Hole Effective Mass Research Articles

First-principles study of SrTe and BaTe: Promising wide-band-gap semiconductors with ambipolar doping

Transparent wide-band-gap (WBG) semiconductors are crucial components in diverse (opto)electronic and energy devices. Using first-principles calculations, we demonstrate that alkaline earth tellurides MTe (M = Sr or Ba) are promising WBG semiconductors that can be ambipolarly doped and transparent to visible light. Because of their large direct band gaps (3.74 eV for SrTe and 3.09 eV for BaTe), 100 nm thick MTe films exhibit significant transmittance (over 80%) for visible light. The effective mass of electrons and holes in MTe is predicted to be small (<1 m0) enough to show high carrier mobilities. From the analysis of the defect properties, we show that the major carrier type (electrons vs. holes) and its concentration can be controlled by adjusting the synthetic conditions. We also find that the valence band maximum of MTe is relatively shallow. Thus, MTe can be utilized as an electron-blocking (hole-transport) layer in emerging perovskite solar cells.

Investigation of Aluminum Arsenide Honeycomb Monolayer via Density Functional Theory

First principle calculations are performed to theoretically predict the physical properties of hexagonal aluminium arsenide planar and buckled monolayers. The structural characteristics showed that the buckled parameter is about 0.32 A°. Cohesive energies have favourable values and it indicates the fabrication possibility. Phonon dispersion properties indicated that the planar aluminium arsenic monolayers are dynamically unstable, while the buckled is less dynamically unstable. The elastic constant parameters achieved the required characteristics of stable hexagonal monolayer structures. The study of electronic band structure prefers to indirect semiconductor band gaps, and the density of states showed strong orbital hybridization in the conduction band. Planar structure has isotropic light electron effective mass and anisotropic heavy hole effective mass. The buckled structure has isotropic light electron effective mass and isotropic heavy hole effective mass. The absorption spectra have high absorption coefficient in various visible and ultraviolet wavelength. The absorption coefficient levels off at about direct and indirect band gaps.

Half-Heusler phase TmNiSb under pressure: intrinsic phase separation, thermoelectric performance and structural transition

Half-Heusler (HH) phase TmNiSb was obtained by arc-melting combined with high-pressure high-temperature sintering in conditions: p = 5.5 GPa, T_{HPHT} = 20, 250, 500, 750, and 1000 ^{circ }C. Within pressing temperatures 20–750 ^{circ }C the samples maintained HH structure, however, we observed intrinsic phase separation. The material divided into three phases: stoichiometric TmNiSb, nickel-deficient phase TmNi_{1-x}Sb, and thulium-rich phase Tm(NiSb)_{1-y}. For TmNiSb sample sintered at 1000 ^{circ }C, we report structural transition to LiGaGe-type structure (P6_3mc, a = 4.367(3) Å, c = 7.138(7) Å). Interpretation of the transition is supported by X-ray powder diffraction, electron back-scattered diffraction, ab-initio calculations of Gibbs energy and phonon dispersion relations. Electrical resistivity measured for HH samples with phase separation shown non-degenerate behavior. Obtained energy gaps for HH samples were narrow (le 260 meV), while the average hole effective masses in range 0.8–2.5m_e. TmNiSb sample pressed at 750 ^{circ }C achieved the biggest power factor among the series, 13 upmuWK^{-2}cm^{-1}, which proves that the intrinsic phase separation is not detrimental for the electronic transport.

Propagation of surface plasmons in a quantum collisional semiconductor plasma

In this work, the propagation of surface plasmons in a quantum collisional semiconductor plasma is theoretically investigated using the quantum hydrodynamic model. The general dispersion equation of surface plasmons is deduced and solved in the presence of collisional effects and quantum tunneling of electrons and holes. It is shown that collisions play a significant role in the decay or growth of surface plasmons. Moreover, it is found that the quantum tunneling of electrons and holes increases the phase velocity, the group velocity, and the growth rate of instability. It is also indicated that the growth rate of instability is decreased with the increase of the ratio of hole density to electron density and the ratio of electron effective mass to hole effective mass.

