Te multilayers thus provide a realistic platform for promisingly facilitating reliable applications in microelectronic devices.

Role of growth temperature on microstructural and electronic properties of rapid thermally grown MoTe2 thin film for infrared detection

In the field of electronic and optoelectronic applications, two-dimensional materials are found to be promising candidates for futuristic devices. For the detection of infrared (IR) light, MoTe2possesses an appropriate bandgap for which p-MoTe2/n-Si heterojunctions are well suited for photodetectors. In this study, a rapid thermal technique is used to grow MoTe2thin films on silicon (Si) substrates. Molybdenum (Mo) thin films are deposited using a sputtering system on the Si substrate and tellurium (Te) film is deposited on the Mo film by a thermal evaporation technique. The substrates with Mo/Te thin films are kept in a face-to-face manner inside the rapid thermal-processing furnace. The growth is carried out at a base pressure of 2 torr with a flow of 160 sccm of argon gas at different temperatures ranging from 400 °C to 700 °C. The x-ray diffraction peaks appear around 2θ= 12.8°, 25.5°, 39.2°, and 53.2° corresponding to (002), (004), (006), and (008) orientation of a hexagonal 2H-MoTe2structure. The characteristic Raman peaks of MoTe2, observed at ∼119 cm-1and ∼172 cm-1, correspond to the in-plane E1gand out-of-plane A1gmodes of MoTe2, whereas the prominent peaks of the in-plane E12gmode at ∼234 cm-1and the out-of-plane B12gmode at ∼289 cm-1are also observed. Root mean square (RMS) roughness is found to increase with increasing growth temperature. The bandgap of MoTe2is calculated using a Tauc plot and is found to be 0.90 eV. Electrical characterizations are carried out using current-voltage and current-time measurement, where the maximum responsivity and detectivity are found to be 127.37 mA W-1and 85.21 × 107Jones for a growth temperature of 600 °C and an IR wavelength illumination of 1060 nm.

2D Tellurium Films Based Self-Drive Near Infrared Photodetector.

To reduce the amount of energy consumed in integrated circuits, high efficiency with the lowest energy is always expected. Self-drive device is one of the options in the pursuit of low power nanodevices. It is a typical strategy to form an internal electric field by constructing a heterojunction in self-drive semiconductor system. Here, a two-step method is proposed to prepare high quality centimeter-sized 2D tellurium (Te) thin film with hall mobility as high as 37.3 cm2 V-1 s-1, and the 2D Te film is further assembled with silicon to form a heterojunction for self-drive photodetector, which can realize effective detection from visible to near infrared bands. The photodetectivity of the heterojunctions can reach 1.58×1011 Jones under the illumination of 400 nm@ 1.615 mW/cm2 and 2.08×108 Jones under the illumination of 1550 nm@ 1.511 mW/cm2 without bias. Our experiments demonstrate the potential of 2D tellurium thin films for wide band and near infrared integrated device applications.

2D Vertical and Horizontal Memristors Based on Large-Area 2D Tellurium Film

2D Vertical and Horizontal Memristors Based on Large-Area 2D Tellurium Film

Thickness‐Dependent Bandgap and Atomic Structure in Elemental Tellurium Films

Elemental tellurium electrical switch, relying on a transient crystal–liquid–crystal phase transition, has recently been proposed as a promising selector candidate for the next‐generation 3D high‐density memory, bridging performance gap in today's computer. Further miniaturization of the switch cell to increase memory density strongly depends on the scalability of the tellurium film, which, however, has not been experimentally studied. Herein, the tellurium films are prepared with the thickness downscaled from 400 to 2 nm and a significant increase is found in the bandgap from 0.29 to 0.91 eV, as predicted by ab initio molecular dynamics. Interestingly, the as‐deposited tellurium films with a thickness above 3 nm are in the crystalline trigonal phase, whereas 2 nm thick film suddenly becomes amorphous, observed by both Raman and transmission electron microscopies. In this finding, since the leakage current of the elemental tellurium switch is determined by both the Schottky barrier between tellurium/electrode interface and the bandgap of the tellurium film, a reduction in leakage current is predicted with further miniaturization.

