Narrow-gap Semiconductors Research Articles

Mid-Infrared, Optically Active Black Phosphorus Thin Films on Centimeter Scale.

Black phosphorus (BP) is a narrow bandgap (∼0.3 eV) semiconductor with a great potential for optoelectronic devices in the mid-infrared wavelength. However, it has been challenging to achieve a high-quality scalable BP thin film. Here we present the successful synthesis of optically active BP films on a centimeter scale. We utilize the pulsed laser deposition of amorphous red phosphorus, another allotrope of phosphorus, followed by a high-pressure treatment at ∼8 GPa to induce a phase conversion into BP crystals. The crystalline quality was improved through thermal annealing, resulting in the observation of photoluminescence emission at mid-infrared wavelengths. We demonstrate high-pressure conversion on a centimeter scale with a continuous film with a thickness of ∼18 nm using a flat-belt-type high-pressure apparatus. This synthesis procedure presents a promising route to obtain optical-quality BP films, enabling the exploration of integrated optoelectronic device applications such as light-emitting devices and mid-infrared cameras on a chip scale.

Optical properties of InSb derived from reflection electron energy loss spectroscopy spectrum

The energy loss function (ELF) of the narrow bandgap semiconductor, InSb, was derived from the reflection electron energy loss spectroscopy (REELS) spectrum measured at 4 keV incident electron energy in an energy loss range of 1.6–200 eV using a high energy resolution electron spectrometer. The experimental spectrum was analyzed with the reverse Monte Carlo (RMC) method, which integrates the classical trajectory Monte Carlo simulation with the simulated annealing method for optimizing the trial ELF. Subsequently, the complex dielectric function ε(ω), refractive index and extinction coefficient were determined from the obtained ELF in the energy loss (i.e. photon energy) range of 1.6–200 eV. The validity of the obtained data was verified by the f-sum rule, ps-sum rule, inertial sum rule and dc-conductivity sum rule. We found that our calculated data of InSb fulfill the sum rules with very high accuracy; therefore, the use of these calculated optical data in material science and surface analysis is highly recommended for further applications.

On semiconductor–metal transition in FeSi induced by ultrahigh magnetic field

At low temperatures, iron monosilicide is a strongly correlated narrow-gap semiconductor. A first order transition to metal state induced by magnetic field was observed for the first time at 355 T in Ref. [Yu. B. Kudasov et al., JETP Lett. 68 (1998) 350]. However, recently a smooth transition from 230 T to 270 T was found under similar conditions in Ref. [D. Nakamura et al., Phys. Rev. Lett. 127 (2021) 156601]. This discrepancy goes far beyond experimental errors and deserves a careful study. A methodological analysis of inductive and RF techniques of conductivity measurements shows that the difference of these critical magnetic field estimations stems from a divergence in dynamic ranges of the techniques. In fact, the above mentioned methods supplement each other. The semiconductor-metal transition under magnetic field in FeSi is a complex phenomenon which occurs at the wide range of magnetic fields.

Deposition of Sn-Zr-Se precursor by thermal evaporation and PLD for the synthesis of SnZrSe3 thin films

