Direct Bandgap Properties Research Articles

The increase of the scattering at high electric fields in multilayer ReS2 FETs: Output characteristics and 1/f noise

Field-effect transistors (FETs) employing two-dimensional (2D) materials have attracted significant attention as a potential alternative to silicon FETs. Among these materials, multilayer rhenium disulfide (ReS2) has emerged as a focal point of interest owing to its distinctive direct bandgap properties. While there is extensive research on the electrical characteristics and doping, studies on the changes in electrical properties during scale-down for practical applications are insufficient. In this study, we investigated the mobility reduction of ReS2 FETs at high drain bias of ReS2 FETs by comparing the different channel lengths of 0.24 μm and 1.5 μm. A reduction in mobility was observed for the shorter channel length, attributed to the enhanced scattering factor at high electric field. To assess the impact of scattering degradation, we conducted a low-frequency noise analysis at drain-source voltage (VDS) = 0.4 V and VDS = 3.0 V for the 0.24 μm length FET. The decrease of the Hooge parameter (αH) at high VDS was observed, which was attributed to an augmentation in Coulomb scattering. This study observed mobility degradation under high electrical fields during scale-down and identified the cause of mobility degradation through low-frequency noise analysis. This contributes to scaling down for practical applications of 2D FETs.

Impact of Carrier Gas Flow Rate on the Synthesis of Monolayer WSe2 via Hydrogen-Assisted Chemical Vapor Deposition.

Transition metal dichalcogenides (TMDs), particularly monolayer TMDs with direct bandgap properties, are key to advancing optoelectronic device technology. WSe2 stands out due to its adjustable carrier transport, making it a prime candidate for optoelectronic applications. This study explores monolayer WSe2 synthesis via H2-assisted CVD, focusing on how carrier gas flow rate affects WSe2 quality. A comprehensive characterization of monolayer WSe2 was conducted using OM (optical microscope), Raman spectroscopy, PL spectroscopy, AFM, SEM, XPS, HRTEM, and XRD. It was found that H2 incorporation and flow rate critically influence WSe2's growth and structural integrity, with low flow rates favoring precursor concentration for product formation and high rates causing disintegration of existing structures. This research accentuates the significance of fine-tuning the carrier gas flow rate for optimizing monolayer WSe2 synthesis, offering insights for fabricating monolayer TMDs like WS2, MoSe2, and MoS2, and facilitating their broader integration into optoelectronic devices.

A study on the Si1-xGex gradual buffer layer of III-V/Si multi-junction solar cells based on first-principles calculations.

III-V/Si multi-junction solar cells have been widely studied in recent years due to their excellent theoretical efficiency (∼42%). In order to solve the problem of lattice mismatch between Si and III-V compounds of III-V/Si solar cells, different hexagonal Si1-xGex buffer layer models on the surface of hexagonal diamond Si(001) were built, and the structural, electronic and optical properties of the proposed models were calculated based on first principles calculations. The results showed that all models of the designed buffer layer could effectively reduce the lattice mismatch, and the buffer layer hex-Si1-xGex (x = 0, 0.75, and 1) is the ideal model and has achieved the best lattice-matching improvement with high defect formation energy, as well as direct band gap properties and a larger light adsorption coefficient. These theoretical models, with their analyzed properties, could offer a promising pathway toward realizing high efficiency and low cost III-V/Si multi-junction solar cells.

Effect of substituting on the transition dipole moment of the double perovskite Cs2AgInCl6

The all-inorganic double perovskite Cs2AgInCl6 with three dimensional structure has attracted much attention due to its direct bandgap property and particular luminescence mechanism, which is self-trapped exciton emission. However, it is a pity that Cs2AgInCl6 exhibits low photoluminescence quantum yield, which affects its application for light-emitting devices. In this paper, the band structure and transition dipole moment of Cs2AgIn(1−x)Sb x Cl6 (x = 0, 0.25, 0.5, 0.75) are calculated using first principle calculation. The calculated results shows that the pure material Cs2AgInCl6 not only has a large band gap but also has the dipole forbidden transition, which means that the electrons cannot be excited from the valence band maximum to the conduction band minimum. However, the substituted Cs2AgIn0.75Sb0.25Cl6 have a good property for the band gap about 3.066 eV and break forbidden transition at point X. The reason for its change is due to the overlap of electron and hole for charge density. Our work provides theoretical guidance for the design of more efficient light-emitting devices.

