V. V. Afanas'ev

Mechanisms responsible for improvement of 4H-SiC/SiO2 interface properties by nitridation

An analysis of fast and slow traps at the interface of 4H–SiC with oxides grown in O2, N2O, and NO reveals that the dominant positive effect of nitridation is due to a significant reduction of the slow electron trap density. These traps are likely to be related to defects located in the near-interfacial oxide layer. In addition, the analysis confirms that the fast interface states related to clustered carbon are also reduced by nitridation.

Temperature and frequency dependent electrical characterization of HfO2/InxGa1−xAs interfaces using capacitance-voltage and conductance methods

Electrical properties of metal-oxide-semiconductor capacitors using atomic layer deposited HfO2 on n-type GaAs or InxGa1−xAs (x=0.53, 0.30, 0.15) epitaxial layers were investigated. Capacitance-voltage (CV) measurements indicated large temperature and frequency dispersion at positive gate bias in devices using n-type GaAs and low In content (x=0.30, 0.15) InxGa1−xAs layers, which is significantly reduced for devices using In0.53Ga0.47As. For In0.53Ga0.47As devices, the CV response at negative gate bias is most likely characteristic of an interface state response and may not be indicative of true inversion. The conductance technique on Pd/HfO2/In0.53Ga0.47As/InP shows reductions in interface state densities by In0.53Ga0.47As surface passivation and forming gas annealing (325 °C).

Electronic properties of two-dimensional hexagonal germanium

The electronic properties of two-dimensional hexagonal germanium, so called germanene, are investigated using first-principles simulations. Consistent with previous reports, the surface is predicted to have a “poor” metallic behavior, i.e., being metallic with a low density of states at the Fermi level. It is found that biaxial compressively strained germanene is a gapless semiconductor with linear energy dispersions near the K points—like graphene. The calculated Fermi velocity of germanene is almost independent of the strain and is about 1.7×106 m/s, quite comparable to the value in graphene.

An electric field tunable energy band gap at silicene/(0001) ZnS interfaces

The interaction of silicene, the silicon counterpart of graphene, with (0001) ZnS surfaces is investigated theoretically, using first-principles simulations. The charge transfer occurring at the silicene/(0001) ZnS interface leads to the opening of an indirect energy band gap of about 0.7 eV in silicene. Remarkably, the nature (indirect or direct) and magnitude of the energy band gap of silicene can be controlled by an external electric field: the energy gap is predicted to become direct for electric fields larger than about 0.5 V Å(-1), and the direct energy gap decreases approximately linearly with the applied electric field. The predicted electric field tunable energy band gap of the silicene/(0001) ZnS interface is very promising for its potential use in nanoelectronic devices.

Influence of Al2O3 crystallization on band offsets at interfaces with Si and TiNx

Experiments on internal photoemission of electrons from Si and TiNx into thin Al2O3 layers suggest that crystallization of amorphous Al2O3 to the cubic γ-Al2O3 is accompanied by only a ≈ 0.5 eV upshift of the conduction band bottom edge. As it is known that in transforming from the amorphous to the γ-phase, the bandgap of Al2O3 increases from 6.2 eV to 8.7 eV, more than 80% of this gap widening must occur at the VB side of the gap. The latter suggests that interaction between electron states of O anions plays the dominant role in the oxide gap widening.

The VO2 interface, the metal-insulator transition tunnel junction, and the metal-insulator transition switch On-Off resistance

Transition metal compounds showing a metal-insulator transition (MIT) show complex behavior due to strongly correlated electron effects and offer attractive properties for nano-electronics applications, which cannot be obtained with regular semiconductors. MIT based nano-electronics, however, remains unproven, and MIT devices are poorly understood. We point out and single out one of the major hurdles preventing MIT-electronics: obtaining a high Off resistance and high On-Off resistance ratio in an MIT switch. We show a path toward an MIT switch fulfilling strict Off and On resistance criteria by: (1) Obtaining understanding of the VO2-interface, a protoypical MIT material interface. (2) Introducing a MIT tunnel junction concept to tune switch resistances. In this junction, the metal or insulating phase of the MIT material controls how much current flows through. Adapting the junctions parameters allows tuning the MIT switchs Off and On resistance. (3) Providing proof of principle of the junction and its...

Energy barriers at interfaces between (100) InxGa1−xAs (0≤x≤0.53) and atomic-layer deposited Al2O3 and HfO2

The electron energy band alignment at interfaces of InxGa1−xAs (0≤x≤0.53) with atomic-layer deposited insulators Al2O3 and HfO2 is characterized using internal photoemission and photoconductivity experiments. The energy of the InxGa1−xAs valence band top is found to be only marginally influenced by the semiconductor composition. This result suggests that the known bandgap narrowing from 1.42 to 0.75 eV when the In content increases from 0 to 0.53 occurs mostly through downshift of the semiconductor conduction band bottom. It finds support from both electron and hole photoemission data. Similarly to the GaAs case, electron states originating from the interfacial oxidation of InxGa1−xAs lead to reduction in the electron barrier at the semiconductor/oxide interface.

Study of leakage mechanism and trap density in porous low-k materials

The field and temperature dependence of the leakage current of low-k material is studied by using planar capacitors. First it is shown that our planar capacitors are suitable test vehicles to analyze the intrinsic properties of low-k materials. Then an evaluation of the trap density of the investigated low-k material is performed. Eventually different models such as Poole-Frenkel emission, Schottky emission and trap-assisted Fowler-Nordheim tunneling are analyzed in a temperature range from 80 K to 473 K and in a field range from 4.2 MV/cm to 6.6 MV/cm. It is found that Poole-Frenkel emission and Schottky emission do not fit the experimental data, whereas trap-assisted Fowler-Nordheim tunneling exhibits an adequate match between the extracted parameters and the theoretical predictions.

Electron band alignment between (100)InP and atomic-layer deposited Al2O3

Energy barriers at interfaces of (100)InP with atomic-layer deposited Al2O3 are determined using internal photoemission of electrons. The barrier height between the top of the InP valence band and bottom of the alumina conduction band is found to be 4.05±0.10 eV corresponding to a conduction band offset of 2.7 eV. An interlayer associated with the oxidation of InP may result in a lower barrier for electron injection potentially leading to charge instability of the insulating stack. A wide-gap P-rich interlayer has a potential to reduce this degrading effect as compared to In-rich oxides.

Defect-induced bandgap narrowing in low-k dielectrics

In this work, core-level X-ray photoelectron spectroscopy was utilized to determine the surface bandgap for various porous and non-porous low-k a-SiCOH dielectrics before and after ion sputtering. By examining the onset of inelastic energy loss in O 1s core-level spectra, the gap narrowing was universally found in Ar+ ion sputtered low-k dielectrics. The reduction of the bandgap ranges from 1.3 to 2.2 eV depending on the film composition. We show that the bandgap narrowing in these low-k dielectrics is caused by development of the valence-band tail as evidenced by the presence of additional electronic states above the valence-band maximum. Electron-spin-resonance measurements were made on a-SiCOH films to gain atomic insight into the nature of the sputtering-induced defects and reveal formation of carbon-related defects as the most probable origin of the gap states.