Lattice constant

The lattice constant refers to the constant distance between unit cells in a crystal lattice. Lattices in three dimensions generally have three lattice constants, referred to as a, b, and c. However, in the special case of cubic crystal structures, all of the constants are equal and we only refer to a. Similarly, in hexagonal crystal structures, the a and b constants are equal, and we only refer to the a and c constants.

For example the Lattice Constant for a common carbon diamond is a = 3.57Å at 300k. It is important to note that the structure is somewhat cubic.

A group of Lattice constants could be referred to as lattice parameters. However, strictly speaking the full set of lattice parameters consist of the three lattice constants and the three angles between them.

As lattice constants have the dimension of length, their SI unit is the meter. Lattice constants are typically on the order of several angstroms (i.e. tenths of a nanometre).

In epitaxial growth, the lattice constant is a measure of the structural compatibility between different materials. Lattice constant matching is important for growth of thin layers of materials on other materials; when the constants differ, strains are introduced into the layer, which prevents epitaxial growth of thicker layers without defects.

Lattice matching
Matching of lattice structures between two different semiconductor materials, allows forming of a region of band gap change in the material without introducing a change in crystal structure. It allows construction of advanced light-emitting diodes and diode lasers.

For example, gallium arsenide, aluminium gallium arsenide, and aluminium arsenide have almost equal lattice constants, making it possible to grow almost arbitrarily thick layers of one on the other one.

Lattice grading
Typically, films of different materials grown on the previous film or substrate are chosen to match the lattice constant of the prior layer to minimize film stress.

An alternative method is to grade the lattice constant from one value to another by a controlled altering the alloy ratio during film growth. The beginning of the grading layer will have a ratio to match the underlying lattice and the alloy at the end of the layer growth will match the desired final lattice for the following layer to be deposited.

The rate of change in alloy must be determined by the weighing of the penalty of layer strain and hence defect density vs. the cost of the time in the epitaxy tool.

For example, Indium gallium phosphide layers with a band-gap above 1.9 eV can be grown on Gallium Arsenide wafers with index grading.

Lattice measurement
Lattice constants can be determined by X-ray diffraction.

Gitterparameter Parámetro de red Constante de réseau Gardelės konstanta 格子定数 Stała sieci krystalicznej Постоянная решётки