Optical Signatures of Phosphorene Chemical Evolvement in Liquid Environment

Phosphorene possesses a unique combination of physical and chemical properties, including tunable direct bandgap, anisotropic electronic and optical properties. However, application of phosphorene is hampered by its fast degradation under ambient conditions. It is believed that H2O and O2 play key roles in the instability of phosphorene, but their exact contributions have not yet been completely understood in experiment. Herein, a technique of probing the mechanism of phosphorene oxidation through the evolvement of its photoluminescence spectra in a liquid suspension is introduced. With the addition of H2O2, the photoluminescence intensity of the suspension surprisingly increases, indicating a passivation effect due to the formation of phosphorene oxide, which is also verified by Raman spectroscopy. In contrast, direct addition of H2O leads to irreversible and rapid degradation as shown by the quenching of photoluminescence. Furthermore, monolayer phosphorene suspension photoluminescence at 2.17 eV is obtained by completely eliminating effects from contaminations, substrate strain, dielectric substrate, and oxidation of samples. Solvent protection in combination with the optical probing method provides an effective approach in investigating the chemistry of active materials like phosphorene. Phosphorene has great potential in the range of sensing applications including high‐resolution H2O2 and humidity sensors.

Synergistic effect of dual-site occupation and activators confinement realizing an efficient ultra-broadband NIR emitter

Broadband near-infrared (NIR) phosphor-conversion light-emitting diode (pc-LED) devices have an extensive application prospect in night vision monitoring, nondestructive testing, and biomedical imaging. In this work, the ultra-broadband NIR emission produced by the chromium-activated NaZn(PO3)3 phosphor with two-site occupation peaks at around 900 nm with a full width at half-maximum (FWHM) of 218 nm. Time-resolved spectroscopic techniques and steady-state fluorescence spectra coupled with crystal structure confirm the multiple occupations of the chromium ion. Notably, the NaZn(PO3)3: Cr3+ phosphors exhibit an internal quantum efficiency (IQY) of 80.6 % and an external quantum efficiency (EQY) of 35.4 %, which may originate from the activators confinement, direct band-gap property, and high absorbance. To better comprehend the connection between structure and luminescence capabilities, the band structure, crystal field strength parameters, fluorescence lifetime, and thermal stability were all thoroughly investigated. Finally, the optimal sample has demonstrated its application in the quantitative analysis of ethanol/water mixtures. The results provide a novel perspective for the rational design of desired emitters.

Enhancing the performance of ZnGa2O4 metal-semiconductor-metal ultraviolet solar-blind photodetectors by surface fluorine plasma sensitization

ZnGa2O4 has a wide direct bandgap and excellent optical properties and is an emerging material for use in solar-blind ultraviolet (UV) photodetectors (PDs). Recently, metal-semiconductor-metal (MSM) based devices have gained popularity because of their low production cost, minimal design, flexible integration, and tunable performance. However, such devices suffer from low sensitivity. Herein, we propose a sensitization engineering strategy that involves surface fluorine plasma (SFP) treatment to enhance the performance of a ZnGa2O4 MSM solar-blind UV PD. Through this strategy oxygen vacancy defects are simultaneously passivated and the ratio of adsorbed oxygen species on the surface of the ZnGa2O4 thin film is changed. These modifications optimize the current under both dark and light conditions. In particular, the dark current has a trade-off relationship between the reduction in the oxygen vacancy concentration and changes in the crystal grain boundaries. As a result, the photocurrent is enhanced because of the increased amount of adsorbed oxygen. The PD prepared using a ZnGa2O4 thin film treated by SFP for 15 min exhibited excellent performance, specifically, it showed a large responsivity (42.22 A/W) and good specific detectivity (5.72 × 1014 Jones). Notably, these values are superior to those of most reported MSM solar-blind PDs. Moreover, the excellent rejection ratio and durability during repeated use demonstrate the benefits of SFP treatment for enhancing operating stability. Our study provides an effective avenue for optimizing the optoelectronic performance of oxide semiconductors.

