Day 1, Monday, March 24, 2008

ENERGY FORUM

No sleepy start at this MRS Spring Meeting! MegaMonday got off to a strong opening with four lectures focusing on the critical problem of energy. The MRS Bulletin has just released a special expanded issue of the MRS Bulletin on the topic of "Harnessing Materials for Energy." The Energy Forum was organized to complement this special issue and featured four talks by scientists and researchers exploring different areas in energy. Using the metaphor of energy as a large puzzle, with various technologies forming pieces of this puzzle, the forum explored how these pieces could fit together to solve what is arguably humankind's greatest near-term challenge, namely energy generation with minimal impact on the environment.

Delivering the opening remarks to a full house, George Crabtree from Argonne National Laboratory emphasized the role of materials in possible solutions to the global energy crisis. He stressed that global energy needs are expected to double by 2050 and triple by 2100, driven especially by the growing energy needs of developing countries.

ENERGY OVERVIEW
George Whitesides from Harvard University delivered the first lecture of the Energy Forum and succinctly outlined the opportunities for materials science in energy, sustainability, and global stewardship. He emphasized that issues such as energy, climate, water, and sustainability are fundamentally interconnected and that these are enormous problems with the potential to bring great conflict. Whitesides proposed the formula, well-being = available energy/number of people, and stressed that there is a great need to both produce more energy as well as conserve energy that is currently being produced. In particular, the scarcity of water threatening several parts of the world has the potential to further exacerbate the energy crisis since significant energy may have to be invested in water production in the future.

Whitesides went on to outline the major sources of energy that are currently used along with the outlook for these energy sources over the next few decades. Fossil fuels such as hydrocarbons, coal, and gas are the mainstay of our current energy needs, but it may well be that their role in inducing deleterious climate change makes them unviable options in the very near future. Furthermore, nuclear energy is plagued by concerns about proliferation and waste disposal, whereas hydroelectric power generation is currently almost at peak capacity. Solar energy represents a promising alternative but very significant improvements in the design and cost of solar cells are required to harness the dilute energy of the sun. Whitesides emphasized that notwithstanding the claims of a hydrogen economy, it is important to realize that hydrogen is a method of transporting energy and not a source of energy. In that sense, hydrogen is really like electricity—it is not a solution by itself to the global energy problem.

In the short term, he proposed that more research attention be focused on the conservation of energy, such as the development of materials that reduce the tremendous power losses during transmission and the design of better illumination sources such as bright light-emitting diodes. There is also a great need to globally develop carbon management strategies to deal with the CO₂ being released into the atmosphere by burning fossil fuels. Whitesides outlined several topics that he believed are fertile grounds for the next generation of material scientists, including catalysis, separations, development of materials with extreme properties, being able to move electrons more efficiently in matter, scaling to nanoscale dimensions, and matching the complexity of biological processes such as photosynthesis. Given the current corporate atmosphere that emphasizes short-term returns, Whitesides stressed that this effort will have to be lead by research universities, perhaps in collaboration with venture philanthropic organizations and local and federal government. “The good news is that this is an issue that the public deeply cares about in one form or the other”, said Whitesides, “the problem is that even if we apply all that we know, we still come up short,” adding that since the basic knowledge does not exist, we have no alternative but to go out and create new knowledge to solve these problems.

BIOFUELS and BIOMASS
Chris Somerville of the University of California—Berkeley discussed the area of biofuels. With the recent world-wide increases in the cost of petroleum and related fuels, ethanol has very much been in the news. However, grain-based ethanol directly affects the food chain. Somerville then focused on cellulosic fuels as an alternative to grain-based sources. In particular, he discussed Miscanthus, which is a perennial grass and which can yield over 26 tons/acre of land without irrigation, which two and a half times as much as switchgrass. He ran though the process of producing syngas using cellulosic sources. He also discussed the problems and issues associated with breaking down cellulose to yield fuel. Somerville touched upon an alternate to ethanol in the form of alkanes.

