2015 MRS Spring Meeting

Apr 6, 2015 ‐ Apr 10, 2015


The 2015 MRS Spring Meeting and Exhibit was held in San Francisco, California. Technical symposia were held within the following topical clusters: Energy, Nanomaterials, Electronics and Photonics, Soft and Biomaterials, and General – Fabrication and Characterization. Access to all of the content below is free – simply login with your MRS username and password for complete access. Don’t have a username and password? Create a free account.


Sessions

Physical Mechanisms Affecting the Reliability of GaN-Based High Electron Mobility Transistors

Apr 8, 2015 8:00am ‐ Apr 8, 2015 8:30am

Identification: CC5.01

GaN-based high electron mobility transistors (HEMTs) offer excellent high-power and high-frequency performance, allowing them to amplify high power signals at microwave frequencies very efficiently. The current in GaN HEMTs flows in a two-dimensional electron gas at the interface between GaN and AlGaN layers, which offers excellent electrical properties (high carrier density and mobility). Understanding the physics of failure in these devices remains an important issue, with both abrupt failures and gradual parametric degradation having been observed. These devices may show sudden and permanent damage when subjected to very high reverse bias-stress. This failure mechanism has been addressed by improvements in processing technology. However, the parametric degradation that occurs in the �semi-ON� state at moderate drain biases remains an issue. In this condition, the device is biased close to pinch-off, but the relatively small numbers of electrons that are flowing are accelerated by a high electric field. The resulting energetic carriers can activate, by dehydrogenation, or reconfigure defects near the interface. The defect generation is greatest at the end of the gate on the gate-drain side, where the lateral electric field is at its maximum. This may lead to significant reductions in drain current and transconductance, as well as shifts in threshold voltage, resulting in poor DC, RF and large-signal performance. GaN/AlGaN HEMTs grown under various conditions (i.e., gallium-rich, nitrogen-rich, and ammonia-rich) have been analyzed and the defects responsible for degradation in each device type have been identified. The atomic-scale nature of the traps that produce changes in threshold voltage, leakage current, and drain current have been related to changes in transconductance and turn-off voltage using a combination of electrical measurements, quantum mechanical calculations, Monte-Carlo device simulations, and accelerated degradation tests. A relatively simple formulation has been developed under the assumption that the hot-electron scattering cross-section is independent of the electron energy. In this case one can relate the change in defect concentration to the operational characteristics of a device, such as the spatial and energy distribution of electrons (electron temperature), electric field distribution and electron energy loss to the lattice. The number of electrons with energy (obtained from device-level Monte Carlo simulations) in excess of that required to activate a defect (obtained from density functional theory) is used to predict the degradation rate. The results of quantum mechanical calculations of candidate defect formation energies as functions of growth conditions and Fermi level position were used to identify the primary defects responsible for the degradation as hydrogenated Ga vacancies or hydrogenated N antisite defects. In each case, degradation occurs by the hot electrons providing the energy needed to release a hydrogen atom. The calculations also yield the activation energy for hydrogen release.

Speaker(s):

Lithium Niobate Based Piezoelectric Transducer

Apr 8, 2015 8:00am ‐ Apr 8, 2015 8:15am

Identification: P4.01

Over the last several decades, the community of nanomaterial chemists have developed exquisite methods for creating gold nanostructures with controlled size, shape, and crystal form. As we move into the 21st century - "The Century of Biology" - biological applications of gold nanostructures abound, from chemical sensing to biological imaging to photothermal therapy. In this talk I will discuss some details in all these areas, and then show that the presence of gold nanoparticles in the extracellular matrix surrounding cells can lead to unanticipated effects; these effects could be used for good, if understood.