Growth-favored nonpolar BAlN digital alloy with cation-order based tunable electronic structure

BAlN, an ultra-wide bandgap semiconductor, has attracted the interest of many due to its potential in high-power electronics and high-efficiency optoelectronics. However, the solid solution alloy (SSA) grown along [0001] direction experiences phase separation, which poses a challenge for device applications. To overcome the issues, we proposed BAlN-based digital alloy (DA) arranged along [10–10] and [11–20] directions. Among the whole range of B composition, the mixing enthalpy of DA is at most 16 % and 13 % of the SSA for [10–10] and [11–20] directions, suggesting an improved growth feasibility of DA. The characteristics of DA, including phonon dispersions, bandgap value, dielectric constant, and potential piezo-response along [0001] direction, were thoroughly investigated. Interestingly, cation orders at the same composition offer a wide range of electric, optical, and mechanical properties, some of which are even better than the conventional SSA. It was possible to achieve a maximum tunability of 25 % for the bandgap, while still maintaining direct bandgap properties for up to 67 % boron component. The DA exhibits excellent thermodynamic stability and its characteristics can be easily adjusted, making it a suitable material for high-quality ultra-wide bandgap BAlN alloy growth and applications.

Review on III–V Semiconductor Nanowire Array Infrared Photodetectors

AbstractIn recent years, III–V semiconductor nanowires have been widely investigated for infrared photodetector applications due to their direct and suitable bandgap, unique optical and electrical properties, flexibility in device design and to create heterostructures, and/or grow on a foreign substrate such as Si with more effective strain relaxation compared with planar structures. In particular, vertically aligned and ordered nanowire arrays have emerged as a promising photodetector platform, since their geometry‐related light absorption and carrier transport properties can be tailored to achieve high photodetector performance and new functionalities. In this article, the state‐of‐the‐art progress in the development of various types of infrared photodetectors based on III–V semiconductor nanowire arrays is reviewed. The nanowire synthesis/fabrication methods are introduced briefly at first, followed by discussions on the working principle and device performance of various types of nanowire array‐based photodetectors and their emerging applications. Finally, we analyze the challenges and present the perspectives for the development of future low‐cost, large‐scale, high‐performance nanowire array infrared photodetectors for practical applications.

Photoluminescence characterization of GeSn prepared by rapid melting growth method

In this work, single crystalline GeSn on insulator (GSOI) were grown by rapid melting growth (RMG). The Sn content distribution along the strip was investigated, reaching to 7.8% near the end of the strip. The GeSn strip had high quality with no threading dislocations, as revealed by cross-sectional transmission electron microscopy (XTEM). Photoluminescence (PL) measurements along the strip were performed to check the indirect-direct bandgap transition Sn content of GeSn alloys. It was found the integrated PL intensity increased about 6.6 times for Sn content 0.86%–7.8%. For GeSn with Sn content 6.1%, the temperature-dependent PL shows that the energy difference between direct Γ-Γ valley and indirect L-Γ valley decreases due to the bandgap narrowing (BGN) effect, and the direct band transition gradually dominates the PL spectra as temperature increases. The temperature-dependent PL spectra have only one peak for GeSn with Sn content 7.8%, suggesting the direct bandgap property. These results indicate that GeSn grown by RMG is very promising for fabricating efficient Si based light sources.

