Semiconductor Energy Gap Research Articles

Two-dimensional heterostars from nanographene and group-III nitrides for efficient spin filters: Spintronic and quantum transport properties

Two-dimensional (2D) heterostructures open the door toward novel applications, such as multiterminal transistors, by integrating the properties of the participant materials. Here we construct 2D heterostars from zigzag-triangular graphene (ZTG) and group-III nitrides. They are investigated using density functional theory and non-equilibrium Green’s function approaches. These heterostars are ferromagnetically spin-ordered devices that originate and distribute on ZTG. Further control over the net spin can be achieved by forming bilayer heterostars where spin can be increased, decreased, or removed. Single-layer heterostars are wide energy gap semiconductors of ∼3.5 eV that decrease to ∼1 eV in bilayers or are kept unchanged in other bilayers. Quantum transport calculations show that different spintronic devices can be constructed by connecting the source/drain voltage to different terminals of the nanostars. These devices are characterized by a low tunneling current through the potential barrier of group-III nitrides that significantly increased by applying gate voltage. A spin filter based on graphene-AlN nanostar with efficient separation of spin-up current is demonstrated. Also, a significant improvement in both the spin-up and spin-down currents can be achieved in the same nanostar but using other terminals. 2D heterostars are then potential candidates for building multi-terminal spintronic devices.

Low-temperature electrical conductivity of ion-beam irradiated Bi–Sb films

Bismuth-antimony alloys are among the most studied topological insulators and also have very promising thermoelectric properties. In addition, in the amorphous state they exhibit superconductivity with critical temperatures in the range 6.0–6.4 K. In this work, we have prepared and studied different polycrystalline films of Bi100–xSbx (x = 0, 5, 10, 15), and we have induced, through ion beam irradiation, significant damage in their internal structure with the aim of amorphizing the material. Specifically, we have irradiated Bi ions in the 10–30 MeV range, exploiting the capabilities of a 5 MV ion beam accelerator of tandem type. We have characterized the Bi–Sb films before and after irradiation from a morphological and structural point of view and measured their electrical resistivity from room temperature to near 2 K, to evaluate the influence of the preparation method and degree of disorder. We have found that the studied Bi–Sb system always behaves as a small energy gap semiconductor that follows the empirical Meyer–Neldel rule, which correlates the conductivity prefactor with the exponential value of the energy gap.

Study of Galena Ore Powder Sintering and Its Microstructure

Galena is a natural mineral enriched with lead sulfide (PbS). It typically forms in hydrothermal veins associated with igneous rocks and can also occur as a gangue mineral in other ore deposits. PbS is of special importance for scientific research applications due to the possibility of tuning its semiconductor energy gap using nanotechnology in conjunction with powder metallurgy as an easy, controllable production route. In this paper, almost pure PbS was successfully produced starting from a high ratio of PbS phase galena ore. As-received galena lumps were roughly pulverized and milled to produce four particle size ranges of 38, 63, 125, and 250 µm prior to compaction and sintering in a vacuum (pre-flushed with argon gas). SEM coupled with the EDAX analysis unit was employed to investigate the microstructure and chemical composition of the as-received galena and the subsequent products after sintering. The chemical analysis confirmed the high ratio of PbS compound in the as-received galena and sintered products with approximately 85% Pb and 13% S mass ratio. The sintering process of the galena powder was carried out at different values of temperature, time, and compaction pressure. Additionally, the effect of length to diameter ratio of compacted and sintered samples was investigated. XRD analysis confirmed the existence of the PbS phase in the as-received and sintered samples at 700 °C with approximately 98 wt.%, as well as a new phase that is formed at 800 °C with a lower percentage. The micro-hardness of the as-received and sintered samples was measured and compared with the as-received galena ore. The results showed a significant reduction in the hardness of sintered galena powder compared with the bulk as-received galena by 52%. Furthermore, a relative sintered density of 99.3% for the as-received galena density signifies a novel result using powder metallurgy techniques.

What's behind their various activities? Discerning the importance of surface terminations in Ag3PO4 semiconductor from DFT calculations

The surface energy and band gap of inorganic semiconductors are essential properties for a wide variety of applications in materials science, and studying morphologies holds significant importance. However, it is challenging to find a correlation between them at the atomic scale when these materials are multifunctional. This motivates the development of predictive computational models that can optimize these materials based on these quantities. Present findings shed light on the structure and electronic properties of (100), (110), and (111) surfaces of Ag3PO4 by using density functional theory calculations. By introducing two concepts, namely, polyhedron energy and polyhedron band gap, we provide compelling evidence for the effects of surface termination of Ag3PO4 photocatalyst. We compare our predictions to experimental crystal habits, and our results not only allow us to explain their evolution for obtaining a given morphology but also offer a coherent explanation, based on atomic-scale insights, for the discrepancies of morphology-photocatalytic activity relationship of Ag3PO4 in the recent literature. Overall, this work opens a novel avenue for first-principles calculations of morphology and provides a roadmap for the design of more effective inorganic semiconductors with desirable physicochemical properties for potential applications.

