Optical Properties

Optical constants (n,k) are measured across a large spectral range from 140 to 2100 nm, depending on the spectroscopic ellipsometer used. 

Optical constants (n,k) of transparent and semi-transparent thin films

Homogeneous films exhibit a single set of optical constants.

Absorption coefficient

The absorption coefficient (a) is simply given by the relation:

Optical constants (n,k) of bulk substrates (solid and liquid)

Spectroscopic ellipsometry is the only technique able to provide optical constants of bulk substrates by a simple measurement. Common bulk substrates are semiconductor materials such as c-Si, GaAs, metals such as Al, Pt, and glass, sapphire, plastic. For liquid measurements a liquid cell is used.

Optical constants (n,k) for graded films

A graded film exhibits a change to its optical constants through the layer. It specifies one value of (n,k) at the bottom and another one for the layer top.

Optical constants (n,k) for anisotropic films

A uniaxial anisotropic film exhibits two different sets of optical constants, commonly called ne (for extraordinary ray) and no (for ordinary ray). For biaxially anisotropic films 3 different sets of optical constants are measured, nx,ny,nz and kx,ky,kz


Optical bandgap (Eg)

The ability to measure optical constants accurately allows the determination of the optical bandgap (Eg). This is an important physical property that is often requested for the characterization of semiconductor and high k materials. The optical bandgap value is directly calculated by the software either by a Tauc-plot calculation or as a calculated parameter from a dispersion formula.

Optical constants shift with varying alloy composition

Increasing Al increases the optical bandgap shifting the absorption edge to higher energies. This is illustrated by the graphic where the AlxGa1-xAs alloy composition is changing from x=0 to x=1.

Optical constants depend strongly on the crystallinity of materials

Amorphous, poly and crystalline silicon exhibit very different optical constants.

Reflectance / Transmittance

R and T can be measured by spectroscopic ellipsometers. It is also possible to simulate the reflectance and transmittance of a sample after its analysis using DeltaPsi2 software.