Crystalline silicon materials

The effects of an isolated defect on material quality (e.g. recombination activity of a clean, undecorated dislocation, capture cross sections of point defects) are well known for many defects present in crystalline silicon material, but the interactions of the impurities or structural defects form a major challenge in getting an improved understanding of the complex situation in the solidified silicon ribbon. Currently it is impossible to list a complete overview of the known interactions, but more information can be found in [49],... 

It may occasion surprise that an amorphous material has well-defined energy bands when it has no lattice planes, but as Street s book points out, the silicon atoms have the same tetrahedral local order as crystalline silicon, with a bond angle variation of (only) about 10% and a much smaller bond length disorder . Recent research indicates that if enough hydrogen is incorporated in a-silicon, it transforms from amorphous to microcrystalline, and that the best properties are achieved just as the material teeters on the edge of this transition. It quite often happens in MSE that materials are at their best when they are close to a state of instability. 

Commercially available PV systems most often include modules made from single-crystal or poly-ciystalline silicon or from thin layers of amoiphous (non-crystalline) silicon. The thin-filni modules use considerably less semiconductor material but have lower efficiencies for converting sunlight to direct-current electricity. Cells and modules made from other thin-filni PV materials such as coppcr-indiuni-diselenide and cadmium telluride are under active development and are beginning to enter the market. 

The next step is the hydrogen reduction of the trichlorosilane (Reaction 2 above). The end product is a poly crystalline silicon rod up to 200 mm in diameter and several meters in length. The resulting EGS material is extremely pure with less than 2 ppm of carbon and only a few ppb of boron and residual donors. The Czochralski pulling technique is used to prepare large single crystals of silicon, which are subsequently sliced into wafers for use in electronic devices.1 1... 

Single-Crystal Silicon. Silicon is still the dominant material in photovoltaic. It has good efficiency, which is 25% in theory and 15% in actual practice. Silicon photovoltaic devices are made from wafers sliced from single crystal silicon ingots, produced in part by CVD (see Ch. 8, Sec. 5.1). However, silicon wafers are still costly, their size is limited, and they cannot be sliced to thicknesses less than 150 im. One crystalline silicon wafer yields only one solar cell, which has an output of only one watt. This means that such cells will always be expensive and can only be used where their high efficiency is essential and cost is not a major factor such as in a spacecraft applications. 

Taramasso, M., Perego, G., and Notari, B. (1983) Preparation of porous crystalline synthetic material comprised of silicon and titanium oxides. US Patent 4,410,501. 

The presence of a dense material with a varying void fraction results in compressive stress, with typical values of 500 MPa. Compressive stress can be determined conveniently by comparing the curvature of a crystalline silicon wafer before and after deposition of an a-Si H film. 

Many different substrates are used fora-Si H deposition. Usually Corning 7059 glass [390] and crystalline silicon are used for materials research, as both have similar thermal expansion coefficients to fl-Si H. Devices are mostly made on... 

In this sub-subsection, the Er doping of amorphous silicon is discussed. The problem of limited solubility of Er in crystalline silicon has been circumvented. However, the electrical properties of pure a-Si are poor compared to c-Si. Therefore, hydrogenated amorphous silicon is much more interesting. Besides, the possibility of depositing a-Si H directly on substrates, i.e., optical materials, would make integration possible. Both low-pressure chemical vapor deposition (LPCVD) [664] and PECVD [665, 666] have been used to make the a-Si H into which Er is implanted. In both methods oxygen is intentionally added to the material, to enhance the luminescence. 

The conclusion to be drawn from these studies as far as the suitability of different ion-implanted hydrogen standards is concerned is that pre-amorphized silicon is the best target material, though crystalline silicon is an acceptable alternative if room temperature is never exceeded. 

When classifying chemical products, Seider et al. [3] identify three categories (1) basic chemicals (commodity and specialty chemicals, bio-materials, and polymeric materials) (2) industrial chemicals (films, fibers, paper,. ..) and (3) configured consumer products (dialysis devices, post-it notes, transparencies, drug delivery patches,. ..). In the manufacture of epitaxial silicon wafers, a thin film of crystalline silicon is often deposited on a polished crystalline silicon... 

Epitaxial thin films of silicon, to be deposited on crystalline silicon wafers, require no materials development. Other semi-conductor materials are possible, but silicon continues to be most cost-effective, principally due to the relatively... 

Pure crystalline silicon is a brittle material with a gray metallic appearance. Its mechanical properties, such as Knoop hardness (950-1150 kg mm-2), Young s modulus (190 GPa for (111), 170 GPa for (110), 130 GPa for (100)), torsion modulus (4050 kg mm-2) and compression breaking strength (5000 kg cm-2) vary slightly with crystal orientation. Silicon has a low thermal expansion coefficient (2.33x 1(T6 K-1) and a high thermal conductivity (148 W K-1m-1). Crystalline silicon melts at 1413 °C (1686 K). 

We now turn to the issue of physical accuracy. Just as accuracy should not be considered as a single topic, physical accuracy is too vague of an idea to be coherently considered. It is much better to ask how accurate the predictions of DFT are for a specific property of interest. This approach recognizes that DFT results may be quite accurate for some physical properties but relatively inaccurate for other physical properties of the same material. To give just one example, plane-wave DFT calculations accurately predict the geometry of crystalline silicon and correctly classify this material as a semiconductor,... 

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