Lab 102-B — Hold the Material: Silicon Wafers and a Crystal Model
Goes with: Units 1, 3, 4. Time: 2–3 hours. Cost: about \(60–\)130 (estimate).
Goal
Handle real silicon: measure a wafer's thickness, mass and density and compare with the value computed from the lattice constant (2.329 g/cm³); cleave a wafer and see crystal planes in its straight fracture lines; build a diamond-cubic model and see the tetrahedral bonding and (111) planes with your own hands. Look at broken solar cells to see texture, coatings and metallisation.
Shopping list
| Item | Spec / example | Qty | Approx. cost (estimate) | Where |
|---|---|---|---|---|
| Silicon wafers, 100 mm | test or reclaim grade, (100) orientation, ~525 µm thick; optionally one (111) wafer for comparison | 2–5 | $2–20 each | University Wafer, eBay |
| Broken/"B-grade" solar cells | mono, any size | a few | $5–15 | eBay, AliExpress, Amazon |
| Molecular model kit | with ≥ 18 tetrahedral (4-hole) atoms and bonds (e.g., Molymod or similar inorganic/organic kit) | 1 | $20–35 | Amazon |
| Precision scale | 0.01 g resolution, 100–200 g capacity | 1 | $15–25 | Amazon |
| Digital caliper (and ideally a micrometer) | caliper 0.01 mm; micrometer 0.001 mm for thickness | 1–2 | $15–25 / $25–40 | Amazon, Harbor Freight |
| Diamond scribe | carbide/diamond-tip pen | 1 | $8–12 | Amazon |
| Cut-resistant gloves (ANSI A4+), safety glasses | — | 1 each | $15 | hardware store |
| Rigid sharps container, lint-free cloth | — | 1 | $8 | pharmacy |
| Optional USB microscope | 50–200× | 1 | $25–50 | Amazon |
Safety
- Silicon shards are razor-sharp and fly when cleaving: safety glasses and cut-resistant gloves; cleave inside a cloth or plastic bag.
- Never grind, sand or saw silicon dry (fine dust is a respiratory hazard). Wipe up fragments with a damp cloth; put shards in a rigid sharps container.
- Solar-cell fragments may carry silver/aluminium/lead-containing solder: wash hands; don't heat them.
Procedure
Part 1 — Density. 1. Weigh the wafer (0.01 g). Measure diameter (caliper) and thickness at five points (micrometer preferred). Account for the flat: a 100 mm wafer's primary flat is ~32.5 mm long (SEMI standard; approximate), removing a small segment of area. 2. Compute volume = area × mean thickness; density = mass/volume. Compare with 2.329 g/cm³.
Part 2 — Cleaving. 1. On a (100) wafer, scribe a short (2–3 mm) nick at the edge, perpendicular to the primary flat (the flat marks a ⟨110⟩ direction on standard (100) wafers). 2. Place the wafer face-up on a clean cloth with a thin rod (e.g., a toothpick or wire) under the scribe line; press gently on both sides — it snaps along a straight line. 3. Make a second cleave at right angles. Observe: straight edges at 90° to each other. 4. If you have a (111) wafer, repeat: cleavage lines form at 60°/120° — triangles. 5. Look at the fractured edge under magnification: mirror-smooth cleavage planes.
Part 3 — Model. 1. Build the 8-atom diamond-cubic cell (Unit 3 figure): FCC corners and face centres plus four interior atoms at (¼,¼,¼)-type positions, each bonded tetrahedrally. 2. Extend to a few cells; find a (111) plane (atoms with three bonds below and one above) and a (100) plane (two below, two above). Relate to why KOH etches (100) faster. 3. Measure the bond angle with a protractor (≈ 109.5°).
Part 4 — Solar-cell anatomy. Under the microscope: pyramids on the textured surface (a few µm), the blue SiNₓ colour (interference), silver fingers (~20–40 µm wide in modern cells; estimate) and busbars or pads; rear side aluminium (PERC) or grid (bifacial).
What to measure
- Wafer mass, thickness (mean ± spread), density.
- Angle between cleavage lines.
- Finger width and pitch on a cell (microscope + ruler image).
Expected results
- Mass of a 100 mm, 525 µm wafer ≈ 9.5 g; measured density within ~1–2 % of 2.329 g/cm³ if thickness is measured carefully (thickness uncertainty dominates).
- (100) wafers cleave in straight lines at 90°; (111) at 60°.
- Fracture surfaces are flat and shiny — cleavage along crystal planes, a hallmark of a brittle covalent crystal.
How to interpret
Density from the lattice constant matching your measurement is a direct check that the diamond-cubic picture is right. Straight cleavage shows anisotropy — properties depend on direction — which is also why wafers break in predictable patterns during handling, why laser-cut half cells have defined edge quality, and why texture pyramids have fixed angles.
Sources
- SEMI M1 (wafer dimensions; named, values approximate).
- Lattice constant/density: Unit 3.