Nikolas J. Podraza

University of Toledo, Toledo, Ohio, United States

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Publications (108)193.51 Total impact

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    ABSTRACT: Excimer laser assisted re-oxidation for reduced, crystallized BaTiO3thin films on Ni-foils was investigated. It was found that the BaTiO3 can be re-oxidized at an oxygen partial pressure of ∼50 mTorr and substrate temperature of 350 °C without forming a NiOx interface layer between the film and base metal foil. The dielectric permittivity of re-oxidized films was >1000 with loss tangent values <2% at 100 Hz, 30 mVrms excitation signal. Electron Energy Loss Spectroscopy indicated that BaTiO3thin films can be re-oxidized to an oxygen stoichiometry close to ∼3 (e.g., stoichiometric). High resolution cross sectional transmission electron microscopy showed no evidence of NiOx formation between the BaTiO3 and the Ni foil upon excimer laser re-oxidation. Spectroscopic ellipsometry studies on laser re-oxidized [001]C and [111]C BaTiO3single crystals indicate that the re-oxidation of BaTiO3single crystals is augmented by photo-excitation of the ozone, as well as laser pulse induced temperature and local stress gradients.
    Preview · Article · Jan 2016 · Journal of Applied Physics
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    ABSTRACT: The fundamental challenge for designing transparent conductors used in photovoltaics, displays and solid-state lighting is the ideal combination of high optical transparency and high electrical conductivity. Satisfying these competing demands is commonly achieved by increasing carrier concentration in a wide-bandgap semiconductor with low effective carrier mass through heavy doping, as in the case of tin-doped indium oxide (ITO). Here, an alternative design strategy for identifying high-conductivity, high-transparency metals is proposed, which relies on strong electron-electron interactions resulting in an enhancement in the carrier effective mass. This approach is experimentally verified using the correlated metals SrVO3 and CaVO3, which, despite their high carrier concentration (>2.2 × 10(22) cm(-3)), have low screened plasma energies (<1.33 eV), and demonstrate excellent performance when benchmarked against ITO. A method is outlined to rapidly identify other candidates among correlated metals, and strategies are proposed to further enhance their performance, thereby opening up new avenues to develop transparent conductors.
    No preview · Article · Dec 2015 · Nature Materials
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    ABSTRACT: Spectroscopic ellipsometry (SE) is used to study the growth evolution of the bottom cell p-type doped and intrinsic hydrogenated silicon (Si:H) layers in p-i-n amorphous/nanocrystalline Si:H (a-Si:H/nc-Si:H) tandem photovoltaic (PV) devices. SE data collected in situ during the growth enables the identification of crystallite coalescence transitions in the surface roughness evolution, as well as the variations in optical response in the form of the complex dielectric function (ε = ε1 + iε2). The latter indicates relative amounts of the a-Si:H and nc-Si:H components during mixed-phase Si:H growth. A growth evolution diagram for the i-layer in the tandem PV device configuration is constructed. Device properties are related to p- and i-layer structure and can be understood on the basis of the growth evolution diagram.
    No preview · Article · Oct 2015 · IEEE Journal of Photovoltaics
  • Kiran Ghimire · Hamna F. Haneef · Robert W. Collins · Nikolas J. Podraza
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    ABSTRACT: Optical properties of commercially available gadolinium gallium garnet (Gd3Ga5O12) single crystals have been studied over a spectral range from 0.034 to 5.887eV using spectroscopic ellipsometry and transmittance measurements via a near infrared to ultraviolet spectral range multichannel ellipsometer and a Fourier transform infrared (FTIR) ellipsometer. The complex dielectric function (ε{lunate}=ε{lunate}1+iε{lunate}2) spectra from 0.034 to 5.887eV have been determined. Transmission measurements have been performed to more accurately extract the absorption coefficient near the band edge and identify sub band gap features. Analysis of ε{lunate} for Gd3Ga5O12 indicates a direct band gap at 5.66±0.01eV, sub band gap absorption features from 3.95 to 5.06eV, and nine transverse optical phonon modes at 288, 313, 329, 361, 370, 467, 577, 609, and 670cm-1 and longitudinal optical phonon modes at 288, 327, 338, 370, 461, 490, 592, 663, and 707cm-1.
    No preview · Article · Oct 2015 · physica status solidi (b)
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    ABSTRACT: Molybdenum oxide (MoOx ) and nickel oxide (NiOx ) thin films were deposited by reactive biased target ion beam deposition. MoOx thin film resistivity varied from 3 to 2000 ω· cm with a temperature coefficient of resistance (TCR) from ?1.7% to ?3.2%?K, and NiOx thin film resistivity varied from 1 to 300ω· cm with a TCR from ?2.2% to ?3.3%?K, both easily controlled by varying the oxygen partial pressure. Biased target ion beam deposited high TCR MoOx and NiOx thin films are polycrystalline semiconductors and have good stability in air. Compared with commonly used vanadium oxide thin films, MoOx or NiOx thin films offer improved process control for resistive temperature sensors. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE).
    No preview · Article · Mar 2015 · Optical Engineering
