GeSe Films Research Articles

Regulating the crystal orientation of vapor-transport-deposited GeSe thin films by a post-annealing treatment

Recently, GeSe has emerged as a highly promising photovoltaic absorber material due to its excellent optoelectronic properties, nontoxicity, and high stability. Although many advantages make GeSe well suited for thin-film solar cells, the power conversion efficiency of the GeSe thin-film solar cell is still much below the theoretical maximum efficiency. One of the challenges lies in controlling the crystal orientation of GeSe to enhance solar cell performance. The two-step preparation of GeSe thin films has not yet been reported to grow along the [111] orientation. In this work, we study the effect of a post-annealing treatment on the GeSe thin films and the performance of the solar cells. It was found that amorphous GeSe films can be converted into polycrystalline films with different orientations by changing the post-annealing temperature. [111]-oriented and [100]-oriented GeSe thin films were successfully prepared on the same substrate by optimizing the annealing conditions. With the structure of Au/GeSe/CdS/ITO cell devices, PCEs of 0.14% and 0.16% were ultimately achieved.

Thermal atomic layer deposition of ternary Ge-S-Se alloy for advanced ovonic threshold switch selectors in three-dimensional cross-point memory array

Two-terminal ovonic threshold switch (OTS) selectors based on chalcogenide alloys have attracted significant attention as alternatives to conventional silicon-based transistors. These selectors offer outstanding features that enable the mass production of three-dimensional cross-point arrays for advanced memory technologies. In this study, we developed a thermal atomic layer deposition (ALD) process for ternary Ge-S-Se alloys. By utilizing ALD super-cycles of Ge-Se and Ge-S, a broad range of film compositions can be achieved. We investigated the electrical characteristics and film properties of the deposited films, with a focus on their smooth surfaces and excellent conformality, which are indispensable characteristics for future vertical cross-point structures. By incorporating S into the Ge-Se film via an ALD super-cycle, we achieved a significant reduction in off-current of Ge0.50S0.15Se0.35 to approximately 35 nA. Additionally, the thermal stability of Ge0.50S0.15Se0.35 film is enhanced up to 420 ℃ due to the higher bandgap and shorter bonding length of S, exhibiting a cyclic endurance of more than 5 × 107. This research not only provides insight into the control of the film composition to improve and modify device characteristics but also constructs a foundation of ALD-based OTS research on ternary Ge-S-Se for a future X-point memory application.

Preparation of Mixed Few-Layer GeSe Nanosheets with High Efficiency by the Thermal Sublimation Method.

Two-dimensional (2D) GeSe has been proven promising in fast and broadband optoelectronic applications for its complicated band structure, inert surface property, and excellent stability. The major challenge is the deficiency of the effective technique for controllably prepared large-scale few-to-monolayer GeSe films. For this purpose, a layer-by-layer thinning method by thermal sublimation for manufacturing large-scale mixed few-layer GeSe with direct bandgaps is proposed, and an optimized sublimation temperature of 300 °C in vacuum is evaluated by atomic force microscopy. Scanning electron microscopy, transmission electron microscopy, energy-dispersive spectra, and fluorescence mapping measurements are performed on the thinned GeSe layers, and results are well-indexed to the orthorhombic lattice structure with direct bandgaps with an atomic ratio of Ge/Se ≈ 5:4. Raman and fluorescence spectra show an α-type crystalline structure of the thinned GeSe films, indicating the pure physical process of the sublimation thinning. Both the bulk and few-layer GeSe films demonstrate broadband absorption. Conductivity of the few-layer GeSe device indicates the overall crystalline integrity of the film after thermal thinning. Given the convenience and efficiency, we provide an effective approach for fabrication of large-scale 2D materials that are difficult to be prepared by traditional methods.

