Conduction Band Minimum Research Articles

Electronic properties of InBixP1-x alloy in the P-rich range calculated using first-principles calculations

First-principles calculations are carried out to explore the structural and electronic properties of the P-rich InBixP1-x versus bismuth fraction. It is found that the bowing coefficient for the lattice constant is minor (b=0.042 Å) due to the structural relaxation effect of the bond lengths in InBixP1-x. In the P-rich range, the InBixP1-x alloy has a direct bandgap. The result for the bond length shows that the average bond length in the InBixP1-x alloy becomes long with increasing bismuth fraction, resulting in weakening the total p−p coupling interaction and the total s−s coupling interaction. The descending of the conduction band minimum (CBM) is due to the weakened total p−p coupling interaction while the ascending of the valence band maximum (VBM) is owing to the weakened total s−s coupling interaction. Because of the descending of the CBM and the ascending of the VBM, the bandgap energy is shrunk. It is also found that the initial bismuth level is introduced below the VBM of InP due to the large strain of the In-Bi bond. In order to represent the bandgap energy in the P-rich range, the combination of a linear equation and the fusion-repelling model is proposed. It can offer an accurate description by setting V=1.39 eV2 and γ=1.61 eV. Besides, the positive formation energy shows that the Bi incorporation in InP is energetically unfavorable. This work is helpful for understanding the structural and electronic properties of the P-rich InBixP1-x alloy.

Surface defects related to polishing cycle in ß-Ga2O3 crystals grown by floating zone

We report an investigation on ß-Ga2O3 Schottky barrier diodes performed on substrates grown by floating-zone method using structural characterization techniques (secondary-ion mass spectrometry, inductively coupled plasma-mass spectroscopy, and atomic force microscopy) and electrical measurements (current-voltage, capacitance-voltage, Hall effect, and capacitance deep-level transient spectroscopy). Four distinct electron trap levels labeled ES, E1, E2, and E3 were found in the range of 1 eV below the Ga2O3 conduction band minimum. Among them, E1, E2, and E3 show signatures similar to those reported in the literature for Czochralski and edge-defined film-fed grown ß-Ga2O3 substrates. Trap ES was found near the surface, and we could establish a link between this defect and the damage induced by the substrate polishing technique. The level related to ES was identified at ∼0.31 eV below the conduction band minimum. An energy band above 0.31 eV was also detected and is associated with states at the metal–semiconductor interface. We demonstrated that the interface states and surface deep traps are not uniformly distributed on the ß-Ga2O3 surface. Furthermore, they contribute to the reverse leakage current and the on-state conduction degradation of the diodes.

Atomic scale localization of Kohn–Sham wavefunction at SiO2/4H–SiC interface under electric field, deviating from envelope function by effective mass approximation

To clarify the cause of the low channel conductivity at the SiO2/4H–SiC interface, the wavefunction at the SiC conduction band minimum was calculated using density functional theory under an applied electric field. We found that the wavefunction for a 4H–SiC (0001) slab tends to be localized at the cubic site closest to the interface. Importantly, because the conduction electrons are distributed closer to the interface (<5 Å) than expected from the effective mass approximation (EMA), they are more frequently scattered by interface defects. This is expected to be the reason why the channel conductivity for the (0001) face is particularly low compared with that for other faces, such as (112¯0). The breakdown of the EMA for the (0001) interface is related to the long structural periodicity along the [0001] direction in 4H–SiC crystals.

Unfolding the Impact of H2-Reduction Treatment in Enhancing the Photocatalytic Activity of Rutile TiO2 Based on Photocarriers Dynamics

