Valence Band Maximum Research Articles

Defect control for high efficiency antimony selenosulfide solar cells by interface Engineering of buried monoatomic aluminum oxide layer

Antimony selenosulfide (Sb2(S,Se)3) reveals excellent optoelectronic characteristics, positioning it as a propitious light-absorbing substance with potential applications in photovoltaic technology. However, a multitude of deep-level defects significantly limit the efficiency of Sb2(S,Se)3 solar cells. In this study, the density of the surface and deep-level defects was reduced by adding a monoatomic Al2O3 layer on the surface of CdS film. Compared to the as-deposited film, highly [hk1]-oriented Sb2(S,Se)3 film having significantly enhanced charge separation was obtained. The addition of a monoatomic Al2O3 layer effectively modified the energy band structure by increasing the valence band maximum and more intrinsic Sb2(S,Se)3 film between n-type CdS layers and p-type spiro. This contributed to increased carrier collection and reduced carrier recombination. Ultimately, 9.39 % efficiency of the device were obtained as a result of the optimized orientation and reduced defects. Consequently, this research demonstrates an effective method for optimizing crystallographic orientation and minimizing defect density, thereby maximizing the efficiency of Sb2(S,Se)3 solar cells.

High photovoltaic performance of lead-free Cs2AgInCl6-xBrx perovskite solar cell using DFT and SCAPS-1D simulations

The search of the lead-free highly efficient photovoltaic material is a key challenge for next generation solar cells. The structural, electronic, and optical properties of lead-free double perovskite Cs2AgInCl6-xBrx (x= 0,1,2,3,4,5 and 6) were investigated using density functional theory and its photovoltaic performance by using one-dimensional solar cell capacitance simulator SCAPS-1D. By increasing Br content in Cs2AgInCl6, the lattice parameter increases and the electronic band gap decreases without affecting the direct nature of band gap at Г- point. By mixing Br in Cs2AgInCl6 also provides an effective way to tune the energy band gap. In addition, increasing the Br content in mixed halide composition Cs2AgInCl6-xBrx (x= 0,1,2,3,4,5 and 6) reproduces more dispersive conduction band minima and valence band maxima result in lower effective masses of both charge carriers and consequently higher bipolar carrier mobility. The optical properties of Cs2AgInCl6-xBrx, such as absorption coefficient, reflectivity, dielectric constants, refractive index, and extinction coefficient were investigated with different Br concentrations. Among all compositions, the Cs2AgInClBr5 exhibits better optical properties and behaves as poor reflector and good absorber in the visible region. The photovoltaic performance of different absorber layers of Cs2AgInCl6-xBrx compositions in the perovskites solar cell module with configuration FTO/ETL/Cs2AgInCl6-xBrx /HTL/Au is analysed by using the SCAPS-1D. It is also observed that Cs2AgInClBr5 has high bipolar carrier mobility, relatively higher stability and direct band gap value of 1.75 eV with the PCE of 21.95% in the visible region indicate the suitability of the Cs2AgInClBr5 as a promising lead-free photovoltaic material.

Investigation of the surface band structure and the evolution of defects in β-(AlxGa1−x)2O3

This study examines the electronic and luminescent properties of β-(AlxGa1−x)2O3 (0 ≤ x ≤ 0.42) thin films grown on (0001) sapphire using laser-MBE, with a focus on the evolution of defect energy levels and their impact on surface Fermi level pinning and luminescence. X-ray photoelectron spectroscopy (XPS) and cathodoluminescence (CL) have been employed to analyze surface band bending and defect evolution as a function of aluminum content. The results have revealed a pinned Fermi level at 3.6 eV above the valence band maximum despite the increase in the bandgap. The consequent upward band bending has been confirmed by a peak shift in the core level XPS. The defects that lead to the Fermi level pinning effect are attributed to E2*, which is related to a Ga vacancy or Ga vacancy-O vacancy complex. In addition, CL spectroscopy and depth-resolved CL have demonstrated consistent blue and ultraviolet emissions across the Al content range and a similar suppression of electron concentration on blue and ultraviolet emissions in β-(AlxGa1−x)2O3 and β-Ga2O3. Based on the observed evolution of defects with Al content, the blue band emission is attributed to electron transition in the donor–accepter pair.