Ultrasonication‐Assisted Seed Screening Enables Oriented and Efficient Low‐Dimensional Crystal‐Structural Thin‐Film Photovoltaics

AbstractUltrasonication‐assisted liquid exfoliation of layered materials is widely used to produce a large number of 2D nanosheets due to its simplicity, universality, and mass production. Here, the utility of ultrasonication is extended from liquid exfoliation to seed screening in the case of layered GeSe and GeS. It is found that the ultrasonic stripping force can also be used to selectively remove the lying seeds interacting with substrate through weak van der Waals (vdW) forces while retaining the standing seeds connected with substrate via tightly covalent bonds. The standing seeds then induce the growth of 2D crystal‐structural films with standing orientation. This thereby enables efficient carrier transport within covalently bonded layers, instead of poor transport between layers held together by vdW forces. The resulting standing‐oriented GeSe films exhibit a 21‐times increase in carrier mobility compared to lying‐oriented films. As a result, this study demonstrates the highest power conversion efficiencies of 6.1% and 10.5% under AM1.5G 1‐ and 0.01‐sun illumination reported for GeSe solar cells, respectively.

Nonlinear optical response of polycrystalline GeSe films

GeSe exhibits in-plane optical anisotropy similar to black phosphorus and other two-dimensional materials, so it is believed to have remarkable nonlinear optical (NLO) properties. In this work, amorphous GeSe films were prepared by vacuum thermal evaporation. After annealing at 450 °C, GeSe films exhibited a polycrystalline phase. The NLO properties of these GeSe films were investigated by the Z-scan technique. The results indicated that GeSe films exhibited typical reverse saturable absorption (RSA) and self-focusing phenomenon. It was also found that the nonlinear optical parameters and laser-induced damage threshold of annealed recrystallized GeSe films were significantly improved. The nonlinear absorption and refractive coefficients were β = (2.90 ± 0.06)×10−7 m/W and n2= (1.06 ± 0.07)×10−13 m2/W at the incident laser intensity of 0.4 GW/cm2, respectively. The polycrystalline GeSe film showed remarkable RSA-induced optical limiting properties, which exhibited an optical limiting starting threshold of 0.018 GW/cm2 and an optical limiting threshold of 0.98 GW/cm2. These results strongly indicate that GeSe films have potential applications in solid-state optical limiting devices and laser protection.

Ab initio study of the piezoelectric effects of the 2D semiconductors of IV group monochalcogenides (GeSe, GeS)

In this article, the piezoelectric effects of the 2D semiconductors of IV group monochalcogenides (GeS, GeSe) are studied by methods of the density functional theory and first-principles pseudopotential based on own program code. The spatial distribution of valence electron density, density of states, and Coulomb potential along transverse direction are calculated. It was found that a more significant rearrangement of the electron density was observed during compression of both GeS and GeSe bi-layer films than mono-layer films, but more significant in GeS films.

Transmittance oscillation in amorphous semiconductors: Revisited

Transmittance self-oscillation discovered in amorphous GeSe2 films has attracted considerable interest, while its underlying mechanism remains controversial. We suggest that the oscillation is governed by at least two kinds of changes; one occurs in absorption for thin ([Formula: see text]m) films and the other in refractive-index for thick ([Formula: see text][Formula: see text]mm) plates. We also detail a model, which assumes recoverable micro-crystallization. An idea for increasing the oscillation frequency is proposed for possible applications.

Synthesis of Large-Area GeS Thin Films with the Assistance of Pre-deposited Amorphous Nanostructured GeS Films: Implications for Electronic and Optoelectronic Applications

Synthesis of Large-Area GeS Thin Films with the Assistance of Pre-deposited Amorphous Nanostructured GeS Films: Implications for Electronic and Optoelectronic Applications

Solution-processed GeSe/CdS heterogenous film for self-powered photodetectors

Two-dimensional (2D) materials display excellent photoelectric behaviors which makes them promising candidates for photodetectors and related fields. Germanium selenide (GeSe), as an advanced photosensitive 2D material, owns remarkable photo absorption and fast carrier mobility. However, its photo-electron conversion efficiency suffers from high carrier recombination. Building heterojunctions is one effective strategy to facilitate the separation efficiency of photoinduced electron-hole. Here, hybridization of GeSe and CdS is realized and prepared by assembling CdS nanodots on the electrodeposited GeSe film in an ethanol solution, to form a p-n type heterogeneous film on the ITO substrate. A PEC-type photodetector is fabricated based on the GeSe/CdS heterogeneous film, which displays obviously strengthened photocurrent density and photoresponsivity. Meanwhile, such GeSe/CdS PEC photodetector exhibits the capability of self-powered photoresponse and good stability. This heterogeneous structure paves an accessible and practical way for promoting GeSe hybrids toward high-efficient photoelectric conversion devices.

