Understanding Lasers

Chapter 9.4.1: Doping Layers and Substrates

9.4.1 Doping Layers and Substrates

A diode laser must contain at least three layers: a p-type layer, an n-type layer, and an intermediate active or junction layer where recombination causes light emission. The essential components of these layers are semiconducting compounds such as gallium aluminum arsenide and dopants that create either extra carriers or holes.

Lasers are grown by depositing a series of thin layers on a substrate usually made of a semiconductor or sometimes an insulator such as sapphire. The most common diode-laser substrates are gallium arsenide and indium phosphide. Binary semiconductors are preferred as substrates because they can be produced in quantity at relatively low cost. Layers deposited on them must have an atomic spacing (often called lattice constant) closely matched to the spacing of atoms in the substrate. Typically, these thin layers are made of semiconductors with compositions slightly different from the substrate, such as aluminum gallium arsenide on GaAs or InGaAsP on InP. The lasers are doped with elements with extra electrons or with fewer electrons to produce n or p carriers.

Typically, substrates are n-doped, with one or more n-doped layers deposited on them, then an undoped active layer, and, finally, multiple p-doped layers deposited on top of the active layer. Compositions and doping are typically varied to produce the desired properties.

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