InSe Films Research Articles

Self-assembly of a lateral quasi-Ohmic CuInSe2/InSe isotype heterojunction for flexible devices by pulsed laser deposition

The contacts between two-dimensional InSe films and metal electrodes play an important role in nanoelectronics flexible devices. Generally, there is a large work function difference between the Au and InSe films, which would form a Schottky contact to deteriorate device performances. Herein, we designed a lateral self-assembled CuInSe2/InSe isotype heterojunction on a flexible mica substrate by pulsed laser deposition, which could improve the contact performances between electrodes and InSe films. By combining the X-ray photoelectron spectroscopy and Kelvin probe force microscopy results, the In rich CuInSe2 and InSe regions could act as quasi-n+-n junctions for reduction of the contact resistance with electrodes. Compared to the InSe films with Au electrodes, the CuInSe2/InSe isotype heterojunction presents approximately half channel resistance and four times photocurrent values. Moreover, the heterojunction devices can still maintain relatively good performance under bending states by restraining the dark current. The present work proves the potential of CuInSe2/InSe isotype heterojunctions for flexible applications.

Morphological and nonlinear optical properties of Al:InSe thin films

Herein, an Al:InSe thin film is designed and synthesized by the direct current-radio frequency sputtering method. The obtained Al:InSe films possess smooth surface morphology with Al:InSe particles dispersion on sapphire substrates. Via aluminum (Al) doping, two-photon absorption and the doping energy level together induced free-carrier absorption, established on the conduction band. Al:InSe thin film displays a broad-band nonlinear absorptive band arising from 532 nm to 1064 nm, and exhibits effective third-order (βeff) and fifth-order nonlinear absorption coefficients (γeff) and relaxation time (τ) under femtosecond pulsed laser. This study provides a potential method for preparing Al:InSe thin film to investigate their nonlinear optical properties.

Инверсия типа проводимости тонких пленок n-InSe под действием лазерного излучения

AbstractConductivity inversion in thin n -InSe films under intense pulsed laser irradiation was obser. A p – n structure based on indium selenide formed between irradiated and nonirradiated regions of a thin-film sample. It was confirmed by EDAX analysis that the composition of the sample remained the same after irradiation. The conductivity inversion is attributed to a change in the dynamics of lattice defects under heating.

Pulsed-laser deposition of InSe thin films for the detection of thickness-dependent bandgap modification

Layer-structured InSe is one of the intensively studied two-dimensional monochalcogenide semiconductors for optical and electrical devices. Significant features of the InSe device are the thickness dependent bandgap modification resulting in a peak shift of photoluminescence and a drastic variation of electron mobility. In this study, by applying the pulsed-laser deposition technique, we investigated the optical and electrical properties of c-axis oriented InSe films with the thickness varying from a few to hundred nanometers. The energy at the absorption edge systematically shifts from about 3.3 to 1.4 eV with the increasing thickness. The InSe films on Al2O3(0001) are highly resistive, while those on InP(111) are conductive, which probably originates from the valence mismatch effect at the interface. The electron mobility of the conducting charge carrier at the interface of InSe/InP is enhanced in thicker samples than the critical thickness of about 10 nm, corresponding to the bandgap modification characterized by the optical measurement. Therefore, the substrate and the film thickness are critically important factors for the materialization of InSe optical and electrical device applications.

Infrared-to-violet tunable optical activity in atomic films of GaSe, InSe, and their heterostructures

Two-dimensional (2D) semiconductors—atomic layers of materials with covalent intra-layer bonding and weak (van der Waals or quadrupole) coupling between the layers—are a new class of materials with great potential for optoelectronic applications. Among those, a special position is now being taken by post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been found (Bandurin et al 2017 Nat. Nanotechnol. 12 223–7) that the band gap in 2D crystals of InSe more than doubles in the monolayer compared to thick multilayer crystals, while the high mobility of conduction band electrons is promoted by their light in-plane mass. Here, we use Raman and PL measurements of encapsulated few layer samples, coupled with accurate atomic force and transmission electron microscope structural characterisation to reveal new optical properties of atomically thin GaSe preserved by hBN encapsulation. The band gaps we observe complement the spectral range provided by InSe films, so that optical activity of these two almost lattice-matched PTMC films and their heterostructures densely cover the spectrum of photons from violet to infrared. We demonstrate the realisation of the latter by the first observation of interlayer excitonic photoluminescence in few-layer InSe/GaSe heterostructures. The spatially indirect transition is direct in k-space and therefore is bright, while its energy can be tuned in a broad range by the number of layers.