In the pursuit of efficient titania photocatalysts for water splitting, several strategies have been employed. One of these is reduction of titania (TiO2) via treatment under hydrogen atmosphere, a promising technique to improve the photocatalytic performance. It has been proposed that electron injection induced by reduction on TiO2 demonstrated an improved photocatalytic activity. However, the dynamical processes involving the photogenerated charge carriers in reduced TiO2 have not been fully explored and understood yet. In this work, we employ time-resolved absorption spectroscopy (TAS) and photoluminescence (PL) measurements to unravel the impact of H2 reduction treatment on the photocarriers in rutile TiO2 (R-TiO2). TAS results revealed that the photoexcited electrons are preferentially filling the shallower trap states of reduced R-TiO2, with the lowest energy limit of ∼0.25 eV below the conduction band minimum, much shallower than that of nonreduced R-TiO2 (∼0.62 eV). Furthermore, these favorable effects of H2-reduction resulted in notable increase of long-lived shallow-trapped electrons, substantially extending the lifetimes of electrons and holes. PL measurements strongly support the positive impact of H2 reduction treatment: the NIR emission (∼850 nm) is largely quenched, indicating that the radiative recombination of deep trapped electrons and holes is significantly reduced. The combined PL and TAS results shed light on the impact of H2 reduction on R-TiO2, and this strategy can be potentially useful for further development of other photocatalytic materials.

Studies on the Light-Induced Phase Transition of CsPbBr3 Metal Halide Perovskite Materials.

We investigate the internal mechanism of the light-induced phase transition of CsPbBr3 perovskite materials via density functional theory simulations. Although CsPbBr3 tends to appear in the orthorhombic structure, it can be changed easily by external stimulus. We find that the transition of photogenerated carriers plays the decisive role in this process. When the photogenerated carriers transit from the valence band maximum to conduction band minimum in the reciprocal space, they actually transit from Br ions to Pb ions in the real space, which are taken away by the Br atoms with higher electronegativity from Pb atoms during the initial formation of the CsPbBr3 lattice. The reverse transition of valence electrons leads to the weakening of bond strength, which is proved by our calculated Bader charge, electron localization function, and integral value of COHP results. This charge transition releases the distortion of the Pb-Br octahedral framework and expands the CsPbBr3 lattice, providing possibilities to the phase transition from the orthorhombic structure to tetragonal structure. This phase transition is a self-accelerating positive feedback process, increasing the light absorption efficiency of the CsPbBr3 material, which is of great significance for the widespread promotion and application of the photostriction effect. Our results are helpful to understand the performance of CsPbBr3 perovskite under a light irradiation environment.

DFT study of electronic and structural properties of single‐walled gallium nitride nanotubes

AbstractThe electronic and structural properties of the armchair and zigzag single‐walled GaN nanotubes (GaNNTs) are studied by using density functional theory. The effects of tube diameter on the GaN bond lengths, the axial lattice constant, the Fermi energy, the buckling separation, and the bandgap of the tube are investigated. It is revealed that there is a correlation between the bandgap and the buckling; the smaller the bandgap, the higher the buckling. The results show that by increasing diameter of tubes the buckling separation decreases. The 4p‐orbitals of Ga atoms and 2p‐orbitals of N have the main role to the valence band maximum and the conduction band minimum, respectively. All armchair and zigzag GaNNTs are semiconductors having indirect and direct bandgap, respectively. It is also found that by increasing the diameter of nanotubes the bandgap increases, for both armchair and zigzag GaNNTs. The consequences of this study can undoubtedly be useful in future experimental works on optoelectronic devices.

Thermodynamic, optical, and morphological studies of the Cs2AgBiX6 double perovskites (X = Cl, Br, and I): Insights from DFT study

In contrast to Pb-based perovskites, Cs2AgBiX6 (X = Cl, Br, I) double silver-bismuth perovskites have unveiled a promising future for the advancement of low-risk solar energy due to their high durability and non-toxicity of their constituents. Despite the focus on the optoelectronic properties of Cs2AgBiX6 double perovskites, research into their structural properties behavior has been limited. So in this paper, we investigated in depth the cubic structure of Cs2AgBiX6 double perovskites. The newly acquired cubic phase is more in line with current experimental results in optoelectronic characteristics, exhibiting higher optical properties useful for optoelectronic device design. All three halide compounds exhibit an indirect band gap character due to the highly dispersive nature of the valence band maximum and conduction band minimum, resulting in reduced effective mass values, as well as the disordering of the Ag+ and Bi3+ cations in their sublattices. The thermodynamic potentials of phonon modes include enthalpy (H), free energy (F), entropy (S), and heat capacity (C). The optical and thermodynamic properties of pure Cs2AgBiCl6 and Cs2AgBiBr6 samples imply that they have promising qualities for optoelectronic and photovoltaic applications.

