Third SMASH Newsletter published

The third SMASH Newletter has been published and sent to those of you who are registered as subscribers. The SMASH Newsletter is published twice a year. The third SMASH newsletter can also be downloaded as a pdf-file here.

SMASH Training Workshop at ILC

The SMASH Training Workshop “Analytics and Simulation” was organized on 30 January – 1 February 2011 by ILC Bratislava. 29 people participated to the workshop in total.

The Training Workshop was organized to share the knowledge in physical and chemical analytical techniques gained within the SMASH project to the scientific community participating to the project and to present some non-confidential results to people from academia.

The workshop comprised 12 tutorial lectures of specialists from eight different SMASH partner institutions. The lectures covered overview of SMASH objectives and goals, advanced nano-LED structures and technologies, modeling and simulation of progressive quantum-sized structures, and principles, techniques, sample requirements and application examples of various physical and chemical analytical techniques intensively applied to diagnostics of various nanostructured materials and devices developed within the SMASH project.

Most of the lecture materials were collected from the presenting authors and these are available upon the request to SMASH partners.

To promote the workshop and spread information about the program a specific webpage was created and published at http://www.ilc.sk/smash/.

A short summary and the impact of SMASH consortium on IWN-2010

The International Workshop on Nitride Semiconductors 2010 (IWN2010) was held at the Marriott Tampa Waterside Hotel & Marina in Tampa, Florida, U.S.A., September 19 – 24, 2010. This biennial workshop was the sixth following those in Nagoya, Aachen, Pittsburgh, Kyoto and Montreux and complements the biennial series of the International Conference on Nitride Semiconductors (ICNS). This workshop received 719 abstracts which gave rise to 385 oral presentations, 239 poster presentations and 793 registrations.

From my point of view, IWN-2010 was an interesting and fruitful conference on the new developments in nitride research. It was impressive to see how fast nitride research moves both in width and depth. People are looking for new nitride material and structures with superior properties, for example, the hot topics on non-polar and semipolar GaN based devices from UCSB (University of California, Santa Barbara) and bulk GaN from Kyma. More exploration on new applications of III-nitrides for electronic devices, solar cells and chemical/bio- sensors were reported during the workshop, besides the traditional applications in LEDs and laser diodes. On the other hand, detailed studies were presented that reveal the properties of III-nitride material, structure and devices. One of the keynote talks from Chris Van de Walle from UCSB discussed the loss mechanisms in nitride LED and lasers, especially the mechanism of efficiency ‘droop’, which has been disputed for several years. Based on their first-principle studies, he suggested that phonon-assisted Auger recombination is responsible for the efficiency droop. The Auger coefficient, which is mainly contributed by phonon-assisted Auger processes, increases with decreasing bandgap, which would be an explanation for the occurrence of the ‘green gap’. On the other hand, Prof. Hiroshi Amano (from Nagoya University, Japan) stated in his keynote talk that at not very high injection, the main factor for low efficiency was the defect density (dislocations, stacking faults, etc..). Besides LED related topics, a lot of activities concerning N-polar GaN for electronic devices were reported. For example, in an III-nitride HEMT, the channel of a 2DEG can be flipped upside down by using N-polar GaN GaN/AlGaN heterostructures, which has many advantages over traditional AlGaN/GaN structures. Also, there was emerging interest on new fields in IWN-2010, reflected by new sessions on photovoltaic and sensors, as well as MEMS device, which were not included in former ICNS-2009 and IWN-2008 conferences.

GaN nanorods (or nanowires) gained more and more attention, especially concerning material growth and device applications in IWN-2010. Beyond the many talks and posters in the nanostructure session, where nanorods played an important role, there were already many talks about the novel applications of GaN nanowires in other sessions, as well. For example, the talk from Prof. Palacois from MIT showed that GaN nanowires can be used as a channel to overcome the limitation of nitride based transistors, i.e. access resistance and short channel effect. III-nitride nanowires were also suggested by Jonathan J. Wierer from Sandia National Laboratory to be used in solar cells in order to achieve better performance. Many of the presentations in the nanostructure sessions were impressive and were of high quality. Prof. Kishino from Sophia University in Japan reported very nice results on selective area growth of GaN nanorods by MBE. They showed optically-pumped stimulated emission and electroluminescence from their nanoLED arrays. Dr. Lymperakis from the Max-Planck-Institute in Germany showed nice results on the simulation of nanowire growth, which explained some basic phenomena during GaN nanorod growth.