Speaker(s):

Multiple Stoichiometries in Organic Single Crystals of Charge Transfer Compound

Apr 8, 2015 8:00am ‐ Apr 8, 2015 8:15am

Identification: MM4.01

Single crystals of organic charge transfer compounds with low concentration of imperfections and high purity are outstanding objects for the study of relation between charge transfer and physical properties. The degree of charge transfer in compounds impacts antiferromagnetism, superconductivity, electron-phonon interactions and energy storage. Crystal growth of binary compounds made from organic donors and acceptors is more complicated than that of monomolecular crystals. For studies of charge transfer degree, compounds with multiple stoichiometries are especially desired. However, systems with multiple stoichiometries are rare. Due to the different physical properties (solubilities, sublimation temperatures) of donors and acceptors, the stoichiometry of the synthesized compounds may differ from the stoichiometry of the starting materials. In this study we observed an interesting phenomenon: the final stoichiometries of single crystal P1T1 (perylene1:TCNQ1) and P3T1 (perylene3:TCNQ1) are not affected by the stoichiometry of the starting materials, but by the solvent in which the perylene-TCNQ crystals were grown. The P1T1 crystals were grown from toluene, whereas the P3T1 crystals were grown from benzene regardless of the acceptor/donor stoichiometry in the starting materials. The solubility data were employed to analyse the effect of solvent on the stoichiometry of the perylene-TCNQ charge transfer single crystals. Steady-state optical spectra and time-resolved fluorescence measured in a mixture of perylene and TCNQ in toluene and benzene, confirmed selective crystallization of P1T1 and P3T1 from toluene and benzene. In contrast to solution growth, when utilizing the physical vapour transport (PVT) method, a mixture of monomolecular crystals, P1T1 (perylene1:TCNQ1), P2T1 (perylene2:TCNQ1) and P3T1 (perylene3:TCNQ1) is obtained. P2T1 is a new discovered structure. The charge transfer degrees of P1T1, P2T1 and P3T1 have been measured and calculated. Field-effect transistors on the single crystals� surfaces of P1T1, P2T1 and P3T1have been made. The results reveal thatP1T1 is typically an n-type semiconductor, P3T1 showed p type behaviour, whereas P2T1 showed ambipolar properties.

Speaker(s):
  • Peng Hu, Nanyang Technological University

Characterization of Highly Resistive Nanoscale RRAM Contacts

Apr 8, 2015 8:15am ‐ Apr 8, 2015 8:30am

Identification: AA5.02

Resistive random access memory (RRAM) is a promising candidate for next generation data storage due to its potential for performance, scalability and compatibility with CMOS processing. The switching of metal oxide films (HfOx, TiOx, AlOx) has been studied [1] and several mechanisms proposed for the formation of low resistance pathways of such films [2,3]. However, the performance of the memory device is also dependent on the contact resistance, especially at sub-5 nm dimensions [4]. Measuring the contact resistance in highly resistive films is challenging due to low current levels, variability and interface ionic migration. Similar studies exist for phase change materials such as Ge2Sb2Te5 (GST) [5,6] but a comprehensive study for the more resistive metal oxide based RRAM materials is still lacking. In this study, we introduce a methodology to measure highly resistive thin films and their interfacial electrical resistances. We carry out low current four-probe measurements to measure contact resistance with a Cross Bridge Kelvin [6] structure. The overlap parameter for the structure is ~100 nm to enhance the signal-to-noise ratio for the low-current level typically used in these resistive devices. This allows accurate extractions of contact resistance and film resistivity for structures with specific contact resistance values greater than 100 ??�cm2 and underlying film resistivity value above 10 ?�cm. Circular transfer length measurements (TLM) are carried out with a Ti/Pt contact stack on 20 nm HfO2 films prepared via atomic layer deposition (ALD) and sputtered GST films � yielding specific contact resistivity values of 7.5 ?�cm2 and 3.2 x 10-4 ?�cm2 at room temperature, respectively. For RRAM films which involve ion migration in the bulk and at the metal oxide interfaces, we utilize a Greek Cross [7] structure for sheet resistance measurement to avoid the injection effect at the interface. We find transfer lengths for contacts to HfO2 to be in the range of 200 nm and for amorphous GST in the range of 4 ?m at room temperature. Linear TLM structures with metal line widths below 200 nm are also fabricated and measured for stoichiometric ALD HfO2 and sputtered HfOx films, to extract the carrier mobility under the metal contact [8]. This measurement platform can be used to precisely measure the interfacial and bulk characteristics for highly resistive films. It is also a useful tool in exploring the switching mechanism, as a single electrode-memory bit interface can be measured accurately, with low current and low noise measurement capability. [1] H.-S.P. Wong et al, Proc. IEEE (2012) [2] R. Waser et al, Adv. Mater. (2009) [3] D. Ielmini, IEEE-TED (2011) [4] C.-L. Tsai, E. Pop, et al ACS Nano (2013) [5] S. Savransky, I. Karpov, MRS Proceedings (2008) [6] D. Roy et al. IEEE-EDL(2010) [7] S. Enderling et al, IEEE-TSM (2006) [8] D. Schroeder, 3rd ed. (2006)

