Symposium FJ
Development and Application of New Functional Transparent Conducting and Semiconducting Inorganic Materials

Session FJ-1 - Fundamentals

FJ-1:IL01  Interfaces and Defects in Semiconducting Oxides
A. KLEIN, Technische Universität Darmstadt, Institute of Materials Science, Darmstadt, Germany

The electrical and functional properties of semiconducting oxides are strongly influenced by defect concentrations, which can reach values of several percent. Due to the contribution of charged defects to the space charge layer formation, defects can also influence barrier formation at interfaces of semiconducting oxides. Defects might (i) be present before interface formation, (ii) be induced during interface formation, and (iii) migrate to the interface after completion of interface formation if they are sufficiently mobile. Apart from oxygen vacancies, also dopants, impurities and multivalent cations can modify barrier formation. The presentation gives an overview how defects affect interface formation of semiconducting oxides and how this can be studied using photoelectron spectroscopy with in-situ sample preparation. The effects are illustrated using transparent conductors, dielectrics and ferroelectrics as examples.


FJ-1:IL02  Excitonic Effects and Dielectric Screening in Transparent Conducting Oxides
A. SCHLEIFE, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA

High-performance computing enables quantum-mechanical studies of material properties with unprecedented accuracy: Many-body perturbation theory is capable of predicting electronic and optical properties in excellent agreement with experiment. More accurate approximations become feasible and will allow for computational materials design. In this talk I will provide insight into how the GW+BSE approach can be used to study the impact of dielectric screening contributions due to free carriers and lattice polarizability on optical and excitonic properties of oxide semiconductors. These materials have exciting optoelectronic and photovoltaic applications, which justifies the large interest in their optical properties. It will be quantified how screening due to free carriers and lattice polarizability reduces excitonic effects, tremendously changing the shape of the optical absorption spectrum and reducing exciton binding. For indium oxide, I will show that contributions due to lattice polarizability are important to obtain agreement with experiment. These contributions strongly modify the shape of the absorption onset. Applying these techniques to semiconductor nanocrystals, allowed us to apply computational spectroscopy, to optically distinguish semiconductor nanocrystal polymorphs.


FJ-1:IL03  A Non-oxide p-type Transparent Semiconductor CuI
NAOOMI YAMADA, Department of Applied Chemistry, Chubu University, Kasugai, Japan

CuI is a p-type semiconductor with a bandgap of 3.1 eV. Recently, study of CuI as a transparent p-type semiconductor has been initiated.[1] A single crystal CuI was reported to show a hole mobility of 44 cm2 V-1 s-1,[2] which is much higher than those in p-type oxides. Besides, p-CuI/n-ZnO was reported to behave as transparent p-n junction diodes with high rectification ratios.[3] Thus, CuI is a promising transparent p-type semiconductor. For the fabrication of polycrystalline films, we developed a new simple method, in which CuI polycrystalline films is synthesized by the chemical reaction between Cu3N thin films and solid-phase iodine.[4] This method does not need high temperature, so that it can be applied to the fabrication of CuI films on plastic substrates. In this presentation, we show electrical properties of CuI films on plastic sheets. Furthermore, we report the fabrication of transparent p-n junction diodes composed of p-type CuI and n-type amorphous In-Ga-Zn-O on flexible plastic sheets.[5]
1) Grundmann et al., Phys. Status Solidi A 210, 1671 (2013). 2) Chen et al., Cryst. Growth Des. 10, 2057 (2010). 3) Yang et al., Sci. Rep. 6, 21937 (2016). 4) Yamada et al., Chem. Mater. 28, 4971 (2016). 5) Yamada et al., Adv. Electron. Mater. (2017), doi: 10.1002/aelm.201700298.


FJ-1:IL04  Atomic Layer Deposition of Transparent Conducting and Charge Transport Layers for Photovoltaic Applications
M. McCARTHY, L. RYAN, S. O'BRIEN, I.M. POVEY, Tyndall National Institute, University College Cork, Cork, Ireland

Organometallic halide perovskite solar cells have gained considerable interest in recent times. Despite the devices’ low cost, reported power conversion efficiencies have risen rapidly and now exceed 22 %. [1] However, in addition to stability issues, scale up and improving efficiency through tandem structures are key aspects for commercial development. Here we examine atomic layer deposition, an industry compatible large area low temperature growth technique, to generate the charge transport and transparent conducting layers. For electron transport layers the most commonly used material is TiO2. Here we examine both nominally undoped and doped TiO2, and in addition SnO2 a possible replacement material. Regarding hole transport layers V2O5, NiOx and MoOx are considered. For the transport conducting material ZnO and its doped variants are assessed as replacements for high temperature FTO and ITO. With all these studies the materials are assessed in terms of their physical properties and their performance in perovskite solar cell structures.
[1] https://www.nrel.gov/pv/assets/images/efficiency-chart.png


FJ-1:IL06  Thio- and Seleno-cyanates: Theory and Applications for an Emerging Class of Multi-functional Materials
L. TSETSERIS, Department of Physics, National Technical University of Athens, Zografou Campus, Athens, Greece 

