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The family of 2D chalcogenide semiconductors has been growing rapidly. Metal monochalcogenides, for instance, can enable new possibilities in functional electronics and optoelectronics. A Gallium Selenide (GaSe) thin flake is used to fabricate a back gated field effect transistor (FET) with n‐type conduction behavior and wide hysteresis at the ambient conditions. The device shows high mobility up to 28 cm2 V−1 s−1 with Ion/Ioff ratio over 103. Under the laser exposure, the device shows a decrease in the threshold voltage and a left‐shift of the transfer characteristic with a slight increase in the current. The transfer characteristic exhibits a hysteretic behavior with hysteresis width linearly dependent on the applied gate voltage. Moreover, the GaSe‐based FET shows a photo response with a photoresponsivity of 475 mAW−1 and detectivity of 4.6 × 1012 Jones. The photocurrent rise and decay times are 0.1 and 1.3 s, respectively. Furthermore, the GaSe FET device can be used as a performant memory device with well separated states and memory window enhanced by the laser exposure, confirming an optoelectronic memory class. [ABSTRACT FROM AUTHOR]

The study analysed the impact of radiation defects on p-GaSe single crystal implanted with H+ ions (70 keV) on its charge transport mechanism. The research was conducted at 100 K and 300 K in an electric field of 102-104 V/cm. The study found that the activation energy of charge carriers injected at low temperatures and electric fields E < 103 V/cm ranged from 0.23-0.39 eV. This was observed due to the trapping of charge carriers in concentration traps of approximately 9·1013 cm-3, leading to monopolar injection. In the fields, E > 103 V/cm, a sharp increase in current was observed, which was explained by the thermal ionisation of local levels following the Frenkel mechanism. The study determined that the charge transport mechanism in GaSe:H+ crystals at low temperatures has a non-activated character.

Group IIIA metal chalcogenides are an auspicious material system due to their variability of properties and hence the multitude of application options, for example, in the fields of optoelectronic, thermoelectric, piezo‐, and ferroelectric devices. Indium and gallium selenide films are innovatively grown in a sequential PVD (physical vapor deposition) process starting from metal precursor layers of various thicknesses, which are then subject to chalcogenization in different selenium contents. The resulting thin films are investigated for structural and optical properties by Raman, XRD (X‐ray diffraction), and UV–Vis–NIR spectrometry, revealing that all the compounds In2Se3, InSe, In4Se3, Ga2Se3, and GaSe as well as different polytypes can be achieved depending on the metal/chalcogen ratio. Results from Raman and XRD spectroscopy are highly consistent, and also from the optical measurements changes in absorption characteristics can be correlated. The results indicate, that by fine‐tuning the selenium content, deliberately growing ultra‐thin layers of the different indium and gallium phases will be possible, thus opening up a promising route for 2D material fabrication. Given the scalability of the fabrication method, it is highly promising for large‐scale deployment of the materials. [ABSTRACT FROM AUTHOR]

Gallium selenide (GaSe) has become increasingly popular in the field of optoelectronics due to its suitable band gap. The layered structure of bulk crystal GaSe makes it easy to create a monolayer of GaSe. However, this also results in an increase in the band gap. In this paper, we used the full-potential periodic density functional theory (DFT) method to examine the electronic structure of both pure and Cu-doped GaSe. We modeled Ga1-xCuxSe with x = 0.02, 0.03, 0.056, and 0.125 with and without Ga vacancy. We compared the structure, electronic properties, and magnetic properties of the pure bulk and monolayer with those of the Cu-doped samples. The density of state graphs clearly demonstrate that Ga-defected and Cu-doped GaSe have p-type characteristics. The presence of a Ga vacancy causes the GaSe monolayer to become half-metallic, which has potential applications in spintronics. When the Cu concentration reaches approximately 3%, there is a 20% reduction in the band gap. Both the Ga vacancy and Cu doping create deep defect levels that lead to carrier recombination. The position of these levels is highly dependent on the concentration of Cu. Increasing the Cu concentration from% 1 to 2.78% results in a decrease in recombination. [ABSTRACT FROM AUTHOR]

Abstract The family of 2D chalcogenide semiconductors has been growing rapidly. Metal monochalcogenides, for instance, can enable new possibilities in functional electronics and optoelectronics. A Gallium Selenide (GaSe) thin flake is used to fabricate a back gated field effect transistor (FET) with n‐type conduction behavior and wide hysteresis at the ambient conditions. The device shows high mobility up to 28 cm2 V−1 s−1 with Ion/Ioff ratio over 103. Under the laser exposure, the device shows a decrease in the threshold voltage and a left‐shift of the transfer characteristic with a slight increase in the current. The transfer characteristic exhibits a hysteretic behavior with hysteresis width linearly dependent on the applied gate voltage. Moreover, the GaSe‐based FET shows a photo response with a photoresponsivity of 475 mAW−1 and detectivity of 4.6 × 1012 Jones. The photocurrent rise and decay times are 0.1 and 1.3 s, respectively. Furthermore, the GaSe FET device can be used as a performant memory device with well separated states and memory window enhanced by the laser exposure, confirming an optoelectronic memory class.

The electrical and photoelectric properties of anisotype n-Si−p-GaSe heterojunctions obtained as a result of the deposition of a GaSe thin layer on a cold n-Si single crystal substrate by the thermal evaporation method were studied. It was determined that the height of the potential barrier in thermal annealing structures at T = 200 °C during t = 3 hours occurs due to the decrease in the density of states of local levels located near the Fermi level in the amorphous layer. The mechanism of photosensitivity in an isotype heterostructures was analyzed and it was found that the photosensitivity of the heterojunction increases as a result of a decrease in the surface density of state at the contact boundary of the components, by thermal means. The spectral distribution of the quantum efficiency in the n‑Si – p‑GaSe heterojunction was studied and their perspective was determined.

