Variation of Energy Bands with Alloy Composition
As III-V ternary and quaternary alloys are varied over their composition ranges, their band structures change.
Variation of Energy Bands with Alloy Composition:
- Fig. given below illustrates the band structure of GaAs and AlAs, and the way in which the bands change with composition x in the ternary compound AlxGa1-xAs.
- The binary compound GaAs is a direct material, with a band gap of 1.43 eV at room temperature.
- For reference, we call the direct (k = 0) conduction band minimum .
- There are also two higher-lying indirect minima in the GaAs conduction band, but these are sufficiently far above that few electrons reside there.
- We call the lowest-lying GaAs indirect minimum L and the other X.
- In AlAs the direct minimum is much higher than the indirect X minimum, and this material is therefore indirect with a band gap of 2.16 eV at room temperature.
- In the ternary alloy AlxGa1-xAs all of these conduction band minima move up relative to the valence band as the composition x varies from 0 (GaAs) to (AlAs).
- However, the indirect minimum Amoves up less than the others, and for compositions above about 38 percent Al this indirect minimum becomes the lowest-lying conduction band.
- Therefore, the ternary alloy AlGaAs is a direct semiconductor for Al compositions on the column III sublattice up to about 38 percent, and is an indirect semiconductor for higher Al mole fractions.
- The band gap energy Eg is shown in color on Fig. c.