The outstanding properties of SiO2, which include high resistivity, excellent dielectric strength, a large band gap, a high melting point, and a native, low defect density interface with Si, are in large part responsible for enabling the microelectronics revolution. The Si/SiO2 interface, which forms the heart of the modern metal–oxide–semiconductor field effect transistor, the building block of the integrated circuit, is arguably the worlds most economically and technologically important materials interface. This article summarizes recent progress and current scientific understanding of ultrathin (<4 nm) SiO2 and Si–O–N (silicon oxynitride) gate dielectrics on Si based devices. We will emphasize an understanding of the limits of these gate dielectrics, i.e., how their continuously shrinking thickness, dictated by integrated circuit device scaling, results in physical and electrical property changes that impose limits on their usefulness. We observe, in conclusion, that although Si microelectronic devices...

Ultrathin (<4 nm) SiO2 and Si-O-N gate dielectric layers for silicon microelectronics: Understanding the processing, structure, and physical and electrical limits

We have developed a Kramers–Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (e1,e2) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon–germanium (a-Si1−xGex:H) and silicon–carbon (a-Si1−xCx:H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (e1,e2) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where α increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (e1,e2) can be simulated assuming a single ...

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Analytical model for the optical functions of amorphous semiconductors from the near-infrared to ultraviolet: Applications in thin film photovoltaics

Self-assembled indium linear chains on the Si(111) surface are found to exhibit instability of the metallic phase and 1D charge density wave (CDW) The room-temperature metallic phase of these chains undergoes a temperature-induced, reversible transition into a semiconducting phase The 1D CDW along the chains is observed directly in real space by scanning tunneling microscopy at low temperature The Fermi contours of the metallic phase measured by angle-resolved photoemission exhibit a perfect nesting predicting precisely the CDW periodicity

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Instability and charge density wave of metallic quantum chains on a silicon surface

It is a well-known phenomenon that two solids with sufficiently flat surfaces can stick to each other when brought into intimate contact in ambient air at room temperature. The attraction between the two bodies is primarily mediated through van der Waals forces or hydrogen bonding. Without a subsequent heating step, that type of bonding is reversible. Annealing may increase the energy of adhesion up to the cohesive strength of the materials concerned. The wafer bonding phenomena in brittle materials systems, especially in silicon, is reviewed in the experiment. The focus is on low temperature bonding techniques. The pivotal influence chemical species on the surfaces have on the subsequent type of bonding (van der Waals, hydrogen, covalent bonding, mechanical interlocking) is discussed. Methods of modifying the surface chemistry for tailoring bonding properties are addressed. The paper is aimed at providing an overview of the current understanding of the factors determining the bondability and strength of the bonding obtainable. The authors assess the present state of the experimental methods for determining basic parameters governing the adhesion. A number of examples illustrate the applicability of fusion bonding for as diverse fields as opto-electronics, microsystems technology, and fabrication of advanced substrates like silicon-on-insulator wafers.

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Wafer direct bonding: tailoring adhesion between brittle materials

Metal oxide semiconductor field-effect transistors, formed using silicon dioxide and silicon, have undergone four decades of staggering technological advancement. With fundamental limits to this technology close at hand, alternatives to silicon dioxide are being pursued to enable new functionality and device architectures. We achieved ferroelectric functionality in intimate contact with silicon by growing coherently strained strontium titanate (SrTiO3) films via oxide molecular beam epitaxy in direct contact with silicon, with no interfacial silicon dioxide. We observed ferroelectricity in these ultrathin SrTiO3 layers by means of piezoresponse force microscopy. Stable ferroelectric nanodomains created in SrTiO3 were observed at temperatures as high as 400 kelvin.

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A Ferroelectric Oxide Made Directly on Silicon

Surface phases on silicon

This handbook attempts to review the data available in literature through 1993 on surface phases on silicon substrates. In the introductory section, the authors present the notion of surface phases, the basics of two-dimensional crystallography, and the experimental background used to study these phases. The authors review 64 absorbates on silicon substrate, including silicon itself. For each absorbate, data are reported, when available, for the three main substrate orientations. For each surface orientation, the structures are described, and the overall information is conveniently summarized in a formation-phase diagram. A brief summary of the results obtained from the formation of thicker films is also reported. Finally, when appropriate or available, information is included on surface properties and processes or phenomena related to the formation and occurrence of the surface phases of absorbates on silicon substrate such as self-diffusion, desorption, electromigration, chemical passivation, electronic structure modification, and enhanced reactivity of the silicon surface by the presence of the absorbate. The prototypical low-energy electron diffraction patterns encountered for each known superstructure for a single domain or a multidomain structure can be found in the final supplement.

V.G. Lifshits

Papers

Surface phases on silicon

Surface Phases on Silicon: Preparation, Structures, and Properties

Si (111) -α-3×3-Au phase modified by in adsorption: Stabilization of a homogeneous surface by stress relief

Restructuring process of the Si(111) surface upon Ca deposition

Mn clusters on Si(1 1 1) surface: STM investigation