9.11 DIODE LASER MATERIALS AND WAVELENGTHS

Earlier in this chapter you learned that the wavelengths emitted by diode lasers depended on the composition of the semiconductor, and learned which families of semiconductors could emit light. Now that you have learned about the general workings of semiconductor lasers, let us look in more detail at the materials and their implications.

The two parameters that most directly affect the structure of diode lasers are the band gap of the material and the lattice constant or spacing between atoms.

When an electron and hole recombine, the energy they release- and thus the laser wavelength-depends on the gap between the conduction band occupied by the free electron and the valence band where the hole is. The band gap depends on the atomic composition of the semiconductor; in general, change the composition and you change the band gap, although there are some exceptions. Table 9-2 lists important types of semiconductor lasers and their usual wavelengths.

The band gap is also important in controlling electron behavior in a diode laser. If a material with a low band gap energy is sandwiched between layers with higher band gap energy (forming a double heterojunction), conduction electrons can be trapped in the low band gap material. Band gap energy is often given in electron volts; to calculate the equivalent wavelength (in nanometers) that a diode made of that material would emit, divide 1240 by the band gap energy E in electron volts:

As you learned in Section 9.4.4, the lattice constant is important in semiconductor fabrication. Adjacent layers should have nearly identical lattice constants so the atoms line up properly. If the atomic spacings do not match, the differences causes strain in the layers. Semiconductors can withstand a little strain, particularly in a series of thin layers, called a strained layer superlattice,

but too much strain causes defects in the material, leading to device failure.