Opening a Band Gap in Biphenylene Monolayer via Strain: A First-Principles Study.

A biphenylene network is a novel 2D allotropy of carbon with periodic 4-6-8 rings, which was synthesized successfully in 2021. In recent years, although the mechanical properties and thermal transport received a lot of research attention, how to open the Dirac cone in the band structure of a biphenylene network is still a confused question. In this work, we utilized uniaxial and biaxial lattice strains to manipulate the electronic properties and phonon frequencies of biphenylene, and we found an indirect band gap under 10% biaxial strain through the first-principles calculations. This indirect band gap is caused by the competition between the band-edge state A and the Dirac cone for the conduction band minimum (CBM). Additionally, the lightest carrier's effective mass in biphenylene is 0.184 m0 for electrons along x (Γ→X) direction, while the effective mass for holes shows a remarkable anisotropy, suggesting the holes in the tensile biphenylene monolayer are confined within a one-dimensional chain along x direction. For phonon dispersion, we discovered that the Raman-active Ag3 phonon mode shows a robust single phonon mode character under both compressive and tensile strain, but its frequency is sensitive to lattice strain, suggesting the lattice strain in biphenylene can be identified by Raman spectroscopy.

Fluorinated 2-Phenylethylammonium Spacer Cations for Improved Stability of Ruddlesden–Popper Pb/Sn Halide Perovskites: Insight from First-Principles Calculations

Improving the stability of hybrid halide perovskite solar cells is essential for their commercialization in emerging cutting-edge technology. Here, we report the results of state-of-the-art first-principles calculations to achieve and understand enhancement in the stability of Ruddlesden–Popper (RP) lead/tin halide perovskites with fluorinated 2-phenylethylammonium (PEA) spacer cations. It is found that the substitution of up to five F atoms in the phenyl ring makes C slightly positively charged due to ∼0.5e charge transfer to electronegative F. This leads to the formation of electric dipoles in the spacer cation region and polarization of the charge density. This improves the formation energy and enhances the stability of the layers. The charge transfer makes some of the C 2p orbitals partly empty so that they hybridize with F 2p orbitals near the conduction band minimum. This is interesting for the charge transport perpendicular to the layers. Our study further shows that changing the metal cations Pb with Sn does not affect the orientation of the cation molecules in the pure as well as chemically tuned RP layers whereas changing the halide ions I with Br has an impact on the orientation of the organic molecules. The calculated structures of the layers agree well with the available experimental results and suggest structures of some unexplored layers that would be interesting to fabricate. The calculated electronic structure and optical properties show that the absorption coefficient is high (∼105 cm–1) for the layers having heavily fluorinated spacer cations. Furthermore, we find higher stability and a desirable lower direct band gap for Sn-based green perovskite layers compared to the Pb counterparts. Our results provide computational insight into the surface engineering of RP perovskites for further developments in this fast-growing area.

Modeling stoichiometric and oxygen defective TiO2 anatase bulk and (101) surface: structural and electronic properties from hybrid DFT.

We present a periodic hybrid DFT investigation of the structural and electronic properties of both stoichiometric and oxygen-defective TiO2 anatase bulk and (101) surface, in singlet and triplet spin states. In all cases, an excellent agreement with available photoelectron spectroscopy data has been obtained, reproducing the offsets of the deep defect levels positions from the conduction band minimum of TiO2 created upon oxygen vacancy (VO) formation. For the bulk, different local structural polaronic distortions around the VO site have been evidenced depending on the spin state considered. Although a similar conclusion has been drawn for the defective surface for the nine different vacancy positions which have been considered, large migration of the twofold coordinated surface O atom has also been evidenced, up to the initial vacancy site in some cases. The very good agreement obtained with available experimental data regarding the offsets from the conduction band minimum of the deep defect levels positions both for the bulk and the (101) surface of TiO2 anatase is encouraging for the application of the proposed hybrid-based computational strategy to TiO2 surface-related processes such as TiO2-based photocatalysis in which oxygen vacancies are known to play a key role. All calculations have been performed with Crystal17, considering different hybrid functionals with both effective core pseudopotentials and all-electron atom-centered basis sets, as well as additional empirical dispersion effects with the D2 and D3 models.