Speaker(s):

Growth and Characterization of Tin Oxide Low Dimensional Structures and Tin Oxide- Graphene Oxide Nanocomposites for Battery Applications

Apr 8, 2015 8:15am ‐ Apr 8, 2015 8:30am

Identification: TT6.02

Lithium-ion batteries are widely used so far due to their high energy density, safety and long cycle lifetimes, however, current Li-ion battery electrodes are usually made from graphitic carbon which theoretical capacity is limited. SnO2 has attracted increasing attention as an alternative anode material [1] because of its higher Li storage capacity than carbonaceous electrodes. Different approaches are considered in order to solve problems such as the SnO2 volume expansion during charge/discharge processes, as the use of nanostructured SnO2 [2] and the synthesis of SnO2 composited with graphene [3]. In this work, different SnO2 based compounds have been fabricated and characterized, with special interest focused on the effects induced by Li and Cr doping. Therefore, rods, tubes, nanoparticles and graphene-based compounds have been fabricated following different approaches. Low dimensional SnO2 doped structures in forms of nanowires and microtubes have been grown at 800-1400 �C by a catalyst free evaporation-deposition method using either metallic Sn or SnO2 mixed with Cr2O3 and Li2CO3 as precursors. SnO2 nanoparticles doped with Cr and Li have been synthesized via a modified Pechini method which allows to reach high control in size and composition. SnO2-graphene oxide composites have been grown by a modified Hummer method. In this work the effect of Cr and Li on the structural and luminescent properties of rutile-type SnO2 low dimensional structures (nanoparticles, nanowires, microtubes, and composites) is studied by means of transmission electron microscopy (TEM), cathodoluminescence (CL), energy dispersive x-ray spectroscopy (EDS) and Raman spectroscopy. The thermal parameters and the corresponding precursor determine the morphology of the as grown structures which dimensions vary from 5 nm to tens of microns width and up to hundred of microns length. In the case of SnO2, chromium is usually incorporated as substitutional Cr3+ in octahedral coordination, therefore anionic vacancies and/or cationic interstitials are generated during doping as well as a decrease in conductivity is also observed. However, the Cr3+ characteristic emission at 1.79 eV is not observed for all the samples and the luminescence of Cr doped SnO2 highly differs from that characteristic from undoped SnO2. The incorporation of Li in SnO2 and its influence on the luminescence properties has scarcely been studied in nano and microstructures. The codoping of Cr and Li causes an enhancement of the Cr emission meanwhile the conductivity of the samples is increased. [1] Y.D. Ko, J.G. Kang, J.G. Park, S. Lee, D.W. Kom, Nanotechnology, 20, 455701 (2009) [2] J.Y. Huang, L. Zhong, C.M. Wang, J.P. Sullivan, W. Xu, L.Q. Zhang, S.X. Mao, N.S. Hudak, X.H. Liu, A. Subramaniam, H. Fan, L. Qi, A. Kushima, J. Li. Science, 330, 1515 (2010) [3] J. Lin, Z. Peng, C. Xiang, G. Ruan, Z. Yan, D. Natelson, J.M. Tour, ACS Nano, 7, 6001 (2013)

