Real Time Optics of Photovoltaic Materials and Structures:

From Mueller Matrices to Phase Diagrams

Robert W. Collins

Department of Physics, Materials Research Institute,

and Center for Thin Film Devices

The Pennsylvania State University, University Park, PA 16802

A set of spectroscopic probes based on ellipsometric principles has been developed with the goal being to characterize thin film photovoltaic materials and device structures in situ and in real time during their fabrication. In these optical probes, polarized white light from a polarization generation system is directed through a reactor window onto the film surface during growth. The obliquely reflected beam exits the reactor through a second window and is analyzed by a multichannel polarization detection system.

Instruments of increasing complexity have been designed to characterize thin film structures of increasing complexity. Incorporating a single rotating polarizer as the polarization detection system leads to an instrument that provides the tilt and ellipticity angles of the reflected beam polarization ellipse versus wavelength and time. This approach is sufficient for determining the time evolution of optical properties and layer thicknesses for uniform, isotropic materials. If instead a rotating compensator (phase shifter) followed by a fixed polarizer is incorporated as the detection system, information on the degree of polarization of the reflected beam can be obtained, as well. This is a useful parameter in characterizing the growth of inhomogeneous, isotropic materials. Finally, if rotating compensators are incorporated on both the polarization generation and detection sides of the instrument, then the 4x4 Mueller matrix of the growing film is accessible versus wavelength and time. This is the ultimate measurement, and is useful in characterizing the evolution of optical properties and microstructure of materials that are both inhomogeneous and anisotropic.

We have applied the single rotating compensator version of real time spectroscopic ellipsometry to characterize the time and thickness evolution of the optical properties and microstructure of silicon thin films from spectra collected during deposition. From the optical properties, one can easily distinguish hydrogenated amorphous silicon (a-Si:H) growth from microcrystalline silicon (mc-Si:H) growth. In fact, depth profiles in the volume fraction of microcrystallinity can be determined. This capability has led to the development of deposition phase diagrams that can guide the fabrication of silicon thin films at low temperatures (<300°C) for highest performance solar cells. The simplest phase diagrams identify a single transition from an amorphous growth regime to a mixed-phase (amorphous + microcrystalline) growth regime versus accumulated film thickness [the a®(a+mc) transition]. These phase diagrams have shown that optimization of a‑Si:H intrinsic layers for solar cell applications is achieved by rf plasma-enhanced chemical vapor deposition using the maximum possible flow ratio of H₂ to SiH₄ that can be sustained while avoiding the a®(a+mc) transition.