Impact of Zr substitution on the electronic structure of ferroelectric hafnia

HfO2-based dielectrics are promising for nanoscale ferroelectric applications, and the most favorable material within the family is Zr-substituted hafnia, i.e., Hf1−xZrxO2 (HZO). The extent of Zr substitution can be great, and x is commonly set to 0.5. However, the bandgap of ZrO2 is lower than HfO2, thus it is uncertain how the Zr content should influence the electronic band structure of HZO. A reduced bandgap is detrimental to the cycling endurance as charge injection and dielectric breakdown would become easier. Another issue is regarding the comparison on the bandgaps between HfO2/ZrO2 superlattices and HZO solid-state solutions. In this work, we systematically investigated the electronic structures of HfO2, ZrO2, and HZO using self-energy corrected density functional theory. In particular, the conduction band minimum of Pca21-HfO2 is found to lie at an ordinary k-point on the Brillouin zone border, not related to any interlines between high-symmetry k-points. Moreover, the rule of HZO bandgap variation with respect to x has been extracted. The physical mechanisms for the exponential reduction regime and linear decay regime have been revealed. The bandgaps of HfO2/ZrO2 ferroelectric superlattices are investigated in a systematic manner, and the reason why the superlattice could possess a bandgap lower than that of ZrO2 is revealed through comprehensive analysis.

Effective interface treatment by zirconium acetylacetonate for inverted methylammonium-rich perovskite solar cells

Surface passivation of halide perovskites using suitable agents is crucial to enhance the performance of corresponding solar cells as surface defects dominate nonradiative recombination in the devices. Meantime, it is important to reduce surface recombination as well as improve the contact nature at the interface between perovskite and transport layers by introducing such an agent in devices. Many works have achieved remarkable results by designing novel interfacial modifiers, but it is still a challenge to realize excellent interface treatments by less-complicated small molecules. Here, we demonstrate that zirconium acetylacetonate (ZrAcac) provides both effective methylammonium-rich perovskites surface passivation and excellent interfacial charge transport channel between the perovskite and fullerene-based electron transport layer in inverted cell structure. We theoretically find that the deep in-gap states from the PbI and IPb anti-site defects are effectively eliminated by the strong interaction between ZrAcac and the perovskite surface, which is further confirmed by experimental Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Meantime, the suitable conduction band minimum and deep valance band maximum of the ZrAcac on perovskites surface made a better electron transporting channel and interfacial anti-transfer barrier. Consequently, the corresponding device without encapsulation shows a power conversion efficiency (PCE) of 22.1% and still retains 80% of its initial PCE after 400 h of exposure to a constant environment (∼10% RH and 30 °C). This work provides a simple and cheap alternative perovskite interface treatment for future large-scale commercialization of perovskite solar cells.

Layered–Control Approach to Tune The Mobility of Perovskite SrTiO3: A Density Functional Theory Prospects

The advancement of epitaxial technology has enabled the simulation of oxide heterostructures (HS) with unique interfacial material characteristics not found in bulk materials. Recent discoveries of emergent phenomena of definite oxide interfaces have attracted much attention to oxide HS. This work explored the possibility of tuning the electron mobility of SrTiO3 (STO) through CaSnO3/SrTiO3 and ZnSnO3/SrTiO3 HSs, based on density functional theory (DFT). Own to the Sn–5s states of CSO and ZSO with more substantial band dispersion than Ti–3d states of STO, near conduction band minimum (CBM), our simulated results suggest that the bandgaps of CSO/STO (0.502 eV) and ZSO/STO (0.349 eV) HS systems are much smaller than bulk STO (1.802 eV). The effective electron masses also show much smaller values (0.31 and 0.40 m0) and (0.38 and 0.52 m0) for (CSO)7/(STO)4 and (ZSO)1/(STO)4 for HS systems compared to bulk STO (7.03 and 0.94 m0) along Γ–X and Γ–M direction. The bandgap and effective electron masses results suggest that the bandgap of STO can be well controlled and tuned by the thin film layer numbers of CSO and ZSO with better electron transportability.

