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Preferential Positioning, Stability, and Segregation of Dopants in Hexagonal Si Nanowires
摘要： We studied the physics of common p- and n-type dopants in hexagonal-diamond Si –a Si polymorph that can be synthesized in nanowire geometry without the need of extreme pressure conditions– by means of first-principles electronic structure calculations and compared our results with those for the well-known case of cubic-diamond nanowires. We showed that i) as observed in recent experiments, at larger diameters (beyond the quantum confinement regime) p-type dopants prefer the hexagonal-diamond phase with respect to the cubic one as a consequence of the stronger degree of three-fold coordination of the former, while n-type dopants are at a first approximation indifferent to the polytype of the host lattice; ii) in ultra-thin nanowires, because of the lower symmetry with respect to bulk systems and the greater freedom of structural relaxation, the order is reversed and both types of dopant slightly favor substitution at cubic lattice sites; iii) the difference in formation energies leads, particularly in thicker nanowires, to larger concentration differences in different polytypes, which can be relevant for cubic-hexagonal homojunctions; iv) ultra-small diameters exhibit, regardless of the crystal phase, a pronounced surface segregation tendency for p-type dopants. Overall these findings shed light on the role of crystal phase in the doping mechanism at the nanoscale and could have a great potential in view of the recent experimental works on group IV nanowires polytypes.
关键词： density functional theory,Nanowires,formation energy,dopants,2H-Si,hexagonal diamond silicon
更新于2025-09-23 15:23:52
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Density Functional Theory Calculations of Oxygen-Vacancy Formation and Subsequent Molecular Adsorption on Oxide Surfaces
摘要： The surface oxygen vacancy formation energy (EOvac) is an important parameter in determining the catalytic activity of metal oxides. Estimating these energies can therefore lead to data-driven design of promising catalyst candidates. In the present study, we determine EOvac for various insulating and semiconducting oxides. Statistical investigations indicate that the band gap, bulk formation energy, and electron affinity are factors that strongly influence EOvac. Electrons enter defect states after O desorption, and these states can be in the valence band, mid-gap, or in the conduction band. Subsequent adsorption of O2, NO, CO, CO2, and H2 molecules on an O-deficient surface is also investigated. These molecules become preferentially adsorbed at the defect sites, and EOvac is identified as the dominant factor that determines the adsorption mode as well as a descriptor that shows good correlation with the adsorption energy.
关键词： catalytic activity,molecular adsorption,oxygen vacancy formation energy,metal oxides,density functional theory
更新于2025-09-23 15:21:21
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Defect levels induced by double substitution of B and N in 4H-SiC
摘要： Studies of substitutional impurities in 4H-SiC play a major role in identifying the most enhanced defect levels induced. N and B substitution in 4H-SiC on separate studies reveal that the induced negative-U charge state ordering while the induced shallow acceptor level. In this report, in order to predict the most stable defect and its electrically active defect levels, the Heyd, Scuseria and Ernzerhof (HSE06) hybrid functional was used to model and predict the energetics and defect levels induced by double substitution of B and N pair (NBCC, NBSiC, NBCSi, NBSiSi) in a 4H-SiC. While both B and N substituted for C under equilibrium conditions is the most energetically favourable pair, the N and B both substituted for Si has a relatively high formation energy. The energies of the various configurations suggest that the substitution of B and N pair in 4H-SiC is always stable, and NBCC is the most energetically favourable defect. While all the defects studied induced acceptor levels, only the NBSiC and NBCSi induced shallow donor levels. The (0/?1) acceptor level induced by the defects is close to the conduction band minimum for the case of NBSiC and deep lying middle of the band gap of 4H-SiC for the NBCC and NBCSi.
关键词： Defect,Charge state,Formation energy,Substitution
更新于2025-09-19 17:15:36
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Codoping Er-N to Suppress Self-Compensation Donors for Stable <i>p</i> -Type Zinc Oxide
摘要： Stable p-type doping of zinc oxide (ZnO) is an unsolved but critical issue for ultraviolet optoelectronic applications despite extensive investigations. Here, an Er-N codoping strategy for defect engineering of ZnO to suppress the self-compensation of the donor-type intrinsic point defects (IPDs) over the acceptor-type ones is proposed. Via first-principles calculations, the influence of nitrogen and erbium concentration on the stability of ZnO is investigated. The complex (ErZn-mNO) consisting of multiple substitutional N on O sites and one substitutional Er on Zn site is a crucial stabilizer. With an increase of the concentration of N, the absorption edges redshift to lower energy due to the impurity band broadening in the bandgap. The results suggest that codoping Er-N into the ZnO matrix is a feasible way to manufacture stable p-type ZnO.
关键词： p-type ZnO,Er-N codoping,first principles,defect formation energy
更新于2025-09-10 09:29:36
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Density-functional-theory calculations of structural and electronic properties of vacancies in monolayer hexagonal boron nitride (h-BN)
摘要： We have carried out the density functional theory calculations of vacancies in monolayer h-BN. We model five configurations, two configurations of monovacancies (VB and VN) and three configurations (VBB, VNN, and VBN) of divacancies. In the case of monovacancies, we find that VB and VN form a C3V symmetry. In the divacancy case, we find that VNN produces a heart-like configuration having two pentagons and leaving two dangling bonds, while VBB leaves four dangling bonds. As for VBN, it produces two pentagons and has no dangling bond. The calculated formation energies of VB; VN; VBB; VNN, and VBN are 11.65 eV, 12.05 eV, 17.59 eV, 22.32 eV, and 16.89 eV, respectively. These energies show that VB is more stable than VN, while VBB is more stable than VNN. We conclude that the N-rich h-BN sheet is energetically more favorable to be formed rather than the B-rich one. However, the most stable configuration of the divacancies belongs to VBN compensating from the absence of the dangling bond, which obeys the dangling-bond-counting-model. Furthermore, we calculate band structures of the most stable mono- and divacancies. We find that monovacancy somewhat changes the electronic structure, shown by localized states near the Fermi level, while the divacancy produces two new states above the Fermi level.
关键词： B-N vacancies,Electronic structure,Formation energy
更新于2025-09-04 15:30:14