Thio- and seleno-cyanate groups (SCN and SeCN), are well-known ligands in a plethora of coordinated metal complexes [1] that are used in diverse applications, such as molecular magnetism, solid-state dye-sensitized photovoltaics and solid-state electrolytes. In recent important developments, [2] perovskite-based solar cells with a prominent thio-cyanate material, CuSCN, as the hole transport layer were shown to have photo-conversion efficiency as high as 17.5%-20%. Here, we review the advantages that thio- and seleno-cyanates can bring to various applications, using first-principles calculations to link them to atomic-scale details of their structure. Besides CuSCN and CuSeCN as p-type transport materials, we highlight the potential SCN and SeCN groups hold as building blocks of two-dimensional materials, [3] for example, novel variants of MoS2-like systems with versatile electronic and magnetic properties.
[1] J. L. Burmeister, Coord. Chem. Rev. 105, 77 (1990); X. Y. Wang et al., Chem. Commun., 281 (2008). [2] N. Wijeyasinghe et al., Adv. Funct. Mater. 27, 1701818 (2017); N. Arora et al., Science, doi:10.1126/science.aam5655. [3] L. Tsetseris, Phys. Chem. Chem. Phys. 18, 7837 (2016); D. Kaltsas and L. Tsetseris, J. Phys. Condens. Matter 29, 085702 (2017).


FJ-1:IL08  Thermal Transport in Transparent Conductive Oxide Films
NOBUTO OKA¹, Y. SHIGESATO², ¹Kindai University, Iizuka, Fukuoka, Japan; ²Aoyama Gakuin University, Sagamihara, Kanagawa, Japan 

A thermal design for various optoelectronic devices driven by current, such as organic light emitting diodes, is quite significant because the heat generating in the device can damage itself. For this purpose, highly accurate thermophysical properties for component layers are required, including transparent conductive electrodes. We have reported the thermal conductivity analysis for various transparent conductive oxide (TCO) films, i.e. Sn-doped In2O3 (ITO), amorphous indium zinc oxide (IZO), Al-doped ZnO (AZO), Antimony-doped SnO2 (ATO), Ta-doped SnO2 (TTO), and Nb-doped TiO2 (NTO) films with a thickness of 200, 300 nm [1,2]. Thermal conductivity of various TCO films has been measured using pulsed light heating thermoreflectance methods. Concerning ITO films deposited by dc magnetron sputtering in this study, the electrical conductivity was (0.3–3.4)×10^5 S m^-1 [1]. The thermal conductivity was 4.0–6.0 W m^-1 K^-1, proportional to the electrical conductivity. Thermophysical properties of all the TCO films can be discussed based on the heat transport by electrons (Wiedemann-Franz law) and phonons.
1) T. Ashida, N. Oka, Y. Shigesato, et al.: J. Appl. Phys. 105 (2009) 073709. 2) N. Oka, Y. Shigesato, et al.: J. Mater. Res. 29 (2014) 1579.


FJ-1:IL09  First-principles Modeling of Complex Oxide Interfaces
C.G. VAN DE WALLE, Materials Department, University of California, Santa Barbara, CA, USA; A. JANOTTI, University of Delaware, Newark, DE, USA

Perovskite oxides have received significant attention in recent years, in part due to their ability to generate very high density two-dimensional electron gases (2DEGs) at interfaces between polar and nonpolar materials [1]. Most of these oxides have degenerate conduction bands composed of transition-metal d states, leading to large effective masses and low mobilities, a detriment for applications. BaSnO3 has emerged as an alternative: it crystallizes in the perovskite structure but its conduction band is nondegenerate and composed of Sn s states, resulting in high mobility, favorable for a transparent conductor. I will show how cutting-edge first-principles calculations shed light on the multiple aspects of this problem: band alignment and confinement of the 2DEG [1,2], mobility [3], and doping.
Work performed in collaboration with L. Bjaalie, B. Himmetoglu, A. Janotti, Y. Kang, K. Krishnaswamy, H. Peelaers, and L. Weston.
[1] L. Bjaalie, B. Himmetoglu, L. Weston, A. Janotti and C. G. Van de Walle, New J. Phys. 16, 025005 (2014). [2] K. Krishnaswamy, L. Bjaalie, B. Himmetoglu, A. Janotti, L. Gordon, and C. G. Van de Walle, Appl. Phys. Lett. 108, 083501 (2016). [3] K. Krishnaswamy, B. Himmetoglu, Y. Kang, A. Janotti, and C. G. Van de Walle, Phys. Rev. B 95, 205202 (2017).


FJ-1:IL10  Ab Initio Design of P-type Transparent Conductors: From Oxides to Oxide Chalcogenides
G. TRIMARCHI, Department of Chemistry, Northwestern University, Evanston, IL, USA

Transparency and p-type conductivity seldom emerge in the same material, making p-type transparent conductors (TCs) very rare. Here, we apply state-of-the-art band structure methods and defect theory to investigate select oxides and oxide chalcogenides as novel p-type TCs. From the family of known Ag and Cu oxides we predict that Ag3VO4 and KAg11(VO4)4 are p-type materials at the verge of transparency (transparent to the red light). Our calculations show that these Ag oxides have a hole concentration of ~10^14 cm-3 at room temperature, a low content of hole-killing defects (such as oxygen vacancies), and a lower hole effective mass than CuAlO2, the prototypical p-type TCO. The oxide sulfides, on the other hand, can potentially exhibit lower hole effective masses than the oxides owing to the strong hybridization between the S p and metal orbitals that can produce more dispersive bands at the valence maximum and lighter hole masses than in oxides. We propose La5Cu6O4S7, a compound structurally analogous to the prototypical p-type oxide sulfide LaCuOS, as an example of multi-anion intrinsic TC, i.e., not requiring doping, while, so far, only oxides have been proposed as intrinsic TCs.

