### 3.4.2 Thin Film Structure

| This will be short, because we will discuss this topic mostly when we come to deposition methods.
| Let's distinguish to separate points Geometry and morphology. Internal structure.
| The first topic has been mentioned before, but now we go beyond the points made there.
|  | Let's consider to grow a thin film on a substrate that has a small hole in it - say half a µm in lateral size and 1 µm deep. We deposit a thin film (atomic) layer by layer (symbolized by various shade of yellow). What we might get could have one of the topologies shown below or anything in between
|  | The only difference between these quite different topologies is the deposition method; we will cover that in more detail in chapter 6.
| There is much more along this line, but we cover it when we get to it.
| As far as the internal structure goes, the situation is similar. Everything known from bulk materials goes:
|  | Poly-crystalline thin films with grain sizes ranging from a few nanometers to cm (Are the Zn-covered steel lamp posts, letter boxes, etc. with huge grains products of the thin film industry?)
|  | Single crystalline thin films, but full of defects like dislocations, precipitates, point defects.
|  | Nearly perfect single crystalline thin films - what we often would like to have, but not always get.
| If we just look at polycrystalline thin films, we may have just regular grains, or all kinds of textures. Again, we deal with it when we run across it.
| Then we have some thin film specialities:
|  | Amorphous thin films, like amorphous Si (a-Si) or many other materials. You just can't have amorphous bulk Si or most everything else that usually likes to form a crystal.
|  | Mixtures of amorphous and crystalline phases; truly nanocrystalline structures (i.e. grain size around 10 nm) - practically never found in bulk.
| A case in point is Silicon (what else?). We have:
|  | Amorphous Silicon, used, e.g., in microelectronics. If it is heavily mixed (= "doped" with Hydrogen (> 15 %), we have the crucial thin film for a-Si:H solar cells or for the transistor matrix of liquid crystal displays (LCD). Another example is amorphous SiO2, the work-horse of microelectronics.
|  | Amorphous-crystalline mixes, like a-Si:H containing nanometer-sized embedded islands of crystalline silicon (c-Si and then called µc-Si:H. This is the base of the so-called "microcrystalline Si thin-film solar cell", one of the hottest contender for the solar cell market of the future.

© H. Föll (Semiconductor Technology - Script)