GeSe Films Research Articles

PHOTOBLEACHING IN GeSe2 AND SiO2/GeSe2 THIN FILMS

We prepared GeSe2 film and GeSe2 covered by a layer of thin SiO2 (SiO2/GeSe2) to examine photo-induced effects. We found, upon sub-gap light illumination, transmission edge of both the films exhibit a blue shift, but the amount of the shift in SiO2/GeSe2 is less than that in GeSe2. Kinetics measurements of the photo-induced effects indicate that the both films exhibit PB behavior after more than few hundreds seconds illumination. Raman scattering measurements show that some of Ge-Ge and Se-Se bonds are broken and convert to Ge-Se bonds. All the results indicate that the change of the intrinsic structure upon illumination is dominated in the PB process.

Enhancement of photodetection by PbSe quantum dots on atomic-layered GeS devices

Hybrid structures of quantum dots (QDs) on two-dimensional materials have aroused great interest because of their high absorbance properties and tunable wavelength detection ranges. In this work, 1.44 times the photoresponse bandwidth of PbSe QDs on atomic-layered GeS hybrid structure devices is achieved compared with pure GeS devices due to the transfer of photogenerated carriers between the PbSe QDs and the GeS film. A doubling of the peak photoresponsivity is obtained at a wavelength of 635 nm, and the detectivity of the hybrid devices increases by 39.5 and 27.4 times under 808 and 980 nm illumination, respectively. Additionally, tripling of the carrier mobility is measured in the hybrid devices (165.2 cm2 V−1 s−1) compared with that of pure atomic-layered GeS devices (54.2 cm2 V−1 s−1). The concentration of PbSe QDs on the GeS films is optimized for the highest photoresponsivity and carrier mobility of the hybrid devices. The results indicate that a hybrid structure of QDs on atomic-layered materials is a promising way to enhance photodetection.

Atomic Layer Deposition of GexSe1-x Thin Films for Endurable Ovonic Threshold Selectors with a Low Threshold Voltage.

An ovonic threshold switch (OTS) based on amorphous chalcogenide materials possesses several desirable characteristics, including high selectivity and fast switching speed, enabling the fabrication of one selector-one resistor (1S-1R) crossbar array (CBA) for random access memory. Among the several chalcogenide materials, GeSe offers high selectivity and a strong glass-forming ability with environment-friendly, simple binary composition. In this report, the GeSe thin films were deposited via atomic layer deposition (ALD) using Ge(N(Si(CH3)3)2)2 and ((CH3)3Si)2Se for its envisioned application in fabricating three-dimensional vertical-type phase-change memory. Highly conformal GexSe1-x films were obtained at a substrate temperature ranging from 70 to 160 °C. The unique deposition mechanism that involves Ge intermediates provided a way to modulate the composition of the Ge-Se films from 5:5 to 7:3. Low threshold voltages ranging from 1.2 to 1.4 V were observed depending on the composition. A cycling endurance of more than 106 was achieved with the Ge0.6Se0.4 composition with 104 half-bias nonlinearity. This work presents the foundations for the future development of vertical-type 1S-1R arrays when combined with the ALD technique for Ge2Sb2Te5 phase-change materials.

Post-annealing treatment of a-GeSe thin films for photovoltaic application

p-GeSe is a promising absorber material for thin-film solar cell. In this letter, p-GeSe thin films were fabricated by post-annealing treatment of a-GeSe films. We systematically studied the effects of annealing time on the crystallinity, surface morphology and photoelectric properties of GeSe films, and we found the bandgaps, optical constant and conductivity of GeSe films have shown significant variations when GeSe films transformed from amorphous to polycrystalline. The bandgap of GeSe film can be tuned in a range of 1.17–1.42 eV and the hole mobility can reach as high as 15.02 cm2 V−1 s−1. Finally, a thin-film solar cell based on FTO/CdS/p-GeSe/Au configuration was fabricated and has realized an efficiency of 0.65% and a short circuit current of 12.61 mA/cm2.

Thermal annealing of Ge-Se thin films and its influence on waveguide performance

Ge-Se thin film waveguide is used in optical devices because of its excellent optical properties. We investigated the structural and optical properties of as-deposited and thermally annealed Ge18Se82 films and the associated waveguides. The optimized annealing condition at 170 °C was determined for Ge18Se82 films. This study reveals that the annealing process can reduce the density of homopolar bonds and voids in the films. After the annealing process, Ge18Se82 waveguides with the dimensions of 1.0 µm×4.0 µm and 1.5 µm×4.0 µm present 0.22 dB/cm and 0.26 dB/cm propagation loss reduction, respectively. This finding suggests that thermal annealing is an appropriate method for improving the performance of chalcogenide glass devices.