 
Session FJ-2 - Material Design and Device Development

FJ-2:IL01  Amorphous Semiconductor Mobility Physics
J.F. WAGER, School of EECS, Oregon State University, Corvallis, OR, USA

The flat-panel display industry appears to have an insatiable desire for increasing semiconductor mobility. This is a prime reason why amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) are now replacing amorphous hydrogenated silicon (a-Si:H) TFTs in commercial flat-panel display backplanes. Thus, the objective of this presentation is to do a deep dive into the physics of amorphous semiconductor mobility. Understanding band tail states is key. Band tail states are localized electronic states existing near conduction and valence band edges. Band tail states arise as a consequence of disorder and exhibit an exponential distribution defined by a characteristic (Urbach) energy. Our first objective is to formulate an appropriate near-band-edge density-of-states model accounting for both delocalized (free carrier) and localized (band tail) electronic states. Next, we derive both real- and reciprocal-space properties of band tail states and compare their tendencies for two crystalline (c-Si and c-GaAs) and two amorphous (a-IGZO and a-Si) semiconductors. Finally, we simulate electron and hole transport trends for a-IGZO and a-Si TFTs in order to elucidate how band tail state properties are the prime determiner of mobility performance in an amorphous semiconductor.


FJ-2:IL02  High-throughput Development of Wide Bandgap Conductive Sulfides
A. ZAKUTAYEV, National Renewable Energy Laboratory, Golden, CO, USA

Transparent conducting and semiconducting inorganic materials play many important roles in emerging technologies, such as light-emitting diodes, photovoltaic solar cells, and transparent electronics. Historically, many of these applications have been dominated by oxide transparent (semi)conductors, with few notable exception of non-oxide materials. In particular in chalcogenide thin film photovotlaics based on Cu(In,Ga)Se2 and CdTe absorbers, n-type CdS contact layers have been historically used, and p-type ZnTe contact layers have more recently emerged. This presentation will discuss development of wide bandgap conductive sulfide materials, in the context of photovoltaic contact applications. High-throughput experimental material development methods will be illustrated on the examples of p-type BaCu2S2 compounds [1], as well as (Zn,Cu)S alloys. High-throughput device experiments will be exemplified by n-type In2S3 [2] and CdS [3] contact layers for Cu2ZnSnS4 and Cu(In,Ga)Se2 solar cells. In addition, recent advances in applying combinatorial approaches to Sb2Se3/ZnS interface band offset measurements will be reported [4].
[1] Chem. Mater. 29 8239 (2017). [2] ACS Appl. Mat. Int. 8 14004 (2016). [3] ACS Combi. Sci. 18 583 (2016). [4] Adv. Mater. Int., 3, 1600755 (2016).


FJ-2:L03  Low-dimensional Multi-layer Metal Oxide Semiconductors for Transistor Applications
T. ANTHOPOULOS, King Abdullah University of Science and Technology (KAUST), Division of Physical Sciences and Engineering, Thuwal, Saudi Arabia

Research on metal oxide semiconductors (MOx) is rapidly making progress towards the goal of providing viable alternatives to silicon technologies for various emerging applications. Many MOx possess important qualities that are absent from several other emerging semiconductor technologies. Currently significant effort has been directed towards the development of novel compounds that combine superior charge transport with processing versatility and high operating stability. In this presentation we will discuss an alternative strategy to materials development based on low-dimensional, solution-processed multilayer MOxs and their application in TFTs with performance characteristics beyond the state-of-the-art. In particular, we will describe how ultra-thin layers (<10 nm-thick) of various MOx can be grown from solution at temperatures ranging from room temperature up to 200 C using a combination of conventional thermal annealing and advanced photonic processing techniques. Emphasis will be placed on the use of photonic-based processing for the rapid (sub-second) chemical conversion of various precursor systems to high quality semiconducting layers and their implementation in high electron mobility (36 cm2/Vs), low-operating voltage (<2 V) multilayer MOx channel transistors.


FJ-2:IL07  Alternative Transparent Conductors for Flexible CIGS Thin Film Solar Cells
Y.E. ROMANYUK, L. GREUTER, T. FEURER, R. CARRON, S. NISHIWAKI, S. BUECHELER, A.N. TIWARI, Empa - Swiss Federal Laborotaries for Materials Science and Technlology, Dübendorf, Switzerland

Thin film solar cells based on Cu(In,Ga)Se2 (CIGS) semiconductor feature conversion efficiency of 22.6% on rigid substrates, and up to 20.4% efficiency on flexible polymer foils. The classical thin film structure Substrate Mo/CIGS/CdS/i-ZnOAl:ZnO/Ni-Al grid/MgF2 is often employed in the highest efficiency devices, however, many alternatives, especially for the CdS buffer and ZnO-based transparent conducting layer, are being investigated in an attempt to reach a comparable or even higher performance. The talk will review several alternatives for the buffer and the transparent contact for flexible CIGS solar cells including material and fabrication aspects, and their performance will be compared to the reference devices.