Background ABX3 (X=Se, S) chalcogenides are an emerging class of materials for sustainable photovoltaics. Among ABX3 materials, BaZrS3 has gained the highest community interest. BaZrS3 is the wide bandgap absorber (> 1.7 eV) and therefore is intended for application as a top sub-cell in multijunction devices. However, narrow band gap ABX3 compounds have drawn little attention although this could potentially open the path for fabrication of multijunction solar cells based entirely on ABX3 materials. SnZrSe3 is a narrow bandgap semiconductor with an absorption edge located at around 1.0 eV, but there are no reports on the formation of SnZrSe3 thin films thus far. In this work, therefore, we aim to obtain SnZrSe3 thin films by sublimation methods. Methods Thermal evaporation and pulsed laser deposition (PLD) techniques were used to deposit Sn-Zr-Se precursor films. SnZrSe3 powder was synthesized and used as a source material for evaporation and PLD target preparation. Precursor films were deposited by PLD from single-phase and binary selenide targets. Results We found that using SnZrSe3 powder, only SnSe films were deposited under various conditions by thermal evaporation. Precursor films obtained by PLD from single-phase targets were amorphous and comprised SnSe2 and a-Se phases whereas using binary targets crystalline SnSe and a-Se were detected. Thermogravimetric analysis revealed that SnZrSe3 was thermally stable up to 450 °C and afterwards decomposed into SnSe, Se and ZrSe2-x. Conclusions Using methods described in this work, we were not able to achieve congruent sublimation of SnZrSe3 because of the following reasons: (i) upon energetic excitation, SnZrSe3 decomposes into compounds with very different vapour pressure; (ii) inability to substitute O with Se due to very high chemical affinity of Zr and O. Direct sublimation methods are challenging for formation of SnZrSe3 thin films and other techniques, such as co-evaporation should be explored.

Quasi-type-II Cu-In-Zn-S/Ni-MOF heterostructure with prolonged carrier lifetime for photocatalytic hydrogen production

Visible-driven photocatalytic hydrogen production using narrow-bandgap semiconductors has great potential for clean energy development. However, the widespread use of these semiconductors is limited due to problems such as severe charge recombination and slow surface reactions. Herein, a quasi-type-II heterostructure was constructed by combining bifunctional Ni-based metal–organic framework (Ni-MOF) nanosheets with BDC (1,4-benzenedicarboxylic acid) linker coupled with Cu-In-Zn-S quantum dots (CIZS QDs). This heterostructure exhibited a prolonged charge carrier lifetime and abundant active sites, leading to significantly improved hydrogen production rate. The optimized rate achieved by the CIZS/Ni-MOF heterostructure was 2642 μmol g−1 h−1, which is 5.28 times higher than that of the CIZS QDs. This improved performance can be attributed to the quasi-type-II band alignment between the CIZS QDs and Ni-MOF, which facilitates effective delocalization of the photogenerated electrons within the system. Additional photoelectrochemical tests confirmed the well-maintained photoluminescence and prolonged charge carrier lifetime of the CIZS/Ni-MOF heterostructure. This study provides valuable insights into the use of multifunctional MOFs in the development of highly efficient composite photocatalysts, extending beyond their role in light harvesting and charge separation.

Time-resolved mid-infrared photoluminescence spectroscopy of an undoped InAs substrate

Time-resolved mid-infrared photoluminescence (PL) spectroscopy of an undoped InAs substrate has been achieved with wavelength upconversion and time-correlated single photon counting methods. The substrate exhibits multiple PL peaks at photon energies of around 0.415 eV, and the peak positions and intensities change as the temperature is varied from 3.7 to 80 K. The dominant PL peaks are attributed to free and donor-bound excitons and radiative recombination between electrons at the Fermi edge in the conduction band and holes in the valence band edge. The PL lifetime of the excitons is 12 ns, which is four times longer than that of GaAs. The band edge electron–hole recombination has a longer PL lifetime of 60 ns at 20 K. The unveiling of luminescence dynamics in narrow bandgap semiconductors will contribute to the development of mid-infrared light-emitting devices.

Mechanical and dynamical stability, electronic and bonding properties of a new narrow-gap semiconductor YPdAs Half-Heusler: DFT and QTAIM study