Dual Effect of Light Irradiation for Surface Relief Gratings Formation in Se-rich Ge-Se Thin Films

Dual Effect of Light Irradiation for Surface Relief Gratings Formation in Se-rich Ge-Se Thin Films

Broadband Photodetection of GeSe Films of Vertically Grown Nanoflakes

GeSe films formed with vertically grown nanoflakes were synthesized by a chemical vapor deposition (CVD) method. The GeSe films display good photoresponse over a broad range of wavelengths from ult...

GeSe nanosheets modified surface plasmon resonance sensors for enhancing sensitivity

Abstract Germanium selenide (GeSe) nanosheets are stable and inexpensive and considered to have a great potential for photovoltaic applications, however we have demonstrated that GeSe nanosheets are also promising for sensing technology, in this paper. By spin-coating the GeSe nanosheets on the surface of noble metal (Au), we have obtained a surface plasmon resonance (SPR) sensor with significantly enhanced sensitivity, and the performance of the sensor is closely related to the thickness of the GeSe film. By detecting different refractive index solutions, we have obtained the optimized sensitivity with 3581.2 nm/RIU (which is nearly 80% improvement compared to traditional SPR sensors) and figure of merit with 14.37 RIU−1. Moreover, the proposed SPR sensor was vastly superior in sensing Pb2+ heavy metal ions after coating it with chitosan and GeSe composite. A maximum sensitivity of 30.38 nm/μg/l has been verified, which is nearly six times better than that of conventional SPR sensor. Our results demonstrated that GeSe nanosheets overlayer with modified SPR sensor has its great potential in heavy metal detection and chemical-specific molecular identification.

Study of Se and Ge Solutions at Different Phs to Form Gese Via E-ALD

GeSe is a two-dimensional chalcogenide with ovonic threshold switching properties. Its properties strongly depend on the stoichiometry and the quality of the film.1 Electrochemical atomic layer deposition (E-ALD) is an electrochemical method that utilizes surface limited reactions to deposit materials layer by layer.2 Using E-ALD will allow growth of higher quality GeSe films. Before attempting deposition, cyclic voltammetry studies of the individual solutions were done to determine the best deposition potentials. Ge solutions made at different pHs show maximum deposition in more basic solutions, such as pH 9. Studies of Se solutions at pH 1.6, 3, 5, 7, and 9 were done to find complimentary deposition potentials to the basic Ge solution. By investigating the behavior of Se and Ge using cyclic voltammetry, better understanding of the chemistry was garnered for the development of an E-ALD cycle. Further research will evaluate the deposition patterns of Ge on Se and vice versa. Finally attempts at depositing a stoichiometric GeSe deposit will be made using E-ALD.

Promising GeSe Nanosheet-Based Thin-Film Photocathode for Efficient and Stable Overall Solar Water Splitting

Compound semiconductor GeSe nanosheet-based thin films were systematically researched for application in solar water splitting. p-type GeSe films obtained from the rapid thermal sublimation method ...

Highly oriented GeSe thin film: self-assembly growth via the sandwiching post-annealing treatment and its solar cell performance.

GeSe is considered as a potential absorber material for thin film solar cells owing to its ideal band gap, strong light absorption, remarkable air durability, Earth-abundance and non-toxic constituents. However, the high vapor pressure of GeSe at a temperature below its melting point makes it difficult to synthesize a high-quality GeSe film. To alleviate this limitation, in this work, a thermal evaporation combining a novel sandwiching post-annealing method was introduced to deposit high quality GeSe thin films with (100)-orientation. The self-assembling mechanism of the highly oriented GeSe film was carefully investigated by the systematic experiments and confirmed by the lowest total energy of the (100) crystal plane. Finally, the fully-inorganic, low-cost and non-toxic planar device with the superstrate configuration of FTO/TiO2/GeSe/carbon/Ag was also successfully fabricated. Notably, as a result, an impressive open circuit voltage (VOC) of 340 mV (maximum: 456 mV) was achieved, which is the highest VOC of GeSe solar cells reported so far. Furthermore, through current-voltage, capacitance-voltage profiling and drive level capacitance profiling measurements, it was demonstrated that the limiting factors of the GeSe solar cell performance were the narrow depletion width (138 nm) and the drastic recombination at the TiO2/GeSe interface.

