Solid State Materials: Structure, Properties, and Fabrication

Solid State Material Classification

Three types of solids are classified according to their atomic arrangement:

• Crystalline Materials: Atoms are arranged in a highly ordered, repeating pattern.
• Amorphous Materials: Atoms lack a long-range ordered structure.
• Polycrystalline Materials: Composed of many small, randomly oriented single-crystalline regions.

Microscopic views illustrate crystalline and amorphous materials, while polycrystalline structures are shown through a more macroscopic view of adjacent single-crystalline regions.

Diamond Structure Discovery and Bragg Diffraction

The discovery of the diamond structure is closely linked to Bragg diffraction. This phenomenon occurs when two beams with identical wavelength and phase approach a crystalline solid and are scattered off two different atoms within it. The lower beam traverses an extra length of 2dsinθ. Constructive interference, which is crucial for diffraction, occurs when this extra length is equal to an integer multiple of the radiation's wavelength.

Applications of Solid Materials

The distinct atomic arrangements of solid materials lead to varied properties and applications:

• Crystalline Materials: Used in high-performance semiconductor devices such as bipolar transistors, FETs, MOSFETs, LEDs, and Lasers, enabling high speed, high gain, high power, light-emission, and light detection.
• Amorphous Materials: Commonly found in passive components like resistors, dielectrics, and inductors.
• Polycrystalline Materials: Offer a balance between crystalline and amorphous properties, enabling device fabrications such as thin-film transistors (TFTs) for LCD displays and solar panels, especially where single crystal formation is limited by material and process conditions.

Crystal Planes and Directions: Miller Index

The Miller Index is a notation system used to describe crystallographic planes and directions. To determine the Miller Index (hkl) for a plane, follow these steps:

1. Find Intercepts: Determine where the plane intercepts the crystal axes. Express these intercepts as integral multiples of the basis vectors. (The plane can be moved from the origin, retaining its orientation, until integral intercepts are found on each axis).
2. Take Reciprocals: Take the reciprocals of the three integers found in step 1.
3. Reduce to Smallest Integers: Reduce these reciprocals to the smallest set of integers (h, k, l) that maintain the same relationship to each other.
4. Label the Plane: Label the plane as (hkl).

Understanding the Diamond Lattice

The diamond lattice is a fundamental crystal structure with key characteristics:

• It is characteristic of elements like Silicon (Si) and Germanium (Ge).
• In compound semiconductors, atoms are arranged in a basic diamond structure but differ on alternating sites.
• This variant is commonly known as the zincblende lattice.

Bulk Crystal Growth Process

The fabrication of semiconductor wafers often begins with a bulk crystal growth process, involving several critical steps:

1. Ingot Grinding: Mechanically grind the ingot into a perfect cylinder.
2. Crystal Plane Identification: Use X-ray crystallography to identify crystal planes within the ingot.
3. Notch Grinding: A small notch is ground on one side of the cylinder to delineate a specific crystal face, such as a {110} face.
4. Wafer Sawing: The Si cylinder is sawed into individual wafers, typically about 775 μm thick, using a diamond-tipped inner-hole blade saw or a wire saw.
5. Chemical-Mechanical Polishing (CMP): The front surface of the wafer undergoes Chemical-Mechanical Polishing using a slurry of very fine SiO₂ particles in a basic NaOH solution to achieve a mirror-like finish.
6. Impurity Doping: Impurity doping may be added to modify the electrical properties of the wafer, tailoring it for specific semiconductor applications.