Surface plasmon-enhanced photodetection of monolayers MoS2 on an ion beam modified functional substrate

Monolayer molybdenum disulfide (1L-MoS2) is considered a potential optoelectronic device material due to its ultrathin and direct bandgap properties. However, the absorption of incident light by 1L-MoS2 has shown to be relatively low and is not sufficient to implement high photoelectric conversion efficiency, limiting its practical applications in photodetectors. Due to the local surface plasmon resonance effect, the integration of plasma nanoparticles (NPs) with 2D materials may provide a promising method for enhancing light–matter interactions. Nevertheless, MoS2 may undergo fold deformation when transferred to the plasma structure when prepared via conventional strategies, resulting in the introduction of larger defects. In this work, we reported on a photodetector with enhanced MoS2 photoresponsivity on a flat plasmon functional substrate, in which the Ag NPs were embedded into fused silica (SiO2) by ion implantation. Using MoS2/Ag NPs:SiO2 architecture, the photocurrent of the MoS2-based photodetector was significantly improved under incident light of 375, 532, and 635 nm, with a maximum increase of 72.8 times, while the response time also decreased to a certain extent. Furthermore, the plasma functional substrate had the advantages of environmental stability and repeatable recycling, allowing it to be easily integrated with different 2D materials. Thus, this work offered a viable path for realizing efficient photodetectors based on 2D material.

First-principles determination of high thermal conductivity of PCF-graphene: A comparison with graphene

Poly-cyclooctatetraene framework (PCF)-graphene, an emerging all-sp2 hybridized two-dimensional (2D) carbon allotrope, possesses an intrinsic direct bandgap (0.77 eV) and excellent mechanical properties, indicating great potential in nanoelectronics. Understanding the thermal transport behavior of PCF-graphene is of vital importance for determining the reliability of related devices based on it. In this work, the thermal transport in PCF-graphene is systematically studied using the Boltzmann transport theory combined with first-principles calculations. The results show that the room-temperature thermal conductivity of PCF-graphene with only considering three-phonon scattering is as high as 1587.3 W/m K along the zigzag direction, and decreases by 27.1% (1157.4 W/m K) when including four-phonon scattering, indicating the four-phonon scattering plays a non-negligible role in in thermal transport. Although the thermal conductivity of PCF-graphene is not as large as that in graphene, it still exceeds most common 2D materials and makes it suitable for applications in the thermal management of microelectronics. Analyses of phonon group velocity and phonon scattering rates are conducted to reveal the high thermal conductivity of PCF. Moreover, as the temperature increases to 800 K, the reduction of thermal conductivity is close to 50% after including four-phonon scattering. The analysis of phonon group velocity and phonon scattering rates are conducted to reveal the underlying mechanism. Our results provide insights for constructing high-thermal-conductivity materials based on 2D carbon allotropes.

Group IV element allotropes in the Fmmm phase: First-principles calculations

Three new Group IV element allotropes Fmmm C72, Si72, and Ge72 are proposed in this paper, and their physical properties are researched by density functional theory (DFT). Each novel allotrope has mechanical, dynamical, and thermodynamic stability. The study of the elastic modulus shows that Fmmm Si72 has greater elastic anisotropy than Fmmm C72 and Ge72. Fmmm C72 is a superhard material with the hardness of 47.8 GPa. The investigation of the electronic band structures of Fmmm Si72 and Ge72 exhibits they have semiconducting properties, while Fmmm C72 shows metallicity. Fmmm Si72 has a direct band gap (1.41 eV), and it is very suitable for solar cells. In addition, Fmmm Si72 has good absorption capacity in the visible light region, so Si72 has possible applications in photovoltaic applications due to its direct band gap property and good absorption capacity in the visible light region.