Tuning the Optical Absorption Property of GeSe Thin Films by Annealing Treatment

As an emerging promising photovoltaic absorber material, GeSe has attracted significant interest recently due to its simple binary composition, attractive optical and electrical properties as well as earth‐abundant and low‐toxic constituents. However, no systematic study on the absorption property tuning of GeSe has been reported. Here, first it is shown that the crystallization temperature of amorphous GeSe is about 330 °C through differential thermal analysis and temperature‐dependent X‐ray diffraction. Next, GeSe films with tunable absorption property in the range of 1.79–1.14 eV are fabricated by annealing amorphous GeSe films at different temperatures. Finally, through the combined analysis of bandgaps measured by transmission spectroscopy and nanosheet thickness characterized by cross‐sectional high‐resolution transmission electron microscopy, the blue shift of bandgap with decreasing thickness of nanosheets has been attributed to the quantum confinement effect. Such continuously tunable absorption property of GeSe films makes it more useful for further optoelectronics.

Atomic layer deposition of GeSe films using HGeCl3 and [(CH3)3Si]2Se with the discrete feeding method for the ovonic threshold switch

The ovonic threshold switch (OTS) based on the voltage snapback of amorphous chalcogenides possesses several desirable characteristics: bidirectional switching, a controllable threshold voltage (Vth) and processability for three-dimensional stackable devices. Among the materials that can be used as OTS, GeSe has a strong glass-forming ability (∼350 °C crystallization temperature), with a simple binary composition. Described herein is a new method of depositing GeSe films through atomic layer deposition (ALD), using HGeCl3 and [(CH3)3Si]2Se as Ge and Se precursors, respectively. The stoichiometric GeSe thin films were formed through a ligand exchange reaction between the two precursor molecules, without the adoption of an additional reaction gas, at low substrate temperatures ranging from 70 °C–150 °C. The pseudo-saturation behavior required a long time of Ge precursor injection to achieve the saturation growth rate. This was due to the adverse influence of the physisorbed precursor and byproduct molecules on the efficient chemical adsorption reaction between the precursors and reaction sites. To overcome the slow saturation and excessive use of the Ge precursor, the discrete feeding method (DFM), where HGeCl3 is supplied multiple times consecutively with subdivided pulse times, was adopted. DFM led to the saturation of the GeSe growth rate at a much shorter total injection time of the Ge precursor, and improved the film density and oxidation resistance properties. The GeSe film grown via DFM exhibited a short OTS time of ∼40 ns, a ∼107 ON/OFF current ratio, and ∼104 selectivity. The OTS behavior was consistent with the modified Poole–Frenkel mechanism in the OFF state. In contrast, the similar GeSe film grown through the conventional ALD showed a low density and high vulnerability to oxidation, which prevented the OTS performance. The ALD method of GeSe films introduced here will contribute to the fabrication of a three-dimensionally integrated memory as a selector device for preventing sneak current.

GeSe Thin-Film Solar Cells Fabricated by Self-Regulated Rapid Thermal Sublimation.

GeSe has recently emerged as a promising photovoltaic absorber material due to its attractive optical and electrical properties as well as earth-abundant and low-toxic constituent elements. However, no photovoltaic device has been reported based on this material so far, which could be attributed to the inevitable coexistence of phase impurities Ge and GeSe2, leading to detrimental recombination-center defects and seriously degrading the device performance. Here we overcome this issue by introducing a simple and fast (4.8 μm min-1) rapid thermal sublimation (RTS) process designed according to the sublimation feature of the layered structured GeSe. This new method offers a compelling combination of assisting raw material purification to suppress deleterious phase impurities and preventing the formation of detrimental point defects through congruent sublimation of GeSe, thus providing an in situ self-regulated process to fabricate high quality polycrystalline GeSe films. Solar cells fabricated following this process show a power conversion efficiency of 1.48% with good stability. This preliminary efficiency and high stability, combined with the self-regulated RTS process (also extended to the fabrication of other binary IV-VI chalcogenide films, i.e., GeS), demonstrates the great potential of GeSe for thin-film photovoltaic applications.

On the thickness dependence of both the optical band gap and reversible photodarkening in amorphous Ge-Se films

Amorphous Ge30.5Se69.5 and Ge39.5Se60.5 films were prepared by thermal evaporation from previously synthetized bulk glasses. We did not observe a shift in the optical band gap values associated with the change in film thickness for the virgin and annealed states of the thin films. Certain minor changes in the optical band gap values were explained by the change in the chemical composition and by the change in the sample disorder. Both ultraviolet and visible and far infrared spectra indicate that annealed Ge39.5Se60.5 thin films are practically insensitive to the illumination while Ge30.5Se69.5 thin films show a reversible photodarkening. The observed reversible changes of the optical band gap for Ge30.5Se69.5 thin films were also not dependent on the film thickness. The network structural rigidity, which can be combined with a quick relaxation of the light induced excitation directly back to the ground state, was suggested as the reason for the absence of reversible photodarkening in Ge39.5Se60.5 thin films.

