My research is centered around advanced 2D materials – these include semiconductor freestanding membranes as well as materials like graphene and TMDs. I am interested in synthesis, fabrication, and application of these materials and structures with a long standing history of using molecular beam epitaxy, self-assembly, lithography, various microscopy techniques, x-ray diffraction and optical characterization.
Deterministic release and rearrangement of nanomembranes or thin films
In the last years, freestanding membranes were established as a building block for nanotechnology. Investigated structures include stretchable electronics, solar cells, wrinkled semiconductor membranes for various applications as well as rolled-up and folded three-dimensional micro- and nano-objects.
Rolled-up tubes as building blocks for nanotechnology
We are working on emerging device structures based on rolled-up membranes (supported by FAPESP and CNPq projects). In the last decade, these structures were used as optical resonators, sensors or suggested as x-ray wave guides. The structures are fabricated by growing a strained heterostructure by molecular beam epitaxy, defining an pattern using optical lithography followed by selective underetching releasing the layer system. The tube forms by self-assembly due to strain relaxation of the initial layer system. We want to understand, how the special strain state of the tube wall influences the physical properties of integrated light emitters, e.g. quantum wells. Using the the possibility to form hybrid radial superlattices of unique material combinations, we want to integrate new magnetic or plasmonic functionality. Our aim is to promote these structures for the future use as active device in semiconductor technology.
The scanning electron microscopy images above shows a rolled-up micro-tube with a quantum well integrated into the tube walls. The photoluminescence spectrum on the left demonstrates (obtained in collaboration with the GPO of the IFGW), how the emission of the quantum well shifts due to the roll-up process and strain relaxation.
Semiconductor membranes as virtual substrates for epitaxy
We overgrow of freestanding semiconductor membranes to investigate the influence of these compliant substrates to the formation of self-assembled nanostructures (supported by FAPESP and CNPq). We are interested to understand the growth dynamics during material deposition by molecular beam epitaxy . We investigate the surface diffusion by ex-situ methods like scanning electron microscopy and atomic force microscopy. We carry out x-ray diffraction to quantify the strain transfer from deposited material to the underlying substrate. Our goal is to demonstrate the possibilities of membranes as virtual substrate to tailor optical or electrical properties of heterostructures grown on top of them.
The image above depicts an AFM study of a overgrown, wrinkled InGaAs nanomembrane with InAs islands formed in the released areas of the sample (Nanotechnolgy 25, 455603 (2014)).
Growth of semiconductor hetero- and nanostructures
III-V heterostructures grown by molecular beam epitaxy are fundamental building blocks for research and technological applications. Being able to grow such structures requires a continuous development of the growth strategies to obtain structures with high optical or electrical quality.
Unstrained quantum dot structures
We have an ongoing research line for growing various kinds of III-V special tailored heterostructures for internal use or to provide for our collaborators. These include standard samples like GaAs quantum wells, InAs dots, and more advanced structures like Bragg reflectors or micro-cavity. Furthermore, we explore advanced heterostructures, and study – besides the membrane overgrowth – the fabrication of unstrained quantum emitters for optical applications or the fabrication of InAs flash dots for single photon sources.
The AFM images above depict structures made by Ga assisted deoxidation and local hole etching followed by the overgrowth with AlGaAs/GaAs/AlGaAs to develop unstrained mesoscopic GaAs structures (Nanoscale Research Letters 12, 61 (2017)).