Influence of substrate temperature on the properties of ZnTe:Cu films prepared by a magnetron co-sputtering method

Copper-doped Zinc Tellurium (ZnTe:Cu) films were deposited on borosilicate glass using magnetron co-sputtering technique. The influence of the substrate temperature on the structural, morphological, optical and electrical properties of ZnTe:Cu films was investigated by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), UV–Vis spectrophotometer and Hall effect measurement system. The results indicate that substrate temperature significantly affects the properties of the ZnTe:Cu films. When the substrate temperature increases from room temperature to 600 °C, the (111)-preferred orientation of ZnTe:Cu films is gradually replaced by the (220)-preferred orientation. At high substrate temperatures (≥500 °C), the CuxTe phase appears in the ZnTe:Cu films, resulting in higher carrier concentration (>1019 cm−3) and lower resistivity (<10−2 Ω cm) of the prepared films.

Electrical conductivity, Seebeck coefficient, and crystal growth of p-type tellurium films through temperature dependent RF sputtering

Te thin films have recently received considerable attention owing to its superior electrical and thermoelectric properties. During the deposition process, if the temperature of the substrate is raised, high crystallinity and improved electrical properties can be expected. In this study, we used radio frequency sputtering for Te deposition to study the relationship between the deposition temperature, crystal size, and electrical performance. As the deposition temperature is increased from room temperature to 100 °C, we observed an increase in crystal size from the x-ray diffraction patterns and full-width half maximum calculations. With this grain size increment, the Hall mobility and Seebeck coefficient of the Te thin film increased significantly from 16 to 33 cm2 V−1 s−1 and 50 to 138 μV K−1, respectively. This study reveals the potential of a facile fabrication method for enhanced Te thin films using temperature control and highlights the importance of the Te crystal structure in determining the electrical/thermoelectrical properties. These findings are particularly significant for the development of semiconductor material systems for various applications, including thermoelectric devices, CMOS, FET, and solar devices.

Critical behavior in the epitaxial growth of two-dimensional tellurium films on SrTiO3 (001) substrates

Materials’ properties may differ in the thin-film form, especially for epitaxial ultra-thin films, where the substrates play an important role in their deviation from the bulk quality. Here by molecular beam epitaxy (MBE) and scanning tunneling microscopy/spectroscopy, we investigate the growth kinetics of ultra-thin tellurium (Te) films on SrTiO3 (STO) (001). The MBE growth of Te films usually exhibits Volmer–Weber (VW) island growth mode and no a-few-monolayer film with full coverage has been reported. The absence of wetting-layer formation in the VW growth mode of Te on STO (001) is resulted from its low diffusion barriers as well as its relatively higher surface energy compared with those of the substrate and the interface. Here we circumvent these limiting factors and achieve the growth of ultra-thin β-Te films with near-complete coverages by driving the growth kinetics to the extreme condition. There is a critical thickness (3 monolayer) above which the two-dimensional Te films can form on the STO (001) substrate. In addition, the scanning tunneling spectra on the ultra-thin Te film grown on STO exhibits an enormously large forbidden gap compared with that grown on the graphene substrate. Our work establishes the necessary conditions for the growth of ultra-thin materials with similar kinetics and thermodynamics.

Critical behavior in the epitaxial growth of two-dimensional tellurium films on SrTiO3 (001) substrates

Materials’ properties may differ in the thin-film form, especially for epitaxial ultra-thin films, where the substrates play an important role in their deviation from the bulk quality. Here by molecular beam epitaxy (MBE) and scanning tunneling microscopy/spectroscopy, we investigate the growth kinetics of ultra-thin tellurium (Te) films on SrTiO3 (STO) (001). The MBE growth of Te films usually exhibits Volmer–Weber (VW) island growth mode and no a-few-monolayer film with full coverage has been reported. The absence of wetting-layer formation in the VW growth mode of Te on STO (001) is resulted from its low diffusion barriers as well as its relatively higher surface energy compared with those of the substrate and the interface. Here we circumvent these limiting factors and achieve the growth of ultra-thin β-Te films with near-complete coverages by driving the growth kinetics to the extreme condition. There is a critical thickness (3 monolayer) above which the two-dimensional Te films can form on the STO (001) substrate. In addition, the scanning tunneling spectra on the ultra-thin Te film grown on STO exhibits an enormously large forbidden gap compared with that grown on the graphene substrate. Our work establishes the necessary conditions for the growth of ultra-thin materials with similar kinetics and thermodynamics.

Liquid Metal-Templated Tin-Doped Tellurium Films for Flexible Asymmetric Pseudocapacitors.

