G. Tallarida

Local Electronic Properties of Corrugated Silicene Phases

IO N Exploring new physical-chemical properties of atomically thin graphitic materials is retaining a booming interest and it is expected to have a tremendous impact on the development of future nanoelectronic devices. So far, the enormous consideration for graphene [ 1 , 2 ] has overshadowed other two dimensional (2D) counterparts which may overcome the intrinsic limitations of graphene as active material. Indeed, despite the recent advances either in graphene bandgap opening [ 3–5 ] or integration in ad hoc device structures, [ 6 , 7 ] the zero-gap character of free standing graphene poses a severe drawback for an electric fi eld modulation suitable to graphene based logic devices. A strong effort is currently made to provide theoretical and experimental evidence of stable graphene-like nanolattices of the other group-IV semiconductors, the so called silicene and germanene, [ 8–11 ] which might take benefi t from being naturally compatible with the Si technology and from an intrinsically higher spin orbit coupling. Unlike graphene, the spontaneous formation of silicene is not expected to be energetically favorable because the sp 3 hybridization of Si atoms is more stable than the sp 2 one. However, a graphene-like form of silicene has been theoretically predicted [ 8 , 9 , 12 ] and, in just a few years, experimental investigations have moved up this fascinating hypothesis to concrete evidence. Recent experiments [ 13 , 14 ] report on the epitaxial growth of silicene on Ag(111) substrates with a non-trivial surface arrangement and relevant electronic features such as a linear electronic dispersion at the K points and an estimated Fermi velocity v F = 1.3 × 10 6 ms − 1 . [ 13 ]

In situ chemical and structural investigations of the oxidation of Ge(001) substrates by atomic oxygen

The exposure of Ge(001) substrates to atomic oxygen was studied in situ to establish the stability of the germanium oxide. After preparing chemically clean and atomically flat Ge(001) surfaces, the Ge samples were exposed to atomic oxygen in a wide temperature range from room temperature to 400°C. The chemical composition of the so-formed oxides was studied by means of x-ray photoelectron spectroscopy, while the structure was observed by reflection high energy electron diffraction. At low substrate temperatures the atomic oxygen is efficiently chemisorbed and suboxides coexist with the dioxide, which in turn is remarkably promoted in the high temperature range.

Atomic-layer deposition of Lu2O3

Rare earth oxides could represent a valuable alternative to SiO2 in complementary metal–oxide–semiconductor devices. Lu2O3 is proposed because of its predicted thermodynamical stability on silicon and large conduction band offset. We report on the growth by atomic-layer deposition of lutetium oxide films using the dimeric {[C5H4(SiMe3)]2LuCl}2 complex, which has been synthesized for this purpose, and H2O. The films were found to be stoichiometric, with Lu2O3 composition, and amorphous. Annealing in nitrogen at 950°C leads to crystallization in the cubic bixbyite structure. The dielectric constant of the as-grown Lu2O3 layers is 12±1.

ZnO grown by atomic layer deposition: A material for transparent electronics and organic heterojunctions

We report on zinc oxide thin films grown by atomic layer deposition at a low temperature, which is compatible with a low thermal budget required for some novel electronic devices. By selecting appropriate precursors and process parameters, we were able to obtain films with controllable electrical parameters, from heavily n-type to the resistive ones. Optimization of the growth process together with the low temperature deposition led to ZnO thin films, in which no defect-related photoluminescence bands are observed. Such films show anticorrelation between mobility and free-electron concentration, which indicates that low n electron concentration is a result of lower number of defects rather than the self-compensation effect.

Poly(3-hexylthiophene)/ZnO hybrid pn junctions for microelectronics applications

Hybrid poly(3-hexylthiophene)/ZnO devices are investigated as rectifying heterojunctions for microelectronics applications. A low-temperature atomic layer deposition of ZnO on top of poly(3-hexylthiophene) allows the fabrication of diodes featuring a rectification ratio of nearly 105 at ±4 V and a current density of 104 A/cm2. Electrical characteristics are discussed taking into account the chemical structure of the stack and the energy band diagram.

