Judges’ Queries and Presenter’s Replies

  • Icon for: Jon Kellar

    Jon Kellar

    Judge
    May 20, 2013 | 12:47 p.m.

    What is the ultimate resolution of KRIPES?

  • Icon for: Haofei Wei

    Haofei Wei

    Presenter
    May 20, 2013 | 01:54 p.m.

    Hi Jon,

    Thanks for the question. Initially, we plan to have a resolution of 400 meV using CaF2 windows and acetone as the ionization gas, which is a fairly standard design. This will be able to measure features such as the bandgap of photovoltaic materials, which is in the 1-2 eV range, as well as any strongly dispersive features in the energy spectrum.

    One of the first improvements we plan to make to the chamber is to fit the detectors with an additional Sr0.7Ca0.3F2 window, which has a slightly lower pass energy than CaF2 and can increase our energy resolution to 85 meV, as is described in the paper by Maniraj et al in Rev. Sci. Inst. 82, 093901 (2011). The main obstacle to installing these windows immediately is the lack of a supplier for the window material, an issue which we are currently looking into. We will actually build the detectors will a mount for the new windows in the first generation design, so that as soon as we find a source for the Sr0.7Ca0.3F2 windows we can install them without having to fabricate completely new detectors.

    By having the Sr0.7Ca0.3F2 windows in front of the CaF2 windows and movable via a UVH wobble stick, we can select between high and low resolution modes to optimize for either counts or energy resolution, depending on the feature we wish to study. This can all be done in situ, so that with one sample we can make both kinds of measurements while preserving its surface quality by keeping it inside the chamber.

    With Geiger-Muller detectors, which rely on matching the bandpass energy of a window with the ionization energy of a gas for the energy resolution, it’s difficult to achieve the kinds of resolution currently reachable in ARPES (~1-2 meV). In addition, the significantly lower count rates in KRIPES compared to ARPES (smaller than ARPES by factor of 10^5 due to interaction cross sections) means that a relatively low energy resolution compared to ARPES is necessary to reach high enough counts in order to eliminate statistical Poisson-type counting noise as a significant source of systematic error.

    I hope this answered your question without being overly verbose.

    Best,
    Haofei

  • Icon for: Marc Porter

    Marc Porter

    Judge
    May 20, 2013 | 02:33 p.m.

    Why use FeO2- as the p-type layer?

  • Icon for: Haofei Wei

    Haofei Wei

    Presenter
    May 20, 2013 | 08:59 p.m.

    Hi Marc,

    The LaFeO3 structure itself is just a photovoltaic material with a bandgap larger than that of LaVO3 to more efficiently absorb higher-energy photons in a heterojunction design that has become fairly standard in more recent PV cells (see, for example, those made by Solar Junction, Inc). The p-type layer actually refers to the metallic layer which exists at the surface of the material whose origins I felt best to leave out of the poster to keep it from becoming too wordy, but a description of which I will provide below.

    This metallic layer, along with the n-type conducting layer which similarly exists at the interface between our heterostructure and the substrate, serve as natural electrodes to carry away both positive and negative photocurrents. There are existing technologies to make electrical contacts with conducting layers buried under insulating materials in GaAs heterostructures as well as oxides, so contacting these interface and surface layers will not be a problem. In addition, the surface metallic layer is expected to be extremely thin (the interface layer in LaAlO3/SrTiO3 is <4 nm), and so should not absorb incoming photons and reduce efficiency in a significant way, though that, among many other things, remains to be experimentally tested.

    In the figure, the p-type and n-type conducting layers actually refer to the topmost layer of the structure and the interface layer between the LaVO3/LaFeO3 heterostructure and the SrTiO3 substrate, respectively. These conducting layers are the result of an electronic reconstruction to avoid the polar catastrophe in the LaVO3/LaFeO3 heterostructure.

