Tellurium Thin Films Research Articles

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.

Correlation between structural, morphological, and electrical transport properties of nano-dimensional Tellurium thin film by low energy ion beam irradiation

Tellurium (Te), an emerging semiconductor material, exhibits intriguing physical properties in low-dimensional forms. This study investigates the structural, morphological, and transport properties of low-dimensional Te thin film deposited by physical vapor deposition after 400 keV Kr2+ ion irradiation with varying fluences of 1 × 1013, 5 × 1013, and1 × 1014 ions-cm−2. The X-ray diffraction (XRD) pattern reveals the hexagonal phase of pristine Te. As ion fluence increases, intensity of the XRD peaks decreases and the full-width half maxima (FWHM) increases, indicating a reduction in crystallinity. The intensity of the Raman peak decreases upon irradiation, resulting in a red shift in the spectra of the irradiated sample. The evolution of morphology at different fluences has been examined with atomic force microscopy (AFM) measurements. The results indicate the segregation of grains and an increase in root mean square (RMS) roughness from 2.59 pm to 5.11 pm with ion fluence up to 5 × 1013 ions-cm−2. At the highest fluence, i.e., 1 × 1014 ions-cm−2, there is an agglomeration of grains, and hence a decrease in RMS roughness to 3.92 pm is observed. The DC resistivity measurement indicates the semiconducting nature of all films. Additionally, this measurement signifies the rapid decrease in activation energy at both band-to-band conduction and nearest neighbour hopping (NNH) conduction. Following irradiation, Hall measurement demonstrates a decrease in the concentration of charge carriers and an increase in resistivity due to scattering originating from grain boundaries. However, when the fluence is increased to 1 × 1014 ions-cm−2, there is a reduction in resistivity and an increase in carrier concentration with enhanced Hall mobility in comparison to pristine Te. Therefore, this study showcases that low-energy ion irradiation has a significant impact on the modification of the structure, morphology, and electrical transport properties of semiconducting Te thin film.

Atomic Layer Deposition Route to Scalable, Electronic-Grade van der Waals Te Thin Films.

Scalable production and integration techniques for van der Waals (vdW) layered materials are vital for their implementation in next-generation nanoelectronics. Among available approaches, perhaps the most well-received is atomic layer deposition (ALD) due to its self-limiting layer-by-layer growth mode. However, ALD-grown vdW materials generally require high processing temperatures and/or additional postdeposition annealing steps for crystallization. Also, the collection of ALD-producible vdW materials is rather limited by the lack of a material-specific tailored process design. Here, we report the annealing-free wafer-scale growth of monoelemental vdW tellurium (Te) thin films using a rationally designed ALD process at temperatures as low as 50 °C. They exhibit exceptional homogeneity/crystallinity, precise layer controllability, and 100% step coverage, all of which are enabled by introducing a dual-function co-reactant and adopting a so-called repeating dosing technique. Electronically, vdW-coupled and mixed-dimensional vertical p-n heterojunctions with MoS2 and n-Si, respectively, are demonstrated with well-defined current rectification as well as spatial uniformity. Additionally, we showcase an ALD-Te-based threshold switching selector with fast switching time (∼40 ns), selectivity (∼104), and low Vth (∼1.3 V). This synthetic strategy allows the low-thermal-budget production of vdW semiconducting materials in a scalable fashion, thereby providing a promising approach for monolithic integration into arbitrary 3D device architectures.

A step toward calculating the uncertainties in combined GIXRF‐XRR

AbstractThe combination of X‐ray reflectivity (XRR) and grazing incidence X‐ray fluorescence (GIXRF) is a surface sensitive analytical method, which can be used for the characterization of thin films and multilayered materials. Both of these techniques are implemented on the same experimental setup and make use of similar mechanical processes and the same fundamental physical concept required for a combined data analysis. The combination of these techniques removes ambiguous results for the characterization of nanometer layers, as well as nanometer depth profiles, resulting in more accurate characterization of thickness, roughness, density, and elemental composition. Due to the vast number of fitting parameters, the estimation of the thin film sample structure is a challenging task. In this paper, we propose a recursive method for estimating the uncertainties of data from GIXRF‐XRR analysis, based on a Bootstrap statistical method. This approach relies on re‐sampling a dataset to estimate statistics on a population by applying random weights. We applied this method on an as‐deposited chalcogenide germanium, antimony, and tellurium (GST) thin film with a carbon‐capping layer. We found good agreement between the experimental and the theoretical XRR‐GIXRF values for a sample structure model, of which the parameters were determined within a confidence interval using the bootstrap method. We also propose an approach for calculating the uncertainty on the solid angle of detection based on Monte Carlo simulations.