Electron-induced effects in Ge-Se films studied by Kelvin probe force microscopy

The processes of surface relief formation on GexSe100-x films under their point irradiation by an electron beam have been studied. Electron-induced effects in GexSe100-x films are interpreted in framework of a two-layer charge model. The distribution of the surface potential in the locally irradiated regions of the GexSe100-x film have been established using the Kelvin probe force microscopy (KFPM). The KPFM measurements reveal that the driving force behind mass transfer is not localized charge in the film itself, but rather the gradient of the electric field created by the charge. The potential distribution derived from the measured KPFM illustrates the distribution of gradient of the electric field. We find that the direction of the field gradient sets the direction of mass transfer during point exposure with electron beam.

Enhanced performance of GeSe thin-film solar cells via bifacial charge transport materials design

Owing to non-toxic, earth abundant and stable properties, GeSe has become a promising photovoltaic absorber material in thin-film solar cells (TFSCs). Currently GeSe film is mainly prepared by rapid thermal sublimation (also called one-step method), where the evaporation source and substrate are heated together, thus it is difficult to regulate their temperatures independently, leading to the re-sublimation of GeSe film and diffusion of Cd element. In this work, a two-step method is introduced to prepare the GeSe film. Moreover, a synergic interface design is applied to enhance the performance of GeSe TFSCs. On one hand, the conductivity of CdS buffer layer is increased by post-annealing to reduce the series resistance and promote the electron transfer from GeSe to CdS, validated via linear sweep voltammetry curves. On the other hand, Cu2O is inserted as a hole transport layer for the first time. The Cu2O layer has sufficiently improved the hole extraction efficiency and suppressed the charge recombination, demonstrated by transient photovoltage spectroscopy and electrochemical impedance spectroscopy. By incorporating these bifacial modifications, the power-conversion efficiency (PCE) of the device has been improved by about four times (0.16%–0.63%), comparable to the highest PCE among all the reported GeSe TFSCs fabricated via a two-step method.

MXene-contact enhanced broadband photodetection in centimeter level GeS films

The persistent pursuit of broadband photodetectors derives from their unique properties for promising applications such as optoelectronic devices, imaging sensors, and neuromorphic computing. Here, we fabricated high-performance, ultra-wide spectral response (250–1064 nm) and easy-processing spin-deposition photodetector based on amorphous germanium sulfide (a-GeS) films by using Ti3C2T x transparent electrodes as contacts. In addition, photodetectors based MXenes contacts have larger photocurrent compared with Au contacts because MXenes conductive films have larger photo-responsive active areas and the surface plasmon-assisted hot carriers generated by the laser irradiation on MXenes. As-prepared photodetectors based on MXenes electrodes exhibit a photo-to-dark current ratio of up to 3.91 × 102 under a bias of 8 V, coupled with the response speed of 59 ms, photoresponsivity (166 mA W−1) and high detectivity (4.41 × 1010 Jones). This work combining amorphous materials with highly conductive MXene film has excellent application prospects for ultra-wide spectral response optoelectronic devices.

Raman study of photoinduced changes in Cd-doped amorphous GeSe2 films

For Cd-doped amorphous GeSe2 films Raman spectra reveal a considerable decrease of the amount of edge-sharing GeSe4 tetrahedra and an increasing number of ethane-like Se3Ge–GeSe3 structural units compared to the undoped films. A Raman peak with a frequency close to that of the CdSe LO phonon observed for the Cd-doped films at increasing laser power density (Pexc ≥ 100 kW/cm2) provides evidence for the photoinduced formation of CdSe nanoparticles in the GeSe2:Cd films due to photoenhanced diffusion.