Surface-Potential-Based Drain Current Model for Ambipolar Organic TFTs

A surface-potential-based drain current model is presented for ambipolar organic thin-film transistors (OTFTs). First, following the multiple trapping and release (MTR) conduction mechanism, a drain current model is presented for unipolar OTFTs considering exponentially distributed trap state density in the energy gap of an organic semiconductor. Next, from the model for unipolar OTFTs, analyzing electrons or (and) holes in different regimes, the model for ambipolar OTFTs is presented. The presented model can describe the drain current by compact expressions and can estimate the trap states density. The model is verified by available experimental data considering temperature characteristics.

Effect of oxygen vacancy defects on electronic and magnetic properties of copper pyrophosphate material

The effect of oxygen vacancies (VO) on the electronic and magnetic properties of copper pyrophosphate dihydrate Cu2P2O7*2H2O (CuPPD) semiconductors is studied employing the DFT-based ab initio pseudopotential method. Site preference for vacancy formation on the selected oxygen sublattices is analyzed. Investigations revealed that the VO vacancies led to narrow bands of hybridized Cu-3d and O-2p states within the energy gap of perfect CuPPD semiconductors, significantly reducing the gap width from 2.17 eV to 0.19 eV and changing its character from indirect to direct one. Calculations have shown that the vacancies have no significant influence on the ferrimagnetic structure of CuPPD. The magnetic properties are driven by local magnetic moments of Cu atoms pointed oppositely. The oxygen vacancies slightly modify the spin polarization of d-orbitals of Cu atoms neighboring the vacancy, resulting in the non-zero total magnetic moment of 0.19 µB/FU (0.0 µB/FU in perfect CuPPD). The study's main finding, which can be exploited in nanoelectronics, is that the controlled implementation of oxygen vacancies can regulate the energy gap of CuPPD semiconductors.

Discovery of new potential magnetic semiconductors in quaternary Heusler compounds by addition of lanthanides

Magnetic semiconductors have attracted a lot of attention by having both electronic charge and spin degrees of freedom. In this paper, we obtained twenty magnetic semiconductors such as FeVLaSb, FeVPrSb, FeCrTbSi, CoVDySi, and CoVHoSi by adding lanthanides to quaternary Heusler compounds based on the Slater–Pauling law and orbital hybridization theory. The relationship between the lattice constants and energy gaps of the magnetic semiconductors with lanthanide elements is investigated by in-depth analysis. These magnetic semiconductors of quaternary Heusler compounds are promising candidates to find applications as spin filtering materials in spintronics devices.

Analysis of Temperature-Dependent Photoluminescence Spectra in Mid-Wavelength Infrared InAs/InAsSb Type-II Superlattice

Photoluminescence (PL) is one of the commonly used methods to determine the energy gap ({E}_{mathrm{g}}) of semiconductors. In order to use it correctly, however, the shape of the PL peak must be properly analyzed; otherwise, the value of {E}_{mathrm{g}} is burdened with a large error. {E}_{mathrm{g}} is often mistakenly attributed to the PL peak position, which in type-II superlattices (T2SLs) exhibits typical “S-shaped” behavior as a function of temperature, significantly different from the Varshni model used to define the energy gap of III-V compounds. The position peak of the PL relative to the real {E}_{mathrm{g}} in T2SLs is red-shifted because of the carrier localization at low temperatures and blue-shifted because of the free carrier emission at high temperatures. To correctly determine {E}_{mathrm{g}}, the shape of the PL peak should be analyzed using the theoretical PL line shape model that takes into account both localized (below the bandgap) and free carriers (above the bandgap) emissions. This work shows that the use of such a model to analyze the shape of the PL signal gives the correct results of determining {E}_{mathrm{g}} for mid-wave infrared InAs/InAsSb T2SL, which showed a significant contribution of localized states in optical transitions and characteristic “S-shaped” PL peak behavior. This allowed us to determine the correct values of the Varshni coefficients for a given T2SL. The result also agrees with the theoretical calculations of {E}_{mathrm{g}} made using the k·p method.

Dynamics of surface-plasmon lasing in planar metal gratings on semiconductor.

