Measurement of superlattice optical properties by variable angle spectroscopic ellipsometry

Variable angle of incidence spectroscopic ellipsometry is a sensitive, nondestructive technique for determining optical constants, layer thicknesses, microstructure, and other parameters. We have applied this technique to the study of AlAs-GaAs and Al (x)Ga(1-x)AsGaAs superlattices. For a sample with Al (0.5)Ga(0.5)As barrier layers and 20 periods, sharp spectroscopic features were observed at the first electron to heavy hole, e-hh(1), first electron to light hole, e-lh(1), and second electron to heavy hole, e-hh(2) transition energies. Cross sectional transmission electron microscopy (XTEM) showed this superlattice to be of good quality. Ellipsometric data for two other samples, with AlAs barriers, did not contain any sharp features due to quantized level transitions, and XTEM of these samples revealed poor quality superlattice structure. An advantage of ellipsometry is that the complex refractive index can be obtained without Kramers-Kronig analysis. The effective refractive index for a 20 period superlattice was solved using ellipsometric data at three angles of incidence, near 74°. The real part is increased by about 2% at the e-hh(1) peak, and the imaginary part (extinction coefficient) is increased by 0.05.