Platform-agnostic waveguide integration of high-speed photodetectors with evaporated tellurium thin films

Many attractive photonics platforms still lack integrated photodetectors due to inherent material incompatibilities and lack of process scalability, preventing their widespread deployment. Here, we address the problem of scalably integrating photodetectors in a photonics-platform-independent manner. Using a thermal evaporation and deposition technique developed for nanoelectronics, we show that tellurium, a quasi-2D semi-conductive element, can be evaporated at low temperatures directly onto photonic chips to form air-stable, high-speed, ultrawide-band photodetectors. We demonstrate detection from visible (520 nm) to short-wave infrared (2.4 µm), a bandwidth of more than 40 GHz, and platform-independent scalable integration with photonic structures in silicon, silicon nitride, and lithium niobate.

High-Performance Hexagonal Tellurium Thin-Film Transistor Using Tellurium Oxide as a Crystallization Retarder

This study investigates the effect of oxygen plasma (PO) on the crystalline structure of tellurium (Te) thin films during reactive sputtering. Introduction of oxygen radicals suppresses uncontrolled rapid growth of hexagonal Te crystals, amorphizing the deposited Te thin film. This amorphous phase changes to the hexagonal phase upon alumina encapsulation. A 4-nm-thick Te transistor with a PO of 7% exhibits outstanding device performances, with a field-effect mobility up to 40.8 cm2V−1s−1 and an on/off current modulation ratio up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.1\times 10^{{6}}$ </tex-math></inline-formula> . These behaviors originate from alleviated random polycrystallinity in the corresponding thin film. However, when PO increases above 7%, amorphization progresses further, and remnant oxygen ions hamper the growth of the hexagonal phase in Te thin film. Consequently, hole transport is degraded. This study suggests tellurium oxide as a crystallization retarder for high-performance p-channel Te transistors.

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.

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.

Growth Mechanism, Structural and Photoelectrochemical Study of Zinc Tellurium Thin Film

Chalcogenides II-VI group of crystalline material zinc tellurium thin films have been synthesized by chemical bath deposition (CBD) method in which glass substrates have been used for deposition technique in presence of triethanolamine solution. The as-synthesized films were characterization by X-ray diffraction scanning electron microscopy (XRD), optical spectroscopy and thermoelectric techniques. The deposited material was orthorhombic in shape. The 2.01 eV is the optical band gap energy of the sample and the electrical conductivity of the thin film was originating in the order of 10-8 (Ω cm)-1, which showed n-type thermoelectrical conductivity. The solar cell can be represented as a n-ZnTe|NaI (0.1M) + I2 (0.1M)|C(graphite). The solar efficiency of the cell was found to be 1.03%.

Stable two-dimensional pentagonal tellurene: A high ZT thermoelectric material with a negative Poisson’s ratio

Motivated by the recent experimental exfoliation of tellurium thin film, and the amazing physical and chemical properties of various theoretically tellurene allotropes, we design a novel and stable pentagonal structure of tellurene (SP-type Te), and make a detailed comparison with other possible Te allotropes, related to their mechanical, electronic and thermoelectric properties. SP-type Te shows the higher expandability than other Te allotropes, due to its lower layer modulus and Young’s modulus. More interestingly its Poisson’s ratio along x (y) direction is unusually negative (~ −0.01) which can be further decreased by applying coaxial strain. Three-layers SP-type Te can turn into a topological insulator (TI) with a nontrivial band gap ~ 0.03 eV when uniaxial compressional strain σx = −7%. Moreover the higher the number of layers in SP-type Te, the lower σx inducing its topological transition requires. At room temperature, lattice thermal conductivity kL of SP-type Te is the lowest in all Te allotropes, and meanwhile its peak ZT for n-type doping even can reach up to 2.84. Thus if SP-type Te can be synthesized in future, these extraordinary properties would promote it with great potential in designing low-dimensional mechanical, spintronic and thermoelectric devices.

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.

Gas-sensing features of nanostructured tellurium thin films.

