Valence band splitting in Cu2(Sn,Ge, Si)S3: effect on optical absorption spectra

in Physica Status Solidi. Rapid Research Letters (2017)

We perform a detailed analysis of the valence band splitting (VBS) effect on the absorption spectra of monoclinic Cu2(Sn,Ge,Si)S3 combining theory and experiment. We cal- culate the imaginary part of the ... [more ▼]

We perform a detailed analysis of the valence band splitting (VBS) effect on the absorption spectra of monoclinic Cu2(Sn,Ge,Si)S3 combining theory and experiment. We cal- culate the imaginary part of the dielectric function for all three compounds using hybrid functionals and maximally lo- calized Wannier functions in remarkably dense k-meshes to ensure an accurate description of the low energy spectral regime. We find that the VBS will affect the absorption spectra of these materials leading to multiple absorption onsets. Our experimental spectra on Cu2(Sn,Ge)S3, analysed using both Tauc plots and inflection points, verify this prediction. A good agreement between theory and experiment in terms of VBS values is recorded. [less ▲]

Topological states in multi-orbital ​HgTe honeycomb lattices

in Nature Communications (2015), 6(6316),

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice. However, the quantum spin Hall effect predicted at the K point could not be observed in graphene and other ... [more ▼]

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice. However, the quantum spin Hall effect predicted at the K point could not be observed in graphene and other honeycomb structures of light elements due to an insufficiently strong spin–orbit coupling. Here we show theoretically that 2D honeycomb lattices of ​HgTe can combine the effects of the honeycomb geometry and strong spin–orbit coupling. The conduction bands, experimentally accessible via doping, can be described by a tight-binding lattice model as in graphene, but including multi-orbital degrees of freedom and spin–orbit coupling. This results in very large topological gaps (up to 35 meV) and a flattened band detached from the others. Owing to this flat band and the sizable Coulomb interaction, honeycomb structures of ​HgTe constitute a promising platform for the observation of a fractional Chern insulator or a fractional quantum spin Hall phase. [less ▲]

Preparation and study of 2-D semiconductors with Dirac type bands due to the honeycomb nanogeometry

in Proceedings of SPIE (2014, March 07), 8981

The interest in 2-dimensional systems with a honeycomb lattice and related Dirac-­type electronic bands has exceeded the prototype graphene [1]. Currently, 2-­dimensional atomic [2,3] and nanoscale [4-­8 ... [more ▼]

The interest in 2-dimensional systems with a honeycomb lattice and related Dirac-­type electronic bands has exceeded the prototype graphene [1]. Currently, 2-­dimensional atomic [2,3] and nanoscale [4-­8] systems are extensively investigated in the search for materials with novel electronic properties that can be tailored by geometry. The immediate question that arises is how to fabricate 2-­D semiconductors that have a honeycomb nanogeometry, and as a consequence of that, display a Dirac-­type band structure? Here, we show that atomically coherent honeycomb superlattices of rocksalt (PbSe, PbTe) and zincblende (CdSe, CdTe) semiconductors can be obtained by nanocrystal self-­assembly and facet-­to-­facet atomic bonding, and subsequent cation exchange. We present a extended structural analysis of atomically coherent 2-­D honeycomb structures that were recently obtained with self-assembly and facet-­to-­facet bonding [9]. We show that this process may in principle lead to three different types of honeycomb structures, one with a graphene type-­, and two others with a silicene-­type structure. Using TEM, electron diffraction, STM and GISAXS it is convincingly shown that the structures are from the silicene-­type. In the second part of this work, we describe the electronic structure of graphene-­type and silicene type honeycomb semiconductors. We present the results of advanced electronic structure calculations using the sp3d5s* atomistic tight-­binding method10. For simplicity, we focus on semiconductors with a simple and single conduction band for the native bulk semiconductor. When the 3-­D geometry is changed into 2-­D honeycomb, a conduction band structure transformation to two types of Dirac cones, one for S-­ and one for P-­orbitals, is observed. The width of the bands depends on the honeycomb period and the coupling between the nanocrystals. Furthermore, there is a dispersionless P-­orbital band, which also forms a landmark of the honeycomb structure. The effects of considerable intrinsic spin-­orbit coupling are briefly considered. For heavy-­element compounds such as CdTe, strong intrinsic spin-­‐orbit coupling opens a non-­trivial gap at the P-orbital Dirac point, leading to a quantum Spin Hall effect [10-­12]. Our work shows that well known semiconductor crystals, known for centuries, can lead to systems with entirely new electronic properties, by the simple action of nanogeometry. It can be foreseen that such structures will play a key role in future opto-­electronic applications, provided that they can be fabricated in a straightforward way. [less ▲]