We investigate the dynamics of surface plasmon (SP) lasing in Au gratings fabricated on InGaAs with a period of around 400 nm, which locates the SP resonance near the semiconductor energy gap and facilitates efficient energy transfer. By optically pumping the InGaAs to reach the population inversion required for the amplification and the lasing, we observe SP lasing at specific wavelengths that satisfy the SPR condition depending on the grating period. The carrier dynamics in semiconductor and the photon density in the SP cavity was investigated from the time-resolved pump-probe measurement and the time resolved photoluminescence spectroscopy, respectively. Our results reveal that the photon dynamics is strongly correlated with the carrier dynamics and the lasing build-up is accelerated as the initial gain proportional to the pumping power increases, and this trend is satisfactorily explained using the rate equation model.

Investigating the structural, electronic, and elastic properties of Li-based quaternary Heusler alloy semiconductors using hybrid functional – HSE06 bandgap recalculations

In this work, we introduce a new class of stable semiconducting equiatomic quaternary Heusler alloys. The 21 new semiconductors reported in this work contain 18 valence electrons per unit cell.Three different structural arrangements, referred Y1,Y2 and Y3 are taken into consideration. The bandgaps are investigated and significantly underestimated by GGA-PBE functional. We recalculated them by means of the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional to achieve the correct ones. This study shows that the HSE06 correction mainly acts on the lowest conduction bands. The energy gaps of the quaternary Heusler semiconductors are in the range of 0.766–1.94 eV, which suggests that these semiconductors are prospective materials for a variety of applications, including thermoelectric, optoelectronic, and photovoltaic. Furthermore, formation energy, elastic constants, and phonon dispersion curves are calculated to verify the mechanical and dynamical stabilities of these alloys.

Going ballistic: a novel characterization for the electronic energy gap.

The energy gap of molecular semiconductors is a critical parameter in molecule-based devices because it fundamentally determines the operating principle and device performance, such as the charge transfer driving force. Hence, an accurate quantification of the energy gap of a molecular semiconductor is essential. The hot electron technique, which is implemented using a hot electron transistor and ballistic electron emission microscope, has been verified as the most powerful characterization in recent years. By monitoring the charge transport on the metal/molecule interface, an electron-injection barrier (or LUMO) and hole-injection barrier (or HOMO) can be achieved, which contributes to the electronic energy gap determination. In this review, a comprehensive comparison of these two techniques was made. It helps us to select a suitable method in specific situations and provides a profound comprehension of the charge transport process in molecular semiconductor and molecule-based devices.

Influence of biaxial strain on the electronic and thermoelectric properties of Sb2Te3 monolayer

We investigate the influence of biaxial strain on the electronic and thermoelectric properties of Sb2Te3 monolayer by first-principles calculation and semiclassical Boltzmann transport theory. The calculated results of formation energy and phonon spectrum suggest there is a limited strain for Sb2Te3 monolayer. The effective mass of hole increases with the increase of tensile strain, but decreases with the increase of compressive strain. However, the change trend of electron effective mass with strain is opposite to that of hole effective mass. The results of energy band structures show that the Sb2Te3 monolayer is an indirect energy gap semiconductor. Three conduction band valleys almost degenerate together at 5% compressive strain. At this strain, Seebeck coefficient and power factor are improved for n-type doped Sb2Te3 monolayer, suggesting that strain engineering is an effective way to improve the thermoelectric properties of Sb2Te3 monolayer.

InAs/MoRe Hybrid Semiconductor/Superconductor Nanowire Devices

Implementing superconductors capable of proximity-inducing a large energy gap in semiconductors in the presence of strong magnetic fields is a major goal toward applications of semiconductor/superconductor hybrid materials in future quantum information technologies. Here, we study the performance of devices consisting of InAs nanowires in electrical contact with molybdenum-rhenium (MoRe) superconducting alloys. The MoRe thin films exhibit transition temperatures of ∼10 K and critical fields exceeding 6 T. Normal/superconductor devices enabled tunnel spectroscopy of the corresponding induced superconductivity, which was maintained up to ∼10 K, and MoRe-based Josephson devices exhibited supercurrents and multiple Andreev reflections. We determine an induced superconducting gap lower than expected from the transition temperature and observe gap softening at finite magnetic field. These may be common features for hybrids based on large-gap, type II superconductors. The results encourage further development of MoRe-based hybrids.

