Thin Amorphous Material Research Articles

Free-Standing Molecularly Thin Amorphous Silica Nanosheets.

Recent progress in 2D materials has initiated new fields of molecularly thin amorphous materials with mysterious properties and structures. However, designed synthesis of molecularly thin amorphous silica still remains a challenge; whether free-standing molecularly thin amorphous silica nanosheets can exist is unclear. Here, this issue is addressed by using a new chemical protocol; solid-state surfactant lamellae withordered alkyl-chain arrangements can serve as superior templates guiding free-standing amorphous silica nanosheets. Simple sonication of the lamellar hybrids allows exfoliation into monolayer amorphous silica nanosheets with 0.9nm thickness. In addition, the nanosheets show thedistinctive feature ofhigh colloidal stability that enables atomic layer engineering of silica nanocoatings and dielectric nanofilms. The approach may shed new light on the properties and applications of old silica.

Medium-range order in amorphous silicon investigated by constrained structural relaxation of two-body and four-body electron diffraction data

The structures of four types of amorphous silicon are examined by an experimentally constrained structural relaxation method (ECSR). Experimental selected area electron diffraction data and fluctuation electron microscopy normalized diffraction variance data were used as constraints to guide a Monte Carlo relaxation procedure towards best fit models. A Tersoff potential was also used to further restrict the space of possible solutions. The materials examined were self-ion-implanted silicon and pressure-amorphized silicon, both in their as-prepared and thermally annealed states. In the fitted models for these materials regions containing two types of medium-range order were identified. One type involves formation of paracrystallites with cubic and hexagonal structures, where both short-range crystalline and medium-range order are present. The other type of medium-range order appears in the form of extended crystalline planes without associated short-range crystalline order. These two types can coexist. It is observed that the best fit models for both as-prepared samples contain approximately 10–15% paracrystalline ordered regions, reducing to about 5–10% in the annealed materials. None of the models are true continuous random networks. We conclude that, with long computational times and with a suitable potential function, the ECSR procedure provides a powerful, although at present semi-quantitative, tool for determining the structural form of medium-range order in thin amorphous materials.

An Automated Procedure for On-Line Measurement of Spherical-Aberration Coefficient of High-Resolution Electron Microscopes

Spherical aberration coefficient (Cs) of the objective lens and electron wavelength ultimately determine the point-resolution of a high resolution electron microscope (HREM). Accurate measurement of Cs has become increasingly critical for reconstruction of structural information well beyond the point-resolution by means of either electron holography or focal series methods with a field emission gun (FEG) microscope. There are two main existing procedures for Cs measurement, i.e. (1) using diffractograms from a thin amorphous material, and (2) using beam-tilt-induced image displacement (BID). Since these procedures generally involve intensive data measurement, it is highly desirable to have an automated procedure. With an image pickup system such as CCD camera and appropriate software, we have developed an automated procedure for on-line Cs measurement. The procedure is based on analyzing diffractograms from a thin amorphous material such as amorphous carbon or germanium. The use of CCD camera allows for on-line measurement, and also for magnification to be calibrated with high precision, which is critical in Cs measurement.

Hopping transport through amorphous junctions

We study electron tunneling through thin amorphous material and obtain the relative strength of different hopping channel contributions to tunneling compared to the elastic resonant contribution under high magnetic field. The order of magnitude of these contributions are in good agreement with the recent experimental data on amorphous silicon tunnel junctions.

Low temperature crystallization of amorphous silicon using an excimer laser

Low temperature processing is a prerequisite for compatible technologies involving combined a-Si and poly-silicon devices or for fabricating these devices on glass substrates. This paper describes excimer-laser-induced crystallization of thin amorphous silicon films deposited by plasma CVD (a-Si:H) and LPCVD (a-Si). The intense, pulsed UV produced by the laser is highly absorbed by the thin amorphous material, but the average temperature is compatible with low temperature processing. The process produces crystallites whose structure and electrical characteristics vary according to starting material and laser scan parameters. The crystallized films have been principally characterized using x-ray diffraction, TEM, and transport measurements. The results indicate that crystallites nucleate in the surface region and are randomly oriented. The degree of crystallization near the surface increases as the doping level and/or deposited laser energy density is increased. The crystallite size increases with a power law dependence on deposited energy, while the conductivity increases exponentially above threshold for unintentionally doped PECVD films. The magnitude of the Hall mobility of the highly crystallized samples is increased by two orders of magnitude over that of the amorphous starting material.

Dielectric properties of thin aluminium fluoride films

The dielectric properties of vacuum-deposited aluminum fluoride thin films have been investigated in the frequency range 10 −1-10 6 Hz and at different temperatures in the range 77°–413°K. These properties are found to be similar to those of other thin amorphous materials. Below room temperature an activation energy of 0.004 eV is found, while at higher temperatures an activation energy of 0.028 eV is suggested. At low temperatures in the frequency range 10 −-10 3 Hz, the conductance varies as f n where n is of the order of 0.80, a value consistent with the theory of hopping in amorphous materials. The conductance also appears to follow the relationship δ = A exp (−BT − 1 4 ) , which is characteristic of phonon-assisted hopping conduction. At higher frequencies the conductivity is approximately proportional to the square of the frequency.