The electrical and photoelectric properties of anisotype n-Si-p-GaSe heterojunctions obtained as a result of the deposition of a GaSe thin layer on a cold n-Si single crystal substrate by the thermal evaporation method were studied. It was determined that the height of the potential barrier in thermal annealing structures at T = 200°C during t = 3 hours occurs due to the decrease in the density of states of local levels located near the Fermi level in the amorphous layer. The mechanism of photosensitivity in an isotype heterostructures was analyzed and it was found that the photosensitivity of the heterojunction increases as a result of a decrease in the surface density of state at the contact boundary of the components, by thermal means. The spectral distribution of the quantum efficiency in the n-Si -- p-GaSe heterojunction was studied and their perspective was determined. [ABSTRACT FROM AUTHOR]

Abstract Group IIIA metal chalcogenides are an auspicious material system due to their variability of properties and hence the multitude of application options, for example, in the fields of optoelectronic, thermoelectric, piezo‐, and ferroelectric devices. Indium and gallium selenide films are innovatively grown in a sequential PVD (physical vapor deposition) process starting from metal precursor layers of various thicknesses, which are then subject to chalcogenization in different selenium contents. The resulting thin films are investigated for structural and optical properties by Raman, XRD (X‐ray diffraction), and UV–Vis–NIR spectrometry, revealing that all the compounds In2Se3, InSe, In4Se3, Ga2Se3, and GaSe as well as different polytypes can be achieved depending on the metal/chalcogen ratio. Results from Raman and XRD spectroscopy are highly consistent, and also from the optical measurements changes in absorption characteristics can be correlated. The results indicate, that by fine‐tuning the selenium content, deliberately growing ultra‐thin layers of the different indium and gallium phases will be possible, thus opening up a promising route for 2D material fabrication. Given the scalability of the fabrication method, it is highly promising for large‐scale deployment of the materials.

At present, the available experimental data on the optical properties of layered III–VI monochalcogenide compounds are scattered and somewhat contradictory, although they are the parent materials for promising two-dimensional (2D) structures. This work is devoted to optical studies of bulk GaSe crystals, whose perfect structural properties are confirmed by Raman studies, observation of singlet-triplet splitting of 1.5 meV, and the polarized photoluminescence measurements from the sample edge. We analyze the band structure of GaSe, namely the sequence and energies of direct and indirect exciton transitions, using cw and time-resolved micro-photoluminescence measurements with variation of temperature. It turns out that the direct band gap in bulk GaSe is at 2.13 eV, close to calculated values. The indirect exciton transition is located ∼15 meV below the direct exciton (2.11 eV). Its intensity quickly quenches and characteristic decay time strongly shortens with increasing temperature, while the contribution of the direct exciton is relatively enhanced. [ABSTRACT FROM AUTHOR]

The gallium monochalcogenides family, comprising gallium sulfide (GaS), gallium selenide (GaSe), and gallium telluride (GaTe), is capturing attention for its applications in energy storage and production, catalysis, photonics, and optoelectronics. This interest originates from their properties, which include an optical bandgap larger than those of most common transition metal dichalcogenides, efficient light absorption, and significant carrier mobility. For any application, stability to air exposure is a fundamental requirement. Here, we perform a comparative study of the stability of layered GaS, GaSe, and GaTe nanometer-thick films down to a few layers with the goal of identifying the most suitable Ga chalcogenide for future integration in photonic and optoelectronic devices. Our study unveils a trend of decreasing air stability from sulfide to selenide and finally to telluride. Furthermore, we demonstrate a hydrogen passivation process to prevent the oxidation of GaSe with a higher feasibility and durability than other state-of-the-art passivation methods proposed in the literature. [ABSTRACT FROM AUTHOR]

In recent years, two-dimensional (2D) type-II van der Waals (vdW) heterojunctions have emerged as promising candidates for high-performance photodetectors. However, direct experimental evidence confirming the enhancement of photoelectric properties by the heterojunction's type and structure remains scarce. In this work, we present flexible photodetectors based on individual GaSe and MoS2, as well as a vertically stacked type-II GaSe/MoS2 vdW heterojunction on polyethylene terephthalate (PET) substrate. These devices demonstrate outstanding responsivities and rapid response speeds, ensuring stable and repeatable light detection. It is notable that the GaSe/MoS2 heterojunction photodetector exhibits the highest on-off ratio and fastest response speed, attributed to the formation of type-II band alignment. Furthermore, the GaSe/MoS2 heterojunction photodetector maintains robust stability even in a bent state, highlighting remarkable flexibility. This work exemplifies the type-II vdW heterojunctions in enhancing photoelectric properties through direct in-situ experimentation, laying the groundwork for practical applications of 2D flexible photodetectors. [ABSTRACT FROM AUTHOR]