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    ABSTRACT: We present the optical function spectra of Cu 2SnSe3 determined from 0.30 to 6.45 eV by spectroscopic ellipsometry (SE) at room temperature. We analyze the SE data using the Tauc-Lorentz model and obtain the direct-bandgap energy of 0.49 ± 0.02 eV, which is much smaller than the previously known value of 0.84 eV for the monoclinic-phase Cu 2SnSe3. We also perform density-functional theory calculations to obtain the complex dielectric function data, and the results show good agreement with the experimental spectrum. Finally, we discuss the electronic origin of the main optical structures.
    No preview · Article · Jan 2015 · Applied Physics Letters
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    ABSTRACT: Assessment of the performance of single-junction hydrogenated amorphous silicon (a-Si:H) p-i-n configuration solar cells has been developed with a combination of real-time spectroscopic ellipsometry (RTSE) and current-voltage (I-V) measurements. For each layer, RTSE measurements enabled the determination of thickness and optical properties in the form of the complex dielectric function (ε = ε1 + iε2) spectra. RTSE tracked changes in a as a function of depth and was used to extract profiles in the i-layer bandgap and crystallite fraction in the n-layer. Through mapping I-V characteristic measurements, spatial variations in device performance were determined. By comparing individual devices at the location of the RTSE beam spot, the influence of a and thickness for each layer on device performance was identified through simulations of quantum efficiency yielding the shortcircuit current. This study compares two devices prepared with different superstrate preheating processes and finds that the combination of RTSE and I-V measurements along with quantum efficiency simulations were able to identify plasma damage to the transparent conducting oxide as the likely cause for variation in device performance. This comparison serves as one example of how the optically obtained information, such as thickness and a for each layer, can be used to understand the final device performance.
    No preview · Article · Jan 2015 · IEEE Journal of Photovoltaics
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    ABSTRACT: Ellipsometric spectra have been analyzed using a procedure whereby the complex dielectric function (epsilon = epsilon(1) + i epsilon(2)) is parameterized using different, physically realistic models in isolated regions of a single spectrum while layer thicknesses are kept common. The isolated spectral ranges correspond to regions where the material of interest is either non-absorbing or heavily absorbing and physically realistic models describing epsilon are available. Common structural parameters (thicknesses) obtained by this divided spectral range approach are used to extract epsilon by numerical inversion over the full spectral range. The divided spectral range analysis has been applied to study amorphous hydrogenated silicon, nanocrystalline zinc oxide, and epitaxial bismuth stannate thin films, as well as single crystal bismuth germanate. Layer thicknesses obtained from the divided range analysis are compared to those using continuous, but not necessarily physically supported, parameterizations of epsilon over the full spectral range. The divided spectral range analysis yields inverted spectra in epsilon which are free of discontinuities beyond noise present in experimental data and whose determination does not require any assumption to be made about the line shape of epsilon in weakly absorbing regions.
    No preview · Article · Nov 2014 · Thin Solid Films
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    ABSTRACT: Real time spectroscopic ellipsometry (RTSE) from the near-infrared to ultraviolet has been applied for analysis of the deposition of polycrystalline thin films that form the basis of two key photovoltaic heterojunction configurations, superstrate SnO2/CdS/CdTe and substrate Mo/Cu(In1-xGax)Se-2/CdS. The focus of this work is to develop capabilities for monitoring and controlling the key steps in the fabrication of these device structures. Analysis of RTSE data collected during sputter deposition of CdS on a rough SnO2 transparent top contact provides the time evolution of the CdS effective thickness, or film volume per unit substrate area. This thickness includes interface, bulk, and surface roughness layer components and affects the CdS/CdTe heterojunction performance and the quantum efficiency of the solar cell in the blue region of the solar spectrum. Similarly, analysis of RTSE data collected during co-evaporation of Cu(In1-xGax)Se-2 (CIGS; x similar to 0.3) on a rough Mo back contact provides the evolution of a second phase of Cu2-xSe within the CIGS layer. During the last stage of CIGS deposition, the In, Ga, and Se co-evaporants convert this Cu2-xSe phase to CIGS, and RTSE identifies the endpoint, specifically the time at which complete conversion occurs and single-phase, large-grain CIGS is obtained in this key stage. Published by Elsevier B.V.
    Full-text · Article · Nov 2014 · Thin Solid Films
  • Hamna F. Haneef · Nikolas J. Podraza
