V. V. Afanas’ev

Trap-assisted tunneling in high permittivity gate dielectric stacks

The electrical characteristics of SiOx/ZrO2 and SiOx/Ta2O5 gate dielectric stacks are investigated. The current–density JG in these dielectric stacks is shown to be strongly temperature dependent at low voltage (below about 2 V), the more so in the ZrO2 stack. On the other hand, JG is much less temperature dependent at higher voltage. These results are consistent with a model which takes into account the direct tunneling of electrons across the SiOx layer and the trap-assisted tunneling of electrons through traps with energy levels below the conduction band of the high permittivity dielectric layer. The energy levels and densities of these electron trapping centers are estimated by fitting this trap-assisted tunneling model to the experimental results.

Band offsets and electronic structure of SiC/SiO2 interfaces

The electronic structure of SiC/SiO2 interfaces was studied for different SiC polytypes (3C, 4H, 6H, 15R) using internal photoemission of electrons from the semiconductor into the oxide. The top of the SiC valence band is located 6 eV below the oxide conduction band edge in all the investigated polytypes, while the conduction band offset at the interface depends on the band gap of the particular SiC polytype. In the energy range up to 1.5 eV above the top of the SiC valence band, interface states were found. Their electron spectrum is similar to that of sp2‐bonded carbon clusters in diamond‐like a‐C:H films suggesting the presence of elemental carbon at the SiC/SiO2 interfaces.

Internal photoemission at interfaces of high-κ insulators with semiconductors and metals

Internal photoemission spectroscopy provides the most straightforward way to characterize the relative energies of electron states at interfaces of insulators with metals and semiconductors by measuring the spectral onset of electron/hole photoemission from one solid into another. The article reviews the application of this technique for characterization of advanced nanometer-thin insulators prospected to be used in microelectronic devices. Fundamental aspects and technical features of the internal photoemission experiments are discussed together with basic electronic properties of a number of investigated high-permittivity insulating films and their interfaces in semiconductor heterostructures. Significant differences are found in the electronic properties of nanometer-thin amorphous insulating layers as compared to the known bulk phase characteristics. The band alignment at the interfaces of these insulators with metals is found to be highly sensitive to the surface preparation procedures. By contrast, ...

Variation in the fixed charge density of SiOx/ZrO2 gate dielectric stacks during postdeposition oxidation

The effect of postdeposition oxidation of SiOx/ZrO2 gate dielectric stacks at different temperatures (500–700 °C) on the density of fixed charge and interface states is investigated. It is shown that with increasing oxidation temperature the density of negative fixed charge is reduced, but the density of interface states increases. The net positive charge observed after oxidation at T>500 °C resembles the charge generated at the Si/SiO2 interface by hydrogen in the same temperatures range. This association is supported by the resistance of both types of charge against molecular hydrogen anneal but their fast removal in the presence of atomic hydrogen at 400 °C. Therefore, we propose that the observed oxidation-induced positive charge in the SiOx/ZrO2 gate stack may be related to overcoordinated oxygen centers induced by hydrogen.

Band alignments in metal-oxide-silicon structures with atomic-layer deposited Al2O3 and ZrO2

The energy barrier height Φ for electrons at the interfaces of various metals (Mg,Al,Ni,Cu,Au) with nanometer-thin Al2O3 and ZrO2 layers grown on (100)Si by atomic layer deposition has been directly measured using internal photoemission of electrons into the insulator. The behavior of the metal/Al2O3 contacts with increasing metal electronegativity XM resembles that of the metal/SiO2 interfaces with ideality factor dΦ/dXM≈1. The metal/ZrO2 contacts exhibit a less ideal behavior with dΦ/dXM≈0.75. The metal–silicon work function differences in structures with Al2O3 and ZrO2 insulators appear to be considerably larger than in the structures with thermally grown SiO2, suggesting the presence of a negative dipole layer at the metal/deposited oxide interface.