Nanocrystalline and amorphous nanostructured tellurium (Te) thin films were grown and their gas-sensing properties were investigated at different operating temperatures with respect to scanning electron microscopy and X-ray diffraction analyses. It was shown that both types of films interacted with nitrogen dioxide, which resulted in a decrease of electrical conductivity. The gas sensitivity, as well as the response and recovery times, differed between these two nanostructured films. It is worth mentioning that these properties also depend on the operating temperature and the applied gas concentration on the films. An increase in the operating temperature decreased not only the response and recovery times but also the gas sensitivity of the nanocrystalline films. This shortcoming could be solved by using the amorphous nanostructured Te films which, even at 22 °C, exhibited higher gas sensitivity and shorter response and recovery times by more than one order of magnitude in comparison to the nanocrystalline Te films. These results were interpreted in terms of an increase in disorder (amorphization), leading to an increase in the surface chemical activity of chalcogenides, as well as an increase in the active surface area due to substrate porosity.

Evaporated tellurium thin films for p-type field-effect transistors and circuits.

There is an emerging need for semiconductors that can be processed at near ambient temperature with high mobility and device performance. Although multiple n-type options have been identified, the development of their p-type counterparts remains limited. Here, we report the realization of tellurium thin films through thermal evaporation at cryogenic temperatures for fabrication of high-performance wafer-scale p-type field-effect transistors. We achieve an effective hole mobility of ~35 cm2 V-1 s-1, on/off current ratio of ~104 and subthreshold swing of 108 mV dec-1 on an 8-nm-thick film. High-performance tellurium p-type field-effect transistors are fabricated on a wide range of substrates including glass and plastic, further demonstrating the broad applicability of this material. Significantly, three-dimensional circuits are demonstrated by integrating multi-layered transistors on a single chip using sequential lithography, deposition and lift-off processes. Finally, various functional logic gates and circuits are demonstrated.

Towards flexible CMOS circuits.

Tellurium thin films evaporated at cryogenic temperatures facilitate the realization of high performance wafer-scale flexible p-type field-effect transistors and various types of logic gates.

(Invited) High-Performance 2D Tellurium Transistors Towards CMOS Logic Applications

The thirst for novel two-dimensional (2D) materials for ultra-scaled electronics keeps growing during the last decade as the downsizing trend of silicon-based conventional semiconductors is approaching its physical limit. An ideal paradigm of 2D materials suitable for logic device applications should possess high carrier mobility for both type of carriers, good air-stability, controllable doping scheme and a reasonable bandgap. Recently reported hydrothermal growth technique of 2D tellurium (Te) thin films, which meet all the above criteria, offers a new option for 2D material-based CMOS applications. Tellurium is a narrow bandgap p-type semiconductor (0.35 eV) with nearly symmetric mobilities for both electrons and holes of around 700 cm2/Vs at room temperature. It has one-dimensional chiral crystal structure and can be grown into 2D films under right conditions. Just like other 2D materials, these 2D facets of Te also contain no dangling bonds and therefore the 2D films can preserve the material properties and have great potential for miniaturizing device geometry in principle. Here we first report systematic investigation of 2D Te p-type transistor performance approaching atomic-thin limit. The device key parameters, such as field-effect mobility, on/off ratio and stability were thoroughly studied as a function of film thickness. The device performance was then optimized through contact and dielectric engineering, and large drive current over 1 A/mm was achieved. We further employed photo-current mapping method to show that rare accumulation-type ohmic contacts were formed using high work function metal (palladium). Finally, we present an atomic layer deposited (ALD) dielectric doping technique to effectively dope the intrinsically p-type channel into n-type with almost symmetric operations for both NMOS and PMOS. Prototypical inverters based on Te CMOS demonstrate great potential of constructing Te-based high-speed CMOS logic circuits. Our work not only established a comprehensive guideline for designing and optimizing 2D Te based CMOS devices, but also provided a device platform to explore versatility of 2D Te from many other perspectives, such as condensed matter physics and thermoelectronics.