    For anyone reading this who may not be familiar with the polar catastrophe phenomenon, here’s a figure which illustrates the concept quite well: http://www.nature.com/nmat/journal/v5/n3/fig_ta.... In the structure presented in the poster, the nominal valence of the metal oxide VO2/FeO2 (found by setting the valence of the oxygen to – 2 and the valence of the rare earth La to +3) is – 1, and the nominal valence of LaO is +1, so that there is an inherent electric field throughout the whole structure whose direction is indicated by the vertical arrows in the schematic diagram. This is why LaVO3 and LaFeO3 are both called polar materials. To avoid an extremely large potential difference created by the electric fields between the layers, all pointing in the same direction (the so-called polar catastrophe), electrons move from the surface of the material into the interface so that the potential throughout the structure oscillates around zero instead of constantly increasing or constantly decreasing. The result then, is an excess of electrons at the interface between our heterostructure and the non-polar SrTiO3 substrate and an excess of holes at the surface of our structure, resulting in a p-type conducting layer at the surface and an n-type conducting layer at the interface. This phenomenon is well-known in traditional semiconductors, and has been well documented at the LaAlO3/SrTiO3 interface in oxides, where LaAlO3 is another polar material and SrTiO3 is the same non-polar substrate we will be using in our proposed structure. In the LaVO3/LaFeO3 photovoltaic cell design, the p- and n-type metallic layers will serve as electrical contacts, and the natural electric field in the polar materials will separate the photoexcited carriers and reduce electron-hole recombination, which is a major source of efficiency loss in photovoltaic cells.

    I hope this answers your question.

    Best,
    Haofei

  • May 21, 2013 | 11:21 a.m.

    What other techniques will be or can be used to examine the properties of the complex oxides you are studying?

  • Icon for: Haofei Wei

    Haofei Wei

    Presenter
    May 21, 2013 | 04:21 p.m.

    Hi Adriane,

    Photoemission spectroscopy will be the primary probe we use, as that is the expertise of our group, but there will be a variety of other measurements we will make to characterize the samples.

    As an example, for the solar cell heterostructure, atomic force microscopy will be used to ensure that the samples we’ve grown are indeed epitaxial and being grown layer-by-layer, as is necessary to create the PV cell structure. Electronic transport and Hall effect measurements will be used to test for the existence of the conducting interface and surface layers which function as the electrodes in the proposed structure. Optical absorption spectroscopy will be used to characterize the absorption coefficient of the PV cell, as well as provide another measure of the electronic band gap in the material. X-ray diffraction will be used to measure the lattice constants of the heterostructure and x-ray photoelectron spectroscopy will be used to measure the stoichiometry and valence of the samples, and both are important tests to ensure that the samples we have grown are actually perovskite LaVO3 or LaFeO3 and not some other competing phase.

    For the materials we are interested in, their electronic structure is the feature we would like to study, and so our measurements are focused on either quantifying the sample quality, in the case of the x-ray and AFM measurements, or studying their electronic properties, such as with photoemission, optical spectroscopy, or electrical transport measurements. Other complex oxides exhibit a variety of other interesting magnetic and structural properties, but our group does not have the equipment or expertise to study those in depth, but can collaborate with other groups which specialize in magnetic or structural measurements if our samples turn out to have other interesting properties besides electronic ones.

    Best,
    Haofei

  • May 21, 2013 | 07:56 p.m.

    Will the proposed solar cell (layers of LaVO3/LaFeO3) be more robust that CdTe, e.g. last longer in use?

  • Icon for: Haofei Wei

    Haofei Wei

    Presenter
    May 21, 2013 | 10:15 p.m.

    Hi Peter,

    That’s a great question, but to be honest I can only speculate on the answer, as the study of complex oxides to date has mainly focused on their intrinsic properties and less on their robustness in applications.

    The proposed solar cell structure has the advantage of not requiring any metallic contacts to be part of the structure due to the conducting interface and surface layers, reducing its complexity. These interfacial layers and surface layers are sensitive to oxygen vacancies in the crystal, however, and so may be a weak point in the structure and make it less robust than CdTe-based technology. The actual stability of similar oxides in actual use, however, is something that I don’t know of any studies having looked into. SrTiO3 is quite a robust and stable substrate, but the stability of LaVO3 and LaFeO3, especially in the thin film form which they have in the proposed solar cell structure, is not well understood.