Natural band alignment of MgO1−xSx alloys

We have calculated formation enthalpies, bandgaps, and natural band alignment for MgO1−xSx alloys by first-principles calculation based on density functional theory. The calculated formation enthalpies show that the MgO1−xSx alloys exhibit a large miscibility gap, and a metastable region was found to occur when the S content was below 18% or over 87%. The effect of S incorporation for bandgaps of MgO1−xSx alloys shows a large bowing parameter (b ≃ 13 eV) induced. The dependence of the band lineup of MgO1−xSx alloys on the S content by using two different methods and the change in the energy position of the valence band maximum (VBM) were larger than those of the conduction band minimum. Based on the calculated VBM positions, we predicted that MgO1−xSx with S content of 10%–18% can be surface charge transfer doped by high electron affinity materials. This work provides an example to design for p-type oxysulfide materials.

Electronic properties and passivation mechanism of AlGaN/GaN heterojunction with vacancies: a DFT study

A significant issue for GaN-based high-electron-mobility transistors (HEMTs) in high power devices is the material defect, particularly the defect states generated by the defects, which has a negative impact on the device carrier concentration and carrier transport. Based on density functional theory (DFT), we investigate the microscopic properties of different type point vacancies in the AlGaN/GaN heterojunction. It is found that N vacancy introduces defect states near the conduction band minimum (CBM) of the GaN layer and AlGaN/GaN interface. Ga and Al vacancies introduce defect states near the valence band maximum (VBM) in bulk and interface of AlGaN/GaN heterojunction. Moreover, Al vacancy is more likely to be an effective candidate for acceptor defect than Ga vacancy. We further study several AlGaN/GaN interface passivation schemes by introducing F, V group element P, and III group element B at the AlGaN/GaN heterojunction interface to analyze the passivation mechanism. According to the results of the passivation models, B passivation of Ga and Al vacancies is an effective method to completely remove the defect states from Ga and Al vacancy defects. Combining the III and V groups elements into the passivated process may be effective in achieving high-quality AlGaN/GaN heterojunction interface for the future GaN-based HEMTs fabrication.

First principle calculations: The electronic, optical, mechanical, and vibrational properties of TiS3

The computational predictions of transition-metal tri-chalcogenide (TMTCs) were performed using ab initio density functional theory (DFT) to investigate the electronic band structure, the partial density of states (PDOS), optical absorptions, dielectric functions, complex conductivity, reflectivity, refractive index, electron loss, the Poisson’s ratio, Young’s modulus, bulk-to-shear ratio, and phonon dispersion. The bandgap is measured from the valence band maximum (VBM) to the conduction band minimum (CBM) with the G–Z transitions. This suggests that the material is an indirect bandgap semiconductor. The electronic bandgap ([Formula: see text] is significantly improved with nonlocal hybrid functionals, especially in HSE0s, with [Formula: see text] of 1.0[Formula: see text]eV, which is in excellent agreement with the experimental data. However, our data shows that the HF-LDA exchange correlations significantly overestimate the [Formula: see text] with 7.33[Formula: see text]eV. Also, our optical absorption data indicates a high absorption coefficient of about [Formula: see text][Formula: see text]cm[Formula: see text]. The absorption peak of 7.4[Formula: see text]eV indicates TiS3 can be applied in vacuum ultra-violet (VUV) applications. The reflectivity is also shown to be high, with over 90% of light being reflected. The mechanical stability of the monoclinic system can be testified by our elastic coefficients and the phonon dispersions.

Large ferroelectric photovoltaic effect of a wurtzite-structure SnC/ScN superlattice

A high bandgap is the most important factor impeding the development of ferroelectric photovoltaic (FE-PV) materials. Finding a new FE-PV material with a narrow bandgap is thus very urgent. Here, a wurtzite-structure SnC/ScN (w-SSCN) superlattice was constructed to obtain light absorption properties using first-principles calculations based on the narrow bandgap of group IV and their alloys. The mechanic and dynamic calculations show that w-SSCN is stable. It has a lower band gap (2.796 eV) and a larger visible absorption coefficient than traditional ferroelectric material BaTiO3, and is even close to Si. In addition, ferroelectric polarization switching can be achieved due to the small barrier (0.215 eV). The polarization decreases under tensile strain, but the visible light absorption is enhanced due to the decrease in the bandgap. Meanwhile, the switching barrier reduces. The w-SSCN with 4% tensile strain exhibits a remarkable visible light absorption property due to the narrow bandgap (1.216 eV), and its spontaneous polarization is relatively large (0.535 C/cm2) under a low switching barrier (0.011 eV). The density of states diagram shows that the conduction band minimum (CBM) state and the valence band maximum (VBM) state come from the Sn 5 s and C 2p states, respectively. The contributions of electrons and holes originating from the different atoms suppress recombination, which enhances the solar power conversion efficiency. This work provides a new way to promote the development of FE-PV materials.

