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

Straight and Vertically Aligned Palladium-Seeded GaAs Nanowires

Apr 7, 2015 9:45am ‐ Apr 7, 2015 10:00am

Identification: S1.06

The use of a foreign metal seed particle as a catalyst for nanowire growth is an established technique used to achieve growth for a wide range of growth temperature and precursor flows. The predominantly used material is gold and it is well known that it is suitable over a wide range of growth parameters. It enables growth of nanowires with a well-controlled diameter and length; in addition, it is possible to grow heterostructures and switch crystal structure. We are currently investigating the use of alternative seed particle materials to learn more about how the seed-particle affects the properties of the nanowire, such as its morphology, structure and growth mechanism. We will present our current findings of palladium seeded GaAs, where we have done a systematic study over a large growth parameter window. The palladium particles were generated by an aerosol-phase deposition using a spark discharge generator (SDG), able to generate pristine spherical particles of metals with a high dose and size control. A closed coupled showerhead MOCVD from AIXTRON was used to grow the nanowires.The explored parameter space is, growth temperature 350-600�C, V/III ratio 0.1-73, growth time 5-30 minutes, particle size, 10-40 nm and particle dose in the range of 1-10 ?m-2. The analysis is performed by high-resolution transmission electron microscopy (HREM), energy dispersive x-ray spectroscopy (XEDS) and scanning electron microscopy (SEM). We will demonstrate how to grow vertically aligned wires. In addition, we will also demonstrate how the growth parameters affect typical growth characteristics such as nucleation, crystal structure, crystal growth direction, morphology and growth rate. Other groups have succeeded in growing straight wires in non-(111)B directions for InAs [1]�[3]. However, this is the first time that epitaxial (111)B GaAs nanowires seeded by palladium are reported. The XEDS analysis reveals that the gallium content in the seed particle after growth can be between 45-80 at%. High gallium content may allow for vapor-liquid-solid (VLS) growth at higher growth temperatures and is suspected to be the key to the epitaxial (111)B wires. This and other interesting discoveries will be discussed, such as how the particle composition is affected by growth parameters and how the palladium seeded growth compares to gold seeded growth. References [1] H. Xu, Y. Wang, Y. Guo, Z. Liao, and Q. Gao, �Defect-free< 110> zinc-blende structured InAs nanowires catalyzed by palladium,� Nano Lett., pp. 5744�5749, 2012. [2] S. Heun, B. Radha, and D. Ercolani, �Pd-assisted growth of InAs nanowires,� Cryst. Growth �, vol. 10, no. 9, pp. 4197�4202, Sep. 2010. [3] R. Perumal, Z. Cui, P. Gille, J.-C. Harmand, and K. Yoh, �Palladium assisted hetroepitaxial growth of an InAs nanowire by molecular beam epitaxy,� Semicond. Sci. Technol., vol. 29, no. 11, p. 115005, Nov. 2014.

Speaker(s):

Fundamentals and Technology of Silicon Heterojunction Solar Cells

Apr 7, 2015 10:30am ‐ Apr 7, 2015 11:00am

Identification: E1.04

Although the price of solar panels has recently decreased considerably, the production cost of solar cells is still too high for the generated electricity to compete with bulk electricity prices. The Dutch STW program FLASH aims at reducing the production costs of photovoltaic (PV) modules by improving the solar cell conversion efficiency, reducing materials consumption and using abundant materials only, and by applying low-cost and low-temperature processing methods, which lowers the energy costs in manufacturing as well. We focus on silicon heterojunction (SHJ) solar cells, because they combine the high efficiency of crystalline silicon (c-Si) wafer technology with the high throughput, low-cost production technologies for amorphous silicon thin-film solar cells. Thin film technology provides excellent methods for surface passivation and junction formation at low processing temperatures. Surface passivation, a method to electronically de-activate defects at the interface, is crucial for reducing losses in SHJ solar cells and leads to very high efficiencies. At present the world record efficiency of 25.6% by Panasonic for SHJ cells is even higher and is obtained on larger area than that of wafer-based technologies using conventional diffusion processes for junction formation. SHJ devices have lower mechanical stress, which facilitates the use of thinner wafer material. This in turn facilitates a major contribution to cost reduction and improves the environmental profile of solar electricity significantly. We aim to further develop this by (i) smart defect engineering. In this approach new TCO�s deposited by Atomic Layer Deposition are implemented; and (ii) the development of new silicon heterojunction cell structures and their production technology.

