Electronic transport in self assembled nanostructures for infrared devices

 

J. Smoliner

E. Gornik

 

 

In P08, nanowires (NWs) and nanocrystals shall be studied with respect to their usability as infrared detectors and emitters. The material systems, which we will focus on, are strained Ge-NWs and colloid nanocrystals.

In detail, we propose a direct band gap in ultra-strained Ge-NWs monolithically integrated into micromechanical modules. Ge is normally recognized as a poor light emitting material due to its indirect band structure. To tailor the optical properties of Ge, however, tensile strain can be used. It has been theoretically shown that Ge becomes a direct band gap material for a 2% tensile strain, and that the direct band gap of Ge will narrow down to 0.5 eV or a wavelength of 2.5 μm.

We explore a special setup enabling the application of pure tensile strain up to 5% without any shear component while measuring simultaneously the optical properties of thereby ultra-strained Ge-NWs. Attention will be focused initially on intrinsic Ge-NWs, on which there is considerable activity today. Later on in the project, the scope will be broadened to n-doped Ge-NWs as well as Ge/Si alloy-NWs. Thus, adequate strain and n-type doping engineering can effectively provide population inversion in the direct bandgap of Ge. The ultimate goal is to demonstrate an increased functionality of ultra strained NWs. We expect to obtain a general understanding that will lead to prototype devices such as light emitting device or detectors and can be extended in the future to other material systems.

The second main goal of the project is to further develop characterization techniques for single nanostructures on AFM platforms. The topic we want to concentrate on in this project is the investigation of the photo-capacitance properties of single nanostructures embedded in an insulating host material. Such samples are hardly accessible by photoconductivity or regular capacitance (C(V)) measurements. Measuring the sample capacitance as a function of the incident wavelength should give access to parameters as the band gap and eventually the energies of quantized levels inside the nanocrystals.

The application of AFM based methods also opens up the unique possibility to study the behaviour of nanostructures subjected to high AFM induced strain fields. Force dependent photocurrent investigations should enable a rather direct investigation of the strain distribution inside the material underneath the AFM tip. On NWs, it is straightforward to use an AFM tip either to pressurize a wire device, or alternatively to apply tensile strain. In this way, strain or pressure dependent photoconductivity studies are possible.

 

Report 2005-2008 (.pdf)

 

Report 2009-2012 (.pdf)

 

 

This project part ended with the end of phase II.