THE REFRACTIVE-INDEX NEAR THE FUNDAMENTAL ABSORPTION-EDGE IN ALXGA1-XAS TERNARY COMPOUND SEMICONDUCTORS

The real part of the complex refractive index is calculated for photon energies below and above the fundamental absorption edge in terms of experimentally available band structure parameters. No singularity is predicted. Rather, a change in the functional form of n occurs as the photon energy passes through the band-gap energy G, leading to a local maximum, or more accurately, a plateau region in n as a function of frequency which is observed experimentally. The effect of a finite spin orbit-splitting energy and the effect of higher conduction bands are included. The former effect smoothes the discontinuity in the slope of n as the photon energy passes through G, while the latter effect causes a further increase in n as a function of frequency at photon energies greater than G, leading to a higher absolute maximum in n at a photon energy equal to the energy separation of the heavy-hole valence band and the higher conduction band E1. This is in accordance with experimental observation. Theoretical and experimental results are compared for Al(x)Ga(1-x)As over a range of photon energies from 1.2 to 1.8 eV and mole fractions x from x = 0 to x = 0.198.