FJ-2:IL08  Flexible, Transparent and Conductive Ag Nanowire Networks
D. BELLET¹, T. SANNICOLO^1,2, S. AGHAZADEHCHORS^1,3, T. PAPANASTASIOU¹, H. VIET-NGUYEN^1,4, D. MUÑOZ-ROJAS¹, C. JIMÉNEZ¹, N.D. NGUYEN³, ¹Univ. Grenoble Alpes, LMGP, CNRS, Grenoble, France; ²Univ. Grenoble Alpes, CEA LITEN, Grenoble, France; ³Univ. of Liège, Département de Physique, Liège, Belgium; ⁴CEA-INES, LITEN, Le Bourget-du-Lac, France

There has been lately a growing interest into flexible, efficient and low cost transparent electrodes which can be integrated for many applications, including several related to energy technologies (photovoltaics, lighting, electro-chromism…) or displays (touch screens, transparent heaters…). Metallic nanowires deposited by solution methods can form random percolating networks which constitute efficient transparent electrodes [1]. Such networks exhibit very good electro-optical properties and appear as alternative to the reference material, i.e., indium tin oxide (ITO) which is often used. However the scarcity of indium and the lack of ITO flexibility have prompted the search for alternative materials. The main properties of metallic nanowire networks as well as the influence of post treatments, network density, nanowire size and their stability [3] will be discussed.
1. T. Sannicolo, M. Lagrange, A. Cabos, C. Celle, J.-P. Simonato and D. Bellet, Small 12, 6052 (2016). 2. S. Sorel, D. Bellet, J.N. Coleman, ACS Nano 8, 4805 (2014). 3. M. Lagrange, T. Sannicolo, D. Muñoz-Rojas, B. Guillo Lohan, A. Khan, M. Anikin, C. Jimenez, F. Bruckert, Y. Bréchet and D. Bellet, Nanotechnology 28, 055709 (2017).

 
FJ-2:IL09  Computational Approach to Synthesis of Functional Polymorphs
D. GINLEY¹, K. PERSSON², L. GARTEN¹, P. SELVARASU¹, J. PERKINS¹, WENHAO SUN², K. POPOV², S. DWARAKNATH², G. CEDER², J. MANGUM³, B. GORMAN³, L. SCHELHAS⁴, M. TONEY⁴, M. AYKOL², Z. CHAN⁵, D. NOCERA⁵, J. HAGGERTY⁶, O. AGIRSEVEN⁶, J. TATE⁶, D. KITCHAEV⁷, W. TUMAS¹, ¹National Renewable Energy Laboratory, Golden, CO, USA; ²Lawrence Berkeley National Laboratory, USA; ³Colorado School of Mines, USA; ⁴SLAC National Accelerator Laboratory, USA; ⁵Harvard University, USA; ⁶Oregon State University; ⁷Massachusetts Institute of Technology, USA

Computational materials design has become proficient at the identification of new structural polymorphs and identifying their functionality. A key challenge is the actual synthetic realization of these polymorphs. Transition metal oxides tend to have rich polymorphs manifolds. Here we show how computational materials design coupled to iterative experiment can begin to address the question of synthesizability. We examine the synthesis of the brookite polymorph of TiO2 and look at the factors for the nucleation and growth of this illusive phase. We also look at the synthesis of the of the MnO2 polymorphs where the traditional approach of PLD or sputtering does not work effectively. Rather we examine computationally and experimentally the solution based approaches and find that by addressing the pH, the particle size and the chemical potential of the other ions in the solution we can predict the synthetic range for phase formation and we have successfully synthesized phase pure birnessite and todorokite which potentially have unique functionality.


FJ-2:IL10  Graphene Films as Transparent Electrodes
D. NEUMAIER, AMO GmbH, Aachen, Germany

Graphene, the two-dimensional carbon crystal, has been considered since about 10 years as a promising material for transparent electrodes because of the low light absorption in combination with a very high electrical conductivity. In this lecture I will review and discuss the current state of research in this field and highlight specific bottlenecks and challenges. On a lab scale the outstanding potential of graphene has been demonstrated already by showing sheet resistance values below 10 Ohm at a transparency of 85%, outperforming ITO based coating. However, a transfer of those results to large scale production has not been achieved yet. Nevertheless, there was significant progress on the large scale fabrication of graphene and square-meter sized layers can nowadays be synthesized routinely. In addition different demonstrator devices including, OLED, solar cells and touch screens, have been realized with graphene films as transparent electrodes. Especially for devices requiring mechanical flexibility, graphene offers already now a clear performance advantage.


FJ-2:L11  Response to Mechanical Bending Stress of AZO/Ag/AZO Thin Films
G. TORRISI^1,2, I. CRUPI³, S. MIRABELLA^1,2, A. TERRASI^1,2, ¹University of Catania, Italy; ²CNR-IMM, Catania, Italy; ³University of Palermo, Italy

We report the effect of mechanical bending cycles on the electrical and optical properties of ultra thin AZO/Ag/AZO multilayers (45nm/10nm/45nm) and, for comparison, of AZO and ITO single layers whose thickness was 100 nm and 700 nm, deposited at room-temperature on flexible polyethylene naphthalate (PEN) plastic substrates. The electrical stability of the films after several cycles of bending were evaluated by monitoring the relative variation of the electrical resistance with respect to the as prepared sample; crack size and density were detected by Scanning Electron Microscopy (SEM). We observed excellent electrical stability and high mechanical flexibility in the AZO/Ag/AZO sample even after 100 cycles, whereas for the single AZO films the resistivity rapidly increases. Computer simulations were also used to better understand the basic mechanisms for which this kind of materials show superior robustness under bending stress.


