N. J. Podraza

University of Toledo, Toledo, Ohio, United States

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Publications (88)121.39 Total impact

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    ABSTRACT: The origin of the deep subgap states in amorphous indium gallium zinc oxide (a-IGZO), whether intrinsic to the amorphous structure or not, has serious implications for the development of p-type transparent amorphous oxide semiconductors. We report that the deep subgap feature in a-IGZO originates from local variations in the oxygen coordination and not from oxygen vacancies. This is shown by the positive correlation between oxygen composition and subgap intensity as observed with X-ray photoelectron spectroscopy. We also demonstrate that the subgap feature is not intrinsic to the amorphous phase because the deep subgap feature can be removed by low-temperature annealing in a reducing environment. Atomistic calculations of a-IGZO reveal that the subgap state originates from certain oxygen environments associated with the disorder. Specifically, the subgap states originate from oxygen environments with a lower coordination number and/or a larger metal-oxygen separation.
    Applied Physics Letters 06/2014; 104(23):232108-232108-4. · 3.52 Impact Factor
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    ABSTRACT: In the scale-up of Cu(In1-xGax)Se2 (CIGS) solar cell processing for large-area photovoltaics technology, the challenge is to achieve optimum values of layer thicknesses, as well as CIGS Cu stoichiometry and alloy composition x within narrow ranges and simultaneously over large areas. As a result, contactless metrologies - those that provide such information in real-time or in-line process step by step, with the capabilities of large-area mapping - are of great interest in this technology. We have demonstrated high-speed multichannel spectroscopic ellipsometry (SE) in a number of modes for CIGS metrology, including 1) single-spot real-time SE monitoring of (In1-xGax)2Se3 as the first stage in multisource evaporation of three-stage CIGS; 2) control of Cu stoichiometry in the second and third stages of the process; 3) single-spot in situ SE analysis of alloy composition and grain size averaged through the thickness for the final CIGS film; 4) offline mapping of the CIGS thickness and composition over large areas, as well as mapping after each device fabrication step for correlation with local small area cell performance; 5) ex situ single-spot analysis of alloy composition profiles in CIGS and of completed solar cell stacks to extract thicknesses and properties of semiconductor and contact layers; and 6) predictive capability for quantum efficiency based on the results of SE multilayer analysis. With the future development of new instrumentation, the offline and ex situ capabilities in multilayer analysis and mapping will be possible in-line for both rigid and roll-to-roll flexible substrates.
    IEEE Journal of Photovoltaics 01/2014; 4(1):333-339. · 3.00 Impact Factor
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    ABSTRACT: An expanded-beam spectroscopic ellipsometer has been developed and applied toward in situ high-speed imaging/mapping analysis of large area spatial uniformity for multilayer coated substrates in roll-to-roll thin-film photovoltaics (PV). Slower speed instrumentation available in such analyses applies a 1-D detector array for spectroscopic mapping and involves width-wise translation of the ellipsometer optics over the moving coated substrate surface, measuring point-by-point in a time-consuming process. The expanded-beam instrument employs instead a 2-D detector array with no moving optics, exploiting one array index for spectroscopy and the second array index for line imaging across the width of a large area sample. Thus, the instrument enables imaging width-wise and mapping length-wise for uniformity evaluation at the high linear substrate speeds required for real-time, in situ, and online analysis in roll-to-roll thin-film PV. In this investigation, we employ the expanded beam technique to characterize the uniformity of the Ag, ZnO, and n-type hydrogenated amorphous silicon (a-Si:H) layers of an a-Si:H n-i-p structure deposited on a flexible polyimide substrate in the roll-to-roll configuration. Spectroscopic ellipsometry data across a line image were collected as the substrate was translated by a roll-to-roll mechanism. Coated areas as large as 12 cm × 45 cm were analyzed in this study for layer thickness and optical properties by applying the appropriate analytical models for the complex dielectric functions of the Ag, ZnO, and n-type a-Si:H layers.
    IEEE Journal of Photovoltaics 01/2014; 4(1):355-361. · 3.00 Impact Factor
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    ABSTRACT: Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temperature gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near ultraviolet (UV-Vis) range at room temperature yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy critical point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coefficients (α ∼ 102 cm−1) of BiI3 that renders reflection-based ellipsometry insensitive to the indirect gap for this material. Further, electronic-structure calculations of the band structure by density functional theory methods are also consistent with the presence of an indirect band gap of 1.55 eV in BiI3. Based on this, an indirect band gap with a value of 1.67 ± 0.09 eV is considered to best represent the band gap structure and value for single crystal BiI3.
