National Renewable Energy Laboratory
Measurements and Characterization
Spectroscopic Ellipsometry
NREL's ex situ ellipsometer is capable of mapping samples up to 10 × 10 cm with spatial resolution of 100 × 300 µm. This figure shows a 149 point map of a thin a-Si:H layer used as the intrinsic layer in a silicon heterojunction solar cell. Points were measured every 2.5 mm.
Spectroscopic ellipsometry (SE) measures the change in polarization upon reflection of polarized light from a surface. Because SE measures two quantities at each wavelength, namely the amplitude ratio and phase change, the technique obtains more information than available with standard optical reflection techniques. Because SE measures ratios of intensities rather than absolute intensities, it obtains a much higher degree of precision and accuracy than standard optical reflection techniques. Because of its nature as an optical measurement, SE is non-destructive, rapid, and capable of in situ measurements during growth and processing of devices.
SE is particularly adept at measuring film thickness and roughness in very thin layers and multilayer samples. This makes the technique ideal for many applications in photovoltaics (PV). SE is also well-suited to measurement of the optical constants, n & k, of the layers that make up PV cells. Accurate optical constants are essential for modeling the optical performance of solar cell devices. The complex dielectric function is an alternate expression of the optical properties that directly relates to the electronic properties of the material. Through measurement of the complex dielectric function, SE can provide detailed knowledge of the electronic properties of the material such as band gap, absorption coefficient, and critical point energies. This in turn provides insight into material properties such as composition, degree of ordering or crystallinity, hydrogen content, temperature, and many other physical properties that affect the electronic structure.
In-situ ellipsometry measurements of surface roughness and film thickness in the earliest stages of the deposition process shed light on the film nucleation process.
A rapidly growing application of SE is for in situ or in-line measurement of materials growth and processing. Real time SE, or RTSE, measures film roughness, film thickness, and optical constants as a function of deposition time, providing knowledge of film nucleation and growth processes, as well as how the material properties evolve with film thickness. Such information can be of great value when developing new deposition processes or troubleshooting existing deposition and processing steps. The figure at right illustrates RTSE data from a-Si:H deposition. The peak in surface roughness at early time indicates that growth occurs through nucleation followed by coalescence of nucleation centers into a contiguous film.
Applications
Material Parameters
Determines the composition, crystallinity, temperature, electronic structure, and other variables that affect optical properties.
Thickness of Thin Films and Multilayer Structures
Determines thickness, interface roughness, and homogeneity in multilayer structures with thicknesses from sub-monolayer to tens of micrometers.
In-line or In-situ Monitor of the Growth and Processing
Monitors bulk or thin-film materials in real time inside the growth or processing chamber.
For additional information contact Brian Keyes, 303-384-6695.