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Semiconductor Materials (HMA)

Head of group: Professor Sebastian Lourdudoss

Research Overview

The Laboratory of Semiconductor Materials carries out research and education in the area of materials, structures and devices needed for advanced photonic and electronic applications. With a focus on semiconductor hetero- and nanostructure fabrication, including crystal growth, wafer processing and characterization, we develop novel devices and applications. Our decade-long tradition of strong research on devices in III-V materials for fiberoptic communications is continued while other areas of applications steadily increase in importance. The major current activities are:

• Epitaxy for integrated photonics
• Gallium nitride growth technology
• Silicon based heterostructures
• Vertical cavity devices
• Photonic crystals
• Nanoscale characterization

The rapidly maturing crystal growth technology for III-V heterostructures has moved our research focus from conventional, nearly lattice matches structures in gallium arsenide (GaAs) and indium phosphide (InP) towards strongly lattice mismatched material combinations. One reason for this is that increased photonic integration is becoming imperative to improve functionality and decrease cost of photonic subsystems. We are exploring novel techniques for growing III-V structures on silicon to facilitate integration with silicon waveguide technology as well as electronics. This work is based on our worldwide unique Hydride Vapor Phase Epitaxy (HVPE) facilitiy which combines large growth rate and substrate selectivity and possibility for epitaxial lateral overgrowth.

The novel and fascinating properties offered by gallium nitride (GaN) heterostructures provides the second reason to study strongly lattice mismatched semiconductor systems, since there are no suitable lattice-matched substrates available for these materials. During the last year we have adopted equipment and technology developed for high temperature growth of silicon carbide to growth of GaN with excellent results in crystal quality of GaN on sapphire. This work is now extended towards heterostructures and device applications in terms of ultrafast modulators for 1.55 mm wavelength, white light emitting diodes (LED), nanophontonic structures for improved light extraction in LED´s and GaN/ SiC heterostructures for electronic devices.

Silicon germanium technology is now well established in the world of high performance electronics. To reach a greater flexibility when choosing materials it is necessary to incorporate additional elements to provide not only compressive but also tensile strain. Hence, the growth and characterization of SiGeC-layers is the subject of intensive studies. As for III-V materials, selective area growth is a key technique to push material limits and to allow useful integration in device structures.

The traditional strong focus on Metal-Organic Vapor Phase Epitaxy (MOVPE) for quantum wells in GaAs and InP based structures is currently concentrated on growth and characterization of GaInNAs/GaAs for 1.3 mm wavelength. There is a pressing need for new light sources in this wavelength range and GaInNAs should in theory be well suited provided this metastable alloy could be grown with sufficient quality. However, it is a considerable challenge to control the composition and defect formation at the low growth temperatures required. Applications for both vertical cavity surface emitting lasers (VCSEL) and edge emitting lasers are pursued. The vertical cavity laser is a key device in many large volume communication systems. Manufacturable technology is well established for the 0.8 mm wavelength range but unfortunately fiber communication requires at least 1.3 mm wavelength. We are studying alternative approaches to reach this goal including DBR mirror technology, active layer material (GaInAs or GaInNAs).

Photonic crystals is one of the hottest scientific topics today. Crudely speaking, a photonic crystal could make it possible to handle light in much the same way as we handle electrons in a semiconductor crystal. Realization of photonic crystals in semiconductors would open the way towards photonic integration with ensuing increases in functionality and compactness by several orders of magnitude. The primary fabrication technology is dry etching of nanometer sized holes with >1:10 aspect ratio. We have developed such processes for InP and shown some of the worlds best results in this material. The results pave the way for direct integration with active devices such as edge emitting long wavelength lasers.

With increasing complexity and decreasing dimensions of structures the need for advanced characterization is obvious. Methods to characterize a whole range of properties of nanoscale structures have to be developed. We have focused on scanning probe techniques, in particular scanning capacitance measurements which provide information about the electrical properties which are crucial to semiconductor structures. In addition, the use and interpretation of more common tools such as high resolution x-ray diffraction is continuously refined and applied to all of the project areas mentioned above.

Last updated 2005-01-27 by rmlo