Optical absorption spectra of 9 MeV electron-irradiated GaSe crystals were studied. Two absorption bands with the low-photon-energy threshold at 1.35 and 1.73 eV (T = 300 K) appeared in the transparency region of GaSe after the high-energy-electron irradiation. The observed absorption bands were attributed to the defect states induced by Ga vacancies in two charge states, having the energy positions at 0.23 and 0.61 eV above the valence band maximum at T = 300 K. The optical pump-terahertz probe technique (OPTP) was employed to study the dark and photoexcited terahertz conductivity and charge carrier recombination dynamics at two-photon excitation of as-grown and 9 MeV electron-irradiated GaSe crystals. The measured values of the differential terahertz transmission at a specified photoexcitation condition were used to extract the terahertz charge carrier mobilities. The determined terahertz charge carrier mobility values were ~46 cm2/V·s and ~14 cm2/V·s for as-grown and heavily electron-irradiated GaSe crystals, respectively. These are quite close to the values determined from the Lorentz–Drude–Smith fitting of the measured dielectric constant spectra. The photo-injection-level-dependent charge carrier lifetimes were determined from the measured OPTP data, bearing in mind the model injection-level dependencies of the recombination rates governed by interband and trap-assisted Auger recombination, bulk and surface Shockley–Read–Hall (SRH) recombination and interband radiative transitions in the limit of a high injection level. It was found that GaSe possesses a long charge carrier lifetime (a~1.9 × 10−6 ps−1, b~2.7 × 10−21 cm3ps−1 and c~1.3 × 10−37 cm6ps−1), i.e., τ~0.53 μs in the limit of a relatively low injection, when the contribution from SRH recombination is dominant. The electron irradiation of as-grown GaSe crystals reduced the charge carrier lifetime at a high injection level due to Auger recombination through radiation-induced defects. It was found that the terahertz spectra of the dielectric constants of as-grown and electron-irradiated GaSe crystals can be fitted with acceptable accuracy using the Lorentz model with the Drude–Smith term accounting for the free-carrier conductivity. [ABSTRACT FROM AUTHOR]

A novel nanocomposite material based on Ga2 Se3 and β-Ga2 O3 crystallites photosensitive in the wave- length range from 500 nm to 600 nm of the electromagnetic spectrum was obtained by heat treatment in air at 900 °C of GaSe plates for 30 min. Upon increasing the duration of the heat treatment up to 6 h, complete oxidation of the GaSe plates occurs with the formation of the β-Ga2 O3 layer with bandgap width of 4.60 eV and photosensitivity in the ultravi- olet region (UV-C). The photosensitivity bands of photoresistors based on Ga 2 Se3 /Ga2 O3 composite and nanostructured β-Ga2 O3 oxide fall within the absorption band in the green-red and UV-C region, respectively, and the respective photoresistors can serve as airborne ozone detectors.

2D materials with van der Waals (vdW) gaps between covalently bonded blocks are in the focus of modern semiconductor science and technology. Of special interest is the possibility to control their properties through the vdW gap reconfiguration. Herein, using first‐principles simulations it is demonstrated that uniaxial compression of GaSe, counterintuitively, does not serve to close the gap but, instead, changes the dimensionality from 2D to quasi‐1D, with flat GaSe chains oriented along the applied pressure. The structural change is accompanied by drastic changes in electronic structure. The inability to close the vdW gap to the antibonding nature of Se–Se interactions is attributed. [ABSTRACT FROM AUTHOR]

GaSe0.75S0.25 having layered structure is a potential semiconductor compound for optoelectronics and two-dimensional materials technologies. Optical and structural measurements of the GaSe0.75S0.25 thin film grown on the glass substrate showed that the compound has hexagonal structure and band energy of 2.34 eV. GaSe0.75S0.25 thin film was also grown on the silicon wafer and p-GaSe0.75S0.25/n-Si heterojunction was obtained. To make the electrical characterization of this diode, temperature-dependent current–voltage (I–V) measurements were carried out between 240 and 360 K. Room temperature ideality factor and barrier height of the device were determined from the analyses of I–V plot as 1.90 and 0.87 eV, respectively. Temperature-dependent plots of these electrical parameters showed that the ideality factor decreases from 2.19 to 1.77, while barrier height increases to 0.94 from 0.71 eV when the temperature was increased from 240 to 360 K. The conduction mechanism in the heterojunction was studied considering the Gaussian distribution due to presence of inhomogeneity in barrier height. The analyses presented the mean zero-bias barrier height, zero-bias standard deviation, and Richardson constant. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) van der Waals (vdW) AIIIBVI semiconductor materials, such as InSe and GaSe, are of considerable interest due to their potential use in various microelectronics applications. The range of properties of materials of this class can be extended further through the use of quasi-binary alloys of the InSe(Te)-GaSe(Te) type. In this work, we study the effect of compositional and structural disorder in 2D In(Ga)Se(Te) on the band structure and electronic properties using first principles modeling. The results for In(Ga)Se demonstrate a noticeable decrease in the band gap for structures with a random distribution of indium and gallium cations, while for In(Ga)Te with a random cation distribution, metallization occurs. Changes in the compositional arrangement of chalcogens (there can be either the same or different atoms on each side of the vdW gap) lead to pronounced changes in the band gap, but no significant changes in topology are observed. In addition, a significant effect of the distance between the layers on the band gap under compression along the c axis was found for both alloys under study. An important point of our study is that van der Waals gap engineering is a very powerful tool to control the properties of 2D materials and its alloys. [ABSTRACT FROM AUTHOR]

Bandgap engineering and quantum confinement in semiconductor heterostructures provide the means to fine‐tune material response to electromagnetic fields and light in a wide range of the spectrum. Nonetheless, forming semiconductor heterostructures on lattice‐mismatched substrates is a challenge for several decades, leading to restrictions for device integration and the lack of efficient devices in important wavelength bands. Here, it is shown that the van der Waals epitaxy of 2D GaSe and InSe heterostructures occur on substrates with substantially different lattice parameters, namely silicon and sapphire. The GaSe/InSe heteroepitaxy is applied in the growth of quantum wells and superlattices presenting photoluminescence and absorption related to interband transitions. [ABSTRACT FROM AUTHOR]