He concluded by describing his visions for the field. This includes replacing grain-based fuels by cellulosic fuels thereby disassociating biofuels from the food chain. It also includes greater use of sugarcane which reduces the area of land used and which yields both sugar and cellulose. He hopes that biodiesel can be obtained from cellulosic sources rather than vegetable oils. He also believes that ethanol as a fuel can be replaced by other options such as alkanes. Finally, synthetic catalysts can make a big difference in the business of producing fuels from biological sources.

CATALYSIS
The broad field of catalysis was discussed by Daniel G. Nocera of the Massachusetts Institute of Technology. In terms of the environmental issues relating to fossil fuels, it is the CO₂ content that is of concern. The only solution is to cut the tie between energy use and carbon. Solar energy is the only sustainable, renewable, and carbon-neutral source of energym, whether it is biomass or photovoltaics. The highest energy density is achieved in chemical bonds. Nocera described photosynthesis and attempts to create artificial photosynthesis. At its very essence, solar energy conversion boils down to water splitting. The basic science for this requires multi-electron proton-coupled electron transfer. He described in some detail work done by his group as well as other groups on O-O bond manipulation which is important in biological photosynthesis.

In conclusion, Nocera offered several take-home thoughts. The current need for energy is so enormous that conventional, long-discussed sources will not be sufficient. New science is required for this purpose. Solar (direct) + water (indirect) has the capacity to meet future energy needs. However, large expanses of fundamental science need to be discovered. According to him, renewable energy research not an engineering problem, rather, it needs to be tackled as a basic science problem and new materials, catalysts and many new modes of reactivity await discovery. Finally, as per Nocera, Chemistry is the central science of energy because it involves light capture and conversion with new materials, and energy storage in bonds or new materials.

SOLAR TECHNOLOGY
The fourth presentation of the MRS Energy Forum was on Solar Technology by Prof. Martin Green, Research Director at the Photovoltaic Centre of Excellence at the University of New South Wales, Australia. Green gave a comprehensive overview on the current and future opportunities of photovoltaics, predictions and new trends in solar technology including inorganic, organic and hybrid systems thereof. “Can Photovoltaics power the Future?” was his provocative question to introduce his talk in front of a packed auditorium with over 400 attendees and commenced with a clear “Yes”! The green energy market is booming with growth rates around 40% over the last decade with direct water heating being the largest application due to an Australian process that the Chinese adopted and continue to implement rapidly which represents by far the largest market. The picture on solar cell production gives a similar picture: Japan with 36%, followed by Germany with 20% and China with 15%. Asia overall has a market share of over 63% in 2006 compared to the USA with just 6.8%. The world energy needs could be satisfied by a square with a 700 km baseline (10% efficiency). Comparing concentrator cell technology and just looking at the mirror cost (troughs at $250/m2; flat mirrors at $40/m2) not to mention durability in a dessert climate with significant erosion indicates that direct conversion with low cost thin film solar cells appears the solution to become economical competitive. However, current installation costs are too high and only competitive in remote areas where grid connection would be even more expensive. PV power cannot compete on wholesale but at retail price. In Italy for example, the current power cost per kW is with 22 cts already overlapping with PV power cost. The demarcation line of profitability has started to move up through Europe. The cost of power in Germany has actually dropped over the years and the government has tapped into that funding source by raising taxes and using some of those funds to support green energy development and implementation especially wind and solar power. In the US, Macy's, as the first large corporation, has gone green with 26 stores.

