Das Seminar findet üblicherweise Donnerstags am
Wilhelm-Conrad-Röntgen Campus, Dep. Si Photovoltaik EE-IS im
Seminarraum 227, 12489 Berlin, Kekuléstr.5, im 1. OG. statt.
Beginn: 10:15 Uhr
(ab10:00 Uhr Kaffee und Kekse).
Dr. Boris Naydenov
“Electron Paramagnetic Resonance (EPR) – a spectroscopy tool for studying material and device properties”
EPR is a method for studying unpaired electrons (e. g. radicals or dangling bonds) with a very broad range of applications in physics and chemistry. This technique measures the electron spin, which allows to explore its local magnetic environment, both statically and dynamically at an atomic scale. In this talk the principles of EPR spectroscopy and its application to solid state physics and devices will be reviewed. The current progress in the Berlin Joint EPR lab (BeJEL) will be given, where our capabilities to perform EPR and Electrically Detected Magnetic Resonance (EDMR) at various magnetic fields will be presented. The two new highlights at our lab are the EPR on a chip (EPRoC) and single atomic quantum sensors in diamond. In the former project we develop in a collaboration with Uni Stuttgart a miniaturized EPR spectrometer, which has not only increased spin sensitivity, but can be utilized for in-situ and operando experiments. The latter topic deals with the nitrogen-vacancy color centers in diamond (NVs), an extraordinary physical system with unique properties. Single NVs can be observed optically and the emitted fluorescence depends on the state of the electron spin, allowing to perform single spin EPR (Optically Detected Magnetic Resonance, ODMR) at ambient conditions. This feature makes the NV a high sensitive sensor with nano-scale spatial resolution not only for magnetic fields, but also for electric fields, pressure and temperature. At the end of the talk the possible application of these methods for studying photovoltaic materials and devices will be discussed.
Dr. Heike Angermann
“Fifty years of Surface PhotoVoltage (SPV) investigations in Berlin Adlershof:
Chemical conditioning of Silicon substrates to improve surface morphology and electronic interface properties”
The deposition of ultra-thin films on Si surfaces requires the application of extremely clean, smooth, undamaged and well-conditioned substrate as starting point, depending upon on the details of the device structure. Therefore the monitoring and investigation of electronic interface properties received increasing attention in the recent years. A very sensitive method to determine the energetic distribution of rechargeable interface states Dit(E), measuring the surface photovoltage (SPV) as a function of the external bias voltages Uf, was described firstly in 1968 by K. Heilig  in Berlin Adlershof. Up to now, the contactless field-modulated SPV technique is employed in our institute as an excellent non-destructive and very surface-sensitive tool to provide detailed information on surface charges and interface state densities on Si substrates and devices. This seminar summarizes applications of SPV investigations for the optimization of electronic interface properties of Si devices and solar cells and reports also on approaches with spectral-dependent SPV techniques for the characterization of broad classes of other semiconductors.
SPV measurements were combined with spectroscopic ellipsometry (SE) measurements in the ultra-violet and visible (UV-VIS) region, with Fourier-Transform infrared ellipsometry (FTIR SE), scanning electron microscopy (SEM), and atomic force microscopy (AFM) to establish correlations between preparation induces micro-roughness, interface charge and the energetic distribution of defect densities Dit(E) on differently treated polished Si(111) and Si(100) interfaces as well as on solar cell substrates. In cooperation with various universities, institutes and companies the results of these investigations were successfully used to improve wet-chemical Si surface conditioning methods for different solar cell applications:
By close collaboration with CiS Erfurt saw damage etch, texturization, cleaning and passivation procedures for a-Si/c-Si Heterojunction solar cells were optimized (J. Kegel, HTW Berlin). Moreover, a wet-chemical oxidation method in diluted HCl solutions prior to the deposition of all-PECVD AlOx/a-SiNx passivation stacks was developed in order to replace cost intensive pre-treatments in solar cells manufacturing (A. Laades, CiS Erfurt). As an alternative to traditional approaches with concentrated acidic and alkaline solutions RCA and H2SO4/H2O2, we also investigated the application of ultra-pure deionized water (DIW) with dissolved ozone (O3) with admixtures of hydrochloric acid (HCl) and fluoric acid (HF), to reduce chemical consumption and disposal costs as well as to improve cleaning efficiency of solar cell substrates (MKS ASTex GmbH, Fraunhofer ISE Freiburg). In cooperation with the University of Freiburg, IMTEK, advanced oxidation techniques were analyzed by combined x-ray photoemission (XPS) and SPV investigation. Optimizing a rapid thermal oxidation (RTO) and subsequent forming gas annealing (FGA) process, chemically abrupt SiO2/Si interfaces with low Dit could be also be fabricated either at low temperatures, at short times, resulting in each case in excellent interface passivation (B. Stegemann, HTW Berlin).
 K. Heilig, Experimentelle Technik der Physik 14 (1968) 135