Can silicon behave like graphene? A first-principles study

The electronic properties of two-dimensional hexagonal silicon (silicene) are investigated using first-principles simulations. Though silicene is predicted to be a gapless semiconductor, due to the sp2-hybridization of its atomic orbitals, the weak overlapping between 3pz orbitals of neighbor Si atoms leads to a very reactive surface, resulting in a more energetically stable semiconducting surface upon the adsorption of foreign chemical species. It is predicted that silicene inserted into a graphitelike lattice, like ultrathin AlN stacks, preserves its sp2-hydridization, and hence its graphenelike electronic properties.

Band alignment between (100)Si and complex rare earth∕transition metal oxides

The electron energy band alignment between (100)Si and several complex transition∕rare earth (RE) metal oxides (LaScO3, GdScO3, DyScO3, and LaAlO3, all in amorphous form) is determined using a combination of internal photoemission and photoconductivity measurements. The band gap width is nearly the same in all the oxides (5.6–5.7eV) yielding the conduction and valence band offsets at the Si∕oxide interface of 2.0±0.1 and 2.5±0.1eV, respectively. However, band-tail states are observed and these are associated with Jahn-Teller relaxation of transition metal and RE cations which splits their d* states.

Ternary rare-earth metal oxide high-k layers on silicon oxide

Ternary oxides, GdScO3, DyScO3, and LaScO3, deposited by pulsed laser deposition using ceramics targets of stoichiometric composition, were studied as alternative high-k gate dielectrics on (100) Si. Their physical characterization was done using Rutherford backscattering, spectroscopic ellipsometry, x-ray diffraction, and transmission electron microscopy on blanket layers deposited on (100) Si, and electrical characterization on capacitors. It is found that DyScO3 and GdScO3 preserve their amorphous phases up to 1000°C. Other encouraging properties for high k applications were demonstrated, including k-value ∼22, almost no hysteresis or frequency dispersion in C–V curves, and leakage current reduction comparable to that of HfO2 of the same equivalent oxide thickness.

Electron spin resonance features of interface defects in thermal (100)Si/SiO2

Electron spin resonance (ESR) on thermal (100)Si/SiO2 predominantly exhibiting either the Pb0 or Pb1 interface defect confirms the Pb1 point symmetry as monoclinic-I with g1=2.0058, g2=2.00 735±0.00 010, and g3=2.0022, where the g2 direction is at 3°±1° (towards the interface) with a 〈111〉 direction at 35.3° with the interface plane. Its line width is found weakly dependent on magnet angle, exhibiting a strain induced spread σg⊥∼0.00 035 in g⊥ about 2–3 times less than typical for Pb in (111)Si/SiO2. For Pb0, an axially symmetric g matrix is observed, with g∥=2.0018 and g⊥=2.0081, and σg⊥∼0.0009. From comparison of salient ESR data, it is concluded that Pb and Pb0 are chemically identical; however, systematic fabrication-induced variations in defect environment will lead to second order systematic shifts in average properties. The Pb1 defect is provisionally pictured as an unpaired Si bond on a defect Si atom at slightly subinterface plane position in the Si substrate, possibly facing an oxygen atom.

Electron energy barriers between (100)Si and ultrathin stacks of SiO2, Al2O3, and ZrO2 insulators

Electron energy barriers between the valence band of (100)Si and the conduction bands of ultrathin SiO2, Al2O3, ZrO2 insulators and their stacks were determined using internal photoemission of electrons. For SiO2, the barrier of 4.25±0.05 eV was found unchanged down to the oxide thickness of ≈1 nm. The barriers for Al2O3 and ZrO2 are substantially lower: 3.25±0.08 and 3.1±0.1 eV, respectively. Thermal oxidation at 650–800 °C enhances the barriers at the Si/Al2O3 and Si/ZrO2 interfaces but does not reduce the high density of band tail states in the insulators, suggesting the formation of silicates.