In-depth evolution of tellurium films deposited by Frequency Assisted Thermal Evaporation in Vacuum (FATEV)

In order to enlighten the in-depth organization of thin tellurium films, deposited by the frequency assisted thermal evaporation in vacuum (FATEV) approach, morphological, structural and spectroscopic investigations were performed. Spectroscopic ellipsometry (SE) and cross-section scanning electron microscopy (SEM) showed a change of the growth mechanism upon application of higher vibrations’ input frequencies during the films deposition. The films, deposited by conventional thermal evaporation in vacuum with no vibrations applied, as well as these, deposited by FATEV with application of 50 Hz vibrations, were characterized by initial densification followed by 2D nanoparticles growth when a certain threshold thickness was reached. On the other hand, the high-frequency vibrations of 4 and 10 kHz preconditioned growth of tellurium nanoribbons oriented towards the z-axis from the very beginning of the film formation. The topography changes, observed by atomic force microscopy (AFM) and scanning electron microscopy, showed highly porous surfaces (with high root mean square surface roughness) formed by distinct nanoblades for the films, deposited at low input frequencies, while the films, deposited under the impact of vibrations in the kilohertz range, were much more ordered, and hence their surface was significantly smoother. The structural parameters of the samples were investigated by X-ray diffraction (XRD) analysis.

Thermoelectric Performance of 2D Tellurium with Accumulation Contacts.

Tellurium (Te) is an intrinsically p-type-doped narrow-band gap semiconductor with an excellent electrical conductivity and low thermal conductivity. Bulk trigonal Te has been theoretically predicted and experimentally demonstrated to be an outstanding thermoelectric material with a high value of thermoelectric figure-of-merit ZT. In view of the recent progress in developing the synthesis route of 2D tellurium thin films as well as the growing trend of exploiting nanostructures as thermoelectric devices, here for the first time, we report the excellent thermoelectric performance of tellurium nanofilms, with a room-temperature power factor of 31.7 μW/cm K2 and ZT value of 0.63. To further enhance the efficiency of harvesting thermoelectric power in nanofilm devices, thermoelectrical current mapping was performed with a laser as a heating source, and we found that high work function metals such as palladium can form rare accumulation-type metal-to-semiconductor contacts to Te, which allows thermoelectrically generated carriers to be collected more efficiently. High-performance thermoelectric Te devices have broad applications as energy harvesting devices or nanoscale Peltier coolers in microsystems.

Uticaj brzine depozicije i mikrostrukture substrata na gasnu osetljivost Te - tankih filmova

Tellurium thin films have been prepared using different rates (0.1 ÷ 30 nm/s) by physical deposition in vacuum on glassy, sintered alumina and electrochemically nanostructured Al2O3 substrates. The sensitivity to nitrogen dioxide of fabricated films was tested at room temperature. It is shown that the deposition rate strongly influences the microstructure of the films in question, as well as their gas sensing properties. The increasing of deposition rate results in transformation of microcrystalline structure of the film into an amorphous one. Simultaneously, both the gas - sensitivity and the response time decrease. The results are explained in terms of interaction between gas molecule and lone - pair electrons of tellurium atoms.

Rice-like tellurium thin films deposited by a galvanic displacement reaction and ultra-high sensing response to hydrogen sulfide (H2S) gas at room temperature

In this study, we present the synthesis of rice-like tellurium (Te) thin films using a galvanic displacement reaction (GDR, a substrate–sacrificial, electro-less deposition technique) on a silicon wafer and their response to hydrogen sulfide (H2S) gas at room temperature. The thin films were composed of rice-like Te nano grains, whose thickness and crystallinity were controlled by adjusting the reaction time of the GDR. As the reaction time increased, the as-fabricated rice-like Te-based gas sensors showed full response and recovery. The sensors with an optimized GDR showed ultra-high sensing response to H2S (∼1000% at 100 ppm of H2S) under ambient sensing conditions, with excellent response toward H2S gas (ranging from 15 ppb to 90 ppm). Additionally, the optimized sensor shows relatively high response toward H2S gas as compared to other gases such as NO2, NH3, and CO. Together, these results indicate that the rice-like tellurium (Te) thin films have great potential for applications in high-performance H2S gas sensors that are capable of operating at room temperature.

Investigation of fast and sizeable photostriction effect in tellurium thin films using fiber Bragg grating sensors

We report a sizeable photoinduced strain in 90 nm tellurium (Te) thin film, coated on a fiber Bragg grating (FBG) sensor. The Bragg wavelength shift of the FBG sensor is used as the probe, to understand the photostrictive properties of Te thin film under illumination with laser light of visible wavelengths (405, 532 and 633 nm), at varying optical power density (5–161 mW/mm2), and also during cyclic exposure. An induced elastic strain of the order of 10−5 to 10−4 has been observed which is found to increase with increasing wavelength of laser illumination. The high (1 pm) resolution of the FBG interrogator used facilitates the accurate detection of the elastic strain induced in Te thin films even at a short exposure time of 0.1 s.