Speaker(s):
  • Ana Cremades, Dpt. Fisica de Materiales, Facultad de CC. Fisicas, Universidad Complutense de Madrid

Design and Optimization of Transition Metal Oxide-Based Resistive Switching Devices for Data Storage and Computing Systems

Apr 8, 2015 8:30am ‐ Apr 8, 2015 9:00am

Identification: AA5.03

There is presently an extensive effort to develop non-volatile resistive random access memories (RRAM) based on transition metal oxides [1-4]. These generally work by the formation of a conductive filament of oxygen vacancies between the two electrodes. In one model, the ‘hour glass model’ [2], the oxygen vacancies migrate between two ‘reservoirs’, allowing the filament neck to increase or decrease in size during the SET and RESET process. HfO2, TiO2, Ta2O5, and Al2O3 are the typically used oxides, together with a scavenging metal that allows the vacancy creation process during the initial filament forming process. However, there has been no clear materials selection criteria given in terms of how to maximize memory endurance or retention lifetime. Here, we determine the standard operating conditions in terms of O chemical potential and Fermi energy. It is shown how the choice of scavenger metal can be used to fix the O chemical potential. This then fixes the O vacancy formation energy, because that formation energy varies between ~6.1 eV at pO2 = 0 eV, to ~ 0.2 eV at for example the Hf/HfO2 equilibrium potential pO2 = -5.8 eV. Setting this formation energy then ensures that the total number of O vacancies is conserved during memory cycling, maximizing endurance, and stops formation of any O interstitials. Choice of the O chemical potential/scavenging metal allows the endurance and retention time to be optimized. The various migration energies and charge state energies are calculated for the oxides. It is argued that Ta2O5 has the preferred properties, having lower migration energies, charge state energies at the preferred Fermi energies, and having a wider stability zone of its amorphous phase than HfO2. On the other hand, TiO2 has sub-oxide phases which complicate electrode processes, while Al2O3 has too high defect formation and migration energies. It can only be used as an oxide modifier. 1. R Waser, et al, Adv Mats 21 2632 (2009) 2. R DeGraeve, et al, Tech Digest VLSI (2013)p8.1; Tech Digest VLSI (2012); 3. S Clima et al, APL 100 133102 (2012) 4. J J Yang et al, Nature Nanotechnol 3 429 (2008) 5. L Goux et al, ECS Solid State Lets 3 Q79 (2014)

Speaker(s):

Piezoelectric Potential Enhanced Oxygen Evolution Reaction

Apr 8, 2015 8:45am ‐ Apr 8, 2015 9:00am

Identification: P4.04

Lowering the overpotential required for driving oxygen evolution reaction (OER) is crucial for practically generating hydrogen from water splitting process. Besides developing effective and non-noble electrocatalysts, integrating the catalyst with silicon and III-IV semiconductors based photoanodes is a popular strategy to achieve this goal. However, they are always suffering from the poor stability and induce light limited anodic current, which is substantially smaller than the diffusion limited anodic current. Piezotronics have been frequently adopted to engineer the interfacial electronic band structure of heterojunctions to enhance charge generation, separation and transportation. Here, we propose to use the piezoelectric potential generated by sputtering ZnO film to decrease the external overpotential required for the OER with e-beam evaporated nickel film as the electrocatalyst. The piezo-induced positive charge on the adjacent surface of nickel film can presumably lower the electron energy level of Ni/NiOx film and thus facilitate the oxidizing reaction. It has been found that piezoelectric potential could also enhance charge delivering between ZnO film and ITO. Significantly larger current density and long term stability are expected by replacing ZnO with other ceramic piezomaterials like PMNPT or PZT. This approach could render a new pathway for efficient and practical water splitting.

