Novel Real Time Optics for Thin Film Materials Research - I Robert W. Collins The Pennsylvania State University, DMR New optical spectroscopies based on the interaction of polarized light with the reflect- ing surfaces of solids and thin films have been developed. These probes have been applied to analyze materials fabrication and processing in real time. The following research has been undertaken: ►development and optimization of nanostructured polarizing filters along with the models that describe their optical performance; and ► development and optimization of structured surfaces for thin film photovoltaics applications along with new concepts that relate surface roughness and optical characteristics (see right). micro macro geometric (non-unif) 10   8 10  6 10  4 10  2 in-plane scale (m) microscopic roughness 0.1  50 nm macroscopic roughness 50 nm  5  m Mueller matrix analysis AFM and profilometry geometric optical scale roughness 5  m  5 mm  g (nm)  s (nm) d s (nm) time (s) Three characteristics of the polarized light beam provide surface roughness layer thick- ness evolution on three different in-plane scales. These characteristics include: (i) polarization ellipse azimuth and ellipticity; (ii) irradiance; and (iii) degree of polarization. Shown here is the configuration used for polarized light spectro- scopy performed in situ and in real time during surface structure optimization of ZnO for light trapping in thin film photovoltaics. Final roughness thickness (nm) Chi Chen et al., Phys. Rev. Lett. 90, Art. no (2003)

Novel Real Time Optics for Thin Film Materials Research - II Robert W. Collins, The Pennsylvania State University, DMR The ability to measure and control the surface and bulk structure of thin films over a wide range of in-plane scales simultaneously is important for the fabrication and optimization of thin film optical and optoelectronic devices. In this research, new polarized light spectroscopies have been developed that span a wide spectral range, can be applied in real time during thin film growth and processing, and employ different beam characteristics as probes of film structure at different in- plane scales. These beam characteristics include the polarization ellipse shape, the beam irradiance, and the degree of beam polarization, in order to characterize surface and bulk morphology from the sub-nanometer scale to the millimeter scale. Using such probes, thin film circular polarization filters have been developed and optimized having nanoscale chiral structure and applications in biosensing. In addition, surface structure with in-plane scales from nanometers to microns have been incorporated into ZnO films for optimized light trapping that improves the efficiency of thin film photovoltaic devices.

Educational impact of this award: 4undergraduates 8grad. students 1postdoc. fellow 6visiting students & faculty participated in the development of novel optical instru- mentation for real time analysis and optimization of thin films. Novel Real Time Optics for Thin Film Materials Research - II Robert W. Collins, The Pennsylvania State University, DMR Interdisciplinarity & outreach under this award: increased awareness of the importance and broad applications of real time measurement, monitoring, and control of materials fabrication among researchers throughout thin film industries; and educated undergraduates and public about clean, inexpensive power generation by thin film silicon photovoltaics (above and left). G=0 E g =1.8 eV G=0.083 E g =1.5 eV G=0.167 E g =1.35 eV (Above) Amorphous roughening transition observed during the growth of a-Si:H films plotted along with precursor diffusion length versus the H 2 /SiH 4 flow ratio; (right) deposition phase diagrams including amorphous roughening and amorphous-to-(mixed- phase-microcrystalline) transitions used to guide (a- Si:H, a-Si 1-x Ge x :H) multijunction solar cell fabrication. (Left) Multijunction a-Si:H solar shingles fabricated by collaborator Uni-Solar Corp. of Auburn Hills, MI, and installed on Collins residence in Monclova, OH.

This research program has involved undergraduates at all stages from project conception to computer modeling of optical properties to materials and device fabrication. Among the undergraduates involved in this project, Jorge M. Flores (thin film photovoltaics) has joined a funded research project to assess science education in the villages of his native Mexico; Sarah Johnson (optical modeling of thin films) has joined the U.S. Defense Department; Scott Thaller (optical properties of ZnO) is a graduate student at the University of Minnesota; and Nikolas Podraza (nanoscale chiral thin films) is a graduate student at the University of Toledo focusing on thin film photovoltaics. As part of this multidisciplinary research program, deposition phase diagrams were conceived and developed to optimize thin film silicon based solar cells in collaboration with Prof. C. R. Wronski at Penn State University. This effort (i) identified a new regime of silicon film growth called protocrystalline silicon with enhanced ordering and (ii) prescribed methods for fabricating solar cells within this regime. The solar cell optimization procedures specified through the phase diagram are now being used worldwide for the fabrication of the highest efficiency and stability solar cells. It is expected that thin film photovoltaics and in particular protocrystalline silicon will play a significant role in future renewable energy development. These new technological developments would not have been possible without the instrumentation and basic science developments supported by the National Science Foundation. Novel Real Time Optics for Thin Film Materials Research - II Robert W. Collins, The Pennsylvania State University, DMR