Effects of the oxygen precursor on the electrical and structural properties of HfO2 films grown by atomic layer deposition on Ge

We report on the growth by atomic layer deposition of HfO2 films on HF-last treated Ge(001) substrates using HfCl4 as a Hf source and either O3 or H2O as oxygen sources. The choice of the oxygen precursor strongly influences the structural, chemical, and electrical properties of the HfO2 films: Those grown using H2O exhibit local epitaxial growth, a large amount of contaminants such as chlorine and carbon, and a large frequency dispersion of the capacitance-voltage (C–V) characteristics. Films grown using O3 are good insulators and exhibit well-shaped C–V curves with a minimum frequency dispersion of the accumulation capacitance. Moreover, they are smoother, less crystallized, and with a lower contaminant content than those grown using H2O. However, the use of O3 leads to the formation of a 2nm thick layer, possibly GeOx, at the HfO2∕Ge interface.

Nanoscale morphological and electrical homogeneity of HfO2 and ZrO2 thin films studied by conducting atomic-force microscopy

The morphological and electrical evolution of HfO2 and ZrO2 thin films is investigated on the nanoscale using conducting atomic-force microscopy in ultrahigh vacuum. Films of different thicknesses have been grown by atomic layer deposition. With increasing film thickness the film structure changes from amorphous to polycrystalline. By conducting atomic-force microscopy using local current–voltage curve statistics and two-dimensional current imaging it is found that the formation of crystallites has different effects on the electrical properties of the two dielectrics. In the case of HfO2, the crystalline fraction causes weak spots in the oxide, whereas for the ZrO2 films the crystallites exhibit lower leakage currents compared to the amorphous matrix and leakage is mainly determined by thickness fluctuations.

Structural and Electrical Properties of HfO 2 Films Grown by Atomic Layer Deposition on Si, Ge, GaAs and GaN

HfO 2 thin films were grown by atomic layer deposition on Si, Ge, GaAs and GaN substrates, using Hf(O t Bu) 2 (mmp) 2 and HfCl 4 . The results show that this combination of precursors promotes a conformal and smooth growth of HfO 2 films on all substrates. As grown films in the thickness range of 10–20 nm have the same electronic density and smooth surfaces. Films 20 nm thick are polycrystalline with the monoclinic structure, whereas the crystallized fraction in the 10 nm thick layers is much lower. The HfO 2 /Ge interface is remarkably sharp. The dielectric constant of the HfO 2 films is 15. Low density of interface states and oxide fixed charges are obtained for the films grown on Si. The optimization of the HfO 2 interface with the other substrates requires more effort.

Structural and electrical characterization of ALCVD ZrO2 thin films on silicon

Abstract We report on the structural and electrical properties of ZrO 2 thin layers grown on Si by atomic layer chemical vapour deposition. Atomic force microscopy, X-ray diffraction, X-ray reflectivity and time-of-flight secondary ion mass spectrometry have been used to characterize as-grown and annealed samples. High frequency capacitance–voltage measurements have been performed to determine the capacitance of the gate dielectric stack. The ZrO 2 film is found to be polycrystalline. Electrical and structural data suggest a coherent picture of film modification upon annealing.

Cubic-to-monoclinic phase transition during the epitaxial growth of crystalline Gd2O3 films on Ge(001) substrates

Thin crystalline films of Gd2O3 are grown on an atomically flat Ge(001) surface by molecular beam epitaxy and are characterized in situ by reflection high energy electron diffraction and x-ray photoelectron spectroscopy, and ex situ by x-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy. The first stage of the growth corresponds to a cubic (110) structure, with two equiprobable, 90° rotated, in-plane domains. Increasing the thickness of the films, a phase transition from cubic (110) to monoclinic (100) oriented crystallites is observed which keeps the in-plane domain rotation, as evidenced by XRD and AFM.