Crystal structure, electronic devices

The most characteristic feature of any crystal is its symmetry. It not only serves to describe important aspects of a structure, but is also related to essential properties of a solid. For example, quartz crystals could not exhibit the piezoelectric effect if quartz did not have the appropriate symmetry this effect is the basis for the application of quartz in watches and electronic devices. Knowledge of the crystal symmetry is also of fundamental importance in crystal stmcture analysis. 

Silicon s tetravalent pyramid crystalline structure, similar to tetravalent carbon, results in a great variety of compounds with many practical uses. Crystals of sihcon that have been contaminated with impurities (arsenic or boron) are used as semiconductors in the computer and electronics industries. Silicon semiconductors made possible the invention of transistors at the Bell Labs in 1947. Transistors use layers of crystals that regulate the flow of electric current. Over the past half-century, transistors have replaced the vacuum tubes in radios, TVs, and other electronic equipment that reduces both the devices size and the heat produced by the electronic devices. 

Because a book of this size could not cover all topics in solid state chemistry, we have chosen to concentrate on structures and bonding in solids, and on the interplay between crystal and electronic stracture in determirring their properties. Examples of sohd state devices are used throughout the book to show how the choice of a partictrlar solid for a particular device is determined by the properties of that solid. 

Electronic. Diamonds have been used as thermistors and radiation detectors, but inhomogeneities within the crystals have seriously limited these applications where diamond is an active device. This situation is rapidly changing with the availability of more perfect stones of controlled chemistry from modem synthesis methods. The defect structure also affects thermal conductivity, but cost and size are more serious limitations on the use of diamond as a heat sink material for electronic devices. 

In molecular beam epitaxy (MBE) [317], molecular beams are used to deposit epitaxial layers onto the surface of a heated crystalline substrate (typically at 500-600° C). Epitaxial means that the crystal structure of the grown layer matches the crystal structure of the substrate. This is possible only if the two materials are the same (homoepitaxy) or if the crystalline structure of the two materials is very similar (heteroepitaxy). In MBE, a high purity of the substrates and the ion beams must be ensured. Effusion cells are used as beam sources and fast shutters allow one to quickly disrupt the deposition process and create layers with very sharply defined interfaces. Molecular beam epitaxy is of high technical importance in the production of III-V semiconductor compounds for sophisticated electronic and optoelectronic devices. Overviews are Refs. [318,319],... 

There exists a considerable literature on CVD (2) but relatively few attempts have been made to combine chemical and physical rate processes to give a complete representation of the deposition process. Most CVD studies have focused on demonstrating the growth of a particular material or crystal structure. However, the combined analysis is necessary in order to design CVD reactors where it is possible to deposit thin films of constant thickness and uniformity across an entire wafer. This is particularly important in the realization of submicron feature sizes for Very Large Scale Integrated Circuits. The further development of devices based on III-V compounds also depends on CVD reactor design improvements since the composition and thus the electronic properties of these materials vary considerably with process conditions. 

Single-crystal x-ray or neutron diffractometers equipped with a low-temperature device were sparse in the 1970s. At present, several tens exist in the world, but very few are capable of handling temperatures below 10 K. As stated in Section III, until now crystal structure studies have been performed only by means of the angle-dispersive method. We discuss below the three main kinds of cryostats that can be used on the instrument geometries described in Section III.B. More details can be found, for example, in Ref. 79, which compares LT setups suitable for accurate electron density map studies. 

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