Summary of Current Research Interests

Current research interests include the physical mechanisms of film growth that control the electromagnetic, optical, and infrared-vibrational properties of thin film materials used in opto-electronic device applications.  These areas involve studying film growth mechanisms and structure to identify nucleation and growth behavior as well as phase changes in thin films; the optical properties pertaining to solid state physical properties including the electronic band structure of materials, as well as the phonon or vibrational modes in solids; electromagnetic and optical behavior and how it can be manipulated by structure such as in metamaterials and plasmonics; and finally the total impact that all of these areas upon device physics and functionality.

Specific research interests include utilizing real time spectroscopic ellipsometry in situ to study the growth evolution of hydrogenated silicon, germanium, and silicon-germanium alloy thin films prepared by plasma enhanced chemical vapor deposition for use in thin film infrared sensing microbolometers (collaborators: T. Jackson, M. Horn-PSU; E. Dickey-North Carolina State U.; D. Drabold-Ohio U.), thin film photovoltaic applications (collaborators: T. Jackson, C. Wronski, T. Mayer-PSU; R. Collins-Univ. Toledo, Applied Materials Inc., Solasta Inc., and Air Products and Chemicals Inc.), and nanostructured / nanowire photovoltaics (collaborators: J. Redwing, T. Mallouk, C. Wronski, T. Mayer-PSU; E. Dickey-North Carolina State U.); spectroscopic ellipsometry studies of vanadium oxide and nickel manganese oxide materials used in microbolometers (collaborators: T. Jackson, M. Horn, S. Trolier-McKinstry-PSU; E. Dickey-North Carolina State U.); real time spectroscopic ellipsometry in order to track changes in microstructure of ionic block co-polymer films for proton exchange applications as a function of post-deposition processing conditions such as relative humidity (collaborator: M. Hickner-PSU); real time spectroscopic ellipsometry to track polymer absorption on surface (collaborator: A. Palamino-PSU); optical properties and microstructure of multiferroic and strain enhanced ferroelectric materials (collaborators: V. Gopalan-PSU; D. Schlom-Cornell Univ., J. Musfeldt-U. Tennessee); the optical properties and structure of sculptured thin films (collaborator: M. Horn-PSU); component materials and their integration in device structures for metamaterials (collaborators: D. Werner; T. Mayer-PSU); and optical properties and microstructural evolution of anodized tantalum oxide thin films for electrolytic capacitors (collaborators: D. MacDonald-PSU; E. Dickey-North Carolina State U.). A short description of several key research activities follows, grouped in terms of materials and applications.

Hydrogenated Group IV Amorphous Semiconductor Thin Films for Opto-electronic Devices

This work has entailed using real time spectroscopic ellipsometry in situ to study the growth evolution of hydrogenated silicon, germanium, and silicon germanium alloy thin films for amorphous and microcrystalline thin films as functions of germanium content and other processing conditions (degree of hydrogen dilution, doping, etc.) for photovoltaic and infrared sensing microbolometer applications.  Control of the microcrystallite evolution from the amorphous phase and grading of the germanium content in alloy materials of interest for thin film hydrogenated silicon photovoltaics have been demonstrated.  The growth mechanisms and surface roughness evolution in hydrogenated amorphous materials have been studied and indicate that a greater diffusion of surface precursors is observed with increasing hydrogen dilution, which also results in more electronically stable material for photovoltaic applications.  The dependence of the electrical properties of interest for microbolometers (resistivity, temperature coefficient of resistance, noise characteristics) has been identified to vary with dopant (n- or p-type), hydrogen dilution during deposition, and germanium content.  In general, for a given resistivity range of device interest (~0.1-10 kWcm) the temperature coefficient of resistance (a) increases in magnitude when films are prepared under high hydrogen dilution conditions while remaining amorphous prior to microcrystallite nucleation, (b) is higher for n-type amorphous material compared to p-type, and (c) is higher for materials incorporating germanium.  Some selected publications on hydrogenated silicon germanium include:

i)     “Analysis of Si1-xGex:H Thin Films with Graded Composition and Structure by Real Time
               Spectroscopic Ellipsometry,”
by N. J. Podraza, J. Li, C. R. Wronski, E. C. Dickey, M. W. Horn, and
               R. W. Collins, Physica Status Solidi a 205, 892 (2008).

ii)    “Analysis and Control of Mixed-Phase Amorphous+Microcrystalline Silicon Thin Films by
               Real Time Spectroscopic Ellipsometry,”
by N. J. Podraza, J. Li, C. R. Wronski, E. C. Dickey, and R. W.
               Collins, Journal of Vacuum Science and Technology A 27, 1255 (2009).

iii)    “Experimental and Theoretical Study of the Evolution of Surface Roughness in Amorphous Silicon
                Films Grown by Low-Temperature Plasma-Enhanced Chemical Vapor Deposition,”
by Y. A.
                Kryukov, N. J. Podraza, R. Collins, and J. G. Amar, Physical Review B 80, 085403 (2009).

iv)    “Microstructural Evolution in Si1-xGex:H Thin Films for Photovoltaic Applications,” by N. J.
                Podraza, D. Saint John, J. Li, C. R. Wronski, E. C. Dickey, and R. W. Collins, Proceedings of the 35th
                Photovoltaic Specialists Conference
, 000158 (2010).

