Short and long term scientific perspective



The Spezialforschungsbereich Infrared Optical Nanostructures (IR-ON) has been established to utilize the rapid progress in nanoscience and -technology to overcome the shortage of reliable semiconductor emitters and detectors for the 2-20 µm infrared wavelength region. Nanostructures offer an elegant solution to the "infrared problem". The carrier confinement results in quantized energy levels, enabling optical intersubband transitions at energies in the mid-infrared regime as well as size tuning of interband transitions. The ambitious research program to create novel infrared optical nanostructures and devices and develop a compre-hensive knowledge about their physical properties provides a unique opportunity for Austria to define and emphasize its position in this exciting research field.

The objectives of the IR-ON research program and its long term goals have been to

●    advance nanofabrication through self assembled growth of quantum dots, nanowires, and nanocrystals, as well as their assembly in ordered superstructures,
●    obtain new physical insights in infrared response, carrier dynamics, energy level schemes, and many body effects in quantum confined structures,
●    advance nanocharacterization by developing analysis techniques with very high spatial, spectral, and temporal resolution,
●    develop novel infrared quantum dot and nanocrystal devices,
●    train young researchers in fields of photonics, nanotechnology, and quantum physics,
●    improve the significance and relevance of Austrian science on the international level.

By intense cooperations and fruitful interactions between theory and experiments rapid progress and important breakthroughs have been achieved by the SFB in all fields. Growth of quantum cascade structures, quantum dots and nanowires as well as chemical synthesis of nanocrystals was the starting point of the IR-ON research program. Novel growth techniques have been devised to control the geometry, location and ordering of nanostructures. Vertical alignment and embedding in 2D superlattices have been used for band-structure engineering and development of novel devices. Based on the obtained physical knowledge derived by spectroscopy, nanocharacterization and novel theoretical methods, new infrared opto-electronic devices that make use of semiconductor nanostructures can now be realized. This not only yields drastic improvements of the performance but also boosts the implementation of practical devices with tunable emission as versatile tool for sensing applications. The IR-ON program has generated tremendous synergistic effects and has made significant impact on the educational program of the participating institutions by introducing novel topics into the teaching program and by providing excellent training of young scientists and engineers.

To achieve these goals a cooperative research effort over a period far exceeding the typical lifespan of individual grants is needed to design novel materials with tailored infrared properties, develop advanced characterization methods, and combine them with state of the art theoretical models both for a profound understanding of the underlying physics and for device engineering. The research is based on the intense collaboration of renowned groups from the Johannes Kepler University Linz, Vienna University of Technology, University of Vienna, Technical University of Munich, and Friedrich-Schiller-University of Jena. These groups aggregate a large body of expertise and experience in nanofabrication, analysis, spectroscopy and theory of nanostructured infrared materials, as well as in design and fabrication of optoelectronic devices, which forms a sound basis to achieve the IR-ON objectives. The sharing of expensive infrastructure and facilities such as clean rooms, processing equipments, experimental setups, simulation tools as well as personal resources generates tremendous synergistic effects as well as substantial savings in the required project funding. This has been achieved through the close cross-linking and joint research activities of the IR-ON partners which was established and implemented during the first funding period and has been one of the key factors for the success of this SFB program. This is complemented by a large number of additional national and international collaborations with leading research groups in the field to complement the expertise for IR-ON research. In particular, the incorporation of German participants and other European collaborations has increased the value of IR-ON significantly beyond the national Austrian level.

The IR-ON program has also made significant impact on the educational program of the participating institutions by introducing its topics into the teaching program and by helping to set up new specialized master course program as well as by training young talented scientists and engineers to boost the Austrian activities in innovative research and development. In total, the ambitious research program to create novel infrared optical nanostructures and devices and develop a comprehensive knowledge about their physical properties has provided a unique opportunity for Austria to define and emphasize its position in this exciting research field.
Owing to intense cooperation among the SFB participants, collaborators in Europe and abroad as well as due to the fruitful interaction between theory and experiments, in the first funding period substantial progress and important breakthroughs were achieved. The IR-ON SFB has therefore strengthened the significance of Austrian science as a whole in the emerging field of nanoscience and technology.

Scientific Development of the SFB



The scientific research of the IR-ON SFB was focused on four topical fields, namely, nano-fabri¬cation, new physical insights, nanocharacterization, and novel infrared devices. By intense collaborations and interactions between the IR-ON partners, important break-throughs were achieved in all four topical fields of the SFB.

Electron beam and nanoimprint lithography was implemented in projects P02 and P12 for site-controllled growth of ordered Ge islands on silicon substrates and a process sequence was developed for precise positioning of self-organized Ge islands in registry with photonic crystal structures on silicon-on-insulator substrates. Strong enhancement of photoluminescence was obtained in these structures. In project P03, a novel technique enabling ultra-high strain levels on monolithically integrated VLS-grown silicon nanowires was developed and anomalous piezoresistive phenomena were demonstrated in these wires. GaAs/AlAs nanowires on Si nanowires were demonstrated, forming hierarchical star-like structures with 6-fold symmetry. In addition the InGaAs/GaAsSb material system was developed for fabrication of efficient intersubband devices. In project P04, novel IV-VI quantum dots embedded in II-VI materials with intense mid-infrared emission were fabricated and in project P05 a new synthesis method for silver-chalcogenide nanocrystals was demonstrated. This resulted in infrared emitting and photoconducting materials with extremely small particle sizes.

Physical insights
In projects P12, P02, and P04 the mechanisms of self-organized ordering of nanoislands on prepatterned substrates were unraveled and the polytypism in epitaxial nanowires was clarified by projects P06 and P07 by combining experimental results and ab initio calculations. Project P06 has built a bridge between semi-empirical methods and full ab initio methods, allowing seamless integration of these methods in a unified framework to model realistic nanostructures containing a large number of atoms. In project P13 a sophisticated theory of quantum cascade lasers has been developed which allows to treat incoherent and coherent transport. The time-resolved stimulated emission experiments in P11 showed that hot electron effects play a limiting role for quantum cascade laser performance.

Structural analysis of single nanowires and single SiGe islands in a device structure using focused X-ray radiation was demonstrated in P06. In P08 spectrally resolved scanning photocurrent spectroscopy on single nanostructures was achieved for InAs quantum dots and colloidal PbS nanocrystals. In projects P04 and P02, in situ growth studies of self assembled formation of SiGe and IV-VI nanostructures were performed using scanning tunneling and transmission electron microscopy. Photoluminescence of few SiGe islands and room temperature luminescence of SiGe islands integrated in 2-D photonic crystal structures was demonstrated in P12.


Figure 1: Extended spectral coverage of the infrared and THz region provided by devicesand structures developed within IR-ON



Novel infrared quantum dot devices

Project P04 has demonstrated the first quantum dot lasers emitting in the mid-infrared spectral region. Highly efficient hybrid bulk heterojunction photodetectors based on colloidal PbS nanocrystals, C60 derivatives, and conjugated polymers were developed in project P05. These were integrated into infrared imagers, capable to make real time movies. Project P12 showed quantum well intersubband infrared detectors based on light hole (LH) transitions operating in the THz an mid-infrared spectral region. P03 developed a novel material system for intersubband devices, which was utilized for the realization of novel mid-infrared and THz quantum cascade lasers (P11). Moreover, quantum dot infrared photo detectors combined with photonic crystal structures were demonstrated.