Liquid metals can be surface activated to generate a controlled galvanic potential by immersing them in aqueous solutions. This creates energized liquid-liquid interfaces that can promote interfacial chemical reactions. Here we utilize this interfacial phenomenon of liquid metals to deposit thin films of tin-doped tellurium onto rigid and flexible substrates. This is accomplished by exposing liquid metals to a precursor solution of Sn2+ and HTeO2+ ions. The ability to paint liquid metals onto substrates enables us to fabricate supercapacitor electrodes of liquid metal films with an intimately connected surface layer of tin-doped tellurium. The tin-doped tellurium exhibits a pseudocapacitive behavior in 1.0 M Na2SO4 electrolyte and records a specific capacitance of 184.06 F·g-1 (5.74 mF·cm-2) at a scan rate of 10 mV·s-1. Flexible supercapacitor electrodes are also fabricated by painting liquid metals onto polypropylene sheets and subsequently depositing tin-doped tellurium thin films. These flexible electrodes show outstanding mechanical stability even when experiencing a complete 180° bend as well as exhibit high power and energy densities of 160 W·cm-3 and 31 mWh·cm-3, respectively. Overall, this study demonstrates the attractive features of liquid metals in creating energy storage devices and exemplifies their use as media for synthesizing electrochemically active materials.

Effect of additives and optimized Cyclic voltammetry parameters on the morphology of electrodeposited Tellurium thin film

• The CV curve of Tellurium shifts with increasing deposition time. • Mechanism of Tellurium displacement and effect of additives. • Dense Tellurium film was fabricated under optimized deposition potentials. • Various dendritic morphologies of Tellurium were obtained by different additives. Thin film technologies in terms of preparation and fabrication are of great importance in various fields, such as integrated circuits (IC), LCD monitors, photovoltaics, solar cells and sensors. Tellurium, as a narrow bandgap semiconductor, has attracted much attention due to its rich optical and electrical properties. Taking advantage of these properties, its films have been explored in recent years as saturable absorbers, gas sensors, and thermoelectric materials. However, it is challenging to control the growth of the specific morphology of tellurium films. In this work, based on the electrodeposition challenge, we systematically investigated the electrodeposition process of tellurium films. It was found that the CV curves of electrodeposited tellurium shift in the positive direction with increasing deposition time. In addition, the stabilized potential was determined based on the stable deposition peak potential after a certain deposition time. Moreover, three different additives, namely polyvinyl alcohol (PVA), tartaric acid (TA) and sodium lignosulfonate (SLS), were adopted to manipulate the morphology of as-deposited tellurium films. A rational route to deposit tellurium films with various morphologies have been established.

Desorption characteristics of selenium and tellurium thin films

The temperature-dependent desorption behavior of selenium and tellurium is investigated using a heated quartz crystal microbalance. Prior to heating the quartz crystal microbalance, selenium and tellurium films with varying thickness were deposited using thermal effusion cells in a molecular beam epitaxy system for subsequent determination of temperature-dependent mass loss of the deposited films. The desorption rate for tellurium was found to exhibit one sharp peak around 190 °C, indicating the loss of the entire film irrespective of film thickness within a temperature window of 20 °C, which was completely evaporated at 200 °C. Similar experiments for selenium revealed that the thermal desorption took place via a two-stage process with a smaller portion of the material desorbing within an even narrower temperature window of 5 °C at a much lower peak temperature of 65 °C, while most selenium desorbed within a temperature range of 10 °C around 90 °C. This two-stage behavior indicated the presence of at least two chemically distinct selenium species or binding states. The direct and quantitative determination of the chalcogen desorption process provides important insights into the kinetics of chalcogenide-based film growth and is in addition of applied benefit to the research community in the area of Se/Te capping and decapping of air sensitive materials as it provides temperature ranges and rates at which full desorption is achieved. Our work furthermore points toward the need for a more detailed understanding of the chemical composition state of atomic and molecular beams supplied from thermal evaporation sources during growth.

Robust Piezoelectricity with Spontaneous Polarization in Monolayer Tellurene and Multilayer Tellurium Film at Room Temperature for Reliable Memory.