Speaker(s):
  • Ruud Schropp, ECN - Solliance, Eindhoven University of Technology

Zinc Oxide Nanorod P-N Junction-Based Piezoelectric Energy Generators: Metrological Considerations of Origin of Charge Generation

Apr 7, 2015 10:45am ‐ Apr 7, 2015 11:15am

Identification: P1.06

Self-powered piezoelectric systems are vital components to harvest ambient waste energy for applications such as autonomous self-powered sensors. ZnO nanorod-based devices are gaining wide attention for energy harvesters as they are easily synthesized at low temperature onto a range of substrates - including flexible ones. However, losses related to screening of piezoelectric polarisation charges by free carriers in ZnO nanorods can significantly reduce the output of these devices. The surface chemisorbed and physisorbed species on ZnO reduces the piezoelectric voltage generation and reliability by increasing the carrier concentration and therefore the internal screening. In this presentation we discuss approaches to reduce this internal screening through surface passivation of the ZnO nanorods. The electrical field which can be delivered using a strained ZnO nanorod energy harvester is related to the rate at which the depolarising field is set up and the rate at which piezoelectric polarisation charges are separated in the material. This balance between charge leakage and stored charge is related to the surface and bulk conductivities of the materials system and we show how this may be both modelled and optimised. Though there is still some controversy in the community concerning the origins of the charge measured in many ZnO systems and by many labs, in our work, we demonstrate that the effect is very likely to be piezoelectric in nature. Controlled vibration testing of the devices provides strong evidence that the effect results from a piezoelectric response in the material. The generated voltage increases linearly with the pressure applied to the sample, which is expected for a piezoelectric effect. When the devices are illuminated the voltage output drops significantly. This is attributed to the photo-induced conductivity of the ZnO, which is known to reduce the piezoelectric coefficient due to screening by conduction electrons. The input force to the hybrid device is measured using controlled bending. By simultaneously measuring the output power of the devices the energy conversion efficiency is calculated to be 0.0067 % with slow bending, increasing exponentially with bending rate up to ~8.5 % with much faster bending. This increase agrees with our proposed model of screening-limited energy output. These results not only demonstrate an alternative approach to the design of a ZnO nanorod energy harvesting design, but also contribute to the understanding of the factors that limit the device performance. Finally, we summarise some of the issues surrounding the importance of how measurements are made in these nanosystem energy harvesters.

Speaker(s):

Influence of Lamination Process and Materials on the Residual Stress in the Silicon Solar Cells as Revealed by Synchrotron X-Ray Microdiffraction

Apr 7, 2015 11:00am ‐ Apr 7, 2015 11:15am

Identification: E1.05

In this study, we investigate the effect of two polymer encapsulation with different material properties such as Young�s modulus (E), yield strength etc...on the residual stress gradients of silicon. We observe through synchrotron X-ray microdiffraction that, solar photovoltaic (PV) module laminated with encapsulants A and B which have Young�s modulus of 6.34 and 28.32MPa respectively, reveals distinct variations in residual stress of silicon. The residual stress of silicon near the solder (stress concentration region), showed a maximum quantitative value of ~300 MPa with encapsulant A whereas for the solar PV with encapsulant B, it showed a much higher value of ~450 MPa. Further, this residual mechanical stress and its relation to fracture/crack initiation events of silicon were understood using three point bending tests. The result shows that with encapsulant A, crack initiation of silicon at force of 37KN is observed whereas for the PV with encapsulant B, silicon cracked at much lower force of 10KN. These studies confirm that encapsulant materials have a significant effect on the residual stress of silicon which directly affects the working efficiency and reliability of the solar PV.

