Yifeng Duan

Hybrid density functional theory studies of AlN and GaN under uniaxial strain

The structural stability, spontaneous polarization, piezoelectric response, and electronic structure of AlN and GaN under uniaxial strain along the [0001] direction are systematically investigated using HSE06 range-separated hybrid functionals. Our results exhibit interesting behavior. (i) AlN and GaN share the same structural transition from wurtzite to a graphite-like phase at very large compressive strains, similarly to other wurtzite semiconductors. Our calculations further reveal that this well-known phase transition is driven by the transverse-acoustic soft phonon mode associated with elastic instabilities. (ii) The applied tensile strain can either drastically suppress or strongly enhance the polarization and piezoelectricity, based on the value of the strain. Furthermore, large enhancements of polarization and piezoelectricity close to the phase-transition regions at large compressive strains are predicted, similar to those previously predicted in ferroelectric fields. Our calculations indicate that such colossal enhancements are strongly correlated to phase transitions when large atomic displacements are generated by external strains. (iii) Under the same strain, AlN and GaN have significantly different electronic properties: both wurtzite and graphite-like AlN always display direct band structures, while the the bandgap of wurtzite GaN is always direct and that of graphite-like GaN always indirect. Furthermore, the bandgap of graphite-like AlN is greatly enhanced by large compressive strain, but that of wurtzite GaN is not sensitive to compressive strain. Our results are drastically different from those for equibiaxial strain (Duan et al 2012 Appl. Phys. Lett. 100 022104).

Strain-assisted structural transformation and band gap tuning in BeO, MgTe, CdS and 2H-SiC: A hybrid density functional study

Structural transformations and electronic structures of (0001) BeO, MgTe, CdS and 2H-SiC films under equibiaxial in-plane strains are studied using the HSE06 range-separated hybrid functionals. The main results are summarized as follows: I) The structural transition from the polar wurtzite to the nonpolar graphite-like phase is predicted for BeO, MgTe and CdS but not for 2H-SiC, which is more covalent in nature. II) Either a direct or an indirect band structure is displayed in wurtzite BeO and 2H-SiC based on the values of strain, while only a direct band gap is displayed in wurtzite MgTe and CdS. At large tensile strains, the band gaps of graphite-like BeO and CdS are always indirect, whereas the graphite-like MgTe undergoes a direct-to-indirect band gap transition. Furthermore, the decrease in band gap is observed for both types of strain, thereby enabling a number of important technological applications.

Anomalous structural transformation, spontaneous polarization, piezoelectric response, and band structure of semiconductor aluminum nitride under hydrostatic pressure

Structural phase transition, spontaneous polarization, piezoelectric response, and band structure of aluminum nitride under hydrostatic pressure are systematically studied via first-principles calculations. The band structures are obtained from the HSE06 range-separated hybrid functional. Our calculated results exhibit interesting behaviors: (i) Just like the cases of uniaxial and in-plane strains, the material undergoes a structural transition from the equilibrium wurtzite phase to a pseudographitic h-MgO phase at large pressure. (ii) Although the new phase is nonpolar, the spontaneous polarization of wurtzite phase is greatly enhanced by pressure and reaches the maximum value at the phase transition. (iii) The appropriately applied pressure remarkably enhances the piezoelectric response for wurtzite phase, with the strongest behavior appearing at the phase transition. This is consistent, in that the wurtzite structure becomes markedly soft along the polar axis as pressure increases and similar to the st...

Different evolutionary pathways from B4 to B1 phase in AlN and InN: metadynamics investigations.

Pressure-induced B4-B1 phase transitions of AlN and InN at ambient temperature are systematically investigated using density functional-based metadynamics simulations. A homogeneous deformation path, which is energetically favorable, is through a hexagonal structure for AlN, and through a tetragonal structure for InN. Furthermore, the dynamical stability, instead of the mechanical stability, is crucial to determining the phase-transition paths: the intermediate hexagonal structure can remain stable, whereas the tetragonal structure is always unstable. The B4 phase always shows the direct band gap before the occurrence of structure transition, while the band gap of stable intermediate hexagonal phase is indirect for AlN. Finally, the band gap of the ultimate cubic phase is direct for AlN and indirect for InN, due to the strong p-d repulsion at the R point.

Strain-induced structural, band-structure and piezoelectric evolutions in Al0.5Ga0.5N alloy

Structural phase transition, band structure, and piezoelectric response of Al0.5Ga0.5N alloy under uniaxial and biaxial strains are systematically investigated using first-principle calculations. The main findings are summarized as follows: (I) Although the wurtzite structure transforms to an intermediate graphite-like structure for both uniaxial and biaxial strains, the second-order phase transition is found for uniaxial strain and the first-order transition for biaxial strain. The transition is driven by the mechanical and dynamical instabilities for uniaxial strain, and by the mechanical instability for biaxial strain. (II) The wurtzite phase always remains the direct band structure, whereas the band gap of graphite-like phase is always indirect. The band gaps of wurtzite and graphite-like phases are greatly reduced by internal strains. (III) The drastic enhancements in piezoelectric response are observed near phase transition, which is attributed to the flat and shallow local energy minima associated ...

