| Titre : | caractérisation des défauts profonds dans le silicium Amorphe hydrogéné et autres semiconducteurs photo-Actifs de type III-Vpar la méthode de photocourant constant:CPM |
| Auteurs : | Toufik Tibermacine, Auteur ; Amar Merazga, Directeur de thèse |
| Type de document : | Thése doctorat |
| Editeur : | Biskra [Algérie] : Faculté des Sciences Exactes et des Sciences de la Nature et de la Vie, Université Mohamed Khider, 2011 |
| Format : | 1 vol. (146 p.) / couv. ill. en coul / 30 cm |
| Langues: | Anglais |
| Mots-clés: | a-Si:H ; c-Si:H ; SI-GaAs:Cr ; ac-CPM ; dc-CPM ; Optical Absorption Spectrum ; Deep Defect Density ; Light Soaking |
| Résumé : |
We present in this thesis the optical and electronic properties of a number of semiconductor materials namely undoped and P-doped hydrogenated amorphous silicon a-Si:H prepared by Plasma Enhanced Chemical Vapour Deposition (PECVD), hydrogenated micro-crystalline silicon (c-Si:H) prepared by Very High Frequency Plasma-Enhanced Chemical Vapor Deposition (VHF-PECVD) and semi-insulating Cr-doped GaAs (SI-GaAs:Cr) prepared by the Liquid Encapsulated Czochralski (LEC) method. Sub-band gap optical absorption spectra (h) of all samples have been measured by the constant photocurrent technique in dc and ac excitation (dc-CPM and ac-CPM). Then, these absorption coefficients are converted into electronic density of states (DOS) distribution within the mobility gap by applying the derivative method of Pierz et al. We present in this thesis the relationship between the optic al excitation frequency and the optical and electronic properties of semiconductors materials in particular a-Si:H, c-Si:H and GaAs. We have developed a code program to simulate the dc and ac-CPM sub-band-gap optical absorption spectra. This numerical simulation includes all possible thermal and optical transitions between extended states and gap states. Our numerical results shows that (i) a discrepancy between dc mode and ac mode in absorption spectrum and gap state distribution particularly in defect region; (ii) extraction of DOS distribution using ac mode is better than using dc mode specially at high frequency (iii) DOS distribution can be reasonably reconstructed over a wide range of energy, especially at ultra high frequency, using both n(h) p(h) corresponding to optical transitions associated with free electrons and free holes creation, respectively. In addition and to validate our simulation results, we have measured (h) for all samples at several frequencies. Our experimental results prove the simulation ones and showed that a significant difference between dc- and ac-absorption spectra is observed in defect region and that the determination of the density of the occupied states within the gap mobility of the material is better for high frequencies than for low frequencies. The evolution of the sub-band-gap absorption coefficient (h) and the CPMdetermined density of gap-states distribution within the gap versus the illumination time leads to: (i) an increase in the deep defect absorption without any significant changes in the Urbach tail (exponential part), (ii) a presence of more charged than neutral defects as predicted by thevi defect pool model, and (iii) a saturation point of the degradation of both optical absorption coefficient and density of deep states of slightly P-doped sample measured by dc-CPM. The constant photocurrent technique in dc-mode as a spectroscopy method for the defect distribution determination is, therefore, most reliable to study the light soaking effect on the stability of hydrogenated amorphous silicon layers used in solar cells manufacturing. The constant photocurrent method in the ac-mode (ac-CPM) is also used in this work to determine the defect density of states (DOS) in microcrystalline silicon (c-Si:H) and to investigate the defect levels of semi-insulating Cr-doped GaAs from the optical absorption spectrum. The microcrystalline absorption coefficient spectrum (h) is measured under acCPM conditions at 60Hz and then is converted by the CPM spectroscopy into a DOS distribution covering a portion in the lower energy range of occupied states. By deconvolution of the measured optical absorption