Nonlinear optical crystals of gallium selenide are efficient up- and downconverters of infrared and terahertz frequencies. Their nonlinear properties have been investigated at wavelengths within the main transparency window. However, insufficient attention has been paid to studies at the telecommunication wavelength, especially for sulfur-doped crystals. Closing this gap, we report on the optical and electro-optical properties of GaSe(1–x)Sx crystals (where x = 0, 0.03, 0.12, 0.16, and 0.22). For this purpose, the refractive indexes of the ordinary waves at terahertz frequencies and at a wavelength of 1.55 μm have been measured. The detection efficiency of the subterahertz waves in the crystals was studied using Er-fiber laser pulses and compared with that of GaAs, the etalon electro-optical crystal, at this wavelength. This allows us to estimate the dependence of the electro-optic coefficient r22 of GaSe(1–x)Sx on the sulfur concentration. It was shown that the sample with x = 0.12 has the largest value of the electro-optical coefficient r22 = 1.26 pm/V and provides the highest detection efficiency among the samples. The potential of employing S-doped GaSe crystals as nonlinear optical converters for photonic devices operating at telecom wavelengths is discussed. [ABSTRACT FROM AUTHOR]

Optical absorption spectra of 9 MeV electron-irradiated GaSe crystals were studied. Two absorption bands with the low-photon-energy threshold at 1.35 and 1.73 eV (T = 300 K) appeared in the transparency region of GaSe after the high-energy-electron irradiation. The observed absorption bands were attributed to the defect states induced by Ga vacancies in two charge states, having the energy positions at 0.23 and 0.61 eV above the valence band maximum at T = 300 K. The optical pump-terahertz probe technique (OPTP) was employed to study the dark and photoexcited terahertz conductivity and charge carrier recombination dynamics at two-photon excitation of as-grown and 9 MeV electron-irradiated GaSe crystals. The measured values of the differential terahertz transmission at a specified photoexcitation condition were used to extract the terahertz charge carrier mobilities. The determined terahertz charge carrier mobility values were ~46 cm2/V·s and ~14 cm2/V·s for as-grown and heavily electron-irradiated GaSe crystals, respectively. These are quite close to the values determined from the Lorentz–Drude–Smith fitting of the measured dielectric constant spectra. The photo-injection-level-dependent charge carrier lifetimes were determined from the measured OPTP data, bearing in mind the model injection-level dependencies of the recombination rates governed by interband and trap-assisted Auger recombination, bulk and surface Shockley–Read–Hall (SRH) recombination and interband radiative transitions in the limit of a high injection level. It was found that GaSe possesses a long charge carrier lifetime (a~1.9 × 10−6 ps−1, b~2.7 × 10−21 cm3ps−1 and c~1.3 × 10−37 cm6ps−1), i.e., τ~0.53 μs in the limit of a relatively low injection, when the contribution from SRH recombination is dominant. The electron irradiation of as-grown GaSe crystals reduced the charge carrier lifetime at a high injection level due to Auger recombination through radiation-induced defects. It was found that the terahertz spectra of the dielectric constants of as-grown and electron-irradiated GaSe crystals can be fitted with acceptable accuracy using the Lorentz model with the Drude–Smith term accounting for the free-carrier conductivity.

In recent years, two-dimensional (2D) type-II van der Waals (vdW) heterojunctions have emerged as promising candidates for high-performance photodetectors. However, direct experimental evidence confirming the enhancement of photoelectric properties by the heterojunction’s type and structure remains scarce. In this work, we present flexible photodetectors based on individual GaSe and MoS2, as well as a vertically stacked type-II GaSe/MoS2 vdW heterojunction on polyethylene terephthalate (PET) substrate. These devices demonstrate outstanding responsivities and rapid response speeds, ensuring stable and repeatable light detection. It is notable that the GaSe/MoS2 heterojunction photodetector exhibits the highest on-off ratio and fastest response speed, attributed to the formation of type-II band alignment. Furthermore, the GaSe/MoS2 heterojunction photodetector maintains robust stability even in a bent state, highlighting remarkable flexibility. This work exemplifies the type-II vdW heterojunctions in enhancing photoelectric properties through direct in-situ experimentation, laying the groundwork for practical applications of 2D flexible photodetectors.

The objective of this research is to look at how anisotropic mechanical stresses affect the structural, elastic, and electronic properties of various GaSe polytypes. Density functional theory (DFT) was employed in all calculations. Consistent and precise calculations as well as descriptions of trade-off and correlation effects were performed using the LDA-PBE approximation. The uni-axial compressive stresses in the GaSe layer planes, in addition to the perpendicularly applied uni-axial compressive stress, are taken into account (along the c-axis). The gap distance and the GaGaSe ^ bond angle, both of which determine the thickness of crystal layers and the size of the unit cell in the basal plane, have the largest influence on changes in the lattice parameters a and c. Compression along the c-axis causes the interlayer spacing to shrink and the crystal anisotropy to decrease, whereas compression in the layer planes has no effect on gallium selenide's property. The stability conditions reflect that the different polytypes of GaSe are mechanically stable at zero pressure and under pressure. Our results at zero pressure show that the B/G ratio is of order of 1.33, 1.44, 1.67, and 1.44 for ε-GaSe, β-GaSe, γ-GaSe, and δ-GaSe, respectively, which means that all polytypes GaSe crystal are fragile. The calculated band structure shows that the structure semiconductors ε-GaSe, β-GaSe, and δ-GaSe have a direct band gap of 0.751 eV, 0.818 eV, and 0.888 eV respectively and an indirect band gap of 0.819 eV for γ- GaSe. At bi-axial strains up to 8 GPa, calculations of the electronic band structure demonstrate a progressive growth in the band gap. Inter-band transition energy appears to be decreasing with loading under uni-axial compressive stress along the c-axis. A transition to the metallic type of conductivity may occur when the projected pressure dependences of the direct and indirect band gaps are taken into account at uni-axial pressures of roughly 10 GPa. [ABSTRACT FROM AUTHOR]