    Journal of Applied Physics 07/2013; 114(3). · 2.19 Impact Factor
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    ABSTRACT: Biaxial strain induces a phase transition from a pseudo-rhombohedral (R) to pseudo-tetragonal (T) phase in BiFeO3 (BFO) thin films. Using optical second harmonic generation, we measure the nonlinear optical dij coefficients at a fundamental wavelength of 1550 nm for R and T-BFO thin films. A large increase of the dij magnitudes is observed for T-BFO in comparison to R-BFO. The dij magnitudes for T-BFO were measured to be: |d33| = 18.1±2.4, |d31| = 60.8±8.1, and |d15| = 47.0±4.2, and for R-BFO: |d33| = 15.1±2.1, |d31| = 8.5±1.2, |d15| = 0.9±0.1, and |d22| = 18.7±2.6 (pm/V). The strain-enhanced nonlinear optical properties of T-BFO thin films make them potentially useful for optical applications.
    Applied Physics Letters 07/2013; 103(3). · 3.52 Impact Factor
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    ABSTRACT: Tin monusulfide (SnS) is an absorber with promising optoelectronic properties and low environmental constraints of interest for high-efficiency solar cells. The optical properties of SnS thin films are investigated to assess their compatibility with the solar spectrum. SnS thin films were RF magnetron sputter-deposited at target powers of 105-155 W and total pressures of 5 to 60 mtorr in argon at room temperature. X-ray diffraction patterns confirmed a dominant tin monosulfide herzenbergite phase. The absorption coefficient was determined by spectroscopic ellipsometry and unpolarized spectrophotometry measurements. Both methods show that the films have absorption coefficients above the band gap in the range of 105 -106 cm-1. The direct gap, indirect gap, and forbidden direct gap for the films were found to be in the range of 1.2-1.6 eV, indicating a strong match with the solar irradiance spectrum.
    IEEE Journal of Photovoltaics 07/2013; 3(3):1084-1089. · 3.00 Impact Factor
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    ABSTRACT: In this study, real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CuIn1-xGaxSe2; CIGS) thin film deposition by the three-stage co-evaporation process used for high efficiency photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1-xGax)2Se3 (IGS) at a temperature of 400°C on substrates consisting of soda lime glass coated with thin film molybdenum (Mo). This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result, precise composition and thickness control is desired in order to achieve the optimum open circuit voltage (Voc) and fill-factor (FF) of the resulting CIGS solar cell. In fact, RTSE has been applied broadly in previous studies for the characterization of complicated thin film deposition processes used in PV device fabrication - with the potential for process monitoring and control in many situations. In the case of the first-stage IGS deposition of this study, RTSE has been used to characterize the time evolution of (i) the Mo/IGS interface filling fraction, (ii) the IGS surface roughness layer thickness, and (iii) the IGS bulk layer thickness, as the depositing layer covers the rough Mo surface. In addition, one can extract the evolution of the bulk layer optical properties, expressed in the form of the complex dielectric function, which can serve as a fingerprint for IGS composition and also provide information on relative void vol.% and grain size (or defect density) in the IGS. Overall the structural and compositional information can assist in understanding the growth of three-stage CIGS absorbers for solar cells and in optimizing cell performance.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); 06/2013
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    ABSTRACT: Radial junction Si pillar array solar cells based on the heterojunction with intrinsic thin layer (HIT) structure were fabricated from p-type crystal Si (c-Si) wafers of different doping densities. The HIT structure consisting of intrinsic/n-type hydrogenated amorphous Si (a-Si:H) deposited by plasma-enhanced chemical vapor deposition (PECVD) at low temperature (200°C) was found to effectively passivate the high surface area of the p-type Si pillar arrays resulting in open circuit voltages (Voc>0.5) comparable to that obtained on planar devices. At high c-Si doping densities (>1018 cm-3), the short-circuit current density (Jsc) and energy conversion efficiency of the radial junction devices were higher than those of the planar devices demonstrating improved carrier collection in the radial junction structure.
    Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th; 06/2013
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    ABSTRACT: Real time spectroscopy ellipsometry (RTSE) has been applied to study the evolution of thin film optical structure during sputter deposition of polycrystalline CdS/CdTe solar cell stacks on transparent conducting oxide (TCO) coated glass substrates optimized for high efficiency. RTSE provides information on (i) interface formation to the underlying high resistivity transparent (HRT) layer during initial CdS growth, (ii) bulk layer CdS growth and its surface roughness evolution, (iii) CdS/CdTe interface formation when the overlying CdTe layer is deposited on the CdS, and (iv) CdTe bulk layer growth and its roughness evolution. Structural/optical models developed in the analysis of RTSE data acquired at a single point are also applied in the analysis of ex situ mapping SE data obtained over the area of the completed solar cell stack. As a result, maps of the structural parameters can be extracted, which then can be correlated with maps of the small area device performance. When uncorrelated non-uniformities exist over the area, optimization by combinatorial methods is possible.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); 06/2013
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    ABSTRACT: A growth evolution diagram has been developed to guide plasma-enhanced chemical vapor deposition (PECVD) of n-type hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) for use as the n-layer component of p-i-n a-Si:H superstrate solar cells. The substrates for such growth evolution diagram development by real time spectroscopic ellipsometry (RTSE) are crystalline silicon wafers that have been over-deposited with intrinsic a-Si:H layers. This diagram was applied to guide n-layer depositions on 15 cm × 15 cm glass/TCO/p/i superstrates in order to relate the diagram to the performance parameters of single-junction a-Si:H solar cells. Over the 15 cm × 15 cm TCO coated glass superstrate area, a 16 × 16 array of p-i-n dot cells has been fabricated, and this area has been mapped at high resolution by spectroscopic ellipsometry (SE). Analysis of such SE data over the full area provides maps of the p-layer effective thickness, i-layer thickness and band gap, and n-layer thickness and nanocrystalline Si:H vol. fraction. In addition, J-V measurements were performed on the 16 × 16 array of dot cells. The goal of this study is to identify and understand the relationships between basic materials property and thin film solar cell performance variations over large areas, and to evaluate impacts of non-uniformities on module performance.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); 06/2013
  • Laxmi Karki Gautam, Nikolas J. Podraza
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    ABSTRACT: The effect of silane + disilane gas mixtures on the growth evolution of hydrogenated silicon (Si:H) thin films have been studied using in situ, real time spectroscopic ellipsometry during plasma enhanced chemical vapor deposition. Growth evolution diagrams were generated for fixed values of S (disilane-to-silane gas ratio) with variable R (hydrogen-to-reactive-gas ratio). Amorphous Si:H (a-Si:H) quality was found to degrade with increasing S, however R could be increased to maintain film quality. The growth rate of optimized a-Si:H material was stable with S, but a wider process parameter range for improved, but not fully-optimized, material was available at lower R with higher rates. Crystallinity was suppressed at higher values of S and lower values of R, but promoted with higher R at increased rates by potentially increasing the initial crystallite nucleation density.
    2013 IEEE 39th Photovoltaic Specialists Conference (PVSC); 06/2013
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    ABSTRACT: The effect of dimensional confinement on the optical band gap of SrTiO3 is investigated by periodically introducing one extra SrO monolayer every n SrTiO3 layers. The result is the n = 1–5 and 10 members of the Srn+1TinO3n+1 Ruddlesden-Popper homologous series. Spectroscopic ellipsometry, optical transmission, and cathodoluminescence measurements reveal these Srn+1TinO3n+1 phases to have indirect optical band gaps at room temperature with values that decrease monotonically with increasing n. First-principles calculations suggest that as n increases and the TiO6 octahedra become connected for increasing distances along the c-axis, the band edge electronic states become less confined. This is responsible for the decrease in band gaps with increasing n (for finite n) among Srn+1TinO3n+1 phases.
    Applied Physics Letters 03/2013; 102(12). · 3.52 Impact Factor
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    ABSTRACT: An understanding of the relationship between materials property and thin-film solar cell performance variations over large areas is of interest for evaluating the impact of macroscopic nonuniformities in scale-up from laboratory cells to production modules. In this study, we have spatially correlated the properties of the hydrogenated silicon (Si:H) i- and p-layers-as mapped over a 13 cm×13 cm substrate area-with device performance parameters from an array of a-Si:H based n-i-p dot cells. To evaluate materials and device nonuniformities, a 16 × 16 array of dot cells has been fabricated over the substrate area, and this same area has been mapped by spectroscopic ellipsometry (SE).Analysis of the SE data over the full area provides maps of i-layer thickness and band gap, p-layer thickness and band gap, and p-layer surface roughness thickness for the n-i-p solar cell structure. The mapped values adjacent to the devices have been correlated with photovoltaic (PV) device performance parameters. When sufficient nonuniformity exists, these correlations enable optimization based on specific values of the fundamental properties. Alternatively, if the optimum set of properties has been identified, the impact of deviations due to macroscopic uniformities can be evaluated.
