|
|
Gamma- ray and X- ray detection by detectors based on (CdZn)Te single crystals.
Drobný, Miloslav ; Belas, Eduard (advisor) ; Grill, Roman (referee)
In this thesis, the influence of metal contacts, prepared on semiconductor detectors made of high resistive Cd0,85Zn0,15Te, on the quality of detected signal was investigated. The goal of thesis was to prepare ohmic contacts. Two materials (gold, platinum) and two methods of the preparation (chemical deposition, evaporation) were tested. It was found that improved detection properties were obtained on detectors prepared with gold contacts. Current-voltage characteristics of these contacts were found to be pseudoohmic. Moreover, when the Au contacts are prepared by evaporation, the value of the surface leakage current is lower.
|
|
|
Magnetooptical properties of semiconductor quantum structures
Uxa, Štěpán ; Grill, Roman (advisor)
Title: Magnetooptical Properties of Semiconductor Quantum Structures Author: Štěpán Uxa Department: Institute of Physics of Charles University Supervisor: Assoc. Prof. Roman Grill, PhD Supervisor's e-mail address: grill@karlov.mff.cuni.cz Abstract: In this theoretical work, a detailed study of optical properties of excitons in double quantum wells subject to magnetic and electric fields is presented. Starting from the well-known Luttinger formulae describing the real valence-band structure of III-V semi- conductor compounds, we developed an efficient way to solve the Schrödinger equation of a coulombically-bound pair electron-hole forming an exciton. Derived formulae were illus- trated on relevant figures, giving us an opportunity to better understand the effects of external fields on the studied system. Dispersion relations, the shift of energy levels in an electric field, absorption and photoluminescence spectra, and charge density distribution were calculated and achieved results were discussed in detail. Keywords: exciton, double quantum well, coupled quantum wells, Luttinger (-Kohn) hamiltonian, valence-subband mixing
|
|
|
High mobility two-dimensional electron gas in CdTe quantum wells:High magnetic field studies
Kunc, Jan ; Grill, Roman (advisor) ; Smrčka, Ludvík (referee) ; Knap, Wojciech (referee)
KurHigh mobility two-dimensional electron gas in CdTe quantum wells: High magnetic field studies Experimental studies of two-dimensional electron gases confined in CdTe and CdMnTe quantum wells are presented. The data analysis is supported by numerical calcula- tions of the band structure of confined states, using the local density and envelope func- tion approximations. Four by four, k.p calculations have been performed to justify the parabolic approximation of valence bands. Samples were characterized by Raman scatter- ing spectroscopy and far infrared cyclotron resonance absorption measurements. Low-field magneto-transport shows the dominant contribution of the semi-classical Drude conduc- tivity and three orders of magnitude weaker contributions of weak localization, electron- electron interaction and Shubnikov-de Haas oscillations. The contribution of electron- electron interactions is explained within a semi-classical model of circling electrons. The shape of Landau levels, broadening, transport and quantum lifetimes and dominant long- range scattering mechanism have been determined. High-field magneto-transport displays fractional quantum Hall states at Landau levels N = 0 and N = 1. The ground states 5/3 and 4/3 have been determined to be fully spin polarized, in agreement with the approach of composite...
|
|
|
Zeemanův jev v polovodičových kvantových strukturách
Stráský, Josef ; Grill, Roman (advisor) ; Výborný, Karel (referee)
This theoretical thesis presents detailed study of negatively charged excitons - trions - confined in single quantum well in presence of perpendicular magnetic field. Complex valence band of GaAs/GaAlAs compound is described within Luttinger Hamiltonian framework. Singlet and triplet states of negative trion are introduced. Advanced theoretical analysis of Zeeman effect for different states of trion is performed. Landau gauge of magnetic field and unusual wavefunctions basis is chosen and its accuracy is tested. Evolution of ground state energy and photoluminescence spectra with magnetic field is evaluated for different values of Landé g-factors. Probability of occurrence of electrons with respect to the hole position and their spatial correlation function are investigated.
|
|
| |
|
| |
|
| |
|
| |
|
|
Luminescence spectroscopy of two-dimensional quantum structures in GaAs/AlGaAs system
Grohoľová, Adela ; Grill, Roman (advisor) ; Oswald, Jiří (referee)
The aim of this work is the study of photoluminescence properties of GaAs/Al0.33Ga0.67As double quantum well. Low-temperature luminescence spectra of this sample are measured in dependence on electric and magnetic eld and dierent excitation power. The temperature dependencies of photoluminescence especially of the indirect excitons in in-plane magnetic eld are gauged as well. The simple model of localized indirect excitons is discussed to explain the discrepancy concerning the damping of indirect exciton photoluminescence in in-plane magnetic eld. The eective g-factors of indirect, neutral and charged excitons are calculated from observed Zeeman splitting. Few simple models are proposed to explain the behavior of eective g-factors. The possible agreement or contradiction with other published experimental data is discussed.
|
|
| |
guest ::
RSS feed.