Strain Engineering of Cd0.5Zn0.5S Nanocrystal for Efficient Photocatalytic Hydrogen Evolution from Wasted Plastic.

The challenge of synthesizing nanocrystal photocatalysts with adjustable lattice strain for effective waste-to-energy conversion is addressed in this study. Cd0.5Zn0.5S (CZS) nanocrystals are synthesized by a simple solvothermal method, regulation of the ratio between N, N-dimethylformamide, and water solvent are shown to provoke expansion and contraction, inducing an adjustable lattice strain ranging from -1.2% to 5.6%. With the hydrolyzed wasted plastic as a sacrificial agent, the 5.6% lattice-strain CZS exhibited a robust hydrogen evolution activity of 1.09mmol m-2h-1 (13.83mmol g-1h-1), 4.5 times that of pristine CZS. Characterizations and density functional theory calculation demonstrated that lattice expansion increases the spatial distance between the valence band maximum and conduction band minimum, thus reducing carrier recombination and promoting charge transfer. Additionally, lattice expansion induces surface S vacancies and adsorbed OH groups, further enhancing redox reactions. This study focuses on the synchronous regulation of crystal structure, charge separation/transport, and surface reactions through lattice strain engineering, which providing a reference for the rational design of new photocatalysts for effective waste-to-energy conversion.

The Energy Level Alignment at the Buffer/Cu(In,Ga)Se2 Thin‐Film Solar Cell Interface for CdS and GaOx

AbstractSputter‐deposited GaOx (i.e., oxygen‐deficient gallium oxide) films are evaluated as a potential replacement for the standard CdS buffer layers in Cu(In,Ga)Se2 (CIGSe) based thin‐film photovoltaics. The energy level alignment at the GaOx/CIGSe and CdS/CIGSe interfaces are compared by means of direct and inverse photoemission. For the GaOx/CIGSe a (0.04 ± 0.07) eV (i.e., a small spike‐like) conduction band offset (CBO) and a (−3.21 ± 0.19) eV (i.e., a large cliff‐like) valence band offset (VBO) are found, which suggests a nearly ideal charge‐selective contact. The derived GaOx band gap of (4.80 ± 0.25) eV confirms its utility as a highly transparent buffer layer. However, the GaOx (with x derived to be 1.1 ± 0.1) exhibits considerable (presumably) defect‐related occupied states above the valence band maximum. It is proposed that these states may increase charge carrier recombination and decrease open circuit voltage in respective devices; also explaining why solar cells with standard CdS buffer outperform devices with GaOx buffer, despite less ideal electronic interface properties (CBO: (−0.18 ± 0.07) eV, VBO: (−0.98 ± 0.15) eV) and the smaller CdS band gap of (2.35 ± 0.22) eV.

First-principles simulations of scanning tunneling microscopy images exhibiting anomalous dot patterns on armchair-edged graphene nanoribbons

To interpret our recent scanning tunneling microscopy (STM) experiment of nanographene sheets, in which the existence of rectangularlike lattices is confirmed, we conducted first-principles calculations of both simple and wrinkled armchair-edged graphene nanoribbons (AGNRs). We focused on the origin of this unique lattice and simulated STM images in the occupied band for the above-mentioned ribbons with different widths. Combined with the band structures of AGNRs, we investigated the features of electron distribution in detail under various magnitudes of applied sample biases (−1 to −0.05 V). In contrast to the well-known hexagonal structure of graphene at a large bias of −1 V, a rectangularlike lattice emerges in the STM images of AGNRs with specific widths when they are generated solely by electronic states occupying the valence band maximum. Furthermore, we find that a wrinkle in graphene parallel to the edges in AGNR significantly affects the STM images of a neighboring flat graphene region, even when its height is only several angstroms. By comparing the partial density of states of the wrinkle with flat graphene, we discussed the role of a wrinkle in functionalizing graphene sheets. Published by the American Physical Society 2024

