Questions and answers for all topics
Chapters about nanomaterials
Technology of nanomaterial preparation, AIIIBV heterostructures (QD, QW) - preparation, properties, applications - Eduard Hulicius,
Questions and answers
Heterostructures: Semiconductor heterostructures in some of devices (you can choose one)?
New effects – tunnel diode, quantum cascade laser, Ohm normal (quantum Hall effect), ...
Fundamental improvement of parameters – LD (CW at room temperature), LED (high efficiency, colors), planar waveguides, ... Localization of electrons and holes and light, ...
Technology in general: Name three main reasons for using of epitaxial technologies!
It is possible to prepare material of better quality than from melted material – the lower temperature the lower entropy and the lower solubility of undesired impurities).
Higher reproducibility of the heterostructure preparation – more controlled structure= better devices Possibility of nanostructure preparation – new effects.
Possibility of separation of photons and electrons in the device structure – LD, LED.
Possibility of separation of electrons and their donors – high mobility = HF devices.
In-situ nanocharacterization and diagnostics: Describe difference between RAS and RHEED!
RAS = Reflectance Anisotropy Spectroscopy, is optical non vacuum method, which is suitable despite much bigger size of photons than lattice constant. It is working because of „full surface“ atom arrangement during different stages of layer or structure growth and polarized photons can „see“ surface arrangement of atoms. RAS can monitor growth of individual monolayers via ML oscillation. It can give information about QD and QW growth. But it has lower resolution than vacuum RHEED.
RHEED = electron vacuum method of the surface study, it can work only in the ultrahigh vacuum, there are possibility of surface study by electron methods, possibility of monitoring of growth of individual monolayers via ML oscillation. It is possible to prepare sharp and define heterojunctions layer thickness is controlled with accuracy of fractions of ML, because the electron size (= de Broglie wavelength – space of probability of electron location) is comparable with crystal lattice constant.
Photovoltaic conversion of solar energy and the use of nanostructures - Martin Ledinský
Questions and answers
The use of nanostructured materials for thin-film solar cells applications
Preparation of thin films and observation of their inner structure. The motivation for using nano-characterization methods.
Principles of scanning probe microscopy (SPM).
SPM Modes for characterization of nanostructures and their electronic properties: Scanning tunneling microscopy (STM), conductive atomic force microscopy (C-AFM), Kelvin force microscopy (KFM) and near-field optical microscopy (SNOM). How the surface state affects the measurement. Combination of SPM technique with scanning electron microscopy (SEM) or optical spectroscopic methods (micro-Raman spectroscopy, luminescence spectroscopy).
Question A: What is the main advantage of nanostructured thin-film cells compared with crystalline silicon based?
Question B: How the multi-junction solar cells work and what is their main advantage?
Surfaces and interfaces in nanostructures - Pavel Jelínek, Martin Švec
Elementary concepts of crystallography, types of 3D periodic lattices. Description of crystal planes, effect of crystal cut on the symmetry of a surface, reconstruction. Electronic states in the bulk, metal, semiconductor, and insulator. Interaction of adsorbates with a surface, physisorption, chemisorption, precursor, diffusion. Diffraction, Bragg law, reciprocal space, Ewald construction. Description of electron density by harmonic analysis, Brillouin zone, local density of states.
Experimental methods for analysis of surfaces and interfaces. Low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS, UPS), photoelectron diffraction. Scanning probe microscopy (STM, dynamic AFM), scanning tunneling spectroscopy (STS). Photoelectron emission microscope (PEEM), low energy electron microscope (LEEM).
Atomic manipulation using STM or dynamic AFM. Calculations of electron density, interpretation of STM and AFM images.
Optical properties of the semiconductor nanomaterials – Jiří Oswald
Questions and answers
1. Describe difference between direct and indirect forbidden gap! Show examples of semiconductors with direct and indirect forbidden gap!
For semiconductors with direct forbidden gap maximum of valence band and minimum of conduction band are at the same position for the same k. Kval for maximum of valence band and kcond for minimum of conduction band are not the same for indirect semiconductors.
Direct semiconductors: GaAs, InAs, GaN, AlN (AIIIBV)
Indirect semiconductors: Si, Ge
2. Describe radiative channels in semiconductors!
Channels of radiative recombination in semiconductors are:
a) Free electron-hole radiative recombination (band-band recombination).
b) Radiative recombination of free electron (hole) to neutral acceptor (donor) e-A0 (h-D0).
c) Radiative recombination of donor-acceptor pairs (D0-A0).
d) Radiative recombination of electron-hole pairs (excitons X).
4. Why are optical effects much stronger in nanostructures than in bulk semiconductors?
a) Effective capture of nonequilibrium charge carriers /electrons and holes) in potential wells.
b) Higher overlap of wave functions of electrons and holes.
c) Higher bonding energy of excitons - they are observable at room temperatures also.
d) Impurity influence is suppressed.