YPdAs is a semiconductor with an indirect band-gap of a value close to that of the direct one. Despite its important semiconductor behaviour, this half-Heusler compound has not yet been synthesized. Therefore, the main objective of this work is to confirm its energetic, mechanical and dynamical stability to prove that it can be formed experimentally and exploit it in optoelectronic applications. We have adopted two calculation methods to ensure accuracy of the results: FP-LAPW and PPs. Both methods gave negative formation energy values, which confirmed the energetic stability of YPdAs. Furthermore, YPdHCAs structure is the most stable. In this structure, the estimated elastic stiffness constants Cij perfectly satisfy the mechanical stability criteria. The analysis of phonon dispersion curve has shown a similarity between the two adopted methods (Direct method and DFPT). Given the absence of soft modes, the dynamical stability is confirmed for the first time. From the above, we confirm the possibility of synthesizing this compound experimentally. The electronic results show that the band-gap is indirect. There is no band inversion between VBM and CBM, which shows the trivial nature of this semiconductor. The detailed analysis of the bonds between the different atoms of this compound using QTAIM has shown the presence of only two bonds (Y–Pd and Pd–As) that have a similar character of a dative nature.

Perspective on defect characterization in semiconductors by positron annihilation spectroscopy

This Perspective focuses on experimental and theoretical aspects of positron annihilation spectroscopy. This set of methods is highly suitable for identifying and quantifying vacancy-type defects in semiconductors and also allows for analyzing their physics characteristics. We present selected examples from the past decade, where the methods have been used for obtaining timely and useful insights into the defect-controlled phenomenon in narrow-gap (Ge, GaSb) and wide-gap (III-nitride, oxide) semiconductors. We also discuss possible future developments that may allow more detailed studies in novel semiconductor materials and devices with ever more complex lattice structures.

Enhanced minority carrier lifetime in bulk hydrogen-passivated InAsSbBi

We investigate the bulk passivation of the dilute bismide alloy InAsSbBi by plasma-assisted hydrogenation. InAsSbBi is of significant interest for mid- to long-wave infrared photodetection due to its bandgap flexibility and potential integration with heterostructured photodetector architectures. Epitaxially grown InAsSbBi samples are characterized by photoluminescence and time-resolved photoluminescence measurements for a range of hydrogenation conditions. Increases in the minority carrier lifetime of over 3× are reported, with no degradation over a period of months following the treatment. Photoluminescence measurements confirm that the hydrogenation process improves the InAsSbBi optical properties. These results offer a path toward the improved performance of InAsSbBi-based photodetectors and potentially other narrow bandgap semiconductor materials and material systems.

Controlled and automatic fabrication of ultra small SnO2-CdS nano-heterojunction via continuous flow method designed for photochemical treatment of dye contaminated natural water resources

Contaminated organics such as dyes represent a most important and rising source of environmental pollution. A capable approach to remedy this utilizes semiconductors to catalytically photodegrade the stable bonds in these toxic dye molecules. Heterostructured photocatalysts composed of two dissimilar semiconductors can conquer difficulties of rapid electron-hole recombination and inefficient light harvesting whilst reducing dependency on either constituent independently. At present fabrication of scalable, non-agglomerated, and monodispersed semiconductor photocatalysts by using continuous flow microreactor technology pays severe concern since it presents a range of advantages compared to the conventional batch reactor technology. In this contribution, we advance the prior art by introducing a simplistic, commercial, and environmentally benign continuous flow microreactor synthetic approach for designing a SnO2-CdS nano-heterojunction at a reasonably low reaction temperature and very short reaction period. Up-to-date, very few researchers are able to prepare the heterojunction nanocomposites such as SnO2-CdS through a microfluidic approach and as well the corresponding ultra-low particle size is not reported either. A PTFE (polytetrafluoroethylene)-based helical tubular microreactor is being exploited that comprises a compact structure and offers improved heat and mass transport as well as better intermixing among the reactants. Heterostructure formation is confirmed by X-ray diffraction (XRD), FTIR spectroscopy, and transmission electron microscopy (TEM) analysis. Mott-Schottky (M−S) and Diffused Reflectance (DRS) analyses jointly demonstrate that the heterostructured nanomaterial, comprising a narrow bandgap semiconductor (CdS) and a wide band-gap semiconductor (SnO2), possesses significant potential as a photocatalyst. Furthermore, when applied in the degradation of Congo red (CR) dye under simulated solar conditions, it clearly exhibits superior photocatalytic activity compared to its individual SnO2 and CdS components. The efficient charge separation in this heterostructure is confirmed by the time resolved spectroscopic study. It is monitored that a strong type-II interaction is exhibited in the heterostructure and a plausible mechanism has been utilized to clarify this behaviour.