Discovery of Clustered-P1 Borophene and Its Application as the Lightest High-Performance Transistor

The two-dimensional network of boron atoms (borophene) has attracted attention for its ultralow molar mass and remarkable polymorphism. Synthesized polymorphs of borophene (striped, β12, χ3, and honeycomb), so far, are all found to be metallic. Employing a genetic algorithm-based structure searching technique, here we discover an allotrope, clustered-P1, which is located very close to the global energy minimum. Clustered-P1 exhibits a bulk silicon-like band gap (1.08 eV) with symmetric effective masses (∼0.2 m0) for electrons and holes along the transport direction. Phonon dispersion and beyond room-temperature ab initio molecular dynamics studies further confirm its excellent dynamic and structural stability. Since two-dimensional semiconductors are promising silicon alternatives for complementary metal-oxide semiconductor (CMOS) technology extension, we further investigate the characteristics of clustered-P1-based transistors using self-consistent quantum transport models for channel lengths of 10-3 nm. The performance of these devices has been found to be balanced for p- and n-type transistors and meets the requirements of the International Roadmap for Devices and Systems (IRDS). Our study may aid in the experimental realization of the lightest high-performance transistor.

Investigation of size and barrier dependent efficiency in InAs quantum dot solar cells

Semiconductor Quantum Dots have unique properties, such as the quantum size effect that provide the breakthrough in optoelectronic devices. The small effective mass of electron and hole open-up an avenue of wide bandgap tunability in InAs Quantum Dots (QDs), making them an attractive material for solar cell application. Therein, 4x4 Luttinger Kohn Hamiltonian method is used to calculate the absorption co-efficient of epitaxially grown InAs QDs. Further, we studied the size-dependent absorption co-efficient and the barrier material model of InAs QDs capping with InP and GaAs. Our theoretical simulation predicts the efficiency of InAs/GaAs and InAs/InP system in the intrinsic layer of p-i-n structure solar cells.

Exploring the effect of C6H5-x/FxBr (x = 0-3) passivating agent on surface properties at different termination ends: first principles.

To prevent further decomposition of organic-inorganic hybrid perovskite by defects, in this work density functional theory was applied to explore the electronic properties, carrier surface mobility and theoretical photoelectric conversion efficiency (PCE) of passivating molecules with different fluorine atom content at the symmetric site of the benzene ring at different termination ends of MAPbI3, which shed light on the control of perovskite surface passivation by different element atoms in the same molecule. We found that the same molecule acts as a different passivation agent at different termination faces. Passivating molecules on the surface termination end by MAI play a Lewis acid role, with molecules with stronger dipole moments narrowing the band gap from the original 1.77 to 1.73 eV. The exciton binding energy of molecules with stronger dipole moments (0.187-0.292 meV) is significantly lower than that of MAPbI3 (0.332 meV), so the effective separation of interface electrons and holes can be realized. Bromopenta-fluorobenzene has a lower adsorption energy of -0.17 eV, which can stably adsorb on the surface of perovskite and increase visible light absorption. Ultimately, the theoretical PCE increased from 15.8% to 16.16%. In addition, on the surface terminated by PbI2, BrB with a strong dipole moment can provide electrons for Pb2+ and act as a Lewis base. At the surface end, it can form an ionic bond with Pb2+, while the antibonding molecular orbital characteristic is dominant, which increases the band gap from 1.76 to 1.87 eV. After increasing to 4-F-BrB, the fluorine atom has strong electronegativity and can easily bond with Pb2+. The conjugate π cycle intensifies the promotion of electron transfer, reducing the work function from 5.262 to 4.703 eV, reducing the effective electron and hole mass (0.514, 0.204 m0), and improving the photovoltaic performance. Finally, increasing the number of passivation molecules resulted in a decrease in the PCE from 15.93% to 14.75%.

Flattening conduction and valence bands for interlayer excitons in a moiré MoS2/WSe2 heterobilayer.

We explore the flatness of conduction and valence bands of interlayer excitons in MoS2/WSe2 van der Waals heterobilayers, tuned by interlayer twist angle, pressure, and external electric field. We employ an efficient continuum model where the moiré pattern from lattice mismatch and/or twisting is represented by an equivalent mesoscopic periodic potential. We demonstrate that the mismatch moiré potential is too weak to produce significant flattening. Moreover, we draw attention to the fact that the quasi-particle effective masses around the Γ-point and the band flattening are reduced with twisting. As an alternative approach, we show (i) that reducing the interlayer distance by uniform vertical pressure can significantly increase the effective mass of the moiré hole, and (ii) that the moiré depth and its band flattening effects are strongly enhanced by accessible electric gating fields perpendicular to the heterobilayer, with resulting electron and hole effective masses increased by more than an order of magnitude - leading to record-flat bands. These findings impose boundaries on the commonly generalized benefits of moiré twistronics, while also revealing alternative feasible routes to achieve truly flat electron and hole bands to carry us to strongly correlated excitonic phenomena on demand.