Stacking effect on the electronic structures of hexagonal GaTe

Employing first-principles density functional theory calculations with HSE06 hybrid functional, we investigate stacking-driven modifications on geometric and energy band structures of hexagonal GaTe (h-GaTe) with thickness ranging from bilayer to bulk. Our results show the most energetically favorable stacking order is AB′ type, regardless of the number of layers. The energy band structure of bulk h-GaTe exhibits indirect, quasi-direct and direct band gap for AA′, AB and AB′ stacking, respectively. In particular, as the thickness increases from single- to eleven-layers, both the conduction band minimum and valence band maximum are shifted to the point for AB′ stacking, leading to an indirect-to-direct band gap transition. Further analysis suggests the origin of the band gap transition can be attributed to the enhancement of interlayer coupling introduced by the stacking order.

Janus PdSTe nanosheet as promising contender for detection of volatile organic compounds (VOCs) in human breath: A first principles investigation

Using the density functional theory and nonequilibrium Green’s function formalism, recent work report brings the interaction between the two-dimensional (2D) Janus PdSTe nanosheet and volatile organic compounds (VOCs) (i.e., acetone, ethanol, methanol, and propanol) to investigate the latent application of Janus PdSTe for excellent performance of VOCs sensors used for human breath analysis. The pristine Janus PdSTe monolayer is semiconducting with band gap of 0.32 eV, having the valence band maxima (VBM) at the Γ-point while, conduction band minima (CBM) at k-point in the Brillouin zone. The adsorption energies are revealed that all the targeted VOCs molecule are physiosorbed on the Janus PdSTe monolayer. We found that the acetone has greater sensitivity compare to other considered VOCs. To check the sensing mechanism of VOCs, the Mulliken charge transfer (CT), interaction distance and type of adsorption were analyzed. The calculated Mulliken charge is also reveals that the stronger electron exchange interation between ethanol and Janus PdSTe monolayer. Simultaneously, the I-V relation for before and after adsorption of VOCs molecules were also obtained using the Green’s function (NEGF) for experimental set up. Finally, our outcome indicates that Janus PdSTe monolayer is a potential and feasible candidate for aformentioned VOCs sensors for applications in room temperature breath analysis of VOCs.

Conversion of p-type SnO to n-type SnO2 via oxidation and the band offset and rectification of an all-Tin oxide p-n junction structure

p-Type Tin monoxide (SnO) is a metastable phase and can be oxidized into n-type SnO2 in an O2 environment during growth and post-growth annealing. Here we first grow SnO thin films by RF magnetron sputtering with different oxygen partial pressure in the sputtering gas followed by rapid thermal annealing (RTA) in O2 to obtain n-type conducting SnO2 films. We found that while after RTA in N2 stoichiometric SnO film is p-type conducting, films sputtered with excess O and RTA in O2 crystallize in rutile SnO2 structure and exhibit n-type conductivity. Exploiting the transformation of p-SnO to n-SnO2 through oxidation, we fabricate an all-Tin Oxide transparent p-n quasi-homojunction (p-SnO/n-SnO2) and compare it to a p-SnO/n-ZnO heterojunction structure in term of their rectification behavior. XPS measurements reveal that both SnO2 and ZnO at the Γ point exhibit a type II band offset with SnO with their respective valence band (conduction band) offset of 2.8 eV (1.9 eV) and 2.4 eV (1.33 eV). On the other hand, the indirect conduction band minimum of SnO shows a type I band offset in both junctions with a small conduction band offset of ∼0.1–0.17 eV. The SnO/SnO2 p-n structure shows a reasonable rectification with an ideality factor of ∼12.3, which can be potentially useful in many transparent p-n junction based devices.