1,721,008 research outputs found
Ferromagnetic versus antiferromagnetic interaction in Co-doped ZnO
No full textBased on first-principles spin-density functional calculations, we find that Co-doped ZnO energetically favors a spin-glass-like state due to antiferromagnetic interactions between transition metal atoms, while ferromagnetic ordering is stabilized by electron doping. We find a short range nature in both antiferromagnetic and ferromagnetic interactions, and suggest that a very high doping level of Co ions is required to achieve ferromagnetism, together with a sufficient supply of electron carriers. Our results explain experimental features such as the low reproducibility of ferromagnetic samples and the very low saturation magnetization per Co ion.
Electrically inactive nitrogen complex in Si oxynitride
No full textBased on first-principles theoretical calculations, we find a very stable nitrogen complex in oxynitrides, which consists of two N atoms at O sites and one O vacancy. This N complex is electrically inactive without bonding with hydrogen, removing the electrical activity of O vacancies, and the stability of this complex is greatly enhanced as going from pure oxide to oxynitride films. We suggest that charge traps involving a single N atom, such as a bridging N center, can be deactivated by reactions with O or NO interstitials, and resulting N interstitials are easily depleted into the interface, in good agreement with experiments.
P-type doping with group-I elements and hydrogenation effect in ZnO
No full textWe investigate the defect properties of group-I elements such as Li and Na in ZnO through fi rst-pri nci pies calculations. Compared with group-V elements such as N, P, and As, Li and Na dopants at substitutional sites have shallower acceptor levels, but, these acceptors are mostly compensated by coexisting interstitial donors. Our calculations show that a codoping technique with hydrogen severely suppresses the concentration of interstitial donors, and greatly enhances the solubility of group-I dopants via the formation of hydrogen-acceptor complexes. The hydrogen-passivated acceptors easily recover the electrical activity by post-annealing, and thus lowresistivity p-type ZnO is achievable with dopants different from group-V elements. (c) 2005 Elsevier B.V. All rights reserved
First-principles study of the structural phase transformation of hafnia under pressure
No full textWe investigate the phase transformation of HfO2 under hydrostatic pressure through first-principles pseudopotential calculations within the local-density-functional approximation (LDA) and the generalized gradient approximation (GGA). We find that with increasing of pressure, HfO2 undergoes a series of structural transformations from monoclinic to orthorhombic I and then to orthorhombic II, consistent with experiments. The calculated transition pressures within the GGA are in good agreement with the measured values, while they are severely underestimated by the LDA. Analyzing the distribution of electron densities for the high-pressure phases, we find that the electron densities of the orthorhombic-II phase are more homogeneous than for the orthorhombic-I phase. Due to this distinct difference in the homogeneity of electron densities, the energy difference between the orthorhombic-I and orthorhombic-II phases is enhanced in the GGA; thus, the transition pressure between the two phases increases significantly.
Electrically deactivating nearest-neighbor donor-pair defects in Si
No full textBased on first-principles density-functional calculations, we propose a class of nearest-neighbor donor pairs that are energetically favorable in highly n-type Si. These donor pairs comprise dopant atoms either fourfold coordinated at the nearest-neighbor distance or threefold coordinated through bond-breaking relaxations. For P and As dopants, the two defect states are very close in energy, less than 0.1 eV, while the threefold coordinated state is more stable by 0.24 eV for Sb dopants. The former state has a very deep donor level close to the valence band maximum, while the defect level lies deep inside the valence band for the latter. Thus, both the donor pairs are electrically inactive at very high doping levels, and they are suggested to be responsible for the observed saturation of carriers.
Theoretical study of the relative stability of wurtzite and rocksalt phases in MgxZn1-xO alloys
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