by Michael A. Fury, Techcet Group

April 13, 2010 – The opening day of the week-long MRS Spring 2010 meeting (April 5-9, San Francisco’s Moscone Center) consisted of 10 tutorial sessions related to specific symposia topics, and a light load of four symposia that were scheduled to get an early jump on the week. An eight-hour tutorial on chemical mechanical planarization (CMP) kept the room full all day — even though CMP can no longer be considered a new kid on the block. Other tutorial topics included phase-change materials, organic electronics, and three related to solar photovoltaics. (Underscored codes at the beginning of papers reviewed refer to the symposium, session and paper number; additional presentation details can be found in the MRS Spring 2010 program.)

First, a few quick facts about this year’s MRS spring meeting:

• Forty-three technical symposia, run concurrently
• Over 4,500 attendees from 50 countries
• 2,500 oral papers, 1,700 posters, 115 exhibitors, and 14,000 entries in the authors directory (comparable to the total MRS membership of ~15,500)
• Running a symposium of this nature requires 38,000 chairs
• Attendees consume ~1,000 gallons of coffee, for which the Moscone Center’s coffee service bills the MRS $100,000. (Even by Starbuck’s standards, $100/gallon is a precious brew!)

And now, the papers:

G1.1. Thierry Baron of the Laboratoire des Technologies de la Microélectronique (LTM), CNRS in Grenoble opened the Materials and Physics of Nonvolatile Memories symposium with a review of nanocrystal memories, charge-trapping devices that have found their way into the fabs of collaborators STMicroelectronics, Leti, and Atmel. The structures discussed have been fully integrated with CMOS processing, and the active trapping materials vary from nitrided polysilicon to nanocrystalline platinum.

G2.1. Yakov Roizin of Tower Semiconductor reviewed the first 10 years of local charge-trapping non-volatile memory. NROM (nitrided read-only memory) devices can be configured to trap up to 4 bits in a single transistor, based on spatial separation of charge-trapping regions. Such devices are based on a SONOS structure (silicon/oxide/nitride/oxide/silicon) in which the nitride is configured as a high activation energy charge trap. Designs capable of 10 million cycles have been achieved commercially.

G2.2. Sabina Spiga from Laboratorio MDM in Italy discussed HfO₂ gate-stack engineering for charge-trapping NV RAM. She showed a gate stack migration from SONOS to TANOS (Ta/Al₂O₃/nitride/oxide/Si) to TAHOS (Ta/Al₂O₃/HfO₂/oxide/Si). Good stack performance requires an RTA at 900°-1030°C to form the requisite Al₂O₃ crystal structure.

G2.3. DongSeog Eun of Yale U. characterized a charge-trapping device with a TiO₂ storage layer. This structure comprised Au/Al₂O₃/TiO₂/Al₂O₃/p-Si/Al. Fast program/erase cycles and memory endurance suggest this may be a viable candidate for charge trapping.

HH2.1. Proof-of-concept work on a number of unconventional concepts for printed organic photovoltaic (OPV) devices was presented by Jan Kroon of ECN Solar Energy in Petten, Netherlands. Among their innovations: replace the conventional LiF/Al low work function contacts with ZnO/Ag; replace the ITO conductor with a printed Ag grid; and make these transitions in a manner compatible with roll-to-roll (R2R) high-speed production. In addition to the materials property challenges, the task difficulty is increased by the imposed ban on the use of halogenated solvents, making the chlorobenzene traditionally used for depositing these materials off-limits.

HH2.2. Gang Li of Solarmer in El Monte, CA, described their high-efficiency polymer solar cells. They have recently achieved a cell efficiency of just under 8%, and modeling of this system suggests that 12% is possible. Integration of cell into panels 200cm² gives module efficiencies close to 5%. (Here is a useful reference for the various printing technologies.)

HH2.3. Darin Laird of Plextronics in Pittsburgh, PA, talked about commercial scaling of OPV to the module level. One enabling element is the volume availability of their organic photoactive Plexcore materials, which have been NREL-certified as high as 5.98% efficiency. Light degradation of OPV materials is still an issue. A 500-hour Xe lamp light soak brought a test module from 4.95% down to 4.33%, showing much improvement over the performance of earlier materials.

S2.1. James Hone from Columbia discussed the mechanics and tribology of graphene, a material that was very well represented throughout the meeting. The breaking strength of graphene needs to be tested with a diamond AFM tip — Si AFM tips are not strong enough, and will break. A non-linear model indicates a break strength of 130GPa, making it the strongest material ever measured.

S2.2. Jeremy Robinson at the Naval Research Lab demonstrated a graphene-based nanomechanical resonator, and reported a graphene Young’s modulus of ~1TPa — yes, that is tera-Pascals! His resonator exhibited a very high Q value of 9,315 at 18.7MHz.

S2.3. Rui Huang at UT Austin described the mechanical properties of graphene nanoribbons. The strain behavior differences between the zigzag and armchair edges suggests that extra attention should be paid to edge orientation in graphene-based devices for electron and thermal transport effects.

S2.4. Yunqi Liu at the Chinese Academy of Sciences presented methods for controllable preparation of p-type and n-type graphene and their resulting electronic properties. Undoped graphene is naturally a p-type semiconductor, while nitrogen doping produces n-type. Graphene ribbons were grown by CVD on ZnS ribbons as a template and catalyst. Dissolving the ZnS left free-standing graphene ribbons with high mobilities of 200-450 cm²/Vsec.

Michael A. Fury, Ph.D, is senior technology analyst at Techcet Group, LLC, P.O. Box 29, Del Mar, CA 92014; e-mail [email protected].