Synthesis of Large-Area InSe Monolayers by Chemical Vapor Deposition.

Recently, 2D materials of indium selenide (InSe) layers have attracted much attention from the scientific community due to their high mobility transport and fascinating physical properties. To date, reports on the synthesis of high-quality and scalable InSe atomic films are limited. Here, a synthesis of InSe atomic layers by vapor phase selenization of In2 O3 in a chemical vapor deposition (CVD) system, resulting in large-area monolayer flakes or thin films, is reported. The atomic films are continuous and uniform over a large area of 1 × 1 cm2 , comprising of primarily InSe monolayers. Spectroscopic and microscopic measurements reveal the highly crystalline nature of the synthesized InSe monolayers. The ion-gel-gated field-effect transistors based on CVD InSe monolayers exhibit n-type channel behaviors, where the field effect electron mobility values can be up to ≈30 cm2 V-1 s-1 along with an on/off current ratio, of >104 at room temperature. In addition, the graphene can serve as a protection layer to prevent the oxidation between InSe and the ambient environment. Meanwhile, the synthesized InSe films can be transferred to arbitrary substrates, enabling the possibility of reassembly of various 2D materials into vertically stacked heterostructures, prompting research efforts to probe its characteristics and applications.

Thickness-dependent transition of the valence band shape from parabolic to Mexican-hat-like in the MBE grown InSe ultrathin films

Using molecular beam epitaxy, InSe films of thicknesses from one to six quadruple layers were grown on Si(111). The surface morphology and structure of the InSe films were monitored using reflection high-energy electron diffraction and scanning tunneling microscopy observations. Angle resolved photoemission experiments revealed that the bulk-like parabolic shape of the valence band of InSe/Si(111) changes for the so-called “Mexican hat” shape when the thickness of the InSe film reduces to one and two quadruple layers. The observed effect is in a qualitative agreement with the reported calculation results on the free-standing InSe films. However, in the InSe/Si(111) system, the features used to characterize the Mexican hat dispersion appear to be more pronounced, which makes the one- and two-quadruple InSe layers on Si(111) promising candidates as thermoelectric materials.

Electrodeposition and Photocharacterization of In2Se3 Thin Films Using PP-ALD for Use As a Potential Photoanode

Indium (III) selenide is an n-type semiconductor that displays 4 main crystal structures. While the more prevalent α and β structures have a direct bandgap of 1.62 eV, the γ structure has been found to have a bandgap of 1.9 eV.1,2 In current work, In2Se3 films were deposited using a pulse potential atomic layer deposition (PP-ALD) technique derived from E-ALD that relies upon the principles of ALD and underpotential deposition (UPD) to create ordered atomic layers. In PP-ALD both precursors, In and Se, are present in one solution that is pumped through the flow cell introducing the ions to the substrate. A cathodic pulse is then used to reductively deposit limited amounts of In and Se on the substrate surface, followed by an anodic pulse used to oxidatively strip any bulk In away from the surface leaving atomic layers of In2Se3. At pH=1, the deposition process creates bubbles in the flow cell that lead to regions of limited deposition. However, at pH=2, less bubbles are formed during the deposition process, leading to more uniform samples. Further investigations into the optimal cathodic and anodic deposition potentials for the PP-ALD program at pH=2 need to be done. These In2Se3 films have been demonstrated to have good photocurrent and have potential as an n-type photoanode in photoelectrochemical cell applications. Czerniawski, J.; Stickney, J.; Electrodeposition of In2Se3 Using Potential Pulse Atomic Layer Deposition. J. Phys. Chem. C 2016. 120, 16162-16167.Chang, K.; Lahn, S.; Chang, J. Growth of single-phase In2Se3 by using metal organic vapor deposition with dual-source precursors. Appl. Phys. Lett. 2006. 89, 182118.

