Why growth needs a starting point
Crystals do not form from nothing in a controlled way. Atoms need a surface to attach to, and the arrangement of that surface influences how new atoms line up. In diamond synthesis, the starting surface is either a seed crystal, which is a small piece of diamond, or a different material called a substrate. Which one is used, and how well it is prepared, largely decides whether the result is a single crystal, a mass of small grains, or a flawed mixture.
Seeds in high pressure growth
In an HPHT cell, a small diamond seed sits at the cooler end of the capsule while a carbon source sits at the hotter end. Carbon dissolved in the molten metal solvent moves toward the seed and deposits on it, extending the existing crystal. Because new carbon follows the seed’s lattice, the crystal grows with the same orientation. The seed’s orientation also affects which crystal faces grow, and growers often choose a seed face that gives stable growth.
Without a seed, carbon in the cell still forms diamond, but it nucleates at many places at once. That is the usual approach for producing large quantities of small crystals for industrial diamond grit: many tiny crystals instead of one big one. Seeded growth is chosen when larger single crystals are the goal, and the process is generally slower and more controlled. The machines involved are HPHT presses of several designs.
Seeds in gas-phase growth
In chemical vapor deposition, the seed is usually a flat plate cut and polished from a diamond. This plate is often called the substrate. Growth on a diamond substrate is called homoepitaxy: new diamond continues the lattice of the old, keeping it a single crystal. Plates for this purpose may come from HPHT-grown diamond or from earlier CVD runs. Because each run adds a layer to a plate, growers can sometimes remove the new layer and reuse the plate, or slice a thick layer into several new plates.
The orientation of the plate surface, meaning which crystal plane it exposes, matters. Common choices are the cube-type surface, which tends to grow smoothly under suitable conditions, though growers also use other orientations depending on the purpose. The orientation influences growth speed, how impurities are taken up and which defects appear.
Foreign substrates
Growing diamond on a non-diamond surface is called heteroepitaxy when the film aligns with the substrate, and simply film growth when it does not. Silicon, metals, cemented carbide and some ceramics are typical substrates, especially for coatings. The difficulty is that diamond does not readily nucleate on most of these. A common fix is seeding, in which the surface is scratched with diamond powder or coated with a suspension of tiny diamond particles so there are many starting points. Without seeding, nucleation can be sparse and the film patchy.
Films grown on foreign substrates are generally polycrystalline. Each nucleus has its own orientation, and the grains grow into each other and leave grain boundaries. For many coatings and for heat spreading, that is acceptable. For electronics and quantum devices, the grain boundaries are usually undesirable. The differences between single crystal diamond and polycrystalline diamond largely come down to this question of starting surface.
How seed quality is carried into the crystal
Any defect in the seed can propagate. Scratches and subsurface damage left by polishing can become sources of dislocations, which are line defects that run through the grown layer. Growers therefore treat seed preparation as a critical step. Typical measures include:
- careful polishing to produce a very smooth surface,
- a short plasma etch to remove damaged material before growth,
- inspection of the seed, for example with optical methods or x-ray diffraction, to check for strain and defects,
- precise alignment so the surface is cut at a controlled angle to the crystal planes.
Small intentional angle offsets are sometimes used to promote step-flow growth, where atoms attach at the edges of atomic steps, which can give smoother surfaces.
Substrate considerations beyond the crystal
Even with the right crystal, practical points matter.
- Size. Larger seeds allow larger finished plates but cost more and are harder to find free of flaws.
- Edges. Seed edges can grow differently, sometimes producing unwanted polycrystalline rims.
- Mounting and temperature. The seed must sit flat on a holder that keeps its temperature even, as hot spots lead to uneven growth.
- Thermal and mechanical match. For coatings on non-diamond substrates, differences in thermal expansion produce residual stress, and preparation affects coating adhesion.
What the choice means in practice
A prepared seed or substrate is effectively a template. Single crystal seeds yield single crystals, usually at greater cost and lower volume. Many-point seeding on a foreign surface gives practical coatings. Understanding that link helps explain why gem-quality and electronic-grade diamond is slower and costlier to make than wear-resistant coatings, and why growers treat the starting surface with so much care.