Extraction of gap states in AlSiO/AlN/GaN metal-oxide-semiconductor field-effect transistors using the multi-terminal capacitance–voltage method

Direct extraction of gap states from a metal-oxide-semiconductor field-effect transistor (MOSFET) in which inversion electrons and holes in a p-type body coexist is challenging. We demonstrate gap-state extraction in lateral-type GaN MOSFETs with high channel mobilities using multi-terminal capacitance–voltage (C–V) methods. The gate stack of the MOSFET was composed of AlSiO/AlN/p-type GaN formed on a p+/n+ GaN tunnel junction structure. The substrate electrode was short-circuited to a p-type body layer through the tunnel junction. The MOSFET was equipped with gate, source, drain, body, and substrate electrodes. When the gate was the high side and the other electrodes were the low side in the AC circuit, a V-shaped C–V curve was obtained because of electron inversion and hole accumulation. When the body/substrate electrodes were connected to the ground level (i.e., split C–V method), the inversion electrons between the gate and source/drain electrodes could be evaluated. We proposed a “reverse” split C–V method in which the source/drain electrodes are grounded and the body/substrate electrodes are connected to the low side. This method enabled extraction of gap states near the valence-band maximum of GaN, with exclusion of the overlap capacitance and the capacitance due to inversion electrons. The proposed method demonstrated overall gap states in the GaN MOSFET with a wide bandgap. The results suggest that hole traps with discrete energy levels caused negative bias instability (NBI) in the GaN MOSFET. Furthermore, NBI and discrete gap states were consistently suppressed by Mg doping at >1018 cm−3 into a p-type body.

Tunable Emission of Low-Dimensional Organic Metal Halides by Stoichiometric Ratio and Metal Center.

Low-dimensional (LD) organic metal halides (OMHs) have a bright future due to their excellent photoelectric characteristics and unique structure. However, the synthesis and emission control of LD-OMHs are still unclear. Herein, the different dimensional (zero-dimensional (0D), one-dimensional (1D), and three-dimensional (3D)) of OMHs were obtained by the reaction of 1,4-diazabicyclo (2.2.2) octane with PbBr2 in different stoichiometric ratios. This discovery shows that the structure and properties of OMHs can be regulated while maintaining the functional organic cations of OMHs, which broadens the path for the development of functional LD-OMHs. Among them, 0D-OMH 1 and 1D-OMH 3 have narrow-band (full width at half-maximum (fwhm) = 74 nm) and broad-band (fwhm = 201 nm) emission, respectively. We found that when organic cations have no contribution to the formation of conduction band minimum and valence band maximum, and the distances between polyhedrons are larger than the van der Waals diameter of the halogen atom, the effect of phonons on exciton transitions can be reduced to achieve a narrow-band emission. Further, Cu(I)- and Mn (II)-based 0D-OMHs were synthesized, which have high photoluminescence quantum yield (PLQY) (33.97 and 47.33%, respectively). When the emitting of 0D-OMHs produced by the interaction of the metal-center and halogens, the asymmetric planar metal-halogen structure will result in a higher PLQY.

Material Design of Ultra-Thin InN/GaN Superlattices for a Long-Wavelength Light Emission.