Silicon is still the predominant solar cell material now outpacing computer applications by volume, which has put pressure on supplies. This has led to efforts for making solar cells as thin as possible, currently at 200 microns and targeting 100 micron thickness in the near future. In addition, metallurgy grade still works well for solar cells moving away from the much more expensive semiconductor grade material. This has led to the search for cheaper alternatives such as amorphous, microcrystalline, and polycrystalline materials, however, they do come with an efficiency penalty (~10% efficient) Other promising systems are CIS and CIGS [Cu(In,ga)(Se,S)2] with ~18% efficiency and CdTe (however, produced a Te shortage), dye sensitized (Grätzel) and organic solar cells. Green pointed out desirable properties such as low materials cost, abundance, non-toxic, large manufacturing capability, fully integrated modules, ruggedness and durability as well as high efficiency. He then took a look at reduction in cost per kW over the last 20 years to estimate future competitiveness: The early PVs were at 20,000/kW (1981) down to 3,000/kW (2002) with thin film systems having the potential to actually reach competitive prices with current gas turbines at some future point. Silicon is unlikely to get us there since the wafer costs are just too high and will not come down much further. He closed his presentation by touching upon several new concepts that show promise such as circulator approaches, hot carrier systems, thermal PV, thermionics, quantum dot systems, interband converters. Due to the high installation cost, market support is needed to help commercialize PVs with projections of 1% solar cell power generation by 2020 (Bavaria, Germany, not exactly a desert with a lot of sun, is already at 1.4%!), 25% by 2050 and 64% by 2100. We are already ahead of the first prediction. We can do our part to help continue to accelerate that trend.

PLENARY SESSION

The major highlight of the day was the plenary session held in the evening. This was unusual for a Spring Meeting for the plenary session to be held on a Monday rather than the usual Wednesday, however, this was in line with the overall theme of energy with the plenary talk focusing on energy as well. First, 2008 MRS President Cynthia Volkert welcomed everyone to the session and the conference. She mentioned some facts about MRS and its membership which has touched 15,000 recently. In addition, this Spring conference already has a record number of attendees reflecting the recent growing trend for the Spring Meeting. She mentioned the upcoming retirement of John Ballance, the Executive Director of MRS, who has played a key role in the success of the organization. A search is under way for the next Executive Director.

The officers of the Society were then recognized. The four chairs of this Spring Meeting, Jeffrey C. Gelpey (Mattson Technology), Robert J. Hamers (University of Wisconsin-Madison), Paul Muralt (Swiss Federal Institute of Technology) and Christine A. Orme (Lawrence Livermore National Laboratory), were recognized for their efforts in organizing this successful conference. The volunteers responsible for putting together the special MRS Bulletin issue on Energy as well as the Editor, Betsy Fleischer, were specially recognized. Finally the newest MRS Student Chapter at the Univ. of Texas at El Paso was officially initiated. This was followed by the main draw of the evening, the plenary talk by Michael Graetzel.

Power from the Sun - The Advent of Mesoscopic Solar Cells
Solar power is clearly going to be an important piece of the energy solution in the near future, said Michael Graetzel of Ecole Polytechnique Federale de Lausanne, Switzerland, beginning his plenary lecture. While most of the focus has been on photovoltaics, there is a new paradigm, mesoscopic solar cells based on interpenetrating network junctions. These include dye sensitized nanocrystalline solar cells (DSC). This was the focus of Graetzel's talk. The workhorse dye used is a Ruthenium based dye. The dye-sensitized nanocrystals can achieve a quantitative conversion of photons into electric current. He described various details of the process involved. He made the point that charge separation in DSC is by kinetic competition similar to natural photosynthesis. Currently the solar to electric power efficiency has been demonstrated to be above 11%. In particular, DSC outperform silicon at low light levels. The efficiencies of DSC can still be vastly improved according to theoretical predictions.

Graetzel then described examples of various materials that can be used for energy harvesting and conversion and that are under investigation, including advanced nanostructures, new sensitizers, and quantum dot sensitized cells. Solar cells with different colors, aesthetically pleasing can now be produced. A commercial production plant for DSC is now in operation in Cardiff, Wales, and the first product is a light-weight power supply for mobile phones. He also mentioned an educational component where these types of solar cells can be made by school children. Finally, Graetzel described solar hydrogen generation using rust (Fe₂O₃) as the catalyst. He concluded by suggesting that DSC have a crucial role to play in future solar power generation, helping with the overall energy problem.