Nanostructured / Nanowire Silicon for Photovoltaics Devices

This work has involved using spectroscopic ellipsometry and transmission electron microscopy to accurately characterize the microstructure of thin (< 10 nm) hydrogenated silicon material deposition onto planar and pillared-structure hydrogen terminated crystalline silicon to identify deposition conditions whereby no microcrystallite epitaxy occurs.  These materials are of interest for nanostructured heterojunction photovoltaic devices.  Nanostructured crystalline silicon p-n junction devices with ~8 mm diameter pillars spaced ~3 mm apart have been fabricated from heavily doped crystalline silicon which exhibit significantly higher efficiencies (~8%) compared to their planar counterparts (~4%), which simulations have indicated are attributed to enhanced optical absorption due primarily to geometric scattering within the pillar structure with contributions also from diffraction effects.  A selected publication is:

i)     “High Aspect Ratio Silicon Solar Cells,” by H. P. Yoon, Y. Yuwen, G. D. Barber, N. J. Podraza, J.
               M. Redwing, T. E. Mallouk, C. R. Wronski, and T. S. Mayer, Applied Physics Letters 96, 213503

Vanadium Oxide and Nickel Manganite Materials for Uncooled Microbolometers

Vanadium oxide, a material currently used in commercial uncooled microbolometers, and nickel manganite, a high temperature coefficient of resistance thermistor material of interest for future generation devices, have been studied via spectroscopic ellipsometry in order to track correlations between the material structure on the optical properties, and how these optical properties can in turn be used to diagnose material properties in device configurations.  Complex dielectric function spectra (e = e1 + ie2) have been obtained in the far infrared to near ultraviolet spectral range from 0.04 to 5.15 eV or 33 to 0.24 mm, respectively.  Pulsed dc magnetron sputtering has been used to fabricate vanadium oxide with electrical properties comparable to that produced via reactive ion beam sputtering in commercial settings.  Absorption features in the optical properties for vanadium oxide prepared using either deposition technique have been identified in the infrared and visible ranges and are observed to increase in amplitude with decreasing oxygen content, decreasing resistivity, and increasing degree of crystallinity.  Nickel manganite materials have been fabricated using chemical solution deposition and spin spray processing at low temperatures.  The optical properties for nanocrystalline nickel manganite exhibit variations that correlate with film density and crystallite grain size.  The variations in the optical properties with electrical and material properties for both vanadium oxide and nickel manganite has indicated that this is a promising technique for rapid characterization and early disqualification of films during fabrication processes.  Selected publications include:

i)     “Nickel Manganite Thermistor Films Deposited by Spin Spray for Microbolometer Applications,”
               by S. W. Ko, J. Li, N. J. Podraza, E. C. Dickey, and S. Trolier-McKinstry, Journal of the American
               Ceramic Society
94, 516 (2011).

ii)     “Optical Properties of Solution Deposited Nickel Manganite Thin Films,” by N. J. Podraza,, D. B.
                Saint John, S. W. Ko, H. M. Schulze, J. Li, E. C. Dickey, and S. Trolier-McKinstry, Thin Solid Films
519, 2919 (2010).

iii)    “Influence of Microstructure and Composition on Hydrogenated Silicon Thin Film Properties for
                Uncooled Microbolometer Applications,”
by D. B. Saint John, H.-B. Shin, M.-Y. Lee, S. K. Ajmera,
                A. J. Syllaios, E. C. Dickey, T. N. Jackson, and N. J. Podraza,
Journal of Applied Physics 110, 033714

Complex Oxides

Spectroscopic ellipsometry has been used to study the optical properties of complex oxide materials of interest due to their nature as multiferroics or potential as strain enhanced ferroelectrics.  The complex dielectric function spectra have been obtained for molecular beam epitaxy or pulsed laser deposited films including strained SrTiO3, EuTiO3, PbVO3, BiFeO3, CaTiO3, Ruddleson-Popper series, and Aurivelius series films.  This work has resulted in obtaining the optical properties and identifying the band gap of many of these materials for the first time, as well as detecting variations as a function of phase or processing, such as differences between rhombohedral and tetragonal BiFeO3.  Selected publications include:

i)     “Optical Band Gap of BiFeO3 Grown by Molecular-Beam Epitaxy,” by J. F. Ihlefeld, N. J. Podraza,
                Z. K. Liu, R. C. Rai, X. Xu, T. Heeg, Y. B. Chen, J. Li, R. W. Collins, J. L. Musfeldt, X. Q. Pan, J.
                Schubert, R. Ramesh, and D. G. Schlom, Applied Physics Letters 92, 142908 (2008).