GaSehCS(NH2)2hC14H10ii clathrate with a hierarchical subhosthhosthguestii type architecture was formed under illumination and its electrically conductive properties were studied. The method of impedance spectroscopy studied the frequency behavior of real and imaginary parts of complex total impedance in the range of 10-3-106 Hz. Measurements were performed under normal conditions, in a permanent magnetic field (220 kA/m), or under light illumination (for a standard solar spectrum of AM 1.5 G total available power is 982 W/m2). Structure of the impurity energy spectrum at the Fermi level was investigated by the method of thermostimulated discharge in the temperature range from 240 to 340 K. Using Jebol-Pollack theoretical approaches based on impedance spectra, parameters of the impurity energy spectrum were calculated, such as the density of states at the Fermi level, the jump radius, the scatter of trap levels near the Fermi level and the real density of deep traps. As evidenced by studies, illumination during clathrate synthesis forms an internal electret polarization, which leads to abnormal behavior of the photoresistive effect and to the appearance of the memristive effect. The imposition of a permanent magnetic field during measurement of complex resistance leads to the appearance of quantum capacitance. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) van der Waals (vdW) AIIIBVI semiconductor materials, such as InSe and GaSe, are of considerable interest due to their potential use in various microelectronics applications. The range of properties of materials of this class can be extended further through the use of quasi-binary alloys of the InSe(Te)-GaSe(Te) type. In this work, we study the effect of compositional and structural disorder in 2D In(Ga)Se(Te) on the band structure and electronic properties using first principles modeling. The results for In(Ga)Se demonstrate a noticeable decrease in the band gap for structures with a random distribution of indium and gallium cations, while for In(Ga)Te with a random cation distribution, metallization occurs. Changes in the compositional arrangement of chalcogens (there can be either the same or different atoms on each side of the vdW gap) lead to pronounced changes in the band gap, but no significant changes in topology are observed. In addition, a significant effect of the distance between the layers on the band gap under compression along the c axis was found for both alloys under study. An important point of our study is that van der Waals gap engineering is a very powerful tool to control the properties of 2D materials and its alloys.

Abstract The two-dimensional materials have the thickness of an atomic layer level and are expected as alternative materials for future electronics and optoelectronics due to their specific properties. Especially recently, transition metal monochalcogenides and dichalcogenides have attracted attention. Since these materials have a band gap unlike graphene and exhibit a semiconductor property even in a single layer, application to a new flexible optoelectronics is expected. In this study, the photovoltaic characteristics of a GaSe/MoSe2 heterojunction device using two-dimensional semiconductors, p-type GaSe and n-type MoSe2, were investigated. The heterojunction device was prepared by transferring GaSe and MoSe2 onto the substrate which the titanium electrodes were fabricated through a mechanical peeling method. The current–voltage characteristics of the GaSe/MoSe2 heterojunction device were measured in a dark condition and under light irradiation using a solar simulator. The irradiation light intensity was changed from 0.5 to 1.5 sun. It was found that when the illuminance was increased in this illuminance range, both the short-circuit current and the open-circuit voltage increased. The open-circuit voltage and the energy conversion efficiency were 0.41 V and 0.46% under 1.5 sun condition, respectively.

GaSe> clathrate with a hierarchical subhost> type architecture was formed under illumination and its electrically conductive properties were studied. The method of impedance spectroscopy studied the frequency behavior of the real and imaginary parts of the complex total impedance in the range of 10 -3-10 6 Hz. The measurements were performed under normal conditions, in a permanent magnetic field (220 kA/m), or under light illumination (for a standard solar spectrum AM 1.5 G total available power is 982 W/m 2). The structure of the impurity energy spectrum at the Fermi level was investigated by the method of thermostimulated discharge in the temperature range from 240 to 340 K. Using Jebol-Pollack theoretical approaches based on impedance spectra, the parameters of the impurity energy spectrum were calculated, such as the density of states at the Fermi level, the jump radius , the scatter of trap levels near the Fermi level and the real density of deep traps. As evidenced by studies, illumination during clathrate synthesis, forms an internal electret polarization, which leads to abnormal behavior of the photoresistive effect and to the appearance of the memristive effect. The imposition of a permanent magnetic field during the measurement of complex resistance leads to the appearance of quantum capacitance.

A selective area growth of hexagonal GaSe on prepatterned graphene substrate and fabrication of a photodetector array is reported. The GaSe film is grown on the graphene substrates using a molecular beam epitaxy system and characterized for structural and morphological properties using X‐ray diffraction, Raman, and atomic force microscopy techniques. Preliminary observations indicate the growth of hexagonal GaSe layers along (001) direction on graphene film under certain optimum growth conditions. Subsequently, GaSe‐based Schottky diode photodetectors are fabricated in a single‐element and 16 × 16 crossbar array geometries. Current–voltage characteristics of the diodes show a rectifying behavior with a turn‐on voltage and a low leakage current of ≈2 V and ≈20 nA, respectively. Photoresponse characteristics of the diode are investigated by measuring wavelength‐dependent spectral response and temporal photocurrent response under illumination of different lights, including 532 nm laser and white light, with varied incident intensities. The spectral responsivity of the detector is found to be as high as 98 A W–1 at a wavelength of 500 nm with a turning‐on profile corresponding to the band edge of GaSe. The photo mapping capability of the detector array is also demonstrated in this report. [ABSTRACT FROM AUTHOR]