GaN heterostructure is a promising material for next-generation optoelectronic devices, and Indium gallium nitride (InGaN) has been widely used in ultraviolet and blue light emission. However, its applied potential for longer wavelengths still requires exploration. In this work, the ultra-thin InN/GaN superlattices (SL) were designed for long-wavelength light emission and investigated by first-principles simulations. The crystallographic and electronic properties of SL were comprehensively studied, especially the strain state of InN well layers in SL. Different strain states of InN layers were applied to modulate the bandgap of the SL, and the designed InN/GaN heterostructure could theoretically achieve photon emission of at least 650 nm. Additionally, we found the SL had different quantum confinement effects on electrons and holes, but an efficient capture of electron-hole pairs could be realized. Meanwhile, external forces were also considered. The orbital compositions of the valence band maximum (VBM) were changed with the increase in tensile stress. The transverse electric (TE) mode was found to play a leading role in light emission in normal working conditions, and it was advantageous for light extraction. The capacity of ultra-thin InN/GaN SL on long-wavelength light emission was theoretically investigated.

Vacancy‐Type Defects and Their Trapping/Detrapping of Charge Carriers in Ion‐Implanted GaN Studied by Positron Annihilation

Annealing behaviors of vacancy‐type defects in ion‐implanted GaN are studied by positron annihilation. Mg+ and N+ ions are implanted to obtain 700 nm deep box profiles with Mg and N concentrations of 1 × 1018 cm−3, and the samples are annealed using an ultrahigh‐pressure annealing system. For as‐implanted samples, the major defect species is identified as Ga‐vacancy (VGa)‐type defects. For N‐implanted GaN, the size of the vacancies increases as the annealing temperature increases up to 1100 °C and then shrank above 1200 °C. This behavior is attributed to recombinations between N‐vacancy (VN)‐type defects and excess N. For Mg‐implanted GaN, the major defect species after annealing above 1000 °C is vacancy clusters such as (VGaVN)3. Some of them act as nonradiative recombination centers for blue and ultraviolet luminescence. Their energy levels corresponding to the transition from positive to neutral are located between 2.6 eV above the valence band maximum and the conduction band minimum. The thermal activation process of electron detrapping from the vacancy clusters is also studied. For Mg‐implanted GaN, one of the major secondary defects is collapsed vacancy disks forming dislocation loops. They are eliminated by additional N implantation, which is associated with vacancy agglomerations under the annealing in VGa‐rich condition.

Structural, electronic and thermoelectric properties of monolayer TiSe2.

In this work the first-principles calculations of the structural, electronic and thermoelectric properties of monolayer TiSe2 are presented. The optimized lattice parameter of monolayer TiSe2 shows excellent agreement with the experimental value. The computed band structure and density of states calculations predict metallic nature of monolayer TiSe2 with overlapping of 0.44eV between the lowest conduction band and top valance band at high symmetry point M. The position of pseudogap formed by Ti-3d orbitals near the Fermi level confirms the mechanical stability of monolayer TiSe2. Due to the influence of positive strain (tensile strain), the Ti-Se bond length increases and the layer height decreases. The applied tensile strain changes the metallic nature of TiSe2 into a semiconductor with opening of bandgap. It has also been observed that the positions of conduction band minima and valance band maxima change with strain. The charge analysis shows that charge transfer from Ti to Se atom increases when tensile strain is applied, while an opposite trend is observed with compression. The computed thermoelectric coefficientsi.e. Seeback coefficient, power factor and figure of merit are in good agreement with the experimental data.The temperature dependence of these coefficientsis also reported. The density functional theory based calculations are reported employing the PBE-GGA ansatz using the plane wave-pseudopotential method embodied in the Quantum ESPRESSO package. The self-consistent field calculations are performed over a dense Monkhorst-Pack net of 12 × 12 × 1k-points. The energy convergence criteria for the self-consistent field calculation were set to 10-6 Ry/atom with a cutoff energy of 90 Ry. The thermoelectric properties are computed by combining the band structure calculations with the Boltzmann transport equation using Boltztrap2 peckage.

Visualizing the Low-Energy Electronic Structure of Prototypical Hybrid Halide Perovskite through Clear Band Measurements.

