Nanotechnology and nanoscale materials and devices exhibit fascinating prospects for innovations in the 21th century. Ultrathin layers or tiny crystals show extraordinary properties which can be tailored in a controlled way to match the requirements for new electronic applications. Within this promising research field, the design of nanoscale materials and devices as well as their microscopic understanding are key issues. Applications in micro- or optoelectronics are more and more dominated by nanostructures. In addition, also completely new research fields, like, e.g., spintronics (the use of the spin degree of freedom as an information carrier in semiconductors) or quantum information technology (the use of quantum mechanical properties for information processing) have emerged.
Within this field, the research activities at the chair of electronic materials and nanostructures can be divided into the following areas:
Graphene and 2D-Materials
Graphene and 2D semiconductors have emerged in recent years as extremely promising candidates for novel electronics and optoelectronics based on 2D materials. Graphene is now being produced industrially in large quantities and on a large scale, and already finds application in nanotechnology and materials technology, as well as in devices. In contrast, 2D semiconductors are still in the phase of intensive research and the investigation of possible applications.
Attractive features of 2D materials are, for example, their high strength combined with flexibility, as well as the possibility of producing complex material combinations, structures and functionalizations using a wide variety of manufacturing processes. In addition, there are interesting properties, such as excellent electrical and thermal conductivity in graphene and high binding energies between electrons and holes in 2D semiconductors.
Our research area includes the fabrication of graphene on various substrates up to 4 inches in diameter and the use of graphene e.g. as a large-scale transparent contact material. Similarly, we are exploring large-area transition metal dichalcogenides (TMDC) as 2D semiconductors. These large-area 2D materials can no longer be fabricated using scotch tape and "exfoliation" of graphite or crystals, but require industrial processes such as chemical vapor deposition (CVD). In various cooperative projects, we optimize the fabrication processes with respect to low-defect 2D materials, embed them in device architectures and use them e.g. for ultrathin and flexible light emitters and photodetectors.
Nano-Optoelectronics: Materials and Devices
Modern optoelectronic devices contain functional layers and structures with extensions down to a few nanometers. Examples are zero-dimensional semiconductor quantum dots, one-dimensional nanowires or two-dimensional quantum wells. If the size of semiconductor crystals is reduced down to a few nanometers in at least one dimension, the energy states will be subject to the laws of quantum mechanics. The modified density of states and the possibility of adjusting the bandgap simply by size enable fascinating prospects for innovative applications. Efficient light emitters and solar cells or devices, which deliver ‘photons on demand’ are examples.
Our goal is to understand the fundamental electronic and optical properties of such nanomaterials with respect to future applications. Based on a detailed physical understanding, such quantum materials will be embedded into device architectures in our clean room in order to elaborate the application potential in optoelectronics. Our multiple analysis techniques – electron microscopy, nano-optics and scanning force microscopy – give us the opportunity to visualize the current and potential distribution inside the devices and relate them to the local optical and structural properties. This finally allows a correlation between material properties and device functionality
Spintronics and Magnetic Semiconductors
In microelectronics, the charge of the electron is used as an information carrier. An innovative concept in information technology will in addition use the quantum mechanical property ‚spin‘ for the storage and transmission of information. Modern read heads in hard disks are e.g. based on the fact that the current transport in nanometer thin ferromagnet-insulator heterostructures depends on the relative orientation between the electron spin and the magnetization of the metallic ferromagnets. ‚Spintronics‘ in a narrow sense denotes the usage of the spin degree of freedom in semiconductors.
Our goal is to combine electronic, optical and magnetic functionality in novel nanomaterials. We investigate magnetically doped semiconductor nanostructures, like quantum dots, clusters or nanoribbons, with the intention to control magnetism and magneto-optics by optical and/or electrical stimulation. Micro-coils are used to achieve local magnetic fields, switchable on a sub-nanosecond time scale for dynamically controlling spin states in semiconductors. The research activities in our group cover the design and development of suitable micro- and nanoscale systems in our clean room and their analysis with modern techniques of time- and spatially resolved magneto-optics.