Robust neuromorphic computing in the Big Data era calls for long-term stable crossbar-array memory cells; however, the elemental segregation in the switch unit and memory unit that inevitably occurs upon cycling breaks the compositional and structural stability, making the whole memory cell a failure. Searching for a novel material without segregation that can be used for both switch and memory units is the major concern to fabricate robust and reliable nonvolatile cross-array memory cells. Tellurium (Te) is found recently to be the only peculiar material without segregation for switching, but the memory function has not been demonstrated yet. Herein, apparent piezoelectricity is experimentally confirmed with spontaneous polarization behaviors in elementary 2D Te, even in monolayer tellurene (0.4nm), due to the highly oriented polarization of the molecular structure and the non-centrosymmetric lattice structure. A large memory window of 7000, a low working voltage of 2V, and high on switching current up to 36.6 µA µm-1 are achieved in the as-fabricated Te-based memory device, revealing the great promise of Te for both switching and memory units in one cell without segregation. The piezoelectric Te with spontaneous polarization provides a platform to build robust, reliable, and high-density logic-in-memory chips in neuromorphic computing.

Growth of high-quality semiconducting tellurium films for high-performance p-channel field-effect transistors with wafer-scale uniformity

Achieving high-performance p-type semiconductors has been considered one of the most challenging tasks for three-dimensional vertically integrated nanoelectronics. Although many candidates have been presented to date, the facile and scalable realization of high-mobility p-channel field-effect transistors (FETs) is still elusive. Here, we report a high-performance p-channel tellurium (Te) FET fabricated through physical vapor deposition at room temperature. A growth route involving Te deposition by sputtering, oxidation and subsequent reduction to an elemental Te film through alumina encapsulation allows the resulting p-channel FET to exhibit a high field-effect mobility of 30.9 cm2 V−1 s−1 and an ION/OFF ratio of 5.8 × 105 with 4-inch wafer-scale integrity on a SiO2/Si substrate. Complementary metal-oxide semiconductor (CMOS) inverters using In-Ga-Zn-O and 4-nm-thick Te channels show a remarkably high gain of ~75.2 and great noise margins at small supply voltage of 3 V. We believe that this low-cost and high-performance Te layer can pave the way for future CMOS technology enabling monolithic three-dimensional integration.

Impacts from triple phases of a germanium–antimony–tellurium film coating on thermal emission from SiO2 and boron doped Si

This work experimentally demonstrates mid-infrared emittance spectra of dielectric and semi-conductor substrates with and without a germanium–antimony–tellurium (GST) film coating. The film experiences non-volatile phase changes at 140°C and 300°C. Impacts from amorphous, face-centered cubic, and hexagonal close packed phases on spectral emittance are demonstrated within the spectral range from 4 μm to 18 μm. The spectra are measured at 100°C, 200°C, 300°C, and 400°C to show temperature dependence. Close-to-total emittance is calculated for comparison. The GST film can reduce emittance from a SiO2 substrate, but it raises close-to-normal emittance as well as the spectral emittance at wavelengths 5 μm ≤ λ ≤ 18 μm for the doped Si substrate.

Tellurium Single-Crystal Arrays by Low-Temperature Evaporation and Crystallization.

Thermally evaporated tellurium possesses an intriguing crystallization behavior, where an amorphous to crystalline phase transition happens at near-ambient temperature. However, a comprehensive understanding and delicate control of the crystallization process for the evaporated Te films is lacking. Here, the kinetics and dynamics of the crystallization of thermally evaporated Te films is visualized and modeled. Low-temperature processing of highly crystalline tellurium films with large grain size and preferred out-of-plane orientation ((100) plane parallel to the surface) is demonstrated by controlling the crystallization process. Tellurium single crystals with a lateral dimension of up to 6µm are realized on various substrates including glass and plastic. Field-effect transistors based on 5°C crystallized Te single grains (6-nm-thick) exhibit an average effective hole mobility of ≈100 cm2 V-1 s-1 , and on/off current ratio of ≈3 × 104 .

Strong bulk photovoltaic effect and second-harmonic generation in two-dimensional selenium and tellurium

