Materials for Semiconductor Lasers
We have discussed the properties of the junction laser largely in terms of GaAs and AlGaAs. However, the InGaAsP/InP system is particularly well suited for the type of lasers used in fiber-optic communication systems. Lattice-matching is important in creating heterostructures by epitaxial growth.
Materials for semiconductor lasers:
- The fact that the AlGaAs band gap can be varied by choice of composition sublattice allows the formation of barriers and confining layers.
- The quaternary alloy InGaAsP is particularly versatile in the fabrication of laser diodes, allowing considerable choice of wavelength and flexibility in lattice-matching.
- By choice of composition, lasers can be made in the infrared range 1.3-1.55 μm required for fiber optics.
- Since four components can be varied in choosing an alloy composition, InGaAsP allows simultaneous choice of energy gap (and therefore emission wavelength) and lattice constant (for lattice-matched growth on convenient substrates).
- In many applications, however, other wavelength ranges are required for laser output.
- For example, the use of lasers in pollution diagnostics requires wavelengths farther in the infrared than are available from InGaAsP and AlGaAs.
- In this application the ternary alloy PbSnTe provides laser output wavelengths from about 7 μm to more than 30 μm at low temperatures, depending on the material composition.
- For intermediate wavelengths, the InGaSb system can be used.
- Materials chosen for the fabrication of semiconductor lasers must be efficient light emitters and also be amenable to the formation of p-n junctions and in most cases the formation of heteroj unction barriers.
- These requirements eliminate some materials from practical use in laser diodes.
- For example, semiconductors with indirect band gaps are not sufficiently efficient light emitters for practical laser fabrication.
- The II-VI compounds, on the other hand, are generally very efficient at emitting light but junctions are difficult to form.
- By modern crystal growth techniques such as MBE and MOVPE it is possible to grow junctions in ZnS, ZnSe, ZnTe, and alloys of these materials, using N as the acceptor.
- Lasers can be made in these materials which emit in the green and blue-green regions of the spectrum.
- In recent years much progress has been made in the growth of large band gap semiconductors using GaN, and its alloys with InN and A1N.
- The InAlGaN system has direct band gaps over the entire alloy composition range, and hence offers very efficient light emission.
- Band gaps range from about 2 eV for InN, to 3.4 eV for GaN and 5 eV for AlN.
- This covers the wavelength range from about 620 nm to about 248 nm, which is from blue to UV.
- The resurgence of interest in this field was triggered by the work of Nakamura at Nichia Corporation in Japan who demonstrated very high-efficiency blue light emitting diodes (LEDs) in GaN.