Growth of high-quality large-sized crystals using the traditional chemical vapor transport (CVT) or vertical Bridgman (VB) technique is costly and time-consuming, limiting its practical industrial application. Here, we propose an ultrafast crystal growth process with low energy consumption and capability of producing crystals of excellent quality, and demonstrate that large-sized GaSe crystals with a lateral size of 0.5 to 1 cm can be obtained within a short period of 5 min. X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) studies clearly indicate that the as-grown crystals have a good crystallinity. To further show the potential application of the resulting GaSe crystals, we fabricate the few-layer GaSe-based photodetector, which exhibits low dark current of 21 pA and fast response of 34 ms under 405 nm illumination. Our proposed technique for rapid crystal growth could be further extended to other metallenes with low-melting point, such as Bi-, Sn-, In-, Pb-based crystals, opening up a new avenue in fulfilling diverse potential optoelectronics applications of two-dimensional (2D) crystals. [ABSTRACT FROM AUTHOR]

GaSe, a two-dimensional layered metal monochalcogenide, has recently attracted growing interest due to its unique electronic properties and potential technological applications. In this study, we investigate the oxidation mechanisms and properties of GaSe exposed to air for different durations, with the intensive use of Raman spectroscopy, combined with atomic force microscopy (AFM), photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). Raman analysis reveals the oxidation of GaSe, resulting in the formation of a thin layer comprising Ga 2 Se 3 , Ga 2 O 3 , and amorphous selenium. Utilizing these signatures, oxidation is then tracked. Raman spectroscopy reveals that GaSe layer becomes oxidized almost immediately after exposure to air. However, the oxidation is a self-limiting process, taking roughly 15 min to construct an 8 Å thick layer of Ga₂O₃. XPS analysis shows a good agreement with Raman analysis. The polarized Raman study suggests that the Ga₂Se₃ and Ga₂O₃ layers tend to reach an oriented structural state over time. In ambient conditions, the intensity of all Raman modes and the luminescence decreases, linked to reduction in GaSe thickness. By using various Raman excitation wavelengths, we highlight the depth-dependent oxidation dynamics in this 2D layered GaSe material. [Display omitted] • Oxidation of GaSe-2D materials. • GaSe oxidation is monitored using Raman spectroscopy combined with AFM, PL, and XPS. • Thickness of GaSe is estimated by Raman and PL. • The thickness of Ga 2 O 3 is monitored using pARXPS. [ABSTRACT FROM AUTHOR]

Electronic devices based on bidimensional materials (2DMs) are the subject of an intense experimental research, that demands a tantamount theoretical activity. The latter must be hold up by a varied set of tools able to rationalize, explain and predict the operation principles of the devices. However, in the broad context of multi-scale computational nanoelectronics, there is currently a lack of simulation tools connecting atomistic descriptions with semi-classical mesoscopic device-level simulations and able to properly explain the performance of many state-of-the-art devices. To contribute to filling this gap we present a multi-scale approach that combines fine-level material calculations with a semi-classical drift-diffusion transport model. Its use is exemplified by assessing 2DM field effect transistors with strained channels, showing excellent capabilities to capture the changes in the crystal structure and their impact into the device performance. Interestingly, we verify the capacity of strain in monolayer GaSe to enhance the conduction of one type of carrier, enabling the possibility to mimic the effect of chemical doping on 2D materials. These results illustrate the great potential of the proposed approach to bridge levels of abstraction rarely connected before and thus contribute to the theoretical modeling of state-of-the-art 2DM-based devices. [ABSTRACT FROM AUTHOR]

We demonstrate the conversion to quasi two-dimensional (2D) β -Ga2O3 by thermally oxidizing layered GaSe of different thicknesses (from bilayer to 100 nm). GaSe flakes were prepared by mechanical exfoliation onto Si with a 300 nm SiO2 layer, highly oriented pyrolytic graphite, and mica substrates. The flakes were then annealed in ambient atmosphere at different temperatures ranging from 600 °C to 1000 °C for 30 min. Raman spectroscopy confirmed the formation of β -Ga2O3 in the annealed samples by comparison with the Raman spectrum of a β -Ga2O3 reference crystal. Atomic force microscopy was employed to study the morphology and the thickness of the β -Ga2O3 flakes. In addition, we used energy dispersive x-ray spectroscopy together with scanning electron microscopy to investigate the evolution of the composition, especially Se residuals, and the sample topography with annealing temperature. β -Ga2O3 appears at temperatures above 600 °C and Se is completely evaporated at temperatures higher than 700 °C. The thicknesses of the resulting β -Ga2O3 flakes are half of that of the initial GaSe flake. Here we therefore present a straightforward way to prepare 2D β -Ga2O3 by annealing 2D GaSe. [ABSTRACT FROM AUTHOR]