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    ABSTRACT: Optical properties in the form of the complex dielectric function spectra (epsilon = epsilon(1) + epsilon(2)) of commercially produced single crystal bismuth germanate (Bi4Ge3O12) have been studied using spectroscopic ellipsometry from 0.033 to 6.478 eV. Transmission measurements have also been performed in order to more accurately obtain low values of the absorption coefficient close to the band edge. Critical point parameters have been determined by analyzing the interband transitions. Infrared extended measurements yield vibrational modes corresponding to chemical bonding environments and the lattice structure. The overall analysis yields e from 0.033 to 6.478 eV and provides information about the crystal such as the energy of the band gap at 4.16 +/- 0.01 eV; interband transitions at 4.49, 4.75, 4.81, 5.08, 5.59, and 6.08 eV; transverse optical phonon modes at 283, 364, 395, 447, 702, 729, and 778 cm(-1) and longitudinal optical phonon modes at 292, 386, 445, 458, 710, 779, and 817 cm(-1). (C) 2014 AIP Publishing LLC.
    No preview · Article · Oct 2014 · Journal of Applied Physics
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    ABSTRACT: Four applications of real-time spectroscopic ellipsometry (RTSE) and ex-situ mapping spectroscopic ellipsometry (SE) in thin-film hydrogenated silicon (Si:H) photovoltaics (PV) technology are reviewed with the common theme being the development and application of SE-derived growth evolution diagrams. The goals of these applications are to understand and consequently further advance this technology. In the first application, fabrication of engineered thin films consisting of periodic arrays of silicon (Si) nanocrystallites in an amorphous Si:H (a-Si:H) host matrix has been guided by a growth evolution diagram developed by RTSE for radio-frequency plasma-enhanced chemical vapor deposition (PECVD) using SiH4+H-2 mixtures. Such precisely controlled microstructures are of interest as possible intrinsic-layer components of p-i-n and n-i-p thin-film PV devices, and RTSE is shown to be a key technique for guidance in fabrication and for structure verification. In the second application of growth evolution diagrams, very-high-frequency PECVD intrinsic a-Si:H, hydrogenated amorphous silicon-germanium alloys (a-Si(1-x)Gex:H), and hydrogenated nanocrystalline silicon (nc-Si:H) have been investigated for use as the top, middle, and bottom-cell i-layer components, respectively, of triple-junction n-i-p solar cells. The growth evolution diagram for the bottom-cell i-layer, starting from an underlying mixed-phase amorphous + nanocrystalline silicon [(a + nc)-Si:H] n-layer, reveals a bifurcation at a critical H-2-dilution flow ratio R (R=[H-2]/[Si2H6], in this application) between mixed-to-amorphous phase evolution [(a+nc)-> a] at low R and mixed-to-nanocrystalline phase evolution [(a+nc)-> nc] at high R. The highest performance single-step nc-Si:H solar cell is found at minimal R while remaining on the nanocrystalline side of the identified bifurcation where suitable grain boundary passivation can be assured. Because of the importance of the roll-to-roll flexible substrate configuration in such multi-junction Si:H-based PV technology, RTSE has been demonstrated in a third application for monitoring PECVD of a-Si:H n-i-p solar cell structures on back-reflector-coated flexible roll-to-roll polymer substrates. RTSE has been used for probing along the center line of the moving substrate during deposition, and ex-situ mapping SE has been used over the full substrate area after deposition. Detailed studies of the top-most p-layer of the n-i-p solar cell have been performed, with the goal being to develop spatially-dependent (in contrast to R-dependent) growth evolution diagrams in order to evaluate uniformity across the width of the substrate and thus to enable optimization of the resulting a-Si:H PV modules. In this study, efficiency optimization occurs at the p-layer transition region in which a-Si:H nucleates from the i-layer surface, but evolves to predominantly nc-Si:H for improved contact to the top-most In2O3:Sn layer. In the fourth and final application reviewed here, the mapping-SE-deduced properties of the Si:H i and p-layers have been spatially correlated with device performance parameters from an array of n-i-p a-Si:H-based dot cells over a 13 x 13 cm(2) substrate area. Analysis of the SE data acquired over the full area provides property 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 structure. The mapped values adjacent to the PV devices have been correlated with the device performance parameters. When sufficient non-uniformity exists, these correlations enable optimization based on specific ranges of values that characterize the fundamental properties of the material and the film structure.
    No preview · Article · Oct 2014 · Solar Energy Materials and Solar Cells
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    ABSTRACT: Vanadium oxide (VOx) and hydrogenated silicon germanium (SixGe1-x) are the two predominant thin film material systems used as the active layer in resistive infrared imaging. Thin films of VOx used in microbolometers have a resistivity typically between 0.1 and 1 Omega-cm with a temperature coefficient of resistance, TCR vertical bar between 1.4%/K to 2.4%/K, while SixGe1-x: H thin films have a resistivity between 200-4,000 Omega-cm with a vertical bar TCR vertical bar between 2.9%/K to 3.9%/K. Future devices may require higher TCR materials, however, higher TCR is loosely associated with higher resistivity and therefore also with high noise. This work compares 1/f noise of high resistivity VOx and Ge:H thin films having vertical bar TCR vertical bar > 3.6%/K. The high TCR thin films of VOx were found to be amorphous while, depending on the deposition conditions, the Ge:H thin films were either amorphous or mixed phase of amorphous + nanocrystalline. Evaluation of these VOx and Ge: H thin films indicates a prospects for a superior process-property relation of 1/f noise in Ge: H thin films in comparison with thin films of VOx