Effect of substitution doping and surface adsorption of Al atoms on photocatalytic decomposition of water and oxygen from BiVO&lt;sub&gt;4&lt;/sub&gt; (010) crystal surface

Using solar photoelectrochemical decomposition of water to produce hydrogen and oxygen is one of the most feasible approaches to obtaining renewable energy. Compared with hydrogen-evolution reaction (HER), the oxygen-evolution reaction (OER) is very complex, there are four sluggish proton-coupled electron transfer processes. It is critical to improve OER performance. The BiVO&lt;sub&gt;4&lt;/sub&gt; (010) facet possesses low surface energy, strong visible absorption, and good activity for OER, and is considered as one of the most suitable PEC catalysts. However, its poor electron conductivity, low charge carrier mobility, and high charge recombination rates significantly limit its practical applications. To achieve highly active OER photocatalysts, we modify BiVO&lt;sub&gt;4&lt;/sub&gt; (010) facet by substitutial doping with Al atom and surface adsorption with Al atom. According to density functional theory calculations, we compare OER performances of these two modified BiVO&lt;sub&gt;4&lt;/sub&gt; (010) facets. The results show that both approaches can effectively regulate the electronic structure of BiVO&lt;sub&gt;4&lt;/sub&gt; and then tune OER activity resulting from the change of the structure. Though Al substitutional doping reduces the band gap of the (010) facet and enhances the visible light absorption, the improvement of OER performance is not significant because the doping site is inside and has little influence on the surface active site. Importantly, the surface adsorption of Al atom is considered as an efficient means to improve the OER activity on BiVO&lt;sub&gt;4&lt;/sub&gt; (010) facet due to the combined action between surface adsorbed Al and active site Bi atoms. Al adsorbed (010) facet exhibits excellent OER catalytic activity: 1) the induction of localized states and the reduction of band gap are conducive to the electronic transition, optical absorption, thus increasing the electrical conductivity; 2) there is lower hole effective mass, and thus effectively enhancing the ability to transfer from anode surface to electrolyte surface, thereby increasing the difference between the effective mass ratio of electron−hole pairs and 1 and effectively reducing the electron-hole recombination; 3) the nteraction between the active sites and oxygen-containing intermediates is reinforced in the OER process, therefore the potential determining step of OER decreases effectively. This work provides an important reference for designing efficient and stable two-dimensional semiconductor-based photocatalysts for OER. We believe that it will arouse great interest of the BiVO&lt;sub&gt;4&lt;/sub&gt; community and motivate numerous experimental researches.

Photoluminescence studies of non-toxic monoclinic yttrium oxide quantum dots synthesized at low temperature for live cell imaging applications

Synthesis of monoclinic Y2O3 remains a challenging task due to complex formation conditions such as high pressure or high temperature environments. Monoclinic yttria exhibits better photoluminescence characteristics compared to the cubic phase and hence can be a better host for nanophosphors. Reports are available predicting the stability of monoclinic phase at low temperature in extremely fine particles. But experimental preparation of this phase at low temperature is not yet reported. Here we report the first-time preparation of monoclinic Y2O3 at 90 °C using simple laboratory hot air oven. The synthesized nanoparticles exhibit intense PL emission in the blue region and hence can find applications in display industry. The band structure calculations were carried out using density functional theory and the excitonic Bohr radius was estimated using the electron as well as hole effective masses determined from band structure. The cytotoxicity studies reveal non-toxic nature the particles. The fluorescence imaging experiments show that the material is capable of emitting blue fluorescence in L6 skeletal cell lines and it ensures the applicability of this material in the biomedical field.

Optical and thermoelectric properties of square lattice phases of alkali halide compounds

The geometrical, vibrational, electronic, optical and thermoelectric properties of the 16 compounds of the I-VII square lattice monolayer structure such as s-MX (M = Li, Na, K, Rb and X = F, Cl, Br, I) are studied in a systematic manner through the first principles. First-principles computations using the density functional theory is applied to analyze the series of group I–VII materials. The frequency dependent phonon dispersion relations show that every member of the group I–VII under consideration are dynamically stable. All the sixteen dynamically stable alkali halides possess band gap ranging from 3.99 eV to 7.03 eV through the calculation from PBE functional, whereas using HSE06 functional, the energy bandgap are obtained in the range of 4.98 eV to 9.20 eV. We obtained the effective mass, deformation potential, relaxation time and mobility for electron and holes for all the considered compounds of group I-VII. The optical properties of the compounds I-VII are also examined in the context of parallel and perpendicular field polarizations. According to the current observation, square lattice phases exhibit anisotropic optical behavior with respect to the plane of polarizations (parallel and perpendicular). The lower the value of reflectivity and absorption coefficient, the greater the dominance in the partially visible and ultraviolet energy ranges, indicating that these materials are transparent in this range and hence suitable for anti-reflection coating. Finally, we computed the thermoelectric characteristics of each I-VII (s-MX) material with respect to temperature. All of the s-MX materials of LiI, KBr, KI, and RbCl display excellent thermoelectric properties over a broad temperature range of 300 K–800 K. The present study indicate that the alkali halides in square lattice phase (s-MX) appears to be prominent materials for future thermoelectric 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.

