The structure of second and third order twin boundaries in silicon

Direct evidence of 2H hexagonal Si in Si nanowires

Hexagonal Si (2H polytype) has attracted great interest because of its unique physical properties and wide range of potential applications. For example, it might be used in heterojunctions based on hexagonal and cubic Si. Although hexagonal Si has been reported in Si nanowires, its existence is doubted because structural defects of diamond cubic Si can produce structural signals similar to those attributed to hexagonal Si. Here, through the use of atomic resolution high-angle annular dark-field scanning transmission electron microscopy imaging, we unambiguously report the coherent intergrowth of diamond cubic (3C polytype) and 2H hexagonal Si in Si nanowires grown by chemical vapor deposition. A model describing the intergrowth of 3C and 2H Si is proposed and the reasons for the generation of 2H Si are discussed in detail.

Structural defects in cubic semiconductors characterized by aberration-corrected scanning transmission electron microscopy

The development of new electro-optical devices and the realization of novel types of transistors require a profound understanding of the structural characteristics of new semiconductor heterostructures. This article provides a concise review about structural defects which occur in semiconductor heterostructures on the basis of micro-patterned Si substrates. In particular, one- and two-dimensional crystal defects are being discussed which are due to the plastic relaxation of epitaxial strain caused by the misfit of crystal lattices. Besides a few selected examples from literature, we treat in particular crystal defects occurring in GaAs/Si, Ge/Si and β-SiC/Si structures which are studied by high-resolution annular dark-field scanning transmission electron microscopy. The relevance of this article is twofold; firstly, it should provide a collection of data which are of help for the identification and characterization of defects in cubic semiconductors by means of atomic-resolution imaging, and secondly, the experimental data shall provide a basis for advancing the understanding of device characteristics with the aid of theoretical modelling by considering the defective nature of strained semiconductor heterostructures.

Silicon nanowire polytypes: identification by Raman spectroscopy, generation mechanism, and misfit strain in homostructures

Silicon nanowires with predominant 9R, 27T, 2H and other polytype structures with respective hexagonalities of 50, 40 and 35.3% were identified by Raman microscopy. Transmission electron microscopy indicates that intrinsic stacking faults form the basic building blocks of these polytypes. We propose a generation mechanism in which polytypes are seeded from incoherent twin boundaries and associated partial dislocations. This mechanism explains observed prevalence of polytypes and trends in stacking for longer period structures. The percentage of hexagonal planes in a polytype is extracted from its Raman spectrum after correcting the zone-folded phonon frequencies to account for changes of the in-plane lattice parameter with respect to diamond cubic (3C) Si. The correction is found to be (i) of the same order of magnitude as frequency differences between modes of low period polytypes and (ii) proportional to the hexagonality. Corrected phonon frequencies agree with experimentally found values to within 0.4 cm(-1). Homostructures in which a central polytype region is bounded by 3C regions, with the planes (111)(3C)║(0001)(polytype) parallel to the nanowire axis, are found in <linear span>112<linear span> oriented nanowires. Strain-induced shifts of the Raman modes in such structures enable a rough estimation of the lattice misfit between polytypes, which compares favorably with first-principles calculations. Considerations presented here provide a simple and quantitative framework to interpret Raman frequencies and extract crystallographic information on polytype structures.

Grain Boundary Dislocations in GaAs

A preliminary survey of the dislocation structures of grain boundaries in Bridgman grownpolycrystalline GaAs suggests that many of the phenomena observed in grain boundaries inface centered cubic metals, silicon and germanium also occur in boundaries of a polar semiconductor. Our experiments show that low angle grain boundaries are comprised of crystal lattice dislocations and deviations from coincidence orientations are accommodated bysecondary dislocations; specifically, in twins, the a-fringe technique reveals that there are multiple structures where the different domains are separated by partial grain boundary dislocations.

Computed Structures of [001] Symmetrical Tilt Boundaries in Covalently Bonded Materials

The dependence of the structure of (210) and (310) symmetrical [001] tilt boundaries in silicon, germanium and diamond on the Keating covalent force field (potential) has been investigated by computer modelling. We have found that the sensitivity of grain boundary structure to variations of the Keating potential depends on the local atomic arrangement at the grain boundary.

Fabrication and Properties of Single, Double, and Triple Gate Polycrystalline-Silicon Thin Film Transistors

Polysilicon based Thin Film Transistors (poly-Si TFT's) with superior electrical performance can be achieved by maximizing the number of intrinsic point defect injected into the material during high temperature processing. These point defects will migrate to grain boundaries (GB's), enhance their mobility by facilitating climb, and allow the boundary to achieve a low energy configuration with a minimum of electrically active broken bonds. Proper processing of poly-Si TFT's therefore requires a redesign of the conventional processing cycle where, working with single crystal silicon, one minimizes the concentration of intrinsic point defects which otherwise precipitate out as Oxidation induced Stacking Faults (OSF's).

TFT's were fabricated under nine different processing cycles to study the relationship between device performance and fabrication conditions. Device performance increased with higher gate oxidation temperature, elimination of HCI flow during gate oxidation, post hydrogenation, and multiple gates. Using conventional MOS processing steps only, n-type (p-type) devices were fabricated, which were capable of handling 40 volts VDS with a leakage current of 2×10−11 (6×10−12) A/μm and effective electron (hole) channel mobilities of 130 (50) cm2/Vs.

Anti-Site Bonds and the Structure of Interfaces in SiC

High resolution electron microscopy has been used to study the structure of the 3C/6H interface, Σ,=3 {111}and Σ.=3 {112}grain boundaries in 3C-SiC. In SiC, as in other compound semiconductors, anti-site bonds occur in a variety of defects. These are high energy bonds comparable to that of dangling bonds. But, while dangling bonds at the grain boundaries may be eliminated by reconstruction just as in elemental semiconductors, it may not be possible to avoid anti-site bonds.These problems are discussed for the Σ=3 {112} grain boundary, where the structures proposed for Ge and Si are used as starting models for SiC.

Energies and Atomic Structures of Grain Boundaries in Silicon: Comparison Between Tilt and Twist Boundaries

The energies and atomic structures of tilt and twist boundaries in Si have been examined by using the tight-binding electronic theory, and the reason why twist boundaries are seldom found in polycrystalline Si has been investigated. About the frequently observed {122} Σ=9 and {255} Σ=27 tilt boundaries, the configurations without any coordination defects consistent with the electron microscopy observations have relatively small interfacial energies with small bond distortions. About the <111> Σ=7, <011> Σ=3 and <001> Σ = 5 twist boundaries, the configurations contain larger bond distortions or more coordination defects, and much larger interfacial energies than those of the tilt boundaries. The <001> twist boundaries have very complex structures as compared with the other twist boundaries, which can be explained by the morphology of the ideal surfaces. The stability of the tilt boundaries in Si can be explained by the viewpoint of the stable structural units consisting of atomic rings.

Extending the limit of atomic level grain boundary structure imaging using high-resolution electron microscopy

It has been possible to image sigma = 21/[111]21.8 degrees tilt boundaries in thin Au films and to deduce their atomic arrangements. These results represent an electron microscopic resolution level of 1.43 A, attainable with a small amount of image processing, which produces interpretable structure images. This substantial improvement over other recent grain boundary studies, which required about 1.9-2.0 A resolution, clearly demonstrates that many more tilt grain boundary orientations are now accessible instead of a limited subset.