Modern nanotechnologies are designed to create functionally hybrid inorganic/organic materials with extraordinary properties. Great interest in this direction was gained by clathrates – inclusion compounds built on the host principle. Applying intercalation techniques, the supramolecular structures thus formed can be ordered in a certain way using inorganic matrices, forming clathrates of subhost> type. According to this principle, we formed GaSe2>> clathrate. By forming this structure using weak interactions, it was possible to observe a giant magnetocapacitance effect at room temperature. Its obtained values indicate the prospects of using GaSe2>> clathrate as a material for creating capacitive analogues of resistive storage devices. The observed effect is related to a special state of the impurity energy subsystem, which at room temperature will have a decisive influence. Thus, when 2>> supramolecular complex is introduced into the GaSe semiconductor matrix, impurity levels are split into bands and a quasi-continuous spectrum is formed in two temperature ranges: 248-252 and 298-332 K. The formation in this case also of deep quantum wells at the semiconductor matrix/supra-molecular complex interface led to the effect of negative capacitance, which, in turn, can be used to form non-gyratory delay lines that can be directly incorporated into the structure of micro-and nanoelectronics. The magnitude of this effect, according to the research results, can be controlled by illumination and an applied constant magnetic field. The theoretical calculations of the impurity energy spectrum, carried out based on impedance spectroscopy data according to the Geballe-Pollack theory, show good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The two-dimensional materials have the thickness of an atomic layer level and are expected as alternative materials for future electronics and optoelectronics due to their specific properties. Especially recently, transition metal monochalcogenides and dichalcogenides have attracted attention. Since these materials have a band gap unlike graphene and exhibit a semiconductor property even in a single layer, application to a new flexible optoelectronics is expected. In this study, the photovoltaic characteristics of a GaSe/MoSe2 heterojunction device using two-dimensional semiconductors, p-type GaSe and n-type MoSe2, were investigated. The heterojunction device was prepared by transferring GaSe and MoSe2 onto the substrate which the titanium electrodes were fabricated through a mechanical peeling method. The current–voltage characteristics of the GaSe/MoSe2 heterojunction device were measured in a dark condition and under light irradiation using a solar simulator. The irradiation light intensity was changed from 0.5 to 1.5 sun. It was found that when the illuminance was increased in this illuminance range, both the short-circuit current and the open-circuit voltage increased. The open-circuit voltage and the energy conversion efficiency were 0.41 V and 0.46% under 1.5 sun condition, respectively. [ABSTRACT FROM AUTHOR]

2D GaSe is a semiconductor belonging to the group of post‐transition metal chalcogenides with great potential for advanced optoelectronic applications. The weak interlayer interaction in multilayer 2D materials allows the formation of several polymorphs. Here, the first structural observation of a new GaSe polymorph is reported, characterized by a distinct atomic configuration with a centrosymmetric monolayer (D3d point group). The atomic structure of this new GaSe polymorph is determined by aberration‐corrected scanning transmission electron microscopy. Density‐functional theory calculations verify the structural stability of this polymorph. Furthermore, the band structure and Raman intensities are calculated, predicting slight differences to the currently known polymorphs. In addition, the occurrence of layer rotations, interlayer relative orientations, as well as translation shear faults is discussed. The experimental confirmation of the new GaSe polymorph indicates the importance of investigating changes in the crystal structure, which can further impact the properties of this family of compounds. [ABSTRACT FROM AUTHOR]

[Display omitted] • Most non-metal dopants from Group A can only introduce p -type carriers. • Metal dopants can introduce both p - and n -type carriers. • Most transition metal dopants from groups IVB to VIIIB10 introduce magnetism in GaSe and InSe. Monolayer gallium selenide (GaSe) and indium selenide (InSe) have demonstrated promise in applications of ultra-thin electronics and optoelectronics. Based on high-throughput first-principles calculations, we investigate the stability and electronic and magnetic properties of substitutionally doped monolayer GaSe and InSe with the aim of broadening their application prospects. Most non-metal dopants from Group A can only be p -type carriers, whereas metals can be both p - and n -type carriers. In addition, considerable magnetism is predicted in GaSe doped with transition metals from periods four to six in groups ⅣB–VIIIB10, with relatively high binding energies, indicating high potential for spintronic applications. Our findings provide theoretical support for experimentally extending the utilization of metal chalcogenides, such as GaSe and InSe, in spintronic, electronic and optoelectronic applications. [ABSTRACT FROM AUTHOR]

The electrical properties and behaviour in constant magnetic field of fourfold expanded GaSe matrix intercalated with SmCl 3 guest were investigated by means of impedance spectroscopy and cyclic voltammetry. It was determined that the synthesized GaSe< SmCl 3> clathrate of 4-fold expansion demonstrates the coexistence of mechanisms of generation, transformation and accumulation of electric energy on a quantum level. These mechanisms are driven from external sources of magnetic, thermal and electric field without Faradaic reactions. Therefore, investigated GaSe< SmCl 3> structure is of great attraction in multivoltaics as a prototype of new class of materials. Quantum mechanical model of electro motive force of spin nature is proposed. The main focus of work lies in the prospects of synthesized clathrates for the development of power nanosources and gyrator-free delay nanolines controlled by means of magnetic field.

The electrical properties and behaviour in constant magnetic field of fourfold expanded GaSe matrix intercalated with SmCl3 guest were investigated by means of impedance spectroscopy and cyclic voltammetry. It was determined that the synthesized GaSe clathrate of 4-fold expansion demonstrates the coexistence of mechanisms of generation, transformation and accumulation of electric energy on a quantum level. These mechanisms are driven from external sources of magnetic, thermal and electric field without Faradaic reactions. Therefore, investigated GaSe structure is of great attraction in multivoltaics as a prototype of new class of materials. Quantum mechanical model of electro motive force of spin nature is proposed. The main focus of work lies in the prospects of synthesized clathrates for the development of power nanosources and gyrator-free delay nanolines controlled by means of magnetic field. [ABSTRACT FROM AUTHOR]

Gallium selenide (GaSe), a layered semiconductor of Group‐III monochalcogenides, has been recognized by the scientific community in recent years as an appealing material in the fields of photonics and (opto)electronics. Thanks to its pseudodirect bandgap and its thickness‐dependent (opto)electronic properties, GaSe has emerged as a promising candidate for the implementation of thin‐film transistors (TFTs) and photodetectors with fast response and high sensitivity. Solution processing of 2D materials provides low‐cost inks that allow the design and realization of printed electronic devices, enabling this technology to move from the laboratory to the industry. In this work, a solution‐processed GaSe‐based light‐driven transistor is presented. Liquid phase exfoliation (LPE) is used to exfoliate bulk GaSe in isopropanol, formulating a functional ink that is subsequently deposited by spray coating onto Si/SiO2 substrates. The GaSe phototransistor exhibits a p‐channel behavior with a high on/off ratio (≈103) that is gate‐voltage dependent. Moreover, the device response also depends on the illumination with a maximum responsivity of 13 A W–1 to UV–visible light and a fast response time of 35 ms. This study demonstrates that liquid phase exfoliated GaSe is a promising candidate for the design and realization of next‐generation (opto)electronic devices. [ABSTRACT FROM AUTHOR]