Comparative study of atomic arrangements in equiatomic GeSe and GeTe films before and after crystallization

EXAFS analysis is performed on equiatomic GeSe and GeTe films to elucidate changes in the local atomic environments induced by crystallization. It reveals for both the films that the 4:2 configuration dominates in their as-deposited amorphous state. In their crystallized state, however, the 3:3 configuration prevails together with quasi-crystalline Ge clusters. The two chalcogenide films exhibit very similar characteristics such as hidden glass transition phenomenon during heating, dominance of the 3:3 configuration in their melt state, decreased optical bandgap upon crystallization, and reduced temperature dependence of electrical conduction accompanied with crystallization. These common properties are partly attributed to the structural changes identified through our EXAFS analysis. In particular, their previously reported non-ohmic current-voltage behaviors associated with the phase switching and the threshold switching are explained in terms of the local structural information. The special phase-change properties of the equiatomic GeSe film is explicated by its amorphous structure.

Detection of orbital angular momentum using a photonic integrated circuit.

Orbital angular momentum (OAM) state of photons offer an attractive additional degree of freedom that has found a variety of applications. Measurement of OAM state, which is a critical task of these applications, demands photonic integrated devices for improved fidelity, miniaturization, and reconfiguration. Here we report the design of a silicon-integrated OAM receiver that is capable of detecting distinct and variable OAM states. Furthermore, the reconfiguration capability of the detector is achieved by applying voltage to the GeSe film to form gratings with alternate states. The resonant wavelength for arbitrary OAM state is demonstrated to be tunable in a quasi-linear manner through adjusting the duty cycle of the gratings. This work provides a viable approach for the realization of a compact integrated OAM detection device with enhanced functionality that may find important applications in optical communications and information processing with OAM states.

Electron conductivity in GeTe and GeSe upon ion implantation of Bi

This paper presents results on ion implantation of bismuth in GeTe and GeSe films. The conductivity and the thermopower of amorphous chalcogenide films are investigated. Electron conductivity in the films is attained at the Bi implantation doses higher than (1.5–2) × 1016 cm−2. In conjunction with the structural modification in the films as revealed Raman spectroscopy, the results suggest the structural re-arrangement of the amorphous network occurs via weakening the bonds of a lower energy. The onset of electron conductivity is hindered by a stronger bond in an alloy. In GeTe, this is the Ge-Ge bond.

X‐ray radiation induced effects in selected chalcogenide glasses and CBRAM devices based on them

Conductive bridge resistance change (CBRAM) memory devices are one of the premier emerging technologies for non‐volatile memory. The application of these devices could overlap possible situations where they are expected to perform in environments containing X‐ray radiation. This poses the question, how X‐ray radiation affects the materials comprised within these devices, as well as the performance of the CBRAM devices. In this work, we studied the structural changes caused by a wide range X‐ray radiation over thin Ge–Se films with composition ranging from Se rich to Ge rich, as well as X‐ray induced Ag diffusion within these films. The results show that after the cessation of radiation, the Ge rich films undergo considerable structural modification while the other compositions did not exhibit substantial changes. X‐ray stimulated Ag diffusion with formation of Ag–Se by‐products occurred predominantly in the Se and Ge rich films. These effects influence the performance of the CBRAM devices, based on these films and their I–V characteristics, threshold voltage and endurance are presented and discussed in the context of the materials characterization findings of this work, performed by Raman spectroscopy, Energy dispersion spectroscopy and X‐ray diffraction.

Linear and nonlinear optical properties of Sb-doped GeSe2 thin films**Project supported by the National Key Basic Research Program of China (Grant No. 2012CB722703), the National Natural Science Foundation of China (Grant No. 61377061), the Young Leaders of Academic Climbing Project of the Education Department of Zhejiang Province, China (Grant No. pd2013092), the Program for Innovative Research Team of Ningbo City,

Sb-doped GeSe2 chalcogenide thin films are prepared by the magnetron co-sputtering method. The linear optical properties of as-deposited films are derived by analyzing transmission spectra. The refractive index rises and the optical band gap decreases from 2.08 eV to 1.41 eV with increasing the Sb content. X-ray photoelectron spectra further confirm the formation of a covalent Sb–Se bond. The third-order nonlinear optical properties of thin films are investigated under femtosecond laser excitation at 800 nm. The results show that the third-order nonlinear optical properties are enhanced with increasing the concentration of Sb. The nonlinear refraction indices of these thin films are measured to be on the order of 10−18 m2/W with a positive sign and the nonlinear absorption coefficients are obtained to be on the order of 10−10 m/W. These excellent properties indicate that Sb-doped Ge–Se films have a good prospect in the applications of nonlinear optical devices.