    IEEE Journal of Photovoltaics 01/2013; 3(1):387-393. · 3.00 Impact Factor
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    ABSTRACT: A spectroscopic ellipsometry (SE) capability having the potential to scan production-scale areas at high speed has been developed and successfully applied tomap the alloy composition of copper–indium–gallium–diselenide (CuIn1−xGaxSe2: CIGS) thin films. This technique not only generates a compositional map but simultaneously provides maps of the more typical SE-determined properties as well, including bulk layer and surface roughness layer thicknesses. As a result, the methodology is suitable for characterization in online production-scale applications. In order to develop the mapping capability, CIGS films having different molar Ga contents x and fixed copper stoichiometry were deposited and measured in situ by SE in order to extract the complex dielectric functions (ε = ε1+iε2 ) of these films. For mathematical interpolation between the available alloy contents, the (ε1 , ε2 ) spectra were parameterized using an oscillator sum. Best-fitting equations were obtained that relate each oscillator parameter to the Ga content x, as determined by energy dispersive X-ray analysis. This approach reduces the number of fitting parameters for (ε1 , ε2 ) from several to just one: the Ga content x. Because (ε1 , ε2) is now represented by this single parameter, the chances of parameter correlations during fitting are reduced, enabling production-scale compositional mapping of chalcopyrite films by SE.
    IEEE Journal of Photovoltaics 01/2013; 3:359. · 3.00 Impact Factor
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    ABSTRACT: Single-phase epitaxial films of the monoclinic polymorph of BiVO4 were synthesized by reactive molecular-beam epitaxy under adsorption-controlled conditions. The BiVO4 films were grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates. Four-circle x-ray diffraction, scanning transmission electron microscopy (STEM), and Raman spectroscopy confirm the epitaxial growth of monoclinic BiVO4 with an atomically abrupt interface and orientation relationship (001)BiVO4 ∥ (001)YSZ with [100]BiVO4 ∥ [100]YSZ. Spectroscopic ellipsometry, STEM electron energy loss spectroscopy (STEM-EELS), and x-ray absorption spectroscopy indicate that the films have a direct band gap of 2.5 ± 0.1 eV.
    APL Materials. 01/2013; 1(4):042112.
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    ABSTRACT: A new orthorhombic phase of the multiferroic BiFeO_{3} has been created via strain engineering by growing it on a NdScO_{3}(110)_{o} substrate. The tensile-strained orthorhombic BiFeO_{3} phase is ferroelectric and antiferromagnetic at room temperature. A combination of nonlinear optical second harmonic generation and piezoresponse force microscopy revealed that the ferroelectric polarization in the orthorhombic phase is along the in-plane ⟨110⟩_{pc} directions. In addition, the corresponding rotation of the antiferromagnetic axis in this new phase was observed using x-ray linear dichroism.
    Physical Review Letters 12/2012; 109(24):247606. · 7.73 Impact Factor
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    ABSTRACT: Vanadium oxide (VO{sub x}) thin films have been deposited by pulsed-DC magnetron sputtering using a metallic vanadium target in a reactive argon and oxygen environment. While the process parameters (power, total pressure, oxygen-to-argon ratio) remained constant, the deposition time was varied to produce films between 75 {+-} 6 and 2901 {+-} 30 A thick, which were then optically and electrically characterized. The complex dielectric function spectra ({epsilon} = {epsilon}{sub 1} + i{epsilon}{sub 2}) of the films from 0.75 to 5.15 eV were extracted by ex situ, multiple-angle spectroscopic ellipsometry (SE) measurements for the series of varied thickness VO{sub x} samples. Significant changes in {epsilon} and resistivity occur as a function of thickness, indicating the correlations exist between the electrical and the optical properties over this spectral range. In addition, in situ measurements via real time SE (RTSE) were made on the film grown to the largest thickness to track optical property and structural variations during growth. RTSE was also used to characterize changes in the film occurring after growth was completed, namely during post sputtering in the presence of argon and oxygen while the sample is shielded, and atmospheric exposure. RTSE indicates that the exposure of the film to the argon and oxygen environment, regardless of the shutter isolating the target, causes up to 200 A of the top surface of the deposited film to become more electrically resistive as evidenced by variations in {epsilon}. Exposure of the VO{sub x} thin film to atmospheric conditions introduces a similar change in {epsilon}, but this change occurs throughout the bulk of the film. A combination of these observations with RTSE results indicates that thinner, less ordered VO{sub x} films are more susceptible to drastic changes due to atmospheric exposure and that microstructural variations in this material ultimately control its environmental stability.