The paper defined the laser-induced damage threshold from the fluence and the peak power of GaSe and GaSe:In single crystals upon exposure to nanosecond radiation in the two micron range and assessed the influence of test radiation energy parameters (pulse repetition rate, pulse duration) on the damage threshold. Laser-induced damage threshold was determined with the parameters of the incident radiation close to the pump radiation parameters of promising dual-wavelength optical parametric oscillators (effective pump sources for THz difference frequency oscillators): wavelength was ≈2.1 μm; pulse repetition rates were 10, 12, 14, and 20 kHz; and pulse durations were 15, 18, 20, and 22 ns. The obtained results made it possible to conclude that the value of GaSe damage threshold at a wavelength of 2.091 μm of the incident radiation was influenced by the accumulation effects (the damage threshold decreased as the pulse repetition rate increased). The accumulation effects were more significant in the case of the In-doped sample, since a more significant decrease in the damage threshold was observed with increasing frequency in terms of the peak power and the fluence. [ABSTRACT FROM AUTHOR]

Interface interactions between two distinct layered materials of vertically stacked van der Waals (vdW) heterostructures play a vital role in the modulation of their electrical/optoelectronic properties. Meanwhile, intriguing properties, e.g., ultrafast charge transfer (Jin Nat Nanotechnol. , 2018 , 13 , 994 − 1003) and moiré excitons (Tran et al Nature , 2019 , 567 , 71 − 75), have been discovered in strongly coupled heterobilayers composed of transition metal dichalcogenides; post-transition metal chalcogenides GaSe and InSe remain unexplored. Here, we use photoluminescence measurements and a scanning photoelectron microscope for electronic characterization of multilayer vertically stacked GaSe/InSe samples coupled with two-probe electrical measurements to reveal the carrier transport and optoelectronic properties at a strongly coupled GaSe/InSe vdW interface. A significant PL quenching and a shift of the valence band with respect to the Fermi level occurred at the heterojunction, evidencing the strong coupling of charge transfer between the GaSe and InSe layers. The band alignment at this heterointerface was determined as a type-II band alignment, similar to a weakly coupled system; however, charge transfer led to charge redistribution in the vicinity of the heterointerface, resulting in an interface dipole. Strong interface coupling also made the origin of the forward current different from that of the weakly coupled system; it is governed by interlayer recombination between the majority carriers rather than carrier injection over conduction band offsets. Moreover, tuning the band alignment via modulation of the majority carriers could further change the origin of the photocurrent. Our results not only present the first charge transport study of a strongly coupled multilayer GaSe/InSe system but also open up the way to further utilizing vdW interfaces in the applications of next-generation optoelectronics. [ABSTRACT FROM AUTHOR]

Built-in electric fields appear during the molecular-beam epitaxy of GaSe on a GaAs(001) substrate at the GaSe/GaAs interface, the presence of the fields is evidenced by Franz–Keldysh oscillations observed in photoreflectance spectra. The varying values of the strength of these fields (from 9.8 to 17.6 kV/cm) can be associated both with the diffusion of Se atoms into the GaAs substrate (buffer layer) and with the formation of transient submonolayers during epitaxial growth. Built-in fields are not observed at the interface of structures grown on GaAs(111)B and GaAs(112) substrates, which can be explained by the lower propagation efficiency of Se into the substrate with mentioned orientations when compared with GaAs(001). [ABSTRACT FROM AUTHOR]

In this study, Al/p-GaSe/In Schottky diodes (SDs) were fabricated by using a p-type GaSe semiconductor grown by the Brigdman-Stockbarger method. Current–voltage (I–V) measurements of the Al/p-GaSe/In diode were carried out from 100 K to 400 K with steps of 20 K in the +2 V- (−2) V voltage range and in the dark. The characteristic parameters of SDs were calculated from ln(I)-V characteristics according to the thermoionic emission (TE) theory and from modified Norde's functions depending on the sample temperature. The barrier height (BH) values calculated from ln(I)-V and Norde's functions were in agreement. The values of BH and ideality factor (IF) calculated from ln(I)-V characteristics for the SD ranged from 0.97 eV and 1.01 at 400 K to 0.45 eV and 2.18 at 100 K, respectively. The value of serial resistance (Rs) calculated from modified Norde's functions of the SD decreased with increasing temperature. Also, the reverse bias current value of the Al/p-GaSe/In SD under light illumination was higher than the current value in the dark. Thus, the obtained results show that the prepared Al/p-GaSe/In diode might be used as a photodiode in the optoelectronic applications. [ABSTRACT FROM AUTHOR]

The nonlinear absorption in GaSe crystals at high optical excitation levels is experimentally studied. A Nd:YAG laser (first and second harmonics, 1064 and 532 nm) and a liquid laser (tuning range 594–643 nm) were used as excitation sources. It is shown that the features observed in the exciton resonance region of the absorption, luminescence, and photoconductivity spectra of GaSe crystals at high optical excitation levels can be explained by exciton–exciton interaction and Coulomb interaction screening by free carriers. [ABSTRACT FROM AUTHOR]