Speaker(s):

Direct Synthesis of Advanced van der Waals Heterostructures Based on Graphene, MoS2, and WSe2

Apr 7, 2015 11:00am ‐ Apr 7, 2015 11:15am

Identification: O1.06

Heterogeneous engineering of two-dimensional layered materials, including metallic graphene and semiconducting transition metal dichalcogenides, presents an exciting opportunity to produce highly tunable electronic and optoelectronic systems. The direct growth of such heterostructures is desirable to engineer pristine layers and their combination as van der Waals heterostructures. We reported the direct and epitaxial growth of crystalline tungsten diselenide (WSe2) and molybdenum disulfide (MoS2) monolayer on epitaxial graphene (EG) grown on SiC and the applications on these two single-junctions, WSe2/EG, and MoS2/EG, respectively. With an atomically sharp interface, the vertical transport measurement across the WSe2/EG junction provides evidence that the interlayer gap between the layers adds a barrier to carrier transport, while the photosensing measurements carried out on the MoS2/EG photosensors shows a 20 x increased photon responsibility comparing with bare 1L MoS2 photosensors. In the end, the possibility to directly grow MoS2/WSe2/EG and WSe2/MoS2/EG, the double-junction having strong interlayer coupling due the type-II band alignment is presented.

Speaker(s):

Dynamic Coupling at the Nanoscale

Apr 7, 2015 11:15am ‐ Apr 7, 2015 11:30am

Identification: L1.08

Understanding collective and emergent behaviors of active colloidal assemblies provides useful insights into the statistical physics of out-of-equilibrium systems, thereby enabling researchers to better engineer and utilize many body dynamics at the submicroscopic regime. Recently, there has been a surge in the development of model systems to investigate controlled transfer of energy from self-propelled microswimmers � such as bacteria, algae, and inorganic catalysts to their immediate surroundings. Herein with a series of experiments, we demonstrate that nanoscale catalytic swimmers also can distribute momentum around their vicinity, significantly influencing the motion of nearby passive tracers. We measured diffusion of polymer tracers during enzymatic catalysis using various analytical techniques such as dynamic light scattering, fluorescence correlation spectroscopy, and diffusion nuclear magnetic resonance spectroscopy. In all the measurements, the diffusion of tracers was found to enhance substantially during enzymatic turnover of substrates. The increase in tracer diffusion was found not only to depend on the tracer size but also on the total rate of reaction, which is similar to the observations reported for particles near active micron-scale swimmers.

Speaker(s):