    Compared to other semiconductors such as Si, Ge, GaAs, etc, complex oxides are still an extremely new field of materials research. As a result, the majority of research on them has been focused on understanding their properties and behavior and being able to reliably predict and control these properties for applications. If the theoretical model for the solar cell were shown to be correct by our experiment, it would actually be an important step in demonstrating our understanding of these materials which, by nature of their strong electronic correlations, are quite difficult to theoretically model.

    Were this experiment to demonstrate the viability of such an oxide-based solar cell design, it could spur further research into how to design and manufacture complex oxide based technologies to be robust and long-lasting for a variety of applications, and hopefully provide a more quantitative and definitive answer to your question.

    Best,
    Haofei

  • Icon for: Antal Jakli

    Antal Jakli

    Judge
    May 22, 2013 | 09:32 p.m.

    What is the expected efficiency of the complex oxides solar cells?

  • Icon for: Haofei Wei

    Haofei Wei

    Presenter
    May 22, 2013 | 10:26 p.m.

    Hi Antal,

    There are no published values for the expected efficiency of complex oxide solar cells, and I myself do not know enough about their physics to be able to comment with any certainty, so I’m afraid that I can’t provide a quantitative answer to that question.

    The absorption coefficient of the complex oxide solar cells is shown below the schematic of the solar cell structure, and it’s closely related to the efficiency. The coefficient alpha in the figure is the coefficient in Beer’s law, I~e^(alpha*r), where I is the light intensity and r is the penetration distance into the material. Alpha essentially measures how far light is able to penetrate into the material before being absorbed, and as you can see from the figure the complex oxide heterostructure absorption coefficient, shown in blue, compares quite favorably to that of CdTe, shown in green, in the region of the spectrum where solar radiation is the strongest. I don’t know enough about solar cell design to comment on how to convert from the absorption coefficient to an expected efficiency, but the fact that the absorption coefficient is so high means that the electronic structure of the material is quite favorable to its use as a photovoltaic material, as the absorption coefficient is dependent strongly on the structure of the bandgap.

    There are various other factors affection absorption, such as the rate of electron-hole recombination in the material, and after fabricating the junction we can experimentally determine the efficiency of such a solar cell design.

    I hope this in some way answers at least part of your question.

    Best,
    Haofei

  • Further posting is closed as the competition has ended.

Presentation Discussion

  • May 23, 2013 | 09:02 p.m.

    Thanks for the nice video, the scientific concepts were clearly presented and explained.
    Small question, is there a distinction between K-resolved and Angle-resolved (ARIPES)?

  • Icon for: Haofei Wei

    Haofei Wei

    Presenter
    May 23, 2013 | 09:10 p.m.

    Hi Aleksandra,

    I’m glad you found it interesting! There’s actually no difference between the two beyond the name. K is the standard letter for momentum, and angle resolution gives us momentum resolution, so it’s just a matter of which term you prefer and which acronym rolls of your tongue easier.

    Best,
    Haofei

  • Further posting is closed as the competition has ended.

Icon for: Haofei Wei

HAOFEI WEI

Cornell University
Years in Grad School: 2

Designer Materials for Next Generation Electronics: Studying Complex Oxides using Inverse Photoemission Spectroscopy

Understanding the properties of complex oxides and their physical origins is key to utilizing their unique properties in technical applications. These properties include high-temperature superconductivity, metal-insulator transitions, and many more that make them ideal candidates for the generation of electronics. In this project, my collaborator and I design and have begun construction on an momentum (K)-resolved inverse photoemission spectroscopy system (KRIPES) which will be used to study to behavior of electrons in these fascinating materials. The KRIPES system will specifically study the structure of unfilled electronic bands and thus the nature of electronic excitations in complex oxides, of which there is a significant gap in existing experimental data. We are in a collaboration with the Schlom group at Cornell, which specializes in the growth of high-quality, atomically-precise complex oxide films. Through growing and characterizing a variety of materials with different properties, we hope to gain a fuller understanding of the physics that leads to the many useful properties in complex oxides and be able to control them.