Hexagonal boron phosphide and boron arsenide van der Waals heterostructure as high-efficiency solar cell

The rapid development of two-dimensional (2D) materials offers new opportunities for 2D ultra-thin excitonic solar cells (XSCs). The construction of van der Waals heterostructure (vdWH) is a recognised and effective method of integrating the properties of single-layer 2D materials, creating particularly superior performance. Here, the prospects of h-BP/h-BAs vdW heterostructures in 2D excitonic solar cells are assessed. We systematically investigate the electronic properties and optical properties of heterogeneous structures by using the density functional theory (DFT) and first-principles calculations. The results indicate that the heterogeneous structure has good optoelectronic properties, such as a suitable direct bandgap and excellent optical absorption properties. The calculation of the phonon spectrum also confirms the well-defined kinetic stability of the heterstructure. We design the heterogeneous structure as a model for solar cells, and calculate its solar cell power conversion efficiency which reaches up to 16.51% and is higher than the highest efficiency reported in organic solar cells (11.7%). Our work illustrates the potential of h-BP/h-BAs heterostructure as a candidate for high-efficiency 2D excitonic solar cells.

Theoretical study of M 6 X 2 and M 6 XX′ structure (M = Au, Ag; X,X′ = S, Se): Electronic and optical properties, ability of photocatalytic water splitting, and tunable properties under biaxial strain

MoS2, a transition metal dichalcogenide (TMDC), has attracted significant amount of attention due to its direct bandgap, tunability and optical properties. Recently, a novel structure consisting of MoS2 and noble metal nanoclusters has been reported. Inspired by this, first principle calculations are implemented to predict the structures of M 6 X 2 and M 6 XX′ (M = Au, Ag; X, X′ = S, Se). The calculated bandgap, band edge position, and optical absorption of these structures prove that the silver compounds (Ag6 X 2 and Ag6 XX′) have great potential for catalytic water splitting. In addition, biaxial strain (tensile strain and compressive strain) is applied to adjust the properties of these materials. The bandgap presents a quasi-linear trend with the increase of the applied strain. Moreover, the transition between the direct and indirect bandgap is found. The outstanding electronic and optical properties of these materials provide strong evidence for their application in microelectronic devices, photoelectric devices, and photocatalytic materials.

Flexible Sb0.405Te0.595 photodetectors with broadband spectral response up to 4.5 µm

As one of the third-generation topological insulators (TIs), antimony telluride (Sb2Te3) is considered as a prospective candidate for extremely sensitive and broadband photodetectors ascribed to its extremely narrow direct band gap and exotic optical properties. However, due to the ultrafast carrier recombination time and small carrier lifetime of Sb2Te3, it continues to be an enormous challenge to achieve broadband detection in the mid-wave infrared (MWIR) region. Here, distinctive broadband photodetectors based on stable and scalable high-quality Sn-catalyzed Sb0.405Te0.595 films grown by physical vapor deposition (PVD) was described, and they were found to be photoconductive and sensitive in the visible light (VL) - MWIR region (405–4500 nm). The optimized responsivity (Ri) and specific detectivity (D*) of photodetectors achieved 588 A/W and 6.435 × 108 Jones, respectively. Additionally, these devices exhibit fantastic mechanical flexibility, durability and stability following 200 bending cycles without obvious degradation of photoelectric performance. These excellent properties provide new strategy and insight into the manufacture of high-performance and wearable photodetectors for energy efficient nanoelectronics.

Localized surface plasmon resonance-enhanced solar-blind Al0.4Ga0.6N MSM photodetectors exhibiting high-temperature robustness

