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See detailReconstructions and electronic structure of (11-22) and (11-2-2) semipolar AlN surfaces
Kalesaki, Efterpi UL; Lymperakis, Liverios; Kioseoglou, Joseph et al

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 ▲]

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See detailScrew threading dislocations in AlN: Structural and electronic properties of In and O doped material
Kioseoglou, Joseph; Kalesaki, Efterpi UL; Belabbas, Imad et al

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 ▲]

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See detailElectronic structure of 1/6⟨20-23⟩ partial dislocations in wurtzite GaN
Kioseoglou, Joseph; Kalesaki, Efterpi UL; Lymperakis, Liverios et al

in Journal of Applied Physics (2011), 109

The I1 intrinsic basal stacking faults (BSFs) are acknowledged as the principal defects observed on {11-20} (a-plane) and {1-100} (m-plane) grown GaN. Their importance is established by recent ... [more ▼]

The I1 intrinsic basal stacking faults (BSFs) are acknowledged as the principal defects observed on {11-20} (a-plane) and {1-100} (m-plane) grown GaN. Their importance is established by recent experimental results, which correlate the partial dislocations (PDs) bounding I1 BSFs to the luminescence characteristics of GaN. PDs are also found to play a critical role in the alleviation of misfit strain in hetero-epitaxially grown nonpolar and semipolar films. In the present study, the energetics and the electronic structure of twelve edge and mixed 1/6⟨20-23⟩ PD configurations are investigated by first principles calculations. The specific PD cores of the dislocation loop bounding the I1 BSF are identified for III-rich and N-rich growth conditions. The core structures of PDs induce multiple shallow and deep states, attributed to the low coordinated core atoms, indicating that the cores are electrically active. In contrast to edge type threading dislocations no strain induced states are found. [less ▲]

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See detailEffect of edge threading dislocations on the electronic structure of InN
Kalesaki, Efterpi UL; Kioseoglou, Joseph; Lymperakis, Liverios et al

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 ▲]

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See detailMorphology and strain of self-assembled semipolar GaN quantum dots in (11-22) AlN
Dimitrakopulos, George; Kalesaki, Efterpi UL; Kioseoglou, Joseph et al

in Journal of Applied Physics (2010), 108

GaN quantum dots (QDs) grown in semipolar (11-22) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy ... [more ▼]

GaN quantum dots (QDs) grown in semipolar (11-22) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy techniques. The embedded (11-22)-grown QDs exhibited pyramidal or truncated-pyramidal morphology consistent with the symmetry of the nucleating plane, and were delimited by nonpolar and semipolar nanofacets. It was also found that, in addition to the (11-22) surface, QDs nucleated at depressions comprising {10-11} facets. This was justified by ab initio density functional theory calculations showing that such GaN/AlN facets are of lower energy compared to (11-22). Based on quantitative high-resolution TEM strain measurements, the three-dimensional QD strain state was analyzed using finite-element simulations. The internal electrostatic field was then estimated, showing small potential drop along the growth direction, and limited localization at most QD interfaces. [less ▲]

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