Modeling optical energy gap of thin film cuprous oxide semiconductor using swarm intelligent computational method

Cuprous oxide (Cu2O) is a p-type metal oxide semiconducting material with potential in photovoltaic and photocatalysis applications due to its excellent absorption capacity in visible region and tunable energy gap. Experimental synthesis and energy gap characterization of thin film cuprous oxide semiconductor with desired dopants and varying experimental conditions for enhanced photocatalytic as well as photovoltaic activities are laborious and consume appreciable precious resources. This work hybridizes particle swarm optimization method with support vector regression algorithm for computing energy gap of thin film cuprous oxide semiconductor using the thickness of thin film and distorted lattice parameter as descriptors. The predictions of swarm-based support vector regression (S-SVR) model are compared with estimates of stepwise regression (SR) model while S-SVR shows superior performance of 39.47 %, 36.20 % and 114.41 % on testing data samples over SR model using root mean square error (RMSE), mean absolute error (MAE) and correlation coefficient (CC), respectively. The developed S-SVR model is characterized with 0.9559 CC, 0.0586 MAE and 0.028 RMSE on the basis of training samples. The developed S-SVR and SR models were further validated using external data samples while the developed S-SVR demonstrates excellent agreement with the measured values. The convincing precision demonstrated by S-SVR model would be of indispensable significance in determining energy gap of cuprous oxide semiconductor (for photocatalytic applications in pollutant removal, solar cell, gas sensors and thin film transistors) with appreciable quickness and reduced cost coupled with experimental difficulty circumvention.

Bending Pyrenacenes to Fill Gaps in Singlet-Fission-Based Solar Cells

Singlet fission is envisaged to enhance the efficiency of single-junction solar cells beyond the current theoretical limit. Even though sensitizers that undergo singlet fission efficiently are known, characteristics like low-energy triplet state or insufficient stability restrict their use in silicon-based solar cells. Pyrenacenes have the potential to overcome these limitations, but singlet-fission processes in these materials is outcompeted by excimer formation. In this work, bent pyrenacenes with a reduced propensity to stack and thus form excimers are computationally evaluated as singlet-fission materials. The energies of the S1, T1 and T2 states were estimated in a series of bent pyrenacenes by means of time-dependent density functional theory calculations. Our results show the opposite trend observed for perylene diimides, namely, an increase in the energy of the T1 and S1 states upon bending. In addition, we show that the energy levels can be tuned on demand by manipulating the bend angle to match the energy gap of various semiconductors that can be used in single-junction solar cells, making pyrenacenes promising candidates for singlet fission.

Bismuth oxychloride photocatalytic wide band gap adjustment through oxygen vacancy regulation using a hybrid intelligent computational method

Bismuth oxychloride is a lamellar semiconductor with fascinating features in photocatalysis due to its excellent charge storage capacity, sufficient reduction/oxidation potentials, stable chemical properties, wide distribution of elements, and low toxicity. The wider application of this semiconductor for many technological and industrial applications is hindered by the resistive factors of wide band gap and the ease of recombination of photo-generated electrons and holes. Oxygen vacancy creation in bismuth oxychloride semiconductor controls and adjusts the band gap to the desired spectrum region and subsequently acts like electron trapping centers for the prevention of carrier's re-union. The formation of oxygen vacancy peaks for the lattice and adsorbed oxygen species in the de-convoluted O 1s spectra of x-ray photoelectron spectroscopy (XPS) at a particular value of binding energy experimentally confirms oxygen vacancy creation for photocatalytic activity enhancement. This work focuses on modeling the energy gap (Eg) of bismuth oxychloride semiconductor with oxygen vacancies using the binding energy (BE) of the de-convoluted O 1s lattice oxygen and oxygen vacancy as descriptors through hybrid particle swarm-based support vector regression (PSVR) model as well as stepwise regression (STR) model. The developed PSVR-Eg model shows performance improvement of 23.38%, 55.96%, and 66.54% over the STR-Eg model on the basis of the coefficient of correlation, root means square error, and mean absolute error. The developed PSVR-Eg further predicts the influence of lattice oxygen and oxygen vacancy on the energy gap of the semiconductor. The characteristic precision, accuracy, and quickness of the developed models are highly meritorious in determining the level of oxygen vacancy that adjusts the semiconductor energy gap to the desired value for photocatalytic activity enhancement at a reduced cost and without experimental stress.

