2. Semiconductor Materials and Products

2.1 General Chemistry and Structure

2.1.1 General Considerations and Elemental Semiconductors

Some General Statements

	All semiconductors come in a certain structural form. We will look at the possibilities by just comparing extremes:
	Large in three dimensions or Small in at least one dimension?
		Large: Can you see it and only it (i.e. not its substrate) with the naked eye? Touch it, hold it, break it? Examples: Single crystals of e.g. Si (300 mm diameter, 1 m long!), GaAs, InP, GaP, SiC, .... Si wafers (up to 450 mm diameter, about 1 mm thick), GaAs, etc.
		Small:, at least in one dimension. Can you see it with the naked eye? But, maybe, not hold it? Examples: Small in one dimension: All thin films; always on a substrate. This contains most optoelectronic and all "thin film" solar cell uses. Many usable semiconductors exist only as thin films! Small in two dimensions: Micro- or Nanowires. A big research field right now (2007) and in use as connections on ICs. Small in three dimensions: Micro- or Nanoparticles. Semiconducting micro- or nanoparticles are not yet part of products coming out of semiconductor technology (unless you count an integrated transistor as a microparticle which we do not), but a big research item, e.g. for "Nano" solar cells
	Crystalline or Amorphous?
		So you need a (thin film) semiconductor on a really large area - for a flat panel display, for example. Or a solar cell module made from "one piece". There is no single crystal big enough for that and it would be prohibitively expensive to make on (provided you could).
		You then must try live with a poly-crystalline thin film or, maybe with an amorphous one. Be prepared to spend some 10.000 man-hours in getting it to work. Class Exercise: Ponder the history of "LCD" flat panel displays.
	Here are some alternatives:
		Mono Crystalline or Poly Crystalline Contains dislocations and other defects or is (almost) perfect? Large Grains or Small Grains? Electronic parameters are adjustable or Fermi level pinning is observed? Grain boundaries problematic or tolerable?

	By now you get the drift: This may turn out to be quite complicated. Thank God, there are some specialists who have to know all this stuff; the rest of us can forget about it and just be good consumers.
		Right. Those specialists, by the way, are called Materials Scientists and Engineers. Sorry. But it will be up to you (and a few others) to save the world - your world.
		Class Exercise: Why does the world need saving? How shall it be done?

	Elemental Semiconductors
	There isn't much. All we need to do is to look at a rather small part of the periodic table:
		Here is a part of the complete periodic table accessible by the link. Semiconductor are outlined a reddish background and big letters. The redder, the better!
		IIA The Rest IIIA IVA VA VIA VIIA VIII He Be B C N O F Ne Mg Al Si P S Cl Ar Ca Ga Ge As Se Br Kr Sr In Sn Sb Te J Xe Ba Tl Pb Bi Po At Rn
	What we have, in (subjective) order of importance, are
		Silicon (Si) It's so obviously of top importance that we are not going to say anything more to it at this point.
		Germanium (Ge) A true fine semiconductor. Good single crystal can be grown, doping etc. is easily possible, but the band gap is a bit too small for most applications. Far worse: There is no "good" Germanium Oxide (GeO2) The first semiconductor put to commercial use in the early sixties - and then phased out almost completely. In the last few years Ge experiences a kind of "come back"; we take that as a reason to start an "advanced" page at some point.
		Selenium (Se) An often overlooked semiconductor. Historically of some interest, and in particular because it made "Xeroxing", i.e. photo copying possible. We take that as a reason to start an "advanced" page at some point.
		Diamond (C); metastable fcc form There are some technical uses (besides the obvious non-technical ones in (hetero) human relations, but nothing we have to concern ourselves with at present.
		Tin (Sn); a - Sn (below 13 oC) Forget it!
		Boron (B) Forget it!
		Phosphorous (P); Forget it!
		Arsenic (As) Forget it!
	Are we going for Crystalline or Amorphous?
		To make it short: In case of doubt we use crystals, preferably single crystals, preferably "perfect" single crystals. Class Exercise: Why?
		Applications on large areas, however, use amorphous thin films or poly crystals for obvious (???) reasons.

	Chemical Element - Technical Semiconductor
	We finally must concern ourselves a little with what exactly it is we are looking for when we consider semiconductor technology. To a small extend, we have already discussed some points of interest above.
		To make the issue clear, consider that you can buy a kg of e.g. "Silicon" from a chemical company like Merck. What you will get is a bottle full of a greyish powder, which will be of no use whatsoever for semiconductor technology. We are obviously looking for more than just the element.
		Let's look at some material parameters that are of interest to us when we want to make a product or component by doing some semiconductor technology. Here we just list key words (hoping that they will strike a chord). In the next sub-chapter we will take a closer look:
		Properties Remarks Crystal Crystal structure fcc, bcc, hex,.. Lattice constant Will turn out to be very important General structure single-, poly-crystal, thin layer, .. Defect densities dislocation density, impurity concentration, .. Defect properties Formation-, migration enthalpies of point defects, .. Unit weight [mol], Density [g/cm3]   Mechanical properties Yield strength (as f(T)), fracture strength, surface energies, ... Electronic Properties Band gap [eV] Gives ni(T) Type direct, indirect, dispersion function Effective mass of electrons and holes [m*/m] Important, but beyond the scope here. Neff in C and V; ni Needed for calculating n(T) Mobility (undoped) Very important for speed Lifetime; Diffusion coefficient of electrons and holes; Diffusion length Appear in any equation! Mechanism of luminescence Important, but beyond the scope here. Deep levels of impurities and defects Important if you can't be perfect Dielectric properties Dielectric constant Appears in most equations! Break through field strength Obviously important Specific intrinsic resistance Not so important Electron affinity Thermal Properties Therm. expansion coefficient Very important in many cases Therm. conductivity, Specific heat, Melting point of some interest, important in power applications Economical / Ecological Propertiess Supply / Price Potentially important; depends on product. Poisonous / polluting; directly or indirectly Si you can eat; GaAs is poisonous!
		Class Exercise: Supply examples for critical parameter - component couplings

© H. Föll (Semiconductor Technology - Script)