Organic-inorganic hybrid perovskites (OIHPs) are a promising class of materials that rival conventional semiconductors in various optoelectronic applications. However, unraveling the precise nature of their low-energy electronic structures continues to pose a significant challenge, primarily due to the absence of clear band measurements. Here, we investigate the low-energy electronic structure of CH3NH3PbI3 (MAPI3) using angle-resolved photoelectron spectroscopy combined with ab initio density functional theory. We successfully visualize the electronic structure of MAPI3 near the bulk valence band maximum by using a laboratory photon source (He Iα, 21.2 eV) at low temperature and explore its fundamental properties. The observed valence band exhibits a highly isotropic and parabolic band characterized by small effective masses of 0.20-0.21 me, without notable spectral signatures associated with a large polaron or the Rashba effect, subjects that are intensely debated in the literature. Concurrently, our spin-resolved measurements directly disprove the giant Rashba scenario previously suggested in a similar perovskite compound by establishing an upper limit for the Rashba parameter (αR) of 0.28 eV Å. Our results unveil the unusually complex nature of the low-energy electronic structure of OIHPs, thereby advancing our fundamental understanding of this important class of materials.

Origin of hole mobility anisotropy in 4H-SiC

Hole mobility anisotropy in 4H-SiC was investigated based on both experimental and theoretical approaches. First, the authors established a complete database of the anisotropic hole mobility along both directions parallel and perpendicular to the c-axis in 4H-SiC over the wide acceptor density and temperature ranges by preparing Hall bar structures on p-type SiC(112¯0) epitaxial layers. Empirical equations for the mobility along each direction vs the acceptor density and temperature were determined, which should be useful for the simulation and designing of any SiC devices. In addition to that, the anisotropy in the hole mobility was extracted from the experimental results, and its origin was discussed focusing on that in the effective mass (m∗) of holes. The obtained mobility ratio was far from the m∗ ratio at the valence band maximum, and an averaged m∗ along each direction was determined by theoretical calculation taking into account the energy distribution of holes. Consequently, the authors revealed that the anisotropic hole mobility is explained quantitatively by the anisotropic m∗ considering the E–k dispersion over the entire first Brillouin zone.

Immobilizing Triphenylamine with Photoredox Inert Sr2+ Forming Sr‐MOF with Controlled Electron Migration for Photocatalytic Oxidation of Thiols to Disulfides

Comprehensive SummaryThe photocatalytic oxidative coupling of thiols is one of the most popular methods to synthesize the disulfides. Triphenylamine and its derivatives (TPAs) are promising for the above reaction, but suffer from the easy polymerization and difficult separation. To overcome these obstacles while controlling the photogenerated electrons transfer directly to target substrates, herein, we constructed one TPA‐based metal‐organic framework (MOF), (Me2NH2)[Sr(TCBPA)]·DMA·3H2O (1), by direct self‐assembly of tris(4′‐carboxybiphenyl)amine (H3TCBPA) and photoredox inert strontium ion (Sr2+). DFT calculations revealed that the valence band maximum (VBM) and the conduction band minimum (CBM) are mainly located on TCBPA3–, successfully inhibiting the undesirable electron migration to metal nodes. Experimental results indicated that 1 displays superior performance than homogeneous H3TCBPA, which may result from the abundant π···π and C—H···π interactions between the well‐arranged TCBPA3– and the build‐in electric field between the anionic framework and the Me2NH2+. This work highlights that immobilizing TPAs into MOFs is one promising approach to designing heterogeneous photocatalysts for the synthesis of disulfides by oxidative coupling of thiols.

The Substitution of Rare-Earth Gd in BaScCuTe3 Realizing the Band Degeneracy and the Point-Defect Scattering toward Enhanced Thermoelectric Performance.