Electron-Induced Effects in Ge-Se Films Studied by Kelvin Probe Force Microscopy

Electron-Induced Effects in Ge-Se Films Studied by Kelvin Probe Force Microscopy

Raman Spectra of Bulk and Few-Layer GeSe From First-Principles Calculations

Raman spectra play a significant role in the study of polar materials. Herein, we report the influence of strain and interlayer shift on vibration responses in bulk and few-layer ferrovalley material GeSe in different polarization states (ferroelectric/FE and antiferroelectric/AFE) based on density functional theory and density functional perturbation theory calculations. We find Ag1 mode shifts by about 10 cm−1 from monolayer to bilayer and trilayer due to the interlayer coupling. The Ag3 mode on behalf of FE mode is observed that is consistent with the experiments in bulk and few-layer GeSe. Meanwhile, in our calculations, with the transition between AFE and FE state in the bilayer and trilayer, the Raman frequency of Ag2 and Ag3 mode decrease obviously whereas that of Ag1 mode increases. Interestingly, the Raman peaks shifted a lot due to the strain effect. We expect these variations in the Raman spectroscopy can be employed to identify the status of GeSe films, e.g., the AFE or FE state, and the number of layers in experiments.

Surface Diffusion and Epitaxial Self-Planarization for Wafer-Scale Single-Grain Metal Chalcogenide Thin Films.

Although wafer-scale single-grain thin films of 2D metal chalcogenides (MCs) have been extensively sought after during the last decade, the grain size of the MC thin films is still limited in the sub-millimeter scale. A general strategy of synthesizing wafer-scale single-grain MC thin films by using commercial wafers (Si, Ge, GaAs) both as metal source and epitaxial collimator is presented. A new mechanism of single-grain thin-film formation, surface diffusion, and epitaxial self-planarization is proposed, where chalcogen elements migrate preferentially along substrate surface and the epitaxial crystal domains flow to form an atomically smooth thin film. Through synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy, the formation of single-grain Si2 Te3 , GeTe, GeSe, and GaTe thin films on (111) Si, Ge, and (100) GaAs is verified. The Si2 Te3 thin film is used to achieve transfer-free fabrication of a high-performance bipolar memristive electrical-switching device.

Valence Band Structure of Chalcogenide Obtained by X‐Ray Photoelectron Spectroscopy and Etching Technique

Chalcogenide glass, such as GeSe, has been widely used in the selector device for high‐density vertically stackable memory application due to their nonlinear electrical property. The electronic structure of the valence band, from which the nature of the bonding, the short‐range structure, and the Fermi level can be obtained, is of great importance to understand their unique electrical behavior. However, the surface oxidation issue makes it difficult to obtain the accurate valence band structure by X‐ray photoelectron spectroscopy (XPS). Herein, XPS depth profiles using the combination of the monatomic and cluster ion etching are performed to determine the valence band structures of amorphous Ge–Se films capped with a thin carbon layer. The completely different etching behavior dependent on composition in the depth profiles may be closely associated with the intrinsic bonding configurations of the amorphous films. After obtaining the fresh surface, the intrinsic valence band structures of amorphous Ge–Se samples demonstrate the different bonding behavior and short‐range structure. Most importantly, the Fermi level of amorphous GexSe100−x compounds is also determined.

Boosting the efficiency of GeSe solar cells by low-temperature treatment of p-n junction

Germanium monoselenide (GeSe) is an emerging promising photovoltaic absorber material due to its attractive optoelectronic properties as well as non-toxic and earth-abundant constitutes. However, all previously reported GeSe solar cells rely on a superstrate configuration coupled with a CdS buffer layer, and suffer from unsatisfactory performance. Here we demonstrate that this low efficiency arises from the inevitable high-temperature treatment of p-n junction in superstrate configuration. This results in the diffusion of Cd atoms from CdS layer into GeSe film that introduces detrimental deep trap states inside the bandgap of GeSe (∼0.34 eV below conduction band minimum). We adopt therefore a substrate configuration that enables the deposition of CdS atop pre-deposited polycrystalline GeSe film at room temperature, avoiding the Cd diffusion. By optimizing the annealing temperature of complete devices via a high-throughput screening method, the resulting substrate solar cells annealed at 150°C achieve an efficiency of 3.1%, two times that of the best previously reported superstrate GeSe results.