Extreme learning machine computational method of modeling energy gap of doped zinc selenide nano-material semiconductor

Zinc selenide (ZnSe) semiconductor belongs to a class of wide energy gap material with low visible region optical absorption, high optical coefficient (nonlinear) and fascinating optoelectronic as well as photocatalytic features which foster and strengthen its applications in resisting thermal shock, photoelectric fields (which include dielectric mirrors, shot wavelength lasers and light emitting diode), buffer layer of solar cell and many photocatalytic environmental cleanliness applications. Nanostructured ZnSe semiconductors occupy a special place in application domain due to the characteristic features attached to the crystallite size of the material which include high fluorescence quantum yields, solvent dispersibility and adjustable emission color among others. Effective utilization of nanostructured ZnSe semiconductor involves tuning of its energy gap through doping mechanisms which is accompanied with distortion in the parent lattice structure. In this contribution, extreme learning machine (ELM) computational intelligent method is proposed for predicting the band gap energy of doped ZnSe nanostructured semiconductor using the material crystallite size and lattice parameter as descriptors to the models. The developed ELM based model using sine (Sine), sigmoid (Sig) and transig (Trang) activation functions were compared with the existing SVR-GA and SPR models in the literature using various performance metrics. The developed Sine-ELM model shows superior performance over Sig-ELM, Trang-ELM, SVR-GA (2021) and SPR (2021) models with performance improvement of 12.74%, 64.89%, 68.87% and 75.62% using mean squared error (MSE) metric for testing data samples. The superiority of the model developed was further justified through validation with external set of data. The precision and superiority of the ELM based models developed would eventually ease quick characterization and tuning of energy gap of ZnSe nanostructured semiconductors for various applications.

Modeling Energy Gap of Doped Tin (II) Sulfide Metal Semiconductor Nanocatalyst Using Genetic Algorithm‐Based Support Vector Regression

Tin (II) sulfide (SnS) is a metal chalcogenide semiconducting material with fascinating and admirable physical features for practical applications in solid‐state batteries, photodetectors, gas sensors, optoelectronic devices, emission transistors, and photocatalysis among others. The energy gap of SnS semiconductor nanomaterial that facilitates its usefulness in many applications can be adjusted through dopant incorporation which results in crystal lattice distortion at various crystallite sizes of the semiconductor. This work employs lattice parameter descriptors to develop a hybrid genetic algorithm (GA) and support vector regression algorithm (SVR) intelligent model for determining the energy gap of doped SnS semiconductors. The predictive strength of the developed GA‐SVR model is compared with the stepwise regression algorithm‐ (STRA‐) based model using different performance evaluation parameters. The developed GA‐SVR model performs better than STRA model based on root mean square error, mean absolute error, and correlation coefficient with performance improvement of 70.68%, 67.63%, and 20.98%, respectively, using the testing set of data. Influence of different dopants and experimental conditions on energy gap of SnS semiconductor were investigated using the developed model, while the obtained values for the energy gaps agree with the measured values. The developed models demonstrate high degree of potentials in terms of accuracy, precision, and ease of implementation that fosters their real‐life applicability in estimating the energy gap of doped SnS semiconductor with experimental stress circumvention.

Opportunities for energy level tuning at inorganic/organic semiconductor interfaces

The aim of this Perspective is to provide an overview of approaches that can be employed to tune the energy level alignment at interfaces between inorganic and organic semiconductors for use in electronic and optoelectronic devices. The approaches include tailoring intramolecular dipolar bond distribution, controlling molecular orientation at interfaces, and the insertion of a molecularly thin interlayer that abruptly shifts the electrostatic potential between the two semiconductors and, thus, affords level tuning. With these state of the art methods, the frontier energy levels at an inorganic/organic heterojunction can be varied up to ca. 3 eV, i.e., covering the energy gap of most semiconductors. By combining two or more of these approaches or by employing interfacial molecular switches, it is envisioned that unconventional and dynamically switchable interfacial energy level scenarios can be created, enabling expanded or superior device functionality.

Measuring Energy Gaps of Organic Semiconductors by Electron Energy Loss Spectroscopies

Herein is explored the use of electron energy loss due to π–π* transition to measure energy gaps of organic semiconductors. Sources of kinetic electrons studied include an external electron gun (reflection electron energy loss spectroscopy (REELS)) and an internal core shell excitation by X‐rays (i.e., X‐ray photoelectron spectroscopy (XPS)). To obtain the bandgap accurately, a data analysis method is proposed to extract the optical bandgap from the π–π* inelastic electron energy loss spectra. This method uses a Gaussian function to fit experimental data yielding a peak width w and position E0. The energy gap of an organic semiconductor, i.e., the onset of the π–π* transition peak, can then be calculated by . Through examination of 11 organic semiconductors, it is found that the bandgaps measured by REELS agree well with optical bandgaps. The bandgaps measured by XPS, however, do not always agree with optical gaps. This indicates that the XPS π–π* inelastic peak in some material systems may convolute with other core shell processes such as shake‐up. In addition, it is shown that all key energy structures of an organic semiconductor can be measured concurrently by REELS and ultraviolet photoemission spectroscopy.