Electronic structure of atomically coherent square semiconductor superlattices with dimensionality below two

in Physical Review. B, Condensed Matter (2013), 88(11), 9

Reconstructions and electronic structure of (11-22) and (11-2-2) semipolar AlN surfaces

in Journal of Applied Physics (2012), 112

The energetics, atomic geometry, and electronic structure of semipolar (11-22) and (11-2-2) AlN surfaces are investigated employing first principles calculations. For metal-rich growth conditions ... [more ▼]

The energetics, atomic geometry, and electronic structure of semipolar (11-22) and (11-2-2) AlN surfaces are investigated employing first principles calculations. For metal-rich growth conditions, metallic reconstructions are favoured on both polarity surfaces. For N rich to moderate Al rich conditions, the (11-22) planes promote semiconducting reconstructions having 2 × 2 or c(2 × 2) periodicity. In contrast, under the particular range of the Al chemical potential the (11-2-2) surfaces stabilize reconstructions with excess metal and it is only at the extreme N rich limit that the semiconducting c(2 × 2) N adatom structure prevails. The present study reveals that the reconstructed (11-22) surfaces do not contain steps in contrast to (11-2-2) where surface steps are inherent for N rich to moderate metal rich growth conditions and may result in intrinsic step-flow growth and/or growth of parasitic semipolar orientations. [less ▲]

Screw threading dislocations in AlN: Structural and electronic properties of In and O doped material

in Journal of Applied Physics (2011), 110

Density functional theory calculations were performed on undoped AlN screw threading dislocations (TDs) as well as TDs doped by indium and oxygen, prompted by integrated experiments through transmission ... [more ▼]

Density functional theory calculations were performed on undoped AlN screw threading dislocations (TDs) as well as TDs doped by indium and oxygen, prompted by integrated experiments through transmission electron microscopy and spectroscopic techniques demonstrating enhanced In and O concentrations in screw dislocation cores. It is revealed that screw TDs act as conduction pathways to charge carriers, introducing multiple levels in the bandgap due to overstrained, dangling, and “wrong” bonds formed even in the undoped cores. The presence of impurities and especially metallic In elevates the metal-like electronic structure of the distorted material and promotes the conductivity along the dislocation line. Hence screw dislocations in AlN are established as highly prominent conductive nanowires in semiconducting thin films and prospects for novel, highly functional nano-device materials through exploitation of screw TDs are attested. [less ▲]

Effect of edge threading dislocations on the electronic structure of InN

in Applied Physics Letters (2011), 98(7), 072103

The open issue of the n-type conductivity and its correlation to threading dislocations (TDs) in InN is addressed through first principles calculations on the electronic properties of a-edge TDs. All ... [more ▼]

The open issue of the n-type conductivity and its correlation to threading dislocations (TDs) in InN is addressed through first principles calculations on the electronic properties of a-edge TDs. All possible dislocation core models are considered (4-, 5/7-, and 8-atom cores) and are found to modify the band structure of InN in a distinct manner. In particular, nitrogen and indium low coordinated atoms in the eight-atom core induce states near the valence band maximum and above the conduction band minimum, respectively. The formation of a nitrogen–nitrogen “wrong” bond is observed at the 5/7-atom core resulting in a state inside the band gap. The 4- and 5/7-atom cores induce occupied states resonant in the conduction band due to In–In strain induced interactions and wrong bonds, respectively. These occupied states designate TDs as a source of higher electron concentrations in InN and provide direct evidence that TDs contribute to its inherent n-type conductivity. [less ▲]