    No preview · Conference Paper · Jun 2014
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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.
    Full-text · Article · Jun 2014 · Applied Physics Letters
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    ABSTRACT: Mapping spectroscopic ellipsometry (M-SE) has been applied for optimization of polycrystalline CdS/CdTe solar cell fabrication on transparent conducting oxide (TCO) coated glass superstrates. During fabrication of these solar cells, the structure undergoes key processing steps after the sputter-deposition of the CdS/CdTe. These steps include CdCl2 treatment of the CdTe layer and subsequent deposition of ultrathin Cu. Additional steps involve final metal back contact layer deposition and an anneal for Cu diffusion that completes the device. In this study, we have fabricated cells with variable absorber thickness, ranging from 0.5 to 2.5 μm, and variable CdCl2 treatment time, ranging from 5 to 30 min. Because both CdS window and Cu back contact layers are critical for determining device performance, an understanding of their deposition processes and process-property-performance relationships is important for device optimization. We have applied M-SE to map the effective thickness (volume/area) of the CdS and Cu films over 15 cm × 15 cm substrates prior to the fabrication of 16 × 16 arrays of dot cells. The model for M-SE analysis has been established using single-spot real time SE (RT-SE). We report correlations of cell performance parameters with the CdCl2 treatment time and with the effective thicknesses from M-SE analysis. We demonstrate that correlations between optical/structural parameters extracted from M-SE analysis and device performance parameters facilitate process optimization.
    No preview · Conference Paper · Jun 2014
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    ABSTRACT: Wide band-gap, p-type doped, hydrogenated amorphous silicon-carbon alloy (a-Si1-xCx:H:B) layers deposited by plasma enhanced chemical vapor deposition (PECVD) under conditions that yield efficient hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells have been applied as back contacts to sputter-deposited CdTe superstrate solar cells. We report a maximum observed Voc of 0.78 V and a best initial efficiency of ∼ 7.7 % (relative to an ∼ 12% standard cell baseline) without the introduction of Cu into the back contact region. We studied the stability of the best performing cells over a two year time period and found that although Voc is relatively stable, the series resistance of the cells increased significantly leading to fill-factor degradation. The role of hydrogen loss from the back contact layer via diffusion into the CdTe absorber is explored as a possible cause of this degradation. In related investigations, we have found that the effect of in-diffusing H on the CdTe solar cell performance is detrimental, as observed from a brief (∼ 15 s) exposure of CdTe to a low power H2 plasma between the treatment by CdCl2 and the application of standard Cu/Au back contacts. This detrimental effect of H in the fabrication of the back contact layer from hydride gases, and the subsequent instability of the back contact itself, were found to be significant challenges encountered in this investigation.
    No preview · Conference Paper · Jun 2014
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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.
    Full-text · Article · Jan 2014 · IEEE Journal of Photovoltaics
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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.
    Full-text · Article · Jan 2014 · IEEE Journal of Photovoltaics
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    ABSTRACT: Electrical properties for resistive microbolometer sensor materials including resistivity, temperature coefficient of resistance (TCR), and normalized Hooge parameter were explored in n-type a-Si:H and a-Si 1−xCx:H prepared by plasma enhanced chemical vapor deposition. The complex dielectric function spectra (ε = ε 1 + iε 2) and structure were measured by spectroscopic ellipsometry. Two-dimensional drift-diffusion simulations were used to understand the band-tail slope dependency of TCR and 1/f noise.
    No preview · Article · Nov 2013 · Journal of Applied Physics
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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.
    No preview · Article · Oct 2013 · APL Materials
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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.
    No preview · Article · Jul 2013 · Journal of Applied Physics

Publication Stats

964 Citations
193.51 Total Impact Points

Institutions

  • 2006-2015
    • University of Toledo
      • • Department of Physics and Astronomy
      • • Center for Photovoltaics Innovation and Commercialization
      Toledo, Ohio, United States
  • 2011
    • William Penn University
      Worcester, Massachusetts, United States
  • 2004-2011
    • Pennsylvania State University
      • • Department of Electrical Engineering
      • • Department of Materials Science and Engineering
      • • Materials Research Institute
      • • Department of Physics
      University Park, MD, United States