Plasmonically-boosted exciton-photon coupling strength in a near-infrared LED based on a ZnO:Ga microwire/GaAs heterojunction with surface-coated Au&Ag alloy nanorods.

The development of electrically-driven low-dimensional coherent light sources via highly-polarized polariton emission behavior has been extensively researched, but suffers from limited modulation of the exciton-photon coupling strengths. Herein, an electrically-biased near-infrared exciton-polariton light-emitting diode (LED), which includes a Ga-doped ZnO microwire (ZnO:Ga MW) and p-type GaAs substrate, is demonstrated. The well-designed LED structure is conducive to producing strong coupling between excitons and cavity photons, thus yielding highly-polarized light-emissions due to the optical birefringence in the ZnO:Ga MW microcavity. In particular, when the LED device is modified using Au&Ag alloy nanorods (AuAgNRs) with desired plasmonic properties, the electroluminescence (EL) performance is significantly boosted, especially the Rabi-splitting energy, which increases from 96 to 285 meV. The current-injection exciton-polariton emission from the LED undergoing a strong coupling regime is confirmed through angle-resolved EL measurements. This study exhibits a performance-boosted near-infrared exciton-polariton LED at room temperature, which provides a new scheme toward the realization of highly energy-efficient polariton coherent light sources. Further, the significantly lower density of polariton states induced by the incorporated metal nanostructures highlights a bright future of realizing ultralow-threshold polariton lasers much more feasibly, in comparison to conventional lasers based on narrow bandgap semiconductors.

Beyond metals: theoretical discovery of semiconducting MAX phases and their potential application in thermoelectrics.

MAX phase is a family of ceramic compounds, typically known for their metallic properties. However, we show here that some of them may be narrow bandgap semiconductors. Using a series of first-principles calculations, we have investigated the electronic structures of 861 dynamically stable MAX phases. Notably, Sc2SC, Y2SC, Y2SeC, Sc3AuC2, and Y3AuC2 have been identified as semiconductors with band gaps ranging from 0.2 to 0.5 eV. Furthermore, we have assessed the thermodynamic stability of these systems by generating ternary phase diagrams utilizing evolutionary algorithm techniques. Their dynamic stabilities are confirmed by phonon calculations. Additionally, we have explored the potential thermoelectric efficiencies of these materials by combining Boltzmann transport theory with first-principles calculations. The relaxation times are estimated using scattering theory. The zT coefficients for the aforementioned systems fall within the range of 0.5 to 2.5 at temperatures spanning from 300 to 700 K, indicating their suitability for high-temperature thermoelectric applications.

Janus layers and electronic structure of 1T-(TiSeS)2.

TiS2-TiSe2 is one of the most studied titanium based solid solution systems. However, so far, all research on it has only focused on its disordered phase. Here, we systematically investigate its ordered phases. Using a structure search method based on the particle swarm optimization (CALYPSO) algorithm, we identify TiSeS-156 and discover a new structure (1T-(TiSeS)2). Based on first principles theory, their phonon spectra, formation energy, mechanical, electronic, thermal, and optical properties, as well as chemical bond analysis and synthetic pathways, have been investigated. The primitive cell of TiSeS-156 has three atoms and has a space group of P3m1 (no. 156). 1T-(TiSeS)2 has six atoms and has P3̄m1 symmetry (no. 164). TiSeS-156 and 1T-(TiSeS)2 are constructed by stacking the S-Ti-Se Janus layer materials. TiSeS-156 and 1T-(TiSeS)2 are narrow-gap semiconductors. The localized nature of the Ti(3d) states of TiSeS-156 and 1T-(TiSeS)2 leads to their semiconductor properties. 1T-(TiSeS)2 and TiSeS-156 have very similar mechanical, electronic, thermal, and optical properties of 1T-TiS2 and 1T-TiSe2, and are members of the 2D hexagonal lattice transition metal dichalcogenide layered material family. However, compared with 1T-TiS2 and 1T-TiSe2, TiSeS-156 and 1T-(TiSeS)2 have a wider range of potential applications, such as photovoltaic devices and photocatalysis, due to their S-Ti-Se Janus layer structure. They also provide a pathway for the preparation of Janus TiSeS monolayer and multi-layer materials. Moreover, our findings provide crucial insights for understanding the rich and complex crystal structures of the TiS2-TiSe2 system, which have broad implications for further exploration of this class of promising materials.