Zintl compounds have continuously received significant attention, primarily due to their structural characteristics that align with the properties of the electron crystal and phonon glass. In this study, the crystal structure and thermoelectric properties of the quaternary Zintl chalcogenide BaScCuTe3 are investigated. The band structure calculations for BaScCuTe3 reveal a slight energy split of 0.08 eV between the second valence band and the valence band maximum, suggesting the presence of multiband-transport behaviors. Substitution of rare earth Gd for Sc is conducted, which significantly increases the hole concentration from 4.1 × 1019 cm-3 to 8.2 × 1019 cm-3 at room temperature. Meanwhile, the Seebeck coefficient increases because of the participation of the second valence band. A maximum power factor of 6.56 μW/cm·K2 at 773 K is obtained, which is 72% higher than that of the pristine sample. Moreover, the lattice thermal conductivity decreases from 0.57 W/m·K for BaScCuTe3 to 0.48 W/m·K for BaSc0.97Gd0.03CuTe3 at 773 K, owing to the introduction of point-defect scattering. As a result, there is a noteworthy improvement in the thermoelectric figure of merit zT, increasing from 0.44 for the undoped sample to 0.85 for BaSc0.98Gd0.02CuTe3. Considering these findings, BaScCuTe3 exhibits great potential and holds promise for further investigation in the field of thermoelectric materials.

Time evolution of the defect states at the surface of MoS2

MoS2 has generated significant attention due to its unique electronic properties and versatile applications. Being a van der Waals material, MoS2 is expected to exhibit an inert surface due to lack of dangling bond. However, our photoemission study finds MoS2 to be highly sensitive toward residual gases. The position of the valence band maximum (VBM) shifts even in a vacuum of 10−10 Torr. We find this to be due to CO adsorption causing unintentional electron doping. The time evolution of the position of VBM is exponential, and it reaches two different saturation points, depending on whether the sample is exposed to ultraviolet (UV) radiation or not. Our XPS (x-ray photoemission spectroscopy) study shows no time-dependent escape of sulfur, which was in a previous study attributed to a VBM shift. The VBM shift can be reversed by annealing, sputtering, and UV light, which desorb CO gases. The study shows that the MoS2 surface is easily doped, which offers the possibility of using it as a sensor but in many other applications could diminish device performance and needs to be considered.

A first-principles investigation of internal energy and entropy of formation of charged defects in Th1−xUxO2 (x ≤ 0.5)

Mixed thorium/uranium dioxide, (Th,U)O2, is under consideration for advanced nuclear fuel applications. Investigating the point defect structure and energy in this oxide is important for predicting its behavior as fuel. In this work, we use first-principles calculations based on the generalized gradient approximation (GGA)+Hubbard U approach to investigate the internal energy and entropy of the formation of point defects in Th1−xUxO2 at various compositions below x ≤ 0.5. Point defects including O vacancies, O interstitials, Th vacancies, Th interstitials, U vacancies, and U interstitials have all been considered with their charges ranging from neutral to the maximum nominal values. The observed trends have been explained in terms of electronic density of states. The valence band maxima of crystals that contain defects play a crucial role and exhibit variations depending on the U content and the applied charge. The temperature dependence of internal energy and entropy of formation of defects have also been examined. The internal energy of formation of defects was found to exhibit slowly varying or constant values with respect to changes in the U content, except at low values of x and low temperatures. The entropy of formation of defects was observed to decrease with increasing U content. It was additionally observed that the entropy of formation of vacancies increases with temperature, while that of interstitials decreases. This investigation further revealed that at 0 K, the cation vacancies and anion interstitials become increasingly favorable with increasing U content, while cation interstitials and anion vacancies become less favorable.

From Molecules to Devices: Insights into Electronic and Optical Properties of Pyridine-Derived Compounds Using Density Functional Theory Calculations.