FJ-2:IL12b  Transparent Diluted Magnetic and Plasmonic Metal Oxide Nanocrystals
P.V. RADOVANOVIC, Department of Chemistry, University of Waterloo, Waterloo, ON, Canada

Simultaneous control of multiple functionally relevant properties of metal oxides could lead to the development of radically new information and photonic technologies. In this talk I will discuss examples from our recent work, focusing on using structure, doping, and native defects to manipulate different degrees of freedom in transparent metal oxide nanocrystals (NCs). I will briefly review our studies of in situ phase transformation of In2O3 NCs during their colloidal growth. The consequence of the NC phase on defect-induced dilute magnetic ordering of transition metal dopants and localized surface plasmon resonance will be discussed. Using a combination of magnetic and magneto-optical techniques, we demonstrated two distinct mechanisms of long range ordering of magnetic dopants in nanocrystalline films prepared from colloidal NCs as building blocks, both of which rely on extended structural defects. I will also discuss the synthesis and spectroscopic properties of new plasmonic In2O3-based NCs, and comparative investigation of their electronic structure using combined Drude-Lorenz model and density functional theory. Prospects of these NCs as multifunctional materials with truly interacting degrees of freedom will also be discussed.


FJ-2:IL13  Physical Properties and Applications of Doped BaSnO3 Semiconductors with High Electrical Mobility and Optical Transparency
KEE HOON KIM, Center for Novel States of Complex Materials Research and Institute of Applied Physics, Department of Physics and Astronomy, Seoul National University, Seoul, South Korea

In 2012, we have reported that BaSnO3 doped with a few percent of La exhibits unusually high electrical mobility of 320 cm2(Vs)-1 at room temperature and superior thermal stability at high temperatures. Following that work, we have studied physical properties of (Ba,La)SnO3 and Ba(Sn,Sb)O3 single crystals and epitaxial films including temperature-dependent transport and phonon properties, optical properties and first-principles calculations. We have found that almost doping-independent mobility of 200-300 cm2(Vs)-1 is realized in the single crystals in a broad doping range from 1.0x10^19 to 4.0x10^20 cm-3. Moreover, the conductivity of ~10^4 (ohm cm)-1 reached at the latter carrier density is comparable to the highest value previously reported in the transparent oxides. Meanwhile, there have been significant progresses in the thin film fabrications based on the doped BaSnO3. They include pulsed laser deposition, solution growth, sputtering, and molecular beam epitaxy, all of which are targeting to improve electrical mobility at room temperature. On reviewing those efforts in recent years, I naturally extend discussions toward realizing a transparent pn junction, field effect transistors with the doped stannates, and other optoelectronic device applications.


FJ-2:IL15  Photonic Processing for Metal Oxide Thin Films
D.C. KOUTSOGEORGIS, Nottingham Trent University, Nottingham, UK

Humankind has always been fascinated by light. But, besides just mesmerising us, light can also be a powerful tool for manipulating matter and its characteristics. Light is no longer limited to just a diagnostic for probing materials’ characteristics, but has also become an engine for manipulating materials’ properties. This presentation is about using light in order to process thin films and manipulate their characteristics. As an alternative to conventional thermal annealing, photonic processing enables the use of temperature sensitive substrates without any loss in the effectiveness of a high temperature treatment. A highly localised and ultra rapid thermal treatment can be achieved, targeting the material of choice only and with minimal influence onto the surrounding materials. The 10 parameters that affect the outcome of photonic processing will be presented and some case studies of its successful application will be analysed. These include the processing of thin films for effective dopant activation (AZO), control of crystalline structure (ZnO), creation of ohmic contacts in TFTs (IGZO) and photo conversion of sol-gel precursors to oxides of high quality (In2O3).


FJ-2:IL16  Interface Chemistry for Organic Electronics and Opto-electronics
S.R. MARDER, School of Chemistry and Biochemistry, School of Materials Science and Engineering, and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA, USA

Organic semiconductors have attracted interest for electronic applications due to their potential for use in low-cost, large-area, flexible electronic devices. Here we will report on recent developments pertaining to surface modifiers and dopants that could impact the charge injection/collection processes in organic light emitting diodes, organic field effect transistors, and organic photovoltaic devices. In particular, we will examine how phosphonic acids assemble on ITO substrates, the impact of the surface dipole on the work function of the ITO and electron transfer kinetics across surface modifiers. We will also discuss the development of metallocenes-based dimers as n-dopants and very briefly described metal dithiolene complexes as p-dopants for organic semiconductors and their impact of device performance.


