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

ISAT 436ISAT 436Micro-/Nanofabrication Micro-/Nanofabrication

and Applicationsand Applications

Crystals and Crystal Crystals and Crystal GrowthGrowth

D. J. Lawrence

James Madison University

CrystalsCrystals

In perfect crystalline materials, all the atoms occupy well defined and regular positions known as “lattice sites”.

Most microelectronic devices are manufactured from crystalline material that is nearly perfect and is referred to as “single crystal”.

Examples of such materials are Si, Ge, GaAs, InP, and CdTe.

From Crystalline Ingot to ChipFrom Crystalline Ingot to ChipOverview of the integrated circuit (IC)

fabrication process.

Today we will emphasize the growth of crystallineingots.

This course will emphasize the process steps that take place here.

Crystals: ExamplesCrystals: Examples Crystals are described by their most basic structural element,

called the “unit cell”. A unit cell can be thought of as the most basic building block of

a crystal. For example, there are three basic cubic crystal structures:

Crystalline SiliconCrystalline Silicon Silicon is by far the most commonly used semiconductor. The silicon crystal structure is face-centered cubic, but there

are two Si atoms at each “lattice site”. 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.

III-V Compound SemiconductorsIII-V Compound Semiconductors Gallium Arsenide (GaAs) is an example of a III-V compound

semiconductor. Gallium is in column III of the periodic table. Arsenic is in column V of the periodic table. The crystal structure is similar to that of silicon.

Notice that each gallium atom is bonded to four nearest neighbor arsenic atoms, and conversely.

More SemiconductorsMore SemiconductorsOther examples of III-V semiconductors:

InP, GaP, InAs, InSb, GaN, AlN. III-V alloys: AlxGa1-xAs, In1-xGaxAsyP1-y ,

AlxGa1-xN.

There are a tremendous number of compound semiconductors.

Examples include ZnSe, CdS, Hg1-xCdxTe, ZnO, SnO2 , In2O3 , …

Some of these compounds have non-cubic crystal structures.

SolidificationSolidification

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?

It is polycrystalline. See Photovoltaic Fundamentals, pp. 25-26.

Crystal GrowthCrystal Growth Special care must be taken in order to

produce a (nearly) perfect crystalline ingot of silicon (or other materials).

Such an ingot is called a “single crystal,” e.g., “single-crystal silicon.”

The most common method for preparing single-crystal semiconductor ingots is the Czochralski (Cz) crystal growth process.

See Photovoltaic Fundamentals, pp. 18-21, and other handouts.

Silicon Wafer ManufacturingSilicon Wafer Manufacturing

Amorphous MaterialsAmorphous Materials

Not all solids are crystalline or polycrystalline. For example, consider glass.An amorphous solid is a material in which

there is no recognizable “long-range order” in the positioning of atoms.

E.g., some solar cells are made from amorphous silicon.

Amorphous SiliconAmorphous Silicon In amorphous silicon, even though there is no

long-range order, there is “short-range order.” 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.

However, there tend to be “dangling bonds,” which can be neutralized by terminating them with hydrogen.

See Photovoltaic Fundamentals, pp.28-30.

Crystalline Silica = Quartz ( SiOCrystalline Silica = Quartz ( SiO2 2 ))

(2-D representation)

(2-D representation)

Amorphous Silica = Silica Glass ( SiOAmorphous Silica = Silica Glass ( SiO2 2 ))