In this study, we delve into the electronic structure, spectroscopic, and optical properties of five benzo derivatives of pyridine, namely, 5-(4-chlorophenyl)-2-fluoropyridine (1), 2-fluoro-5-(4-fluorophenyl)pyridine (2), 4-(2-fluoropyridin-5-yl)phenol (3), 5-(2,3-dichlorophenyl)-2-fluoropyridine (4), and 5-(5-bromo-2-methoxyphenyl)-2-fluoropyridine (5). Utilizing quantum chemical density functional theory calculations at the B3LYP and Perdew-Burke-Ernzerhof levels of theory combined with the 6-311G(d,p) and 6-311++G(d,p) basis sets, we investigated the electronic and optical characteristics of these compounds. Band structure calculations were conducted for their crystalline structures, revealing a direct band gap varying from 3.018 to 3.558 eV, with the valence band maximum and conduction band minimum located at the G point in the Brillouin zone. The optical properties were analyzed, including the dielectric functions, reflectivity, and refractive index. Notably, reflectivity was found to be minimal in the photon energy range of 0.0-3.0 eV, and the static refractive index, n(0), ranged from 1.55 to 1.70. The research also involved assessing the reactivity of the compounds through calculation of the frontier orbital energy gaps (ΔE), indicating a significant charge transfer and high reactivity. Additionally, we performed frequency analysis to unveil the Fourier-transform infrared spectra of compounds 1-5 at room temperature. Molecular electrostatic potential surfaces of the optimized structures were employed to map the electrophilic and nucleophilic regions of the compounds. This investigation provides a comprehensive understanding of the electronic and optical properties of these pyridine derivatives, shedding light on their potential applications in optoelectronics.

Defect engineering and alloying strategies for tailoring thermoelectric behavior in GeTe and its alloys

GeTe exhibits excellent p-type medium-temperature thermoelectric properties with low toxicity and good mechanical characteristics, making it highly promising for development in the thermoelectric field. However, GeTe is prone to producing Ge vacancies, leading to high p-type carrier concentration, which results in elevated electronic thermal conductivity and a low Seebeck coefficient. This study systematically analyzes intrinsic and extrinsic defects in GeTe and its alloys, focusing on reducing p-type carrier concentration through first-principles calculations. The results reveal that substituting Ge-sites with Bi (BiGe) yields lower donor defect formation energy, effectively reducing p-type carrier concentration of GeTe and its alloys compared to other elemental doping. Additionally, alloying with certain elements, such as Pb, proves favorable for decreased p-type carrier concentration due to lowered energy levels of valence band maximum (VBM). Inspired by this, screening divalent elements for alloying on Ge-sites reveals that Sr, Ba, Eu, and Yb substantially reduce the VBM of GeTe. Further calculations for Ba and Yb-alloyed GeTe confirm changes in formation energies for donor (favorable) and acceptor (unfavorable) defects. Our work provides a systematic investigation of intrinsic and various extrinsic doping defects in GeTe and its alloys, shedding light on possible strategies of optimizing carrier concentration in these compounds.

Anisotropic Band Evolution of Bulk Black Phosphorus Induced by Uniaxial Tensile Strain

We investigate the anisotropic band structure and its evolution under tensile strains along different crystallographic directions in bulk black phosphorus (BP) using angle-resolved photoemission spectroscopy and density functional theory. The results show that there are band crossings in the Z–L (armchair) direction, but not in the Z–A (zigzag) direction. The corresponding dispersion-k distributions near the valence band maximum (VBM) exhibit quasi-linear or quadratic relationships, respectively. Along the armchair direction, the tensile strain expands the interlayer spacing and shifts the VBM to deeper levels with a slope of −16.2 meV/% strain. Conversely, the tensile strain along the zigzag direction compresses the interlayer spacing and causes the VBM to shift towards shallower levels with a slope of 13.1 meV/% strain. This work demonstrates an effective method for band engineering of bulk BP by uniaxial tensile strain, elucidates the mechanism behind it, and paves the way for strain-regulated optoelectronic devices based on bulk BP.