spectrum of SI-GaAs: Cr, we have extracted the distribution of the deep defect states. Independently, computer simulations of the ac-CPM for both materials are developed. Using a DOS model for microcrystalline which consistent with the measured ac-CPM spectra and a previously measured transient photocurrent (TPC) for the same material, the total ac-(h) is computed and found to agree satisfactorily with the measured ac-(h). Using a DOS model for gallium arsenide which consistent with the measured ac-CPM spectra and a previously measured modulated photocurrent (MPC) for the same material, the total ac-(h) is computed and found to agree satisfactorily with the measured ac-(h).The experimentally inaccessible components n(h) and p(h), corresponding to optical transitions associated, respectively, with free electron and free hole creation, are also computed for both semiconductors. The reconstructed DOS distributions in the lower part of the energy-gap from n (h) and in the upper part of the energy-gap from p (h) fit reasonably well the DOS model suggested by the measurements. The results are consistent with a previous analysis, where the sub-gap ac-(h) saturates to a minimum spectrum at sufficiently high frequency and the associated DOS distribution reflect reliably and exclusively the optical transitions from low energy occupied states. |
| Sommaire : |
Title i Acknowledgements ii Abstract v Table of contents viii Chapter 1. General introduction 1.1. Introduction 1 1.2. Background and Motivation 3 1.3. Objectives and outline of the thesis 6 References Chapter 2. Introduction to amorphous semiconductors 2.1. Introduction 11 2.2. Fundamental properties of hydrogenated amorphous silicon 11 2.2.1. Structural properties 11 2.2.2. Electronic structure 18 2.2.3. Effects of doping 22 2.2.4. Metastability and Staebler-Wronski effect 24 2.2.5. Optical properties of a-Si:H 25 2.2.6. Mechanisms of transport 29 2.2.7. Phenomena of recombination 30 2.3. Fundamental properties of hydrogenated micro crystalline silicon 31 2.3.1. Deposition methods of hydrogenated micro crystalline silicon 31 2.3.2. Atomic structure 31 2.3.3. Electrical and optical properties 32 2.3.4. Advantages of μc-Si:H 33 Referencesix Chapter 3. Characterization techniques of amorphous semiconductors 3.1. Introduction 37 3.2. Dark conductivity 38 3.3. Steady state photoconductivity 40 3.4. Constant photocurrent measurement 44 3.5. Photothermal deflection spectroscopy 49 3.6. Dual beam photoconductivity 52 References Chapter 4. Constant photocurrent technique: Theory and modeling 4.1. Introduction 57 4.2. Density of states distribution models for a-Si:H 57 4.3. Trapping and recombination 63 4.4. Absorption and generation 65 4.5. Function occupancy 70 4.5.1. Thermal equilibrium 70 4.5.2. Steady state equilibrium 71 4.5.3. Dynamic equilibrium 73 4.6. Implementation of numerical model 74 4.6.1. Steady state equilibrium: DC contribution 74 4.6.2. Dynamic equilibrium: AC contribution 76 4.6.3. Deconvolution procedure 78 Referencesx Chapter 5. Constant photocurrent technique: Results and discussion 5.1. Introduction 81 5.2. Experimental procedure 82 5.3. Dark conductivity and activation energy measurements 85 5.4. Reflectance-Transmittance measurements 88 5.5. Optical absorption spectrum measurement: Constant Photocurrent Method 91 5.6. Density of states 95 5.7. Results and discussion 96 5.7.1. Hydrogenated amorphous silicon a-Si :H: Undoped sample Intersolar ISB4 96 5.7.2. Hydrogenated amorphous silicon a-Si :H: Doped sample : P272 104 5.7.3. Hydrogenated micro-crystalline silicon c-Si :H : OCC354 sample 109 5.7.4. Semi-insulating Cr doped Gallium arsenide :SI- GaAs :Cr sample 1713 117 5.7.5. Ultra high frequency effect 121 5.7.6. Light Soaking effect 123 References Chapter 6. Summary and conclusions 6.1. Conclusions 129 6.2. Suggestions for further work 132 Appendix A 13 |
| En ligne : | http://thesis.univ-biskra.dz/3493/1/Thesis%20%20Toufik%20Tibermacine.pdf |
Disponibilité (2)
| Cote | Support | Localisation | Statut |
|---|---|---|---|
| TPHY/06 | Théses de doctorat | bibliothèque sciences exactes | Consultable |
| TPHY/06 | Théses de doctorat | bibliothèque sciences exactes | Consultable |