TUTORIALS

Tutorial C: GaN, GaAs, SiC and Related Alloys on Silicon Substrates
Hongxing Jiang of Kansas State University and Edward Yi Chang of the National Chiao Tung Unviersity, Taiwan taught Tutorial C. The authors divided their workshop into three parts: The first and second parts focused on materials challenges one faces when trying to grow GaN and GaAs respectively onto Si substrates due to lattice mismatch and the relative large thermal expansion mismatches. They then discussed various approaches taken to overcome those challenges and how they affect electrical and optical properties, followed by device performance. The third part discussed basics of high speed III-V electronic devices, comparing their performance and advantages with Si. Jiang introduced the topic by reviewing the rapid growth of GaN devices and applications in the last 15 years fueled by the success in LED development. Now, GaN devices have applications in almost all fields of modern technology such as automotive, aircraft, wireless communication, energy, medicine, defense and of course lighting. The applications are driven by HB-LEDs (>95%) with the projected revenues surpassing $4B this year.

The second section started by reemphasizing photonics applications such as optical circuits and very large scale integration for key applications of III-V on Si. A Si substrate would significantly reduce manufacturing cost compared to InP and GaAs substrates. The combination would allow for the realization of the ultimate vision of high switching activity factor low-voltage high–speed III-V based logic circuit blocks coupled with the functional density advantages provided by the Si-CMOS platform. Current challenges to overcome for GaAs growth on Si is the >4% lattice mismatch that leads to large number of threading dislocations and the large thermal expansion mismatch. This scenario requires a heterogeneous integration approach. Several solutions have been developed and were discussed in detail: Two step SiGe buffer (NCTU), metamorphic GaAs on Si (Intel), selective area growth combined with thermal cycle annealing (MIT), SrTiO₃ , ZnSe and compositionally graded buffers.

The third section focused on high speed III-V devices introducing HEMTs, high electron mobility transistors, modulation doping and quantum well structures. The devices evolved from GaAs-based to InP-based HEMTs and HBTs leading to next generation Sb based devices. In turn, various concepts were introduced, and examples were discussed, including fabrication processes and the challenges of producing good ohmic contacts due to the doping limitations in GaAs. The tutorial closed by discussing challenges and technological trends. Three key challenges were identified which are the need for high k-gate dielectrics to minimize leakage current and stand-by power consumtion; low hole mobility, which has led to renewed interest in Ge having the highest hole mobility; lastly, heterogeneous integration needs to be achieved to reduce manufacturing cost and to realize the ultimate vision of high switching activity factor low-voltage and high–speed III-V based logic circuit blocks coupled with the functional density advantages provided by the Si CMOS platform.

Additional information
Since the III-nitrides cover the wavelength range from IR to UV with bandgaps ranging from 0.7 eV (InN) to 6.3 eV (AlN), which is of great interest for photonics and communications for integration of optical communication and high power computing. Silicon is the dominant computing material and cheap (6” <$20/wafer) with very large wafer processing (12”). Therefore, III-nitride –Si integration is highly desired because it would open up a multitude of possibilities. Therefore, many universities are working in this area. MOCVD and MBE growth techniques are being employed to grow GaN on Si. Due to the 13% lattice mismatch, a 30% rotation of the GaN (0001) plane about the c axis is observed. GaN (0002) grows on Si(111) for example. The difference in thermal expansion leads to tensile stresses during cool-down, which causes broadening of the Bragg peaks in the XRD spectrum. Wafers larger than about 2” and layers thicker than about 1 micron lead to significant bowing, which prevents further processing, but 1 micron is typically not enough to reduce dislocation density to an acceptable level. One widely used solution is the use of buffer layers such as Ga, HT/LT AlN or GaN by MOVPE or an oxidized amorphous/fine grain alumina layer. In this way, most lattice mismatch can be accommodated.