    Journal of Applied Physics 11/2012; 112(9). · 2.19 Impact Factor
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    ABSTRACT: The conformational behavior of polymers in clay-polymer nanocomposites (CPN) is not fully understood because of the many factors involved. The purpose of the present study was to investigate the conformational behavior of a polymer at the micro- and meso-scales in order to predict the behavior of tunable CPN. The study used a pH-responsive polymer, polyacrylamide, which has time-dependent hydrolysis response properties, to examine micro-scale conformational behavior of the polymer adsorbed on representative clay-mineral surfaces, SiO2 and Al2O3. A nanocomposite and a microcomposite were used to link meso-scale CPN behavior to micro-scale polymer conformation. The conformational behavior was characterized using in situ, real-time spectroscopic ellipsometry. The contracted coil conformation of polyacrylamide was observed at pH = 3, while extended conformation was observed at pH = 11.5 on both SiO2 and Al2O3 surfaces. At pH = 11.5, the polymer conformation changed from expanded coil to extended conformation over time. The polymer conformation changed more rapidly with the Al2O3 surface due to mineral dissolution at pH = 3 and 11.5. Swelling tests were conducted as functions of pH and time to link the micro-scale phenomena to meso-scale CPN behavior. The results indicated that the swelling potential of CPN corresponded to the conformation of adsorbed polyacrylamide, which varied with pH and time. The swelling potential of CPN was maximized at pH = 11.5 and decreased with decreasing pH, corresponding to the observed micro-scale conformational behavior.
    Clays and Clay Minerals 08/2012; 60(4). · 1.40 Impact Factor
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    ABSTRACT: The n = 8 member of the Aurivillius complex oxide superlattice series of phases, Sr5Bi4Ti8O27, was synthesized by pulsed-laser deposition on (001) SrTiO3 single-crystal substrates. This phase, with a c-axis lattice parameter of 7.25 ± 0.036 nm, and its purity were confirmed by x-ray diffraction and transmission electron microscopy. The film is observed to be single phase and free of intergrowths of other-n members of the series. Using spectroscopic ellipsometry, Sr5Bi4Ti8O27 was determined to exhibit an indirect band gap of 3.53 eV at room temperature.
    Applied Physics Letters 05/2012; 100(22). · 3.52 Impact Factor
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    ABSTRACT: Amorphous vanadium oxide (VOx) is a component found in composite nanocrystalline VOx thin films. These types of composite films are used as thermistors in pulsed biased uncooled infrared imaging devices when containing face centered cubic vanadium monoxide phase crystallites, and substantial fractions of amorphous material in the composite are necessary to optimize device electrical properties. Similarly, optoelectronic devices exploiting the metal-to-semiconductor transition contain the room-temperature monoclinic or high-temperature (>68 °C) rutile vanadium dioxide phase. Thin films of VOx exhibiting the metal-to-semiconductor transition are typically polycrystalline or nanocrystalline, implying that significant amounts of disordered, amorphous material is present at grain boundaries or surrounding the crystallites and can impact the overall optical or electronic properties of the film. The performance of thin film material for either application depends on both the nature of the crystalline and amorphous components, and in this work we seek to isolate and study amorphous VOx. VOx thin films were deposited by pulsed dc reactive magnetron sputtering to produce amorphous materials with oxygen contents ≥2, which were characterized electrically by temperature dependent current-voltage measurements and optically characterized by spectroscopic ellipsometry. Film resistivity, thermal activation energy, and complex dielectric function spectra from 0.75 to 6.0 eV were used to identify the impact of microstructural variations including composition and density.
    Journal of Applied Physics 04/2012; 111(7). · 2.19 Impact Factor

Publication Stats

541 Citations
121.39 Total Impact Points

Institutions

  • 2005–2013
    • University of Toledo
      • Department of Physics and Astronomy
      Toledo, Ohio, United States
  • 2004–2011
    • Pennsylvania State University
      • • Department of Electrical Engineering
      • • Department of Materials Science and Engineering
      • • Department of Physics
      University Park, MD, United States