Speaker(s):

Radial Heterojunction Crystalline Silicon Nanowire Solar Cells with 11.8% Conversion Efficiency

Apr 8, 2015 8:45am ‐ Apr 8, 2015 9:00am

Identification: S4.03

Band diagrams are a tool of fundamental importance for properly understanding the properties of heterostructures [1]. They condense crucial information about electronic properties, including band alignments, built-in fields, insulating and metallic regions and space-charge formation. The band diagram expected for an electronically reconstructed LaAlO3/SrTiO3 heterostructure is readily identified on the base of simple electrostatic arguments. It includes among it’s features a confining potential at the SrTiO3 side of the interface and a large electric field within LaAlO3. The present literature on the topic is highly controversial since the absence of the abovementioned features has been claimed in a number of publications. This contributed to shed many doubts on the origin of the interfacial 2D electron gas. In the course of our work, measurements performed of different types of heterostructures hosting a 2D electron gas are reported. As a first step, growth conditions for the fabrication of metallic LaAlO3/SrTiO3, LaGaO3/SrTiO3 and NdGaO3/SrTiO3 interfaces are addressed and compared. By resorting to several complementary techniques, including STEM-EELS [2], X-ray photoemission spectroscopy, second harmonic generation [3] and photoconductivity [4] we address the presence of intrinsic electric fields within all these heterostructures. We argue that doping effects taking place under a probing radiation in the VIS, UV or X-ray range might well affect the output of many experiments. We suggest that a thorough understanding of the steady state achieved by these systems under a photon beam is crucial for the correct interpretation of available experimental data. [1] Herbert Kroemer (Nobel Lecture), Rev. Mod. Physics 73, 783 (2001); [2] C. Cantoni, F. Miletto Granozio et al., Adv. Mater. 24, 3952 (2012); [3] E. Di Gennaro, F. Miletto Granozio et al, Adv. Opt. Mat. 1, 834 (2013); [4] G. De Luca, F. Miletto Granozio et al., Appl. Phys. Lett. 104, 261603 (2014)

Speaker(s):

Elucidating and Controlling Biotic/Abiotic Interfacial Interactions for Enhancing Material Properties

Apr 8, 2015 9:00am ‐ Apr 8, 2015 9:30am

Identification: GG3.04

Biomimetic materials design holds enormous potential for the generation of technologically advanced materials by exploiting the inherently specific and programmable nature of biomolecular interactions. For example, peptides that have specific affinities for materials surfaces have been utilized to control the assembly and interfacial properties of a wide range of abiotic chemistries. The complex structure and functionality native to biomolecules enables one to envision a future in which materials properties can be controlled by designing biotic/abiotic interactions. To attain this advanced level of control, a more thorough understanding of the interfacial interactions between biomolecules and materials is required. Undertaking both extensive experimental and computational studies, we have begun to unravel the influences and effects that drive biotic-abiotic interactions. In this talk, I will cover our approaches in understanding how biomolecules interact with abiotic surfaces, to control physiochemical properties by modulating these interactions, and in developing new routes for the synthesis and assembly of functional hybrid materials.

Speaker(s):

Nanowire Based Energy Applications of Wurtzite and Zincblende Semiconductors

Apr 8, 2015 9:15am ‐ Apr 8, 2015 9:45am

Identification: P4.06

Rod-shaped semiconductor nanowires provide today highly promising materials for applications in areas like energy scavenging as well as in energy conservation. The ability to form each nanowire into a designed three-dimensional device structure, and ways in which such units can be combined into ideal arrays thus forming a complete device, lends great promise for many areas of applications. The ability to control the crystalline structure in-between the cubic zincblende and the hexagonal wurtzite structures lends another handle to optimize performance, and may sometime also be an issue in the ability to control things. In this talk I will concentrate on two such examples: (a) one in which III-V NWs, like InP or GaAs and ternaries based on these, can form the basis for a highly efficient photovoltaic technology, one that can easily constitute an add-on to standard silicon solar cells, and (b) one based on the GaN family of hexagonal materials, in which one aims to master and control the effect of piezo-electric effects in axial and radial LED device structures. I will conclude by discussing how one may take advantage of the built-in piezo-electric fields in specially designed tunnel device structures to increase the interband tunneling probability.

Speaker(s):
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