JARA-FIT Lab Course Nanoelectronics - Experiments

J1/J2: Fabrication/Characterization of a Si Device

Tutor: Q. Zhao

MOSFET (metal-oxide-semiconductor field effect transistor), the key device for integrated circuits, has been scaled down to nanometer regime. A high gate oxide capacitance is required in order to improve the electrostatics and the device performance. The traditional SiO2 which has been used as gate oxide for very long time should be as thin as <1nm for the nanometer device. High gate leakage currents by tunneling through such thin SiO2 layer degrade the device performance. Therefore, alternative oxides with higher permittivity have been investigated and used for nanometer MOSFETs. HfO2 deposited by atomic layer deposition (ALD) is now the standard high-k dielectrics for devices of today. In this project you will fabricate MOS devices in the HNF clean room. From the fabrication you will learn some basic semiconductor processing technologies, like optical lithography, dry and wet etching, metal deposition for contacts. The fabricated MOS devices will be characterized with capacitance-voltage (C-V) measurements. The goal of this project is to extract the dielectric constant, the equivalent oxide thickness (EOT), and the density of interface states by characterization of the fabricated MOS devices.

J4: Quantum Transport in Semiconductor Nanostructures

Tutor: Th. Schäpers

The experiment "transport in nanostructures" will be performed in a He-3 cryostat. This cryostat allows to reach temperatures below 1 K. By means of electrical transport measurements at low temperatures information on quantum states and scattering processes in nanostructures can be gained. The cryostat is equipped with an 8 T magnet, which allows to study the transport properties as a function of a magnetic field. During the experiment the working principle of a He-3 cryostat will be explained. The students will learn how to mount a sample into the cryostat and how to cool down the sample. Finally, magnetotransport measurement will be performed at various temperatures and magnetic fields.

J6: Redox Based Non-Volatile Memory Devices

Tutor: V.Rana, S.Hoffmann-Eifert

This experiment deals with current research on new materials and devices for next generation non-volatile memory technology. Redox-based random access memory (ReRAM) is considered a potential candidate not only for the new type of storage class memory (SCM) but also for an artificial synapse element in neuromorphic computing. These devices allow to realize beyond von-Neumann type concepts, like for example computation-in-memory, and provide an energy-efficient solution for future artificial intelligence systems. In contrast to the volatile charge-based dynamic random access memories (DRAM), the non-volatile ReRAM stores information in form of its resistance state. For the SCM applications, these states can be interpreted as “0” (high resistance state) and “1” (low resistance state), but also intermediate states, as required for the neuromorphic computation, are possible. Due to the high scaling potential down to a few nanometer, ultra-dense 3D passive crossbar arrays can be realized. During the experiment, we will demonstrate all required steps for manufacturing a ReRAM device, including the growth of ultrathin metal-oxide layers with Atomic Layer Deposition (ALD), patterning the device structure with optical lithography in a clean room, and subsequent DC (continuous) and AC (pulse) electrical characterization.

J10: Atomically Thin Semiconductors

Tutor: B. Kardynal

The aim of the project is to gain insight into optical techniques such as photoluminescence and reflectance measurements as non-invasive and powerful methods to investigate band structures and many body effects in semiconductor nanostructures. The experiments are performed on a WSe2 monolayer, a representative of the emerging material class of two-dimensional semiconductors. These materials are promising candidates for new optoelectronic devices, due to their rich excitonic physics resulting from strong spin-orbit interactions, very weak Coulomb screening, as well as a lack of inversion symmetry. The students will perform photoluminescence and reflectance spectroscopy measurements at liquid Helium temperatures in order to determine optical transitions of different excitonic states in the sample. The sophisticated experimental setup allows for measurements on the micrometer scale with varying excitation and detection parameters (laser intensity, light polarization, spectral and spatial filtering). Intrinsic material properties such as the optical and electronic band gap, exciton binding energies, optical selection rules and valley coherence will be derived from the results.

J11: X-ray Diffraction of Semiconductor Heterostructures

Tutor: G. Mussler

The aim of this practical course is to gain an insight into the x-ray diffraction technique to analyze crystal properties. In particular, the students will carry out a reciprocal space map on a SiGe pseudosubstrate in order to determine the Ge content and the degree of relaxation. X-ray reflectivity scans will be performed on a single Bi2Te3 film grown on a Si substrate to determine the thickness of the epilayer. Finally, a SiGe/Si superlattice will be analyzed by means of a 2/ scan to obtain the superlattice parameters. The students will subsequently analyze the x-ray data analytically as well as by means of a software.