The Effect of ALD Temperature on Border Traps in Al2O3 InGaAs Gate Stacks

Apr 7, 2015 11:30am ‐ Apr 7, 2015 11:45am

Identification: AA1.10

For future high performance III-V n-channel MOS devices, In0.53Ga0.47As is a promising material for the channel due to its high electron mobility. Atomic layer deposited (ALD) Al2O3 has a large conduction band offset to InGaAs and can form a low defect-density interface with InGaAs.1 Therefore, Al2O3 has received attention as either a candidate dielectric layer for InGaAs nMOSFETs, or as a large band-offset interface layer interposed between the InGaAs channel and a higher-k dielectric such as HfO2.2 Apart from the well-known oxide/InGaAs interface charge traps that may pin the Fermi level of the channel, traps in the oxide layer, called border traps, may also reduce the charge in the channel and thus degrade the on-state performance of InGaAs MOSFET devices. In this presentation, we study the effects of various approaches to reduce the border trap density, such as variation of ALD temperature, post-gate metal forming gas (5% H2/95% N2) anneals (FGA). Experimental methods employed include quantitative interface trap and oxide trap modeling3, 4 of MOS capacitor data obtained over a range of frequencies and temperatures. With the application of these models, we find that MOS capacitors fabricated using trimethylaluminum (TMA)/H2O at an ALD temperature of 120�C have a considerably lower border trap density (Nbt) while maintaining a similarly low interface trap density (Dit) compared to samples prepared with a more standard 270�C Al2O3 ALD temperature. Large-dose TMA exposure (pre-dosing) of the InGaAs(100) surface prior to Al2O3 ALD is also found to be an important step to guarantee stable electrical quality of the low temperature-deposited samples. To understand the nature of this ALD temperature effect, composition and bonding characterization methods such as XPS and SIMS are employed to probe the origin of the Nbt variation as a function of the structure of the Al2O3 layer. Besides altering the ALD temperature, the impact of other treatment methods on the Nbt, such as variations of H2/N2 forming gas anneal time and temperature, and application of bias-temperature stress, will also be discussed. References 1. J. Ahn, T. Kent, E. Chagarov, K. Tang, A.C. Kummel, and P.C. McIntyre, Applied Physics Letters 103, 071602 (2013). 2. V. Chobpattana, T.E. Mates, W.J. Mitchell, J.Y. Zhang, and S. Stemmer, Journal of Applied Physics 114, 154108 (2013). 3. H. Chen, Y. Yuan, B. Yu, J. Ahn, P.C. Mcintyre, P.M. Asbeck, M.J.W. Rodwell, and Y. Taur, IEEE Transactions on Electron Devices 59, 2383 (2012). 4. Y. Yuan, B. Yu, J. Ahn, P.C. Mcintyre, P.M. Asbeck, M.J.W. Rodwell, and Y. Taur, IEEE Transactions on Electron Devices 59, 2100 (2012).

Speaker(s):

Synthesis of Siox/C Nanocomposite by Mechanochemically Activated Sol-Gel Process Combined with Dry Ball Milling and Its Electrochemical Properties

Apr 7, 2015 11:30am ‐ Apr 7, 2015 11:45am

Identification: I1.09

Silicon-based materials are an emerging anode technology for the use in the next generation electrochemical enegy storage, because it is cheap, widely distributed on our planet and non-toxic to the environment [1]. However, the development of commercial silicon-based anode progresses slowly due to the properties inherent to pure silicon, such as safety and capacity limiting volume expansions during cycling and poor conductive nature [2]. Functionalization of the nanostructured silicon oxide with conductive carbon coating is one of the possible strategies to overcome these problems. Moreover, oxide matrix of silicon benefits to alleviate large volume changes [3-4]. The present work is aimed to develop a novel simple preparation technique of SiOx/C nanocomposite through mechanochemically activated sol-gel process combined with dry-ball milling (DBM) process. In this synthesis, tetraethyl orthosilicate (TEOS), aqueous ammonia and acetylene black(AB) were mixed by a high-energy ball milling at 800 rpm for 4 hours. The resulting slurry was dried at 110 oC for 2 hour in an air oven and then annealed at 600 oC for 2 hours in N2 atmosphere. The obtained sample could be identified as amorphous SiOx/C composite by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) analysis. It could be also seen a field emission scanning electron microscopy (FE-SEM) observation that the SiOx/C composite was composed of large agglomerated micro-sized particles. In order to reduce the particle size, the composite was mechanically milled at 800 rpm for 4 hours by high-energy DBM with additional AB as an additive. The as-milled sample was the SiOx/C nanocomposite consisting of highly dispersed carbon nanoparticles on fine amorphous SiOx particles. Electrochemical properties of the SiOx/C nanocomposite were evaluated by assembling a CR2032 coin-type cell. The galvanostatic charge-discharge test was performed on multichannel battery testers (Hokuto Denko,HJ1010mSM8A) between 0.01 and 3.0 V versus Li/Li+ at a current density of 50 mA g-1. The cell exhibited a highly stable reversible capacity of 620 mAh g-1 at a current density of 50 mA g-1 after 30 cycles with no capacity fading. The SiOx/C nanocomposite was also annealed at different temperatures for in 3%H2+N2 atmosphere and then evaluated its battery performance. References 1) Chang et.al., Enengy Environ. Sci, 5(2012)6895. 2) Yao et al., J. Power Sources, 196(2011)10240. 3) Yan et. al., Sci. Rep. (2013)1568. 4) Guo et al., Electrochim. Acta, 74(2012)271.