In IWN-2010, there were 2 keynote talks and 4 invited talks on GaN nanorods (or nanowires). One of the two keynote talks was given by Prof. Enrique Calleja (Universidad Politécnica de Madrid), the leader of SMASH work package 4. Three of the 4 invited talks were given by SMASH partners. Besides these, there were many contributed presentations from the SMASH consortium members.

Prof. Calleja, gave a very nice summary keynote talk on the mechanism of self-assembled and selective area growth of GaN nanorods by MBE. He addressed the current status of GaN nanorod research, the achievement and problems in nanorod growth and applications. His talk gave some examples of efficient white light generation from self-assembled arrays of nanoLEDs, particularly from Sophia University and Michigan University. Finally, he concluded that the GaN nanorod has reached a level of maturity although there are still many open questions.

Prof. Waag (Braunschweig University of Technology, WP4 partner) had an invited talk on nanorod applications in solid state lighting. He pointed out that the development of GaN nanorod devices is now heading into the “quantitative” phase, where real devices with real figures of merit are to be expected. He presented the growth and characterization of core/shell GaN nano-emitters by selective area MOCVD growth. This result was highlighted during the last day of the conference in the summary session.

Dr. Geelhaar (Paul-Drude-Insitute, partner in WP1 and WP4) gave an invited talk about MBE growth of GaN nanowires, where he compared collector-induced and self-assembled GaN nanowires. Based on detailed investigations, they suggested that collector-induced nanowire growth leads to more stacking faults, lower and broad PL emission, which means worse nanowire quality.

Prof. Witzigmann (University of Kassel, WP2 leader) was invited to give a talk in the theory and simulation session on the modeling and analysis of III-nitride nanorod LEDs.

Besides these, there were many interesting contributed presentations from our SMASH consortium. For example, Dr. Hans J. Lugauer (OSRAM, leader of WP5 and partner in WP2) reported systematic investigations on selective area growth and characterization of N-polar GaN nanorods as well as theoretical work on photonic crystal effect in GaN nanorod ensemble. Dr. Philip Shields (from University of Bath, WP3 leader) presented his work on coalesced layer growth using nanorod templates. Both presentations were highlighted during the Friday summary sessions. There were still many other contributed presentations from our SMASH partners which are not possible to mention them all in detail due to the length of this article.

During IWN-2010, there was also a rump session on nanostructures, where, Prof. Calleja, Prof. Waag and Dr. Geelhaar (all 3 from the SMASH consortium) were three of the session chairs. In the rump session, the benefits of GaN nanorods were addressed. Three challenging issues which people are facing currently in GaN nanorods growth and applications were discussed:  GaN nanorod growth (growth method, doping and defects), InGaN growth (strain effect, diameter dependence) and GaN nanorods based devices. At end of the session, Prof. Waag proposed to set up a road map for future GaN nanorod applications considering the problems we face now and possibly will have in future. The results were in general quite optimistic. With those present believing that there is a bright future for GaN ‘nanoLEDs’.

In general, I found the nitride community has moved very fast both in research and applications. Our SMASH partners have made a strong impact on this conference by their impressive work.

Shunfeng Li was born in Hebei, China in 1974. He received his B.S. degree in Physics from Peking University, Beijing, China in 1997, and his M.A. degree from the institute of semiconductors, Chinese Academy of Science in Beijing in 2001. In 2005, he got his Ph.D degree from Paderborn University in Germany. His master and Ph.D work were devoted in cubic III-nitride MOCVD and MBE growth and characterization. Before joining the Braunschweig team, he worked as a postdoc in Karlsruhe Institute of Technology (KIT), focusing on MBE growth of InAs quantum dots based optical amplifier, microcavity and spin-injection LED. Currently, he is a postdoc researcher in Braunschweig University of Technology (TU-BS). His research interest includes MOCVD growth of GaN nanorods, nanoLEDs and their characterization.

General Assembly Meeting in Bath

During October 7th and 8th 2010 all partners of the SMASH project gathered in the worderful city of Bath for a General Assembly Meeting. The activities for the first year of the project was reviewed and progress was evaluated. Before ending the meeting a detailed plan for the coming 6 months was set up. The meeting was hosted by Duncan Allsopp and Philip Shields from the University of Bath.