Hybrid k·p tight-binding model for intersubband optics in atomically thin InSe films

We propose atomic films of n-doped $\gamma$-InSe as a platform for intersubband optics in the infrared (IR) and far infrared (FIR) range, coupled to out-of-plane polarized light. Depending on the film thickness (number of layers) of the InSe film these transitions span from $\sim 0.7$ eV for bilayer to $\sim 0.05$ eV for 15-layer InSe. We use a hybrid $\mathbf{k} \cdot \mathbf{p}$ theory and tight-binding model, fully parametrized using density functional theory, to predict their oscillator strengths and thermal linewidths at room temperature.

A flexible, transparent and high-performance gas sensor based on layer-materials for wearable technology

Gas sensors play a vital role among a wide range of practical applications. Recently, propelled by the development of layered materials, gas sensors have gained much progress. However, the high operation temperature has restricted their further application. Herein, via a facile pulsed laser deposition (PLD) method, we demonstrate a flexible, transparent and high-performance gas sensor made of highly-crystalline indium selenide (In2Se3) film. Under UV–vis-NIR light or even solar energy activation, the constructed gas sensors exhibit superior properties for detecting acetylene (C2H2) gas at room temperature. We attribute these properties to the photo-induced charger transfer mechanism upon C2H2 molecule adsorption. Moreover, no apparent degradation in the device properties is observed even after 100 bending cycles. In addition, we can also fabricate this device on rigid substrates, which is also capable to detect gas molecules at room temperature. These results unambiguously distinguish In2Se3 as a new candidate for future application in monitoring C2H2 gas at room temperature and open up new opportunities for developing next generation full-spectrum activated gas sensors.

Calculated carrier mobility of h-BN/γ-InSe/h-BN van der Waals heterostructures

Recent experiments reported excellent transport properties of two-dimensional (2D) van der Waals (vdW) heterostructures made of atomically thin InSe layers encapsulated by two hBN capping layers (ISBN). The carrier mobility of the ISBN films exceeded at low temperature, much higher than that of pristine InSe films. It has been puzzling why the relatively inert hBN capping layer could so drastically enhance mobility of the ISBN composite. Using a state-of-the-art first principles method, we have calculated phonon limited carrier mobility of 18 different ISBN films and 6 pristine InSe films with different thicknesses, the largest system containing 2212 atoms. The hBN capping layer significantly alters the elastic stiffness coefficient as compared with pure InSe—thus the acoustic phonons in the ISBN composite—giving rise to the observed large mobility of ISBN films. Of the 18 calculated ISBN films, the ones with no strain at the hBN/InSe interface possess the highest electron mobility, reaching at room temperature, which could easily go over at low temperatures. We conclude that the mechanical properties of the composite 2D vdW ISBN material play the crucial role for inducing the large carrier mobility, a principle that could be applied to many other 2D vdW heterostructures.

Growth and characterization of InSe/Ge/InSe interfaces

In the current study, we report the effect of insertion of a 200nm thick Ge film between two layers of InSe. The Ge sandwiched InSe films are studied by means of X-ray diffraction technique, energy dispersion X-ray spectroscopy attached to a scanning electron microscope, optical spectrophotometry and light power dependent photoconductivity. It was observed that, The InSe prefers the growth of InSe monophase when deposited onto glass and the growth of γ−In2Se3 when deposited onto InSe/Ge substrate. The three layers interface (InSe/Ge/γ−In2Se3) exhibits a Ge induced crystallization process at annealing temperature of 200°C. The optical analysis has shown that the InSe films exhibit a redshift upon Ge sandwiching. In addition, the conduction and valence bands offsets at the first interface (InSe/Ge) and at the second (Ge/γ−In2Se3) interface are found to be 0.55eV and 1.0eV, and 0.40eVand 1.38eV, respectively. Moreover, the photocurrent of the Ge sandwiched InSe exhibited higher photocurrent values as compared to those of InSe. On the other hand, the dielectric spectral analysis and modeling which lead to the identifying of the optical conduction parameters presented by the plasmon frequency, electron scattering time, free electron density and drift mobility have shown that the Ge sandwiching increased the drift mobility values from 10cm2/Vs to ∼42cm2/Vs. The main plasmon frequency also increased from 1.08 to 1.68GHz.