Electrical conductivity of germanium selenide nanosheets in oxygen and butane

As a type of two-dimensional (2D) semiconductor material, 2D germanium selenide (GeSe) exhibits excellent optoelectronic properties, and has potential applications in optoelectronic devices. The GeSe is a layered material with weak van der Waals interaction. Because of the high brittleness of GeSe, it is not easy to obtain 2D GeSe samples only by mechanical peeling technique. In order to obtain a thinner GeSe sheet, we use heat treatment to thin the bulk GeSe at a high temperature in vacuum. The GeSe samples obtained by mechanical peeling are placed in a tubular furnace with a pressure of 5 × 10<sup>-4</sup> Pa for high temperature heating and thinning. In order to explore the better thinning effect, we set four temperatures to be at 320, 330, 340 and 350 ℃, respectively. After high temperature thinning, the samples are characterized and observed by atomic force microscope (AFM), scanning electron microscope (SEM), Raman spectrometer and photoluminescence (PL) spectrometer. From the above experiments, the GeSe nanosheet with a thickness of about 5 nm is prepared by mechanical peeling and high temperature thinning technology. Then, the electrical conductivities of GeSe nanosheets in oxygen (O<sub>2</sub>) and butane (C<sub>4</sub>H<sub>10</sub>) with different concentrations are evaluated by our designed experimental device. The results show that with the increase of oxygen concentration, the electrical conductivity of GeSe nanosheets increases. When the GeSe nanosheet is in butane gas, its conductivity under the same voltage decreases with the increase of the concentration of butane gas. In order to further analyze the mechanism of gas adsorption on GeSe nanosheets, we carry out the first-principles calculations. Our calculation results show that the adsorption energy of GeSe nanosheets for oxygen and butane is –4.555 eV and –4.865 eV, respectively. It is shown that both adsorption systems have a certain stability. The adsorption energy of C<sub>4</sub>H<sub>10</sub> is smaller than that of O<sub>2</sub>, which corresponds to the smaller layer spacing of C<sub>4</sub>H<sub>10</sub> than that of O<sub>2</sub> on GeSe surface. From Bader analysis, it is shown that 0.262<i>e</i> is transferred from the surface of GeSe nanosheet to O<sub>2</sub> molecule, which is much larger than 0.022<i>e</i> transferred from GeSe to C<sub>4</sub>H<sub>10</sub> molecule. It can be inferred that the bond formed between GeSe and O<sub>2</sub> molecule is covalent bond, while GeSe adsorption C<sub>4</sub>H<sub>10</sub> is very fragile hydrogen bond adsorption. In an ideal condition (single atomic GeSe layer, no Se vacancy, and the device preparation process is vacuum), our calculation results show that C<sub>4</sub>H<sub>10</sub> still has a weak ability to obtain electrons from the GeSe nanosheet. However, the complex conditions such as the actual layer thickness, the appearance of Se vacancy and the adsorption of O<sub>2</sub> molecules on the surface leads to the difference between the experimental results and the theoretical calculations, which can be attributed to the adsorption of O<sub>2</sub> molecules on the GeSe surface from the air during the processing of GeSe thinning and device fabrication. Owing to the high density of Se vacancies in the thin film, the high density of O<sub>2</sub> adsorption is caused. Thus, butane gas is easy to lose electrons on the GeSe surface due to the O<sub>2</sub> adsorption. In other words, electrons are transferred from butane gas molecules to the surface of GeSe film and neutralized with holes, which reduces the concentration of carriers and the concentration of holes in GeSe film, thus reducing the conductivity. Our research will contribute to the application of GeSe nanosheets in optoelectronic devices at the atmosphere of oxygen and butane.

Deposition and characterization of solution processed Se-rich Ge-Se thin films with specular optical quality using multi-component solvent approach

The Ge25Se75, Ge20Se80 and Ge15Se85 thin films were deposited in specular optical quality from n-propylamine - methanol solvent mixture by spin-coating technique. As-prepared solution processed thin films were thermally stabilized to reduce the content of organic solvent residuals and optical properties, surface topography, composition, structure and chemical resistance of prepared Ge-Se thin films were studied in dependence of annealing temperature. Suitable thermal stabilization temperatures were found for each studied chalcogenide glass composition with respect to maintaining of thin films’ low surface roughness and targeted elemental composition. Stabilized thin films exhibited high refractive index, high chemical resistance, low surface roughness and structure close to source bulk glasses. The experiments proved that used n-propylamine - methanol solvent offered suitable way for preparation of high optical quality Ge-Se thin films by solution based deposition route.