Morphology and optical properties of porous stuctures on the basis of II-VI semiconductor compounds
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StatusThe thesis was presented on the 30 July, 2009Approved by NCAA on the 1 October, 2009 Abstract – 0.48 Mb / in romanian |
Keywords
porous ZnSe, porous CdSe, semiconductor nanotemplates, anodic etching, photoluminescence, Raman spectrum, electrochemical deposition, metal nanotubes.Summary
Dissertation work is devoted to the development of technology for the fabrication of CdSe and ZnSe porous layers by electrochemical etching, as well as the study of photoelectric and optical properties. The influence of such parameters as the concentration of electrolyte, anodization voltage and the concentration of free carriers on the morphology of porous CdSe was studied. It was found that in the CdSe samples with low conductivity under the influence of ultraviolet light, the nucleation of pores is more homogeneous. The technology for obtaining of CdSe porous layers in a neutral electrolyte based on NaCl was proposed and developed. Thermal annealing of ZnSe crystals in the melt of Zn with Al concentration from 0 to 40 % was found to modify the concentration of free carriers and, as a result of electrochemical etching, porous ZnSe layers with diameter of pores ranging from 40 nm to 1 μm, transparent in the visible region, were obtained.
It was found that CdSe and ZnSe are characterized by the propagation of pores along the current lines in the process of growth and, in contrast to the elementary semiconductors (Si, Ge) and III-V compounds (GaP, GaAs, InP, etc.), they do not exhibit growth of pores along certain crystallographic directions. A regularity was evidenced, according to which the anisotropy of the electrochemical etching of semiconductor material decreases with the increase of the ionic component of chemical bonds.
The exploration of photoluminescence spectra of porous CdSe showed that the decrease of the skeleton size down to 10–20 nm results in a blueshift of the excitonic emission lines by 10 meV which was attributed to quantum-size effects in the nanocrystalline CdSe porous skeleton.
The analysis of micro-Raman spectra revealed that the concentration of free electrons in the bulk ZnSe, calculated from the position of L+ mode, corresponds to the concentration derived from the Hall effect. At the same time analysis of L+ mode in porous ZnSe samples indicates a two-fold decrease in the concentration of free electrons in comparison with bulk material. The reduction of the concentration of charge carriers is due to the capture of free electrons by the energy states on the huge surface of the porous matrix.
Using micro-Raman spectroscopy and FTIR, the Fröhlich mode in porous ZnSe layers was identified. The position of the Fröhlich mode is found to be identical in porous samples with different diameters of pores and skeleton wall thicknesses, in accordance with the effective medium theory when applied to porous materials with identical semiconductor skeleton relative volume concentration.
The study of the relaxation of photoconductivity in porous CdSe membranes showed that the photovoltaic properties of porous cadmium sulfide are determined by potential barriers, the relief of the potential being controlled by the morphology and the degree of porosity of the material.
The thermal annealing in air of porous ZnSe matrices opened the possibility for obtaining porous ZnO layers with sufficient quality for random laser applications.
The technology for the fabrication of ZnSe-Pt metamaterial was elaborated, consisting of metal nanotubes embedded in semiconductor envelope with large bandgap. This nanocomposite material is promising for photonic applications.
The main results of the Doctoral Thesis were published in 19 scientific papers. The Thesis is written in Romanian and consists of 158 text pages, 94 figures, 4 tables and 187 references.
Oficial Reviewers
- Mihail Caraman
doctor habilitat, professor, Moldova State University - Mihail Iovu
doctor habilitat, professor, Institute of Applied Physics of the
Theses