Most common is now the use of HT and LT AlN buffer layers, which protects the Si substrate form nitridation due to higher AlN bond energy. Other strain engineering and compensation of tensile strain by the application of strained layer superlattices (SL), graded AlGaN layers and SOI as complaint layer. Nitronex developed a very thin graded AlN to GaN transition layer, which resulted in almost no cracking, but changing the growth conditions for such a system continuously is not easy. SLs act as a filter for threading dislocations as well as changing growth conditions, which helps to stop stacking faults.

A particular problem poses the fact that the highly dope Si substrate commonly used is rather conductive (impurity doping) preventing the testing of the GaN layer. Highly pure Si again is very expensive. However, Hall measurements are possible when using insulating SOI substrates. After the discussion of different growth schemes of films and columns, Prof. Jiang showed examples of the electrical and optical properties that have been achieved on Si, which still lags behind that of GaN materials deposited on sapphire.

The first UV and violet GaN LEDs were fabricated by IBM in 1998 after p-GaN was demonstrated. Multicolored LEDs were realized by using organic fluorescent materials.

This first part closed by reemphasizing that materials improvements are needed to reduce dislocation densities for computing applications.

Tutorial P: Synthesis, Characterization, and Applications of Carbon Nanotubes and Related One-Dimensional Nanostructures
Tutorial P was focused on the synthesis, characterization, and applications of one-dimensional nanostructures and provided attendees with a snapshot of the current excitement in these emerging areas. The first instructor, Li-Chyong Chen from National Taiwan University gave an overview of the growth mechanisms and unique properties of 1D inorganic nanowires that make them promising candidates for applications in areas such as lasing, sensing, and high-brightness light-emitting diodes (LEDs). Chen described the various growth mechanisms thought to be involved in the synthesis of the nanowires by vapor transport and chemical vapor deposition. She also provided experimental advice for those considering establishing such setups, illustrating the various practical issues involved with results from her own group on the growth of GaN and InN nanowires. Notwithstanding the contradictory mechanisms reported in the literature, catalyzed growth using metal nanoparticle catalysts provides substantial control over the size, precise location, and diameter of the resulting 1D nanowires. Chen and her collaborators have been able to synthesize nanowires with dimensions significantly below the Bohr radius, thus allowing measurement of interesting electrical and optical properties arising from quantum confinement.

The second speaker, Zhong Lin Wang from Georgia Institute of Technology focused on the catalyst-free growth of nanostructures with various morphologies including nanobelts, nanohelices, nanosaws, and herring-bone structures. Wang emphasized the use of high-resolution electron microscopy and electron diffraction to obtain insight into the structural details of these novel nanostructures. Detailed structural characterization provides insight into the nanowire growth process, as Wang illustrated with examples of GaN and ZnO nanostructures grown by his research group. In the second half of his talk, Wang focused on practical applications of these nanostructures as piezoelectric energy generators, UV photon detectors, and high-sensitivity sensing elements. Wang also discussed the use of in situ electron microscopy to study the remarkable mechanical properties of these nanostructures.

The final speaker, Meyya Meyyappan, from NASA gave an overview of the state-of-the-art in the synthesis and applications of carbon nanotubes. Meyyappan started by discussing the structure and remarkable properties of carbon nanotubes that make them promising for applications in areas as diverse as electronics, interconnects, field emission, retinal transplantation, chemical and biological sensing, and high-strength composites. He also outlined the challenges that have hindered their implantation in many of these applications including lack of sufficient synthetic control in growing these structures, the difficulties in engineering their interface with other materials, and the urgent need to scale up the production of nanotubes to lower cost. The speaker described the various growth phenomena involved in the preparation of nanotubes and nanotube arrays, illustrating this discussion with results from his research group and emphasizing the sensitivity of the growth process to various experimental parameters. Meyyappan concluded with a detailed discussion of the outlook for the use of carbon nanotubes in various applications.

MYMRS IPOD WINNERS

- Manijeh Razeghi, Northwestern University
- Jia Huang (Lutherville, Maryland)

© Materials Research Society, 2008