The appealing properties of tunable direct wide bandgap, high-temperature robustness and chemical hardness, make Al x Ga1−x N a promising candidate for fabricating robust solar-blind photodetectors (PDs). In this work, we have utilized the optical phenomenon of localized surface plasmon resonance (LSPR) in metal nanoparticles (NPs) to significantly enhance the performance of solar-blind Al0.4Ga0.6N metal–semiconductor–metal PDs that exhibit high-temperature robustness. We demonstrate that the presence of palladium (Pd) NPs leads to a remarkable enhancement by nearly 600, 300, and 462%, respectively, in the photo-to-dark current ratio (PDCR), responsivity, and specific detectivity of the Al0.4Ga0.6N PD at the wavelength of 280 nm. Using the optical power density of only 32 μW cm−2 at −10 V, maximum values of ∼3 × 103, 2.7 AW−1, and 2.4 × 1013 Jones are found for the PDCR, responsivity and specific detectivity, respectively. The experimental observations are supported by finite difference time domain simulations, which clearly indicate the presence of LSPR in Pd NPs decorated on the surface of Al0.4Ga0.6N. The mechanism behind the enhancement is investigated in detail, and is ascribed to the LSPR induced effects, namely, improved optical absorption, enhanced local electric field and LSPR sensitization effect. Moreover, the PD exhibits a stable operation up to 400 K, thereby exhibiting the high-temperature robustness desirable for commercial applications.

Lead-Free Ultra-Wide Direct Bandgap Perovskite EACaI3

Recently, perovskite-based light-emitting devices (LEDs) have been invested with near-infrared, red, green, and blue emissions. And, short-wavelength perovskite LEDs targeted at violet emission remain a significant challenge. Moreover, the majority of previously reported devices were focused on the lead-containing halide perovskites. In this research, for the first time, EACaI₃ (270 nm) was proposed as ultraviolet-C potential lead-free perovskite material with direct ultra-wide bandgap property with 0.03% imaginary part of phonon density of state. The perovskite EACaI₃ has dynamic stability and could be used for devices demonstrated with excellent working stability. These results suggest that the lead-free EACaI₃ are potentially attractive candidates for preparing environment-friendly and stable ultra-wide band gap device.

Two-dimensional monolayer B2P6 with anisotropic elastic properties and carrier mobility

A novel two-dimensional (2D) monolayer B2P6 allotrope is proposed by the first-principles calculations. The monolayer B2P6 is demonstrated as stable by the combination of cohesive energy, phonon dispersion and elastic constants. Moreover, monolayer B2P6 has anisotropic Young's modulus and negative Poisson ratio. The carrier mobility is also anisotropic and can reach thousands of cm2V−1s−1, which is close to that of monolayer black phosphorene. Furthermore, monolayer B2P6 has a direct band gap of 0.71eV (HES06) and effective optical absorption over both infrared and visible light range.The combined direct band gap, ultrahigh mobility and optical properties render monolayer B2P6 a broad application prospect in the field of nano-electronic devices and thin-film solar cells. Finally, calculations for OH adsorption show the possible synthesis of monolayer B2P6 in water environment and also potential applications such as electrocatalytic hydrogen production.

Novel double-perovskite SrLaLiTeO6:Mn4+ far-red phosphor with superior thermal stability for indoor plant growth LED

A series of red-emitting phosphors SrLaLiTe1-xO6: xMn4+ were prepared via high-temperature solid-state reaction technique. The structure of SrLaLiTeO6 (abbreviated as SLLT) is a typical cubic double-perovskite structure with the space group of Fm-3m (225). Based on the density functional theory (DFT) calculation, the SrLaLiTeO6 presents a direct band gap property with the band gap energy (Eg) of 3.22eV. The estimated Eg of SrLaLiTeO6 and SrLaLiTe0.988O6: 0.012Mn4+ were evaluated to be 3.08 eV and 2.98 eV derived from diffuse reflectance spectra, respectively. The emission spectra show a narrow emission band from 650 nm (15384 cm−1) to 770 nm (12987 cm−1) peaking at 696 nm and 708 nm, which exhibits a greater overlap with the phytochrome (PR and PFR) absorption. Furthermore, the nephelauxetic effect of Mn4+ in the SLLT host were estimated in detail by using the crystal field strength (Dq) and Racah parameters (B, C). Finally, the intensity remained at 79.01% and 81.44% of the original intensity when the temperature was elevated at 423 K, respectively, which illustrates good thermal stability. All these results manifest that SrLaLiTeO6: Mn4+ is expected to be a candidate for indoor plant cultivation LED.