Electron beam lithography of GeTe through polymorphic phase transformation.

We report two previously undiscovered phases of GeTe including the sphalerite (c-) phase and the hexagonal (h-) phase with interlayer van der Waals gaps. A polymorphic phase transformation from rhombohedral α-GeTe to c- and h-GeTe at near room temperature is first realized via electron beam irradiation. Their underlying thermodynamics and kinetics are illustrated using the in situ heating experiments and molecular dynamics simulations. Density-functional theory calculations indicate that c-GeTe exhibits typical metallic behavior and h-GeTe is a narrow-gap semiconductor with a strong spin-orbital coupling effect. Our findings shed light on a strategy for designing GeTe-based quantum devices compromising nanopillars and heterostructures via an atomic-scale electron beam lithography technique.

NiCo-MOFs in situ anchored on graphdiyne with metal-like properties form a strongly coupled electron transport interface and construct an ohmic contact to achieve efficient charge-hole spatial separation.

Metals exhibit unique characteristics in photocatalysis, and their incorporation into semiconductors can result in remarkable features. This study focuses on the preparation of graphdiyne with Cu (CG) by using Cu powder as a catalyst. The addition of Cu reduces the narrow band gap of graphdiyne and imparts metal-like properties to the material. By leveraging the electronegativity of CG, a spherical NiCo-MOF (NC) is grown and in situ anchored on CG, forming a strongly coupled electron transport interface. In addition, the CG with metal-like properties also displays distinct characteristics. The integration of CG and NC through an ohmic contact significantly enhances the spatial separation of photogenerated carrier holes. Efficient hydrogen evolution is achieved through a synergistic effect of the strongly coupled electron transport interface and the spatial separation of photogenerated carrier holes. This research provides a new perspective on the design and development of metal-like narrow band gap semiconductors.

Discovery of Rb-based auric–aurous halide perovskites as promising narrow bandgap semiconductors for energy conversion applications

Inorganic gold halide perovskites, owing to their excellent stability and tunable bandgaps, are poised to serve as environmentally benign alternatives to lead halide perovskites in the field of energy conversion applications.

Unveiling the potential of a BCN-biphenylene monolayer as a high-performance anode material for alkali metal ion batteries: a first-principles study.