Effect of Au nanosandwiching on the structural, optical and dielectric properties of the as grown and annealed InSe thin films

In the current work, the structural, optical and dielectric properties of the InSe/Au/InSe nanosandwiched structures are investigated by means of X-ray diffraction and UV–visible light spectrophotometry techniques. The insertion of a 20 and 100nm thick Au metal slabs between two InSe layers did not alter the amorphous nature of the as grown InSe films but decreased the energy band gap and the free carrier density. It also increased; the absorption ratio and the values of dielectric constant by ~ 3 times. The insertion of 100nm Au layers as a nanosandwich enhanced the drift mobility (31.3cm2/Vs) and plasmon frequency (1.53GHz) of the InSe films. On the other hand, upon annealing, a metal induced crystallization process is observed for the InSe/Au (100nm)/InSe sandwiches. Particularly, while the samples sandwiched with a layer of 20nm thickness hardly revealed hexagonal γ−In2Se3 when annealed at 300°C, those sandwiched with 100nm Au slab, displayed well crystalline phase of hexagonal γ−In2Se3 at annealing temperature of 200°C. The further annealing at 300°C, forced the appearing of the orthorhombic In4Se3 phase. Optically, the annealing of the InSe/Au(100nm)/InSe at 200°C improved the absorption ratio by ~ 9 times and decreased the energy band gap. The nanosandwiching technique of InSe seems to be promising for the engineering of the optical properties of the InSe photovoltaic material.

Architecture of monophase InSe thin film structures for solar cell applications

Control of microstructural evolution during the crystallization of InSe thin films is an inevitable strategy to mold their fundamental properties and potential for the fabrication of solar cells. Impact of annealing as well as substrate temperature on the crystallization progress and physical characteristics of thermally evaporated InSe was examined systematically, which eventually dictates the overall performance of resulting device. Structural and compositional characterizations have been thoroughly investigated by X-ray diffraction and energy dispersive X-ray analyses. InSe films form hexagonal structure with a preferred orientation of crystallites along the (004) direction upon crystallization. The layer of InSe is formed by two concomitant processes, deposition and recrystallization. Application of heat treatment resulted in topographical modification, which was probed by an atomic force microscope. Surface roughness was enhanced due to the influence of temperature and thereby the growth of grains. Investigations of electrical and optical properties, thus provided ample evidence for the use of crystallized monophase InSe as an absorber layer in photovoltaic conversion devices. Carrier concentration and mobility of charge carriers estimated from the Hall measurements were found to be 19.43×1020cm−3 and 2.01cm2V−1s−1 respectively. Moreover, this research work explores power conversion efficiency of p-InSe/n-CdS heterojunction solar cells.

Indium selenide microrod films: chemical bath deposition from acidic bath

Films of indium selenide was deposited onto glass and indium tin oxide coated glass substrates in acidic medium using chemical bath deposition at room temperature. Indium sulphate and sodium selenosulphate were used as precursors of In3+ and Se2−, respectively. The structural, surface morphological and optical properties of the deposited films were studied. Diffractograms in structural study revealed the deposited material is In2Se3 films. Controlled bath conditions resulted in the evolution of the In2Se3 microrod-like morphology. The optical band gap of the film was found to be 1.7 eV.

Optical properties of In2Se3 thin films

In2Se3 films are produced by ion-beam evaporation at substrate temperatures of 313 and 623 K. As the target, In2Se3 single crystals grown by the vertical Bridgman method are used. The composition and structure of the crystals and films are determined by the X-ray spectral analysis and X-ray diffraction techniques, respectively. It is established that the crystals and films crystallize with the formation of a hexagonal structure. The band gap and refractive index of the In2Se3 films are determined from the transmittance and reflectance spectra. It is found that, as the substrate temperature is increased, the band gap increases.