FJ-2:L17  In-Ga-Zn-O Thin Films with Tunable Optical and Electrical Properties Prepared by Reactive High-power Impulse Magnetron Sputtering
J. REZEK, J. HOUSKA, M. PROCHAZKA, S. HAVIAR, Department of Physics and NTIS - European Centre of Excellence, University of West Bohemia, Plzen, Czech Republic

High-power impulse magnetron sputtering (HiPIMS) is a very suitable and progressive method for preparing high-quality oxide layer. This is mainly because of a high degree of ionization of target material particles in the discharge plasma and the associated high ion-to-atom ratio in particles flux going toward a substrate. These could result in formation of very dense films structure. Moreover, enhanced kinetic energy of particles impacted onto growing film could substitute thermal heating of the substrate which is very important in the case of deposition on heat sensitive substrates. In spite of these advantages, the use of reactive HiPIMS for the preparation of In-Ga-Zn-O (IGZO) layers has been rarely reported only. In this work, we show the use of reactive HiPIMS is an effective way to produce IGZO layers, and that the value of pulse-averaged target power density being in the range of 100-1020 Wcm-2 (which is two orders of magnitude higher compare with the conventional dc or RF magnetron sputtering) is a suitable parameter for controlling the optical and electrical properties of the layers. We explain the correlation between plasma discharge parameters, and electrical and optical properties of formed films.


FJ-2:IL18  Growth and Properties of Ga2O3 Thin Films
R. FORNARI^1,2, A. BARALDI¹, V. MONTEDORO¹, A. PARISINI¹, M. PAVESI¹, M. BOSI², C. FERRARI², E. GOMBIA², D. KLIMM³, F. MEZZADRI⁴, G. CALESTANI4, I. CORA⁵, B. PÉCZ⁵, ¹Dept. of Mathematical, Physical and Computer Sciences, University of Parma, Parma, Italy; ²Institute of Electronic and Magnetic Materials (IMEM-CNR), Parma, Italy; ³Leibniz Institute for Crystal Growth (IKZ), Berlin, Germany; ⁴Dept. of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Parma, Italy; ⁵Centre for Energy Research, Hungarian Academy of Sciences, Institute for Technical Physics and Materials Science, Budapest, Hungary  

Semiconducting sesqui-oxides (In2O3, Ga2O3, Al2O3 and related alloys) have become an important subject of semiconductor physics. Among them, beta-Ga2O3 is by far the most investigated, owing to its wide bandgap of about 4.9 eV and extremely high Baliga FOM. Power electronic devices as well as solar-blind UV detectors are the targeted application of this material. Ga2O3 presents a number of polymorphs, and our team decided to focus the attention on the epsilon phase, which has proved to be thermodynamically stable up to 700°C and has a “hexagonal” structure. This offers a better matching to usual sapphire substrates and/or nitride templates, and prevents the anisotropy problems of the monoclinic beta polymorph. In this presentation, the developed epitaxial process for (001)-oriented epsilon-Ga2O3 will be described, along with an overview of its crystallographic and physical characteristics. It will be shown that the hexagonal lattice in reality is constituted of orthorhombic domains separated by 120° twins. Finally, the results of preliminary attempts to utilize epsilon-Ga2O3 for preparing UV photo-detectors (for wavelengths shorter than 270 nm) will be reported.


FJ-2:IL19  Photosensitive TCO-based Hybrid Materials for Gas Sensor Applications
M.N. RUMYANTSEVA, A.F. NASRIDDINOV, E.V. PAKHOVA, E.V. LUKOVSKAYA, O.A. FEDOROVA, A.M. GASKOV, Moscow State University, Moscow, Russia

Upon detection of oxidizing gases by semiconductor gas sensors, the signal generation occurs due to adsorption of analyte molecules on the metal oxide surface, which is accompanied by the localization of electrons on the adsorbed species. At room temperature this process is kinetically irreversible. The desorption of analyte and recovery of electrophysical properties of the sensitive material to the initial state occurs under elevated temperature. The replacement of thermal heating with visible light photoactivation can significantly reduce the power consumption of the sensor. Bulk TCO – SnO2, ZnO, In2O3, are transparent in this spectral range. Photosensitive TCO-based hybrid materials contain photosensitizers, the role of which consists in shifting the optical sensitivity range towards larger wavelengths. Selected organic dyes – Ru(II) complexes with macrocyclic organic ligands, are characterized by absorption in the visible spectral range with high extinction coefficients. The composition and design of the ligand platform play a critical role of controlling the activities of the photosensitizers. Sensor measurements demonstrated that TCO-based hybrid materials can be used for oxidizing gases detection under visible light illumination without thermal heating.


FJ-2:L20  Use of Electrografted Aryl-layers to Control the Conductivity of ZnO Surfaces
A.R. McNEILL, A.J. DOWNWARD, M.W. ALLEN, University of Canterbury, New Zealand

Zinc oxide (ZnO) and tin oxide (SnO2) are transparent, earth-abundant, wide band-gap semiconductors with a high surface sensitivity that must be controlled for use in electronic applications. In ambient conditions, ZnO and SnO2 surfaces are terminated by hydroxyl groups that cause the conduction and valence bands to bend downwards, creating a 2-dimensional electron accumulation layer that renders the surface highly susceptible to unwanted atmospheric adsorbates. The surface hydroxyl termination can be deliberately replaced with covalently bonded organic functional groups. In doing so, the surface band bending and surface conductivity can be directly manipulated. In this work, nitro- and trifluoromethyl-phenyl multilayered films have been attached to ZnO and SnO2 substrates by electrografting from aryldiazonium salt solutions. Synchrotron X-ray photoemission spectroscopy confirms that both surface modifiers remove the native downward band bending on ZnO surfaces, with the nitro-phenyl termination producing a large upwards band bending consistent with an electron-depleted surface. It was also observed that X-ray-induced reduction of the nitro-terminated film serves to further increase the upward band bending, which is an unexpected and as yet unexplained phenomenon.