Inspired by a freshly synthesized two-dimensional biphenylene carbon network, which features a captivating combination of hexagonal, square, and octagonal rings, we explored a similar biphenylene network composed of boron, carbon, and nitrogen (bpn-BCN) using first-principles calculations. There are six possible phases of borocarbonitrides, which are isoelectronic to biphenylene carbon networks with a stoichiometric ratio of 1 : 1 : 1 for boron (B), carbon (C), and nitrogen (N) atoms. All possible isoelectronic structures of the BCN combination of biphenylene networks are found to be stable, according to first-principles calculations. However, the geometry has a relatively large number of robust C-C and B-N bonds and strong partially ionic-covalent B-C and C-N bonds inside these bpn-BCN monolayers are effectively more stable. Furthermore, we employed first-principles calculations to investigate the electrochemical properties of the most stable geometry of BCN biphenylene as a potential anode material for alkali metal (AM) ion batteries. A global search has been made to find the most favourable alkali metal ion adsorption sites. The biphenylene monolayer has octagonal, square, and hexagonal motifs with different adsorption strengths. Furthermore, the partially ionic bond of B-N (due to the electronegativity difference) also supports the alkali metal ions for adsorption. The electronic properties of the stable phase of bpn-BCN reveal its narrow bandgap semiconductor nature. The ion diffusion calculations show a low activation barrier for Li, Na, and K of 0.65 eV, 0.26 eV, and 0.23 eV, respectively, indicating a fast charge/discharge rate. Furthermore, the theoretical capacities of the BCN biphenylene monolayer for Li (1057.33 mA h g-1), Na (647.27 mA h g-1), and K (465.98 mA h g-1) are found to be greater than those of commercial graphite. The average open-circuit voltage for AM decreases with increasing metal ion concentrations. It falls within a reasonable range of 0.34-1.89 V. Our results show that the BCN biphenylene monolayer could be a promising anode material in alkali metal ion rechargeable batteries.

Functionalized hexagonal boron nitride bilayers: desirable electro-optical properties for optoelectronic applications.

Structural, electronic, and optical properties of functionalized hexagonal boron nitride (h-BN) bilayer were deeply explored by carrying out the PBE + G0W0 + BSE calculations. Hydrogenation/hydrofluorination/fluorination can cause the planar h-BN bilayer to form a novel diamane-like monolayer by interfacial sp3 atom bonding. These functionalized h-BN bilayers are estimated to be stable dynamically due to their phonon dispersions. The functionalization on h-BN bilayer can induce its electronic nature to be transformed from an indirect wide-gap insulator to direct narrow-gap semiconductor, which is desirable for its application in optoelectronics. In particular, hydrogenated and hydrofluorinated h-BN bilayers have strong absorbance coefficients for the near-infrared and visible part of the incident sunlight (larger than 105 cm-1). More interestingly, the binding energy of the observed first bright exciton can achieve a value beyond 1 eV, which can effectively reduce the recombination of photogenerated electron-hole pairs. These results are potentially important for extending the applications of the h-BN bilayer in optoelectronic devices.

Quantum Conductivity in Single and Coupled Quantum-Dimensional Particles of Narrow-Gap Semiconductors

Quantum Conductivity in Single and Coupled Quantum-Dimensional Particles of Narrow-Gap Semiconductors

A 2D low-buckled hexagonal honeycomb Weyl-point spin-gapless semiconductor family with the quantum anomalous Hall effect.

Spin-gapless semiconductors (SGSs), serving as superior alternatives to half-metals, open up new avenues in spintronics. Specifically, Weyl-point SGSs (WPSGSs) with ideal Weyl points at the Fermi energy level represent an optimal amalgamation of spintronics and topological physics. Moreover, considering spin-orbital coupling (SOC), most two-dimensional (2D) WPSGSs undergo transformation into half Chern insulators (HCIs) with the emergence of the quantum anomalous Hall effect (QAHE). The 2D I-II-V half-Heusler compounds, constituting a broad family of narrow-bandgap semiconductors with low-buckled hexagonal honeycomb crystal structures akin to silicene, aptly function as SGSs and serve as nontrivial topological parent materials. Through first-principles calculations, we propose that the Li12X10Cr2Y12 (X = Mg, Zn, Cd; Y = P, As) monolayers, derived by substituting certain X atoms in the LiXY (X = Mg, Zn, Cd; Y = P, As) monolayers of I-II-V half-Heusler compounds with Cr atoms, emerge as potential candidates for ideal 2D WPSGSs. These monolayers exhibit stable thermodynamic properties and 100% spin polarization. With SOC taken into account, the Li12X10Cr2Y12 monolayers transition into HCIs with a Chern number of +1, giving rise to the QAHE. These intriguing findings lay the groundwork for a promising material platform for the development of low-power spintronic and topological microelectronic devices.