The research of the Group-IV Heterostructures group is dedicated to Si-based semiconductor heterostructures. Growth, structural, electrical and optical properties of these heterostructures are investigated, and their application potential is assessed by processing them into electronic, spintronic and optical nanodevices.
The heterostructures are grown in a Si molecular beam epitaxy reactor (Riber Siva 45), which provides the group-IV matrix materials Si, Ge, C and Sn. Almost arbitrary n and p doping profiles are realized with Sb and B doping sources, respectively. Molecular beam epitaxy (MBE) is a physical vapor deposition technique, for which matrix and doping atoms or molecules are deposited epitaxially in an ultra high vacuum. Heating the substrate to moderately high temperatures (100 - 600°C) assures sufficiently high surface mobility of impinging atoms or molecules to form a perfect continuation of the underlying Si or SiGe substrate lattice. The advantage of this conceptually very simple technique lies in its excellent control of layer thicknesses (down to single atomic layers) and layer composition (from arbitrary alloys to controlled doping in the ppb range). Compared to the CVD (chemical vapor deposition) techniques, which are usually employed for industrial applications, MBE offers a much higher flexibility with respect to growth rates and temperatures. Thus, the technique is especially suitable for a fast implementation and optimization of novel layer sequences and heterostructure concepts. The versatility of MBE allows a broad spectrum of topics, e.g.:
- Self-organization phenomena for the creation of quantum dots during epitaxial growth
- Deposition of Si1-xGex and Si1-x-yGexCy quantum wells for optical and electrical investigations
- Growth of direct-bandgap Ge1-xSnx alloys
- Growth of complete layer sequences for transport- and optoelectronic device- and for spintronics applications
For the characterization of epitaxial layers a wide variety of analytical techniques is available within the institute. Surface morphologies are examined with atomic force microscopy (AFM). Structural properties, layer compositions and interfacial morphologies of heterostructures can be determined by x-ray diffraction (XRD), with transmission electron microscopy (TEM) and with electron dispersive x-ray spectroscopy (EDX). The investigation of optical properties is conducted with a photoluminescence setup, and several Fourier-Transform spectrometers for the near and middle infrared. Transport measurements under high magnetic fields (up to 16T) and at cryogenic temperatures (down to 300mK) can be carried out and provide fundamental information on intrinsic electronic and spin properties of the epitaxial layer sequences. Standard techniques such as Hall measurements, a parameter analyzer for measurements of I-V and C-V characteristics or a facility for DLTS (deep level transient spectroscopy) are also implemented. Two-dimensional electron gases in strained Si channels are also investigated by electron spin resonance (ESR).
The cleanroom of our institute offers all technological and analytical tools for the manufacturing of nanodevices based on group-IV heterostructures. A key installation for these means is an electron-beam lithography tool with a highly accurate laser stage for the implementation of nanostructures with feature sizes and overlay accuracies in the 10nm range.
Combining self-organized Ge quantum dot growth with lithographic nano-patterning of the substrates allows us to realize perfectly site-controlled quantum dot light emitters in the near infrared frequency range between 1.3 and 1.5 µm. In this way, we demonstrated, e.g., photonic crystal resonators with single and multiple quantum dot light sources positioned with an accuracy of better than 20nm within the cavity of the resonator. The long term aim of this research field is the implementation of group-IV light sources in the telecom frequency band that can be monolithically integrated into standard CMOS devices.