CN2-crystals - Semiconductor Crystals properties and growth...

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1 1 Semiconductor Crystals: properties and growth 2 Outline basic solid types crystal structure: general discussion crystal structure of most semiconductors Notation to denote planes and directions: “Miller indices” (jargon of the trade) Semiconductor crystal growth and wafer fabrication
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2 3 Solid-types There are three basic solid types: ¾ Crystalline (ordered) … as, for example, diamond ¾ Amorphous (disordered) … as, for example, window glass ¾ Polycrystalline (many small single crystalline/ordered regions randomly oriented to each other) … as, e.g., polysilicon Figure 1—1 Three types of solids, classified according to atomic arrangement: (a) crystalline and (b) amorphous materials are illustrated by microscopic views of the atoms, whereas (c) polycrystalline structure is illustrated by a more macroscopic view of adjacent single-crystalline regions, such as (a). 4 A semiconductor device may contain all three types as is the case for the “semiconductor” Industry workhorse, the m etal (actually typically highly conductive poly-Si) o xide (SiO 2 ) s emiconductor (Si) f ield-e ffect t ransistor (MOSFET). Figure 6—33* Cross section of a MOSFET. This high resolution transmission electron micrograph of a silicon Metal–Oxide Semiconductor Field Effect Transistor shows the silicon channel and metal gate separated by a thin (40Å, 4nm) silicon–dioxide insulator. The inset shows a magnified view of the three regions, in which individual rows of atoms in the crystalline silicon can be distinguished. (Photograph courtesy of AT&T Bell Laboratories.) amorphous SiO 2 crystalline Si metal in this case … … but more often poly Si
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3 5 We will be principally, but not exclusively, interested in the properties of charge carriers in semiconductor crystals in this course; … … metals or polysilicon are most often used simply as contacts (reservoirs of carriers) and amorphous materials are most often simply used as insulators (barriers to charge transport). 6 Hess, 2.4 modified: one view of a representation of, e.g., a GaAs cyrstal. Crystal structure A crystal is, simply put, a periodic array of atoms. However, to distinguish among the many possible types of crystal structures and the analyze their properties requires a bit more elaborate description of their structure … … employing terminology such as “Bravais lattice,” “atomic basis,” “translation/basis vectors,” “unit cells,” and “primitive” versions of all of the above.
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4 7 Crystal structure is described via a “Bravais lattice ” and an “atomic basis ,” where … … a “ Bravais lattice ” is a periodic array of “points”—a mathematical construct—such that the lattice looks identical from each point. … and an “ Atomic basis ” is a fixed array of one or more atoms associated with each Bravais lattice point—a physical object.
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