9.4.4 Lattice Matching and Strain

A crucial element in diode-laser fabrication is matching the atomic spacing of successive layers. Perfect crystals are arrays of regularly spaced atoms, but atomic spacing differs among compounds. Failure to match the atoms in successive layers can produce defects in the crystal, which degrade its optical, electronic, or mechanical properties.

Semiconductors can accommodate small differences in atomic spacing, which produce some strain within the crystal but not enough to cause damage. However, developers try to minimize strain by limiting differences in lattice spacing. This is particularly important in matching a deposited material to a substrate. As mentioned in Section 9.4.1, simple semiconductor compounds

such as GaAs are preferred for substrates because they are much easier to make in quantity than compounds containing three or four elements, so layer compositions must be chosen to match an available substrate, typically GaAs or InP.

Somewhat larger differences in lattice spacing can be accommodated by depositing a series of layers with incremental steps in atomic spacing. The small increments produce small amounts of strain between adjacent layers, which the semiconductor can tolerate. Using such strained-layer superlattices broadens the range of materials usable in diode laser structures.

The lattice spacing depends on the nature and size of the atoms in the semiconductor, and varies with composition. Making heterostructures requires a way to change the band gap without introducing large changes in the lattice spacing. A major reason that GaAlAs compounds were widely used in the early stages of diode laser development is that the lattice spacing changes very little with aluminum content, so adding aluminum to increase the band gap did not cause lattice mismatches between layers of GaAlAs and GaAs.

The lattice spacing of other III V compounds differs more with composition, so it is not possible to add just one element to change the band gap without also changing the lattice constant. Fabricating lattice-matched structures with different band gaps on an InP substrate requires a compound with four elements, In-GaAsP. Adjusting both the In/Ga ratio and the As/P ratio makes it possible to produce InGaAsP layers with different band gaps but lattice spacing matched to each other and the substrate.