1. ISAT 436 Micro-/Nanofabrication and Applications Crystals and Crystal Growth D. J. Lawrence James Madison University

2. Crystals H In perfect crystalline materials, all the atoms occupy well defined and regular positions known as “lattice sites”. H Most microelectronic devices are manufactured from crystalline material that is nearly perfect and is referred to as “single crystal”. H Examples of such materials are Si, Ge, GaAs, InP, and CdTe.

3. From Crystalline Ingot to Chip H Overview of the integrated circuit (IC) fabrication process. Today we will emphasize the growth of crystalline ingots. This course will emphasize the process steps that take place here.

4. Crystals: Examples H Crystals are described by their most basic structural element, called the “unit cell”. H A unit cell can be thought of as the most basic building block of a crystal. H For example, there are three basic cubic crystal structures:

5. Crystalline Silicon H Silicon is by far the most commonly used semiconductor. H The silicon crystal structure is face-centered cubic, but there are two Si atoms at each “lattice site”. H Diamond and germanium have the same structure -- the “diamond structure”. Also see page 11 of Photovoltaic Fundamentals. Notice that each silicon atom is bonded to four nearest neighbors.

6. III-V Compound Semiconductors H Gallium Arsenide (GaAs) is an example of a III-V compound semiconductor. H Gallium is in column III of the periodic table. H Arsenic is in column V of the periodic table. H The crystal structure is similar to that of silicon. Notice that each gallium atom is bonded to four nearest neighbor arsenic atoms, and conversely.

7. More Semiconductors H Other examples of III-V semiconductors: InP, GaP, InAs, InSb, GaN, AlN. H III-V alloys: Al x Ga 1-x As, In 1-x Ga x As y P 1-y, Al x Ga 1-x N. H There are a tremendous number of compound semiconductors. H Examples include ZnSe, CdS, Hg 1-x Cd x Te, ZnO, SnO 2, In 2 O 3, … H Some of these compounds have non-cubic crystal structures.

8. Solidification H If we melt Si (or Ge, or GaAs, or …) and pour the molten material into a mold and let it solidify, what can we say about the crystal structure of the resulting ingot? H It is polycrystalline. H See Photovoltaic Fundamentals, pp. 25-26.

9. Crystal Growth H Special care must be taken in order to produce a (nearly) perfect crystalline ingot of silicon (or other materials). H Such an ingot is called a “single crystal,” e.g., “single-crystal silicon.” H The most common method for preparing single-crystal semiconductor ingots is the Czochralski (Cz) crystal growth process. H See Photovoltaic Fundamentals, pp. 18-21, and other handouts.

10. Silicon Wafer Manufacturing

11. Amorphous Materials H Not all solids are crystalline or polycrystalline. H For example, consider glass. H An amorphous solid is a material in which there is no recognizable “long-range order” in the positioning of atoms. H E.g., some solar cells are made from amorphous silicon.

12. Amorphous Silicon H In amorphous silicon, even though there is no long-range order, there is “short-range order.” H I.e., most of the silicon atoms bond with four neighbor silicon atoms at distances and bond angles nearly the same as in a crystal. H However, there tend to be “dangling bonds,” which can be neutralized by terminating them with hydrogen. H See Photovoltaic Fundamentals, pp.28-30.

13. Crystalline Silica = Quartz ( SiO 2 ) (2-D representation)

14. Amorphous Silica = Silica Glass ( SiO 2 )