# 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 n i(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)