FJ-2:L21  Reactive Dip-coating of Rhombohedral Delafossite CuAlO2 Based on Mesoporous Alumina Nanofibers
A. SAFFAR SHAMSHIRGAR¹, M. AGHAYAN¹, T.S. TRIPATHI², M. KARPPINEN², M. GASIK³, I. HUSSAINOVA^1,4, ¹Department of Mechanical and Industrial Engineering, Tallinn University of Technology, Estonia; ²Department of Chemistry, Aalto University, Aalto, Finland; ³School of Chemistry, Material Science and Engineering, AALTO University, Aalto, Finland; ⁴ITMO University, St. Petersburg, Russia

The development of p-type transparent semiconductors with large optical bandgaps and high electrical conductivity is needed for a wide range of applications in optoelectronics and solar cell technologies. The experimental techniques currently used are limited to complex procedures and time-consuming processing. In this work, we propose a versatile, straightforward, and reproducible method of rapid reactive dip-coating using a mesoporous network of highly aligned gamma-alumina nanofibers for the synthesis of delafossite CuAlO2 by a time-effective process of 2 hours duration. The rhombohedral CuAlO2 was densified with the help of spark plasma sintering in a vacuum. Electrical conductivity improves with an increase in annealing temperature while its room temperature value for a sample annealed at 1100 °C was 0.07 S m-1 measured with the four-probe method. The direct optical bandgap of 3.35 eV was estimated with the help of diffuse reflection data for the sample sintered at optimal temperature. Both Seebeck coefficient and Hall measurements confirmed the p-type conductivity of the material.

 
Session FJ-3 - Applications

FJ-3:IL01  Phonon Engineering on In2O3- and ZnO-based Thin Films
JUNJUN JIA, YUZO SHIGESATO, Graduate school of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa, Japan

Growing technological demand for In₂O₃- and ZnO-based thin films stems from their large use as transparent conductive electrodes in various optoelectronic devices, and understanding the physical of the phonon thermal transport at nanoscale is important for fundamental advances in the device performance and longevity. We have studied the phonon thermal conduction in In₂O₃- and ZnO-based films. This talk involves the fabrication of the In₂O₃- and ZnO-based films, the measurement of the phonon thermal conductivity, and the theoretical calculation for the phonon conduction. Thermal conductivities for the In₂O₃- and ZnO-based thin film with various carrier densities were described well by Wiedemann-Franz law, and the existence of dopants and/or vacancies obviously lowers the thermal conductivity in ZnO-based thin film. Besides the point defects, the existence of the interface between InO₂ layer and ZnO layer causes an obvious decrease in thermal conductivity in layered In₂O₃ (ZnO)_m films. In order to further understand phonon transport in In₂O₃- and ZnO-based films, we also estimated the influence of the point defect and the interface on the thermal conduction by theoretical calculations. These results are believed to improve the understanding for the phonon transport in oxide films.


FJ-3:L02  Solution Synthesized Delafossite Nanoparticles for Hole Transport Layer in Organic and Perovskite Solar Cells
T.B. DAUNIS, JIAN WANG, BOYA ZHANG, D. BARRERA, JULIA W. P. HSU, University of Texas at Dallas, Richardson, TX, USA; W. DUNLAP-SHOHL, D. MITZI, Duke University, USA

We investigate the applications of Cu(1+)-based delafossite compounds, CuIMIIIO2 (M= Al, Ga, Cr, etc.), which are p-type transparent conducting oxides, as hole transport layer (HTL) materials in organic photovoltaic and perovskite photovoltaic devices. Compared with the commonly used NiOx HTL, delafossite compounds have deeper valence band edge, higher hole mobility, and better optical transparency. We report the synthesis of CuGaO2 and CuCrO2 nanoparticles using microwave-assisted and regular hydrothermal synthesis. Well-dispersed suspensions made from the particles are used to spin coat thin films at room temperature. These delafossite films are smooth and highly transparent. Ionization energy and work function of the films determine whether the HTL would be efficient for a given active layer, and are measured using photoelectron spectroscopy in air and Kelvin probe, respectively. Separating material synthesis and film formation opens up possibilities to optimize the materials properties (e.g. crystalline phase, particle size, composition) independently from the temperature restriction, substrate property dependence of device processing. In the presentation, details in film preparation, optoelectrical property characterization, and PV device characteristics will be discussed.