Few-layer selenium and tellurium films have been recently prepared, and they provide a new platform to explore novel properties of two-dimensional (2D) elemental materials. In this work, we have performed a systematic first-principles study on the electronic, linear, and nonlinear optical (NLO) properties of atomically thin selenium and tellurium films within the density-functional theory with the generalized gradient approximation plus scissors correction using the band gaps from the relativistic hybrid Heyd-Scuseria-Ernzerhof functional calculations. Interestingly, we find that few-layer Se and Te possess large second-harmonic generation (SHG), linear electro-optic effect, and bulk photovoltaic effect. In particular, trilayer (TL) Te possesses large SHG coefficient, being more than 65 times larger than that of GaN, a widely used NLO material. Bilayer (BL) Te has huge static SHG coefficient, being more than 100 times larger than that of GaN. Furthermore, monolayer (ML) Se possesses large SHG coefficient. Moreover, we predict that TL Te exhibits strong bulk photovoltaic effect (BPVE), being greater than that of GeS, a polar system with the largest BPVE found so far. Although the shift current conductivities of bulk and 2D Se are comparable, the shift current conductivities of TL Te are five times larger than those of bulk Te. Finally, an analysis of the calculated electronic band structures indicates that the strong NLO responses of 2D Se and Te materials are primarily derived from their low-dimensional structures with high anisotropy, directional covalent bonding, lone-pair electrons, and relatively small band gaps. These findings provide a practical strategy to search for excellent NLO and BPVE materials.

Visible nonlinear optical properties of tellurium and application as saturable absorber

Tellurium, a novel two-dimensional (2D) Group-VI material, has attracted much attention benefitting from its excellent electrical and optical properties. In this contribution, tellurium nanosheets were directly dispersed in toluene in the existence of polymer and were easily fabricated into flexible thin films. The nonlinear optical absorption properties of tellurium film at the visible band were studied by the open-aperture Z-scan method. For the application of laser modulation, we realized the passively Q-switched Pr: YLF laser over the spectrum of orange (605 nm), red (639 nm), deep red (721 nm) with tellurium film as the saturable absorber (SA). Furthermore, we also realized the passively Q-switched mode-locked laser at 639 nm with the tellurium SA. The research results indicate the tellurium thin film has remarkable nonlinear optical absorption properties and could be a promising optical modulator in the visible domain.

Thermally stable tungsten and titanium doped antimony tellurium films for phase change memory application

Sb2Te3 alloy, as one of the representative phase change base materials, is largely limited by its low thermal stability in the practical application. Here, both tungsten and titanium are used to improve the thermal stability of materials. After co-doping, the crystallization temperature is significantly increased, and the increment will be leveled up if more tungsten content was doped in the W-Ti-(Sb2Te3). Specifically, the amorphous-to-metastable FCC phase transition temperature is about 120 degrees C for pure Sb2Te3, approximately 230 degrees C for W0.03Ti0.045-(Sb2Te3)(0.925), and around 304 degrees C for W0.07Ti0.045-(Sb2Te3)(0.885), respectively. Beyond that, the second FCC-to-hexagonal phase transition temperature is also improved due to the addition of tungsten atoms, as obtained from the XRD analysis result. As observed in the bright images of W0.03Ti0.045-(Sb2Te3)(0.925), tungsten dopant can refine the crystalline grains effectively further, by utilizing transmission electron microscope. In the final, electrical characterization of phase change memory devices proves W0.03Ti0.045-(Sb2Te3)(0.925) owns excellent operation properties, such as higher-speed reversible write/erase, and better resistance stability of both high and low states, compared with the widely used Ge2Sb2Te5 phase change material.

“All-crystalline” phase transition in nonmetal doped germanium–antimony–tellurium films for high-temperature non-volatile photonic applications

Phase-change materials have attracted much attention in the past three years due to its wide applications in non-volatile optical fields. Although the amorphous-cubic phase transition of Ge2Sb2Te5 is used most commonly, it has problems such as poor thermal stability and large density variation because the amorphous phase is metastable and of low density. Since these issues have not been well addressed, non-volatile optics are currently limited to applications in mild environments (≤120 °C). In this paper, we replace the traditional phase transition (amorphous-cubic phase) with the “all-crystalline” phase transition (cubic-hexagonal phase) that has been ignored in the past, achieving combination of high optical contrast (ΔR=10.3%), high thermal stability (TL = 240 °C) and ultra-small density variation (Δρ = 0.5%). The maximum service temperature of non-volatile optics increases from 120 to 240 °C. Our results from experiments, theoretical calculations and spectral fittings are in good agreement, proving that the key to achieve “all-crystalline” phase transition with high optical contrast is to increase the difference in structural disorder between cubic and hexagonal phases. An effective method for increasing this disorder difference is the small atom (N or C) doping rather than large atom (Ag) doping that previously thought. The new insights have been well explained by discussing relationships among doping, formation energy, structural disorder and optical contrast. Therefore, our research not only proposes new methods and mechanisms for achieving high-performance “all-crystalline” phase transition, but also extends the applications of non-volatile optical devices from mild to high-temperature environments in the fields of aerospace and military.