Instability of the magnetic state of MPX3(M=Mn,Ni;X=S,Se) monolayers induced by strain and doping

The instability of magnetic states in two-dimensional magnetic materials plays a crucial role in the application of spintronics. However, the instability mechanism and magnetic state diagram of $M\text{P}{X}_{3}$ ($M=\mathrm{Mn}$ and Ni and $X=\mathrm{S}$ and Se) monolayer with multiple magnetic states remains elusive. In our paper, we investigate the instability of magnetic states using ab initio calculation combined with an effective Heisenberg model. We obtain a series of magnetic state diagrams of $M\text{P}{X}_{3}$ ($M=\mathrm{Mn}$ and Ni and $X=\mathrm{S}$ and Se) monolayer under the approaches of strain and carrier doping. We clarify the instability mechanism that is mainly determined by the nearest-neighbor exchange interaction between magnetic ions, find a higher N\'eel temperature in ${\mathrm{NiPSe}}_{3}$ monolayer, and unveil a considerable increase in hole effective mass along the armchair direction in the ${\mathrm{NiPSe}}_{3}$ monolayer, which leads to giant anisotropy of carrier mobility. Our results provide a useful guidance to design high-performance spintronics devices based on the $M\text{P}{X}_{3}$ monolayer.

The transparency and p-type performance in B6P: The hybrid functional study

The transparency and p-type performance for B6P is investigated by the hybrid functional calculations. It is found that B6P is an indirect band gap semiconductor with a band gap of 3.23 eV. The hole effective mass along the T to Γ direction is only 0.35 m0, where m0 is the mass of the free electron. The B6P is a promising high transparent material due to its low absorption, low reflection, and high transmittance. For intrinsic B6P, BP (PB(B)) is the main defect under B-rich (P-rich) condition. The formation energy of the dominant defect is calculated. The thermodynamic simulation method is performed to quantitatively study the Fermi energy, hole concentration, and defect density. Our results show that quenching the intrinsic B6P from 1900 K to room temperature can increase the holes' density from 9.86 × 102 to 2.80 × 1013 cm−3. These results provide essential insight for B6P as a potential transparent p-type material.

Formula omitted] electronic structure and strain-dependence of the crystal field splitting and inter-band transition energy of W–InN

In this paper, we study the electronic structure of wurtzite Indium Nitride (W–InN) using a k→∙π→ theory. Here π→ is the relativistic momentum operator. It is based on an eight-band model; three valence bands and one conduction band, each Kramers’ split by spin-orbit interaction. We diagonalize the k→∙π→ Hamiltonian following a two-band model, where each valence band is treated separately with the conduction band by using a suitable basis set in the presence of spin-orbit interaction. Effects of other valence bands on the dispersion of a given valence band are incorporated using second order perturbation theory. Valence band dispersions are plotted and compared with a previous calculation and our own results obtained by using a different method. Agreement is found to be satisfactory. Electronic and hole effective masses are calculated and compared with previously reported values, which are substantially different from each other. Fair agreement is noticed. Discrepancies are discussed. Effect of strain is calculated on the electronic structure by calculating the crystal field energy and transition energy for small variations of strain. Although results could not be compared with any previous calculation, due presumably to their absence in literature, trends and systematics are found to be in accord with the same found in case of other wurtzite semiconductors.

Optical properties of InI3: Density functional theory calculations and experimental results

The electronic and optical properties of InI3 have been investigated by both experimental and theoretical approaches. The First Principle electronic structure and optical properties were calculated by Density Functional Theory (DFT) formalism using Full Potential Linearized Augmented Plane Wave plus local orbitals (FP-LAPW + lo) implementing different appropriate exchange-correlation functionals in WIEN2k code. The calculations were also carried out implementing other functionals and only TB-mBJ results tally the closest with experimental results. The optical measurements of a well characterized InI3 films indicate the band gap as of direct type and residual strain dependant. The stress free intrinsic band gap Eg = 2.8 eV (average of Eg⊥c and EgIIc) has been determined from the residual strain dependence of the band gap in closest agreement with theoretically calculated value of 2.9 eV. Both theory and experiment show that the band gap is anisotropic along and perpendicular to c-axis. The electron effective mass me*=0.79m0 and the hole effective mass mh*=0.85m0 have been determined from the band structure. The experimental and theoretical optical properties match reasonably well.