J13: Hybrid Semiconductor Nanowires

Tutor: A.Pawlis

The goal of this practical course is to gain insight into the fabrication and characterization of self-organized nanowire (NW) structures grown by the powerful technique of molecular beam epitaxy (MBE). Using the unique possibilities of our ultra-high vacuum nanocluster to combine different common semiconductor materials, self-catalyzed GaAs/ZnSe core/shell NWs will be grown in-situ by selective epitaxy on pre-structured SiO2/Si-(111) substrates. Such semiconductor hybrid NWs represent novel botton-up nanostructures with strong potential for the realization of electrically or optically controlled modern devices for quantum technologies. After the NW growth, the second part of this course is focused on the optical characterization of the as-grown NW structures at room-temperature and 4 K by micro-photoluminescence (µPL) measurements. The results will be analyzed regarding the temperature dependence of the spectral response and intensity of the emission of the NWs. At low temperatures, quantum confinement effects are revealed by the µPL emission. Finally, at 4 K time-resolved single photon counting technique will be applied to investigate the lifetime of the relevant optical transitions in the core/shell NWs.

J14: Growth of Multifunctional Oxide Thin Films

Tutor: C. Bednarski-Meinke

One of the main components in nanoelectronics is the use of two dimensional thin films. Controlled and defect free growth of thin film heterostructures hold the key to its large scale technological applications. In the course of this experiment one gets to understand how thin films are grown and structurally characterized using both in-situ and ex-situ techniques. Oxide thin films, having multifunctional properties, e.g., ferroelectric, ferromagnetic, etc., will be grown epitaxial on single crystalline substrates using a state-of-the-art Molecular Beam Epitaxy (MBE) system. The surface morphology of the film/substrate heterostructure will be studied in-situ (i.e., during growth) using reflected high and low energy electron diffraction (RHEED and LEED) techniques. The prepared sample will then be characterized structurally ex-situ using X-ray diffraction (XRD) and reflectometry (XRR) with regards to its out-of-plane crystalline orientation, determination of the film thickness and roughness.

J15: Transmission Electron Microscopy

Tutor: A. Kovacs

Tranmission electron microscopy measurements are carried out in the Ernst Ruska-Centre using an FEI Tecnai electron microscope working at 200 kV. The aim is to study the morphology, structure and chemical composition of a thin layer of magnetic semiconductor that was deposited on GaAs substrate. The students will determine the chemical composition of the layer using X-ray energy dispersive spectroscopy. The specimen is pre-thinned using focused ion beam to a thickness of ~ 100 nm. During the experiments, the students will learn the basic operation of TEM e.g. to tilt the specimen using Kikuchi-lines to zone axis, to record bright-field and dark-field images, to record electron diffraction patterns that will be used to determine the zone axis and strain in the system, and to record high-resolution TEM images of the atomic columns of Ga and As.

J16: Spectro-Microscopy with Low-Energy Electrons (LEEM / PEEM)

Tutor: C. Kumpf

A “Low Energy Electron Microscope” (LEEM) and “Photo-Emission Electron Microscope” (PEEM) allows to investigate the surfaces of solids with a very high spatial resolution (better than 2 nm). In particular, it is possible to observe kinetic processes such as layer or crystal growth in situ and in real time. A broad variety of contrast mechanisms are available in LEEM/PEEM (amplitude and phase contrast, bright and dark field microscopy, work function and chemical contrast), which makes the method very flexible and applicable to many different sample systems.
In the practical exercise we observe the growth of an organic monolayer film on a metal surface in situ (i.e., during deposition of the molecules). After deposition, the geometric structure is investigated in detail by using electron diffraction and microscopy. Different rotational and mirror domains are identified and characterized. The final result is a full structural characterization of the submonolayer film.

J17: Scanning Probe Microscopy with Single Molecules

Tutor: C. Wagner

Low-Temperature Scanning Probe Microscopy (LT-SPM) is becoming the versatile tool of nanotechnology research. Using LT-SPM, single molecules and atoms can be imaged and manipulated with unprecedented precision. In the course of this lab course experiment the students will perform experiments using the combined Non-contact Atomic Force/Scanning Tunneling Microscope (NC-AFM/STM) operating at the base temperature of 5 Kelvin. The experiment will be conducted on a monolayer molecular film and single perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules adsorbed on an atomically clean Au(111) surface. The students will be able to get familiar with the most advanced techniques of high-resolution imaging as well as single molecule manipulation.

J18: Band Structure Imaging by Angle-Resolved Photoelectron Spetroscopy

Tutor: L. Plucinski

Angle-resolved photoelectron spectroscopy (ARPES) is the most powerful technique for imaging the electronic band structure of crystalline solids. The surface sensitivity of this technique requires the use of ultra-high vacuum while best results are obtained on mirror-like surfaces of materials that are not heavily insulating. Samples must be prepared under vacuum to avoid contamination. The goal of the practical exercise will be to characterize the electronic band structure of a topological insulator from the family of Bi2Se3, Bi2Te3, and Sb2Te3. The students will learn the surface preparation technique by exfoliation using the scotch tape. A prepared sample will be transferred to the analysis chamber and the ARPES spectra will be measured at room temperature using the He-I source (photon energy 21.2 eV) and the Xe source (photon energy 8.4 eV). In the second step the sample will be cooled down to approx. 20 K and the ARPES spectra will be measured again. During the data analysis, the students will learn how to convert the measured data into energy-momentum maps, how to disentangle the surface and bulk electronic structure, and how to estimate the Fermi velocity in the topological bands. They will also establish a relation between the sample temperature and the Fermi level broadening in ARPES spectra.