ii)     “Optical Band Gap and Magnetic Properties of Unstrained EuTiO3 Grown by Molecular-Beam
by J. H. Lee, X. Ke, N. J. Podraza, L. Fitting Kourkoutis, T. Heeg, M. Roeckerath, J. W.
                 Freeland, C. J. Fennie, J. Schubert, D. A. Muller, P. Schiffer, and D. G. Schlom, Applied Physics
94, 212509 (2009).

iii)     “Optical Properties of Quasi-Tetragonal BiFeO3 Thin Films,” by P. Chen, N. J. Podraza, X. S. Xu,
                 A. Melville, E. Vlahos, V. Gopalan, R. Ramesh, D. G. Schlom, and J. L. Musfeldt, Applied Physics
96, 131907 (2010).

Sculptured Thin Films for Photon Management in Opto-electronic Devices

Mueller matrix spectroscopy has been used to characterize the anisotropic optical properties and engineered microstructure of chiral sculptured thin films fabricated from MgF2 and TiO2 and columnar sculptured thin film films fabricated from SnO2 and ZnO.  New analysis procedures and software has been developed in order to simultaneously extract the indices of refraction in each direction and identify depth profile in the microstructure arising from variations such as void fraction gradients in a single spectroscopic measurement.  The chiral films exhibit Bragg resonance phenomena at wavelengths dependent upon film processing conditions and the measurement angle of incidence.  Significantly sharper resonance features were observed for a TiO2 film prepared by a serial bi-deposition process compared to a MgF2 film made by continuous substrate rotation.  Series of columnar SnO2 and ZnO films were fabricated at different glancing angles of incidence which result in bulk-like average indices of refraction (n~1.9 at l = 700 nm) at low glancing angles which could be tailored to values near n ~ 1.5 at l = 700 nm for high glancing deposition angles.  These variations are of interest for the fabrication of low-index ZnO and SnO2 layers which may be incorporated into multilayer anti-reflection coatings or enhanced back reflector structures in thin film photovoltaics without introducing other compositional gradients which may degrade the electrical properties of the top contact or back contact. Selected publications include:

i)     “Transparent Conducting Oxide Sculptured Thin Films for Photovoltaic Applications,” by N. J.
               Podraza, C. Chen, D. Sainju, O. Ezekoye, M. W. Horn, C. R. Wronski, and R. W. Collins, Materials
               Research Society Symposia Proceedings
865, F.7.1.1 (2005).

ii)    “Analysis of the Optical Properties and Structure of Serial Bi-Deposited TiO2 Sculptured Thin
               Films using Mueller Matrix Ellipsometry,”
by N. J. Podraza, S. M. Pursel, C. Chen, M. W. Horn,
               and R. W. Collins, Journal of Nano-Photonics 2, 021930 (2008)

Tantalum Oxide for Electrolytic Capacitors

The complex dielectric function spectra and microstructure have been extracted for Ta2O5 films of interest for electrolytic capacitor applications that have been prepared under different anodization conditions. Spectroscopic ellipsometry analysis has proven to be sensitive to the presence of a mono-layer or bi-layer structure dependent on the electrolyte used during anodization of metallic tantalum substrates.  Films prepared in a phosphate electrolyte exhibit a bi-layer structure consisting of an inner layer at the oxide / tantalum interface exhibiting a higher band gap than observed in mono-layer films prepared in sodium sulfate and an outer layer at the oxide / electrolyte interface exhibiting a lower band gap than the mono-layer material. Variations in the optical properties and band gap as functions of other anodization conditions such as voltage or time have also been established, and indicate that the band gap of the inner and outer layers of a bi-layer structure increase with formation voltage, but the band gap of the outer layer decreases with anodization time while that of the inner layer remains relatively constant.  Changes in the anodization process have also been studied as a function of the surface treatment of the tantalum substrate and post deposition processing, including the growth of a thermal oxide prior to or post anodization.  A selected publication is:

i)     “Complex Dielectric Functions of Anodic Bi-Layer Tantalum Oxide,” by J. D. Ray Sloppy, N. J.
               Podraza, E. C. Dickey, and D. D. MacDonald, Electrochemica Acta 55, 8751 (2010).