Trends in opto-electronic properties of MgxZn1-xSnN2 using first principles methods

We have computationally studied MgxZn1-xSnN2 alloy system in wurtzite structure using density functional theory and beyond. Cationic disorder of the random limit is chosen for modeling the crystal structure. Lattice mismatch of less than 1 % in the alloy system helps in the miscibility of Mg and Zn, thus making it widely useful in tandem solar cells. Electronic band gaps increase linearly with higher Mg concentration. These materials have hole effective masses significantly lower than electron effective masses. Considerable absorption together with a lower reflectivity can render this alloy an effective absorber. All the alloys are mechanically stable, but (x=0.25) and (x=0.50) are dynamically unstable. These alloys also exhibit large hardness.

Transparency and p-Type Conductivity of BeSe Doped with Group VA Atoms: A Hybrid Functional Study

Utilizing a hybrid functional method, the transparency and p-type conductivity of BeSe are investigated. Our studies confirm that N- and P-substituted Se (labeled as NSe and PSe) are promising p-type defects due to their smaller ionization energy. BeN2 and BeP2 are efficient dopant sources for their moderate formation energy. Based on the thermodynamic equilibrium fabrication method together with the rapidly quenching scheme, we find the hole density, induced by NSe (PSe) defects, can reach 4.44 × 1018 (3.83 × 1016) cm–3. A high density of holes, smaller hole effective mass (along the Γ-X and W-X directions, the hole effective masses are 0.466 and 0.759m0 (m0 is the electron’s static mass)), wide band gap, and weak plasmonic effect show that BeSe with NSe defects is an excellent transparent p-type semiconductor. These findings provide significant insight to explore a transparent p-type semiconductor.

Nanoscale View of Engineered Massive Dirac Quasiparticles in Lithographic Superstructures

Massive Dirac fermions are low-energy electronic excitations characterized by a hyperbolic band dispersion. They play a central role in several emerging physical phenomena such as topological phase transitions, anomalous Hall effects, and superconductivity. This work demonstrates that massive Dirac fermions can be controllably induced by lithographically patterning superstructures of nanoscale holes in a graphene device. Their band dispersion is systematically visualized using angle-resolved photoemission spectroscopy with nanoscale spatial resolution. A linear scaling of effective mass with feature sizes is reported, underlining the Dirac nature of the superstructures. In situ electrostatic doping dramatically enhances the effective hole mass and leads to the direct observation of an electronic band gap that results in a peak-to-peak band separation of 0.64 ± 0.03 eV, which is shown via first-principles calculations to be strongly renormalized by carrier-induced screening. The methodology demonstrates band structure engineering guided by directly viewing structurally and electrically tunable massive Dirac quasiparticles in lithographic superstructures at the nanoscale.

Determination of the Strain Influence on the InAs/InAsSb Type-II Superlattice Effective Masses.

A3B5 materials used for the superlattice (SL) fabrication have properties that enable the design of devices optimized for infrared (IR) detection. These devices are used in the military, industry, medicine and in other areas of science and technology. The paper presents the theoretical assessment and analysis of the InAs/InAs1-xSbx type-II superlattice (T2SL) (grown on GaSb buffer layer) strain impact on the bandgap energy and on the effective masses of electrons and holes at 150 K. The theoretical research was carried out with the use of the commercial program SimuApsys (Crosslight). The k·p method was adopted in T2SL modeling. Luttinger coefficients (γ1, γ2 and γ3) were assessed assuming the Kane coefficient F = 0. The bandgap energy of ternary materials (InAsxSb1-x) was determined assuming that the bowing parameter (bg) for the above-mentioned temperature is bg = 750 meV. The cutoff wavelength values were estimated based on the theoretically determined absorption coefficients (from approximation the quadratic absorption coefficient). The bandgap energy was calculated according to the following formula: Eg = 1.24/λcutoff. The theoretical simulations allowed us to conclude that the strain in T2SL causes the Eg shift, which also has an impact on the effective masses me and mh, playing an important role for the device's optical and electrical performance. The T2SLs-simulated results at 150 K are comparable to those measured experimentally.