FJ-3:IL04  Wide Band Gap ZnO Applications
TETSUYA YAMAMOTO, Materials Design Center, Research Institute, Kochi University of Technology, Kami-shi, Kochi, Japan

My talk discuss current status and future prospect of wide-band-gap ZnO applications. ZnO has several advantages over nitride semiconductors such as gallium nitride (GaN) in the application range, however, the most important being its larger exciton binding energy, the ability to grow single crystal substrates and deposit highly oriented polycrystalline films at low temperatures at amorphous glass or polymer substrates. Other favorable aspects of ZnO include its broad chemistry leading to many opportunities for wet chemical etching, low power threshold for optical pumping, radiation hardness, and biocompatibility such as antibacterial substances. Together, these properties of ZnO make it an ideal candidate for a variety of devices ranging from chemical sensors through to deep and vacuum ultra-violet applications and nanotechnology-based devices such as displays. We, very recently, reported low-optical-loss transparent conductive Ga-doped ZnO films for plasmonics in the near-infrared spectral range.[1] We achieved 200-nm-thick GZO films with the cross-over wavelength adjusted to 1.476 micron meter, exhibiting a very low imaginary part of the dielectric function of 0.434 at the telecommunication wavelength of 1.5 micron meter. Precise control of carrier concentration together with tailor-making of carrier transport enables us with successful achievement above. We will discuss the issue to be resolved, especially, the improvement of carrier transport of ZnO films by clarifying a key factor to achieve very high Hall mobility In2O3 codoped with Ce and H atoms.[2]
References: [1]Junichi Nomoto, Hisao Makino and Tetsuya Yamamoto, "Low-optical-loss transparent conductive Ga-doped ZnO films for plasmonics in the near-infrared spectral range", Science of Advanced Materials, 9 (2017) pp. 1815-1821. [2]Eiji Kobayashi, Yoshimi Watabe and Tetsuya Yamamoto, "High-mobility transparent conductive thin films of cerium-doped hydrogenated indium oxide", Appl. Phys. Express, 8 (2015) pp. 015505-1-4.


FJ-3:IL05  Transparent Materials for Perovskite (Opto-)electronics
T. RIEDL, University of Wuppertal, Wuppertal, Germany

Perovskite solar cells (PSCs) are candidates for tandem devices with crystalline silicon to unlock efficiencies beyond 30%. Aside from that, semitransparent PSCs are envisaged for building integration. For both, transparent electrodes that afford a conductivity and transmittance better than indium-tin-oxide are needed. Semi-transparent electrodes based on metal nanowires or ultra-thin metal layers are considered. Unfortunately, metals like Ag are extremely susceptible to corrosion by the halide moieties of the PSC. On top of that, PSCs still suffer from substantial stability issues in the presence of moisture or heat. I will discuss avenues to overcome these issues. E.g., the introduction of ALD-grown tin oxide (SnOx) affords air resilient and temperature stable PSCs with a lifetime > 4500h. Being conductive, SnOx can be placed inside the device stack, e.g. between the metal electrode and the perovskite. Its outstanding permeation barrier properties protect the perovskite against ingress of moisture or migrating metal atoms, while simultaneously the metal electrode is shielded against leaking halide compounds. Thus, SnOx is also excellently suited to sandwich and protect ultra-thin metal layers (Ag or Cu) in Indium-free semitransparent electrodes (SnOx/metal/SnOx) for PSCs.


FJ-3:IL06  Low Damage Sputtering of TCOs for LEDs
M. MAUTE, Osram Opto Semiconductors GmbH, Regensburg, Germany

Transparent conductive oxides (TCO) like Indium Tin Oxide (ITO) are a key feature for blue/white sapphire LEDs based on GaN. However, this semiconductor material is extremely sensitive to damage caused by ions during e.g. a magnetron sputter process. Therefore, special precautions must be taken in order to achieve low contact resistance. In addition, low absorption and low sheet resistance are essential for the LED performance. Here, we present a low damage sputter approach using a so-called Facing-Target-Cathode (FTC). We will show data regarding electrical contact, absorption, and resistance and we will summarize how this adds up to a high-performance LED for e.g. general lighting applications.


FJ-3:IL07  Toward Realization of Ga2O3 Transistors for Power Electronics Applications
MAN HOI WONG, Y. NAKATA, C.-H. LIN, National Institute of Information and Communications Technology, Koganei, Tokyo, Japan; K. SASAKI, Tamura Corp., Sayama, Saitama, Japan, and National Institute of Information and Communications Technology, Koganei, Tokyo, Japan; Y. MORIKAWA, Silvaco Japan Co., Ltd., Yokohama, Kanagawa, Japan; K. GOTO, Tamura Corp., Sayama, Saitama, Japan, and Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan; A. TAKEYAMA, T. MAKINO, T. OHSHIMA, National Institutes for Quantum and Radiological Science and Technology, Takasaki, Gunma, Japan; A. KURAMATA, S. YAMAKOSHI, Tamura Corp., Sayama, Saitama, Japan; H. MURAKAMI, Y. KUMAGAI, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan; M. HIGASHIWAKI, National Institute of Information and Communications Technology, Koganei, Tokyo, Japan

β-Ga2O3 is being actively pursued for power switching and harsh environment electronics. State-of-the-art Ga2O3 MOSFETs were realized on unintentionally-doped (UID) β-Ga2O3 (010) epilayers by employing Si-ion implantation doping for the channel and ohmic contacts. Field-plated depletion-mode devices delivered a high off-state breakdown voltage of 755 V, a large drain current on/off ratio of over nine orders of magnitude, stable high temperature operation at 300°C, and dispersion-free pulsed output characteristics. Bulk Ga2O3 exhibited high gamma-ray tolerance, while radiation-induced dielectric damage and interface charge trapping limited the overall radiation hardness of these devices. Accumulation-mode normally-off operation was realized by gating a UID β-Ga2O3 (010) channel with low background carrier density. The design and operation of vertical Ga2O3 MOSFETs engineered with a current aperture will also be presented. This work was partially supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics” (funding agency: NEDO).