Phase transition and band-structure tuning in InN through uniaxial and biaxial strains.

The phase transitions and band structure of InN under uniaxial and biaxial strains are systematically investigated using first-principles calculations. The main findings are summarized as follows: (I) although graphite-like phases are observed for both types of strain, the phase transitions are drastically different: second order for uniaxial strain and first order for biaxial strain. Furthermore, the second-order transition is driven by elastic and dynamical instabilities, whereas the first-order transition is driven only by elastic instability. (II) The wurtzite bandgap is always direct and that of the graphite-like phase is always indirect. Furthermore, the wurtzite bandgap is drastically enhanced by compressive uniaxial strain but reduced by tensile uniaxial strain. However, both biaxial strains greatly reduce the bandgap and eventually the semi-metallic phases are achieved.

Comparing the effects of uniaxial and biaxial strains on the structural stability and electronic structure in wurtzite ZnS

Structural stability and electronic structure of wurtzite ZnS under uniaxial and biaxial strains are systematically studied using the HSE hybrid functional. The two types of strain display the markedly different influences on the structural and electronic properties: (I) The newly predicted graphite-like phase is observed at large compressive uniaxial strains, not at large tensile biaxial strains, which is attributed to the different elastic responses to uniaxial and biaxial strains. (II) The direct band structures are obtained in wurtzite ZnS under uniaxial and biaxial strains, whereas the indirect band gaps are only observed in graphite-like ZnS under large uniaxial strain. Our results are different from the widely accepted conclusion but are in good agreement with the available experimental data.

Effects of hydrostatic pressure and biaxial strains on the elastic and electronic properties of t-C8B2N2

The effects of hydrostatic pressure and biaxial strains on the elastic and electronic properties of a superhard material t-C8B2N2 have been studied using first-principles calculations. The structure is proven to be mechanically and dynamically stable under the applied external forces. All the elastic constants (except C66) and elastic modulus increase (decrease) with increasing pressure and compressive (tensile) biaxial strain exx. A microscopic model is used to calculate the Vickers hardness of every single bond as well as the crystal. The hardness of t-C8B2N2 (64.7 GPa) exceeds that of c-BN (62 GPa) and increases obviously by employing pressure and compressive exx. Furthermore, the Debye temperature and anisotropy of sound velocities for t-C8B2N2 have been discussed. t-C8B2N2 undergoes an indirect to direct bandgap transition when exx > 2%; however, the indirect bandgap character of the material remains under pressure.The effects of hydrostatic pressure and biaxial strains on the elastic and electronic properties of a superhard material t-C8B2N2 have been studied using first-principles calculations. The structure is proven to be mechanically and dynamically stable under the applied external forces. All the elastic constants (except C66) and elastic modulus increase (decrease) with increasing pressure and compressive (tensile) biaxial strain exx. A microscopic model is used to calculate the Vickers hardness of every single bond as well as the crystal. The hardness of t-C8B2N2 (64.7 GPa) exceeds that of c-BN (62 GPa) and increases obviously by employing pressure and compressive exx. Furthermore, the Debye temperature and anisotropy of sound velocities for t-C8B2N2 have been discussed. t-C8B2N2 undergoes an indirect to direct bandgap transition when exx > 2%; however, the indirect bandgap character of the material remains under pressure.

Metadynamics investigations of the AlN/GaN superlattice

Pressure-induced phase transitions of the AlN/GaN superlattice at ambient temperature are systematically investigated using density-functional–based metadynamics simulations. Accompanied with the hexagonal-to-tetragonal phase transition, the coordination number increases to six from four. The homogeneous deformation pathway is energetically favorable and driven by the dynamical instability. Furthermore, no stable intermediate structure of five-fold coordination appears during the evolutionary process of phase transition. The band gap of the hexagonal phase is always direct, and that of the tetragonal phase always indirect due to the strong p-d repulsion. As the metastep number increases, the band gap is enlarged to an ultraviolet-spectrum range.

First-Principles Investigations of Pb

Crystal structure predictions of Pb0.5Ba0.5TiO3 alloys under different pressures are performed based on the particle swarming optimization algorithm. The predicted stable ground-state and high-pressure phases are tetragonal ferroelectric (I4mm) and cubic para-electric (Fmm), respectively, whose structural details have not been reported. The pressure-induced colossal enhancements in piezoelectric response are associated with the mechanical and dynamical instabilities instead of polarization rotation. The band gap of the tetragonal phase is indirect and that of the cubic phase is always direct. As pressure increases, the alloy displays the similar band-gap behaviors to PbTiO3, while different from BaTiO3, which is attributed to the different orbital contributions to the valence bands. Our calculated results are in good agreement with the available data.