Sensitive Electronic-Skin Strain Sensor Array Based on the Patterned Two-Dimensional α-In2Se3

Two-dimensional (2D) layered semiconductors have emerged as a highly attractive class of materials for flexible and wearable strain sensor-centric devices such as electronic-skin (e-skin). This is primarily due to their dimensionality, excellent mechanical flexibility, and unique electronic properties. However, the lack of effective and low-cost methods for wafer-scale fabrication of these materials for strain sensor arrays limits their potential for such applications. Here, we report growth of large-scale 2D In2Se3 nanosheets by templated chemical vapor deposition (CVD) method, using In2O3 and Se powders as precursors. The strain sensors fabricated from the as-grown 2D In2Se3 films show 2 orders of magnitude higher sensitivity (gauge factor ∼237 in −0.39% to 0.39% uniaxial strain range along the device channel length) than what has been demonstrated from conventional metal-based (gauge factor: ∼1–5) and graphene-based strain sensors (gauge factor: ∼2–4) in a similar uniaxial strain range. The integrated ...

Deposition conditions, composition, and structure of chemically deposited In2Se3 films

In2Se3 films up to 300 nm thick have been obtained for the first time by hydrochemical deposition on glass, glass ceramic, and molybdenum substrates in the In(NO3)3–C4O6H6–CSeN2H4 system with the use of selenourea as a chalcogenizing agent. The phase and element composition and morphological features of layers obtained at 353 and 363 K have been studied by X-ray photoelectron spectroscopy, energy-dispersive electron probe X-ray microanalysis, and scanning electron microscopy. The optical band gap width has been determined.

Absorption and optical conduction in InSe/ZnSe/InSe thin film transistors

In this work, (n)InSe/(p)ZnSe and (n)InSe/(p)ZnSe/(n)InSe heterojunction thin film transistor (TFT) devices are produced by the thermal evaporation technique. They are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion X-ray spectroscopy and optical spectroscopy techniques. While the InSe films are found to be amorphous, the ZnSe and InSe/ZnSe films exhibited polycrystalline nature of crystallization. The optical analysis has shown that these devices exhibit a conduction band offsets of 0.47 and valence band offsets of 0.67 and 0.74[Formula: see text]eV, respectively. In addition, while the dielectric spectra of the InSe and ZnSe displayed resonance peaks at 416 and 528[Formula: see text]THz, the dielectric spectra of InSe/ZnSe and InSe/ZnSe/InSe layers indicated two additional peaks at 305 and 350[Formula: see text]THz, respectively. On the other hand, the optical conductivity analysis and modeling in the light of free carrier absorption theory reflected low values of drift mobilities associated with incident alternating electric fields at terahertz frequencies. The drift mobility of the charge carrier particles at femtoseconds scattering times increased as a result of the ZnSe sandwiching between two InSe layers. The valence band offsets, the dielectric resonance at 305 and 350[Formula: see text]THz and the optical conductivity values nominate TFT devices for use in optoelectronics.

Properties of Se/InSe Thin-Film Interface

Se, InSe, and Se/InSe thin films have been prepared by the physical vapor deposition technique at pressure of ∼10−5 torr. The structural, optical, and electrical properties of the films and Se/InSe interface were investigated by means of x-ray diffraction (XRD) analysis, ultraviolet–visible spectroscopy, and current–voltage (I–V) characteristics. XRD analysis indicated that the prepared InSe films were amorphous while the Se films were polycrystalline having hexagonal structure with unit cell parameters of a = 4.3544 A and c = 4.9494 A. Spectral reflectance and transmittance analysis showed that both Se and InSe films exhibited indirect allowed transitions with energy bandgaps of 1.92 eV and 1.34 eV, respectively. The Se/InSe interface exhibited two energy bandgaps of 0.98 eV and 1.73 eV above and below 2.2 eV, respectively. Dielectric constant values were also calculated from reflectance spectra for the three layers in the frequency range of 500 THz to 272 THz. The dielectric constant exhibited a promising feature suggesting use of the Se/InSe interface as an optical resonator. Moreover, the Au/Se/InSe/Ag heterojunction showed some rectifying properties that could be used in standard optoelectronic devices. The ideality factor and height of the energy barrier to charge carrier motion in this device were found to be 1.72 and 0.66 eV, respectively.