Speaker(s):

Sulfuration Resistance Study on the Ag and Ag-Pd Reflectors of GaN-Based LEDs

Apr 7, 2015 11:45am ‐ Apr 7, 2015 12:00pm

Identification: CC2.06

Owing to its high reflectance in the visible region, Ag reflector has been widely applied in LED device and package. However, peoples have found that the Ag reflector would suffer sulfuration. As a result, the reflectance of the Ag reflector film will decrease and the lighting performance of the entire LED device would drop significantly. In this study, we first studied the sulfuration resistance of the Ag reflector. The sulfuration resistance test was done with an immersion process in the 0.01M Na2S solution for 10 minutes. The reflectance was measured by PerkinElmer Lambda35 UV/VIS Spectrometer. We found that the Ag reflector would be tarnished seriously by sulfuration and the reflectance of the tarnished Ag reflector drops below 40%. An Ag-Pd alloy reflector was produced by the electroless plating process. Then, the Ag-Pd alloy reflector was also tested with the sulfuration resistance. The preliminary results show that the Ag-Pd alloy reflector only drops about 15% after the sulfuration test. Its reflectance is still above 70% Compared to the pure Ag reflector, the Ag-Pd alloy reflector layer shows a good sulfuration resistance in this work. In this talk, we will also present the XPS and XRD study on the Ag and Ag-Pd reflectors after the sulfuration resistance tests. Also, both the Ag and Ag-Pd reflectors would be processed as the reflector layer in GaN light-emitting diodes (LEDs). The light output power of LEDs with the Ag and Ag-Pd reflectors would be analyzed after the sulfuration resistance tests.

Speaker(s):

One-Step Synthesis of Van der Waals Heterostructures between Multi-Layer Transition Metal Dichalcogenides, MoS2 and WS2

Apr 7, 2015 11:45am ‐ Apr 7, 2015 12:00pm

Identification: O1.08

Transition metal dichalcogenides (TMDCs) are a promising class of two-dimensional (2D) materials for use in 2D electronics. This is due to an inherent band-gap that can be direct or indirect, depending on the number of layers, unlike graphene which has no bandgap in its pristine condition. For a wide range of applications, TMDC heterostructures or more broadly van der Waals (vdW) heterostructures are expected to show interesting properties due to interfacial effects. Furthermore, vdW heterostructures avoid any problems arising from lattice mismatch that affect other types of heterostructures. Current studies on heterojunctions between TMDCs have primarily focused on mechanical (or chemical) exfoliation or co-vapor growth methods. These methods are limited by the size and non-uniform distribution of the heterostructures on the substrate. Any application of TMDC heterostructures needs a viable method to produce large-area, patternable heterostructures. Recently we have demonstrated a large-area synthesis of TMDC thin films via vapor transport. In this presentation, I will discuss a one-step synthesis procedure we have developed for stacked heterostructures of multi-layer MoS2 and WS2. This heterostructure forms a p-n junction, so potential applications include the formation of thin, flexible 2D transistors. This method has the key advantage over current methods that it can be patterned, is large-area, and is suitable for scaled synthesis of devices. We have characterized our synthesized heterostructures with Raman Spectroscopy and cross-sectional transmission electron microscopy. Our device shows transport measurement consistent with what we would expect for a p-n junction device. This work lays the ground work for further studies on patterned synthesis of TMDC heterostructures.

Speaker(s):
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