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Raman Investigation of Silicon Nanocrystals: Quantum Confinement and Laser-induced Thermal Effects

Author: DuanYu
Tutor: ShenWenZhong
School: Shanghai Jiaotong University
Course: Condensed Matter Physics
Keywords: Silicon Nanocrystals Thermal Effect Quantum SizeEffect Raman Spectra and Laser Density
CLC: TB383.2
Type: Master's thesis
Year: 2012
Downloads: 91
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Abstract

This paper concentrates on the quantum confinement and laser-inducedthermal effects to the Raman spectroscopy of silicon nanocrystals. In thispaper, micro-Raman with a spatial resolution of micrometer has beencarried out on hydrogenated nanocrystalline silicon (nc-Si:H) thin film ofdifferent sizes under various laser power. The phonon dynamic propertieshave been analyzed in this paper.First we review the studies of the silicon nanocrystals, including thetypes of silicon nanocrystals, as well as the mechanisms of growth andanalysis instrument. We report the principle and selection rule of Ramanspectroscopy, together with some types of Raman spectroscope. ThoughRaman analyses, we can get information about the structures of siliconnanocrystals prepared by PECVD system,i.e., the average grain sizes,crystalline volume fraction and strain of the films.Nanocrystalline Si thin film, a mixed-phase material consisting ofnanocrystal embedded in amorphous tissue, have attracted much attentionin recent years because of their importance in the fundamentalunderstanding of physics and potential applications in siliconoptoelectronics. It should be noted that there is no detailed study on therelationship between the Raman spectra related to the nanocrystal sizeand the thermal effect due to the laser irradiation. To investigate the properties of crystal vibration,temperature-dependent Raman scattering has been measured on siliconnanocrystals. In fact, Si nanostructures are embedded in an environmentmedium, which can have poor thermal conductivity. This establishes thebase for the experimental determination of the local temperature throughmonitoring the Raman band shift and the Stokes/anti-Stokes intensityratio (IS/IA) under different laser power densities. Therefore, therelationship between lattice vibration and thermal effect can be achievedstraightforwardly. Moreover, a comprehensive investigation of thethree-phonon and four-phonon decay processes under laser irradiation isvery important for the understanding of the phonon dynamic behavior insemiconductor nanostructures.In this dissertation, we have presented that the downshift offrequencies and broadening of linewidth are mainly due to theanharmonic effect. Through detailed theoretical modeling, we havedetermined the decay process of the TO2. The anharmonic constantsincrease with crystal sizes and the lifetime reduces with increasinganharmonicity. Furthermore, it is found that the contribution offour-phonon process increases while that of three-phonon precess reduceswith increasing hole density, which can be attributed to the increase ofsilicon nanocrystals DOS. We predicted theoretically the quantumconfinement effect mainly for sizes smaller than ~6nm.However, ourresearch about the two contributions to the Raman shift and linewidth arequalitative analysis, it still needs further investigations to get moreaccurate conclusion.This dissertation tightly combines the application of investigationmethod on semiconductor and the analysis of obtained physical characteristics of these semiconductor materials. On the one hand, wecarefully explore how to employ the modern spectroscopy methods tostudy semiconductor materials; and on the other hand, the obtainedphysical properties of semiconductors are very important for furtherstudies or applications, which have been deeply analyzed in thisdissertation. From the viewpoint of the investigation methodology, wehave well combined the theoretical analysis together with the experimentalresults, that is, the most suitable theories and models are selected tosimulate the experimental data, while the experiments are designed basedon the requirement and guidance of the theories. Therefore, the experimentdata are more reliable and the theoretical analysis is more appropriate. Allthese achievements have been published in SCI journals.This work was supported by the National Major Basic ResearchProject of 2012CB934302, and Natural Science Foundation of Chinaunder contracts 10734020, 11074169, and 11174202.

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